Metal Clusters in Chemistry
Edited by P. Braunstein L. A. Oro P. R. Raithby
Further Titles of Interest
A. Togni, R. L. Halterman (Eds.) Metallocenes Synthesis, Reactivity, Applications (2 Volumes) 1998.3-527-29539-9 Industrial Inorganic Products An Ullmann’s Encyclopedia (6 Volumes) 1998, 3-527-29567-4 M. Beller, C. Bolm (Eds.) Transition Metals for Organic Synthesis Building Blocks and Fine Chemicals (2 Volumes) 1998.3-527-29501-1 E. C. Constable Metals and Ligand Reactivity 1996, 3-527-29277-2
Metal Clusters in Chemistrv J
Edited by
P. Braunstein L. A. Oro P. R. Raithby
Weinheim New York Chichester Brisbane Singapore Toronto
Prof. Dr. P. Braunstein Lab. de Chimie de Coordination UniversitC Louis Pasteur 4, rue Blaise Pascal F-67070 Strasbourg CCdex
Prof. L. A. Oro Dept. of Inorganic Chemistry I. C. M. A. - Faculty of Science University of Zaragoza-CSIC E-50009 Zaragoza
Dr. P. R. Raithby Dept. of Chemistry University of Cambridge Lensfield Road GB-Cambridge CB2 1EW
This book was carefully produced. Nevertheless, editors, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Cover picture: Prof. Dr. A. Miiller, University of Bielefeld, Germany Library of Congress Card No.: applied for
A catalogue record for this book is available from the British Library. Deutsche Bibliothek Cataloguing-in-Publication Data: Metal clusters in chemistry / ed. by P. Braunstein ... - Weinheim ; New York ; Chichester ; Brisbane ; Singapore ; Toronto : Wiley-VCH 1999 ISBN 3-527-29549-6
0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Asco Typesetters, Hong Kong. Printing: betz-druck gmbh, D6429 1 Darmstadt. Bookbinding: Wilhelm Osswald & Co., D-67433 Neustadt. Printed in the Federal Republic of Germany.
Preface
Metal clusters occupy a central role in chemistry. Aspects of cluster chemistry impinge on branches of the subject ranging from organometallic chemistry, through coordination chemistry, homogeneous and heterogeneous catalysis to solid-state chemistry and catalysis. Research in metal clusters is an interdisciplinary enterprise that has exploded rapidly in importance over the past twenty years because metal clusters exhibit increasingly interesting bonding as well as electronic, structural and physical, and chemical and catalytic properties. Several reviews and books have appeared in the past emphasising the key role that metal clusters play in chemistry, materials science, nanotechnology and catalysis, but no up to date single source dealing with the recent developments in metal cluster chemistry is currently available. The purpose of this book is to provide a general source of references for workers including experimental and theoretical chemists and physicists in this interdisciplinary field, and covering both fundamental and applied science. It is, however. also directed at graduate students and scientists who have not previously been involved in research into metal clusters. The vast majority of the metal clusters considered in this book follow the definition offered by Cotton and others, namely, complexes containing three or more metal atoms held together, at least in part, by metal-metal bonds, although sometimes for the sake of convenience dinuclear complexes with metal-metal bonds are included. Thus, the existence of metalmetal bonds distinguishes cluster complexes from polynuclear cage complexes. This book originates from the Scientific Network of the European Science Foundation (ESF), which was initiated in 1992, with the aim of promoting the interactions between European laboratories working in the field, as well as stimulating multidisciplinary efforts and identifying frontier areas. Several workshops and Euroconferences associated with this Network were held during the period 19921998. The three volumes present a coordinated set of contributions written by leading European scientists who have been involved in the ESF Network activities. We are aware that research into metal clusters has also been very active in non-European countries. Therefore, in order to pay tribute to these contributions, leading scientists
vi
Preface
from outside Europe have been invited to add their personal views to the different sections covered by this book on Metal Clusters in Chemistry. The first volume of this book covers the topics of molecular metal clusters, the second volume contains sections on metal clusters in catalysis and dynamic and physical properties of metal clusters. Volume 3 covers the chemistry of nanomaterials and of solid-state clusters. It also contains an update of the reviews published on metal clusters in the last ten years, and finally presents a view on further developments in cluster chemistry leading into the next millennium. In general, the sections within the book contain a blend of experimental results together with an analysis of theoretical aspects of the work. In some instances, closely related areas have been discussed by several authors and here a complementary diversity of views is offered. The editors wish to thank the authors for their contributions to this book and the numerous scientists who have been involved in Network activities, they have all greatly assisted in the project by their vitality and enthusiasm for this developing area. We hope that this book will offer a source of ideas for future research developments. Cluster chemistry is still a young science that continues to grow rapidly. The diversity within the subject area is so large that it is now possible, with time and effort, to identify emerging patterns, such as structure-reactivity relationships, and to analyse them. Clearly, the apparent cooperative phenomena between two or more metal centres depend on the ability of the metal to “talk” to each other, either by direct metal-metal interactions or through ligand bridges. We firmly believe that the search for applications in quite different fields including the construction of metal clusters with desired electronic and catalytic properties will be of fundamental importance in the coming years. In this book it is our aim to show that metal cluster chemistry is an active, multidisciplinary and international research area with a rich past and a bright future, and to stimulate further interest in and enthusiasm for the subject .
May, 1999
P.B. L.A.O. P.R.R.
Contents
Preface ...............................................................
v
Volume I:
Molecular Metal Clusters
1
Molecular Clusters. ..................................................
1
General Introduction ................................................. F A . Cotton
3
1.1
Molecular Clusters - An Overview .................................. M. H. Chisholm
8
1.2
Expanding, Degrading, and Rearranging Hexametal Boride ... Clusters .............................................. C.E. Housecvoft Introduction .... ... ................................... Expansion. .......................... .................. Expansion beyond Mh during syntheses and ligand substitution reactions of Mg borides. ............................... Expansion from Mg cages using group 11 metal frag Degradation ........................................................ Rearrangements ............................... ................ Conversion of trigonal prismatic to octahedra Interconversion between cis- and trans-isomers of octahedral R u ~ M ~( M B = Ir or Rh) borides .......................
1.2.1 1.2.2 1.2.2.1 1.2.2.2 1.2.3 1.2.4 1.2.4.1 1.2.4.2 1.3 1.3.1
10
10 12
18 20
Steric Effects in Metallacarboranes................................. 26 A.J. Welch Introduction ..................... .................. 26
viii
Contents
1.3.2 1.3.3 1.3.3.1 1.3.3.2 1.3.4
Minor consequences ................................................ Major consequences ................................................ Polyhedral deformation ............................................ Low-temperature isomerization .................................... Conclusions and outlook ...........................................
1.4
Heteronuclear Clusters Having Transition Metals and Metals of Group 1 4 . , ............................................... D .J . Cardin Introduction ........................................................ The coordination of diorganogroup-14 element compounds ...... Synthetic approaches ............................................... Compounds in which the group 14 ligand is bonded terminally to a cluster .......................................................... Compounds in which the group 14 ligand doubly bridges metals in a cluster without further reaction ........................ Clusters in which the Group 14 ligand or the heterometallic cluster has undergone further reaction .............................
1.4.1 1.4.2 1.4.3 1.4.4 1.4.5 1.4.6 1.5 1.5.1 1.5.2 1.5.3 1.6
1.6.1 1.6.2 1.6.2.1 1.6.2.2 1.6.3 1.6.4 1.7
1.7.1 1.7.2
Hetero-Metal Clusters by Assembling Amino Substituted Subvalent Main Group Metals and their Ligand Reactions ........ M . Veith Introduction ........................................................ Syntheses of clusters by using MezSi(NtBu)zM ( M = Ge( 11), Sn(I1))............................................................... Conclusion ..........................................................
32 34 34 39 44 48 48 49 50 51 54 65
73 73 74 89
Synthetic Pathways to a Neglected Class of Compounds: 91 Organobimetallics of Aluminium and cobalt ........................ J .J . Schneidev Introduction ........................................................ 91 Synthesis of mixed group 13/transition metal complexes .......... 92 Transition metal bonds to Ga, In and T1 .......................... 92 Transition metal (tm) bonds to B and A1 .......................... 95 Synthetic routes to organobimetallic compounds of Co and A1. . 100 Outlook and trends ................................................ 106 Group 5 and 6 Bimetallic Complexes with Phosphido Bridges: Syntheses and some Structural Features ........................... G. Boni, M . M . Kiibicki and C. Moibe Introduction ....................................................... Synthesis of metallophosphines ...................................
110 110 110
Contents
ix
1.7.3 1.7.3.1 1.7.3.2 1.7.4 1.7.5
Bimetallic complexes from metallophosphines. . . . . . . . . . . . . . . . . . . . Monobridged complexes .......................................... Dibridged complexes .............................................. Bimetallic complexes from metallodiphosphines ................. Molecular structures of phosphido bridged complexes . . . . . . . . . . .
1.8
Polythiometalates and Polyoxothiometalates Based on AcidoBasic Condensation Processes ..................................... 124 F. Skheresse, E. Cudot and C. Sinwnnrt-Jeyat Introduction .......................... .................. 124 Thiometalates and catalysis ........................ Academic research induced by catalysis ........... Sulfido-ligands ................ .......................... 127 Disulfido-ligands . . . . . . . . . . . Vanadium systems.. ...................................... The [MzS2X2I2+X-0, S fr CS2 activation ............. Reactions of thiometalates Selective sulfuration of fully oxygenated polyoxoanions ......... 136 Addition of a thiometallic fragment on a lacunary polyanion Self-condensation of the [Mo2S202I2' fragment. ................ 139
1.8.1 1.8.2 1.8.3 1.8.3.1 1.8.3.2 1.8.3.3 1.8.3.4 1.8.3.5 1.8.4 1.8.4.1 1.8.4.2 1.8.4.3
1.9 1.9.1 1.9.2 1.9.2.1 1.9.2.2 1.9.2.3 1.9.2.4 1.9.2.5 1.9.3 1.9.4 1.10 1.10.1 1.10.2
1 14
1 15 116 1 19 120
Electronic Effects on the Shape of M2X2 Frameworks ( X = Naked Chalcogenide Atom, M = Late Transition Metal). . 143 C. Mealli and A . Ovlirndini ...................... 143 Introduction .......................... .............................. ........ 146 n less electron rich M2X2 pl etons . . 146 Puckering of the MlX2 skeletons ............................. 150 Coupling/ uncoupling of dichalcogenide within puckered ...................... ........................ 152 Coupling of chalcogenide atoms in electron rich M2X2 ............................ planar frameworks. plexes of the type L2MX2 Uncharged and dic Conclusions ........................... .................. 160 ................................... Appendix Towards Transition Metal Clusters by Reaction of Simple Metal Carbonyls with Chalcogenides and Chalcogenolates G. Henkel and S. WriJgruber Introduction . . . . . . . . . . . . . . . . . ...................... Synthetic aspects.. .................................................
164
X
Contents
1.10.3 1.10.4 1.10.5 1.10.6 1.10.7 1.10.8 1.10.9
Topological aspects ................................................ Complexes containing up to two metal atoms .................... Complexes containing three metal atoms ......................... Complexes containing four metal ions ............................ Complexes containing five metal ions ............................. Complexes containing six metal ions .............................. Complexes of higher nuclearities ..................................
1.11
Low-Nuclearity Iron and Ruthenium Selenido-Carbonyl Clusters Derived from Phosphine and Diphosphine Selenides ..... 193 D . Cauzzi. C. Graifi G. Predievi and A. Tiripicchio Introduction ....................................................... 193 Clusters derived from Ph3PSe ..................................... 194 Ph3P-substituted selenido iron clusters ............................ 194 196 Ph3P-substituted selenido ruthenium clusters ..................... 197 Cluster derived from (Ph2PSe)2X ................................. (PhlP)2X-substituted selenido iron clusters ....................... 199 ( PhzP)2X-substituted selenido ruthenium clusters ................200 Structural features ................................................. 201 M3E2 nido clusters ................................................. 201 Ru~E closo ~ clusters ............................................... 202 Cluster growth reactions ........................................... 204 207 Concluding remarks ...............................................
1.11.1 1.11.2 1.11.2.1 1.11.2.2 1.11.3 1.11.3.1 1.11.3.2 1.11.4 1.11.4.1 1.1 1.4.2 1.11.5 1.11.6 1.12
1.12.1 1.12.2 1.12.3 1.13
1.13.1 1.13.2 1.13.3 1.13.4 1.13.5
A New Supporting Ligand for p 0 x o Metal Derivatives: N.N.dialky1carbamato. 02CNR2 .................................. D . Belli Dell’ Amico. F. Culderazzo. F. Mavchetti and G. Pampaloni Hydrolytic processes ............................................... Non-hydrolytic processes .......................................... Conclusions ........................................................
Alkyne Scission on Metal Cluster Frameworks.................... M.J. Morris Introduction ....................................................... Molybdenum-ruthenium clusters .................................. Molybdenum-cobalt clusters ...................................... Alkyne scission in dinuclear metallacyclopentadiene complexes .......................................................... Conclusions ........................................................
165 167 170 177 180 181 187
209 210 215 218 221 221 225 227 229 233
Contents 1.14
1.14.1 1.14.2 1.14.3 1.14.4 1.14.4.1 I . 14.4.2 1.14.4.3 1.14.4.4 1.14.4.5 1.14.5 1.14.5.1 1.14.5.2 1.14.5.3 1.14.5.4 1.14.5.5 1.14.6 1.15
1.15.1 1.15.2 1.15.3 1.15.4 1.15.5
1.16 1.16.1 1.16.2 1.16.3 1.16.4 1.16.5
xi
Multiple Interactions Between Arenes and Metal Atoms. ......... 236 A .J. Deeming Introduction ....................................................... 236 Arenes bridging two metal atoms employing only n interactions .237 Arenes brid n interactions ...................................................... 239 Aryls in metal clusters.. . . . . Terminal aryls in metal clus Bridging aryls without n-co Bridging aryls with n-coordination through a single carbon ............................................................ 246 aryls with n-coordination through two carbon atoms.. 247 Bridging aryls with n-coordination through a11 six carbon atoms. .............................................................. 248 Arynes in metal clusters ........................................... 250 Terminal q'-arynes ................................. 252 Arynes bridging tw Arynes bridging three metal atoms ............................... 254 255 Arynes bridging four metal atoms ................................ ................261 Arynes bridging five metal atoms Some recent developments with nap From C-H-Activation to Arene Clusters and Back Organometallic Cluster Chemistry with Cyclopentadienyl ....... .... 269 Cobalt Fragments and Olefins ... H. Wudepohl and A . Metz Introduction ....................................................... 269 Ethylene: cluster complexes with pu,-ethylidyneligands .......... 271 Substituted styrenes: cluster complexes with face-capping arene ligands.. ..................................................... 278 Cycloalkenes: cluster complexes with bridging cycloalkyne ................................... 285 ligands ........ 287 Conch Cyclopentadienylnickel Clusters ................................... S. Pasynkiewicz and A . Pietrzykowski Introduction ....................................................... (Cyclopentadieny1)trinickel clusters ............................... (Cyclopentadieny1)tetranickel clusters. ............................ (Cyclopentadieny1)pentanickelclusters. ........................... (Cyclopentadienyl)hexanickel clusters. ............................
290 290 290 302 305 306
xii 1.17
1.17.1 1.17.2 1.17.3 1.17.4 1.17.5
1.18 1.18.1 1.18.1.1 1.18.1.2 1.18.2 1.18.2.1 1.18.2.2 1.18.2.3 1.18.2.4 1.18.3 1.18.4 1.19
1.19.1 1.19.2 1.19.3 1.19.4 1.19.5 1.19.5.1 1.19.5.2 1.19.5.3 1.19.5.4 1.19.5.5 1.19.6
Contents
Ligand Orientation Effects on Metal-Metal Bonding ............. 308 S . Alvarez and G. Aulldn Introduction ....................................................... 308 Pyramidality and metal-metal bond strength in binuclear complexes ............................................... 309 Ligand orientation effects in unsupported [Fe2(CO)8]*- and derivatives .......................................................... 311 Ligand orientation effects in bridged Fe-Fe bonds, including 314 triangular Fe2M clusters .......................................... Clusters with a Fe4C skeleton ..................................... 318 Synthesis and Properties of Metal Carbonyl Clusters Containing Nitrido Ligands ........................................ A. Furnagalli and R.D . Pergola Introduction ....................................................... Generalities on interstitial nitrido clusters ........................ Synthesis of the nitrido ligand in clusters ......................... Synthesis of nitrido clusters ....................................... Iron clusters ........................................................ Ruthenium clusters ................................................ Cobalt and rhodium clusters; some clues to the pyrolytic cluster growing mechanism ........................................ Mixed-metal clusters ............................................ Structural features .............................................. Spectroscopic data associated with the interstitial nitrides ....
323 323 323 324 327 327 328 329 335 338 342
High Nuclearity Osmium . Gold Clusters ...................... 348 J. Lewis and P.R. Raithby Introduction .................................................... ... 348 Electron counting schemes and the rationalization of cluster geometries ....................................................... 349 Synthetic routes to higher nuclearity clusters .................. 352 Characterization of the new clusters ........................... 353 Reactions of high nuclearity osmium cluster carbonyl anions towards monodentate and bidentate gold phosphine cations . 355 Additions to hexanuclear osmium cluster anions .............. 355 Additions to heptanuclear osmium cluster anions ................359 Additions to octanuclear osmium cluster anions . . . . . . . . . . . . . . . . . 362 Additions to nonanuclear cluster anions .......................... 364 365 Additions to decanuclear cluster anions .......................... Synthesis of poly-aurated high nuclearity osmium carbonyl cluster .............................................................. 368
Contents
...
Xlll
1.19.6.1 1.19.6.2 1.19.6.3 1.19.6.4 1.19.6.5 1.19.7
Hexaosmium cluster systems ...................................... Heptaosmium cluster systems ..................................... Octaosmium cluster systems. ...................................... Nonaosmium cluster systems.. .................................... Decaosmium cluster systems ...................................... Conclusion.. .......................................................
1.20
Novel Imido Rhodium Clusters: Synthesis and Perspectives. . . . . . . 38 1 L,A . Oro, M. A . Ciriuno, C. Tejel, Y.-M.Shi and J . Modrego Introduction .................. .............................. 381 The challenge of synthesizing m imido complexes.. ....... 382 A synthetic strategy for rhodium triangular cores ................ 382 Butterfly rhodium imido clusters ...................... A “Rh3( p-N-p-tolyl)z” core with tuneable donicity . . Exploiting the ambivalent donor character of the “Rhi( p-N-p-tolyl)z” core: heterometallic clusters ................ 389 Trinuclear anionic rhodium imido clusters . . The bonding in imido-rhodium clusters.. .... Fluxional behavior .......... .................................. 395 Conclusions and outlook ............ ...................... 396
1.20.1 1.20.2 1.20.3 1.20.4 .20.5 .20.6 .20.7 .20.8 .20.9 1.20.10
1.21.1 1.21.2 1.21.2.1 1.21.2.2 I .21.2.3 1.21.2.4 1.21.3 1.21.5 1.21.5.1 1.21.5.2 1.21S . 3 1.21S.4 1.22
1.22.1
368 370 370 372 373 377
Synthesis and Reactivity of Tripalladium Clusters ................ 399 D.Kovalu-Demertzi Introduction ......................... .......................... 399 ................. ................ 400
Synthesis of neutral mixed-v Synthesis of tripalladium Pd(0)clusters. ........................... 403 Synthesis of tripalladium clusters in the formal oxidation state of Pd( 11) ........................ ....................... Bonding and structure in tripalladium clusters ............ ................. ..................... 406 ............ 407 eactions ................. Oxidative addition reactions and electron-transfer reactions.. . . . 410 Ligand addition reactions . . . . . . Host-guest chemistry ........... ..................... 412 Platinate (11) Complexes as Building Blocks for Complexes with Pt +M (Donor- Acceptor) bonds. ............................ J. Forniis and A . Martin Introduction .......................................................
4 17 4 17
xiv
Contents
1.22.2 1.22.3 1.22.4 1.22.5 1.22.6 1.22.7
Ag complexes .................................. 418 Synthesis of Pt Attempts to prepare Pd + Ag complexes......................... 428 Structural types of Pt + Ag complexes ........................... 431 0-X. . .Ag secondary interactions .................................. 434 Other Pt-M complexes ............................................ 436 Attempts to prepare complexes containing Pt + Pt bonds ....... 438
1.23
Complexes of Keggin-Type Monolacunary Heteropolytungstates: Synthesis and Characterization .................................... 444 A.M. V. Cavaleiro. J.D.Pedrosa de Jesus and H .I. S. Nogueira Introduction ........ ............................. 444 lated species................ 445 Keggin type heterop Polyoxotungstates with formula [XWllM(L)039In- ...... The 1 : 1 complexes ................................................ 446 Synthesis and stability in solution ................................. 447 Structural characterisation ........................................ 448 Electronic and vibrational spectra ................................ 448 NMR and EPR spectra ........................................... 449 Powder and single crystal X-ray diffraction studies .............. 451 451 Electrochemical properties ........................................ Thermal stability ................................................... 452 Polyoxotungstates with the formu M(XW11039)2In- ..........453 ............................. 453 The 1 : 2 complexes ............... .......... 453 Synthesis and stability in solution .................. 453 Structural characterisation ........................................ 453 Vibrational spectra ................................................ 454 NMR spectra ...................................................... Single crystal X-ray diffraction studies ...................... Electrochemical properties ........................................ 455 The 1 : 2 complexes as oxidation catalysts ........................ 455 ................................ 456 Concluding remarks ...........
1.23.1 1.23.2 1.23.3 1.23.3.1 1.23.3.2 1.23.3.3 1.23.3.3.1 1.23.3.3.2 1.23.3.3.3 1.23.3.3.4 1.23.3.3.5 1.23.4 1.23.4.1 1.23.4.2 1.23.4.3 1.23.4.3.I 1.23.4.3.2 1.23.4.3.3 1.23.4.3.4 1.23.4.4 1.23.5 1.24
1.24.1 1.24.2 1.24.2.1 1.24.2.2
--f
.
Reactivity of Diauracycles A Way to Prepare Chains of Gold Atoms ........................................................ M . Laguna and E. Cerrada Introduction ....................................................... Reactivity of [ A u ~ ( C H ~ P P ~ ~............................... CH~)~] Reactions of [ A u ~ ( C H ~ P P ~ ~with C H gold( ~ ) ~ ]111) or silver(I) complexes .......................................................... Reactions of [ A u ~C( H Z P P ~ ~ C H with ~ )mono ~ ] and binuclear gold( I) complexes .................................................
459 459 461 463 466
Contents
xv
1.24.2.3 1.24.3
Reactions of [ A u ~ ( C H ~ P P ~ ~with C Hprotic ~ ) ~ ]acids ............ 470 Mixed-valence linear chains of gold atoms ...................... ,471
1.25
Aurophilicity at Chalcogenide Centers. Synthesis of Polynuclear Chalcogenido-centeredComplexes with Gold-Gold Interactions. .............. ............... 477 0. Crespo, M.C. Gimeno Introduction .............................................. Oxygen-centered complexes ....................................... 478 ............................ 481 Sulfur-centered complexes Double bridging sulfido li ................. 482 ......................... 482 Triply bridging sulfido lig Quadruply bridging sulfido ligands .................. Quintuply and sextuply bridging sulfido ligands. .................487 Selenium-centered complexes. ..................................... 488 ............................. 488 Double bridging selenido ligands Triply bridging selenido ligands ............................... Quadruply bridging selenido ligands .............................. 490 ....................... 490 Tellurium-centered complexes
1.25.1 1.25.2 1.25.3 1.25.3.1 1.25.3.2 1.25.3.3 1.25.3.4 1.25.4 1.25.4.1 1.25.4.2 1.25.4.3 1.25.5 1.26
.26.1 .26.2 .26.3 .26.4 .26.5 .26.6 1.27 1.27.1 1.27.2 1.27.2.1
Au(1). . .Au(I) and Au(1). . .Ag(I) Loose Clusters.. ............... 493 J. Vicente, M-T. Chicote, I. Saura-Llamas, M-C. Lagunas, M. C. Ramirez de Arellano, P. Gonzblez-Herrero, M-D. Abrisqueta and R. Guerrero. Introduction ........................... ................... 493 Assisted loose clusters .......................... ...... 494 ............................ 499 Unassisted loose clusters . . . Mixed loose clusters .............. ............................ 502 Aurophilic coordination ................. ...............504 ....... 504 Conclusions ........................................ The Heteronuclear Cluster Chemistry of the Group 11 Metals Some Recent Advances ............................................ 509 I. D.Salter Introduction ....................................................... 509 Investigations into the fluxional behaviour exhibited by group 51 1 11 metal heteronuclear clusters in solution ....................... Dynamic behavior involving a rearrangement of the metal cores of clusters containing square-based pyramidal A u ~ R u ~ 51 1 units.. ..............................................................
xvi
Con tents
1.27.2.1.1 1.27.2.1.2 1.27.2.1.3 1.27.2.1.4 1.27.2.2 1.27.2.2.1 1.27.2.2.2
1.27.3 1.27.3.1 1.27.3.2 1.27.3.3
1.28 1.28.1 1.28.2 1.28.2.1 1.28.2.2 1.28.3 1.28.3.1
I .28.3.2 1.28.3.3 1.28.3.4 1.28.3.5 1.28.4
Introduction ..................... .............................. 511 Dynamic behavior of the clusters [ A u ~ R u ~ ( ~ ,u,-COMe)( -H)( p-L2)(CO)9]........................... 51 1 Dynamic behavior of the cluster [ A u ~ R u ~ ( ~ ~ - H )p-1,2-Ph2PC6H4PPh2)(CO)12] (~-H)( . . . . . . . . . . . . . 515 Dynamic behavior of other clusters with metal frameworks containing square-based pyramidal A u ~ R units. u~ Dynamic behavior involving the bidentate diphos 1,1 '-bis(dipheny1phosphino)ferrocene (dppf) attached to the group 11 metals in heteronuclear clusters.. ....................... 518 Introduction ....................... ............................. 518 The dynamic behavior of the clust [AU2RU3(p-H )(pI,-COMe)(P-dPPf)(CO)y1> [ M ~ R u ~ H ~ ( ~ - ~ P P ~[M ) (= C CU, O ) ~Ag Z ]or Au], [ A U ~ R ~ ~ ( ~ ~ - S ) ( ~ - ~and PP~)(CO)~] [ A U C U R U ~ ( ~ ~ - H ) ~ ( ~ - ~............................... P P ~ ) ( C O ) ~ ~ ] . 519 The use of sterically demanding bidentate diphosphine ligands to control the metal core geometries adopted by clusters containing Au~Ru3units ...................... Introduction ................................... ............. 528 Structures of clusters of general formula [AUZ (p-H)(p-L2)(CO)12]..... ....................................... 529 Structures of clusters of general formula [Au2Ru3(p3 (p-L*)(CO)g].......................................... Homonuclear and Heteronuclear Cluster Compounds of Gold .... 535 J. Strahle Introduction ....................................................... 535 Synthesis of homonuclear gold cluster compounds. .............. 536 Photolysis of R3PAuN3 ........................................... 536 Synthesis of [Au(AuCl)(AuPPh3)8]+.............................. 537 Synthesis of heteronuclear gold cluster compounds .............. 539 Photolysis of Ph3PAuN3 in the presence of a metal carbonyl ................................................. 539 s of [Re(CO)5Au(AuPPh3)6]2+ ................ 543 Photolytic synthesis of [ ( C ~ H ~ ) M O ( C ~ ) ~ ( A U........... P P ~ ~ ) ~543 ]+. Co-photolysis of Pd or Pt azido complexes with metal carbonyl complexes .......................................................... 544 Co-photolysis of Ph3PAuN3 and L2M( N3)2 ( M = Pd, Pt; L2 = dppe, ( PPh3)2)............................................... 546 Properties and reactions of heteronuclear gold cluster ..................................................... 547
Contents
xvii
1.28.5 1.28.6
Structures of the heteronuclear gold cluster compounds ......... 549 Bonding in the homonuclear and heteronuclear gold cluster 551 compounds.........................................................
1.29 1.29.1 1.29.2 1.29.2.1 1.29.2.2 1.29.2.3 1.29.3 1.29.3.1 1.29.3.2 1.29.3.3 1.29.3.4 1.29.3.5 1.29.4 1.29.4.1 1.29.4.2 1.29.4.3 1.29.4.4 1.29.4.5 1.29.4.6 1.29.4.7 I .29.5
Naked Clusters of the Post-Transition Elements ...... 561 S. Ulvtdund and L. Bcnytsson-Kloo Introduction and scope of review ................................. 561 Synthesis and stabilization of cationic clusters .................... 562 Historic overview and fundamen Molten salts and related routes . Superacids and other low-temper ................... 567 Synthesis and stabilization of anionic clusters .................... 571 Liquid ammonia and amine solutions ............................ 571 ..... 572 Cluster stabilization by sequestering agents Electrochemical methods .......................................... 573 574 Recent synthetic developments .................................... A note on Zintl phases .................................. 574 Structure and bonding .. . . . . . . . . . 575 Electron-precise polyhedra . 5n-electron clusters .................. 576 Electron-rich clusters with electron counts < 6n . . . . . . . . . . . . . . . . . . 577 Electron-rich clusters with electron counts >6n .................. 581 . . . . . . . . . . . . . . . . . . 582 Electron-poor clusters. electron counts < 5n The bonding of cluster species in group 12 .......... Em-cluster bonding . Are the naked clusters really n Cluster condensation and extended structures .................... 589 Reactions and extensions ........ ...................... 591
Volume 11:
Catalysis and Dynamics and Physical Properties
2
Metal Clusters in Catalysis ........................................
2.1
Metal Clusters in Catalysis . An Overview........................ 605 R.J . Puddephatt Introduction ....................................................... 605 Clusters as models for heterogeneous catalysis ................... 606 Stabilization of metal clusters for catalysis ....................... 608 Homogeneous catalysis with cluster complexes ................... 610 Homometallic clusters for homogeneous catalysis ................ 610 Heterometallic clusters for homogeneous catalysis ............... 611 Heterogeneous catalysis involving cluster complexes ............. 612 Conclusion ......................................................... 613
2.1.1 2.1.2 2.1.3 2.1.4 2.1.4.1 2.1.4.2 2.1.5 2.1.6
603
xviii
2.2 2.2.1 2.2.2 2.2.3
2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.4
2.4.1 2.4.2 2.4.2.1 2.4.2.2 2.4.3 2.4.4 2.5
2.5.1 2.5.2 2.5.2.1 2.5.2.2 2.5.2.3 2.5.2.4 2.5.2.5 2.5.2.6 2.5.2.7 2.5.3
Contents
Heterometallic Clusters in Catalysis ............................... P. Braunstein and J. Rosd Introduction ....................................................... Catalytic properties of heterometallic metal-metal bonded cluster compounds of transition metals ........................... Conclusion .........................................................
616 616 622 665
.
ortho-Metalated Dinuclear Rhodium(11) Compounds Synthesis. Structure and Catalytic Applications .............................. 678 P . Lahuerta and I? Estevan Introduction ....................................................... 678 The chemical reaction ............................................. 680 Selectivity of the metalation reaction ............................. 687 Chiral ortho-metalated compounds ............................... 689 NMR spectroscopy ................................................ 690 Crystal data ........................................................ 691 ortho-Metalated dinuclear compounds in catalysis ............... 692 The Role of Co-catalysts in the Reductive Carbonylation of Aromatic Nitro Derivatives Catalyzed by Transition Metal Carbonyl Clusters .................................................. S. Cenini and F. Ragaini Introduction ....................................................... Synthesis of carbamates and ureas ................................ Neutral ligands as co-catalysts .................................... Halide anions as co-catalysts ...................................... Synthesis of heterocycles .......................................... Conclusions ........................................................
697 697 699 699 700 706 713
Homogeneous Catalysis with Ruthenium Carbonyl Cluster Complexes: Hydrogenation of Alkynes ............................ 715 J.A. Cabeza Introduction ....................................................... 715 Survey of catalyst precursors ...................................... 716 Clusters containing only hydride and carbon monoxide ligands . 720 Clusters containing P-donor ligands .............................. 721 Clusters containing cyclopentadienyl ligands ..................... 724 Clusters containing edge-bridging N-donor ligands .............. 724 Clusters containing face-bridging N-donor ligands ............... 725 Clusters containing face-bridging N-donor ligands and phosphine ligands .................................................. 732 Heteronuclear clusters containing ruthenium ..................... 736 Concluding comments ............................................. 737
Contents 2.6
2.6.1 2.6.2 2.6.3 2.6.4 2.6.5 2.6.5.1 2.6.5.2 2.6.6 2.6.7
2.7
2.7.1 2.7.2 2.7.3 2.7.3.1 2.7.3.2 2.7.3.3 2.7.4 2.8
2.8.1 2.8.1.1 2.8.2 2.8.2.1 2.8.2.2 2.8.3 2.8.3.1 2.8.3.2
xix
Metal Clusters as Models for Hydrodesulfurization Catalysts .... 741 M. Brorson, J. D. King, K. Kiriukidou, F. Prestopino and E. Nordlander Introduction ....................................................... 741 Industrial hydrodesulfurization ................................... 742 The active phases in heterogeneous The mechanism(s) of HDS catalysis 746 Model clusters and model reactions.. ............................. 747 Low-valent transition metal clusters .............................. ..................... Transition metal sulfido clusters ... Chevrel phases: heterogeneous HDS catalysts containing molecular clusters. ................................................. 776 Summary. ...... ..................................... 777 Synthesis with Supported Metal Particles by Use of Surface Organometallic Chemistry: Characterization and some ..................................... 782 Applications in Catalysis F. Lefehvre. J.-P. Cundy and J.-M. Basset Introduction ......................................... Synthesis and characterization of bimetallic catalys orted metal particles ...... 783 reaction of organometallics with ..................788 Applications in catalysis. ........ Ligand effect: Hydrogenation of aldehydes[' '1 ............................................ ....... 788 ........................ 789 Adatom effects.. .... ........................... 791 The site isolation e Conclusion ........ ........................ 793 Activation of Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters. Homogeneous and Surface-mediated ... 796 Reactions. .............................................. S. Deahute, P. J. King and E. Suppa Introduction ..................... ............... 796 ............... 798 The aim of this work ............ The role of surfaces in CO and water activation ....... Activation of cluster-bound carbonyls .................. Nucleophilic-electrophilic attack at coordinated carbonyls.. .... 804 Reactions leading to clusters containing water or water ................................... 805 components ............... fragments of water.. ........... 805 Clusters containing water Surface-mediated splitting of water into its components ... 810 (hydration and dehydration reactions) .............
xx 2.8.3.3 2.8.4 2.8.4.1 2.8.4.2 2.8.4.3 2.8.5 2.9
2.9.1 2.9.2 2.9.3 2.9.3.1 2.9.3.2 2.9.4 2.9.5 2.10
2.10.1 2.10.2 2.10.2.1 2.10.2.2 2.10.2.3 2.10.3 2.10.3.1 2.10.3.2 2.10.3.3 2.10.4 2.10.4.1 2.10.4.2 2.10.5
Contents Clusters with ligands which could come from water ............. 812 Hydration and dehydration reactions of cluster-bound propargyl alcohols ................................................. 816 Mononuclear complexes........................................... 819 Bimetallic complexes .............................................. 821 Reactions of propargyl alcohols with metal carbonyl clusters of the iron triad .................................................... 822 Concluding remarks ............................................... 832 Solid-gas Reactions Involving Metal Carbonyl Clusters .......... 844 S. Aime, W. Dastru, R. Gobetto and A. Vide Introduction ....................................................... 844 Solid-gas reaction pathways ....................... ....... 845 Solid-gas reactions involving unsaturated transition metal 846 clusters ............................................................. Reactions of (,u-H)20s3(CO)l~ with CO, NH3. H2S and HCl ..................................................... Selective incorporation of l3C0 in ( p-H)Os3(C0)9(p3-q2-4-Me-CgHgN)............................. 851 Solid-gas reactions involving lightly stabilized transition metal clusters ................................................... Conclusions ........................................................ 855 The Surface of Inorganic Oxides: an Unusual Reaction Medium for the High-yield and Selective Synthesis of Various Osmium Carbonyl Clusters .................................................. 860 E. Cariati, E. Lucenti. D . Roberto and R. Ugo Introduction ....................................................... 860 Synthesis of neutral osmium carbonyl clusters.................... 861 Synthesis of [ H ~ O S ~ ( C O........................................ ),~] 861 Synthesis of lOs3(CO)12]from [Os(CO)3C12]2or OsC13 .......... 863 Syntheses of [ H O S ~ ( C O ) ~( ~ YY =]OH, OR. C1. Br. 02CR) from [Os3(CO)l2................................................ Synthesis of anionic osmium carbonyl clusters ................... 865 Surface-mediated syntheses of [H30s4(C0)12]- and [H20s4(C0)12I2-................................................... 866 Surface-mediated syntheses of [ O S ~ & ( C O ) ~.................. ~]~869 Surface-mediated syntheses of [Os5C(CO)14]*- ...................869 The understanding of the process of nucleation of surface osmium(11) carbonyl species to osmium carbonyl clusters ....... 870 On the surface of magnesia........................................ 870 On the surface of silica added with alkali metal carbonates ..... 871 Conclusion ......................................................... 874
Contents
xx1
2.1 1
The Role of Interstitial Atoms in Transition Metal Carbonyl
2.11.1 2.1 1.2 2.11.2.1 2.11.2.2 2.1 1.2.3 2.1 1.2.4 2.1 1.2.5 2.1 1.2.6 2.1 1.2.7 2.11.3 2.11.3.1 2.1 1.3.2 2.11.3.3 2.11.3.4 2.1 1.3.5 2.11.3.6 2.11.3.7 2.11.4 2.1 1.4.1 2.1 1.4.2 2.1 1.4.3 2.11.5
........................................ . . . . . 877 and C.M. Martin Introduction ................... ......................... 877 ............... 880 Types of interstitial atom.. ................ Three-coordinate interstitial sites. . . . . . . . . . . . . . . . . . . . Four-coordinate interstitial site ......................... 884 Five-coordinate interstitial sites . Six-coordinate interstitial sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 886 Eight-coordinate interstitial sites ................. Ten-coordinate interstitial sites. ................................... 892 892 Twelve-coordinate interstitial sites ................................ Mechanistic insights into interstitial atom formation. ............ 893 Interstitial hydrogen atoms. ................... Interstitial boron atoms ........................... Interstitial carbon atoms ........ Interstitial nitrogen atoms. ........ ........................ 901 Interstitial oxygen atoms ................. ............... 904 Interstitial phosphorus (arsenic a ..................................... 905 Interstitial sulfur atoms. ........................ 907 Why the interest? ................ ............... 907 Binary phases ............................. The surface-cluster analogy ........................... Enhanced cluster stability .................................. 909 Concluding remarks ............. ........................ 909
2.12
2.12.1 2.12.2 2.12.2.1 2.12.2.2 2.12.3
Potential Applications of Nanostructured Metal Colloids H. B6nnemann and W. Bvijoux Introduction ....................... .................... ............... 914 .................... .............................. 924 Fuel cells.. . . . . . . . . . . . . . . . . . .
3
Dynamics and Physical Properties. ................................
3.1
Dynamic and Physical Properties - An Overview ................. 935 J.R. Shupley
3.2
The Ligand Stereochemistry of Transition Metal Carbonyl Clusters ............................................................ A . Sironi Introduction .......................................................
3.2.1
933
937 937
xxii 3.2.2 3.2.2.1 3.2.2.2 3.2.3 3.2.4 3.2.4.1 3.2.4.2 3.2.4.3 3.2.4.4 3.2.5 3.2.5.1 3.2.5.2 3.2.6 3.2.6.1 3.2.6.2 3.2.7 3.3
3.3.1 3.3.2 3.3.3 3.3.3.1 3.3.3.2 3.3.3.3 3.3.4 3.3.5 3.3.6 3.3.7 3.3.8 3.3.9 3.3.10 3.3.11 3.3.12 3.3.13
Contents
Favorable ligand packing ................. .................... .............. The ligand polyhedral model The equal potential surface ( Stereochemical regularities in BMCCs ............................ Charge equalization . The formal local char
938 941
................................................. 946 ................... .............. 946 yl clusters ...................... . . 949 .................................. 949 [( Ind)3Ir3(P ~ - C O )........... ~] [MM'(CO)4Cp2] .............. .................................. 950 951 Intermolecular interactions ........................................ Solid-state structure of [Cr2(CO)lo][2,2,2.Crypt-M]2 ( M = Na, K) ..................... ............................ Solid-state dynamics of Fe3(CO)12................................ Conclusions ........................................................
952 953 955
Multinuclear NMR Studies on Homo- and Heterometallic Rhodium Clusters Containing 6 or More Metal Atoms ........... 960 B. T. Heaton. J .A . Iggo. I .S. Podkorytou, D .J . Smawfield, and S.P. Tunik Introduction ....................................................... 960 NMR measurements ............................................... 960 Hexanuclear homo- and heterometallic clusters containing Rh .................................................................. 963 [Rhs(CO)16] and related homo- and heterometallic derivatives .......................................................... 963 Substituted derivatives of [Rh6(CO)16]............................ 967 Hexanuclear, homo- and heterometallic rhodium-containing clusters with an interstitial atom (B, C, N ) ....................... 972 Heptanuclear Rh-containing clusters ............................. 979 Octanuclear Rh-containing clusters ..................... Nonanuclear Rh-containing clusters .................... Decanuclear Rh-containing clusters ............................... 986 987 Undecanuclear Rh-containing clusters ............................ Dodecanuclear Rh-containing clusters ............................ 988 Tridecanuclear Rh-containing clusters ............................ 990 Tetradecanuclear Rh-containing clusters ......................... 992 Higher nuclearity Rh-containing clusters ( 2 15 metals) ....... Concluding remarks ............................................... 995
Contents 3.4
3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.4.5.1 3.4.5.2 3.5
3.5.1 3.5.2 3.5.3 3.5.4 3.5.5 3.5.6 3.5.7 3.5.8 3.5.9 3.5.10
3.6 3.6.1 3.6.2 3.6.3 3.6.4 3.6.5 3.6.6 3.6.7
3.7 3.7. I
Structure and Dynamics in Metal Carbonyl Clusters: NMR. EXAFS and Crystallographic Studies ............................ L.J . Farrugia and A . G. Orpen Introduction ...................................................... Dynamic NMR spectroscopy of clusters......................... General mechanisms of ligand exchange......................... Fluxionality in the archetypal clusters M3(C0)12 M = Fe, Ru. 0 s ................................................................. X-ray absorption spectroscopy studies of metal carbonyl clusters ............................................................ Cluster structures in solid and solution .......................... Cluster reaction studies ...........................................
xxiii
1001 1001 1003 1007 1008 1018 1020 1021
Reversible Skeletal Rearrangements in Transition Metal Clusters ........................................................... 1028 P.J . Dyson Introduction ...................................................... 1028 Stereochemical non-rigid behavior of metal cluster polyhedra . . 1029 Polyhedral rearrangements in trinuclear clusters ................ 1030 Polyhedral rearrangements in tetranuclear clusters .............. 1032 Square pyramidal-bridged butterfly interconversions ............1035 Octahedral-trigonal prism interconversions ..................... 1040 Octahedral-capped polyhedral interconversions ................ 1042 Polyhedral rearrangements of high-nuclearity osmium clusters . I045 Polyhedral rearrangements of heteronuclear group 8-platinum clusters ............................................................ 1048 Concluding remarks .............................................. 1049 Skeletal Isomerism in Transition Metal Clusters .................1053 0. Rossell. M. Seco and G. Segalks Introduction ...................................................... 1053 1054 Trinuclear clusters ................................................ Tetranuclear clusters .............................................. 1056 1059 Pentanuclear clusters ............................................. Hexanuclear clusters .............................................. 1061 Hepta- and higher nuclearity clusters ............................ 1065 1069 Conclusions ....................................................... Bond Length-Bond Enthalpy Patterns in Metal Carbonyl Cluster Chemistry ................................................. A .K. Hughes and K. Wade Introduction ......................................................
1073 1073
xxiv 3.7.2 3.7.2.1 3.7.2.2 3.7.3 3.7.4 3.7.5 3.7.6 3.7.7 3.7.8 3.7.9
3.8 3.8.1 3.8.2 3.8.2.1 3.8.3 3.8.3.1 3.8.3.2 3.8.3.3 3.8.3.4 3.8.4 3.8.4.1 3.8.4.2 3.8.5 3.8.5.1 3.8.5.2 3.8.6 3.8.6.1 3.8.7
3.9
Contents
The method ....................................................... 1074 1074 Basic assumptions ................................................ Bulk metals: rates at which metal-metal bonds change in 075 strength with length .............................................. Applications to metal carbonyls with known heats of 077 formation ......................................................... Applications to neutral osmium carbonyl clusters. 080 0S~.-(co)y .......................................................... Applications to osmium carbonyl anions [Os.v( CO),.]2- and neutral and anionic osmium carbonyl . hydrides . [Os,( CO),.H,] '-................................................... 1088 Applications to rhenium carbonyl clusters ....................... 1093 1095 Applications to rhodium carbonyl clusters ...................... Core carbon atoms: the relevance of metal carbides ............1097 101 Concluding remarks ....................................... Bimetallic Effects on the Redox Activity of Transition-metal 104 Carbonyl Clusters ................................................. P . Zanello and F. Fubrizi de Biuni 104 Introduction ...................................................... 104 Trinuclear clusters ................................................ Triangular complexes [M3(C0)12]( M = Fe. Ru) ................ 1104 1112 Tetranuclear clusters .............................................. Tetrahedral complexes ............................................ 1112 1117 Butterfly complexes ............................................... [ M ~ ( C O ) I ~ CECR)].'~ +~(R ( M = Fe. Ru. Co) ................... 1118 Planar complexes ................................................. 1122 Pentanuclear clusters ............................................. 1122 Bow-tie complexes [Fe&l(CO)16]"- ( M = Pt. Au) .............. 1123 Spiked triangulated rhomboidal complexes [M3(Co)lo+.(,U.X)(,U.HgCMo(~~)3(C5H5)~ )I- ( M = Mn. n = 2. X = H; M = Fe. n = 0. X = CO) ........................ 1123 1127 Hexanuclear clusters .............................................. Octahedral complexes ............................................ 1127 1131 Raft-like complexes ............................................... Octanuclear clusters .............................................. 1133 Bicapped octahedral complexes [R~~(C~)~~(C){,U~.R~(CO)~}{~~.M(CO)~}] ( M = Re. Rh) ... 1133 Conclusions ....................................................... 1134
'-
Electron-sink Features of Homoleptic Transition-metal Carbonyl Clusters ................................................. G. Longoni. C. Femoni. M . C. Iupulucci and P . Zanello
1137
Contents
3.9.1 3.9.2 3.9.3 3.9.4 3.9.5
3.10 3.10.1 3.10.2 3.10.3 3.10.4 3.10.5
3.11 3.11.1 3.11.2 3.11.3 3.1 1.3.1 3.11.3.2 3.11.4
3.12 3.12.1 3.12.2 3.12.3 3.12.3.1 3.12.3.2 3.12.3.3 3.12.4 3.12.4.1 3.12.4.2 3.12.4.3 3.12.5
Introduction ...................................................... Metal carbonyl clusters featuring only two chemically and electrochemically reversible oxidation states ..................... HTMCC featuring.three chemically and electrochemically reversible oxidation states ........................................ HTMCC displaying electron-sponge features .................... Conclusions .......................................................
xxv 137 139 1141 1150 1154
Modeling of Electrode Interactions with Metal Clusters ......... 1159 A . Ignaczak and J .A .N.F. Gomes Introduction ...................................................... 1159 Historical background ............................................ 1159 The B3LYP method applied to the cluster-model calculations .. 1162 Electrostatic effects ............................................... 1170 Conclusions ....................................................... 1175 The Transition from Low- to High-nuclearity Molecular Metal Clusters Followed by X-ray Photoelectron Spectroscopy .........1179 R. Zanoni Introduction ...................................................... 1179 A brief introduction to applications of photoemission spectroscopy to clusters........................................... 1180 Case studies ....................................................... 1182 Experimental resolution of surface and bulk atoms in ligated 1182 metal clusters ..................................................... From molecular clusters to semiconducting particles - high nuclearity Cu-Se clusters ......................................... 1186 Perspectives ....................................................... 1192 Structure. Morphology. and Interface Structure of Supported Metal and Alloy Particles . HRTEM Studies ................... S. Giorgio. H . Graoui. C. Chapon and C.R. Henry Introduction ...................................................... Sample preparation ............................................... Shape and structure............................................... Pd-MgO (001).................................................... Pd-ZnO (100) .................................................... Pd-ZnO (001) .................................................... Characterization of the interface ................................. Pd-MgO (001) .................................................... Pd-ZnO (100) .................................................... Pd-ZnO (001) .................................................... Alloying effect ....................................................
1194 1194 1196 1196 1196 1198 1198 1202 1202 1204 1204 1205
xxvi
Contents
3.12.6 3.12.7
Annealing in gas atmospheres .................................... Discussion ........................................................
3.13
Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry ......................................................... 1213 J . Belloni and M . Mostufuvi Introduction ...................................................... 1213 Pulse radiolysis principle ......................................... 1214 Nucleation and growth mechanisms ............................. 1215 Principle of radiolytic formation of atoms and clusters .........1215 Atom and charged dimer formation ............................. 1217 Coalescence processes ............................................ 1221 1222 Hydrated clusters ................................................. Ligand effect on cluster growth .................................. 1223 Effect of dose rate on cluster growth ............................. 1224 1225 Cluster formation from multivalent ions ......................... Coalescence of bimetallic clusters ................................ 1225 Principle of redox potential determination of transient species ............................................................. 1228 Reactivity of metal atoms ........................................ 1228 1228 Redox potential of metal atoms .................................. Comparison between ionization potential and optical absorption of atoms .............................................. 1231 Reactivity of metal clusters ....................................... 1232 Monometallic clusters ............................................ 1232 Reactivity of bimetallic clusters .................................. 1239 Mechanism of electron transfer catalyzed by clusters ........... 1241 The thermodynamics of catalysis ................................ 1241 Autocatalytic growth ............................................. 1243 1243 Kinetics of electron-transfer catalysis ............................ Conclusion ........................................................ 1244
3.13.1 3.13.2 3.13.3 3.13.3.1 3.13.3.2 3.13.4 3.13.4.1 3.13.4.2 3.13.4.3 3.13.4.4 3.13.4.5 3.13.5 3.13.6 3.13.6.1 3.13.6.2 3.13.7 3.13.7.1 3.13.7.2 3.13.8 3.13.8.1 3.13.8.2 3.13.8.3 3.13.9 3.14 3.14.1 3.14.2 3.14.3 3.14.4 3.14.5 3.14.6
1207 1209
The Investigation of Heavy Metal Cluster Structures in Powder Samples by the Radial Distribution Function Method ............ 1248 V.Z. Korsunsky Introduction ...................................................... 1284 Simple manifestation of nearest metal-metal distances ......... 1251 More detailed structure manifestation in RDFs ................. 1254 Polynuclear chains with direct interactions between heavy atoms .............................................................. 1257 . ligands . Heavy atoms in ........................................... 1259 1261 Conclusion ........................................................
Contents
xxvii
3.15.1 3.15.2 3.15.3 3.15.4
The Spectrum of an Exciton in Nan0 crystal Semiconductor Structures . Theory ............................................... S.I . Pokutnyi and V.V. Kooukhuk Introduction ................................................. The Hamiltonian of an exciton in a nano crystal ........... The spectrum of an exciton in a nano crystal ............... Bulk exciton in a nano crystal ...............................
Volume 111:
Nanomaterials and Solid State Chemistry
4
Nanomaterials.....................................................
4.1
Metal Clusters and Nanomaterials: an Overview ................. 1273 M. Ichikawa Introduction ...................................................... 1273 Ship-in-a-bottle synthesis of metal clusters in micropores ....... 1274 Robust metal clusters in mesopores .............................. 1284 Bimetallic clusters in zeolites ..................................... 1285 Cluster tranformation to nanoparticles in mesopores ........... 1287 Nanowires in mesoporous channels and their unique properties ......................................................... 1289 Metal Catalysts derived from zerolite-entrapped metal clusters . 1292 Water-gas shift reaction ( WGSR) ................................ 1293 NO + CO reaction ................................................ 1296 CO hydrogenation ................................................ 1296 Methane homologation reaction ................................. 1297 1298 Future prospect ...................................................
3.15
4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.1.7 4.1.7.1 4.1.7.2 4.1.7.3 4.1.7.4 4.1.8 4.2
4.2.1 4.2.2 4.3
4.3.1 4.3.2 4.3.3
1263 26 3 264 265 266
1271
Silver-Tellurium Clusters from Silylated Tellurolate Reagents . . 1302 D . Fenske and J.F. Corrigan Introduction ...................................................... 1302 Silver-tellurium clusters .......................................... 1303 Nanosized Clusters on and in Supports . Perspectives for Future Catalysis .......................................................... 1325 G. Schmid Introduction ...................................................... 325 Between molecule and bulk . the position of nanosized 326 clusters ............................................................ The effect of the ligand shell in immobilized clusters on activity and selectivity ............................................ 329
xxviii
Contents
4.3.4 4.3.5 4.3.6
Bimetallic shell-structured particles .............................. Clusters in nanotubes ............................................. Conclusions and perspectives .....................................
4.4
On the Possibility of Single Electronics Based on LigandStabilized Metal Clusters ......................................... 1342 U. Simon Introduction ...................................................... 1342 1343 What is single-electron tunneling? ................................ Single-electron tunneling in nanoparticles ....................... 1347 Evidence of SET in ligand-stabilized metal clusters ............. 1349 1349 Single cluster properties .......................................... One-dimensional arrangements .................................. 1351 Two-dimensional arrangements .................................. 1353 1354 Three-dimensional cluster networks .............................. Clusters as building blocks for SET devices ..................... 1356 Doubts and chances .............................................. 1356 First devices....................................................... 1356 Summary and outlook ............................................ 1359
4.4.1 4.4.2 4.4.2.1 4.4.3 4.4.3.1 4.4.3.2 4.4.3.3 4.4.3.4 4.4.4 4.4.4.1 4.4.4.2 4.4.5 4.5
4.5.1 4.5.2 4.5.2.1 4.5.2.2 4.5.2.3 4.5.3
4.6
4.6.1 4.6.2 4.6.3 4.6.4 4.6.4.1 4.6.4.2 4.6.4.3 4.6.4.4 4.6.4.5
1332 1336 1339
Strategies for Assembling Pd and Pt Atoms ...................... 1364 M .N . Vargafik, N . Yu. Kozitsyna, N . V. Cherkashina. R.I . Rudy. D .I . Kochubey and I. I. Moiseev Introduction ...................................................... 1364 1367 Giant clusters ..................................................... Polymeric precursors of giant clusters ........................... 1367 Giant Pd clusters ................................................. 1372 Platinum clusters and colloids .................................... 1379 Conclusions ....................................................... 1388 Electronic Structure of Naked. Ligated and Supported Transition Metal Clusters from ‘First Principles’ Density Functional Calculations ........................................... 1392 G. Pacchioni. S. Kruger and N . Rosch Introduction ...................................................... 1392 Density functional theory ........................................ 1393 Naked clusters .................................................... 1395 Clusters in ligand shells ........................................... 1402 Low-nuclearity gold compounds - complexes or clusters? ...... 1402 Metal-ligand interaction ......................................... 1404 Magnetic quenching in carbonylated Ni clusters ................1407 Bimetallic clusters ................................................ 1411 Interstitial atoms in clusters ...................................... 1417
xxix
Contents
4.6.5 4.6.5.1 4.6.5.2 4.6.6 4.7
4.7.1 4.7.2 4.7.3 4.7.4 4.7.5
Supported metal clusters ......................................... Ni4. Cu4. and Pd4 clusters on MgO(001) ........................ Ni clusters deposited on A1203 ................................... Concluding remarks ..............................................
1422 1424 1427 1429
Physical Properties of Metal Cluster Compounds. Model Systems for Nanosized Metal Particles ........................... 1434 L.J . de Jongh Introduction: Why are metal nanoparticles of interest? ......... 1434 Giant magic-number metal clusters .............................. 1437 Evolution to magnetic metallic properties . Some examples . . . . . 1440 Odd-even electron numbers and energy level statistics in cluster assemblies ................................................. 1445 Energy-level statistics in assemblies of small metal particles: summary of theoretical background ............................. 1449
4.8.4 4.8.4.1 4.8.4.2 4.8.5
On the Size-Induced Metal-Insulator Transition in Clusters and Small Particles ............................................... P.P . Edwards, R .L. Johnston and C.N .R. Rao Introduction - divided metals .................................... The electronic structure of divided metals ....................... The metal-insulator transition in mesoscopic and macroscopic systems ............................................................ Experimental studies.............................................. Single particles .................................................... Arrays of particles ................................................ Concluding remarks ..............................................
5
Solid-state Cluster Chemistry ....................................
1483
5.1
Solid-state Cluster Chemistry . An Overview .................... T. Saito
1485
5.2
Transition Metal Clusters - The Relationship between Molecular and Crystal Structure ................................. M .J . Calhorda, D . Braga and F. Grepioni Introduction ...................................................... Experimental and theoretical methods ........................... Molecular and crystal structures of transition metal carbonyls Conclusions .......................................................
4.8
4.8.1 4.8.2 4.8.3
5.2.1 5.2.2 5.2.3 5.2.4 5.3 5.3.1
1454 1454 1457 1463 1467 1468 1476 1478
1491 1491 1492 . 1494 1503
Discrete and condensed clusters in low Valent Niobium Oxides . . I509 G. Svensson. J . Kohler and A . Simon Introduction ...................................................... 1509
xxx
Contents
5.3.2 5.3.3 5.3.3.1 5.3.3.2 5.3.3.3 5.3.3.4 5.3.3.5 5.3.3.6 5.3.3 5.3.4 5.3.6
Oxoniobates containing discrete Nb6012 clusters ................ 1509 Reduced oxoniobates with condensed Nb6012 clusters.......... 1519 Intergrowths between perovskite and NbO ...................... 1521 The formation of intergrowth compounds ....................... 1526 Structural considerations ......................................... 1529 Two pairs of compounds ......................................... 1534 Attempts to change the properties by doping .................... 1538 What is the valence of the niobium atoms? ...................... 1539 Oxotantalates containing discrete Ta60 1 2 clusters ............... 1542 Reduced oxoniobates containing Nb208 clusters ................1543 Superconductivity in low-valent oxoniobates .................... 1545
5.4
Cluster Compounds Containing Mixed-Charge Cluster Units: [M6X12]2+ and [Msx.. 14+ or [ M ~ X I ~ and ]~+ [ M o ~ C I ~ (M=Nb. ]~+ Ta; X=CI. Br) ............................ 1551 N . BrniteviC Introduction ...................................................... 1551 Hexanuclear [M,jX12].+ ( M = Nb. Ta; X = Cl. Br; n = 2. 3. 4) cluster units and reactions in non-aqueous solvents .............1552 Behavior of [M6X12I2+in methanol .............................. 1552 The [(M6X12)X2].6ROH( R = Me. Et) clusters as precursors . . 1553 Clusters with mixed-charge cluster units ......................... 1555 [Ta6C112(PrCN)6][(Ta6C112)C16].2PrCN, a compound with homonuclear mixed-charge cluster units - [Ta6C112]2+ in 1555 the cation and [Ta6C112I4+in the anion .......................... [M6X12(EtOH j6][( Mo6Cls)C14X2].nEtOH.mEtzO,compounds with heteronuclear mixed-charge cluster units [MgX12l2+and [M06C18]~+( M = Nb. Ta; X = C1. Br) .......................... 556 Conclusion ........................................................ 561
5.4.1 5.4.2 5.4.2.1 5.4.2.2 5.4.3 5.4.3.1 5.4.3.2 5.4.4 5.5
5.5.1 5.5.2 5.5.3 5.5.3.1 5.5.3.2 5.5.4 5.5.4.1 5.5.4.2 5.5.5
M&14 and M6L18 Units in Early Transition Element Cluster 563 Compounds........................................................ C. Perrin Introduction ...................................................... 1563 1564 Preparation and characterization ................................ The M6L18 based compounds . M ~ M ~ M................... ~ L I ~ 1565 Electronic properties of the M6L18 unit .......................... 1565 Dependence of M6L18 packing on the size, charge, and stoichiometry of the countercations .............................. 1566 M6L14 based compounds - MlM6L14 ........................... 1573 1573 Electronic properties of the M6L14 unit .......................... M6L14 stacking in the various structures ......................... 1574 Evolution of the intra-unit distances with electronic or steric factors - comparison of M6L14 and M6L18 units ................ 1577
Contents 5.5.5.1 5.5.5.2 5.5.5.3 5.5.5.4 5.5.6 5.5.7
5.5.8 5.6
5.6.1 5.6.2 5.6.2.1 5.6.2.2 5.6.2.3 5.6.3 5.6.3.1 5.6.3.2 5.6.4
5.7
5.7.1 5.7.1.1 5.7.1.2 5.7.2 5.7.2.1 5.7.2.2 5.7.2.3 5.7.2.4
xxxi
Evolution of the metal-metal intracluster bond depending 1579 on the cluster oxidation state ..................................... Relativistic effects on M-M intracluster distances .............. 1581 Matrix effects of ligand size ...................................... 1581 Effects of charge on intra-unit distances ......................... 1583 1584 The oxyhalides in Mg cluster chemistry .......................... Physical properties of discrete M6L18- and M6Ll4-based compounds ........................................................ 1585 Concluding remarks .............................................. 1588
Ternary Rhenium and Technetium Chalcogenides Containing Re6 or T c ~Clusters ............................................... 1591 W Bronger Introduction ...................................................... 1591 The structural systems of the ternary chalcogenides of rhenium and technetium ................................................... 1591 Ternary chalcogenides of rhenium and technetium containing isolated clusters ................................................... 1594 a ternary rhenium chalcogenide with Cs6Re& . two-dimensionally linked clusters ................................ 1595 Ternary chalcogenides of rhenium and technetium with three1599 dimensionally-linked clusters ..................................... Experiments and calculations for the characterization of the binding in clusters of the ternary chalcogenides of rhenium and technetium ................................................... 1604 Magnetochemical and vibrational spectroscopic investigations . 1604 Molecular orbital calculations for the determination of the relative stabilities of Mg clusters ................................. 1605 The 24 valence electron configuration of the Re6 cluster and the explanation of contradictory experiments ................... 1607 Discrete and Linked Homoatomic Clusters of the Elements Ge. Sn. and Pb .................................................... 1612 T.F. Fassler Introduction ...................................................... 1612 Scope .............................................................. 1616 Historical background - Zintl ions and Zintl phases ............ 1616 Molecular clusters ................................................ 1617 Wade’s Rules ..................................................... 1617 Structurally characterized polyhedral clusters of group 13-1 5 elements ........................................................... 1618 Soluble polyhedral Zintl ions of group- 14 elements ............. 1618 Structures of homoatomic nine-atom clusters ................... 1623
xxxii
Contents
5.7.2.5 5.7.3 5.7.3.1 5.7.3.2 5.7.3.3 5.7.3.4 5.7.4
Crystal packing in compounds with nine-atom clusters .... 628 Cluster units in Ge.. Sn.. and Pb-rich intermetallic phases . 630 630 lntermetallic phases containing discrete polyhedra . . . . . . . . . lntermetallic phases containing linked polyhedra . . . . . . . . . . . . . . . 1630 Superconductivity................................................. 1633 Theoretical investigations of the electronic structures ...........1633 Conclusion ........................................................ 1637
5.8
Hexacapped Cubic Transition Metal Clusters and Derivatives . a Theoretical Approach ........................................... 1643 R. Guutier, J.-F. Hulet and J.-Y. Suillard Introduction ...................................................... 1643 Empty hexacapped cubic species ................................. 1645 Species containing interstitial transition-metal atoms ........... 1650 Distorted metal-centered cubic Mg architectures ................ 1653 Interstitial main group atoms .............................. 655 Cubic cluster condensation ................................. 656 659 Conclusions and comments ................................
5.8.1 5.8.2 5.8.3 5.8.4 5.8.5 5.8.6 5.8.7 5.9
5.9.1 5.9.2 5.9.2.1 5.9.2.2 5.9.2.3 5.9.3 5.9.3.1 5.9.3.2 5.9.3.3 5.9.3.4 5.9.4 5.9.4.1 5.9.4.2 5.9.4.3 5.9.4.4 5.9.4.5
Metallocarbohedrenes MgC12 (M =Ti. Zr. Hf. V. Nb. Cr. Mo. Fe) and Ti.M’. C12 (M’=Y. Zr. Hf. Nb. Ta. Mo. W. Si. x + y = 8) - from Mass Spectrometry to 664 Computational Chemistry ................................... M ..M . Rohmer. A4. Binard and J ..A4. Poblet 664 Metallocarbohedrenes - organometallic fullerenes? ....... Formation. growth and dissociation of met-cars ................ 666 Methods of preparation .......................................... 1666 1671 Crystal growth .................................................... Dissociation pathways ............................................ 1674 Physical properties of met-cars and related clusters .............1676 1676 Ionization potentials .............................................. Electron affinity ................................................... 1677 Collective electronic properties: delayed ionization and 1678 delayed atomic ion emission...................................... Ion chromatography studies ...................................... 1679 Chemical reactivity of met-cars and related M,C, clusters . . . . . 1681 Methods of investigation ......................................... 1681 1681 Association reactions of Ti8C$ .................................. Reactions of niobium-contamlng met-cars and titanium carbide clusters with acetone ..................................... 1683 Reaction of TixCl2+and other met-cars with methyl iodide .... 1683 Oxidation-induced reactions of TiXC12 and other metal-carbide clusters ............................................................ 1685
Contents
5.9.4.6 5.9.4.1 5.9.5 5.9.5.1 5.9.5.2 5.9.5.3 5.9.6 5.9.6.1 5.9.6.2 5.9.6.3 5.9.7 6
6.1 6.2 6.2.1 6.2.2 6.2.3 6.3 6.4 6.5
xxxiii
Reactivity ofVgC12’ and NbgC12+. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1685 Reactivity of vanadium-carbon nanocrystals . . . . . . . . . . . . . . . . . . . 1686 Theoretical models of the pentagonal dodecahedron.. . The earliest theoretical studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1686 The quest for Jahn-Teller distortion.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1689 The electronic ground state of dodecahedra1 met-cars . . . . . . . . . . 1690 A conformer proposed by theory - the tetracapped tetrahedron of metal atoms.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1693 Looking for different cage structures.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1693 The tetracapped tetrahedron of metal atoms . . . . . . . . . . . . . . . . . . . . 1693 Pentagonal dodecahedron or capped tetrahedron the controversy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1704 Conclusion - more than a peak in a mass spectrum?. . .. ... . . ... 1707 -
Metal Clusters in Chemistry - Bibliography of Reviews 1988-1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M.I. Bruce Introduction . . . . . . . . ... . . . ..... ...... ........ ... .. . . . 1711 Books . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................... 1711 Books on metal clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1712 ................................... Conferences . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 1716 .............. 1718
7
Retrospective and Prospective Considerations in Cluster . . . . . . . . . . . . . . 1755 Chemistry . . . . . . . . . . . . . . . . . . . . . . . .
7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8
Introduction .
..................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1756
Structure of carbonyl clusters . . . . . . . . . . .
. . . . . . . . . . . . . . 1760
. . . . . .. . ... . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . .
1779
..............1781
Index. ... . . ...... . . . ..... .. ... .... ..... ... . . . . . . _ . .... ..... ........ 1783
List of Contributors
Maria-Dolores Abrisqueta Grupo de Quimica Organometilica Departamento de Quimica lnorginica Facultad de Quimica Universidad de Murcia Aptdo. 4021 E-3007 1 Murcia Spain Silvio Aime Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali Universita di Torino Via Pietro Giuria 7 1-10 125 Torino Italy Santiago Alvarez Departament de Quimica Inorganica Universitat de Barcelona Av. Diagonal, 647 E-08028 Barcelona Spain Gabriel Aullon Departament de Quimica Inorganica Universitat de Barcelona Av. Diagonal, 647 E-08028 Barcelona Spain
Jean-Marie Basset Laboratoire de Chimie Organometallique de Surface UMR CNRS-CPE 9986 43 Bd du 11 Novembre 1918 F-696 16 Villeurbanne Cedex France Daniela Belli Dell‘ Amico Dipartimento di Chimica e Chimica Industriale Universita di Pisa Via Risorgimento 35 1-56126 Pisa ltaly Jacqueline Belloni Laboratoire de Physico-Chimie des Rayonnements UMR 8610 Universite Paris-Sud - B2t. 350 Centre d’Orsay F-9 1405 Orsay Cedex France ~
Marc Benard Laboratoire de Chimie Quantique UMR 7551 CNRS Universite Louis Pasteur 4 rue Blaise Pascal F-67070 Strasbourg France
xxxvi
List of Contributors
Lars Bengtsson-Kloo Inorganic Chemistry Royal Institute of Technology S-10044 Stockholm Sweden Gilles Boni Laboratoire de Synthese et d’Electrosynthese Organometalliques Universite de Bourgogne 6, Boulevard Gabriel F-21000 Dijon France Helmut Bonnemann Max-Planck-Institut fur Kohlenforschung Kaiser-Wilhelm-Platz 1 D-45470 Mulheim a.d. Ruhr Germany Dario Braga Dipartimento di Chimica G. Ciamician V. Selmi, 2 40126 Bologna Italy Pierre Braunstein Laboratoire de Chimie de Coordination (UMR 7513 CNRS) Institut Le Be1 Universite Louis Pasteur 4 rue Blaise Pascal 67070 Strasbourg France Werner Brijoux Max-Planck-Institut fur Kohlenforschung Kaiser-Wilhelm-Platz 1 D-45470 Mulheim a.d. Ruhr Germany Nevenka BrniEevit Ruder BoSkovib Institute
BijeniEka 54 HR-10001 Zagreb Croatia Welf Bronger Institut fur Anorganische Chemie Technische Hochschule Aachen Professor-Pirlet-Stral3e 1 D-52056 Aachen Germany Michael Brorson Haldor Topwe Research Laboratories Nym~llevej55 DK-2800 Lyngby Denmark Michael I. Bruce Department of Chemistry University of Adelaide Adelaide South Australia 5005 Australia Javier A. Cabeza Departamento de Quimica Organica e Inorganica Instituto de Quimica Organometalica ‘Enrique Moles’ Universidad de Oviedo-CSIC E-3307 I Oviedo Spain Emmanuel Cadot Institut Lavoisier Universite de Versailles-St-Quentin 45, Avenue des Etats-Unis F-78035 Versailles France Fausto Calderazzo Dipartimento di Chimica e Chimica Industriale Universitii di Pisa Via Risorgimento 35
List of Contributors
1-56126 Pisa Italy Maria Jose Calhorda ITQB - Instituto de Tecnologia Quimica e Biologica R. da Quinta Grande, 6 Apartado 127 P-2780 Oeiras Portugal Jean-Pierre Candy Laboratoire de Chimie Organometallique de Surface UMR CNRS-CPE 9986 43 Bd du 1 1 Novembre 1918 F-696 16 Villeurbanne Cedex France David J. Cardin Department of Chemistry University of Reading Whiteknights Reading Berkshire RG6 6AD UK
xxxvii
Ana M. V. Cavaleiro Department of Chemistry University of Aveiro P-38 10 Aveiro Portugal Sergio Cenini Dipartimento di Chimica Inorganica Metallorganica e Analitica and CNR Center Via G. Venezian 21 1-20133 Milano Italy Elena Cerrada Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain Claude Chapon CRMC2-CNRS Campus de Luminy, Case 9 13 F-13288 Marseille Cedex 9 France
Elena Cariati Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro CNR Universita di Milano via G. Venezian 21 1-20133 Milano Italy
Natalia V. Cherkashina N. S. Kurnakov lnstitute of General and Inorganic Chemistry Russian Academy of Sciences 3 1 Leninski Prospect 117907 MOSCOW, GSP-1 Russia
Daniele Cauzzi Dipartimento di Chimica Generale ed Inorganica, Chimica Analytica, Chimica Fisica Universita di Parma Viale delle Scienze 1-43100 Parma Italy
Maria-Teresa Chicote Grupo de Quimica Organometalica Departamento de Quimica Inorghnica Facultad de Quimica Universidad de Murcia Aptdo. 4021 E-3007 1 Murcia Spain
xxxviii List o j Contributors Malcolm H. Chisholm Department of Chemistry Indiana University Bloomington, IN 47405 USA Miguel A. Ciriano Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain John F. Corrigan Department of Chemistry The University of Western Ontario London Ontario N6A 5B7 Canada F. Albert Cotton Chemistry Department Texas A&M University P.O. Box 300012 College Station, TX 77842 USA Olga Crespo Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain W. Dastrh Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali Universita di Torino Via P. Giuria 7 I- 10125 Torino Italy Stefano Deabate Dipartimento di Chimica IFM
Universita di Torino Via P.Giuria 7 I- 10125 Torino Italy Antony J. Deeming Department of Chemistry University College London 20 Gordon Street London WCI H OAJ UK L. Jos de Jongh Kamerlingh Onnes Laboratory Leiden University P.O. Box 9506 N L 2300 RA Leiden The Netherlands Roberto Della Pergola Dipartamente di Scienze dell’ Ambiente e del Territorio Universita di Milano Bicocca Via G. Emanueli 15 1-20126 Milano Italy Paul J. Dyson Department of Chemistry The University of York Heslington, YOIO5DD UK Peter P. Edwards School of Chemistry The University of Birmingham Edgbaston Birmingham B15 2TT UK Francisco Estevan Departamento de Quimica Inorganica Universitat de Valencia Dr. Moliner 50 E-46100 Burjasot-Valencia Spain
List of Contributors
Fabrizia Fabrizi de Biani Dipartimento di Chimica dell’Universita di Siena Pian dei Mantellini 44 1-53100 Siena Italy
xxxix
Universita degli Studi dell’ Insubria Via J. H. Dunant 3 1-21100 Varese Italy
Louis J. Farrugia Department of Chemistry University of Glasgow Glasgow G12 8QQ UK
Regis Gautier Laboratoire de Chimie du Solide et Inorganique Moleculaire UMR CNRS 651 1 Universite de Rennes I F-35042 Rennes Cedex France
Thomas F. Fassler ETH Zurich Laboratorium fur Anorganische Chemie Universitatsstr. 6 CH-8092 Zurich Switzerland
M. Concepcion Gimeno Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain
Dieter Fenske Institut fur Anorganische Chemie der U niversi tat D-76128 Karlsruhe Germany
Suzanne Giorgio CRMC2-CNRS Campus de Luminy, Case 913 F-13288 Marseille Cedex 9 France
Cristina Femoni Dipartimento di Chimica Fisica ed Inorganica Universita degli Studi di Bologna viale del Risorgimento 4 1-40136 Bologna Italy
Roberto Gobetto Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali Universitii di Torino Via P. Giuria 7 1-10125 Torino Italy
Juan Fornies Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain
Jose A.N.F. Gomes CEQUP/Faculdade de CiCncias Universidade do Porto Rua do Campo Alegre 687 P-4 150 Porto Portugal
Alessandro Fumagalli Dipartimento di Biologia Strutturale e Funzionale
Pablo Gonzalez-Herrero Grupo de Quimica Organometalica Departamento de Quimica Inorganica Facultad de Quimica
XI
List of Contributors
Universidad de Murcia Aptdo. 4021 E-3007 1 Murcia Spain
University of Liverpool P.O. Box 147 Liverpool L69 7ZD UK
Claudia Graiff Dipartimento di Chimica Generale ed Inorganica, Chimica Analytica, Chimica Fisica Universita di Parma Vide delle Scienze 1-43100 Parma Italy
Gerald Henkel lnstitut fur Synthesechemie Gerhard-Mercator-Universitiit Lotharstrasse 1 D-47048 Duisburg Germany
Houda Graoui CRMC2+-CNRS Campus de Luminy Case 9 13 F-13288 Marseille Cedex 9 France Fabrizia Grepioni Dipartimento di Chimica G. Ciamician V. Selmi, 2 1-40126 Bologna Italy Rita Guerrero Grupo de Quimica Organometalica Departamento de Quimica lnorganica Facultad de Quimica Universidad de Murcia Aptdo. 4021 E-3007 1 Murcia Spain Jean-Franqois Halet Laboratoire de Chimie du Solide et Inorganique Moleculaire UMR CNRS 651 I Universite de Rennes I F-35042 Rennes Cedex France Brian T. Heaton Department of Chemistry
Claude R. Henry CRMC2-CNRS Campus de Luminy, Case 913 F-13288 Marseille Cedex 9 France Catherine E. Housecroft lnstitut fur Anorganische Chemie Universitat Basel Spitalstrasse 51 CH-4056 Basel Switzerland Andrew K. Hughes Department of Chemistry University of Durham Science Laboratories South Road Durham DHI 3LE UK Maria Carmela Iapalucci Dipartimento di Chimica Fisica ed lnorganica Universita degli Studi di Bologna viale del Risorgimento 4 1-40136 Bologna Italy Masaru Ichikawa Catalysis Research Center Hokkaido University Kita-ku, N-11. W-10
List of Contributors
Sapporo 060 Japan Jonathan A. Iggo Department of Chemistry University of Liverpool P.O. Box 147 Liverpool L69 7ZD UK Anna Ignaczak Department of Theoretical Chemistry University of t o d i ul. Narutowicza 68 90-136 Lodz Poland Brian F. G. Johnson Dept. of Chemistry University of Cambridge Lensfield Road Cambridge CB2 1EW UK Roy L. Johnston School of Chemistry The University of Birmingham Edgbaston Birmingham B 15 2TT UK Jason D. King Inorganic Chemistry 1 Chemical Center Lund University Box 124 S-22100 Lund Sweden Philip J. King School of Chemistry University of Bristol Cantock’s Close Bristol BS8 1TS UK
xli
Keranio Kiriakidou Inorganic Chemistry 1 Chemical Center Lund University Box 124 S-22100 Lund Sweden Dmitry I. Kochubey G. K. Boreskov Institute of Catalysis Siberian Branch of the Russian Academy of Sciences 5 Prospect Akademika Lavrent’eva 630090 Novosibirsk Russian Federation Jiirgen Kohler Max-Planck-Institut fur Festkorperforschung HeisenbergstraIje 1 D-70569 Stuttgart Germany Vladimir I. Korsunsky Institute of Chemical Kinetics and Combustion Siberian Branch of the Russian Academy of Sciences Novosibirsk 630090 Russia Dimitrd Kovala-Demertzi Inorganic Chemistry Department of Chemistry University of Ioannina G-45 110 Ioannina Greece Volodymyr V. Kovalchuk 270045 Odessa 90, Velyka Arnautska apt. 16 Ukraine Natalia Yu. Kozitsyna N. S. Kurnakov Institute of General and Inorganic Chemistry
xlii
List of’ Contributors
Russian Academy of Sciences 3 1 Leninski Prospect 117907 MOSCOW, GSP-1 Russia
Universitat de Valencia Dr. Moliner 50 E-46100 Burjasot-Valencia Spain
Sven Kruger Lehrstuhl fur Theoretische Chemie Technische Universitiit Munchen D-85747 Garching Germany
Frederic Lefebvre Laboratoire de Chimie Organometallique de Surface UMR CNRS-CPE 9986 43 Bd du 1 1 Novembre 1918 F-696 16 Villeurbanne Cedex France
Marek M. Kubicki Laboratoire de Synthese et d’Electrosynthese Organomttalliques Universite de Bourgogne 6, Boulevard Gabriel F-21000 Dijon France
Jack Lewis Robinson College Grange Road Cambridge CB3 9AN UK
Antonio Laguna Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain
Giuliano Longoni Dipartimento di Chimica Fisica ed Inorganica Universita degli Studi di Bologna viale del Risorgimento 4 1-40136 Bologna Italy
Mariano Laguna Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain
Elena Lucenti Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro CNR Universita di Milano via G. Venezian 21 1-20133 Milano Italy
Maria-Cristina Lagunas Grupo de Quimica Organometalica Departamento de Quimica Inorganica Facultad de Quimica Universidad de Murcia Aptdo. 4021 E-30071 Murcia Spain Pascual Lahuerta Departamento de Quimica Inorganica
Fabio Marchetti Dipartimento di Chimica e Chimica Industriale Universita di Pisa Via Risorgimento 35 1-56126 Pisa Italy Antonio Martin Departamento de Quimica Inorganica
List of Contributors
Instituto de Ciencia de Materiales de Arag6n Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain Caroline M. Martin Dept. of Chemistry University of Cambridge Lensfield Road Cambridge CB2 IEW UK
Ilya I. Moiseev N. S. Kurnakov Institute of General and Inorganic Chemistry Russian Academy of Sciences 3 1 Leninski Prospect GSP-1 117907 MOSCOW, Russia Michael J. Morris Department of Chemistry University of Sheffield Sheffield S3 7HF UK
Carlo Mealli Instituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione Consiglio Nazionale delle Ricerche Via J. Nardi 39 1-50132 Firenze Italy
Mehran Mostafavi Laboratoire de Physico-Chimie des Rayonnements - UMR 8610 Universite Paris-Sud-Bgt. 350 Universitk Paris-Sud Centre d’Orsay F-91405 Orsay Cedex France
Alexander Metz Anorganisch-Chemisches Institut Ruprecht-Karls-Universitat Im Neuenheimer Feld 270 D-69 120 Heidelberg Germany
Helena I. S. Nogueira Department of Chemistry University of Aveiro P-3810 Aveiro Portugal
Javier Modrego Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain Claude Morse Laboratoire de Synthese et d’Electrosynthese Organometalliques Universite de Bourgogne 6, Boulevard Gabriel F-2 1000 Dijon France
xliii
Ebbe Nordlander Inorganic Chemistry 1 Chemical Center Lund University Box 124 S-22100 Lund Sweden Annabella Orlandini Instituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione Consiglio Nazionale delle Ricerche Via J. Nardi 39 1-50132 Firenze Italy
xliv
List o j Contributors
Luis A. Oro Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain
Christiane Perrin Laboratoire de Chimie du Solide et Inorganique Moleculaire UMR CNRS 65 11 Universite de Rennes I Avenue du General Leclerc F-35042 Rennes Cedex France
A. Guy Orpen School of Chemistry University of Bristol Bristol BS8 ITS UK
Antoni Pietrzykowski Faculty of Chemistry Warsaw University of Technology Koszykowa 75 00-662 Warsaw Poland
Gianfranco Pacchioni Dipartimento di Scienza dei Materiali Istituto Nazionale di Fisica della Materia Universita di Milano Via Emanueli 15 1-20126 Milano Italy Guido Pampaloni Dipartimento di Chimica e Chimica Industriale Universita di Pisa Via Risorgimento 35 1-56126 Pisa ltaly Stanislaw Pasynkiewicz Faculty of Chemistry Warsaw University of Technology Koszykowa 75 00-662 Warsaw Poland Julio D. Pedrosa de Jesus Department of Chemistry University of Aveiro P-38 10 Aveiro Portugal
Josep-M. Poblet Departament de Quimica Universitat Rovira i Virgili Pc Imperial Tarraco 1 E-43005 Tarragona Spain lvan S. Podkorytov S. V. Lebedev Central Synthetic Rubber Research Institute Gapalskaya 1 St. Petersburg, 198035 Russia Sergey I. Pokutnyi State Maritime University Nikolaev 327025 Ukraine Giovanni Predieri Dipartimento di Chimica Generale ed Inorganica, Chimica Analytica, Chimica Fisica Universita di Parma Viale delle Scienze 1-43100 Parma ltaly Fabio Prestopino lnorganic Chemistry 1
List of Contributors
Chemical Center Lund University Box 124 S-22100 Lund Sweden Richard J. Puddephatt Department of Chemistry University of Western Ontario London, Ontario N6A 5B7 Canada Fabio Ragaini Dipartimento di Chimica Inorganica Metallorganica e Analitica and CNR Center Via G. Venezian 21 1-20I33 Milano Italy Paul R. Raithby Department of Chemistry University of Cambridge Lensfield Road Cambridge CB2 1EW UK M. Carmen Ramirez de Arellano Grupo de Quimica Organometalica Departamento de Quimica Inorganica Facultad de Quimica Universidad de Murcia Aptdo. 4021 E-30071 Murcia Spain C.N.R. Rao CSIR Centre of Excellence in Chemistry Indian lnstitute of Science Bangalore 560012 India Dominique Roberto Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro CNR
xlv
Universita di Milano via G. Venezian 21 1-20133 Milano Italy Marie-Madeleine Rohmer Laboratoire de Chimie Quantique UMR 7551 CNRS Universite Louis Pasteur 4 rue Blaise Pascal F-67070 Strasbourg France Notker Rosch Lehrstuhl fiir Theoretische Chemie Technische Universitat Munchen D-85747 Garching Germany Jacky Rose Laboratoire de Chimie de Coordination (UMR 7513 CNRS) Institut Le Be1 Universite Louis Pasteur 4 rue Blaise Pascal F-67070 Strasbourg France Oriol Rossell Departament de Quimica Inorganica Universitat de Barcelona Marti i Franques 1 1 1 E-08028 Barcelona Spain ~
Rimma I. Rudy N . S. Kurnakov Institute of General and Inorganic Chemistry Russian Academy of Sciences 3 1 Leninski Prospect 117907 MOSCOW, GSP-I Russian Federation Jean-Yves Saillard Laboratoire de Chimie du Solide el Inorganique Moleculaire
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List o j Contributors
UMR CNRS 651 1 Universitk de Rennes I F-35042 Rennes Cedex France Taro Saito Department of Chemistry Faculty of Science Kanagawa Univeristy Hiratsuka, Kanagawa 259-1293 Japan Ian D. Salter Department of Chemistry University of Exeter Stocker Road Exeter EX4 4QD UK Enrico Sappa Dipartimento di Chimica IFM Universiti di Torino Via P. Giuria 7 1-10125 Torino Italy Isabel Saura-Llamas Grupo de Quimica Organometalica Departamento de Quimica Inorginica Facultad de Quimica Universidad de Murcia Aptdo. 4021 E-30071 Murcia Spain Gunter Schmid Institut fur Anorganische Chemie FB 8 Universitat-GH Essen Universitatsstr. 5-7 D-45 1 17 Essen Germany Jorg J. Schneider Institut fur Anorganische Chemie Universitat GH-Essen UniversitatsstraBe 5-7
D-45 117 Essen Germany Francis Secheresse Institut Lavoisier Universite de Versailles-St-Quentin 45, Avenue des Etats-Unis F-78035 Versailles France Miquel Seco Departament de Quimica Inorganica Universitat de Barcelona Marti i Franques 1-1 1 E-08028 Barcelona Spain Gloria Segales Departament de Quimica Inorginica Universitat de Barcelona Marti i Franques 1-1 1 E-08028 Barcelona Spain John R . Shapley School of Chemical Sciences University of Illinois 505 South Mathews Avenue Urbana, IL 6 1801 USA You-Mao Shi Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Aragon Universidad de Zaragoza-CSIC E-50009 Zaragoza Spain Arndt Simon Max-Planck-Institut fur Festkorperforschung HeisenbergstraBe 1 D-70569 Stuttgart Germany
List of Contributors
Ulrich Simon Universitat G H Essen Institut fur Anorganische Chemie Schutzenbahn 70 D-45141 Essen Germany Corine Simonnet-Jegat Institut Lavoisier Universite de Versailles-St-Quentin 45, Avenue des Etats-Unis F-78035 Versailles France Angelo Sironi Dipartimento di Chimica Strutturale e Stereochimica Inorganica Via Venezian 21 1-20133 Milano Italy Daniel J. Smawfield Department of Chemistry University of Liverpool P.O. Box 147 Liverpool L69 7ZD UK Joachim Strahle Institut fur Anorganische Chemie Eberhard-Karls-Universitat Auf der Morgenstelle 18 D-72076 Tubingen Germany Gunnar Svensson Arrhenius Laboratory Stockholm University S- 1069I Stockholm Sweden Cristina Tejel Departamento de Quimica Inorganica Instituto de Ciencia de Materiales de Arag6n Universidad de Zaragoza-CSIC
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E-50009 Zaragoza Spain Antonio Tiripicchio Dipartimento di Chimica Generale ed Inorganica, Chimica Analytica, Chimica Fisica Universita di Parma Viale delle Scienze 1-43100 Parma Italy Sergey P. Tunik St. Petersburg University Department of Chemistry Universitetskii pr. 2 St. Petersburg, 198904 Russia Renato Ugo Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro CNR Universita di Milano via G. Venezian 21 1-20133 Milano Italy Stefan Ulvenlund Department of Explorative Pharmaceutics Astra Draco AB P.O. Box 34 S-221 00 Lund Sweden Michael N. Vargaftik N. S. Kurnakov Institute of General and Inorganic Chemistry Russian Academy of Sciences 3 1 Leninski Prospect 1 17907 MOSCOW, GSP-1 Russia Michael Veith Anorganische Chemie
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List of Contributors
Universitat des Saarlandes D-66123 Saarbrucken Germany
Ruprecht-Karls-Universitat Im Neuenheimer Feld 270 D-69 120 Heidelberg Germany
Alessandra Viale Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali Universita di Torino Via P. Giuria 7 I- I0 125 Torino Italy
Stefan WeiBgraber Institut fur Synthesechemie Gerhard-Mercator-Universitat LotharstraRe 1 D-47048 Duisburg Germany
Jose Vicente Grupo de Quimica Organometalica Departamento de Quimica Inorganica Facultad de Quimica Universidad de Murcia Aptdo. 4021 E-3007 1 Murcia Spain
Ken Wade Department of Chemistry University of Durham Science Laboratories South Road Durham D H l 3LE UK Hubert Wadepohl Anorganisch-Chemisches Institut
Alan J . Welch Department of Chemistry Heriot-Watt University Edinburgh EH14 4AS UK Piero Zanello Dipartimento di Chimica Universita di Siena Pian dei Mantellini 44 1-53100 Siena Italy Roberto Zanoni Dipartimento di Chimica Universita degli Studi di Roma ‘La Sapienza’ piazzale Aldo Moro 5 1-00185 Rome Italy
1 Molecular Clusters
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
General Introduction F. Albert Cotton
When I was asked to write a general introduction to this book, at first I was hesitant. It was not my natural modesty, but a sense of the vastness of the subject that made me pause. When I introduced the term “metal atom cluster” in 1964”’ to designate “a finite group of metal atoms that are held together mainly, or at least to a significant extent, by bonds directly between the metal atoms, even though some nonmetal atoms may also be intimately associated with the clusterLz1,” the number of compounds that were known to satisfy this definition was small. I would estimate that there were no more than a dozen well-characterized examples of such compounds and perhaps fewer. Today I would hesitate to guess how many thousand cluster compounds there are, for fear of underestimating, and the range of types is now great. Over the past thirty years, growth in the field has been exponential. Perhaps even more than in other areas of inorganic chemistry, little if any of this growth could have occurred were it not for the enabling technique of X-ray crystallography. Unlike Werner complexes or certain other areas of chemistry (much of organic chemistry, for example), there are no simple structural prototypes to which the majority of clusters could be a priori assigned. For much of the field, structural knowledge, which of course is indispensable in understanding the chemistry, has to be obtained on a case-by-case basis by X-ray crystallography. As an historically interesting illustration of the impossibility of making progress in metal atom cluster chemistry without X-ray crystallographic structural data, I would draw attention to the case of the now largely forgotten chemist Kurt Lindner. In the 1920s Lindner and his students at Berlin University carried out many studies on the chemistry of the compounds of Mo”, W”, and Ta”, and in 1927 Lindnerl3I published a summary account of all their work in which he attempted to assign structures that he concocted by fusing together tetrahedra and/or octahedra via p 2 - and p,-halide ligands. These structures, of course, bear no resemblance to reality. A few more words about the definition of a cluster are appropriate. At the time the term was proposed, there had been occasional use of the word “cage”. This
4
I Molecular Clusters
1
2
3
term has to be (and has been) rejected for several good reasons. The word cage suggests the ideas of containment and encapsulating of something, but this is not characteristic, nor even possible, for many clusters. Clearly a triangle of metal atoms, being two dimensional, or even some three-dimensional clusters, such as butterflies, cannot surround anything and small closo-clusters such as tetrahedra do not have sufficient space inside to do so. If one wishes to consider a pair of bonded metal atoms to be a logical part of the cluster family, the term cage would clearly be inappropriate. Of course, there are some metal atom clusters that really do encapsulate non-metal atoms ( H , C, N usually), but this is a special characteristic of some rather than a general characteristic of all clusters. Another important point concerning the definition of a metal atom cluster compound is the requirement that there is significant direct bonding between the metal atoms. In the absence of this there is no justification for using a special word (cluster) since the molecule or ion would be simply the kind of polynuclear Werner complex already well known at the turn of the 19thCentury. Such a species is little more than the sum of its parts, apart from some weak, chemically insignificant, magnetic interactions. To illustrate, a molecule such as 1 is not a metal atom cluster compound, whereas 2, with its direct M-M bond, is. There are, of course, some ambiguous cases, such as the iron-sulfur clusters, 3. Whether there is significant “bonding directly between the metal atoms” of such clusters, at the Fe-Fe distances concerned, 2.6-2.7 A, is a moot point. Indeed, the word “cluster” is often used more broadly to designate aggregates in which any direct metal-to-metal bonding is unlikely. As so often happens in chemical nomenclature, and indeed in linguistic questions more generally, borderline areas arise, grow, and require elasticity in terminology. I cannot help saying that I do not think a compound with only metal to ligand contacts is appropriately called a “metal cluster”. But it happens. Despite its spectacular present, it should be remembered that metal atom cluster chemistry also has a very early origin. The earliest point to which I have been able to trace it is the period 1857-1861 when a Swedish chemist, Christian Bloomstrand, discovered the dichloride and dibromide of molybdenum. 14] Based on the significant observation that these contained two kinds of halogen, one precipitable by silver ion and the other not, in a 1 : 2 ratio, he concluded that the minimum acceptable molecular formula for these compounds was M03X6, and that this consisted of a
General Introduction
5
4
5
non-dissociating M O ~ X core ~ ~ and + two dissociable X- ions. It was not until many years later that suggestions were made by Werner”’ and WeinlandL6]as to the structures of these “dihalides.” Their suggestions are shown as 4 and 5. Both were based on the assumption that metal atoms could be joined together only by bridging ligands, direct metal-metal bonding being a non-existent concept in that period. The later suggestions of Lindner, which have already been mentioned, also assumed that the compounds are trinuclear, which we now know to be untrue. Other early discoveries in the field were reported in the period 1907-1913.”~s~91 There were the [ M ~ X I ~ J X ~compounds, . ~ H ~ O where M = Nb, Ta and X = C1, or Br. Even at this early time Chapin was able to show that “Ta6C114.7H20” in solution dissociated into a TagC112~+core and two C1- ions. Much later, X-ray work supplied structural details. One of the most impressive contributions to the early development of metal atom cluster chemistry was the work of C. Brosset,’12’a Swedish crystallographer who determined the structures of several compounds derived from “MoC12” and showed that they were based on a M06Clg4+ core, consisting of an octahedron of directly bonded Mo atoms (each to four others) with a p3-C1atom lying above each triangular face. It is remarkable that these complex structures were resolved as long ago as the early 1940s. For a number of years these octahedral Mg clusters remained the largest ones known. The advent of the first metal carbonyl cluster, Fe?(C0)12, was also long ago.“” Here again there was long uncertainty about its structure, resolved only by the work of Dahl in 1966,[141who was also responsible for clarifying the nature of the analogous Ru.,(C0)12 and O S ~ ( C O ) ~During ~ . [ ~ ~the ] same period of time Chini and coworkers[’”] prepared the first four-atom metal carbonyl cluster, Coq(C0)12, whose Rh and Ir analogues were soon to follow, and in 1963 Dahl showed the true formula and structure of the first six-atom cluster compounds of the carbonyl type, 1 6. Rh6 (co) Also in the 1960s the [Re3C112I3- cluster was discovered, and the name cluster was generally adopted. Thus, by about 1970, the stage had been set for the explosive development that has occurred over the past quarter of a century. The field is now so broad that an all-inclusive catalogue may no longer be feasible. We can, ‘‘‘9’
6
1 Molecular Clusters
however, mention a number of the major classes, as they are represented in the present contribution to the secondary literature. The two “classical” types of metal atom cluster chemistry that already existed (embryonically) some thirty years ago have both prospered and grown enormously. One of these encompasses clusters with the metal atoms in low to medium oxidation states together with halide, oxygen or chalcogenide ions. In addition to MogX8, Nb6X12, Re3X9 and their derivatives, there are now M03 and W3 based clusters, Chevrel phases, Re6 based clusters, Nb3 and Nb4 based clusters, and others. The other classical type embraces those containing metal atoms in very low (ca. 0) oxidation states, in which the ligands are most commonly CO. For this class some useful if not rigorous relationships between structures (closo, nido, arachno) and electron counts have been developed, beginning with “Wade’s rules”. To this second class a new subdivision has been added, namely, those into which main group elements have been incorporated. Sometimes they are encapsulated (C, N ) or they may be part of the polyhedron, for example in RCCo3(C0)9, where the C and Co atoms together define a tetrahedral cluster. Hydrido clusters and the corresponding anionic clusters have also become very numerous. Of course, metal atom clusters have become an important part of transition metal organometallic chemistry. Thus, isolobal species such as v6-C6H6Mo and Mo(CO)3 can be substituted for one another. Mixed metal clusters have also become very numerous and well known. These range from simple cases, where obvious isoelectronic replacements are made, as exemplified by C04(CO)12 and RhCo3 (CO)12, to very large and complex species such as [Ni9Pt3(C0)21HI3-. There is no end in sight to the possibilities for more mixed metal clusters. A recent innovation is the creation of very large clusters that have solid cores of close-packed metal atoms. An example is one containing a Pd33Ni9 core, covered with CO and PR3 molecules. The Pd atoms form a trigonal stack of hexagonally close packed atoms, with the Ni atoms at exterior corners. Another unusually interesting example[’71is a compound containing the anion [A177 {N(SiMe3)2)20l2-. This is not only a very large cluster but is an opening into the realm of main group metals. Another more recent development is the occurrence of clusters that are integral components of extended solid state structures. The Chevrel phases, already mentioned, are one example but there are numerous others, of which in some the clusters have a hetero atom in the center of the octahedron, e.g., ZrgNC115. Recently, it has been shown that the individual clusters can often be extracted and dissolved in suitable solvents. A major new sub-field of cluster chemistry is that of nanoparticles of metals in which there are no intentionally appended ligands. These are not molecular and thus neither homodisperse nor crystalline. Interest in all these newer types of clusters is often driven by their potential in terms of heterogeneous catalysis, and the unusual electrical, magnetic and spectro-
General Introduction
7
scopic properties they may possess. Therefore, the chemistry of metal clusters is a subject of practical as well as scientific interest. It is thus timely that this wideranging survey of the field should appear. The editors are to be congratulated for having assembled a group of authoritative reviewers, able to write as experts on the many areas of the subject. These 77 contributions constitute a remarkable encyclopedia on a fascinating, active and important subject.
References [I] F. A. Cotton, Inorg. Chem. 1964, 3, 1217. [2] This definition was later slightly modified to read “. . . . held together entively, mainly or at least ’ F. A. Cotton, Quurt. Reu. 1966, 20, 389; F. A. Cotton, to a significant extent ....” C McGruw-Hill Yeurbook of Science and Technology,1966. 131 K. Lindner, Zeits. Anorg. Allg. Chem. 1927, 162, 203. 141 C. W. Bloomstrand, J. Prukt. Chem 1857, 71, 449; 1859, 77, 88; 1861, 82, 433. [ 5 ] A. Werner, “Neuere Anschuuungen uuf dem Gebiete der anorganischen Chemie,” Braunschweig, 1905. [6] R. Weinland, “Einjuhrung in der Chemie der Komplexuerbindungen,” 2”d Ed., 1924. [7] M. C. Chabrie, Comptes Rend. 1907, 144, 804. [8] W. A. Chapin, J. Am. Chem. Soc. 1910, 32, 323. [9] H. S. Harned, J. Am. Chem. SOC.1913, 35, 1078. [lo] H. S. Harned, L. Pauling and R. B. Corey, J. Am. Chem. Soc. 1950, 72, 5477. [ l l ] P. A. Vaughn, J. H. Sturtivant and L. Pauling, J. Am. Chem. Soc. 1950, 72, 5477. [ 121 C. Brosset, Arkiu f o r Kemi, Miner. Geol. Band 20A, No. 7 (145); Band 22A, No. 11 (1946); Akriu, Band No. 1 (1949). [ 131 J. Dewar and H. 0. Jones, Proc. Roy. Soc. (London), 1905, A76, 558; 1907, A79, 66. 1141 C. H. Wei and L. F. Dahl, J. Am. Chem. Soc. 1966,88, 1821; 1969, 91, 1351. [15] E. R. Corey and L. F. Dahl, J. Am. Chem. Soc. 1961, 83, 2203. [16] P. Chini, V. Albano and S. Martinengo, J. Organomet. Chem. 1969, 16, 471. [I71 E. Ecker, E. Weckert and H. Schnockel, Nature 1997,387, 379.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.1 Molecular Clusters - An Overview Malcolm H. Chisholm
The term cluster takes on many meanings in chemistry. However, for the inorganic chemist the definition provided by F. A. Cotton‘’] some 30+ years ago still forms the basis for origins of this field. “A group of three or more metal atoms linked together at least in part by metal-metal bonding.” Today inorganic chemists have expanded the field of “clusters” to include complexes that contain a group of metal atoms that are electronically or magnetically interacting. In this way the term finds common usage in bioinorganic chemistry. Metal clusters abound in organometallic chemistry and one of the earliest fields of endeavor was in the area of metal carbony1 chemistry where clusters such as Fe3(C0)12 and C04(C0)12 have been known for 50 years or more. Interestingly these seemingly simple homoleptic species still find themselves topics of current debate in terms of their dynamic solution and solid-state behavior. 12,31 The subtle changes in their coordination properties within a series such as M3(C0)12, where M = Fe, Ru and Os, and mixed metal complexes there of, M2M’(C0)12, are not fully understood. The formation of unsaturated metal carbonyl fragments under thermolysis or photolysis led to the formation of higher nuclearity clusters often incorporating interstitial atoms such as carbide, nitride, hydride or oxide. Also it soon became apparent that the high reactivity of metal carbonyl cluster fragments could activate a wide variety of small organic molecules, particularly unsaturated species such as alkenes and alkynes. Indeed, carbon-carbon, carbon-hydrogen, and carbon-heteroatom bond cleavage reactions at metal carbonyl clusters became a field of study in itself. With access to increasingly higher nuclearity clusters the cluster-surface bonding analogy emerged. [41This is well exemplified by the bonding modes of ethene and benzene to two and three metal atoms, respectively. As the field of metal carbonyl cluster chemistry was expanding rapidly many other new classes of clusters joined the fold, in particular, gold phosphine clusters, metal-alkoxide, -thiolate, and -halide clusters of the early transition elements. The theories of bonding in metal clusters rapidly advanced aided by both computational procedures and conceptual ideas. Without a doubt the most significant in the latter
1.1 Molecular Clusters - An Overview
9
area was the isolobal principle, advanced by Hoffman and his coworkers.151Two species are isolobal when their frontier molecular orbitals have the same symmetry and the same number of electrons. So H+ and Au(PPh3)+ are isolobal, as are ( l ) , H, CH3, Mn(C0)5, CpW(CO)3; (2), :CH2, Fe(C0)4, PtL2, and Mo(OR)4 and (3) CH, CpM(CO)?,where M = Mo, W, CO(CO)I,P, W(OR)3. This view of bonding in clusters, one based on a fragment molecular orbital approach wherein fragments combine, leads not only to retrosynthetic analysis but also to new synthesis. This synthetic strategy was recognized early on by Stone and his coworkersL6]and is widely in use today in the synthesis of mixed main-group-transition-metal clusters and all types of hybrid clusters built from organometallic and non-organometallic fragments. As the classes of cluster compounds became more and more diverse, the forms of their chemistry expanded rapidly. Heteronuclear clusters could be tethered to supports and used as precursors to heterogeneous catalysts. Large clusters could be used in the synthesis of nanoparticles or linked together in an orderly manner to make new materials. As clusters grew in size and complexity and their characterization techniques became more routine then questions of their electronic structure took on new significance - how many metal atoms does it take to mimic the properties of a bulk metal? Meanwhile there are even more interesting properties quantum effects not recognized previously. The area of metal cluster chemistry starts to merge with that of colloids and indeed with solid-state chemistry as well as surface chemistry. For these and other reasons it is no wonder that metal cluster chemistry has attracted the attention of some of the brightest minds in science over the past several decades and continues to flourish in so many ways. In this volume some of the leading cluster chemists provide up-to-date reviews of their fields of research. The topics covered include virtually every aspect of cluster chemistry with the notable exception of early transition metal clusters with x-donor ligands, which was the subject of a recent book.”]
References [ I ] F. A. Cotton, Acc. Chem. Res. 1969, 2, 240. 121 L. J. Farrugia. Dalton Trans. 1997, 1783. [3] (a) B. F. G. Johnson, Dalton Tram. 1997, 1473; (b) B. E. Mann, Dalton Trans. 1997, 1457. [4] (a) J. T. Yates, Jr. and M. R. Albert, “The Surface Scientist’s Guide to Organometallic Chemistry”, ACS, Washington, D.C.; 1987; (c) E. L. Muetterties, Chem. Soc. Rev. 1982, If, 283. (51 R. Hoffmann, Angeiv. Chem. Int. Ed. Engl. 1982, 21, 711. [6] F. G. A. Stone, Angeiv. Chem. Int. Ed. Engl. 1984,23, 89. [7] Early Transition Metal Clusters with n-Donor Ligands, M. H. Chisholm, Ed., VCH Publishers, Inc., New York, NY; Weinheim, Germany and Cambridge, England.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.27 The Heteronuclear Cluster Chemistry of the Group 11 Metals - Some Recent Advances Iun D.Sulter
1.27.1 Introduction The first examples of compounds containing a bond between copper, silver or gold and another, different transition metal were a series of dimers reported by Nyholm and c o - w ~ r k c r s [in' ~1964. ~ ~ Although Nyholm rt ul['] also described the first mixedmetal gold cluster, [Au?Fe(C0)4(PPh?)?]," in the same year, the heteronuclear cluster chemistry of the Group 11 metals was very poorly developed before 1981, despite the great interest shown in the field of transition metal cluster compounds as a whole during that time period.[51However, since 1981, the chemistry of mixedmetal clusters containing copper, silver or gold has attracted considerable interest and a very large number of Group 1 1 metal heteronuclear clusters have now been synthesized and studied.[6p1 Most examples that are currently known contain one or more M(ER3) ( M = Cu, Ag or Au; E = As or P; R = alkyl or aryl) units and, in the vast majority of cases, the Group 11 atom(s) are ligated by organophosphine(s).["15' The M( ER3) fragments in this class of cluster compound can adopt either edge-bridging or face-capping bonding modes on subunits of transition metals (M') other than Group 11 metals (Fig. 1). These subunits normally either consist of a discrete trigonal planar Mi unit or an M i face of a larger polyhedron of M' atoms.[6-101 When two or more M( ER3) units are present in a mixed-metal cluster, the situation is complicated by the tendency for the coinage metals to adopt adjacent sites in the metal framework of the cluster. This tendency has been attributed to bonding interactions between the M atoms.[6 91 However, a reasonable number of examples
*The existence of a bonding interaction between the gold atoms [Au-Au 3.028(1) A] was not recognized until later.'"]
5 10
I Moleculur Clusters
(a)
PR3
(b)
M*-
M'
PR3
/"\ M'
M' M'
Figure 1. The (a) edge-bridging and (b) face-capping bonding modes adopted by M(PR3)
( M = Cu, Ag or Au; R = alkyl or aryl) units in heteronuclear clusters. M' is a transition metal other than a Group 11 metal.
of heteronuclear clusters which do not have the Group 11 metals in close contact have also been The combination of the fact that the M(ER3) units can adopt two different bonding modes and the fact that close contact between the Group 11 metals may or may not occur in species in which there is more than one coinage metal means that heteronuclear clusters containing M( ER3) moieties can adopt a wide variety of different skeletal geometries. Much of the interest shown in heteronuclear Group 11 metal clusters arises from the unusual bonding properties of the M( ER3) units, which are in marked contrast to those of almost all other transition metal fragments that are found in cluster compounds. Theoretical studies['7p191of the bonding capabilities of the M( ER3) units suggest that the differences in energy between the various structural types can be very small for mixed-metal clusters that contain these fragments. This theoretical prediction that the metal skeletons of Group 11 metal heteronuclear clusters should often show an unusual degree of flexibility is supported by experimental evidence obtained both from the solid state and from solution For example, heteronuclear Group 11 metal clusters with very similar stoichiometries can often exhibit markedly different metal core structures, skeletal isomerism can occur, both in the solid state and in solution, and the metal frameworks of this class of cluster compound are frequently stereochemically non-rigid in solution. This article describes some recent results in the field of Group 11 metal heteronuclear cluster chemistry from my group and co-workers. We have been particularly interested in the following topics: (i) Attempts to discover new types of stereochemical non-rigidity exhibited by the metal cores of heteronuclear Group 11 metal clusters in solution. (ii) Studies of the fluxional behavior of the bidentate diphosphine ligand 1,l '-his(dipheny1phosphino)ferrocenewhen it is attached to Group 11 metals in mixedmetal clusters. (iii) Investigations into how the nature of bidentate diphosphine ligands attached to gold atoms in heteronuclear clusters affect the skeletal geometry adopted by these cluster compounds.
1.27 The Heteronuclear Cluster Chernistrj? of the Group I 1 Metals
51 1
1.27.2 Investigations into the fluxional behaviour exhibited by group 11 metal heteronuclear clusters in solution 1.27.2.1 Dynamic behaviour involving a rearrangement of the metal cores of clusters containing square-based pyramidal A u ~ R uunits ~ 1.27.2.1.1 Introduction As stated in the previous section, dynamic behaviour involving intramolecular rearrangements of the metal skeletons of Group 11 metal heteronuclear clusters is relatively common, in marked contrast to the situation observed for almost all other transition metal clusters. which have metal frameworks that are stereochemically rigid in The mechanisms of these metal core rearrangements are, therefore, of considerable interest. A recent re-examination of some low-temperature H N M R spectroscopic datac2'] p3-H)(p-H)(p-Ph2ECH2E' Ph?)(CO)121 previously obtained for the clusters [Au~Ruq( ( E = E' = As or P; E = As, E' = P),[21,221 which all adopt a capped square-based pyramidal metal core geometry, suggested that these clusters all exhibit dynamic behavior, which involves an intramolecular rearrangement of the metal frameAlthough dynamic behaviour involving intramolecular metal core rearrangements is commonly observed for heteronuclear clusters containing two Au( ER3) units,[6.8.''I skeletal non-rigidity of a metal framework containing a square-based pyramidal AuzRui unit had not been previously reported. We were interested in studying this novel fluxional process further, but unfortunately no NMR spectra consistent with the ground state structures of any of the above hexanuclear clusters could be obtained, even at low temperatures. Therefore, we decided to attempt to synthesize other clusters with metal frameworks containing square-based pyramidal A u ~ R ufragments. ~
'
1.27.2.1.2 Dynamic behaviour of the clusters [AU2RU3(1UU-H)(1U~-COMe)(1U-L2)(C0)91 [Lz = Ph2P(CHz),PPh2 {n = 1 (1) or 5 (2)) or dppf (3) (dppf = 1,l '-bis(dipheny1phosphino)ferrocene)l The clusters [Au2Ru3(p-H)(p3-C0Me)(p-L2)(CO)9] [L? = PhzP(CH?),PPh2 {n = 1 (1) or 5 (2)}[231or dppf (3)[241] can be synthesized in ca. 30-55'%1 yield by treating a diethyl ether solution of [Ru3(p-H)3(,u3-COMe)(C0)9] with the complex [Au?(p-L~)Mez].They all adopt square-based pyramidal metal core structures, with the
5 12
I Moleculur Clusters
Figure 2. The molecular structure of [A~~Ru~(p-H)(p&OMe)(p-dppf)(C0)9] (3).L'41The C2, C' and C4 carbons on each of the phenyl rings have been omitted for clarity. The clusters [AuzRu3(p-H)(pu,-COMe){p-Ph2P(CH2),PPh2}(C0)9] [n = I (1) or 2 (2)] adopt similar square-based pyramidal skeletal geometries.[231
basal plane defined by the two Au atoms and two of the Ru atoms and the third Ru atom forming the apex of the pyramid (e.g. Fig. 2). In the ground state structure of [AuZRu~(p-H)(pu,-COMe)(p-Ph2PCHZPPh2)(CO),] ( l ) the , two methylene hydrogens are inequivalent and the two phenyl rings attached to each phosphorus atom are inequivalent, since there is no plane of symmetry through the two Au atoms in the square-based pyramidal A U Z R Umetal ~ NMR framework of the cluster (Fig. 3).[231At low temperatures, 'Hand I3C-{'H} spectra consistent with the ground state structure can be obtained. However, as the temperature is raised, the two separate methylene hydrogen signals in the 'HNMR spectrum broaden and then coalesce until, at the high-temperature limit, a single methylene proton resonance is observed. The two sets of signals in the I3C-{ 'H} NMR spectrum for each different type of phenyl carbon atom also broaden and then coalesce as the temperature increases and a single resonance is observed for each different type of phenyl carbon atom at the high temperature limit. Clearly, a fluxional process renders the two methylene hydrogens and the two phenyl rings attached to each phosphorus atom equivalent at high temperatures. This fluxional coupling is observed on the hydrido process must be intramolecular; since 31P-1H ligand signal in the ' H NMR spectrum at the high-temperature limit. The fluxional
1.27 Tlw Hetrronucliw C'luster Clieniistrj),of' tlze Group I I Met&
Figure 3. The metal core of the cluster [ A u 2 R u ~(p-H )( ,ul-COMe i(~(-Ph2PCH2PPh2(COlq] I l), showing the two inequivalent methylene proton environments and the inequivalence of the two phenyl rings attached to each phosphorus atom in the groundstate structure of the cluster. The carbonyl. hydrido and methoxycarbyne ligands have been omitted for clarity (reprinted by permission of the Royal Society of Chemistry from ref. 23).
5 13
U
Ru3
process occurring is thought" to involve an actual rearrangement of the squarebascd pyramidal AuzRui metal core of the cluster, together with a concomitant migration of the hydrido ligand around the Ru-Ru edges of the trigonal planar R u ~ fragment. The proposed mechanism for the rearrangement of the metal skeleton of [Au2Ru3(,u-H)(p3-COMe)(p-Ph2PCH2PPh2)(C0)9] involves a series of separate "windscreen-wiper'' movements of the diphosphine-digold unit, each of which must use one of the two gold atoms as a stationary pivot, while the other gold atom moves between the two remaining adjacent edge-bridging Ru-Ru sites (i.e. the second gold atom and the hydrido ligand exchange their edge-bridging sites). Successive movements of this type, with each gold atom acting as a pivot in turn results in a migration of the diphosphine-digold unit (and also the hydrido ligand) around ~ (Fig. 4). The the three edge-bridging Ru-Ru sites of the trigonal planar R u moiety proposed dynamic behaviour of the metal core of the cluster creates an effective mirror plane through the Ph2PCHZPPhz ligand, which is attached to the two gold atoms, and hence renders the two methylene hydrogens and the two phenyl rings attached to each of the phosphorus atoms equivalent on the NMR timescale. It is interesting that the edge-bridging Au( PPhl) unit in the cluster [AuRu3 {pi(Me2P)3CH}(CO)9( PPh3)J-[O?SCF3)is known to undergo migration around the *The observed variable-tempcrature ' H and " C - - { ' H ] N M R spectra can also bc explained by a fluxional process involving the two phosphorus atoms of the bidentatc diphosphine ligand Ph2PCH2PPh2 undergoing intramolecular site-exchange between the two gold atoms in the metal skeleton of the cluster. IIowever. this type of fluxional process is thought to be extremely unlikely.'231
514
1 Moleculur Clusters
Au fragment
0
RU
fragment
Figure 4. The mechanism proposed for the intramolecular metal core rearrangement of the squarebased pyramidal metal skelctons of the clusters [Au2Ru3(p-H)(p3-COMe)(p-Ll)(CO)9] [L? = Ph?P(CH2),PPhl (n = I or 5 ) or dppf]. This mechanism involves a series of separate “windscreenwiper” movements of the diphosphine-digold unit, each of which must use one of the two gold atoms (labeled with * on the figure) as a stationary pivot, while the other gold atom moves between the two remaining adjacent edge-bridging Ru-Ru sites (ie.the second gold atom and the hydrido ligand exchange their edge-bridging sites). Successive movements of this type, with each gold atom acting as a pivot in turn, result in a migration of the diphosphine-digold fragment (and also the hydrido ligand) around the three edge-bridging Ru-Ru sites in the trigonal planar R u j unit. The fluxional process creates an effective mirror plane through the bidentate diphosphine ligand, which is attached to the two gold atoms (reprinted by permission of the Royal Society of Chemistry from ref. 23).
three edges of the trigonal planar R u unit ~ in solution.[251This fluxional process is fast on the NMR timescale at ambient temperatures, but a 31P-{1H}N MR spectrum consistent with the ground state structure of the cluster was obtained at low temperatures. The variable-temperature NMR spectra of the clusters [Au2Ru3(pU-H)[Lz = PhzP(CH2)5PPh2 (2) or dppf (3)]are consistent with (p3-COMe)(p-L2)(CO)g] the A u ~ R umetal ~ frameworks of both compounds undergoing similar dynamic behaviour to that proposed for It is of interest to determine and compare the thermodynamic parameters for the ( l ) . [ 2 3 3 2 4 1
1.27 The Heteronuclecir Cluster C l ~ e ~ ~ i softlze t r j ~ Group I I Metals
5 15
intramolecular metal core rearrangements of the clusters ( l ) , (2) and (3). Bandshape analysis of variable-temperature "C-{ ' H ) NMR spectra for (1)-(3) and also of ' H NMR spectra for (1)* affords the thermodynamic parameters presented in Table 1. It is well established that energies quoted in terms of AGI values are less prone to systematic errors than the other parameters calculated by band-shape analysis and, therefore, AGT values are normally used for comparison purposes.[261 Table 1 shows that the magnitude of AGt for the metal core rearrangement of (1) determined from H NMR spectroscopy is identical within experimental error to that obtained from I3C-{ ' H ) NMR spectroscopy. Interestingly, the values of AGt for the skeletal rearrangements of (2) (58.6 ? 0.2 kJmol-') and (3) (58.9 ? 0.1 kJ mo1-I) are identical within experimental error, whereas the magnitude of AGt for the same fluxional process in (1) (67.1 & 0.2 kJmol-') is ca. 8 kJ mol-' greater. It is possible that the greater stereochemical demands of the Ph2PCH2PPh2 ligand compared with those of dppf and Ph?P(CH2)5PPh2hinder the migration of the two Au atoms around the trigonal planar Ru-i unit and so cause the magnitude of AG+ for this fluxional process to be considerably greater for (1) than the AG' values observed for (2) and (3). If this is the case and the relative stereochemical demands of the bidentate diphosphines are important, then it is perhaps rather surprising[271that the ferrocenyl backbone of dppf would seem to be exhibiting a very similar degree of flexibility to that of the aliphatic pentamethylene backbone of the Ph?P(CHZ)TPPh2 ligand. The magnitudes of AS: for intramolecular fluxional processes in organometallic complexes are frequently found to be between +20 and -20 J K-' mo1-'[281and all of the ASt values calculated for 11)-(3) (Table 1) lie in this range. This observation is consistent with the proposed intramolecular nature of the fluxional processes which (1)-(3) undergo in solution. The skeletal rearrangement mechanism proposed for (1)-(3) (Fig. 4) would also explain the single methylene proton environments observed in the low-temperature ' H N M R spectra of each of the clusters [Au2Ru4(p3-H)(p-H)(p-Ph2ECH2E'Ph2)(CO)l?]( E = E' = As or P; E = As, E' = P) (see Sec. 1.27.2.1.1). It is interesting that the proposed metal core rearrangement in each of the above clusters must have a very much smaller value of AG* than those observed for (1)-(3), since a ' H NMR spectrum consistent with the ground-state structure could not be obtained for any of the hexanuclear clusters. even at -90 0C.[201
'
1.27.2.1.3 Dynamic behaviour of the cluster [ A u z R u ~ ( ~ u ~ - H ) ( ~ - H ) ( ~ ~ ( - ~ , ~ - P ~ z P C ~(4) H~PP~~)(CO)IZI The metal core of the recently synthesized hexanuclear cluster [Au2Ru4(p3-H)(p-H)(, L L - ~ , ~ - P ~ ? P C CO) ~ H I~21P(4) P~ contains ~ ) ( a square-based pyramidal Au2Ru3
* It was not possible to derive the thermodynamic parameters for the skeletal rearrangements of (2)
and (3) from variable-temperature ' H NMR spectra.""241
ASt(JK-' mol-I)
7.5 10.5b 14.8 & 15.2 -19.9 & 11.1 20.2 i 8.0
AGt(kJ mol-')
67.1 f 0.2b 61.0 & 0.2 58.9 f 0.1 58.6 & 0.2
69.3 f 3.3b 71.4 f 4.1 52.9 f 3.2 64.1 i 2.2
AHt(kJmo1-I)
"Calculated at 298.15 K by band-shape analysis of selected phenyl carbon signals in variable-temperature I3C-{'H} NMR spectra, unless otherwise stated. bCalculated at 298.15 K by band-shape analysis of the methylene hydrogen signals in variable-temperature ' H NMR spectra.
(2) [ A u z R u(p-H)(p3 ~ -COMel { P - P ~ ~ P ( C H Z ) , P1(COhI P~Z (3) [AuzRu3(p-H)(p3-COMe)(~-dppf)(CO)91
(1) IAurRu3(p-H)(p3-COMe)(pu-Ph2PCH2PPh2)(C0)9I
Cluster
Table 1. Energy parameters" for the intramolecular metal core rearrangement observed in solution for the clusters [Au2Ru3(pH)(p3-COMe)(pL-L2)(C0)p] [L2 = Ph*P(CH*),PPhZ ( n = 1 or 5)[231 or dppf {dppf = I . l'-bis(diphenylphosphino)ferr~cene}~~~~] (Fig. 4).
2:
n,
5
Q 2
E
n
55
m
2
1.27 The Hetrronucleur C1u.ster Chemistrjy of the Group I I Metuls
5 17
Figure 5. The molecular structure of IA u2 Ruq ( p3-H ) (p-W ) ( p- 112-Ph?PCs Hq PPh2)(CO)I 21 (4).'2 'I There is no planc of symmctry through the two gold atoms in the capped square-based pyramidal AuzRuq metal core of the cluster, so the two phenyl rings on each phosphorus atom of the bidentate diphosphine ligand are not equivalent in the ground-state structure. The two hydrido ligands have been omitted for clarity. One of these hydrido ligands is thought to occupy an edge-bridging site across the Ru(2)-Ru(3) vector and the other is thought to cap the Ru(l)Ru(2)Ru(4)face of the cluster's metal skeleton.'2y1
unit and the two sets of phenyl rings on each phosphorus atom are inequivalent (Fig. 5).[291However, at ambient temperatures, the I3C-{ 'H} NMR spectrum of (4) shows a single resonance for each different type of phenyl carbon atom, which is consistent with the metal core of the cluster undergoing a similar rearrangement in solution to that proposed for (1)-(3) (Fig. 4). The intramolecular metal core rearrangement of (4) must be accompanied by a concomitant fluxional process involving site-exchange of the two hydrido l i g a n d ~ .Although ~ ~ ~ ] some broadening* is observed for the phenyl carbon atom signals in the I3C- { ' H } NMR spectra at low temperatures, no spectrum consistent with the ground-state structure of (4) could be obtained. Thus, the magnitude of AGI for the metal core rearrangement in (4) must
*The broadening observed in the low-temperature 13C-{ 'H} N M R spectra may be associated with the slowing down of the rotation of the phenyl rings around the P-C bonds rather than the slowing down of the intramolecular metal core rearrar~gement.'~~]
518
I Molecular Clusters
be considerably lower than those observed for (1)-(3), which is consistent with the {E = observations for the clusters [Au2Ru4(p3-H)(p-H)(p-Ph2ECH2E'Ph2)(C0)12] E' = As or P; E = As, E' = P) (Sec. 1.27.2.1.2),[231which are closely related to (4).
1.27.2.1.4 Dynamic behaviour of other clusters with metal frameworks containing square-based pyramidal A u ~ R uunits ~ Although to the best of our knowledge, the paper describing the clusters [Au2Ru3(p-H)(p3-COMe){p-Ph2P(CH2),PPh2}(C0)~] (n = 1 or 5)[231was the first specific report of the stereochemical non-rigidity of a cluster containing a squarebased pyramidal A u ~ R uunit, ~ it is interesting to look back at the NMR data described in the literature for other clusters with metal skeletons containing similar A u ~ R uunits ~ to see if there is any evidence for the novel fluxional process that (1)(4) undergo. 161 has a metal core structure conThe cluster [Au2RugC(pu-Ph2PCH2PPh2)(Co) sisting of a square-based pyramidal A u ~ R uunit ~ and an Rug octahedron, containing an interstitial carbido ligand, fused together by sharing a common R u face.[301 ~ Although the two methylene hydrogens and the two phosphorus atoms in the Ph2PCH2PPh2 ligand, which is attached to the two gold atoms, are both inequivalent in the ground-state structure, a triplet is reported for the methylene hydrogen signal in the ' H NMR spectrum of the cluster and a singlet is reported for the 3'P-{1H} NMR spectrum. The authors[301do not comment on their NMR spectroscopic data, but an intramolecular rearrangement of the square-based pyramidal A u ~ R umoiety ~ in the octanuclear cluster similar to that observed for (1)-(4) (Fig. 4) (Sec.s 2.1.2 and 2.1.3) would explain the apparent equivalence of the two methylene hydrogens and the two phosphorus atoms on the NMR timescale in solution. The variable-temperature I3C-{ 'H} NMR spectra observed for the cluster [Au2Ru3(p-H)(p3-COMe)(C0)9( PPh3)2] are also consistent with the square-based pyramidal A U Z R Umetal ~ skeleton undergoing an intramolecular rearrangement in However, the spectra can equally well be explained by carbonyl ligand site-exchange, which is a well established fluxional process for transition metal organometallic
1.27.2.2 Dynamic behaviour involving the bidentate diphosphine ligand 1,l '-bis(dipheny1phosphino)ferrocene (dppf ) attached to the group 11 metals in heteronuclear clusters 1.27.2.2.1 Introduction Our interest in studying the fluxional behaviour of the dppf ligand attached to coinage metals in Group 11 metal heteronuclear clusters was prompted by a report of a
1.27 The Heteronuclcw Cluster Chemistrj?of the Group 11 Metals
519
variable-temperature NMR spectroscopic study on the cluster (Au?Ruq(p-dppf)(C0)12BH].[341The dppf ligand attached to the gold atoms in the hexanuclear cluster undergoes dynamic behaviour involving inversion at the two phosphorus atoms and concomitant twisting of the two cyclopentadienyl rings. This dppf ligand fluxionality was observed to be in concert with a “rocking” motion of the two Au atoms with respect to the Ru4B core of the cluster and the two processes were found to be mutually dependent. Therefore, we wished to investigate the dynamic behaviour of the dppf ligand when it bridges two Group 11 metals in a heteronuclear cluster further. As well as the compound [AuzRu3(p-H)(p3-COMe)(p-dppf)(C0)9] (3):which is discussed in Sec. 1.27.2.I .2, we were interested in studying systems with metal frameworks containing trigonal bipyramidal or distorted trigonal bipyramidal M2Rul ( M = Cu, Ag or Au) units, since these species are also known to have stereochemically non-rigid metal cores in solution.[3 5 , 3 6 1 However, the intramolecular core rearrangements of the latter type of cluster involve a very different mechanism to that previously discussed for (3).The two coinage metals in the trigonal bipyramidal or distorted trigonal bipyramidal MzRu3 units in the ( M = Cu; Ag or Au)[”] and metal cores of the clusters [M2Ru4H2(pu-dppf)(C0)12] ~~’ between the two distinct sites ( i e . axial [Au2Ru3(p3-S)(p-dppf) ( C O ) S ] [exchange and equatorial in the ground-state metal framework structures) via a mechanism which is thought to involve a restricted Berry pseudo-rotation (Fig. 6).[35p371
1.27.2.2.2 The dynamic behaviour of the clusters IAU2RU3(iu-H)(iu3-COMe)(iu-dPPf)(C0)91 (31, [ M ~ R ~ ~ H ~ ( ~ ~ - ~ PIM P= ~ )cu ( C(51, O Ag ) ~ (6) ~ Ior A U (711, [AU2RU3(iu3-S)(iu-dPPf)(C0)91(8) and IAuCuRu4(iu3-H)2(iu-dPPf)(C0)121 (9) The clusters [M2RuqH+dppf)(C0)12] [ M = Cu (S), Ag (6) or Au (7)][27,381 and [A~?Ru3(p~-S)(p-dppf)(C0)9][~~’ (8) all have metal frameworks which contain trigonal bipyramidal or distorted trigonal bipyramidal M z R u ~groups (Figs. 7, 8 and 9). Variable-temperature ‘ H and 3’P-( ‘ H f N M R spectroscopic studies show that the two coinage metals in each cluster still undergo intramolecular site-exchange, even though they are linked together by the dppf ligand.[24*27,381 For clusters 15)-(7), this metal core rearrangement must be accompanied by a concomitant process involving the site-exchange of the two hydrido ligands. The spectra also show that, in each case, the dppf ligand is undergoing a second fluxional process, involving inversion at the phosphorus atoms and a concomitant twisting of the cyclopentadienyl rings. Figure 10 illustrates how the two fluxional processes exchange the hydrogen atoms of the cyclopentadienyl rings between different environments and a typical series of variable-temperature spectra are illustrated for (6) in Fig. 1 1 .[381 The 2D EXSY ‘ H N M R spectrum of (5) iFig. 12) also clearly shows how the cyclopentadienyl hydrogen atoms in the copper-containing cluster are exchanged by a combination of the metal core rearrangement and dppf ligand
520
1 Molrculur Clusters
Figure 6. The restricted Berry pseudo-rotation mechanism proposed for the intramolecular metal core rearrangements observed in solution for the clusters [ M ~ R U ~ H ~ ( C O ) ~=~Cu, L ~Ag ] or Au; (M L = a variety of monodentate phosphine ligands or L2 = a variety of bidentate diphosphine ligands). The mechanism exchanges the two coinage metals in sites M( 1) and M(2) of the trigonal bipyramidal M2Rul unit in the metal skeletons of the clusters oio a square-based pyramid a1 inter' mediate (reprinted by permission of the Royal Society of Chemistry from ref. 36).
fluxional process.[271The dppf fluxion is also occurring in addition to the intramolecular metal core rearrangement (Sec. 1.27.2.1. l ) for (3). The thermodynamic parameters for the two distinct fluxional processes in (3) and (5)-(8) have been calculated by band-shape analysis of variable-temperature NMR spectra where possible (Table 2).[243271 All of the values of AS$ in Table 2 are between -20 and +20 J K-' mol-', which supports the proposed intramolecular nature of the dppf ligand fluxionality and the metal core Table 2 shows that the values of AGi obtained for the intramolecular metal core rearrangements of (5)-(7) decrease in the order of descending Group 11 metal congeners, as has been previously observed for a number of analogous clusters. The value of AGI obtained for ( 5 ) is significantly larger than the ranges of ca. 40-43 kJ mol-'
Figure 7. The molecular structure of [ C U ~ R U ~ [ ~ - H ) ~ ( p d p pf )(CO),r](5).showing the capped trigonal bipyramidal skeletal geometry. Thc structure of the analogous silver-containing cluster [A~~RUJ(~I,-HI~(//-~~~~)(CO)I~~ (6) is thought to be very similar and that of the closely related trimetallic species [AuCuRul (/ci-H)li/i-dppf)(<’0)121 (9) is also very similar, except that there is a gold atom in the site of Cu(2). rather than a copper atom (reprinted by permission of‘ Elsevier Science Ltd. from ref. 38).
Figure 8. The molecular structure of [ A u ~ R u ~ ( / / I -/ H ~ -)HI ) ( / L dppfhC0)121 ( 7 ) .One of the equatorial Au-Ru [Au(1 )-Rur2)] distances is too long (Au. .Ru 3.558 A! for any significant bonding interaction, so the A u ~ R u ~ fragment in the metal core of the cluster is distorted towards a square-based pyramidal geometry Therefore, the A u z R u ~skeletal geometry of the cluster is intermediate between capped trigonal bipyramidal and capped squarebased pyramidal (i.c,. intermediate between gold atom arrangements A and B in Fig. 13) 1 reprinted by permission of the Royal Society of Chemistry from ref. 27).
d
522
1 Molecular Clusters
Figure 9. The molecular structure of [ A U ~ R U ~ ( ~ ~ - S ) ( / L(S),‘”’ -~P~ show~)(CO)~] ing the trigonal bipyramidal Au2Ru3 skeletal geometry.
Figure 10. Schematic representations of the two dynamic processes which the clusters [ M ~ R u ~p-H z ( dppf)(CO),z][M = Cu ( 5 ) ,Ag (6) or Au (7)]undergo in solution. In each case, the iron atom and the phenyl rings of the dppf ligand have been omitted for clarity. (a) The intramolecular metal core rearrangement, which exchanges the two coinage metals ( M and M’) in each of (5)--(7)between the two distinct sites [eg Cu(1) and Cu(2) in Fig. 71 in the capped trigonal bipyramidal or distorted capped trigonal bipyramidal metal skeletons of the clusters, shown when the dppf ligand is stereochemically rigid, (b) the effect of the intramolecular metal core rearrangement on the cyclo-
1.27 The Heteronudear Cluster Chemistry of the Group 11 Metals
523
HA
Figure 10 (continurtl) pentadienyl (Cp) hydrogens when the dppf ligand is stereochcmically rigid, (c) the inversion at the phosphorus atoms and twisting of the cyclopentadienyl rings observed for the dppf ligand. At the low-temperature limit. Cp hydrogens HA--HHare all inequivalent, so eight signals are observed in the ' H NMR spectra of (5)and (6) iey. Fig. 11). However. at intermediate temperatures, the skeletal rearrangements of (6! and (7) are fast on the NMR timescale, but the dppf ligand fluxional process cannot be observed. Thus, the site-exchange of Group 1 1 metals M and M'causes exchange also to occur between the following pairs of Cp hydrogens, HA HH, Hg ++ HG, Hc HF and Ho 4 HE and four Cp hydrogen peaks are observed in the ' H NMR spectra of (6) (Fig. 1 1 ) and (7) at -60 or -50 "C. At the high-temperature limit, both the metal core rearrangement and the dppf ligand fluxion are fast on the NMR timescale. The latter dynamic process produces effective mirror planes through both of the two C S H ~ units P of the dppf ligand and exchanges the pairs of protons HA HD, HB tf Hc. HE Hf13HI. HG. Therefore, the combination of the two fluxional processes renders the set of four hydrogens H A , HI), Hb and HH all equivalent and the set of four hydrogens He. Hc. HI. and HG all equivalent on the NMR timescale and so two signals due to Cp hydrogens are visible in the ' H NMR spectra of(5)-(7) at ambient temperatures and above (q. Fig. 1 I ) (reprinted by permission of Elsevier Scicnce Ltd. from ref. 38).
-
-
-
-
524
I Moleculur Clusters
Ii
(c)
50
,
v--
45
40
35
,
3 0
Figure 11. The signals due to the Cp hydrogens in the ' H N M R spectra of IAgZRLi4(p3-H)( p d p p f )(CO)12]($4 recorded at various temperatures. (a) The high-temperature limiting spectrum at +40 " C , with both the dppf ligand fluxion and the skeletal rearrangement fast on the N M R timescale, (b) at -50 "C, with the skeletal rearrangement still fast on the N M R timescale. but the dppf ligand fluxion not observed, (c) the low-temperature limiting spectrum at -100 " C , when both the skeletal rearrangement and the dppf ligand fluxion are not observable on the N M R timescale (reprinted by permission of Elsevier Science Ltd. from ref. 38).
1.27 The Heteronuclear Cluster Chemistry of the Group 11 Met&
525
3.5
1.0
1.5 6
i.0
i.5
5.5
5.0
4.5
4.0
3.5
s
Figure 12. The signals due to the cyclopentadienyl hydrogens in the 2D EXSY ' H NMR spectrum of the cluster [Cu2Ru4(p3-H)2(p-dppf)(C0)12] ( 5 ) in CD2Clz solution, recorded at -70 "C. The spectrum clearly shows which cyclopentadienyl hydrogens are exchanged by a combination o f the intramolecular metal core rearrangement process and dppf ligand fluxion. All of the hydrogen atoms (group I ) which produce signals A, B. F and H are being exchanged by a combination o f the two fluxional processes and so are all of the hydrogen atoms (group 2) which give rise to peaks C , D. E and G. However, there is clearly no exchange between any of the hydrogen atoms belonging to group 1 and any of those in group 2. The signals marked with * are due to the solvent. and the other very small peaks which are visible are either spinning sidebands of the solvent signals or they are due to trace impurities (reprinted by permission o f the Royal Society of Chemistry from ref. 27).
and ca. 40-45 kJmol-', which have been previously reported for skeletal rearrangements in the analogous clusters [Cu2Ru4(p3-H)2(CO)12L2]( L = a variety of monodentate phosphine and phosphite l i g a n d ~ ) ' ~and ~ ] [Cu2Ru4(p3-H)2(p-L2)(C0)12][L2 = PhzE(CH2),,PPhl ( E = P, n = 1, 5 and 6; E = As, n = 1 or 2) or c ~ s - P ~ ~ P C H = C H P Prespectively. ~ ~ ] , [ ~ ~ ] However, the size of AGI measured for ( 5 ) is comparable to that of 48.1 0.2 kJ mol-' calculated by band-shape analysis at 298 K for the similar fluxional process in [Cu2Ru4(p3-H)2{p-Ph2P(CH2)2PPh2}-
AGt(kJ mol-')
AS7 (J mo1-l K-I )
ASi(Jmol-lK-l)
AHt(kJ mol-')
AH~(kJmol-')
"Calculated at 298.15 K by band-shape analysis of the cyclopentadienyl hydrogen signals in variable-temperature 'HN M R spectra, unless otherwise stated. bThe dppf ligand undergoes a fluxional process, which involves inversion at the phosphorus atoms together with a twisting of the cyclopentadienyl rings (Fig. 10). 'Calculated at 298.15 K by band-shape analysis of variable temperature 31P-{IH} N M R spectra. "The free energy of activation for this process in [Au2R&(p3-H)(p-H)(p-dppf)(C0)l~] (7) is too low for either the low-temperature limiting ' H NMR spectrum or a low-temperature limiting "P-{'H} N M R spectrum with narrow linewidths to be observed, even at 173K. Therefore. band-shape analysis of the variable-temperature ' H or 31P--{' H } N M R spectra to obtain parameters for the metal core rearrangement was not possible for (7).However, the value of AGi for the metal core rearrangement in (7) has been estimated from the coalescence temperature in the variable temperature ilP-{ 'H}N M R spectra as ca. 33 kJ mol-' at the coalescence temperature of ca. 193 K.L3*1 'Thc metal skeleton of [AuCuRuq(~3-H)z(p-dppf)(C0)12] ( 9 )is stereochemically rigid in solution at ambient temperatures. 'No NMR spectra consistent with the ground-state structure of [Au2Ru3(p3-S)(p-dppf)(CO)q](8, could be obtained, even at low temperatures. Therefore, no value of AGI for the intramolecular metal core rearrangement of (81could be obtained.["1 'Calculated at 298.15 K by band-shape analysis of variable-temperature "C-{ IH} NMR spectra.
Cluster
f
AGi (kJ mol-' )
(b) Intramolecular rearrangement of the cluster's metal core.''
Cluster
(a) I , 1'-Bis(dipheny1phosphino)ferrocene(dppf) ligand fluxion.b
Table 2. Energy parameters" for the fluxional processes observed in solution for the clusters [MMrRuH2(p-dppf)(C0)12]( M = M ' = Cu, Ag or Au; M= Cu, M' = Au)~"], [Au2Ru3(p-H)(p3-COMe)(p-dppf)(C0)9] i24i and [Au2R~~(p~-S)(dppf)(CO)s].~~~j
'r
cr\
1.27 The Hetesonuckecir Cluster Chernistsj?of the Group I I Metals
527
(CO)12].[351 The only previously reported values of AGi for metal core rearrangements in copper-containing clusters, which are larger than that observed for (5) are 52.8 f 0.1 kJmol-I (for n = 3) and 52.1 k 0.1 kJmol-' (for n = 4) for I C U ~ R U ~ The magnitude of AGI for the skeletal (p3-H)2(~-P~~P(CH~),,PP~~}(CO)I~].[~~] rearrangements in the silver-containing cluster (6) is comparable to those of 40 k 1 kJmol-' and 40.4 k 0.1 kJmol-I measured for (Ag2Ru4(pc,-H)2(CO)12( PPh3)2]'361and [Ag2Ru4(/i3-H)2(,u-Ph2P(CH2)3PPh2}(C0)12],[351 respectively. It is significantly larger than the range of AGi values of ca. 32-36 kJ mol-' observed for [Ag2Ru4(p3-H)2(p-L~)(C0)12] [Lz= Ph?P(CHz),PPh2 (n = 1 or 2) or cis-PhzPCH= C H P P ~ Z ]51. [The ~ only previously reported values of AGt for skeletal rearrangements in silver-containing clusters that are larger than that observed for (2) are 41.5 f 0.1 kJmol-' (for n = 4) and 42.2 f 0.1 kJmol-' (for n = 6) for [AgzRuq( p u , - H ) 2 ( ~ i - P h ~ P ( C H ~ ) n P P h ~ } j C 0and ) 1 ~45 ] ' 3ks 11 kJ mol-' for [Ag2Ru4(p3-H)2(CO)l2(PR3)2] ( R = Pr' or Cy).[""' Accurate quantitative comparisons of the magnitudes of A@ for metal core rearrangements in gold-containing clusters are very difficult, since the free energies of activation are normally too small for wellresolved low-temperature NMR spectra to be ~ b t a i n e d . ~ ~ ~ ~ ~ ~ ~ ~ ~ ] The AGi values calculated for the dppf ligand fluxion (Table 2) clearly show that this process and the metal core rearrangement are independent for each of the clusters, (3) and (5)-(8), which is in marked contrast to the two mutually dependent processes reported[341for [Au2Ru4ipu-dppf)(C0)12BH]. The values of AGi for the dppf ligand fluxionality in clusters (5)-(8) all lie in the range 47.0 _+ 0.2 to 51.5 f 0.1 kJ mol-', so it would appear that the nature of the coinage metals that the dppf ligand is attached to and the exact nature of the structure of the metal framework of a cluster do not have a great influence on the dynamic behaviour of the dppf ligand. However, the magnitude of AGt for the dppf fluxion in (3) (45.2 0.1 kJmol-I) is ca. 2 kJ mol-' lower than the bottom end of the above range. This may reflect the fact that the dppf ligand is attached to a square-based pyramidal A u ~ R uunit ~ (Fig. 2) rather than to a trigonal bipyramidal or distorted trigonal bipyramidal M2Ru3 ( M = Cu, Ag or Au) moiety (Figs. 7-9). We also wished to establish whether or not the dppf ligand fluxion would occur in a cluster with a stereochemically rigid metal core, so we synthesized the trimetallic 121 (9) ( Fig. 7).[271 The variable-temperature species [AuC~Ru~(p~-H)~(p-dppf)(CO) IH NMR spectra of (9) demonstrate that the dppf ligand undergoes the same fluxional process as that observed for (3) and ( 5 ) - ( 8 ) . Band-shape analysis of these NMR spectra afforded a value of A($ of 49.2 0.2 kJmol-' for the dppf ligand fluxion (Table 2). This ACi value lies in the range observed for the same fluxion in (5)-(8) (Table 2), so it would appear that whether or not the dppf ligand is bonded to a cluster metal framework that is stereochemically non-rigid in solution does not have a large effect on the magnitude of ACT for the inversion and twisting process.
528
1 Moleculur Clusters
1.27.3 The use of sterically demanding bidentate diphosphine ligands to control the metal core geometries adopted by clusters containing A u ~ R u ~ units 1.27.3.1 Introduction As discussed in Sec. 1.27.1, the energy differences between different metal core structures can be very small for some Group 11 metal heteronuclear clusters. Species containing two Au( ER3) ( E = As or P; R = alkyl or aryl) units bonded to other transition metals exhibit a wide range of skeletal geometries,[6-s1 but four main structural types can be identified. These structural types depend on whether the two Au(ER3) groups are edge-bridging or face-capping and whether or not there is close contact between the gold atoms. The most commonly observed metal core structure has two face-capping Au( ER3) fragments in close contact (Gold atom arrangement A in Fig. 13).[6-81Digold heteronuclear clusters containing two Au(ER3) units bridging different metal-metal edges with the gold atoms in close contact (Gold atom arrangement B in Fig. 13) are much rarer and most known examples have the Au atoms linked together by bidentate ligands.[6-81We wished to investigate how digold clusters could be forced to adopt skeletal geometries containing gold atom arrangement B (Fig. 13) by synthesizing two series of analogous clusters ligated by bidentate diphosphines of varying stereochemical demands.
Au fragment
0 Other transition metals fragment M' Figure 13. Two possible arrangements for the two Au atoms in a digold heteronuclear cluster. The trigonal planar M; fragment can either be a discrete three-metal unit or one face of a larger polyhedron of transition metals M' other than Group 11 metals. (a) Au arrangement A. (b) Au arrangement B (reprinted by permission of the Royal Society of Chemistry from ref. 23).
1.27 The Hcteroriuclear Cluster Chemistrj~q f t h e Group I I Metals
529
1.27.3.2 Structures of clusters of general formula [Au2Ru4(p3-H)(p-H)(,u-L2)(C0)12] [L2 = Ph2P(CH2),PPh2 (n = 1 or 2), cis.-Ph2PCH=CHPPh2, 1,2-Ph2PC~H4PPh2or 1,l '-bis(dipheny1phosphino)ferrocene (dppf )] Table 3 lists the metal core structures adopted by the clusters [Au2Ru4(p3-H)(p-H)(CO)I ? ( PPh3)2]'"' and [Au2Ru4(p3-H)(p-H)jp-L2)(CO) 121 [L2 = Ph2P(CHZ),PPh2 (n = 1 or 2),[2'1C ~ S - P ~ ~ P C H = C H P 1,2-Ph2PC6H4PPh2[291 P ~ ~ , ' ~ ~ , ~ ~ ]or dppflZ7]].It can be seen that the formal replacement of the two PPhi groups bonded to the gold PPh3)zj by any of the bidentate diphosphine atoms in [Au2Ru4(p3-H)(p-H)(COlj2( ligands L2 results in a change in the metal framework structure of the cluster. The ligands Ph2PCHlPPh2, cis-Ph?PCH=CHPPhl and 1, ~ - P ~ ~ P C ~ all H ~force PP~I ~ observed in the metal skeleton of the PPh3the trigonal bipyramidal A u ~ R uunit containing species (Fig. 14) to change to gold arrangement B (e.g. Fig. 5 ) . However, the ligands Ph2P(CH2)2PPh2 and dppf seem to be less stereochemically demanding in this context and they both distort the trigonal bipyramidal AuzRui unit in [Au2Ru4(p3-H)(p-H)(C0)12( PPh3)2]towards a square-based pyramid, so that one of the equatorial Au-Ru (Au(1). . .Ru(2) in Fig. 8) distances is too long for any
Table 3. The different gold atom arrangements adopted by the Au2Rui units in the A u ~ R umetal ~ f r a m e w o r k s o f t h e c l u s t e r s [ A ~ ~ R ~ ~ ( ~ ~ , - H ) ~ ~ ~[Lz - H=~ Ph>P(CH2J,PPhl ( ~ ~ - L ~ ) ~ C O ~( ~ n= ~ ]1 or 2),I2I1 dppf,["] tu-PhzPCH=CHPPh? Ii5 41' and 1,2-Ph2PC6H4PPh2["1] dnd IAu2Ru4(p7-H)(pU-H)(CO)12(PPhi),] 1 5 Cluster
Gold atom arrangement for the AuzRu3 unit (Fig. 13)
IAU~R~?(/~,-H)(~-H)(CO)I~~PP~~)~~ [Au2Ru4(pu,-H)(pu-H) (pu-PhzP(CH?)zPPhz) (CO)12 I [AU~RU~(~~-H)(~U-H,(~-~P~~)(CO),~~
A (trigonal bipyramidal) intermediate between A and B" intermediate between A and Bb IAU~RQ(/~~-H)(~-HJ(/~-P~~PCH>PP~~)(CO)I~]' B (square-based pyramidal) [A~?R~J(~~-H)(~-H)(/C-C~S-P~~~CH=CH=CHPP~~)(CO)~~] B (square-based pyramidal) [ A U ~ R U ~ ( / ~ , - H ) ( ~ - H ) ( ~ ~ - ~ . ~ - P ~ ? P C ~ H ~ P P ~ B~ (square-based ) ( C O ) I ~ ~ pyramidal) "One of the equatorial Au-Ru distances is too long (Au. .Ru 3.446A) for any significant bonding interaction, so the AuzRu? unit in the Au2Ru4 metal core of the cluster is distorted towards a square-based pyramidal geometry.'"] of the equatorial Au-Ru [Au(1). . .Ru(2) in Fig. 81 distances is too long (Au.. .Ru 3.558 A) for any significant bonding interaction, so the AuzRu3 unit in the AuzRu4 metal core of the cluster is distorted towards a square-based pyramidal g e ~ m e t r y . i ~ ~ ] "The closely related clusters [Au~Ru4(/c,-H)(p-H)(p-Ph~AsCH~EPh~)(CO)~~] (E = As or P) also adopt very similar skeletal geometries. with the Au2Ru3 fragment exhibiting gold atom arrangement B.1221
530
1 Moleculuv Clusters
Ppl Figure 14. The molecular structure of [Au~Ru~(IIu,-H)(II-H)(CO) 12( PPh3 121. The gold atoms in the capped trigonal bipyramidal A u ~ R metal u ~ framework adopt gold atom arrangement B (Fig. 13). The phenyl groups have been omitted for clarity (reprinted by permission of the Royal Society of Chemistry from ref. 36).
significant bonding interaction (Au-Ru 3.446 A for Ph2P(CH2)2PPh2[21 and 3.558 A for dppfIZ7]).Therefore, the metal core structures adopted by [Au2Ru4(p3H)(p-H)(p-L2)(CO)12][L2 = PhzP(CH2)2PPh2 or dppf] can be best described as intermediate between capped trigonal bipyramidal and capped square-based pyramidal (i.e. intermediate between gold atom arrangements A and B in Fig. 13).
1.27.3.3 Structures of clusters of general formula [Au~Ru~(u~-S)(,U-L~)(CO)~] [L2 = Ph2P(CH2),PPh2 (n = 1 or 2), &-Ph2PCH=CHPPh2, 1,2-Ph2PC6H4PPh2 or 1,l'bis(dipheny1phosphino)ferrocene (dppf )] Table 4 lists the metal core structures adopted by the clusters [Au2Ru3(p3-S)(C0)9( PPh3)2lL4']and [Au2Ru~(p3-S)(,u-L2)(CO)9] [L2 = Ph2P(CHz),PPh2 (n = 1[431 or 2[291),C ~ S - P ~ ~ P C H = C H 1,2-Ph2PCsH4PPh2[291 P~~,[~~I or dppf1241].It can be seen that the bidentate diphosphine ligands generally have a much smaller effect on the metal core structures of the pentanuclear Au2Ru3 clusters compared with that observed for the hexanuclear A u ~ R usystem ~ (Sec. 3.2). In fact, none of the bidentate diphosphine ligands studied are capable of distorting the trigonal bipyramidal AuzRu3 unit to a square-based pyramidal geometry ( c j Table 3). The formal replacement of the two PPh3 groups attached to the gold atoms in
1.27 The Heteronuclrar Cluster Chemistry o f t h e Group I 1 Metals
531
Table 4. The different gold atom arrangements adopted by the AuzRu3 metal frameworks of the clusters I A ~ ? R ~ ~ ( ~ I , - S ) ( ~ - L ?[L? ) ( C=OPhlP(CH?),,PPhz )Y] (n = 1 ~ 4 i ]or 2120'), dppf, cu-Ph?PCH= "'l] and [AuzRu3(pi-S)(CO)y(PPhj )>]I3' 42' CHPPh?1441and 1,2-PhzPC6H~PPh2 Cluster
Gold atom arrangement for the AuzRu3 metal framework (Fig. 13)
[AU2RuI(pLJ-S!(CO)Y(PPh3 h1 [Auz R u (pj-S){ ~ /L-Ph2P(CH?)2PPh?}(CO10 1 [AuzRu?(p,-S)(pc-d~~f)(CO!~l [Au?Ruj ( p j-S)(p-cb-Ph?PCH=CHPPh:)(CO)9] [A uR ~ U? ( ~-S) 3 (p-Ph2PCH zPPh2)iCO)S1
A (trigonal bipyramidal) A (trigonal bipyramidal) A (trigonal bipyramidal) A (trigonal bipyramidal) Intermediate between A and Ba [ A u ? R u ~ ( ~ ~ - S ) ( / L - I , ~ - P ~I~ P C ~ H ~ P PIntermediate ~ ~ ) ( C O )between ~ A and Bb "One of the equatorial Au-Ru distances is too long ( A u . ..Ru 3.335 A) for any significant bonding interaction, so the Au2Ru3 metal core of the cluster is distorted towards a square-based pyramidal geometry.14" bThere are two independent molecules in the asymmetric unit of this cluster. In each of these molecules, one of the equatorial Au-Ru [ry. Au(2j.. .Ru(3) in Fig. 151 distances is too long (Au. ' .Ru 3.487 and 3.515A) for any significant bonding interaction, so the AulRuj metal core of the cluster is distorted towards a square-based pyramidal ge~metry.:'~]
[Au2Ru3(p3-S)(C0)9( PPh3)2] by the bidentate diphosphine ligands PhZP(CH2)zPPh2, cis-PhzPCH=CHPPh2 and dppf does not significantly alter the trigonal bipyramidal skeletal geometry observed for the PPh3-containing species (e.g. Fig. 9). Only two of all of the bidentate diphosphines used cause any significant change to the skeletal geometry observed for the PPh3-containing species, which is in marked contrast to the results obtained for the hexanuclear A u ~ R usystem ~ (Sec. 3.2). The 1,2-Ph?PChHqPPhz and PhzPCH2PPh2 ligands both cause a change from the trigonal bipyramidal gold atom arrangement A (Fig. 13) in [Au2Ru3(p3-S)(C0)y(PPh3)2] by causing the elongation of one of the equatorial Au-Ru [eg. Au(2).. .Au(3) in Fig. 151 distances, so that it is too long for any significant bonding interaction. This elongation is 3.335 A for [Au2Ru3(p3-S)(p-Ph2PCH2PPh2)( C O ) Y ] [and ~ ~ ]3.487 and 3.515 A for the two independent molecules, which are observed in the asymmetric unit of [ A u ~ R u ~ ( ~ ~ - S ) ( ~ - ~ , ~ - P ~ ~ P C ~ H ~ P (Fig. 15). Therefore, the Au2Ru3 metal frameworks of [Au2Ru3(p3-S)(p-L~)(C0)9] ( Lz=PhzPCHzPPh2 or 1 , ~ - P ~ Z P C ~ Hboth ~ P adopt P ~ ~ geometries ) which are intermediate between trigonal bipyramidal and square-based pyramidal (i.e. gold atom arrangements A and B in Fig. 13). It is interesting that the sterically demanding bidentate diphosphine ligands that are capable of forcing the A u ~ R uunit ~ in the metal cores of the A u ~ R uclusters, ~ which are described in Sec. 1.27.3.2 (Table 3), to adopt the square-based pyramidal gold arrangement B (Fig. 13) are not able to cause the same distortion in the metal skeletons of the sulfur-containing Au2Ru3 clusters (Table 4). This observation is
532
1 Molecular Clusters
E4.3 \Lj
Lb
Figure 15. The molecular structure of one of the two molecules in the asymmetric unit of the cluster [Au*Ru? (,u?-S)(p-l,2-Ph2PC6H4PPh2)(CO)91.[29] In each of these molecules, one of the equatorial Au-Ru [eg. Au(2). . .Au(3)] distances is too long (Au.. .Ru 3.487 and 3.515 A) for any significant bonding interaction, so the AuzRu3 metal core of the cluster is distorted towards a square-based pyramidal geometry. Therefore, the gold atoms in the metal framework of the cluster adopt a structure, which is intermediate between gold arrangements A and B (Fig. 13).
especially surprising in view of the fact that gold arrangement B is actually the preferred skeletal geometry for the series of similar pentanuclear clusters [Au2Ru3(p-H)(p3-COMe)(pu-L2)(CO)9] [L2 = Ph2P(CHz),PPh2 (n = 1 or 5)[231 and dppfIz4]](Sec. 1.27.2.1.2.) (e.g. Fig. 2) and also for [Au2Ru3(p-H)(p3-COMe)(C0)9(PPh3)2],L311 in which the gold atoms are not even bridged by a bidentate diphosphine ligand.
Acknowledgements I would like to acknowledge the following co-workers for their invaluable help in the recent investigations of the chemistry of Group 1 I metal heteronuclear clusters: Dr. Steven Williams (synthesis and NMR spectroscopy), Drs. Vladimir Sik and Keith Orrell ( N M R spectroscopy), Ms Catriona Collins (NMR spectroscopy) and Drs. Trushar Adatia and Jon Steed (X-ray crystallography). My thanks are also due to Drs. Steven Williams and Kevin Young for the helpful advice that they offered concerning this article. In addition, I am very grateful to John Matthey plc for supporting my work with generous loans of gold, silver and ruthenium salts.
1.27 The Heteronuclear Cluster Cliernistry of the Group 1 I Metuls
533
References [ I I C.E. Coffey, J. Lewis, R.S. Nyholm; J. Cliern. Soc.. 1964, 1741. [2] A.S. Kasenally, R.S. Nyholm, R.J. O’Brien, M.H.B. Stiddard, Nuture, 1964, 204. B71. [3] AS. Kasenally, R.S. Nyholm, M.H.B. Stiddard, J. Am. Chenz. Soc.. 1964, 86; 1984. 141 J.W. Lauher. unpublished results. See ref. 9 for details. [ 5 ] For example, D.A. Roberts. G.L. Geoffroy in Cornprehensiae Oryunometallic Chemistry. eds. G. Wilkinson. F.G.A. Stone, E.W. Abel, Pergamon. Oxford, 1982, vol. 6, p. 763. [6] I.D. Salter in Coniprehensir~e Orquriornetallic Chemistry II. eds. G. Wilkinson, F.G.A. Stone and E.W. Abel, Pergamon, Oxford, 1995. vol. 10, p. 255. 171 D.M.P. Mingos. M.J. Watson. Adc. h o r y . Chern.. 1992, 39. 237. [8] I.D. Salter. Adti. Orgunornet. C l i m . . 1989, 29. 249. 191 K.P. Hall. D.M.P. Mingos, Proy. Inory. Chiwi., 1984, 32, 327. I 101 P. Braunstein. J. Rose, Gold Bull. 1985. 18, 17. I 11 I I.D. Salter, Adc. Dynumic Stereocheni., 1988, 2. 57. [I21 M. Melnik, R.V. Parish, Coortl, Chern. Reaie\r.s. 1986, 70, 157. [ 131 J.J. Steggerda, J.J. Bour, J.W.A. van der Velden. Reel. Truc. Chim. Pu~s-Bus.1982. 101, 164. (141 P.G. Jones, Gold Bull., 1986, 19, 46. [ 151 P.G. Jones, Gold Bull., 1983, 16. 114. (161 P.G. Jones, Gold Bull.. 1981, 14. 102. 1171 D.J. Wales. D.M.P. Mingos, L. Zhenyang, Inorg. Chem., 1989. 28, 2754. 1181 D.M.P. Mingos, Polj~liehon.1984, 3. 1289. 1191 D.G. Evans, D.M.P. Mingos. J. Orgcrnorne~.Chern.. 1982, 232. 171. [20] S.S.D. Brown, I.D. Salter. unpublished results. [21] S.S.D. Brown. I.D. Salter. A.J. Dent. G.F.M. Kitchen, A.G. Orpen, P.A. Bates, M.B. Hursthouse, J. Cliern. Soc.. Dulton Truns. 1989, 1227. 1221 S.S.D. Brown. I.D. Salter. D.B. Dyson, R.V. Parish. P A . Bates. M.B. Hursthouse, J. Chevn. Sot, Dulton Truns., 1988. 1795. (231 C.A. Collins, I.D. Salter, V. Sik, S.A. Williams, T. Adatia. J. Cheni. Soc,., Dultun Trcmx., 1998. 1107. 1241 C.A. Collins, I.D. Salter. V. Sik. S.A. Williams, J.C. Jeffery J. Chetn. Soc.. Dulton Trans., submitted for publication. [25) J.T. Mague. C.L. Lloyd, 0rgunoiiietcillic.s. 1992, I I . 26. [26] E.W. Abel, S.K. Bhargava. K.G. Orrell. Prog. Inorg. Clieni., 1984, 32, I . [27] I.D. Salter, V. Sik, S.A. Williams, T. Adatia, J. Cheni. Soc., Dulton Trans.. 1996, 643. 1281 B.E. Mann in Comprc~hen.c.ice Or~junometrrllic~ Chrmi.stry, eds. G. Wilkinson, F.G.A. Stone and E.W. Abel, Pergamon. Oxferd, 1982. vol. 3. p. 89. [29] I.D. Salter, J.W. Steed. V. Sik. S.A. Williams. unpublished results. 1301 P.J. Bailey. M.A. Beswick. J. Lewis, P.R. Raithby. M. Carmen Ramizez de Arellano, J. Oryrrnoriiet. Clietn.. 1993. 459. 293. [31] L.J. Farrugia. M.J. Freeman, M. Green, A.G. Orpen, F.G.A. Stone. I.D. Salter, J. Orgcmomet. Cliein., 1983. 249. 273. [32] L.W. Bateman. M. Green. K.A. Mead, R.M. Mills. I.D. Salter. F.G.A. Stone. P. Woodward, J. Chivn. Soc.. Dulton Trcrns., 1983. 2599. 1331 For example. B.F.G. Johnson. R.E. Benfield in Trunsition Metul Clusters, ed. B.F.G. Johnson, John Wiley and Sons Ltd.. 1980. ch. 7; B.F.G. Johnson. A t h ~ .D y n ~ n i i cStweochen?., 1988. 2, 207: L.J. Farrugia in Coriiprrl7en.r.i1~~, Or~iunornetullicClzeniistry II. eds. G. Wilkinson, F.G.A. Stone and E.W. Abel, Pergamon. Oxford. 1995, vol. 10. p. 187. [34] S.M. Draper. C.E. Housecroft. A.L. Rheingold. J . Orgcmoriwr. Chmi., 1992, 435, 9.
534
I Molecular Clusters
[35] C.P. Blaxill, S.S.D. Brown, J.C. Frankland, I.D. Salter and V. Sik, J. Chem. Soc., Dalton Trans., 1989, 2039. [36] M.J. Freeman, A.G. Orpen, I.D. Salter, J. Chem. Soc., Dalton Trans., 1987, 379. [37] A.G. Orpen, I.D. Salter, Orgunometallics, 1991, 10, 111. [38] I.D. Salter, S.A. Williams, T. qdatia, Polyhedron, 1995, 14, 2803. [39] P.J. McCarthy, I.D. Salter, V. Sik, J. Organomet. Chem., 1988, 344, 41 1 . 1401 C.J. Brown, P.J. McCarthy, I.D. Salter, J. Chem. Soc., Dulton Trans., 1990, 3583. [41] T. Adatia, Acta Crystallogr., Sect. C, 1993, 49, 1926. [42] M.I. Bruce, 0. bin Shawkataly, B.K. Nicholson, J. Organomet. Chem., 1985, 286, 427. [43] S.S.D. Brown, S. Hudson, I.D. Salter, M. McPartlin, J. Chem. Soc., Dalton Trans., 1987, 1967. [44] I.D. Salter, S.A. Williams, T. Adatia, unpublished results.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.28 Homonuclear and Heteronuclear Cluster Compounds of Gold Joaclzim Strahle
1.28.1 Introduction Since the pioneering work of Malatesta and his coworkers,['-31many new homonuclear gold clusters have been synthesized. These clusters can be divided into two groups. A smaller group of non-centered clusters consists of four to seven gold atoms, while a larger group of cluster compounds is characterized by one central and seven to twelve peripheral gold atoms. M i n g o ~ [ ~ classified ,~' the centred clusters with respect to their structure into toroidal and spherical types. Even a high nuclearity gold cluster compound, Au55( PPh3)12C16, consisting of 55 gold atoms is known.[61The results have been reviewed in several article^.[^.^^*] Most of the gold cluster compounds are synthesized from mononuclear gold( I ) complexes like RjPAuX ( X = halide, NO,, SCN-, etc.) by reduction with NaBH4, Ti(q-tol)*,CO, or other reducing agents. We have recently found that the photolytic decomposition of the azido complexes R3PAuN1 in T H F is another convenient method.['] Furthermore, the photolysis of azido complexes can be used for the synthesis of heteronuclear gold cluster compounds, if a source for the heteroatom is present during the photolysis in form of a carbonyl or azido complex.['1 A great variety of heteronuclear gold cluster compounds are known. In many of these clusters the AuPR3 moiety behaves like an isolobal proton"'.' 'I and bridges an edge of a transition metal cluster. While another type of heteronuclear clusters is built up by one central transition metal atom and up to twelve gold atoms, as described in several reviews.[*,' 14]
536
I Molecular Clusters
1.28.2 Synthesis of homonuclear gold cluster compounds 1.28.2.1 Photolysis of R3PAuN3 The controlled thermal decomposition of azido complexes[151has been used for the synthesis of metal nitrogen compounds such as nitride," 61 N-halogenoimido'' 'I or phosphorane iminato complexes.[' *I This type of decomposition, which presumably proceeds with a cleavage of the N,-Nb bond of the coordinated azido ligand, could not be observed for the gold(I ) azido complexes R3PAuN3. Thermal treatment resulted in the formation of gold metal. The photolytic decomposition of R3PAuN3 by irradiation with UV light at low temperature, however, afforded homonuclear gold cluster compounds. In this case a reductive elimination of the complete azido ligand in the form of molecular nitrogen occurs. The type of homonuclear gold cluster compound formed by the photolysis of R3PAuN3 is influenced by the steric demand of the ligand bound to the gold atom. If Ph3PAuN3 is used for the photolytic reaction in THF then smaller clusters like [(Ph3PAu)812+are obtained (Eq. 1). 6Ph3PAuN3
+ 2 Ph3PAu'
hc
+
+[(Ph3PAu),12+ 9N2
(1)
This cluster has already been synthesized by the reaction of [A~(AuPPh3)81"~ with PPh3, and has been structurally c h a r a c t e r i ~ e d . [ ~ The , ~ ~metal ] core has a structure which can be described as a capped centred chair (Fig. 1). In the presence of smaller ligands like Cl-, which can be abstracted from a chlorinated solvent like CH2C12 or can be added as Ph3PAuCl to the reaction mixture, the larger, and presumably more stable cluster [Aull (PPh3)8C12]+is usually observed as a result of the photolysis (Eq. 2, Fig. 2).Iz1]
6"
Figure 1. Structure of the inner core of the homonuclear gold cluster cation [(AuPPh3)8l2+. Only the P atoms (open circles) of the phosphine ligands are shown.
Figure 2. Structure of the inner core of thc homonuclear gold cluster cation [Au(AuPPh7)8(A~Cl)~l+ C1 atoms hatched circles, P dtonx open circles
8 Ph3PAuN3
+ 2 Ph3PAuClf
Ph3PAu'
h-t,
(PPhj),C12]+
+ 3 PPh3 + 12 N2
(2)
The same Aull cluster was obtained by Vollenbroek et ~ 1 by an . aggregation ~ ~ ~ reaction of [AuiAuPPh3)xl3+ with C1-. Other Aul I clusters showing an identical skeleton of metal atoms, however, with the composition Au(AuPAr3)7(AuX)3 ( X = I, SCN or C N ) have been isolated previously by Malatesta and coworkers, and were the first examples of homonuclear gold c l ~ s t e r s . [ ~ , ~ ~ . ~ ~ ~ A reduction of the bulkiness of the phosphine ligand by using MezPhPAuN3 as the starting material for the photolysis results in the formation of the Au13 cluster It [Au13(PMe2Ph)loC12]3+,[251 which has been already obtained by Mingos et exhibits a centred icosahedron (Fig. 3) structure. These homonuclear clusters are usually also obtained as by-products of the photolytic synthesis of the heteronuclear cluster compounds (see Sec. 1.28.3).
1.28.2.2 Synthesis of [Au(AuCl)(AuPPh3)$ While the homonuclear clusters obtained from the photolysis had already been synthesized using other methods,[',*' as described above, an unknown cluster cation was obtained by an unexpected reaction.[''] In the course of our attempts to synthesize heteronuclear vanadium gold clusters c.W a photolysis of a mixture of Ph3PAuN3/PhiPAuC1 ( 3 : 1 ) and N ~ ~ [ ( C S H S ) V ( C O in) ITHF/toluene ] (2: 11, the homonuclear cluster cation [Au(AuCl)(AuPPh3)p,]+was discovered. As this new cluster cation cannot be detected if the photolysis is performed without the vanadium complex, and the known cluster [Au(AuCl)2(AuPPh3)8lf is obtained instead, the vanadium complex probably directs the reaction to yield the new cluster cation.
~
538
I Molecular Clusters
Figure 3. Structure of the inner core of the homonuclear gold cluster cation [Au(AuPMe2Ph)lo(AuC1)212f Cl atoms: hatched circles; P atoms: open circles.
The structure of the spherical cluster [Au(AuCl)(AuPPhs)x]+(Fig. 4)consists of a centered eight membered crown of gold atoms with an additional AuCl molecule bound to the central gold atom. The cluster is isoelectronic and isostructural with [ P ~ ( C O ) ( A U P P ~ ~and ) ~ []P~~+( [ C~ O~) (~A~U ~ P] P ~ ~ with )~]~ the+ [CO ~ ~ligand ] occupying the position of the AuCl molecule. They result from the reaction of toroidal clusters [ M ( A U P P ~ ~ ) *(M ] ~= + Pt[149291,PdfZE1) with carbon monoxide. Interestingly, the corresponding homonuclear cluster [ A U { A U P ( C ~ H ~ O M ~ )' ] ~ } ~ ] ~ + , with a toroidal structure of a centred crown, is also known. Its transformation to a spherical cluster by a reaction with a molecule of AuCl would, however, require a
Figure 4. Structure of the inner core of the homonuclear gold cluster cation [Au(AuCl)(AuPPh?) 8 ] CI atom: hatched circle; P atoms: open circles.
+
1.28 Homonuclear and Heteronuclear Cluster Compounds of Gold
539
simultaneous reduction by two electrons, as AuCl, unlike CO, is not an electron donor.
1.28.3 Synthesis of heteronuclear gold cluster compounds 1.28.3.1 Photolysis of Ph3PAuN3 in the presence of a metal carbonyl complex If the photolysis of Ph3PAuN1 is carried out in the presence of a metal carbonyl complex, the Ph3PAu(0) moieties can substitute CO ligands affording heteronuclear cluster compounds of the composition [M(CO),(AuPPh3),]'+. To maintain the electron count of the transition metal M requiring a stable 18 electron configuration, each CO ligand must by substituted by two PhlPAu(0) fragments. Therefore an electron balance of n + 2x + y z = 18 is necessary, where n is the number of valence electrons of the transition metal M. Usually a series of cluster compounds with a different degree of substitution of the CO ligands results from the photolysis. The photolytically induced substitution stops, however, when the stable M(CO)3 step is reached. In addition to the substitution reaction, one or two Ph3PAu+ cations can be added to form a cluster cation. The photolysis of Mo(C0)6 and Ph3PAuN3 in THF, for example, yields the series of cluster compounds [Mo(CO)5(AuPPh3)1]+,[Mo(C0)4(AuPPh3)5]+,and [ Mo( CO)3( A U P P ~ ~ ) ~ ](Eq. + [ ~3).~ - ~ ~ ' ~
Mo(CO),
+ 2n Ph3PAuN3 + Ph3PAu'
The structures of [ M O ( C O ) ~ ( A U P P ~ ~and ) ~ ] +[ M [ ~O~ (I C O ) ~ ( A U P P ~ ~are )~]+[~~I shown in Fig. 5. The structure of [Mo(CO)5(AuPPh3)3]+was not determined. It can, however, be assumed that it has the same structure as the isoelectronic cluster V(C0)5(A~PPh3)3.[351 Homonuclear gold cluster compounds are always formed as by-products, but the amount can be reduced by using an excess of the metal carbonyl complex. The yield of the different heteronuclear cluster compounds can be influenced by the time of irradiation. Longer reaction times favour the formation of the larger clusters. The wavelength of the UV radiation also has a strong influence. It has, however, not been studied in detail. The various products formed from the photolysis can be separated chromatographically on an A1203 column. For this, the cluster cations are transformed in their readily soluble PF; salts.
540
I Molecular Clusters n
Figure 5. Structures of the inner core of thc heteronuclear gold molybdenum cluster cations a) [Mo(CO)d(AuPPh3)5]+,and b) [Mo(C0)3(AuPPh3)7]+.
The analogous tungsten gold clusters [W(C0)4(AuPPh3)5]+ and [W(CO)3(AuPPh3)7]- as well as [W(c0)4(A~PPh3)6]~+ have been synthesized by Steggerda et al. from the homonuclear gold cluster [(AuPPh3)g12+and W(C0)3(NCEt)3or w(c0)~.[361
The photolysis of a mixture of Ph3PAuN3 and the iron carbonyl complex Fe(C0)5 in THF yields [Fe(C0)3(A~PPh3)5]+.[~~] The structure of its FeAuj skeleton is the same as that of [Mo(C0)4(AuPPh3)5]+(Fig. 5a). The known cluster Fe(C0)4(A~PPh3)2,[~~] which can be obtained from [Fe(C0)4l2- and Ph3PAu+, was not detected as a product of the photolysis reactions. When Fe2(C0)9 or Fe3(CO)l2 are used in place of Fe(CO)5 analogous results are observed. On the contrary R u ~ ( C O12,) behaves differently. Presumably because of the stronger metal-metal bonds photolysis with Ph3PAuN3 does not result in a cleavage of the Ru3 cluster. Oxidative addition of Ph3PAuN3 at one Ru-Ru bond takes place instead, and simultaneously the azido group reacts with one CO ligand to form a cyanato ligand, so that Ru3(CO)1O(p-AuPPh3)(pu-NCO) is formed.[371 Co2(CO)g proved to be the most reactive carbonyl complex. It is the only one that reacts without irradiation to afford the known[391heterobinuclear complex Ph3PAuCo(C0)4 (Eq. 4):[401 Co2(CO),
+ 2 Ph3PAuN3 + 2Ph3PAuCo(CO), + 3 N2
(4)
The photolysis of Ph3PAuCo(C0)4 with Ph3PAuN3 yields the cluster cation [ C ~ ~ ( C O ) ~ A U ( A ~ P Pwhose ~ ~ )inner ~ ] +skeleton ,[~~' consists of two CoAu4 trigonal bipyramids sharing one axial Au atom (Fig. 6). It can be assumed that this cluster is a condensation product of [Co(C0)3(AuPPh3)4]+ and Co(C0)3(AuPPh3)3, which are presumably present as intermediate products in the reaction mixture.
1.28 Hornonuclear and Heteronuclear Cluster Compounds of Gold
Figure 6. Structure of the inner core of the condensed cluster ICo?(CO)hAu(AuPPhj)fi]+.
V
541
U
[Co(CO)3(AuPPh3)4]+can be obtained in good yield, if [ C O Z ( C O ) ~ A U ( A U P P ~ ~ ) ~ ] + or Ph3PAuCo(C0)4 is reacted with [Ph3PAu]PFs in refluxing CHzClz without irrad i a t i ~ n . [ ~ ~It' ~will ' ] be shown in Sec. 1.28.3 that the neutral cluster Co(CO)3(AuPPh3)3 (Fig. 7a) is obtained from the reaction of [Co(CO)?(AuPPh3)4]+with
c1-,[341
The photolysis of a mixture of Mn2(CO)lo and Ph3PAuN3, in THF, yields [Mn(C0)4(AuPPh3)4]+ (Fig. 8), together with small amounts of [Mn(C0)3(AuPPh3)6]+ (Fig. 9a).r4'1Better yields of [Mn(C0)3(AuPPh3)6]+are obtained starting from P ~ ~ P A U M ~ ( C OPh3PAuN3, ) ~ , [ ~ ~ Iand Ph3PAuNCO in the photolysis (Eq. 5j.[421In this case, the neutral cluster M ~ ( C O ) ~ ( A U P P(Fig. ~ ~ )7bj ~ [is~ the ~] main by-product. Ph3PAuMn(CO),
+ [Ph3PAuIf + 4 Ph3PAuN3
hf
+
[Mn(C0)3(AuPPh3)s]+ 2CO
+ 6N2
(5)
Figure 7. Structures of the inner core of the neutral clusters a) [Co(CO)j(AuPPhi)-i], and b) IMn(C0)4(AuPPh3)31.
542
I Molecular Clusters
e
Figure 8. Structure of the inner core of the heteronuclear gold manganese cluster cation [Mn(C0)4(AuPPh3)4I1.
The neutral cluster Mn(C0)4(AuPPh3)3had been synthesized previously by Ellis[441 from [Mn(C0)4l3- and [Ph3PAu]+. Surprisingly, the photolysis of P ~ ~ P A U V ( C Oand ) ~ [Ph3PAuN3 ~~] did not give the expected heteronuclear vanadium gold cluster. Only homonuclear gold cluster cations with [V(co)6]- as the counterion could be obtained.[461Chromatographic separation of the reaction mixture dissolved in CHZC12 afforded [( Ph3PAu)g(AuCI)~AU][V(CO)~] as the main product. The photolysis was therefore carried out substituting some of the Ph3PAuN3 by Ph3PAuNCO to avoid an excess of the azide, which probably favours the formation of homonuclear gold clusters. The best results were obtained with a 3 :2 molar mixture of Ph3PAuNCO/Ph3PAuN3 together with Ph3PAuV(C0)6, giving approximately 35% yield of [V(CO)4(AuPPh3)6]+ (Fig. 9b),[461 in addition to 30% of V ( C O ) ~ ( A U P P ~and ~ ) ~30Y0 [ ~ ~of’ the homonuclear gold clusters.
Figure 9. Structures of the inner core of the cluster cations a) IMn(C0)3(AuPPh3)6]’~,and b) IV(C0)4(AuPPhi)6]+.
1.28 Honlonucleur and Hrteronucleur Cluster Compounds of Gold
543
Figure 10. Structure of the cluster [Re(CO)sAu(AuPPh3)6]z+. Only the P atoms of the phosphine ligands are shown.
1.28.3.2 Photolytic synthesis of [Re(C0)~Au(AuPPh3)6]~$ The photolysis of a mixture of Re2(CO)lo and Ph3PAuN3 parallels the results obtained with M n l ( C 0 )10. The clusters Re(C0)4(AuPPh3)3,[Re(C0)4(AuPPh3)4If, and [Re(C0)3(AuPPh3)6]+have been obtained and identified by mass spectrometry.[471However, no single crystal for structure determinations could be obtained. Therefore, in addition, the photolysis of Rez(C0)10 with Ph3PAuNCO was studied. Crystallisation of the product of the photolysis from methanol yields the new cluster cation [Re(CO)sAu(AuPPh3)6]'+ (Fig. lo), which crystallizes with [Re3(C0)9(pu-OMe)3(p,-OMe)]and C1- as counter ion^.[^^^ The structure of this new cluster cation contrasts the results described in Sec. 1.28.3.1, where the heteronuclear gold clusters are characterized by a transition metal atom M located in the center of a hemispherical fragment of gold atoms, where all the Au-M bond lengths are similar. The new cluster cation, however, is more closely related to the homonuclear gold clusters. As the Re(C0)S moiety is isolobal to a Ph3PAu fragment['O,'ll it occupies the position of a peripheral gold atom, and, in fact, the cluster has a structure comparable with that of '
ALI(AUPC~~)~(AUSCN)~.'~"]
1.28.3.3 Photolytic synthesis of [(CsHs)Mo(C0)2(AuPPh3)4]+ In addition to the heterobinuclear complexes Ph3PAuM(CO), ( M = Co, Mn, V ) , heteroleptic cyclopentadienyl carbonyl complexes Ph?PAuM(Cp)(CO),can be used
544
1 Moleculuv Clusters
6
[ C ~ M O ( C O ) ~ ( A U P P ~Only ~ ) ~the ] + .P atoms of the phosphine ligands are shown.
as materials for the photolysis with Ph3PAuN3. As a first result we obtained [CpMo(CO)*(AuPPh3)4]+ from the photolysis of CpMo(C0)3(AuPPh3) and Ph3PAuN3 (Eq. 6).'501
+
+ [P~~PAu]' [ C P M O ( C O ) ~ ( A U P P ~+~3N2 )~]+ + CO
C P M O ( C O ) ~ ( A U P P ~2~Ph3PAuN3 ) +
(6)
As with [Mn(CO)4(AuPPh3)4]+(Fig. 8), the five metal atoms of the cluster cation [CpMo(CO)2(AuPPh3)4]+form a trigonal bipyramid with the Mo atom in equatorial position (Fig. 11). By analogy with the heterometallic gold clusters discussed in Sec. 1.28.3.1, the composition of the cluster cation is also determined by the electronic requirements of the transition metal Mo. With the electrons from the bonds to the cyclopentadienyl and CO ligands and the three electrons from the I(Ph3PAu)4]+ fragment the Mo atom reaches a stable configuration of 18 electrons.
1.28.3.4 Co-photolysis of Pd or Pt azido complexes with metal carbonyl complexes The palladium and platinum azido complexes L2M(N3)2 ( M = Pd, Pt; L2 = dppe, (PPh3)2), like Ph3PAuN3, undergo a reductive elimination of the azido ligands upon UV irradiation forming palladium( 0) and platinum( 0) complex fragments L2M. Unlike gold atoms, however, Pd and Pt atoms do not show the same tendency to cluster together. Therefore, the photolysis of palladium( 11) and platinum( 11) azido complexes in the presence of metal carbonyl complexes does not give the comparable heteronuclear clusters. Most of the metal carbonyl complexes do not react at all with the Pd or Pt azido complexes. Only with Co2(CO)8 is a reaction observed. With (dppe)Pt(N3)2 the photolysis affords the trinuclear cluster (dppe)PtCoz(C0)7(Eq. 7),[51a1 which was previously synthesized by Dehand and
1.28 Homotiucl~wrlitid Heteronucleur Cluster Compound.\ of Gold
545
Figure 12. Structure of the heteronucledi du\tei (dppe)PtCo,(CO), Only the C atoms of the CH2 groups and P atoms are shown of the bis(dipheny1phosphine) ligniid
Nennig.[j2]in 1974, from (dppe)PtCI?and [Co(CO),]-, and also by Hidai et a/.[s31 using (dppe)Pt(C=CPh)?and C O ~ ( C O ) ~ .
If
+
(dppe)Pt(Ni)? C O ? ( C O ) ~
I
(dppe)PtCo:!(CO),
+ 3 N ? + CO
(7)
The structure determination["'"] shows, that one of the bridging CO ligands in Co?(CO)x is substituted by a (dppe)Pt(O) fragment forming a triangular CozPt cluster (Fig. 12).This is similar to the situation found in [(PhiP)(OC)PtCo2(CO)7].[51 h1 When [ (dppe)PtCo?(CO),] is used in the photolysis reaction with PhiPAuNi, the triangular ColPt cluster is cleaved and the heteronuclear cluster cations [Pt(dppe)(AuPPhi)4I2+(Fig. 13) and [ C O ~ ( C O ) ~ A U ( A U P P(Fig. ~ ? ) 6) ~ ]are + formed (Eq. 8).[s1"1
+
[(dppe)PtCo?(CO),] 8 Ph?PAuN3+ 3 PhiPAu+ hI + [Pt(dppe)( AuPPhi),] 2+
+ P P h i + C O + 12N2
Figure 13. Structure of the heteronuclenr cluster cation [(dppelPt(AuPPh&
+ [ C O ? ( C O ) , A U ( A U P P '~ ~ ) ~ ]
546
1 Molecular Clusters
Figure 14. Structure of the inner core of the heteronuclear gold platinum cluster [Pt(CO)(AuPP&)61if. C1 atoms: hatched circles; P atoms: larger open circles.
1.28.3.5 Co-photolysis of Ph3PAuN3 and L2M(N3)2 (M = Pd, Pt; L2 = dppe, (PPh3)2) The photolysis of (Ph3P)zPt(N3)2,Ph3PAuN3, and Ph3PAuC1 in THF affords a mixture of products, which, after chromatographic separation, yields the neutral clusters Pt( PPh3)(AuPPh3)6(AuC1)3 and Pt (CO)(AuPPh3)6(AuC1)3.[5 The former has already been synthesized and structurally characterized by Steggerda et al. [ j 4 ] Our structure determination of the CO derivative shows that the skeleton of both clusters is identical, consisting of an icosahedral fragment of nine Au atoms with the Pt atom in its center (Fig. 14). The origin of the CO ligand is not obvious, since no carbon monoxide or carbonyl complex was used during the synthesis. In accordance with the one can assume that it originates from coordinated ethanol, which was used as an eluant during the chromatographic separation. Additional gold platinum clusters obtained by other methods have been reviewed by Steggerda.[l4] The photolysis with the palladium( 11) azido complexes (Ph3P)*Pd(N3)2 and (dppe)Pd(N3)zin the presence of Ph3PAuN3 and Ph3PAuCl yields different products than those formed from the analogous Pt complexes. In this case the PdAu12 clusters Pd(AuPPh3)8(AuC1)4(Fig. 15)[j6] and Pd(AuPPh3)6(Au2dppe)(A~C1)4[~’~ are obtained (Eq. 9).They are analogous to the homonuclear Au13 cluster [ A ~ ( A U P M ~ ~ P ~ ) ~ O ( A Uand C ~ represent ) ~ ] ~ +the , [ first ~ ~ ,examples ~ ~ ~ of heteronuclear clusters involving a complete icosahedron of twelve Au atoms with the heteroatom in its center.
+
-
(dppe)Pd(N3), 8 Ph3PAuN3 l1.U
+ 4 Ph3PAuCl
Pd(AuPPh3),(Auzdppe)(AuCl), + 15 N2 + 6 PPh3
(9)
1.28 Homonuclear und Heteronucleuu Cluster Compounds of Gold
547
Figure 15. Structure of the inner core of the icosahedral gold palladium cluster IPd(AuPPh3)s(AuC1)AJ. C1 atoms: hatched circles; P atoms: open circles
1.28.4 Properties and reactions of heteronuclear gold cluster compounds In the crystalline form the heteronuclear gold cluster compounds are air-stable at ambient temperature and not sensitive to light. Solutions, however, decompose slowly under action of light to form gold metal. A comparison of the wavenumbers of the CO stretching vibrations of the substituted carbonyl complexes shows that the (AuPPh?),,fragments have a good donor ability. This ability increases with the number of AuPPh3 groups bound to the transition metal M. The CO stretching vibration for Mo(C0)6 is 2112 cm-1,[583 and for while for [Mo(C0)4(AuPPh3)5]+values of 1890, 1915, and 1975 c111-',[~~] [Mo(CO)3(AuPPh3)7]+values of 1835 and 1895 are found. Analogously, the stretching vibrations of the terminal CO ligands of C o z ( C 0 ) are ~ in the region between 2001 and 21 12 cm-' .[581 In [Co(CO)3(AuPPh3)4]+the corresponding lower values are 1915, 1925, and 1980 ~ m - ' , [ ~ 'and ] in [Co(CO)z(AuPPh3)6]+they are 1873 and 1904 An even stronger donor ability is observed in the neutral clusters, as can be seen from the frequencies shown by Co(CO)3(AuPPh3)3,at 1891, 1905 and 1963 ~ r n - ' . [ ~ ~ ] As discussed above, the stepwise photolytic substitution of CO ligands by R3PAu fragments stops once the M(C0)3 stage is reached. We, therefore, used the long known nucleophilic attack of OH- on the CO group, which was first discovered by Hieber and Leutert in 1932,[601when they reacted Fe(CO)5 with OH- to obtain the metallate [Fe(CO)4]*-.Correspondingly, [Co(CO)3(AuPPh3)4]+reacts with ethanolic
548
I Molecular Clusters
Figure 16. Structures of the inner core of the heteronuclear gold cobalt cluster cations d) [Co(C0)2(AuPPh3)slt, and b) [Co(C0)2(AuPPh3)7I2+
NaOH in the presence of Ph3PAuCl to afford the larger cluster [Co(CO)2(AuPPh3)6]+(Fig. 16a) in approximately 83% yield, in addition to small quantities of the dication [Co(CO)2(AuPPh3)7l2+(Fig. 16b).[591In accordance with the findings of Hieber and Leutert it can be assumed that the metallate [Co(CO)2(AuPPh3)4]- is formed as an intermediate product, which subsequently adds two or three cations [PhsPAu]+ (Eqs. 10, 11).
+
[ C O ( C O ) ~ ( A U P P ~ ~4) OH ~]+ +
[Co(CO),(AuPPh3),]-
+ C0:- + 2 H20
(10)
[CO(CO)2(AuPPh3)6]+
(11)
[Co(CO),(AuPPh3),]-
+ 2 Ph3PAu'
+
The analogous rhodium clusters [Rh(C0)2(AuPPh3)6]+and [Rh(CO)2(AuPPh3)7l2+ with the same composition and structure had already been obtained by Mingos et a1.[611from [Ph3PAu]+ and [BH4]- in the presence of [Rh(CO)*(MeCN)2]+. The observation that the dication [Co(C0)2(AuPPh3)7I2+is obtained in addition to [Co(C0)2(AuPPh3)6]+shows that an equilibrium mediated by [Ph3PAuIf exists and can be used to build up and to degrade the clusters. This equilibrium can furthermore be influenced by C1- or PPh3, which are able to remove a [Ph3PAu]+ cation from the cluster in the form of the very stable products Ph3PAuCl and [Au(PPh3)2]+. We found that the cluster cations [Co(C0)3(A~PPh3)4]-~ and [Mn(C0)4(AuPPh3)4If react with PkPCl in CH2C12 to yield the neutral clusters Co(C0)3(AuPPh3)3 (Eq. 12)1341 and Mn(C0)4(AuPPh3)3[431 containing one less [AuPPh3]+ group. While Mingos et a1.[611used the reaction of PPh3 with [Rh(CO)2(AuPPh3)7l2+to obtain the monocation [Rh(CO)z(AuPPh3)6]+. The re-
1.28 Honzonuchr rind Heteronurleur Clustcr Coinpouncl..t of Gold
549
verse reactions with [Ph3PAu]+ to yield the clusters with one additional Au atom have also been shown to occur.
[Co(CO),(AuPPh3),]+ fC1-
+
k Co (C O),(AuPPhi), PhqPAuCl
( 12)
Another interesting reaction is the condensation of [Co(CO)i(AuPPh3)4]+and Co(C0)3(AuPPhl)? to form [Co2(CO)sAu(AuPPhi)6]+(Fig. 6) in high yield (Eq. 13).[341
This reaction has, however, so far only been observed in the case of the cobalt-gold clusters. The condensed cluster [Coz(CO)hAu(AuPPh3)6]+. on the other hand, can be transformed with PhiPAu+ and PPh3 to two cluster cations [Co(CO)3(AuPPh3)4]+(Eq. 14).[34'
1.28.5 Structures of the heteronuclear gold cluster compounds With the exception of [Re(C O ) S A U ( A U P P ~ ~ ) ~all] ' +the [ ~ heteronuclear ~~ gold transition metal clusters described above are characterized by a core structure consisting of one central heteroatom M and three to twelve gold atoms at approximately equal distances from the heteroatom. The peripheral gold atoms tend to from close contacts with each other and, therefore, build a close-packed surface composed of triangular faces. For the larger clusters these surfaces can best be described as fragments of an icosahedron (Fig. 17), which is complete in the case of the PdAuIZ clusters Pd(AuPPh~)~(Au~dppe)(A~Cl)4[~~~ and Pd(AuPPh3)s(A~C1)4[~~] (Fig. 15). This results in a close-packing of MAu3 tetrahedra sharing MAu2 faces and having the central heteroatom M as a common corner. The smaller cluster C O ( C O ) ~ ( A U P P ~has ~ )a~ skeleton [ ~ ~ I of a single tetrahedron (Fig. 7a), while the skeletons of the MAu4 clusters exhibit a trigonal bipyramidal structure, which can be regarded as a double tetrahedron (Figs. 8, 13). An exception to the rules given above is the MnAu3 cluster Mn(C0)4(AuPPh3)3. Its skeleton has a structure of a planar MnAu3 rhombus with the Mn atom in an equatorial position (Fig. 7b).[43,621
550
I Molecular Clusters
Figure 17. Representation of the idealized metal atom skeletons of the heteronuclear gold clusters as fragments of centred icosahedra.
1.28 Homonuclear and Heteronuclear Cluster Compounds of' Gold
55 1
Despite of the fact that M ~ ( C O ) ~ ( A U P P ~and ~ ) C~ O [ ~( C~ O . ~) ~ (] A U P P ~ ~ ) ~ [ ~ ~ ] have the same number of cluster valence electrons, isomeric structures of a rhombus and a tetrahedron are found. The analogous observation of isomeric structures formed for clusters with the same total number of cluster valence electrons was made for the Au6M clusters. The metal skeleton of [ M ~ ( C O ) ~ ( A U P P ~ ~ ) ~ ] + [ ~ ~ I (Fig. 9a) takes the spherical form of a pentagonal bipyramid with the Mn atom in axial position. The clusters [ V ( C O ) ~ ( A U P P ~ ~ (Fig. ) ~ ] +9b) [ ~ ~and ] [Co(CO)2( A U P P ~ ~ ) ~ ](Fig. + [ ~ ' ]16a), on the other hand, exhibit an ellipsoidal skeleton in form of a double-capped trigonal bipyramid with the heteroatom in an equatorial position (Fig. 17). The occurence of two different structures in the case of both A u ~ Mand Au6M clusters may be explained in terms of the stereochemistry and bonding.[421Model studies show that the ellipsoidal structure of Au6V results in a lower repulsion between the four CO ligands and the (AuPPh3)6 fragment. While for the smaller Mn(CO)3 group in [Mn(C0)3(AuPPh3)6]+also with a spherical framework only a minor repulsion results. The spherical framework, however, allows for more effective gold-gold interactions. On the other hand, the different structures for [Mn(C0)3(AuPPh3)6]+ and [V(C0)4(AuPPh3)6]+as well as [Co(C0)2(AuPPh3)6]+obviously lead to a better orbital overlap in each case (see Sec. 1.28.6). This probably also accounts for the presence of isomeric A u M ~ clusters: although the stereochemical repulsion is less effective. Other examples for Au3M clusters with a tetrahedral skeleton are [( PhMe2P)3Re( H ) ~ ( A U P P ~ ~ ) ~ [Rh( ] + , H)(CO)( [ ~ ~ ] P P ~ ~ ) ~ ( A L I P P ~ ~[()triphos)Rh( ~ ] + , [ ~ ~H)2] ( A u P P ~ ~ ) ~ ] ~ and + , [ ~V' I( C O ) ~ ( A U P P ~ Whereas ~ ) ~ . [ ~ ~the ] clusters (PhMe2P)3P P a~ planar ~ ) ~ ]skeleton +[~~I Re( H)2(A~PPh3)3,[631 and [( P ~ ~ P ) ~ I ~ ( N O ~ ) ~ ( A Uhave Au3M in form of a rhombus (Table 1). Examples for clusters with a trigonal bipyramidal MAu4 core are summerized in Table 2. Table 3 contains selected cluster compounds with a MAus skeleton. Heteronuclear clusters with a AusPt core show a variety of structures. Of the cluster cations [Pt(PPh3)(AuPPh3)6j2f,[671 [Pt(PP~~)(CC-~-BU)(AUPP~~)~]+,[~*~ and only the last one has an inner skeleton comparable [Pt(PPh3)(CO)(AuPPh3)6]2+,[671 to the one of [V(C0)4(AuPPh3)6]+ or [Co(C0)2(AuPPh3)6]+,while for clusters with a pentagonal pyramidal AU6 fragment [Mn(CO)3(AuPPh3)6lf is the only example (Table 4). Selected higher nuclearity cluster compounds with seven to twelve gold atoms are given in Tables 5 and 6.
1.28.6 Bonding in the homonuclear and heteronuclear gold cluster compounds The structure and bonding of the homonuclear and heteronuclear cluster compounds has been discussed by M i n g ~ s . [ ~ . ~W . ~e , have ' ~ ] also described, from a
M-Au distances [pm]
258.4(4)-262.0(4) Mn(CO)d(AuPPh3)3 ( P ~ M c z P ) ~ R ~ ( H ) z ( A u P P ~ ~ ) ~ 268.8(2)-273.0(2) [(Ph3P)ZIr(N@)(AuPPh3)31+ 259.3( 1)-267.5( 1)
Co(CO)i(AuPPhi13 [(PhMezP)iRe(H)i(AuPPh3)3]+
250.4( 1)-254.0(2) 272.3(2) 264.0f3)-272.2(3) [(P~~P)~R~(H)(CO)(AUPP~~)~]’ [(triphos)Rh(H)*(AuPPhj)j]’+ 269.5(2) V(CO)5(AuPPh3)j 270.9(1)-275.6( 1 )
Cluster compound
[341 1631 1641 1651 1351 1431 I631 [661
tetrahedron tetrahedron tetrahedron tetrahedron tetrahedron rhombus rhombus rhombus
279.5( 1)-285.1(1) 293.1(2) 281.412)-291.4(2) 288.7( I ) 276.8( 1)-285.5( 1) 276.6(2)-28 1.3(2) 278.7( 2 ) ,28 1.2(2) 272.7( l), 280.6(1 )
Ref.
Structure
Au-Au distances [pm]
Table 1. Metal-metal bond lengths for MAu3 heteronuclear cluster compounds with structures of a tetrahedron or a rhombus.
2
Bi;
9
r;
5
n
$2
‘r
h,
v1
cn
Table 2. Metal-metal bond lengths for heteronuclear cluster compounds with a trigonal bipyrdmidal MAu4 core. Cluster compound [Co(CO)q(AuPPhi)i]' [ Ml1(C0)4IAuPPh: '41
'+
[Pt(dppe)(AuPPh?J] [CpMor CO).( AuPPh? 141 Ilr(PPh: ~ 2 ( H ~ ~ ( A i i P P l i : ) ~ ] [Rh(P(O-r-Pr~~)~iHI~ AuPPli? A ] +
M-Au distances [pml
Au-Au distances [pm]
Ref
258 Of2) 264 7( 1 ) 261 3(3)-273 ?(?' 266 1(1)-2689(11 275 5i2) 280 8\21 261 7\21-275 3f21 265 I i 1 1 272 5 1I )
278 41 1)-293 4( 1 I 277 3( I ,-292 2' 1, 276 O( 1) 283 912) 281 2(1)-285 6(21 279 4(2\-314 2(21 281 if 1)-308 4(1 J
[401 [411 [5161 I 501 [741 1751
qualitative basis. some aspects of the bonding in heteronuclear gold transition metal cIusters.['] Mingos[" distinguished the centered higher nuclearity gold cluster compounds with regard to their topology. The spherical clusters have the peripheral gold atoms lying approximately on the surface of a sphere. They are characterized by four bonding cluster orbitals. Whereas the toroidal clusters with only three bonding cluster orbitals have the peripheral gold atoms lying approximately on a ring or torus. The homonuclear gold clusters [ ( A U P P ~ ~ ) ~ ] ' + , [[Au(AuPPh3)x~.~~I ) ~ ][AU(AUCI)(AUPP~~)~]+[")~ ~+.[~~.~~~ ( A u C I ) ~ ] + , [ ~[ (~A. ~U ~PIM ~ I P ~ ) I O ( A U C ~and described in Secs. 1.28.2.1 and 1.28.2.2 belong to the spherical type. Of the spherical type are also the heteronuclear clusters centred by a Pt or Pd atom, Pt(PPh3)(AUPP~~)~(AUCI)~,'~'".~~I Pt(CO)(AuPPh?)h(AuCI)j,[' la' Pd(AuPPh3)x( A u C I ) ~ , " ~ ] which are described in Sec. 1.28.3.4. The and Pd(AuPPh3)6(A~2dppe)(AuCl)4,~'~~ eight electrons for the four bonding orbitals are provided by the Ph3PAu(O) fragments, and in the case of the PtAuq clusters in addition by the ligands bound to the Pt atom. As discussed in Sec. 1.28.3.2, the heteronuclear cluster cation [Re(C0)5A u ( A u P P ~ ~ ) ~ ] ' is+ [ comparable ~~] to a homonuclear gold cluster. Since the Re(C0)5 group is isolobal to a Ph;PAu moiety, it occupies the position of a peripheral gold atom. The resulting cluster l, Fig. 10) forms a toroidal topology, and therefore, has three occupied bonding cluster orbitals. The centred homonuclear gold clusters are characterized by strong radial bonds between the central and the peripheral atoms. The bonds between h e peripheral gold atoms are noticeably weaker and have longer Au A U distances. These findings are also reflected by the structure of j Re(CO)jAiiuAuPPli~ jhl'-. where the radial Re-Au bond length is 287.0 pin, while the distance to the neighboring peripheral gold atoms is 297.0 pm. For the heteronuclear gold clusters j M ( C 0 ),iAuPPh;),.]'+ with a central heteroatom M and three to seven peripheral gold atoms. as described in the previous
277.6( 1)-308.8( 1) 272.2(4)-301.3(4) 276.3(5)-293.2(3) 279.5(3)-300.5(3) 281.1 (5)-317.6(3)
Au-Au distances [pm] 1371 [341 ~361 [611 1761
Ref.
279.2(1)-295.1(1) 280.4( 1)-288.4( 1) 275.3(5)-296.2(4) 281.0(2)-297.6(2)
261.8( 1)-272.3(2) 254.9(1)-257.1(2) 273( 1 j-284( 1j 265.9(2)-271.4(2)
[Mn(CO)3(AuPPh3161' [Co(CO)2(AuPPh3)6]+ [V(C0)4(AuPPh3)6]+
[Pt(PPh3)(CO)(AuPPh3)6I2+
Au-Au distances [pm]
M-Au distances [pm]
Cluster compound
Pb btb btb btb
Structure
[421 [591 [461 ~ 7 1
Ref.
Table 4. Metal-metal bond lengths for MAu6 heteronuclear cluster compounds with the structures of a pentagonal bipyramid (pb) or a bicapped trigonal bipyramid (btbj.
259.0(1)-265.1(1) 279.9(5j-284.6(7) 275.2(3)-284.9(3) 266.9(4)-273.1(4) 259.0(3)-267.6(3)
[Fe(C0)3(AuPPh3)s]+ [Mo(CO)&4uPPh3)sIf [W(C0)4(AuPPh3) s ] + [Rh(CNCsHs)3(AuPPh3I s ] 2+ [Pt (PPh3) ( CO)(AuPPhjIs]
+
M-Au distances [pm]
Cluster compound
Table 3. Metal-metal bond lengths for MAus heteronuclear cluster compounds with the structure of a capped trigonal bipyramid.
0 P
%
$:
0
52
k
P
wl
wl
279 6(2)-300 4(2) 280(3)-300(3) 286 O(2)-306 O(2) 285 4(2)-293 S(2) 277.1(2)-298 3(2)
267 5(2)-273 812) 268(2)-278 9(4) 269 O(2) 273 4(2) 272 2(2)-276 7( I ) 271 0(3)-278 2(4)
[Pt(CO)\AuPPhi)b(AuCl\?] I Pt(PPh?)(AuPPh?)h( AUCI)i] [P~(CN)(AUCN)(AUPP~~)~]*
[ Pd(AuPPhi)b(Au?dppe)(AuCI 141 [Pd(AuPPhi) ~ ( A L I C I ) ~ ]
Au-Au distances [pm]
M-Au distances [pml
Cluster compound
Table 6. Metal-metal bond lengths for MAu9 and MAUL?heteronuclear cluster compounds.
151al [541 1771 ~ 7 1 1561
Ref.
1271
2 8 3 . 3 l)-307.9( I )
[Pt(C0i(AuPPhi)~]'+
[Co(C0)z(AuPPh3)jI2+ [Rh(C0)z(A~PPhi)j]~' [Mo!CO\~(AUPP~?)~]+ [W(CO)?!AuPPh3)7If
265.1(1)-270.3( I )
Ref.
[591 1611 [32, 331 [361 [611 1281 1291
Au-Au distances (pm]
274.813)-332.6(3) 278.8(4)-311.8(5) 283.8(1)-3O3.1( 1 ) 28S.7(3)-303.5(4) 278.1(2)-300.9(2) 276.5(2)-280.5(2) 280.0(4)-286.4(4)
M-Au distances [pm]
2.56.8(6)-264.7(5) 264( 1 )-276.5(8) 277.1( I )-286.0( 1 ) 276.0(4)-285.6( 3) [ R ~ ( C N C ~ H ~ ) ~ ( A U P P ~ ~ ) ~ ( A U C265.4(3)-281.9(3) I)~]+ [Pd(AuPPh3)*l2' 261.1(2)-262.4!2) [Pt(AuPPh?)x]'+ 263.1(31-264.0(3)
Cluster compound
Table 5. Metal-metal bond lengths for MAuj and MAu8 heteronuclear cluster compounds.
wl wl wl
e. 9 c
3
x
2E:
B
2 >
2 E
Y
5
2
2
7.
Q 3 Q
3
c
3
5
tu 00
'I.
556
I Moleculur Clusters
sections, a different bonding scheme must be considered. The number of bonding cluster orbitals depends on the number and type of frontier orbitals of the M(CO), or M(C5H5)(CO),group. The gold atoms of the Ph3PAu groups on the other hand can be assumed to be sp; hybridized. One of the hybrid orbitals is binding the phosphine ligand, while the other one is available for metal-metal bonding. Within the cluster it points radially to the center. The remaining pair of degenerate ps and pjJ orbitals are empty and energetically high-lying, and therefore can be neglected for the bonding. An M(CO), group has three frontier orbitals; they can be classified according to Wade[691and Owen[701into one radial orbital pointing at the center of the cluster, and one set of two degenerate tangential orbitals. They combine with the radial orbitals of the Ph3PAu groups to form three bonding MOs. One of them, where all of the group orbitals involved contribute with the same sign, can be characterized as being of 0 type, while the other two contain one nodal plane each, and therefore, are of the n type (Fig. 18). An M(CO)4 or Mo(CgH5)(C0)2group on the other hand, has only two frontier orbitals, one radial, like a M(CO)3 group, but only one tangential orbital. Therefore, only two cluster MOs are formed, one of 0 and one of n type (Fig. 18). The existence of only two bonding cluster MOs also means that for the relatively large clusters like [ V ( C O ) ~ ( A U P P ~ ~only ) ~ ] the + [ ~astonishingly ~] small number of four electrons is available for the metal-metal bonds in the cluster. An additional stabilization by gold-gold interactions must, however, be considered, and the high stability of these clusters is another indication for the importance of this unprecedented affinity between gold atoms, that was termed aurophilicity by S ~ h r n i d b a u r1,721 .~~ The isomeric structures of the MAu6 and MAu3 clusters (see Sec. 1.28.5) are presumably influenced by the different number of n MOs in these clusters. Since the two tangential orbitals of the M(CO)3 groups are arranged perpendicular to one another, a hemispherical (AuPPh3)6 or (AuPPh3)3 is preferred. It is interesting to note that the bonding in [ M ~ ( C O ) ~ ( A U P P ~ ~ is ) ~similar J + [ ~ ’to] that in the cyclopentadienyl complex CpMn(CO)3, since the hemispherical, pentagonal pyramidal (AuPPh3)6 unit behaves like a cyclopentadienyl ring. When there is only one tangential orbital as in the M(C0)4 group of [ V ( C O ) ~ ( A U P P ~ ~ or )~M ] + ~[ ~( ~ C ]O ) ~ ( A U P Pthe ~ ~elliptical ) ~ [ ~ ~ VAu6 I or planar, rhombic MnAu3 structure obviously leads to a more effective overlap. Three of the frontier orbitals of the C o ( C 0 ) t group,[731as present in [Co(CO)2( A u P P ~ ~ ) ~ ]are + ,unoccupied [~~] and suitable for a combination with the orbitals of the (AuPPh3)6 fragment, to form three bonding cluster MOs. In this case the two tangential orbitals of the Co(C0): group are not degenerate, and are of different form (b2 and 2b1, Fig. 18). -The orbital 2bl would not achieve an effective overlap with a hemispherical (AuPPh3)h fragment, while with the elliptical form present in [Co(COj?(AuPPh3)6]+a better overlap is possible. In the cyclopentadienyl complex c p C o ( c O ) ~where , the cyclopentadienyl ligand is comparable to a
1.28 Homonuclear
uiid
Heterorzuclear Cluster Conipoundc o j Gold Mn(C0);
Mn(C0)j
it3
6
557
co (CO):
co ( C O ) ;
2b'
dL
C O (C 0 )11 ( C ) [ (P11jP.411)fi
Figure 18. Representation of the orbital interactions in the cluster skeletons of a) [V(CO)J(AUPP~&,]'.b'i [Mn((:O~;,AuPPh;)(,l+and c) [Co(CO)r(AuPPhi)hl+.The radial sp hybrid orbitals of the PhlPAu groups are represented as filled and open circles.
558
I Moleculur Clusters
hemispherical (AuPPh3)6fragment, this 2bl orbital is also assumed to only overlap weakly with the cyclopentadienyl ring.[731 As discussed in Sec. 1.28.3.1, the number of Ph3PAu(O) groups in the heteronuclear clusters of the type [ M ( C O ) , ( A U P P ~ ~ )is~ ,determined ]~+ by the electronic demand of the transition metal M, that requires a stable 18 electron configuration. The cluster cation [Pt(dppe)(A~PPh~)~]~+[~'] is an exception to this rule, since the central Pt atom only achieves 16 electrons by adding to the 10 d electrons of the Pt(0) atom the four electrons of the two Pt-P bonds, and the two electrons of the (AuPPh3)if fragment. For platinum it is, however, not unusual to have a 16 electron configuration.
Acknowledgements The generous support of the Deutsche Forschungsgemeinschaft and the Verband der Chemischen Industrie is gratefully acknowledged. Thanks are also due to my co-workers for their dedicated and enthusiastic cooperation. I also thank Mrs. E. Niquet for preparing the SCHAKAL diagrams.
References [I] L. Malatesta, L. Naldini, G. Simonetta, F. Cariati, J. Chem. Soc., Cliem. Commun. 1965, 212213. [2] M. McPartlin, R. Mason, L. Malatesta, J. Chem. Soc., Clzem. Commun. 1969, 334. [3] M. Manassero, L. Naldini, M. Sansoni, J. Chem. Soc., Chem. Commun. 1979, 385-386. [4] D. M. P. Mingos, Polyhedron 1984,3; 1289-1297. [5] D. M. P. Mingos, D. J. Wales, Introduction to Cluster Chemistry, Prentice-Hall, London, 1990. [6] G. Schmid, R. Pfeil, R. Boese, F. Bandermann, S. Meyer, G. H. M. Calis, J. W. A. Van der Velden, Chem. Ber. 1981, 114, 3634-3642. [7] D. M. P. Mingos, Gold Bull. 1984, 17, 5-12. [8] K. P. Hall, D. M. P. Mingos, Progr. lnorg. Chem. 1984, 32, 237-325. 191 J. Strahle, J. Orgunomet. Chem. 1995, 488, 15-24. [lo] R. Hoffmann, Anyew. Chem. Znt. Ed. Engl. 1982,21, 71 1-725. [ 111 D. G. Evans, D. M. P. Mingos, J. Orpnon7et. Chen7. 1982, 232, I7 1 - 191. [12] D. M. P. Mingos, M. J. Watson, A h . Inory. Chem. 1992, 3Y, 327-399. [13] P. Braunstein, J . RosC, Gold Bull. 1985, 18, 17-30. [14] J. J. Steggerda, Comments Inorg. Chem. 1990, 11, 113-129. [15] J. Strahle, Comments Inory. Chem. 1985, 4 , 295-321. [16] a) K. Dehnicke, J. Strahle, Angew. Chem. lnt. Ed. Enyl. 1981, 20, 413-426; b) K. Dehnicke, J. Strahle, Anyew. Chem. Int. Ed. Engl. 1992, 31, 955-978. 1171 K. Dehnicke, J . Strahle, Chem. Rev. 1993, 93, 981-994.
1.28 Honionucleav und Heteronucleav Cluster Compounds of Gold
559
[ 181 K. Dehnicke. J. Strihle, Polyhedron 1989, 8. 707-726. [I91 J. Pethe, C. Maichle-Mossmer, J . Striihle, Z. Anarg. Allg. Chem. 1998, 624, 1207-1210. [20] F. A . Vollenbroek, W. P. Bosman, J. J. Bour. J. H. Noordik, P. T. Beurskens. J. Chem. Soc., Chem. Commun. 1979. 387-388. [21] J. Mielcke, Diploma Thesis, University of Tubingen. 1989. [22] F. A . Vollenbroek, J. J. Bour, J. W. A. van der Velden, Red. Truc. Chim. Puy.s-Bus 1980, 99, 137- 141. (231 F. Cariati, L. Naldini, Inorg. C h h . A d a 1971, 5, 172- 174. [24] P. Bellon, M. Manassero. M. Sansoni, J. Chew. Sor..,Dulton Truns. 1972, 1481L1487. (251 G. Beuter, Thesis, University of Tubingen, 1990. [26] C. E. Briant, B. R. C. Theobald, J. W. White, L. K. Bell, D. M. P. Mingos: A. J. Welch, J. Chiw. Soc., Chcrn. Conitnun. 1981, 201-202. [27] R. P. F. Kanters, P. P. J. Schlebos. J. J. Bour, W. P. Bosman, H. J. Behm. J. J. Steggerda, Inorg, Chem. 1988. 27. 4034-4037. [28] L. N. Ito, B. J. Johnson, A. M. Mueting, L. H. Pignolet, Inorg, Chem. 1989, 28, 2026-2028. 129) J. J. Bour, R. P. F. Kanters, P. P. J . Schlebos, W. P. Bosman, H. Behm, P. T. Beurskens, J. J. Steggerda, Recl. Trai>.Chim. Ptrys-Bus 1987. 106; 157-1 58. [30] K. P. Hall. B. R. C. Theobald, D. 1. Gilmour, D . M. P. Mingos. A. J. Welch, J. Chem. Soc., Cheni. Cornmiin. 1982. 528 -530. 1311 C. E. Briant, K. P. Hall. D. M. P. Mingos, J. Chem. Soc., Cheni. Cotnmun. 1984. 290-291. [32] G. Beuter, J. Striihle. A m p i . . C h t w , In/. Ed. Et7
560
I Molecular Clusters
[56] M. Laupp, J. Strahle, Z. Naturforsch. B 1995, 50, 1369-1372. [57] M. Laupp, J. Strahle, Angew. Chem. Inf. Ed. Engl. 1994, 33, 207-209. [58] D. M. Adams, Metal-Ligand and Related Vibrations, E. Arnold, London, 1967. 1591 M. Holzer, J. Strahle, G. Baum, D. Fenske, Z. Anorg. AEIg. Chern. 1994, 620, 192-198. [60] W. Hieber, € 7 . Leutert, Z. Anovg. Allg. Clwm. 1932, 204, 145- 164. [6l] S. G. Bott, H. Fleischer, M. Leach, D. M. P. Mingos, H. Powell, D. J. Watkin, M. J. Watson, J. Chem. Soc., Dalton Trans. 1991, 2569%2578. (621 J. E. Ellis, Adti. Organoinent. Chem. 1990, 31, 1-51. 1631 B. R. Sutherland, K. Folting, W. E. Streib, D. M. Ho, J. C. Huffman, K. G. Caulton, J. Amer. Clienz. Soc. 1987, 109, 3489-3490. 1641 P. D. Boyle, B. J. Johnson, A. Buehler, L. H. Pignolet, Inorg. Chern. 1986, 25, 5-7. 165) A. Albinati, F. Demartin, P. Janser, L. F. Rhodes, L. M. Venanzi, J. Amer. Chem. SOL..1989, 111, 2115-2125. 1661 A . L. Casalnuovo, L. II. Pignolet, J. W. A. van der Velden, J. J. Bour, J. J. Steggerda, J. Amer. Clicwz. SOL..1983, 105, 5957-5958. (671 L. N. Ito, J. D. Sweet, A. M. Mueting, L. H. Pignolet, M. F. J. Schoondergang, J. .J. Steggerda, Inory. Chem. 1989,223, 3696-3701. 1681 D. E. Smith, A. J. Welch, I. Treurnicht, R. J. Puddephatt, Inorg. Cliern. 1986, 25, 4616-4617. 1691 K. Wade, Adu. Inovg. Chem. Radiochem. 1976, 18, 1-66. 1701 S. M. Owen, Poljhedron 1988, 7, 253.~283. (711 F. Scherbaum, A. Grohmann, B. Huber, C. Kruger, H. Schmidbaur, Anyew. Chem. Int. Ed. Engl. 1988, 27, 1544-1546. 1721 H. Schmidbaur, Gold Bull. 1990, 23, 11-21. 1731 T. A. Albright, J. K. Burdett, M. H. Whangbo, Orbital Interaction in Chemi,sfry,John Wiley, New York, 1985. 1741 A . L. Casalnuovo, J. A. Casalnuovo, P. V. Nilsson, L. H. Pignolet, Inorg. Chem. 1985, 24, 2554--2559. 1751 B. D. Alexander, A. M. Mueting, L. H. Pignolet, Inorg. Chem. 1990, 29, 1313-1319. 1761 L. N. Ito, A. M. P. Felicissimo, L. H. Pignolet, Inorg. Chem. 1991, 30, 387-396. 1771 J. J. Bour, P. P. J. Schlebos, R. P. F. Kanters, W. P. Bosman, J. M. M. Smits, P. T. Beurskens, J. J. Steggerda, Inorg. Chim. Acfa 1990, 171, 177-181.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.29 Naked Clusters of the Post-Transition Elements Stcfun Ulvenlund and Lars Bengtsson-Kloo
1.29.1 Introduction Compared with the transition-metal cluster chemistry, the post-transition element counterpart may appear as a bewildering jungle inhabited by bizarre molecular objects. In Figure 1 a typical transition-metal cluster“’ is compared with two examples from the cluster chemistry of mercury’21and sulfur.[31The selection of clusters for Fig. 1 is somewhat biased in order to clearly demonstrate the following points: 1. Whereas the transition-metal clusters are ligand-supported (in the typical case by
carbonyls), the post-transition element clusters are most often unsupported and/ or destabilized by the interaction with ligands; hence the use of the term “nuked cluster”. 2. The transition-metal clusters often display highly symmetrical metal frameworks, normally with symmetries derived from the platonic or archimedian solids or variations thereof. The post-transition element clusters, on the other hand, are not necessarily affected by such confinements. However, as will be explained in Sec. 1.29.4, relationships between the structure and the electron count do exist for the naked clusters. and regular, closed geometries are found for a rather large group of these species. For instance, the trigonal prismatic symmetry of the Rhh framework in Fig. I A is also found in the naked tellurium cluster Te64+ 141 From the structures shown in Fig. I , the need for a broad definition of the term “cluster” is obvious. Throughout this review, a “cluster” will therefore be defined as an aggregate oj t1i-o or more like atonzs connected by direct and substuntiul elementelwmwt bonds. This sweeping definition allows for an unscrupulous inclusion of molecular and ionic rings, cages and chains under the common heading “cluster”.
562
A
I Moleculav Clusters
W
f
B
C
Figure 1. A biased comparison between transition and posttransition element clusters. The figure shows the cluster frameworks and surrounding atoms at bonding distances in (A) the carbon-centered carbonyl cluster RhsC(CO)is, (c)S I ~ A S F and ~)~, ( B ) the infinite and crystallographically incommensurate mercury chains in Hg3-,y(AsFg).
With reference to Fig. 1 it should be obvious that such broad-mindedness is desirable when main-group clusters are concerned. However broad and sweeping, the definition does not account for the large number of hetrvoatomic aggregates which do, intuitively, belong to the class of “clusters”. Since the bulk of this text concerns homoatomic species, this problem is a bearable one. Another term which will frequently be used is “sub-valence”. This term will hereafter be defined as it normally (but implicitly) is, namely as a n unusually low, positive oxidation state, which generally will not be stable in aqueous solution. The two terms “sub-valence” and “cluster” are intimately connected in the sense that all naked, cationic post-transition element clusters are sub-valent species.
1.29.2 Synthesis & and stabilization of cationic clusters 1.29.2.1 Historic overview and fundamental concepts The first synthesis of a naked cluster was probably performed around 1798, when Klaproth reported that the newly discovered element tellurium slowly dissolves in
1.29 Nuked Clusters of the Post-Transition Eletwents
563
sulfuric acid to give a red solution.[" It took more than 150 years until the true nature of the species responsible for this coloration was revealed. Finally, in 1968 Gillespie and co-workers unambiguously showed that the species is the sub-valent cluster Te4'+, 1.[61 By that time, however, such cationic clusters had already been isolated and structurally characterized in the solid state. In 1961 Corbett and coworkers reported the unprecedented Big5+ cluster, 2, which was found to be present in the sub-chloride Bi6C17, isolated from molten Bi-BiClj mixtures.['] The structural characterization of this compound ended a long discussion about the subchlorides of the Bi-BiCIj system that had started with Heintz' studies on the system in 1844.[81Historically, the chemistry of the cationic, naked clusters of the posttransition elements may thus be said to be an off-spring of molten-metal salt chemistry['] and of the chemistry of very strong mineral acids (later to be supplemented by the "superacids").
,-
1
2
The observation that the media used in these pioneering syntheses of cationic clusters ( o h m and BiC13) are both very strong acids is important. Indeed, the concepts of Lewis acidity and aci~-.srahili=ation['o,' are central to the understanding of the formation of subvalent species in general and naked cationic clusters in particular. In order to understand why, consider the simple reaction ( I ) , in which a subvalent species M,,"+ disproportionates to the normal-valent compound MXh and the parent metal M in the presence of the Lewis base X-.
The heart of the acid-stabilization concept is a consequence of the thermodynamics of reaction (1): if X forms strong bonds with M h + ( i e . if X- acts as a strong Lewis base), then disproportionation of M,,"' will be strongly driven by the favorable change in enthalpy. Weak Lewis bases ('non-coordinating' anions)" would therefore be expected to be beneficial for the stability of sub-valent species. In other words, the thermodynamic stability of M,7"t in a given system would be expected to increase upon the addition of a Lewis acid, which is stronger than the
564
1 Molecular Cliislevs
normal-valent oxidation state of the element M in reaction (1). These expectations are nicely borne out by numerous experimental observations, and the reactions in the Sb-SbC13 and Sb-SbC13-AIC13 systems serve as excellent examples. Antimony metal has a very low solubility in liquid SbC13 (0.018 mole-%) and the sub-valent species formed upon dissolution disproportionates to metallic antimony and SbCl3 upon solidification of the solution."31 The nature of the sub-valent species and the stoichiometry of the choroantimonate( 111) ions formed when SbC13 acts as a chloride acceptor are both unknown. However, the general symproportionation reaction taking place can be written as in (2).
( n - ~ / 3 ) S b+ (WZ+ a/3)SbC13 4 Sb,"'
+ Sb,,C13,n+uUp
(2)
The addition of an excess of the strong Lewis acid AlC13 results in a 200-fold increase of the solubility of the metal and blocks the disproportionation of the subvalent species observed in neat SbC13.[l4]In the Sb-SbCl3-AICl3 system, the favorable enthalpy of formation of Al,,C13,+lp ions might be considered as facilitating the dissolution of the metal and stabilizing the sub-valent compound [reaction (3)].""] The formation of chloroantimonate( 111) ions in the Sb-SbC13 system [reaction (2)]; on the other hand, is not sufficiently favorable to allow the dissolution to proceed to an appreciable degree or to stabilize the sub-halide. (U
-
n/3)Sb
+ (a/3)SbCI3 + (a . m)AlC13
+
Sb,"'
+ ~ A l ~ C 1 3 ~ , +(3) ,~
Born-Haber cycles have been used by Passmore et al. to rationalize the thermodynamic stability of a large number of AsF6- salts of cluster cations.[15' The calculations correctly predict the stability of most characterized compounds and also give indications that some compounds, which never have been synthesized in spite of great efforts (ey. AsF6- salts of monoatomic halogen cations), are unstable. This success confirms the argument that the formation of sub-valent cations is mainly enthalpy driven and that a Coulombic treatment of their solid compounds often is a reasonable approximation. To summarize, the naked post-transition element clusters normally present themselves in strongly Lewis-acidic solutions and in solid compounds, which are ionic in the sense that their structures contain large, weakly coordinating anions separated from the cluster cations at van der Waals or weakly bonding distances. The latter fact has given rise to the expression "anti-coordination chemistry" in connection with such cluster compounds.[' 'I
1.29.2.2 Molten salts and related routes The synthesis of cluster compounds and sub-valent species in molten salt solution was mainly developed by Corbett and his co-workers.["] In this section and
1.29 Nnkcd Clusters of the Post-Trunsition Elements
565
throughout the review. the term “molten salt” is used liberally to denote molten halides whose melting points are above room temperature. Many of these (e.q. SbClI, AlCll, GaClI) are low-melting and not appreciably dissociated in the liquid state; they are, strictly speaking, molecular solvents. Historically, however, the solution studies of halides with elevated melting points have been considered a part of molten salt chemistry,‘”’ irrespective of the microscopic constitution of the melts. The most obvious and frequently used route to sub-valent species in molten halide solution is the reduction of a normal-valent halide with its parent metal, i.e. symproportionation reactions such as (2). This methodology is natural, since it disfavors disproportionation of the sub-valent compound to the zero-valent state according to ( 1 ) . In addition, the methodology minimizes the number of components in the system. Post-transition metal-molten salt systems from which solid, subvalent compounds have been isolated through symproportionation reactions are summarized in Table 1. Table 1. Metal-molten salt systems in which solid, sub-valent compounds with metal-metal bonds
have been synthesized by symproportionation or similar reactions. The many sub-halides of gallium and indium have been omitted. These compounds contain either sub-valent monoatomic cations [ c . ~ .(Ga-)!GaCl;)l””* and (In+)(C1-:]1172~ and/or ligand-stabilized metal-metal bonded nnioizs [c,.y. (Ga+)2(Ga2Xh2-).X = B r , For a review of the sub-valent chemistry of the group 13 metals, see refs. 155, 174, 175, and 1761.
*”
Metal-molten salt system Cd -CdC12-AICI: Hg-Hg2Clz -AlCl: Gd SbCIq GaCI: BI-BIX~,X = C1. Br BI-BIXJ, X = Br, I Bl-CaCl: BI BICII-AICI: Bi-BiCIq-GaCI: Bi-In BiCli BI-BICI:-HfC14 BI- BiCIq-ZrCI4 Bi-BiBr:-ZrBra Se-SeCIJ- AICI: Se SeCIJ NbCls Se SeBr4 TaBr, Te TeCIq- AICI: I2
ICI AICIq
Isolated solid phases
Ref. 1211, 212,300-3021 [213. 215, 2161 [291 [ 16, 68, 259. 3031 1259, 260. 3041 [281 [200, 2481 1200, 201 I [291 [3051 [247I [ 306)
PO51 1234, 3071 1308I ~ 9 1 1191 1307, 3091 1307, 3091 12351
W1
566
1 Molecular Clusters
Table 2. Gaseous heats of formation, dH,b[AX-], for a number of Lewis acid-base AXcomplexes of relevance for the synthesis of cationic clusters.
Lewis acid-base reaction
AH,b[AX- ]/kJ mol-'
Ref.
AIC13 + C1- + AlC1, GaC13 + C1- + GaC1; PF5 + F- 4PF, A s F ~+ F- 4AsF; SbFs + F- + SbF,
-348 -335 -423 -465 -465
[310, 3111 [310, 3111
[3121 13121 [3 131
Many molten halides are insufficiently Lewis-acidic to allow for the formation of appreciable amounts of sub-halides through symproportionation reactions with the parent metal. As described above, the addition of an excess of a more acidic halide is the obvious way to increase the yield. The most commonly used acids of this type are GaC13 and AlC13, which are both very potent halide acceptors (Table 2). The use of inert halides as solvents for symproportionation reactions seems not to have been investigated extensively. However, such solvents may provide effective, alternative routes to sub-valent compounds. This is illustrated by the improved synthesis of Bi6C17 from Bi and BiC13 in liquid antimony( 111) chloride.[l6] A major drawback connected with the symproportionation reaction is that the number of redox reactions that can take place is limited. A more recent development, which shows obvious similarities to the traditional molten salt route, but which avoids the limitations of symproportionations, is the solid-state technique developed by Beck in which a volatile, high-valent transition-metal chloride acts both as halide acceptor and as oxidizing agent.[17]The synthesis of Tes2' by oxidation of tellurium with WC16 according to (4) is representative."']
The fact that Tes2+ has never been synthesized by symproportionation nicely proves that new, unprecedented cluster species may be synthesized if some imagination is used in the choice of oxidizing agent and Lewis acid. Other chalcogenide clusters, low-dimensional bismuth iodides and mixed clusters such as Te& 2+ have recently been synthesized by similar vapor transport r n e t h o d ~ . [ ~ ~ - ~ ' ] Similarly, liquid GaC13 can be used as an oxidizing agent for post-transition elements and affords a new synthetic route to cationic clusters.[271Thus, (Bi53+)(GaC14-)3can be synthesized from Bi-GaCls melts, 3.[2'1Investigations of the solution chemistry of the Bi-GaC13 system show that GaC13 is reduced to Ga+ by bismuth metal, while concommitantly acting as a chloride acceptor. The forma-
1.29 Nuked Clusters of the Post-Trunsition Elements
567
3
tion of Bi5(GaC14)3can be summarized according to reaction ( 5 ) 10Bi
+ (9 + 3m)GaC13
-
2Bis(GaCl4)3+ 3Ga’
+ 3Ga,,,C13,n+1-
(5)
Another way to vary the metal-molten salt theme is to change the reducing agent. This variation does not seem to have been exploited, although it implies an increase in the number of possibilities in cluster synthesis; a stronger reducing agent might be expected to be required in systems where the symproportionations fail. The attempts to synthesize antimony clusters by varying the reducing agent is a promising example.[291Thus, the Sb- SbC13, Sb-SbClI-AlCl3, and SbbSbC13-GaCl3 systems do not yield any isolable cluster species, whereas the Ga-SbC11-GaC13 one does.r291Preliminary data on the black solid phase isolated from this system are consistent with Sbs(GaC14)1, the bismuth analogue of which has been synthesized and characterized.[2*,z91
1.29.2.3 Superacids and other low-temperature systems A superacid can most conveniently be defined as a protic acid of a strength comparable with or more acidic than unhydrous sulfuric acid.[301Examples of such acids are fluorosulfuric acid (HSO3F), oleum and, more notably, mixtures of strong protic acids with strong Lewis acids such as HS03F SbFS (the “magic acid”) and HF + SbF5. On the Hammett acidity scale ( these two mixtures are the strongest acids known. A book by 0’ Donne11 gives an excellent review on solute speciation, acid-base equilibria, and the inorganic chemistry related to superHowever, most of the work performed in superacidic media to date concerns organic chemistry; such media are very suitable for the synthesis of otherwise highly unstable organic radicals and carbocations as reviewed by Olah et ul. in A~ summary l of superacids connection with the general aspects of ~ u p e r a c i d s . [ ~ which have been used in the synthesis of post-transition element clusters is given in Table 3.
+
568
I Molecular Clusters
Table 3. Superacids which have been used in the synthesis of post-transition element clusters. Superacid
Comments
bS04
High viscosity makes crystallization difficult; insufficient acidity for most cluster synthesis. High viscosity; higher acidity than anhydrous H2SO4.
H2S207
Representative reaction
HS03F
The most acidic simple mineral acid.
Oleum
Variable composition; high vapour pressure of SO3. Difficult to handle; high viscosity of reaction mixtures. Easier to handle and better crystallization medium than neat S206F2. The “magic acids”; extremely acidic.
S206F2 HS03FPS206F2 HS03F-AsFS/SbFS HF-AsF~/S~F~ HS03FPS03-SbF5
Acidity comparable to the magic acids. Used to enhance the stability of bromine cations.
+ 3AsF5 + Ss2+ + 2AsF6See magic acids. See HSO3F.
Sg
+ AsF3r3’]
The superacid route to naked clusters was pioneered and developed by Gillespie, Passmore and co-workers and has mainly contributed to the synthesis and understanding of the chalcogen and halogen cluster corn pound^.[^^-^^^ Originally, the syntheses were perfomed in accordance with Klaproth’s original but accidental synthesis of Te42+ in concentrated sulfuric acid (Sec. 1.29.2.1); i.e. one-component superacids were used both as the oxidizing agent and as the source of the necessary weak Lewis base. The synthesis of Se42+by oxidation of selenium in disulfuric acid according to (6) is typical.r371 4 Se
+ 6 H2S207
+
Se42+
+ 2 H S 3 0 K + 5 H2S04+ SO2
(6)
An inherent aspect of reaction (6), which is worth pondering on at this point, is the extreme stability of cluster compounds towards oxidation in suficiently acidic media. The stability of Se42+ towards oxidation to the Se(1V) and Se(V1) species known from aqueous solution chemistry is truly amazing considering the presence of the potent oxidizing agent H2S207. This illustration of the concept of acid-
1.29 Nuked C1uster.s of the Post- Trrmsitioti E1mctit.v
569
stabilization is perhaps more striking than those from molten salt chemistry. The stability of course disappears if the acidity of the solvent is decreased; sulfur clusters, for instance, are stable in HSO1F-SbF5 mixtures but are slowly oxidized further to give SO? in the weaker fluorosulfuric The formation of stable solutions and compounds of the halide cluster cations generally demands stronger oxidation agents and stronger acids than the chalcogens. A versatile oxidizing agent in this respect is peroxydisulphuryl difluoride, S206F2, which readily oxidizes halogens and chalcogens to cationic clusters in HSO3F or other superacidic solvents.[3y1 The oxidizing superacids are versatile and effective for the generation of clusters in solution but severe difficulties are associated with crystallization and recovery of solid cluster phases. These difficulties were to a great extent overcome by the observation that the pentafluorides of antimony and. even better, arsenic in superacidic or inert solvents quantitatively oxidize post-transition elements to cationic clusters. The method was presented in 1969 and was first used in the synthesis of the (AsF6)- salt of (Sg)’+ 4 in HF solution, according to reaction (7).i401 Sg
+ 3 ASFS
+
S ~ ( A S F+~A) s~F ~
(7)
4
The use of AsF5 as the oxidizing agent for the elements in cluster synthesis has several advantages over the molten salt route and the syntheses in “neat” superacids: the reactions may be performed in inert solvents (most often liquid S02) and the syntheses may be performed at ambient or low temperature rather than at elevated temperatures. In addition, the reaction by-products and the solvents are volatile and easily separated from the reaction mixture. Furthermore, the AsF6- ion is thermodynamically more stable than the AlCL- and GaC14- ions (Table 2), which are the protagonists of the molten salt scene. A considerable number of cluster compounds of mercury, bismuth, the halogens [ ~and, ~ to a and the chalcogens have now been prepared by the use of A s F ~ 361 lesser degree, SbF5 and other oxidizing fluorides such as NbF5 and TaF5.[41*421 More recently, the use of catalysts to promote reactions in SO2 solutions has also been developed as an additional synthetic tool.i431
570
I Molecular Clusters
In contrast to the traditional molten salt route, the superacid route has also produced numerous examples of hetevoatomic clusters, most notably mixed polychalcogen cluster cations and such aggregates containing both halogen and chalcogen atoms. For instance, mixed chalcogen A,Mn2+ clusters ( a n = 6 or 10) can be synthesized either by direct reactions of A/M alloys with AsF5 in liquid SO2 or by reaction of (Mq2+)(AsFgP)2or (Mg2+)(AsF6-)2with another chalcogen A in SO2 s o l ~ t i o n . Similarly, ~ ~ - ~ ~ reactions between sulfur, halogens and AsFs or SbFS in SO2 produce species such as S71+and S7Br+.49 5 1 An attractive extension to the possibilities of producing heteroatomic clusters in SO2 solutions of strong Lewis acids is the synthesis of new, unusual coordination compounds in the same media. and The synthesis and characterization of the triazidotellurium( IV), Te(N3)3}f,[521 pentabromodiselenium, Se2Br5+,[531 cations are but two examples. The use of Lewis-acidic metal halides dissolved in organic solvents as synthetic media is a novel approach to post-transition element cluster chemistry, and the only two solvent systems which have been studied so far are the GaCl3-benzene and, to a lesser extent, GaBr3-benzene ~ y ~ t e m . [ The ~ ~ use , ~ of~ benzene , ~ ~ - as~ solvent ~ ~ in the synthesis of clusters is convenient since the solvent is cheap, easy to handle and can be used at ambient temperatures without specially designed glassware or other dedicated laboratory equipment. Furthermore, the recovery of solid, sub-valent compounds from the benzene solutions is conveniently performed by extraction with an alkane. The sub-valent chemistry of gallium, indium, antimony, bismuth and mercury has been investigated in GaXs-benzene systems. Both gallium and indium metal dissolve in GaX3-benzene solutions to form colorless solutions of the corresponding sub-valent ions M+.[55,56,591 In contrast, bismuth metal dissolves to yield highly colored or opaque solutions in which Bi5,+, Bi+, Gat and Ga3ClloP have been identified as the reaction p r o d ~ c t s . [ ~From * , ~ ~such ] solutions, Bi~(GaC14)3may be recovered. The overall reaction is the same as that between bismuth and molten GaC13, i.e. ( 5 ) . Similarly, reaction between Bi and GaBr3 in benzene produces Bis(GaBr4), or Bis(GaBr4)z depending on reaction conditions.[591 Symproportionation reactions between Bi and BiC13 dissolved in GaC13-benzene solutions are also possible and ultimately yield Bis3+ in both chloride and bromide media.[54,591 However, strongly emerald green solutions of Bi+ can be identified as intermediates.[591Similarly, the synthesis of the alleged SbS(GaC14)3 in molten Ga-SbC13-GaC13 mixtures (vide supra) seems to be paralleled in benzene soluti~n.['~] Hg, 2+ can be synthesized in GaC13-benzene solution either by a symproportionation of Hg22+and mercury metal or by direct oxidation of mercury metal by GaC13.[581The latter route has interesting implications, since mercury metal is only very sparingly soluble in neat, molten GaC13. It therefore seems plausible that interactions with benzene stabilizes Hg, 2f in such solutions and therefore renders mercury metal more susceptible to oxidation. Cluster-arene interactions are discussed in more detail in Sec. 1.29.4.6.
+
1.29 Nuked Clusters of the Post-Transition Elements
57 1
1.29.3 Synthesis and stabilization of anionic clusters As we have seen, syntheses of cationic clusters are to a large extent a matter of keeping ligands (Lewis bases) out of the picture; similarly, syntheses of anionic. clusters are a matter of keeping electrons on a short leash. The why and how of this
struggle is the object of the following sections.
1.29.3.1 Liquid ammonia and amine solutions Joannis used solutions of alkali metals in liquid ammonia to reduce post-transition elements for the first time in 1891,[h01and subsequent contributions by Kraud6'] and Zint1[621clarified the chemistry involved. By carrying out careful electrochemical studies Zintl unambiguously showed that the post-transition elements dissolve in these media to form solutions of reduced "polyanionige Saltze" or, to use a more modern terminology, naked anionic clusters. From potentiometric titration data he also managed to give the correct stoichiometry of a rather large number of such clusters. r y . Pbg" and Sng4-, 5 . To acknowledge Zintl's contributions, such clusters were named "Zintl ions" by Corbett in 1976.[631 Zintl also found that the most convenient way to produce the solutions of the anionic clusters in liquid ammonia is to extract alkali metal/post-transition element alloys in the solvent. However, detailed solid-state characterization of the clusters is very difficult using this technique, since poorly crystalline and often pyrophoric solids are obtained once the solvent is evaporated. These troublesome solids are alkali-metal ammoniates of cluster ions,[641of which only [Li(NH1)4]3[Li2(NH1)2Sbj] ' 2 NHI seem to have been completely structurally characterized, 6.[651 Furthermore, the ammoniates most often slowly revert back to the alloy upon further loss of ammonia. The last step involves transfer of electrons from the strongly reducing cluster anion back to the alkali-metal ion and thus represents a major synthetic obstacle.
5
6
572
I Molecular Clusters
The problems of properly characterizing the compounds from ammonia solutions caused a major hiatus in the exploration of the cluster anions. However, in 1970 Kummer and Diehl reported that liquid ammonia may be replaced by more easily handled and considerably more stable polyamines, most conveniently by tetraethylenediamine (en).[66%671 Extraction of sodium-tin alloys in en and subsequent precipitation with T H F or monoglyme yields the reasonably stable compound ( Na+)4(Sng4-). 6-8en.[661 Kummer and Diehl pointed out the analogy between Sng4- and the previously characterized[681cluster cation Big5+, and a partial structural characterization was reported for ( Na+)4(Sng4-)‘ 7en.[691However, in terms of the synthesis of well-defined, stable cluster compounds, the route pioneered by Kummer and Diehl left a lot to be desired. Nevertheless, supercritical amines have recently been found to be good reaction media for the synthesis of extended chalcogenide structures.[701 Remarkably little effort has been made to identify the predominant cluster ions in ammonia and amine solution, apart from the early work of Zintl and the theses by Wilson and Lambert.[71,721 In the latter work, Sng4- and Pbg4- were characterized by spectroscopic methods although their conformations could only be inferred.
1.29.3.2 Cluster stabilization by sequestering agents A way to block the problematic back-donation of electrons from the cluster anion to the cation and thereby a way to prevent the reformation of an alloy, is to sequester the latter ion in a more effective way than is possible with neat ammonia or en. In the 1970s these sequestering agents became available in the form of macrocyclic ligands (cryptands). The formidable ability of cryptands to selectively bind and stabilize alkali metal ions is perhaps most strikingly displayed in the amazing A number of cryptands have been used in “natride” salt (cryptand-Na+)( the syntheses of Zintl ions, but the most successful is 4,7,13,16,21,24-hexaoxa-1,10diazabicyclo-[8.8.8]hexa-cosane,( N ( C I H ~ O C ~ H ~ O C ~ Hwhich ~)~N is ) hereafter, , for obvious reasons, abbreviated “crypt”, 7.
The addition of crypt to en during the dissolution of alloys of sodium and post-
1.29 Nuked Clusters of the Pnst-Trunsition Elements
573
transition elements not only greatly enhances the dissolution rate of the alloy, but also allows isolation of crystalline, well-defined compounds (salts) containing Zintl ions and sequestered sodium ions.[641The thermodynamic significance of the sequestering agent is, of course, the extremely strong complex formation between crypt and Na+, which pushes the cluster-forming reaction (8) to the right.
Most, if not all, solids containing Zintl ions and sequestered alkali metal ions are infinitely stable in the absence of moisture and oxygen. Since the original report of the solid state structures of (crypt-Na+)2(Pb5'-) and (crypt-Na+)4(Sng4-) in 1975,[741the synthetic use of crypt has now produced compounds containing Zintl ions of germanium, tin, lead, arsenic, antimony, bismuth and t e l l ~ r i u m . [ ~ ~ , ~ ~ ~ ~ and T1Sng3-[S21have In addition, a number of mixed clusters such as been synthesized by extraction of ternary alloys. This type of synthesis has the additional potential of producing unusual coordination compounds as was first shown by Rudolph and Wilson in the synthesis of the tetratellurostannate(1V) ion, SnTed4-, by extraction of Na/Te/Sn alloys.L831Numerous group 15/16[s41and 14/16 185-871 mixed clusters (q. [Sb2Se42-])have been obtained in similar ways and the Sng4- cluster has been used as a starting material for the synthesis of the trigonal bipyramidal Sn2Tei'- anion.["] Also the nickel( 11)-phosphine adduct of Ge94-, was made by the extraction of a K4Ge9 alloy with crypt in the presence of Ni(C0)2(P P h 3 ) ~ . [ ~ ~ l
1.29.3.3 Electrochemical methods Electrochemical syntheses of Zintl ions were proposed and preliminarily investigated by E. Zintl himself as early as 1933,[901but was not revitalized until the 1980s.[83.91.921 This synthetic route, as currently applied by Haushalter et (11.,[93-9s1 involves the application of a cathodic current to an electrode made from the alloy under study in a supporting electrolyte of large cations (typically. solutions of quarternary ammonium or phosphonium salts). The cathodic production of polyanions is rapid and normally allows for the isolation of crystalline compounds in a few days. An obvious advantage of the method is that the alloy used does not necessarily have to involve a reactive alkali or alkaline-earth metal; for instance, a number of Sb,,Tei,"- species are produced in the cathodic dissolution of an SbzTe3 a l l ~ y . [ ~ ~ * ~ ~ I The Pbg4- ion has also been identified in ammonia solutions of KI by the cathodic electrolysis of lead electrodes.[961
574
I Molecular Clusters
1.29.3.4 Recent synthetic developments Recent developments have enabled stable compounds containing Zintl ions to be isolated from media, which do not comprise expensive sequestering agents such as crypt. This seems to open up wide possibilities for the synthesis of new, unprecedented cluster species. In 1983, Haushalter et al. reported that some binary alkalimetal/post-transition alloys were (quite surprisingly!) extractable in aqueous solution in the presence of (Bu4N)Br and to yield highly colored solutions.[971This observation lead to the isolation and subsequent structural characterization of the polychalcogenide salts ( Bu4N+)2(Mn2-)(M = S, Se, Te; IZ = 5-6)[971and has been of immense importance in the development of polyanionic coordination chemistry of the heavier chalc~genides.[~~l Compounds containing the Sng4- ion, previously were also isolated in the same pioneering characterized as (~rypt-K+)4(Sng~-),~~,~~ study, but they were not structurally characterized. Common solvents other than water can also be used in the extraction of alloys to produce polychalcogenide ions. For instance, the extraction of K4SnTe4 in methanol solutions of (Ph4P)Br yields . 2 CH30H.['001Extraction of the same alloy in aqueous solutions (Ph4P+)~(Te4~-) of (Me4N)Br yields (Me4N+)4(Sn2Te64-),['011which clearly demonstrates that the formation of polyanions is highly dependent on solvent and counterion and that a large arsenal of various synthetic routes is essential for the full exploitation of polychalcogenide chemistry. This area of cluster chemistry is now under rapid development." 02-1041 Extraction of alloys in non-amine solution has so far been concentrated on the production of novel metal-telluride p o l y a n i o n ~ . [Metalates ~~] of group 15 and 16 elements have been produced by solventothermal methods and superheated solvents with great success.~46~105~1061
1.29.3.5 A note on Zintl phases There is a close relationship between the solution-based syntheses of the naked clusters, as described above, and pure solid state chemistry. This relationship stems from the fact that homonuclear bonding and discrete clusters in many cases also exist within alloys of the type used in the synthesis of Zintl ions in solution. Indeed, such alloys are often to be regarded as well-defined intermetallic compounds."071 A class of intermetallics of special relevance here is the "Zintl phases" (not to be confused with non-intermetallic solids containing Zintl ions)." 08] The cluster structures in Zintl phases often have direct counterparts among the naked clusters discussed in this review and they generally follow the same structure-bonding relationships (Sec. 1.29.4.4). For instance, the classical, discrete Zintl clusters Mg4-, 5 , have been structurally characterized in Cs4Geg and K12Snl7, isolated from melts of alkali metals and the group 14 element^,^'"^*^'^^ as well as in compounds synthe-
1.29 Naked Clusters qf the Post-Transition Elements
575
sized by the use of sequestering agents as described in Sec. 1.29.3.2. However, the description of the clusters in Zintl phases as 'naked' can certainly be challenged. The very special conditions for bonding and stability, which exist within these phases (and intermetallics in general), often enable stabilization of cluster species that clearly would be unstable in most other chemical environments.[291Clusters with extremely high charge or extreme bond strain are but two examples.* Nevertheless. several molten alloys between the alkali metals and the post-transition metals display short-range ordering, i. e. Zintl clustering, analogous to those found in the corresponding solids. This provides some support to the notion that the clusters in solid Zintl phases are to be regarded as isolated or 'naked' in some sense." 12-118] However, since the Zintl anion has been acclaimed to be heavily involved in the physical properties (e.y. the "paddle-wheel" mechanism of conductivity) of the molten alloys, the interaction with cations is most likely substantial and possibly not exclusively electrostatic in character." 6,1 "I When appropriate, Zintl phases will be alluded to in the following section, but the sheer quantity of structural and other data collected in this field of chemistry['081 prohibits an extensive review within the boundaries of this work. In the following, the emphasis will continue to be on clusters which are stable in both solution and the solid state.
'
1.29.4 Structure and bonding One of the most fascinating topics in cluster chemistry is the relationship between the structure of a cluster and the number of electrons involved in the clusterskeleton bonding. In this respect, cluster chemistry is in general indebted to borane chemistry, since it was in this field that the first glimpses of such a relationship were caught. In this section, the way the electron count (hereafter abbreviated ec) of posttransition element clusters is related to their structure is described. Qualitative or semi-quantitative aspects will be put forward rather than intricate molecular orbital (MO) considerations, in order to make the arguments more general. The section is subdivided into areas dealing with electron-precise, electron-rich, and electron-poor clusters, respectively. The definitions of the words precise, rich and poor in this context follow the original ones by Mingos" 91 and are clarified along the way. The determination of the ec of naked post-transition element clusters is generally very easy (as compared to the transition-metal case) because of the lack of occupied valence d orbitals, interfering ligands and interstitial atom^.^^^^-^^^^ The ec of
'
* It should be mentioned that unusual curionic clusters may be stabilized in certain host-guest structures in the solid state. For instance, the extremely electron-poor clusters In3 and Ins have been claimed to form in zeolite A (see ref. 11 I ) .
'+
'+
516
I Molecular Clusters
a post-transition element cluster will hereafter be defined as the total number of valence s- and p-electrons of its atomic constituents minus its charge.
1.29.4.1 Electron-precise polyhedra. 5n-electron clusters The most convenient starting-point for a description of the relationship between the ec and the cluster structure is the electron-precise polyhedral clusters. The notion of these clusters as "electron-precise'' stems from the fact that the number of electrons is just correct to form traditional, localized two-center, two-electron (2c,2e) bonds along all edges of the relevant polyhedron, with one lone pair situated at each apex." 91 The electron-precise polyhedra are always three-connected; i. e. each atom in the cluster framework (corresponding to an apex in the polyhedron) is bonded to three other framework atoms. Furthermore, these polyhedra are possible only for an even number of vertices (cluster atoms), and the number of vertices must be larger than or equal to four. Balaban has given the matter a thorough treatment and has compiled a listing of possible geometries.['261It is clear that the ec in electron-precise M, polyhedra is 5n (see Fig. 2).
ec = Srz
ec = S / l
ec = 511
+2
+2
ec = S r i
+ 4 = 6n
ec = 5n + 6 = 611
Figure 2. The expected bond breakings occuring upon successive two-electron reductions of electron-precise tetrahedral and trigonal prismatic clusters. The lone-pairs of the prismatic cluster and its reduced derivatives are not shown.
1.29 Nuked Clustcw of the Post-Trunsitior.1 E1cwent.s
577
Chemical examples of electron-precise clusters include the many polyhedral C,,H, 20 (the so-called p o l y l ~ e d r ~ ? ~2 7s1)and [ ' their silicon, germanium molecules with tz I and tin analogues.['28 I 3 O 1 Also related are the organostibine clusters.[' 3 1 - 1 3 2 1 H 0moatomic examples from the chemistry of the naked post-transition element clusters are considerably more scarce. Except for the well-known P4 and As4 molecules and the Md4- ions of group 14 (which are exclusively found in Zintl none seems to have been structurally characterized. By taking hc.feroatomic Zintl ions into consideration only two more examples are added: Pb~Sbz'-['"~ and and is best described in Sn2Bi2'- .[*'I T12Tez2- has a butterfly terms of two telluride ions coordinated to two T1(I ) ions.['391
1.29.4.2 Electron-rich clusters with electron counts
< 6n
The elements of group 16 in the Periodic Table have a comparatively large number of valence electrons to be distributed over the cluster framework in which they are incorporated. Their homoatomic clusters are therefore often electron-rich in the sense that they have an electron count higher than 5n. The more electron-poor character of the elements of group 15 results in a borderline situation where cationic clusters are found to be electron-poor (Sec. 1.29.4.4), whereas the anionic ones are electron-rich. The structurally characterized, electron-rich naked clusters of the post-transition elements are listed in Table 4. The structure and bonding of naked, electron-rich clusters have been thoroughly described and discussed by Gillespie,["] Pa~smore,["~as well as by Lynes and Mingos,[' 1 9 . 1 2 1 . 1 4 0 ] R elated bonding models are the 8-N rule generalized by Grimm and Sommerfeld['"'] and the Zintl-Klemm concept normally applied to Zintl phase^.['^)*.'^^ 1441 The topology (connectivity) of M,, clusters having an electron count >517 can often be derived from the electron-precise polyhedra using a simple valence-bond In a two-electron reduction of an electron-precise polyhedral cluster, the extra two electrons must be localized as a lone pair if the overall pattern of localized 2c,2e M-M bonds and the octet configurations are to be preserved. In this valence-bond model, a two-electron reduction thus induces a cleavage of one M-M bond and consequently opens the polyhedral cluster skeleton as schematically shown in Fig. 2 for the case of the tetrahedron and the trigonal prism. In MO terms this process can be seen as the filling of an anti-bonding orbital and a subsequent rupture of an M-M bond. For each additional two-electron reduction, one M-M bond is broken; the cluster framework successively becomes more open and ut tlie limit o f a n electron count oJ'hn UIZ open ring results (Fig. 2 ) . According to this model, a further reduction, beyond an electron count of 612, produces chains (Sec. 1.29.4.3) or causes a complete disintegration of the M, moiety. Additional features must be included in the model in order to accommodate clusters with odd numbers of atoms.[361 At this point, it should be emphasized that the simple model outlined here serves
518
1 Molecular Clusters
Table 4. Characterized, homoatomic post-transition element clusters with electron counts (ec) > Sn (excluding polychalcogenides). Cluster
ec
Structure
5n + 3
Ass ring with two 3connected As atoms Two linked As11 units Square planar Trigonal prism with one capping Sb atom See As1 I 3Square planar Rather obvious . . . Square planar
5n + 4 5n + 2 Sn + 3 5n + 3 5n + 2 Sn+ 1 5n + 2
+
5n 6 5n+ 17 5n 2
Ricyclic S8 ring See Fig. 1 Square planar
5n + 6
See sS2+
+
+8
5n+ 15
Six-membered boat bridged by four Se Analogous to S1g2+
5n + 2
Square planar
5n+4
Boat
5n + 2
Trigonal prism
5n + 6 5n + 6 6n + 2
Bicyclic Tex ring Biconnected squares V-shape
5n
Characterized compounds
1.29 Nuked Clusters
of
the
Post-Transition Elements
579
Table 4 (continued) Cluster
Is
+
7+
Xe:+
ec
Structure
6n + 1 6n + 2 6n + 4 612 + I 6n + 2 6n + 2
Rather obvious . . . V-shape Planar Z-shape Rather obvious . . . V-shape See Sec. 1.29.4.3
6n + 4 6 n t 12
Z-shape Weakly bound Z-shaped Is* moieties forming zigzag chains Rather obvious
611 + 3
Characterized compounds
as a rationalization of cluster structures, not as an a priori prediction. The model deals only with the expected number of bond-breakages or bond-formations and the resulting connectivity (topology) of the clusters, not with the expected "real" structures of the reduced or transformed species (i.e. the cluster conformation). The real structure of a cluster (its conformation) is a consequence of many factors, including the obvious electron-electron and core-core repulsion, core-electron attraction, and (in the solid state) packing effects. Also, and more interestingly, the possibilities of weak intra-cluster bond formation must be taken into account. Such bonds may stem from the possibility of delocalizing electron density from anti-bonding MOs to empty AOs (i.e. unoccupied d orbitals in the case of post-transition elements)[361or from the formation of n*-n* bonds. The theory of the latter type of bonds were first developed by Bannister[1451 and later refined by G i l l e ~ p i e [ 'and ~ ~ ]Gleitner.[I4'] For instance, it is possible to explain the weak cross-ring interactions of Ss2+ and Ses" (Fig. 3) in a satisfactory way using the concept of n*-n* bonds, 4.[15] This argument that a two-electron reduction of a given cluster results in a more open cluster framework has been experimentally verified numerous times. However, some electron-rich cluster species refuse to conform to the simple models and instead adopt structures which are blatantly rule-breaking. Thus, the Tef,4f cluster is found to adopt the trigonal prismatic structure of an electron-precise 5n-electron c l ~ s t e r , [ ~ *in' ~spite ~ , ' of ~ ~the ~ fact that its ec is 5n + 2. An ec of 5n + 2 suggests that one of the bonds should be broken and that a Czr boat conformation would be more stable (Fig. 2). The anomalous structure of Te,j4+ has been explained by arguments of varying sophistication. The bonds within the triangular faces of the Te64t prism (2.68 A)are found to be considerably shorter than those between these faces (3.13 A), which led Gillespie to describe the anomalous structure of Te,j4+ as
580
1 Molecular Clusters
Figure 3. The incommensurate crystal structure of Hgi-,F(AsF6).
a resonance hybrid of resonance structures which all conform to the expected structure of a 5n 2 cluster (for instance 8 and 9).[4,361 Passmore et al. has elaborated on the Te64f theme using MO theory.['51 They pointed out that the cluster may be described as a Te32+ dimer in which the two monomeric units are bound by a 6c,4e TC* - TC' bond, thus giving support to the general idea that electron delocalization plays an important r61e in the structure of Finally, semi-empirical and ab initio calculations have also been used in 511 Density functional attempts to understand the structure of Te64+ . studies suggest that the relatively high charge on the cluster is the main reason for its anomalous structure, since the charge is more evenly distributed when it is delocalized in the prismatic conformation than when localized in the boat conformation.[I5 Obviously, subtle electronic factors are sometimes of great importance in the structure of naked clusters.
+
[140915031
8
9
The four-membered 22-electron clusters of the post-transition elements seem to further emphasize this trivial observation. According to the simple valence-bond model, the conformation of these 5n + 2 clusters is expected to be the butterfly
1.29 Ncikcd Clusters
of'the Post-Transition
Elements
58 1
shape (CI", Fig. 2). On the other hand, detailed ah initio and EHMO calcul a t i o n ~ ' show ~ ~ . that ~ ~ two ~ stable conformations are possible for both 20- and 22electron M4 clusters: Td and C,, for the 20-electron case and Czv and D4h for the 22-electron case. In reality. the only characterized 4n 2 electron M4 cluster that conforms with the valence bond predictions is Si46p.['j 3 ] However, the iso-electronic M4 clusters of sulfur, selenium, tellurium, antimony and bismuth are squares (D4h) and their bonding is best described in terms of delocalized p7c-p7c bonds, ~0,[15.140,154-1~6]
+
10
As pointed out by Passmore ef d.''51 these species are both validating and violating the "double-bond rule". This rule states that elements of the second row form thermodynamically stable double bonds, whereas elements of lower rows do not."571 The M41t cations ( M = S, Se. Te) and M41p anions (M = Sb, Bi) obviously contain thermodynamically stable 7c-bonds (albeit delocalized) and therefore violate the rule. At the same time, however, the dimerization of [M=M]' units to M4*+ results in a lowering of the number of double bonds per M, which is in agreement with the rule. On the other hand, the stubborn reluctance of 0 2 ' to dimerize to O 4 I t is probably a reflection of the strength of the 0=0double bond relative the corresponding single bond.[151
1.29.4.3 Electron-rich clusters with electron counts >6n The cationic clusters formed by chlorine, bromine and iodine are invariably Xnf ( n = 2, 3 or 5 ) chain fragments, which occasionally interact to form more elaborate structural motifs (Table 4). Chains are also a dominant topology for the polyphosphides and polyphosphanes,"581 as well as of the polyhalidesr'591 and poly~ h a l c o g e n i d e s . [ ~ ~ ~I' ~ ~The ~ ' " ~propensity ~' for chain formation rather than rings or closed structures forming can be ascribed as a direct consequence of the ec of these catenated species. All known cationic halogen clusters as well as the polychalcogenides and polyhalides have electron counts larger than 6n and would therefore. as discussed above, be expected to be open chains, in accordance with what is observed.
582
I Molecular Clusters
+
The Br2+ and 12’ ions are paramagnetic (ec = 13 = 6n 1) species, which have been isolated as their fluoroantimonate(V) salts (Table 4). In terms of MO theory, the unpaired electron resides in a singly occupied n* orbital (a n* SOMO:, and these ions may therefore be termed n*-radicals.[’’I Whereas Iz(Sb2FI1) is paramagnetic,[’611the salts I*(AsF6) and I*(SbF6)(Sb3F14)are found to be diamagnetic and to contain ( I 2 + ) 2 d i m e r ~ . [ ’ ~This ~ ” ~dimerization ~] is hardly surprising; 12’ has a bond order of 1.5 and is thus violating “the double-bond rule”. However, due to the electron count of 142f a bonding situation analogous to the chalcogen M42f rings is out of question; in 142+there are too many electrons to allow for the formation of a delocalized 5pz-5pn system. Furthermore, the 1-1 distances of 142+ vary greatly, Passmore et al. have described the bonding in the 142’ dimer in terms of a z*-n* bond formed by a positive overlap of the n*-SOMO’s of the 12’ units, ll.“sl Or
1 0
I 0 0 1
11
1.29.4.4 Electron-poor clusters. Electron counts <5n Polyhedral clusters in which the electron count is less than 5n are “electron-poor’’ in the sense that the number of skeleton electrons is insufficient to form localized, 2c,2e bonds. Instead, models involving 3c,2e bonds or full global electron delocalization (’three-dimensional a r o r n a t i ~ i t y ) [ ’ ~ ~must - ’ ~ ~be ] invoked to describe the bonding c~nditions.[’’~~ Electron-poor naked clusters are, as would be expected, formed by the early post-transition elements in groups 13 and 14. In addition, the cationic clusters of group 15 are electron-poor. Examples, derived from the compilation in Table 5, include the iso-electronic Pbg4- and Big” clusters and the 22-electron M5 species of thallium, tin, lead, antimony, and bismuth. A notable feature of the formally naked, electron-poor clusters of group 13 is their absence in solution and in solids synthesized by solution techniques; they seem to present themselves exclusively in intermetallics (Zintl phase^).['^*^'^^' For instance, the TIs7- cluster is found together with T177- in the ternary Zintl-phase Na2K21T11g,[1681but the necessarily high charge of such group 13 clusters presumably makes them unstable in solution, as alluded to in Sec. 1.29.3.5.[291 In solution and non-intermetallic solids, the only stable sub-valent species of the group 13 elements are the atomic ions Ga+ and In+ and ligand-supported c l ~ s t e r s . [ ” ~ ’ ~ ~ ~ ’ ~ ~ ~ Most of the understanding of the bonding and structural relationships in electronpoor clusters has its origin in the chemistry of the boranes. L i b s ~ o r n b ’ s ~and ’~~~’~~~ William’s[’951 original systematization of the structures and bonding in these com-
1.29 Nuked Clusters of the Post-Transition Elements
583
Table 5. Characterized naked, homoatomic post-transition element clusters with electron counts (ec) < 5n. Cluster
ec
Structure
Characterized compounds
4n + 2 4n + 2 4n + 3
Trigonal bipyramid Tricapped trigonal prism See See. 1.29.4.4
4n+4
Monocapped square antiprism
411 + 2
Tetrahedron
4n + 2 4t1 + 2 4tz + 3 4n + 4
Trigonal bipyramid Tricapped trigonal prism See See. 1.29.4.4 Monocapped square antiprism
4n + 4 4n + 2 4FI + 2 4n + 3 4n + 4
Tricapped trigonal prism Trigonal bipyramid Tricapped trigonal prism See Sec. 1.29.4.4 See Sec. 1.29.4.4
4n 4n 4n 4n
+4 +5 +2 +2
Tricapped trigonal prism Puckered ring Trigonal bipyramid? Trigonal bipyramid
4n 4n
+6 +4
Square antiprism Tricapped trigonal prism
pounds has been developed and generalized to account for the bonding patterns found in both main-group and transition-metal clusters, mainly by Rudolph ("paradigm for the electron requirements of cluster^"),[^ 241 Wade ("Wade's rules")" 2 3 1 and Mingos." The structural relationship between closo-, nido-, arachno- and hypho-boranes (with ccs of 4n 2. 4n 4, 4n + 6, and 4n 8, respectively) as well as its bearings on general cluster structures are well-known and need not be described here. Sufficient to say is that these relationships are directly applicable to naked clusters if the electrons of the B-H exobonds of the boranes are supposed to correspond to lone-pairs in the clusters. The great energy separation between the valence np- and ns-electrons in heavier post-transition elements means that the
+
+
+
584
1 Moleculiir Clusters
ns-electrons generally can be assumed to fill the r61e as l ~ n e - p a i r s . " ~However, ~] the exact energy difference between these electrons can have substantial impact on the bonding conditions and degree of delocalization within the cluster framework, as has recently been shown by calculations on the closo-M5 clusters of T1, Sn, Pb, Sb, and BiLz9] The relationship between the structure and bonding of boranes and that of naked post-transtion element clusters was noticed and discussed for the first such species ever to be characterized, uiz. the Big5+ ion.[681Ironically, this cluster is an anomaly in the sense that its ec is 4n + 4, rather than 4n 2 as would be expected from its structure (vide infra). Naked, post-transition clusters with ec = 4n 2 that are adhering to the rules are exemplified by the recently synthesized, octahedral Sn62cluster core in {crypt-K+},{ (Snz-)[Cr(C0)5]3},[1971 and by the previously mentioned 22-electron MS species of thallium, tin, lead, and bismuth. The latter species all adopt the expected trigonal pyramidal structure, 3. Recent developments in the field include the isolation of the group 14 analogue Ges2- with the aid of sesquestering agents[751and a confirmation of trigonal bipyramidal structure of Bii3+ in solutions and Fluxional behavior of the closo-M5 clusters in solution has been shown to be unlikely, since the square pyramidal (C4") structure is not energetically available.[291 Nice examples of nido-clusters from the world of naked clusters are provided by the (4n 4)-electron clusters Geg4- and Sn94-,[9931993b 0th of which are monocapped square antiprisms obtained by removing one of the capping vertices of the MI" closo-structure, the bicapped square antiprism. The squareantiprismatic (4n+6)-electron cluster Big 2+ provides an example of an arachnocluster, 12.['6,200,2011
+
+
+
f k
A few electron-poor naked clusters containing an odd number of electrons are known, namely the 39-electron clusters Geg3-, Sng3-, and Pbg3- (ec = 4n 3).[76-791 These M9 clusters present a challenge for the closo/nido structural relationship since they are electronic intermediates between the two types. However, the closolnido concept also seems to be valid also for the 39-electron clusters; the structure of all three clusters are found to be intermediate between the "pure" closo-
+
1.29 Naked Clusters of the Post-Transition Elements
Figure 4. Two different views of the tlo\o-Ms [tricapped trigond prism) dnd m d o - M ~(monocnpped square antiprism) clusters showing the striking geometrical similarities between the two cases.
Moiioc,ippwl \(I.
aiilipri\ni,
C4,,
585
Tricapped triganal pi-ism, qh
(tricapped trigonal prism) and nido-structures (monocapped square antiprism), albeit somewhat closer to the &.so-structure. The family of naked 40-electron (4n 4) Mg clusters display two puzzling features: firstly, the structural chw/nido anomaly of B & - alluded to above and, secondly, the fluxionality on the NMR-timescale of the Mg4- clusters of group 14.17' These features suggest the existence of energetically accessible isomers in addition to the expected C4, nido-structure observed in the solid state for Geg4and Sn94-.[2023 The two different anomalies are closely related and boil down to the relation between the C4, and D 3 h symmetries. Geometrically, the C4v and D3h geometries are very alike and can be converted into one another by simple atomic displacements ( Fig. 4).['04] Calculations employing EHM0,['501relativistic EHM0,'2051 and CND0[",i991 methods suggest the expected C4v symmetry to be the more stable, but they also show that the energy barrier of a C4v + &h rearrangement is low. Furthermore, the rearrangement energy was shown to decrease in the order Ge > Sn > Pb > Bi.[2051Big" may thus be expected to be particularly prone to a distortion from the ideal C4, towards the D3h doso-structure. Simple packing effects may therefore be the cause of the deviation from the expected structure of Big5' in the solid state.['991Similarly, the energy separation between the C4,, and D 3 h forms is expected to be low enough to explain the fluxionality in solution,[2021 although recent ab initio calculations have shown that the Dih closo structure is only a transition state in the rearrangement of the C4, structure.[2061 A dependence of the cluster geometry on the polarity of the solvent is also to be expected since species with C4v symmetry have an appreciable dipole moment, whereas clusters with Dih symmetry do not.[641Ultimately, the sterical non-rigidity of the Mg clusters can be traced back to the electron configuration of these species.[2071An analysis of the HOMOS and LUMOs of Ms and Mg closo-clusters show that a two-electron
+
20232031
586
I Molecular Clusters
+
reduction or a two-electron oxidation ( i e . the transformation to [4n 41- and 4nelectron clusters, respectively) in both cases result in clusters with stable electron configurations, albeit with a slightly lower overall bond order than the original c l o s o - c l ~ s t e r . This [ ~ ~ is~ ~not ~ ~true ~ ~ for other M, cfoso-clusters (n < 1 l), where such changes in the ec produce clusters which are susceptible to Jahn-Teller distortion. Thus, 4n- and (4n 4)-electron h& and M9 species are also electronically stable in the closo-conformation, whereas other M, clusters with such ecs are not. A nice confirmation of this is the stability of the BsC18 and BgC19 molecules, both with ec = 4n.L2081 Certain types of heteroatoms are readily incorporated in the nine-membered, electron-poor clusters of group 14 as shown by multinuclear NMR spectroscoSn9-xGex 4- ,[71,83,2031 T1Sns5-,[831 and pic studies of Sng-xPbx4-,[71*91,202,2031 Sn8-xPbxT15-[2031 (x = 0-9) in en solution. All these species are found to be fluxional on the NMR timescale and a definite elucidation of their structure in solution is therefore not possible. However, the clusters T1Sng3- and T 1 s b ~ ~have been structurally characterized by crystallography and investigated by theoretical method^.[*^,^^^' The clusters adopt the cfoso-structures expected form their ec (4n + 2): bicapped square antiprismatic for T1Sng3- and tricapped trigonal prisThe stability of heteroatomic, electron-poor cluster frameworks matic for TlSns may seem surprising considering the delocalized nature of the bonding; superficially, a heteroatom can be expected to disturb the delocalization and thereby lower the stability. However, as pointed out by Corbett, many deltahedra and deltahedral fragments have non-equivalent positions “whereby heteroatoms presumably can be better accommodated”.[641
+
’-.
1.29.4.5 The bonding of cluster species in group 12 The principles of the bonding in the linear cations of group 12 are quite different from what has been discussed so far. The electron configuration of the group 12 metals zinc, cadmium, and mercury is (n - l)d”ns2, which means that these elements to a large extent have to rely only on s electrons for their bonding and that they consequently, as far as cluster chemistry is concerned, behave as being extremely electron-poor. The number of valence electrons is not sufficient for the formation of closed structures and the option is to form linear chains and chain fragments, as observed. The bonding in the dimers Mz2+ (M = Zn,[209,2101 Cd,[211,2L21 Hg) as well as in the more reduced species Hg32+r41,214-2161 and Hg42+,[41,217,2181 is easily accounted for by a simple valence-bond model and does not seem to call for further comment. However, the inter-chain Hg-Hg distances found for Hg42+ suggest an appreciable bonding and there is a tendency to form infinite zigzag chains. This tendency is fully developed in the fascinating, infinite mercury chains found in the non-stoichiometric, crystallographically incommensurate phases Sb,r2211 Nb,r2221 Ta (Fig. 1).[2221 The chains behave Hg3-s(MF6), M = As,[2,21932203
1.29 Nuked Clusters qf the Post-Trunsition Elements
587
as one-dimensional metal fragments and the bonding (however unique and interesting) is more akin to metallic bonding than to cluster bonding.[2232271 Possibly, a similar view also applies to the little studied compound Hg,(MF6) ( M = Nb, Ta), into which Hg3-n( MFc,) can be transformed.[421In Hg3(MF6) the one-dimensional structural motif of Hgip0(MFh) is replaced by two-dimensional Hg-nets. Recently, low-dimensional mercury clusters have also been characterized by means of X-ray diffraction and electron microscopy in Hgx(Ti&) compounds with close relations to the fluoroniobate and - t a n t a k e corn pound^.^^^^^^^^^ The triangular HgJ4+ and Hg2Agi+ clusters have been isolated with the aid of ~ ~ ~Hg-Hg ~] distances are typically diphosphines,[230-2321 and as a r s e n a t e ~ . The 0.15-0.20 A longer than in the linear Hg,,’+ clusters, which is logical considering the formally smaller number of 6s electrons available for Hg-Hg bonding.
1.29.4.6 Em-cluster bonding. Are the naked clusters really naked? Specific cluster-counterion and/or inter-cluster interactions are evident in many solid structures containing formally naked clusters. With the exception of the mercury clusters, these interactions are generally weak enough to validate the notion of the cluster as “naked”. However, they are nevertheless interesting since in some cases they influence the cluster structures. In addition, the cation-Zintl ion and inter-cluster interactions are of interest since they represent bridges to the structures found in Zintl phases and other inter metallic^.[^^] The charge-transfer interaction between fluoroantimonate( V ) anions and M4’’ cluster cations (M = Se, Te) has been thoroughly discussed[2341and 35Cl NQR studies have been undertaken in order to elucidate the interaction between chloroaluminate( 111) ions and cluster cations in salts of I3+, 15’: Te42f, Hg3’’ and Bi5 .12 3 . 2 3 s 1 The results imply that the anion-cluster interaction is predominantly ionic in 13(AlC14),IS(AIC14),Te4(AlC14)2,and Bi5(AlC14)3,whereas an appreciable covalent nature is evident in Te4(A12C17)2 and, especially, in Hg?(AIC14)2.In general, the covalent nature of the cluster-counterion interaction is very pronounced in cluster compounds of mercury. Thus, the nearest-neighbor Hg-Cl and Hg-F disand tances in tetrachlorometallate and hexafluorometallate compounds of Hg, Hg4’+ are short and do not lend support to the idea that these compounds can be 0n the contrary, the structures are better described described as ionic.[41.s8.214~2181 in terms of molecular entities and an essentially linear Hg-Hg-X coordination. Indeed, it is well known that the group 12 element dimers M2’+ in general and Hg2’+ in particular are only metastable in the absence of ligands along the M-M vector direction. The necessity of a reasonably electronegative ligand to stabilize Hg2‘+ was explored by Schwerdtfeger and c o - w o r k e r ~ . [ ~However, ~~’ Hg2?+ forms complexes with arenes in SO2 solution,[2371and recent experimental data for the Hg-GaC13-benzene systems (Sec. 2.3) are best rationalized in n = 2 or 3, moieties in a quasi-trihapto coordination mode, terms of Hg,,(C6H6)2~’,
’
’+
588
I Molecular Clusters
13.[581 Such moieties are closely akin to the Hg22+-mesitylene structure in [Hg(CgH12)A1C14]2[23x1 and presumably also to the (Hg22+)(CsH6)2 complexes in the structurally uncharacterized [Hg(C6H6)AlC14]2.[2"1 Thus, it seems that weak Hg-C bonds may help to stabilize the mercury chain clusters, an idea that is lent further support by the spectroscopic data of [ H ~ ~ ( C O ) ~ ]1)2.[2401 (S~~FI
Cluster-benzene interactions have also been proposed to be of importance in solutions of Bi53+.[2x1Bearing the configuration of the HOMO of Bis" in mind,[29.1981 the trigonal bipyramidal structure of As2(AICp*)3(Cp*=C5Me5)with the Cp*s coordinated to the equatorially located A1 atoms, is in good agreements with these However unsupported by solid-state structural data, the proposed cluster-arene interactions are also in good agreement with the extensive literature on complexes between arenes and sub-valent ions,[2421 recently highlighted by the spectacular C s ~ ( c p ) ~entity[2431 and buckyball complex T l ( C 6 0 P h ~ ) . [ ~ ~ ~ ] Specific interactions between Zintl ions and cations in salts of cryptated cations are very limited because of the effective sequestering of the cation. Without the presence of effective sequestering agents the situation changes considerably. In Rb3As7.3en (en = ethylenediamine),[2"51N a 4 P 1 4 . 6 e n , [ ~and ~ ~ ]Na&19.7en,[~~] the cations are found to bridge between the cluster anions. In the case of Na4Sn~.7en, the bridging results in severe distortion of the approximately tricapped trigonal prismatic Sng4- clusters. A bridging function for the cations is even more pronounced in the compound (crypt-K+)3(KSng3-), which was synthesized from a solution where the amount of crypt (accidentally) was insufficient to ensure a stoichiometric sequestering of the cations.[24h1The structure contains nominal Sng4ions linked into infinite chains by K+ ions. The nature of the counterion in solid cluster compounds can have a major impact on cluster stability, even in compounds where the cluster-counterion contacts are found at van der Waals distances and the cluster-counterion interaction thus can be regarded as non-specific. For instance, Bile( HfC16)3[2471 and Bi5(MC14)3(M = Al, Ga)[293200324x1 all contain bismuth in the formal oxidation state +0.6, but the former compound is made up of the sub-valent species Bi+ and Big5+,whereas the two
1.29 Nuked Clusters oj the Post-Tvcmsition Eletnents
589
latter contain only Bis". The species are related to one another by the disproportionation reaction 19). 2 Bi5'+
i
+ Bi'
Big''
(9)
The formal disproportionation of Bi53 ' , which occurs in the presence of HfC16'-, is obviously caused by the higher charge and larger size of the anion as compared to MCL-. Dikarev rt al. have treated this problem with a model based on a hypothetical close-packing of anions around the clusters.[24y1A situation reminiscent (but even more intricate) of the formal solid-state disproportionation of Bi53+ is found in the three compounds (crypt-K)3Geg,2,5en,['9y1 (crypt-K)?Geg,PPh3 ," " and (crypt-K)3Geg.0.5en,[781which have already been mentioned (Sec. 1.29.4.4). Although the K : Ge ratio is identical in these compounds, the former compound is a mixed-valence compound containing two types of diamagnetic clusters (Geg4and Geg'-), whereas the two latter only contain the paramagnetic Ge9'- ion. This series of compounds seem to illustrate that rather subtle changes of the composition of a solid phase indeed heavily affect the stability of a given cluster, further emphasized by the analogous mixed-valence Sn94-/2p and Pbg4-12- compounds.r801
1.29.4.7 Cluster condensation and extended structures Inter-cluster bonding between the post-transition element clusters is rather common. Both Hg4" and 12 chain fragments inter-connect, as already discussed in the Secs. 1.29.4.3 and 1.29.4.5, and the cationic cluster chemistry of tellurium has been shown to contain examples of similar, but more elaborate inter-cluster bonding.i17.2501 In (Teg'-)(WC16-)2 the inter-cluster Te-Te distance is found to be short (3.4 A) and indicative of substantial bonding.["] This intercluster bonding pattern may be said to be fully developed in Te7(WOBr4),[2s and T ~ ~ ( A S F ~where )?[~~~' polymeric (Te7'+ ) cations are found, 14.
'
14
Such low-dimensional chalcogenide compounds are discussed in a recent review by Beck.'2s01Also, a large number of chalcogenide and a smaller number of pnic-
590
I Molecular Clusters
tide metallates display low-dimensional b e h a v i ~ r . [ ' 3~- 2.5~6 1~ S'imilarly, the As214cluster consists of two linked As11 and the compound BaSn? formally contains chains of face-sharing s1-16~~ octahedra.[' Weak inter-cluster bonding is also evident in structures such as (Bi~2+)(A1C14-)2.['6,2011 Some sub-halides contain extended structural motifs, which are derived from the extended structures of the cluster-forming element itself. The one- and two-dimensional structures of the bismuth sub-bromides and sub-iodides BiX,[259,2601 Bi 1414, 15,[251 and Bi1814[2611 (reviewed and discussed by Dikarev et a1.)[2491 comprise Bi-Bi chains and nets, which are clearly related to the structure of metallic bismuth. Similarly, the lower halides of tellurium (Te3C12 and Te2X; X = C1, Br, 1)[262-2641 show structural similarities with elemental tellurium. Inclusion of heteroatoms into such sub-halides gives rise to very intriguing structural chemistry, as shown by the recently reported mixed bismuth sub-halides BiS gNi51,[26s1 B ~ ~ ~ R U ? B and I - ~the ~ . highly [ ~ ~ ~exciting I Bi7RhBrg, containing a molecular, pentagonal bipyramidal (RhBi7)Brs cluster with an interstitial Rh atom.[26x1 Thus, the bonding features in sub-valent compounds not only provide a link to the structures of Zintl phases, but also to the structures of the elements. This latter aspect is also emphasized by the note on the structure of amorphous red phosphorous.[2691
15
1.29 Nuked Clusters qf the Post-Transition Elements
591
1.29.5 Reactions and extensions The reaction chemistry of the naked post-transition element clusters is still a rather white spot on the map of cluster chemistry. To a large extent this is a consequence of experimental difficulties and the often extreme redox conditions. Thus, the reactions of anionic clusters are often trivial results of their strong reducing capacity[681and their cationic counterparts are often exceedingly strong oxidizers. An extreme example is of course (Oz+)(BF4-),which oxidizes xenon to its unipositive oxidation state.[2701 However, the reactivity of other chalcogenide and halide cluster cations can be harnessed. Thus, ME’+, M = S, Se, react with tetrafluoroethylene to ‘-2721 and Ifl+, n = 2,3, yield his( perfluoroethy1)-polychalcogenides,(C2F5)2Mn[27 reacts quantitatively with his( perfluoroethyl)diselenide, (C2F5)2Se2,to give the corresponding salts of the [Se(C*H5)I# cation.[273113’ also reacts with various nitrogen-containing organic molecules to form novel organic cations such as CH3NI+ and cationic derivatives of pyridine, C ~ H S N I + . ‘ ~ ~ ~ ] Perhaps of more general interest are reactions involving transition-element species. The high solubility of many transition metal complexes in SO2 make reaction between such compounds and cationic clusters possible. Thus, Se4” reacts with The ] *cluster .[~~~] W(C0)g in SO2 solution to yield the cluster ion [ W ~ ( S ~ ~ ) ( C O ) I O was isolated as its SbFb- salt in 1985 and shown to contain [WSe2(C0)5+]2dimers weakly interacting through Se-Se bonds. At the same time, the molybdenum analogue was independently synthesized and structurally characterized by another research Although synthesized by a different route, the [M(C0)3($The cations in these Te3)](SbF6)2 (M = Mo, W ) compounds are compounds comprise the first ever characterized three-membered chalcogen rings and have a “piano-stool” geometry with the M(CO)3 moiety sitting below the center of the Te32t ring plane, 16. Other examples include the As6 and As4 structures stabilized by Cp*Co and Cp*Fe.r27y.2801 Sub-valent post-transition species have been isolated also with the aid of transition-metal clusters. The groups of Venanzi, with the spectacular Hg2 unit in Hgl[Pt?(CO)i(PPh-i-Pr2)3]2,[2811 and Puddephatt seem to be the most successful in this game.[2822841
592
1 Molecular Clusters
Many interesting transition metal complexes are also soluble in en, which enables reactions between such species and Zintl ion to occcur. This field of reaction chemistry was pioneered by Teixidor et al. who reacted Pt( PPh3)4 and Pd( PPh3)4 with Sng4- and Pb94- in en to yield PtMg4- and PdMg4- cluster cores.[285-2861 Using a similar route, the Ni( PPh3)2+ adduct of Geg4-, containing an additional interstitial Ge atom, has recently been isolated and structurally characterized, 17.[891 However, the first structurally characterized cluster formed by direct reaction between a Zintl ion and a metal complex was [CrSng(CO)3I4-,isolated as its cryptK+ salt in 1988.[2871 The CrSbg4- cluster core was found to have the closo-structure expected from its ec. Similar incorporation of transition-metal complexes into previously known Zintl anions has been accomplished for Sn94-,287and M73- ( M = P, As, Sb).[2881 However, metal-carbonyl fragments can be used also to stabilize enigmatically elusive post-transition cluster species. A nice example is the Pbg4- ion, which was unambiguously detected in solution as early as the 1930s (Sec. 1.29.2.1),but eluded isolation in the solid state for almost 60 years. In 1990, the Pbg4- core was finally isolated as the stable closo-cluster [CrPbg(C0)3]4-,[2891 iso-structural with the previously described [CrSng( CO)3]4- tetraanion. Stabilization by metal carbonyls has also produced the octahedral Sn62- closo-cluster core found in [Sns{ Cr(CO),}6I2-, the first ever synthesized and characterized six-membered, electron-deficient naked cluster,
17
18
The incorporation of transition-metal atoms into post-transition clusters may be a relatively new area of research, but the opposite process ( i e . the incorporation of
I 29 Nuked Cliwter\ of tlir Post-Trunsitiori Elements
593
post-transition atoms into transition-metal clusters) is not. This type of chemistry has been separately reviewed by Whitmire and and the bonding in transition-metal compounds containing E2 units, E = group 15 or 16 element, has been rather extensively i n ~ e s t i g a t e d . [ ~ ” ’ ~In~ “the ~ majority of cases the posttransition fragment can be regarded as a simple electron donor, a ligand. However, as pointed out by Whitmire, this simple description is not applicable to species such as [ Bi4Fe4(CO)131’- , [ 2 y 5’2y61 which can just as well be described as a tetrahedral Bi4 unit coordinating Fe(CO),,groups. It thus seems as if the cross-fertilization between transition and post-transition chemistry represented by the Zintl-metal carbonylates and their cationic counterparts can rightfully be regarded as one step on the way to a continuous cluster chemistry, which starts with the naked post-transition element clusters and ends with the well-known transition-metal counterparts. Finally, it should be mentioned that Zintl ions have found a (still) limited application in the production of surface-modified materials. Certain organic polymers can be metallated efficiently in a non-electrochemical route employing Zintl ions of tin In en Similarly, many anisotropic inorganic materials, for instance metal single-crystals and graphite, can be surface-metallated by use of Zintl ions.[2ys1 The surface reaction between Sng4- and gold crystals has been studied by electron m i c r ~ s c o p y . ’ ~ ” ~
Acknowledgements The Swedish Natural Science Research Council ( N F R ) and Human Capital & Mobility (contract no. CHRX CT93 0277) are gratefully acknowledged for their financial support.
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594
I Molecular Cluster5
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1.29 Nuked Clusters ojthe Post-Transition Elements [46] [47] 1481 1491
595
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596
I Molrculur Clusters
1921 B. Eisenmann, Angeiv. Chem. Int. Ed. Enyl. 1993, 32, 1693. 1931 C.J. Warren, D.M. Ho, R.C. Haushalter, A.B. Bocarsly, Angeiv. Chem. Int. Ed. Eng. 1993, 32, 1646. 1941 C.J. Warren, S.S. Dhingra, D.M. Ho, R.C. Haushalter, A.B. Bocarsly, Inory. Chew!. 1994, 33, 2709. [95] J.L. Shreeve-Keyer, R.C. Haushalter, Polyhedron 1996, 15, 1213. [96j J.B. Chlistunoff, J.J. Lagowski, J. Phys. Chem. 1997, BIOI, 2867. 1971 R.G. Teller, L.J. Krause, R.C. Haushalter, Inorg. Chem. 1983, 22, 1809. [98] L.C. Roof, J.W. Kolis, Chem. Rev. 1993, 93, 1037. [99] J.D. Corbett, P.A. Edwards, J. A m . Chem. Soc. 1977, 99, 3313. [loo] J.C. Huffman. R.C. Haushalter, Z. Anory. Ally. Chem. 1984, 518, 203. [loll J.C. Huffman, J.P. Haushalter, A.M. Umarji, G.K. Shenoy, R.C. Haushalter, Inorg. Chem. 1984,23, 2312. [lo21 M.A. Ansari, J.C. Bollinger, J.A. Ibers, Inory. Chem. 1993, 32, 231. [I031 T.M. Martin, P.T. Wood, J.W. Kolis, Inorg. C h m . 1994, 33, 1587. [lo41 S.S. Dhingra, R.C. Haushalter, Inorg. Chem. 1994, 33, 2735. [lo51 W.S. Sheldrick, M. Wachold, Angeiv. Chem. Int. Ed. Eng. 1997, 36, 206. [ 1061 C. Wang, R.C. Haushalter, Inary. Clzem. 1997, 36, 3806. [lo71 R. Nesper, Anyriv. Chem. Int. E d Enyl. 1991, 30, 789. I 1081 H.G. von Schnering, Anyew. Chem. Int. Ed. Enyl. 1981, 20, 33; H. Schafer, B. Eisenmann, Reo. Inor<]. Chem. 1981, 3, 29; H.G. von Schnering, Nova Actu Leopold 1985, 59, 165; H. Schafer, Ann. Rev. Muter. Sci. 1985, 15, I; H.G. von Schnering, Bol. Soc. Chil. Quim. 1988, 33, 41; S.M. Kauzlarich, Comments Inorg. Chem. 1990, 10, 75; R. Nesper, Prog. Solid State Cliem. 1990, 20, I ; J.D. Corbett, Pure & Appl. Chem. 1990, 62, 103 J.D. Corbett, Sfruct. & Bondincg 1997, 87, 157; Chemistry, Structure and Bonding of Zintl Phuses und Ions, Ed. S.M. Kauzlarich, VCH, Weinheim 1996. [lo91 V. Queneau, S.C. Sevov, Anyew. Chem. Int. Ed. Eny. 1997, 36, 1754. [110] H.G. von Schnering, M. Baitinger, U. Bolle, W. Carrillo-Cabrera, J. Curda, Y . Cirin, F. Heinemann, J. Llanos, K. Peters, A. Schmeding, M. Somer, Z. Anorg. Allgem. Cheni. 1997, 623, 1037. 11111 N.H. Heo, H.C. Choi, S.W. Jung, M.Park, K. Seff,J. Phys. Chem. 1997, 101B, 5531. [I121 H.T.J. Reijers, W. van der Lugt, M.-L. Saboungi, D.L. Price, J. Non-Cryst. Solids 1990, 117/ 118, 56. [ 1131 H.T.J. Reijers, M.-L. Saboungi, D.L. Pricc, W. van der Lugt, Phys. Rev. 1990, 41B, 5661. [I141 H.T.J. Reijers, W. van der Lugt, M.-L. Saboungi, Phys. Rev. 1990, 42B, 3395. [115] W. van der Lugt, Phys. Scr. 1991, T39, 372. [116] M.-L. Saboungi, J. Fortner, W.S. Howells, D.L. Price, Nature 1993, 365, 237. [ 1171 R. Winter, 0. Leichtweiss, M.-L. Saboungi, R.L. McGreevy, W.S. Howells, J. Non-Cryst. Solids 1996, 205-207, 66. [118] R.D. Stoddard, M.S. Conradi, A.F. McDowell, M.-L. Saboungi. D.L. Price, J. Non-Crysf. Solids 1996, 205-207, 203. [119] D.M.P. Mingos, Nuture (Phys. Sci.) 1972, 236, 99. [120] A. Simon, Angew. Chem. Int. Ed. Enyl. 1988, 27, 159. [121] D.M.P. Mingos, Acc. Chem. Rex. 1984, 17, 1984. [122] B.K. Ted Znorg. Chem. 1984,23, 1251. 11231 K. Wade, Adz.. Inory. Chem. Radiocheni. 1976, 18, 1 . 1124) R.W. Rudolph, Acc. Chem. Res. 1976, Y, 447. [125] K. Wade, Chem. Brit. 1975, 11, 177. [126] A.T. Balaban, Rev. Roum. Chim. 1966, 11, 1097. [127] H. Prinzbach, K. Weber, Angew. Chem. Int. Ed. Engl. 1994, 33, 2239. [128] M. Weidenbruch, Angew. Chem. Int. Ed. Engl. 1993, 32, 545. [129] A. Sekiguchi, C. Kabuto, H. Sakurai, Anyew. Chem. Int. Ed. Engl. 1989, 28, 55.
1.29 N ~ l k r dCluste~sof the Post- Transition Elements
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598
I Molecular Clusters
[168] Z. Dong, J.D. Corbett, J. Am. Chem. Soc. 1994, 116, 3429. [I691 Y. Dumas, Bull. Soc. Clzirn. France 1969, 2634. [I701 H. Schmidbaur, R. Nowak, W. Bublak, P. Burkert, B. Huber, G. Muller, Z. Natuvfbrsch. 1987, 42B, 553. [I711 D. Mascherpa-Corral, Thesis, Montpellier U. S. T. L., 1975. [ 1721 J.M. van den Berg, Acta Cryst. 1966, 20, 905. [I731 C.P. van der Vorst, G.C. Verschoor, W.J.A. Maaskant, Acta Cryst. 1978, 34B, 3333. [I741 A.J. Carty, D.G. Tuck, Prog. Inorg. Chem. 1975, 19, 243. [175] P. Paetzold, Angeiv. Chem. Int. Ed. Engl. 1991, 30, 544. [176] D.G. Tuck, Polyhedron 1990, 9, 377. [I771 M.J. Taylor, P.J. Brothers, in Chemistry of Aluminium, Gallium, Indium and Thallium, Ed. A.J. Downs, Blackie Academic & Professional, 1993, ch. 3, p. 1 1 I . [I781 C. Dohmeier, D. Loos, H. Schnockel, Angew. Chem. Int. Ed. Engl. 1996,35, 129. [I791 D. Loos, H. Schnockel, D. Fenske, Anyew. Chem. Int. Ed. Engl. 1993, 32, 1059. [I801 D. Loos, E. Baum, A. Ecker, H. Schnockel, A.J. Downs, Angew. Chrm. Int. Ed. Engl. 1997, 36, 860. [I811 C.D. Doriat, M. Friesen, D. Loos, E. Baum, A. Ecker, H. Schnockel, Angew. Chem. Int. Ed. Engl. 1997, 36, 1969. [182] W. Kostler, G. Linti, Angew. Chem. Int. Ed. Eng. 1997, 36, 2644. [ 1831 P.J. Brothers, K. Hubler, U. Hubler, B.C. Noll, M.M. Olmstead, P.P. Power, Angew. Chem. Int. Ed. Engl. 1996, 35, 2355. [ 1841 K.W. Hellmann, L.H. Gade, A. Steiner, D. Stalke, F. Moller, Angew. Chem. Int. Ed. Engl. 1997, 36, 161. [ 1851 L. Bengtsson-Kloo, S. Ulvenlund, Spectrochim. Actu 1997, ,453, 2129. 11861 G. Gerlach, W. Honle, A. Simon, Z . Anorg. Allg. Chem. 1982, 486, 7. (1871 W. Honle, G. Gerlach, W. Weppner, A. Simon, J. Solid State Chem. 1986, 61, 171. [l88] W. Uhl, M. Layh, T. Hildebrand, J. Organornet. Chem. 1989, 364, 289. [I891 M. Tacke, H. Kreinkdmp, L. Plaggenborg, H. Schnockel, Z. Anorg. Allg. Chem. 1991, 604, 35. [I901 D. Loos, H. Schnockel, J. G d U S S , U. Schneider, Anyew. Chem. Int. Ed. Engl. 1992, 31, 1362. 11911 W. Uhl, W. Hiller, M. Ldyh, W. Schwartz, Angew. Chem. Int. Ed. Engl. 1992, 31, 1364. [I921 W. Uhl, Angew. Chem. Ilzt. Ed. Engl. 1993, 32, 1386. [I931 W.N. Lipscomb, Boron Hydrides, Benjamin, New York, 1963; W.N. Lipscomb, in Boron Hydride Chemistry, Ed. E.L. Muctterties, Academic Press, New York, 1975, ch. 2, pp. 30-78. [I941 W.N. Lipscomb, Angew. Chem. 1977,89, 865. [I951 R.E.W. Williams, Inorg. Chem. 1971, 10, 210. [I961 F. Klanberg, D.R. Eaton, L.J. Guggenberger, E.L. Muetterties, Inorg. Chem. 1967, 6: 1271. [I971 B. Schiementz, G. Huttner, Anyew. Chem. Int. Ed. Engl. 1993, 32, 297. [I981 K. Ichikawa, T. Yamanaka, A. Takamuku, R. Glaser, Inorg. Chem. 1997, 36, 5284. 11991 C.H. Belin, J.D. Corbett, A. Cisar, J. Am. Chem. Soc. 1977, 99, 7163. 12001 J.D. Corbett, Inorg. Chem. 1968, 7, 198. [201] B. Krebs, M. Hucke, C. Brendel, Angew. Chem. Int. Ed. Engl. 1982, 21, 445. 12021 R.W. Rudolph, W.L. Wilson, F. Parker, R.C. Taylor, D.C. Young, J. Am. Chem. Soc. 1978, 100, 4629. [203] W.L. Wilson, R.W. Rudolph, L.L. Lohr, R.C. Taylor, P. Pyykko, Inorg. Chem. 1986, 25, 1535. 2041 L.J. Guggenberger, E.L. Muetterties, J. Am. Chem. Soc. 1976, 98, 7221. 2051 L.L. Lohr, Inorg. Chem. 1981,20, 4229. 2061 L. Bengtsson-Kloo, A.N. Kouznetsov, B.A. Popovkin, J. Rosdahl, to be published. 2071 M.E. O’Neill, K. Wade, J. Mol. Struct. 1983, 103, 259. 2081 J.A. Morrison, Chem. Reo. 1991, 91, 35. 2091 B. Gaiek, F. Proshek, Russ. J. Inorg. Chem. 1964, 9, 256.
1.29 Nuked Clusters of the Post-Transition Elements
599
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600
I Molecular Clusters
12511 J. Beck, Angeie. Chern. Int. Ed. Enyl. 1991, 30, 1128. 1252) G.W. Drake, G.L. Schimek, J.W. Kolis, Inory. Chem. 1996, 35, 1740. [253] J. Li, Z. Chen, T.J. Emge, D.M. Proserpio, Inorg. Chern. 1997, 36, 1437. 12541 S.S. Dhingra, D.-K. Seo, G.R. Kowach, R.K. Kremer, J.L. Shreeve-Keyer, R.C. Haushalter, M.-H. Whangbo, Angeie. Chem. Int. Ed. Engl. 1997, 36, 1087. [255] M. Asbrand, B. Eisenmann, Z. Anorg. Allgem. Chem. 1997, 623. 561. [256] M.J. Ferguson, R.W. Hushagen, A. Mar; Inorg. C%em.1996,35, 4505. (2571 R.C. Haushalter, B.W. Eichhorn, J. Chenz. Soc., Chem. Cornrn. 1988, 1027. 12581 T.F. Fassler, C. Kronseder, Anyew. Chem. Int. Ed. Engl. 1997, 36, 2683. 12591 H. von Benda, A. Simon, W. Bauhofer, Z. Anorg. Allcg. Chem 1978, 438, 53. 12601 H.G. von Schnering, H. von Benda, C. Kalveram, Z. Anorg. Allg. Chem. 1978, 438, 37. 12611 E.V. Dikarev, B.A. Popovkin, Dokl. Acad. Nauk SSSR, 1990, 31, 90. 12621 R. Kneip, D. Mootz, A. Rabenau, Angeiv. Chem. Int. Ed. Engl. 1973, 12, 499. [263] M. Takeda, N.N. Greenwood, J. Cliem. Soc., Dalton Trans. 1976, 631. [2641 R. Kneip, D. Mootz, A. Rabenau, Angel;. Chenz. Int. Ed Engl. 1973, 13, 403. 12651 M. Ruck, Z. Anory. Allyrm. Chem. 1995, 621, 2034. 12661 M. Ruck, Z. Anory. Allgcm. Clzem. 1997, 623, 243. 12671 M. Ruck, Z. Anory. Allgem. Chern. 1997, 623, 1591. 12681 M. Ruck, Angav. Chem. Int. Ed. Engl. 1997, 36, 1971. (2691 H. Hartl, Angeiv. Chem. Int. Ed. Engl. 1995, 34, 2637. 12701 C.T. Goetschel, K.R. Loos, J. Am. Chem. Soc. 1972, 94, 3018. 12711 C.D. Desjardins, J. Passmore, J. Chem. Soc., Dalton Trans. 1973, 2314. 12721 H.L. Paige. J. Passmore, Inorg. Chem. 1973, 12, 593. 12731 J. Passmore, P. Taylor, J. Chem. Soc., Dalton Trans. 1976, 804. 12741 I. Tornieporth-Oetting, T. Klapotke, J. Passmore, Z. Anorg. Allg. Chern. 1990, 586, 93. 12751 C. Belin, T. Makani, J. Roziere, J. C/zem.Soc., Chern. Comrn. 1985, 118. 12761 M.J. Collins, R.J: Gillespie, J.W. Kolis, J.F. Sawyer, Inorg. Chem. 1986, 25, 2057. 12771 R. Faggiani, R.J. Gillespie, C. Campana, J.W. Kolis, J. Clzem. Soc., Chem. Comm. 1987, 485. 12781 A. Seigneurin, T. Makani, D. Jones, J. Roziere, J. Clzern. Soc., Dalton Trans. 1987, 2111. 12791 C. von Hiinisch, D. Fenske, F. Weigend; R. Ahlrichs, Chem. Eur. J. 1997, 3. 1495. 12801 G . Friedrich, O.J. Schercr, G. Wolmcrshauser, Z. rmory. ally. Chem. 1996. 622, 1478. [281] A. Albinati, A. Moor, P.S. Pregosin, L.M. Venanzi, J. Am. Chem. Soc. 1982, 104, 7672. 12821 K.-H. Dahmen, D. Imhof, L.M. Venanzi, T. Gcrfin, V. Gramlich, J. Organornet. Chenz. 1995, 486, 37. [283] L. Hao, J.J. Vittal, R.J. Puddephatt, Orycznomet. 1996, 15, 3115. [284] A. Albinati, K.-H. Dahmen, F. Demartin, J.M. Forward, C.L. Longley, D.M.P. Mingos, L.M. Venanzi, Inorg. Chetn. 1992, 31, 2223. [285] F. Teixidor, M.L. Luetkens, R.W. Rudolph, J. Am. Clirm. Soc. 1983, 105, 149. 12861 M.L. Luetkens, F. Teixidor, R.W. Rudolph, Inorg. Chim. A m 1984, 83. L13. 12871 B.W. Eichhorn. R.C. Haushalter, W.T. Pennington, J. Am. Clzem. Soc. 1988, 110, 8704. 12881 S. Charles, B.W. Eichhorn, A.L. Rheingold, S.G. Bott, J. Am. Chem. Soc. 1994, 116, 8077. 12891 B.W. Eichhorn, R.C. Haushalter, J. Chem. Soc., Chenz. Conzm. 1990, 937. (2901 K.H. Whitmire. J. Coord. Chern. 1988, 1 7, 95. 12911 J.R. Eveland, J.-Y. Saillard, K.H. Whitmire, Znorg. Chem. 1997, 36, 330. [292] S. Kahlal, J.-F. Halet. J.-Y. Saillard, K.H. Whitmire; J. Organornet. Chenz. 1994, 478, I . 12931 P.D. Mlynek, L.F. Dahl, Or(janomer. 1997, 16, 1655. 12941 K.H. Whitmire, J.R. Eveland, J. Chem. Soc., Chrm. Comm. 1997, 1335. 12951 K.H. Whitmire, M.R. Churchill, J.C. Fettinger, J. Am. Chem. Soc. 1985, 107, 1056. 12961 K.H. Whitmire, T.A. Albright, S.-K. Kang, M.R. Churchill, J.C. Fettinger, Inorg. Chem. 1986,25, 2799. [297] L. Krause, R.C. Haushalter, Thin Solid Films 1983, 102, 161. 12981 R.C. Haushalter, Angeiv. Chern. Int. Ed, Engl. 1983, 22, 558.
I 2 9 Naked Clusters of the PoJt-Tvrinsition Element5
601
[2991 M.M.J. Treacy, R.C. Haushalter, S.B. Rice, L'/troriiicro.sL.op~1987, 23, 135. [3001 J.D. Corbett. hiorg. C ' h i i . 1962. I , 700. [301I R.A. Potts. R.D. Barnes. J.D. Corbett. fnorg. C'/ietir. 1968. 7. 2558. [3021 J.D. Corbett. W.J. Burkhard, L.F. Drudig. J. A m . C/iem. Soc. 1961. 83, 76. [3031 R.M. Friedman. J.D. Corbett, Inorg. Chini. Actu 1973, 7, 525. [3041 B. Predel, D. Rothacker. T/ier-rnoc/iir?i.Act[/ 1970, I . 477. [3051 A.N. Kouznetsov. A.V. Shevelkov. S.I. Troyanov. B.A. Popovkin. submitted for publication. [306] A.N. Kouznetsov, A.V. Shevelkov. S.I. Troyanov. B.A. Popovkin. Zliur. Nrorg. Khim 1996. 4 / , 958. 13071 J.D. Corbett. D.J. Prince. B. Garbisch. Itiorg. Clietn. 1970, 9. 2731. [3081 R.K. McMullan. D.J. Prince. J.D. Corhett, h o r y . C%eni. 1971, 10, 1749. [309] T.W. Couch. D.A. Lokken. J.D. Corbett, /norg. C/ieni. 1972. I f , 357. [310] J.D. Beck, R.H. Wood, N.N. Greenwood, Inorgi. Chei~i.1970. 9, 86. [31 I ] R.G. Pearson. R.J. Mawby. in "Huloyerz Chetni.s/rj,".ed. V. Gutmann. Academic Press. New York, 1967, vol. 3. p.67. [312] T.E. Mallouk. G.L. Rosenthal, G. Miiller. R. Brusasco. N. Bartlett. I/ior
602
1 Molecular Clusters
[343] H. Hartl, J. Nowicki, R. Minkwitz, Anyew. Chem. Int. Ed. Engl. 1991, 30, 328. [344] T. Birchall, R.D. Myers, J . Passmore, W.A.S. Nandana, G. Sutherland, Can. J. Chem. 1982, 60, 1264. [345] J. Passmore, G. Sutherland, P.S. White, Inorg. Chem. 1981, 20, 2169. [346] A. Aplett, F. Grein, J.P. Johnson, J. Passmore, P.S. White, Inorg. Chem. 1986, 25, 422. [347] J. Passmore, P. Taylor, T. Whidden, P.S. White, Cun. J. Chem. 1979, 57, 968. [348] T. Drews, K. Seppelt, Anyew. Chem. Int. Ed. Eng. 1997, 36, 273. [349] T. Birchall, R.C. Burns, L.A. Devereux, C.J. Schrobilgen, Inorg. Chem. 1985, 24, 890. [350] R.C. Burns, R.J. Cillespie, W.C. Luk, Inory. Chem. 1978, 17, 3596. (3.511 R. Fehrmann, N.J. Bjerrum, H.A. Andreasen, Inorg. Clzem. 1976, 15, 2187. [352] I.D. Brown, D.B. Crump, R.J. Gillespie, Inory. Chem. 1971, 10, 2319.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.2 Expanding, Degrading, and Rearranging Hexametal Boride Clusters Catherine E. Housecroft
1.2.1 Introduction Our work over the last decade on boron-containing clusters has detailed a number of strategies for assembling metal cages while at the same time denuding the boron atom of hydrogen atoms.“] While metal clusters containing fully interstitial carbon or nitrogen atoms (particularly the former) have been well documented in the literature for a good number of years,12-61those containing fully interstitial boron atoms are more limited in number. The first example to be reported was [Co6(CO)18B]‘71but was not structurally characterized. Since that report, the application of electron counting schemes[8-111has become a useful method either of predicting or rationalizing metal-cluster structures possessing either single or condensed metal skeletons. The count of 93 valence electrons for “[Cog(C0)18B]”is not in keeping with a closed hexacobalt cage, suggesting that this formulation must be viewed with some skepticism. Fourteen years passed between the report of the boride from Schmid and coworkers,17]and that of the next fully encapsulated boron atom in a discrete molecular environment; Shore’s group described [HRu,j(CO)17B], the octahedral structure (Fig. 1) being confirmed by the results of an X-ray diffraction study.[12]The metalbound H atom is readily removed, and Shore also reported the isolation of [( P ~ ~ P ) ~ N ] [ R u ~ ( CI1 O21) Independently, ~~B]. we reported the synthesis and spectroscopic characterization of the salt [Me3NH][Ru6(CO) I ~ B []l.31 These studies opened the door to the development of the chemistry of interstitial borido clusters. At this stage, we should stop to clearly define what is meant in this review by the term “interstitial”. Figure 2 shows the environment of a boron atom in M4butterfly, Ms-square-based pyramidal, and M6-octahedral and trigonal prismatic frameworks, i.e. the most common M4 to M6 cavities in discrete clusters. The exposure of the boron atom in the M4B-species means that the reactivity of this site is ~ignificant,”~~,‘~,’ 51 whereas in the M6B-systems, the boron atom is protected
1.2 Expanding, Degrading, and Rearranging Hexametal Boride Clusters
11
Figure 1. The structure of [HRuh(C0)17B] (X-ray diffraction study). The H atom was not located.
from attack. A pc,-boron atom within a square-pyramidal metal core is also partially exposed, and for these reasons we have chosen to define “interstitial” or, more correctly, “fully interstitial”, as describing an atom “fully encapsulated within a metal cage”. For the examples presently available, this essentially restricts it to a p 6 n
Figure 2. Semi- and fully-interstitial boron environments in Mq, MS and Mg cages having butterfly, square-based pyramidal, octahedral, and trigonal prismatic structures respectively.
Butterfly M4 P4-B semi-interstitial
Square-base pyramid M5 P5-B semi-interstitial
Octahedron M6
Trigonal prism Mg
P6-B
interstitial t
P6-B
interstitial
12
1 Molecular Cluster3
Figure 3. The core structures of (a) [HOss(C0)16Bland (b) [ C O ~ ( C O ) BH)], ~ ~ B (determined by X-ray diffraction mcthods.
mode. There are some ambiguities, notably in Shore's [ H O S ~ ( C ~ ) ~(Fig. ~ B ]3a) "~~ and Fehlner's [ C O ~ ( C O ) ~ BH)]'17' ~ B ( (Fig. 3b), but such geometries will not concern us in this review. Further ambiguity arises for capped metal-cages where the distance of the boron atom to the capping atom is usually longer than a normal bonding interaction; such examples will be discussed later in this article.
1.2.2 Expansion 1.2.2.1 Expansion beyond M6 during syntheses and ligand substitution reactions of M6 borides In most of our studies of ruthenium boride clusters, the strategy has been to build up the cage starting from R u ~ Bor Ru4B p r e c u r s ~ r s . ' ' ~ ' ~High ~ ' ~ ~nuclearity, homometallic products have consisted predominantly of RUG-cages;for example, photolysis of [ R U ~ ( C O ) ~ Bgives H ~ ]the butterfly (Fig. 2) cluster [HRuq(C0)12BH2] and the octahedral boride [ H R u ~ ( C O ) ~ ['~ B ] However, . limited examples of aggregation beyond six ruthenium atoms have been observed, and, perhaps significantly, photolytic reaction conditions were common to these occurrences. In characterizing the products of the photolysis of [ R u (C0)9BH5], ~ mass spectrometric B ~ ]a, 124-valence evidence pointed towards the formation of [ R u ~ ( C O ) ~ ~with electron count, which is consistent with the structure shown in Fig. 4a.['*] When PPh3-for-CO substitution in [Rug(C0)17B]- was carried out using photolysis, a range of products was obtained including [HRu7(C0)19B] and [HRu7(C0)18( PPh3)BI. Capped-octahedral structures, consistent with the 98 valence-electron counts, have been proposed for both these products (Fig. 4b).[l9] The formation of iron borides with metal nuclearity > 6 has been achieved by the Fehlner group,1201 but, as in the Ru7 and Rug clusters cited above,[18,'91the actual cavity containing the boron atom is a hexametal one. The results of a crystallo-
1.2 Expanding, Degrading, and Rearranging Hexameta1 Boride Clusters
Figure 4. Proposed core structures of' (a) [ R u ~ ( C O ) ~(faced-sharing ~B~] octahedra), and (b) [HRu7(C0)19B]and [HRu7(CO)18 ( PPhl)B] (capped octahedron).
13
(b)
graphic study have confirmed that [HFe7( C0)20BI2- contains a capped-trigonal prismatic metal core (Fig. 5) with the boron atom essentially in contact with only six metal atoms, the seventh Fe-B separation being significantly longer than the other six.
Figure 5. The core structure of [HFe7(C0)2oBI2-, determined by single crystal X-ray diffraction methods.
W
14
I Molecular Clusters
Figure 6. The core structure of [ R ~ ~ R u ~ ( C O ) ~determined ~ B Z ] - , by single crystal X-ray diffraction methods.
Heterometallic borides of core-composition RmM2B ( M = Ir or Rh) may be prepared by the reaction of [HRu4(C0)12BH]- with suitable group 9 reagents, for example [Rh2(C0)4C12],12'] [Rh2(nbd)2C12](nbd = norbornadiene), [221or [Ir2L4C12] In each case, no expansion of ( L = cyclooctene or L2 = cycloocta-l,5-diene).~2'1 the cluster cage beyond six metal atoms has been observed; these reactions are further discussed in Sec. 1.2.4. However, the reaction between [ R u ~ ( C O ) ~ B Hand ~][Rh2(C0)4C12] led to the formation of [ H R ~ ~ R u ~ ( C O )[~R~~B~]R, L I ~ ( C O ) ~ ~ B ] and [ R ~ ~ R L I ~ ( C O ) ~Characterization ~B~]-. by X-ray diffraction methods of the [( Ph3P)2N]+ salt of [ R ~ ~ R u ~ ( C O )has ~ ~ shown B ~ I - that this possesses the capped, This anion belongs to double trigonal prismatic structure shown in Fig. 6.[231 a family that includes [ H ~ R U ~ ( C O ) ~ ~ B[HFe7(C0)20BI2-, ] - , ' ~ ~ ~ ~ ~ ] [201 and the (CO)~ s C ] ~1271 - , [C06Ni2(CO)16c2]~-, '281 carbides [Ni7(CO)&I2-, [261 [Ni40~3 [Ni,o(CO)1gC2I2-,[291 [Nil I (C0)15C2l2-,[261 and [Ni12(CO)1 6C2I2-.'261 The pathway by which [Rh3Rug(C0)23B$ arises from the combination of [ R U ~ ( C O ) ~ B Hand ~]{Rh(C0)2}+has not been established, but it is noteworthy that the tetrahedral Ru3B unit that is present in the starting ruthenaborane is retained (twice over) in the product as illustrated by Fig. 6. There is no evidence to indicate that the pathway involves cage expansion beginning from a hexametal cage, and we have not been able to isolate single trigonal prismatic borides from this reaction mixture.
1.2.2.2 Expansion from M6 cages using group 11 metal fragments The expanded-cage products, (i.e. more than six metal atoms), described in the previous section arose serendipitously. A designed method of expanding the Mg cage involves reactions between hexametal boride anions and group 11 metal fragments, provided in the form of, for example, LAuCl or ClAu(L-L)AuC1 in which L = monodentate phosphine and L-L = bisphosphine. Such general reactions are well established in cluster chemistry, '301 and an extension to boride chemistry is an
1.2 Expanding, Degrading, and Rearranging Hexametal Boride Clusters
15
obvious route to higher nuclearity, boron-containing clusters. Generally, cluster anions undergo simple addition of LAu+ fragments. However, a comparative study starting from the RugB anions [Rug(C0)17B]- and [H2Rug(C0)18B]- (Fig. 7) has produced the following, interesting results. The reactions of [Rug(CO)17B]- with [LAuCl] ( L = PPh3 or P(CsHdMe-2)3) gave, as expected, [Rug(CO)I ~ B ( A u L )in ] good yield. Structural characterization confirmed the attachment of the gold(1) phosphine unit in an edgebridging position,"51 the same site occupied by the cluster-hydrogen atom in [HRU~(CO)~~B and ] ~a' ~result " ~ ~upholding the isolobal relationship between H+ and LAu+.[~']In principle, the trigonal prismatic boride [H2Rug(CO) should also react with LAuCl with the addition of an LAu+ unit. However, we found that this reaction was accompanied by elimination of Hz and CO to give a mix] , [Rug(CO)1gB(AuL)3] ture of [Rug(CO)17B(AuL)], [HRug( CO)I ~ B ( A u L ) ~and when L = PPh3, and [Ru6(C0)17B(AuL)] and [HRug(C0)1gB(AuL)2] when L = P(CsH4Me-2)3. This result exemplified a novel trigonal prismatic + octahedral cage-rearrangement; the role of the boron atom in maintaining core-integrity during the transformation is clearly important. A similar situation has been observed in nitride chemistry, but under quite different reaction conditions: trigonal prismatic [Cog(CO)lsN]- loses carbon monoxide when heated in THF to give octahedral [Cog(CO)I ~ N ] ~ Figure . ' ~ 7 illustrates the cluster structures of [Rug(C0)17B(AuL)], [HRug(C0)1gB(AuL)2], and [Rug(C0)1gB(AuL)?],each being confirmed by the results of single crystal X-ray diffraction determinations (for different L). While the Rus-cage is essentially octahedral (and this geometry is consistent with the 86-valence electron count), distortion in the digold and trigold derivatives is particularly severe. In [ H R u ~ ( C O ) ~ ~ B ( A U the P P ~Ru-Ru ~ ) ~ ] ,edge most intimately associated with the two gold centers is lengthened to 338.9(2) pm (compared to 281.0 lengthened to 299.2(3) pm for the remaining Ru-Ru distances), and in [Rug(C0)1gB(AuPPh3)3], the two opposing Ru-Ru edges. which are bridged respectively by one or two gold centers are similarly elongated (327.6(1) and 313.0(3) pm). The distortion in the core of [HRu6(C0)1gB(AuPPh3)2]is illustrated are in Fig. 8a. In the solid state, the three AuPPh? units in [Rug(C0)1gB(AuPPh~)3] non-equivalent, but the 31P NMR spectrum (for a CD2C12 solution) at 180 K reveals only two signals in a 1 : 2 ratio. This is rationalized in terms of a dynamic process involving the "rocking motion" shown in Fig. 8b.'251Of course, this assumes that the 31PNMR spectroscopic handle is valid as a probe for the movement of the entire gold( I ) phosphine fragment. Although this assumption has commonly been invoked in solution studies of clusters with peripheral AuPR3 units, some caution is needed since Au-P bond cleavage is not unknown.1321 The best route theP ~reaction for forming [ R ~ ~ ( C O ) ~ ~ B ( A is UP ~ ) ~ ] between [Rug(C0)17B]- and [(Ph3PAu)30][BF4],and this strategy proved to be the only method of preparing [ R u ~ ( C ~ ) ~ ~ B { A U P ( C ~lower H ~ Myields ~ - ~of)these ~ } ~trigold ] ; clusters were obtained when the precursor was [H2Rug(C0)18BIp.'251 The reactions of [Rug(C0)17B]- with derivatives in the family [ClAu(L-L)AuCl] (L-L = bisphosphine) have led to three types of products: [{Rug(C0)17B}2(p-
16
I Molecular Clusters
1 PRODUCTS
Figure 7. Schematic representation of the formation of mono-, di- and trigold(1) phosphine derivatives formed from [Rug(C0)17B]- or [ H ~ R u c , ( C O ) ~ ~Product B ] - . distributions are discussed in the text.
1.2 Expanding, Degrading, and Rearranging Hexametul B o d e Clusters
17
Figure 8. (a) The core structure of [HRub(CO)lsB(AuPPh?)2],determined by single crystal X-ray diffraction methods, illustrating the elongation of the gold-associated Ru-Ru edge. ib! Proposed solution fluxional process for [Rug(CO),gB(AuPPh7)3].
Au( L-L)Au)], [fRug(COjl7B](Au(L-L)AuCI)], and [ { R U ~ ( C O ) ~ ~ B } ~ A In U]-.[~~] [ { R U ~ ( C O ) ~ ~ B } ~L-L)Au)], ( ~ - A U ( two RugB-cages are linked by the Au( L-L)Au fragment, while in [ { Rug(C0)17B}(Au(L-L)AuCl)], the boride cluster interacts with one gold center from which a pendant (L-L)AuCl chain extends. The third type of cluster consists of two octahedral boride cages, which are fused by a common gold(1) center (Fig. 9). The exact geometry about the gold atom
Figure 9. The proposed structure of [(Rub(C0)17B}ZAu]-; the geometry at the gold( I ) center is not necessarily pseudo-square planar, but may possess a spiro-twist as has been observed in related, structurally characterized species.
18
1 Molecular Clusters
has not been crystallographically confirmed, but such fusion is not unprecedented; the anions [ { R U ~ ( C O ) I & ) ~ Tand ~ ] - [ ( R U ~ ( C O ) ~ ~ C }are ~ Hboth ~ ] ~ -related to [ { R u ~ ( C ~ ) ~ ~ B } and ~AU have ] - been structurally characterized. In addition, we have reported the preparation and structures of the anions [ { H F ~ ~ ( C O ) ~ ~ B H } ~and A U[]{-H R u ~ ( C O ) ~ ~ B H } ~ Aboth U ] - , of which show a spiro-twist about the gold(1) center.[32b1The relative yields of the three types of product depended upon the nature of L-L, with yields of [ ( R u ~ ( C ~ ) ~ ~ B } ~ A U ] being high for conformationally restricted ligands, for example (2)-bis(dipheny1phosphin0)ethene. [331 [3433s1
1.2.3 Degradation Pentametal boride clusters feature far less frequently in the literature['6'17,36,371 than do tetrametal or hexametal species, and systematic expansion of an M4B to M5B or M4M'B cage is difficult; reaction pathways tend to continue through to the species in which the boron atom is fully encapsulated.[22s361 During the formation of [Fe4Rh2(CO)16Blp,Fehlner had observed the formation of an intermediate species proposed as an Fe4RhB cluster, but it eluded isolation.[381 Relatively early work in our studies of [ H R U ~ ( C O ) I ~and B ] [Rug(C0)17B]- set out to mimic what was already known for the isoelectronic carbide and nitride clusters: under 1 atm of CO at 25 "C, [Rug(C0)16N]~ is converted quantitatively to [ R U ~ ( C O ) I ~ N ] -while , [ ~ ~ ]the quantitative conversion of [Rug(C0)17C] to [ R u ~ ( C O ) ~ occurs ~ C ] under 80 atm of CO at 70 0C.[401Our attempts to achieve a similar conversion from an octahedral Ru6B to square-pyramidal Ru5B skeleton were unsuccessful. However, the reaction between CO (55 atm) and [Ru~(CO)~~B(AUP (see P ~Sec. ~ ) ] 1.2.2.2) for 14.5 h resulted in the formation in good yield of the blue compound [Ru5(CO)l5B(AuPPh3)];the remaining material was unreacted [ R u ~ ( C O ) ~ ~ B ( A U PThe P~~ reaction )]. can be represented by Eq. ( I ) , and Fig. 10 illustrates the structural changes in the cluster core that accompany the reaction. Of significant interest is the way in which the AuPPh3 unit swings down to interact directly with the boron, and thus, might be considered as a protecting group since the Rug + Rus degradation does not occur in its absence. Attempts to isolate [Ru~(C0)15B]-by removal of the {AuPPh3}+ fragment (ie. by treatment with [( Ph3P)2N]Cl) have not been successful.
+
[ R u ~ ( C O ) ~ ~ B ( A U P P3~CO ~)] +
+
[ R u ~ ( C O ) I $ ( A U P P ~ ~Ru(C0)5 )]
(1)
The application of "B NMR spectroscopy has been a routine and important part of our studies of metal boride clusters.['51The degradation of the Ru6-cage results
1.2 Expanding, Degrading, and Rearranging Hexametal Boride Clusters
19
CO, 55 atrn
Figure 10. The formation of [Rus(CO)lsB(AuPPh3)]by degradation of [Rus(CO)17B(AuPPh3)]; each structure has been crystallographically confirmed.
in the boron atom becoming more exposed (fully to semi-interstitial as is shown in Fig. 2), and the IlB NMR spectroscopic resonance shifts upfield from 6 + 194.4 P ~ ~signal ) ] . also in [Rus(C0)17B(AuPPh3)]to 6 172.5 in [ R u ~ ( C O ) ~ ~ B ( A U PThe becomes significantly broader (wi= 25 Hz compared to 175 Hz). A similar structural motif as that observed for the core of [Ru~(C0)15B(AuPPh3)] has also been confirmed by single crystal X-ray crystallography for [HRhRu4(nbd)(CO)12B(AuPPh3)] (nbd = norbornadiene).'361However, this boride was prepared by the controlled cluster-expansion reaction shown in Eq. (2); the point during the reaction at which the [Ph3PAuCl] is added is critical; if it is too late the major product is [ R ~ ~ R U ~ ( ~ ~ ~ ) ~ ( C O ) I ~ B ( A U P P ~ ~ ) ] . [ ~ ~ , ~
+
The cluster skeletons of the many octahedral borides that we have now reported are particularly stable with respect to degradation; the isomerizations that are discussed in Sec. 1.2.4 support this point and emphasize the fact that the ,u,-boron atom acts as "atomic glue" in stabilizing each cluster core. However, the reaction of [Rh2Ru4(CO)16B]- with [ClAu(dppm)AuCl] (dppm = bis(diphenylphosphin0)methane) has provided an unusual example of cluster degradati~n.'~'] We[32c,33,421 and other^[^^-^'' have made use of a number of compounds in the family
20
I Molecular Clusters
(a)
(b)
Figure 11. (a) Proposed structure of IRh~Ry(CO)~sBAu(dppm)l. (b) The structure of [RhRu4(CO)14B{Auz(dppm)11, determined by single crystal X-ray diffraction methods; Ph and CO groups, and H atoms are omitted for clarity.
[ClAu(L-L)AuCl] (L-L = bisphosphine ligand) to link metal centers and clusters. The reaction between [ R ~ ~ R U ~ ( C O ) and I ~ B[ClAu(dppm)AuCl] ]has been found but in addition to lead to the formation of [{Rh2Ru4(CO)16B}2(pU-Au(dppm)Au] two novel species were formed: [Rh2R~(CO)lsBAu(dppm)](Fig. 1 la) and [RhRu4(CO)14BAu2(dppm)](Fig. 1 lb). Although we have not investigated the the fact that we mechanism of the formation of [RhRu4(C0)14BAu2(dppm)], have also been able to isolate [Rh2Ru4(CO)1sBAu(dppm)]from this reaction may indicate that phosphorus-to-rhodium coordination plays a role in the abstraction of the rhodium atom from the Rh2Ru4 cage and thus, in cage degradation. As in the structures of [Rus (CO)l~B(AuPPh3)land [HRhRu4(nbd)(CQ)12B(AuPPh3)] described above, the crystallographically confirmed structure of [RhRu4(CO)14BAuz(dppm)] shows a square-based pyramidal group 8/9 metal core containing a semi-interstitial boron atom with the gold( I ) phosphine moiety attached asymmetrically to the base of the square pyramid.
1.2.4 Rearrangements 1.2.4.1 Conversion of trigonal prismatic to octahedral Ru6B clusters The trigonal prism and octahedron may be described in terms of two eclipsed or two staggered equilateral triangles, respectively. The conversion of one to
1.2 Expanding, Degrading, and Rearranging Hexarnetal Boride Clusters
21
Figure 12. A transformation from a trigonal prismatic Ru6B to octahedral Ru6B cage formally involves rotation of one Rui triangle with respect to the other.
the other involves mutual rotation of these triangles. Figure 12 shows this cluster core rearrangement as it applies to the transformation of [H2Ru6(C0)18Blp ( LL=) ~ ] into [ R u ~ ( C ~ ) ~ ~ B ( A[UHLR)u]~, ( C O ) ~ ~ B ( A Uand L ) ~[] R , u~(CO)~~B(AU phosphine), details of which were described in Sec. 1.2.2.2 and Fig. 7. A similar metal-cage rearrangement has been observed during attempts to prepare phosphine-substitution products of [ H * R U ~ ( C O ) ~ ~ B ]In- . ' fact, ~ * ~ we have been unsuccessful in obtaining any derivatives of this trigonal prismatic boride which retain the latter framework, cage-rearrangement (accompanied by the loss of H2 and CO) apparently being facile.
1.2.4.2 Interconversion between cis- and trans-isomers of octahedral R u ~ M ~(M B = Ir or Rh) borides The reaction of [HRu4(C0)12BH]- with [Rh*(C0)4C12] has proved a successful route to the formation of [ R ~ ~ R u ~ ( C O ) I and ~ B ]converts -, a semi-interstitial p4-B atom into a fully interstitial one. The anion [Rh2Ru4(C0)16BIp has also been isolated as one product in the reaction of [Ru3(C0)9BH4Ip and [Rh2(C0)4C12] (see ( Cbeen O ) de~~B] Sec. 1.2.2.1), and the crystal structure of [( P ~ ~ P ) ~ N ] [ R ~ ~ R u ~has t e ~ m i n e d . ' The ~ ~ ] metal cage in [Rh2Ru4(C0)16BIp is octahedral (as expected from the 86 valence-electron count) with the rhodium atoms mutually trans. However, in solution, we have shown1211 that the product of the reaction of [HRu4(C0)12BHlp with [Rh2(C0)4C12] consists of a mixture of cis- and trans-isomers of [Rh2Ru4(C0)16BIp ; the "B NMR spectrum (in ['HgJTHF) exhibits two triplets at 6+194.2 (cis) and 6+197.3 (trans) ( J ~ h = p 26 Hz). Similar differences in "B NMR chemical shift have also been observed for the cis and trans-isomers of [Fe4Rh:(C0)16B]-, although in this case the cis-isomer was not observed as a persistent species in solution but rather as an intermediate in the formation of tran.s-[Fe4Rh~(CO)16B]- from [HFe4(CO)12BHIp with [Rh2(C0)4Cl*].[381 For comparison. we have also studied the formation of [IrlRu4(CO)16B]-; this octahedral boride has been prepared by reacting [HRu4(C0)12BHIp with [Ir2L4C12] ( L = cyclooctene or L2 = cycloocta-1,5-diene) followed by treatment with CO at
22
I Molecular Clusters
Figure 13. The formal addition of {Ir(C0)2} fragments to the butterfly framework of [ H R ~ ( C O ) I ~ B H(with ]concomitant loss of two hydrogen atoms) is proposed to give cis[Ir2R~4( C0)16B]- prior to rearrangement to the truns-isomer. When rhodium carbonyl fragments are added, both cis- and truns-products have been observed in solutions of the final product.
atmospheric pressure.” Here, the isolated crystalline product was the [ (Ph3P)2N]+ salt of trans-[Ir2Ru4(CO)lsB]-, and in solution, “B NMR spectroscopic data showed only one species, assumed to be the trans-isomer. It seems likely, however, that the cis-isomer does form first (Fig. 13) and then undergoes rapid cage isomerization. The preference between cis- and trans-isomers of the octahedral Rh2Ru4B and Ir2Ru4B borides can be altered by derivatizing the clusters. We first noted such changes in the products of the reactions of [ R u ~ M ~ ( C O ) ~ (~M B= ] - Ir or Rh) with gold( I ) phosphines.’’’] The reaction of trans-[Ir2Ru4(CO)16B]- with [R~PAuCI] ( R = Ph or C6Hll) led exclusively to ~ ~ S - [ I ~ ~ R U ~ ( C O )AI ~ single B A ~crystal PR~]. structure determination of C~S-[I~~RU~(CO)~~BAUP(C~H~ 1)3] confirmed the relative positions of the iridium atoms, and also showed that the gold(1) phosphine unit occupied an edge-bridging site, interacting with the iridium rather than ruthenium atoms (Fig. 14a). The octahedral cage suffers from severe distortion; although the Ir-Ir edge is significantly lengthened (327.3(2) pm) as Fig. 14a1illustrates, the range of other edge lengths is quite large (280.3(4) to 306.2(5) pm). Any reactions of [Rh2R~(C0)16B]-may be presumed to begin with a mixture of trans- and cis-isomers. Treatment with [R~PAuCI]( R = Ph or C6H11) has been found to give green (major) and brown products, and on standing, the brown product converted to the green material. When an analogous reaction was carried out with [(2MeC6H4)3PAuCl], only a green product was obtained. Each green material was shown to be ~ ~ U ~ ~ - [ R ~ ~ R U ~ ( C Owhile ) ~ ~spectroscopic B A U P R ~data ] , for the brown compounds were consistent with their formulation as c ~ ~ - [ R ~ ~ R u ~ ( C O ) I ~ B A U P R ~ An X-ray diffraction study of trans-[Rh2Ru4(C0),6BAuP(c6H11)3] confirmed the structure shown in Fig. 14b, with the gold(1) phosphine unit capping an RhRu2face; of course, all faces in the trans-RhzRu4 cage consist of RhRuz triangles. In the light of the skeletal isomerism exhibited by [ R ~ ~ R u ~ ( C O ) ~and ~B][ R ~ ~ R ~ ~ ( C O ) ~ ~ Bwe A were UPR interested ~], to investigate the effects of introducing ligands other than carbonyls. The first study undertaken looked at the reaction of [HRQ(CO)~~BH]with [Rhl(nbd)2C12] (nbd = norbornadiene).[”] In this case,
I .2 Expanding, Degrading, and Rearranging Hexametal Boride Clusters
23
P Figure 14. The core structures (confirmed by single crystal X-ray diffraction of (a) cis[ I ~ ~ R ~ ( C O ) I ~ B A ~131, P (and C ~ (b) HII trans-[RhzRu4(CO)I~BAUP( C6H1I )3].
the product was ciL~-[HRh2Ru4(nbd)2(C0)12B], deprotonation of which yielded the conjugate base that retained the cis-Rh2Ru4 skeleton. Both clusters have been characterized by X-ray crystallography. We saw no evidence for these cis-species to convert to their respective trans-isomers. Similarly, the gold( I ) phosphine derivatives [ R ~ ~ R u ~ ( ~ ~ ~ ) ~ ( C (OR)=IPh, ~ BC6Hll A U P or R ~2-MeCsH4) ] each possessed cis-arrangements of the Rh~Ru4-framework,with no tendency being shown for cis + trans skeletal isomerism. In a more recent report,[491we have described the effects of introducing triphenylphosphine and trimethylphosphite ligands. Reactions between [RhzRuq(CO)l6B]- and PPh3 demonstrated that PPh3 substitutes for CO at rhodium rather than ruthenium centers, and that in the derivative, [RhzRu4(CO)14( PPh3)ZB(AuPPh3)], the rhodium atoms (each bearing a PPh3 ligand) were mutually cis in the octahedral cage. Up to three P(OMe)3 ligands may be introduced into [RhzRu4(C0)16B]- and we have shown that cis + trans Rh~Ru4-cageisomerization for becomes less facile across the series [Rh2Ru~(CO)~~-,{P(OMe)~},B(AuPPh~)] x = 1 to 3 . Further, migration of P(OMe)3 from Rh to Ru accompanies cis + trans x = 2 or 3. cage rearrangement in [R~~R~~(CO)I~_,{P(OM~)~},B(A~PP~~)],
Acknowledgements Coordinates for some structures determined from X-ray diffraction studies were obtained using the Cambridge Crystallographic Database,1501implemented through
24
1 Molecular Clusters
the ETH in Zurich. The results reported here from my own group could not have been carried out without the dedication of the students and postdoctoral associates who have worked on the project; their names are cited in relevant references. Funding for this section of our research has come from the Petroleum Research Fund (administered by the ACS) and the SERC/EPSRC (for studentships).
References [ I ] C.E. Housecroft, Chem. Soc. Rev., 1995, 24, 215. 121 J.S. Bradley, Adv. Organomet. Chem., 1983, 22, 1. 131 M.I. Bruce in Comprehensive Organornetallic Chemistry I , Eds. E.W. Abel, F.G.A. Stone, and G. Wilkinson, Pergamon, Oxford, 1982, Vol. 4, Ch. 32.6, p. 889; M.P. Cifuentes and M.G. Humphrey in ComprehensiveOryanometallic Chemistry 11, Eds. E.W. Abel, F.G.A. Stone, and G. Wilkinson, Pergamon, Oxford, 1995, Vol. 7, Ch. 16, p. 907. [4] W.L. Gladfelter, Ado. Organomet. Chem., 1985, 24, 41. [5] M.D. Vargas and J.N. Nicholls, Adv. Znory. Radiochem., 1986, 30, 123. [6] C.E. Housecroft in Inorganometallic Chemislry, Ed. T.P. Fehlner, Plenum Press, New York, 1992, Ch. 3, p. 73. [7] G. Schmid, V. Batzel, G. Etzrodt, and R. Pfeil, J. Organomet. Chem., 1975, 86, 257. [8] D.M.P. Mingos and D.J. Wales, Introduction to Cluster Chemistry, Prentic Hall, Englewood Cliffs, 1990. [9] D.M.P. Mingos and A.S. May in The Chemistry of Metal Cluster Complexes, Eds. D.F. Shriver, H.D. Kaesz, and R.D. Adams, VCH; 1990, p. 11. [ 101 C.E. Housecroft, Metal-metal Bonded Carbonyl Dimers and Clusters, Oxford University Press, Oxford, 1996, Ch. 3. [ 111 C.E. Housecroft, Structure and Bonding, 1997, 87, 137. [I21 F-E. Hong, T.J. Coffy, D.A. McCarthy, and S.G. Shore, Inorg. Chern., 1989, 28, 3284. [ 131 A.K. Chipperfield, C.E. Housecroft, and P.R. Raithby, Oryanometullics, 1990, 9, 479. [14] C.E. Housecroft, Ado. Organomet. Chem., 1991, 33, 1. [15] C.E. Housecroft, Coord.Chem. Rev., 1995, 143, 297. [16] C.S. Jun, T.P.Fehlner, and A.L.Rheingold, J. Am. Chem. Soc., 1993, 115, 4393. [17] J-H. Chung, D. Knoeppel, D. McCarthy, A. Columbie, and S.G. Shore, Inorg. Chenz., 1993, 32, 3391. [18] S.M. Draper, C.E. Housecroft, A.K. Keep, D.M. Matthews, X. Song, and A.L. Rheingold, J. Organomet. Chem., 1992, 423, 241. 1191 C.E. Housecroft, A.L. Rheingold, A. Waller, and G.P.A. Yap, Polyhedron, 1998, 17, 2921. [20] A. Bandyopadhyay, M. Shang, C.S. Jun, and T.P. Fehlner, Inorg.Chem., 1994, 33, 3677. [21] J.R. Galsworthy, A.D. Hattersley, C.E. Housecroft, A.L. Rheingold, and A. Waller, J. Chem. Soc., Dalton Trans., 1995, 549. [22] A.D. Hattersley, C.E. Housecroft, and A.L. Rheingold, J. Cluster Sci., 1997. 8, 329. [23] J.R. Galsworthy, C.E. Housecroft, and A.L. Rheingold, J. Chem. Soc., Dalton Trans., 1994, 2359. 1241 C.E. Housecroft, D.M. Matthews, A.L. Rheingold, and X. Song, J. Chem. Soc., Chem. Commun., 1992, 842. [25] C.E. Housecroft, D.M. Matthews, A. Waller, A.J. Edwards, and A.L. Rheingold, J. Chem. Soc., Dalton Trans., 1993, 3059.
1.2 Expunding. Degrading, und Rearranging Hexumetul Boride Clusters
25
[26] A. Ceriotti, G. Piro, G. Longoni, M. Manassero, N. Masciocchi, and M. Sansoni, New J. Chem., 1988, 12, 501. 1271 G.B. Karet, R.L. Espe, C.L. Stern, and D.F. Shriver, Inorg. Cliem., 1992, 31, 2658. 1281 A. Arrigoni, A. Ceriotti. R.D. Pergola, G. Longoni, M. Manassero? N. Masciocchi, and M. Sansoni, Angew. Cliem., Int. Ed. Engl., 1984, 23, 322. [29] A. Ceriotti. G. Longoni, M. Manassero, N. Masciocchi, L. Resconi, and M. Sansoni, J. Cllem. Soc,.,Chem. Commun., 1985, 181, 1301 See for example: K.P. Hall and D.M.P. Mingos, Puog. Inorg. C/zern., 1984, 32, 237; I.D. Salter, A r k Organomet. Chem.. 1989, 32, 237 and references therein. [31] G. Ciani and S. Martinengo, J. OrGqunomet. Chem., 1986, 306, C49. 1321 Within our own metallaborane and boride cluster systems, see for example: (a) K. S. Harpp and C. E. Housecroft, J. Urgunoniet. Chem., 1988, 340, 389; (b) S.M. Draper, C.E. Housecroft, J.E. Rees, M.S. Shongwe, B.S. Haggerty, and A.L. Rheingold, Organometallics, 1992, 11, 2356; (c) J.R. Galsworthy, C.E. Housecroft. and A.L. Rheingold, J. Chem. Soc., Dalton Truns., 1995; 2639. [331 C.E. Housecroft. A.L.Rheingold, A. Waller, and G.P.A. Yap, J. Organomet. Chem., 1998, 565, 105. [34] C.B. Ansell, M.A. Modrick, and J.S. Bradley, Acta Crystallogr., Sect. C, 1984, 40, 1315. 1351 R.F.G. Johnson, W.-L. Kwik, J. Lewis, P.R. Raithby. and V.P. Saharan. J. Chem. Soc., Dulton Trans. 1991, 1037. [36] A.D. Hattersley, C.E. Housecroft, and A.L. Rheingold, .I. Chem. Soc., Dalton Trans., 1996, 603. [37] C.E. Housecroft, D.M. Matthews, and A.L. Rheingold, Organometallics, 1992, 11, 2959. (381 R. Khattar, J. Puga, T.P. Fehlner, and A.L. Rheingold, J. Amer. Chem. Soc., 1989, 111, 1877;
A.K. Bandyopadhyay, R. Khattar, J. Puga, T.P. Fehlner, and A.L. Rheingold, Inorg. Cliem., 1992. 31, 465. 1391 M.L. Blohm and W.L. Gladfelter, Orgunometallics, 1985, 4, 45. 1401 B.F.G. Johnson, J. Lewis, W.J.H. Nelson, J.N. Nicholls, J. Puga, P.R. Raithby, M.J. Rosales, M. McPartlin, and W. Clegg, J. Chem. Soc., Dalton Truns., 1983, 277. 1411 A.D. Hattersley, C.E. Housecroft, and A.L. Rheingold, Inorg. Chim. Acta, 1999, in press. [42] S.M. Draper, C.E. Housecroft, and A.L.Rheingold, J. Organomet. Cliem., 1992, 435, 9. 1431 C.E. Briant, K.P. Hall, and D.M.P. Mingos, J. C h m . Soc., C h i . Comniun., 1983, 843. 1441 M. Ferrer, R. Reina, 0. Rossell, M. Seco, S. Alvarez, E. Ruiz, M.A. Pellinghelli, and A. Tiripicchio, Orgunometallics, 1992, 11, 3753. [45J J.E. Goldberg, D.F. Mullica, E.L. Sappenfield, and F.G.A. Stone, J. Chem. Soc., Dalton Truns.. 1992, 2495. 1461 R. Reina, 0. Rossell, M. Seco, J. Ros, R. Yaiiez, and A. Perales, Znorg. Chem., 1991, 30, 3973. 1471 R.I. Brice, S.C. Pearse, I.D. Salter, and K. Hendrick, J. Chem. Soc., Dalton Truns., 1986, 2 18 1. 1481 C.E. Housecroft and A. Waller, unpublished results. [49] A.D. Hattersley, C.E. Housecroft, L.M. Liable-Sands. A.L. Rheingold, and A. Waller, pol^)hedron, 1998, 17, 2957. [ S O ] F.H. Allen, J.E. Davies, J.J. Galloy, 0. Johnson, 0. Kennard, C.F. Macrae, E.M. Mitchell, G.F. Mitchell. J.M. Smith, and D.G. Watson, J. Chem. Znj: Cornp. Sci., 1991, 31, 187.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.3 Steric Effects in Metallacarboranes Alan J. Welch
1.3.1 Introduction Metallacarboranes are cluster compounds in which the cluster vertices comprise one or more metal atoms (main group metal, transition metal or rare earth [lanthanide or actinide] metal), one or more (most often two) carbon atoms, and boron atoms, the last usually in the majority. The total number of cluster vertices may vary but the vast majority of metallacarboranes have between 6 and 14 vertices; moreover, the cluster may be either open or closed. However, the single largest group of metallacarboranes has 12 vertices and is structurally based on the closed icosahedron, 1. Boranes and heteroboranes, of which metallacarboranes are a constituent group, have fascinating, non-classical structures involving multicenter bonding. In the simplest terms this propensity for multicenter bonding arises directly from the fact that their component { BH} fragments are electron dejicient, having 3 available valence orbitals but only 2 valence electrons. By multicenter bonding the relatively few valence electrons available are efficiently shared by many vertices. Whilst molecular orbital ( MO) studies of (hetero)boranes adequately describe the bonding within individual molecules, the significant breakthrough in the area came in 1971 with the development of the polyhedral skeletal electron pair (PSEP) theory, or “Wade’s rules”.‘’] These simple rules rationalized the structural patterns in (hetero)boranes that had been recognized by Rudolph”] and by W , i l l i a m ~by [ ~re~ lating the cluster structure to two factors; the number ( n )of cluster vertices and the number of skeletal electron pairs (seps). Briefly, if there are ( n 1) seps then the cluster adopts the structure of a Platonic solid (icosahedron if n = 12, octadecahedron if n = 11, and bicapped square antiprism if n = 10, etc.) and is termed closo. If there are ( n + 2) seps then the cluster adopts a structure based on one of these closed structures but with a high connected vertex missing, and is termed nido, and if there are ( n + 3) seps then the structure, termed arachno, is that of a closed form with two (adjacent) missing vertices. Still more open forms, hypho, klado, . . . are
+
27
1.3 Steric EfJits in Metallacarboranes
1
+
+
found with (n 4), (n 5 ) , etc. seps, but are less numerous. At the other extreme a few heteroboranes, including metallacarboranes, have only n seps, and display structures derived from the corresponding closo geometry that has undergone a single diamond-square-diamond (dsd) transformation (see later). Such species are termed pileo, or more commonly hypercloso. Figure 1 shows part of what has been termed the Wade-Williams structural matrix, illustrating the geometrical relationship between closo n vertex, nido (n - 1) vertex, and arachno (n - 2) vertex polyhedra that arises because all have (n 1)
+
Figure 1. Part of the WadeWilliams structural matrix.
hypercloso
closo
nido
arachno
28
1 Molecular Clusters
seps. Counting seps is a straightforward procedure in most cases; if v is the number of valence electrons of the vertex element and x is the number of electrons formally provided by exo-polyhedral atoms or groups, the skeletal electron contribution is (v + x - 2) for a main group element vertex, (v x - 12) for an 18-e transition metal vertex, and (v + x - 10) for a 16-e transition metal vertex. The vast majority (but not all) of the metallacarboranes discussed in this review are considered as 18-e species. The study of steric effects in metallacarboranes is essentially a study in structural chemistry. In the early 1990s we recognized that, for a systematic study of the effects of steric crowding on a family of molecules, metallacarboranes were ideal candidates. Within the cluster structure a metallacarborane clearly has isomeric possibilities; rearrangements between different heteroborane isomers do take place (usually at high temperatures), but the mechanisms by which they occur are poorly understood. The possibilities for variation in substituents at the carbon and metal vertices are obvious, since metallacarboranes are most easily synthesized by metallation of carboranes, themselves usually prepared by reaction between a preformed borane cluster and an alkyne; thus judicious choice of alkyne and metal fragment affords the desired C and M substitution, respectively. In addition, however, substitution reactions at boron vertices, themselves known for many years, are growing in popularity and complexity, as became evident by the recent synthesis of Finally, metallacarboranes 2,3,4,5,6,7,8,9,10,1l-(CHC12)lo-I,12-clos0-C2BloH2.[~] are, for the most part, thermally and chemically stable compounds attainable by well-established and straightforward synthetic routes. For all these reasons they are eminently suitable for a systematic study of the effects of steric congestion on molecular structure and reactivity. The triangulated surface of (hetero)borane clusters presents the perfect template on which to study steric crowding. We reasoned that, if bulky substituents X, Y and Z could be bound to mutually adjacent cluster atoms, then local steric crowding would be efficiently established, as shown in 2. In practical terms, the family of icosahedral metallacarboranes designated 3,1,2-closo-MC2B9 are suited to such a study. This is the heteroatom pattern readily afforded by metallation of a 7,8nido-CzB9 carborane ligand, itself the product of straightforward deboronation of a 1,2-closo-CzBlo carborane parent, Scheme 1. If the cage carbon atoms carry bulky groups R' and R2, and L, is a sterically demanding ligand set bonded to the metal atom, the local crowding desired would be set up. How might such a crowded metallacarborane behave?
+
2
1.3 Steric Efects in Metallacarboranes
29
Scheme 1. Deboronation of 1,2-closo-C~B~0 carborane to 7,8-nidoo-C2B9carborane, and metallation giving 3,1,2-c/oso-MC2B9 metallacarborane.
The vast majority of our work in this area has involved 3,1,2-MC2Bg metallacarboranes in which both cage carbon atoms carry phenyl substituents. We wanted to use the steric bulk of these phenyl substituents to produce crowded metallacarboranes that might accordingly have unusual or interesting properties, but it was first necessary to try to get some feel for the electronic consequences of C-phenyl substitution on the cage. This is conveniently achieved by comparing the IIB{'H} NMR spectra of 1,2Fig. 2. closo-C2BloHlz, l-Ph-l,2-closo-C2BloH~1and 1,2-Ph2-1,2-closo-C2BloHlo, The IlB resonances of all three compounds occur between 0 and -15 ppm, but progressive phenyl substitution does cause a slight overall movement of the reso-
I 0
-5
PhCZBloHII
I
I -5
0
-10
-15 ppm
Ill I -10
0
-5
-10
<S"B>
-8.5
<6"B>
-7.6
-15 ppm
I
PhzCzBIoHlo
<6I1B> -10.7
-15 ppm
Figure 2. Stick representations of the "B{ 'H} NMR spectra of 1,2-c/oso-C2BloHlz,I-Ph-1,2-c/osoC2B loH II and 1,2-Phz-1,2-c/oso-CzBloH10.
30
1 Molecular Cluster3
*
L
6.4 A
+ *
3.7A
Figure 3. The effective width (6.5 A) and effective thickness (3.7A) of a phenyl group.
nances to high frequency. The weighted average "B chemical shift, (611B), conveniently quantifies this movement. For the three carboranes considered, (dl' B) is -10.7, -8.5 and -7.6ppm, respectively. The effect is measurable but small; replacement of C H by CPh has relatively little electronic effect on the carborane, at least in as much as it is reflected in (dI1B). On the other hand, replacement of H by Ph is a significant steric change and moreover, it is one that is highly dependent on the conformation. Figure 3 shows "face-on" and "edge-on" space filling diagrams of a C6H5 group. The effective width is ca. 6.4A (based on van der Waals radii) but the effective thickness is ca. 3.7A (based on n MOs). Clearly, the steric demands of a phenyl group, which is a substituent to a heteroborane cluster, depend markedly on its conformation. For a molecule with Ph attached to one of two adjacent cage C atoms, this conformation is conveniently described by the angle 0, defined as the modulus of the average Ccage-Ccage-Cphenyl-Cpheny] torsion angle. 1' Thus, for the simplest phenyl carborane, 1-Ph-1, ~ - c I o s o - C ~ B ~ there O H ~are ~ , two limiting conformations defined by 0 = 0" and 0 = 90°, Fig. 4. In collaboration with
6 = oo
Figure 4. The limiting conformations of 6 = 90'
I-Ph-1,2-closo-CzB,oH, I.
1.3 Steric Efsects in Metullucarboranes
31
Table 1. The Structure of l-Ph-1,2-closo-C2Bl,-,H11.
SCXRD cc-modification at 200 K P-modification at 150 K Electron Diffraction HF/6-31G*
67.7(4) 71.7(2) 54 65
1.640(5) 1.649(2) 1.628(5) 1.626
[61 [71 [61 [61
colleagues at the Universities of Edinburgh and Erlangen, we have determined the structure of this molecule by low-temperature single-crystal X-ray diffraction on two crystalline modification^,["'^ by electron diffraction in the gas phaseL6]and by ab initio MO calculation^.'^^ The essential results are summarized in Table 1. The optimum value of 19is in the range ca. 55-70", but the theoretical study reveals a very shallow potential well with the conformation at 8 = 90" only 0.3 kJ mol-' less stable, and even that at I9 = 0" only 2.1 kJmol-' less stable, than the optimum conformation. Clearly, with two CPh fragments at adjacent positions in a heteroborane, such high 0 values could not be sustained without significant Ph . . . Ph congestion. Accordingly, a structural studyis1 shows that the phenyl rings in 1,2-Ph2-1,2-cZosoC2B10H10 (Fig. 5a) have conformations defined by low 8 values (average 0 = 5.5"). Moreover, when this species is deboronated to afford the anion [7,8-Ph2-7,8-nidoC2BgH101- the same basic conformation is found'9J(Fig. 5b). Deprotonation of this monoanion by removal of the endo-H atom attached to B( 10) affords the dianionic 3, in which the phenyl rings are carborane ligand [7,8-Ph~-7,8-nido-C2BsHg1~-, ideally oriented to sterically crowd an incoming metal fragment. We have found that such crowding can have either relatively minor or relatively major consequences.
a
b
Figure 5. The structures of (a) 1,2-Phz-l,2-closo-C2BloHlo and (b) [7,8-Phz-7,8-nido-C2BsH 101showing low 0 values for the phenyl rings
32
I Molecular Clusters
@ c-c
Ph
2-
Ph
3
1.3.2 Minor consequences In this section we discuss species derived from metallation of the ligand 3 that have somewhat distorted cluster structures as the result of steric crowding. However, the extent of the crowding is not sufficient to cause either a major polyhedral deformation (e.g. the breaking of a cage connectivity) or an internal polytopal rearrangement (either to an isomer of the molecule expected or to a different structural form). Examples of such major changes do exist, and are described in the following section. When a transition metal fragment {ML,} bonds to the C2B3 face of a 7,8nidu-CzBg ligand, an important feature of the structure of the resulting metallacarborane is A, the slipping parameter, describing the lateral displacement of the metal atom from a position above the center of the carborane ligand face, 4.To a first approximation, A is inversely related to the quality of the match between the frontier MOs of the metal fragment with those of the ligand, with the result that A{lmear ML} (poor match) > A{angular ML,} (intermediate match) > A{conical ML,} (good match; A = O).ilO1 Reaction of 3 with [Ph3PHgC12]2 affords 7,8-Ph2-1O-endu-Ph3PHg-7,8-niduC2B9H9, 5.'"' The {HgPPh3}2+fragment is isolobal with H+. Thus, this fragment is expected to simply endo-o bond to B( 10) of the anion 3, affording a direct structural analogue of [7,s-Ph2-7,X-nido-Cz Bs H 101- (large, electronically -driven, A), which is precisely what is observed. However, A for compound 5 is l.lOA, M-
A
4
5
1.3 Strvic. Effkcts in Metullucurboranes
33
significantly greater than that measured (0.92 A) in the non-Cpheny1analogue 10endo-Ph3PHg-7,8-nido-C2B9H1 I ,I121 suggesting some degree of steric repulsion between the Ph cage substituents and the PPh3 group. Consistent with this, the average 0 value for the phenyl rings in 5 is 29.1", which is much greater than in [7,8-Ph2-7,8-nido-C2BgH101- . Evidence for rather more steric congestion exists in the metallacarborane 1,2-Ph2-3-cod-3,1,2-c/oso-PdC~B~H9, 6, (cod = cyclo-octa-l,5-diene). In this compound, formed by reaction between 3 and PdC12(cod), the orbital match between metal fragment (now formally {MLz}) and cage is better, whereby the resulting smaller A ensures a more crowded molecule. The manifestation of this crowding is molecular distortioni131; principally, the Pd atom is slipped by 0.52 A away from the cage C atoms (c.f: 0.24A in the analogous compound without Ph subs t i t ~ e n t s " ~the ~ ) , C( 1)-B(4) and C(2)--B(7)distances are abnormally long, and the Ph groups are twisted to an average H value of 48.8", towards the upper limit calculated for an adjacent diphenyl heteroborane before massive destabilization. Is'It is of considerable interest to note that reaction between 3 and PtClz(cod) under the same conditions yields only an isomerized platinacarborane, uide infra. Although a linear {ML} fragment such as {HgPPh3}2+or {AuPPh3}+ suffers a large slipping distortion when bonded to a carborane ligand, a tetrahedral analogue such as {CuPPh3}+ does not, having essentially three frontier orbitals with which to bond to the cage. Thus, in [3-PPh3-3,1, ~ - c / ~ ~ ~ - C U C ~ the B ~Cu(3) H ~ ~atom ] ~ is ,~"~ more-or-less central over the C2B3 pentagon (A = 0.27 A), and this situation barely alters (A = 0.25 A) in the dicopper species 3,4,8-exo-Ph~PCu-3-PPh~-3,1,2-c/osoC U C ~ B ~1,H7,I in which a second {CuPPh3}+ fragment is exopolyhedrally bonded to the Cu(3)B(4)B(8) and supports a Cu-Cu bond. Compound 8 is the di-CPh analogue of 7. To avoid undue crowding between the cage Ph groups and the endo-{Cu(3)PPh3} fragment the latter is pushed back to A = 0.62A. Presumably this distortion precludes 3,4,8 face capping by a second {CuPPh3}+ fragment, which instead is located over the B(2)B(8)B(9)face, there being no direct CU-CU interaction. Thus, steric crowding is responsible for exo-skeletal isomerism in this (and related) di-cupracarboranes. ' l 71
6
7
8
34
1 Molecular Clusters
R' R~small
R~P'
closo catalytically inactive
Y
exo-nido catalytically active
Scheme 2. Closo and exo-nido rhodacarboranes.
1.3.3 Major consequences More serious steric overcrowding in metallacarboranes can have a dramatic effect on molecular structure. Broadly three such major consequences have been recognized; polyhedral deformation including connectivity breaking, low-temperature isomerization (both of the 1,2 + 1,7 and 1,2 + 1,2 variety) and vertex extrusion yielding exo-nido species. Most of our work in the area has focused on the first two aspects and these are described below. However, vertex extrusion is important in that it can produce metallacarborane structures which are important in understanding their homogeneous catalytic activity. [' *] A typical rhodacarborane system is shown in Scheme 2. If the substituents R ' and R 2 on cage-C are small then the molecule exists primarily as the closo 12-vertex Rh"' species shown on the left. However, an equilibrium between this and the exo-nido Rh' tautomer shown on the right must exist, with this latter form responsible for catalytic behaviour. Increasing the bulk of R' and R 2 progressively increases the proportion of the exo-nido form as steric crowding is avoided. When R' = Me and R 2 = Ph the compound exists solely in the exo-nido form and can be crystallized as such.
1.3.3.1 Polyhedral deformation If the carborane ligand 3 is capped by an {M(y-L)} fragment (y-L is an y-bonded polyene) a major polyhedral deformation occurs. The first system we studied, 1,2Ph2-3-Cp*-3,1,2-RhC2BgHg 9 [Cp* = (y-C5Me5)]is typical."'] The steric demands of the capping metal fragment force both cage bound phenyl rings to adopt high values (ca. 82.5") causing them to move apart to avoid unacceptable P h . . . P h
1.3 Steric Effects in Metallacarboranes
35
9
crowding so prizing open the C(1)-C(2) connectivity. The C(1)-C(2) distance is stretched to ca. 2.5A (cJ ca. 1.7A in a non-deformed, e.g. mono-Ph, analogue) and, at the same time, the M(3) . . . B(6) distance is contracted to ca. 3 A (cJ: ca. 3.5A in a non-deformed analogue), producing an essentially square M(3)C(1)B(6)C(2)face. This polyhedral deformation appears to be quite general for a variety of 1,2-Ph2-3-(q-L)-3,1,2-MC2B9H9 metallacarboranes studied, Table 2. Moreover, analysis of the MC2B9 skeletons of deformed and non-deformed anal o g u e ~ by ‘ ~ the ~ ~ “Root Mean Square (RMS) Misfit” methodf241shows that the deformation is essentially localized to the M(3)C(1)C(2)B(6)face. We have termed MC2B9 metallacarboranes with square M(3)C(1)C(2)B(6)faces as “pseudocloso”. Although they are not represented on the Wade-Williams matrix (Fig. I), structurully they appear to lie intermediate between closo and hypercloso 12-vertex shapes (Fig. 6); since these have, respectively, ( n 1) and n seps, the electronic description of pseudocloso metallacarboranes is clearly of interest, and MO studies designed to investigate this are currently in hand.[251 Spectroscopically, pseudocloso metallacarboranes are characterized by (6”B) values shifted ca. 15 ppm to high frequency relative to analogous, non-deformed (6”B) is analogues, e.g. for 1,2-Ph2-3-Cp*-3,1,2-pseud0closo-RhC~B~H~”~~
+
Table 2. Structural Parameters (A) and (d” B) (ppm) for Pseudocloso Metallacarboranes 1,2-Ph23-(?-L)-3,1,2-MC2BgHg.
w-I
C(1).. C(2)
M(3).. .B(6)
2.92 2.95 2.99 2.96 3.01
(dl1B)
Ref.
36
I Moleculuv Clusters
Figure 6. Closo._useudo. closo and hypereloso 12-vertex polyhedra. Y
closo (n+l) seps
pseudocloso
hypercloso (n) SePS
+6.0 ppm whereas for 3-Cp*-3,1,2-~loso-RhC2B9H11~'~~ it is -8.6 ppm. Moreover, following assignment of the B NMR spectra of pseudocloso metallacarboranes, first by individual gauge for localized orbital (IGLO) calculations'201and subsequently by B-" B correlated spectroscopy (COSY) experiments, [''] we have been able to show that all boron resonances are shifted to high frequency, albeit by differing amounts, in moving from a closo to an analogous pseudocloso species (Fig. 7). Recent work has extended the family of pseudocloso metallacarboranes: Reaction of 3 with a source of the {Rh[9]aneS3}2+ fragment yields 1,2-Ph2-3,3,3-[9]ane S3-3,1,2-pseudocloso-RuC2B9H9, 10, the first pseudocloso metallacarborane with a o-bonded { ML3} fragment; [''I metallation of the tetra-anionic carborane 11 with { Ru(q-cymene)}2+ affords the unique closo-pseudocloso species 12 in which one 12vertex metallacarborane has a normal closo geometry and the other a pseudocloso geometry (although in solution at room temperature both clusters become equivalent on the NMR time-scale and (611B) is 1.1 ppm, intermediate between that ex-
4,7
.1
40
30
20
10
0
5.
-10 -20
-30
-40ppm
1,2-Ph2-3-Cp*-3,1,2-pseudocloso-RhC2B9H9
Figure 7. Stick representations of the 'lB{ 'H} NMR spectra of 3-Cp*-3,1,2-closo-RhC~B9H11 and B9H9. 1,2-Ph2-3-Cp*-3,1,2-pseudocloso-RhC~
1.3 Steric. Ejfects in Mrtullucarbovunes
37
11
10
12
13
eSMe
Ph
CIAu,
c-c
14
15
16
pected for closo and pseudocloso forms):['*' metallation of the zwitterionic nido carborane 13 with {Ru(r-cymene)}2+ (cymene = C6H4MeiPr-1,4) or {RhCp*)*+ gives rise to pseudocloso 14 or its RhCp* equivalent, the first examples of carbaborylphosphine ligands r5-bonded to transition metals:[291finally, reaction of the triply substituted monoanionic carborane ligand 15 with a source of the {RuCp*}+ fragment yields the pseudocloso species 16.[30i
38
I Molecular Clusters
18
17
Although pseudodoso metallacarboranes are afforded by {M(v-L)} capping of nido diphenylcarborane, use of a carborane with slightly less sterically demanding substituents yields partially deformed “semipseudodoso” species such as 17 ( Ph and C-CPh cage s u b s t i t u e n t ~ [ ~ and ~ ~ )18 (SPh substituents; steric crowding between S lone pairs of ele~trons[~’’). In 17 and 18 the C ( l ) . . . C(2) connectivity is 2.18 and 2.10A respectively, with Ru(3) . . .B(6) 3.17 and 3.20A respectively, and the (611B) values, 2.4 and -2.3 ppm respectively, are indicative of a limited shift to high frequency relative to closo analogues. Figure 8 reveals essentially an inverse linear relationship between C( 1) . . . C(2) and M(3) . . . B(6) for closo, semi-
4.0
I
closo
1.5
2.0
2.5
3.0
C(1).,,C(2)
Figure 8. Plot of M ( 3 ) . - . B ( 6 )and cp, ’ ‘ ’ C(2) distances (A) in metallacarboranes.
1.3 Steric Efects in Metallacarboranes
1
semipseudocloso
39
~
pseudocloso
hypercloso
closo
nido
Figure 9. Structural continuum between closo and hypercloso 12 vertex shapes.
pseudocloso, pseudocloso, and hypercloso metallacarboranes, implying a continuum of structure type on the Wade-Williams matrix (Fig. 9) with full steric control of intermediate structures. The change from closo to hypercloso involves a single dsd transformation (of the 3,1,6,2 face). A complementary view of the deformed metallacarborane structures we have produced via steric crowding is that they constitute frozen “snapshots” of this dsd transformation.
1.3.3.2 Low-temperature isomerization The isomerization of closo carboranes at elevated temperatures has been known for B ~ ~ H ~ cleanly ~ to the 1,7 isomer more than 30 years. Thus, ~ , ~ - c I o s o - C ~rearranges at ca. 450 0C[321whilst at ca. 700 “C the 1,7 form converts to the 1,12 isomer, although accompanied by substantial decompo~ition‘~ 31 (Scheme 3). Metallacarboranes undergo analogous isomerizations at similar temperatures (e.g. Scheme 4),1341although it has previously been recognized that arranging for the starting metallacarborane to be sterically crowded can reduce the isomerization temperature (e.g. Scheme 5).[351Work in our group has shown that low-temperature isomerizations are common in metallacarboranes with C-phenyl substituents; starting
40
1 Molecular Clusters
@ -@ 4-@ 7OO0C
45OoC
Scheme 3. Isomer-
from 3,1,2-MCzBg precursors two quite different isomerizations have been noted, the 1,2 i 1,7C atom isomerization of Schemes 4 and 5 , and the so-called 1,2 i 1,2C atom isomerization (videin&). Typical of the former is the isomerization of I-Ph-3,3-( PMe2Ph)2-3,1,2-cfosoPtC2BgH10, 19, to an approximately equimolar mixture of 1-Ph-2,2-(PMe2Ph)zin warm 1,2,8-closo-PtC2BgH1o and 8-Ph-2,2-(PMe2Ph)2-1,2,S-closo-PtC2BgHlo (55 "C) CDC13 (Scheme 6), the identities of both products being established by diffraction studies.1361 Even more dramatic is the result of the reaction between {Pt(PMe2Ph),}2+ and the diphenylcarborane ligand 3. On warming a frozen (-196 "C) mixture of Pt(PMe2Ph)zCl2 and 3 to room temperature the only isolatable product is the C-separated species 20.[361Thus, the (transient) product 1,221, must undergo a net 1,2 + 1,7 Ccage Ph2-3,3-(PMe2Ph)2-3,1,2-closo-PtC2BgHg, atom isomerization at < 18 "C.
-
Ph'
20
19
21
PR, = PMezPh
I
I
500°C,
@ C
Scheme 4. Isomerization of 3-(q-CsH5)-3.1,2closo-CoC2BgHI1,
I.3 Steric Eflects in Metallucurboranes
41
65'C
Scheme 5. Low-temperature isomerization of I-'Bu-3,3,3-( PPh?)l(H)3.1 ,2-closo-IrC2BgHlo.
$Bu fc,
4-
It is tempting to suggest that such low-temperature isomerization is driven by the relief of steric crowding in the pre-isomerized molecule, but whilst overcrowding is likely to be an important contributory factor there are several clues that it is not solely responsible: metallacarborane 19 does not appear to be particularly overcrowded, yet it isomerizes easily; reaction of 3 with {Pt(cod)}2+affords a direct analogue of isomerized 20, yet use of {Pd(cod)}'+ yields only the distorted, non-isomerized, 6, suggesting that the nature of the metal atom is important in determining the course of the isomerization;" 31 moreover, this idea is further supported by the observation that the reaction of 3 with {Ni(PMe'Ph),}*+ reNevertheless, whateverthe true sults in a "1,2 + 1,2" Ccdgeatom i~omerization.'~'~ nature of the reason or reasons for low-temperature isomerizations of metalla( phenyl)carboranes, the simple fact that such changes can reliably be produced under mild conditions offers the important possibility of an experimental study of the isomerization mechanism(s). The precise mechanism(s) by which carboranes and heterocarboranes isomerize has been the subject of considerable speculation over many years. Notable suggestions have included the hextuple-concerted dsd mechanism uia a cubeoctahedral intermediate (Scheme 7),I3'l the triangle rotation1391and extended triangle rotationl4'1 mechanisms, the pentagonal twist p r o c e ~ s ,involvement ~ ~ ~ , ~ ~ of ~ a nido interrr~ediate[~'Iand the involvement of an anticubeoctahedral intermediate.'421 Equally, there have been a number of theoretical investigations of the phenomeIn a significant recent study144JWales concluded that the 1,2 + 1,7 and
Ph' 17, PR,
= PMe,Ph
Scheme 6. Low-temperature isomerization of l-Ph-3,3-(PMe2Ph)2-3,1.2-clo.ro-PtC~B~Hlo, 17
42
I Molecular Clusters
Scheme 7. Proposed hextuple-concerted dsd rearrangement of ~ , ~ - C I O S O - C ~ B ~ O H ~ ~ to 1,7-closo-C2B I o H ~ ~ .
1,7 + 1,12 rearrangements of C2B10H12 occur via high energy intermediates linked to precursor, product, and each other by a series of sequential dsd steps. For the 1,2 + 1,7 process, one intermediate, 22, of C, symmetry, was however found to be relatively stable. Two fundamental problems have restricted the impact of experimental study of the isomerization mechanism(s). Firstly, until very recently, it has not proved possible to isolate an intermediate in a heteroborane isomerization, which would have provided an important signpost for the rearrangement mechanism. Secondly, the fact that such isomerizations tend to require high temperatures has meant that vertex labeling studies ( i e . substitution of B H by BX) have been of limited use because of the possibility of B-X bond cleavage and positional scrambling. Recent studies at Heriot-Watt University have had some success in overcoming both these essential problems and thus have been shedding direct light on the true 1,2 + 1,7 isomerization mechanism, but in addition we have also recently been able to show that, at least in one particular system, the hextuple-concerted dsd mechanism is not operating. As previously noted, transient 21 isomerizes to 20 at < 18 "C, a net 1,2 + 1,7 Ccageisomerization. The operation of the hextuple-concerted dsd process on 21 affords 23, but since this species is not observed it, too, must be transient, undergoing further rearrangement to yield 20. We have synthesized 23 by a different route; reaction of the C-separated carborane ligand 24 with the {Pt(PMe2Ph)2}2f fragment.[451Compound 23 does not isomerize in refluxing toluene, therefore it is not an intermediate in the low-temperature rearrangement of transient 21 to observed 20, and the hextuple-concerted dsd mechanism is inappropriate.
22
23, PR3 = PMeZPh
24
1.3 Steric Effects in Metullucurborunes
43
26
27
Scheme 8. (Proposed) Transformation of the transient species 27 into non-icosahedral 25, followed by (established) transformation of 25 into 26.
Whilst evidence against one mechanism is of some value, evidence for another is clearly better. Recently we have been fortunate in isolating, for the first time, an intermediate in the isomerization of one icosahedral metallacarborane into another. Thus, reaction of 3 with a source of the {Mo(v,K~H~)(CO),}' fragment affords the anion 25, which is closed but not icosahedral.[461Gentle warming of 25 ( T H F reflux) rapidly yields C-separated 26. We propose that 27 is the transient first product of the metallation of 3 but is overcrowded, rearranging spontaneously to 25. The relationship between 27 and 26 is the same as that between 1,2- and I , ~ - C ~ B I O H ~ ~ , thus identifying 25 as an isomerization intermediate (Scheme 8). Interestingly 25 has the same structure as Wales' predicted C, symmetric intermediate 22. [441 Very recently we have begun to develop analogous chemistry with vertex labeled analogues of 3. Following treatment of carborane 15 with the Mo fragment cation the neutral compound 28, a second structural analogue of 22, has been isolated. 14'' Subsequent gentle thermolysis affords a mixture of the C-separated icosahedra 29 and 30 (Scheme 9). Whilst the relationship between 28 and 29 is clear (and consistent with theory'441)that between 28 and 30 is not, and we believe that 28 is formed along with a second intermediate, 31, which gives rise to 30. The predicted structure of 31 is shown.
28
29
44
1 Molecular Clusters
Ph
Scheme 9. Metal dependency of the metallation of cdrborane ligand 3 ( R = variety of phosphine ligands).
1.3.4 Conclusions and outlook Pre-organizing for metallacarboranes to be deliberately overcrowded can have significant structural effects. Work in our group has focused on polyhedral deformution and on low-temperature isomerization. Polyhedral deformation can result in unusual, open (pseudocloso) structures, which, although not represented on the Wade-Williams matrix, appear structurally to lie between closo and hypercloso forms. The further discovery of partially deformed (semipseudocloso)species supports the idea of a continuum of structure type, which can, at least in part, be controlled by judicious use of substituents with differing steric demands. Low-temperature isomerization of 3,1,2-MC2Bg icosahedra into 1,2,8-MC~Bg
1.3 Steric' Efects in Metallacarhoranes
45
icosahedra (net 1,2 41,7 Ccdgeatom isomerization) can routinely be achieved. We have shown, at least in one system, that the hextuple-concerted dsd mechanism is inappropriate and have demonstrated in another the first isolation of an intermediate species. Current studies with vertex labeled carborane precursors should afford additional mechanistic information and may lead to a complete experimental mapping of all vertices in the net 1,2 41,7 isomerization of metallacarboranes. In the future we would expect to be able to extend this study to embrace the net 1,7 41,12 isomerization. Whilst it is very encouraging that our recent work with metallacarborane isomerization has provided experimental support for the conclusions of previous theoretical studies on carborane isomerization, the inclusion of metal vertices can clearly have a moderating effect. This is dramatically demonstrated by examples of metal-dependency. Thus, reaction of 3 with { Pt( PR3)2]2i instantaneously produces a 1.2 i 1,7 Ccageisomerization, whereas reaction with {Ni(PR3)2}*+ results in one Ccageatom moving from upper to lower pentagons of the icosahedron, away from the metal atom, whilst remaining directly connected to the other Ccageatom1371 (Scheme 9). Such a process is known as a 1,2 1.2 Ccageisomerization. Considerable further work is necessary to fully understand the nature of the metal dependency of the isomerization processes and to track the movement of every vertex through labeling. In these future studies our ability to drive these isomerizations routinely at low temperatures through steric crowding is likely to continue to be important. ---f
Acknowledgements The chemistry described above which has originated from our laboratories, originally at the University of Edinburgh and latterly at Heriot-Watt University, would not have been possible without the dedication and skill of a number of postdoctoral and postgraduate co-workers, in rough chronological order Zoe Lewin, Natalia Douek, Gwenda Kyd, Tom McGrath, Jill Cowie, David Donohoe, Kerry Adams, Rhodri Thomas, Andrew Weller, Anna McWhannell, Georgina Rosair, Shirley Dunn, Rhona Garrioch, Kristen Low, and Catherine McAvoy. Recent collaboration with Francesc Teixidor, and Clara Viiias and their group at the ICMAB-CSIC, Bellaterra, Spain, has been stimulating and fruitful, and financial support from the EPSRC, NATO, the Royal Society, the University of Edinburgh, and Heriot-Watt University is also gratefully acknowledged. Last, but by no means least, continued support for our work by the Callery Chemical Company, PA, has been invaluable and is considerably appreciated.
46
1 Molecular Clusters
References [I] Wade K. (1971) J. Chem. Soc., Chem. Commun. 792; (1976) Adv. Inorg. Chem. Rudiochem. 18 1 [2] Rudolph R.W. (1976) Acc. Chem. Res. 9 446 [3] Williams R.E. (1971) Inorg. Chem. 10, 21 I; (1976) Adv. Inory. Chem. Radiochem. 18 67 [4] Jiang W., Knobler C.B., Hawthorne M.F. (1996) Angew. Chemie Int. Ed. Engl. 35 2536 [5] Cowie J., Reid B.D., Watmough J.M.S., Welch A.J. (1994) J. Organometal. Chem. 481 283 [6] Brain P.T., Cowie J., Donohoe D.J., Hnyk D., Rankin D.W.H., Reed D., Reid B.D., Robertson H.E., Welch A.J., Hofmann M., von Rag& Schleyer P. (1996) Inorg. Chem. 35 1701 [7] Thomas Rh.LI., Rosair G.M., Welch A.J. (1996) Acta Crystallogr. C52 1024 [8] Lewis Z.G., Welch A.J. (1993) Acta Crystullogr. C49 705 [9] Cowie J., Donohoe D.J., Douek N.L., Welch A.J. (1993) Actu Crystallogr. C49 710 [lo] Mingos D.M.P., Forsyth M.I., Welch A.J. (1978) J. Chem. Sac. Dalton Trans. 1363 [I I] Lewis Z.G., Welch A.J. (1993) Acta Crystallogr. C49 715 [I21 Colquhoun H.M., Greenhough T.J., Wallbridge M.G. (1979) J. Chem. Soc. Dalton Trans. 619 [I31 Kyd G.O., Yellowlees L.J., Welch A.J. (1994) J. Chem. Soc. Dalton Trans. 3129 [I41 Smith D.E., Welch A.J. (1986) Acta Crystallogr. C42 1717 [I51 Do Y., Kang H.C., Knobler C.B., Hawthorne M.F. (1987) Inorg. Chem. 26 2348 [I61 Kang H.C., Do Y., Knobler C.B., Hawthorne M.F. (1988) Inorg. Chem. 27 1716 [I71 Adams K.J., Cowie J., Henderson S.G.D., McCormick G.J., Welch A.J. (1994) J. Organometal. Chem. 481 C9 [I81 Hawthorne M.F. et a1 (1984), J. Am. Chem. Soc. 106 2979; 2990; 3004 [I91 Lewis Z.G., Welch A.J. (1992) J. Organometal. Chem. 430 C45 [201 Brain P.T., Buhl M., Cowie J., Lewis Z.G., Welch A.J. (1996) J. Chem. Soc. Dalton Trans. 231 [21] Gradler U., Weller A.S., Welch A.J., Reed D. (1996) J. Chem. Soc. Dalton Trans. 335 [22] Donohoe D.J. (1996) PhD Thesis, University of Edinburgh 1231 Thomas Rh.Ll., Welch A.J. (1997) J. Chem. Soc. Dalton Trans. 631 [241 Macgregor S.A., Wynd A.J., Moulden N., Gould R.O., Taylor P., Yellowlees L.J., Welch A.J. (1991) J. Chem. Soc. Dalton Trans. 3317 [25] Macgregor S.A., Welch A.J. (l998),work in progress [26] Bown M., Plesek J., Base K., Stibr B., Fontaine X.L.R., Greenwood N.N.; Kennedy J.D. (1989) Mugn. Reson. Chem. 27 947 [27] Welch A.J., Weller A.S. (1996) Inorg. Chem. 35 4548 [28] Thomas Rh.Ll., Rosair G.M., Welch A.J. (1996) J. Chem. Soc. Chem. Commun. 1327 [29] McWhannell M.A., Rosair G.M., Welch A.J., Teixidor F., Vifias C. (1999) J. Organometul. Chem., 573 165 [30] Rosair G.M., Welch A.J., Weller A S . (1998) Organometullics, 17 3227 [31] Teixidor F., Vifias C., Flores M.A., Rosair G.M., Welch A.J., Weller A.S. (1998) Inorg. Chem., 37 5394 [32] Grafstein D., Dvorak J. (1963) Inorg. Chem. 2 1128 [33] Papetti S., Heying T.L. (1964) J. Am. Chem. Soc. 86 2295 [34] Kaloustian M.K., Wiersema R.J., Hawthorne M.F. (1972) J. Am. Chem. Soc. 94 6679 1351 Baker R.T., Delaney M.S., King R.E., Knobler C.B., Long J.A., Marder T.B., Paxon T.E., Teller R.G., Hawthorne M.F. (1984) J. Am. Chem. Soc 106 2965 [36] Baghurst D.R., Copley R.C.B., Fleischer H., Mingos D.M.P., Kyd G.O., Yellowlees L.J., Welch A.J., Spalding T.R., O’Connell D. (1993) J. Organometal. Chem. 447 C14 [37] Garrioch R.M., Kuballa P.J., Low K.S., Rosair G.M., Welch A.J. (1999) J. Organomet. Chem., 575 57 [38] Lipscomb W.N. (1966) Science 153 373 1391 Zakharkin L.I., Kalinin V.N. (1966) Dokl. Akad. Nauk SSSR 169 590
1.3 Steric Effects in Metallucarborunes
47
[40] Wu S., Jones M. (1989) J. Am. Chem. Soc. 111 5373 [41] Edvenson G.M., Gaines D.F. (1990) Inorg. Chem. 29 1210 [42] Roberts Y.V., Johnson B.F.G. (1994) J. Chem. Soc. Dalton Trans. 759 [43] Gimarc B.M., Ott J.J. (1986) Inorg. Chem. 25 83, 2708; Wales D.J. Stone A.J. (1987) h o r g . Chem. 26 3845; Gimarc B.M., Warren D.S., Ott J.J., Brown C. (1991) Inorg. Chem. 30 1598 [44] Wales D.J. (1993) J. Am. Chem. Soc. 115 1557 [45] Welch A.J., Weller A.S. (1997) J. Chem. Soc. Dalton Trans. 1205 1461 Dunn S., Rosair G.M., Thomas Rh.Ll., Weller A S . , Welch A.J. (1997) Angew. Chem. Int. Ed. Engl. 36 645 1471 Dunn S., Rosair G.M., Weller A.S., Welch A.J. (1998) J. Chem. Soc. Chem. Commun. 1065
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.4 Heteronuclear Clusters Having Transition Metals and Metals of Group 14 David J. Curdin
1.4.1 Introduction The chemistry of heterometallic clusters has given us many beautiful molecules, and continues to provide us with surprises. The majority of work has concentrated on homonuclear clusters, though heteronuclear derivatives are receiving increasing attention. In the heteronuclear field, most work has concentrated on compounds having two or more transition metals, though work with main group metals has been a continuing if small area. It was authoritatively reviewed by Whitmire"] but there has not been a comprehensive survey since. Transition metal clusters having a group 14 element have been studied by a number of workers; for recent references see Mackay,(21Lewis,13Jand Braunstein.141There are, in addition, many pure group 14 element clusters, (5, and references therein.) but these will only be mentioned in passing in this article. Heterometallic clusters have been the subject of synthetic studies, attempts to account for the structure and bonding in such systems; and with a view to catalytic activity. Despite the many claims of clusters acting as catalysts, there is still much dispute concerning the precise nature of the active sites in such systems. Clusters incorporating group 14 elements have been part of this activity, not least because the main-group elements often enhance cluster stability considerably. In particular carbido clusters have received much attention, stemming from the view that carbido-metal intermediates are important in the initiation of Fischer-Tropsch catalyses.I6]In this short article we shall concentrate on the use of low-valent derivatives of germanium, tin, and lead in the build-up of heterometallic clusters from small homonuclear transition metal ones, and focus mainly on the work carried out in our laboratories in recent years.
1.4 Heteronuclear Clusters Having Transition Metals and Metals of Group 14
49
1.4.2 The coordination of diorganogroup-14 element compounds The coordination chemistry of the now large variety of species of the type R2M, where R is an organic group, (or an amido ligand) and M a group 14 element has been reviewedF7'and been the subject of several subsequent publications.ls-lO1The coordination arises through the nucleophilic lone pair of electrons and vacant orbital on M.'11-141illustrated below: '
1
in which the shading indicates double occupancy of the orbital. In valence bond language, M can be described as sp2 hybridised, and the vacant orbital as having mainly p-character. In keeping with this, gas-phase electron diffraction data for the species where R = (Me3Si)ZCH and M = Ge or Sn, for example show the molecules to be m o n o r n e r i ~ . "It~ ~is the considerable bulk of the ligands in these species which prevents oligomerisation. The first ionization potential for these species is generally low, for example 7.42 eV in {(Me3Si)2CH},Sn,F'51and 7.32 eV in the lead Accordingly the stannylene behaves as- a good nucleophile, and perhaps surprisingly for a sixelectron species, has only low electrophilic character. Coupled with this nucleophilicity, is the strong tendency of tin(I1) to become tin(IV), so these compounds in addition to Lewis base behaviour also insert [oxidatively as far as the tin is concerned] into other bonds; behaviour which is not the main concern here. Since the stannylene is diamagnetic, and behaves as a singlet carbene analogue,F161it can donate to a metal atom as a terminal ligand (2), [M, M', C, and C (of R) coplanar]
L 2
in which the metal M' is assumed to have octahedral coordination by M and five
50
1 Moleculur Clusters
ligands L. A further possibility is that the R2M entity becomes bonded as a bridging, rather than terminal ligand, as in 3,
L" 3
in which it is assumed that the M' metals form a triangle, and are coordinated by n ligands L, which would typically be carbonyls. Indeed, concerning coordination to clusters, this is by far the preferred mode of attachment, only a single example each of the terminal mode being known for germylenes or stannylenes. In the bridging mode, when coordinated to a planar metallic array, the R groups are generally orientated well out of the metal plane, though the ligating carbons are frequently not perpendicular to this plane, and characteristic angles are observed (see below). Nevertheless, coordination around M is not very far removed from a regular tetrahedral. Many of these compounds, (except derivatives of amido group 14 species) in contrast to the group 14 element precursors, are air and moisture stable, and resist attack by quite strong aqueous acids. The presence of silyl or other lipophilic substituents confers substantial solubility in organic solvents on most of these species, and makes a variety of NMR and other spectroscopic measurements in solution possible.
1.4.3 Synthetic approaches The precise method of synthesis of these compounds depends on the group 14 reagent, and the transition metal cluster. Here we will concentrate on clusters formed by addition to tri- or tetra-nuclear clusters of iron, ruthenium, osmium, or iridium. For these metals, activation may be required for osmium or iridium species, whereas ruthenium carbonyl reacts readily. Triiron dodecacarbonyl reacts very readily, but is generally accompanied by cluster breakdown; even retaining a dimetallic fragment requires prior activation, and low temperatures. These patterns are related to the strength of the MI-CO bond, and of the M'-M' bond, both of which increase on descending the group. When activation is required, the well-known amine oxide method generally works well. Trimethylamine N-oxide reacts with many coordinated carbonyl groups
1.4 Hrteronucltw Clusterx Having Transition Metals and Metals of Group 14
51
affording amine and carbon dioxide."" If the reaction is carried out in acetonitrile, the vacant coordination site is occupied by the nitrile, a ligand which is much more readily displaced by nucleophiles than is carbon monoxide. L,M'-CO
+ MeiNO + MeCN
L,M'-NCMe
4
+ C02 + Me3N
Not all CO ligands can be so removed, but an estimate can be made in individual cases from t(C-0): when this falls below a critical value, the M-CO interaction is too strong, and it is unlikely that the reaction will take place. A factor with low-valent group 14 reagents is their extremely ready oxidation. After formation of the nitrile adduct, excess Me3NO must be removed; and this can usually be achieved by filtration through a small pad [ca. 2 cm] of dry, oxygen-free silica gel. The literature contains some interesting reports concerning reactions with the much less sensitive triorganophosphines, where this precaution was not taken, and it is clear that oxidation of the phosphine has been the main problem. Acetonitrile is by no means the only nitrile that can be used, but it is convenient and easily removed. It is interesting in this connection, that while no group 14 adduct of the Fe3 triangle has been reported, the complex [Fe3(CO)1 1 (NC-C6H4-Me-4)] was obtained (as were several group 15 adducts) in good yield, and structurally characterized"81 from addition of the nitrile following treatment of [Fe3(CO)121 with Me3NO at low temperature. The group 14 reagents vary considerably in their reactivity. Compounds [{Me3Si)2CH},M},] - ( M = Ge or Sn) are the most reactive: displacing carbon monoxide from triruthenium dodecacarbonyl at only slightly above ambient temperatures. The trimeric aryl reagent [{C6H2Pr\-2,4,6)2Sn},]' I 9 ' requires moderate heating, usually in toluene, and the amido species frequently have lower reactivity. Apart from hydrido species (see below) the products can normally be isolated either following chromatography on silica, or by crystallization from a hydrocarbon solvent. The hydrido species (especially where there is bridging to the group 14 metal) may be air and water sensitive, and may not survive chromatography, at least using protic columns.
1.4.4 Compounds in which the group 14 ligand is bonded terminally to a cluster Addition of [{S ~ I { C H ( M ~ ~ S to ~ )a~toluene } ~ } ~solution ] of [Osj(C0)11 (NCMe)] at room temperature or below, gave the adduct [Os3(CO)1 I (SnRz)] in excellent yield, after recrystallization from toluene at -20°C. The product, which is stable in air for short periods, undergoes slight decomposition over a few days, and forms mono-
52
I Molecular Clusters
clinic, orange crystals. The crystal structure shows that the tin group occupies an equatorial position on the osmium triangle, as expected for a bulky substituent. The structure is shown in Fig. 1 .C2O] A further reason for the equatorial position of the tin, may be that in this site, there is no competition for overlap with 71-electrons of the metal from a transcarbonyl group (backbonding electrons). Although the n-acceptor character of SnR2 is less than that of a carbonyl group,L211 there is evidence that it is still appreciable. In the present case for example, the measured Sn-0s distance 2.573(3)A, is significantly less than the sum of the single-bond radii [2.648~%]. The osmium triangle is very close to equilateral, though the measured Os-0s vector trans to the tin [2.891(1)A]is higher than the distance in [Os3(CO)12] or the remote Os-0s vector here [2.877(3)A]. Examples of terminal coordination of osmium, and particularly osmium clusters are rare, but one intriguing example is provided by the group 14 adduct [(OC)sOs+ Os(C0)3(GeC13)Cl], though in this case the donating species is the formally eighteen electron Os(C0)s fragment, and not the germanium group. The Os-0s distances (from 2.916(2) to 2.913( 1) A, for the three independent molecules of the unit cell) are slightly longer than those of [Os~(C0)12], 2.877(13)& (mean), though this is difficult to interpret because of the presence of the presumably high trans-influence group 14 ligand. Our only other similar example of terminal coordination with an osmium cluster is with the germanium analogue { (Me3Si)zCH),Ge, which forms the di-adduct shown below. ' 2 2 1 In this case the structural data are not of particularly high quality, though the heavy atom framework is not in doubt. There are two hydrogen atoms associated with the molecule, but the data do not allow us to locate these directly. However, based on the related [Os3(C0)lo(pu-H)2SnR2],(see below) these can be tentatively placed as shown in Fig. 2, from the positions adopted by the carbonyl and germanium ligands. The supporting bond-length and angle data for these placements are shown in Fig. 3. It can be seen that one of the Os-0s vectors and one of the Os-Sn vectors is
1.4 Heteronuclear Clirsters Hwiny Transition Metals and Metals OJ' Group 14
53
0
Figure 2. The structure of [Osj(ilc-GeR2)(GeRz)(CO),(/c-H)2]
0
significantly longer than the other, resulting in, we suggest, a form accommodating a bridging atom. In both cases, the in-plane carbonyl ligands are placed much closer to the non-bridged framework bonds (angles 114", 118", and 118") than to the bridged, supporting the proposed hydrogen locations. The more normal mode for the R I M fragments coordinated to clusters is bridging two metal centers. However, we may visualise the formation of the bridged structure from the reactants. the stable terminal species mentioned above do not appear to be convertible into the bridged. Neither the terminal GeR2 nor SnR2 compounds gave rise to detectable quantities of bridged isomers on heating, though other forms of decomposition occurred at high temperatures.
Figure 3. The structure of [[Os?(,pGeR2)(GeR2)(CO)~~( p H ) 2 ] . K = CH(SiMe3):; giving sclected bond lengths. and the hydrogen locations suggested.
.Ge
54
I Molecular Clusters
1.4.5 Compounds in which the group 14 ligand doubly bridges metals in a cluster without further reaction This bonding mode is by far the commonest for simple addition of MR2 species to reactive metal clusters. For example in the group 8 dodecacarbonyl series, the lighter two metals react with a variety of MR2 species readily, while the osmium carbonyl requires activation. In the ruthenium case, the first isolated product is generally the pentametallic species [ R u(CO)s( ~ p-CO)(MR2)2],though it is possible to obtain the hexametallic clusters. These reactions can be conducted in high yields in aprotic aliphatic or aromatic solvents, heptane and toluene being convenient. Typical product structures are shown in Figs. 4-7. In the ruthenium reactions it is interesting that no hydrido species could be obtained when reactions were conducted under hydrogen. The isolation, by Bonnet and coworkers,1231of the species [ R U ~ N ~ ( ~ H ) ~ ( C O ) ~ (from V - Cthe ~ Hreaction ~)] of ruthenium dodecacarbonyl with [{Ni(q-C5H5)(CO)2}2] under hydrogen, suggested the intermediacy of the unstable [ R u ~ ( C OI )o H ~in ] , these reactions, however, the presence of a dry dihydrogen atmosphere made no difference to the products
Figure 4. The structure of [ O S(~-snR2)2(CO)s(~-CO)]; ~ R = CH(SiMe3)2.
1.4 Heteronucleur Clusters Huviny Trunsition Metuls and Metuls of Group 14
55
Figure 5. The structure of [Ru?(~-SnR2)1(C0)91; R = C6HzPr;-2.4.6.
when adding MR2 species to ruthenium clusters, and no hydrido species were isolated. )]; (Fig.4) are Structurally related to [ O s ~ ( p U - S n R 2 ) ~ ( C 0 ) ~ ( pR- C=0CH(SiMe3)2, both the ruthenium analogue, and the ruthenium and osmium compounds prepared with the amide Sn{N(SiMe3)2},, which is isoelectronic with Sn(CH(SiMe3)2},.
Figure 6. The structure of [Rui(pSnR2 ) 2 ( /r-SnRi)(C0)9;; R = ChHlPr\-2.4.6. R’ = CH(SiMe3)r
56
1 Moleculuv Clustcrs
The use of this and the related germanium amide as ligands for both clusters and single transition metal atoms has been described. [ 9 1 The compound [OS~{~-S~{N(S~M~~)~}~(CO)~(~-CO)] has the same metal framework as the alkyltin analogue, and similar parameters, though the preparation requires more forcing conditions, because of the lower nucleophilicity of the his(amido)- compared with the bis(alky1)-tin species. As expected the product is moisture sensitive [ Sn-N bonds] and appears to decompose readily in solution, even on standing in an inert atmosphere. A noteworthy feature of the structure is that the Sn-0s bonds are significantly longer in the amido-compound, but the decomposition is complex. since osmium dodecacarbonyl is a significant product. Structurally, the addition of a group 14 bridge leads to the lengthening of the M-M vector, (significantly more than by a bridging carbonyl) and a coplanar array of metal atoms, with the bulky alkyl or aryl substituents oriented in a plane close to, but not precisely perpendicular to the metal plane. The Sn-Ru distances are significantly shorter than the appropriate sum of covalent The turning away from the perpendicular direction becomes much more pronounced in triangular clusters of the type [Fe2(CO)h(p-MR2)](see below). Nevertheless, it is responsible for a unique stacking feature in certain clusters. For example the cluster [ R u(C0)9 ~ ( ~ - S ~ { C H ( S ~ M(p-Sn{ ~ ~ ) ZC~H2Pr\-2,4,6}~)2], }~) generated as follows:
+
[Ru3(C0)12] 2Sn{C6H2P~\-2,4,6}~ +
[ R u(c0) ~ lo(p-Sn{C6H2Pr;-2,4,6),)2]
[RU~(CO)IO(,LL-S~(C~H~P~;-~,~.~}~)~] + sn(CH(SiMe3)~)~ +
[ R u(C0)s(p-Sn{CH(SiMe3)2}2)(p-Sn{C6H2Pr;-2,4.6}2)2] ~
crystallises in space group P61 or P65 for individual crystals. The chirality results
I . 4 Hetc~ronuclcur Cltr~ter,Huriny Trunsition Metii1.T [mu' Metals o f Group I4
57
Figure 8. The structure of [Fe,(CO)*(p-SnR?)]; R = ChHiEt2-2,6.
from the helical packing of molecules within the crystal, forced by the different ligands surrounding the metal framework. and their twist. The product overall, is of course, a racemic mixture of the two forms. Whereas. as described under synthesis, the ruthenium carbonyl reacts without activation, and osmium requires the introduction of a labile ligand, but triiron dodecacarbonyl, with its weaker M-M and M-C bonds, generally undergoes some cluster breakdown on reaction with M R ? reagents, though not (see above) with a variety of group 15 donors.'"' The tin reagents, Sn(C6H2Prl,-2,4,6j2, Sn{CH(SiMe3)2}2, and Sn{C6H2Ph3-2.4,6jz all afforded the trinuclear clusters [Fez(C0)g(p-SnR2)], whose structures are shown in Figs. 8- 10.
Figure 9. The structure of [Fe2(C0)8(,u-SnR2)]; R = CsH?Pr;-2.4.6.
58
I Moleculur Clusters
Figure 10. The structure of [Fe2(C0)s(p-SnRz)], R = CsH*Ph3-2,4,6.
These molecules have the expected tin group bridging the Fe-Fe vector, but the disposition of the R groups is unexpected in that in each case the C-Sn-C plane is tilted significantly away from the perpendicular to the metal plane. This deviation can be quantified in terms of a “twist” angle, the dihedral angle between the metal and the C-Sn-C planes.‘251While this is difficult to explain in electronic terms, it is remarkably constant (73.8-77.3’ for a range of clusters) for the aryl substituted ligands, but rather more variable for the (less bulky) Sn{CH(SiMe3)2}, derivatives. Although it has not yet proved possible to add group 14 metals to a triangular iron framework, careful, low-temperature activation of [Fes(CO)121, using Me3NO and MeCN, followed by Sn{CH(SiMe3)2},, has allowed the isolation and characterization of an FezSnz cluster (Fig. 1 1).[261 This molecule is related to [Fe2(,~-SnMe2)2(CO)s],[~’] but both the stoichiometry and geometry are different in [Fe2(,~-sn{CH(SiMe3)2>,)2(CO)~]. In the dimethyl compound the Sn atoms can approach each other closely, so that the Fe atoms are forced apart and they acquire an 18-electron configuration from the 4 carbonyls and the bridging tin groups. This close approach of the tins is precluded with the bulky alkyl substituents, and the Fe atoms are now forced close, eliminating two carbonyls, so that they acquire an 18-electron configuration through a formal Fe=Fe bond, 3 carbonyls and the bridging groups. Carbonyls of the group 9 elements have been explored mainly with iridium, since rhodium analogues proved difficult to obtain, at least employing Rh4(C0)12 as a starting material. Iridium carbonyl is notoriously insoluble, and consequently difficult to substitute. We have employed two methods; activation with Me3NO/MeCN. or conversion to the anion [Ir4(CO)11Br]-, which is very convenient as Br- is
I,4 Neteronuclear Clusters Huving Trunsition Metals and Metuls of Group 14
59
Figure 11. The molecular structure of [Fe2(1c-SnR1)2(C0)6];R = CH(SiMe3)I
readily lost on treatment with a variety of nucleophiles, and which was used for the majority of the clusters described here. Although the amine oxide method works well, it requires low temperatures and we have not been able to characterize the acetonitrile adduct. Even with the most reactive of the MR2 species, e.g. Sn{CH(SiMe3)2},, Ir4(CO)12 failed to react, even at elevated temperatures in toluene. Perhaps more surprisingly, the nitrile adduct [Ir4(CO)IINCBU']also failed to give a tin cluster, though in this case decomposition occurred at higher temperatures. Activation with Me 3 N 0 followed by MeCN gave only an unstable brown solid, which was heat and light-sensitive. Treatment of this with Sn{CH(SiMe3)2}, in toluene afforded two iridium clusters of formulae [ I ~ ~ ( C O ) C I { S ~ { C H ( S ~ and M~~)~}~}~] [Ir3(CO)lo{Sn{CH(SiMe,),}2}2]. The first has a planar hexametallic metal framework, determined by a single crystal X-ray study, in which the Ir-Ir distances are significantly elongated over the parent carbonyl and other substituted derivatives. However, the crystals show disorder, and a full refinement was not possible. The other compound was also isolated using the bromide derivative, and is described below. Activation of the parent carbonyl using the bromo derivative has enabled us to isolate a much wider range of heterometallic clusters, the composition of which now depends on the group 14 species employed. In the case of Sn{CH(SiMe3)2},,
60
I Molecular Clusters
Figure 12. The molecular structure of [h(CO)9(p-C0)2( S n h ) ] ; R = CH(SiMe3)Z.
several clusters can be obtained depending both on the conditions and the stoichiometry.r281 When a single equivalent of the tin reagent is used in diethyl ether at low temperature, yellow-orange crystals of the expected [IT4(C0)g(p-C0)2(SnR2)] are obtained. This compound shows two distinct IR bands at 1872 and 1836 cm-', characteristic of bridging carbonyls, and implying the bridging mode for the tin addend. This structure was confirmed by an X-ray structure (Fig. 12). Curiously, when the reaction employs two equivalents of tin reagent, a similar orange-yellow crystalline material was the major product. Elemental analysis was identical to the earlier product, as was the ' H NMR, and the IR spectrum in solution. However, the crystals had a quite distinct solid-state IR spectrum: exhibiting four well-resolved bridging carbonyl bands at 1871, 1864, 1841, and 1832 cm-l. A possible structure for this isomer, has an axial, terminal SnR2, (the same orientation as the bromo-substituent in the starting anion), and four bridging carbonyls disposed as shown in the Fig. 13. Addition of a substantial excess of Sn(CH(SiMe3)2}, to the bromo anion in either ether or toluene gives a mixture of products as well as some unreacted [Ir4(CO)121. These products can be separated by careful chromatography, giving five new clusters of which two, formed in moderate quantities are described here. A yellow crystalline cluster having a single v(C0) in the bridging region (1800 cm-I) was paramagnetic and the elemental analysis showed an Ir : Sn ratio of 3 : 2. The structure is shown in Fig. 14.
1.4 Ht~teronuclcwC1ustrr.i Hacitig Tramition Metals and Metals of Group 14
Figure 13. Proposed structure for the second isomer of composition [Irq(c'O)I I (SnRz)].
61
CO R = CH(SiMe3)2
Once again the pentametallic structure is planar, with significantly lengthened lr-lr distances. The carbonyl bridged distance is 2.873(2)A, and the tin-bridged separations are 2.957(2) and 2.967(2)A. Not only are the tin bridging distances significantly longer than the carbonyl-bridged, as is normally found, but these distances are significantly greater than other bridged Ir-Ir lengths in planar clusters, (see for example below). The average value for those in Irq(CO)12 is 2.693& but comparisons are complicated with the tetrahedral structure in comparison to the planar array of the tin-iridium compound. The increases can be traced to the fact does not have the expected 48that the present cluster, [Ir~(C0),(p-CO)(pu-SnR2)2] electron-precise count, but is a 5 1 -electron cluster, with corresponding occupancy of orbitals having M-M antibonding character.
LJ
Figure 14. The molecular structure of [Iri(C0)9(~r-CO)(~-SnRz)z]; R = CH(SiMe?)>.
62
I Molecular Clusters
Q Figure 15. A molecule of [ h ( C O ) h(~-SnR2)2 ( ~ ~ - 0Sn-RSn03)I; 3 {R = CH(SiMe3)2}, with the methyl groups omitted for clarity.
The other main products of this reaction is the Ir3Sn3 compound referred to above, and a blue-green cluster with a complex IR carbonyl spectrum, (but no bridging carbonyls) both suggesting an unusual type of product. The structure of this material is shown in the Fig. 15. In this compound the Ir4 nucleus has been retained, but an Ir-Ir bond has been opened up to give the butterfly, two sides of which are tin-bridged. The third tin entity has lost an alkyl group, and bonds to the Ir4Snz raft by one direct Sn-Ir bond, and three oxygen bridges. The oxygen atoms must have been acquired during silica chromatography as the reaction conditions require the strictest exclusion of air and moisture because of the sensitivity of the SnR2 species, however the mechanistic details are not k n o ~ n . ' ' ~ ] When the same tin/iridium stiochiometry is employed, but a small amount of T H F is added to the solvent, the major product is now an air-sensitive purple cluster, again with an Ir$3n2 framework. This material has no bridging carbonyl stretches, is diamagnetic, and shows a ' H NMR peak in solution at 6 - 9.61; a region typical of late transition metal hydrides. The structure of this species, [Ir3(pU-SnR2)2(CO)*H], is shown in Fig. 16.[301 Hydrogen abstraction, which must occur from the added THF, allows the cluster to acquire the expected, (diamagnetic) electron precise 48-electron count. In agreement with this, the Ir-Ir bonds are now significantly shorter than in the electronrich cluster, (2.842(2), 2.834(2), and 2.838(2)A). It is noteworthy, however, that the iridium triangle is now considerably closer to equilateral than in the electron rich cluster, despite the presence of the high trans-influence hydride ligand, and the absence of the group bridging the third Ir-Ir vector.
1.4 Hrtrronucleur Clusters Having Transition Metals and Metals of Group 14
63
Figure 16. Molecular structure of [Ir3(/i-SnR~)2(CO)~H].
Use of the bulky aryltin( 11) monomer/dimer mixed species [SnArz/ArzSnSnArz], with Ar = C6H*Pri-2,4,6,in reactions with [Ir4(CO)11BrIpsalts, results in a different series of clusters. Again a mixture of products was obtained with the main components being a blue-green species believed to be an analogue of the raft described above, together with four other crystalline clusters, and trace materials. All of these clusters have been characterized by elemental analysis, and molecular weight measurements, and can be formulated as [Ir?(SnAr2)2(CO)6], [Ir3(SnArz)z(CO)s], [Ir4(p-SnAr2)4(CO)x],and [Ir4(,~~-SnAr2)5(CO)j]. The first, a red crystalline species, is air-sensitive, diamagnetic, and lacks carbonyl bridges; and almost cercontaining an Ir- Ir bond, tainly has the structure [(OC)3Ir(p-SnArz)zIr(C0)3], which confers an eighteen-electron configuration on the iridium atoms. The spectroscopic data for [Ir3(SnArz)~(CO)~] indicate that the structure is analogous to the electron-rich Sn{CH (SiMe3)2}, derivative, but lacking the bridging carbonyl. Spectroscopic data (IR and NMR) for the remaining compounds together with the analytical, molecular weight, and mass spectrometric data indicate the structures shown in Fig. 17, and, as implied, the species are both diamagnetic. X-ray structures are currently not completed for these clusters. While tin clusters form the widest range of heterometallic species reported here, it has proved possible to isolate and characterize some mixed lead compounds. These are both light- and air-sensitive, frequently precluding chromatographic separation, but in favorable cases, crystallisation using Schlenk techniques has given pure products. Few lead-transition metal clusters were known at the start of this work, and only for combinations with including a single structurally characterized species, [Pb{Fez(CO)s},].1341 We first introduced the lead( 11) species Pb(CsHzPh3-2,4,6)2 as a reagent having '
64
1 Moleculur Clusters
Figure 17. Proposed structures for the species [Ir4(pSnAr2)4(CO)g],and [Ir4(p-SnAr2)5(CO)j].
extremely bulky ligands, in order to protect the M-Pb bonds from chemical attack. The lead species is an extremely air- and light-sensitive purple solid, which can be stored with rigorous exclusion of air in the dark. Using this ligand, the sensitive species [Ru3(p-PbR2)2(CO)~(pu-CO)], [Ru~(~-P~R~)(CO)~(~-CO)~], and [Ir4(p-PbR2)(C0)s(p-C0)2]were prepared, and characterized spectroscopically but were not sufficiently stable for structural work to be carried out.[351However, using the reagent Pb{CH(SiMe3)2},, with [Ru3(C0)12]the clusters [Ru3(pU-PbR2)2(CO)9(p-CO)]and [ R u(p-PbR2) ~ (C0)g(p-CO)] were obtained, again as light- and airsensitive species.[361Reaction occurs on reflux in hexane, and the compounds must be separated by fractional crystallization, since chromatography leads to decomposition. The structure of [Ru3(p-PbR2)2(C0)9(pU-CO)] is shown in Fig. 18.
Figure 18. The molecular structure of [Rus(~-PbR2)2(C0)9 (P-CO)].
1.4 Heteronuclear Cliistei.s Huring Trunsition Metuls und Metuls of Group 14
65
In this molecule, the Ru-Ru vectors are significantly lengthened in comparison to the dodecacarbonyl as expected, and there is a single bridging carbonyl. The Ru-Pb distances (the first reported) range from 2.765(2)A to 2.790(2)A. In each case the lead bridges are asymmetrical, with the longer Pb-Ru separation being adjacent to the bridging carbonyl, and is possibly caused by a pseudo tvuns-influence of this ligand at the ruthenium center.
1.4.6 Clusters in which the Group 14 ligand or the heterometallic cluster has undergone further reaction The reaction of SnRz ( R = (Me3Si)lCH) with the reactive osmium hydride [ O S ~ ( P - H ) ~ ( C O )affords I O ] an Os3Sn cluster retaining both hydrogen at0ms:1~~1 [SnR21 + [ O S ~ ( P - H ) ~ ( C O ) I[~O] S ~ ( C O ) I O ( ~ ~ - H ) ~ S ~ R ~ ] +
The X-ray data, coupled with N M R data reveal that the hydrogen atoms are both in bridging positions, but one of them is between tin and osmium, a unique feature. The hydrogens were not directly located in the X-ray data, but the disposition of the other atoms around the bridged tin and osmium, the appropriate bond lengths, and intermediate 'J( '"Sn-'H) coupling all confirm this location (Fig. 19). As with many other compounds having main-group atoms bridged by hydrogen, [Os3(CO)lo(,u-H)2SnR2] is highly reactive, and certainly compared with any of the other clusters described here. Indeed on warming in solution, or even on standing in the solid state for prolonged periods brings about a rearrangement reaction.1381 An alkyl group of the tin moiety migrates to the carbon of a coordinated carbonyl producing an acyl group, and a hydrogen migration occurs so that both adopt the more usual position bridging two osmium atoms. The rearranged product has the structure shown in Fig. 20, in which methyl groups have been omitted for clarity. The tin atom now has trigonal planar coordination involving the remaining alkyl group and two of the osmium atoms. If we regard this as sp2 in character, the oxygen of the acyl group donates into the vacant, orthogonal p-orbital of the tin. This geometry of the cluster is seen to be in accord with this picture. A further reaction of the starting cluster is the coordination of his(dimethy1)acetylene dicarboxylate, which again involves the unique bridging hydride. In this case, both of the cluster hydrides are transferred to the same acetylenic carbon of the alkyne, so that the coordination of the resulting dihydro-derivative is through a single carbon, giving the carbene-complex with the structure shown in Fig. 21.
Figure 19. The structure of [Ow(CO)~o(~-H)zSnRzl (R = CH(SiMe3)2), showing the hydride positions.
1.4 Hetevonucleur C1uster.r Hcrcing Transition Metuls und Metal., of Group 14
67
Figure21. Thc structure of [Os;(~~-SnR~){pC(COOMe)CH~COOMe}(CO)l~]. [R = CH(SiMei)l]. Hydrogen atoms are omitted for clarity.
The compound can then be represented as [0s3(p-SnR2){ p-C( CO0Me)CH2COOMe}(CO)lo]. [R = CH(SiMc3)2]. This mode of coordination of an alkyne to a cluster has no precedent. An unexpected feature of the structure is the close approach of one of the oxygen atoms of a COOMe group to one of the osmium atoms of the cluster (see Fig. 21).[381 The tin reagent [ S I I { ~ - C ( S ~ M ~ ~ ) ~ Cand ~ Hrelated ~N>~ species, ], although potentially good donors and moderately electron-rich at the metal centre, have four coordinate tin atoms, and therefore have a highly reduced tendency to add to clusters. However, with the reactive [Os3(pU-H)2(CO)lo], one of the tin ligands can be lost as the substituted picoline:
This elimination of the picoline, generates the Sn{2-C( SiMe3)2C5H4N} fragment, which coordinates to the cluster as a p-stannyne complex, as shown in Fig. 22. The remaining hydride remains coordinated to the cluster, bridging two of the framework osmium atoms. In terms of the electron count, this electron-precise cluster has 45 cluster valence electrons from the Os3(CO)loH fragment, and thus three are contributed, as required, by the stannyne group. Notable features of the structure are (i) the appreciable lengthening of the 0 s - 0 s distance bridged by the
68
1 Molecular Clusters
Figure 22. The structure of [Osj(,u-H)(CO)lo{Sn{2-C(SiMej)zCsH4N)], omitting hydrogen atoms for clarity.
stannyne (3.001(4)A) compared to other tin-bridged osmium atoms, (ii) the correspondingly short distances of the Sn-0s bonds (2.641(2) and 2.642(2)A) compared to related separations, and (iii) the short Sn-N bond in the stannyne [2.235(14)A] compared to the starting reagent [Sn-N 2.449(7)A]1391.The first of these is perhaps surprising for an electron precise cluster, but the tin-ligand distances are easily understood in terms of the loss of a picolyl ligand, and donation of three electrons to the cluster framework. The stannyne complex is unstable in solution, losing a further picoline molecule, and affording a highly insoluble cluster, for which we have no structural data at present. The M(I1) reagents [ M ( C H ( P P ~ Z ) ~ }(M ~ ]= , Sn or Pb), introduced by Karsch,[401have three coordinate metal atoms, and again are expected to retain their composition when bonded to clusters. In this case though, reactions afford no isolable heterometallic clusters, but after work-up, give cluster adducts of the parent bis(phosphine), PhzPCH2PPh2. The hydrogen in the product must be an artifact of the experimental procedures emp10yed.l~'~ For example, in reaction with [ R u ~ ( C O121, ) the adduct [ R u ~ ( C Olo(dppm)] ) was obtained rapidly and quantita-
1.4 Hcteronucltw Clustrrc Hur inq TrunJition Metals ond Metuls of Group 14
69
Figure 23. The structure of [Ru?(CO)h(pSnK2)(pPh2PCH2PPh2)1, prepared by ligand transfer from [ Sn { CH (PPh2 ) 2 }?I.
tively using the tin reagent (lower yields were obtained from the lead analogue), whereas the free bis( phosphine) requires prolonged heating with chemical activation. This disappointing result can, however, be turned to good account. in preparing phosphine derivatives of existing clusters. The examples below illustrate this point:
Structures of these compounds are shown in Fig. 23 and 24.
70
I Molecular Clusters
Figure 24. The structure of the paramagnetic, electron-precise cluster [Os, (pSnR2):,(C0)g(,u-P~~PCH~PP~~)].
Acknowledgements The author wishes to acknowledge the outstanding contribution to this work of Dr. Christine Cardin, who has provided all the structural data on which this paper relies so heavily. He would also like to thank the coworkers cited in the references for their hard work and many helpful discussions, Dr. Paul R. Raithby for the provision of diffractometer time in the early years and for helpful discussions, the EPSRC, the Krieble fund, and the Reading University Research Endowment Fund for support.
References [l] K. H. Whitmire, J. Coord. Chem., 17, 95 (1988). [2] S. G. Anema, K. M. Mackay, and B. K. Nicholson, J. Chem. Soc., Dalton Truns., 3853 (1996). [3] J. Lewis and J. R. Moss, Can. J. Chem., Rev. Can. Chimie, 73, 1236 (1995). [4] P. Braunstein, C. Charles, G. Kickelbick, and U. Schubert, Chem. Cornrnun., 2093 (1997).
1.4 Heteronuclmr Clusters Hiwing Transition Metals and Metals of Group 14
71
[5] L. R. Sita, R. M. Xi, G. P. A. Yap, L. M. LiableSands, and A . L. Rheingold. J. Am. Chetn. Soc., 119, 756 (1997). 161 C. J. Su, P. C. Su. Y. Chi, S. M. Peng, and G. J. Lee, J. Am. Chem. Soc., 118, 3289 (1996). [7] M. F. Lappert and R. S. Rowe, Coord. C'henz. R ~ L I100, . , 267 (1990). [8] R. Bohra, P. B. Hitchcock, M. F. Lappert, S. C. F. Auyeung, and W. P. Leung. J. Chem. Soc.; Dalton Trans., 2999 (1995). [9] S. L. Ellis. P. B. Hitchcock, S. A. Holmes, M. F. Lappert, and M. J. Slade, J. Oryanomet. Chem., 444. 95 (1993). [lo] M. Veith, S. Weidner. K. Kunze. D. Kafer, J. Hans, and V. Huch. Coord. Chem. Rec., 137, 297 [ 1994). [ 1 I ] T. Fjeldberg, A. Haaland, B. E. R. Schilling, H. V. Volden, M. F. Lappert, and A. J. Thorne, J. Oryanornet. Chem., 276, C 1 (1984). [I21 T. Fjeldberg, A . Haaland. B. E. R. Schilling. H. V. Volden, M. F. Lappert, and A. J. Thorne. J. Orgunomet. Chem., 280, C 43 (1985). [13] T. Fjcldberg, A. Haaland. B. E. R. Schilling, M. F. Lappert, and A. J. Thorne, J. Chem. Soc., Dalton Trans.. I 5 5 1 ( 1 986). [14] D. E. Goldberg, P. B. Hitchcock, M. F. Lappert, K. M. Thomas, A. J. Thorne, T. Fjeldberg, A . Haaland, and B. E. R. Schilling, J. Chem. Soc.. Dalton Trans., 2387 (1986). I 151 P. J. Davidson and M. F. Lappert, J. Chem. Soc., Chenz. Commun., 317 (1973). [ 161 M. F. Lappert, J. Oryanornet. Chrm.. 100, 139 (1975). [I71 H. Alper and J. T. Edward, Cun. J. Chem., 48, 1543 (1970). [18] C. J. Cardin, D. J. Cardin, N. B. Kelly. G. A. Lawless, and M. B. Power, J. Oryunomet. Chcni., 341. 447 (1988). [I91 F. J. Brady, C. J. Cardin, D. J. Cardin. M. A. Convery, M. M. Devereux, and G. A. Lawless, J. Oryanomet. Chem., 241. 199 (1991), and references therein. 1201 C. J. Cardin, D. J. Cardin, M. B. Hursthouse, and G. A. Lawless, unpublished work. [ Z l l J . D. Cotton, P. J. Davidson, and M. F. Lappert. J. Chem. Soc., Dalton Trans., 2275 (1976). [22] C . J. Cardin, D. J. Cardin; J. M. Power, P. P. Power. and M. M. Olmstead, Unpublished ohsrruations (1985). (231 G. Lavigne, F. Papageorgiou. C. Bergounhou. and J. J. Bonnet, Inory. Chem., 22, 2485 (1983). 1241 C. J. Cardin, D. J. Cardin, G. A. Lawless. J. M. Power, M. B. Power, and M. B. Hursthouse, J. Oryanoniet. Chenz.,325, 203 ( 1987). 1251 C . J. Cardin. D. J. Cardin, M. A. Convery, M. M. Devereux, B. Twamley, and J. Silver, J. Chem. Soc., Dalton Truns., 1145 ( 1996). [26] C. J. Cardin, D. J. Cardin, G. A . Lawless, and M. B. Powcr, Proc~edinysy f t h e Royul Irish Acudemy Section B Biological Geological and Chemicul Science, 89, 399 (1989). 1271 C . J. Gilmore and P. Woodward, J. Chmm.Soc., Dalton Trans., 1387 (1972). 1281 C. J. Cardin, D. J. Cardin, and M. B. Power, unpuhlishrd ohsertations (1990). 1291 C . J. Cardin, M. B. Power, and D. J. Cardin. J. Oryanomet. Chem.,462, C 27 (1993). [30] C. J. Cardin, D. J. Cardin. H. Henke, and M. B. Power. unpuhlished ohservations (1990). [31] T. J. Marks, and A. R . Newmann, J. A m . Chem. Soc.. 0.5; 769 (1973). [32] J. D. Cotton, S. A. R. Knox, I. Paul. and F. G. A. Stone, Journal of the Chemical Society, A , 264 (1967). 1331 J. Dalton. I Paul, and F. G. A. Stone, Journul oj'tlzr Chemical Society> A , 1215 (1968). [34] K. H. Whitmire, C. B. Lagrone. M. R. Churchill, J . C. Fettinger, and B. H. Robinson, Inory. Chcm., 26, 3491 (1987). 1351 D. J . Cardin and B. Twamley, unpublished observations (1994). [36] N. C. Burton, C . J. Cardin, D. J. Cardin, B. Twamley, and Y. Zubavichus, Organornetallic~.s, 14. 5708 (19951. ~
12
1 Molecular Clusters
1371 C. J. Cardin, D. J. Cardin, H. E. Parge, and J. M. Power, J. Cllrm. Soc., Chem. Commun., 609 (1984). 1381 C. J. Cardin, D. J. Cardin, J . M. Power, and M. B. Hursthouse, J. Am. Chem. Soc., 107, 505 (1985). [39] L. M. Engelhardt, B. S. Jolly, M. F. Lappert, C. L. Raston, and A. H. White, J. C'hern. Soc., Chem. Commun. 336 (1988). [40] H. H. Karsch, and G. Mueller, Orgunomrtnllics, 5 , 1664 (1986). [41] D. J. Cardin and M. M. Devereux, unpuhlishedohsevautions (1991). ~
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.5 Hetero-Metal Clusters by Assembling Amino Substituted Subvalent Main Group Metals and their Ligand Reactions
1.5.1 Introduction Over the last fifteen years we have been interested in assembling several metallic elements around a central transition metal to create a metal cluster by this strategy. Our approach has been very simple, and we have used his(amino)carbene analogues (related to molecules containing ns’-elements) such as Me?%(NtBu)zM ( M = Ge, Sn, Pb) to replace standard nonmetallic ligands in a transition metal complex M’L,, ( L = CO, PR3, organic ligands). A brief review of the molecular clusters synthesized in our laboratory is given here. Because in the final clusters the metallic elements like Ge or Sn are only three coordinates (for tetra-coordinate G e or Sn see reviewsl’.’I or recent literaturei3’)we were curious to know if we could use this specific property for further reactions. Within this general theme there were two main questions: ( 1 ) Would the inner metallic skeleton survive in oxidation reactions‘? (2) Could we use the amino ligands bonded to the main group metals for nucleophilic substitution without destroying the inner metallic arrangement? As some of the structural results have not been published so far, we have added some brief experimental details in footnotes. It is not within the scope of this article to cover the whole literature on this topic; we would nevertheless like to point out that clusters similar to those dealt with in this article have also been prepared by M.F. Lappert’s g r ~ u p . ~Recently, ~ , ’ ~ one of my former coworkers. L. Stahl, has extended the amino stabilization concept to new transition metal comp1exes.I” A general review concerning the synthesis of hetero-metal clusters with ns’-main group metals and covering some of the general literature appeared in 1992.’”
74
1 Molecular Clusters
1.5.2 Syntheses of clusters by using MezSi( NtBu)2M (M = Ge(II), Sn(I1)) Syntheses of metal clusters with CO as coligand In 1981‘’’ we reported for the first time that the bis(amino)stannylene 2 can replace CO ligands in hexacarbonyl complexes of chromium or molybdenum. It has since been found that this type of reaction can be expanded to either the bis(amino)germylene 1 or to W(CO)6 as summarized in Eq. (1).
M
= G e(l), Sn
(2)
M
= Ge, Sn; M’ = Cr, Mo, W
(1)
The substitution of CO by the his(amino)germylene or -stannylene is carried out thermally in refluxing toluene or, alternatively, by displacement of tetrahydrofuran ( T H F ) by germylene or stannylene from the intermediate complex (CO)SM’(THF)’~ in] ,a photochemical reaction. The lead analogue of 1 and 2, Me2Si(NtBu)2Pb[’01,does not seem to react similarly as we have been unable to isolate well characterized products. When an excess of M’(CO)6 is used mostly the mono-substituted derivatives with n = 1 and the general formula Me*%(NtBu)2M-M’(C0)5 are generally formed. Using an excess of 1 or 2, besides the mono complexes, complexes with a higher substitution number (n = 2 or 3) are also formed. This is not only observed when W(CO)6 is used as the starting material but also with the metal carbonyls of chromium or molybdenum. Until now, we have not carried out a systematic study, but we have isolated one of the triply substituted complexes of chromium, [MeZSi(NtBu)2Sn]3Cr(CO)3 (3), in the crystalline form. The result of the single crystal X-ray determination of this compound“ is depicted in Fig. 1. In the crystal, two different molecules with the same composition and metal arrangement are found in the asymmetric unit, but they differ in their point symmetry (one is C2, the other C l ) and by the mutual orientation of the planar SiNzSn four membered rings to one another (only one molecule is shown in the figure). The four metal atoms are in a plane forming a T-shape with Cr occupying the central position; this mer configuration is common to the two symmetrically independent molecules. Even in solution we have found no evidence (13C, 29Si NMR), for the presnce of a f u c isomer. The Cr-Sn distances fall into two categories: there are short distances for the Sn-Cr bonds that are trans to one another (2.504-2.515 A) and longer Sn-Cr bonds for all those that are in a cis position to two other Sn-Cr bonds and trans to a CO group (2.542-2.543 A). The shorter trans Sn-Cr bonds are consistent with longer Sn-N bonds in the SnN2Si rings (mean
I . 5 Hetero-Metal Clusters
75
Figure 1. One of the two independent molecules in the crystal structure of 3 is shown." ' I Hydrogen atoms are omitted for clarity: carbon and oxygen atoms are not labeled.
value: 2.06(1)A) compared to the longer cis Sn-Cr bonds, which are associated with shorter Sn-N bonds (mean: 2.03( 1) A). The Cr-C bonds show no special effect and the atoms in the cis and trans positions have similar bond lengths (1.84(3)A). Another metal cluster, which also contains four metal atoms is formed when the carbene analogues 1 or 2 are reacted with either Mn2(CO)lo or FeZ(C0)g (Eqs. 2a and 2b).1'23'31
+
2 Me?Si(NtBu)zM Mn?(CO)lo 1.2 +
2 CO
+ [Me2Si(NtBu),M]2[Mn(C0)4]2 M
= Ge
(2a)
(4), Sn ( 5 )
+
Me?Si(NtBu)?M FeZ(C0)o 132 +
Fe(C0)5 + I/n [Me2Si(NtBu)2M-Fe(C0)4In n = 1 M = Ge (6). Sn (7) n = 2 M = Ge (S), Sn (9)
(2b)
As in Eq. (1). in the reactions of 1 or 2 with dimanganesedecacarbonyl, the carbony1 groups are displaced by an equal number of the 1 or 2 groups. The crystal structure determinations of 4 and 5 reveal that the germanium and tin atoms do not simply replace the carbon atoms of the carbonyl groups, but also lead to a rearrangement of the structure: in each case a four-membered planar MzMn? ring is formed. In Fig. 2 the result of the X-ray structure determination of 4 is shown, the structure of 5 is quite similar but nevertheless different in some details.
76
1 Molecular Clusters
Figure 2. The centrosymmetric molecule [Me?%(NtBu)zGe]*[Mn(CO)412 14) from an X-ray structure determination."3' See also caption to Fig. 1. The Mn -Mn bond is represented as a dashed line.
The most striking difference between the two compounds 4 and 5 both of which have the same point symmetry C , and are isomorphous,[l3]are the unsymmetrical Ge-Mn bonds in 4 (2.460(2)A and 2.600(2)A) while in 5 the corresponding Sn-Mn bonds are quite similar (2.61 13(4)A and 2.693(1)A). Compared to Mn?(CO)s(pC O ) ( / L - G ~ M ~ ~ )which , " ~ I has Mn-Ge distances in the range of 2.432-2.4437(2) A, one of the two bonding distances in 4 is clearly elongated. We think this phenomenon could be most easily explained by steric requirements and not by electronic effects. On this line Mn-C distances tr-uns to the Mn-Ge bonds show no significant differences (1.798(5)and 1.808(5)A). the Mn-Mn distances In comparison to MnZ(C0)lo ( M n . . .Mn 2.923(3) in 4 and 5 are systematically elongated (4: Mn-Mn = 3.025(2)& 5 : Mn-Mn = 3.184A). Whereas the expansion of the bond in the germanium derivative 4 is quite small, in the tin derivative 5 it is more important. From N M R measurements we deduce that 4 and 5 are diamagnetic. We therefore propose an intramolecular Mn-Mn bond in these derivatives or a stable singlet state of a diradical. It is interesting that despite the different Mn-Ge/Mn-Sn bond lengths. the equivalent angles within the M2Mn2 rings at Ge or Sn (4:73.4( l)', 5 : 73.7( 1)") or at Mn (4: 106.6(l)", 5 : 106.3(1)") are almost equal. As may be deduced from Eq. (2b) similar M2M2' rings are obtained by the reaction of 1 and 2 with Fe2(C0)9. Nevertheless, the situation is complicated by an equilibrium between the monomeric entities [MezSi(NtBu)2M-Fe(C0)4], 6 and 7, and the dimers 8 and 9 (Eq. 3), respectively. 2Me2Si(NtBu)2M-Fe(C0)4 M = G e (6),Sn (7)
+ [ M ~ ~ S ~ ( N ~ B U ) ~ M - F ~ ( C O( 3) )~ ] ~ M
= Ge
(8),Sn (9)
While the equilibrium for 7/9 is relatively quickly attained at room temperature with a 1 : 1 ratio of 7 to 9, the corresponding 6/8 equilibrium is very slow. As a matter of fact, in a freshly prepared solution of 6 only NMR spectral data due to the monomer is present. When a toluene solution of 6 is allowed to stand for several
1.5 Hetero-Metul Clusters
77
Figure 3. The centrosymmetric molecule [Me: Si(N tBu)z Sn]?[Fe(CO:d]2 (9j from an X-ray structure determination.[' 31 For further details see caption to Fig. 1.
days, the signals of 8 become prominent attaining a 3 : 1 ratio at room temperature. Both the equilibria 6/8 and 7/9 are temperature dependent, as shown by NMR, shifting in both cases to the monomer at higher temperatures. The structure of 9 has been determined by X-ray analysis and the arrangement of atoms is depicted in Fig. 3.[l3IThe molecule again has C, point symmetry (compare 4, 5 ) which means that the central four membered metal cluster is strictly planar and crystallizes isotypically in a centro-symmetric lattice. As in 4 and 5 the two SiNzSn rings are almost perpendicular to the central Sn?Fe:! ring; the two tin atoms can thus be considered as spiro-centers. The most remarkable properties of the compound are the following: 1) Although there is a monomer/dimer equilibrium in solution, the two Fe-Sn distances are closer to one another (Sn-Fe: 2.712(2)A; Sn-Fe': 2.685(2)A)than the corresponding Ge-Mn or Sn-Mn distances in 4 and 5 . [(Cp)?SnFe(C014]2has a structure similar to that of 9, the FezSn:! ring displaying shorter Fe-Sn bonds (2.651 and 2.670!~)"". 2) The Fe-Fe distance (4.116(1)A) within the Fe:!Snz ring is much larger than a typical Fe-Fe bond length and the Sn-Sn separation (3.492(1)A) is smaller than in compound 5 . As expected, no metal-metal interactions across the ring diagonals need to be considered. As in other transition metal complexes of Me?%(NtBu)zSn, 2 , the coordination of the tin atom to the transition metal leads to a shortening in Sn-N bond length (9: Sn-N (mean): 2.06819)A) compared with the uncoordinated compound ( 2 :2.091(5)AJ.'17' Another type of reaction of 1 and 2 with simple metal carbonyls has been observed with Co:!(COis (Eq.(4)). Me:! Si (N t Bu)?M t Coz (CO), M = Ge ( I ) . Sn ( 2 )
+
Me:! Si(N rBu):!M [Co(CO),]z M = Ge (10),Sn (11)
(4)
In these reactions the carbene analogues 1 and 2 insert into the Co-Co bond of the dicobaltoctacarbonyl forming a triatomic MCo2 metal cluster. I' 81 The reactions
78
I Moleculur Clusters
Figure 4. The molecular structure of [MezSi(NtBu)&1][Co(C0)4]2 (11) as determined by X-ray diffraction techniques'"] (see also caption to Fig. I ) .
are almost quantitative. The compounds obtained from these reactions are diamagnetic and all spectroscopic data are in accord with an insertion product of the carbene analogue into the transition metal-metal bond. The result of the X-ray structure analysis on 11 is shown in Fig. 4. In the crystal structure of 11 the asymmetric unit contains two independent molecules that are nevertheless quite similar in their structures. The two Sn-Co distances [2.632(2)A and 2.638(1) A] are almost equal. They are longer than in the comparable compound ClzSn[Co(CO)& in which the Co-Sn distances are in the range of 2.533-2.535A,L191but are shorter than the Fe-Sn distances in 9 (see above). Even in the compounds C lS n [ C 0 ( C 0 ) 4 ] 3 [ ~ ~ ~ and S ~ S ~ ~ [ C O ( Cthe O Sn-Co ) ~ ] ~ distances ~ ~ ~ ~ (2.606& 2.59 A) are shorter than in 11. The Sn-N distances in 11 (2.059A) are again shortened with respect to the free bis(amin0)stannylene 2 (see above). The Co-Sn-Co bond angles in 11 are 115.4(1)" and 116.6(1)",respectively, and confirm that there is no Co-Co bonding interaction.
Reactions of 1 and 2 with cyclopentadienyl-carbonylmetal complexes The insertion ability of carbene analogues 1 and 2 into metal-metal bonds (see above) can be expanded to include many molecules with metal-metal or metalcarbon bonds. As some of these results have been published previously, we just briefly mention the products obtained in these reactions.i221 Cp(CO)lFe-Fe(CO)zCp
+ MezSi(NtBu)zGe 1 i
Me2Si(NtBu)zGe[Fe(CO)2Cp]2 ( 5 ) 12
1.5 Hetwo-Mrtul Clusters
Cp(C0)2Fe-Fe(CO)2Cp
79
+ 2Me*Si(NtBu)zSn [Cp(CO)2Fe]2[Me*%(NtBu),Sn]z 13
4
(6)
Cp(C0)2Fe-Me + MelSi( N tBu)?M M = Ge ( l ) , S n (2) +
Me2 Si ( NtBu) 2 M (Me)[ Fe(C0)2 CP] 14 ( M = Ge). 15 ( M = Sn)
(7)
As may be seen from inspection of Eqs. ( 5 ) and (6) the two his(amino)carbene analogues react differently with [ Cp(C0)2Fe]2. While only one germylene inserts into the iron-iron bond forming 12, two stannylene units are inserted to form 13. In the first case a triatomic heterometal cluster is obtained with an electron count similar to that in compounds 10 and 11 and in the second case (Eq. 6) a tetraatomic Fe-Sn-Sn-Fe zigzag chain with point symmetry C, is obtained with Sn-Fe distances of2.605(2) A and a Sn-Sn bond length of 2.991(2) A; the Fe-Sn-Sn angle is 117.9(1)". In contrast to compound 9, the Sn-N distances are not shortened in 13 and are found to be almost equal to that in the free stannylene (13: Sn-N 2.099(2) A). In Eq. ( 7 ) the results of the insertion of the low valent elements in 1 and 2 into an iron-carbon bond are summarized. The two products 14 and 15 are obtained in almost quantitative yield and are similar in structure.'221 Another reactivity pattern of the carbene analogues 1 and 2 with cyclopentadienyl metal carbonyls was established with C O ( C ~ ) ( C O ) ~As. "can ~ ~ be concluded from Eq. (8), carbon monoxide is smoothly eliminated and the cobalt atom in Co(Cp)(CO)bonds to a germanium or tin atom.
+
2Me*Si(NtBu)zM 2Co(Cp)(CO)2 M = Ge (1),Sn (2) +
[Me2Si(NtBu),M-Co(Cp)CO], M = Ge (16),Sn (17)
+2 C 0
(8)
In both cases the simple M-Co compound is not obtained but a dimeric species with a MICo2 ring. This can unambiguously be established by an X-ray structure analysis, which has been performed on single crystals of 17. The molecule is centrosymmetric (Fig. 5 ) and has the point symmetry C Z h , the two fold axis running through the silicon and tin atoms and the mirror plane intersecting the cyclopentadienyl ligands and running through the cobalt atoms and the carbonyl group. The Sn-Co bonds are thus equal (2.567(1)A) and the angles at Sn (98.2') and Co(8l.X)'' add up to 180". It is interesting to compare this compound 17 with compound 5 in which a
80
1 Molecular Clusters
Figure 5. The molecular structure of the trans isomer (mutual orientations of the Cp ligands with respect to the ring) of [MezSi! NtBu)2Sn]*[Co(Cp)CO]z (17) as determined by X-raydiffraction techniques['" (see also caption to Fig. 1). The molecule has Czi, symmetry in the crystal.
Mn-Mn bond seems to be present, and with 9 where the Co(Cp)CO unit is replaced by the isoelectronic Fe(C0)4 unit. A superposition of 17 and 9 shows, that the two molecules are quite similar, the bonds within 17 being about 95% shorter than in 9 (this holds for M'-M', Sn-M' and Sn-Sn distances in the two molecules). As a consequence the Cp(C0)Co group is more tightly bonded to the main group metals than is the Fe(C0)4 unit. The central four membered ring in 5 is, on the other hand, very different from the M'2Sn2 rings in 9 and 17. Here the Mn-Mn distance is obviously much smaller than the corresponding M' -M' distances in 9 and 17. The probable Mn-Mn bonding interaction is nicely reflected by this comparison (see also Fig. 6). Besides the formation of the dimers 16 and 17, there is an equilibrium between two forms in solution. The two cyclopentadienyl groups of the dimer can be either trans oriented to one another (as in the crystal) or cis oriented (Eq. 9). The cis compound is easily recognized by its 'H- and "C-NMR spectra that show two different tert-butyl groups, whereas the trans compound has only one symmetrically independent tert-butyl group. The equilibrium is temperature dependent, the cis: trans ratio for 16 and 17 at room temperature is approximately 1 : 2 and at 330 K it approaches 1 : 1, the cis isomer growing at the expense of the trans isomer. I*'1
Figure 6. Comparison between MrM; rings of equal size: 17 with 9 (SnzFcz) and 17 with 5 (SnzMnz).
1.5 Hetevo-Metal Clu.sters
cis
81
trans
Syntheses of metal clusters with phosphine, cyclopentadienyl ligands and homoleptic complexes The Wilkinson catalyst Rh( PPh3)3C1 has been allowed to react with the germylene Me2Si(NtBu)zGe (1) and the stannylene MezSi(NtBu)zSn (2).The products of these reactions are quite different and are strongly dependent on the carbene analogue (Eqs. 10, 1 l a and 1 l b ) . 1 2 3 1 4 MezSi(NtBu)lGe 1
+ Rh(PPh3)3Cl +
3 PPh3
+ [MezSi(NtBu)2Ge]4RhCl
(10)
18 2 MelSi(NtBu)zSn 2
+ Rh(PPh3)3Cl
4
PPh3 + ci,r-[Me2Si(NfBu)zSn]2Rh(PPh3)2Cl 19
(1 la)
5 MezSi(NtBu)zSn + Rh(PPh3)3Cl 2 +
3 PPh3 + [Me~Si(NtBu)zSn]5RhCl 20
(1 l b )
The reaction of 1 with Rh( PPhl)3C1 proceeds directly to the homoleptic complex 18, which has been fully characterized, and we have not been able to detect any intermediate in this reaction.1241 The analogous reaction of 2 with Rh( PPh3)3C1can be separated into two steps: first a trinuclear metal entity 19 is formed, and at higher molar ratios of 2 the homoleptic tin substituted rhodium(1) complex 20 is formed. which contains six metal atoms clustered at the center of the m o 1 e c ~ l e . l ~ ~ ~ The compounds 18, 19 and 20 have been characterized by X-ray structure determination techniques. In 18, the central unit is an almost square planar germanium arrangement in the middle of which the rhodium atom is situated. One of the edges
82
I Molecular Clusters
of this central Ge4-square is bridged by a chlorine ligand. Compound 20 is different from 18 as all five tin atoms are coordinated around the rhodium atom in a trigonal bipyramidal fashion, the chlorine ligand again bridging two of the tin centers. The difference in structures may be attributed to the lower steric demands of ligand 2. Finally the tin atoms in compound 19 are in a cis arrangement with the rhodium atom occupying the center of a distorted square planar coordination polyhedron, which is made up of two tin and two phosphorus atoms. Again two tin atoms are bridged by the chlorine ligand. The Ge-Rh and Sn-Rh distances in 18, 19 and 20 are in the expected range (Ge-Rh: 2.337-2.376; Sn-Rh: 2.526-2.571 A) and reflect the coordination numbers of the elements under consideration. The reaction of Pd(PPh3)4 with MezSi(NtBu)zSn is somewhat reminiscent of that with Rh( PPh3)3Cl as again the stannylene replaces the phosphine l i g a n d ~ . ' The ~~] product 21, which can be isolated from this reaction (Eq. 12), still contains one triphenyl-phosphine ligand bonded to each palladium center. Pd(PPh3)4
+ 3/2 Sn(NtBu)zSiMez +
1/2(PPh3)Pd[Sn(NtBu)2SiMez]$d(PPh), + 3 PPh3
(12)
21
The PdzSn3 trigonal bipyramidal metal cluster formed in this reaction (X-ray structure evidence) is new for Pd,Sn, systems but has previously been found for platinum-tin systems in (Ph3P)Pt(Snacac)3Pt(PPh3).'251 An interesting feature of the structure is that the SiNzSn rings in 21 are oriented in a paddle wheel fashion and thus create chirality along a three-fold axis (two different molecules crystallize in the asymmetric unit of the lattice). The Pd-Sn bond lengths are found between 2.688 and 2.725A. The Pd-Pd separation in 21 is only 2.590A, which suggests that Pd-Pd bonding may be present. In accord with this model the Pd-Sn-Pd angles are extremely acute (56.94-57.54'). A very elegant way of forming a NiSn4 cluster saturated by amide ligands involves the displacement of cyclooctadiene (cod) from the Ni(cod)z complex by the bis(amin0)stannnylene 2 (Eq. 13).lz6' Ni(cod)z
+ 4Sn(NtBu)2SiMe2 2
+
2cod
+ Ni[Sn(NtBu)2SiMe2]4
(13)
22
The blood-red compound 22 contains five metal atoms of which the four tin atoms tetrahedrally coordinate the central nickel atom, as has been confirmed by X-ray structure analysis. The tin-nickel distances range from 2.388(2)-2.399(2) A. Because of the steric crowding of the tert-butyl groups of the amino ligands, the Sn-Ni-Sn angle deviates considerably (up to 12") from the ideal value of 109.5". The same coordination number at nickel, but in a different geometry, is found when NiBrz is reacted with 2. A similar reactivity and similar structures are found
1.5 Hetero-Metal Clusters
83
for PdC12 and PtC12 while other transition metal halides (CrC12, FeC12, CoC12, ZnClz) form Lewis acid/base adducts with 2 without any direct transition metal-tin bonding (Eq. 14).‘241
+
4 Me.ISi(NtBu)*Sn M’X2 2
i
M’[Sn(NtBu)2SiMe2]4X, M’ = Ni,X = Br (23) M’ = Pd, Pt; X = C1 (24,25)
(14)
The molecules 23, 24 and 25 have an almost planar M’Sn4 cross in common with the transition element occupying the central site in all cases. The two halogen ligands are in between two tin atoms forming almost symmetrical Sn-X-Sn bridges with acute angles (74-81”) at the halogen atoms. The change in the coordination geometry of Ni in going from molecule 22 to 23 can be understood by a change in the oxidation state from formal 0 (d”) to +2 (d’). The Sn-Ni distances are elongated in 23 when compared to 22: they are found between 2.459(4) and 2.466(4)A and reflect the higher coordination number at tin in 23. A quite spectacular ‘insertion reaction’ of the carbene analogues 1 and 2 into transition metal ligand bonds is found with nickelocene [NiCpz] (Eq. 15).’”’ NiCp,
+ 2 MezSi(NtBu)zM 1 (Ge).2 (Sn)
+
[CpNi(M(NtBu)zSiMe2},(lu-Cp)] M = Ge (26).Sn (27)
(15 )
The crystal structures of 26 and 27 reveal a novel type of cluster. As a matter of fact, the three metallic atoms are connected to one another to form an open triangle whereby the nickel atom occupies the central position. This triangular unit is intermediate between two parallel cyclopentadienyl ligands (Ni is penta-hapto coordinated to one of the cyclopentadienyl units, while the two main group metals coordinate to the second cyclopentadienyl) and the structure can thus be regarded as a Cp, ‘sandwich’ centered by a triatomic cluster. The germanium compound differs from the tin compound by the asymmetric cyclopentadienyl bridge, which becomes symmetrical in the tin derivative. The Ge-Ni distances thus differ considerably (2.085(3) and 2.258(3)A) while the Sn-Ni distances are almost equal (2.326(2) and 2.369(2)A). The metal-metal distances are very short as may be seen from the comparison of the bond lengths in 22 and 23 (see above).
Trimetallic clusters with bis(amin0)germylenes and stannylenes As a consequence of our versatile assemblage routes, which had so far led only to clusters with two different metallic elements, we thought it to be useful to apply this strategy to obtain clusters with three different metallic elements. In collaboration with P. Braunstein and M. Knorr we have explored the reaction of heterodinuclear complexes, stabilized by the ‘diphos’ ligand and with a Si(OMe)3group attached to iron, with 1 and 2, the results being illustrated in Eqs. (16), (17) and (18).[2x1
84
1 Molecular Clusters
P h 3 e Pt
Fe(C0)3
~
+ MqSi(NtSu),M
-----)
Ph3-F-'
I
H
Si(OMe)3
phcx'.'"
Fe(C0)3 + {H-Si(OMe)3}
(16)
'd
M = Gem, Sn (2) Si Me2 M = Ge (23,Sn (29) X = CH2, NH
ph2ri"'ph[Ar Fe(C0)3 + Me2Si(NtBu)2Ge--t Me$i(Nm~)~Ge-p
Ph3+Pt-
Fe(C0)3 + PPh,
\/
\/
A tSu-N\r-tBu
71
mu-r TrnU Si
Si Me2
Me2 29
30
phrr
Me-M
*\
/ Me
,e(CO)3+ Me2Si(NtBu)2M
0-
(17)
Si(OMe)2
-----)
' " ' I *"' Me-T-r (18)
Me2siwBu)2\
/si(OMe),
0 M
= Ge
(I), Sn (2)
Me
M = Ge, M' = Pd (30) M = Sn, M' = Pd (31) M = S n , M'=F't(32)
In Eq. (16) a ligand on Pt and on Fe ( H and Si(OMe)3)is completely displaced by a bridging main group metaldi(amide): the compounds 28 and 29 have been fully characterized by several methods, multinuclear NMR being the most diagnostic. So
85
1.5 Hetero-Metul Clustrcs
far we have not analysed the products which result from the formally displaced trimethoxysilane. Nevertheless. in the two cases a trimetallic cyclic cluster is obtained with three different metal atoms. Unfortunately we were not able to add a further stannylene ligand to 28 by displacing the triphenylphosphine group. As illustrated in Eq. (17) this reaction also occurs with the bis(amino)germylene (1) leading to the tetra-metallic compound 30 which contains three different metallic elements. Another synthetic route to trimetallic clusters is shown in Eq. (1 8): here a formal insertion of the germylene 1 and stannylene 2 into the 0-Pd( Pt) bond takes place and the metal clusters 31, 32 and 33 are obtained. An X-ray crystal structure determination of 31 reveals a rather short Ge-Pd bond (2.36(5)A)and a remarkably long Fe-Pd bond (2.74(6)A). Variable temperature NMR spectroscopy on 31, 32 and 33 is consistent with a rapid exchange of the methoxy groups on germanium and tin, respectively. In summary, we have synthesized various heterobimetallic and heterotrimetallic clusters. The different structural motifs of these clusters are shown in Fig. 7 for general comparison (the ligand on the main group metals M is exclusively
M/ M \ M
RhSn, (19) NiGe, (26) NiSn, (27)
&CO, (10) SnCo, (11) GeFe, (12)
FeSn,Fe
GePdFe (31) SnPdFe (32) SnPtFe (33)
PtFeGe (28) FVFeSn (29)
CrSn,
(13)
(3)
\M PtFeGe, (30)
MnzGe, (4) Mn,Sn, (5)
M M
M
.M, M
NiSn,
I
\M
M’ ’ M
M
(22)
RhGe,
‘M (18)
NiSn, (23)
PdSn, (24) PtSn, (25)
Pd,Sn,
(21)
I
M
M
RhSn,
(20)
NiSn, (35)
Figure 7. General structural motifs of metal atoms in clusters using M( NtBuiZSiMel [M = G e ( I ) , Sn (2)jas “ligands” to transition metals (M’). The numbers in parcntheses refer to the molecules specified in the text.
86
1 Molecular Clusters
Ligand reactions of amino substituted heterometal clusters With the syntheses of metal complexes of the bis(amino)germylene, 1, and -stannylene, 2, we have made available a huge number of different metal clusters (see above). We have also begun to study the properties of these metal centered molecules in view of addition and substitution reactions that occur without destroying the central metal part of these molecules. Our first results in this area are outlined in this chapter. All the reactions have the homoleptic compound Ni[Sn(NtBu)2SiMe2]4,22, as the starting material. The first attempt was to oxidize 22 using simple oxidation reagents such as Br2 or PhMe3NBr3. Compound 22 is very sensitive to oxidation and reacts immediately with elemental halogens. As the reactions proceed very quickly we were not able to isolate pure products, and a mixture of different oxidized species as shown in Eq. (19) was obtained.'241
+
[Me2Si(NtB~)~Sn]4NiX2 22
+ NiBr2
+ SnBr2 +
x Me2 Si(N tBu)zSn(NtBu)2SiMe2 t
+
y [Me2Si(NtBu)2Sn]4NiBr2 z Me2Si(NtBu),SnBr2 23
(19)
X2 = Br2, PhMe3NBr3
The formation of the compounds NiBrz and Me&( NtBu)2SnBr2 indicates that oxidation has proceeded too far and has led to an irreversible destruction of the cluster. MezSi(NtBu)zSn(NtBu)2SiMe2 and SnBr2 are known to originate from the reaction of Me2Si(NtBu)zSnBrz with the bis(amino)stannylene 2. The only product, which still has the NiSn4 cluster unit in the molecular skeleton, is [MezSi(NtBu)2Sn]4NiBr2 which by comparison of its spectral data with those of 23, which is obtained by an alternative route (see above), can be easily characterized. Thus, it has been shown that an oxidation of 22 to a small extent is possible without destroying the cluster. It is important to note that this oxidation is accompanied by a structural change of the cluster and the tetrahedral Sn4Ni core of the molecule 22 changes to an almost planar Sn4Ni arrangement, which is typical of a formal Ni2+ with ds electron configuration. We believed that replacement of the amino ligands on the nickel complex 22 by alkoxo or siloxo ligands is feasible. Nevertheless, the treatment of 22 with alcohols poses in itself problems and the steric bulk of the groups attached to the hydroxyl moiety plays an important role. The type of products which can be obtained are shown in Eqs. (20)-(22).'291
1.5 Hetero-Metul Clusters
H
+
I
[MezSi(NtBu)zSn]4Ni 4 CH3OH 22
+
[MezSi(NtBu)2Sn]4Ni niPrOH 22
+
87
OCH3 I
[Me2Si(NtBu)(NtBu)Sn]4Ni (20) 34
x Mez[NtBu(H)]z
4
H I
0-iPr I
+ y [MezSi(NtBu)2Sn]2[Me2Si(NtBu)(NtBu)Sn] [(iPrO)zSn]2Ni
(21)
35 [Me2Si(NtBu)2Sn]4Ni+ 8 Ph3SiOH +4
[(Ph3SiO)zSn]2+ 4 Me$3[NtBu(H)]2
+ Ni
(22)
The reaction in Eq. (22) is an example of the complete destruction of the original cluster; the amine ligands on tin were replaced by tri( pheny1)siloxy groups with quantitative formation of the amine MezSi[NtBu(H)]2, but the nickel-tin bonds are lost and bond fission was observed. Besides the formation of elemental tin, the dinuclear complex [( Ph3SikO)zSnlz was obtained as a white crystalline product and characterized unambiguously by an X-ray structure determination. [291 While tert-butanol does not react with 22 (presumably due to the steric bulk of the tert-butyl group), methanol reacts smoothly to form the tetra-alcohol adduct 34 in quantitative yield (Eq. (20)).The molecule 34 crystallizes in a tetragonal space group and its molecular structure is depicted in Fig. 8. The molecule has S4 crystal
Figure 8. The molecular structure (S4 symmetry) of Ni[Sn(OMe)(NtBu){N(H)tBu}SiMe& (34) from an X-ray structure determination.12y1 Hydrogen atoms are omitted and carbon atoms are not labeled. There are hydrogen bridges (0. .N = 2.89( 1) A) between the oxygen atoms and the nitrogen atoms N( 1 ) which in contrast to N(2) are pyramidally coordinated.
88
1 Moleculur Cluster3
b
Figure 9. The molecular structure of [MezSi(NtBu)2Sn]2[Me2Si(NtBu)(NtBu{H})Sn][(iPrO)2Sn]zNi (35) as determined by X-ray diffraction techniques. 1291 For further details see caption to Fig. 1 and text.
point symmetry. On each of the four Sn(NtBu)2SiMe2 ligands of 22 one methanol molecule has been added in such a way that the methanolate ligand is coordinated to the tin center increasing the coordination number of the tin atoms to four, and the proton has been trapped by one of the two nitrogen atoms in the ring. As can be deduced from inspection of the Fig. 8 an oxygen...nitrogen hydrogen bond is clearly distinguishable (0.. .N = 2.89( 1) A) between an alcoholate group and a nitrogen atom of neighboring bis(amino)stannylene ligand. Compared to 22 the Ni-Sn distances in 34 are elongated by 0.02 A (Sn-Ni in 23: 2.463(2)A). This is in accord with an increase in the coordination number at the tin atom from 3 to 4. A complex structure is obtained from the reaction of the nickel stannylene cluster 22 with iso-propanol. In this case, the compound 35 can be isolated in reasonable yields and the structure of the molecule as found by an X-ray diffraction analysis is shown in Fig. 9. The central nickel atom is coordinated by five tin atoms in a very distorted trigonal bipyramidal arrangement. Two of the tin atoms still have an unchanged MezSi(NtBu)z ligand and one further ligand has a proton coordinated to one of the ring nitrogen atoms resulting in a MezSi(NtBu)[NtBu(H)] unit. The latter tin (Snl) and Sn3 are connected by an iso-propyloxy group. Finally, there are two tin atoms (Sn4 and Sn5) which have exclusively iso-propoxy ligands coordinated to them. The amino ligand on Sn4 and Sn5 have been completely substituted by alkoxo ligands. The question arises as to why Ni(0) should have five stannylene ligands? By examining the tin-nickel bond distances in more detail a very surprising answer can be given to this question. Whereas the Ni-Sn distances from Ni to Snl, Sn2, Sn3 and Sn5 range from 2.456(2)-2.465(2)A (all tin atoms being tetra-coordinate) the distance to Sn(4), which is the only tin atom with three bonds to either nickel or oxygen, is found to be 2.623(2)A. Furthermore, the coordination polyhe-
1.5 Hetero-Metal Clusters
89
dron around Sn4 is trigonal pyramidal with acute Ni-Sn(4)-0 angles of about 86" and an 0-Sn-0 angle of 94.2'. We therefore propose the following description of N( H)tBu)Sn][(iPr0)2Sn]Ni the cluster 35: to [Me?Si(NtBu)lSn]2[Me2Si(NtBu)( a Sn(0iPr)z molecule is added in such a way that two oxygen atoms ( 0 2 and 0 3 ) coordinate to two different tin centers (Sn2 and Sn5) while Ni, through a filled dz2 orbital, coordinates to Sn4, which uses its empty pz-orbital as acceptor (dn - pn bonding). This description would explain the geometry of the cluster as well as the trigonal pyramidal geometry around the tin atom Sn(4).
1.5.3 Conclusion We have been able to demonstrate that the simple concept of assembling clusters by facile addition reactions of carbene analogues to transition metals is working well. It leads to a variety of different clusters (see Fig. 7). Because of the reactive properties of the amido substituted carbene analogues, addition and substitution reactions may easily be performed. We are currently exploring this field in more detail. The results might also be of significance for catalysis of 0x0 derivatives of metal clusters because of their reduced chemical reactivity, which would be of general interest. Another aspect could be to use clusters terminated by 0x0 ligands as precursors for novel metal oxides.
References / I ] M. Veith, Chmi. Rec. 1990, YO, 3. 121 M. Veith, S. Weidner, K . Kunze. D. Kiifer, J. Hans, V. Huch, Coord, Chem. Rev. 1994, 137, 297. 131 M. Veith, C. Mathur. V. Huch, J. Cheni. Soc.. Dulton Trun.~.1997. 995. 141 M. F. Lappert, R. S. Rowe. Coord. Chern. Rev. 1990. IOO, 267. [5] T. B. Hitchcock, M. F. Lappert. M. J. McGeary. Organomefu//ics 1990, 9, 884. [6] L. Grocholl, V. Huch. L. Stahl, R. J. Samples, P. Steinhart. A. Johnson, Inorg. Cheni. 1997, 36, 4451. [7] M. Veith, Muyrczin Forschung, Universitiit des Saarlandes, 1992, I . 33. [S] M. Veith, H. Lange. K. Brauer. R. Bachmann. J. Oryunornef. Chcni. 1981. 216, 377. [9] W. Stohmeier. D. Von Hobe, Chem. Bcv. 1961, 94. 2031. [lo] M. Veith, M. Grosser, Z. Nufu$rsch. 1982, 37h, 1375. [l 11 M. Veith, L. Stahl, V. Huch. unpublished data. [ M e ~ S i ( N t B u ) ~ S i i ] ~ C r ( C3. O)~, CixH72CrNh03Si3S113, 1093.3 g/mol; monoclinic, space group C2/c, ~1 = 34.403(7), h = 18.260(4). c = 26.147(5)[A], /l = 113.68(3); V = 15043(5) A', Z = 12, Siemens AED 4-circle ~ parameters, R I = 0.042 diffractometer. 7858 reflections (5206 with F > 2 1 7 ~ )665 (wR? = O . l I l j , CCDC 118887.'"'
90
1 Molecular Clusters
[I 21 Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no CCDC118887-1 18892. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ UK [Fax: int. Code +44(1223)336-033; E.mail
[email protected]]. 1131 M. Veith, M. Olbrich, M. Notzel, C. Klein, L. Stahl, V. Huch, unpublished results. [Me$( NtBu)&e]2Mn2(C0)8, 4, C28H48GezMn2N408Si2, 879.9 g/mol; monochic, space group P21/n, u = 8.876(5), b = 11.703(6), c = 18.858(1])[A], 8, = 91.91(2)"; V = 1958(2)A, Z = 2, Siemens AED 4-circle diffractometer, 2833 reflections (2528 with F > 2 a ~ . ) ,233 parameters, R = 0.0349 (R, = 0.0332) CCDC 118888.L'21[Me2Si(NtBu)2Sn]2Mn2(co)8, 5 , C28H48Mn2N4OsSi2Sn2, 972.14 g/mol; monoclinic, space group P21/n, a = 8.899(8), b = 11.940(9), c = 19.188(10)[A], p = 92.75(3)"; V = 2037(3)A3, Z = 2, Siemens AED 4, parameters, R1 = 0.030 circle diffractometer, 3570 reflections (3282 with I > 2 0 ( ~ ) ) 209 (wR2 = 0.086), CCDC 118889.['21[MezSi(NtBu)2Sn]z [Fe(C0)4]2,9, C28H48Fe2N408Si2Sn2, 973.96g/mol; monoclinic, space group P2l/n, a = 8.959(2), b = 12.033(2), c = 19.042(4)[A], p = 91.80(3)"; V = 2051.8(7) A3, Z = 2, Siemens AED 4 circle diffractometer, 1570 reflections 206 parameters, R1 = 0.056 (wR2 = 0.1518) CCDC 1 18890.L'21 (1436 with F > ~CJF), [I41 K. Triplett, M. D. Curtis, J. Amer. Chem. Soc. 1975, 97, 5747. [I51 L. F. Dahl, R. E. Rundle, Acta Crystullogr. 1963, 16, 419. [16] P. G. Harrison, T. J. King, J. A. Richards, J. Chem. Soc., Dalton Truns. 1975, 16, 419. [I71 M. Veith, Z. Naturforsch. 1978, 33b, 7. [IS] M. Veith, M. Olbrich, C. Klein, unpublished data. MezSi(NtBu)lSn[Co(C0)4]2, 11, C I8H24Co2N208SiSn, 661.03 g/mol; triclinic, space group = 97.89(3), P-I, u = 8.971(2), b = 17.872(4), c = lS.688(4)[A], CI = 110.82(3), y = 97.57(3)["],V = 2720(1) A', Z = 4, Image Plate System (STOE-IPDS), 8146 reflections , parameters, R1 = 0.057 (wR2 = 0.167), CCDC 1 18891."21 (7570 with F > 2 a ~ . )577 [MezSi(NtBu)2Sn]2[Co(CO)cp]2,17, C ~ ~ H ~ ~ C O ~ N ~ O (crystallies ~ S ~ ~ Sas~a ~toluene .C,H ad-~ duct), 1034,44g/mol; monoclinic, space group C2/m, u = 14.704(11), b = 17.857(11), c = 9.181(7)[A], ,8 = 101.98(6)", V = 2358(3)A3, Z = 2, Siemens AED diffractometer, 1600 . ) ,1 parameters, R I = 0.024 ( w R ~ = 0.073) CCDC 1 18892.r'21 reflections (1572 with F > 2 ~ 7 ~11 [I91 K. M. Mackay, B. K. Nicholson, M. Service, Actu Crystallogr. 1990, C46, 1759. [20] P. Klufers, Z. Naturforsch. 1991, 46b, 187. [21] K. Merzweiler, H. Kraus, L. Weisse, Z. Nutuvfbrsch. 1993, 48B, 287. [22] M. Veith, L. Stahl, V. Huch, Orgunometallics 1993, 12, 1914. [23] M. Veith, L. Stahl, V. Huch, Inorg. Chem. 1989, 28, 3278. [24] M. Veith, A. Muller, L. Stahl, M. Notzel, M. Jarczyk, V. Huch, Inorg. Chem. 1996, 35, 3848. [25] G. W. Bushnell, D. T. Eadie, A. Pidcock, A. R. Sam, R. D. Holmes-Smith, S. R. Stobard, J. Amer. Chem. Soc. 1982, 104, 5837. 1261 M. Veith. L. Stahl, V. Huch, J. Chem. Soc., Chem. Comm. 1990, 359. [27] M. Veith, L. Stahi, Angew. Chem. 1993, 105, 123; Angew. Chem. Int. Ed. Engl. 1993, 32, 106. [28] M. Knorr, E. Hallauer, V. Huch, M. Veith, P. Braunstein, Orgunometullics 1996, 15, 3868. [29] M. Veith, M. Jarczyk, V. Huch, M. Opsolder, to be published.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.6 Synthetic Pathways to a Neglected Class of Compounds: Organobimetallics of Aluminum and Cobalt Jovy J. Schneider
1.6.1. Introduction Preparative organometallic chemistry of mixed main group metal (mgm)-transition metal (tm) compounds has recently grown significantly and spans nearly the whole range of possible element combinations of tms and mgms. Although there are many examples of these bimetallic complexes, and some have been obtained via unexpected synthetic routes, certain combinations of mixed tm/mgm complexes are still rare due to the lack of synthetic pathways to these compounds. Hence, some tm/ mgm combinations have not yet been explored in detail, or more importantly, have not even been synthesized and characterized. One of these tm/mgm combinations is Co/AI, which is the main topic of this article. To unravel new and yet unexplored synthetic pathways for these types of complexes, is a vital task for the experimentalist working in the field. From a qualitative theoretical point of view a description of chemical bonding in mixed tm/mgm complexes, in which the mgm acts just as a simple donor ligand within a bimetallic complex, is often appropriate. However, when the nuclearity of the compounds increases the mgms become more and more incorporated in the overall description of chemical bonding of bimetallic tm/mgm systems. The description of the mgm as a simple donor atom then no longer holds. Intriguing synthetic examples that lend support to this more extended view of the mgm/tm bonding in bimetallic complexes have been reported."] Within this article we will focus on new synthetic pathways, which have, for the first time, provided routes to the synthesis and structural characterization of organobimetallic Co/Al-complexes exhibiting unusual and even, from a theoretical point of view, interesting bonding arrangements between A1 and Co. First of all, to put the experimental results presented in this account in a broader context, a brief insight into synthetic routes, which allow for the selective formation of tm bonds to heavier group 13 elements (Ga, In, T1) in bimetallic complexes, will be given. This
92
1 Molecular Clusters
will be followed by a brief discussion of synthetic methodologies leading to the preparation of the first and second row group 13 congeners, B and Al. However, within the limited space of a multitopic cluster book a review of previous work is not necessarily intended to be comprehensive but rather should make the reader aware of developments and trends, so that he is able to arrange and classify results presented within the subject area. Our own synthetic work in the field of Co/Al organobimetallic complexes is presented within this context. In a concluding section possible applications of organobimetallic complexes containing tm/group 13 element combinations and trends for future work in this area will be discussed.
1.6.2 Synthesis of mixed group 13/transition metal complexes 1.6.2.1 Transition metal bonds to Ga, In and T1 A huge number of different synthetic approaches to generate mgm/tm bonds in organometallic complexes exist. The most widely applicable can be categorized as follows: I. Elimination reactions: i) with extrusion of an alkali metal salt ii) with extrusion of gaseous by-products iii) with extrusion of neutral non-gaseous molecules iv) bimolecular elimination reactions 11. Direct reaction of mgm with tm compounds or bare tm Alternative methods, which are not explicitly mentioned above, exist and lead to other mgm/tm combinations. These have been the subject of several excellent reviews.L21 I. Elimination reactions i) with extrusion of an alkali metal saltL3]
+
Na[Co(C0)4] EX3 + [E{Co(C0)4}3] (E = In, T1, X
+ NaX
= Br, C1)
The salt elimination route is widely applicable to a huge variety of tm/mgm complexes.
1.6 Synthetic P a t h a y s to a Neglected Cluss of Conzpounds
93
ii) with extrusion of gaseous by-product^[^'
+
+
3 [HMoCp(C0)3] 3 [Me3Ga] + [Ga{M o C ~ ( C O ) ~ } 3~CH4 ]
(2)
I
iii) with extrusion of neutral, non gaseous molecules[51
Thermodynamically stable by-products are released in these two reactions and allow for the high yield formation of mixed tm/mgm complexes. iv) bimolecular elimination reactions This synthetically promising type of reaction can be described by the general reaction sequence shown in Eq. (4). R MR+L,M'
R
= alkyl,
M
I
--f
L,MM'
= mgm
compound, L,M'
(4) = tm
compound
94
1 Molecular Clusters
A number of examples of the addition of alkyltin,[61or --leadF7]compounds to give organoplatinum complexes have been reported. However, examples of this type of reaction leading to the formation of tm/group 13 elements are rare and its general utility has not been established so far (see Eq. 5 and 6).[81
-PMe3
[IrMe(PMed4]
+
[BrlnMed
Me I ,.$Me3 Me- lr -1nMeBr
benzene
orether
Me3P
'I
or
Me I .,,\Me Me3P- Ir -1nMeBr Me3P'I
(benzene)
-PMe3
[IrMe(PMe3)4] + [InMe3]
PMe3
PMe3
(ether)
Me
I *,\\\Me Me3P- li-lnMe2 4 Me3P I PMe3
Me
1 %\,\& pMe3
+
Me-
4
Me3P
Ir'-lnMe2
I
PMe3
11. Direct reaction of mgm with tm compounds or bare tm. In contrast to group 14, 15, and 16 elements, which have been introduced in a number of cases as naked mgms or elements,['] usage of bare group 13 metals for the preparation of mixed group 13/tm complexes is not widespread. Nevertheless, as early as in 1942, Hieber showed that bulk group 13 metals and a finely divided tm can be combined directly together in the presence of CO to form mixed mgm/Co clusters (mgm = Ga, In, Tl).[91These compounds represent the first examples of this reaction type.
M = Ga, In , TI
3
Homonuclear unbridged tm bonds exhibit a high reactivity towards insertion or addition reactions when the ligand periphery is sterically crowded. This results in a
1.6 Synthetic Puthwuys to u Neglected Class of Compounds
95
substantial weakening of the tm (M-M) bond, an increase in reactivity, and consequently a homolytic scission of the particular tm (M-M) bond and an insertion of metallic Ga (Eq. 8)."01
4
1.6.2.2 Transition metal (tm) bonds to B and A1 tm-B bonds Although known since 1963," the first crystallographic characterization of tm to boron o-bonds was not reported until 1990."21 Synthetic pathways to compounds containing o-bonds between tms and boron have been reviewed and categorized according to the elements which are bonded to boron.[131 i) compounds containing halogen substituted B to tm-o-bonds:
BC13
+ Na[MCp(C0)3]
M
Mo,W
=
+
[C12BpMCp(C0)3] NaCl
4
(9)
Similar salt elimination between various other anionic tm organometallics ( Ni, Fe, Co) and BC13 can lead to crystalline complexes containing direct tm-B bonds." 31 ii) compounds containing carbon-substituted B to tm-o-bonds: Schmid and Noth have synthesized a variety of compounds containing B-tm bonds (tm = Mn, Mo) by another general reaction sequence (Eq.
+
+
R2BC1 Na[M(CO)3R*]4 R2BpM(CO)3R* NaCl R
= various
alkyl substituents
R* = Cp (M = Mo), CO(PPh3) (M = Mn)
(10)
96
I Molecular Clusters
However, this early work lacked a crystallographic characterization of tm-B bonds in such borylene metal complexes, which might be responsible for the fact that this work had fallen into oblivion until recently." Synthesis and X-ray crystallographic characterization of the first bridged borylene tm complexes in which BR fragments bridge two metal centers were reported more then 30 years after this early ground laying work in tm/borane chemistry (Eq. 1 l).[lG1 r
2K
R
x
dR I
X = NMe2, f-Bu
R = H, Me, Me
tm-A1 bonds Compared to boron, complexes with aluminum/tm bonds are scarce. Nevertheless, several reaction pathways leading to the synthesis of these compounds exist. i) via reaction of tm carbonyl anions with neutral organo aluminium compounds:[171
5
This reaction has led to the formation of the first structurally characterized Fe-A1 bond. Because of the basic character of the carbonyl fragment, which is reflected in the v(C0) IR stretching frequency, and the soft basic characteristics, attributed to the low oxidation state of the tm atom, tm-mgm bonding is most attractive towards
1.6 Synthetic Patlzwciys to u Neylectetl Clu.vs oJ Compounds
97
the relatively soft group 13 Lewis acid [(C6Hs)iAl]and is responsible for the formation of direct Fe-A1 bonding in this complex.['*] [ClAl{(CH2)3NMe?}(i-Bu)] +
+ [CpFe(CO)?]-
[Cp(CO)?Fe-Al((CH*)3NMe*(i-Bu)}]
(13)
6
Taking this knowledge into account, the same principle coupled with a kinetically hindered situation around the A1 center (Eq. 13, 6) has afforded the synthesis of a number of transition metal substituted alanes with direct tm-mgm bonding thus, opening a new synthetic route into the field of tm/Al bimetallic compounds." 9h1 The general trends, exemplified by Eq. (12) and Eq. (13), can be rationalized through the principle of hard and soft acids and bases."9c1 Tuning the electronic situation at the acidic and basic sites is the key to tm-mgm bonding in these type of complexes. ii) with extrusion of neutral gaseous by-products As early as 1971, Grcen and coworkers reported that reaction of tm hydride complexes with trimethylaluminium results in the formation of the first structurally characterized organobimetallics with direct tm-A1 bonds ( Eq. 14).['01 [(q5-Cp)2MoH2]+ AIMe3
70' C
toluene
98
1 Molecular Clusters
iii) via salt
7
The complex [Al{Co(C0)3)3]was reported more then 25 years ago by Schwarzhans and Keller and described as an a intriguing tetrahedral bimetallic Co/Al cluster complex with direct Co-A1 bonding bearing an unusual naked ligand free A1 atom (7). Independent work to reproduce these results had been, so far, unsuccessful. We, like others working in this field, were not able to reproduce their work.[',221Repeated attempts to follow Schwarzhans' procedure gave predominantly [Co4(CO)12].However, in addition a minor product was found, which indeed turned out to be a Co/Al cluster 8, but one in which the A1 is coordinated via the carbonyl oxygen rather than via a direct bond to the transition metal center (Fig. 1). For this type of complexation two characteristic vco stretching frequencies Analogous ~J M-O=C-M isocarbonyl at 1555 and 1405 cm are o b ~ e r v e d . [ ~
-'
Figure 1. Molecular structure of [C04(CO)i 3 N T H F h CO?(CO)101 8 in the solid state determined by X-ray crystallography.[231 The Co3(CO)9cluster fragment of 8 attached to CSOhas been omitted for clarity owing to crystallographic disorder of this part of the
1.6 Synthetic Patliwclys to a Neglected Class of Compounds
99
bridges are found for early/late transition metal complexes (Co-C=O-Ti at 1268, 1233 cm-1,1241Co-C=O-Zr at 1373 cm-1[24J)and in Al/W complexes with Al-O=C-W bridges ( 1 540 cm-’).[25J A similar coordination environment to that found for A1 as in 8 is found in 9, which is formed as the product of a metal exchange r e a ~ t i o n : ” ~ ] 3 [Hg{WCP(C0)3)21
+ 2A1
+
2 [Al{WCP(C0)3l(THF)31
(16)
/\
The reason for isocarbonyl bridging rather than for Al-tm bonding in these complexes can be found in the interaction of the small and therefore hard group 13 metal A1 with the CO group. The A1 is better suited to the harder basic site in the tm compound, and this is the carbonyl oxygen rather than the metal center. iv) via reaction of {RAl} alanediyl fragments with tm-organometallics:r26J
+
3 [(Me5Cs)AI] 2 [CpzNi] +
[{CpNiAl(MesCs))2]
+ “ [ ( M ~ ~ C S ) C ~ , A I ] ”(17)
This interesting reaction uses a high temperature technique employing free A1 atoms to generate low valent {Al(Me5Cg)) fragments, which are metastable in etheral solution at -78 “C. Reactive alanediyl fragments may attack the Lewis acidic Ni atom of [CpzNi] via insertion into the Cp-Ni bond and formation of mixed Al/Ni-organobimetallic
10
100
1 Molecular Cluster5
1.6.3 Synthetic routes to organobimetallic compounds of Co and A1 As has been shown in the preceding section, so far only a few heteronuclear organometallic compounds with transition metal-aluminium bonds have been synthesized and characterized unequivocally. Before we started our work in the area of Co/A1 organometallics, apart from the solid state intermetallic phases like Co2Alj and CoZA19, no transition metal complexes with discrete Co-A1 bonds were known. Our entry into this chemistry originally stemmed from an interest in the synthesis of homonuclear hydridocobalt clusters.[271In the course of these studies we have learned that diethylaluminum hydride reacts with the bis ethylene complex 11 to form a series of homonuclear paramagnetic clusters 12, 13, and 14 when the initial reaction mixture is quenched with ethanol at -78 "C (Scheme 1).[281 However, an aprotic workup of such a reaction mixture yields the first Co/Al cluster complexes (Scheme 1, Fig. 2 and 3).["] Since gaseous by-products are formed, this reaction route can be classified into category I.ii. Reaction of diethylaluminiumhydride with 11 can be considered as an addition reaction of a Lewis acid complex to a coordinately unsaturated alkene. This type of addition is usually found either for mgm[291or tm complexes[301alone, and represents a synthetically useful route to the formation of dimetallacycloalkanes of mgms or t m ~ . [ ~To ' ] the best of our knowledge, the formation of 15 (CjMe5) and 17 (CjMe4Et) in Scheme 1 shows, for the first time, that mgms and tms can both coordinate to alkenes simultaneously. The length of the olefinic C=C bond in 17 is comparable with that of other dimetalated olefins and is only slightly different from that in free ethene (1.363(5) us. 1.34 A). This indicates a stronger contribution of the alkene-o-donor portion than that of the metal-a-n-acceptor portion to the bond of the Co-dimetallioalkene molecular fragment in 17. An almost ideal planar ordering of all four atoms C1, C1*, A1 and Al* is achieved by the dialuminioolefin unit in 17. This indicates that no significant rehybridization of the sp2 carbon atoms CI and C1" towards sp3 hybridization occurs in the formation of the dialuminioethene unit.["] At that point we turned our attention to the use of the EXAFS technique (Extended X-ray Absorption Fine Structure) which can provide valuable structural information on matter regardless of whether it is in the solid- or solution state. Already widely employed as analytical tool in solid state and material sciences,[321the method has only been employed in a few cases to analyze crystalline organometallic compounds. Our EXAFS measurements on 15 and its Me4EtC5 derivative 17 were carried out at the Co-K edge under an argon atmosphere at the ROMO I1 station at the Hamburger Synchrothronstrahlungslabor HASYLAB at DESY.[''] The Fourier transforms of the k3-weighted X(k) functions reveal two coordination shells for the
1.6 Syntlirtic Path\iq>.s to a NrcqltJcted CIas.s of Compounds
6
P
+
5 9
5 t
v
I
+
I
I
10 1
102
1 Molecular Clusters
Figure 2. Molecular structure of 17 in the solid state determined by X-ray crystallography.[281Selected bond lengths [A] and angles ["I: Co-A1 2.569, CO-C1 1.992(4),CO-CI* 1.986(4), Al-Cl 2.088(4), Co-C6 2.030(4) and ref. [281.
Figure 3. Molecular structure of 16 as determined in the solid state as determined by X-ray crystallography.[281 Selected bond lengths [A] and angles ["I: AI-Al* 2.663(3), Co-Al 2.336(l ) , Co-Al* 2.333(I ) , Co-Al*-Co 110.4(I ) , AI*-Co-Al 69.6(1 ) and ref. [28].
1.6 Synthetic Pathwuys to a Neglected Class of Cornpowids
103
10 50 10 00 9 50
9 00 8 50 800
7 50 700
6 30
600
-
..
550
500 4 50
400
3 30
300 2 50 200 1 50
100
0 so
000 OM)
100
200
300
400
500
600
spectrum
of
r[Al
Figure 4. Fourier transform of the Co K-EXAFS d(Co,C)= 2 03 A, d(Co, Al) = 2 56 A. Fit index 0 024 12’]
15
Fit
parameter
Co atoms in 15 and 17. The first coordination shell is formed by C atoms at a distance of 2.06(1) A to Co, and the second coordination shell by A1 atoms at a distance of 2.56(3) A (15) and 2.59(3) A to Co (17). However, an interpretation of the remaining small intensity peaks in the Fourier transformed g(r) functions is not possible at the moment (Fig. 4 and 5). Considering the dominant main peaks at 2.061( 1 ) A due to carbon backscatterers and the interatomic distances, which can be retrieved from them, they are typical for Co-C5 ring contacts in which the metal atom is r5 coordinated. Therefore, these are assigned to the C O -C ~ R( R = Me5, EtMe4) distances in 15 and 17. Bonding distances obtained from single crystal X-ray analysis in compounds containing tm-A1 bonds are in the range of 2.456(1) A (Fe[”I)-2.974 A ( M O [ ~ ~The ] ) . second set of distances determined for 15 and 17 by EXAFS nicely fits into the lower part of that region, indicating direct Co-A1
104
I Molecular Clusters
I
000
100
200
300
400
500
600
rIAi
Figure 5. Fourier transform of the Co-K-EXAFS spectrum d jCo,C) = 2.03 A, d (Co, Alj 2.57 A, Fit index 0.021.[281
of 17. Fit
parameter:
bonding in 15 and 17. Therefore, EXAFS studies indicate that direct Co-A1 bonding must be present in 15 and 17. This shows the analytical value of such measurements in characterizing prototypes of organometallic compounds such as 15 and 17. As can be seen from Scheme 1, a second mixed tetranuclear organocobalt/ aluminum cluster 16, albeit in low yield, could be isolated as red-orange crystals from the product mixture of the reaction between 11 and [EtzAIH]. An essential feature of its molecular structure is a Co2Al~rectangle with a short Co-Aldistance (2.334 A),indicating direct Co-A1 bonding. With reference to the bimetallic phase [A17Telo] (Al-A1 2.660 and the organometallic complexes ([{CH(SiMe3)2}4A12](Al-A1 2.660( l)'""]), [Kz{Al(i-Bu))1 4 (AI-A1 2.685 A[351), and [{(r5-C,Me5)A1)4](Al-A1 2.773(4) A[361)the assumption of A1-A1 bonding in 16 would appear to be justified. This then should result in the formulation of 16 as
bicyclo[ 1.1.0]-1,3-dicobalta-2.,4-dialuminacyclobutanecontaining a tetracoordinated Al. However "counting" electrons using the EAN rule would leave the complex without any Al-Al bond. This exemplifies that care should be taken in using bond distances as absolute criteria for such compounds. Interannular bonding distances in planar four membered rings, as they are present in 16, are usually Reaction of the his ethylene precursor complex [(q5-Cp)Co(ethene),]with [(C?H5)2AIH]leads to formation of two other structurally different Co-A1 cluster compounds under similar reaction conditions, which have already favored formation of the CpR-substituted Co/AI clusters 15, 16 and 17.
18 Co-Co 2.55A p3-AI - CO 2.356 A Al - A l 2.686 A
19 Co-Co 2.44A p3-Al - Co 2.337 A pz-AI - Co 2.435A
Besides small amounts of the well known tetracobalt hydrido cluster [ { C ~ C O H ) ~ ] which , ' ~ ~ ]is the only product when the initial reaction mixture is quenched with ethanol, formation of two mixed bimetallic Co/AI clusters with different tm/mgm polyhedra are isolated when strictly aprotic conditions are maintained in this reaction. In 18 an octahedral cluster framework in which four {CpCo) fragments are bonded to two (AI(C2Hs)) units is formed. The closo structure of the cluster is completed by two hydrido ligands, which show up in the 'H-NMR spectrum but could not be detected by X-ray crystallography. N o information so far has been obtained as to whether the hydrido ligands are doubly or triply bridging the Co atoms, or both Co and A1 atoms. However, a purely Al bridging situation can be excluded judging from the observed chemical shift
106
I Molecular Clusters
(- 14.17 ppm) of the hydrido ligands, which is typical for hydride ligands bridging only tm’s. In 19 a Co3 triangle is doubly and triply bridged by a CAl(CzH5))and a {A1(C2H5)2}fragment. The “lost” C2H5 unit of the former Al(C2H5)2 fragment, which is now complexed as a ,uu,-ethylidyneunit on the opposite site of the Co3A12 cluster polyhedron of 19. In both complexes, 18 and 19, alkyl transfer reactions from the organoaluminum source are observed. This has already been observed in earlier work in organoaluminum chemistry. We have found that formation of all mixed Co/Al cluster compounds by the [(CpR)Co(C2H4)2]( R = H5, Me5, MeeEt) route depends critically on the organoaluminum source used in the reactions with this type of organocobalt compound. If diisobutylaluminumhydride (dibah) instead of [Et2AlH] is used, no formation of mixed Co/Al clusters is observed. In contrast to Scheme 1 and Eq. (18) the mixed hydrido ethylidyne Co3 cluster 20 is the only isolable product under analogous conditions (Eq. 19).
/”\#
/ /
+
-
80’ C [(i~o-butyl)~AIH] toluene
20
Obviously dibah serves as a source of hydrogen and allows for the decrease in formation temperature of 20 since formation of 20 is observed when 11 alone is thermolyzed in n-octane (l10°C).r391Hence, the source of hydrogen is the precoordinated ethylene in 11, whereas in reaction 19 it is obviously the more easily available hydrogen ligand of the organoaluminum source dibah, which drastically reduces the reaction temperature necessary to form 20.
1.6.4 Outlook and trends Without doubt, work remains to be carried out in order to gain a better understanding of the processes leading to the formation of unusual Co/Al complexes such as 15-19. However, the identification of new reaction pathways may make other
1.6 Syiitlietic P u t h w y s to a Neglected Class of Compounds
107
Co/Al bimetallics accessible, which will take this interesting class of compounds further out of oblivion. As a general result of the work presented here the Co/Al bond in organometallic complexes is accessible by conventional synthesis routes and is sufficiently stable to be studied in more detail in the future. Besides a fundamental interest in characterizing and understanding the reaction pathways, which leads to complexes with still unknown tm/mgm element combinations like Co/Al, there is a growing impetus for experimental work in the organometallic chemistry of bimetallic compounds, which is directed towards possible applications in material science. For specific tm/mgm combinations, one attractive potential application of such compounds is their usage as single source precursors in MOCVD processes. A number of different tm/mgm Combinations do in fact have intrinsic semiconducting properties[401and show promise in possible applications as bimetallic conducting films for semiconductors, which are important in microelectronic applications.[411 Binary mixed organometallic single source precursors have already shown that they can offer a route to pure bimetallic contacts (Al/Co, Al/Fe, Ga/Fe)[421.By taking this bimetallic single source concept further to other tm/mgm combinations and by tailoring the ligand environment to the special application demands. the field for new tm/mgm combinations, which is attractive in materials applications, holds promise for the future. Besides the potential of such complexes in material science, there is always the search for catalytic activity in mixed bimetallic complexes containing tm's and mgms. One promising example of this is the combination tm/group 13 metals, where bimetallics containing Ni and A1 are highly active in ethene dimerization reactions. These were studied for the first time 45 years ago by Ziegler and Holzand later their reactivity was mechanistically clarified by Wilke and coworker~.["~] Inspired by this work, the search for the catalytic active intermediates bearing direct Ni-A1 bonds in ethene dimerization reactions has been very active over the last few years.[451Nevertheless, only recently the first organobimetallic Ni-A1 complex 10, with a completely different ligand environment, was synthesized and structurally characterized, but so far its catalytic activity has not been investigated.12']
Acknowledgement The author would like to thank Prof. Dr. C. Kriiger and his coworkers at the MaxPlanck-Institut fur Kohlenforschung, Miilheim a.d. Ruhr for X-ray crystallographic investigations, the group of Prof. H. Bertagnolli, University of Stuttgart for making the EXAFS measurements possible, the DFG for a Heisenberg Fellowship, and the
108
1 Molecular Clusters
Fonds der Chemischen Industrie and ESF for additional financial support for the research presented here.
References [ 11 K.H. Whitmire, J. Coord, Chem. 1988, 17, 95. [2] see e.y. a) N.A. Compton, R.J. Errington, N.C. Norman, Adv. Orgunomet. Chem. 1990, 31, 91; b) D.G. Tuck, Comprehensive Orgunometallic Chemistry, Vol. 1 (G. Wilkinson, F.G.A. Stone, F. Abel, Eds.) Pergamon, Oxford, 1982, 954. [3] a) D.J. Patmore, W.A. Graham, Inorg. Chem. 1973, 5, 1536; b) W.R. Robinsonj D.P. Schussler, Inorg. Chem. 1973, 12, 848; ( c ) P. Braunstein, M. Knorr, M . Shampfer, A. DeCian, J. Fischer, J. Chem. Soc., Dalton Trans. 1994, 117. [4] a) J.N. Denis, W. Butler, M.D. Click, J.P. Oliver, J. Organornet. Clzem. 1977, 129, 1; b) A.J. Conway, P.B. Hitchcock, J.D. Smith, J. Clicm. Soc. Dalton Truns. 1975, 1945. [S] W. Uhl, M. Pohlmann, Organometallic.s, 1997, 16, 2478. [6] a) C. Eaborn, A. Pidcock. B.R. Steele, J. Cliem. Soc. Dalton Trans. 1976, 767; b) C. Eaborn, K. Kundu, A. Pidcock, ibid, 1981, 1223; c j T.A.K. AIAllaf, C. Edborn, K. Kundu, A. Pidcock, J. Chem. Soc. Chem. Cornmun. 1981, 55, T.A.K. All-Allaf, C. Butler, C. Eaborn, A. Pidcock, J. Organoniet. Chem. 1980, 188, 7. [7] J.K. Jawad, R.J. Puddephatt, Inorg. Chim. Acta; 1978, 31, L391. [8] D.C. Thorn, R.L. Harlow, J. Am. Chem. Soc., 1989, 11, 2575. [9] a) W. Hieber, U. Teller, Z. Anorg. Allg. Chem. 1942, 249, 43; b) H.J. Haupt, F. Neumann, H. Preut, J. Organomet. Chem. 1975, 99, 439. [lo] J.J. Schneider, U. Denninger, J. Hagen, C. Kriiger, D. Blaser, R. Boese, Chenz. Ber. 1997, 130, 1433. 1111 H. Noth, G. Schmid, Angeiu. Chem. 1963, 75, 861; Angew. Chem. Int. Ed. Engl. 1963, 2, 623. 1121 a) R.T. Baker, D.W. Ovenall, J.C. Calabrese, S.A. Westcott, N.J. Taylor, I.D. Williams, T.B. Marder, J. Am. Cliem. Soc. 1990, 112, 9399; b) J.R. Knorr, J.S. Merola, Organonietallics, 1990, 9, 3008. I131 G. Schmid, in Gmelin Handbuch der Anorganischen Chemie 19, Teil 3, 159, Springer Verlag, Berlin, 1975. [I41 a) G. Schmid, H. Noth, J. Orgunomet. Chem. 1967, 7, 129; b) H. Noth, G. Schmid, Z. Anorg. Allg. Chem. 1966, 345, 69. 1151 H. Wadepohl, Anyew. Cliem. 1997, 109, 2547; Angew. Clieni. Int. Ed Engl. 1997, 109, 2441. [I61 H. Braunschweig, T. Wagner, Angew. Chef??.1995, 107. 904; An(jeit.. Chem. Int. Ed Engl. 1995, 34. 825. H. Braunschweig, Angew. Chem. 1998, 110, 1882, Anyen.. Chem. Int. Ed. Engl. 1998, 37, 1786. [ 171 J.M. Burlitch, M.E. Leonowicz, R.B. Peterson, R.E. Hughes, Inorq. Chem. 1979, 18; 1097. [IS] R.A. Fischer, T. Priermeier, Orgunometallics 1994, 13, 4306. [I91 a) R.A. Fischer, J. Behm, T. Priermeier, W. Scherer, A n g c w . Chem. 1993, 105, 776; Anyew. C h ? . Int. Ed. Engl. 1993, 32, 746; b) J. WeiB, D. Stetzkamp. B. Nuber, R.A. Fischer, C. Boehme, G. Frenking, Anyew. Chem. 1997, 109, 95; Anyeit,. Clieni. Int. Ed. Enql. 1997, 36, 70; c ) R.G. Pearson, J. Chen?. Educ. 1968,45, 581, 643. [20] a) R.A. Forder, M.L.H. Green, R.E. Mac Kenzie, J.S. Poland, K. Prout, J. Chem. Soc. Chem. Conimun. 1973, 426; b) R.A. Forder, K. Prout, Acta Cry.rt. 1974, B30, 2312.
1.6 Syntlietic Putliwuys to a Neglected Cluss of Conipounds
109
1211 K.E. Schwarzhans, H. Steiger, Angcn~.Chenz. 1972, 84, 587, Angew. Chem. Int. Ed. Engl. 1972, 11. 535. 1221 G. Schmid, Anyew. Chmi. 1978, 90, 417; Angot,. Climi. Int. Ed. Engl. 1978, 17. 392. (231 J.J. Schneider, U. Denninger, C . Kriiger, Z. Nutu~fi)rsch.1994, 496, 1549. [24] B. Stutte. V. Biitzel, R . Boese: G. Schmid, Chenz. Ber. 1978, 11, 1603. 1251 R.B. Peterson. J.J. Stezowski. Che'ng Wan, J.M. Burlitch, R.E. Hughes, J. Am. Cheni. Soc. 1971. 93, 3532. 1261 C. Dohmeier, H. Krautscheid. H. Schnockel. Anyrw,. C h m . 1994, 106, 2570; Anyaw. Chern. Int. Ed. Engl. 1994, 33, 2482. 1271 a) J.J. Schneider, C. Kriiger, R. Goddard. S. Werner. B. Metz, Chem. Ber. 1991, 124, 301; b) J.J. Schneider, U . Specht. R . Goddard, C. Kriiger, Cliem. Ber. 1997, 130, 161: c) J.J. Schneider. Z. Nutzryforsch. 1994, 49h, 691. 1281 J.J. Schneider, C. Kriiger. M. Nolte. I. Abraham, T.S. Ertel, H. Bertagnoli. Anyew. Chem. 1994, 106, 2537; Angew Chem. Iiit. E d Engl. 1994, 33, 2435. 1291 a ) H. Schnockel. M. Leimkiihler, R. Lotz. R. Mattes. Angew. Cheni. 1986, 98, 929; Angeiv. Chiwi. b i t . Ed Enql. 1993. 32. 973; b) D.L. Thorn. R. Harlow, J. A m Clicm. Soc. 1989. 111, 2575. 1301 a ) F.A. Cotton, P.A. Kibala, Pol.vher/rori, 1987, 6, 645; b) F.A. Cotton, P.A. Kibala. Inory. Cheni. 1990, 29, 3192: c) W. Kaminski, J. Hopf. H . Sinn, H.J. Vollmcr, Angew. Chem. Int. Ed. Enyl. 1976, IS, 629; di W.J. Evans, T.A. Ullibari; J.W. Ziller, J. A m . Chm. Sue. 1990, 112, 219. 1311 a) W. Beck, B. Niemer, M. Wieser. Anyeiv. Chem. 1993, 105, 969; Anyen.. Chem. Int. Ed. Emql. 1993, 32, 923; b) D.L. Thorn. R.L. Harlow. J. Am. Chem. Soc.. 1989, 11, 2575. [321 see e g . J.W. Niemantsverdriet, Spectroscopy in Catalysis. VCH Verlagsgesellschaft Weinheim. Ch.6, 1993. 1331 R. Ncsper, J. Curda, Z. Nutivjhrsclz. 1987, 42h. 557. 134) W. Uhl, Z. Nrrturfi,rsch. 1989. 43h, I 113. [35] W. Hiller, K.W. Klinkhammer, W. Uhl. J. Wagner, Arzgew. Chern. 1991, 103, 182; Angeiv. Chivn. Int. Ed EngI. 1991. 30. 119. [36] C . Dohmeier, C. Robl. M. Tacke, H.G. Schnockel. Anyen. Chein. 1991. 103, 594; Anyen. C/ieni. Int. EN! Engl. 1991. 30, 179. [37] a ) E.A. Zarate. W.J. Youngs, C.A. Tessier-Youngs. J . Am. Chrm. Soc. 1988. 1 1 0 , 4068; b) J.W. Moore. D.A. Sanders. P.A. Scheer. M.D. Click. J.P. Oliver, J. Am. Chern. Soc. 1971, 93, 1035. 1381 J. Miiller. H. Dorner, A n p i t , . Clieni. 1973, 85, 867; Angeii,. C'lieni. Int. Ed. Engl. 1973, 12, 843. [39] a ) C.P. Casey, R.A. Widenhoefer, S.L. Hallenbeck, R.K. Hayashi, D.R. Powell, G.W. Smith, Or~~cinomettillit..(.. 1994. 13. 1521: b) R.B.A. Pardy. G.W. Smith, M.E. Vickers, J. Oryunomet. Cliiwi. 1983. 2.72. 341 . [401 aj C.M. Lukehart. S.B. Milne, S.R. Storck, R.D. Shull. J.E. Wittig in Nanotechnology (G.M. Chow, K.E. Gonsalves. Eds.), ACS Spposium Series, 1996, 622. 1411 b) R.A. Fischer, W. Scheer, M. Kleine. Anyew. C'hcm. 1993, 32. 778. Anqric,. Cheni. h t . Ed Encql. 1993, 32, 748 and references cited therein. 1421 R.A. Fischer, Chem. C'nserer Zeit 1995, 29. 141. 1431 K . Ziegler, Brcnn.c.tofflChe/iii. 1959. 40, 209. 1441 a) G. Wilke. Anyen,. Cheni. 1988. 100, 189, Anyeit,. C h ~ m Int. . Ed En(//. 1988, 27, 185; b) G . Wilke, N o w Actu Leopoldinu N F 65. 1991. 277. 105. [45] K.R. Porschke. W. Klcimann. Y.-H. Tsay. C. Kriiger, G. Wilke, Cheni. Ber.. 1990, 123, 12.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.7 Group 5 and 6 Bimetallic Complexes with Phosphido Bridges: Syntheses and some Structura1 Features Gilles Boni, Mavek M. Kubicki, and Claude Morse
1.7.1 Introduction The concept that the catalytic properties of a late transition metal are significantly altered by the presence of an early transition metal is responsible for the considerable interest in the synthesis of early/late heterobimetallic complexes. Much work has been directed towards the synthesis of this class of compounds and it has been found that the stability of the bimetallic framework is enhanced by the presence of a bridging ligand linking the two disparate metal sites. Our group is interested in a class of p-phosphido heterobinuclear complexes incorporating a group 5 or 6 metallocene derivative and a group 6 or 8 transition metal carbonyl moiety. Terminal phosphido transition metal complexes have been shown to be good precursors for their preparation. The aim of this article is to summarize some synthetic approaches and some structural features of such binuclear compounds.
1.7.2 Synthesis of metallophosphines Metathetical reactions are the common synthetic routes to phosphido derivatives of early transition metals: nucleophilic substitution of halide on the metal by anionic phosphido species has been widely used in group 4 chemistry."] M-C1
+
P-
___)
M-P
+
CI
Scheme 1
An alternative approach, based on a substitution of a halide in a halophosphine by a nucleophilic organometallic fragment, has been developed for d 2 mono-
I . 7 Croup 5 and 6 Bimetallic Complexes with Phosphido Bridges
1 11
hydrides of group 5 CpzM( H ) (L) ( M = Nb, Ta; L = two-electron donor)12' and d2 dihydrides of group 6 Cp2MH2 ( M = Mo, W).[31In a first step, metallophosphonium salts are formed: deprotonation of these salts is achieved by using the hydroxide anion and leads to the corresponding neutral phosphido derivatives. The regeneration of the salts is quantitative in reaction with HCl.
+
c p 2hl.
PRlCl
L
.-*
OH
H+
-
-. /
C-Or 7- l t l .
PR,
L
R = Me, Ph h1 = Nb, Ta ; L = CO, P h l 2 ~H. Phk2Ph M=Mo, W ; L = H Scheme 2
These reactions may be attributed to the basic properties of d2 mono- or dihydrido complexes, already exemplified in other reactions: i) with Bronsted acids
M = Nb, Ta ; L = CO, PMc2Ph h.1 = .110. w : L = H [5]
[4]
Scheme 3
In the cases of niobium and tantalum, two cationic structural isomers are detected depending both on the nature of the ligand L and the reaction conditions. ii) with Lewis acids The following addition compounds containing dative metal-metal bonds have been isolated and characterized.
Scheme 4
112
1 Moleculav Clusters
iii) with acyl chlorides Nucleophilic attack of the lone pair in d2 complexes has also been suggested to account for the formation of aldehydes when acyl chlorides are reacted with carbonyl complexes.[l01
0
CPN.
H
co
+
> RKC' 0 Co
M = Nb. Ta
1
Scheme 5
The reaction products are altered when the transformations outlined in Scheme 2 are carried out starting from phosphito derivatives [L = P(OMe)3 or PPh(0Me)zl. Phosphonato complexes are isolated either as the sole products ( M = Nb, R = Me; M = Ta, R = Ph) or together with the expected metallophosphines in variable yields ( M = Nb, R = Ph). A complete transformation from metallophosphine to the corresponding phosphonato complex occurs over 24 h.
and/or
Cp:bI,
PR'R"(0hIe)
PR'R" II
'
,
PR,
PRlhle
/
Cp$I,
hl=Nb,?'a
Scheme 6
Clearly an intramolecular Arbuzov-like rearrangement explains the formation of metallophosphonates. The phosphorus lone pair attacks the methyl group giving the dealkylation reaction and the conversion of phosphorus ( 111) to phosphorus (V).
Cp?hl,.
/P a
r/
,O-h!e
P
'R'
4
-
Cplbf,
/
PR2hle
PR'R" I1
h l = h b Ta
0
Scheme 7
Steric and electronic effects strongly influence the reaction rate. A fast rearrangement is observed with the more basic and less hindered PMe2 phosphido group." '1
1.7 Group 5 and 6 Bitnrtallic Complexes with Pliosphido Bridges
1 13
With the objective of making other kinds of metallophosphanes, the reaction outlined in Scheme 2 has been extended to dichlorophosphines. Depending on the nature of the basic reagent involved in deprotonation (OH- or OMe-), phosphonyl or phosphinite derivatives are recovered."
?il = Nb, Ta Scheme 8
The do trihydrides Cp2MH3 ( M = Nb, Ta) also give fast reactions with chlorophosphines leading to symmetrical ionic compounds. The central position of the PR2H group has been unambiguously established by ' H NMR spectroscopy." 14'
Scheme 9
Due to the lack of both a metal centred lone pair and an available coordination site, a nucleophilic substitution reaction is obviously inoperative here. Therefore, a direct insertion into the central M-H bond is responsible for the formation of the phosphonium salt. Such a reactivity has been explained recently, by Nikonov et al, in terms of a new and interesting concept, which considers the carbenoid nature of halophosphines." 5 1 Deprotonation of phosphonium salts can give rise either to phosphino or to phosphido derivatives." 31
Cp+4
/
\
/
H
bl = Nb ;R = Me, Ph M = Ta ; R = M e
Scheme 10
H Cp JICI - PR, \ H
PRiH
2(Nbl 2(Ta)
k l = Ta
R = Ph ; Cp = C j H j
R = Me : Cp = C+le-i
3CTa'
114
I Molecular Clusters
Thus, the initial phosphonium salts contain two competing acidic sites located at the metal and at the phosphorus atom, respectively. The strength of these acidic centers depends on the nature of the metal, on the Cp ligands, and on the electron donor capability of the phosphorus substituents. In the case of niobium, only phosphino derivatives are formcd. When M = Ta, R = Me, Cp = C5H5, a phosphino complex is also formed, whereas when R = Ph, Cp = CsHs or R = Me, Cp = C5Me5, phosphido compounds 3(Ta) are obtained. A similar competition occurs during the deprotonation of the molybdenum salts [ CpzMo( H)(PR2H)lf leading to either a Mo( 11) phosphino ( R = Me) or a Mo(IV) phosphido ( R = Ph) Metallodiphosphines may be obtained by using the same procedure as described earlier. These difunctionalized ligands contain a metal r~ bonded phosphido moiety associated with either a dangling phosphorus of a monocoordinated diph~sphine"~] or a phosphine group directly linked to a Cp ligand."'] Thus, these bidentate metalloligands combine the coordination abilities of metallophosphines with the additional properties of organic phosphines. The following examples illustrate these two kinds of phosphorus ligands:
4(Nb) Scheme 11
1.7.3 Bimetallic complexes from metallophosphines Three types of monophosphido derivatives are involved in the synthesis of binuclear systems. PR-
CpzM
CpzTa-/H PPh-
0
\
L' hl=Nb
IZ.I=Ta Scheme 12
1(Nb) l(Ta)
bVI = Mo
iL1 = W
l(kf0) IrWj
H
3(Ta)
I . 7 Gvoup 5 und 6 Binietullic ConiplexeJ with Phosphido Bridges
1 15
1.7.3.1 Monobridged complexes Open chain bimetallic complexes containing a phosphido bridged ligand are easily synthesized by reacting metallophosphines 1(Nb, Ta, Mo, W) with [M'Ln] species (Fig. 1 ) . [ 2 a . 1 9 . 3 a l
M
=
N b Td , M'Ln = Cr(COj5 Mo'CO h,COi, Fe(COI4, MnlCO12Cp 7 (Nb, Ta)
,
Scheme 13
Interestingly, bimetallic complexes including the [ Fe(CO)4] moiety are also recovered after treatment of [ Cp?M(PPhHCl)(CO)]+Cl- salts with Collman's reagent Na?Fei CO)4[201
hl = Nb, Ta
7'(Nb, Ta)
Scheme 14
In contrast to the OH- and MeO- reagents. which are able to achieve both a substitution and a deprotonation reaction (or cice w rs u, see Scheme 8), Collman's reagent, because of its lower basic character, affords only the chloride displacement.
Figure 1. Molecular structure of CpCp*Ta(CO)(pPPh2)Mn(COhCp
116
I Moleculur Clusters
1.7.3.2 Dibridged complexes p-phosphido p-carbonyl structures
A photochemical process is used to convert the monobridged bimetallic systems 7(Nb,Ta) or 7'(Nb, Ta) to p-CO, p-PR2 dibridged complexes. The same structures are also generated directly starting from metallophosphines 1(Nb, Ta) and the carbonyl reagents [M'(C0)4L2] (M' = Cr, L2 = NBD; M' = Mo, W, L = piperidine) or [Fe(CO)3] (Fig. 2). [ 2 PR2
Cp,hI,
'
co
hv -hlyco)" I
PR? 0
c p+ I ,
1 [M'(CO)"-1
co
1
8(Sb, Ta)
h.1 = Nb, Ta ;bl'= Cr, Mo, W, Fc Scheme 15
All these complexes exhibit two characteristic spectroscopic features: an IR vco bridging absorption located near 1700 cm-' , and a dramatically deshielded phosphorus resonance (Ad ~150-200ppm) in the 31PNMR spectrum.
Figure 2. Molecular structure of CpzNb(p-PPh2, p-CO)Fe(CO)3.
p-phosphido p-hydrido structures When metallophosphines l(Mo, W) are reacted with M'(C0)5(THF) (M' = Cr, Mo, W ) , a mixture of mono- and dibridged p-phosphido complexes is obtained.
I . 7 Group 5 and 6 Bimetallic Complexes with Phosphido Bridye.s
%>lo, FV)
h1 = hlo, w M' = Cr, Mo, W
1 17
lO(hl0, FV)
Scheme 16
After chromatographic separation, monobridged complexes ~ ( M o ,W ) easily transformed to the dibridged compounds 10(Mo, W) by photochemical elimination of co.[221 In turn, the tantalum phosphine complex 3(Ta) gives dibridged structures 1 l(Ta), which are accompanied, in some cases, by small amounts of monobridged complexes." 3 3 2 3 1
H / PPh-
Cp ?TL-
- M'(C O ),
11(Ta)
hi' = Cr, \ = 4 hl' = Xlo, \ = 4 hi' = LV, \ = 4 h.1' = Fc, z = 3
Scheme 17
Although a terminal phosphido group is not available in phosphino derivatives 2(Nb, Ta), they behave as metalloligands towards unsaturated organometallic fragments also giving rise to dibridged complexes ll(Ta, Nb) (Fig. 3).[13*231
Figure 3. Molecular structure of Cp2Td(H)ip-PPh? ,u-H,Fel C 0 ) i
118
1 Molecular Clusters H
11(Nb. Ta)
Scheme 18
Because no dihydrido-phosphido species is detected by ‘H and 31 P variable temperature NMR analyses of the starting material 2(Nb, Ta), an electrophilic substitution at the phosphorus atom might account for the formation of the bimetallic complexes. High field resonances of the bridging hydrides and low field 31Presonances are the typical structural features of this kind of dibridged complex.
Reactivity of dibridged complexes Both p-CO and p-H dibridged complexes readily react with L donor ligands including phosphines, phosphites, isonitriles, etc. Open chain structures are restored by a completely regioselective addition of L at the M’ site. Whether the L ligand is cis or trans to the PR2 bridge depends on both the size of the metal M’ arid the bulkiness of the incoming ligand. Thus, for the bulky triphenylphosphine ligand, the trans isomer is formed as the sole product when M’ = Cr, whereas a cisltrans mixture (95/5) is when M’ is the larger tungsten atom.[21C’241
CIS
8(hb. Ta)
trans XI = Nh, TJ
Scheme 19
This possibility of breaking the second bridge allowed us to draw up a strategy for the resolution of a chiral tantalocene as outlined in Scheme 20.[251
1.7 Group 5 and 6 Bimetallic Conzplexes with Phosphido Bridges
119
Two diixstereoisomers Cp" =
x)+ 0
L* = (R)-(+)-PMePh(o-An)
Scheme 20
Each diastereoisomer can be isolated in a pure optically active form. They display quasi mirror image circular dichroism curves which illustrates the enantiomeric relationship between the tantalum asymmetric centers.
1.7.4 Bimetallic complexes from metallodiphosphines Metallodiphosphine 4(Nb) binds a [M'(C0)4] (MI = Cr, Mo, W ) fragment leading to a six membered cyclic structure: which adopts a chair like conformation as revealed by X-ray analysis (Fig. 4). Contrary to the other dibridged systems, 31P NMR resonances correspond well to those of the open chain (monobridged) structures with a large M-P-M' angle.['71
h.1' = Cr, blo, W
4Wb)
lJ(Nb)
Scheme 21
In a similar way, diphosphines 5(Ta) and 6(Ta) act as chelating ligands towards [M'(C0)4] moieties affording two types of Ta(V) or Ta( 111) heterobimetallic systems.[' '1 PPh:
13(Ta) Scheme 22
c
11(Ta)
120
1 Molecular Clusters
Figure 4. Molecular structure of Cp*Nb(p-PPh:!, p-dmpm)Mo(CO)4.
1.7.5 Molecular structures of phosphido bridged complexes Nineteen X-ray analyses have been performed, in our group (Table I ) , on Inonoand di-bridged bimetallic systems with one bent metallocene unit and with one phosphido bridge. A rhodium-niobocene structure with two phosphido bridges has been reported by Nikonov et a1.r261 Their structural parameters are listed in Table 1. The molecules are listed in an order which reflects the number of bridges and the nature of the metallocene (group 5 metals: Nb( 111), Ta( 111),d2; Nb( V), Ta(V), do; group 6 metals: Mo( IV), W( IV), d2). In all dinuclear complexes the metallocene fragments exhibit their usual mononuclear geometries: the pseudo-tetrahedral geometry for d' metals, and the very distorted trigonal bipyramidal (or edge-capped tetrahedral) geometry for five coordinate d o metals. The main question that needs to be answered when considering a dinuclear system, is whether or not a metal (metallocene) -metal (M') bond is present. Two parameters from Table 1, the M-M' separation and the M-P-M' angle, give quite direct answer to this question: i) for monobridged compounds the M-M' distances are in the range of 4.3914.784 A and the M-P-M' angles in the range 122.0 to 129.1". ii) for dibridged compounds the M-M' distance range is 2.869-3.400 A with M-P-M' angles comprised between 73.0 and 84.9". Thus, there should be no direct metal-metal interaction in monobridged complexes. Note that the values of M-P-M' angles in monobridged compounds are the largest observed for sp3 hybridized phosphorus atoms. This is certainly due to the steric
Tabelle 1. Geometrical parameters for phosphido bridged biscyclopentadienyl complexes. Compound
M-M'
M-P
M'-P
M-P-M' M-X-M'
P-M-X P-M'-X
P-M-X/ M-P-M'
M-CP
CP-M-CP Ref. ~
~
Monobridgrd Cuinplexes oJGroiip V Metol.5 (Nb"'. T
Cp?Nb(COJ(p-PMe,)W(CO),PMe,Ph
3.679(2)
2.668(4) 2.610(4) 2.06(2)
124.9(2)
87.3(5J
2 5( IJ
2.06 2.07
139.4
Cp"CpTa(COJ(p-PMeJW(CO),.(PAMP) SS "'
4.7723(7 1 2.685(3) 2.628(3) 2.04( IJ
127.8(1)
83.4(4)
l2.7(1)
2.08 2.08
136.0
Cp"CpTa(COJ(p-PMe2JW(COJ,.(PAMP) RS ' I '
4.7598(9) 2.675(5) 1.99i2)
127.1(2)
82.4(7)
7.2(2)
1.99 2.06
136.4
68.4(1)
2.06
137.5
2.641i.5)
125.94(8) X9.0(2)
2.06
Cp~Nh(P(OMeJ,)(p-PPh2)Cr(CO)~,C,H~~!C2,5,0,0,0,~.7S7S~9J 2.687( 1 ) ,0 2.485( 1 J
2.582( I J
129.11(5) 91.01(5) 71.16(6)
2.06 2.06
135.8
Cp"-CpTa(COJ(p-PPh,JMn(COJ~Cp.1.5C7HI "' 4.541(1)
2.708(2) 2.03( I)
2.397(3)
125 X I )
88.1(2)
69.3(1)
2.05 2.10
136.0
Cp'CpTa(COJ(p-P(OMeJPh)Cr(COJ,' I '
2.647(2) 2.542(2) 2.028(6)
127.1(1)
89.6(2)
69.9(1 J
2.05 2.10
137.1
12S.S0(9J 92.61(7) 92.88(7)
40.4(1)
2.06 2.05
135.0
12447(7J 82(3J
68(3)
1.96
139.8
4.646( 1 J
4.7831(9) 2.715(2) 2.555(2)
2.667(2) 2.5 I7(2)
I96
4.547(1)
123. I(IJ
1.96 I95
141.4
2.584(3) 2.616(3) 12?.0(I)
I.96 1.95
139.9
1.94 I.96
125.5
O(planarJ 2.07
132.2
123.95(9) 82.17(7)
2.884(2) 2.493(2) ?.20X(2) 2.172(7)
2.03 I J
75.42( IJ X5.8(2)
93.2(2)
77.1(4) 88.5(7J
91(1) 103(IJ
O(planar) 2 10
96.3(2j 98 2(2J
O(planar) 2.07 2.07
131.2
94.7(2) I17.9(2)
7.6 7.6
2.1 1 2.09
132.5
90
2.08 2.08
134.4
planar
2.06 2.08
134.8
planar
2.07 2.07
134.9
planar
I.97 1.97
137. I
1.94
125.4 ( I 17.7)
Cp Nb(p-PPh R,p CO)Cr(COJ,
3.07% IJ
2 S46( I) 2.328( I) 78.23(4) 2.148(7) 2.3 I2(6) 87.2(2)
Cp' Nb(p-PPh J,RhEt
2.869(23
2.55.5(6) 2.602(6)
Cp.Ta(HJ(p-PMe,.p-HJCr(COj,
Cp:Ta(H)(p-PPh ,,p-H)Fe(CO)
3.293(1J
2.209(5) 73.7(2) Z.ZIX(6)
41.6(1J
72.5(2)
2.08
105.5(3)
2.284(3) 84.85(8) 64(3) l.X3(8Jbr 1.71(9) 137(5J 7X3J I.60(9),
2.587(2)
3.1 13(2J 2.547(4)
2.I(lJbr
2.168(4) 1.3(1)
82.2(1)
61(3)
134(6J
83(4J
?.464(3J
81 6 ( l )
--
80.73(6) 109(1j
87(2)
134.9
2.08
I.X6(9),
Dihridgrd Ciiinplexe~i f Groyi VI Mrtul\ (Mo, WJ, d2 metrilr
Cp.W(p-PPhl,p-H)W(COJ,
3.2708(8) 2 540(5) -.
[~nsu-CMe:Cp~W(p-PPh.,p~HJW(COJ, ~~
3.2320(5) 2.536(2) 2.02(6)
-.
2.4532) I.95(6)
832)
( I J Cp" = I-'Bu-3.4-Me,-CiH,). . . PAMP = (RJ-(+)-PMePh(o-An):(2) Cp* =C5Me,; (3) Cp' = C5Me,Et.
planar ~
1.98
~~~
122
I Molecular Clusters
.... Figure 5. Shapes of HOMO-1 orbitals for Cp2Nb(pPPh2)(pL-CO)Fc(CO)3 and CpzTa( H)(p-PPh>)(p-H)Fe(C%.
requirements of the bulky organometallic fragments. It is also worth noting that in the monobridged complexes the methyl substituents on the phosphido bridge are in endo positions with respect to the LMP unit, while the bulkier phenyl substituents are exo, pushing the second metal into the endo position. There are two types of dibridged complexes. The M-M' distances and M-P-M' angles are 2.884-3.079 A and 75.4-78.2" in (p-PR2, p-CO) species, whereas the corresponding values for (p-PR2, p-H) rings are 3.113-3.400 A and 80.7-84.9", respectively. This suggests the presence of a direct metal-metal bond for (p-PR2, p-CO) complexes, but the situation is not clear for p-hydrido ones. Extended Huckel calculations confirm the presence of a metal-metal bond in p-CO compounds and show it not to be present in p-H systems. This is illustrated by the shapes and CpzTa( H)(p-PPhz)of HOMO-1 orbitals for Cp2Nb(p-PPh2)(pu-C0)Fe(C0)3 (pu-H)Fe(C0)3 (Fig. 5). The metal (metal1ocene)-phosphorusbonds are shorter and consequently stronger (2.493-2.635 A) in dibridged compounds than in the monobridged ones (2.5842.715 A)suggesting some electron density delocalisation through the second bridge and its lowering on the metallocene unit. This is roughly confirmed by the values of Cp-M-Cp (Cp are the geometrical centers of the C5 rings) angles which are larger (stronger repulsions) in monobridged species than in the dibridged ones (e.y. for group 5 metallocenes: 135.0-139.4" us. 131.2-134.9").
References [I] (a) D. G. Dick, D. W. Stephan, Organometallics 1990, 9, 1911-1916; (b) D. G. Dick, D. W. Stephan, Organornrtallics 1991, 10, 2811-2816; (c) D. G. Dick, D. W. Stephan, Can. J. Chem.
1.7 Group 5 and 6 Bit.rlc.tallic. Complexes with Phospliido Bridges
123
1991, 6 9 , 1146-1 152; (di R. T. Baker. J. F. Whitney, S. S. Wreford, Or
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.8 Polythiometalates and Polyoxothiometalates Based on Acido-Basic Condensation Processes Francis Sicheresse, Emmanuel Caclot, Corine Simonnet-Jegat
1.8.1 Introduction The preparation and characterization of sulfur-containing molecules is of major importance because of their applications in modern problems of chemistry in relation to hydrodesulfurization catalysis, inorganic, and bioinorganic chemistry. In order to define new synthetic routes of S-based coordination compounds, considerable attention has been given to the chalcogenometalates due to their unique ability to function as versatile ligands. Indeed, this field has been extensively reviewed.[' -l5] Our contribution in this field is summarized in Scheme 1, but only results corresponding to part 1 of the diagram will be discussed here. Acidobasic Condensation
Mixed-valence Complexes - Bioinorganic Chemistry - Catalysis
Internal Redox Process
Material with Adjustable Shape Addition of Small Molecules - CuCI, CuNCS - Fragments of Complexes
and Dimensionality - Open Structures - Cubanes - Polymers ID, 2D, 3D
@\ Nucleophilic Reagent with Acceptor Properties
-
Behave as Pseudohalides with Empty d-Orbitals
Scheme 1. Diagrammatic representation illustrating the reactivity of [MS4I2-, M = Mo, W.
1.8 Polytlziometulutes and Polyoxothiometulutes
125
The first preparations of the tetrathiometallates [WS4I2- and [MoSq]*- were reported by Berzelius" 61 and their true composition established by Kriiss" and C o r l e i ~ . ~In " ~ 1883 ~ crystals were obtained"" but their complete structures were not solved until 1963."9q201 In those [MSqI2- anions, the metal ion has a high formal oxidation state (VI), is located at the center of a tetrahedron of sulfides, with M-S distances of about 2.17 A,and has low-lying d-orbitals, as represented in Scheme 2.
-10
-15
-20
Scheme 2. The energy level diagram for [WS,]'-.
The lowest levels located at -21.4 eV and -20.2 eV respectively correspond to the splitting of the four s orbitals of the sulfur atoms; in those levels the contribution of the 6s orbital of the tungsten is very weak. The singlet at -14.3 eV is made up of the total symmetric combination of the 3p orbitals of sulfur, which interact weakly with the 6p tungsten orbitals. The doublet at -14 eV represent the n combination of the d,z and d,zPy' metallic orbitals with the sulfur p-orbitals. The triplet at -13.8 eV is an internal admixture of the p orbitals of the sulfur atoms with small amounts of those of tungsten. The t2 level at -13.7 eV represents the lowest bonding W-S level containing the tungsten d,, and d,, orbitals and forming the 0bonding system with p-orbitals of sulfur atoms. In this level, a weak n-overlap is observed involving p orbitals of sulfur and the tungsten d,z orbital. The HOMO is located at -13 eV and contains only non-bonding sulfur orbitals without any contribution from tungsten. The doublet at -9.1 eV represents the LUMO (the antibonding level corresponding to the bonding level located at -14 eV): in this
126
I Molecular Clusters
level the contribution of the d orbitals of tungsten is very important, higher than in the corresponding bonding level. The level at -8.3 eV represents the antibonding part of the 0-bonding orbital observed at -13.8 eV. These two e and t2 higher energy levels form the acceptor levels of the thiometalate being separated by 0.8 eV (6400 cm-'), which roughly corresponds to the ligand field value of the S2- anion in T d symmetry. The reactivity of the thiometalates and their applications in coordination chemistry, related to their unique ligand properties, were pioneered by A. Muller in about 1970.[211 The first trimetallic complex reported was [Ni(WS4)2]2-[221 in which the thiometalate acts as a bidentate ligand through two sulfur atoms forming an edge of the WS4 tetrahedron. The structure of this compound was confirmed later by a full X-ray diffraction The same type of coordination was observed with oxothiometalates in the isostructural anions [Ni(W0S3)2l2-- and [Ni(W02S2)2]2-[241.Other heterobimetallic compounds having the same or similar architecture were isolated with various transition metal ions, Fe,[203251 C O , ['I~ Ni, Pd, and Pt,[26,271 Zn,[283Cd, and HgC3] The high stability of these complexes was attributed to the delocalizatiori of d electrons of the additional transition metal towards the empty d orbitals (LIJMO) of the central molybdenum or tungsten atom (see above). This delocalization was postulated on the basis of electrochemical experiments,[291these compounds being easily and reversibly reduced through a one or two-electron process. After 1975, thiometalate chemistry was focused in two new directions of research, differing in their purpose as well as in the structural types of the complexes involved. These two new areas of research are related to two fields of chemistry which had apparently not been connected before; heterogenous catalysisr301and bioinorganic ~ h e m i s t r y'I . ~ ~
1.8.2 Thiometalates and catalysis The catalyst generally used in the hydrosulfurization (HDS) process is MoS2, promoted by cobalt or and supported on alumina or activated carbon. The his-thiometalo complex [Ni(WS4]2]2- cited in the introduction was tested as a precursor for an unsupported mixed nickel tungsten sulfide catalyst. Catalytic activity measurements of the thermally-decomposed Ni-complex were found to be better than for catalysts prepared by conventional methods.[331 Metal sulfides are also known to catalyze the hydrogenation of arenes and the hydrogenolysis of thiophene~.[~"' Recent work shows that thiometalates are not necessarily good models for des~lfurization,[~~] their activity being closely dependent on the M : S stoichiometry and on the oxidation state of the metal. A review based on the coniparison of reactions of desulfurization of thiophene in organometallic compounds or on single-crystal metal surfaces has appeared, giving an excellent overview of the state of the art.[361
1.8 Polythiornetalutes and Polyoxothiometulutes
127
Moss and WS5 fragments were shown to be active in hydrogenation reactions, hydrodesulfurization and hydrodenitrogenation;[3j1 so complexes built from fragments of thiometallates were tentatively used as models for the study of processes occurring in heterogenous catalysis[371or even as potential catalysts for HDS or hydrogenation reactions.[”]
1.8.3 Academic research induced by catalysis 1.8.3.1 Sulfido-ligands Referring to catalysis studies, academic research focused on the synthesis of thiocompounds involving MS4 or MS5 “active” cores. Discrete clusters or complexes
Figure 1. a ) Structure of [WS4Pt(dppm)],(adapted from ref. 39). b) Orbital interactions in the {WS4Cu}core. (adapted from ref. 40b).
128
pIos41*-
I Molecular Clusters
PhSSPh
DMF, 90°C
S
S
Figure 2. Reaction scheme showing the formation of [ M o ~ ( S ~ ) ~ ( S ) ~ I : ‘ - , (adapted from ref. 41).
were synthesized and characterized, containing the MS4 fragment, and further complexes containing an additional precious metal, such as [WS4Pt(dppm)j,1391 [M’(bpy)3][(MS4)2M”]( M = Mo, W and M’ = Co, Ni, Mn and M ” = Pt, Pd), [ W S 4 ( R ~ ( b p y ) 2 ) 2 ] ~ and + , [ ~[~A] U ~ ( W S ~ ) ~ ] ~ - . [ ~ ~ ~ ] In these compounds, the MS2M’ central core is highly stabilized by the acceptor properties of the MS4 ligand, which allows these compounds to become involved in ligand substitution reactions and facile redox processes. In contrast the [M’(bpy)2C12] (M’ = Ni, Co) parent complex is irreversibly reduced uia a twoelectron process, [MS4M’(bpy)z]resulting from the substitution of two chlorides by MS4’-, which is reversibly reduced at two clearly separated one-electron steps. These results show that the MS4 ligand can be considered as a good acceptor due to its empty d-orbitals. With M’ = Cu a molecular orbital resulting from the bonding combination of d-orbitals of the thiometalate with d-orbitals of Cu was postulated (see Fig. 1b) based on extended Hiickel calculations.[40b1 The pyramidal MS5 fragment present in homopolynuclear complexes has been obtained by condensation of thiometalates under reducing conditions. For example, the reaction between PhEEPh ( E = S or Se) and [MoS4I2- led to the formation of dinuclear [Mo2S812- complex containing the [Mo2S4l2’ core (Fig. 2).14’] We have also explored the possibility of obtaining homopolynuclear complexes via the electrophilic attack on thiometalates by protons. We have studied these reactions extensively because of the opportunity they offer to observe, and eventually, to trap transient species between the starting thiometalate and the final insoluble and amorphous MS3 compound. Figure 3 shows the general scheme summarizing the acidification of [WS4I2- and detailing the different steps.r531 These condensations represent examples of intricate reactions since the protonation is systematically accompanied by redox phenomena making the metal and/or the sulfido ligand interesting. Moreover, these redox properties are also systematically coupled with the geometrical change of the reduced metal center. A large set of complexes, most of them characterized by X-ray diffraction, were reported. In [W30S8)2-,[443451 and [ W I S ~ ] * - , [the ~ ~sulfido ] ligands [ M 0 3 S s j ~ - , [ [W3Sg]2-,r431 ~~] appear unchanged whereas a metal center is reduced, generally by 2e-. The geometry at the reduced metal center changes from tetrahedral to pyramidal as represented in Fig. 4a. A second MS5 pyramid can be incorporated in the former molecular structure accompanied by a second electronic transfer to give [W4S12]2-,[47d1 which contains two central MSS pyramids sharing a common S-S edge as represented in Fig. 4b. In
129
weakly acid medium
H+ H20
/H+
strongly acid medium
Figure 3. General scheme illustrating the acidification of [WS,]'-, (from ref. 53a)
11
Figure 4. Structures of [WISSX]'- X
=
0, S. and [W4Sl2I2-, (adapted from ref. 43 and ref. 47)
130
1 Molecular Clusters
the [W2S4I2+dicationic fragment the two tungsten atoms are both reduced by one electron (V, d’) and are linked through a metal-metal bond (the complex is diamagnetic, W-W = 2.9 A)) and this ensures its high stability. The molecular structure of [W4S12I2-- can be formulated as [(WS4)(WzS4)(WS4)I2-, the two WS4 groups acting as terminal bidentate ligands towards the {W2S4}*+ central core. This formulation suggests the { W Z S ~ } fragment ~+ exists in acidified solutions as WS42-, probably stabilized by molecules of solvent. Starting from this hypothesis we succeeded in “extracting” the dithio-fragment from acidified mixtures of WS4=- by addition of the 2,2‘-bipyridine, a strongly chelating ligand, in the presence of halides.[47b1In this reaction bipyridine competes with WS42- to stabilize the {W2s4j2+core as summarized below:
By combining acido-basic titration results and UV-vis spectroscopic data a general scheme for the acidification of WS42- can be proposed for a weak acidic medium ( H + : W < 3) showing the reduction of tungsten is accompanied by the formation of the oxidized S2 2- disufido group according to the following equation:
[W3SgI2- is obtained quantitatively for p = 3 and q = 4 ( H + : W = 4 : 3) while [W4S12I2- corresponds to p = 4 and q = 6 (H+ : W = 6 : 5). Note that the expected [W2S6I2- dianion (p = 2, q = 2, H+ : W = 2 : 2) does not exist since it would contain two reduced tetrahedral tungstens, which is forbidden; the reduction of the metal center is always accompanied by the change from tetrahedral to pyramidal geometry. As kinetic studies recently revealed[47c1that the first step of the condensation is the protonation of WS42- according to the following reaction: WS42-
+ H+ + WS4H-
This hydrogeno-anion was recently trapped and isolated in the solid state as a PPh4+ salt (tetragonal, space group Z 4 , a = 13.0130 A, c = 7.1260 A).The structure was determined by X-ray diffraction showing the monoprotonated species has retained the tetrahedral geometry of the W S 2 - precursor.[47C1This new result illustrates that the reduction, and of course the associated geometrical change, take place after the evolution of H2S between two protonated units: the electron transfer occurs in a condensed species, probably a dinuclear intermediate, which remains uncharacterized.
s4
s4
jl Figure 5. a ) Structure of [ W ~ S I I ] ' -(adapted . from ref. 50). b) [W2S11I2- : [ W ~ S I I H relationship. ]
1.8.3.2 Disulfido-ligands Conversely, the selective oxidation of the sulfido ligand of WS4'-, without change in the formal oxidation state of the metal ( V I ) led to quite different structural types, [W2S902]2-,[501 or [ M o ~ S ~ O ~ ] ~ - . [ ~ ~ ] as reported for [W'SIlH] ,[481 [W2S11]2-,[491 In a more acidic medium (H- : W > 3). the oxidation of sulfides into disulfido ligands is accompanied by a complete change in geometry at the metal, as represented in Fig. 5a. The coordination geometry of the metal centers can be described as that of two pseudo pentagonal pyramids sharing an S-corner. The [W2S11]2anion has a twofold axis passing through the apex of the pyramid, which relates the two pyramidal sub-units in a syn-geometry. [ W ~ SI HII can be derived from [ W ~ S I by ~ ] protonation ~ of the S-bridge, which provokes the rotation of a WS5 pyramid around its main axis and changes a p-S bridge into a p-v3-S2 bridge as represented in Fig. 5b.
132
I Moleculur Clusters
In the WS2 terminal three-centered core the n* antibonding orbital of the S2*group splits into two c o m p o n e n t ~ [ ~ .a~ n; * ] :orbital mainly located in the WS; plane and strongly interacting with the metal, and a ni orbital, which is perpendicular to the WS2 plane and interacting weakly with the metal. The absorption observed near 475 nm in the electronic spectrum of [W2S11H]- has been attributed to the n: + d(W) transition.[481A second band related to the n{ + d transition is expected. Because of the strong stabilization of the bonding orbital due to the high oxidation state of the metal, this transition is shifted to higher energies. In less oxidized compounds such as [Mo3S(S2]6I2-,this transition is observed near 365 nm.I4] The formation of disulfido-groups from sulfido-ligands was also achieved by using externul elemental sulfur as the oxidizing agent.[”] The [M(S2)2E] ( E - = 0 ,S, M = W, V ) fragment present in all these complexes was recently trapped from acidic solutions and stabilized by a chelating ligand (2-2’-bipyridine) in the molecular shown in Fig. 6.
1.8.3.3 Vanadium systems The presence of the pyramidal [M(S2)2E](E-0, S) intermediate seems to be a general feature in the acidification of MS4, since this fragment was also trapped and characterized with [V04l3- in a polysulfide-containing medium.I53b1 This intermediate readily reacts with chelating ligands and is stabilized as a mononuclear spe-
s2
Figure 6. Structure of [wO(SZhbpyl, (adapted from ref. 53a).
1.8 Polythiomrtulutrs und Poll’o.uotliiometcilutr~s
1 33
01
s1
Figure 7. Structures of
[(VOfS2)2)2-p2-S4-( VO(S2)2)2]‘
, (adapted from ref. 53c).
cies. With (S,)2- ligands the unexpected templating effect of 4,4‘-bipyridine in the ~ - [ ~ ~ ‘ ] in Fig. 7. solid state gives the tetranuclear anion [ V ~ O ~ S ~ O ]represented
1.8.3.4 The [M2S2X2I2+X = 0, S fragment Between these two extreme redox behaviors (reduction of the metal or oxidation of sulfides), the use of molybdenum led to compounds containing reduced metal, generally Mo(V), and oxidized sulfur ligands, such as (S,7)2-,n = 2,3,4. In a few complexes these polysulfide ligands are present in a single configuration as in [Mo202S2(S2)2],[541( Mo-r2-S2 groups). These polysulfide anions represent good precursors, which, by abstraction of (S,)?- groups through redox processes, lead to cationic fragments, which have been successfully condensed on lacunary anionic species (see below). In other cases the disulfido ligands are present in the complex in various terminal configurations, such as in [Mo2S1ol2- r 5 5 a 1 (Mo-v2-S2 and Mo-v2-S4 groups) as shown in Fig. 8. Disulfido-groups can also be present in a complex as bridging ligands between two metal centers e.y. in [Mo2S& [5 6 1 and [Mo3S13]’- [57a1, which are represented in Fig. 9.
r
Figure 8. Structure of [Mo?Slo]’-, (adapted from ref. %a).
1 2-
134
I Molecular Clusters
s-s 2-
2-
\/
Figure 9. Representations of the structure of [Mol2S,2l2-, (adapted from rcf. 56) and [MoiSl,]*-, (adapted from ref. 57).
These anionic complexes are important for giving, by oxidation of terminal disulfido groups, very reactive, trinuclear cationic species.” h1 In molybdothiocompounds involving M o - ~ ~ -orS4)( ~ moieties, various specific “functional groups”” were clearly identified and engaged in nucleophilic attack on dicarbomethoxyacetylene, leading to the conversion of (S,) 2- sulfido ligands into dithi~lene.[~~~~~
’
1.8.3.5 CS2 activation [(S4)2Mo=S]readily reacts with CS2 to give both cis and trans [(CS4)2Mo=S]complexes, these compounds having the ability to reversibly release CS2 to revert to Mo-q2-S2. This elegant work is obviously closely connected to the activation of the C-S bond in HDS Very recently we have shown that CS2 could be added via reduction to [MoS4I2to give [Mo(CS3)4I3-,the only mononuclear reduced Mo-compound involving CS2 characterized so far[58b1(Fig. 10). This addition of CS2 under reducing conditions represents a facile, in situ synthesis of [CS3I2- thiocarbonate ligands that are difficult to prepare by conventional methods.
1.8.4 Reactions of thiometalates with polyoxometalates The early transition metals such as V, Mo, W, in their highest oxidation state form metal-oxygen macrostructures referred to as polyoxometalates. They represent a rich class of soluble clusters which have been studied for their chemical reactivity,
1.8 Polythiornetulutes and Pol~oxothioinetulutrs
s11
c4
~
135
s12
Figure 10. Structure of [Mo(CS?)4]’-.
physical properties, and also for their use in a large field of applications, including catalysis, medicine and material^.[^^-^^] The macrostructures of these compounds were generally obtained through acido-basic condensation processes[6o1and as a consequence of their reactivity, polyoxometalates can be easily functionalized and their functionalization, for example by organic ligands, not only affects the electronic structure of the parent polyanion but can also stabilize new structural types.[‘ The introduction of sulfur into the polyanionic frameworks is expected to modify the chemical and electronic properties of the 0x0-parents, and owing to the softness of sulfur new architectures are expected to be derived from the various precursors. Three different ways have been developed in our group to prepare the first polyoxothiometalates: J
i) selective sulfuration of a polyoxoanion based on O/S exchange, ii) stereospecific addition of a thiometallic fragment on a geometrically adapted polyvacant polyoxoanion, iii) direct synthesis by condensation of a thiometallic precursor. An example of each type of preparation is given here to illustrate the possibility of adapting the synthetic methology to the geometry of the expected final product.
136
I Molecular Clusters
Figure 11. Reaction scheme for the formation of a-[PWllNbSO39I4-, (from ref. 66).
1.8.4.1 Selective sulfuration of fully oxygenated polyoxoanions Direct reaction of hydrogen sulfide or sulfido ions with a polyoxoanion generally results in the reduction of the metal centers ( M = Mo, W ) and the breaking of the M-0-M bonding scheme of the framework. Therefore, only structures first stabilized aiu redox processes by substitution of tungsten by a less reducible atom can be isolated using this route. The precursor c(-[PW11Nb040]~was obtained by addition of a N b 0 3 + group, resulting from the controlled hydrolysis of NbClS, on the monovacant Keggin heteropolyanion a-[PWll O39I7-. The resulting compound was reacted with variable amounts of paramethoxyphenylthionophosphine sulfide (R2P2S4) to give the first Keggin compound ~-[PW11NbS039]~with sulfide replacing oxide[661(Fig. 11). The O/S substitution in the Keggin structure was clearly proven by IR and Raman studies, together with 31PNMR and voltammetric data.
1.8.4.2 Addition of a thiometallic fragment on a lacunary polyanion Another original method of synthesis is the stereospecific addition of a preformed thiometallic core in a polyvacant polyoxoanion. The [M2S20212+( M = Mo, W ) thioprecursor and y-[SiWlo036]'- represent an excellent example of obtaining the product by matching reactivity and geometry. The thiocation [M2S2O2l2+( M = Mo, W) was obtained aiu selective oxidation of (S,)2- ligands (where y1 = 2 or 4) in [(S4)MoOS2MoO(&)I2- using iodine in aqueous solution or D M F medium[671according to the equation:
+
+
8 [(S,)MOOS~MOO(S,)]*- 2 12 + [M02S202]~+ 2 ~ / Ss
+ 4 I-
This redox reaction is remarkable for changing a nucleophile, namely [(S4)MoOS2MoO(&)I2 ~,into the [Mo2S202I2+,which is a very strong electrophile. The tungsten homologue [W2S202]2Lwas prepared in a similar way in non-aqueous medium. The dithiocation reacts as a Lewis acid with the divacant y-[SiWl0036]'-
1.8 Polythiometulutes und Po1~~osotliion~rtakitc.s 137
Figure 12. Polyhedral view ofthe structure o f I)-[SiW I0 Mo?S?O?s1'- .
anion and fills the polyanion vacancy to yield the metal-saturated y[SiW1oM2S2038]~-( M = MoV, W v ) anion, represented in Fig. 12.[681 W NMR spectroscopy is a powerful tool for characterization of these species in aqueous solution. The number of peaks and the chemical shifts observed in the -110 to -190 ppm range are assigned to W(V1) tungsten atoms with C21.symmetry, while the low frequency (h' = 1041.2 ppm) signal is characteristic of the two equivalent reduced W( V ) atoms (see Fig. 13). The electronic properties of the y-isomer are completely modified by the substitution of only two oxygens by two sulfur atoms. The reduction by two electrons of the [SiW~203o]~parent anion results in the deiocalization of the two electrons over the [SiW 1204,,16- macrostructure, while in the S-substituted [SiW12038S2l6- anion the two additional electrons are localized on two specific metal centers,[68c1which adopt, as expected, a pyramidal geometry. Another possibility exists for incorporating sulfur in a Keggin anion. When the size of the [M202S2I2-' thio-fragment ( M = Mo, W ) and that of the cavity of the lacunary 0x0-precursor are not compatible, the direct stereospecific addition is no longer possible. In that case the acido-basic reaction gives sandwich-type compounds. This is typically the case of the reaction between the trivacant
1060
1040
(PPm)
1020
m -110
-190
(PPm)
Figure 13. I8'W N M R spectra of ; ~ - [ S i W ( V I ) ~ , , W ( V ) ~ (from S ~ o ~ref. ~ ] "68' ~.
138
I Molecular Clusters
Figure 14. a) Polyhedral view of the trivacant ~-[PW9034]~-. b) Polyhedral representation of [P?W24S6074(H20)61'2-.c) View of the (Mot, Mo2, Mo3) equatorial plane: 0 3 4 and 0 3 5 are oxygens of water.
M-(PW~O~ represented ~]~in Fig. 14a and [M202S2I2+( M = Mo, W ) . Two (PWg} Keggin sub-units are doubly bridged by three [M202S2I2+fragments in a sandwich type arrangement, as illustrated in Fig. 14b. The two Keggin units are equivalent, being related by the equatorial plane containing the six sulfur atoms. Seven
1.8 Pol~t1iionietalate.r and Pol~oxothionietulates
139
lines of relative intensities 2 :2 : 2 : 1 :2 : 2 : 1 are observed in I X 3 W NMR spectra. Chemical shifts in the range - 1 10 ppm to - 160 ppm are assigned to tungsten ( V I ) atoms in octahedral 0x0-environment, as reported above. Lines at 1197 ppm and 1376.4 ppm are assigned to the reduced tungsten ( V ) atoms of the three (W2S202) thiometallic fragments. The two resonances suggest the three { W2S202} bridges are not equivalent. The lowering of the symmetry is related to the presence of water molecules, in different environments, linked to the reduced molybdenum (or tungsten): two water molecules and an 0x0 group are directed toward the inner cage between the two {PWg) units, as represented in Fig. 14c.
1.8.4.3 Self-Condensation of the
[Mo2S202I2+fragment
The former strategy of synthesis is really convenient for the preparation of polyoxothioanions derived from well-established structural types, but is inadequate for the design of sulfur-rich frameworks. Therefore we developed another original approach based on the self-condensution under acido-basic conditions of the [Mo202S2I2+ fragment described above.[691The geometry of the molecular compound obtained by addition of potassium hydroxide to an aqueous solution of the dithio-fragment is represented in Fig. 15. Various titrations of solutions of [Mo2S202(H20)6I2+ by standard solutions of KOH allowed the “wheel” to be obtained qualitatively according to the following equation:
A remarkable feature of this structure is the cyclic arrangement of the neutral Mo12cluster with a central cavity of 11 A. Six [Mo2S202] building blocks are connected by hydroxo double-bridges and the coordination at the Mo centers is achieved by a terminal oxygen and a shared molecule of water. These water molecules are very labile, which promises to be a feature of the reactivity of this wheel with anions such as phosphates, acetates. and carboxylates. The self-condensation is reversible since the acidification of [Mo12S12012(OH)12(H20)6]by standard inorganic acids gave the original dithio-precursor. This protocol represents a convenient and productive way of obtaining new types of condensed species from simple and well defined precursors. We have illustrated that sulfur-based macrostructures could be obtained either by reduction of the charge of the cutionic precursor by a base, or by reaction between anionic precursors with acids (the first polynuclear compounds we described at the very beginning of this work have been obtained cia this procedure). The considerable interest of acido-basic condensation processes based on the lowering of the charge of adapted precursors lies in the possibility of designing tridimensional solids having predictable geometries and properties.
140
I Molecular Clusters
Figure 15. Structure of [ M o ~ ~ S , ~ O , ~ ( O H ) I ~a)( ball H~O and )~]: stick view showing the atom bonding scheme. b) Polyhedral representation, (from ref. 69).
References [l] [2] [3] 141
Holm, R.H. Chem. SOC.Reu. 1981, 10, 55. Coucouvanis, D. Acc. Chem. Reu. 1981, 14, 201. Miiller, A,, Diemann. E., Jostes, R., Bogge, H. Anyew. C'hern., Znt. Ed Engl. 1981, 20 934. Miiller, A., Jaegermann, W., Enemark, J.H. Coord. Clzern. Rev. 1982, 46, 245.
1.8 Pol~~thiometulutes and Polyoxothiometalutes
141
[5] a) Averill, B. Struct. Bonding (Berlin) 1983, 53, 59. b) Sarkar, S., Mishra, S.B.S. Cuord. Chem. Rev. 1984, 59, 239. [6] Draganjac, M.. Rauchfuss, T.B. Anyeir. Chern., Int. Ed. Engl. 1985, 24. 742. [7] Muller, A,. Jaegermann. W., Hellmann, W. J. Mol. Struct. 1983, 100, 559. [8] Muller, A. Po/yhedron 1986, 5 , 323. [9] Coucouvanis, D., Hadjikyriacou. A., Draganjac, M.. Kanatzidis, M.G., Ileperuma, 0. P d y hedron 1986. 5. 349. [lo] Muller, A,, Diemann. E. Adv. /nor
142
1 Moleculuv Clusters
Pan, W.H., Harmer, M.A.. Halbert, T.R., Stiefel, E.I. J. Am. Chem. Soc. 1984, 106, 459. Pan, W.H., Leonowicz, M.E., Stiefel, E.I. Inory. Chem. 1983, 22, 672. Miiller, A,, Bhattacharyya, R.G., Koniger-Ahlborn, E., Sharma, R.C., Rittner, W., Neumann, A,, Henkel, G., Krebs, B. Inorg. Chim. Acta 1979, 37, 493. Secheresse, F., Lavigne, G.; Jeannin, Y., Lefebvre, J. J. Coord. Chern. 1980, 11, 1 I . Hanewald, V.K., Kid, G., Gattow, G. Z. Anorg. Allq Chenz. 1981, 478, 215 Bhaduri, S., Ibers, J.A. Inorg. Chem. 1986, 25,4. a) Secheresse, F., Lefebvre, J., Daran, J.C., Jeannin, Y. h o r y . Chem. 1982, 21, 1331. b) Simonnet-Jegat. C.. Toscano, R. A,, Robert, F., Daran, J. C.: Secheresse, F. J. Chem. Soc. Dalton Trans. 1994, 1311. c) Secheresse, F., Miiller, A. 1998, unpublished results. Secheresse, F., Manoli, J.M., Potvin, C. Inorg. Chem. 1986, 25, 3967. Manoli, J.M., Potvin, C., Secheresse, F. Inory. Cliem. 1987, 26, 340. Manoli, J.M., Potvin, C., Secheresse, F. Inorg. Chim.Actu 1987, 133, 27. a) Coucouvanis, D., Hadjikyriacou, A.I. Inory. Chem. 1987, 26, I . b) Hadjikyriacou, A.I., Coucouvanis, D. h o r g . Chem. 1989, 28,2169. Chandrasekaran, J., Ansari, A,, Sarkar, S. Inorg. Chem. 1988, 27, 3663. a) Simonnet-Jegat, C., Jourdan, N.. Robert, F., Bois, C., Secheresse, F. Inorg. Chim. Actu 1994, 216, 201. b) Simonnet-Jegat, C., Robert, F., Skcheresse, F. Truns. Met. Chem. 1994, 19, 379. c) Simonnet-Jegat, C., Delalande, S., Marg, B., Halut, S., Secheresse, F. J. Chrwz. Soc. Chem. Commun. 1996, 423. Rittner, W., Miiller, A,, Neuman, A,, Bather, W.. Sharma, R.C. Angerv. Chem., Int. Eil. Engl. 1979: 18, 530. a) Clegg, W., Christou, G.. Garner, C.D., Sheldrick, G.M. Inory. Chenz. 1981, 20, 1562. b) Xinquan Xin, Guoxin Jin, Boyi Wang, Pope, M. Inory. Chem. 1990, 29, 554. Miiller, A,, Nolte, W.O., Krebs, B. Angeiv. Chem., Int. Ed. Engl. 1978, 17, 279. a) Miiller, A,, Sarkar, S., Bhattacharyya, R.G., Pohl, S.; Dartmann, M. Anyew. C h m . , Int. Ed. Enyl. 1978, 17, 535. b) Miiller, A,, Wittneben, V., Krickemeyer, E., Bogge, H., Lemke. M. Z. Anory. Ally. Chem. 1991, 605, 175. a) Coucouvanis, D., Draganjac, M.E., Koo, S.M., Toupdakis, A,, Hadjikyriacou, A,[. Inorg. Chem. 1992, 31, 1186. b) C. Simonnet-Jegat, C., Cadusseau, E., Dessapt, R. Secheresse, F., submitted for publication. a) Coucouvanis, D., Hadjikyriacou, A.I., Toupadaki, A,. Lane, J.D., Koo, S.M., Ileperuma, O., Draganjac, M.E., Saligoglou, A. Inorg. Chem. 1991, 30, 754. b) Coucouvanis, D., Toupdakis, A,, Lane, J.D., Koo. S.M., Kim, C.G., Hadjikyriacou, A.I. J. Am. Chcm. Soc. 1991, 113. 5271. Pope, M. T. in “Hetero and Isopoly Oxometalates”; Springer-Verlag: New-York, 1983. “Polyoxometallutes: From Platonic Solid~sto Antiretovirul Actiaity” Pope, M. T., Miiller, A. Eds, Kluwer Acad. Pub.: Dordrecht; The Netherlands, 1994. Bussereau, F., Picard, M., Malik, C., TezC, A,, Blancou, J. Ann. Inst. PusteurlVirol. 1998,32,33. Ono, K., Nakane, H., Barre-Sinoussi, F., Chermann C. Nucleic Acids Res. Symp. Sor. 1984, 15. 169. [64] (a) Misono, M. Cutal. Rev.-Sci. Eng., 1987, 29, 269 (b) Haeberle. T., Emig, G. Chem. Eng. Techno/. 1988, 392 (c) Watzenberger, A,, Emig, G., Lynch, D. T. J. Cutul. 1990, 124, 247 (c) Cadot, E., Marchal, C., Fournier, M., TezC, A,, Herve, G. “Polyoxometullate,s: From Plutonic So1id.s to Antiretovirul Activity” Pope, M. T., Miiller, A. Eds, Kluwer Acad. Pub.: Dordrecht, The Netherlands, 1994, p 315. [65] Proust, A,, Thouvenot, R., Yoo, J. K., Gouzerh, P. Inorg. Chem. 1995, 34, 4106. 1661 Cadot, E.; BCreau, V., SCcheresse; F. Inory. Chim. Actu 1995, 132, 1029. [ 671 Coucouvanis, D., Toupadakis, A,, Hadjikyriacou, A.I. Znory. Chem. 1988, 27, 3273. 1681 a) Cadot, E., Bereau, V., Marg, B.. Halut, S., Secheresse, F. Inory. Chem. 1996, 9.5, 3099. b) Cadot, E., Bereau, V., Secheresse, F. Inory. Chim. Actu 1996, 252, 101. c) Bereau, V . These, 1997, Universitc de Versailles. 1691 Cadot, E., Salignac, B., Halut, S., Secheresse, F. Anye,v. Chem.. Int. Ed. En~ql.1998, 37, 612.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.9 Electronic Effects on the Shape of M2X2 Frameworks (X = Naked Chalcogenide Atom, M = Late Transition Metal) Curlo M t d i and Annmhrllu Orlmdini
1.9.1 Introduction Transition metal dimers containing the M2Xz framework ( M = transition metal, X = naked S, Se or Te bridging atom) are numerous. A search in the Cambridge Structural Database”] (no additional bridging ligand is allowed) yields close to 200 hits and structural information is also available from techniques other than X-rays. Some complexes are biologically relevant, in particular the species L ~ F ~ ( , U - S ) ~ F ~ L ~ [ ~ ] whose rhomboidal Fe2S2 core mimics that of important iron proteins. Amongst the transition metal complexes having the general formula [L,M(pX)2MLl,,]” ( m = 3 and n = +2 + -2 or 112 = 2 and n = 0, +2), we have experi- ~ ] present mentally studied those with L3 = CH3C(CHZPPh2)3 = t r i p h o ~ . [ ~The qualitative MO analysis is devoted to some aspects of interpretation of the latter, which are then extended to the whole series of related compounds listed in Table 1. Thus, we will concentrate first on dimers with terminal ML3 fragments and d ” metals ( n 2 6). In these cases, three filled metal orbitals are considered as Zone pairs and the bonding within the skeleton M z X originates ~ from the interactions of the d, and o metal hybridsL6]with the pure AOs of the bridging chalcogenides. Major roles are played by the steric demands of the terminal ligands and by the highly variable electron population. Almost like sponges, the complexes [ L ~ M X Z M Lcan ~ J ”attain up to five different valence elcctron counts. For example, by considering only metals of the first transition row ( F e through Cu) and the different total charges, the species 1-4[7-101and 913] in Table 1 are the least electron rich. Complex IlI3] contains one additional electron, 5-8[”.’21 and lZC3]two, 1314] three and finally 14[13’and 15[14]contain four additional electrons. Structurally, the core MzX2 can be either planar, Scheme la, or hinged at the vector X . . . X, Scheme 1 b.[’ While the puckering favors M-M bond formation, coupling of the two chalcogenide atoms is occasionally observed both in planar (13-15) and puckered (1-4) structures.
x-x M-Xh
2.552(2) 2.007(5) 2.228(2) 2.293(2) 2.364(2) 2.575(2) 2.645(2)b 2.709(9) 2.543(9) 2.498(9) 1.98(2) 2.185( 10) puckered structure, with M-M but not X-X bond (EXAFS) 3.475(2) 2.751(6)b 2.216(2) 3.688(1)b 2.88(2) 2.339(8) 4.093(2) 3.062(2) 2.56(3) undetermined structure, most probably planar undetermined structure. most probably planar 3.434(8) 2.58(2) 2.15(1) 3.598(2) 2.6 13(8) 2.23( I ) 3.865(1) 2.208 (4) 2.225(8) 4.348(2) 2.802( I ) 2.586( 10) 4.028(3) 2.073(4) 2.265(8) 3.02(1) 2.372(3) 3.658(1) 3.568(1) 2.94( I ) 2.37( I). 2.27( I ) " 4.231( I ) 3.066(2) 2.6 12(2j 4.364(1) 2.919(1) 2.62(1 ) 3.555( 1) 3.005(2) 2.333( 1) 3.763(1) 3.136(1) 2.449(2) 3.263( I ) 2.619(2) 4.100(1) 4.104( I ) 3.258(1) 2.620(8) 3.965(1) 3.465(1) 2.632(2) + not available + 121 2.695( 1) 2.636(2) 3.648( 1)
M-M
107
180 180 180
180
180
180 180 180
180 180 180 180 180 180
180 180 180
79.8( 1) 77.1(1) 75.6( 1) 79.7(1)
MXXM
~
aCp* - [C;Mes]-, triphos = CH:(CH2PPh2):, HE2 = H B ( 3 , 5 - P r j p ~ ) ~ ;np: = N(CH2CH2PPh2)3; dppy = PPh2Py: PhrPCHzCHzPPh2. bAveraged values with standard deviations calculated according to the expression: o(d) = [E(dal, - d,)'/n(n I)]'/' 'The two values refer to the average Ir-S distances involving the fragments (Cp*)(MejP)Irand (Cp*)Ir, respectively.
Compound"
Table 1. List of the complexes studied with their salient structural features.
dppe
=
[71
Ref
P P
-
1.9 Electvaizic Effc3ct.v on the Shape qf M z X , Frameivorks
145
u a
h
Scheme I
A key-assumption is that the puckering of LjMX2ML3 is inhibited by short contacts between the substituents of the terminal ligands, even when electronic effects would be favorable. The point is suggested by the pairs of isoelectronic complexes that exhibit either the structure a or b in Scheme 1. For example, in comparing 1 with 9 or 5 with 12, it is evident that the carbonyl ligands permit the formation of the M-M bond through the molecular bending. The latter overcomes 91 1-3, the metal electron deficiency in the d7-d7dimers [(C0)~Fe(,uu-X2)Fe(C0)~],[' as well as in their reduced derivatives, 5.["' The question remains why the two additional electrons in 5 preferentially cleave the X- X bond, which is presumably stronger than the Fe-Fe bond. Simple molecular modeling confirms that the hinge at the X-X vector, observed in structures 1-5. is highly hindered even in presence of the simplest phosphine, PH1. Thus, the imposed planarity in the complexes 6-8 and 12 (apparently with 34 valence electrons as 5) alters the nature of the chemical bonding within the M2X2 skeleton. If one or two electrons are subtracted from some of the latter systems as, for example in 9-11, instability or metastability of the singlet ground state ensues. This is probably the origin of the unique reactivity of the species [(triphos)Rh(pS)2Rh(triphos)3]-', 10, towards the activation of small molecules (e.y. H2 and O Z ) . [ If ~ ' the M2X2 skeletons, that are forced to stay planar, contain one or two additional electrons ( e . q : the (triphos)2NizX:! complexes 13,[4114'' 31), in-situ coupling of the chalcogenide atoms (Scheme 2) is observed.
Scheme 2
Steric constraints similar to those of triphos are apparently also exerted by the tris( pyrazoly1)borate ligands with 3,5 diisopropyl substituents and indeed the dicopper( 11) complex [ ( HBz)Cu(p-S2)Cu(HBz)], 15, is another known example of a S5- bridge in electron rich L6M2 unpuckered framework^."^' The latter species
146
I Molecular Clusters
is particularly interesting for its close relationship with dicopper peroxo analogs, which are important models of dioxygen carrier proteins.[171The peroxo is in equilibrium with a bis(p-0x0) isomer and the X-ray characterizations have indicated that the 0-0 scission is governed by weakening of one terminal Cu-N bond. At the same time, the other two Cu-N bonds reorient towards an overall L2Cu(p-O:2CuL2 planar arrangement. This prompts us to verify whether a similar dichotomy may be expected also for the dichalcogenides. Moreover, the progressive removal of two terminal ligands provides the link to extend the theoretical analysis to the bonding in known L2M(pU-X)2ML2 species ( 18-25[18p251), which are overall planar with no trans-annular linkage. The chemical oxidation of the complex [( Et3P)2Pt(p-Te)2Pt( PEt3)2] 22 induces puckering and Te2 coupling as shown by the structure of the dication 26.["] The electronic underpinnings, which highlight the latter structural trends, can also explain the peculiar behavior of the redox process as observed by cyclovoltammetry . Although several theoretical studies (ab-initio and DFT ) have appeared for specific cases,[21-26p291 th'is chapter exploits qualitative MO theory and, in particular, the undeniable power of the correlation diagrams, to draw the links between molecules different but similar. The arguments are developed with the help of the EHMO method[301and of the graphic capabilities of the package
1.9.2 Discussion 1.9.2.1 Chemical bonding in less electron rich M2X2 planar skeletons It is convenient to start the MO analysis from the model [(PH3)3Co(pu-X)2Co(PH3)3], Scheme la, which has a planar M2X2 skeleton. Many pieces of chemical information are obtainable by applying distortional perturbations to the basic MO picture of the latter. In particular, it may be inferred how the formation of M-M and/or X-X trans-annular bonds in either model a or b in Scheme 1 depends on the electron count. Moreover, some hints can be gained about the potential reactivity of these species. In contrast to the CzVsymmetry of a in Scheme 1, the experimental structures of the isoelectronic complexes 6-8" 21 and l2I3] have quasi-c~hsymmetry (staggered L3M fragments[321).The choice of model a in Scheme 1 allows thc monitoring of the inter-conversion to b in Scheme 1 under the particular symmetry without biasing the theoretical analysis significantly. In fact, the torsional barrier for the ML3 reorientation is small, consistent with both NMR and calculations (the EHMO value for X=S and M=Co is only ca. 2 kcal/mol). Along the pathway, the molecule bends at the X . . . X vector thus forming the M-M bond, while each ML3 group rotates about the axis passing through the metal and parallel to :Y . . . X
1.9 Electronic Effects OM the Sliuppe of M2X2 Fruuneworks
147
(max. rotation up to ca. 15"). Finally, the X-X bond is formed by the hinge of the two MXM planes about the M-M vector. It is worth mentioning that, due to the steric problems arising on reorienting ML3 groups, CO or hydride ligands are more conveniently used than PH3. This can be achieved with confidence as the trends in the macroscopic M O arrangements are fairly constant in all cases. Fig. 1 presents the basic orbital interactions in the planar model a in Scheme 1 . The L6M2 grouping (on the left side) is assembled from two pyramidal L3M fragments, which, for the electronic configurations > d 6 , have six filled non bonding d orbitals (ir. two tZg sets, as each fragment L3M descends ideally from an octahedral For the d7-d7complexes 6-8 and 12, the electron pair in excess is assigned to one of the in-phase and out-of-phase combinations of the metal d, hybrids, which are all sketched on the left hand side of Fig. 1 together with those of the higher CJ hybrids. All of the combinations of chalcogenide atomic orbitals are depicted on the right - hand side. The n-combinations of both the L6M2 or X2 groupings are distinguished as nli or n1 although M2X2 is not an actual mirror plane in the point-
\
/
{with X2 2a}
1a1 *----X
X'
Figure 1. Diagram for the interaction between the frontier MOs for the LhM2 and Xr groupings.
148
1 Molecular Clusters
group C2". The overall ring a-bonding stems from four delocalized interactions of ~ ~ * ) , and (~1110*).[~~' These correspond to the largest the types (alo), ( n ~ ~ * 1 n(a*lnli) Overlap Populations between FMOs and, although the 20*( XZ)combination is already slightly above its metal dn11 partner (bl), X2 is considered as a tetraanion able to donate at least eight a electrons. Figure 1 highlights a fifth donation of the type 711 (see the filled-bonding/emptyantibonding MOs 3al and 4al), which is almost as strong as any of the a interactions. The other two MOs of n ~ type, * 3b2 and 2b1, are localized on MZ and X2, respectively, and are described as non-bonding electron pairs. Thanks to the Z_L donation, the metals reach formal electron saturation, i.e. the electrons count for the dimers 6-8 and 12 is 36, with one resonant M=X double bond, as shown schematically in Scheme 3. The one-electron oxidation of [(triphos)Co(p-S)2Co(triphos)], 12, yields the monocation 11 whose structural features are not dramatically changed.[31 The framework M2S2 remains planar and the M-M distance is still non-bonding although shortened (3.43 us. 3.60 A); only the orientation of the terminal (triphos)Co groups is no more staggered. This is consistent with the localized metal character of the MO, which looses one electron ( i e . M~-zL*,3b2). More intriguing are the possible consequences in the doubly oxidized dication 9 whose structure could not be determined.[31By referring to Fig. 1, the new HOMO-LUMO gap (2bl-3b2) is small (<0.5 eV). Since a triplet ground state cannot be excluded, a reinvestigation of the magnetic properties of 9 would be necessary. In contrast, the Rhz dication 10 is certainly diamagnetic.['] Specific EHMO calculations confirm that 2bl and 3b2 are still the two frontier levels but their gap is large enough (ca.1 eV) due to the intrinsically higher energy of rhodium d orbitals. Complete depopulation of 3b2 magnifies the unsaturation at the rhodium dnL orbitals, which is in part compensated by the delocalized M=S double bond (Scheme 3). In spite of the latter, the Rh2S2 nl framework can be ideally reconciled to the quadrupole, shown in Scheme 4, determined by the pair of empty Rh dnL orbitals that coexist adjacent to the two filled S pnl orbitals.
Scheme 4
Scheme 5
The separation of the nl charges with nucleophilic sulfur and electrophilic rhodium atoms, confers unique reactivity to 10. For example, the reversible activation of two H2 molecules and the formation of the structurally characterized complex [(triphos)HRh(,~-SH)2RhH(triphos)]+~ has been reported.['' As shown in Scheme 5, polarization and heterolytic splitting of two H:, molecules could be induced by the highly polarized Rh"+/S"- linkages, perhaps asynchronously as a ter-molecular reaction is thermodynamically improbable. A comparable M O interpretation of the 0 2 heterolytic splitting over the dipole Ir"'/C"has been previously presented.[351 The ni donor capabilities of the chalcogenide bridges towards the electron deficient metals need not be invoked if a tvcms-annular M-M cr bond could be formed in M2X2 cores similar to some planar M4 systems with the expected electron count. but not in For example, the fifth M-M 0 bond is observed in [Req(C0)16]2-[3h1 Os4(CO)I & ~ ' ] . which has two additional electrons. An MO analysis could highlight the electronic underpinnings of such a remarkable structural difference.r341Here, we simply mention that by applying similar criteria to M:,X? planar frameworks, the proper electronic conditions for the formation of the trans-annular M-M bond cannot be met.["1 Otherwise the latter can be formed on bending the molecule X vector (ride infiva). Before analyzing molecular puckering in detail, we mention other planar cases in which the metal electrophilicity of the skeleton with the same electron count as 10 is relieved by the addition of one terminal ligand. For complexes containing triphos, such a possibility is perhaps limited to the small hydride ligands found in the hydrogenated derivative of Conversely, if the terminal L3M fragment is of the type CpM, there is room also for more sterically demanding donors including phosphines. This is the case of the complexes (Cp*)2L,nIr:,(,u-S)2[L = PMe3, CO, CN-t-Bu, m = l,2].'401Schematics of the latter species, 16 and 17 in Table I , are shown in Scheme 6a and 6b, respectively. Since the fragments [Cp*Ir]'+ and [(triphos)Rh]'+ are i.~olohal,[~'~ it is obvious that in 16 the reoriented dnl orbitals ( F M O s la1 and lb:, in Fig. 1) are used to coordinate the L ligands. Conversely, the removal of only one ligand in 17 (Scheme 6b) liberates one dnl orbital and restores Ir=S double bond at the corresponding metal atom as experimentally confirmed by the two 0.1 A shorter I r 4 distances. If also the second L ligand could be removed (not proven experimentally), molecu-
150
I Molecular Clusters
2.37
L
a
b
Scheme 6
lar puckering could be sterically feasible, especially if Cp* were substituted by less demanding Cp precursors. As will be illustrated next, electronic factors do not allow direct access to the puckering route, which leads to the formation of the M-M linkage.
1.9.2.2 Puckering of the M2X2 skeletons Terminal CO ligands are sterically consistent with the observed puckered struc( X = S, Se, Te).r7p99111 The ture of the iron complexes [(CO),Fe(p-X)2Fe(CO)3]o,p2 analysis of the latter can be conveniently carried out with the help of Walsh diagrams, which start at the well understood MO picture of the planar skeleton (Fig. 1). This strategy has been proven to be very informative for other comparable [(CO)3M(pU-Y2)M(CO),] species [Y = CO, SH, PH2].[421 While the formation of the M-M linkage is induced in all cases by the bending of the molecule at the X . . . X vector, the pathway (Fig. 2a) features the simultaneous formation of the X-X linkage on variation with Fig. 2b. It will be evident that the two final MO pictures require the difference of two units in the total electron count. Both Fig. 2a and 2b have the common feature the crossing between the levels 4al and 3b2, which are LUMO and HOMO of d7-d7 planar complexes (6-8, 12) that have previously been considered to be saturated via the partial M-X nl bonding. Since the metal d, components of 4al are in-phase (see Fig. I ) , this overall M2X2 n ~ antibonding * level converts ultimately into the Mz-a bonding orbital in Fig. 3. The residual X2 contribution in the latter suggests that the classic M-M bent bond[261maintains a reminiscence of the unperturbed d, pz interaction. Analogously, 3b2 (the original HOMO with M2-711* character) converts into the bent M2cr* partner of Fig. 3 in both Fig. 2a and 2b. The basic difference between Fig. 2a and 2b concerns 2bl level, which destabilizes sharply in the former. The reason is as follows; molecular puckering itself mixes the characters of the initially low and filled lbl and 2bl MOs, which avoid the crossing in any case. Recall that in the planar model (Fig. 1) the latter levels, although belonging to the same symmetry, are quasi-orthogonal to in-plane cr annular-bonding ((qla*)) and X2-n1* characters, respectively. Since M2X2 is not a real mirror -
1.9 Electronic Efects on the Shupp of M2X2 Frurne1zwk.Y
a
151
b
Figure 2. Compariwn of the Walsh d i d p m s for the puckering of the MIX? skeleton with (a) or without [b) X-X coupling
plane, even the incipient molecular bending rehybridizes together the pnl and the in-plane p radial orbitals of thc bridging atoms. This also holds for their al combi~ nations so that the results can be schematically represented as follows: the p n orbitals get more and more involved into CT donations to the metals (Fig. 4a and 4b), while the originally out-pointing p orbitals ( 2 1 ~ and 2rr* in Fig. 1) become perpendicular to the planes MXM (Fig. 4c, 4d). At the large X-X separations (as in the dianion 5, which has been shown by EXAFS techniques[""] to contain M-M but not X-X bonds) the 3a1 and 2bl MOs are lone puivs that are somewhat repulsive to each other. Moreover, together with the M-M 1~ bonding orbital 4al (Fig. 3), they represent the three highest occupied MOs, all having X2 contributions. The well known nucleophilicity of the chalcogenide atoms in 5[""] is fully consistent with such an MO picture. When the X- X distance shortens as in Fig. 2a, the pristine 2b1 MO (Fig. 4d) converts into a high lying Xl-cr* MO while the final 2bl orbital is the M-X antibonding counterpart of Fig. 4b. Accordingly. in the complexes with four M-X single bonds plus the X-X and M-M ones, the levels 2b1, 3bl and 3b2 (Mz-a*) must be empty.[431In contrast, the original LUMO 4a1, which has converted into
152
I Molecular Clusters
the M2-a bonding MO, must be populated. If the route (Fig. 2a) is taken starting from planar and stable 36e systems (e.g. 6-8, 12), then one of the two electron pairs in 2bl and 3b2 should be discharged from the system while the other one would be transferred in 4al. In any case, the process is symmetryforbidden[441with the Surther complication that there would be two forbidden electron jumps in a row, as suggested by the Fig. 2a. Interestingly, also molecular puckering, which is not accompanied by X-X bond formation (pathway in Fig. 2b) and does not require variation of the total electron number, is a symmetry forbidden process (crossing of filled 3b2 and empty 4al levels). By disregarding the steric problems and irrespective of lhe coupling of the chulcogenide atoms, the puckering of planur skeletons or its inverse process ( M-M bond cleavage) is not electronicully allowed.
<$ "
M
M
a
b
C
d
Figure 4. Representation of the molecular orbitals of (a) 2a1, (b) lbl, (c) 3a1, and (d) 2bl.
1.9.2.3 Coupling/ uncoupling of dichalcogenide within puckered frameworks Detailed mechanistic studies have demonstrated that the reduction process for [(C0)3Fe(p-Se)2Fe(C0)3],2, ultimately leading to the corresponding dianion 5, is stepwise and not unimolecular.[""] The added electrons are unpaired at the bridging atoms and the radical species combine in a tetrameric intermediate whose scission finally yields the dianonic dimer 5 with Fe-Fe but no Se-Se linkage. Apparently, inconsistent with the latter pathway, the electrochemical oxidation of the bis-sulfido dianion 5 has been quoted[221to lead directly to the uncharged species 1
I . 9 Electronic Effects on the Shape of’ M2X2 Frcrnzeworks
153
. ‘b,
/ 3a,
Figure 5. Evolution of the MOs for elongating the X ~ - Xdistance within MzX2 puckered skeletons.
and to be reversible. While the theoretical analysis of the stepwise reaction seems a rather difficult task, it is interesting to evaluate the problems related to concerted coupling/uncoupling of the two chalcogenides via redox chemistry. The Walsh Scheme ( Fig. 5 ) , which correlates the MO pictures on the right hand sides of Figs. 221 and 2b. respectively, clearly shows the 2bl/4al inter-level crossing. On the left hand side, the model of the uncharged complex 1 has M-M and X-X bonds, hence the corresponding g * levels 3b2 and 3bl are empty and so 2bl exhibits overall M2X?-o* character. The question is, which MO could initially host the additional electrons. Obviously, the ideal candidate to attain the conformer such as 5 upon X-X cleavage would be the X2-cr* M O 3bl. which is too high in energy. The other two LUMO candidates, 2bl and 3b2 are rather close in energy in a region where there are even more closely-packed levels (not shown). In any event, a reduced system maintaining the original structure appears inconsistent with the singlet ground state. Other authors, on the basis of HFS calculations (substantially consistent with the present EHMO results), suggested that 2bl was the acceptor of the new electrons on account of its X?-o* character.[281However, x2-0~ is only one component of 2b1, which is more accurately described as an overall MzX2-o* level. Accordingly, the addition of electrons is expected to weaken the skeletal M-X bonds. The MO 3b2 with its M2-o* character is perhaps a better candidate, not only because it is already slightly lower than 2bl but because it is significantly stabilized by even a small elongation of the M-M bond (the weaker one). The trend is already evident in
154
I Molecular Clusters
Fig. 2a (from right to left) as 3b2 goes down in energy more quickly than 2bl. Moreover, if M-M but not X-X increases (the corresponding diagram is not shown for the sake of brevity), 2bl remains almost constant while 3b2 stabilizes and crosses the ascending 4al level at some point. This suggests that M-M uncoupling is favored over that of X-X. Consistent with the experimental data[""] the MO arguments remove the possibility that the reduction of 1 may lead directly to the dianion 5. It is more likely that the M-M bond elongates while the M2X2 skeleton remains puckered at the X-X linkage. The distortional trend could be inverted if the electrons were removed before 3b2 crosses 4al (fast cyclovoltammetry). On the other hand, the oxidation of 5 by subtracting two electrons from 4al (the M-M bonding MO, at the right hand side of Fig. 6) would soon contrast withfovbidden 2bl/4al inter-level crossing. Only if 2bl was already higher than 4al would the oxidation lead directly to 1. In any case. based on the previous MO arguments, it is hard to see how the process in question can be electrochemically reversible. As a final consideration, the coupling of two naked chalcogenides to generate a formal Xz2- unit in-situ is a rather difficult process for the complexes of the triads of iron and cobalt. A puckered dimer forms only from reactants containing the X2 unit. The X-X linkage cannot be broken directly. Conversely, direct X-X coupling seems possible within L6M2X2 assemblies containing the more electron rich metals such as Ni and Cu.
1.9.2.4 Coupling of chalcogenide atoms in electron rich M2X2 planar frameworks Remarkably, the complexes [(triphos)MX2M(triphos)lnexist with five different electron counts (from M = Co, n = t 2 to M = Ni, n = 0). The electron rich [NizX2]" frameworks (13, n = +1, and 14, n = 0) feature the planar skeleton in Scheme 2 with a somewhat elongated trans-annular X-X linkage (see Table 1). The evolution of the MOs on squeezing the rhombus is presented in Fig. 6. For the sake of continuity, the working symmetry is still C2" but also the C2h labels of real molecules with staggered ML3 fragments are reported on the right-hand side. The MO levels in Fig. 6 correspond to those on the left hand sides of Figs. 2a and 2b. In the uncharged complex 14, the scheme is fully populated up to 4al. Because the latter has overall M2X2 nl* character, not only the previously important X2 + M2 n~ donation is cancelled but significant n l repulsions intuitively disfavor X-X coupling. In contrast, the structure with the latter trans-annular bond is computationally more stable (by ca. 0.5 eV) than the starting structure. Moreover, barriers of a few tenths of an eV separate the two minima so that isoelectronic, but still unreported bis(pcha1cogenide) isomers are not unrealistic. Quantitative EHMO results are questionable especially when bond making/breaking is implied, nonetheless the switch of the HOMOS (4al and 2bl) along the pathway is consistent
1.9 Electronic EfIL’ctson the Shupe of M2Xz Frameworks
155
Figure 6. Evolution of the MOs for squeezing the MzX2 planar skeleton at the X-X vector.
with the two minima and with a symmetrj~cillon*eu’inter-conversion, hence with the dichotomy of the species. Surprisingly, the small total energy variations result from a complex balance of major stabilizing/destabilizing effects on the various populated levels (those evident in Fig. 6 and others at lower energies). In particular, 2bl with M?XZ (nl,la*) bonding character exhibits the greatest destabilization on reducing the X-X distance because of its X?-o* component. Moving towards the right, 2bl crosses 4al but remains at an accessible energy to be the HOMO. The evolving character of 2bl is most critical for the relation between the annular and trans-annular bonds of the M2X2 skeleton. Figure 7 proposes the ring bonding interaction (rill 12n*) already depicted in Fig. 1 and highlights the destabilizing effects on 2bl of the low lc/* combination of chalcogen s orbitals.
\
Scheme 7
I
156
I Molecular Clusters
In the cobalt dimers 6-8 and 12, the Mz-n,,FMO (bl) accepted the electrons from the S2-20* FMO, which, for non-bonding X-X distances, is low and filled. The donor-acceptor roles are now inverted. In fact the d9-d9 metal configuration of 14 implies “back-donation” from bl into the X2-20* FMO, which is forced to be high lying and empty by the short X-X distance. The energy of the resulting MO 2bl (still M2X2-(njl120*) bonding) depends on the weight of its X2-20* component. Ultimately, when the latter FMO is too high, the bonding interaction with the metals vanishes as well as the residual electron density back-donated into X2-20*. Under these circumstances, there remain only three out of four M-X linkages while the X-X bond order becomes practically one. As is often the case in chemistry, the actual system can be described as being between the two limiting situations, i.e.: traizs-annular and annular bonding are in competition and form at the expenses of each other. Recall that the dichalcogenide grouping was the formal 10 electron donor in planar dimers such as [(triphos)2Co2S~](12, as well as 6-8), which were formulated as 36 electron species. The apparently incongruent 38 electron count of [(triph0~)2Ni2Te2],14,[l3]and of 15[l4] as well, is rationalized by the MO arguments. The XZ grouping becomes only a 6 electron donor by virtue of the inactivate nl donor capability and of the inverted electron flow for one of the four annular M-X bonds (from X2 + M2 to M2 + X2). Ultimately, 14 and 15 may be also counted as having 36 electrons (12 from the terminal ligands, 18 from the two d9 metals and 6 from the unit X22-). In a similar way to metals, the chalcogenide atoms tune up the number of electrons made available to the cluster and, accordingly, they are occasionally regarded as integral components of transition metal clusters. Regardless of the substitution of Te with S atoms, 13 has one electron less than 14. The rather long S-S bond of 2.21 A in 13 (approximatly one step before the right hand side of Fig. 6) raises the question whether one electron is missing from 4al or 2bl. The preparation of 13 involves H2S as a reactant and may give, as an alternative product, a complex where a single sulfur atom is linearly bridging the nickel metals.[451Accordingly, 13 is probably formed from an in-situ coupling of two naked S atoms. In this case, 4al is the best candidate over a long range, to have one missing electron. Given the M2X2-n1* character of the latter, the stabilizing n bonding within the ring is partially restored. In contrast, one electron missing in 2bl would weaken the M2 + X2 back-donation, hence the ring 0-bonding. Moreover, the inversion between the SOMO (4al) and second HOMO (2bl) is symmetryfirbidden along the pathway so that the shortening of the S-S linkage must be interrupted before the crossing point. Since such an electronic constraint does not apply to closed shells where both 4al and 2bl are populated, the in-situ S-S coupling may proceed to give the shorter S-S distance of 2.07 A in the Cu2 dimer 15.[’41 Although only dichalcogenide dimers are the topic of this article, we cannot help but point out the peroxo analog of 15, namely ( HBz)Cu(pu-02)Cu( H B z ) , [ ~and ~ ] the very exciting L6Cu202 complexes of Tolman, which exhibit the dichotomy of per-
1.9 Electronic EfSrctJ on the Shappr of MzX? Framettwks
157
Scheme 8
0x0 and bis(p-0x0) forms." 71 Experimental and theoretical a h - i n i t i ~ [ ~studies '] have shown that the latter can be in equilibrium. Moreover, the X-ray structure of the his(p-0x0) derivative [L'pr'Cu(p-oxo)~Cu LIPr3IS 71 indicates that the ( 3 2 0 2 planar skeleton with no trans-annular bond is permitted being accompanied by some rearrangement of the terminal L1M units. As shown in Scheme 8, the latter loose threefold symmetry due to the lengthening of one Cu-L linkage and by the movement of the other two into the Cu202 plane. The connectivity of the tripodal ligand prevents the L3M fragments from transforming into L2M ones. As mentioned, no bis(p-chalcogenide) isoelectronic analog of the ditellurido/disulfido complexes 14 and 15 has been reported. However, a rearrangement of such as shown in Scheme 8 drastically shifts the equilibrium towards the broken X-X linkage and, ultimately, the departure of one terminal donor stabilizes the d8-dX L2Ni(,u-S)2NiL2species by > 3 eV. In fact, recall that in these electron rich species each LIM dnL hybrid (xz + p_)is populated. In going from L3M to L2M fragments, the d, and p, components separate as shown in Scheme 9 with p- becoming the empty uninvolved orbital typical of the square-planar coordination. Conversely, not only is significant energy gained by the electrons in the pure z2 orbital with respect to the d, hybrids but also the overall repulsion of the four 711 electron pairs is strongly mitigated within the M2X2 ring. Another subtle effect arises from the reorientation of the in-plune dnl/ metal hybrids, in particular those contributing to the bl interactions with the bridging atoms. By referring to Scheme 7, the tilted dn,, hybrids of L3M fragmentsc6]lie slightly out of the M2Xz plane whereas the corresponding L2M hybrids are optimally oriented to interact with the 20' combination of the chalcogenides. The
Scheme 9
158
I Molecular Clusters
greater the back-donation into the latter X-X antibonding orbital, the more susceptible is the bond to being broken. This situation recalls the dichotomy of metal olefin complexes.[481In the case of Zeise’s salt, minimum back-donation occurs from a pure d orbital of the fragment L3Pt(II) into the n*c-c level. Conversely, L2Pt(O)fragments have a filled and well hybridized dn orbital that is responsible for a large back-donation, which ultimately favors the cleavage of the C=C 71 bond (metallo-cyclopropane
1.9.2.5 Uncharged and dicationic complexes of the type L2MX2ML2 The electronic features of the complexes [L4M2X2I0,+*( M = Pt, Pd) can be easily derived from the previous MO arguments. The uncharged species 18-24 in Table 1 are planar, whereby the local square coordination is typical of Pd( 11) and Pt( 11) ds atoms. Only one isoelectronic complex 25 is reported to be puckered.r251Recent DFT and ab-initio calculations point out either the energetic preference for the overall planar geometry[”] or a flat potential energy surface on puckering.[291 Conversely, the removal of two electrons as in 26 favors, both experimentally[221 and computationally,[211molecular bending with the formation of the trans-annular X-X bond. The Pd, Pt ds metals have low acidic character at the p; orbital and the axial ligand is eliminated. In turn, the absence of the latter causes the largest stabilization of the z2 orbital (Scheme 9) hence of the whole dimer (additionally, no major dnl/pnl repulsions are active). In a model of 18-24, the interaction (dn111X::-2o*) is more effective than in corresponding L6M2 complexes. In fact, the L2M hybrids are well oriented and, since the metals belong to the second or third transition row, are quite diffuse. At the left-hand side of Fig. 7, the M2X2 o-bonding MO 2bl (donation 2o* i nil) lies below the grouping of 4 4 non bonding d orbitals typical of metals in square planar coordination.r61Similar to the L6M2X2 case in Fig. 2a, 2bi destabilizes on shrinking the X-X distance while the M2X2 skeleton bends. The puckering mixes together the in-plane pa and p n l orbitals of the bridging atoms and produces MOs quite similar to those in Fig. 4a-4d. As shown by the CACAO drawing on the right-hand side of Fig. 7, 2bl is ultimately the M2X2 g * MO. Evidently, the filled Xz-nL* combination has become the major donor towards the reoriented metal dnl1 hybrids. Similar to Fig. 2a but not shown in Fig. 7, a third bl level inherits most of the X2-o* component of the original 2bl and is ultimately found at high energy. In any case, 2bl also looses much energy along the pathway and the final geometry can be attained upon discharge of the electron pair, which populates the level.[501The structure of complex 26, which is the oxidized derivative of 22, is consistent with the latter MO picture, although it remains questionable whether the redox process follows the path shown in Fig. 7. The dashed line in Fig. 7 (from the right to the left hand side) represents 2bl
+
1.9 Electronic Ef(>rtson the Shtrpe of M2X2 Framewwks
159
Figure 7. Evolution of the MOs for the puckering of a L4M2X2 model with simultaneous X-X coupling.
when the M2X2 skeleton unpuckers while maintaining the X-X linkage. The CACAO drawing on the extreme left confirms that 2bl originates now from the backdonation of diqI into the high lying FMO X2-20* as it occurred for the L6M2X2 species (refer to the Fig. 6 at the right-hand side and to Scheme 7). Importantly, 2bl has inverted the character relative to the M2X2 ring bonding (from o* to a) and has obviously gained energy. In no case, however, can it descend below the block of d orbitals on account of the high lying X2-20' component. If populated, the total energy difference with respect to the planar isomer with no trans-annular linkage is large (>2 eV) and the dichotomy invoked for L6M2 skeletons is now less probable. Even so, it is possible that the planar L2M(p-X2)ML2 isomer with trans-annular X-X bond may have a finite lifetime. In fact, according to the experimental data'221 PEt3)2]" ( n = 0, +2), the dication 26, reported for the species [( Et?P)?Pt(p-Te2)Pt( obtained viu the chemical oxidation of 22, exhibits quasi-reversible oxidation waves in cyclic voltammetry. The MO picture outlined suggests that fast addition/removal of electrons to and from 2bl (dashed line in Fig. 7) may be possible as the level remains fairly isolated in the frontier MO region. On the other hand, the reported irreversible electrochemical oxidation of 22 confirms that 2bl is not the best candidate for the removal of the electrons as it lies below the non-bonding d-centered MOs. With a lack of additional pieces of experimental information, it is quite hard to define the MO which actually becomes depopulated and the structural rearrangements, which accompany the irreversible oxidation wave in this case.
160
I Molecular Clusters
1.9.3 Conclusions In the article, a link between the electronic structures of a large class of metal dimers containing two naked chalcogenides as bridges has been established. For the reported L6M2X2 species, the interconversion between planar and puckered M2X2 skeletons (hinged at X . . . X) is electronically prevented even if sterically allowed. Neither can the addition/removal of one or two electrons from the system change the situation. Within the planar or bent frameworks, specific electronic factors govern the in situ formation of the X-X linkage and such a coupling may occur only with electron rich metals, which need not have a M-M bond. The dichotomy of species with or without the trans-annular linkage is in principle possible but a certain energy barrier separates the two isomers. No major steric factors work against the puckering of L4M2X2 complexes because no direct M-M linkage is required. The observed planar/bent rearrangement, which occurs upon chemical oxidation of selected L ~ M z X species, ~ contributes to the X-X coupling as well. However, when carefully evaluated, the underlying electronic effects indicate that the interconversion route is not even in this case the most direct one.
1.9.4 Appendix In performing the extended Huckel calculationsr301care was taken that the geometries were as close as possible to those of the experimental data. The various input files for the CACAO programt3’] are available from the authors on request (e-mail:
[email protected]). The STO parameters used were those standard in a latter package and referenced in its documentation.
1.9.5 Note Added in Proof Since the manuscript was sent to the Editors a few papers have appeared related to species of the type LzM(p-S)zML2. A puckered structure, comparable with that of 25-26, has been determined for a Pt derivative by means of X-ray diffraction,[”] while a Pd analogue has been characterized by computational methods.[521A theoretical analysis of the same framework has emphasized the role of the direct M-M interaction for skeletal puckering.[’ 31 Finally a review article highlights the chemical reactivity promoted by the Pt2S2 core.[s41
References [ 11 Cambridge Crystallographic Data Center, University Chemical LdbOratOry, Lensfield Road, Cambridge CB2 IEW, UK.
121 (a) J. J. Mayerle. S. E. Denmark, B. V. DePamphilis, J. A. Ibers, R . H. Holm, J . A m . Chem. Soc. 1975. Y7, 1032-1045: ib; M. A. Bobrik, K. 0. Hodgson, R. H. Holm. Inory. Chem. 1977. 16, 1851-1858: ( c ) A. Salifoglou. M. G . Kanatzidis, D. Coiicouvanis, Inorg. Chrm. 1988. 27. 3394-3406: (d) N . Ueyama. S. Ueno, T. Sugawara, K. Tatsumi. A. Nakamura, N. Yasuoka. J. Chrni. Soc. Dulton Trcm. 1991. 2723-2727; (el I. Bertini. S. Ciurli, C. Luchinat. Structure rind Bonding. 1995. 83, 1-54. 131 C. A. Ghilardi, C . Mealli, S. Midollini, V. I. Nefedov. A. Orlandini, L. Sacconi, Inor~g.Chem. 1980. 10. 2454 2462. 141 C . Mealli. S. Midollini. Inorg. Chem. 1983. 22, 2785-2786. 151 C. Bianchini. C. Mealli, A. Meli, M. Sabat, Inorg. Cheni. 1986, 25, 4617-4618. [6] T. A. Albright, J. K. Burdett and M. H. Whangbo, Orbital Interuction.s in Cherni.stry, 1985,
John Wiley, New York. [ 7 ] ( a ) C . H. Wei, L. F. Dahl. Inorg. Chem. 1965. 4 , 1-11: 1. L. Eremenko. H. Berke. A. A. H. van der Zeijden, B. I. Kolobkov. V. M. Novotortsev, J. Orguriomet. Chem. 1994, 471. 123-132. [Sl C . F. Campana. F. Y. K. Lo: L. F. Dahl. Inorq. Chem. 1979, 18, 3060- 3064. [9] R . E. Bachman. K. H. Whitmire. J. Organomer. Chem. 1994, 47Y. 31-35. [ l o ] R . Minkwitz, H. Borrmann, J. Nowicki. Z. Ncituif?orsch. 1992, 47b. 915 918. [ I I ] (a) T. D. Weatherill, T. B. Rauchfuss, R. A. Scott. Inorg. Chem. 1986, 25. 1466-1472: (b) D. Seyferth, R. S. Henderson. L. C. Song, Oryu/?ori?et(i//ic,(.1982, I , 1255133. 1121 H. F. Klein, M. GaM. U. Koch. B. Eisenmann, H. Schafer, Z. Nuturfiirsch. 1988, 4 3 , 830838. [ 131 M. DiVaira, M . Peruzzini; P. Stoppioni, J. Chrm, Soc. Clwn. Conimun. 1986, 374-375. [ 141 K . Fujisawa, Y. Moro-oka, N. Kitajima. J. Chem. Soc. Chem. Conzmun. 1994, 623-624. [ I S ] The LhM2 unit in Scheme Ib i s a .suivhorse typical of systems completed by a n-conjugated chelate in the riding bridging mode (e.g. diamidoleiie or dithiolate) and recently investigated by
one of the authors (C. Mealli)"61. Many of the previous MO concepts can be easily extended to the present complexes where the ric1c.r is a pair of naked chalcogenide atoms. [ 161 C . Mealli. A. lenco, A. Anillo, S. Garcia-Granda. R . Obeso Rosete. Inorg. C h f w 1997, 36, 3724- 3729. [ I71 W. B. Tolman, Ace. Choi?. Rex 1997. 30, 227-237 and references therein. [IS] J. G. Brennan, T. Siegrist. S. M. Stuczynski. M. L. Steigcrwald, J. Am. Chetn. Soc. 1990. 112, 9233-9236; K. Matsumoto. C. Nishitani. M. Tadokoro, S. Okeya. J. Chcni. So(,. Drilfori Trrms. 1996, 3 3 3 5-3 3 36. [ 191 C.A. Ghilardi. S. Midollini. A. Orlandini, submitted to Inorg. Chern. Conimzin. 120) V. W. W. Yam, P. K. Y. Yeung. K. K. Cheung. J . Chem. Soc. Cheni. Commuri. 1995. 267269. 1211 A. Bencini, M. DiVaira. R. Morassi, P. Stoppioni, F. Mele. Polyhedron 1996, 15, 2079-2086. 1221 A. L. Ma. J. B. Thoden, L. F. Dahl. J. C/iem. Soc. Chrm.Conzmun. 1992. 1516-1518. 1231 (a) R . D. Adams, T. A. Wolfe. B. W. Eichorn. R . C. Haushalter, Po/y/iedron 1989. 8, 701; (b) D. Fenske. H. Fleischer. H. Krautsheid. J. Magull. C. Oliver, S. Weisberger, Z. Nuturf i w s c h . 1991, 46b, 1384. 1241 .H. Wolkers, K. Dehnicke. D. Fenske, A. Khassanov, S. S. Hafner, Actci CrystuNugr., See. C 1991, 47, 1627. [25] R. Mason. B. Law, D. M. P. Mingos. unpub/i.slzed ivork referenced in C. E. Briant, C. J. Gardner, T. S. A. Hor, N. D. Howells. D. M. P. Mingos. J. Chem Soc. Dalton. Tran.~.1984, 2645-2651. 1261 K. Teo, M. B. Hall. R. F. Fenske. L. F. Dahl, Inorg. Chrni. 1975: 14, 3103 3117. [27] L. DeKock, E. J. Baerends. A. Oskam, Inorg. (%em. 1983. 22; 4158 -4159. [28] L. DeKock, E. J. Baerends. R. Hengelmolen, Orguno/nctallics, 1984, 3, 289-292. (291 M. Capdevila, W. Clegg, P. Gonzalez-Duarte, A. Jarid, A. Lledos, Inorg. Cheni 1996,35,490-497. 1301 (a) R. Hoffmann. J. Chew. Phys. 1963. 3Y, 1397; (b) R. Hoffmann and W. N. Lipscomb, J. Clieni. Phys. 1962.36,2179; [c)R. Hoffmann and W. N. Lipscomb, J. C'hcw. Phys. 1962,37,3489. [31] C. Mealli. D. M . Proserpio. J. Cheni. E(h/c. 1990, 67. 399-402.
162
1 Moleculuv Clusters
[32] In the series [ ( M ~ ~ P ) ~ C O ( , ~ - X ) ~ C O ( Mthe ~ ~Se/Te P ) ~ ]derivatives ,[’*’ differ from those of the S derivatives having one Co-P bond within the plane MzX2 rather than perpendicular to it. Short contacts between the bulky terminal ligands and the more diffuse chalcogenide atoms can cause such a difference. [33] C. Bianchini, C . Mealli, A. Meli, M. Sabat, P. Zanello, J. Am. Chem. Soc. 1987, 109, 185-198. [34] C . Mealli, D. M. Proserpio, J. Am. Chem. Soc. 1990, 112, 5484-5496. [35] P. Barbaro, C. Bianchini, K. Linn, C. Mealli, A. Meli, F. Vizza, Inorg. Chim. Actu. 1990, 112, 5484-5496. [36] (a) M. R. Churchill, R. Bau, Znorg. Clzem. 1968, 7,2606; (b) G. Ciani, G. D. Alfonso, M. Freni, P. Romiti, A. Sironi, J. Organomet. Chem. 1978, 157, 199. [37] V. J. Johnston, F. W. B. Einstein, M. R. K. J. Pomeroy,J. Am. Chem. Soc. 1987,109,8111-8112. [38] By summarizing the arguments of reference 1341, one M-M annular bond stems from the interaction (2a*j.rrll)where, as in Fig. 1, 2a* is a combination of high lying metal hybrids. On squeezing the square along one diagonal, the lower la* (from the filled “tzg” sets) becomes the favored partner of the rill combination at the distal atoms. If the antibonding combination between the two latter FMOs rises enough in energy to be depopulated (here is the difference between 0 s and Re derivatives), then ring bonding stems from the donation of la’ into 7111.By focusing on the FMOs of the shortly separated metals, while la’ and 20 carticipate in ringbonding, the low-filled l a and the high-empty 2a*combinations are only apparently inert. In fact, together they are responsible for the new M-M truns-annular linkage. A similar description was proposed for the Fe-Fe linkage in Fe2(C0)9.[391 In contrast, no M-M bond seems possible in planar M2Xz skeletons with the electron counts of 6-12. In no case does the squeezing of the M-M diagonal (irrespective of the possible steric problems), push the bz antibonding combination between a * ( z 2 )and Xz-nll (refer again to Fig. 1) above the LUMO 4al so that the electrons can be poured from one level to the other. 1391 (a) C. Mealli, D. M. Proserpio, J. Organomet. Chem. 1990, 386, 203-208; (b) C. Mealli, J. Reinhold, E. Hunstock, New J. Chem. 1994, 18, 465-471. [40] D. A. Dobbs, R. G. Bergman, Inorg. Chem. 1994,33, 5329-5336. 1411 R. Hoffmann, Angew. Chenz. Int. Ed. Enyl. 1982,21, 71 1-724. [42] A. R. Pinhas, R. Hoffmann, Inorg. Chem. 1979, 18, 654-658. 1431 The participation of both the p7zI electrons pairs of X2-2 in a donations enables the consideration of the unit as an eight electron donor. Thus, in contrast with previous suggestions,[’*] 1 can be counted a 34 rather than a 32 electron complex. [44] R. B. Woodward, R. Hoffmann, Angew. Chern. Int. Ed. Engl. 1969, 8, 781. 1451 (a) C. Mealli, S. Midollini, L. Sacconi. Chem. Commun., 1975, 765-766; (b) C. Mealli, L. Sacconi, Inorg. Chem., 1982, 21, 2870-2874. [46] N. Kitajima, K. Fujisawa, C. Fujimoto, Y. Moro-oka, S. Hashimoto, T. Kitagawa, K. Toriumi, K. Tatsumi, A. Nakamura, J. A m . Chem. Soc. 1992, 114, 1277-1291. [47] C. J. Kramer, B. A. Smith, W. B. Tolman, J. Am. Chem. Soc. 1996, 118, 11283-1 1287. [48] T. A. Albright, R. Hoffmann, J. C. Thibeault, D. L. Thorn, J. Am. Chem. Sor. 1979, If)!, 3801-3812. [49] J. Chatt, L. A. Duncanson, J. Chem. Soc. 1953, 2039. [50] When puckering is not accompanied by the shortening of the X-X distance (Scheme not shown), the level 2bl is not greatly affected energetically. Also in this case, there is a progressive involvement of X 2 - p q orbitals in a-bonding to the metals, but 2bl remains quite deep in energy as its Xz-a’ character is not magnified. The potential energy surface is consistently low mainly for this reason.[291 [51] M. Capdevila, Y. Carrasco, W. Clegg, R. A. Coxall, P. Gonziles-Duarte, A. Lledos, J. Sola, G. Ujaque, J. Chern. Soc. Chem. Commun. 1998, 597-598. [52] M. Capdevila, W. Clegg, R. A. Coxall, P. Gonziles-Duarte, M. Hamidi, A. Lledos. G. Ujaque, Inory. Chem. Commun. 1998,1,466-468. [53] G. Aullon, G. Ujaque, A. Lledos, S. Alvarez, P. Alemany, Inorg. Chem. 1998, 37, 804-813. 1541 S.-W. A. Fong, T. A. A. Hor, J. Chem. Soc. Dulton Trans., 1999, 639-651.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.10 Towards Transition Metal Clusters by Reaction of Simple Metal Carbonyls with Chalcogenides and Chalcogenolates Gerald Henkel and Sttlfirri Wei&riihrr
1.10.1 Introduction After the exciting development of the coordination chemistry of polynuclear transition metal complexes with sulfur, selenium and tellurium ligands over the past three decades driven, primarily, by their relevance as models for biologically active metal sites, special attention has been devoted to derivatives which contain CO groups as co-ligands in recent years. Besides their importance as a fundamental class of new complexes with unprecedented chemical and structural properties in general, the increasing interest in these compounds can also be traced back to potential applications such as catalysis, the use as precursors for semiconductors with low band-gaps as well as in photovoltaic or in IR detection applications.[’] The general subject has frequently been reviewed from different points of view,[21and the latest accounts, which focus primarily on selenium and tellurium complexes are precedented by reviews dealing more with sulfur-containing species. Within this context. the present article concentrates on transition metal cluster complexes of cobalt, iron and manganese with mixed chalcogen/carbonyl ligand spheres obtained by reaction of simple binary metal carbonyls with alkali-metal sulfides, alkali-metal thiolates or transition-metal thiolate complexes and their selenium or tellurium counterparts. With respect to the large body of complexes with mixed carbonyl/chalcogenide ligand spheres known today, the number of complexes containing chalcogenolate as well as carbonyl ligands is comparatively small. Examples for the latter type of compounds include [Fe2(SC2Hj)2(C0)g],L31 [Fe?(TeR)2(C0)6],[41 [Mn2(ER)?(C0)8] ( E = S, Se, Te),”] [Mn2(ER)3(CO),]- ( E = S, Se, Te),[5361[MnFe(SePh)3(CO)6],[6a’ [MnlCo( ) ~ ] , [ ~TePh)j] [Mn(EPh)2(C0)4]- (E = Te, Se)[s”.6a1[ C O ~ ( S C ~ H ~ ) ~ ( C O ( C0)8],[sa1 [MnzCo(SePh)5( CO)81,[6a1 [ Co5 (SCzH5)j(CO) [Cogs(SC2H5)4(CO)11],[91[Fe?(SC4H,)(EPh)(CO),]-( E = S, Se),“’] whereas the former type of
164
I Molecular Clusters
complexes are represented by species of general formula [M,Ey(CO)Z]"( M = Mn, Fe, Co; E = S, Se, Te; n = -2, -l,O, +1, +2, + 3 ) . In the following sections, we will discuss the results of our recent work in the field of manganese-, iron- and cobalt cluster complexes with mixed chalcogen/carbonyl ligand spheres, taking relevant complexes from the literature into account.
1.10.2 Synthetic aspects The reactions performed in our laboratory were carried out in sealed ampoules at temperatures above the normal-pressure boiling points of the solvents used. These conditions are often referred to as solvothermal conditions, a subject which has been discussed very recently by Sheldrick and Wachhold." This method was developed from the hydrothermal synthesis in water near critical conditions (T, = 374.1 "C, p,. = 221.2 bar), which has been the domain of geochemists and mineralogists with the aim to mimic the formation processes of minerals. In recent years, a pronounced trend to other solvents and substantially less drastic reaction conditions] with temperatures in the range of or slightly above the normal-pressure boiling points, under equilibrium conditions, in sealed reaction vessels has occurred. With these conditions, the reactions can be carried out in thick-walled glass ampoules, and the progress of the reactions can be controlled easily (see [I I]). Generally] the reactants are placed in glass tubes together with small amounts of an appropriate solvent (e.g. methanol or acetonitrile) under strictly anaerobic conditions. After mixing the slurries by shaking, the mixtures are frozen with liquid nitrogen followed by sealing the glass tubes in vucuo with a flame of appropriate temperature. Another important difference to more classical synthetic procedures is the use of very small amounts of solvent. In typical cases, only a maximum of 5 ml of solvent is added to 5-10 mmol of solid reactants. As a result, saturated solutions are obtained in equilibrium with solid material, which dissolves continuously depending on the progress of the reaction and the crystallization of the reaction products. The scope of our work extends to soluble molecular complexes without polymeric character. In this case, it is necessary to work in saturated solutions. If anionic species are formed, high concentrations of appropriate counter-cations (e.g. quaternary ammonium cations) favor crystallization. In most cases, the starting materials used for solvothermal synthesis (as well as for 'normal' reactions) are of simple composition (e.g. binary metal halides, metal chalcogenides or polychalcogenides and simple metal carbonyls). In a specific approach to synthesize mixed chalcogen/carbonyl cluster complexes with unprecedented chemical and structural properties] we start from polynuclear
1.10 Toiiarcl.v Transition Metal C‘1ustei.s
165
transition metal-chalcogenolate complexes in order to make use of the pre-defined metal framework as a building block for clusters of higher nuclearities. In many of our reactions, 2-propane-substituted chalcogenide ions are used as ligands because they can also act as a source for ‘naked’ chalcogenide ions. This is important if ligands are needed which are able to form a larger number of bridging functions than chalcogenolate ligands can do. Sometimes, it is desirable to reproduce the compounds obtained by solvothermal procedures under atmospheric pressure in open reaction vessels. However, the composition of the products formed under solvothermal conditions often differs significantly from the molar ratios of the reactants. In these cases, the reaction conditions have to be optimized in order to improve the yields, and this can more easily be performed by using conventional synthetic procedures. Sometimes, however, it is not possible to ‘translate’ the hydrothermal reaction path into conventional solution chemistry, because the products or some intermediates are not stable under these conditions. Similar difficulties often occur if a reaction leading to a specific complex is optimized by adjusting the reactant ratios according to the product composition. In these cases, reaction pathways involving more complicated intermediates have to be assumed.
1.10.3 Topological aspects The metal ions in mixed chalcogen/carbonyl complexes of manganese, iron and cobalt are often surrounded by five ligand donor functions (normally a { E ~ ( C O ) I } donor set) in a square-pyramidal fashion. A typical example is the binuclear complex [FeZS2(CO),]. which contains a disulfide ligand shown in Fig. 1.[‘*dl In several instances, a trigonal-bipyramidal coordination, as observed in [Fe(C0)5], is preferred instead[l3”-“](Fig. 2). In some cases, however, the coordi-
Figure 1. The butterfly-shaped complex [Fe*&(CO)h]with squarepyramidal coordination sites.
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nation number is expanded to six. The additional ligand then defines the second apex of a pseudo-octahedral ligand environment. This situation is illustrated in Fig. 3 showing the molecular structure of [Mnz(TePh)2(C0)8].[5"1 In polynuclear complexes, the square-pyramidal { ME*(C0)3] sub-units can be linked via common corners or edges. If two of these building blocks are connected via a common E...Eedge with conservation of the coordination number 5 , the
Figure 3. Molecular structure of [Mn:,(TePh)z(CO)s].
I . I0 Towardo Transition Metal Clusters
167
typical 'butterfly'-shaped structure represented by [Fez(CO)&] is formed (see Fig. 1 ) . This fundamental structural motif is the dominant feature in a large number of polynuclear complexes. From another point of view, the butterfly-type structure is an incomplete bioctahedral system with a common face and a vacant bridge. This vacancy has been discussed in terms of a bent bond defined by two orbitals centered on both metals, respectively, and extending in the direction of the vacant In very special cases, the square-pyramidal ligand arrangements in these butterfly-type sub-units are distorted towards trigonal-bipyramidal coordination sites. Within the picture of the bent bond, this transition would disrupt the metal-metal bond and should be directly attributable to electronic variations within the ligand spheres of the metal atoms. In the next sections, we will discuss these topological aspects in more detail taking recent results from our laboratory into account.
1.10.4 Complexes containing up to two metal atoms These compounds do not belong to the family of cluster complexes in a classical sense, but they are mentioned here for systematic reasons. For the synthetic chemist, they are very useful precursors for building up complexes of higher nuclearities. The first mononuclear carbonyl compound of cobalt, iron or manganese known was the pentacarbonyl complex [Fe(CO)51" 3d-e1 with a trigonal-bipyramidal ligand field (see Fig. 2). Though related electroneutral species of cobalt and manganese are not known due to a strict observance of the 18-electron-rule in these cases, the monoanionic manganese derivative [Mn(C0)5]- has also been described as a trigonalbipyramidal species." 3F,g1 Recently, however, it has been shown that [Mn(C0)5]- is able to adopt the square-pyramidal geometry in its [PPh4JS salt as well (Fig. 4). Within the whole series of complex species [M(CO)(]"-, this remarkable pentacarbonyl complex is the only example with a square-pyramidal ligand field known so far. It is formed by a disproportionation reaction of [Mn2(CO)lo] with Na2Se2 together with [Mn3Sel(C0)9I2- in methanolic solution (see below).['41 The coordination polyhedron of [Mn(C0)5]- is a regular square pyramid of nearly perfect symmetry (point group C4\) and as such possesses a spatial distribution of the CO ligands similar to that of the protonated hexacoordinated compound [HMn(CO)5][l'I. Anionic hexacoordinated complex anions of formula [Mn(EPh)2(CO)4)- ( E = Te, Se)[5"36"1 do also exist. 4 large fraction of the binuclear mixed chalcogen/carbonyl transition metal complexes of iron and manganese contain the {M2E,I(C0)6}core with the butterflytype structure (n = 2) or substructure (n == 3, see Section 1.10.3). As an example of a complex with thiolate ligands, the structure of [Fel(SC~H7)2(C0)6]"61 is shown in
168
I Moleculur Clusters
Figure 4. The square-pyramidal complex anion [Mn(C0)5]- in crystals of [Ph4P][Mn(C0)s1.
Figure 5. This complex was obtained by solvothermal reaction of [Fe2(CO)9]with NaSC3H7 in methanol. In contrast to [Fe2S2(C0)6],an S-S bonding interaction is lacking leading to S...S distances as long as 2.919(1) A.The Fe-Fe distance is 2.513(1) A in length indicating a metal-metal bond. The organic residues of the bridging ligands are orientated in an equidirectional sense with respect to the Fe...Fe axis. The {Fe2S2(C0)6} core of [Fe2(SC3H7)2(C0)6] finds its counterpart in the structurally related compound [Fez( SC2H5)2(CO)6].r31 Within the group of binuclear chalcogen/carbonyl complexes, another type of general formula [Mn2(ER)2(CO)8]( E = S[5d1,Ser5h.c1,Te15”]) is established by manganese (see Fig. 3). Besides these chalcogenolate-containing species, corre-
Figure 5. The binuclear complex [Fe2(SGH7MCO)d
1.10 Toilwrd Trmnsition Metcil Clusters
169
Figure 6. Structure of [Mn2Te2(C01s]'-in crystds of (Ph~Pjz[ MnlTe>(COix]
sponding chalcogenide compounds are also known. The complex anions IMnlTe?(CO)g and (Mn,Te( Te2)(CO)s]'- were obtained by solvothermal reaction of [Mnz(CO)lo]with Na2Te2 or [Ph4P]:Te4, respectively, and crystallized as tetraphenylphosphonium Their structures are shown in Figs. 6 and 7. Both species contain manganese as 18-electron-systems in octahedral { Mn(CO)4Tez} ligand fields with telluride (I MnlTe2(CO)s]'-) or mixed telluride/ditelluride ([MnzTe(Te2)(C0)8I2-) ligands in bridging positions. Due to the larger spatial demand of the ditelluride ligand, the Mn. . .Mn distance rises from 4.024( 1) in the former to 4.560( 1 ) A in the latter compound, indicating no metal-metal bonds. About 5% of the [Mn2Te(Te2)(CO)s12- anions in the comhowever. show a different coordination pound [Ph4P]21Mn?Te(Te?)(CO)g].MeOH, pattern of the Te?'- ligand. In this case: only one Te atom is involved in the bridge without any coordination function of the other one. A similar bridging situation has been observed in [Fe,Te( Te2)(CO),j12-['''. A related manganese complex with two diselenide bridges has been identified in crystals of [Ph4P]21Mn2(Se~)2(CO)~].C4H1~~0, and a derivative containing Se4'- is also known.["] In contrast to the butterfly-type structure with its characteristic nonplanar central M2E2 heterocycle, the Mn2Tez core of [Mn2Te2(CO)gl2- is nearly perfectly planar.
Figure 7. Structurc of [Mn2(Te,lTeiCOixl'- i n ciystnls of [Ph3P]2[Mn2Te(Tei.,C0 MeOH
170
1 Moleculur Clusters
0161
A third type of binuclear chalcogen/carbonyl complex is defined by three chalcogen atoms in bridging positions and six carbonyl ligands bonded terminally to the metal atoms. The general formula of these complexes is [Mnz(ER)3(CO),]- ( E = S,[6b,20,211, Se[6”,221, Te[5a1).A mixed-metal derivative of formula [MnFe(SePh)3(CO),][“l is also known. They also contain the metal atoms in octahedral ligand environments. Both coordination octahedra share a common face, which is defined by the three chalcogen atoms. Figure 8 shows the structure of [Mn2(SeC3H7)3(CO)6lpas a typical example for this complex type. The complex anion was obtained by reaction of [Mn2(SeC3H7)6l2- with [Mn2(CO)101 and crystallized as tetraphenylphosphonium salt under solvothermal conditions.[221 The overall structure is an extension of the butterfly-type structure with a third chalcogen atom occupying the vacant bridging site. In this respect, there are striking structural similarities between [Fe2(SC3H7)2(C0)6](Fig. 5 ) lacking this bridge and [Mn2(SeC2H5)3(C0)6Ip.Both complexes are electron-precise with respect to the 18electron-rule, but in the latter case, no metal-metal bond is required in accordance with the long Mn...Mn distance of 3.294(1) A. All three organic residues of the bridging ligands are orientated in an equidirectional sense with respect to the Mn. . .Mn axis.
1.10.5 Complexes containing three metal atoms Within this class of complexes, the metal atoms define triangular frames. With the exception of [ MnzCo( TePh)5(C0)8][5“1 and [ M ~ ~ C O ( S ~ P ~ ) ~ ( Clinear O)~][~”],
1.10 Towards Transition Metal Clusters
171
Figure 9. The complex anion [ F e ~ T e j C O ) ~In] ' crystals ~ of [EtdN]l[Fe;Te(COh]
complexes are not known so far. In the case of cobalt and iron, only species with chalcogenide or chalcogenolate ligands are known, whereas in the case of manganese - besides purely inorganic complexes mixed chalcogenide/chalcogenolate species do also exist. Of particularly widespread occurrence are the triangular complexes of general ( E = S, Se),'"] [Co*FeE(C0)9] ( E = S, Se, Te),[231 formula [CO~E(CO)~)] [CoFe2E(C0)9]'/'- ( E = S, Se, Te)[231,[Fe3E(CO)9]Z- ( E = S, Se, Te),[24.471 [Fe3EZ(CO),] ( E = S, Se: Te)[251and [Mn3E2(C0)9l2- ( E = S, Se),r26,'41 [FeZMnTel(CO)10 and [FelMnSe:,( C0)9]-,[23d1 respectively. As examples, the structures of the iron-containing species [Fe3Te(CO)912 and [Fe,Se,(CO),] are shown in Figs. 9 and 10. In both cases, the 18-electron-rule is obeyed resulting in three ([Fe3Te(CO)9]'-) and two ([ Fe3Sel(CO)g]) metal-metal bonds, respectively. ~
Figure 10. The complex (FelSe2(CO)9].
172
1 Moleculur Clusters
Figure 11. The complex anion [FelSeH(CO)g]-in crystals of [Ph4P I [ F ~ ~ S ~ H ( C1. O ) Y
As a consequence, the metal framework of [Fe3Te(C0)9l2- is a regular triangle, whereas that of [Fe$ez(CO)g] derives from a metal square with one corner being unoccupied. The triangular complexes of the type [Fe3E(CO)912- ( E = S, Se) can be protonated to form [Fe3E(CO)gHI-- species. They contain a hydrido ligand bridging one edge of the Fe triangle.[271As an example, Fig. 11 shows the structure of Complexes of this type are [Fe3Se(CO)gH]- in crystals of [Ph4P][Fe3Se(CO)gH].[27b1 accessible by reaction of mixed chalcogenide/carbonyl complexes with trifluoromethanesulfonic acid in methylene or by solvothermal reaction of binary transition metal carbonyls with simple chalcogenides (Na2S2, Na2Se and NazTe2 respectively) in alkaline s ~ l u t i o n . [ ~ ~ ~ ~ " ~ The hydrido bridge reduces the overall symmetry of the cluster complex by widening the corresponding Fe-Fe distance from the unbridged value of ca. 2.63 A up to the bridged one of 2.72 A. The corresponding {FezSe(C0)6H}framework consists of two trigonal-bipyramidal {FeSe(CO)3H} sub-units with H and Se defining the common edge being in axial and equatorial positions, respectively. Consequently, the substructure does not possess the butterfly-type geometry, which allows for a bent metal-metal bond. It is also possible to synthesize trinuclear complexes with two hydrogen bridges by reaction of mixed chalcogenide/carbonyl complexes with trifluoromethanesulfonic acid under specific In this case, one metal atom is surrounded by a set of six and the other two by a set of five ligands. Compared with the electron-precise compound [Fe3Sez(CO)g],the related complex anion [Mn3Se2(CO)g12-has only 49 valence electrons and thus falls short of the value requested by the 18-electron-rule (PSEP theory[281)by one. The manga-
1.10 Towards Transition Metal Clusters
Figure 12. Structure of [Mn3Se2(CO)OI'- in crystals of [Ph4P]2[Mn?Se2(CO)q].thf
113
M
nese species has been obtained as the oxidized component of the disproportionation reaction of [Mnl(CO)lo]with NazSel (see Section 1.10.4) and characterized as its dark-red tetraphenylphosphonium salt."41 The molecular structure is depicted in Fig. 12. The odd number of valence electrons is manifested by the paramagnetic behavior (pefr= 1.8 p, for LPh4P]2[Mn3Se2(CO)9]at 100 K). The heavy atom framework is a slightly distorted square pyramid with alternating Mn and Se atoms occupying the base and a third Mn atom defining the top. While the heavy atom framework of the 50-electron-species [ Fe?Se2(CO)g](see Fig. 10) is of comparable geometry, the carbonyl ligands around the metal atoms on top of the (M.iSe?} pyramids have different spatial orientations. Based on these observations and a number of other indicators such as Mn-Se bond lengths and formal oxidation states, the unusual contact C(7).. .Mn( 1 ) has been interpreted in terms of a novel unsymmetric carbonyl bridge connecting Mn( 1) and Mn(3), which is lacking in [Fe3Sez(C0)9]. The manganese atoms are surrounded by three carbonyl and two selenide ligands each in a square-pyramidal fashion if the weak interaction of the asymmetric C O bridge (Mn(l)-C(7): 2.726(7) A) is neglected. On the other hand, Mn( 1 j has a distorted octahedral ligand sphere if this bridge is considered as a bond. The binuclear { Mn2Sez(CO)(,) sub-unit containing Mn(2) and Mn(3) adopts the butterfly-type structure. In contrast to [Fe3E?(CO)g]( E = S, Se: Te) with trigonal-bipyramidal coordination of Fe(3) (see Fig. lo), the [Fe3Sz(CO)g] component of the 1 : 1 adduct of composition [Fe2S2(C0)6][Fe3S2(CO)9] has a structure similar to that of [ Mn3Se?(CO)9]z-[291 and thus adopts an alternative conformation. After re-evaluation of the packing situation, it was concluded that both components form charge-transfer adducts such that trinuclear [Fe3S?(C0)9]and binuclear [Fe2(S?)(CO)G]molecules are associated alternatively uici intermolecular S . . . S bridges, which are 3.157 A in
174
1 Molecuhr Clusters
Figure 13. Structure of the adduct [Fe2(S2)(CO)h][FejS2(C0)9].
lengths, to form one-dimensional endless strands (Fig. 13). The structure of the trinuclear [Fe3S2(C0)9] component has been re-interpreted in terms of the novel asymmetric carbonyl bridge recognized for the first time in [Mn3Se2(C0)9I2-, and its occurrence here has been traced back to a transfer of electron density from the trinuclear component to the disulfide group of the binuclear complex. The different electronic situation may also explain why the Mossbauer spectrum of [Fe3S2(C0)9][Fe~( s2)(co)6] differs significantly from that of the individual complexes [Fe&(C0)9] and [Fez(S2)(CO)6].[30i Based on a large body of experimental data, it has been concluded that an asymmetric carbonyl bridge of the type observed in [Mn3Se2(C0)9I2- might be established in complexes of general formula [ M ~ E z ( C O )(~E]=~ S, Se, Te; z = -2, - l , O , + l ) if the two metal atoms in the base of the M3E2 pyramid are electronically non-equivalent. In this case, the metal atom in the higher oxidation state is probably stabilized by a ( pn),-o-dM interaction. The change from the non-bridged ([FelE2(CO)9],Fig. 10) to the bridged state ([Mn3Se2(C0)9I2-, Fig. 12) is accompanied by a 30" rotation of the apical {M(CO)3}fragment around its local symmetry axis followed by a subsequent bend of this axis by about 15" in the direction of M( 1). At the same time, the coordination polyhedron changes from the trigonal bipyramid to the square pyramid. By consequent use of these arguments, it was also possible to solve the problem of the metal distribution in [MnFe2Se2(CO)g]-,[23d1 a related Mn/Fe mixed-metal complex anion possessing the characteristic asymmetric carbonyl bridge identified for the first time in [Mn~Se2(C0)9]2-.['41 The metal positions in the base of the M3Se2 pyramid are most probably filled in an ordered fashion with manganese (position M( 1)) and iron (position M(2)). An interesting extension of the [Mn3Se2(C0)9I2- complex anion has been obtained in the reaction system [MnZ(CO)1o]/Na~Se2/[Ph4P]Clunder solvothermal conditions in methanol. The trinuclear compound [Mn3Se2(SeMe)(C0)9j2-derives from [Mn3Se2(C0)9I2- by addition of a methaneselenolate ligand and removal of one electron from the manganese frame.['41 The electron-precise complex is shown
1.I0 Towards Trunsition Metul Clusters
175
Figure 14. The complex anion [MnJSe?(SeMe)(COjg]’- i n crystals of jPhJP]2[MniSe2iSeMei(C~~y]
in Fig. 14. The additional methaneselenolate ligand (Se(3))bridges the non-bonding Mn(2). .Mn(3) edge of the manganese triangle. Due to a total of 52 valence electrons, only one Mn-Mn bond (Mn(I)-Mn(3)) is needed to achieve the noble gas configuration. This bond is now stabilized by a semi-bridging CO ligand. Each manganese atom is surrounded by six ligands in a pstwboctahedrdl fashion. Both sub-units are extenbinuclear { Mn2(,n-Se)i(CO)hJand { Mn2(pu-Se)1(p-CO)Se(C0)5$ sions of the butterfly-type structure with a third ligand atom occupying the vacant bridging site. From a structural point of view, the {Mn?(p-Se)3(CO)h}sub-unit (see (CO)~]~[~ is closely related to the binuclear complex anion [ M I I ~ ( S ~ C ~ H ~ ) ~ Fig. 8). ln this context, it should also be mentioned that the mixed sulfur/carbonyl complex [MnlS2(SH)(C0)9]’ with coordination properties very similar to those observed in 1 MniSe?(SeMe)(CO)s]’-has recently been prepared.[311It is also possible to cleave the CO bridge as well as the corresponding Mn-Mn bond in this trinuclear complex by replacing the bridging sulfide ligands by disulfide groups. The corresponding complex ion [Mn3(S2)2(SH)(C0)9I2-shown in Fig. 15 was obtained by solvothermal reaction of NaIS? with [MnZ(CO)l o ] in ethanol.[211 The hydrogen atom of the sulfhydryl group, however, could not be located by Xray methods. Another route to [ Mni(S2)2(SH)(CO)9]2p was occasionally found by reaction of [Mn?(SH),(CO)6]-with [NiCI 1 H24NZSZJ under solvothermal conditions S H ) (revealed CO)~] in methanol. The crystal structure of [ P ~ ~ P ] ~ [ M ~ I ~ ( S ~ ) ~ (clearly the position of hydrogen within the SH (see Fig. 15). Each isolated Mn center has a psrudo-octahedral { S3(CO)3) ligand environment with a total valence electron count of 18. The two disulfide ligands are coordinated in different modes. One of them is bonded as a ,u3-q’,q ’ , q ’ , the other one as a p3-q’. q l , i12 bridging ligand.
176
1 Moleculur Clusters
In contrast to iron and manganese, the number of trinuclear cobalt complexes with 2-propane thiolate or with is comparatively small. The reaction of [CO~(CO)S] C3H7SSC3H7 in methanol under solvothermal conditions afforded the electroneutral complex [ C O ~ ( S C ~ H ~ ) ~ (which CO)~ contains ], a triangle of cobalt Its structure is shown in Fig. 16. A similar complex with ethanthiolate ligands has been obtained by conventional synthetic strategies."] The overall structural principles of this complex type can be described as follows. The metal-ligand framework is a trigonal prism with corners defined by the five sulfur donor functions of the isopropanethiolate ligands and one carbon atom of a CO ligand. The metal atoms are shifted from the centers of the
Figure 16. Structure of crystalline
[C~~(SC~H~)S(CO)~I.
1.10 Towards Transition Metal Clusters
177
Figure 17. Structure of crystalline IColTe2 f C 0 )I 1 1.
quadratic faces of the prism slightly towards the axially bonded three remaining CO ligands. One of the three cobalt atoms has a pseudo-square-pyramidal {S,(CO)) ligand set, whereas the other ones are in pseudo-square-pyramidal {S3(C 0 )z ) ligand environments.
1.10.6 Complexes containing four metal ions The reaction of [Co2(CO)8],Na2Te2 and [Ph4P]Cl in methanolic solution under solvothermal conditions afforded black-violet needles of the tetranuclear cluster complex [Co4Te2(CO)11].which is shown in Fig. 17.[341This compound has been discussed as a possible intermediate during the formation of the undecanuclear complex anion [Co~~Te-i(CO)lol’(see Section 1.10.9, Fig. 32). Its trapezoidal metal arrangement is in accordance with the 1 8-electron-rule, which predicts three Co-Co single bonds for the total of 66 valence electrons observed here. Similar metal frameworks have also been observed in structurally related Sb and Bi complexes.[351The four Co atoms are bridged by two p4-Te ligands and additionally bonded to a total of ten terminal and one bridging C O groups resulting in square-pyramidal {Te?(CO)T1 ligand spheres. The cluster complex contains two butterfly-type sub-units, which share a common carbonyl and the two telluride ligands. Other tetranuclear complexes of formula [M2M12E’(CO)11 1 ( e $1 M, M’ = Fe, E = S;[361M. M’ = Ru, E = Te;[371M, M’ = Ru, E = Se;[381 M = Ru. M‘ = Fe, E = Te;[39”1M = Co, M’ = Fe, E = S [39b1) have four metalmetal bonds due to a total of 62 iM = Fe or Ru) or 64 ( M = Co) valence electrons,
178
I Moleculur Clusters
Figure 18. Structure of crystalline [F~&(CO)I11.
respectively. As an example, the structure of [ F ~ ~ S ~ ( C is O )shown ~ I ] in Fig. 18. Square-planar cluster complexes of general formula [ C O ~ E ~ ( C O ( E) = ~ ~S,] Se, Te) that possess only ten carbonyl ligands are also known.[401The structure of [Co4Se2(CO)lo]is shown in Fig. 19. The complex [Mn4(S2)2(C0)15]contains two disulfide ligands and has been obtained by oxidation of [Mn2(SSnMe3)2(CO)S]["' Similar to [Mn3(S2)2(CO)g(SH)I2 (see Chapter 1 . 1 0 3 , the disulfide groups are coordinated in different coordination modes. Each manganese atom is coordinated pseudo-octahedrally by six ligands, but the coordination spheres of the individual atoms differ with respect to the total number of sulfur ligands. Two of them are coordinated to three sulfur atoms, the other ones to two and one, respectively. The family of tetranuclear complexes ~
Figure 19. Structure of crystalline ICo4Se2(CO)1ol.
1.10 Toivuru's Trunsition Metal Clusters
179
Y
Figure 20. The complex anion [Fe4Te2(C0),4]' In crystals of [ P hPI2 [FejTezi C 0 )I 4 1.
[Fe4E4(C0)12] ( E = S. Se, Te) also contain pseudo-octahedrally coordinated metal atoms in { Fe(CO)3Ej} site^.[^^"'^] They can be obtained by thermally or photochemically induced dimerization of [Fe2(E2)(CO)6].[42'1 Another interesting complex was obtained by solvothermal reaction of [Mnz(CO)lo] and Na2Te2 in ethanolic solutionC2 and isolated as [Ph4P]z[Mn4(Te2)3(CO)131. The complex anion [Mn4(T ~ ~ ) ~ ( C O I Iconsists ~ ] ' - of one {Mn(C0)4} and three {Mn(C0)3}fragments, which are bridged by three ditelluride groups acting as bridging ligands in different coordination modes. A variety of other tetranuclear cluster complexes contain two binuclear fM2E?(CO)h}fragments ( M = Mn, Fe, Co), which possess the more or less regular tC4H9[43h1), butterfly-type structure. Examples are [Fe4(SR)2S(COj12]( R = [Fe4(C2S4)(C0)12],[441 and [Fe4Se2(Sez)(CO)1212-.[4s1 In contrast to these species, 1 FeJTel(C0),412- '27c.461 contains only one binuclear { M2Ez(C0)6}fragment which deviates significantly from the regular butterfly-type structure. The structure of [Fe4Te2(C0)14l2- is shown in Fig. 20. The central (FezTe2(CO)h} fragment does not have the ideal butterfly-type structure due to a shift from the square-pyramidal towards a predominantly trigonal-bipyramidal coordination of iron. Each telluride ligand is bonded not only to both iron atoms within the distorted butterfly-type sub-unit but also to an iron atom of a trigonal-pyramidal { Fe(CO)4} fragment completing its ligand environment by forming a nearly perfect FeTe(C0)d trigonal bipyramid. The coordination of two { M(CO),} fragments by the telluride ligands of a {Fe?Tez(CO)h}sub-unit in a direction which coincides with the Te. . .Te axis is a unique feature in complex chemistry. The specific bonding situation of the telluride ligands might be the reason for the unusual coordination geometry observed within the butterfly-type sub-unit. Furthermore, this ligand arrangement might be the result of metal-metal interactions, which differ from those present in but terfly-type units of conventional design. An interesting singularity in polynuclear chalcogen/carbonyl complexes is the tetranuclear iron compound [Fe4Te(CO)1 6 1 ~ ,[471 which has no metal-metal bonds.
180
I Molecular Clusters
The four {Fe(C0)4}fragments of [Fe4Te(C0)16]~are bonded to a central telluride ion in a tetrahedral fashion. The resulting coordination of iron is trigonalbipyramidal with tellurium defining a common axial position for each {FeTe(C0)4} fragment.
1.10.7 Complexes containing five metal ions Within this group, the number of complexes known so far is comparatively small. Recently, the complex anion [FesTe4(CO)1 4 1 ~ - has been described that consists of two { Fe2Te2(C0)6}fragments adopting the butterfly-type structure. Both fragments are bonded to a central {Fe(C0)2}unit via all four tellurium atoms defining apseudooctahedral { FeTe4(C0)2} coordination site with the carbonyl groups in adjacent positions.[481The central {FeTe4(CO)2}unit can also be replaced by a square-planar {NiS4) coordination site leading to the complex anion [Fe4NiS4(CO)12]2-.124h3 Another very interesting complex anion can formally be derived from [ FesTe4(CO)1,l2- by removal of one telluride ion from each of the two butterfly sub-units and replacing those remaining by sulfide ions resulting in the formation of [FeSS2(C0)14]2-.[24h,491 Due to the loss of two telluride ligands, the total number of 86 valence electrons observed in [FesTe4(C0)14I2- is reduced to a value of 76. This drastic decrease is compensated for by the formation of four additional Fe-Fe bonds in [FesS2(CO)14I2- such that an electron-precise complex anion is formed again. This sulfur-containing species is shown in Fig. 21. The novel cluster contains two trinuclear { Fe3S(CO)gS}sub-units which can be structurally derived from the class of [Fe3E(C0),l2- complexes (see Fig. 9) by replacing one CO ligand by a
Figure 21. The complex anion [ F e ~ S z ( C 0 ) ~ 4in] ~crystals of [Ph4Pl2[FesS2(CO)141.
1.10 Tou~urclsTrunsition Metul Clusters
181
Fiqure 22. Structure of [Coj(SEt)i(CO)lo]
chalcogenide ion. Both sub-units are linked viu a common metal atom and thus share a common fFe(C0)zEz)fragment. A pentanuclear mixed chalcogen/carbonyl cluster complex with ethanethiolate ligpnds is the cobalt-containing species [Coj(SEt)j ( C 0 )101, which was structurally characterized by X-ray methods[8J(Fig. 22). The complex contains a trinuclear (Co,(SEt),(CO)j} sub-unit, which is very from a strucclosely related to the trinuclear complex anion [CO~(SC,H,)S(CO),] tural point of view (see Fig. 16, Section 1.10.5). Here again, the metal-ligand frame is a distorted trigonal prism, but now the six corners are defined by four sulfur atoms and two CO ligands defining triangular (S3 f and {S(CO)z}faces, respectively. The main difference on going from [ C O J ( S C J H ~ ) ~ ( CtoO{Co,(SEt)4(CO)j) )~] is the exchange of one thiolate ligand by CO. The trinuclear sub-unit is linked to a binuclear {Coz(SEt)(C0)5 } fragment vitr those three thiolate ligands which define the triangular sulfur face of the prism.
1.10.8 Complexes containing six metal ions With the sole exception of [Fe$3eh(C0)12]2-.hexanuclear mixed chalcogen/ carbonyl cluster complexes of iron and manganese are not known. The structure of is shown in Fig. 23. [Fec,Sec,(CO)lz]'- in crystals of [PPN]2[FehSe6(C0)12][501 This compound has recently been obtained by solvothermal reaction of [Fe(C0)5] with NalSez in methanolic solution and characterized with X-ray crystallographic methods. The complex anion contains two butterfly-type (FezSez(C0)h) sub-units which are bonded to a central planar {FezSe?) fragment. The iron atoms within
182
1 Molecular Clusters
these fragments achieve a tetrahedral ligand environment completely defined by inorganic selenide ions. Prior to our work, the same cluster complex has been isolated as [Ph4P]2[FegSeg(CO)12],r24b3 and the sulfur-containing derivative [FegSg(C0)12I2is also known.[511 In the last paragraph of the previous section, the pentanuclear complex [Cog(SEt)5(C0)10]( Fig. 22) was described as a species containing the trinuclear {Co,(SEt)4(C0)5) sub-unit. Based on the same sub-unit and extending the binuclear {Co,(SEt)(C0)5)fragment towards the trinuclear unit {Co,S(CO)g), the hexanuclear mixed sulfide/thiolate/carbonyl complex [Cogs(SEt)4(CO)111 could also be obtained.[’] Its structure is shown in Fig. 24. In contrast to its CO-rich congener, the hexanuclear mixed sulfide/thiolate/ carbonyl complex [ C O & ( S C ~ H ~ ) ~ O (1- CisOcharacterized )~ by its higher sulfur and drastically smaller CO content. The novel cluster anion was obtained by solvosolution and thermal reaction of [Co2(Co)S] with [ C O ~ ( S C ~ Hin~ )methanolic ~]
Figure 24. Structure of [CO ~S(SE~)~(CO)IO].
I . 10 Towards Trunsition Mctul Clusters
183
Figure 25. The complex anion [CogS?(S'Pr),o(CO)2]-in crystals of [ Ph4P][Co6S?(S'Pr)10 (CO)?1.
characterized as tetraphenylphosphomium salt.[521The structure containing several unprecedented features is shown in Fig. 25. The six cobalt atoms define a common plane. They are grouped in pairs of regular triangles and arranged such that a central metal square is formed. This metal square is capped from both sides by two quadruply bridging sulfide ions, which form a square-bipyramidal {CoqSl) cage. Both edges of the metal square, which connect the triangles, are bridged by 2propanethiolate ligands. In addition, the edges of the triangles, which do not belong to the metal square, are bridged by two 2-propanethiolate ligands each. As a result, the four central Co atoms are surrounded exclusively by five sulfur donor functions in a square-pyramidal fashion. The outer Co atoms have four thiolate ligands. They expand their ligand spheres towards square-pyramidal { CoS4(CO)} coordination sites by binding an additional axial CO ligand each. Within the whole class of complexes described in this article, this is the first example for a cluster complex that contains a butterfly-type substructure completely defined by sulfur donor atoms. They derive from the typical {MzE2(C0)6} fragments by replacement of all six CO ligands by -SC3H7 groups such that the { C O ~ S ~ ( S C ~unit H ~is) formed. ~} Both butterfly-type { C O ~ S ~ ( S C ~units H ~ pres)~) ~ ( C the O)~ sulfur ] - atoms of their cyclic Co2S2 frameent in [ C O ~ S ~ ( S C ~ H ~ ) ~share works as well as the axially bonded thiolate groups. As has been the case with the penta- and hexanuclear complexes [Coj(SEt)5(CO)lo] (see Fig. 22, Section 1.10.7)[s1and [ C ~ ~ S ( S E ~ ) ~ ( C (see O )Fig. I I ] 24, this Section),['] respectively, the two identical trinuclear { C O ~ S ~ ( S C ~ H ~sub-units )~CO} are very closely related to the trinuclear complex of [cO6s2(sC3H~)~o(Co)2]~ [ C O ~ ( S C ~ H ~ ) ~from ( C Oa) ~structural ] point of view (see Fig. 16, Section 1.10.5). The metal-ligand framework of each sub-unit is a distorted trigonal prism, but now all six corners of the prism are occupied by sulfur donor functions. These trigonal prisms share a common edge defined by the sulfide ions. The axial ligands are thio-
184
I Moleculur Clusters Sei6i
Figure 26. The complex anion [Fe&Seg(SMe)2]'- 111 crystals of (PhCH2NEtiI4[Fe+3ey(SMe)2].
late groups for the Co atoms belonging to the central metal square or CO groups for the other cobalt atoms. Although the overall structure of this complex is unprecedented in coordination chemistry, a nearly identical metal arrangement has been observed in hexanuclear iron complex anions of formula [Fe6Es(SR)2I3--( E = S; R = 'Bu, Et,IS3]CH2Ph, Me,[54iE = Se, R = Me, CH2Ph[551).The structure of [Fe6Ses(SMe)2I4- is shown C O ) ~ ] - the , coordination of the in Fig. 26. In contrast to [ C O ~ S ~ ( S C ~ H ~ ) , O (however, metal atoms is tetrahedral resulting in a different ligand distribution, and the higher oxidation state of iron (average value +2.67), as compared to Co (average value +2.17), requires a higher sulfide content. A hexanuclear chalcogen/carbonyl compound of cobalt containing no thiolate S,] Se), is shown in Fig. 27 for E = S[561.The ligands, [ C O ~ E ~ ( E ~ ) ( C(O E )=I ~ S~) cluster complex is assembled from two identical trinuclear { C O ~ S ( C O ) ~sub-units which share the S atoms of the disulfide ligand. They possess principle features already known from the [Fe3E(C0)sl2- ( E = S, Se, Te) series of complexes (see Fig. 9, Section 1.10.5).
Figure 27. Structure of crystalline ~ ~ ~ 6 ~ 2 ( ~ 2 ) 2 ( ~ ~ ) 1 4 ~ ~
The last part of this section is devoted to complexes, which comprise an octahedron of cobalt atoms inscribed into a cube of chalcogenide ions. The octahedral Co faices thus are capped by chalcogen atoms, and the cobalt atoms form additional bonds to exogenous CO ligands resulting in square-pyramidal { CoE4(CO)1 coordination sites. The first complex within this series, the electroneutral species [CogS8(CO)6],was described by Stanghellini et al. in 1984,[s71shortly after the discluster core, namely [Co6&covery of the first complex containing the {Co~,Sxj (€'Etj)6]+ as its [BPh4]While the series of complexes containing tertiary phosphane ligands comprises electroneutral as well as monocationic species>[s9]all CO containing derivatives known so far are uncharged molecules. Besides sulfurcontaining species, uncharged derivatives containing selenium[56h1and tellurium[601 are also known. Within the class of iron complexes, the closely related hexanuclear have also been structurally species of general formula [Fe6E8(PR3)s]o.'+.'t.3+ characterizedr611. It is interesting to note that a complex species of formula [FesEx(CO)b]" is not known. Very recently, the complex anion [CocSeg(CO)(,]- was obtained by solvothermal with NalSe, in methanol/thf solution. The structure of the reaction of [CO?(CO),YJ cluster complex in crystals of [Co6Sep(CO)s][Na(thf) h ] is shown in Fig. 28.[621 The monoanionic species [Co6Sex(CO)s]- is isostructural with [CosSes(CO)6].[s6h1but due to a valence electron count of 90 for the former complex compared to 89 for the latter one. all intramolecular distances are shifted to longer values for the 90-electron species. The most pronounced effect of the oneelectron-reduction is the elongation of the mean Co-Co distance. which increases from 2.910 A up to 2.968 A. It most probably originates from the occupation of a cluster orbital, which is antibonding in origin with respect to the metal framework. Slightly less changes are observed for the Co-Se and Co-(CO) distances. Upon one-electron-reduction, they are as long as 2.35 1 and 1.752 A compared to values of
Figure 28. The complex anion [CobSe~(CO)hl-in crystals of "a( thf ib][Co&g (CO)6].
i
186
1 Moleculur Clusters
2.342 and 1.712 8, in the oxidized case. Similar redox-dependent effects have been observed in the series of phosphane-containing complexes of iron and ~ o b a l t . ~ ~ ~ , ~ ~ ] Solvothermal reaction of [ C O(CO)g], ~ Na2Se2 and [Ph4P]Cl in methanolic solution afforded the formation of hexanuclear cluster complexes, which differ not only in their CO contents but also in their redox states. According to a crystal structure is composed of analysis, the complex salt [P~~P]~[CO~S~~(CO)~][CO$~~(CO)~]CI monoanionic complexes [Co&3es(CO)6]- as described above (Fig. 28), dianionic complexes [Co&es(C0)4j2-, isolated chloride ions and [Ph4P]+ cations in the ratio 1 : 1 : 1 :4. The structure of [Co&es(C0)4I2- is shown in Fig. 29.[631 The novel cluster complex has unprecedented coordination properties, which can be derived from those of [Co&e8(C0)6]- by loss of two opposite CO ligands combined with the introduction of an additional electron. Both cobalt atoms, with their CO ligands removed, move towards the center of the cluster and approach each other inside the Seg cube at a distance of 2.677 A. The Co6 framework has now been transformed from a regular octahedron into a flat square-planar bipyramid with Co-Co distances of 3.045 A within the basal plane and of 2.535 A within the bonds from the axially orientated atoms, respectively, to the atoms in the basal plane. The transformation from [Co6Ses(C0)6]- to [Co,jSes(C0)4~~is also accompanied by a significant shortening of the Co-Se bonds (2.317 us. 2.363 A) within the square-planar coordination sites. In the course of systematic investigations, the sulfur-containing complex salt [P~~P]~[CO~S~(CO)~][CO~S~(C~)~]CI could also be synthesized under solvothermal
1.10 Toit~urdsTrunsition Metul Clusters
187
Fiigure 30. The complex anion [Fe8Te6(CO)z?]’-in crystals of
conditions.[641Here again, more conventional, highly reduced hexanuclear cluster complexes of formula [ Co6sX(Co)6]- coexist with completely unconventional COdeficient species [CohSg(C0)4l2- in even more highly reduced states.
1.10.9 Complexes of higher nuclearities Compared with cluster complexes of lower nuclearities, the number of mixed chalcogen/carbonyl complexes having more than six metal atoms is small. Besides some complexes containing iron and cobalt, no manganese compound within this class of complexes is known. ~ ’ ~ ~ ’ in Fig. 30. The structure of the cluster anion [ F e ~ T e s ( C 0 ) 2 4 ] ~ -IS[ ~depicted This compound is an extension of the tetranuclear complex anion [Fe4Tez(C0)14]2-(see Fig. 20, Section 1.10.6), which can be obtained from [Fe4Te2(C0)14I2by replacement of the axial CO ligand of each of the terminal {Fe(C0)4}fragments by butterfly-type {Fe?Tez(CO)s}units. Here again, the central {FelTe2(CO)h}fragment does not have the ideal butterfly-type structure due to a shift from the squarepyramidal towards a predominantly trigonal-bipyramidal coordination of iron. The coordination of the iron atoms within the two {FeTe3(CO)3}fragments is pseudooctahedral. The second octanuclear mixed chalcogen/carbonyl compound so far known is the complex anion [FegTeg(Te2)(C0)20]2-.’45.481 Th’is novel cage system (Fig. 3 1) contains two cuboidal { Fe4Te4(CO)loTe21 fragments, which share the atoms of the ditelluride group. With a total of 144 valence electrons, the complex anion is electron-precise without any iron-iron bonds. The cluster complex of highest nuclearity containing chalcogen/carbonyl ligands known so far is [CollTe,(CO)lo]’-. The novel cage compound has been obtained by reaction of [Co2(CO)g],Na2Te2 and [Ph4P]CI in methanolic solution under solvothermal conditions[341and isolated as [Ph4P]2[Col,Te,(CO) lo]. The complex anion ‘
188
I Molecular Clusters
Figure 31. The complex anion [FegTes(Te2)(C0)2012-In crystals of [ P ~ ~ P I ~ I F ~ X T ~ X ( T ~ ) Z ( C O ) ~
[C011Te7(CO)lo]~~, shown in Fig. 32, contains the novel { C O ~ O cluster } cage which is a slightly elongated pentagonal-prismatic array of cobalt atoms. The cobalt prism is centered by a single cobalt atom. The only other metal cluster with a related pentagonal-prismatic geometry is the {Pt19} core of [Pt19(CO)2,]4-.r661 The encapsulated Co atom is 2.558 8, away from its Co neighbors. As a result, the average Co-Co distance within the pentagonal-planar faces (2.583 A)is ca. 0.04 A shorter than the corresponding distance between atoms of adjacent metal pentagons (2.622 A). These distances are in the range of normal Co-Co bonds observed e.g. in the tetranuclear complex [Co4Te2(CO)l0](2.58 A).[40c1 Each of the five {Co,} squares is capped by a p4-Te ligand with a mean Co-Te bond distance of 2.510 A.In addition, two ,u,-Te ligands are situated above the centers of the pentagonal Co faces forming Co-Te bonds of 2.569 A in length. These ligands are also bound to the encapsulated Co atom at distances of 2.642(2) and 2.647(2) A,respectively. According to our knowledge, this unusual bridging mode of a chalcogenide ion has been observed for the first time resulting in a linear
Figure 32. The complex anion [Co,lTe7(CO)lo12-in crystals of lPh4Pl2[COI1Te-i(CO)101.
1.10 Towards Transition Metul Clusters
189
TeeCo-Te coordination. The other Co atoms are in a tetrahedral ligand environment due to ten terminally bonded CO groups. They are surrounded by three Te ligands and one CO ligand each. The encapsulated Co atom is the common vertex of a total of five nearly regular octahedral CoCo4Te units, which are connected via common faces to form the complete ColoCo(,u,-Te)z(p4-Te)5 framework. The mean oxidation state of Co is calculated to be +1.09. This value is in agreement with a mixed-valent state in which the encapsulated metal atom is formally divalent and the remaining ones are monovalent. Very recently, a related undecanuclear cluster complex containing selenide ligands has also been obtained by solvothermal reaction of (Coz(CO)8] with [C'o2(SeC?H7)5lp and [Ph4P]Cl in methanolic solution and isolated as [Ph4P]2[c'ol I S ~ ~ ( C O ) ~ ~The ].[~ complex 'I anion [Col 1Se7(CO)loJ2pis isostructural with [c'ol I Te7(CO)I " ] ~and - thus exhibits the same principal structural features including the novel pentagonal cobalt prism as well as the encapsulated cobalt atom. A perspective view, which outlines the body-centered pentagonal-prismatic cobalt (open stncks) and pentagonal-bipyramidal selenium (solid sticks) framework, is given in Fig. 33.
Acknowledgment The authors wish to acknowledge the invaluable collaboration of the diploma and Ph.D. students whose names are cited in the reference list. Special thanks are due to those who gave their assistance in the preparation of this article.
190
1 Moleculur Clusters
Our work has been generously supported by grants from the Deutsche Forschungsgemeinschaft ( DFG) and the German Federal Minister for Education, Science, Research and Technology (BMBF). We also thank the Fonds der Chemischen Industrie for financial support.
References [ I ] a) J.G. Brennan, T. Siegrist, S.M. Stuczynski, M.L. Steigerwald, J. Am. Chem. Soc. 1989, 1 I I , 9240; b) R.C. Haushalter, C.M. O’Connor, J.P. Haushalter, A.M. Umarij, G.K. Shenoy, Ang e l ~ Chem. , 1984,96, 147; AngeiV. Chem. Int. Ed. Enyl. 1984, 23, 169; c) M. Rakowski DuBois, Chem. Rev. 1988, 89, I; d) R.R. Chianelli, Cutal. Rev. Sci. Eng. 1984, 26, 361; e) H. Topsoe, B.S. Clausen, Cutul. Rev. Sci. Eng. 1984, 26, 395; f ) D. Coucouvanis, Ace. Chcm. Res. 1991, 24, 1; g) M.A. Ansari, J. A. Ibers, Coord. Chem. Reo. 1990, 100, 223. [2] a) J.A. McCleverty, Prog. Inorg. Cliem. 1969, 10, 49; b) D. Coucouvanis, Prog. Inorg. Chem. 1970, 11, 233; D. Coucouvanis, Prog. Inorg. Chem. 1979, 26, 301; c) R. Eisenberg, Prog. Inory. Chem. 1970, 12, 295; d) J. Willemse, J.A. Cras, J.J. Steggerda, C.P. Keijzers, Struct. Bonding 1976, 28, 83; e) R.P. Burns, C.A. McAuliffe, Ado. Inorg. Chem. Rudiochem. 1979, 22, 303; f ) R.P. Burns, F.P. McCullough, C.A. McAuliffe, Adv. Inorg. Chem. Rudiochem. 1980, 23, 21 I; g) D. Coucouvanis, Ace. Chem. Re.c 1981, 14, 201; h) M. Draganjac and T.B. Rauchfuss, Anyew. Clzem. 1985, 97, 745; Angew. Chem. Int. Ed. Engl. 1985, 24, 742; j) I.G. Dance, Polyhedron 1986, 5 , 1037; k) A. Miiller and E. Diemann, Ado. Inory. Chem. Radiochem. 1987, 31, 89; 1) P.J. Blower, J.R. Dilworth, Coord. Chem. Rev. 1987, 76, 121; m) A. Miiller, Polyhedron 1986, 5 , 323; n) D. Coucouvanis, A. Hadjikyriacou, M. Draganjac, M.G. Kanatzidis, and 0. Ileperuma, Polyhedron 1986, 5 , 349; o) I.G. Dance, K . Fisher, Prog. Inor(/. Cliem. 1994, 41, 637; p) L.C. Roof, J.W. Kolis, Chem. Rev. 1993, 93, 1037; q) B. Krebs, G. Henkel, Anyeizr. Chen?. 1991, 103: 785: Angew. Chem. Int. Ed. Engl. 1991, 30, 769; r) D. Fenske. J. Ohmer, J. Hachgenei, K. Merzweiler, Angew. Chem. 1988, 100, 1300; Angew. Chem. Int. Ed. Enyl. 1988, 27, 1300; s) M.G. Kanatzidis, S.-P. Huang, Coord. Chem. Rev. 1994, 130, 509; t) J.W. Kolis, Coord. Chem. Rev. 1990, 105, 195. [3] C.H. Wei, L.F. Dahl, Inorg. Chem. 1963, 2, 328. [4] a) R.E. Bachman, K.H. Whitmire, Oryunometullics 1993, 12, 1988; b) M. Shieh, M.-H. Shieh, Orgunometullics, 1994, 13, 920; c) S. Shieh, P.-F. Chen, Y.-C. Tsai, S. Shieh, S.-M. Peng, G.-H. Lee, Inorg. Chem. 1995, 34, 2251. [S] a) W.-F. Liaw, D.-S. Ou, Y.-S. Li, W.-Z. Lee, C.-Y. Chuang, Y.-P. Lee, G.-H. Lee, S.-M. Peng, Inory. Clzem. 1995, 34, 3747; b) C.J. Marsden, G.M. Sheldrick, J. Oryanomet. Chenz. 1972,40, 175; c) W. Eickens, S. Jager, P.G. Jones, C. Thone, J. Oryunomrt. Chem. 1996, 51 I , 67; d) J. Chen, V.G. Young, Jr., R.J. Angelici. Orgunometullics 1996, 15, 325. [6] a) W.-F. Liaw, C.-Y. Chuang, W.-Z. Lee, C.-K. Lee, G.-H. Lee, S.-M. Peng, Inorg. Chenz. 1996, 35, 2530; b) D. Fenske, J. Meyer, K. Merzweiler, Z. Nuturforsch. 1987, 42b, 1207. [7] C.H. Wei, L.F. Dahl, J. Am. Chem. Soe. 1968, 90, 3960. [8] C.H. Wei, L.F. Dahl, J. Am. Chem. Soc. 1968, 90, 3969. 191 C.H. Wei, L.F. Dahl, J. Am. Chem. Soc. 1968, 90, 3977. [lo] A. Winter, L. Zsolnai, G. Huttner, J. Orgunornet. Chenz. 1983, 250, 409 [ I l l W.S. Sheldrick, M. Wachhold, Angew. Chem. 1997, 109, 214; Angew. Clzem. Int. Ed. Enyl. 1997, 36, 206. [12] a) C.H. Wei, L.F. Dahl, Inorg. Chem. 1965, 4, I; b) C.F. Campana, F.Y-K. Lo, arid L.F. Dahl, Inorg. Chem. 1979, 18, 3060; c) D.A. Lesch, T.B. Rauchfuss, Inorg. Chem. 1981,20, 3583.
1.10 Towards Trunsition Metal Clusters
191
[I31 a) B. Beagley, D.W.J. Cruickshank, P.M. Pinder. A.G. Robiette, G.M. Sheldrick, Actu C~JTtullogr. Sect. B 1969, 25, 737; b) J. Donohue; A. Caron, Actu Crystullogr. Sect. B 1964, 17, 663; c ) J. Donohue. A . Caron, Actu Crystullogr. Sect. B 1962. 15, 930; dj R. Boese, D. Blaser, Z. Kristallogr. 1990. 193, 289; e ) D. Braga, F. Grepioni, A.G. Orpen, Organometu1lic.s 1993. 12, 148; f ) B.A. Frenz, J.A. Ibers. Inorg. Chetn. 1972. 11, 1109; g) G. Kong: G.N. Harakas, B.R. Whittlesey, J. ilm. Chem. Sue. 1995, 1 17, 3502. [I41 R. Seidel, B. Schnautz, G. Henkel, Angeiv. Chem. 1996. 108. 1836; Anyew. Chem. Int. EN'. Engl. 1996, 35, 1710. [I51 a ) S.J. La Placa, W.C. Hamilton, J.A. Ibers, A. Davison, Inorg. Chem. 1969, 8. 1928; b) S.J. La Placa, W.C. Hamilton. J.A. Ibers, Inorg. Chem. 1964. 3, 1491. [ 161 B. Schnautz. G. Henkel. in preparation. [ I 71 B. Schnautz. G . Henkel, in preparation. [I81 B.W. Eichhorn, R.C. Haushalter, J.S. Merola, Inorcq. Chem. 1990. 29, 728. [1'3] S.C. O'Neal. W.T. Pennington, and J.W. Kolis, /nor
192
I Molecular Clusters
1411 V. Kiillmer, E. Rottinger, H. Vahrenkamp, J. Chem. Soc. Clzem. Conimun. 1977, 782. 1421 a) L.L. Nelson, F.Y-K. Lo, A.D. Rae, L.F. Dahl, J. Orgunomet. Chem. 1982, 225, 309: b) E. Rottinger, H. Vahrenkamp, J. Oryanomet. Chem. 1981, 213, I; c) L. Bogan, Jr., D.A. Lesch, T.B. Rauchfuss, J. Organornet. Chem. 1983, 250, 429. [43] a) J.M. Coleman, A. Wojcicki, P.J. Pollick, L.F. Dahl, Inorg. Ciiem. 1967, 6, 1236; b) J.A. de Beer, R.J. Haines, J. Orgunometal. Chen?. 1970, 24, 757. [44] P.V. Broadhurst, B.F.G. Johnson, J. Lewis, P.R. Raithby, J. Chem. Soc. Clzem. Commun. 1982, 140. [45] S.-P. Huang, M.G. Kanatzidis, Inory. Clzenz. 1993, 32, 821. [461 R. Seidel, G. Henkel, in preparation. [47\ L.C. Roof, D.M. Smith, G.W. Drake, W.T. Pennington, J.W. Kolis, Inorg. Chem. 1995, 34, 337. (481 L.C. Roof, W.T. Pennington, J.W. Kolis, Angew. Chem. 1992, 104, 924; Angew. Cheni. Int. Ed. Engl. 1992, 31, 913. 1491 R. Seidel, G. Henkel, in preparation. [50] R. Seidel, G. Henkel, in preparation. [51] G.L. Lilley, E. Sinn, B.A. Averill, Inory. Chem. 1986, 25, 1073. [52] S. Weipgriiber, G. Henkel, in preparation. [53] a) G. Christou, R.H. Holm, M. Sabat, J.A. Ibers, J. Am. Chem. Soc. 1981, 103, 6269; b) K.S. Hagen, A.D. Watson, R.H. Holm, J. Am. Chenz. Soc. 1983, 105, 3905. [54] a) G. Henkel, H. Strasdeit, B. Krebs, Angew. Chem. 1982, 94, 204; Angeiv. Chem. Ozt. Ed. Engl. 1982, 21, 201; Angew. Cheni. Suppl. 1982, 489; b) H. Strasdeit, B. Krebs, G. Henkel, Inory. Chem. 1984, 23, 1816. [55] H. Strasdeit, B. Krebs, G. Henkel, Z. Natucfbrsch. 1987, 42b, 565. 1561 a) D.L. Stevenson, V.R. Magnuson, L.F. Dahl, J. Am. Chem. Sue. 1967, 89, 3727; b) G. Gervasio, S.F.A. Kettle, F. Musso, R. Rossetti, P.L. Stanghellini, Inovg. Chem. 1995, 34, 298. 1571 a) G. Gervasio, R. Rossetti, P.L. Stanghellini, G. Bor, Inory. Chim. Actu 1984, 83, L9; b) E. Diana, G. Gervasio, R. Rossetti, F. Valdemarin, G. Bor, P.L. Stanghellini, Inory. Chein. 1991, 30, 294. [58] F. Cecconi, C.A. Ghilardi, S. Midollini, Inorg. Chim. Actu 1981, 64, L47. [59] a) M. Hong, Z. Huang, X. Lei, G. Wei, B. Kang, H. Liu, Polyhedron 1991, 10, 927; b) F. Cecconi, C.A. Ghilardi, S. Midollini, A. Orlandini, P. Zanello, Polyhedron 1986, 5, 2021; c) D. Fenske, J. Ohmer, K. Merzweiler, Z. Nututfbrsch. 1987, 42b, 803; d) D. Fenske, J. Hachgenei, J. Ohmer. Angerv. Ciiem. 1985, 97, 684; Angew. Chem. Int. Ed. Engl. 1985, 24, 706; e) F. Cecconi, C.A. Ghilardi, S. Midollini, A. Orlandini. Inory. Chin?. Acta 1983; 76, L183; f ) D. Fenske, J. Ohmer, J. Hachgenei, Angew. Chem. 1985, 97, 993. Angew. Chenz. Int. Ed. Engl. 1985, 24, 993. [601 M.L. Steigerwald, T. Siegrist, S.M. Stuczynski, h r g . Chem. 1991, 30, 2256. [61] a) A. Bencini, C.A. Ghilardi, S.Midollini, A. Orlandini, U. Russo, M.G. Uytterhoeven, G. Zanchini, J. Chem. Soc. Dalton Trans. 1995, 963; b) F. Cecconi, C.A. Ghilardi, S. Midollini, A. Orlandini, P. Zanello, J. Chem. Soc. Dalton Trans 1987, 831; c) A. Agresti, M. Bacci, F. Cecconi, C. A. Ghilardi, S. Midollini, Inory. Chem. 1985, 24, 689; d) M.L. Steigerwald, T. Siegrist, E.M. Gyorgy, B. €lessen; Y.-U. Kwon, S.M. Tanzler, Inory. Chem 1994, 33, 3389; e) C.A. Goddard, J.R. Long, R.H. Holm, Inorg. Cheni. 1996, 35, 4347. [62] R. Seidel, G. Henkel, in preparation. 1631 R. Seidel, G. Henkel, in preparation. [64] a) S. Weifigraber, G. Henkel, in preparation; b) B. Schnautz, G . Henkel, in preparation. [65] M. Shieh, P.-F. Chen, S.-M. Peng, G.-H. Lee, Inorg. Chern. 1993, 32, 3389. 1661 D.M. Washecheck, E.J. Wucherer, L.F. Dahl, A. Ceriotti, G. Longoni, M. Manassero, M. Sansoni, P. Chini, J. Am. Chem. Soc. 1979, 101, 6110.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.11 Low-Nuclearity Iron and Ruthenium Selenido-Carbonyl Clusters Derived from Phosphine and Diphosphine Selenides D1iniele Cmrri, Cluutlirt Gr-uifl,Giocmni Prdieri,
N
M Antonio ~
Tiripiccliio
1, 11.1 Introduction The chemistry of transition-metal chalcogenido polynuclear clusters has undergone rapid development in the last few years as pointed out in recent reviews.['] The reasons for this increasing interest derive (i) from the usefulness of chalcogenido ligands in cluster growth reactions, (iij from the pursuit of structural models and precursors for the synthesis of new materials and (iii) from the search of new coordinations and geometries.['.'] In this regard, spectacular results have been achieved in the field of nickel tellurides[31and selenides["] and copper s e l e n i d e ~ ,where ~ ~ ' giant molecular clusters have been synthesized and structurally characterized. In order to prepare transition-metal selenido clusters, different synthetic routes are presently available. such as those involving, as starting materials, diphenyl diselenide,[61 phenylselenyl chloride,['] selenofene.[81 trialkylsilyl ~elenides,["*'~ polyselenide anions,'"] and phosphine selenides.[lO1Furthermore, small selenido clusters, such as [Fez(p-Se?)(C0)6]and [Fe3(,u3-Se)z(C0),],can be conveniently used to prepare larger mono- and bimetallic species," ' I as described in Sec. 1.1 1.5. [n particular. the reaction of tertiary phosphine selenides with metal carbonyl co~mplexes[~~' provided a simple. one-step synthetic procedure for the formation of sel'enido clusters through oxidative transfer of selenium atoms to low-valent metal centers, taking advantage of the reactivity of the P=Se bond. Moreover, this method appears suitable for the synthesis of phosphine-substituted molecular clusters of different nuclearity, depending on which phosphine is used. The availability of these clusters offers the opportunity to exert a stoichiometric control in those processes that lead from molecular metal chalcogenides such as [M,E,( PR3),1 to extended inorganic solids like M,E,. The stoichiometric control. combined with the kinetic control, which derives from low activation energies of these processes, often allows the preparation of otherwise inaccessible solid phases or microinhomogeneous materials such as nanocomposites." 'I
194
I Molecular Clustevs
Despite the synthetic potential of the phosphine selenides, their reactions with metal carbonyls have been explored only to a limited extent until the last three years, and most of the previously described chalcogenido-carbonyl clusters containing Group 15 donor ligands had been obtained by substitution reactions.['.' l a , ' 31 For this reason, we started to study the reactions of phosphine and diphosphine selenides with [M3(C0)12]( M = Fe or Ru)[14]with the aim of obtaining new phosphine substituted selenido carbonyl clusters, whose structural characterization and reactivity are the central issue of this article.
1.11.2 Clusters derived from Ph3PSe The carbonyl clusters [M3(C0)12] ( M = Fe or Ru) react with Ph3PSe through oxidative transfer of selenium atoms to the metal cluster (Fig. l), giving a variety of phosphine-substituted selenido carbonyl cluster^,[^^^' whose structural frameworks are shown in Fig. 2. The open-triangular, variously substituted, do-clusters with the M3Se2 core are the major products for both metals, but significant amounts of clusters with dinuclear Fe2Se2, trinuclear Fe3Se and tetranuclear Ru4Se2 cores are also obtained. The product distribution is strongly dependent on the reaction conditions, particularly on the cluster-phosphine molar ratio. In the case of iron the reaction takes place rapidly, upon gentle warming, whereas in the case of ruthenium the presence of Me3N0, as decarbonylation reagent, is required to increase the reaction rate.
1.11.2.1 Ph3P-substituted selenido iron clusters The reaction of [Fe3(C0)12]with Ph3PSe is non-selective. Whatever starting molar ratio is used, the reaction affords six products (1-6, Fig. 2) belonging to the three families of clusters cores Fe2Se2, Fe3Se2 and Fe3Se.['41 The starting Fe3(C0)12/ Ph3PSe molar ratio slightly affects the yields of the individual products, the disubstituted products being abundant only when the ratio is 1 : 8. All these compounds follow the 18-electron rule when pu,-Seand p-Se2 are considered as four- and
1 M = Fe, L, = L, = L, = CO M = Fe, L, = PPh, L, = L3 = CO 3 M = Fe, L, = L, =PPh, L, = CO 2
7 M =Ru,L i z PPh,
b =L3 = CO
8 M = Ru, L , = = PPh, L3 = CO 9 M = Ru, L, = L, = L3 = PPh3
[M3(p3-Se),(CO),S] (50 electrons, d o , 7 s.e.p.s)
6
se
5 L=PPh,
Ph3P
Fe
PPh,
10 L = C O 11 L = P P h 3
[RU4(~~-~),(~~0)2(co)~(L)(Pph3)1
(62 electrons, closo, 7 s.e.p.s) Figure 2. Structural diagrams of the Phi€'-substituted selenido-carbonyl iron and ruthenium clusters (carbonyls omitted) along with the appropriate electron counts according to the I &electron rule and the PSEP theory.'"]
six-electron donors, respectively. The first five products possess the well-known cluster cores Fe2Se2 and Fe3Se2, typical of iron and ruthenium chalcogenido clusPPhl)2] 6 is unprecedented in the neutral ters,"' whereas [Fe~(,uu3-Se)(p-CO)(CO)~( form, only the dianion [Fei(pu,-Se)(CO)g]'-and the corresponding dihydride having been synthesized." 61 The possible sequential steps of these reactions are illustrated in Fig. 3. The first two reactions ( u .h ) , involve oxidative transfer of selenido ligands from the phosphine to the trinuclear clusters and are the key steps in the entire process. The dinuclear compounds are probably produced by decomposition of the FeiSe? cores through an internal redox process (step c), which results in the detachment of metal(0) species. In fact, on heating the pure trinuclear cluster 3 (FejSezP2) under reflux, significant amounts of the dinuclear complexes 4 ( FeZSezP) and 5 (FeZSe2P2)
196
I Molecular Clusters
Figure 3. Possible sequential steps in the reaction between Ph3PSe and [Fe3(C0)12].
are obtained. The released metal(0) fragment could react with trinuclear clusters producing higher-nuclearity species, such as the tetranuclear clusters 10 and 11 found in the case of ruthenium (see 2.2). It is reasonable to think that similar clusters are also formed in the case of iron, but they are probably not sufficiently stable. The variety of Ph3P-substituted products can arise from direct CO substitution by free Ph3P ligands or from disproportionation processes. In this regard, we have found (reaction d ) that the monosubstituted cluster 2 (Fe3SeZP),when dissolved in chloroform at room temperature, is in equilibrium with the corresponding unsubstituted 1 (Fe3Se2) and the disubstituted 3 (Fe3Se2P2) products. The separation of the complex mixtures of selenido-carbonyl iron clusters, derived from the synthetic routes described above, can also be accomplished by high-performance liquid chromatography ( HPLC)." 'I This technique (eventually coupled with mass-spectrometric detection" 71) is particularly convenient for the separation of organometallic species, owing to its superior speed and efficiency when compared with traditional column and thin layer chromatography." *I The order of elution of the iron clusters on silica is strongly dependent on the degree of phosphine substitution and on the cluster framework, which, in the first place, determines the polarity of the molecule.[''1
1.1 1.2.2 Ph3P-substituted selenido ruthenium clusters In the case of ruthenium, with a 1 : 2 Ru3(CO)I*/Ph3PSemolar ratio, the reaction is quite selective giving, in very high yield, the disubstituted trinuclear, 50-electron cluster (nido, seven skeletal electron pairs, s.e.p.s) [Ru3(p3-Se)2(C0)7(PPh3)2] 8, which is just the primary product of the oxidative attack of two Ph3PSe molecules on the metal triangle.[14]The production of large amounts of this compound is probably due to its high thermal stability; in fact, it does not decompose in toluene under reflux, to give dinuclear species, as the isostructural iron cluster 3 does. Minor products are the analogous mono- and trisubstituted derivatives 7 (Ru3Se2P) and 9 (Ru3Se2P3) and the 62-electron (closo, seven s.e.p.s) tetraruthenium clusters
[Ru4(p4-Se)2(p-C0)2(CO)x(PPh3)] 10 and [Ru4(p4-Se),(,u-CO)z(COj7( PPh;)?] 11. The last two species belong to the large family of the bicapped, square-planar clusters of general formula M4Ez L,,, which have attracted considerable interest recently from both the synthetic as well as the structural point of view"91 (see paragraph 1.1 1.4.2). When the starting molar ratio Ru3(CO)Iz/Ph3PSe is 1 :4, the trisubstituted derivative 9 is the main product, but the yield is not as high as that of the disubstituted species 8 in the 1 : 2 reaction. 'The mass spectra, in chemical ionization mode, of the ruthenium clusters"'' show the molecular ions together with several groups of peaks at 28-,LLintervals, arising from the progressive loss of the carbonyl ligands (e.g. [Ru;Sez( PPh3)2(C0)6li, m/z 1153, 100% for 8 and 739'0 for 9). The considerable abundance of the molecular ions, particularly for compound 8 (ca. 50%), testify to the higher stability of the ruthenium clusters compared to the iron homologues, whose mass spectra are generally dominated by mononuclear fragments.
1.11.3 Cluster derived from (Ph2PSe)2X The clusters [M1(CO)12]( M = Fe or Ru) have been successfully reacted with the three diphosphine diselenides (Ph,PSe),X shown in Fig. 4 (along with their parent
Figure 4. Structural diagrams of the diphosphine diselenides along with those of their parent diphosphines.
198
I Molecular Clusters
diphosphines), affording again the substituted open-triangular, 50-electron, nidoclusters [M3(p3-Se)2(CO)7{p-( Ph2P)zX)I as the main products.r20p221 The set of the other products parallels the previous set of Ph3P-derivatives, including the dinuclear species, in the case of iron, and the closo tetranuclear clusters, in the case of ruthenium. The most important aspect of this reaction sequence is the characterization of the new compound [Ruq(,u3-Se)4(CO) lo(p-dppm)] 20, which is the first 72-electrons Ru-Se cubane-like cage complex to be reported. The structural diagrams of the resulting clusters are shown in Fig, 5.
Ph,P
+
‘PPh,
Figure 5. Structural diagrams of the diphosphine-substituted selenido-carbonyl iron and ruthenium clusters (carbonyls omitted).
1.I I Lou-Nuclenrity Iron und Rutheniunz Selenido-Curhonyl Clusters
199
1.1 1.3.1 (PhzP)zX-substituted selenido iron clusters The reactions of [Fe3(CO)12 ] with the diphosphine diselenides dppmSe2 dppeSe2, and dppfcSe2 ( Fig. 4) produce12 various diphosphine-substituted selenido iron complexes, among which the nin'o-clusters [Fei(p,-Se);!(CO)-,Ip-LL)](LL = dppm, 12; dppe, 13; dppfc, 14) should be regarded as the primary products of the oxidative atlack. In particular, the reactions with dppeSe2 and dppfcSe2 afford 13 and 14 respectively, along with other complexes that contain the same Fe3Se2 cluster core, such as the unsubstituted cluster 1 and the derivatives 15 and 16, where dppe and dppfc bridge two cluster units. On the other hand, the reaction with dppmSe2 also gives rise to lower-nuclearity products, particularly to the dinuclear derivative (Fez(p-Se,)(C0)4(p-dppm)l17. Compounds 12 ( Fe3Se2dppm) exhibits an unexpected "P NMR spectrum, at room temperature, consisting of a singlet (6 77.7) and two doublets at higher fields (6 53.0 and 42.6, J(P,P) 58 Hz). Correspondingly, the ' H spectrum shows two methylene triplets (6 4.08 and 3.22, J ( H , P ) 10.5 Hz), which coalesce in toluene at 364 K. Fig. 6 shows the ' H two-dimensional exchange (EXSY) spectrum, which indicates that the two methylene signals are correlated by chemical exchange. These data suggest _ _ the fluxional behaviour depicted in Fig. 7, which consists of the migration of an iron-iron bond to link the two iron atoms bridged by the bidentate
0.0 1.0
2.0 3.0
7 (1 h0 ppni
'
Figure 6. H two-dimensional exchange (EXSY 1 spectrum (303 K , CHCII) of [ Fei i,uI-Se)2iCO)7(pdppm)].
200
I Moleculm Clusters
Figure 7. Proposed dynamic behaviour of [ Fe~(p3-Se)~(C0)7(p-dppm)] in solution.
ligand. In this way the two phosphorus atoms become non-equivalent and give the observed doublets in the 3'P NMR spectrum. The singlet at lower field is attributed to the symmetrical isomer, the only product that could be isolated in the solid state. This isomerism is probably induced by the dppm ligand whose steric demands cause the basal iron atoms to approach each other (see Section 1.11.4.1), promoting linking between them.
1.11.3.2 (Ph~P)2X-substitutedselenido ruthenium clusters The diphosphine diselenide dppmSe2 and [ R u ~ ( C O ) I(1~ :] 1) react in toluene giving I Ru3 (~u,-se)2(c0)7 (p-dppm)] 18, [RU4(L(4-Se)2(p-CO)(CO)s (p-dppm)I 19 and [Rq(p3-Se)4(CO)lo(p-dppm)] 20.[201Cluster 18 has a bicapped, open-triangular, 50electrons nido-core and is the expected primary product of the oxidative attack of dppmSe2 to the starting carbonyl cluster. Cluster 18 exhibits the same NMR spectral patterns as the corresponding iron derivative 12, hence the same fluxional behaviour can be proposed. Also 19 has a known architecture, being formally a substitution derivative of the 62-electron, bicapped square-planar, closo-cluster [RU~(~~-S~)~(,U-CO)~(CO)~].[~"~ Cluster 19 can be regarded as a secondary product, probably deriving from the addition of a mononuclear metal fragment to cluster 18, under the adopted pyrolytic conditions used. The structure of complex 20, outlined in Fig. 5 , consists of a cubane-like, 72electrons M4E4 cage core, which is unprecedented for ruthenium and selenium.'201 Also this complex should be regarded as a secondary product, probably formed by the self-assembly of two dinuclear RuzSez groups, derived from 18 (Ru3Sel) by loss of a mononuclear metal fragment. In the case of dppeSe2 and dppfcSe2 the sole characterized products, obtained by reactions with [ R u ~ ( C O ) ~are ~ ] the , corresponding nido-derivatives [Ru3(p3-Se)2(C0)7(p-dppe)](21) and [Ru3(p3-Se)2(C0)7(dppfc)](22).["] Compound 22 is the
1.I I Loii~-Nucleuritj~ Iron und Rutheniuni S~.l.izi~~-CurhonylClusters
201
only nido-cluster of this family where the diphosphine does not behave as a bridging ligand (Fig. 5 ) . From the reaction of (Ph2PSe)lNH with [Rul(C0)12].Woollins et LII. isolated the cho-cluster [ R u(,u4-Se)2 ~ (u-CO)( C 0 ) s( p - (Ph?PSe)?NH1 ].I2 31
1.11.4 Structural features Most products of the reactions of iron and ruthenium carbonyls with phosphine and diphosphine selenides. described in the preceding sections, display the iiiclo MlSe2 open-triangular and (only for ruthenium) the closo Ru4Se2 square bipyramidal cores. Both these two structural types are characteristic of the chalcogen elements ( S , Se, Te), in combination with transition metals. and compare with a variety of examples in the literature, particularly for iron and ruthenium. The availability of a wide number of data allows an adequate comparison of the relevant structural parameters to be made in order to detect the influence of chalcogen elements and of substituents ( particularly phosphines) on the chalcogenido-carbonyl cluster geomet ry .
1.11.4.1 M ~ E nido z clusters T h e chalcogenido clusters of general formula [M?E?Lg]( M = Fe or Ru, E = S , Se or Te, L = two-electron ligand) exhibit a bicapped, open-triangular array of metal atoms, which can be described as a square pyramidal structure with two metal and
' U p to now, cighteen structures of iiitlo clusters of formula JF;cxip?-E)2L9]( E = S. Se or Te, L = two-electron ligand) are known; thirteen are recorded on the Cambridge Structural Database (CSD, October 1997 release I and five arc reported in recent There are 10 structurcs of the type I R u ; ( j r , - E ~ ~ Lseven ~ ] ; on the CSD, one i n a recent paperL231 and two to be published.'221 L ,= , I PR. S. Se or Te. n = 11; E = Bi. n == 12), 20 With regards to the c / o s o clusters [ R L I J ~ ~ , - E ) ~1,E structures have been considered; 16 of these are found in the CSD, four reported in recent papers. [ I 5 . 2 2.231 The average bond length values, given in Angstrom to two decimal places, have been calculated from distance values with estimated standard deviations lying in the range 0.001~ 0.005 A. CSD reference codes for [ F e ~ ( p , - E ) ~ L BENPIT. ~]: DUJZUD. JIZYUM. KOKGIA, LATRUT. PEDNOB, PEDNUH. PEDPAP, PODCAM, TAQMAZ. VOYWAH. YEDLOI, ZEHQIM. CSD reference codes for J R U J I ~ ~ ~ - EGEJMIR. ] ~ L ~ I : JUVWAY 10, KIHFAI, KUWBAF. KUWBIN. ZEHQUY. ZUVTUF. CSD reference codes for [Ru,l/r4-Ej:L,,1: BIFFIF. FASTEY. FUHYEM. JEYPEI. JUVVURIO. KIHDUA, KIHFEM. KIHFOW. KONFAU. KONFEY. KUWBEJ. PARPIH. TAWKOR. VOTZAF. VOTZEJ. ZEHROT.
202
1 Molecular Clusters
two chalcogen atoms alternating in the basal plane and the third metal atom (Ma,) at the apex of the pyramid (see for example compound 12 in Fig. 5 ) . For the same metal (Fe or Ru) the overall size of these clusters is dependent on the size of the chalcogen element, which affects not only the M-E distances, but also the M-Ma, ones. In the case of Fe, the mean values of the Fe-E bond lengths are 2.24 ( E = S), 2.37 (Se),and 2.55 (Te) A and those of Fe-Fe,, linkages are 2.59, 2.67, and 2.75 A respectively. Correspondingly, the non-bonding E. . .E and Fe. . .Fe diagonal interactions in the basal plane range from the minimum values of 2.84 and 3.37 A, respectively, for the disulfido clusters to the maximum values of 3.31 and 3.85 A of the ditellurido species. The same trend exists in the case of ruthenium, the Ru-E distances being 2.39 ( E = S), 2.52 (Se) and 2.69 &Te) and the Ru-Ru,~ ones 2.78 to 2.83, to 2.96 A,respectively. As regards CO substitution with tertiary phosphines, it is interesting to note that it is regioselective for the basal metal atoms. However, with stronger acceptor ligands, such as PhNC, the apical site is preferred,[241suggesting that Ma, is more electron-rich. Basal substitution with phosphines on a single metal atom M( P) produces a significant lengthening of the M( P)-M,, distance, which differs from the other M-M,, distance by 0.11 A in [FelSe2(CO)sPPh3] 2 and by 0.15 A in [Ru3Sez(CO)-i(dppfc)I22. The case of diphosphines deserves specific comments. Generally (with the exception of compound 22["] and another ruthenium compound[251),diphosphines act as bridging ligands between the basal metal atoms, in such a way that they exert a significant influence on the M. . .M non-bonding distance, producing appreciable modifications of the overall cluster shape. In the case of iron diselenides Fe3Se2(p-LL)["] the Fe.. .Fe separation is 3.489(2) in 12 (dppm), 3.560(4) in 13 (dppe) and 3.626(2) A in 14 (dppfc). The progressive cluster shape modification, by passing from dppm (smallest bite) to dppfc (largest bite), is also apparent from the increase of the Fe-Fe,,-Fe angle, from 81.1(1) to 86.5(1)", and by the corresponding decrease of the Se--Fed,-Se angle, from 89.9( 1) to 79.9( 1)'. The dependence between the angles Se-Ma,-Se and M-Ma,-M, for iron and ruthenium (exhibiting the same trend of iron) are shown in Fig. 8.
1.11.4.2 Ru4E2 closo clusters The structure of closo, square bipyramidal M4E2 clusters (see for example compound 19 in Fig. 5 ) has attracted particular attention from the theoretical point of view, owing to the possibility of different electron counts and of the possiblity of bonding character in the E . . . E interaction. In fact, when E is an aryl- or alkylphosphido ligand ( E = PR), the P. . .P distances are unusually and MO calculations for model iron and rhodium compounds[271indicate that the P. . .P
1.11 Low-Nucharity Iron and Ruthenium Srlenido-Curhonyl Clusters
203
I
Figure 8. Diagram showing the dependence between the angles Se--M,,,-Se and M-Ma,-M in the diphosphine-substituted nick) clusters [M\Se?Lgl( M = Fe or Ru); the two iron and ruthenium subsets give rise to distinct least-squares straight lines, the upper one referring to the first group; points 12 14. 21 correspond to clusters reported in Figure 5,'' 1.221 points [a]-[c] refer respectively to [Rui(p3-Se)2(CO)cj 13[ 6 b1 [ Rui?(,u,-Se)?KO), { p - (Ph2 Pj2C2 }1[6h1
-
84-
v
?!.4(
82-
2
& 80 78 -
76,
I
.
I
.
I
.
I
.
interaction is attractive. With regard to the structures of RuqE2 clusters ( E = S , Se, Te, PR or Bi) the averaged values of the E . . .E separations are 2.78 for PR, 3.08 for S , 3.30 for Se, 3.64 for Te and 3.92 A for Bi. Taking into account that the covalent radii for P and S are 1.10 and 1.04 A respectively,'"] in this family of clusters the P. . .P separation also appears particularly short. The increasing size of the capping element causes significant lengthening of the Ru--Ru bond distances, whether the two metal atoms are CO bridged (the mean Ru--Ru values are 2.75 for S, 2.77 for Se and 2.81 A for Te) or not (2.81, 2.85, and 2.93 A).In the case of PR groups as capping ligands, the mean Ru-Ru distances are 2.71 A in the presence of a CO bridge (lower than the values observed for the chalcogenido clusters) and 2.87 A in the absence of CO bridge (lying in the middle of the range observed for the chalcogenido clusters), resulting in the largest difference between the two types of distances (0.26 A). With regard to phosphine substituted derivatives, the series [Ru4SezLI J offers the widest number (six) of structural examples. In these diselenido ruthenium clusters, substitution with Group 15 donor ligands influences the number and the location of bridging carbonyls, as shown in Fig. 9. Moreover. the presence of a bridging diphosphine (with a small bite like dppm) results in a shortening of the bridged the same mean distance Ru--Ru edge from a mean value of 2.85 to 2.77 A.[22.231 observed in the case of bridging CO. Finally, it is interesting to note that the average value of the Ru-Se distances (2.58 A) observed in the doso clusters Ru4Sez (four Ru-Ru bonds) is significantly higher than the corresponding one (2.52 A) found in the structurally related nido RuJSe2 (two Ru-Ru bonds) species.
204
1 Molecular Clusters
P-Ru-
1 X \
P-Ru-
Ru
Figure 9. Structural types (non-bridging carbonyls omitted) resulting from Group 15 donor ligand substitution (mostly P) on the closo cluster [ R u 4 ( p 4 - S e ) ~ ( p - C O ) ~ ( C O ) ~ ] .
1.11.5 Cluster growth reactions Recent work by other research groups[281showed that small chalcogenido clusters, such as dinuclear and trinuclear Group 8 metal derivatives are useful precursors for cluster growth processes viu reaction with suitable transition metal species especially those belonging to Group 6. For example, [FesSe2(C0)9] reacts with [ C P ~ M O ~ F ~ ~ S ~ ~ ( and C O )[Fez(SeTe)(CO)g] ~][’~~’ inter[ C P ~ M O ~ ( C Oto) ~give ] acts at room temperature with [W(CO)s(thf)] (thf = tetrahydrofuran) yielding [ Fe4Mo(p3-Se)z(pj-Te)z(CO)141. [ 28i1 With regard to phosphine-substituted derivatives, the availability of workable amounts of the trinuclear selenido ruthenium clusters [Ru~(,u3-Se)2(C0)7(pu-dppm)] (18) and [Ru3(p3-Se)2(C0)7( PPh3)2] (8) stimulated us to assess their tendency to give cluster growth reactions. As stated previously, both compounds are nido clusters with 50 electrons and 7 skeletal electron pairs (s.e.p.s), and hence they could be prone to add zero-s.e.p.s metal fragments, such as M(CO)3 ( M = Mo or W ) , to give the hypothetical doso clusters [MRu3(p4-Se)2(CO)lo(PPh3)2], in the case of 8 (species I in Fig. 10). This would have been expected also considering the existence of the analogous monophosphine sulfur derivative [ WRu3(p4-S)2(CO)~ 1 ( PMe2Ph)].[2”1
I.I I Lo\t,-Niic.leurityIron und Rutheniuni Selenido-Ccrrhonyl Clusters
,.
.I
5
205
23 M = M o 24 M = W
Figure 10. Diagrammatic representation jcarbonyls omittcd) of the reactions of cluster 8 and 18 with MjCO); groups derived from [M(CO)3(MeCN)?] ( M = Mo or W); I and I1 are hypothetical intermediates.
The results of the reactions of the two clusters with [M(C0)3(MeCN)3]are summarized in Fig. 10. Cluster 18 does not react, probably owing to steric hindrance of the bridging diphosphine, whereas cluster 8 unexpectedly affords the bicapped, square-planar clusters [M2Ru2(~C14-Se!2(~1-CO)C14(CO)~( PPh3)2] ( M = Mo 23, W 24) as unique products, formally derived from the addition of two M(CO)3groups with removal of a Ru(C0)3 fragment,[301cia the possible intermediates I and 11. Both these bimetallic clusters exhibit electron-deficient (60 electrons), planar, centrosymmei:rical arrays of two Group-6 metals and two ruthenium atoms, bicapped above and below by two quadruply bridging selenium atoms. As a result, the six atoms of the M2Ru2Se2 core form distorted octahedrons, in which four carbonyls asymmetrically bridge the M-Ru edges (Fig. 11, M = W ) . Apart the unusual mixed metal core, cluster 23 and 24 are unique, as they contain only six s.e.p.s instead of
206
1 Molecular ClusterJ
Figure 11. View of the molecular structure of [Ru2W2Se2(,u-C0)4(CO)6(PPh3)2]. Selected bond distances (A) with e.s.d.s in parentheses: W( 1 j-Se( 1 ) 2.595(2), Ru(1)-Se(1j 2.579(2); W( 1)-Ru(1) 2.803(2).
seven, as observed in the clusters of the Ru4El-derived family'6a,23,28c~e,29,3 and as required by the PSEP theory.[321 On the other hand, cluster 18, which appears inert in the previous process, exhibits a particular reactivity, under pyrolytic conditions. In fact, when heated in the presence of the decarbonylation reagent Me3N0, it produces the new cluster [RuhSe4(CO)12(,~-dpprn)z]. This hexanuclear species, shown in Fig. 12, clearly derives from the self-assembling of two cluster units with release of two carbonyl molecules in a sort of condensation process, whose final result is the formation of three new metal-metal bonds.[221The molecular structure of this cluster, solved by X-ray diffraction techniques, has only one precedent, namely that of [OsgS4(CO)161, which was obtained by the photolysis of [ O S ~ S ~ ( C O ) ~ ] . [ ~ ~ ]
Figure 12. Structural diagram (carbonyls omitted) of the hexanuclear cluster [ Ruh SeA(C0)I2 (p-dppm)*]; metal-metal distances (A) with e.s.d.s in parentheses: u 2.826(2j, h 3.015(2).c 2.982(2), d 2.858(2), e 3.002(2),/3.023(3), y 2.81 l(2).
1.11 Lou.-Nuclearitjj Iron und Ruthenium Schido-Ccirhonyl Clusters
207
1.l1.6 Concluding remarks Systematic investigations on the reactions of tertiary phosphine and diphosphine selenides towards iron and ruthenium carbonyl clusters have achieved the following results: ( i )these reactions provide a simple, one-step (sometimes selective) synthetic route to phosphine-substituted, low-nuclearity selenido-carbonyl clusters, through oxidative transfer of selenium atoms; (iij they lead, in some cases, to the formation of new species, such as the monoselenido cluster [Fe~(p3-Se)(~l-CO)(CO)7( PPh?)z] and the cubane-like cage complex [Ru4(p3-Se)4(CO) l&-dppm)]; (iii) some of these cornpounds display particular physico-chemical characteristics, such as the fluxional ( M = Fe behaviour in solution of the dppm derivatives [M3(p3-Se)2(C0)7(p-dppm)] or Ru); (ic) certain compounds of this family exhibit specific activity in cluster growth reactions, such as [Ru3(p3-Se)2(C0)7( PPh3)2] which reacts with M'(CO)3 fragments ( M ' = Mo or W ) , leading to the electron-deficient, closo-octahedral species [M~Ru2(p4-Se)~[p-CO)4(CO)6( PPhj)?]. These findings, along with the results achieved by other research groups, encourage the pursuit of these investigations, particularly by extending the described synthetic strategy to other phosphines, eventually containing other ligand functionalities. An important goal, which directly derives from the results described in Section 1.1 1.5, is to generate a series of phosphine-substituted selenido-carbonyl clusters. with different cluster cores, different ligands and different degrees of substitution, to be tested in cluster growth reactions. In fact, increasing evidence is being acquired that the presence of phosphine ligands can exert a decisive influence on the cluster growth processes.
References / I ] L. C . Roof, J. W. Kolis, C / i m Rrr., 1993. Y3%1037: G. Schmid ( E d . ) , Cluster cind Colloids, VCH, Weinheim, 1994. Ch. 3. (21 M . L. Steigerwald, Pol~~/iedr.on. 1994 13. 1245: M. L. Steigerwald. T. Sicgrist, E . M . Gyorgy; B. Hessen. Y.-U. Kwon, S. M. Tanzler. Inorcq. C%eni.. 1994. 33. 3389. [3] J. G. Brennan. T. Siegrist. S. M . Stuczynski. M.L. Steigerwald, J. h i . Cllerii. S i c . , 1989, I l l . 9240; Z. Nomikou. B. Schubert, R. Hoffniann, M. L. Steigerwald. Inory. Chem., 1992. 31, 2201. 141 D. Fenske, J. Ohmer. J. Hachgenei. Angcw. C/KW..Int. Ed E/iq/.. 1985, 24, 993; D. Fenske. H. Krautscheid, M . Muller. Amqeii.. C/icvi7.,lnr. Ed. En(//..1992. 31, 321. (51 H. Krautscheid. D. Fenske. G. Baurn. M . Semmelmann, A/iqc,it'. Chwn.. I / / / , Ed. Encql.. 1993. 32, 1303. 161 [a)B. F. G. Johnson. T. M. Layer. J. Lewis. A . Martin. P. R . Raithby, J. Orgcinomcf. C'hiwi., 1992. 429. C41; (bl T. M . Layer, J. Lewis. A. Martin, P. R . Raithby. W.-T. Wong, J. Cliern. S i c , . Dcilton Trtr/zs. 1992. 34 1 I . 171 T. Chihara, H. Yamaraki, .I. Oryrrnorm,t. C % e r r i . . 1992. 428, 169. 181 A. J . Arcc. R . Machado. C. Rivas. Y . De Sanctis. A. J . Deeming, J. Orcqcirzonwt. C/7cwi.,1991, 419. 63.
208
I Molecular C1usterLs
[91 J. Hsiou Liao, M. G. Kanatzidis, Inorg. Chem., 1992, 31, 431; J. Hsiou Liao, M. G. Kanatzidis, J. Am. Chem. Soc.; 1990, 112. 7400. [lo] C. Hogdrth, N. J. Taylor, A. J. Carty, A. Meyer, J. Chern. Soc. Chem. Commun., 1988, 834; F. Van Gastel, L. Agnes, A. A. Cherkas, J. F. Corrigan, S. Doherty, R. Ramachandran. N. J. Taylor and A. J. Carty, J. Cluster Sci., 1991, 2, 131; S. M. Stuczynski, Y.-U. Kwon and M. L. Steigerwald, J. Oryanonzet. Chem., 1993, 449, 167; W. Imhof, G. Huttner, J. Oryanomet. Chem., 1993, 448, 247. [ l l ] (a) P. Mathur, Md. M. Hossain, R. S. Rashid, J. Oryanomet. Clzem., 1993, 460, 83; (b) P. Mathur, Md. M. Hossain, R. S. Rashid, J. Oryanomet. C h m . , 1994, 467, 245. [12] M. L. Steigerwald, T. Siegrist, E. M. Gyorgy, B. Hessen, Y.-U. Kwon, S. M. Tanzler, Inorg. Chem., 1994, 33, 3389. [ 131 P. Mathur, B. H. S. Thimmappa, A. L. Rheingold, Znorg. Chem., 1990, 29, 4658; P. Mathur, Md. M. Hossain, R. S. Rashid; P. Mathur, Md. M. Hossain, R. S. Rashid, J. Organomet. Chem., 1993, 448, 2 11. [I41 P. Baistrocchi, D. Cauzzi, M. Lanfranchi, G. Predieri, A. Tiripicchio, M. Tiripicchio Camellini, Inorg. Chim. Acta, 1995, 235, 173. 1151 P. Baistrocchi, M. Careri, D. Cauzzi, C. Graiff, M. Lanfranchi, P. Manini, G. I’redieri, A. Tiripicchio, Inorg. Chim. Actn, 1996, 252, 367. [16] R. E. Bachman, K. H. Whitmire, Inorg. Chem., 1994, 33, 2527; S. N. Konchenko, A. V. Virovets, S. V. Tkachev, V. I. Alekseev, N. V. Podberezskaya, Polyhedron, 1996, 15, 1221. [17] M. Careri, A. Mangia, P. Manini, G. Predieri, V. Raverdino, G. Tsoupras, E. Sappa, J. Chromatogr., 1993, 647, 79; M. Careri, C. Graiff, A. Mangia, P. Manini, G. Predieri, Rapid Commun. Muss Spectrom., 1998, 12, 225. 1181 A. Casoli, A. Mangia, G. Predieri, E. Sappa, M. Volante, Chem. Rev., 1989, 89, 407. 1191 B. H. S. Thimmappa, J. Cluster Sci., 1996, 7, I . [201 D. Cauzzi. C. Graiff, M. Lanfranchi, G. Predieri, A. Tiripicchio, J. Chem. Soc. Dalton Trans.. 1995, 2321. 1211 D. Cauzzi, C. Graiff, M. Lanfranchi, G. Predieri, A. Tiripicchio, J. Organonzer. Chem., 1997, 536-537,497. 1221 D. Cauzzi, C. Graiff, G. Predieri, A. Tiripicchio, J. Chclm. Soc. Dulton Truns., 1999, 237 and results to be published 1231 A. M. Z . Slawin, M. B. Smith, J. D. Woollins, J. Chem. Soc. Dalton Trans., 1997, 1877. [24] H. G. Raubenheimer, L. Linford, G. Kruger, A. Lombard, J. Chem. Soc. D ~ l t o nTrans., 1991, 2195. 1251 H. Shen, S. G. Bott, M. G. Richmond, Inorg. Chim. Acta, 1996, 241, 71. 1261 R. C. Ryan, L. F. Dahl, J. Amer. Clzem. Soc., 1975, 97, 6904. 1271 J.-F. Halet, R. Hoffmann, J.-Y. Saillard, Inorg. Chem., 1985, 24, 1695. 1281 (a) V. Day, D. A. Lesch and T. B. Rauchfuss, J. Am. Chem. Soc., 1982, 104, 1290; (b) R. D. Adams, J. E. Babin, J.-G. Wang and W. Wu, Inorg. Chem., 1989,28, 703; ic) P. Mathur, B. H. S. Thimmappa and A. L. Rheingold, Inorg. Chr~m.,1990, 29, 4658; (d) P. Mathur, I. J. Mavunkal, V. Rugmini and M. F. Mahon, Inorg. Chern., 1990, 29, 4838; (e) P. Mathur, D. Chakrabarty and Md. Munkir Hossain, J. Orgunomet. Chem., 1991, 418, 415; ( f ) P. Mathur, D. Chakrabarty, Md. Munkir Hossain and R. S. Rashid, J. Organomet. Chem., 1991, 420, 79; (g) P. Mathur, Md. Munkir Hossain and A. L. Rheingold, Organornetullics, 1994, 13, 3909; (h) M. Shieh, T.-F. Tang, S.-M. Peng and G.-H. Lee, Inorg. Chem., 1995, 34, 2797; ( i ) P. Mathur and P. Sekar, J. Chem. Soc., Chem. Commun., 1996, 727; (1) S. N. Konchenko, A. V. Virovets and N . V. Podberezskaya, Polyhedron, 1997, 16, 1689. [29] R. D. Adams, T. A. Wolfe and W. Wu, Polyhedron, 1991, 10, 447. 1301 D. Cauzzi, C. Graiff, C. Massera, G. Mori, G. Predieri, A. Tiripicchio, J. Chem. So(.. Dalton Trans., 1998, 32 1. [31] P. A. Eldredge, K. S. Bose, D. E. Barber, R. F. Bryan, E. Sinn, A. Rheingold and B. A. Averill, Inorg. Chem., 1991, 30, 2365. 1321 D. M. P. Mingos, Accounts Chem. Res., 1984, 17, 311. [33] R. D. Adams, I. T. Horvath, J. Am. Chem. SOL..,1984, 106, 1869.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.12 A New Supporting Ligand for p o x 0 Metal Derivatives: N,N-dialkylcarbamato, 02CNR2 Daniela Bdli Dell' Amico. Fausto Caldercrzzo, Fahio Marchetti, Guido Panipaloni
0 x 0 (and sulfido) ligands are ubiquitous in coordination chemistry, more frequently acting as bridging ligands between two ( p z - ) ,three (,LL~-) or four (p4-) metal atoms. They are mainly responsible for the formation of polynuclear systems from compounds of lower nuclearity. In this connection, it is interesting to note that t e m i n a l oxd'] (and sulfido)[21ligands are rare in coordination chemistry. Terminal 0x0 ligands arc mainly found among 0x0 derivatives of metals in their relatively high oxidation states [vanadium(IV) and vanadium( V ) , rhenium( VII), uranium( V I ) , and so on]; when usual or relatively lower oxidation states are considered, normally 0x10 ligands occupy a bridging position. The derivatives of iron( III), the p-0x0 ligand being responsible for the nucleation of these compounds,[31are especially important in coordination chemistry.[31 Terminal hydroxo ligands are even less common.[41Normally, ,LL-OXOderivatives of transition (or main-group elements) are accompanied by supporting ligands, such as carboxylato, alkoxo and so on, which usually reduce the nuclearity of the system. This article mainly concerns the formation of p-oxo metal complexes containing the N, N-dialkylcarbamato group as the supporting ligand. Compounds of moderate nuclearity are therefore produced, whose molecular structures have been solved by X-ray diffraction methods. It will be shown that the p-0x0 ligand more commonly originates from a hydrolytic process, presumably through the intermediacy of a terminal hydroxo-species. However, other p-0x0 derivatives have been identified, the bridging ligand arising from mechanisms different from hydrolysis. The content of this article originates from work carried out in these laboratories on the synthesis and structural characterization of homoleptic N , N-dialkylcarbamates, initiated about twenty years ago. and then expanded to the use of these derivatives as starting material for: a) the preparation of p-0x0 complexes; b) implanting metal cations on inorganic matrices, mainly silica; c) identification of other reaction pathways leading to the genesis of the p-0x0 ligand and, normally, to the corresponding increase of nuclearity. For simplicity. this article is divided into two parts depending on whether the p -
210
I Molecular Clusters
Figure 1. The molecular of the uranium( IV) compound U 4 ( ~ 3 - 0 )(02CNEt2 2 ) I 2 (ethyl groups not shown).
0x0 ligand has derived from: a) a hydrolytic process; or b) a deoxygenation process involving carbon dioxide.
1.12.1 Hydrolytic processes An earlier review article[5a1has pointed out the general aspects of the hydrolytic processes leading to p-0x0 derivatives of metal-carbamates. The first p-0x0 N,N dialkylcarbamato complex to be reported in the literature was the uranium( IV) derivative containing the Uq(p~3-0)2core and twelve supporting N,N-diethylcarbamato as shown in Fig. 1. Moderate to good yields of the p-0x0 derivative could be obtained by careful hydrolysis of the homoleptic precursor.[61 The general equation pertaining to the hydrolytic process of a given N,N-dialkylcarbamato metal complex can be summarized as shown in Eq. (1). mM(02CNR2),
+ pH20
+
M,~(p-O),(02CNR2),,,-2,
+ 2pC02 + 2pNHR2
(1)
The driving force of the reaction is the evolution of carbon dioxide; thus, in most cases, the synthetic procedure leading to the p-0x0 derivative consists of treating the N , N-dialkylcarbamato precursor with a stoichiometric amount of water in an organic solvent. A p-0x0 derivative of titanium( IV), with the formula T i 2 ( ~ - 0 ) ~ (OzCNEt2)2Cp*,, apparently produced by a hydrolytic process, has also been reported.[7a1
1.12 A N e w Supporting Liyandjor p - 0 ~Metal 0 Deriiutiues
N
Figure 2. The molecular ~ t r u c t u r e " ~of' the chromium( 111) compound Cr2(,m-0)(02CNEt2r6C1(NHEt2) (ethyl groupa not shown)
21 1
x Cr (p3-0)(0,CNEt,).CI(NHEt,), Cr:(p3-O)( O,CNEt;),CI(NHEt,),
In addition to the uranium( I V ) derivative, other products, obtained in these Laboratories by hydrolytic processes, have been structurally characterized, namely:
The chromium( 111) compound contains a central tridentate oxide ligand (Fig. 2), an arrangement which is typical of other 0x0-centred derivatives of chromium(111), such as the carboxylato['" and sulfato" 31 complexes. In the diethylcarbamato derivative, the central oxide ligand deviates by only 0.07 A from the plane of the three chromium atoms. It should be noted that the chromium derivative still retains a chloride ligand in its coordination; as the starting material is anhydrous CrCh, this corresponds to incomplete removal of chloride from the kinetically inert 3d' chromium( 111) cation. The aluminum derivative originates from a hydrolytic process of the iso-propyl precursor, shown earlier["] to be dinuclear with bridging carbamato groups, see Eq. (2).
The compound was obtained in good yields by controlled hydrolysis of the homoleptic precursor. The molecular structure of the pu3-0~0derivative is schematically
212
1 Molecular Clusters
Figure 3. The molecular structure'*' of the aluminum compound A14 (p3- 0 ) 2 (OlCNPr 2 ) (alkyl ~ groups not shown).
shown in Fig. 3 and consists of a tetranuclear assembly of aluminum atoms joined by bridging oxide and carbamato groups. The molecule contains five- [Al(l) and Al(3)] and six-coordinate [Al(2) and A1(4)]aluminum atoms with distorted tngonalbipyramidal and octahedral geometries, respectively. This compound thus belongs to the category of alumoxanes, whose probably better known derivatives at present are the alkylal~moxanes,['~~ produced by hydrolytic processes on alkyl-aluminum derivatives. Of the octanuclear derivatives, we first described the heterobimetallic nickel-zinc complex. It was obtained['] in low yields by what appears to be a hydrolytic process involving the homoleptic compounds of zinc( 11) and nickel( II), when the starting material [Ni(NCMe)6]ZnCl4 was made to react with the NHPr12/C02 system in a hydrocarbon as medium over long reaction times. The presence of zinc in the bimetallic derivative was established analytically and on the basis of the diffraction experiment, the latter showing that two of the eight metal atoms are four coordinate, typical of zinc in an oxygen-containing ligand environment (uide injra, data concerning the tetranuclear p4-O derivative of zinc( 11),Zn4(p4-0)(02CNEt2)6). Nickel is five-coordinate in this compound, see Fig. 4. Concerning the formation of the bimetallic compound, it is believed that this originates from a hydrolytic process on the zinc(I1) derivative [the rate ( S K I )of water exchange"61 in the hexa-aquo metal cations is 2 x lo7 for zinc, about three orders of magnitude larger than for nickel( II)], as indicated in Eq. ( 3 ) . The intermediate, presumably unstable hydroxo derivative of zinc, Zn(02CNPr12)0H,thus formed should attack the nickel( 11) carbamate giving the final product. It is interesting to note that the compound crystallizes in the rhombohedra1 space group R3.
+
Zn(02CNPri2)2 H2O + (Zn(OzCNPr'2)OH)
+ NHPr'2 + CO2
(3)
The octanuclear copper( 11 ) compound was obtained by controlled hydrolysis of the
Figure 4. The molecular structure"' of the heterobimetallic nickel-zinc compound Ni6Zn2(pJ-0),(02CNPr12) 12 ialkyl groups not Ehown).
homoleptic precursor Cu(02CNPr;)2, of unknown molecular structure, in good yields. Its molecular structure is schematically shown in Fig. 5. The asymmetric unit contains four copper atoms and six crystallographically independent di-iso-propylcarbamato groups. The primed atoms of Fig. 5 are centrosymmetrically related to those of the other half of the octanuclear unit. An important feature of the molecular structure is the presence of the p4-0x0 ligand, which is separated by 1.906(5)-1.950(6) A from the four copper atoms. The
Figure 5. The molecular 5t1 ucture[")] of the octanuclear copperi 11) compound Cup / I ~ - O I ~ ~ ~ C N (alkyl PI-~),~ groups not shown I
Cu,i~i,-O),iO CNPr
A
214
I Molecular Clusters
Figure 6. The molecular structure" of the octanuclear iron(11) compound Feg(p4-0)2(02CNPr;),, (alkyl groups not shown).
Fe,(~,-O),(O,CNPr',),,
octanuclear units have no significant interactions with the adjacent clusters. It is to be noted that in spite of its high molecular weight of 2370.9, the compound still presents a moderate solubility in hydrocarbons (it can in fact be recrystallized from heptane). The octanuclear 0x0-carbamato derivative crystallizes in the triclinic space group with Z = 1; thus, the molecule, which contains 321 atoms (hydrogens included), has an approximately spherical shape with dimensions of the order of 2 nm. The p4-0~0derivative of iron( 11), of formula Feg(p4-0)2(02CNPr12)12 deserves special attention as it is a very unusual case of a p-0x0 derivative of iron(II), see Fig. 6. Very few examples of authenticated p-0x0 derivatives of iron( 11) have been reported in the literature," 'I although mixed-valence iron( 11)-iron(111) p-0x0 derivatives are known.['81It was obtained in good yields by the hydrolytic process shown in Eq. (4). 8 F e ( 0 2 c N P ~+~ 2~H) 2~ 0 +
+
Fes(pq-0)2(02CNPrk),2 4NHPri2 + 4 C 0 2
(4)
The octanuclear compound of Fig. 6, which is structurally similar to the Zn( 11)-Ni( 11) heterobimetallic derivative, contains four- and five-coordinated iron( 11) centers, while the coordination geometry is approximately tetrahedral for Fe(4) and approximately trigonal bipyramid for the remaining iron atoms. The lattice symmetry is triclinic and is similar to the uranium( IV) derivative discussed above; it has an essentially spherical shape with dimensions of the order of 2 nm.
1.12 A New Supporting Ligundjor p-0x0 Metal Derivatives
215
1.12.2 Non-hydrolytic processes The p-0x0 derivatives obtained by processes which do not involve the intervention of water are (in increasing order of nuclearity): Zr3(~f.I-O)(p-O)Cp2(OzCNEt2)6;'191 Znq (p4-0)(02CNEt2)6;[201
Tax(p-O)12(02CNEt2)16.[2'1 In all three cases, the presence of a metal system in a low oxidation state is required for the formation of the p-0x0 function, namely, ZrCpz(CO)?, zinc metal, and Ta(OzCNEt2)3, respectively. Historically, the first dialkylcarbamato derivative with a p-0x0 functionality not deriving from a hydrolytic process was discovered during attempts to prepare the homoleptic derivative of zinc, Zn(OZCNR2)2, which is still unknown. After some unsuccessful attempts with anhydrous zinc halide^,'^^.^^] the reaction (150 "C, 180 atm of carbon dioxide) of zinc metal with Et2NH/C02 gave the 0x0-derivative of formula Zn4(pq-O)(02CNEt2)6,instead of the expected homoleptic derivative. Our attempts to produce the homoleptic derivative by this method were justified by some precedents in the literature concerning the preparation of the alkali metal and aluminum derivatives starting from the corresponding metals.[243251 The molecular structure of the tetranuclear p-0x0 derivative is shown in Fig. 7. The formation of the tetranuclear Linc derivative is believed to be best described by the stoichiometry shown in Eq. ( 5 ) . 4Zn
+ 7 NHEt2 + 7C02
+
Zn4(p4-0)(0~CNEt2)6 Et2NC(O)H
4
Figure 7. The molecular structurerzo1 of the tetranuclear zinc( I1 compound Z n ~ ( p ~ - O ) ( O ~ C N E (alkyl t 2 ) h groups not shown)
-
+ 3 H?
(5)
2 16
1 Moleculur Clusters
Zr,(ll,-O)(~-CCo)Cp,(O,CNPr,),
Zr,(~3-O)(r.l-O)CP,(O,CNPr ' 2 6
(a)
(b)
Figure 8. The molecular structures["] of the trinuclear zirconium( IV) compounds: (a) the ketenylidene precursor Zr.3(p3-O)(p-CCO)Cp2(02CNPr12)6of: (b) Zr3 (p3-0)(p - 0 ) C p(02CNPr'2)6 ~ (ethyl groups not shown).
The deoxygenation of carbon dioxide is supported by the finding that Et2NI3C(O)H was detected among the products when the reaction was carried out in the presence of labeled carbon dioxide. The molecular structure is highly symmetrical and the diethylcarbamato groups are of one type only, namely bridging bidentate. The zinc atoms form the vertices of a tetrahedron, whose edges are occupied by the bridging carbamato groups, the bridging oxide occupying the center of the tetrahedron. Concerted deoxygenation/reductive coupling of zirconium- (or hafnium)-bonded carbon monoxide in MCp?(C0)2 leads to the trinuclear derivative M3(p3-O) ( , u - C C O ) C ~ ~ ( O ~ C N91R the ~ ) ~molecular ;[~ structure of the zirconium derivative (R = Prl) is shown in Fig. 8a. The reactions leading to the ketenylidene trinuclear species Mi(p3-O)(pu-CCO)Cpz(02CNR2)~and their reaction products with carbon dioxide are shown in Eq. (6) and (7).
Reaction (7) proceeds at room temperature and at one atmosphere pressure of carbon dioxide, thus converting the bridging ketenylidene ligand into the bridging oxide.[19b1The ketenylidene complex reacts with carbon dioxide even in the solid state; the gas produced from the zirconium derivative. promptly evacuated into a mass-spectrometer, was shown to be carbon suboxide C302.[19c1The inorganic reaction product appears to catalyze the polymerization of carbon suboxide to red solid The structure of the product resulting from reaction ( 7 ) , R = Pr’, is shown in Fig. 8b. The tantalum octanuclear derivative Tag(p-O)12(O?CNEt2)16[’11 was serendipitously obtained during unsuccessful attempts to simplify the synthesis of Ta(O?CNEt2)?, by using sodium metal in tetrahydrofuran ( T H F ) , instead of CoCp*, in T H F or toluene, see Eq. (8). The former reducing system actually leads to the octanuclear derivative of tantalum(V ) . the oxidation from tantalum( 111) to tantalum(V ) being realized through the deoxygenation of the corresponding equivalents of carbon dioxide to give CO and HCONEt2, the latter being detected as one of the products. The remaining four oxides of the Tas cage presumably originate from THF.
The molecular structure of the octanuclear tantalum(V ) compound is shown in Fig. 9. The point symmetry is C, and the internal cage has approximately D 4 h symmetry. The eight tantalum atoms are situated at the vertices of a cube whose edges contain the twelve bridging oxides. Once again. the dimensions of this octanuclear species approximately correspond to those of the unit cell, with a diameter of about 2 nm.
Figure 9. The molecular structure‘”] of the octanuclear tantalum( V j compound Tax (p-0) I2jOZCNEtZ)16 (ethyl groups not shown). The compound is triclinic, space group Pi (No. 2)- Z = 1, and cell dimensions u = 15.440(3); h = 15.710(1);c = 16.090(2)A; U = 2971.4 A3.
218
I Molecular Clusters
1.12.3 Conclusions The research on p-0x0 derivatives supported by the N, N-dialkylcarbamato ligands has identified a new class of compounds characterized by a moderate to good solubility in organic solvents. The maximum nuclearity identified so far is eight, but it is quite possible that this can be increased in the future with an appropriate choice of the alkyl groups. The p-0x0 derivatives with the highest nuclearity are characterized by a molecular size around 2 nm, the periphery of the core being constituted by the alkyl groups. In a sense, the latter protect the inner oxygen-containing groups from further attack by water or by the oxidizing agent. It is to be expected that further research will discover new molecular assemblies of this type for several other transition- and non-transition metal cations. Also the formation of clusters of clusters is possible, provided the junction among, for example, the octanuclear clusters will be generated through a hydrolytic process involving some of the peripheral dialkylcarbamato groups. A further development of this chemistry comes from the possibility, already established for some mononuclear N, N-dialkylcarbamato derivatives, of using the carbamato function as a reactive point towards hydroxylated inorganic matrices; this method of chemically implanting metal cations has been successfully of used with tin( IV),[261 platinum( 11),[271 and zirconium( IV),[281 and is now under active investigation for other metal cations. The extension of this technique to polynuclear N,N-dialkylcarbamato derivatives will provide new possibilities, the most evident one being that of chemically implanting small oxide clusters on inorganic matrices.
Acknowledgments Generous support from the Minister0 dell’ Universita e della Ricerca Scientifica e Tecnologica (MURST) and by the Consiglio Nazionale delle Ricerche (CNR, Roma) is gratefully acknowledged.
References [I] For some recent examples of crystallographically established coordination compounds containing terminal 0x0 ligands, see: P. J. Stewart, A . J. Blake, P. Mountford Inorg. Chem., 36, 1982 (1997); F. A. Cotton, G. Schmid, Znory. Chem., 36, 2267 (1997); B. Noll, S. Noll, P. Leibnitz, H. Spies, P. E. Schulze, W. Semmler, B. Johanssen, Znory. Chirn. Actu, 255, 399
1.12 A New Supporting Ligand for p - 0 ~ 0Metal Derivatives
21 9
(1997); K. Kanamori. K. Ino, K.-I. Okamoto. A c f u Crj.sfu/logr., S k t . C, 53, 672 (1997); J. Mizutani, H. Imoto, T. Saito, Actu Crysttillogr., Sect. C, 53, 47 (1997); P. Y. Zavalij, T. Chirayil. M. S. Whittingham. V. K. Pecharsky, R . A. Jacobson, Acta Crysfullogr.. Sect. C, 53. 170 (1997); W. T. A. Harrison, L. L. Dussack, A. J. Jacobson, Acta Crystallogr.., SLW. C, 53. 200 (1997); T. I. A . Gerber, J. Perils. J. G . H. Du Preez, G . Bandoli, Acta Crystullogr.. Scc,t. C. 53. 217 (1997): A. A. Eagle, G . N . George. E. R. T. Tiekink, C. Ci. Young, Inorg. Cliem.. 36, 472 (1997); C. Limberg, M. Biichner, K. Heinze, 0. Walter, Inorg. C/zem.,36, 872 (1997); W. A. Herrmann. W. A. Wojtczak, G. R. J. Artus, F. E. Kuhn. M. R. Mattner, Inorg. Chern.,36, 465 (1997). [2] For some recent examples of coordination compounds containing terminal sulfido ligands, see: H. Pan, M. A. Harmer, L. Wei, M. E. Leonowicz. C. 0. B. Dim, K. F. Miller, A. E. Bruce, S. McKenna. J. L. Corbin. S. Wherland. E. I. Stiefel. 1nor~q.Clzim. Acta. 243, 147 (1997); P. K. Chakraborty, I. Ghosh, R. Bhattacharyya, A. K . Mukherjee, M. Mukherjee, M. Helliwell, Polyhedron, 15, 1443 [ 1996); A. K. Mukherjee, P. K. Das, M. Mukherjee, P. K. Chakraborty, R . Bhattacharya, Acta Crystullogr. , S . C, 53, 209 (1997); Y. Mizobe. M. Hosomizu, Y. Kubota, M. Hidai. J. Or~~unometul. c w . , 507, 179 (1996); J. T. Goodman, S. Inomata, T. B. Rauchfuss. J. A m . Cheni. Soc., 118. 11674 (1996); F. A. Cotton, G. Schmid, Inorg. C/icv7i.,36, 2267 (1997); D. Rabinovich. G. Parkin, Inorg. C/ieni., 34. 6341 11995): S. Thomas, E. R. T. Tieking, C. G . Young. 0rgunori~etuNic.c.15. 2428 (1996); D.-L. Long, S. Shi. X.-Q. Xin. B.-S. Luo, L.-R. Chen. X.-Y. Huang, B.-S. Kang. J. Cliem. Soc.. Dalton Trans., 2617 (1996); C. G . Young. L. J. Laughlin, S. Colmanet. S. D. B. Scrofani, hiorg. Chrm., 35, 5368 (1996); P. J. Lundmark, G. J. Kubas, B. L. Scott. Organon?rrallic.r, 15, 3632 (1996); S. Ogo, T. Suzuki. Y. Ozawa. K. Isobe. Inory. C / i m . , 35. 6093 (1996); W. J. Evans, M. A. Ansari, J . W. Ziller. S. 1. Khan. OrH"tzoriie/ullic,.c.14. 3 (1995 I; S.-P. Huang, M. G. Kanatzidis. Inorq. C/iim, Acro, 230. 9 (1995); H. Kawaguchi. K . Tatsumi. J. Am. Chem. Soc., 117, 3885 (1995); V. J. Murphy, G. Parkin, J. A m . C/7em. Soc., 117, 3522 (1995). 131 S. J. Lippard. Angen.. C/wni., In/. Ed. Emql., 27, 344 (3988); D. M. Kurtz, Chem. Riw, YO, 585 (1990); J. B. Vincent. G . L. Olivier-Lilley. B. A. Averill. Chem. R m . , YO, 1447 (1990); L. Que, A. E. True, Progr. Inorcq. Cliem., 38. 98 (1990; R . G. Wilkins, Chern. Soc. Rev.. 171 (1992). 141 For a recent case of terminally bonded hydroxo ligands in organometallic derivatives of molybdenum(I1) and molybdenum(llI), see: J. C. Fettinger, H.-B. Kraatz. R . Poli, E. A. Quadrelli. Cl7mi. Cornmun., 889 ( 1997). Other coordination compounds of molybdenum containing terminal -OH ligands have been studied crystallographically: P. R. Robinson, E. 0. Schlemper, R. K. Murmann, Inorg. C/iet7i.,14, 2035 (1976); M. R . Churchill. F. J. Rotella, Inorg. Chmi., 17, 668 ( 1978): T. Yamasc, J . C/wm. Soc.. Dalton Trans., 283 (1978); C. T. Kan, P. B. Hitchcock, R. L. Richards. J. Cher77. Soc.. Dalton Truns.. 79 (1982); Inovg. Clzem..22, 2723 (1983);J. Bohmer, G. Haselhorst, K. Wieghardt. B. Nuber, Angew. Clzem., Int. Ed. Engl., 33, 1473 (1994); T. Adachi, D . L. Hughes, S. K. Ibrahim, S. Okamoto, C. J. Pickett, N. Yabanouchi, T. Yoshida, C h i . Commun., 1081 (19953. razzo. F. Marchetti, G. Pampaloni, Sfoichiometric and E.ud.wnate.s, in, 1. T. Horvath, F. JoO. Eds., Aqueous OrgaNATO AS1 series, Paper presented at the NATO Workshop on Aqueous Organometallic Chemistry and Catalysis, Debrecen, Hungary, Aug. 28-Sept. 2nd, 1994; (b) F. Calderazzo, G. Dell' Amico, M . Pasquali, G . Perego, Inorg. Cllen7.. 17, 474 (1978). 161 F. Calderazzo, G . Dell' Amico. R. Netti, M. Pasquali, Inory. Chern.; 17, 471 11978). 1-71 (a) P. Gomez-Sal, A. M. Irigoyen, A . Martin, M. Mena, M. Monge, C. Yelamos, J. Orgunomeial. Cliem.. 4Y4, C19 (1995): (b) D. Belli Dell' Amico, F. Calderazzo. F. Gingl, L. Labella, J . Strahle. Gnzr. C/7ir>i.Ittrl.. 118, 729 (1988). 181 U. Abraham, D. Belli Dell' Amico, F. Calderazzo. S. Kaskel, L. Labella; F. Marchetti, R. Rovai, J. Strahle. Clieni. Commun., 1941 (1997). 1,
220
I Molecular Clusters
191 A. Bacchi, D. Belli Dell’ Amico, F. Calderazzo, U. Giurlani, G . Pelizzi, L. Rocchi, Guzz. Chin?. Itul., 122, 429 (1992). [lo] E. Agostinelli, D. Belli Dell’ Amico, D. Fiorani, G. Pelizzi, Guzz. Chim. Ital.. 118, 729 111988). [l I] D. Belli Dell’ Amico, F. Calderazzo, L. Labella, C. Maichle-Mossmer, J. Strahle, Chem. Commun., 1555 (1994). [ 121 (a) B. N. Figgis, G. B. Robertson, Nuture (London),205, 694 (1965); (b) S. C. Chang, G. A. Jeffrey, Actu Crystallogr., Sect B. 26, 673 (1970); (c) R . C. Paul, P. Kapoor, 0. B. Baidya, R. Kapoor. Z. Naturforsch., B 34, 160 (1979): F. A. Cotton, W. Wang, Inorg. Chenz., 21, 2675 (1982); E. Gonzales-Vergara, J. Hegenauer, P. Saltman, M. Sabat, J. A. Ibers, Inorg. Chim. Actu, 66, 115 (1982). [13] (a) K. Mereiter, H. Vollenkle, Acta Crystallogr., Sect. B, 36, 1278 (1980); (b) K. Mereiter, Acta Crystallogr., Sect. B, 36, 1283 (1980). (141 D. Belli Dell’ Amico, F. Calderdzzo, M. Dell’ Innocanti, B. Guldenpfennig, S. lanelli, G. Pelizzi, P. Robino, Gazz. Chini. Itab, 123, 283 (1993). 1151 (a) J. L. Atwood, M. J. Zaworotko, Chem. Commun., 302 (1983); (b) M. R. Mason, J. M. Smith, S. G. Bott, A. R. Barron, J. Am. Cliem. Soc., 115, 4971 (1993); (c) C. J. Harlan, M. R. Mason, A. R . Barron, Orgunometallics, 13, 2957 (1994). [ 161 M. L. Tobe, Inorgunic Reaction Mechanisms, Nelson, London, 1972. 1171 H. P. Muller, R. Hoppe, Z. Anorg. ANg. Chem., 569: 16 (1989); ibidem, 619, 193 (1993). The preparation of a more recent case [C. M. Che, C. W. Chan, S. M. Yang, C. X. Guo, C . Y. Lee, S. M. Peng, J. Chem. Soc., Dalton Trans, 2961 (1995)l of a p-0x0 derivative of iron( 11) could not be repeated [F. Calderazzo, L. Labella, F. Marchetti, J. Chem. Soc., Dalton Trans., 1485 (1998)l. [I81 For mixed-valence iron( 11)-iron(111) p-0x0 derivatives, see: K. L .Taft, A. Caneschi, L. E. Pence, C. D. Delfs, G. C. Papaefthymiou, S. J. Lippard, J. Am. Chem. Soc., 115, 11753 (1993): K. L. Taft, C. D. Delfs, G. C. Papaefthymiou, S. Foner, D. Gatteschi, S. J. Lippard, J . Am. Chem. Soc., 116, 823 (1994); K. L. Taft, G . C. Papaefthymiou, S. J. Lippard, Inorg. Chern., 33, 1510 (1994); W. Micklitz, V. McKee, R. L. Rardin, L. E. Pence, G. C. Papaefthymiou, S. G. Bott, S. J. Lippard, J. Am. Chem. Soc.. 116, 861 (1994), and refs. therein. [19] (a) F. Calderazzo, U. Englert, A. Guarini, F. Marchetti, G. Pampaloni, A. Segre, Angew. Chem.,Int. Ed. Engl., 33, 1188 (1994); (b) F. Calderazzo, U. Englert, A. Guarini, F. Marchetti, G . Pampaloni, A. Segre, G. Tripepi, Chem. Eur. J., 2, 412 (1996); (c) G . Pampaloni, L. Pandolfo, G. Tripepi, unpublished results; (d) G. Paiaro, L. Pandolfo, CommentsInorg. Chen?.,12, 213 (1991). [20] A. Belforte, F. Calderazzo, U. Englert, J. Strahle, Inorg. Chem., 30, 3778 (1991). [21] P. B. Arimondo. F. Calderazzo, R. Hiemeyer, C. Maichle-Mossmer, F. Marchetti, G. Pampaloni, J. Strahle, Inorg. Chem., 37, 5507 ( I 998). [22] Several unsuccessful attempts have been made to prepare homoleptic N,N-dialkylcarbamato derivatives of zinc: anhydrous ZnX2 (X=Cl, Br) were treated with the R2NH/C02 system leading to dialkylcarbamato derivatives still retaining chloride in their composition 1231, of unknown structure. [23] (a) A. Belforte, PhD thesis, Scuola Normale Superiore, May 1989; (b) R. Alessio, PhD Thesis, Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, February 1997. [24] A. Belforte, F. Calderdzzo, J. Chem. Soc., Dalton Trans., 1007 (1989). [25] T. W. Martinek, US Patent 3,056,820, October 2nd 1962 (to Pure Oil Co.), Chem. Abstr., 58, 6700 g (1963). [26] L. Abis, D. Belli Dell’ Amico, F. Calderazzo, R. Caminiti, F. Garbassi, S. Ianelli, G. Pelizzi, P. Robino, A. Tomei, J . Mol. Catal., 108, L113 (1996). [27] L. Abis, D. Belli Dell’ Amico, C . Busetto, F. Calderazzo, R. Caminiti, C. Ciofi, F. Garbassi, G. Masciarelli, J. Muter. Chem., 751 (1998). [28] L. Abis, F. Calderdzzo. C. Maichle-Mossmer, G. Pampaloni, J. Strahle, G. Tripepi, J. Chem. Soc., Dalton Trans., 841 (1998).
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.13 Alkyne Scission on Metal Cluster Frameworks Michael J. Morris
1.13.1 Introduction The interaction of alkynes with metal carbonyl clusters has been investigated since the earliest days of organometallic chemistry. The 1960s and 70s saw much activity in this area, with the emerging technique of X-ray crystallography used to define the bonding modes of the organic ligands in the products. A review written by Hubel some 30 years ago on the reactions of alkynes with cobalt and iron carbonyls served only to emphasize the bewildering variety of structural types resulting from such reactions."] This diversity arises through the propensity of alkynes to undergo coupling reactions with carbonyl ligands and/or additional alkyne molecules to give ligands such as cyclobutadienes, cyclopentadienones, quinones, and cyclohexadienones. However, sometimes the incorporation of a single alkyne ligand in a well-defined bonding mode can be observed.[21For trinuclear clusters this often 7 ' 1 coordination modes I and 11, and for takes the form of the p3, $-\I and p y , 1 tetranuclear butterfly clusters the alkyne usually bridges all four metals in a p4, 1;1* mode 111. In the case of terminal alkynes, RC-CH, rearrangement of the alkyne ligand is often observed on coordination, for example by oxidative addition to form an alkynyl hydride IV or by 1,2-H migration to form a vinylidene (=C=CHR) V. A much rarer mode of reactivity involves the scission of the alkyne C-C bond and the formation of two alkylidyne ligands VI. Approximately 20 examples of alkyne scission have now been observed over the past 20 years or so and are detailed in Tables 1-3 below, classified according to the type of reaction involved. The reactions have been divided, rather arbitrarily in some cases, into three categories: 1 ) Reaction of an alkyne with a mononuclear metal complex or preformed cluster; 2) Thermolysis of a metal cluster which already contains one or more alkyne ligands; 3 ) Reaction of a metal alkyne complex or cluster with additional metal fragments.
222
1 Molecular Clusters
M
R,
p=
M-
\
M
a
R
II
I
111
R
U
1
M+
I
R IV
V
VI
Scheme 1. Typical bonding modes of alkyne ligands.
Table 1. Alkyne scission in reactions of alkynes with complexes. Precursor
Alkyne
Product
Reference
RI
C-CR~ ( R 1 ,R’ = SiMe3, Ph, Bu, C02Me and others)
3
Me3SiC-CNEt2 or Et2NCzCNEt2
A
MeCKNEt2
5
F~CCGCCF~
6
PhCECPh
I
PhC-CPh
8
PhC-CH
9
PhCECPh
10
223
1.13 AIkjwe Scission on Metal Cluster Frameworks Table 2. Alkyne scission by thermolysis of metal-alkyne complexes A1kyne complex
Product
Reference
C o ( R I C CR')(PPhi)Cp R ' = Ph. R' = Ph. COzMe, CN, R' = Me, R 2 = C02Me
Col(/(;-CR'j(~;-CR')Cpl
11
M?(CO)(v-R'C-CR ' ) C p M = Co, Rh. Ir, R ' , R' = Ph, tol, SiMe3, CF7, C02Me
M; ( p - C R ' ) I p;-CR')Cpj
12
As well as these examples, there are also several recent studies, especially by Chi and co-workers, which involve the related scission of coordinated acetylide ligands to carbide and alkylidyne ligands."yl Reactions are also known which do not give alkylidyne complexes but which imply alkyne scission, e.g. treatment of WFe2(p3Ctol)(CO)gCpwith CIPh2 gave two dimetalla-ally1 complexes, one of which was WFe(pu-CPhCtolCPh)(C0)5Cp with a rearranged chain.[201It should be mentioned that alkyne scission and metathesis can be observed in mono- and dinuclear systems such as Schrock's tungsten alkoxides, but this lies outside the scope of the current article. Historically speaking, the most studied reactions are those involving the forma~ C P ~ ]where M = Co, Rh, Ir. This can be achieved tion of [ M ~ ( ~ u I - C R )clusters from mononuclear units such as CpM(C 0 )l or from preformed alkyne clusters M3(CO)(p3-C2R2)Cpl; in each case the scission step occurs at a trinuclear metal
Table 3. Alkyne scission by reaction of alkyne complexes with cluster-building reagents. Alkyne complex
Other reagent
Product
Reference
224
I Moleculur Clustcw
center. A theoretical analysis of the mechanism (for M = Co, R = H ) by Hoifmann showed that the most likely pathway involved movement of the alkyne to one edge of the metal triangle followed by scission of the C-C bond. In this instance at least, it appears that loss of the CO ligand occurs after alkyne scission; pathways in which the CO was lost first were calculated to be of much higher energy."'] Certain common factors can be gleaned from the Tables. Firstly, virtually all of the reactions take place under thermal conditions or in the presence of decarbonylating agents such as Me3N0, or sometimes both. This strongly suggests that loss of a carbonyl ligand is an important step in many cases, with the resulting electronic deficit being made up by the conversion of a 4-electron donor alkyne into two 3-electron donor alkylidynes. This is borne out by the fact that in some cases the reaction can be reversed simply by placing the product under an atmosphere of CO. The coupling of two alkylidynes to form an alkyne is also a well-known process in systems where the corresponding alkyne scission is not observed. Secondly, it should be noted that virtually all of the reactions involve disubstituted alkynes; only one example of the cleavage of a terminal alkyne, that of Me3SiC-CH at a tricobalt center, had been reported by Vollhardt before our work detailed below. This is clearly because in most cases other more favorable processes, such as oxidative addition or vinylidene formation, intervene. A third notable point involves the disposition of the resulting alkylidyne ligands in heterometallic clusters. In compounds where CpMo, CpW or CpFe fragments are involved, the alkylidyne ligands in the product are always bonded to these vertices, either as p2-bridges or as p3 face-capping fragments. This also means that the alkylidyne ligand will cap a face consisting of two CpM groups and one metal carbonyl unit, in preference to a face involving one CpM group and two metal carbonyl units. In terms of the synthesis of mixed-metal clusters the third approach, addition of metal fragments to alkyne complexes, is one of the most versatile as the use of different metal-ligand combinations and different reaction conditions can greatly affect the outcome. As an illustration, the rhenium acetylide complex Rez(p-H)(p-C-CPh)(CO)p reacted with Mo2(C0)4Cp2 to give the alkyne butterfly cluster M O ~ R ~ ~ ( , U ~ - H C C P ~ )whereas ( C O ) ~with C ~ Fe3(C0)12 ~, rearrangement to a vinylidene ligand occurred to give spiked triangular Fe2Re2(p3-C=CHPh) (CO)15, and with Co2(CO)s, cluster fragmentation gave initially (OC)5Re(C=CPh){Co2(CO)6}, which on exposure to air gave ReCo2(pul-CPh)(CO)loby C r C ~leavage."~] Our own contributions to this area started in late 1994 as a continuation of our studies of the reactivity of the dimolybdenum alkyne complexes MO~(,UR'C-CR2)(C0)4Cp2 (I), which are readily prepared in high yield (>750/(1) by a one-pot reaction from Mo2(C0)6Cp2 (2) via the reactive triply-bonded dimer Moz(C0)4Cp2 (3). We had previously demonstrated that the reactions of these alkyne complexes with phosphines and thiols were very different to those of their precursors; for example, introduction of the alkyne ligand promoted C-S bond cleavage in thiols to give products with sulfido ligands.[211Given that large numbers of novel dimolybdenum species have been derived from 3 through its reactions with
organic molecules, its use in cluster building has been somewhat limited, and the We therefore decided to examine the possible use of 1 results difficult to in cluster building reactions in the hope that the alkyne ligand would be retained in the product.
Scheme 2. Formulae of the dimolybdenum alkyne complexes.
l a 1b IC Id
Rl=R*=H R1=H,R2=Me R1=H,R2=Ph R1 = H, R2 = C02Me
le If 1g
dh
R1=R2=Me Rl=R2=Et R1= R2 = C 0 2 M e R’=RZ=Ph
1.13.2 Molybdenum-ruthenium clusters Reaction of the disubstituted compound le with ruthenium carbonyl in toluene under reflux gave the blue hexanuclear cluster Mo?Ru4(p3-CMe)2(CO)l 2 C p ~4e in moderate yield (28’%1),along with the known carbido cluster Ru6C(C0)14(q-ChHSMe). Changing the solvent to heptane eliminated this product and left the yield of 4e unchanged. The occurrence of alkyne scission in the product was indicated by its spectroscopic properties, particularly the I3C N M R chemical shift of the alkylidyne carbons at b 353.9, and established conclusively by X-ray analysis; the two alkylidyne ligands symmetrically cap the two Mo2Ru faces (Fig. Unlike most octahedral clusters, which have 86 cluster valence electrons in accordance with the rationalization of the Wade-Mingos rules, the product has only 84 electrons. The seat of this unsaturation appears to be the Mo-Mo bond, which is quite short [2.5792(8) A]. Complexes with bulkier disubstituted alkynes (If, lg) failed to give any products. but when the reaction was extended to complexes la-d containing terminal alkynes two compounds could be isolated: the blue alkyne cleavage products Mo~Ru~(,LQ-CH)(~~-CR’)(CO)~~C~~ 4a-d and the orange trinuclear clusters M ~ ~ R u ( ~ ~ - C - C H R ’ ) (5a-d CO) in,which C ~ ~ the alkyne ligand had rearranged to form a vinylidene. The yields of the former were distinctly low (4-16‘%,) whereas those of the latter were quite reasonable (around 40%). As stated before. the cleavage of any terminal alkyne is a rare event; the formation of the hexanuclear clusters 4a-d is unusual in that it is clearly a general process for a number of simple terminal
226
I Molecular Clusters
Figure 1. X-Ray crystal structure of the product 4e. 1
A, toluene or
1
11'
'R* 4a-e
Scheme 3. Clusters formed by reaction of 1 with ruthenium carbonyl
I 5a-d
I H
I 13 Alkjnc Sciuion on Metal Clirrter Fninictrwks
227
alkynes, and the reaction involving l a is particularly noteworthy as it is the first recorded example of the cleavage of acetylene itself into two methylidyne units on a cluster. Predictably, however, it was also the reaction that gave the lowest yield. Since none of the other reactions of 1 involve alkyne scission, we assume that the C=C cleavage step takes place on a mixed-metal cluster and is evidently intramolecular since unsymmetrical alkyne complexes such as l b give rise only to the corresponding unsymmetrical bis(a1kylidyne) clusters (4b) uncontaminated by the symmetrical clusters (4a, 4e), which would be formed by alkyne scrambling. We have not been able to isolate any intermediate species, and it is interesting that we detected no products analogous to the alkyne clusters W ~ R U ( , L Q - C ~ R ~ ) ( C O ) ~ C ~ ~ previously prepared by Stone and coworkers." 31 It is possible that species like this are intermediates in the alkyne scission reaction and possibly also precursors to the vinylidene clusters 5 , but as yet we have no evidence to confirm or deny this. We did attempt to prepare new MoRu clusters of intermediate nuclearity, which could then be treated with alkynes. In the event, however, heating Mo2(C0)4Cp2 with R u (CO) ~ 12 gave another hexaiiuclear cluster, Mo2 R u ~pug-C)( ( ,u-O)(CO)1zCp2, as the only isolable product in a reaction which appears to involve cleavage of a CO ligand into an interstitial carbide and a bridging 0x0
1.13.3 Molybdenum-cobalt clusters Initially we explored the reaction of alkyne complexes 1 with cobalt carbonyl. This proved an excellent route to high yields of the tetranuclear clusters C02M02(p4-R'C2R2)(CO)sCp2( 6 ) ,which contain the alkyne ligand p4-bonded to a butterfly of metal atoms, with the cobalt atoms situated in the hinge of the butterfly and the molybdenum atoms on the wingtips. N o alkyne scission or rearrangement of the alkyne ligand to a vinylidene was observed.[2s1The reaction of l h with C O ~ ( C O ) ~ failed to give an analogous product. the importance of which only became evident later.
A, toluene
30 min
Scheme 4. Reaction of
la-g with CO?(CO)S giving 6a-g.
I
I
co
co 6a-g
228
I Molecular Clusters
cml
Figure 2. X-Ray crystal structure of the product 7.
We then decided to examine the reactions of alkyne complexes 1 with the dicobalt alkyne complexes Co2(R3C2R4)(C0)6.Most of these were rather unproductive: the usual course seemed to be decomposition of the cobalt complex under the reaction conditions (refluxing toluene) or formation of 6. However, we did succeed l h with in isolating a new complex from the reaction of Mo2(pU-C2Ph2)(CO)4Cp2 C02(,u-C2Ph2)(C0)6in 22% yield. This product showed a peak at 6 363.9 in its 13C NMR spectrum, indicating that alkyne scission had occurred to give an alkylidyne group. Full characterization by X-ray crystallography (Fig. 2) revealed it to be another butterfly cluster, C O ~ M O ~ ( ~ ~ - C P ~ ) ~ ( , U ~(7), -C in~which P ~ ~both )(CO)~C~ alkyne ligands of the reagents have been retained: one cleaved into two benzylidyne groups which cap the external Mo2Co faces of the butterfly, and the other as the intact alkyne bridging the internal faces in the same way as in 6.[261One interesting point is that the metal atom distribution in the butterfly is reversed compared to 6: in 7 the molybdenum atoms form the hinge and the cobalt atoms the wingtips. This may be a result of the preference of the alkylidyne ligands to bond to Mo2Co faces rather than MoCo2 ones due to the strong Mo-C bonds: if the metal atom arrangement in 7 was the same as in 6, the available outer faces would both be MoCo2. Another interesting feature concerns the electron counting: complex 7 has only 58 valence electrons, whereas 6 has the 60 electrons expected for a butterfly cluster of this type.[271As noted above for 4, this unsaturation appears to result in a short Mo-Mo bond, 2.5507( 11) A.
1.13 Alkyne Scission on Metal Cluster- Franieworks
229
The mechanism of formation of 7 was rather intriguing since none of the other combinations of alkyne complexes gave a similar product. By preparing Mo2 and Co2 complexes of di-p-tolyl acetylene and combining them with the corresponding complex of CzPhZ, we attempted to discover which of the alkyne ligands was cleaved and which remained intact. However, this would be virtually impossible to ascertain spectroscopically and in any case, the reactions seemed to give mixtures of isomers. We also noticed that in some of the original reactions, small amounts of a purple compound were formed, which we identified as tetraphenylcyclopentadienone (tetracyclone). This raised the interesting possibility that the two alkyne ligands might link up before the cleavage reaction took place. This led us onto the next phase of our work, involving dinuclear molybdenum metallacyclopentadiene complexes.
Ph
8
Scheme 5. Two routes to complex 7.
1.13.4 Alkyne scission in dinuclear metallacyclopentadiene complexes In their well-known paper on alkyne oligomerization at dinuclear metal centers. Knox, Stone, and co-workers showed that complexes of type 1 reacted with further alkynes under rather forcing conditions (refluxing xylene or octane) to give compounds in which up to four alkynes were linked, with the exact species isolated depending on the alkyne substituents.[281In the case of diphenylacetylene it was shown that reaction of l h with more CzPhz gave a green complex Mo*(,u-C4Ph4)-
230
1 Molecular Clusters
(C0)Cpz (8) containing two alkynes joined in a metallacyclopentadiene unit, and possessing a metal-metal triple bond rather like that in Mo2(C0)4Cp2. The compound could be more conveniently prepared in good yield simply by treatment of Mo2(C0)6Cp2 with an excess of C2Ph2 in refluxing octane. Only bulky alkynes such as diphenylacetylene gave this type of product; presumably the steric bulk is required to achieve the right degree of decarbonylation since an analogous compound, Mo2(C0)(pI-C4Et4)Cp2,reported by Muetterties, could be carbonylated to Mo2(C0)3(p-C4Et4)Cp2 whereas the triple bond in 8 is unreactive towards C0.[291 The compound is however, rather air-sensitive, and on stirring in CH2C12 in air is converted within minutes into a brown complex, which they formulated as Mo20(p-O)(pU-C4P~)Cp2 (9), containing the same metallacyclopentadiene unit.
I
Ph 8
9
Scheme 6. Reaction of 8 with atmospheric oxygen.
We prepared 8 and studied its reaction with cobalt carbonyl. Gratifyingly a rapid reaction (2 h) occurred in boiling toluene to give a 49% yield of the alkyne bis(a1kylidyne) butterfly cluster 7. We therefore believe that the reaction of l h with Co2 (p-C2Ph2)(CO)6 probably proceeds by decomposition of the dicobalt complex and reaction of the liberated alkyne with the dimolybdenum species to give 8, which in turn reacts with a dicobalt fragment. This would also explain the failure of the reactions when the alkyne substituents are changed: if either alkyne is not bulky enough, a complex of this type cannot be formed. If the alkyne attached to the molybdenum center is less bulky, the alkyne complex can however react instead with the liberated dicobalt fragment to give a cluster of type 6; if it is C2Ph2 this will not happen and slow decomposition will be observed instead. Formation of 7 from 8 involves cleaving the four carbon chain into fragments of 2 1 1 carbons. We were hopeful that the reaction of 8 with ruthenium carbonyl might give a complex similar to 4 but containing an alkyne ligand as well as two alkylidynes, but in fact it gave a multitude of products, all in low yield. However we also examined the reaction of the 0x0 complex 9 with the two metal carbonyls. In the case of cobalt carbonyl the only isolable product was the known niixedmetal metallacyclopentadiene complex C O M O ( ~ - C ~ P ~ ~ ) ( C In O the ) ~ case C ~ ~of. [ ~ ~ ]
+ +
1.13 Alkyne Scission on Metal Cluster Frameworks
23 1
ruthenium carbonyl the major product was an unusual bow-tie cluster, Mo2Ru3(p3-O)l(p3-CPh)(pu-C3Ph3)(CO)8Cp2 There were several notable features about this compound. Firstly the four-carbon chain has again been cleaved, but in a different way: this time 3 + 1 carbon fragments have been formed in the shape of a p-CIPh3 dimetalla-ally1 ligand that bridges the two molybdenum atoms, and a p3-CPh group, which as expected caps the MozRu face. The other interesting points and MozRu are the highly distorted nature of the bow-tie shape, in which the R u ~ triangles are virtually perpendicular to each other, due to the presence of the two triply-bridging 0x0 ligands, and the unusual loose coordination of one of the phenyl groups of the CIPh3 ligand to ruthenium.
10
Scheme 7. Reaction of 9 with ruthenium carbonyl
In search of an explanation for the different modes of cleavage of the C4Ph4 ligand in the clusters 7 and 10. we were prompted to examine the 0x0 complex 9 rather more closely. Although at first sight we saw no reason to doubt the formulation proposed by Stone, closer inspection revealed one puzzling inconsistency in the spectroscopic data. The I3C NMR spectrum of 8 contains peaks at 200.0 and 106.7 ppm assigned to the p-CPh and CPh carbons respectively, with the former occurring at lower field due to their bridging carbene character. In 9, the corresponding peaks are at 242.8, 197.9, 90.9 and 84.7 ppm, i.e. there is an unexplained shift of approximately 50 ppm in one of the signals assigned to the bridging CPh carbons. Attempts to grow crystals of 9 or its analogues incorporating q-CsH4Me ligands, p-tolyl substituents on the alkyne, or both were all unsuccessful. We therefore examined some reaction chemistry of 9 in an attempt to resolve this question. A common reaction of molybdenum 0x0 complexes is that with organic isocyanates, which effects the replacement of the 0x0 group by the isoelectronic imido ligand. Complex 9 reacted with PhNCO in refluxing toluene to give a 64% yield of
232
I Molecular Clusters
Figure 3. X-Ray crystal structure showing the structural formula of 1 1 .
11 in which only one of the 0x0 ligands has been replaced by an imido group (replacement of both 0x0 ligands did not prove possible under any conditions). This time we were successful in obtaining crystals of the analogue Mo2(0)(p-CqPh4)(p-NPh)(q-CSH4Me)*.Surprisingly, the crystal structure (Fig. 3 ) showed that the pU-C4Ph4ligand adopts an unprecedented coordination mode in which it is bonded to one molybdenum through an interaction similar to a terminal alkylidene. In fact, this requires only the breaking of one Mo-C bond and effectively involves migration of one bridging carbene-like carbon of the metallacyclopentadiene unit to a
9
I
Ph 11
Scheme 8. Reaction of 9 with PhNCO yielding 11.
terminal position. Since the " C NMR spectrum of the imido complex is very similar to that of the 0x0 complex 9, we propose that this coordination mode is also present in the latter. and that the ligand rearrangement occurs during the oxidation process. probably for steric reasons.[321Consequently the structural formula of 9 is not as shown above, but instead, as shown in Scheme 8, analogous to that of 11.
1.13.5 Conclusions The dimolybdenum alkyne complexes 1 are useful starting materials for the synthesis of heterometallic clusters containing hydrocarbon fragments. In the case of the MORUclusters, we have shown that scission of simple terminal alkynes to produce methylidyne groups is possible, albeit in low yield. In the cobalt system we have isolated high yields of alkyne clusters but alkyne scission only occurs in very specific cases, apparently ciu a nietallacyclopentadiene intermediate. Starting from preformed metallacyclopentadiene complexes, we have shown that the scission of four-carbon chains can be achieved in two ways depending on the metal-ligand fragments employed, and in the process have discovered a novel coordination mode for the nietallacyclopentadiene ring. It is interesting to speculate that some of the reactions in Tables 1-3 which involve more than one alkyne, for example of the type 2 C?Ph2 p3-CPh + j(-C3Ph3. might also proceed cia metallacyclopentadiene intermediates which fragment, rather than by alkyne scission followed by alkynealkylidyne coupling. --f
Acknowledgements Virtually all the work described in this article was performed by an excellent postgraduate student, Miss (now Dr.) Louise J. Gill, whose commitment and enthusiasm remained undimmed throughout three years of hard work. All of the crystal structures were solved by Harry Adams (sometimes assisted by groups of undergraduate students) without whom we would still be completely in the dark about many of our products. We would also like to thank the EPSRC for a studentship and Johnson Matthey plc for a generous loan of ruthenium chloride.
234
I Molecular Clusters
References [I] W. Hubel, in Organic Synthesis via Metal Carhonyls, Ed. I. Wender and P. Pino, Wiley, New York, 1968, p. 273. [2] E. Sappa. A. Tiripicchio and P. Braunstein, Chem. Rev., 1983, 83, 203; Coord. Chem. Rev., 1985, 65, 219; P.R. Raithby and M.J. Rosales, Adv. Inorg. Chem. Radiochem., 1985, 29, 169. [3] J.R. Fritch, K.P.C. Vollhardt, M.R. Thomson and V.W. Day, J. Am. Chem. Soc., 19’79, 101, 2768; J.R. Fritch and K.P.C. Vollhardt, Angew. Chem.; Int. Ed. Engl., 1980, 19, 559; B. Eaton, K.P.C. Vollhardt and J.M. O’Connor, Organometallics, 1986, 5, 394. 141 R.B. King, R.M. Murray, R.E. Davis and P.K. Ross, J. Organomet. Chem., 1987, 330, 115; R.B. King and C.A. Harmon, Inorg. Chenz., 1976, 15, 879. [5] E. Cabrera, J.C. Daran and Y. Jeannin, J. Chem. Soc., Chem. Commun., 1988, 607. /6] S.A.R. Knox, K.J. Adams, J.J. Barker and A.G. Orpen, J. Chem. Soc., Dalton Trans., 1996, 975. 171 J.R. Shapley, C.H. McAteer, M.R. Churchill and L.V. Biondi, Oryanometallics, 1984, 3, 1595. [8] J.R. Shapley, M.G. Humphrey and C.H. McAteer, in ‘Selectivity in Catalysis’, Ed. M.E. Davis and S.L. Suib, ACS Symp. Ser., 1993, 517, 127; see also M.C. Comstock and J.R. Shapley, Coord. Chem. Rev., 1995, 143, 501. [9] J.L. Haggitt, B.F.G. Johnson, A.J. Blake and S. Parsons, J. Chem. Soc., Chem. Commun., 1995, 1263. [lo] B.F.G. Johnson. J. Lewis, J.A. Lunniss, D. Braga and F. Grepioni, J. Chem. Soc., Chem. Conzmun., 1988, 972; D. Bragd, F. Grepioni, B.F.G. Johnson, J. Lewis and J.A. Lunniss, J. Chem. Soc., Dalton Trans., 1991, 2223; 1992, 1101. [Ill H. Yamazaki, Y. Wakatsuki and K. Aoki, Chem. Lett., 1979, 1041. 1121 A.D. Clauss, J.R. Shapley, C.N. Wilker and R. Hoffman, Organometallics, 1984, 3, 619; P. Quenec’h, R. Rumin and F.Y. Petillon, J. Organornet. Chem., 1994, 479, 93. (131 Y. Chi and J.R. Shapley, Organometallics, 1985, 4, 1900; F.G.A. Stone and M.L. Williams, J. Chem. Soc., Dalton Trans., 1988, 2467. [14] J.T. Park, J.R. Shapley, M.R. Churchill and C. Bueno, J. Am. Chem. Soc., 1983, 105, 6182; J.T. Park, J.R. Shapley, C. Bueno, J.W. Ziller and M.R. Churchill, Orgunometallics, 1988, 7, 2307. [I51 J.T. Park, B.W. Woo, J.-H. Chung, S.C. Shim, J.-H. Lee, S.-S. Lim and I.H. Suh, Organometallics, 1994, 13, 3384. 1161 M.P. Gomez-Sal, B.F.G. Johnson, R.A. Kamarudin, J. Lewis and P.R. Raithby, J. Chem. Soc., Chem. Conzmun., 1985, 1622; J.M. Fernandez, B.F.G. Johnson, J. Lewis and P.R. Raithby, Acta Crystullogr., Sect. B, 1978, 34B, 3086; C.R. Eady, J.M. Fernandez, B.F.G. Johnson, J. Lewis, P.R. Raithby and G.M. Sheldrick, J. Chem. SOC.,Chem. Commun., 1978, 421. [I71 A.D. Shaposhnikova, M.V. Drab, G.L. Kamalov, A.A. Pasynskii, I.L. Eremenko, S.E. Nefedov, Y.T. Struchkov and A.I. Yanovsky, J. Organomet. Clem., 1992, 429, 109; A.A. Pasynskii, I.L. Eremenko, S.E. Nefedov, B.I. Kolobkov, A.D. Shaposhnikova, R.A. Stadnitchenko, M.V. Drab, Y.T. Struchkov and A.1. Yanovsky, New J. Chem., 1994, IS, 69. [IS] R. Rumin, F. Robin, F.Y. Petillon, K.W. Muir and I. Stevenson, Organometallics, 1991. 10, 2274. 1994, 116, 11181; 1191 S.-J. Chiang, Y. Chi, P.-C. Su, S.-M. Peng, G.-H. Lee, J. Am. Chem. SIC., Y. Chi; P.-C. Su, S.-M. Peng and G.-H. Lee, Organometallics, 1995, 14, 5483; Y. Chi, C. Chung, Y.-C. Chou, P.-C. Su, S.-J. Chiang, S.-M. Peng and G.-H. Lee, Organometallics, 1997, 16, 1702. 1201 F.G.A. Stone, J.C. Jeffery, K.A. Mead, H. Razay, M.J. Went and P. Woodward, J. Chem. Soc., Dalton Trans., 1984, 1383.
I . I3 Allcyie Scission on Mc.tcrl Cluster Frurneicorks
235
1211 G . Conole, K.A. Hill, M. McPartlin. M.J. Mays and M.J. Morris, J. Chern. So(,.,Chern. C(inirnuii., 1989. 688; G. Conole. M. McPdrtlin, M.J. Mays and M.J. Morris, J. Cheni. Soc.. Dalton Tran.?.,1990, 2359; H. Adams. N.A. Bailey, S.R. Gay, T. Hamilton and M.J. Morris, J. Orycinoniet. Chcwi., 1995, 493. C25; H. Adams, N.A. Bailey. S.R. Gay, L.J. Gill, T. Hamilton and M.J. Morris, J. Chmi. Soc., Dalton Trcins., 1996. 2403. 3341. [22] M.D. Curtis, Polj~liedron.1987. 6, 759. 1231 H. Adams, L.J. Gill and M.J. Morris, J. Chrni.Soc., Chnn. Cornmun., 1995. 899 and 1309; Orycinonietnllic,s, 1996. 15. 41 82. [24] H. Adams. L.J. Gill and M.J. Morris. Organorne/aNic~s,1996, 15, 464. [25] H. Adams, N.A. Bailey, L.J. Gill, M.J. Morris and F.A. Wildgoose, J. Chenz. So(,.,Dcilton Truns., 1996, 1437. (261 H. Adams, L.J. Gill and M.J. Morris. J. Client Soc.. D d t o n Truns, 1996, 3909. 1271 E. Sappa, A . Tiripicchio, A.J. Carty and G.E. Toogood, Prog. Inory. Cheni.. 1987, 35, 437. 1281 S.A.R. Knox, R.F.D. Stansfield. F.G.A. Stone, M.J. Winter and P. Woodward. J. Cheni. Soc,.. Dalton Trnns.. 1982, 173. [29] S. Slater and E.L. Muetterties. Inorg. Cheni.. 1980, 19, 3337; E.L. Muetterties and S. Slater, Inorg. Chern.. 1981, 20. 946. 1301 R. Yanez, N. Lugan and R . Mathieu, OrganoriiefaNic.r, 1990. 9) 2998. 131) H. Adams, L.J. Gill and M.J. Morris. J. Orycmoniet. Client, 1997, 533, 117. [32] H. Adams, L.J. Gill and M.J. Morris. J. C'liern. Soc.; Dalton Trcms.. 1998, 2451; L.J. Gill. Ph.D. Thesis, University of Sheffield, November 1997.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.14 Multiple Interactions Between Arenes and Metal Atoms Antony J. Deeming
1.14.1 Introduction Arenes have played a special role in the development and applications of organometallic chemistry. In their q6 mode of coordination, they were among the first known polyhapto ligands and form one of the basic building materials of transition metal chemistry since the first rational synthesis of Cr(ChH6)Z by the FischerHafner method."] The phenyl ligand is another fundamental unit in organometallic chemistry. It is coordinated to metal atoms by particularly resilient 0-bonding, and is particularly resistant to p-elimination to give benzyne. Therefore, aryl complexes are among the more stable compounds containing M-C 0 bonds. However, most arene work has been on mononuclear complexes, and the chemistry of phenyl, benzene, and related ligands as bridges is less developed. Using arenes as bridges, there is scope to make full use of both 0 and 71 interactions and to produce a wide variety of structural and bonding motifs. This review will focus on the coordination of benzene in dinuclear and polynuclear metal compounds where coordination is through M-C 0 bonds in conjunction with n-complexation. There is considerable scope for multiple interaction of arenes with metals and the number of known types of benzene incorporation into clusters is steadily increasing. The number of variations in the modes of interaction is potentially enormous as the number of metal atoms coming into play increases. Even with two or three metal atoms the structural possibilities are extensive. Indeed in this article we can do no more than highlight the main features but by doing so we hope to show how the area might develop. Certainly the study of arenes multiply bonded to several metal atoms will lead to greater understanding of how the arene is chemically modified by such interactions (we know little about this at present) and should provide clear models of chemisorbed arenes at metal surfaces or other polymetallic environments. Indeed, we may eventually be able to switch from the
1.14 Multiple Interactions Betliven Arenes nnd Metal Atoms
(co)30s-
237
-0s
Scheme 1. Photo-isomerization of p 3 - $ , ? I 2 , q' benzene to p3-q'. q ' , q z benzyne by a double oxidative addition.
idea of coordinating arenes to metal atoms to the concept of using an arene as a stable electron-rich core or armature about which to construct a metal shell. Initially we will look briefly at bridging arenes, where only n-interactions are involved, but this introduction will be followed by a consideration of systems where both n and o-interactions operate. The p i - g 2 ,g2, q2-CsHs system, first reported by q'. q2-C6H6),[2.31 has proved to be Johnson and Lewis in 1985 for Osi(CO)~(p3-q', an important organometallic type and a sound model for n-benzene absorption parallel to a ( 1 , I , 1) or other triangulated metal surface. The interesting photochemical isomerization of this cluster leads cia an intramolecular rearrangement to the di-o-bonded benzyne cluster Osl(p-H)z(pi-ql,q ' , g'-CsH4)(C0)9,r41which we first observed as a product of the direct thermal reaction of Os3(CO)12with benzene (Scheme l)."' This is a nice simple example of the involvement of both o and n interactions in arene-cluster partnerships and demonstrates the possibility of isoinerization. Firstly let us look at clusters containing just n interactions between the arene and the metal atom array. We will leave o-bonding until later.
1.14.2 Arenes bridging two metal atoms employing only z interactions Benzene can potentially form bridges between metal atoms using either four or six TC electrons. The simplest situation is where a benzene bridge is formed by q 2 coordination to two metal atoms, as found in the dinuclear compound (('Bu2PCH2CH2P'Bu')Ni}2(~LZ-q2,g2-CsH6), type A (Fig. 1).[61 There is no direct contact between the Ni atoms which lie most favorably over opposite faces of the aromatic ring. A similar form of benzene bridging occurs in the com-
238
I Molecular Clusters M
M
A
6
C
M
D M
M -
M E
F G
H
Figure 1. Modes of benzene bridging between two metal atoms (non-planarity of the rings is exaggerated).
pounds r(CsMes)Re(C0)2)2(,u2-r2,q2-CsHd,7(Pd(AlC14)}2(p2-v2, T2-C6H6),'s'and [{OS(NH3)5)2(p2-q2, q2-C,jHs)]4 .[91 Thermal treatment of the diosmium complex leads to loss of ammonia and the formation of what is believed to be a p2-q2, q6-CsH6 complex, type B, although there has been no crystal structure to confirm this.['] Whereas p2-q2,q2-CsH6 coordination is normally through four adjacent carbon atoms as in type A, it has also been found to occur through opposite C-C ~-q~, type C.['O]Benzene can also bridge two bonds in { ( ' B ~ 3 S i 0 ) 3 T a } ~ ( pq2-CsH6), metal-metal bonded metal atoms as in type D (Fig. 1). An example of this is the 34-electron cluster Ru2(CsMe5)2(p-H)(p-PPhz)(p2-q2, q2-CsH,j), which supports a metal-metal bond and therefore the Ru atoms must be on the same face of the arene." '1 In the 1,4-xylene complex Co2(Cy2PCH2CH2PCy2)2(p2-q3, q3-C&Me2-1,4), all six carbon atoms are coordinated as in type E, which produces a 16-electron configuration for each of the cobalt atoms.['23131 Again there is no metal-metal bond. The metal moieties are on opposite faces of the arene ring, which is distorted into a chair form as shown for E in Fig. 1. On the other hand, a boat form F is adopted if the metal atoms are constrained by metal-metal bonding to be on the same side of the arene as in (CsHs)Ir(p2-q3,q3-CsHs)M(CsHs),where M = Co or Rh.[14] This mode of arene coordination is also found in the bridged paracyclophane system Ru2(CO)6(p2-q3,q3-ClhH16).[151 Another mode of benzene bridging G (p2-q4,q4-C6H6 coordination) is found in {(C5H5)Fe)2(p2-q4, q4-CsMes), formed
I . 14 Multiplp I~~tercrctions Between Arenes and Metal Atoms
239
by cyclotrimerization of but-2-yne.[l 31 Finally benzene is found in triple-decker complexes sandwiched between two metal centers in a p2-v6,q6 fashion H. A recently reported example is found in Hf'I4( PMe2Ph)4(p2-V6.T / ~ - C ~ H ~ ) . [ ' ~ ~ Figure 1 shows that there is now a range of different types of p-benzene available to us. There is no reason to believe that this set is exhaustive but it might be difficult to synthesize other types by design. Little work has been done on the organic chemistry of these benzene rings, although the lack of planarity in several of them ( E to C ) , which is rather exaggerated in Fig. 1, would suggest there is a loss of aromaticity. At present there is no evidence for advantages in producing bridging systems to modify the chemistry of the arene.
1.14.3 Arenes bridging more than two metal atoms employing only 7r interactions Benzene lying parallel to a triangle of metal atoms in a trinuclear or higher nuclearity cluster has been shown to be distorted towards a Kekule cyclohexatriene structure such that the shorter C-C bonds are coordinated.r31 There is some evidence that this relates to the geometry found for the attachment of ben~ ~mpounds ~~'~ with p3-q2, q 2 ,g2-C6H6 zene on Pt or Rh ( l . l > l ) s u r f a c e ~ . [ I Co have now been characterized for Os3(CO)9(,u3-q2, q', 17'-Cr,H6),[~.~~] Ru3(C0)9( p 3 - q 2q', , ?72-FhH6),[Z3 2 5 1 Ru3(p-H)3(q-C5Me5)3(p3-q2, q', q2-C6H6),[z61and Co3(C5H5)3(p3-q-, ri', q'-arene).[27~z81 and at triangular faces of various higher nuclearity clusters.rz91These compounds all adopt the C3v structure in which the centers of three shorter C-C bonds lie directly above the metal atoms (structural type A, Fig. 2). A 30" rotation of the benzene ring on the metal triangle would give an alternative C3v structure in which three carbon atoms now lie directly above the metal atoms. There is no evidence that this orientation can provide a ground stage arrangement, although the facile rotation of the benzene on the top of the metal triangle indicates that both conformations must be close in energy. However, geometries close to this alternative C3" arrangement, but distorted from it, have recently been observed. The complex Ru3(p-H)3(C5Me5)3(1(3-v12, $, $-ChHh) adopts the standard structure A and a one-electron oxidation leads to the corresponding monocation, which has the same basic structure.r261However, a two-electron oxidation leads to the dication with the alternative structure B. As well as benzene rotation by 30°, there has been a sideways slippage of the benzene ring towards one ruthenium atom so that the coordination is best described as q' at one ruthenium atom with an q3 bridge across the other pair of metal atoms. The ligand is ,u3-v3, q', $-benzene. The electronic factors (presumably the control is electronic and not steric), which lead to this change in structure on going from the 48-electron cluster to the corresponding
240
I Moleculuv Clusters
M
M
A
Figure 2. Two orientations of benzene coordinated at a triangle of metal atoms shown from above and in perspective.
B
46-electron cluster, still need to defined properly. By going from A to B there is an increase in the number of M-C contacts and this may be favorable for an 'electrondeficient' system. The A-to-B change is reminiscent of the 30" rotation of p3-alkyne from a conformation with the C-C axis parallel to a metal-metal edge to a perpendicular orientation on going from a 48-electron cluster such as O S ~ ( C O ) I O (p3-JI-alkyne)to a 46-electron system such as Fe3(C0)9(p3-l-alkyne).[301 Clearly both A and B provide models for benzene binding at ( 1, I , 1) metal surfaces, which may prove to be compelling analogues of chemisorbed arene states. The only structural precedent for B is the distantly related system in which one benzene of a Ti(l;16-C6H6)2unit bonds to a triangle of Ti atoms in the B-type manner in Ti4(q6-C6H6)2(p-I)3( IAlR2)3.[3 13321
1.14.4 Aryls in metal clusters Metal clusters incorporate aryl ligands in a variety of ways (A to F, Fig. 3). Some forms only differ by small geometric and energetic changes and are not easily distinguishable other than by diffraction methods. Broadly the aryl binds in such ways as to generate electron precise molecules. In going from A through to F, greater electron donation occurs. The phenyl group in A and B donates one electron, in C and D three electrons while in E and F seven electrons are donated. Often it is therefore possible to account: in part, for the observed mode of bonding in terms of the stoichiometry of the molecule. We believe that, although types B to D may be
1.14 Minltiplr Ititrractions BetLceen Arenes und Metul Atonis
241
Q M A
Figure 3. Modes of aryl coordination that are known for metal clusters.
M
B
C
E
F
M D
distinct in the solid state, they may well be in dynamic equilibrium in solution so that solution geometries may not be so easy to define. One might either argue that a phenyl coordinates in a certain way to satisfy the electronic requirements of the metal atoms or alternatively that a phenyl with a particular bonding type controls the structure adopted by the rest of the molecule. Such discussions are usually unhelpful. Why a particular system chooses between A and B is unclear, or between C and D, or between E and F. It is likely that sterics largely determine the structure adopted out of these pairs. The availability of different modes close in energy in many cases allows ligand migrations and structural variability. At present there is little evidence to substantiate these ideas.
1.14.4.1 Terminal aryls in metal clusters Terminal aryl ligands, common in main group organometallics, are infrequently encountered in cluster compounds of the transition metals. Oddly there are no systematic methods of synthesis from common reagents such as aryllithium. Aryl ligands are generally derived by their cleavage from heteroatoms of the reagents used. For example, phenyl transfer from PPh? can lead initially to clusters containing yIi -Ph and p-PPhl, the latter reinforcing the cluster. For example, thermolysis of Pt( PPh3)4 or Pt(C?H4)(PPh?)2leads to isomers of Pt3(gi-Ph)(PPh2)1(PPh3)2 (Fig. 4).[33,341 Different crystalline modifications contain either two short Pt-Pt interactions [2.758(3) A] and one non-bonding Pt. . .Pt interaction [3.586(2)A1 or, alternatively. there are three bonding interactions of intermediate length [two at 2.956(3) A and one at 3.074(4) A].It is perhaps odd that this molec-
242
1 Molecular Clusters
A
8
Figure 4. Crystalline modifications of Pt3(V'-Ph)(,uu-PPh2)3( PPhl)*. Pt-Pt distances: A, 3.586(2), 2.758(3), 2.758(3) A and B, 3.074(4), 2.95613), 2.956(3) A.
ular upheaval appears to have little effect on the phenyl group other than to change its conformation about the Pt-Ph bond. Another example of q'-Ph derived from PPh3 is the formation of R u (q' ~ -Ph)(p-PPh2)(p3-ampy)(p-PhC=CHPh)(C0)5(PPh3) PPh3)2 with diphenyla~etylene.~~~] In addion treating Ru3(pU-H)(p3-ampy)(CO)7( tion to the expected alkyne insertion into a Ru-H bond, a PPh3 ligand has cleaved into Ph and PPh2 fragments. Phenyl cleavage from dppm (PhzPCHzPPh2) also occurs to form the clusterstabilizing fragment p3-PhPCH2PPh2. This process leads to the phenyl clusters Ru3 (ql -Ph)(p3-PhPCH2PPh2)(CsHs)(C0)5[361and Ru3 ( q -Ph)(p3-PhPCH2PPh2)( , L L ~ - C ~ N C ) ( C ~ N C )'I ( CbyO )reacting ~[~ Ru3 (CO)lo(dppm) with appropriate ligands. The dppm ligand also fragments to give a 11'-Ph cluster Ru3(q1-Ph)( ~ ~ - P ~ P C H ~ P P ~ ~ ) ( , U ~ - C - C P P ~ on ~ )thermolysis ( ~ - P P ~ ~of)the ( C bridged O)~[~~] dicluster system {Ru3(dppm)(CO)~}2(pu-Ph2PC=CPPh2). In a related way the tribut phosphine (Ph2P)3CH cleaves to produce Ir3(ql-Ph)(p3-PPh)(p-dppm)(CO)6,[391 with Ru3(C0)12 this ligand gives several products including Ru2(p-q1-Ph)(C0)5{(Ph2P)2CHPPh}.[401 We have shown that a phenyl migration from the 2-pyridyl phosphine Ph2P(2-CsH4N) can occur to give the terminal phenyl ligand and the robust clusterstabilizing p3-PhP(2-C5H4N ) ligand in the cluster O S ~ ( ~ ' - P ~ ) ( ~ ~ - P ~ P C ~ H ~ N ) (Fig. 5).[411 With ruthenium the phenyl migration to Ru occurs, but there is subsequent migration to CO to give the p-benzoyl cluster Ru3(q1-PhCO)(p3-PhPCjH4N)(C0)9.[42'431 One example of q' -aryl formation not involving tertiary phosphines is the formation of Co3(q'-C6Fj)(p3-S)(CO)gfrom C6F5SSC6F5 and Co2(CO)s by both C-S and S-S bond cleavage.[441 The bridging p-q'-phenyl ligand B and the terminal q'-phenyl ligand A (Fig. 3 ) are both one-electron donor ligands, so it is not easy to predict which would be
'
1.14 Multiple Interactions Between A r e n a and Metal Atoms
243
L 1431
'
Figure 5. Molecular structure of Osq(q -Ph)(p?-PhPCTH4N)(CO)y.
formed in a particular case. Presumably the choice is controlled by steric effects but one might suppose that, if there are no overriding reasons against it, the ligand will adopt mode B to maximize the metal-to-ligand interactions. The phenyl clusters described in this section are all electron precise considering the phenyl as a oneelectron donor. Since both A and B (Fig. 3) are one-electron donors, one might envisage rapid Ph migration between terminal and bridging positions and hence between metal centers. N o experimental evidence for this can be cited. However, the ability of the ligand to be terminal or bridging suggests a potential for phenyl mobility within clusters or at surfaces. The occurrence of terminal rather than bridging Ph depends upon small steric or electronic factors and explanations cannot be given with confidence.
1.14.4.2 Bridging aryls without Ic-coordination These occur more commonly than terminal phenyl ligands, presumably because of the increased M-C contact as indicated (Fig. 3, type B). CopperiI), silver( I ) and gold( I ) compounds with these bridges are particularly well studied.[4s1A number of binary aryl transition metal complexes of the type M,Ary have been synthesized. Those with bulky aryls such as mesityl (mes) or perfluorophenyl are the most tractable, the bulk preventing further oligomerization or polymerization. Some
244
I Molecular Clusters
Figure 6. Representations of the structures of r;e2(p-rl'-rnes)2jrl1-mes)4and Mn3(p-v' -mesh( q -mes)z.
'
'
'
examples are Fez(p-q -mes), ( qI - m e ~ ) * , [ ~and ~ ] Mn3 (p-q' -mes)4(y - m e ~ ) 2 , [ ~ " ~ ) ~ (M~ 'and - C M' ~ F ~are ) ~any ), (Fig. 6) and [ N B U ~ ] ~ [ M M ' ( ~ - ~ ' - C ~ F where combinations of Pd and Pt.[48,49]Related p-y I-phenyl bridges are probably the standard motif in many phenyl species but these are usually polymeric and ill-defined. While the structure of PhCu is unknown, derivatives such as [Li(THF)4] [CugPhs] (Fig. 7) contain such bridges,r50]as does [Cu3(p-q'-Ph)2 ( M ~ ~ N C H ~ C H ~ N M ~ C H ~ C H ~ NinMboth ~ ~ the ) ]cation [ C Uand ~P the~anion.[511 ~] No more than an indication of our knowledge of this type of bridging aryl is presented here. In the transition metal cluster arena, this type of phenyl bridge is also well established, the first example (Fig. 8) being characterized more than 25 years ago by Nyholm et d.[52,531 They obtained product Os?(p-y'-Ph)(p~-PhPC6H4)(p-PPhz)(CO)g as one of many from the direct thermal reaction of Os3(CO)12 with PPh3. I still remember the surprise when this and other triosmium derivatives
Figure 7. Structure of the anion in [Li(THF)4]+[CusI'hs]
I . 14 Multiple Inteructions Between Avenes and Metal Atoms
245
Figure 8. Structure of Os?(p-q -Ph)(p~-PPh~)(p;-PhPC6Hl)jCO)~, the first example of d symmetrical p q ' -phenyl bridge in transition metal clusters
such as benzyne clusters were obtained from this very simple, commonplace reaction. Other examples of compounds containing /r-yl-Ph ligands are Ru3( p-v'-Ph)(p3-ampy)(~l-PPh2)2(C0!6;[35'54' OS~(/L -Ph)( - ~ ,' u - C ~ H ~ N ) ( C1 O 0 , )' ~ ~ ' R u ~ ( ~I -Ph)-v (p3-ampy)(p-PPh2)(C0)7( PPh3)[561(which reacts with CO to give a terminal vl-Ph intermediate prior to Ph migration back to PPhz to reform PPh3), and Ru3-
(p-qI-Ph)(p3-Me~NCONH)(,~r-PPh2)2(C0)6.'~'~ Aryl coordination of this type may be further supported/stabilized by additional interactions. For example, aryl halides are known to act as donors through the halogen atom and in [Pt2Ag(SC4H8)(p-ChF5)2(C6F5)4] the two bridging pentafluorophenyl groups bridge two platinum atoms as p-v'-bridges but there is an additional bonding between ortho-fluorine atoms and the silver atoms. Therefore, these ligands could be considered to be p j coordinated. Figure 9 shows the core of this anionic complex.[581
F
Figure 9. The central core of j Pt?Ap(SC4Hx)(/(I-C~Fj)2(1~ '-C<,Fj)?]-.
F
246
I Molecular Clusters
A
8
orthogonal to the plane of the phenyl, which is the normal situation.
In most p-ql -Ph ligand systems the aryl bridge is approximately symmetrical and the M-M vector is orthogonal to the plane of the arene. This perpendicular arrangement seems to require 3-centered bonding A (Fig. lo), but the observed perpendicular orientation has been used to invoke the contribution of the component B. The alternative in-plane coordination would require a four coordinate planar carbon and this has been observed in the case of the complex V2{2,6-(Me0)2C6H3}4 in which two of the four aryl ligands are constrained to adopt this geometry by the presence of additional coordination through ortho-Me0 groups (Fig. 1 l).[591
1.14.4.3 Bridging aryls with z-coordination through a single carbon atom The bridge C shown in Fig. 3 has been described a semi-bridging and is found in Zn2Ph4,which adopts a different type of bridge from that in A12Ph6, which contains symmetrical p-q'-Ph ligands of type B. The ZnPh2 subunits, distorted from linear, are linked through type C Ph-bridges. The p-q'-Ph bridges could be considered to
j 2
Figure 11. Representation of the structure of V(2,6-(MeO)&$I3}4.
1.14 Multiple Inteructions Between Arcnes and Metal Atoms
241
Scheme 2. Double oxidative addition of PhSeSePh to give bridging Ph and PhCO ligands.
be a-bonded to one Zn atom and, with longer orthogonal interaction to the other Similar situations are Zn atom, could be considered rr-complexed to that atom.*601 encountered in transition metal clusters. The C6F5 bridge in Pt3(p-q1-CgF5)(qIC6F5)3(PPh2)2contains a short a-Pt-C bond length of 2.108(10) A and a long orthogonal Pt-C distance of 2.621(10) A.i611Counting the p -q '- C s F ~ as a 3electron donor, the cluster attains a 44-electron count by an additional q2-interaction to a phenyl of the diphenylphosphido bridge. Another example of this quite derived rare mode of coordination is found in Os3(,uU-ql-Ph)(p-PhC0)(,u~-Se);!(CO)s by a triple oxidative addition of PhSeSePh to [Os,(CO)lo(MeCN)2] (Scheme 2).i621 This compound is saturated if the p-11'-Ph behaves as a 3-electron donor. As in the Pt case above, there is a short a-0s-C bond length of 2.24(2) A and a long orthogonal 0s-C distance of 2.51(2) A. Another example of this type of bridge, but doubled, is present in the 2,2'-biphenyldiyl nickel compound, Ni3(,u3-C12Hs)(,u3C6H4)(P'Prj)3, in which biphenyl is chelated through a-Ni-C bonds at one nickel with secondary interactions of the nickel-bonded carbon atoms at the other two nickel atoms.i631
1.14.4.4 Bridging aryls with n-coordination through two carbon atoms The p - q l , q2-type of coordination, first observed for ligands such as bridging alkenyl or alkynyl, has only recently been shown for aryl ligands (type D, Fig. 3 ) . Treatment of the pentamethylcyclopentadienyl compound Ir2Cpf2(C0)2with the diazonium salt [ 4-MeOCbH4N2][BF4] leads to the addition compound [Ir2Cp*2(puq ' . q2-CsH40Me)(C0)2][BF4]in which the aryl is bridging as shown in Scheme 3.[641Its NMR spectra down to -90 "C are consistent with a symmetrical aryl bridge and not the p - q ' , q'-bonding mode established in the crystal. Of course, there could have been a structural change to the symmetrical form in solution but more likely there is a rapid oscillation of the ligand between the two Ir atoms. The only other case of the p - q ' , q2-bonding mode for phenyl is that in Ru3(p-q', q2-Ph)(p3-S)(p-PPh?)(CO)s[ PPh3) formed by fragmentation of PPh3.[651
248
1 Molrculur Clusters
Rf- J!i Me0
OMe
-
\ -
Cp'lr A c C
A
-I k p *
-
--
,.
Cp'lr
,
. Ir Cp* C 0
0 0
Scheme 3. Rapid p q ' , q2-phenyl oscillation.
1J4.4.5 Bridging aryls with Ic-coordination through all six carbon atoms Bridging phenyl in the compound MnCr(p-y', q6-Ph)(CO)7(PPh3) is a-bonded to the Mn atom and y6-coordinated to a Cr(C0)3 group.[661In effect the complex Mn(y1-Ph)(C0)4(PPh3) is behaving as a six-electron donating y6 arene ligand at the Cr(C0)3 group. There is no significant bonding between the metal atoms. In several osmium and ruthenium clusters of the general type Ms(p-y', y6-Ph)(pU-X)(CO)8 where X is a 3-electron donor (I, SR, C4H3N), the phenyl bridges across the termini of a M3 chain of atoms (Fig. 12). These compounds have been derived in various ways. Oxidative addition of PhSR ( R = Ph or Fc) to R u ~ ( C O ) ~ ~ leads by S-C cleavage to the phenyl products Ru3(pU-q', y6-Ph)(p-SR)(CO)g.[671 Similarly, oxidative addition of iodoarenes (ArI) to Ru3(CO)12 leads directly to the stable products Ru3(p-y1,q6-Ar)(p-I)(CO)g, a reaction successful for a range of different aryl iodides.[681The ' H NMR spectrum of the Ph compound shows five different signals since the p-I atom is out of the R u plane. ~ The cluster is rigid. The iodo group stays on one side of the R u ~plane and the phenyl group is likewise immobile. This rigidity results in non-interconvertible isomers from the addition of 3,4-dimethyliodobenzene as shown in Scheme 4. Although this appears to be a
l z ) o C('O ( Mh, - , M X
Figure 12. Structural formula for clusters of the type M ~ ( P - vqh-Ph)(p-X)!Co)8. ',
I . 14 Mciltijd~Interrrctions Betwwn A~rncIsund Metal Atoms
' Me
+
249
+
Scheme 4. Oxidatiw addition of aryl iodides to R L I ~ ( C O J ~ ~
simple reaction, studies on substituted aryl iodides shows that rearrangements are possible. For example, Scheme 4 shows that 2-methyliodobenzene gives the 3methylphenyl product (as a pair of orientational isomers) and 4-methoxyiodobenzene gives the 3-methoxyphenyl product (also as orientational isomers) in addiWe believe tion to the methoxybenzyne cluster, Ru4(p4-V'-C4H30Me)(CO),~.1~81 that these reactions may occur i-ia dehydroiodination to give benzyne intermediates since this provides a pathway for rearrangement. Loss of two CO ligands from Os3(,uu-V'-Ph)(p-C4H6N)(CO)lo results in a re. '] arrangement of the phenyl bridge to the seven-electron donating p - ~ 'V6-mode.IS To form an electron-precise product a 0 s - 0 s bond is cleaved in the process. Given that phenyl ligands can have the various modes of coordination A to C (Fig. 3) using just the one carbon atom, one can imagine that seven-electron donation could be achieved by v6 in conjunction with different types of q' coordina-
250
I Molecular Clusters
Figure 13. Structure of p j - q ' , q6-Ph in OsdRu ( p j - q ' , q6-Ph)(p-H)3{P(OMe)3}(CO)12.
tion and this has been demonstrated by the formation of O S ~ R U ( P - H ) ~ ( P ~ - P ~ ) (CO)12{P(OMe)3] .[691 This compound is synthesized by cation-anion coupling of [Os4H2(C0)12I2- with [Ru(CgHg)(MeCN)3I2+. Subsequent treatment of the benzene-containing product with trimethylphosphite results in an internal oxidative addition of benzene to give phenyl plus hydride ligands. The spiked tetrahedral structure is shown in Fig. 13. The scope for phenyl coordination is large and we should look forward to the discovery of various new modes of coordination but these discoveries will be largely fortuitous since the very flexibility of the aryl system works against any designed synthesis. An aspect of aryl coordination not covered in this account is where an aryl-to-metal bonding occurs as part of a more complex ligand system, usually including coordinated heteroatoms. For example, we have shown in Scheme 5 a cluster that contains, as a component, a substituted form of the Ph ligand E in Fig. 3.[70*7'1 The cluster Ru3(p3-CgH4CHCH)(CO)sformed by loss of sulfur from benzothiophene contains the same type of p-q',v6-coordinated aryl as in Fig. 12. We will not attempt to describe this more complex chemistry further but an examination of multicomponent ligands of this type could lead to ideas about new motifs that could be applied to 'pure' aryl complexes.
Scheme 5. Sulfur extrusion from benzothiophene at RW(CO)IZ.
1.14.5 Arynes in metal clusters Most studies have been on the parent ligand C6H4, which in the free state is ben~ y n e . [ ~ However, '] the two carbon atoms that are not bonded to hydrogen are
1.14 Multiple Interactions Between Arenes and Metal Atoms
251
normally o-bonded to metal atoms in clusters and therefore there is little chemical resemblance between free and coordinated benzyne. Some authors have preferred therefore to call the ligand ovtho-phenylene to avoid this comparison but in organometallic nomenclature it is usually most convenient to use the name the ligand would have if metal-free. We talk of coordinated alkynes for convenience even though they might be better described as metallacyclopropenes. Describing the bonding of coordinated ChH4 can create problems in some cases and this is apparent in the more complex examples in Fig. 14. Benzyne ligands are known to coordinate to between one and five metal atoms in molecular species. Almost certainly chemists in the course of time will discover additions to the twelve known types A to L we have given in Fig. 14.
8
9 M-
M
M-
M
B
A
M
\
M--M
M E
D
C
-M
F
M
M-
G
J
-M
K
Figure 14. Modes of benzyne interaction with one to five metal atoms.
H
L
252
I Moleculur Clusters
1.14.5.1 Terminal y2-arynes Benzyne formally corresponds to an alkyne and it is not surprising that there are many benzyne and alkyne compounds that correspond directly, in stoichiometry, structure and bonding. However, with benzyne having potentially up to eight electrons for metal binding and alkyne only four, one can account for the greater variety of geometric and bonding types found for benzyne as a ligand. Coordination of benzyne to a single metal atom in a complex is now quite common, especially with early transition metals such as zirconium, but there are examples right across the d-block metals (y2-coordination of type A, Fig. 14). Zr(y2-C6H4)(PMe3)Cp2,[741 Some examples are Ta(y2-C6H4)(y-C5Me5)Me2,[731 Ke(q2-CsH3Me) (q1-C6H4Me)( PMe3)2,r751Ru(y2-C6H4)(PMe3)4,[761Ni(y2-C6H4) (CY~PCH~CH~PCY More ~ ) . ~examples ~'] are provided in reference 72. Alkynes coordinated to transition metals are not linear because the substituents are bent away from the metal. This accounts, to some extent, for the metal-stabilization of arynes since the ligand intrinsically has this geometry. Known mononuclear benzyne complexes are fewer than those of alkyne, not because they are less stable, but mainly because the benzyne ligand must be generated within the coordination sphere by an elimination process, whereas no such problems apply to the synthesis of alkyne complexes. There is a strong driving force favoring bridging modes of benzyne over terminal ones and this has prevented the isolation of stable clusters containing benzyne bonded to just one metal center. This effect is the consequence of the thermodynamic stabilization resulting from forming multiple interactions with metal centers. The same applies to other unsaturated ligands such as alkynes, alkylidenes etc. which occur only very rarely terminally in metal clusters and then usually as reaction intermediates.
1.14.5.2 Arynes bridging two metal atoms One way a bridge can be formed between two metal centers is to generate the dibenzyne ligand C6H2, which links the two nickel atoms in the compound { N ~ ( C ~ ~ P C H ~ C H ~ P,2,4,5-C6H2), C ~ ~ } ~ ( ,although L L - ~this might be properly thought of as two separate metal-to-benzyne entities of the type discussed in Sec. 1.14.5.1.[''I Intuitively one would expect a benzyne ligand to bridge two metal atoms by two 0-bonds as in type B (Fig. 14) and that this would be a more stable than the terminal bonding mode A. The four membered M2C2 ring is essentially unstrained, the longer M-M distance allowing the most favorable angles at the carbon atoms to be accommodated. There are examples in main group chemistry, such as Hg3(C6H4)3which forms a 9-membered ring.[781This compound, the magnesium analogue, and the dilithio compound Liz(C6H4) have be used to transfer the C6H4 ligand by metathesis with metal halides. This method has been used in the
1.14 Multiple Interactions Between Arenrs und Mt.tal Atoms
253
reaction of Mg4(THF)4(C6H4)4 (which contains ,LQ- rather than p2-CsH4 ligands, see below) with InC12 {2,6-(Me2NCH2)C6H3) to give the dinuclear compound
In2(p-C6H4):,-{2,6-(Me2NCH2)2C6H3}.[791 In general, this has not been used as a method in transition metal chemistry where the CsH4 ligand has been derived from phenyl phosphines or from benzene itself by double oxidative addition reactions. For example, the complex Ir:,(q-CgMe5)2(p-O)(PPh3) undergoes an interesting isomerization in benzene at 85 "C to give ~ ( p-C6H4)jp1-OH)(p-PPhl).Triphenylphosphine and not benzene is the Irz ( v - C Me5)2 source of the benzyne.[801This is also the case in the formation of the corresponding formed by treating Ir?(Y-C5Me5)2hydride I~~(~-C~M~~)~(~-C,~H~)(,LL-H)(,LI-PP~:,) HzCI:, with PPh3 and hydroxide ion using phase-transfer methods.["] On the other hand, solvent is the source of benzyne in the formation of I ~ : , C P ~ ( ~ - C ~ H ~by) ( C O ) ~ UV-photolysis of IrCp(CO):, in benzene.[s21Likewise in the treatment of palladium acetate with S'Pr2 in refluxing benzene to give Pd4(p-C,jH4):,(p-S1Pr2)2(p-MeC0:,)4, the bridging benzyne is formed from the ~ o l v e n t . ' Thermal ~ ~ . ~ ~ decomposition ~ of this compounds generates triphenylene. Fig. 15 shows some of these p-CbH4 complexes. The benzyne ligand can bridge across a very wide range of distances between the metal atoms. For example, there is a considerable difference in the Ir-Ir distances in the above diiridium compounds with p-CcH4 bridges: 3.369(1) for Ir2(C5H5)2(p-OH )( p-PPh2)(p-CgH4),['O12.7 17 A for Ir2(C5H5):,(CO):,( , u - C ~ H ~ ) , [ ' ~ ] The hydroxy-bridged and 2.8901(4) A for Ir2(C5Me5)2(p-H)(p(-PPh2)(p-C6H4).['l1 species is a 36-electron species and there is formally a non-bonding Ir-Ir contact whereas in the other two 34-electron species a single Ir-Ir bond results in shorter distances. In the Pd4 compound there are no Pd-Pd bonds and the CsH4-bridged Pd-Pd distance is as large as 3.595(2) the longest distance in this set of compounds. Therefore benzyne can bridge both bonded and non-bonded metal pairs and can accommodate a good range of metal-metal distances.
Figure 15. Examples of pbenzyne bridging two metal atoms.
254
I Molecular Clusterh
L
THF
THF
which includes bridges of the type shown
p.-
Mg
Scheme 6 . Formation of
: Mg
Mg
Mg4(C6H4)4(THF)4
1.14.5.3 Arynes bridging three metal atoms Four geometrically and electronically distinct forms of p3-benzyne, C to F in Fig. 14, have been currently identified. Most examples of p3-benzyne are for transition metals and then there are both a and 71 interactions between the arene and the metal atoms, types D to F. There is just one example of ligand type C, found in a p3-benzyne compound of magnesium, that seems to be supported purely by a-interactions. Scheme 6 shows the transfer of benzyne from the cyclic trimer Hg3(p-CsH4)3 to magnesium to give Mg4(THF)4(p&,jH4)4, which has a cubane structure diagrammatically represented in Scheme 6. The p3-benzyne ligand orthogonally caps a triangle of Mg atoms through a a-bond to one Mg atom and an unsymmetrical three-center 2-electron bridge across the other two manganese atoms, type C (Fig. 14).[781 This type of bridge has not been observed in transition metal compounds where 71 complexation seems to be favored instead. All the other forms of p3-benzyne bridge (D to F, Fig. 14) involve two a-M-C bonds and 71interaction. In the most common form D (p3-q1,q ' , q 2 ) ,the a-M-C bonds can span a bonded or non-bonded pair of metal atoms. Type E (p3-q1,q l , q ' ) involves two 0M-C bonds and a 71-interaction between just one carbon atom and metal, while F (p3-q1,q l , q 6 ) has so far only been found when one metal atom is not bonded to the other two and is really an extension of type B. p3-Benzyne ligands can be derived from various precursors, the simplest being benzene. An example is given in Scheme 7.
1.14 Multiple Interactions Betbt,een Arenes and Metal Atoms
255
H Double oxidative addition L
P-C and C-H cleavage
Scheme 7. A double oxidative addition of dimethylpheiiylphosphine to form a p - q ' , q ' , q2-ChH4 complex.
a. Double oxidative addition of benzene. Direct reaction of benzene with Os3(CO)12 yl, q2-C6H4)at about 460 K gives poor to moderate yields of Os3(p-H)2(,~~(3-q~, ( C O ) I J . ' ~The ~ ] rate of this reaction seems to be controlled by CO dissociation since Os?(CO)lo(MeCN)?reacts at 363 K.[x61The same product may be obtained by the > 290 nm) of Os3(,u3-q2,q 2 ,q2ChH6)(C0)9in toluene at UV-photoisomerization (i 278 K (Scheme l).[41Irradiation of the CbH6 compound isolated in solid Ar or CH4 matrices showed that the isomerization occurs without CO loss and a primary photoproduct of unknown structure was observed spectroscopically. It seems reasonable that the two oxidative additions occur sequentially so one intermediate although this may not be the intermediate is likely to be Os3(pCI-H)(p-C6H~)(C0)g, species that was observed. b. From benzyl alcohol or benzaldehyde. Thermolysis of the benzyl alcohol oxidative addition product Os3(p-H)(pu-OCH2C6H5)(CO)lo leads c i i Os3(p-H)2(p3-q1, q l , q2-C6H4)(C0)9 to benzene, benzaldehyde, Osi(CO)12 and O S ~ H ~ ( C O ) ~ ~Likewise, .["] benzaldehyde reacts with Os3(CO)12 to give Os3(p-H)( ~ - C ~ H ~ C O ) ( Cwhich O ) I ~decarbonylates . to give the same benzyne cluster.[881 c. From p3-phenylmethylidyne. At 493 K the alkylidyne compound Os3(p-H)3( L ( ~ - C C ~ H ~ ) ( isomerizes CO)~, to two isomers of the methylbenzyne cluster Os3(p-H)?(pj-q',q ' , $-C6H3Me)(CO)g.[8q1 This reaction is particularly interesting because of its reversibility. Another more rapid process in O S ~ ( , D - H ) ~ ( ~ ~ - C C ~ H ~ ) (C0)g leads to the intramolecular exchange of phenyl and hydride protons.[901 d. By aryne formation in the coordination shell of the cluster. The alkyne cluster Ru~H~(,u~-~'-7-oxabicyclo[2.2.l]hept-5-en-2-endo-y1 benzoate)(CO)s was synthesized by a double oxidative addition to Ru3(C0)12. On treatment with Me3SiOS02CF3 (TMSOTf),this cluster is reduced and benzoic acid is eliminated 9 ' . q2-C6H4)(C0)9and other products. This seems to be to give Ru3(p-H)2(p3-v1. the only example of benzyne synthesis in . r i f ~ . [ ~ ~ ] e. From phenyl phosphines or arsines. This is the first and most commonly applicable route. Nyholm and co-workers were the first to observe that the triphe-
256
I Molrczilur Clusters
nylphosphine ligand fragments on reaction with O S ~ ( C O ) ~ ~Two . [ of ~ ~ten~ ~ ~ ~ ” ~ and products are the benzyne clusters Osi(p3-q1,q’, q2-C6H4)(p-PPh2)2(CO)~ Os3(p-H)(p3-ql, q’, q2-C6H4)(p-PPh2)(CO)7( PPh3 ). Clearly the fragmentation of PPh3 has occurred and this type of chemistry has been applied to triarylarsine~,[~*] mixed alkylaryl phosphines and a r s i n e ~ , [ ~to~ ~ ferrocenylphenylphos~~] p h i n e ~ , [ ~ ~ and - to phenylphospholes.[’021The cleavage of, for example, PMe2Ph at a triosmium cluster (Scheme 7) requires an ortho C-H and a P-C bond to be cleaved in a double oxidative addition but the sequence of these fragmentations is not always clear, although there seems to be evidence that an orthometallation occurs first in some if not most cases. The evidence for the course of the thermolysis reaction of Osl(CO)11(PMePhz) is clearer. The observation of intermediates (Scheme 8) shows that in this case orthometallation occurs first, followed by benzene elimination (probably via phenyl transfer from phosphorus to metal as in Sec. 1.14.4). The generation of the CsH4 ligand occurs late in the reaction by a final P-C bond cleavage. There is considerable scope for isomerization and formation of isomers in these reactions leading to benzyne. In the conversion of Os3H3(pl-CPh)(CO)g to Os3(p-H)*(p3-q1,q ’ , q2-CsH3Me)(CO)g,the two possible isomers of methylbenzyne are p r o d ~ c t e d . ’ The ~ ~ ] benzyne formed is not always the expected one. For example, the thermolysis of Os3(CO)I1 (MezAsCsHqMe-2) gives the benzyne cluster Osl-
60 ___L
m R
Scheme 8. Established route for the conversion of OS~(,U~-PM~)(,U~-C~H~)(CO)~ from OS~(CO)I 1 (PMePhz). All compounds except that in square brackets have been fully characterized.
I . I 4 Multiple Inteructions Betiteen Arenes ~ i n dMrtul A t o i m
/!SAH*
/ \ I
0s
~
L H I O S
__
257
fi H +u*
Process B
os-
Scheme 9. Mechanism for the exchange of CH with OsHOs hydrogen atoms
(p-H)(~i-AsMez)!p3-q1, q ' . q2C6H4Me-4)(C0)9.The unexpected product, Os3(p-H)(p-AsMe2)(p3-q1,q l ,q2CgH40Me-3)(CO)9 is also obtained from O S ~ ( C O ) ~ I ( M ~ " s C ~ H ~ O M ~ - ~ ) .A' ~ possible '' origin of these rearrangements has been ~UJ-~'~ provided by spin saturation transfer experiments on O S ~ ( , L ~ - H )q~l ,(q2-C6H4)(CO)9.[1"31 In addition, to a very fast intramolecular rotation/flip (process A) of the benzyne and hydride migrations about the cluster edges, a much slower process B involving H-atoms moving between metal and aryne sites has been identified (Scheme 9). Equilibrium between phenyl and benzyne compounds could account for the rearrangements described above. This process allows site exchange of all carbon atoms in the 6-membered ring. The fast process A in Scheme 9 involves both rotation and flipping of the aryne. By introducing an isopropyl group into Os3(,u-H)(p-AsMe2)(p3-q'3 '1' ,q2-C6H;Pr)(C0)9, it was possible to show that the isomerization of the two isomers by aryne rotation occurred concomitantly with exchange of the diastereotopic isopropyl methyl groups MeA and MeB consistent with the flip shown in Scheme 9.r981Notice that the inversion process leads to the exchange of the diastereotopic methyl groups and that the high rate of inversion generally will prevent the separation of enantiomers. This dynamic mobility of the p3-benzyne is a characteristic behavior found in most cases. Although mode D (Fig. 14) is the most common form of p3-benzyne, mode E is observed as an alternative when the aryne bridges an open trinuclear cluster through rr bonds across a bonded pair of metal atoms. The bonding in Os3(p3-X)( ~ ~ - C ~ H ~ ) ( Cwhere O ) Y ,X = PPh,[961PMe,[94.951PFc,"''] PPhCr(CO)3,"041 or As(tolyl),[921 is probably best described as a di-a-bonded benzyne with an additional n-interaction with the third metal atom through one carbon rather than the more common two carbon atoms (Fig. 16). This seems reasonable in view of the open
258
I Molecular Clusten
Figure 16. Structure of the clusters M3(p3-PR)(p3-qi,q i , q i C6H4)(C0)9showing the n interaction in addition to two M-C a-bonds.
R
nature of the cluster but does not explain why the benzyne does not bridge the ends of the open cluster as in other cases. Benzyne is rapidly mobile in these compounds. Mode F of p3-benzyne is quite different since the ligand is an 8-electron rather in than a 4-electron donor. Thermolysis of R u ~ ( C O1){MeZAsCgH5Cr(C0)3), I which a Cr(C0)3 unit is y6-coordinated to the dimethylphenylphosphine ligand, gave as one of the two products the cluster R U ~ ( ~ - H ) ( ~ - A S M ~ ~ ) { ~ - C ~ H ~ C with a type F benzyne, apparently the only known example.['051
1.14.5.4 Arynes bridging four metal atom There are as many a five different modes G to K of p4-benzyne coordination (Fig. 14) but some of these only differ in detail. Thermal treatment of R u ~ ( C O ) ~ ~ ( P P ~ ~ ) leads to the tetra and pentanuclear clusters Ru4(CO)11(p4-PPh)(p4-y1,y I , y4-CsH4) y l , y6-C6H4) (Sec. 1.14.5.5).[1061It is believed that and Ru5(CO)13(p4-PPh)(p5-y1, y ', y1-C6H4), analthese are formed via the intermediate Ruj(CO)s(p3-PPh)(p3-y1, ogous to the known osmium compound from the corresponding reaction. Successive additions of Ru(CO), units would lead to the tetra and pentanuclear products. The benzyne ligand in the tetranuclear compound is coordinated as in G (Fig. 14) and is a 6-electron donor. The related compounds Ru4(CO)11(p4-PR)(p4-y1, y', y4C6H4) where R = F C " ~ 'or ] CH2NPh2['061are also known. These structures are clearly different from those of the known alkyne compounds R u ~ ( C O1)(p4-PR)(p4-y', I y -alkyne) where the alkyne bridges diagonally across the metal square and not parallel to one edge as with benzyne. Having more 71electrons, benzyne has a greater capacity to develop metal-carbon contacts than alkyne. The diagonal mode of benzyne coordination is unknown. Furthermore, mode H is also unknown for benzyne bridges, although this geometry may be an intermediate in the fluxionality of pq-y', y I , y4-C6H4 ligands in a process in which
'
1.14 Multiple Inteructions Between Arenrs and Metul Atoms
4?
iM M - -M\
tilted
M-
259
-M
\ M-M
almost vertical
M-M
tilted
vertical and diagonal
Figure 17. Different geometries of attachment of benzyne, 1,2-naphthyne, 2,3-naphthyne and 2,3thiophyne depending upon their ability to sustain v4 coordination in R u ~ ( C O1)(,ud-PR)(p4-aryne). ,
the benzyne flips while the C-C bond remains parallel to one M - M edge. In q ' , q4the corresponding 1,2-naphthyne species R u ~ ( C OI)(p4-Pnaph)(p4-q1, I C I ~ H ~formed ), from P(l-naphthyl)3, the ligand is still parallel to a M-M edge but much more closely vertical, approaching the geometry H.[ln8]One could argue that the double bonds in the 1,2-naphthyne are not set up for q4 coordination as for benzyne and so that the arene plane is nearly vertical with an angle of 73.5" to the II q ' , q4-C6H4). Conmetal plane compared with 51 . I " in R u ~ ( C O )(p4-PPh)(p4-q1, sistent with this it has recently been shown that the 2,3-naphthyne ligand achieves q4 coordination in a very similar manner to the benzyne compound because this mode of coordination is energetically favorable (Fig. 17).['09] Also the corresponding 2,3-thiophyne derivative is unable to accommodate q4 coordination but in this case the diagonal geometry like that of alkynes is adopted.["'] One would predict that 3,4-thiophyne would adopt the geometry of type G (Fig. 14). It is useful to explore this analogy between known alkyne systems and potential benzyne ones. For example, alkynes react with ruthenium carbonyl to form the Ru4(q2-alkyne)(CO)12, which form an octahedral Ru4C2 arrangement with the metal atoms in a butterfly geometry. Since the synthesis cannot be adapted to arynes, another method is needed. Oxidative addition of aryl iodides to R u ~ ( C O ) ~ ~ gives the p-q',q6 aryl complexes described in Sec. 1.14.4.5. However, we have found that some polycyclic iodoarenes undergo dehydrohalogenation to give clusters of the type Ru4(q2-aryne)(CO)~2. Scheme 10 shows one example of this reaction leading to p4-q1,q'-phenanthryne. Although the parent benzyne compound could not be synthesized, other aryne compounds such as methoxybenzyne, 1,2naphthyne, 9,lO-phenanthryne and 1,2-anthracyne clusters have been generated and shown to have the structure I (Fig. 14) like that of the corresponding alkyne
260
I Molecular Clusters
+
-
9
refluxing octane
I
Scheme 10. Dehydroiodination of iodophenanthrene to give a phenanthryne cluster.
The other types of q4-aryne (J and K in Fig. 14) have all been synthesized by introducing an aryl phosphine to the cluster, which is precoordinated to a Cr(CO), group. For example, PhP{C,jHsCr(CO)3}2,with two of the three phenyl groups q6 coordinated to chromium, reacts with Ru3(C0)12 to give the 50-electron cluster RU~(CO)~(,U~-P{C~H~C~(CO)~}{C~H~C~(CO)~) (Fig. 18). This has structure J (Fig. 14) and is closely similar to E but with the benzyne additionally q6coordinated to the Cr(C0)3 group and with a Cr-Ru bond.["'] Interestingly, if Rq(C0)12 is treated with the analogous tBu compound tBuP{C6H5Cr(C0)3}2, the product Ru3(C0)8(,u3-PtBu){,u~-CgHgCr(C0)3f has one CO fewer.["'] The Cr(CO)3 unit remains y6 coordinated to the benzyne but the R u ~triangle is now closed. Structure K (Fig. 14) with a plane of symmetry through the benzyne ligand is found. Whereas J corresponds to E, structure K corresponds to D. Another cluster with geometry J is formed when Ru3(CO)11{Me2AsCsHsCr(CO)3) is heated. One product Ru3(p-H)(CO)8(,u-AsMez) {,u&jH4Cr(C0)3)has structure J but with a closed R u triangle ~ (Fig. 18).
Figure 18. Examples of types J and K modes of benzyne coordination.
I . I 4 Midtilde Inteructions BetLoeen Arenes rind Metcrl A t o m
26 1
1.14.5.5 Arynes bridging five metal atoms Structure L (Fig. 14) has been found for Rus(CO)l?(pq-PPh)(ps-q', q l , q6-C6H4). It has been described as a model for the activation of benzene at a step on a ( I , 1 , l ) metal surface. The C6H4 ligand is bound parallel to the surface represented by a triangle of atoms. Activation of C-H bonds at metal atoms on the top layer of the step could generate a pj-q',q l , q6-CbH4 group as in L.['O6' Extending the number of metal interactions with the arene beyond this known limit would lead to the hypothetical fully metallated M12C6 unit (Fig. 19). Using reasonable C X and M-C bond lengths, however, leads to the conclusion that the metal shell would need to be extremely stretched to fit around the C6 core. The required M-M distance in M12C6 are about 3.5 A longer than normal M-M bond lengths. However, many of the structures in Fig. 14 are subunits of the M12C6 structure and there may be more structures to be found as subunits with more than five metal atoms. However, very large subunits of M12C6 will only be possible if many of the M-M contacts shown are non-bonding.
1.14.6 Some recent developments with naphthalene and anthracene Our knowledge of how polycyclic aromatics interact with clusters is limited, but we would expect naphthalene, anthracene, pyrene rtc. to be able potentially to form
262
I Molecular Clusters
Scheme 11. Naphthalene derivatives from R u ~ ( C O12) and diphenyl( I-napthy1)phosphine.
more extensive interactions with metals than monocyclics. Most known systems just involve n-interactions and there are examples of this with naphthalene in Mn2(CO)5(p-r4,q6-CloHs)['13] and with acenaphthalene in clusters.[tt41Compounds with both and TC metal-contacts are few. The cluster R u ~ ( C O reacts )~~ P~ q4-C1oH6) ~ ~ ~ ) ( ,(Sec. U~-~', with P(l-naphthyl)3 to give R L I ~ ( C O ) I I ( ~ U ~ -r', 1.14.5.4), the first example of a 1,2-naphthyne."0s1 The corresponding 2,3naphthyne compound is also known.['091In Section 1.14.5.4 we have also described the formation of another type of 1,2-naphthyne coordination corresponding to the phenanthryne cluster shown in Scheme 10. We have followed the tertiary phosphine approach by using Ph2PR where R = 1-naphthyl or 9-anthracyl to introduce these ring systems into clusters (Schemes 1 1 and 12).r11591161 Several products were isolated and characterized in each case. These contain coordinated naphthalene or anthracene but usually stubbornly still attached to phosphorus. Some naphthalene examples are shown in Scheme 11 show the different ways the naphthalene group is coordinated to metal clusters, mostly in the same types of way we have considered previously for benzene. However, in the one compound where naphthalene and phosphorus have separated, a remarkable new type of structure is found, which indicates that naphthalene and other polycyclic aromatics may generate quite different sorts of system to those of benzene. The product Ru6(p3-PPh)(p6-CloH6)(CO)14 is formed in low yield in the reaction of PhZP(1-naphthyl) with Ru3(C0)12 (Fig. 20). In this compound the naphthalene1,s-diyl ligand fits into the fold of a folded raft-like framework of six ruthenium atoms. There are two o Ru-C bonds to ruthenium atoms at the fold and two v 2 and
I . 14 Multiple hitrvuctions Between Arenes und Mctul Atoms
263
Figure 20. Two views of the core of the compound Ruh(CIOH6)(PPh)(CO)14,showing the p6q l , r l ' , q 2 .q'. q i , q 3 coordination of CloHh and the diagonal naphthalene incorporation into the
metal 'step'.
two 17' contacts to the other Ru atoms. Like RuS(p3-PPh)(p4-ChH4)(CO)l3 the ligand could be considered to be a model for arene binding at a step on a ( 1 . 1 . 1 ) metal surface, but unlike the ChH4 ligand, which lies parallel to the surface, the naphthalene ligand slots in at an approximately 45" angle at the step. This new type of arene-metal system is the first of this class of system. The only other examples are the tetraruthenium compound such as that in Scheme 10. We expect that other examples of this type of motif will be discovered before long. ) I ~have In the analogous reaction of PhlP(9-anthacy1)phosphinewith R u ~ ( C O we found a range of products, one of which is illustrated in Scheme 12. The anthracene group is still attached to phosphorus but is n-complexed through eight carbon atoms to a triangle of Ru atoms in a $, q 3 ,'71 manner. One should expect a more extended system to become available where a polycyclic arene lies parallel to a planar triangulated array of metal atoms. However, there is a mismatch of arene ring size and the size of metal-metal bonded triangles, that is a graphite-type layer does not match a (1. 1 . 1) type metal atom array well enough for this parallel arrangement of carbon and metal layers to be extended indefinitely. However, the structures that are appearing from this work illustrate that layers are being formed: a layer of metal atoms n-bonded to a layer of carbon atoms, a layer also containing metal atoms a-bonded to the carbon atoms, and then another layer of metal atoms n-bonded on the other face of the polycyclic aromatic. We predict that new forms of carbon-metal system will be developed based on this principle and that new features will emerge in this area of arene chemistry.
264
I Moleculur Clusters
Ru,(CO),, + Ph,P
fj
Scheme 12. One anthracene derivative from the reaction of R u ~ ( C Oand ) ~ ~diphenyl(9-anthracyl) phosphine.
References 111 E. 0. Fischer, W. Hafner, Z. Natur/iirsch., Ted B, 1955, 10, 665. [2] M. P. Gomez-Sal, B. F. G. Johnson, J. Lewis, P. R. Raithby, A. H. Wright, J. Chem. Soc., Cheni. Commun., 1985, 1682. [3] B. F. G . Johnson, J. Organornet. Ciiem., 1994, 475, 31. 141 M. A. Gallop, B. F. G. Johnson, J. Lewis, A. McCamley, R. N. Perutz, J. Clzeni. Soc., Chem. Commun., 1988, 1071; B. F. G. Johnson et al., Chern. Euu. J.. 1995, I , 252. [51 A. J. Deeming, M. Underhill, J. Organornet. Chem., 1972, 42, C60; J. Cliem. Soc., Dulton Trans., 1974, 1415. [6] I. Bach, K.-R Porschke, R. Goddard, C. Kopiske, C. Kriiger, A. Rufinska, K. Seevogel, Orgnnometallics, 1996, 15, 4959. (71 H. van der Heijden, A. G. Orpen, P. Pasman, J. Cliem. Soc., Chem. Cornniun., 1985, 1576. [8] G. Allegra, G. Tettainanti Casagrande, A. Immirzi, L. Porri, G. Vitulli, J. Am. Chem. Soc., 1970, 92. 289. 191 W. D. Harman, M. Gebhard, H. Taube, Inorg. Chem., 1990,29, 567. [lo] D. R. Neithamer, L. Parkinyi, J. F. Mitchell, P. T. Wolczanski, J. Am. Chem. Soc., 1988, 110. 4421. [ 1 I ] H. Omori, H. Suzuki, Y. Take, Y. Moro-oka, Organonzetallics, 1989, 8, 2270. [12] K. Jonas, J. Organomet.Chem., 1990, 400, 165. [ 131 K. Jonas, G. Koepe, L. Schiefcrstein, R. Mynott, C. Kriiger, Y.-H. Tsay, Angew. Chem.,Int. Ed. Eng., 1983, 22, 620. [I41 J. Miiller, P. E. Gaede, K. Qiao, J. Orgunomet. Clzem., 1994, 480, 213.
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1 Molecular Clusters
[47] E. Solari, F. Musso, E. Gallo, C. Floriani, N. Re, A. Chiesi-Villa, C. Rizzoli, Organometallics, 1995, 14, 2265. 1481 R. Uson, J. Fornies, M. Tomas, J. M. Casas, F. A. Cotton, L. R. Falvello, R. Llusar, Organonietallic.~, 1988, 7, 2279; R. Uson, J. Fornies, M. Tomas, J. M. Casas, R. Navarro, J. Chem. Soc., Dalton Trans., 1989, 169. 1491 R. Uson, J. Fornies, M. Tomis, J. M. Casas, F. A. Cotton, L. R. Falvello, X. Feng, J. Am. Chern. Soc., 1993, 115, 4145; I. Ara, J. M. Casas, J. Fornies, A. J. Rueda, Inorg. Chem., 1996, 35. 7345. [50] P. G. Edwards, R. W. Gellert, M. W. Marks, R. Bau, J. Am. Chem. Soc., 1982, 104, 2072. [51] X. He, K. Ruhlandt-Senge, P. P. Power, J. Am. Chem. Soc., 1994, 116, 6963. 1521 C. W. Bradford, R . S. Nyholm, G. J. Gainsford, J. Cuss, P. R. Ireland, R. Mason, J. Chem. Soc., Chem. Commun., 1972, 87. [53j C. W. Bradford, R. S. Nyholm, J. Cliem. Soc., Dalton Trans., 1973, 529.
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1.14 Multiplcj Interaction5 Betnwn Arrnrs and Metal Atonis
267
1791 M. A. Dam. T. Nijbacker. B. C . de Pater, F. J. J. de Kanter, 0. S. Akkerman, F. Bickelhaupt, W. J. J . Smeets, A. L. Spek. Or~janon~ettillic~s. 1997. 16, 51 I 1801 W. D. McGhee, T. Foo. F. G . Hollander, R. G . Bergman, J. An?. Chem. Soc., 1988, 110, 8543. [Sl] V. V. Grushin, A. B. Vymenits. A. I. Yanovsky. Y . T. Struchkov. M. E. Vol'pin. Or(qunoiT1Ofll//iC.S, 1991, 10, 48. [82j M. D. Rausch. R. G . Gastinger. S. A. Gardner, R. K. Brown, J. S. Wood, J. Am. Cl7m. Soc.. 1977. YY. 7870. 1831 Y. Fuchita, M. Akiyama. Inory. Chivi. Actii, 1991, 189, 129. [841 Y. Fuchita, M. Akiyama. Y. Arimoto, N. Matsumoto, H. Okawa, Inorg. Cllinz. act^. 1993, 20.5. 185. [SSl A. J. Deeming. M. Underhill, J. Cliiwi. Soc.. Drrlton Trans., 1974, 1415. [86l R. J. Goudsmit, B. F. G. Johnson, J. Lewis, P. R. Raithby, M. J. Rosales. J. Cliem. Sw.. I h l f o n Trans.. 1983, 2257. [87] K. A. Azam, A. J. Deeming. J. Cheni. Soc.,C\iern. Con7nzun., 1976. 852. 1881 K. A. Azam. A. J. Deeming. J. Mol. Cut., 1977, 3. 207. K. A. Azam, A. J. Deeming, I. P. Rothwell, J. Orgunonirr. Cheni.. 1979, 178. C20. [89l W.-Y. Yeh, H.-J. Kneuper, J. R. Shapley. Po/j'habon. 1988. 7, 961; H.-K. Kneuper. J. R. Shapley, N K ~J. , Chewi., 1988. 12. 479. 1901 E. Bonfantini. P. Vogel, J. C/ienz. soc.. C'hrvn. C'oniniun.. 1991, 1334. 1911 G. J. Gainsford. J. M. Guss. P. R. Ircland. R . Mason, C. W. Bradford. R. S. Nyholm, J. OU]a/iO/??ef. Chiwi.. 1972, 40, C70. 1921 P. A. Jackson, B. F. G. Johnson. J. Lewis, A. D. Massey. D. Brdga. C. Gradella, F. Grepioni. J. Or~~tinornrt. Clietfi.%1990. 391. 225. 193) A. J. Deeming, R. E. Kimber. M. Underhill. J. Cliim. Soc., Drilton Tran.~..1973. 2589; A. J. Deeming. R. S. Nyholm, M. Underhill. J. Clzeni. Sot,.. C/ienz. Co~zniun..1972, 224. [94] A. J. Deeming, J. E. Marshall. D. N L ~G., O'Brien. N. I . Powell. J. Orgcinonict. Climz., 1990. 384, 347. [951 A. J. Deeming, S. E. Kabir. N. 1. Powell, P. A. Bates, M. B. Hursthouse, J . C h i . Soc., Dulton Trans., 1987, 1529. [961 S. C. Brown. J. Evans. L. E. Smart, J. Cheiu. Soc., Chcwz. Cornniun., 1980, 1021. Arce. A. J. Deeming. J. Clicni. Soc,..Dalton T r i m , 1982, 1 155. Deeming. I. P. Rothwell, M. B. Hursthouse. J. D. J. Backer-Dirks, J . Clicwi. Soc.. Dullon Trimx.. 1981, 1879. 1991 W. R . Cullen. S. T. Chacon. M. I . Bruce. F. W. B. Einstein, R. H. Jones. Orgc~noniefallic..~. 1988. 7, 2273. I1001 W. R. Cullen. S. J. Rettig. T.-C. Zheng. 0rgLinonietcillic.s. 1992, 11. 928. [I011 M. I. Bruce, P. A. Humphrey, 0. bin Shawkataly, M. R. Snow, E. R. T. Tiekink. W. R. Cullen, Orgaiionie/iillic.s, 1990. 9- 2910; T.-C. Zheng, W. R. Cullen. S. J. Rettig, Oryrino~ ~ i e t ~ l l i1994. c s , 13. 3594. 1102) A. J. Deeming. N. I. Powell. A. J. Arce, Y. De Sanctis. J. Manzur, J. Climi. Soc.. Dtilton Traiis., 1991, 3381. [ 1031 H.-J. Kneuper. J. R. Shapley. Orynnornetallics, 1987, 6; 2455. [I041 W. R. Cullen. S. J. Rettig. H. Zhang. Inorg. Clzini. (l05l W. R. Cullen. S. J. Rettig, H. Zhang. Organonieto [I061 S. A. R. Knox, B. R. Lloyd, A. G. Orpen, .I. M . Vifias, M. Weber, J. Cliem. Soc,.. Clieni. C'onmun., 1987, 1498; S. A. R. Knox. B. R. Lloyd, D. A. V. Morton, S. M. Nicholls, A. G. Orpen, J. M. Vidas. M. Weber. G. K. Williams. J. 0r;qcinonzrt. Chein.. 1990, 3Y4, 385. [I071 J. P. H. Charmant, H. A. A. Dickson. N. J. Grist. J. B. Keister. S. A. R. Knox, D. A. V. Morton, A. G. Orpen. J. M. Viiias, J. Clicwi. Soc., Chern. Cornniiin., 1991, 1393. [I081 W. R. Cullen. S. J. Rettig. T. C. Zheng. 0rymnonzetLillie.s. 1995. 14, 1466. 11091 S. A. R. Knox and G . J. McCormick, unpublished work.
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1101 A. J. Deeming, S. N. Jayasuriya, A. J. Arce, Y. De Sanctis, Organometallics, 1996, 15, 786. I 1 I ] W. R. Cullen, S. J. Rettig, H. Zhang, Organometallics, 1991, 10, 2965. 1121 W. R. Cullen, S. J. Rettig, H. Zhang, Organometallics, 1992, 11, 1000. 1131 S. Sun, C. A. Dullaghan, D. A. Sweigart, J. Chem. Soc., Dalton Trans., 1996, 4493. 1141 H. Nagashima, T. Fukahari, K. Aoki, K. Itoh, J. Am. Chem. Soc., 1993, 115, 10430. 115) A. J. Deeming, C. M. Martin, Chem. Commun., 1996, 53. 1161 A. J. Deeming, C. M. Martin, Angew. Clzem. Int. Ed. Eng., 1998, 37, 1691.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.15 From C-H-Activation to Arene Clusters and Back - Organometallic Cluster Chemistry with Cyclopentadienyl Cobalt Fragments and Olefins Hubert Wudepohl and Alexunder Met2
1.15.1 Introduction Activation of C-H bonds by metal complexes one of the “holy grails” of chemistry”] is generally thought to be the realm of the heavier transition metals. This statement is certainly true for cleavage reactions of non-activated C -H bonds on a single metal center (e.~]. in a mononuclear metal complex). Here, the thermodynamics of the C-H activation reactions are governed by the relative magnitudes of the C-H bond energy (typically 400-460 kJ/mol) and the strength of the metalcarbon and metal-hydrogen bonds in the activation product. Experimental and theoretical evidence consistently indicates a general trend of a decrease of M-C and M- H bond dissociation enthalpies as one ascends a group in the periodic table.[’’ For example. the calculated M-CH7 bond dissociation enthalpy in [(C0)4M-CH3] ] is believed to is 214 kJ/mol for M = Ir but only 160 kJ/mol for M = C O . [ ~This be the key factor why the great success of complexes like ((CSMej)M(PMe3)(H)?] ( M = Rh, Ir)[4.5.h1 and [ ( C ~ M e ~ ) I r ( C O ) ~which ] [ ’ ] , are among the classics of C-H activation reagents, has not been paralleled by any cobalt complex of similar composition. However, under certain conditions C-H activation can also be facile and favorable with first-row transition metals. To illustrate this point, just two examples are given here. In 1952, Miller, Tebboth and Tremaine reported the formation of a very stable volatile complex. when cyclopentadiene was passed over reduced iron at 300 0C.[81The product, dicyclopentadienyliron (later called ferrocene) obviously is a C H activation product of cyclopentadiene ( Eq. 1 \. Much later, Timms was able to show that the same reaction could also be brought about at a temperature as low There are many examples of similar reactions in the literature. They as - 196 0C.191 -
-
270
I Molecular Clusters
all profit from the formation of a very stable product (ferrocene in our examples), from the lability of the C-H bond to be activated and/or from the high reactivity of the metal reagent (atomic iron vapor in Timms' reaction). Another variation on the same theme are hydrocarbon reactions on metal surfaces, which are of course of great importance for heterogeneous catalysis. Due to the high reactivity of 'naked' transition metal surfaces a large percentage of their reactions with hydrocarbons are characterized by C-H activation at some stage, usually leading to dehydrogenation of the substrate.["] In this paper, we shall mainly be concerned with oligonuclear metal complexes containing the cyclopentadienyl cobalt fragment (C5H5)Co and its ring substituted derivatives. Being a 14 valence electron (VE) species, (C5H5)Co almost certainly does not exist as a free molecule.[' It is however readily available from a number ~ ] . of the versaof precursors, mainly 18 VE species of the type [ ( C ~ H S ) C O LMost tility of (C5H5)Co comes from its high 'internal' stability, i.e. the low tendency to loose the C5H5 ligand, thus providing a metal site with fairly well defined properties. There are many examples in the literature which illustrate this point. A particularly enlightening study was carried out by Bonnemann et al., who examined the co-cyclooligomerization of alkynes and nitriles to give pyridine derivatives using a In the last 15 years or so, cyclolarge number of [ ( C ~ R ~ ) C Ocatalysts.['21 L~] pentadienyl cobalt chemistry has greatly benefited from the work of Jonas and coworkers, who, with the now famous Jonas reagent [ ( C ~ H ~ ) C O ( C l~aHand ~ ) the ~] made (CsH5)Coreagents even more reactive species [ ( C ~ H ~ ) C O ( C 2, ~M ~ ~ available )] of hitherto unprecedented reactivity." 31 Not unexpectedly from what has been discussed above, Iubile C-H bonds are indeed readily activated by 1. Such bonds may be found in the 'allylic' methylene groups of certain transition metal coordinated unsaturated hydrocarbons, e.9. q6cycloheptatriene or q4-cyclopentadiene. Starting from there, novel dinuclear complexes with cycloheptatrienyl (C7H7) and cyclopentadienylidene (C5H4) bridges were s y n t h e s i ~ e d . ~ 'In ~ ,the ' ~ ~latter case, both C-H bonds of the methylene group of cyclopentadiene are cleaved under surprisingly mild conditions, resulting in a very unusual transformation of the saturated hydrocarbon center to give a carbene
-,
3= I e
\...' CO I
la, R
R, lc, R lb,
co
= = =
H H,Me Me
2
1.15 Froni C-H-Actiziution to Arene Clusters
271
carbon. These reactions and related chemistry have been reviewed[l6] and will not be discussed here any further. Having established the C-H activating power of complexes like 1 we were looking for ways to put this to use with simple non-uctioated C-H bonds, for example those of an olefin. Cyclopentadienyl cobalt is one of the family of 'conical' metal ligand fragments. These fragments, namely (CO)3M and (C,H,?)M (12 = 3 - 8), are important building blocks of many organometallic complexes. An intriguing difference between the two types of conical fragments is the higher effectiveness of the latter to bond with another metal. Although this fact had been deduced from a theoretical study more than 20 years ago," 'I it was not widely recognized until quite recently. Our reasoning was that in a C-H activation product, additional thermodynamic stability might be gained from aggregation of the metal fragments to form a metal cluster complex. Since any gain in energy from metal-metal bonding is expected to be at maximum with (C,,H,,)Mfragments, (CjHj)Co should not be a bad choice. As it turned out, C-H activation accompanied by cluster formation is indeed of paramount importance in the chemistry of 1, and this will be the main topic of the present account. The following discussion will focus on the reactions of some monoolefins with sources of (C5Rs)Cofragments. Only reactions that eventually lead to cluster complexes will be discussed. While concentrating on our own work. including a number of yet unpublished results, we have tried to cover the contributions of others as well. Due to space restrictions, many details could not be dealt with here in depth. In many places, the reader has to be referred to the original literature.
1.15.2 Ethylene: cluster complexes with ,u3-ethylidyne ligands The major part of the chemistry of the complexes [ ( C S R ~ ) C ~ ( C (~RHj = ~ )H~j ]la, H4Me lb, Me5 Ic) is characterized by substitution of one or both ethylene molecules by some other ligand, which may then undergo further chemical transformations. However, the ethylene ligands are not always merely innocent bystanders, just completing the coordination sphere of the cobalt and waiting to be displaced. In a study of the properties of [(CjHj)Co(C2H4)2]l a as a hydrogenation catalyst we noticed a degradation of the catalyst when the reactions were carried out with dihydrogen at 0-20 "C and atmospheric pressure. Some of the ethylene originally present in l a is converted into ethylidyne CCH3 (Eq. 2), which is of course not a stable species by itself but serves as a ligand in a series of newly formed cluster complexes.['*' In the main product [HI ( C S H ~ ) C O ) ~ ( C C H4a, ) ](ca. 50'% yield based on Co) all the components of one former ethylene ligand are still
272
1 Molecular Clusters
H
H
/
\
H/c=c
*
\
H
retained, in the form of a p3-CCH3 and a p3-H ligand. Two molecules of ethylene have been incorporated in the second major product (ca. 20%)yield), [ { ( C ~ H ~ ) C O } ~ (CCH3)2]3a, with formal loss of two hydrogen atoms. Other tetra- and pentanuclear products were identified but could not be isolated."" When our work was in progress, complex 4a was also reported to be the thermal (70 "C) decomposition product of la.[''] The presence of dihydrogen (along with ethylene) in the gas phase was noted at the end of the reaction. This would be consistent with the formation of 3a, which was not mentioned in the publication."" We therefore undertook a reinvestigation of this work. In the absence of dihydrogen, solutions of pure l a are stable for extended periods at ambient temperature. Decomposition starts at around 50-60 "C and leads to a quite similar distribution of tri-, tetra- and pentanuclear cluster complexes as was found in our low temperature reactions with H2. The complexes 3 and 4 have symmetrical structures with cluster cores based on the trigonal bipyramid (Figs. 1, 2). The metal coordinated carbon atoms of the p3-ethylidyne groups are in axial sites on the CojC2 and Co4C clusters, fairly symmetrically disposed over Co3 planes (mean dcoc = 1.85 A). Although not directly located in several X-ray structure analyses,[' 8,191 the hydrido ligand in 4a can safely be assumed to occupy a Coj face capping position."'] In solution, hindered migration of the p3-hydride across the three vacant Co3 faces of the metal cluster takes place. The nuclearity of the products obtained from ring substituted derivatives of 1 CH3
I
CH,
I
I CH3
3a, R = H 3b, R, = H,Me 3c, R = Me
R5
4a, R = H 4b, R, = H,Me
1.15 From C-H-Actitation to Arene Clusters
273
Figure 1. Molecular structure of ~ ( I C F H F ) C O } ; \ ~ I ~ - C3a. CH;)~]
appears to be governed by the steric requirements of the C5R5 ligands. Pyrolysis (1 10 “C) of [(C5Me5)Co(C2H4)2] l c had been reported earlier to give a trinuclear complex. In the original work, the bis(p3-ethylidyne) structure [ { (C5Me5)Co}3(CCH3)2] 3c was assigned to this product on the basis of a crystal structure analysis.[*”] Fairly recently it was shown actually to be the paramagnetic mono(p3-ethylidyne) hydrido cluster complex [H{(C5Me5)Co},(CCH,)] S C . [ ~ ”Crystals of Sc and genuine 3c (Fig. 3) are isomorphous. The molecules of the former (Fig. 4) are disordered with respect to a pseudo-mirror in the Co3 plane, which relates two half p3-CCH3 and two half p3-H ligands.[2’1With Hz, this complex is transformed into the diamagnetic trihydrido complex [( H ) 3{(CsMeS)Co)3(CCH3)]~ C . [ ’ ~ I A confusing product spectrum was found in reactions with the methylcyclopentadienyl complex 1 b. The ‘thermal decomposition’ (50-60 “C) in solution mainly
Figure 2. Molecular structure of [H{ (CjH5)Co)4 ( p - C C H?) 4a. ] The hydrido ligand was not localised in the X-ray structure analyses.
274
1 Molecular Clusters
I
5b, R5 = H,Me 5c, R = Me
6b, R, = H,Me 6c, R = Me
1.15 From C-H-Actitlation to Arene Clusters
275
gave the tetranuclear cluster [H{( C ~ H ~ M ~ ) C O ) ~ ( C4b, CH ~ ) ] with variable along amounts (up to 30%) of the trinuclear trihydrido cluster complex [(H)3C Hwas , ) ~ ]detected.[231 {(C5H4Me)Co}3(CCH3)16b. No [ { ( C ~ H ~ M ~ ) C O } ~ ( C 3b This complex was formed in significant amounts when solutions of 1 b were stirred under 1 bar of H?, 4b still being the main product. When the reaction was carried out under hydrogen pressure (6 bar), the main product was found to be 6b (up to 30'%) along with 4b (up to 20%) and traces of 3b (less than 1%). When heated to 95 "C, 6b slowly lost hydrogen to form 5b (75'%1conversion after 8 days). From these observations it appears that of the two trinuclear hydrido cluster complexes the diamagnetic 6b is the primary thermal product from l b even in the absence of dihydrogen, and is only converted to the paramagnetic 5b at a higher temperature. A similar sequence of reaction steps may well be also involved in the formation of the pentamethylcyclopentadiene derivative 5c. Under the conditions required to generate it from lc, the trihydrido complex 6c is not stable and looses dihydrogen to give the isolated product 5c. Apart from the reactions discussed above, transformation of ethylene into ethylidyne ligands is rarely observed in molecular organometallic chemistry. It is however a well known process in the metal surface chemistry of ethylene. Chemisorbed ethylidyne species were identified as the dominant products of the reactions of several transition metal surfaces with ethylene at room t e m ~ e r a t u r e . ' This ~ ~ ] ligand is mostly in a threefold 'hollow' surface site, corresponding to the p3-coordination geometry, but surface bound p4-ethylidyne is also known. Interestingly, the structure of a surface ethylidyne was firmly established for the first time by the analogy of its vibrational spectrum to that of the molecular cluster complex [I(CO)IC~J~(~~~-CCH~)J.[~~] Practically nothing is known about how the different products 3-6 are formed. It is not even clear if the breaking of the first C-H bond involves a mono- or an oligonuclear intermediate. The observation that the more complicated trihydrido ethylidyne cluster complex 6b appears to be formed first and then transformed into the monohydride 5b illustrates that the actual course of reaction is quite complex. To account for the two additional hydrido ligands in 6, more than one molecule of ethylene (or dihydrogen) must be involved, although the composition of the (thermodynamically less stable) final product 5 only matches the ratio of = 3: 1. (C~RS)CO C2H4 : On the other hand, neither 5 nor 6 appear to be intermediates on the way to the tetranuclear complex 4. In a number of experiments carried out under different reaction conditions neither 5b nor 6b reacted with l a to give the mixed-ligand species [H { (C5H4Me)Col3C(GH5)Co)(CCH3)1."31 With a valence electron ( V E ) count of 46, the complexes 5 are two electrons short of the magic number required by the Wade-Mingos rules for an electronically saturated M3C cluster complex. It is therefore not surprising that these cluster complexes readily react with small inorganic and organic molecules to give elec-
276
I Moleculm Clusters
HCECH
1
I
1
CH3
CH3
CH3
C=NCMe,
/
5c
\
7c N=N=CHR
Scheme 1
tronically saturated 48VE species. The reactions of 5c were studied in some depth by Casey et a1.[261A summary is given in Scheme 1. Analogous products were obtained with the methylcyclopentadienyl cobalt cluster complex 5b and some of the reagents in Scheme 1.r231 The reactions with acetylene are particularly remarkable: formation of the bisethylidyne tricobalt cluster complexes 3 again proceeds with a series of C-H bond breaking and forming processes. Reaction of 5b with CO at room temperature under pressure (6 bar) mainly gave the ‘addition product’ [H{(C5H4Me)Co}3(CO)(CCH3)]7b (SO%,), with a face capping carbonyl and ethylidyne and an edge bridging hydrido ligand (Scheme 2), in close analogy to the pentamethylcyclopentadiene derivative 7c (cf:
I . 15 From C-H-Actitwtion to Arene Clusters
277
Scheme 2
Scheme 1 ) . In addition, a second product [ {(C5H4Me)Co}?(CCH,)ICO)]8b was isolated in low yield.r231 The crystal structures of 7c and 8b have been determined. The crystallographic site symmetry of 3 / m for the molecules of 7c necessarily implies disorder, which was resolved satisfactorily by a mirror-disordered model. No information about the location of the hydrido ligand in the solid could be In solution, symmetry breaking pz-H ligands were detected at low temperature by H-NMR spectroscopy both in 7b and 7c. At higher temperatures, fluxional behavior is observed.r23,261 The spatial arrangement of the two p3-ligands in the hydride-free paramagnetic 47 VE cluster complex 8b is quite interesting (Fig. 5 ) . The p3-ethylidyne caps the Co3 triangle in a symmetric fashion (&c = 1.84. . . 1.86A). The carbonyl ligand is much less symmetrically disposed over the other face of the Co? framework (&c = 1.89. . ’2.04A).[271
’
Figure 5. Moleculnr structure of [ ((C5HqMe)Col;,p;-CCH; ‘m-CO)]8b.
278
1 Moleculur Clusters
1.15.3 Substituted styrenes: cluster complexes with face-capping arene ligands The olefin-to-alkylidyne C-H activation chemistry described in the preceding section is quite uniquc to ethylene. Other terminal olefins do not exhibit a similar reactivity. No reaction takes place at all between l a and iso-butene for example, and the ethylene complex can be recovered unchanged. On the other hand, solutions of l a turn dark red when treated with excess styrene. Ethylene is evolved, indicating ligand exchange. After heating, a complex mixture of products is obtained. Assuming a similar transformation of the phenyl substituted olefin as the one observed for ethylene, cluster complexes with phenylethylidyne ligands would be anticipated. Ph
Ph
I
I
CH2
I
I r\
I
CH2
Ph 9
10
H
I
:,
1
"'r
CH, Ph
co
I
co I
11
12
1.15 From C-H-Activation to Arene Clusters
279
13
However, [ { ( C ~ H S ) C O ) I ( C C H9~ and P ~ )[H{(C,H,)Co}d(CCH*Ph)] ~] 10 were only formed in trace quantities. Two of the mononuclear products could be isolated, H)(Me)(Ph)}] 12, namely [(C5H5)Co(q4-C5Hs)]11 and ((C5H5)Co{q4-exo-CsH5C( which together accounted for about 20% of the cobalt. The formation of the latter complex can be explained by a regioselective addition of the exo-C-H bond of the methylene group of 11 across the vinylic carbon-carbon double bond of styrene. It is not clear how 11 is formed; this may involve cobaltocene as an intermediate. Entirely different products are formed from l a and methyl substituted styrenes. With a- or p-methylstyrene the trinuclear cluster complexes 13 with face-capping arene ligands were isolated in high yields.[281A large number of similar complexes has subsequently been prepared from either of la, l b or 2 and substituted styrenes, allylbenzenes and related phenyl-containing olefins (Table 1 ). Space restrictions do not allow us to discuss the many interesting features of this unique class of cluster complexes in any detail here. Some aspects of the syntheses, structure and dynamic behavior have been reviewed.[29*30,311 Only a brief summary of the salient properties is given here. Invariably, the PI-q' : q2 : q2 (facial) coordination mode of the arene ring to the COI cluster is found in the crystalline state ( C , and Co? rings are in a staggered arrangement). Two examples for the typical molecular structures are given in Figs. 6 and 7. The detailed analysis of a considerable number of crystal structures gave a consistent picture of this novel type of b ~ n d i n g . [ ~ First l . ~ ~ of ] all, the facial arene rings are substantially expanded (dcc 1.44A). both with respect to free benzene (dcc = 1.395A) and to arenes in an apical (q6 to one metal) coordination (dcc 1.41 A). Only a relatively small (but statistically significant) alternation of the lengths of the endocyclic carbon-carbon bonds was found (Adcc about 0.03 A,the shorter bonds being the ones 'on top' of the cobalt atoms). In solution, hindered mutual rotation of the CC,and Co3 rings was observed. This process was investigated in detail with a series of derivative^.[^^^"^ It was shown to involve 60" turns (1,2-shifts of the metals around the arene ring) as the elementary steps. The activation parameters were found, both experimentally and theoretically
280
I Moleculur Clusters
Table 1. Complexes of the type [{(CsRr)Co)i(p,-q2: q2 : q'-arene)] [arene = R3(ChH4){(E)-C(R') = C(H)R2}]13. complex
R'
R2
R?
[ { (C5Hr)Co)?(a-methylstyrene)] [ ( (C5H5)Co}i(P-methylstyrene)]
Me H H H H H H Me Me H H H H H H H Ph H H H H H H Ph H Me H Ph H
H Me H H H H H H H Me Me Me Me Me Me Et H Ph 2-tolyl 4-anisyl Ph Ph Ph Me Me Me Me H Ph
H H 4-Me 4-Et 4-OMe 3-F 4-F 3-F 3-Ph 2-Me 3-Me 4-Me 3-F 4-F 3,5-Mez H H H H H 4-OMe 4-styryl ChMe4-4-CH=CHPh H 4-OMe H H H H
[{(C~Hs)Coj~,p-methyIstyrene)] [{(CrHs)Co)3(p-ethylstyrene)] [ {(C5Hs)Co}?(p-methoxystyrene)] [ {(C,H5)Co}i(m-fluorostyrene)] [ { ( C S H ~ ) Cp-fluorostyrene)] ~)~( [ { (C,H5)Co),(p-fluoro-a-methylstyrene)] [ { (CsH5)Co),( p-phenyl-x-methylstyrene)] [ { (CsH5)Co)3 (o-methyl-[I-methylstyrene)] [ { (C5H5)Co)3 (m-methyl-P-methylstyrene)] [ { ( C ~ H ~ ) C Op-methyl-P-methylstyrene) ),( I [ { (C5H5)Co)3 (m-fluoro-P-methylstyrene)] [ {(C5H5)Co)?fp-fluoro-p-methylstyrene)] [ { (C5Hs)Co) (3,5-dimethyl-P-methylstyrene)] [ { (C5H5)Co)3 (P-ethylstyrene)l [ { ( C ~ H ~ ) C1,l-diphenylethene)] O)~( [ { (CsH,)Co}i(stilbene)] [ { (C~H5)Co)~(o-methylstilbene)] [{(C~H~)Co}~(p-methoxystilbene)] (isomer I) [{(C~H~)Co}~(p-methoxystilbene)] (isomer 11) [ { I C ~ H S ) C Op-distyrylbenzene)] }I( [ ((C~H~)Co)~(p-distyryldurene)] [ { (C5H5)CoJ?( I,l-diphenylpropene)] [{(GH5)Co)3 { 1-( p-dnlsyl)propene) 1 [ { (C-jH5)Co}3(2-phenyl-2-butene)] [ { (CSH4Me)Co)3 (P-methylstyrene)] [ { (CsH4Me)Co}3(I,l-diphenylethene)] [ { (CsH4Me)Co)3 (stilbene)]
( MO[331and molecular mechanics calculations[341)to depend on the steric bulk of the arene substituents. There is ample chemical evidence that the 1-alkenyl substituent on the arene plays a crucial role during the assembly of the metal cluster.[351For example, no arene cluster complexes are formed with alkyl substituted benzene derivatives. Such complexes, 14, are perfectly stable and could be obtained indirectly through hydrogenation of the corresponding p3-alkenylbenzene precursors (Table 2 ) . [ 2 8 3 3 Following a standard procedure, the synthesis of many derivatives 13 usually proceeds quite cleanly. There appeared to be no problems with a number of ringsubstituted styrene derivatives, like p-methyl- and p-ethylstyrene, despite the trouble with styrene itself described above. For example, under our standard conditions, the
1.15 From C-H-Actiwtion to Arene C h t e r J
Figure 6. Molecular structure of [((CjHc)Co)3i/(3-q2 : 7' type 13).
: ri?-~-distyryldurenejl (a
28 1
complex of
Figure 7. Molecular structure of [ ( ( C , H I M e ~ C o ) - I ( ~ ( ~q'- 7 ' q2-l,2-diphenylethanc)] f a complex of type 14)
14
282
I Molecular Clusters
Table 2. Complexes of the type [{(CsRs)Co}3(,u,-v2: v 2: y2-arene)][arene = R3(C~H4){C(H)R'CH2R2}]14. complex
R'
R2
R3
[ {(C~H~)Co}~(iso-propylbenzene)] [ { (C5Hs)Co)3 (n-propylbenzene)] [ { ( C ~ H ~ ) C I, O1-diphenylethane)] }~( [ { (CsH5)Co)3 ( 1,2-diphenylethane)] [ { (CsHs)Co}3(p-diethylbenzene)] [ { (CsHdMe)Co}3(n-propylbenzene)]
Me H Ph H H H Ph H
H Me H Ph H Me H Ph
H H H H 4-Et H H H
[{(CSHdMe)Co}l(1,I-diphenylethane)] [ { (CsHdMe)Co}3(1,2-diphenylethane)]
p3-arene cluster complex [ { (C5H5)Co}3(p3-p-methylstyrene)]was obtained in good yield from p-methylstyrene and However, when just a marginal modification of the reaction conditions was employed, a quite different trinuclear complex, 15a, was formed as the main product, along with only minor quantities of the expected p3-p-methylstyrene cluster complex.[371 Complex 15a is the product of the double C-H activation of two vicinal C-H bonds of the vinyl group (Eq. 3). The cleavedoff hydrogens still remain in the molecule as two hydrido ligands which bridge an O ] ~ (Fig. 8). The so-formed p-tolylacetylene edge and a face of the [ ( C ~ H ~ ) Ccluster ligand bridges the Co3 framework in a p3-q' : q2 : q' fashion. A similar reactivity is observed with ortho-, meta- and para-fluorostyrene, to give the p3-(fluorophenyl)acetylene complexes 15b-d, again as a mixture with the corresponding p3-arene
15a, R 15b, R 15~R , 15d, R
= = = =
pCH, 0-F m-F p-F
1.15 From C-H-Activation to Arene Clusters
283
Figure 8. Molecular structure of [ ( €1)2((C~H~)Co};(p;-q' : q z : q'-p-tolylacetylene)]15a
R
\
R
/
t
\
H
R-C=C-R
+
HD
(3)
cluster complexes. The pi-alkyne complexes 15 cannot be converted into their p3arene isomers 13, and vice versu. It is quite obvious that these two types of trinuclear cluster complexes represent the end products of two distinctly different reaction pathways. The metamorphosis of an olefin to give a pi-alkyne and two hydrides (Eq. 3) is by no means unique to cyclopentadienyl cobalt cluster chemistry. Many olefins, including ethylene, are known to undergo this transformation upon reaction with carbonyl cluster complexes of the heavier metals especially of the iron group. Two types of products, p3-alkyne (16) and/or pi-alkenylidene metal cluster complexes (17) have been found.[381In these reactions, a series of C-H bond breaking reactions takes place on a preformed metal cluster, which is retained during the course of reaction. Some of the intermediates could even be isolated.[381In contrast, in the reactions with la, assembly of the metal cluster and C-H activation are closely associated. No intermediate products have been isolated or were observed by spectroscopy, probably due to an inherent instability. Furthermore, formation of tricobalt clusters with alkenylidene ligands (cJ 17) has never been observed. This is important for the behavior of geminal disubstituted ethylenes, which cannot undergo reaction according to Eq. (3). These substrates usually react with 1 or 2 to give the p3-arene derivatives 13, if one of the substituents is an aryl. Only the highly reactive (CsH5)Co reagent [(C5H5)2Co]/potassium was able to C -H activate an CIsubstituted styrene derivative. The product 18, which was formed in low yield from x-methylstyrene, did however not contain an alkenylidene ligand. Instead, C-H
284
I Molecular Clusters
H R
R'
R'
L/
&
MEl?M M
16
co
17
18
activation of an olefinic and of an aromatic C-H bond had led to the formation of a cobaltaindenyl ring system, which in the final product 18 is stabilized by complexation with another (C5H5)Cofragment (Fig. 9).[391
Figure 9. Molecular structure of [(CSHS)CO { 1 -3,8,9-~-( 1-CsH5-3-CH3-1cobaltaindenyl)}] 18.
1.15.4 Cycloalkenes: cluster complexes with bridging cycloalkyne ligands Styrene derivatives can be considered to be activated olefins. It was of course a challenge to also try and C-H activate simple, less reactive non-functionalized alkenes.
1.15 From C-H-Activution to Arene Clusters
285
Cycloolefins were chosen as substrates to avoid any possible complications generated by double bond shift reactions. When l a was used as the source of (C5H5)Co fragments, C-H activation of the cycloolefin competed with C-H activation of ethylene, the latter leading to the formation of products like [ { ( C ~ H ~ ) C O ) , ( C C H , ) ~ ] 3a and [ H ( ( C S H ~ ) C O } ~ ( C C4a H , (see ) ] above). By using a large excess of the cycloolefin and by judicious choice of the reaction conditions, the reaction with the cycloalkenes could be made dominant. As the C-H activation products of cyclopentene, cycloheptene and cyclooctene, the trinuclear cluster complexes 19a and 19c,d were isolated in moderate y i e l d ~ . [ ~ ' An . ~ ~analogous ] complex, 20, was obtained from cyclopentene and lb.[231 Surprisingly, in several experiments carried out under a variety of conditions, formation of the p-cyclohexyne derivative 19b from cyclohexene and l a was never observed. This complex was prepared from cyclohexene with [(C5H5)2Co]/potassium. Reactions with this reagent usually do not proceed cleanly and generate a large amount of insoluble by-products. Nevertheless, separation of the products is usually less laborious, due to the absence of 3 and 4. As a second type of product, the tetranuclear p4-alkyne cluster complexes 21 were formed in very low yield from the corresponding cycloalkenes and [ ( C ~ H ~ ) ~ C O ] / I ( . [ ~ ~ ' ~ ~ ~ The crystal structures of several of the tri- and tetranuclear cluster complexes 1921 were determined.r411Two typical examples are shown in Figs. 10 and 11. The molecular Co3fpl-alkyne) cores of 19 and 20 are quite similar to that of 15a. The complexes 21 have a butterfly-shaped framework of four metal atoms, which is symmetrically bridged by the cycloalkyne. Due to their multiple metal coordination, the carbon-carbon bonds are comparatively long, about 1.40A in the trinuclear and 1.46. . . 1.50 A in the tetranuclear complexes. There appears to be no sig-
19a, 19b, 19c, 19d, 20,
n = 3, n = 4, n = 5, n = 6, n = 3,
R = H R = H R = H R = H R = Me
21a, n = 4 21b, n = 5 21c. n = 6
286
1 Molecular Clusters
nificant strain in the larger cycloalkyne rings ((26 through C8). This can be derived from a comparison of the endocyclic angles through the alkyne units. In complex 20 with the substantially strained cyclopentyne ring, these angles are considerably smaller (around 110') than those in 19 (about 124"), 21 (127") and the acyclic derivative 15a (127"). The fact that the cyclopentyne complexes are still readily formed nicely illustrates the gain in energy, which is associated with the formation of the metal cluster. The tetranuclear alkyne clusters 21 have a rigid pseudo-octahedral CoqC2 core. In contrast, fluxional behavior is displayed by the trinuclear complexes 15, 19, and 21. In these complexes, the p3-alkyne undergoes a 'windscreen-wiper' type walk on the Co3 frame. This is accompanied by a movement of the pz-hydrido ligand
Figure 11. Molecular structure of [((C5H5)Co}4(p4-q1 : q2 : q2 : q'-cyclohexyne)] 21a.
1.15 From C-H-Actication to Arene Clusters
287
around the metal triangle. Another dynamic process, exchange of the hydrides between the p2 and p? sites, appears to be independent of the alkyne movement.[411
1.15.5 Concluding remarks The studies discussed in this chapter clearly show that even relatively unreactive C-H bonds can be activated with (CsR5)Co reagents. These reactions appear to be driven by the formation of the stable cobalt clusters. Most of the differences in reactivity between plain (C5Hf)Coand its pentamethyl derivative can be explained by the much stronger bonding of the ethylene ligands in the starting material lc. and by obvious steric problems which would occur in the final products. It must be kept in mind, however, that aggregation to give oligonuclear species is only one of several reaction paths for the reactive (C5R5)Co fragments. There is of course an extensive mononuclear chemistry, mostly involving 18 VE complexes of the type [(C~R5)Co(diene)]. In fact. this type of complex is strongly preferred with any ligand, which can offer a (preferably conjugated) diene system. For example, phenyl substituted dienes and trienes do not give ,u?-arene clusters when treated with ~ ~ ) ] metal coordination occurs [ ( C ~ R ~ ) C O ( C 1~ or H ~[ )( C ~ ]~ H S ) C O ( C ~2.MInstead, at a diene unit.["' In some relatively rare cases even the diene system made up from a vinyl group and part of an arene is picked by the cobalt moiety, and the reaction stops there.[391Hence, such situations must be carefully avoided when looking for C H activation. In the C-H activation reactions discussed above, transformation of the hydrocarbons (olefins in the present studies) and aggregation of the metal ligand fragments to eventually form organometallic clusters are closely coupled. Although some reactions are quite specific, control of selectivity is still a major problem. For example, while it has not been possible to force ~icinuldouble C-H activation with ethylene, substrates like styrene do not usually undergo the geminul double C-H activation, which is characteristic for ethylene. In future work, this is the main issue to be addressed. and a deeper mechanistic understanding must be sought.
Acknowledgements Our work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. H. W. gratefully acknowledges the award of a Heisenberg Fellowship.
288
1 Molecular Clusters
References [ I ] B. A. Arndtsen, R. G. Bergman, T. A. Mobley, T. H. Peterson, Acc. Clzem. Res. 1995, 28, 154. 121 P. 0. Stoutland, R. G. Bergman, S. P. Nolan, C. D. Hoff, Polyhedron 1988, 7, 1429, lit. cit.. See also: N. Koga, K. Morokuma, J. Am. Chem. Soc. 1993, 115, 6883; D. M. Musaev, K. Morokuma, J. Am. Chem. Soc. 1995, 117, 799. [3] T. Ziegler, V. Tschinke, A. Becke, J. Am. Chenz. Soc. 1987, 109, 1351. [4] (a) A. H. Janowicz, R. G. Bergman, J. Am. Chem. Soc. 1982, 104, 352; (b) A. H. Janowicz, R. G. Bergman, J. Am. Chem. Soc. 1983, 105, 3929. [5] (a) W. D. Jones, F. J. Feher, J. Am. Chem. Soc. 1984, 106, 1650; (b) R. A. Periana, R. G. Bergman, Oryunornetullics 1984, 3, 508. 161 W. D. Jones, F. J. Feher, Acc. Chem. RPS.1989, 22, 91, lit. cit. 171 J. K. Hoyano, A. D. McMaster, W. A. G. Graham, J. Am. Chem. Soc. 1983, 105, 7190. 181 S. A. Miller, J. A. Tebboth, J. F. Tremaine, J. Chem. Soc. I1952, 632. [9] P. L. Timms, J. Chem. Soc., Chem. Commun. 1969, 1033. [lo] see e.y.: E. L. Muetterties, Pure Appl. Chem. 1982,54, 83; G. A. Somorjai, Phil. Truns. R. Soc. Lond. A 1986, 318, 81; W. F. Maier, Anyew. Chem. 1989, 101, 135. [11] Lightly solvent stabilized species [(C~H~)Co(solv)] have been postulated: C. E. Barnes, J. A . Orvis, Oryunometullics 1993, 12, 1016. 1121 H. Bonnemann, Anyew. Chem. 1985, 97, 264. 1131 K. Jonas, C. Kruger, Anyew. Cheni. 1980, Y2, 513; K. Jonas, Pure Appl. Chem. 1984, 56, 63; Anyew. Chem. 1985, 97, 292; Pure Appl. Chem. 1990, 62, 1169. 1141 H. Wadepohl, W. Galm, H. Pritzkow, Anyew. Chem. 1989, 101, 357; Orgunometullics 1996; 15, 570. [15] H. Wadepohl, H. Pritzkow, Anyew. Chem. 1987, 99, 132; H. Wadepohl, W. Galm, H. Pritzkow, A. Wolf, Chein. Eur. J. 1996, 2, 1453. [16] H. Wadepohl, Comments Znory. Chein. 1994, 15, 369. [17] M. Elian, M. M. L. Chen, D. M. P. Mingos, R. Hoffmann, Inorg. Chem. 1976, 15, 1148. [18] H. Wadepohl, H. Pritzkow, Polyhedron 1989, 8, 1939. [19] S. Gambarotta, C. Floriani, A. Chiesi-Villa, C. Guastini, J. Orgunornet. Chem. 1985,296, C6; S. Stella, C. Floriani, A. Chiesi-Villa, C. Guastini, N e w J. Chenz. 1988, 12, 621. [20] R. B. A. Pdrdy, G. W. Smith, M. E. Vickers, J. Orgunornet. Chem. 1983, 252, 341. [21] C. P. Casey, R. A. Widenhoefer, S. L. Hallenbeck, R. K. Hayashi, D. R. Powell, G. W. Smith, Orgunoinetullics 1994, 13, 1521. [22] (a) C. P. Casey, R. A. Widenhoefer, S. L. Hallenbeck, Orgunometallics 1993, 12, 3788; (b) C. P. Casey, S. L. Hallenbeck, R. A. Widenhoefer, J. Am. Chem. Soc. 1995, 117. 4607. [231 H. Wadepohl, A. Metz, unpublished results. 1241 F. P. Netzer, M. G. Ramsey, Crit. Rev. SolidStute Muter. Sci. 1992, 17, 397, cit. lit. [25] P. Skinner, M. W. Howard, I. A. Oxton, S. F. A. Kettle, D. B. Powell, N. Sheppard, J. Chem. Soc. Furaduy Trans. 2, 1981, 77, 1203. [26] (a) C. P. Casey, R. A. Widenhoefer, S. L. Hallenbeck, R. K. Hayashi, Inory. Chem. 1994, 33, 2639; (b) C. P. Casey, R. A. Widenhoefer, R. K. Hayashi, Inory. Chem. 1995,34, 11 38; (c) C. P. Casey, R. A. Widenhoefer, R. K . Hayashi, Inory. Chem. 1995, 34, 2258. [27] H. Wadepohl, A. Metz, H. Pritzkow, unpublished results. [28] (a) H. Wadepohl, K. Buchner, H. Pritzkow, Anyew. Chenz. 1987, 99. 1294; (b) H. Wadepohl, K. Biichner, M. Herrmann, H. Pritzkow, Orgunometullics 1991, 10, 861. [29] H. Wadepohl, Angew. Chem. 1992, 104, 253. [30] H. Wadepohl, S. Gebert, Coord. Chein. Rev. 1995, 143, 535. [31] H. Wadepohl in: “7he Synergy Between Dynamics and Reuctivity at Clusters and Surfuces”, L. J. Farrugia (Hrsg.), NATO ASI Series C: Muthematical und Physical Sciences, Vol. 465, Kluwer: Dordrecht 1995, p. 175.
I . 1.7 From C-If-Actiiution to Arene Clusters
289
1321 H. Wadepohl, K. Buchner. M. Herrmaiin, C . Jost, S. Muller, manuscript in preparation. [ 3 3 ] H. Wadepohl. S. Muller. unpublished results. . SOC1996, I I K . 11548. 134) P. Mercandelli, A. Sironi. J. A I I Z C/itwi. [35] H. Wadepohl. K. Buchner. H. Pritzkow. Olyrrtzori7cralliis 1989. 8. 2745. 1361 H. Wadepohl. K. Buchner, M. Herrinanii. H. Pritzkow. J. Orqnnonief. C / m t 1999. 573, 22. 1371 H. Wadepohl. T. Borchert. K. Buchner. H. Pritzkow, Clrrm. Bey. 1993. 126. 1615. [38] ( a ) A. J. Deeming, M. Underhill. J. Cliern. Soc.. Ddtorr Trarzs. 1974, 1415; ib) A. J. Canty. B. F. G. Johnson. J. Lewis, J. Oryunornet C/iern. 1972, 43, C35; A. J. Canty. A . J. P. Domingos, B. F. G. Johnson, J. Lewis. J. C / w x Soc., D d t o n Trms. 1973. 2056. Review articles: J. Lewis, B. F. G. Johnson, Pure Appl. Cliern. 1975. 44, 43; R. D. Adams. I. T. Horvath. Proyr. Znory. Chwi. 1985. 33. 127; A . J. Deeming. Adr.. Orgntzornrt. C/irnr. 1986, 26. 1. [39] H . Wadepohl, T. Borchert, K. Buchner. M. Herrmann, F.-J. Paffen, H. Pritzkow, Orqcmonietrillics 1995. 14, 38 I7 1401 H. Wadepohl. T. Borchert, H. Pritzkow, J. C l m i . Soc., Chrnz. Conz~iun.1995, 1447 141] H. Wadepohl. T. Borchert, H. Pritzkow. C/wni. Bcr.:Reciiri/ 1997. 130. 593.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.16 Cyclopentadienylnickel Clusters Stanislaw Pasynkiewicz and Antoni Pietrzykowski
1.16.1 Introduction Cyclopentadienylnickel clusters of general formula Ni,Cp,L,: where Cp = q5-C5H5 or q5-C5CH,R5-, and L, = ligand (most commonly C-R, CO, S, NR etc.) will be discussed in this chapter. Many cyclopentadienylnickel clusters have been prepared recently, their structures determined and their application in organic synthesis and homogeneous catalysis was described. The structure of most of the clusters mentioned in this chapter have been determined by X-ray analysis. Only a few, the most interesting ones, are presented without their crystal and molecular structure having been determined. The scope of this chapter includes tri-, tetra-, penta- and hexanickel clusters. Dinickel complexes ( NiCp)2L,, are not included in this review due to the limitation of space although the number of papers dealing with them is already substantial and the complexes are interesting for many reasons.
1.16.2 (Cyclopentadieny1)trinickel clusters The (q5-cyclopentadienyl)(p~-alkylidyne)trinickel clusters, (NiCp)3C-R, are among the best studied nickel clusters. The three nickel atoms together with the cyclopentadienyl rings form a system of 45 valence electrons (VE) in which each nickel atom possesses 17 VE. Therefore this system tends to achieve the additional three electrons required from an alkylidyne ligand to form a stable cluster of 48 VE in which all nickel atoms fulfill the 18 electron rule. The exeptions are the classic Fischer-Palm 49-electron Ni3Cps (p3-C0)2 cluster"] and the paramagnetic 53electron Ni3Cp3(p3-S)2 species.[21
Cyclopentadienyltrinickel clusters are crystalline solids of high thermal stability, often subliming without decomposition, and are soluble in many organic solvents. Many of these clusters do not react with water but undergo facile oxidation. The main methods of synthesis of cyclopentadienyltrinickel clusters are the reactions of nickelocene with organolithium or -magnesium compounds (Eq. (1))
NiCp, M
=
+ MCH2R
--f
(NiCp)3CR
+ LiCp + CH3R
(1)
Li; MgX
The other methods consist of reactions of NiZCpz(p2-CO)Z with CS2,[31acetyl e n e ~ , [elemental ~] sulfur[21etc. Exchange reactions of various metals, which are substituted by nickel in clusters, have also been describedL5](Eq. (2)). Co3(C0)9CR + (NiCp),(,uuz-CO)? + (NiCp)3CR
(2)
A mixture of many nickel clusters has been obtained by vaporizing nickel atoms into solutions of tert-butyl-substituted cyclopentadienes.[61 Ethylidynetrinickel cluster 1 (NiCp)?CCH3was prepared for the first time from the reaction of nickelocene with methyllithium, in diethyl ether and THF."] 1 sublimes at 60 "C (0.001 Torr), melts without decomposition at 158-160 "C, oxidizes easily in air but does not hydrolyze in water. The mass spectrum shows a parent peak at m/z 396 (58Ni),The " C NMR of 1 established the presence of the p3ethylidyne carbon atom at 6 289.3 (s). The molecular structure of 1 is shown on Fig. 1. The nickel framework is best described as an isoceles triangle. The C2 atom of the apical methylidyne group is tilted slightly towards the Nil atom. Cluster 1 can also be prepared in the reaction of nikelocene with L ~ C H = C H I [ ~ ] (Eq. ( 3 ) ) .
NiCp,
+ LiCH=CH,
--f
(NiCp)jCCH3 1
+ LiCp + H2C=CHCH=CH2
(3)
The unstable {CpNi-CH=CHZ}, formed in the first step of the reaction, undergoes coupling to form intermediate species {(NiCp),} moieties (Eqs. (4) and (5))
+ LiCH=CHZ + (CpNi-CH=CH*} + LiCp coupling { CpNi--CH=CH,} { (NiCp)?} + H2C=CH--CH=CH2
NiCp,
-
(4)
(5)
The species { ( NiCp)2} reacts then with (cyclopentadieny1)vinylnickelforming cluster 1 following a nickel mediated 1,2-hydrogen shift (Eq. (6))
292
1 Molecular Clusters
Figure 1. The molecular structure of ( N ~ C ~ ) ~ ( , U ~ - C C H ~ ) . [ ' '
{CpNi-CH=CHZ}
+ {(NiCp)z}
pcii:T :
A H-transfer
(NiCp)3CCH3
_i
CpNiLNi
1
Because cluster 1 is also formed in the reactions of nickelocene with phenyllithium and phenylmagnesium halides, the source of C-CH3 was inve~tigated.[~.~] It could be assumed that the formation of this group could be achieved by a complex cleavage of phenyl group or of a solvent molecule (THF). Reaction of nickelocene with deuterated phenyllithium was performed to find out whether the ethylidyne group was derived from cleavage of the phenyl ring. It was shown that cluster 1 did not possess any deuterium (Eq. (7)) and therefore, the source of the ethylidyne was not the phenyl ring. NiCp,
+ LiCsDS
THF
(NiCp)3CCH3
(7)
Then the reaction of nickelocene with phenyllithium in THF-dg was examined and it was found that 1 had a composition (NiCp)3CCD3 (Eq. (8)).
NiCp,
-
+ LiChHs THF-dx
(NiCp)lCCD3
(8)
I t was concluded that 1 was formed by the cleavage of the T H F ring. It is postulated that a cleavage of C-H and C-C bonds in T H F is caused by an interaction of coordinatively and electronically unsaturated species { ( NiCp)z}. Ethylidynetrinickel cluster 1 is also formed in the reaction of nickelocene with ethylmagnesium bromide"0' (Eq. (9))
NiCpz
+ BrMgC2HS
(NiCp)3CCHI
4
+ CpMgBr + C Z H ~
(9)
1
The yield of this reaction is low, and this may be explained by the facile occurence of 8-H-elimination and cleavage of Ni-C bond. This problem will be discussed later in this article. Unstable cyclopentadienylethylnickel eliminates 8-H to form the intermediate complex {CpNiH.CH,=CHI). The latter reacts with (CpNiC2HSS to form an active complex { ( NiCp)?.CH?=CH>)and ethane. The insertion of the binuclear complex { ( NiCp)?}into an olefinic C-H bond and a further elimination of ethane leads to the formation of 1 (Scheme 1 ) . It has been shown above that cluster 1 is formed in the reactions of nikelocene with methyllithium, vinyllithium, phenyllithium in the presence of T H F and with ethyllithium. The mechanism of the reaction of nickelocene with methyllithium is complicated and not, as yet, fully explained. The new C-C bond and a number of higher cyclopentadienylnickel clusters are formed in this reaction. The clusters are often unstable and difficult to isolate in a pure state.'"' There is, however, no doubt, that the first step of the reaction is the formation of unstable {CpNiCH?).This complex
{ CpNiC2H5}
p-Helim.
CH? cH2 {CpNiC2H5J CPNi-11 CpNi---l, I CH + CH3CH3 I CH2 CpNi H
I
insertion
Scheme 1
294
1 Moleculur Clusters
is a coordinatively unsaturated, 16-electron species. a-H-elimination results in an increase in the number of valence electrons to 18 ( Eq. (10)). CpNiCH,
CpNi=CH2
I
H The (cyclopentadienyl)(methylene)nickelhydride is very unstable and immediately reacts with another molecule of (CpNiCH3) to form the dinickel complex (Eq. (11)).
CpNi I H
\
kiCp I CH3
4
CH2 +CH4 CpNi-NiCp / \
Further a-H-elimination and reaction of the products with ICpNiCH3) leads to the formation of ( v5-cyclopentadienyl)(,u,-methy1idyne)trinickelcluster 2, which has been recently obtained and spectroscopically characterized" *I (Eq. ( 12)). {(NiCp)ZCH2}
+ {CpNiCH,}
4
(NiCp)3CH 2
+ CH4
(12)
The further reactions of the cluster 2 leading to an exchange of the hydrogen atom for the methyl group to form the fully characterized trinickel cluster 1 have not been explained. It can be assumed, on the basis of the products formed [( NiCp),CCH,, (NiCp)sCCH3, ( NiCp)sC, (NiCp)6Cz], that the cluster 2 reacts immediately with the (CpNiCH3) present in the reaction mixture, to form the cluster 1 and other products (Scheme 2). In order to obtain the cluster 2, the reaction of nickelocene 4 {CpNiCH3)
Scheme 2
-
((NiCp)&)
+ 3 CHq
1.16 Cyclopentudienylnickel Clusters
295
with methyllithium was carried out in the presence of an alkene having a boiling point above room temperature. In this case the complex Cp(CH3)Ni.alkenereacts substantially slower forming, within 7 days, at room temperature a mixture of clusters 1 and 2. The analogous cluster (NiCp)iCPh 3 have been synthesized in the reaction of nickelocene with benzylmagnesium halide and its structure was proposed on the basis of ' H NMR, MS, and IR data.1131 Similar reactions of NiCpz with neopentyland jtrimethy1silyl)methyllithium have been carried out in order to prepare the clusters 4 (NiCp)?CC(CH3)3and 5 ( NiCp)3CSiMe3."41 All the clusters described above (1-5) were prepared in the reaction of nickelocene with organolithium or magnesium compounds lacking P-hydrogens. So in all of these cases the reaction proceeded zGu cr-H-elimination. Previous attempts to prepare (p3-alky1idyne)trinickel clusters from organolithium compounds possessing /?-hydrogens were unsuccessful. Therefore, it was concluded that the decomposition may have been connected to the presence of P-hydrogens in the alkyl group.[14] To check out the above hypothesis, reactions of nickelocene with organolithium and magnesium compounds possessing p-hydrogen were studied. It was found that nickelocene reacts with BrMgCHzCH(CH1)Ph to form trinickel cluster 6 (NiCp)iCCH(CHi)Ph.['" This means that the presence of the P-hydrogen atom and the possibility of [j-H-elimination does not always cause the splitting of the carbon-nickel bond. This was the first example of the synthesis of an alkylidynetrinickel cluster from organonickel compounds possessing P-hydrogen. The cluster 6 has been characterized spectroscopically and by single-crystal X-ray technique. It crystallizes in the monoclinic crystal system. The molecular structure is presented in Fig. 2. After the discovery that { CpNiCH2CH(CH3)Ph}formed an alkylidynetrinickel cluster, reactions of other alkylnickel compounds possessing P-hydrogen atoms were studied (Eq. ( 13)). NiCp, R
=
+ BrMgR + {CpNiR} + BrMgCp
(13)
(CH2)7CH1: C H ~ C ~ HCzH5 S;
The reactions of nickelocene with alkylmagnesium bromide were carried out at 40- 50 " C ; the reaction mixture was then hydrolyzed and the products were separated by column chromatography. In all of these reactions the corresponding (a1kylidyne)trinickel clusters were obtained. The above results show that contrary to previously held opinions, the alkylnickel species { CpNiR}, with R containing Phydrogen, forms trinickel clusters according to Eq. (14) NiCpz + MCH,R
+
{CpNiCH2R} + (NiCp)iCR
R = (CH~)~CH CH(CH1)Ph; I; C2H5 M
=
(14)
Li: MgBr
These observations suggest that the presence of P-hydrogen does not always
296
I Molrculur Clusters
Figure 2. The molecular structure of ( NiCpj3[p3-CCH(CH3)Ph].['01
cause the total splitting of the nickel-carbon bond with the formation of saturated and unsaturated hydrocarbons. This reaction can be regarded as the simplest method of the synthesis of (alkylidyne)(cyclopentadienyl)trinickelclusters. Because all the alkyl ligands studied possessed u- and P-hydrogen it could be assumed that both P-H and a-H-elimination were possible. However, the literature data show that for ligands possessing a- and P-H, the P-hydrogen elimination process proceeds more easily." Mass spectroscopy appears to be a very useful method of identification of ( q 5 cyclopentadienyl)(,uui-alky1idyne)trinickelclusters. The isotope pattern of trinickel species is very characteristic and fragmentation is often identical or very similar. The most characteristic fragments which appear in addition to the parent ions are as follows: m/e 304 (Ni$pz)+, 246 (NizCpZ)+, 188 (NiCp2)+and 123 (NiCp)+. It was assumed that reactions of nickelocene with lithium - or magnesium alkyls possessing /I-hydrogen proceed with the formation of corresponding alkenes. The activation of the alkene by the active dinickel species {(NiCp)z}formed in the reaction leads to the formation of (cyclopentadienyl)(alky1idyne)trinickel clusters. It has been found previously[*' that NiCpz reacts with phenyllithium or phenylmagnesium bromide in THF, in the temperature range -40°C - room temperature, to form biphenyl and a mixture of cyclopentadienylnickel clusters. The active dinickel species {( NiCp)z} is formed in this reaction as an intermediate (Eqs. ( 15), (16)).
NiCpz
+ LiPh
2 {CpNiPh}
-
LiCp
Ph-Ph
+ {CpNiPh}
(15)
+ {(NiCp),}
(16)
Therefore, if the above assumptions are correct, one would expect that the reaction of NiCpz with LiPh carried out in the presence of an alkene should also lead to the formation of trinickel cluster. Reactions of nickelocene with phenyllithium in the presence of I -decene, 1-hexene or (2,4,4-trimethyl-1-pentme) were studied. The reactions of nickelocene with phenyllithium and alkene were carried out in T H F at temperatures ranging from -60°C to room temperature. Products were separated by column chromatography on alumina using hexane/toluene mixture as eluents. In the case of I-decene and I-hexene trinickel clusters: (NiCp)3C(CH,)7CHI 7 and ( NiCp)iC(CH2)4CH38, were isolated and fully characterized. For (2,4,4-trimethyl1-pentem), only traces of corresponding trinickel cluster were obtained." 91 These studies completely confirmed the particular role of coordinatively and electronically unsaturated species { ( NiCp),) in these reactions and allowed a mechanism of the formation of (cyclopentadienyl)(alkylidyne)trinickelclusters to be proposed. The cluster (NiCp)ICCOOCHI 9 should also be included in the class (cyclopentadienyl)(alkylidyne)trinickel clusters. It was prepared in the reaction of Co3(CO)9CR with the known metal exchange reagent (CpNiCO)?."' It5 crystal structure determined by X-ray measurements is shown in Fig. 3.
Figure 3. Thc molecular structure of ~N~c~~KCOOCH~.~~~
298
I Molecular Clusters
In 1958 Fischer and Palm“] reported the synthesis and characterization (dipole moment, magnetic susceptibility measurements and IR spectrum) of a tricyclopentadienyl trinickel dicarbonyl compound 10 (NiCp)3(CO)z. X-ray diffraction studies confirmed the proposed structure showing one carbonyl ligand capping each side of an equilateral triangle of bonded nickel atoms.[”] This triangular metal cluster was of particular importance because it provided the first example of a triply bridging carbonyl ligand and it was a stable, 49 electron, paramagnetic cluster containing one unpaired electron [(NiCp)3 45e and (C0)2 4e]. From electronic considerations Strouse and Dahl[”] proposed that the unpaired electron in the ( NiCp)3(C0)2 cluster occupies the corresponding trimetal-antibonding a2’ HOMO. Further proof for the trimetal-antibonding character of the HOMO was derived from crystallographic data on the diamagnetic monoanion [(NiCp)3(CO)2]-, where a significant increase (0.032 A)of the average Ni-Ni bond length by comparison to the parent Fischer-Palm cluster was observed.[221The crystal structure of pentamethylcyclopentadienyl analogue of the parent Fischer-Palm cluster (NiC5Me5)3(C0)2 was reported by Dahl et al.L221 Analogues to the Fischer-Palm cluster, a 53-electron, S,S-bicapped ( NiCp)3(p3-S)~cluster 11 was obtained from the reaction of (NiCp)2(pz-C0)2with elemental The further studies of Dahl et al. led to the preparation of 52electron [( NiCp)3(p3-S)2]+[SbF& 12 and the determination of its structure[’] (Fig. 4). The cation possesses crystallographically imposed Czv-2rnm site symmetry. The trinickel fragment forms an isosceles triangle with two shorter identical edges and one longer edge. It is capped by two apical sulfur atoms coordinated to the three nickel atoms. Dahl et al. obtained in the reaction of (NiCp)2(p*-CO)2with CS2, two trinickel clusters which are analogous to the Fischer-Palm cluster i. e. the clusters 13 (NiCp)3(p3-CS)2 and 14 ( N~C~)~(~~-CS)(,U~-CO).[~] An interesting trinickel cluster was obtained in the reaction of the ethynyliron complex Cp(C0)2Fe-C-CH with ( NiCp)2(C0)2[41(Eq. (17)). -
-
co
Its molecular structure is shown in Fig. 5 . The tetranuclear adduct 15 contains the “out-of-plane’’ spiked triangular metal core, which interacts with the C2H ligand. In the 13CNMR spectra of 15, even at -80 “C, the two NiCp signals were observed to be equivalent and the two carbonyl ligands bonded to Fe were observed as a broad signal. The fluxional behavior may be explained by a fast ligands rotation.
1.16 Cyclopenta~ienylnickrlC1u.ster.s
299
Figure 4. The molecular structure of [(NiCp)i(/li-S)21i.121
The first (~5-cyclopentadieny1)(~c~-tertiarybutylimido)trinickel cluster ( NiCp)3NBut 16 was synthesized by the reaction of CpNiNO with LiR in T H F solution.[241 An alternative method of preparation consists of the reaction of ( t- c q H9 N ) ~ S with NiCp2.[2s1The molecular structure is shown in Fig. 6.[261The three nickel atoms form an equilateral triangle. The butylamine group is situated perpendicular to this triangle and the three cyclopentadienyl groups are disposed symmetrically. The molecule thus adopts an approximately trigonal pyramidal structure with symmetry C3,{-3m. The electronic configuration of this cluster is therefore close to that of (NiCp)3(C0)2,which also has 49 valence electrons. By vaporizing nickel atoms into a solution of frrf-butyl-substituted cyclopentadienes the cluster (f-BuZC5H3)3Ni.iHZ17 was prepared. Cluster 17 contain
Figure 5. The molecular structure of NiJCp2 [ CO)[ FeCp(CO)?C-CH ] 15.r41
300
I Moleculur Clusters
IN
Figure 6. The molecular structure of (NiCp)3N-'Bu 16.['']
hydrido ligands, which were identified by a single crystal structure determination as p3-H bridges.['] Its structure is shown in Fig. 7. Nickel atoms form an equilateral triangle with a di-tert-butylcyclopentadienyl Iigand q5- bonded to each nickel atom. The metal triangle is bridged by two p3-H atoms to form a trigonal bipyramid. The
c34 (/%c35
Figure 7. The molecular structure of (f-BuzCsHi)jNiiH2 17.'"'
I . 16 Cl'clopenta~i~n~lnickrl Clusters
30 1
cluster 17 shows a rich redox chemistry with reversible redox couples between oxidation states of -2 and +2. Studies on reactions of 17 showed that it easily forms an electronically saturated cluster-anion, which has 48 valence electrons. The first trinickel cluster with an open structure is Ni3Cp2( PhzC2)3 18, which was obtained from the reaction of nickelocene with methyllithium in the presence of diphenylacetylene, with a molar ratio of the reactants of 1 : 1 : 2 (Eq. (18)).["] It had been postulated earlier from spectroscopic evidence that the cluster Ni1CpZ( C F . I C ~ C Fhas ~ ) a~ similar structure.'281 NiCp,
+ LiMe + PhC-CPh
+
Ni3Cp2(C2Ph2)3 18
The crystal structure of 18 is shown in Fig. 8. In the molecule of 18 the three nickel atoms have a bent arrangement. Two of the diphenylacetylene ligands have undergone carbon-carbon bond formation to produce a tetraphenylbutadiene ligand. This ligand chelates the Ni(3) atom with two o-bonds, forming a nickelacyclopentadienyl ring, and interacts with Ni(2) viu two n-bonds. The third alkyne unit forms two n-bonds each to Ni(2) and Ni(1). Two Cp rings are bonded to terminal nickel atoms Ni( 1) and Nii3). All three nickel atoms fulfill the 18 electron rule. The alkyne ligands donate a total of 10 electrons and Cp rings a further 10 electrons to
Figure 8. The molecular structure of Ni?Cp:(Ph?C!)? 18.[271
302
1 Molecular Clusterb
the cluster unit, and in terms of electron counting the complex 18 is a 50 electron cluster, consistent with the presence of two metal-metal bonds.
1.16.3 (Cyclopentadieny1)tetranickel clusters (Cyclopentadieny1)tetranickelclusters have not, so far, been intensively studied. The simplest (~5-cyclopentadienyl)tetranickel cluster (NiCp)4, in spite of many efforts, has not been isolated. This unknown species would have the expected "closed-shell" 60-electron configuration and would be diamagnetic. Such 60-electron species are in fact known, for example (CpCo)4H4.[291 The first (cyclopentadienyl)tetranickel cluster was obtained in the reaction of CpNiNO with LiAlH4/AlC13 in THF soluThe cluster contains three hydride ligands and has the composition tion at 20 0C.[301 (NiCp)4H3 19. Spectral and X-ray s t u d i e ~ [ ~allowed ~ - ~ ' ] the structure shown in Fig. 9 to the proposed. The structure was then confirmed by a neutron diffraction analThe three hydride ligands are situated on three of the four faces of the ysis of 19.[321 Ni4 tetrahedron to give the Ni4H3 core approximate C3" symmetry. Cluster 19 possesses 63 valence electrons and is paramagnetic. The structure of (NiCp)4H3can serve as a model for hydrogen adsorption on nickel metal. Based on the results obtained, it appears that a triply bridging arrangement is indeed the most stable for the H/Ni system.
Figure 9. The molecular structure of (NiCp)4H3 lY.[321
I . 16 Cyclopentuu’ienylnickel Clusters
303
Figure 10. The molecular structure of (NiCp)4H2 20.11y1
It was found that the dihydridotetranickel cluster (NiCp)4H2 20 was present among the products of the reaction of nickelocene with phenyllithium in the presence of terminal alkenes.[”] The cluster 20 has been formed in several reactions of nickelocene with organolithium and magnesium compounds.[331The hydrogen elimination from an organic ligand leads to the formation of unstable {CpNiH} species. These species react further to form the cluster 20. In contrast to 19, the cluster 20 is diamagnetic, which indicates the presence of an even number of hydrido ligands. Its structure was determined by means of ’ H NMR, mass spectroscopy and X-ray diffraction measurements (Fig. 10). Two signals were observed in the H NMR spectrum of this compound: a singlet at 5.34 ppm assigned to the C5H5 protons and a singlet of hydride protons at -19.95 ppm. Establishing the stoichiometry from the relative integrals of the Cp and H signals was not a straightforward task; the large difference in chemical shifts of these two signals requires use of a wide spectral window that results in short acquisition times and a smaller number of data points measured per signal. Also, the differences in proton relaxation time lead to misleading integration values. Several carefully measured experiments using relaxation delays (8-60 s) between FID registration gave a Cp to H proton ratio 20 : 2.1 f 0.2 (expected 20 : 2). Only one signal for the Cp carbon atoms at 82.60 ppm was observed in the I3C NMR spectrum of 20. In EIMS (70 eV) at 150-170 “C a group of peaks at m/e 488-504 was present. These peaks were a result of overlapping of the parent ion at m/e 494 (5XNicalc.)
304
1 Molecular Clusters
and of at least three other fragments at m/e 492 ( NiCp);, 490 [(NiCp)4 - 2H]+ and 488 [(NiCp)4 - 4H]+. Because the parent ion was of low intensity and the computer simulation did not give satisfactory results, a spectrum using the LSIMS(+) technique was recorded (m-nitrobenzyl alcohol was used as matrix). An intensive parent ion at m/e 495( M+H)+ and its fragmentation pattern were observed. The (cyclopentadieny1)tetrdnickelcluster ( NiCp)4Se2 21 was obtained from the reaction of CpNi(PPh3)Cl with Se(SiMe3)2[341 (Eq. (19)). CpNi(PPh3)Cl-
Se(SiMei)z
(NiCp)4Sez 21
+ (NiCp)4Se2(PPh3)2
(19)
The crystal structure of the cluster 21 is shown on Fig. 1 1. It is the first example of tetranickel cluster with square planar arrangement of four nickel atoms. Each nickel atom is bonded to two other Ni atoms, one q5-C5H5 group and two bridging p4-Se ligands. The cluster 21 confirms previous statements that a system of 60 valence electrons in (NiCp)4 is not enough to form a stable cluster. Each selenium atom adds 4 electrons to the system forming 68 VE cluster. The addition of triphenylphosphine to 21 causes splitting of Ni-Ni bonds and the formation of (NiCp)4Se2(PPh3)2.
Figure 11. The molecular structure of (NiCp)4Se2 21.'341
I . 16 CJ.'clopentau'ic.nylnickelClusters
305
Analogous compounds (NiCp)4Tel and ( NiCp)4Tez(PPh3)l were obtained from the reaction of CpNi(PPh3)Cl with Te(SiMe3)2.[341 A related 66 VE tetranickel cluster Cp3Ni4(CO)BrSez 22, in which one C p group was replaced by CO and Br, was also obtained.[351 Several other tetranickel clusters have also been described in the literature but as they do not possess cyclopentadienyl ligands they are not included in this chapter.
1.16.4 (Cyclopentadieny1)pentanickel clusters There have not been any examples of crystallographically determined molecular structures of (cyclopentadieny1)pentanickel clusters up to now, although such a cluster has been obtained and characterized by spectroscopic methods.[361As has already been mentioned in the reaction of nickelocene with methyllithium, besides isolated and fully characterized cluster ( NiCp)3CCH3 1, several higher nuclearity clusters were found. Among them the pentanickel cluster (NiCp)SCCH3 23 was identified. It can be assumed that the cluster 23 is formed in the reaction of metylidynetrinickel clusters 2 with unstable fCpNiCH3) species (Eq. (20), Scheme 2).
+ {CpNiCH,}
(NiCp)lCH
-
{ (NiCp)4C}
+ CH4
(20)
The intermediate compound { ( NiCp)4C) can react with { CpNiCHi } to form cluster 23 (Eq. (21)) {(NiCp)4C}
+ {CpNiCH,}
--f
(NiCp)5CCH3 23
Based on the results of ' H and I3C N M R and mass spectra the following trigonal prismatic structure for cluster 23 has been proposed. NiCp CpNi /C/
/
CpNi- '\ 23
CH3
NiCp
306
I Molecular Clusters
Figure 12. The molecular structure of (NiCp)h 24.[37'
1.16.5 (Cyclopentadieny1)hexanickel clusters The reduction of nickelocene by treatment of a THF solution with an equimolar amount of sodium naphtalenide, in THF, after removal of solvent gives a mixture of five different (cyclopentadieny1)nickelclusters. From this mixture a hexameric cluster ( NiCp)6 24 was isolated and its molecular structure was determined (Fig. 12).[371Cluster 24, which corresponds to a 90 electron metal cluster system, does not conform to the large class of octahedral carbonyl clusters, which invariably possess an electronic configuration of 86 valence electrons for metal-metal and metal ligand bonding. This 90 electron system should then undergo a Jahn-Teller distortion with the four electrons remaining antibonding with respect to the nickel core. The formation of a hexanickel cluster has also been postulated in the reaction of nickelocene with methyllithium (Scheme 2). If the intermediate species {( NiCp)4C}, formed in the reaction (20), reacts with { ( N i c p ) ~ }a, (cyclopentadieny1)carbidohexanickel cluster 25 would be formed (Eq. (22))
The formation of the cluster 25 has been spectroscopically confirmed.[361
1.16 Cyclopentadiienylnickel Clusters
307
References [ I ] E. 0. Fischer, and C. Palm. Chenz. Ber., 1958, Y l . 1725. [2] T . E. North, J. B. Thoden, B. Spencer, and L. F. Dahl, Or(janotnetallics, 1993, I2, 1299. [3] T. E. North, J . B. Thoden, B. Spencer. A. Bjarnson, and L. F. Dahl, Oryunomefallic.s, 1992, I / , 4326. 141 M. Akita, M. Terada, M. Tanaka, and Y. Moro-oka, Or~~tmomrtullie.s, 1992, I / . 3468. [ S ] R. Blumhofer, K. Fischer, and H. Vahrenkamp, Cheni. Ber., 1986, 119, 194. 161 J. J. Schneider. R. Goddard: C. Kriiger. S. Werner. and B. Metz, Chenz. Ber., 1991, 124. 301. 171 H . Lehmkuhl, C. Kriiger, S. Pasynkiewicz, and J. Poplawska, Oryanometullics, 1988, 7. 2038. [S] A. Pietrzykowski, and S. Pasynkiewicz. J. Orgunomer. (%em.,1992, 440, 401. 191 S . Pasynkiewicz, W. Buchowicz, and A. Pietrzykowski. J . 0rycinome.f.Chem., 1997, 531. 121. [ 101 S. Pasynkiewicz, A. Pietrzykowski. L. Trojanowska. P. Sobota, and L. Jerzykiewicz, J. Or<jnnoniet. Chem., 1998, 550, 1 1 I . [ 1 I] S. Pasynkiewicz, J. Oryanornet. Chcvi., 1990, 387. 1. [I21 S. Pasynkiewicz, W. Buchowicz. and A. Pietrzykowski, Trunsif. Metal Chem.; 1998. 23. 301. 1131 T. J. Voyevodskaya, I. M. Pribytkova. and Yu. A. Ustynyuk, J. Organoniet, Cheni.. 1972, 37, 187. [I41 B. L. Booth, and G. C. Casey. J. Orya/zonzef.Chem., 1979, 178, 371. [IS] J. Thomson, and M. C. Baird. Can. J. Chern.. 1970, 48, 3443. [I61 J . Thomson. and M. C. Baird, Inon]. Chini. Acta. 1975, 12. 105. 1171 F. Osawa. T. Ito, Y. Nakamura, and A. Yamamoto, Bull. Cheni. Soc. Jupuri, 1981. 54, 1868. [I81 S . Komiya, Y. Morimoto, A. Yamamoto, and T. Yamamoto, Organometallics, 1982, 1, 1528. [I91 S . Pasynkiewicz, W. Buchowicz, A. Pietrzykowski, and T. Glowiak, J. Or(jarzornet. Chern.. 1997, 536, 249. 1201 A. A. Hock. and 0. S. Mills, Adr. Chern.Coord Cpds. 6"' I. C.. 1961. 640. [21] C. E. Strouse, and L. F. Dahl, J. Am. Chrm. Soc., 1971, 93, 6032. [22] J. J. Maj, A. D. Rae, and L. F. Dahl, J. An?. Chem. Soc., 1982, 104; 3054. [23] H. Vahrenkamp. V. A. Uchtman. and L. F. Dahl, J. An?. Chem. Soc., 1968, 90, 3272. [24] J. Miiller. H. Dorner, and F. H. Kohler, Chrm. Ber., 1973. 106; 1112. [25] S. Otsuka. A. Nakamura, and T. Yoshido, Inovy. Choni.. 1968, 79, 261. 1261 N . Kamijyo, and T. Watanabe. Bull. C/ieni. Soc. Jupmi. 1974, 47, 273. 1271 S. Pasynkiewicz. A. Pietrzykowski. B. Kryzd-Niemiec. and J. Zachara. J. Oryanomef. Cheni. 1998, 566, 217. [28] J . L. Davidson, and D. W. A. Sharp, J. Chmi. Soc.. Dolton Trans., 1976. 1123. 1291 J. Miiller. and H. Dorner, iinyew. Chrm.,Int. Ed Engl.. 1973, 12, 843. 1301 J. Muller, H. Dorner. G. Huttncr, and H. Lorenz, Anqlrn.. Chern., hf.Ed. Eny/. 1973, 12, 1005. [31] G. Huttner, and H. Lorenz, Chern. Bev.. 1974, 107, 996.
1321 T. F. Koetzle, J. Miiller. D. L. Tipton, D. W. Hart, and R. B ~ uJ.. An?. (%ern. Soc., 1979, 101, 563 I . 1331 S. Pasynkiewicz, unpublished result.^. 1341 D. Fenske. A. Hollnagel, and K. Merzweiler. A n y ~ n .Cheni., Int. Ed. En(]/. 1988, 27, 965 [35] D. Fenske. and A. Hollnagel. Angeiv. Cheni., In[. Ed. En'nyl. 1989. 28. 1390. [36] S . Pasynkiewicz. J. Oryunoinet. Chetn., 1990, 387. I . [37] M. S . Paquette. and L. 1:. Dahl, J. Arn. Chenz. Soc., 1980. 102. 6623.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.17 Ligand Orientation Effects on Metal-Metal Bonding Santiago Alvurez and Gabriel Aulldn
1.17.1 Introduction The combined use of theoretical and structural studies allows one to establish the existence of the mutual influence of different structural parameters, usually referred to as structural correlations. Such correlations are useful for predicting or explaining the structural data for new compounds. In particular, they should ultimately allow one to make predictions on compounds with extreme bonding situations, i e . , very weak or very strong bonds. To establish structural correlations from experimental data, a large data set for a family of analogous compounds is needed. In addition, significant structural differences must exist among the different members of the family. Thus, it is clear that in most cases there is not enough data for reliable statistical analysis. In this regard, the structural correlations suggested by a small data set can be confirmed through a theoretical analysis. Conversely, a theoretical study can predict structural correlations that have been hidden among the wealth of structural data that is nowadays available. A theoretical study on a model compound in which one structural parameter (e.g., a bond distance) is optimized for each value of another parameter (e.g., a bond angle or a torsion angle), mimics in some way what is experimentally done by means of chemical substitutions, since small changes in bond and torsion angles can result from electronic or steric effects induced by the ligands or by intermolecular interactions (e.g., hydrogen bonds or packing forces). What is unique to the theoretical studies, when carefully analyzed, is that they can provide simple qualitative explanations to help rationalize a large amount of experimental data, and offer predictions about the structures of compounds yet to be prepared. In this contribution we focus on the structural data for Fe-Fe single bonds, from binuclear compounds to tetranuclear clusters that contain the [Fe2(C0)sl2- fragment. In particular, we will show how the orientation of the ligands affects the Fe-Fe bond distance. First, we give a survey of the pyramidality effect on metal-
1.17 Liquid Orientation Eflects on Metul--Metnl Bonding
309
metal bonds for binuclear complexes. Then we will show that such an effect can also be found for the Fe-Fe bonds in unsupported as well as in bridged binuclear complexes. including those cases with a metal bridging atom (i.e., trinuclear clusters). Finally we discuss ligand orientation effects in tetranuclear iron-carbido complexes and their derivatives.
1.17.2 Pyramidality and metal-metal in binuclear complexes
bond strength
In metal-metal bonded binuclear complexes ( I ) , of the types MzLz,, or M ~ L ~ X ( XZ represents axial ligands). the pyramidality angle x is defined as the average of the MML bond angles.
1
In general, one can describe the effect of pyramidality on the M-M bond length by a parabola['] (Fig. 1 ) : as u increases, the M- M bond distance decreases, eventually reaching a minimum, after which further increase in the pyramidality results in an elongation of the M-M distance. In practice, only a small fraction of the d(u) curve is energetically attainable. and the available structural data for a particular family of compounds correspond to one of three possible situations: (a) a norrnal region (small values of x). in which d decreases with increasing x in an approximately linear way, (b) a turnover point, around which d is practically independent of CA (intermediate u values), and (c) an inverted region (large values of x) in which d increases with Y. Only for a few families is there enough experimental data to make the parabolic behavior evident.[21It is interesting to note that the pyramidality effect on metal-metal bond strength found in the crystal structure data of binuclear complexes is well reproduced at several levels of computation, either qualitatively at the extended Huckel level,["41 or more accurately through density functional,"] MP2,[51 or CASSCF[" calculations. A simple orbital explanation has been proposed[31for the correlation between bond angle and bond distance in the normal regime. In the case of the (7 metal-
310
1 Molecubr Clusters normal region
inverted repn
t
turnover point ~
cx
Figure 1. Schematic representation of the dependence of the M-M bond length on the pyramidality c( for the binuclear compounds M*L,. Reproduced with permission from Ref. [I].
metal bonding, a large degree of pyramidalization ( a > 90") allows mixing of s and p orbitals with d,z, thus hybridizing it toward the other metal atom, whereas a decreased pyramidalization ( a < 90") favors hybridization away from the other metal atom (2).A similar explanation applies to the bonds, while the 6 component appears to be insensitive to the degree of pyramidalization.
2
In general, the experimental trend in the normal region conforms to a leastsquares linear equation, d(M-M) = b c cos a. The independent parameter, b, in such an equation corresponds to the M-M distance for a pyramidality angle of 90" and is referred to as the intrinsic metal-metal distance, whereas the slope c gives a measure of the bond susceptibility to pyramidalization. This simple bond angle-bond length correlation explained some of the unsolved enigmas in the field of metal-metal bonds. For instance, the large variation of the Cr-Cr bond distances found in the quadruply-bonded Cr(11) carboxylates and analogous compounds,[61spanning the whole range of Cr-Cr distances between 1.8 and 2.6 A, is found to correspond to differences in the pyramidality angle. Another case is that of the complexes [RezC14(PMe2Ph)4In+(n= 0-2). In these compounds with Re-Re bond orders 3, 3.5, and 4, respectively, the unconventional relationship between bond order and bond distance is explained by taking into account the different pyramidality angles. An interesting feature of the pyramidality effect is that the existence of axial li-
+
I . I 7 Liyand Orientution Effects on Metal- Metal Bonding
3 1I
gands X has a very small influence on the M-M bond length: the same dependence on 9 is found for compounds with two, one or no axial ligand. The presence of axial ligands favors smaller values of 9, thus indirectly inducing longer M-M bonds. It is worth stressing also that the bond length is found to depend on the average angle c( within a family of analogous compounds, even when the different ligands around a metal atom show clearly different bond angles. This is found both in the structural data and in the results of molecular orbital calculations. Only in a few cases, when the different M-L bonds have markedly different covalent character, do the bond distance seems to depend only on those angles involving the less electronegative ligand. The relationship between pyramidality angle and bond length applies not only to molecules, but also to multiple metal-metal bonds in extended structures in which M2L8 fragments are found. This is the case of the isostructural compounds Ca,Nb@6 (x = 0.95, 0.75) and NaNb305F, in which Nb2O6 or Nb205F clusters show shorter Nb-Nb bonds as the Nb-Nb-0 angle It is remarkable that pyramidality effects are observed for a large variety of M-M interactions, ranging from the quadruple bonds to single Rh-Rhc4] or CO-CO[~] bonds to the non-bonding contacts between square planar ds center^.[^,'^] Similar effects have been detected in organic compounds in which steric strain can induce unusual angles and a neat variation of C-C bond distan~es.["~'~I The main trends that have been established for the metal-metal b o n d ~ [ ~ ~ are: ' - ' ~(a) ] The firstrow transition metals show larger susceptibility to pyramidalization and a wider range of M-M distances than the analogous compounds of second- and third-row metals. (b)The single bonds present a larger susceptibility to pyramidalization than multiple bonds or metal-metal contacts.[91(c) For the first-row transition metals, compounds of the same metal and oxidation state with chelating bridges present clearly shorter intrinsic distances. The chelating nature of the ligand seems to have little influence on the intrinsic M-M distance for second- and third-row transition metals. (d) The intrinsic M-M distances in ds . . . d8 dimers are significantly longer than those of bonded M-M pairs.
1.17.3 Ligand orientation effects in unsupported [Fez(CO)S]2- and derivatives The reactivity of the carbonylate ion [Fe?(CO)sJ'- has been widely explored with regard to the formation of adducts and clusters, due to the nucleophilic character associated with its negative charge. This anion appears in the solid state forming an unsupported Fe-Fe single bond in a structure that can be described by axially fusing two interpenetrated trigonal bipyramids (3).It has been shown, though, that there is an energetically feasible doubly bridged isomer['51 (4) in which the
3 12
1 Moleculur Clusters
3
4
5
coordination spheres of the Fe atoms are two face-sharing octahedra with a vacant bridging position, similar to the situation in the isoelectronic [Co2(CO)s] complex. There is also a hypothetical unsupported isomer for such anion ( 5 ) in which the trigonal-bipyramidal coordination spheres of the metal atoms are fused through equatorial vertices. All three forms of the Fel(C0)8 fragment (3-5)can be found in derivatives with electrophilic groups attached to one or two of the Fe atoms. Some questions arise: What favors either of the three structures for the Fe2(C0)8 fragment in its derivatives? Does the different orientation of the ligands in the three structures affect the Fe-Fe bond distance? For compounds with the same structure, is there a significant variability of the ligand orientation and of the Fe-Fe distance? We start by considering structure 3. The orientation of the ligands in such structure hardly changes for the different salts of the carbonylate dianion (Table 1). Hence, it cannot be established from the experimental data whether there is a bond angle-bond distance correlation. A semiquantitative theoretical study" 'I on the binuclear anion [Fez( C0)sl2- supports the existence of a bond angle-bond distance correlation, with the Fe-Fe distance decreasing upon increasing a in the range 75" < CY < 95" ( i e , a normal pyramidality effect). The bond distances in the range 2.78-2.98 A (with bond angles in the range 86.0 > a > 80.5") are only within 2 kcal/mol from the optimized geometry ( Fe-Fe = 2.874 A and a = 83.2"), in fair Table 1. Structural data for carbonyl complexes of the types [Fe2(CO)& ( 3 ) and [FelEz(CO)s] (6).
~~
2.815 2.770 2.779 2.793 2.844
"71 1181 1191 1201
2.899 2.876 2.840 2.785
1151 [221 1231 1241
I I7 Liycinci Orientation Effects on Metrrl-Metal Bonding
3 13
6
agreement with the experimental data (Table 1 ). Therefore, an interesting synthetic goal would be the preparation of new salts of the [Fe2(CO),12- anion in which the counterions induce different values of a and, consequently, a significant variation of the Fe-Fe distance. It has been shown'"' that two lone pair molecular orbitals in structure 3 are directed in-between the equatorial ligands, thus explaining the formation of two independent bonds with two electrophiles in structure 6. For such unsupported derivatives (6). a normal pyramidality effect is suggested by the data collected in Table 1 at small angles, with a shortest observed Fe-Fe distance of 2.785 A,corresponding to the largest observed angle ( x = 88.2'). Since there is not enough data to firmly establish the existence of such bond angle-bond length correlation, a theoretical study is useful to confirm the proposed trend. Hence, calculations were performed for the model compound [ F ~ ~ ( C H I ) ~ ( Cwith O ) ~ the ] , methyl groups occupying equatorial positions in 6, and varying the orientation of those CO ligands coplanar with the Fe and E atoms.['61 A parabolic dependence of the Fe-Fe distance on the average angle was found (Fig. 2, above), similar to the general pyramidality behavior described by Fig. 1. If one extracts from that parabola the portion that corresponds to energies at most 2 kcal/mol higher than that of the minimum, practically a straight line results (Fig. 2, below, squares), covering a range of angles consistent with the experimentally observed values (Fig. 2, below, circles). Although the calculated curve gives Fe-Fe distances longer than the experimental values, the theoretical correlation between that parameter and the average bond angle x supports the interpretation of the experimental values in Fig. 2 as a normal pyramidality effect, corresponding to the small angle portion of the general parabolic correlation. These results show how one can obtain different values of the structural parameters by adequately varying the nature of the ligands (see Table 1). Note that the turnover point of the pyramidality effect is experimentally and theoretically observed for the M2Lx molecules made by trigonal ML4 groups ( 3 ) , whereas the M ~ E I Lbuilt ~ up from tetragonal MEL4 groups does not show this effect. A similar situation was found for the Mo-Mo multiple bonds, for which the MozLs and Mo2LxX2 complexes (where X represents axial ligands) with tetragonal MoL4 groups show a direct relationship, whereas the MoLi trigonal groups in Mo2L6give an inverted relationship between bond length and bond angle. This fact was explained by means of molecular orbital calculations and attributed to the local symmetry of the ML, fragments."]
314
I Moleculur Clusters
so
85
90
95
100
a ("1
2.5
I
pared below with the experimental data for [FezE2(CO)s]compounds.
1.17.4 Ligand orientation effects in bridged Fe-Fe bonds, including triangular Fe2M clusters In contrast to 3, which can bind one electrophile at each Fe atom as in 6, the doubly bridged anion 4 is best suited to bind only one bridging group as in 7. In effect, the two electron pairs of 4 used to bind to an electrophile can be de~cribed"~] by symmetric and antisymmetric molecular orbitals centered at the metal atoms and directed to the vacant bridging position (9).Hence, a wide variety of acceptor groups
1.I 7 Ligund Orientation Effects on Metal-Metal Bonding
7
8
3 15
9
with two empty orbitals can adequately occupy that position in compounds of type 7. In a structural database search we have found compounds in which the bridging atom is a metal such as Sn, Fe, Ru, 0 s or Au, together with the bridging CO group in [Fez(CO)g]. It is noteworthy that the AuPR~' fragments can act formally as either a terminal two-electron acceptor in 6,'' 51 or as a bridging four-electron acceptor in 7.[251 In compounds of type 7, both the bond angles of the Fe2E skeleton and the orientation of the terminal carbonyls show little variation (54"< Fe-E-Fe < 60°, and 180" < H < 191"). The [Fez(CO)y]compound crystallizes with structure 7,[261has a much shorter Fe-E distance (Fe- C = 2.013 .$) than any member of this family (for which E = Fe, Ru, Os, Sn or Au) but practically the same Fe-Fe distance (Fig. 3 ) . The orientation of the pyramid formed by the three terminal CO ligands and a Fe atom is in this case perfectly aligned with the Fe-Fe vector (0 = 180"), in contrast with other compounds of type 7, for which H is smaller than 186". One can hypothesize about the existence of some degree of correlation between the Fe-E-Fe and 0 angles, but insufficient experimental data is available to draw any clear con-
Figure 3. Scatterplot of the Fe-E bond distance as a function of the Fe-Fe bond distance in the derivatives of the [Fe2(CO)*]*-anion with structures 7 and 8.
24
2.6
2.8
Fe-Fe
(A)
3.0
3 16
1 Molecular Clusters
clusion. A theoretical study on [Fe2(,u-CH2)(,uU-C0)2(C0)6] has shown['61 that the Fe-Fe bond distance in 7 is affected little by changes in 0. The shortest Fe-Fe bond (2.504 A)has been predicted to correspond to H = 180", in excellent agreement with the structural data for the isoelectronic cluster [Fe2(C0)9],r261 which shows a Fe-Fe distance of 2.523 at 8 = 180". In the hypothetical structure 5 of the carbonylate, the molecular orbitals bearing the lone pairs in the equatorial plane are just the symmetric and antisymmetric combinations of d,,-type atomic orbitals, hybridized as the e' set of a trigonal bipyramidal complex.[27]Such orbitals are rehybridized following the movement of the equatorial carbonyl ligands, as schematically depicted in 10 for the antisymmetric combination. Two characteristic cases are those depicted in 6 and 8, in which the two lone pair orbitals are pointing toward two different terminal coordination positions or to the same bridging position, respectively. Groups with two empty lobes (considering an anionic [Fe2(C0),l2- fragment) can interact with the two lone pair orbitals pointing to the same region of space, as shown for the antisymmetric orbital in lob. Hence, a variety of groups are found occupying the bridging position in structure 8, having C, Si, Ge, Sn, Pb, P, As, Sb, In or Pt as the bridging atom E. Of course, the orientation adopted by the equatorial ligands may vary from one compound to another in order to produce the most adequate orientation of the lone pair orbitals for every electrophilic group E. Given the bonding characteristics of those orbitals, one should expect that changes in the orientation of the equatorial ligands might induce changes in the Fe-Fe bond length. The Fe-Fe distance is seen to vary over a wide range (2.59-2.92 A), and so do the Fe-E distances (2.28--2.64A),but no clear correlation between the angular orientation of the terminal carbonyl groups and the Fe-Fe distance has been found. A theoretical study[161has shown that the variations in bond distances in 8 are associated to changes in the orientation of the equatorial ligands, measured here by the Fe-Fe-C angle cis to the Fe-Fe bond ( y ) . The similarity between the experimental data for compounds with either a ~ a r b o n [ ~ *or- ~a~transition l meta1[34,351 bridging atom and those calculated for E = CH2 between 80 and 110" is remarkable (Fig. 4). Note that the Fe-Fe distance is practically constant for large angles, in sharp contrast to the behavior of the unbridged complexes 6, for which the Fe-Fe distance and the total energy increase dramatically for angles larger than 100". However, both curves are practically coincident for smaller angles. Those compounds with Si as a bridging fit well into the general behavior shown by their C analogues. The existence of a similar correlation for those com-
10b
10a
LOc
1.I7 Licjurzd Orientation Effi.c.tson Metal-Metal Bonding
3 17
Figure 4. Relationship between the calculated Fe-Fe distance and the Fe-Fe-CO bond angle ( 7 ) of the carbonyl group trans to the substituents E in the model compound [ F ~ ~ ( , u - C H ~ ) ( C O ) X ] (structure 8, circles, adapted from ref. [ 161). Also shown are the experimental values[ZR~33~411 for [Fe2(pE)(CO)Rjcompounds 8 in which E is a carbene or a vinyl group (circles), and for [Fe2Tz(CO)x]complexes of structure 6 (triangles) in which T is a transition metal.
pounds with Sn, P, As or Sb in the bridging position is not clear from the available experimental data. It is interesting to note that the bridged compounds of type 8 appear to be much more flexible than the unbridged molecules 6, as reflected by the experimental values for the Fe-Fe-C,,, bond angle (from 90 to 110" in 8, from 85 to 93" in 6) and for the Fe-Fe distance (2.59-2.91 A in 8, 2.78-2.90 A in 6) as revealed by a Cambridge Structural Database search[381(see Fig. 3 and Table 1). If such fragment is to be bound by two electrophilic groups as in lla, the antisymmetric combination of the e' orbitals does not have the right symmetry and a distortion (llb) should occur in order to reorient the lone pair orbitals, as schematically shown in 1Oc. This is what happens in the compounds in which E = CuPR?+, with distorted structure^[^^*^^^ of type llb in which the two FelE planes form angles between 31 and 44", and the terminal ligands are rotated around the Fe-Fe axis. It is worth stressing that structure lla has a poor topology to make use of its lone pair orbitals (lOa),whereas a disrotatory motion of the equatorial ligands makes them available for bonding to a bridging group with two empty lobes (lob) to give structure 8. Alternatively, a conrotatory motion of those ligands allows it to bind one electrophilic group to each Fe atom (10c) and give structure llb or, ultimately, structure 6.
318
I Mokculur Clusters
lla
llb
A comparison of the structural data for the three types of bridged compounds (Fig. 3) indicates that the Fe--Fe bond distances are in general larger for compounds 8 and l l b (2.59-2.92 A) than for 7 (2.52-2.61 A).The presence of a bridging carbon atom in compounds with structure 8 favors a short Fe-Fe distance compared to other bridging atoms (Fig. 3). That feature appears both with sp’ carbon atom from CR2 g r o ~ p s [ and ~ ~ with , ~ sp2 ~ . carbon ~ ~ ~ atoms from vinyl g r o ~ p s [ ~ ~ . ~ ~ ] . Apparently, structures 7 and 8 are very close in energy, since [Fe2(CO)9] adopts structure 7, whereas the isoelectronic [Fe2(CR2)(CO)g]compounds appear either in structure S,[28,299411 or as its triple-bridged isomer 7.r42.431
1.17.5 Clusters with a Fe& skeleton A number of derivatives of the tetranuclear cluster [FedC(CO)1 2 1 ~ - with a butterfly structure (12) have been obtained, in which a group is bonded to either the carbido atom or to the opposite Fe-Fe edge (Table 2). A systematic study of the structures of such clusters[441reveals how the carbonyl groups react to the presence of groups attached to these two positions. The position of the three carbonyl ligands coordi-
12
1.17 Ligand Orientation Efects on Metul-Metal Bonding
3 19
Table 2. Orientation of the carbonyl ligands at the wingtip (d) and at the hinge (iof .) the carbido butterfly clusters of type 12. X represents a group linked to the carbide atom. Y is a group bound to the lower Fe- Fe hinge.
X
COOMe C(OMe)? C(OMel2 COMe COMe AuPR~ H H BH2 OMe OMe 0 AuPR~
n
Y
A
153
i 47
155 155 159 169 166 161 163 158 164 159 165 164 I63
150 158 146 146 142 145 146 146 I 60 156 158 160 162
166
H
H H H H AuPEti AuPEti CO
161
Fe-Fe (hinge)
2.533 2.608
2.587 2.558 2.532 2.572
ref. [45l [461 [471 ~481 1491
"W ~481
2.556
[sol
2.578
[441
2.603
~511
2.584 2.586 2.605 2 688 2.649 2.571
1521
[531 154, 471 [55l i551 1561
nated to a particular iron atom can be defined by the angle that the centroid of the three carbon atoms forms with the wingtip Fe atoms, 6, or with the Fe-Fe hinge, A. The values of such angles for the structurally characterized clusters are presented in Table 2. There, it can be seen that whenever there is a group attached to the carbide atom, the angle S is larger (158-167') than for the bare carbido cluster (153-155"). Similarly, the rLangle presents larger values when there is a group bonded to the Fe-Fe hinge (1.58-166") than when there is no such group (142-150"), regardless of the nature of the attached group (Table 2). In other words, the carbonyl ligands open up to allow for bonding to the edges of the cluster. The joint motion of the carbonyl ligands away from the incoming group is not just a steric effect, as was shown by molecular orbital calculations.[351Such motion in the bare cluster produces a rehybridization of the symmetric and antisymmetric donor orbitals of the Fe4C framework that favors interaction with an acceptor group at the wingtip (Fig. 5 , 2a1, and lbl orbitals) or at the hinge (Fig. 5 , l a , , and 2b2 orbitals). Extended Hiickel molecular orbital calculations have shown that the optimum angle b is larger for the wingtip-substituted than for the bare cluster, and that the optimum angle 2 is larger for the hinge-substituted than for the bare cluster, in excellent agreement with the experimental data. The bonding energy between the bare cluster and the H+ or (AUPH?)' groups has been calculated to improve by 8
320
I Molecular Clusters
Figure 5. Changes in the shape of the symmetric and antisymmetric occupied molecular orbitals la1 and 2bz of the [Fe4C(C0)12Ix+cluster when the carbonyl groups of the wingtip Fe atoms open up (increasing CS in 12) and of the molecular orbitals 2al and lbl when the carbonyl groups of the hinge Fe atoms are separated (increasing 1. in 12). Reproduced with permission from Ref. [44].
to 12 kcal when the carbonyl groups are reoriented. Note that the cluster donor orbital 2al centered at the Fe-Fe hinge has strong Fe-Fe 6 bonding character. Upon interaction with an acceptor group, the 2al is partly delocalized and the Fe-Fe bond should be weakened. Such a simple orbital picture allows one to understand the distribution of the hinge bond distances found for different clusters: for the bare hinge, the Fe-Fe distances are in the range 2.53-2.59 A, whereas for the substituted hinge, those distances fall between 2.59 and 2.69 A.
I . I 7 Ligriricl Orientrrtion Eflects on MetulLMetal Bandin
321
Acknowledgments Financial support to this work was provided by DGES. (grant PB95-0848), and Fiindmid Cutulunu per rr la Recercrr through a grant for computing resources in the Centre de Superconiputucib de Ccrtdunyu (CESCA).
References 11 I X.-Y. Liu, S. Alvarez. Diorq. C/zeni. 1997. 36, 1055.
121 F. mot^, J. J. Novoa. J. Losada, S. Alvarez, R . Hoffmann, J. Silvestre, J. Am. Cltern. Soc. 1993, 115, 6216. [3] J. Losada, S. Alvarez. J. J. Novoa, F. Mota, R . Hoffmann. J. Silvestre, J. Atn. C h i . Soc. 1990. 112, 8998. [41 G. Aullon, S. Alvarez. Inory. Clicw. 1993. 32. 3712. 151 G. Aullon, P. Alemany, S. Alvarcz. Inorg. C h n . 1996, 35, 5061. 161 F. A. Cotton. R . A. Walton, Multiple Borzih Betwen Metal A t o m . Clarendon Press, Oxford 1993. [7) P. Alemany. S. Alvarez, V. G. Zubkov, V. P. Zhukov, V. A. Pereliaev, I. Kontsevaya. A. Tyutyunnik, Botll. Soc. Cut. Cikn. 1992, 13, 251. 181 P. Alemany, V. G. Zubkov. S. Alvarcz. V. P. Zhukov. V. A. Pereliaev. 1. Kontsevaya, A. Tyutyunnik. J. Solid Stat(. Clwni. 1993. 105. 27. 191 G. Aullon. S. Alvarez. J. Clieni. Soc... Drrltoii Truns. 1997. 2681. [ l o ] G. Aullon, S. Alvarez. Cltcni. Etrr. J. 1997, 3 , 655. [ 1 I ] V. S. Mastryukov, H. F. Scheffer 111, J. E. Boggs, A N . C/zcm Rex 1994. 27, 242. [ 121 W. A. Shirley, R. Hoffmann. V. S. Mastryukov. J. P/zy.s. Clwn. 1995. YY, 4025. [I31 P. von R. Schleyer. M. Bremer. Ampii.. Client.. h t . Ed. Engl. 1989, 28, 1226. 1141 S. Alvarez, P. Alemany, G. Aullon. A. A. Palacios, J. J. Novoa. in Tllc Sj~ieryjBetiveun Dynurnic.~r i n d Reuctirity (it C/uster.c and Suvfircrs. ( Ed.: L. J. Farrugia). Kluwer Academic Publishers. Dordrecht. 1995. p. 241. 1 151 S. Alvarez. 0. Rossell. M. Seco. J. Valls, M. A. Pellinghelli. A. Tiripicchio, Orljanoriic,tnllics 1991, 10. 2309. [ 161 G. Aullon. S. Alvarez. to be submitted. [I71 H. Deng. S. G. Shore. Itzorq. Clieni. 1992. 31. 2289. [ 181 F. Seel, R. Lehnert. E. Bill. A. Trautwein. %. I V L ~ ~ L I ~ ~ O B I . S1980, ~ ~ / I . 35, 6 3 1. (I91 T. M. Bockman, H.-C. Cho. J. K . Kochi, Organoiiic,ral/ics 1995. 14. 5221. 1201 N . K. Bhattacharyya. T. J. Coffy. W. Quintana, T. A. Salupo, J. C. Bricker, T. B. Shay, S. G. Shore. 0r~jatiorizrtallic.s1990, 9, 2368. idy. K. 13. Whitmire. G. J . Long. J. Oryunontet. C h m . 1992. 427. 355. (22) R. S. Simons, C. A . Tessier. Or~/crriorrietiillic.s1996. 15>2604. 123) D. Luart. M. Sellin. P. Laurent. J.-Y. Salaun, R. Pichon. L. Toupet. H. d. Abbayes, Oryunoniettrllics 1995, 14- 4989. 1241 J. S. Field. R. J. Haimes. C. N. Sampson. J. Clieni. Soc., Dalton Tram. 1987, 1933. [25] 0. Rossell, M. Scco, P. G. Jones. biory. Clienz. 1990, 2Y. 348. 1261 F. A . Cotton. J. M. Troup, J. Cliem. Soc., Dulton Trans. 1974, 800.
322
1 Molecular Clusters
1271 T. A. Albright, J. K. Burdett, M.-H. Whangbo, Orbital Interactions in Chemistry, J. Wiley, New York 1985. [28] C. A. Mirkin, K.-L. Lu, T. E. Snead, B. A. Young, G. L. Geoffroy, A. L. Rheingold, B. S. Haggerty, J. Am. Chem. Soc. 1991, 113, 3800. [29] M. Wiederhold, U. Behrens, J. Organomet. Chem. 1994, 476. 101. [30] 0. S. Mills, A. D. Redhouse, J. Chem. Soc. A 1968, 1282. 1311 W. Schulze, K. Seppelt, Znorg. Chem. 1988, 27, 3872. 1321 M. J. Bennett, W. A. G. Graham, R. P. Stewart Jr., R. M. Tuggle, Znorg. Chem. 1973, 12, 2944. (331 D. Luart, M. Sellin, P. Laurent, J.-V. Salaun, R. Pichon, L. Toupet, H. des Abbayes, Organometallics 1995; 14, 4989. [34] R. Mason, J. A. Zubieta, J. Organomet. Chem. 1974, 66, 289. [35] L. J. Farrugia, J. A. K. Howard, P. Mitrprachachon, F. G. A. Stone, P. Woodward, J. Chem. Soc., Dalton Trans. 1981, 1134. [36] R. S. Simmons, C. A. Tessier, Organometallics 1996, 1.5, 2604. [37] S. G. Anema, G. C. Barris, K. M. Mackay, B. K. Nicholson, J. Organomet. Chem. 1988, 350, 207. [38] F. H. Allen, 0. Kennard, Chem. Des. Autom. News 1993, 8, 31. [39] H. Deng, S. G. Shore, Organometallics 1991, Z0, 3486. [40] H. Deng, D. W. Knoeppel, S. G. Shore, Organometallics 1992, IZ, 3472. [41] C. A. Mirkin, K.-L. Lu, G. L. Geoffroy, A. L. Rheingold, D. L. Staley, J. Am. Chem. Soc. 1989, 111, 7279. [42] C. E. Sumner Jr., P. E. Riley, R. E. Davis, R. Pettit, J. Am. Chem. Soc. 1980, 102, 1752. [43) B. B. Meyer, P. E. Riley, R. E. Davis, Znorg. Chem. 1981,20, 3024. [44] 0. Rossell, M. Seco, G. Segales, S. Alvarez, M. A. Pellinghelli, A. Tiripicchio, D. de Mon1997, 16, 236. tauzon, Organ~metallic~s [4S] J. H. Davis, M. A. Beno, J. M. Williams, J. Zimmie, M. Tachikawa, E. L. Muetterties, Proc. Natl. Acud. Sci. U.S.A. 1981, 78, 668. [46] R. F. Boehme, P. Coppens, Acta Crystullogr., Sect. B 1981, 37, 1914. [47] E. M. Holt, K. H. Whitmire, D. F. Shriver, J. Organomet. Chem. 1981, 213, 125. [48] J. S. Bradley, S. Harris, J. M. Newsam: E. W. Hill, S. Leta, M. A. Modrick, Orgunometullics 1987, 6, 2060. [49] J. S. Bradley, S. Harris, E. W. Hill, M. A. Modrick, Polyhedron 1990, 9, 1809. [SO] J. Wang, A. M. Crespi, M. Sabat, S. Harris, C. Woodcock, D. F. Shriver, Znorg. Cheni. 1989, 28, 697. [51] M. A. Beno, J. M. Williams, M. Tachikawa, E. L. Muetterties, J. Am. Chem. SOC.1981, 103, 1485. [52] H. Wadepohl, D. Braga, F. Grepioni, Organometullics 1995, 14, 24. [S3] X. Meng, N. P. Rath, T. P. Fehlner, J. Am. Chem. Soc. 1989, I l l , 3422. 1541 K. Whitmire, D. F. Shriver, E. M. Holt, J. Chem. Soc., Cizem. Commun. 1980, 780. 1551 C. P. Horwitz, W. M. Holt, C. P. Brock, D. F. Shriver, J. Am. Chem. Soc. 1985, 107, 8136. 1561 0. Rossell, M. Seco, G. Segales, B. F. G. Johnson, P. Dyson, S. L. Ingham, Orgunonzetallics 1996, 15, 884.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.18 Synthesis and Properties of Metal Carbonyl Clusters Containing Nitrido Ligands Alcssandro Funiagalli and Roberto Dellu Pergola
1.18.1 Introduction 1.18.1.1 Generalities on interstitial nitrido clusters It was pointed out, several years ago by Paolo Chini,“] that one of the crucial limitations of cluster growth is the steric requirement of the carbonyl ligands, which becomes more and more relevant as the nuclearity ( n ) increases. In fact, in a small cluster each metal may bond to as many as three- or four carbonyls; in the higher nuclearity species the ratio number of’COjn may drop to as low as one. This is due to the presence of large dihedral angles between flat faces made-up of several atoms, where the steric hindrance between adjacent ligands becomes severe and also because a significant fraction of the metals is internal and not available for ligand coordination. Essentially, for these reasons the largest structures with many Cluster Valence Orbitals (CVO),[21are typical of “electron rich” metals from group 9 and 10, which require fewer ligands. Carbonyl clusters, which may have a number of conventional ligands ( H , halide, phosphine, NO etc.) coordinated, may also contain simple main group “E” atoms (B, C, N , P, S etc.). These latter ligands may be senzi-esposed on the cluster surface or, as is the case of high nuclearity species ( n > 6), in interstitial sites within the metal skeleton. The importance that these interstitial- and semi-interstitial atoms have in the build-up of the largest structures can be seen by an analysis of Table 1, where the largest carbonyl cluster species, so far synthesized from groups 7, 8, 9, and 10, are listed. The majority of these high nuclearity clusters contains interstitial carbides or nitrides. The inclusion of these interstitial main group atoms means that high nuclearity clusters of rhenium and the d8 metals, which have relatively few electrons, can also form. Even first-row transition metals will form high nuclearity clusters if stabilized by interstitial main group atoms despite the relatively low M-M bond energies compared to metal carbonyl bond energies.
324
1 Molecular Clusters
Table 1. The largest homometallic carbonyl cluster species within groups 7-10.
In fact, we may think that the stabilization of these clusters depends essentially on two factors: one is the formation of several M-E,,, interactions, which unfortunately cannot be exactly quantified, since thermodynamic data are not yet available. The second one is that the E atoms, acting as “internal ligands”, contribute to the Cluster Valence Electrons ( C V E )count with no steric requirements at the cluster surface and relatively little “swelling” of the cluster framework. A nice illustration of the influence of the interstitial atom is the comparison of the two hexanuclear ] ~ - [ C O ~ N ( C O ) ~ ~ ]both - ; ” ~specie ] have 86 CVE as reclusters, [ C O ~ ( C O ) ‘~1 3 ]~ and quired for the octahedral geometry, and this is achieved in the monoanion with only 13 carbonyls and significant relief of steric hindrance on the surface, because the interstitial nitrogen can contribute 5 electrons. The nitrogen atom lies within the cage with an average N-Co interaction of 1.85 A,and causes a moderate “swelling” of the metal core, and the Co-Co bond lengths “stretch” to an average dis~ ] 2.509 ~ - A). tance of 2.613 A with respect to those in [ C O ~ ( C O ) ~(av. The chemistry of interstitial nitrido clusters, which is perhaps the second largest group after the carbides, was reviewed several years ago[’51when there were still relatively few examples. Since then, some progress has been made, particularly in the field of high nuclearity clusters of the iron and cobalt sub-groups and several species containing more than one interstitial nitrogen atom have been obtained. This report has been written in the light of these recent results and is also intended to show the distinctive reactivity and the patterns of cluster growth (M-CO or M-M splitting mechanisms), which are modified by the presence of an interstitial nitrogen atom.
1.18.1.2 Synthesis of the nitrido ligand in clusters Here, the term “primary” is intended to refer to those species where the interstitial or semi-exposed nitrido atom has been generated from some precursor, typically a nitrogen-oxygen compound. These primary compounds are listed in Table 2. All those species that are obtained from the reaction of primary nitrido compounds, and contain one or more interstitial nitrido atom(s) will be referred to as “derived” species, and will be discussed in the following sections. Almost all the primary nitrido clusters can be obtained by reaction of a carbonyl anion and the nitrosonium NO+ ion, and this procedure was typically exploited for
I . 18 Sj’ntliesis and Properties of Metul Carhonyl Clusters
325
Table 2. Primary nitrido carbonyl clusters. Compound
Synthesis
Ref.
[Fel(CO)g12-+ NOBF4, diglyme 130°C Feq(CO)I? [Fe(COIqNO]-. THF, RT [Fe?(C0)812+ NOBF4, diglqnie 145 C R L I I I C O+ ) ~[RulCO)INO]-, ~ THF, RT Ruq(CO)l? + PPN(NCO), THF, RT + reflux Rui(CO)lo(/~-N0)? + CO, 1 l O ’ C , 9 h [ H I R U ~ ( C O , I t~ ]NOBF4, CHlC12 Ru?(CO)I?+ PPN(N3), THF, reflux I H ? O S ~ ( C O+) ~ NOBF4, ~~ HzC12 [ M b ( C 0 ) 1 5 ] ~ - NOBF4 [Rh7(CO)l,]’ + NO/CO, MeOH. OH-, RT
+
+
122, 231 I221
the first synthesis of this kind of compounds. However. reactions with other nitrogen sources, such as nitrite (NO?-), azide (N?-) or isocyanate (NCO-) ions have been developed to produce the same clusters much more efficiently under milder conditions. Thus, [ F ~ s N ( C O ) , J was ] - prepared by Muetterties in diglyme at 145 “C, from [Fez(CO)x]’- and [NO]BF4. The same reagents in the same solvent, heated at a slightly lower temperature for a shorter time, yield the tetranuclear cluster [Fe4N(CO)&.[’ 61 The latter was obtained independently by Gladfelter, by condensation of Fel(CO)12 and [Fe(CO)3NO]- at room temperature. In contrast to what might be expected, the two iron complexes do not condense in a 1 : 1 ratio. An excess of Fe3(CO)Iz is necessary, and Fe(CO)5 is produced according to the Eq. (1):
3 Fe;(C0)12
+ 2 [Fe(CO)jNO] +
2 [Fe4N(CO)J
+ 3 Fe(CO), + 2 C 0 2 + CO
(1)
The nitrido ligand, which is coordinated to all four iron atom, is formed by deoxygenation of the coordinated nitric oxide by a carbonyl ligand, with elimination of carbon dioxide:” 71
Fe-NO
+ Fe-CO
4
/N \ Fe-Fe
+ CO?
This step is reminiscent of and possibly mimics the reduction of NO from exhaust gases of combustion engines. The cluster [Ru4N(CO)12]- was also obtained by Gladfelter either with the analogous condensation between [Ru(C0)3NO]- and Ru?(C0)12,or by cleavage of the N-C bond in the isocyanate-substituted cluster
326
I Molecular Clusters
[ R u ~ ( N C O ) ( C O ) I ~which J - , can be prepared by reacting the NCO- ion with R u ~ ( C O ) ~ Condensation ~.[~~I of N3- with excess of Ru3(C0)12, in THF under reflux, yields the octahedral [Ru6N(CO)161- cluster anion via the intermediate formation of clusters such as [Ru3(NCO)(CO)lo]- and [ R u ~NCO)(CO)l ( I]-, The transformation of the azide into a nitride can be schematized as follows:
The closely related hydrido species [HRu4N(C0)12] and [ H ~ R u ~ N ( C O )were II] previously obtained by the more conventional reaction of [ H ~ R u ~ ( C121O ) and NO+, which can also be applied to the preparation of [ H O S ~ N ( C O ) ~ ~ Despite ].[’~I its low yield (lo%),the latter reaction is relevant from a mechanistic viewpoint, since the major product is the nitrosyl-substituted cluster [ H ~ O S ~ N O ( C121, O )which shares the same metal framework and is believed to be the precursor of the nitrido cluster.[201The tetrahedral substrate [H30~4(C0)12]-is transformed in a butterfly cluster, the NO ligand bridging the two wing-tip metal sites, in such an arrangement that the putative nitride atom sits almost exactly in its final position. The transformation of [H30s4NO(CO)121 into the nitrido cluster was indeed observed, by keeping it in solution at room temperature. This strongly suggests that the latter is an intermediate in the formation of the metal-bound nitride and the reaction occurs through the elimination of H20 (instead of CO2). It was, therefore, proposed that the bending of the nitrosyl would enhance interaction between the oxygen atom of the NO and an adjacent hydride or carbonyl, favoring elimination of water or carbon dioxide.[”] [Ru~(CO)IO(~-NO reacts ) Z ] with 1 atm of CO to form the butterfly species [RuN(C0)12(p-NO)]and [ R u ~ N ( C O ) I ~ ( ~ - N in C O30) ]and 5% yield, respectively. These clusters possess 64 CVE, being substituted by three electron donors, which span the elongated hinge edge.[”] The two isostructural clusters [Co6N(CO)1 5]-[”2] and [Rh6N(C0)15]-[’~]were synthesized by reaction of the octahedral clusters [M6(C0),5l2-(M = Co, Rh) with nitrosonium tetrafluoborate. Better yields of the rhodium species were obtained by treatment, in a strongly reducing medium, of [Rh;i(C0)16]3-[241 with NO (diluted 1 : 1 with CO). The reactions are complex and no intermediate step could be identified. However, a nitrosyl derivative is very likely to be involved, and subsequent reduction, by the reaction medium or the starting reduced carbonyl itself, would yield the nitrido cluster. The nitrite ion N02- can conveniently be used as a source of NO ligands. In the form of the [N(PPh3)2]+ salt, it is highly soluble in most organic solvents, and smoothly reacts with carbonyl complexes, yielding either nitrosyl complexes or, in a single step, nitrido clusters.[251 The reduction of a coordinated NO by a CO ligand, with formation of COz has been observed in the synthesis of [MoMzCp3(N ) ( 0 ) ( C 0 ) 4(M ] = Mo, W).[261
1.18 Syntfzesisand Properties (f Metal Curhonyl Clusters
327
1.18.2 Synthesis of nitrido clusters 1.18.2.1 Iron clusters Only tetra- and pentanuclear iron clusters, containing semi-exposed nitrides, were known until 1996. The most relevant studies on their reactivity focused on protonation reactions and CO substitutions (mainly by P-donor ligands). Both [Fe4N(CO),2]- and (Fe5N(CO)14]- can be converted, with strong acids, into the neutral clusters [HFe4N(CO)l2] and [HFe5N(C0)14].['~l The structures of both derivatives have been determined, and in each the hydrides adopt a bridging coordination. Even a large excess of acid was ineffective at forming the cation [HFe4NH(C0)12]+.[161The lack of any residual basicity at the nitrogen atoms is demonstrated by the absence of any direct interaction with the hydrides. The lowlying orbitals of the nitrogen are not susceptible to attack by electrophilic reagents and as a consequence, the cluster does not react with metal complexes with acidic character. It is worth noting that the hydrido-carbide [HFe4C(C0)12]-can be further protonated, to yield the neutral cluster [HFedCH(CO)12 I. The resulting methylidyne fragment displays a direct C-H bond, which models the hydrogenation of carbides anchored on metallic surfaces.[271 A nitrosyl derivative. [Fe4N(CO)IlNO]can be obtained by the direct reaction of the tetranuclear carbonyl cluster and [NO]BF4, in dichlor~methane."~'One CO ligand in [Fe4N(C0)12]- or two in [Fe4N(CO)l,NO]can be substituted by a large variety of P-donor ligands.[281The double substitution in the latter can be accomplished at room temperature, in keeping with the higher reactivity of the nitrosylsubstituted complexes. For CO displacement on the former, two method have been developed (i) the thermal activation,[281and (ii) Electron Chain Transfer Catalysis (ECTC).[291In the second method, catalytic amounts of a reductant such as NaCloH8 form the odd-electron species [ F e 4 N ( C 0 ) 1 2 ] ~whose ~ , carbonyl ligands are labile. Substitution occurs readily, and the electron-rich [Fe4N(CO)I1 LI2- anion is quickly oxidized to the final monoanionic product, propagating the chain reaction. When L = PPh3, yields are comparable with those achieved with the conventional thermal activation. The phosphine always binds at the wing-tip sites, presumably for steric reasons. After substitution by the bisferrocenylphenylphosphine. the redox flexibility of the cluster was increased. Using cyclic voltammetry three different oxidation states for [Fe4N(CO),IPPh(CsH4FeC5H5)2]PcouId be detected, and distinct electrochemical potentials for the oxidation of the two ferrocenyl moieties measured The observations that [Fe4N(CO)121- has no nucleophilic character and its ligands are labilized by reduction, indicated that it might react with anionic metal
328
1 Moleculclr Clusters
complexes. The idea proved correct and, by redox condensation with [Fe2(C0)sJ2-, the octahedral trianion [Fe6N(CO)151" was prepared in good yield. The hexanuclear trianion could not be oxidized to the hypothetical [Fe6N(CO)16j-, but gave the pentanuclear [ FesN(CO)14]- anion instead.[301Thus, the following sequence of reduction and oxidation is the best way to interconvert the three known iron nitride clusters:
1.18.2.2 Ruthenium clusters A higher degree of flexibility was shown by the larger variety of ruthenium nitrido clusters, which can all be interconverted through the addition or removal of Ru(C0)2 units. The smallest member of the series is [RuqN(C0)12]-, which can easily add one or more of such fragments. When treated with Ru(C0)s at room temperature, it forms [Ru5N(C0)14jP within 90 minutes; condensation with Ru?(C0)12 at higher temperature yields [RugN(C0)16j-. The latter, containing a fully encapsulated nitride in an octahedral cage, can be obtained in a single step by reacting the azide and Rul(C0)12 in refluxing T H F (24h).[''] At higher temperature (diglyme, cu. 160 "C), in the presence of excess Ru?(C0)12,other Ru(C0)2 units are added, and the decanuclear [Ru10N(C0)24]- cluster is formed, having a tetracapped octahedral skeleton (Fig. l).[311Therefore, the nuclearity of the final nitrido product can be determined by selecting the reaction temperature and the [Ru4N(CO),2j- : Ru3(C0)12 ratio. The process can be reversed, since
v
Figure 1. The metal skeleton of [RuloN(CO)zd]- with the nitride lodged in the octahedral cavity.
1.I N Sjnthesis irtid Properties of Metal Curhonyl Clusters
329
[ RuloN(C0)24lp and [Ru6N(C0)16]- are almost immediately degraded, under ' ' latter is very stable, and atmospheric pressure of CO, to [ R " N ( C O ) I ~ ] ~ . ' ~The more than 200 atm are required to complete the degradation to a mixture of [Ru4N(CO),2]- and [ R U ~ N C O ( C O ) ~ ~The ] ~ . "stoichiometry ~' and structures of these ruthenium clusters nicely exemplifies the validity of the Polyhedral Electron Pair Theories:"' any "Ru(CO)?" fragment added corresponds to an increase of 12 CVE, exactly as required to reconstruct a closo-cluster by the addition of a vertex (or a trigonal cap) to the arriclzno-cage of [ R u ~ N ( C O ) I ~The ] - . feasibility of the ~ , probably be exwhole process, contrasting the stability of [ F e h N ( C 0 ) 1 5 ] ~can plained by the low charge of the ruthenium clusters, which can be easily dissipated even by a few metal atoms. The [ R U ~ ( C O ) ~ ~ ( , L Q - Ncluster: O M ~ ) ] possessing a triply bridging O-alkylated nitrosyl, was heated to increasingly high temperatures, affording a large variety of clusters with isocyanate, nitrene and nitrido ligands. The most interesting compound is [Rug(CO)13(pj-NH)( pj -N )( p3-OMe) {pi-q'-C(O)OMe j 21 where the nitride is located in a pentanuclear square pyramidal metal framework, which shares one methoxy, two carboxylate, and one nitrene ligand with a mononuclear octahedrally coordinated ruthenium complex.[321
1.18.2.3 Cobalt and rhodium clusters; some clues to the pyrolytic cluster growing mechanism All the cobalt and rhodium nitrido species reported so far derive from the prototypical trigonal-prismatic anions [M6N(CO)15]- ( M = Co or Rh).[22,231 The octahedral cluster [ C O ( , N ( C O ) I ~ ] ~ has [ ' ~been ] obtained by mild pyrolysis of its precursor and is regenerated upon exposure to CO.
It is worth noting that the analogous rhodium derivative has never been obtained, although several attempts have been made to isolate it. This is even more remarkable because in the parallel carbido chemistry both the trigonal-prismatic RhL3j1)and the corresponding octahedral species [M6C(CO)15]2p (M = Co[3.3,341, ( M s C ( C O ) ~ ~species ] ~ - fCoL361. Kh[37.381), with 86 CVE are known. Spectroscopic evidence of an octahedral nitrido Rh cluster, that is perhaps the unsaturated 84 C'VE [Rh6N(C0)12lp anion, has been reported,r3y1but this product has not been isolated and fully characterized. Both the cobalt and rhodium parent hexanuclear nitrido clusters react with the corresponding tetracarbonylmetallate to give in a reaction the heptanuclear species [ C O ~ N ( C O ) ~ ~ ]and ~ ~ [Rh7N(CO)1j12--,[411 ,[""~ which may be reversed by CO. This synthesis also appears as a general method for producing mixed metal species (see later).
330
I Molecular Clusters
Reaction 6 is formally identical for cobalt and rhodium, however, in the cobalt case the octahedral cluster [CogN(CO)13]- is produced first (according to Eq. ( 5 ) ) and then the condensation step occurs. The two compounds, which share the same metal skeleton (a trigonal prism capped on a square face), have also a different ligand disposition which yields an idealized C2 symmetry in the cobalt case and C, for the rhodium compound. The two metals also show a different behavior with regard to reduction in basic media. The anion [RhbN(C0)1~]-requires quite an high concentration of NaOH (0.5-1.3 M in methanol or water) to undergo nucleophilic attack, yielding a hydridic diani~n.[~’] Considerably higher concentrations of base (> 10 M ) are required to give the more reduced species which can be identified as [ R h s N ( C 0 ) 1 4 ] ~ - . ‘ ~ ~ ]
[Co6N(CO)lsJ- is much more sensitive to the basic media and only under carefully controlled conditions, i.e. in buffered solutions in the pH range 8.5-11, does it give [ C O ~ N ( C O ) ~ ~ which H ] ~ - ,is characterized from the IR spectrum (water, 1996, 1828 cm-’), which is reminiscent of that of the rhodium hydrido dianionic species. A higher concentration of aqueous NaOH (1.3 M), yields the anion [ C O ~ O N ~ ( C Owithin ) ~ ~ ] ~minutes, at room temperature,r441together with the mononuclear anion [Co(CO)4]-, a product which is consistent with the formal stoichiometry :
One could speculate whether this decametal cluster with an unprecedented geometry (Fig. 2), containing two interstitial nitrogen atoms, results from a sequence of aggregation and restructuring (2 Co6N t C012N2 C O I O N ~rather ), than a skeletal reduction followed by dimerization (2 Co6N + 2 CosN + Co10N2). The second route seems more consistent with the strong reducing medium, which is very likely to yield, through a two-step reduction, analogously to that of Eq. (7), an intermediate species [ C O ~ N ( C O ) ~ which ~ ] ~ - , is expected to be quite unstable. In fact, such a high anionic charge would further enhance (comparative to rhodium) the EM-CO-EM-Mbond energy gap,[45.461 thus favoring its breakdown with formation of [Co(CO)4]- and possibly a pentanuclear species, [CO~N(CO),]~-, which fits well for the “dimerization” leading to [COIONZ(CO) 1 9 1 ~ ~ . ---f
1.18 Synthesis and Properties of Metul Carhonyl Clusters
331
Figure 2. The metal skeleton of [ColoN2(CO)lol4- with two nitrogen atoms in trigonal prismatic cages.
Now there are quite a few clusters like [ C O ~ " N ~ ( C O )that ~ ~ ]contain ~ - , more than one interstitial nitrogen atom but they are known only for cobalt and rhodium. As can be seen in Table 3 they all are derived from nitrido precursors, mostly [CogN(C0)15]- ( 1 ) or [RhsN(C0)15]- (2), and are generally produced by thermal activation. Scheme I may help to illustrate the different behavior of the Co and Rh compounds. Pyrolysis of 1, under mild conditions (THF, or DGM 50-80 " C ) ,gives essentially only one product, the previously described octahedral anion [CosN(CO)13]-. This is quite stable and only a temperature increase up to 140-150 "C produces, besides some decomposition to cobalt metal, the high nuclearity species [Co14N3(C0)2g]7-.[471 N o evidence of other cobalt nitrido species has been obtained Table 3. Poly-Nitrido Cobalt and Rhodium Clusters. Compound
Synthesis [ C O ~ N ( C O )in~ ~ aqueous ] NaOH ( 1 1 M ) [C014N?(C0)261' in water at pH 11 100°C ( 4 h ) [CosN(CO)lsj- in DGM 140-150 C ( 2 - l h ) reduction or oxidation of [ C O ~ J N ~ ( C O ) ~ ~ J ' IRh6N(CO)I51- in refluxing MeOH t [HCOil /[OH] jRh6N(CO)l,] in MeCN 60-80 "C 1 h) [RhhN[CO)I,J in refluxing MeCN ( 3 h) [RhsN(COrls] i n refluxing i-PiOH ( 1 5 h ) or DGM 140 C(1Oh) (Rh6NfCO115] in refluxing r-PrOH 14 h) [RhhN(CO 151 in DGM 150-170°C (24h) or, better [Rhl4N?(CO)?,I2 in water at pH 11, 100°C (3 5 h)
Ref
[561 [71
332
1 Moleculur Clusters
Scheme 1. Sequence of anions observed in the pyrolysis of [CooN(CO)15] and [RhhN(CO)15]
under these experimental conditions, but minor amounts of [Col4N3(C0)26I2- or [Co14N?(C0)~6]~have been identified.[481The odd electron species, with 195 and 197 CVE respectively, can be obtained by conventional oxidants (molecular oxygen, excess acids, 12 etc.) or reductants (NaOH in MeOH), respectively. The family of anions [C014N3(CO)26]~/~/~is a good example of clusters acting as “sponges” for electrons, because of the possibility of there being several stable electronic configurations with minimal structural variations. The only other large cobalt nitrido species, [ C O ~ ~ N ~ ( C O ) ~has ~ ] ~been -.[~~I obtained using a different approach, that is by reaction of [ C O , ~ N ~ ( C O in ) ~a~ ] ~ buffered aqueous solution ( pH cu. 1 1, 100 “C);there is partial decomposition with a nuclearity reduction and loss of an interstitial nitrogen. Clearly we cannot think of a naked nitride atom set free by the simple breaking of its metallic cage. It is very likely that it leaves attached to the cobalt fragment, which is also cleaved from the cluster. In what form it behaves, whether it is still as a nitride, NO or NCO, has not been established so far. The pyrolysis of 2, even if conducted under relatively moderate conditions (in MeOH, MeCN, i-PrOH or DGM, not exceeding 80 “C), never gives evidence for an octahedral 86 CVE species analogous to the cobalt one. Indeed, the first IR detectable species in solution have been all identified as dodecanuclear clusters, namely [Rhl2N2(C0)24I2- (4),[”’ [Rh12N2(C0)24H]- (5),[5 or (Rh12N2(C0)23HI3 (3),[491 [Rh12N2(CO)ca2312- (6),‘”’ depending on the reaction medium. As already discussed, the choice of a medium with the proper acidity/basicity (inherent or modified by variable amounts of water, buffers, etc.) stabilizes the anions within a
I . 18 Syiztlwsis and Properties
o j Metnl
Curbonyl Clusters
333
particular range of anionic charge/nuclearity ratios ( ~ / n ) . ~ ’ ~In . ’ ~this ’ case, this means that mono-, di- or tri-anionic dodecanuclear species can be obtained almost selectively. Thus [Rh12N2(C0)24J2-is obtained when the pyrolysis of 2 is performed in MeCN, according to the simple formal stoichiometry (9).
In the presence of a base, the trianionic species is produced:
It is very likely that only one Rh12N2 species is produced in the pyrolysis, and subsequently, the compound with the more appropriate c / n ratio, consistent with the reaction medium, is obtained. The interconvertibility of the three species is in fact proved by the original synthesis of 4:[501
+
[ R ~ E N ~ ( C O ) X H ] ’ -H+(excess)
+ CO
+
[Rh12N2(C0)24I2-+ H2 ( 1 1 )
and by the pH dependent equilibrium observed in water between the dianion 4 and its conjugated hydrido monoanion 5:
[ R h I 2N 2 (CO)241
’-& [Rh OH-
12N2(CO)24 H]
(12)
What remains puzzling is the true nature of a recurrent species, which we could track in the pyrolysis of [RhsN(C0)15]-.Several derivatives of this anion have been obtained with different cations ([NMe3CH?Phj+,[NEtiPr]’, PPNS, [K(18-C-6)]+, etc..) and, although crystalline, none was suitable for X-ray structural determination. However, its position in the pyrolytic sequence (it follows [Rh12N2(C0)24]2-and precedes [Rhl4N2(CO)25]”’j1), the characteristic IR (2007 s, 1839 ms, 1797 mcm-I, in acetone) and analytical data, suggest a 12-metal dianion with a possible formula [Rh12N2jCO)ca.23]2-~. This is also in keeping with a cluster that, with respect to the possible precursor [Rh12N2(C0)24]~-, should be more compact with more M-M interactions replacing a reduced number of M-CO bonds. The trianion l R h 2 ~ N ~ ( C O ) 3 ~ ]poses 3 ~ ~ 1some 5 ~ 1questions. This species has been obtained in refluxing 2-propanol, but was never clearly identified in the processes performed in MeCN or DGM. It is very likely that this species would be unstable in more basic synthetic media, due to its low c / n ratio, even lower than that of the starting product [Rh(,N(CO),s]- (0.13 us. 0.17. i.e. a net oxidation). Moreover, it seems somewhat anomalous that this high nuclearity cluster precedes, in the pyrolytic sequence, the smaller [Rh14N2(C0)2jI2- [”I which requires far more severe conditions to be produced ( 3 h in DGM at 140 “C as an alternative to 15 h in refluxing 2-propanol). An explanation may be that this latter cluster, with one 12coordinated inner metal is more “ n ? e t a / k ” in character than [Rh23N4(C0)38l3-,
334
I Molecular Clusters
Scheme 2. Cluster growth through the M-CO (A) or M-M (B) splitting mechanism.
(BI DGM 170’. 30 h
(‘4) H20pH l&li 100°C 5 h
which has two encapsulated metals, but in a 10 atom environment, and therefore does not attain the full compact coordination. But perhaps this case may not be as odd if we focus on what appears to be the growing mechanism of these interstitial clusters. As mentioned before, the interstitial clusters gain a remarkable stabilization from the establishment of several M-EInt interactions. The result, at least in the case of rhodium nitrides, is that two levels of thermal activation may be clearly distinguished: a lower one, which may be roughly placed in the range 50-100 “C, and essentially yields a decarbonylation through M-CO breaking, and the other involving also a fragmentation of the robust M,N, skeleton, which becomes evident at ca. 140 “C. With reference to Scheme 2, the observation of several dodecanuclear species under the mild pyrolysis conditions could be explained with a mechanism, which on passing through an unstable unsaturated species (perhaps an 86 CVE octahedral cluster, which has never been observed), would then give a sort of dimerization of the metal skeleton with its interstitial content. It is worth noting that the “Rh6N” moiety does not necessarily retain its original geometry; the geometry of the resulting (Rh6N)l cluster and its anionic charge, are subject to a number of factors. On the contrary, skeletal rearrangements around the pivotal nitrogen(s) must be expected and, as mentioned earlier, it is essentially the acidity/basicity of the medium that determines the c / n ratio of the product. [Rh14N2(C0)25I2- can only be produced under fiir more severe conditions and this can be argued from a simple consideration: its internal Rh : N ratio has varied with respect to that of the precursor [Rh6N(C0)15]- (7 : 1 us. 6 : I ) , and this implies that an addition of rhodium has occurred. This may only originate from a side process of fragmentation of the metal skeleton involving rupture of both Rh-Rh and Rh-N bonds. If this is true, the introduction of some mono- or dinuclear rhodium ‘yragments”, should give,
1.18 Syntlwsis and Properties of Metal Curhonyl Clusters
335
through a condensation process on a ( Rh6N)2(CO)xsubstrate, the [Rh14N2(C0)25I2anion under milder conditions. This has been indeed verified and, as shown in Scheme 2, the introduction of some Rh4(C0)12, which may be easily disrupted in fragments, considerably lowers the required temperature for the reaction. In these cases, the use of ,cater as a .so/venz for the sodium or potassium salts of the cluster anions is also particularly remarkable; this medium in fact allows the use of appropriate buffers that, as in the present case ( p H ca. 9) may determine the product formation more effectively, reducing the by-products. Moreover, due to minimal solubility of CO, decarbonylation reactions appear particularly favored in water. Further evidence for this route to cluster growth, by coupling of Rh,N, building blocks, is the synthesis of the two giant clusters [Rh28N4(COL,IH,]4/5-."1 The pentaanion was originally obtained in very low yield (<5%) from the exhaustive py- M , 170 "C. 30 h) together with major decomposition rolysis of [ R ~ ~ N ( C O ) I (SD] G to the Rh metal (Scheme 1). This species has a conjugated hydrido tetranion, and addition of acids or bases may shift the equilibrium in either direction:
[ R ~ ~ ~ N ~H(I5-c o& OH) ~[ ~R ~ ~ ~ N ~~( c 4o ) ~~ ~ -
(13)
To verify the role of [Rh14N2(C0)25l2- as a possible precursor we pyrolyzed it at 100 " C , in water, at a pH ca. 11 which presumably would stabilize both the Rh2xN4 anions, and in fact we obtained the high nuclearity anions in ca. 30')/0yield.
On the other hand, pure [Rh6N(CO)lj]-, treated in the same way (i.e. in water at 100 " C , pH ca. 11) did not work, because it could not even give [Rh14N2(C0)25I2-. Finally, in the light of these consideration we postulated that the cluster [Rh23N4(CO)3813-(with a Rh : N very close to the original 6 : 1 ) is derived, through the coupling of two Rhl2N2 sub-units, from a [Rh24N4(CO),IC- intermediate. The detachment of a mononuclear fragment could be justified because of the particular instability of the intermediate. However, with reference to the actual structure of the Rh23N4 cage (Fig. 3), it can be seen that not all Rh atoms are bonded to the interstitial nitrogen atoms. Therefore, it is reasonable to think that in such a large structure as expected for "Rh24N4" a peripheral metal (with minimal or none Rh-N interaction) could well be easily detached even under mild conditions.
1.18.2.4 Mixed-metal clusters The number of heterometallic nitrido clusters so far synthesized is limited, and the majority of these compounds are a result of non-systematic studies. In contrast, many examples of mixed metal carbides are known," 71 the investigations rising from the pioneering work of Muetterties["' on iron-containing species.
336
I Moleculm Clusttw
Figure 3. The metal skeleton of [Rh23N4(C0)38l3-; two different environments of the nitrides are observed.
The high nuclearity anion [ P ~ R ~ ~ O N ( C O ) (Fig. ~ I ] ~4)~was ,['~ first ' obtained as a byproduct in the synthesis of [Rh6N(C0),5]-, from K3RhC16 containing traces of platinum salts. Subsequently, it has been shown that the trianion can be prepared from the pyrolytic condensation of [PtRh4(C0)14]2P[601 and [Rh6N(C0)15]-, which are both compatible with the strong reducing medium previously employed. Two Mo-Co clusters, Cp * Mo3Co2 (C0)s(p3-NH )(p4-N ) and Cp * Mo2Co3(CO)lo(ps-N)(Cp* = q5-C5Me5)have been obtained by the reaction of the unsaturated complex Cp*2M02(CO)4with Co(C0)3NO under photolytic conditions. The two reagents were selected to facilitate the addition of cobalt to the Mo-Mo triple bond, with the aim of producing a face NO-capped triangular cluster. Instead, the nitride and nitrene ligands were produced. In the first cluster, the (p4-N) atom is lodged in a butterfly cavity, formed with two cobalt atoms at the hinge positions, and two molybdenum atoms at the wing-tip sites. The third Mo atom and the NH group define an edge-fused tetrahedron. The second cluster adopts a square pyramidal metal framework, with a cobalt atom at the apical site. Comparison of Mo-N and Co-N bond distances suggests preferential interactions of the nitride with molybdenum.[' * I ,
Figure 4. The metal skeleton of [PtRhloN(C0)21]3-; the nitride is embedded in a cavity made of one platinum (hatched circle) and four rhodium atoms.
1.18 Synthesis und Properties of Metal Curhonyl Clusters
337
Reactivity of triruthenium clusters was particularly useful for mechanistic investigations. Thus, RulHNO(C0)1 0 and Pt(nb)l(P’Pr3) (nb = norbornadiene) lead to the formation, in low yield, of two mixed metal clusters of strikingly similar structure: Ru3PtH(pd-N)(CO)l o ( P’Pr3) and Ru3PtH(p4-Y2-NO)(CO)lo(P’Pr3). They both contain a triangle of ruthenium atoms, spiked by the axial Pt-Ru bond. The (pd-y2-NO)and (pd-N ) ligands are therefore in very unusual environments. The activated N - 0 bond is quite stable, and the nitrosyl cluster is probably a “dead-end product,” rather than an intermediate in the pathway to the nitride.[h21 Rui(C0)12 and [Fe(CO)3NO]-react in a 1 : 1 ratio, yielding in a first fast step the heterometallic anion [FeRu3(C0)12NO]- and then the corresponding nitride anion [FeRu3N(CO)l,j-. Several derivatives of the clusters have been obtained and characterized through X-ray or multinuclear NMR spectroscopy. In the parent carbonyl cluster, the iron is found to be distributed over all different metal sites, but occurs ],[~~] preferentially in the wing-tip positions.[631 In [ H F ~ R U ? N ( C O ) ~ [~FeRu3N(CO)12 { ~ ~ - A u P P ~ ) ~ [HFeRu3N(CO) )],[~~I 10 { P(OMe)3}2], and [FeRu3N(CO)10{P(OMe)3)2]-[6s1 the iron is essentially ordered, either at the wing-tip positions (in the first three clusters) or at the hinge, in the last. While studying the protonation reaction of the phosphite-disubstituted cluster, it was possible to detect the formation of an intermediate, which exists in two isomeric forms. ”N, ‘ H and 3 ’ PNMR spectra all lead to the conclusion that both isomers contained a direct N-H bond and suggested that, at least in this case, the nitrogen atom is the kinetic center of protonation. However. the presence of a v2-N-H ligand, similar to the a methylidyne fragment found in [ H F ~ ~ C H ( C O ) I ~was ] , [ruled ~ ” out. The final stable product, however, contains the usual metal array and the proton is “normally” observed to bridge the Ru-Ru hinge bond.‘‘’] The anion [FeRu4N(C0)14]- has also been characterized and the iron atom occupies the apical site.[631By way of contrast, no X-ray analysis was performed to establish the location of cobalt in [CoRu3N(C0)12], although spectroscopic data strongly suggest a butterfly arrangement for the metal atoms in this mixed metal cluster.[661 The clusters [ R h ~ M N ( C 0 ) 1 j ] ~ and - [ ~ ~[CogMN(C0)1512l (M = Co, Rh, Ir),[681 are members of a homogeneous family of heterometallic nitrido species. They were obtained upon addition of the [M(CO)d]- anions to the trigonal prismatic [M’gN(c0)15]~clusters, (M’ = Co, Rh). In the heptanuclear compounds, the new metal center caps a square face of the prism, and does not interact with the nitrogen, which retains its prismatic coordination. However, the heterometallic atom is redistributed over the capping and the prismatic metal sites, allowing a comparison of metal-to-nitrogen affinity to be made. For example, in the [RhgCoN(C0)1j]~ion, the Co is found only in two sites of the capped face, within the prism, whereas the iridium in [RhgIrN(CO)151’- occupies the capping position almost exclusively (about 90‘%,).Since the M-M and M-CO connectivities (4 and 3, respectively) are identical at the two sites, and the different isomers are probably in equilibrium, it can be inferred that Rh-N bonds are slightly stronger than Ir- N, but much weaker than Co-N:
338
1 Molecular Clusters
E(Co-N) >> E(Rh-N) > E(1r-N) This counterintuitive bond energy order can partially explain the absence of any homometallic iridium nitride, although several potential nitrido precursors have been ~haracterized.'~~] In the [CosMN(C0)15l2- series of compounds, which are isostructural with [Co7N(C0)15I2-,it was much more difficult to establish the distribution of Rh and Ir, owing to cocrystallization of homo- and heterometallic anions. It has already been pointed out that [Fe4N(CO)12]- reacts with anionic carbonyl complexes, allowing reconstruction of the cage around the nitrido atom. We have, therefore, started a systematic investigation devoted to the preparation of mixed-metal clusters, based on iron. The first results were obtained for rhodium and iridium, following the isolation of [Fe5MN(C0),5l2- (M = Rh, Ir) and [Fe4Rh2N(C0)15]-.[701More recently, we have expanded the family, also incorporating manganese and molybdenum, in compounds of formula [Fe5MnN(CO)16l2- and [Fe3Mo3N(CO),8l3-. Some similarities between these clusters -
-
-
-
synthesis: they can be prepared either from [Fe4N(CO)121- and a nucleophilic metal complex, or from [FesN(CO)15]'- and a Lewis acid, in reasonable yield (20-70%). They are always hexanuclear, even if the metal to metal ratio in the reagents does not fit with this. reactivity: they are fairly stable to CO, but are demolished by oxidation, both chemically and electrochemically.~701 We could never detect any tetra- or pentanuclear heterometallic clusters. structural: the ligands are inhomogeneously distributed on the surface, to equilibrate electronic differences at the metal centers. Only in [ F ~ ~ M O ~ N ( C Ocan ),~]~a preferential Mo-N interaction be inferred. spectroscopic: despite their different ligand architecture, they have strikingly similar IR carbonyl spectra. The 15N NMR spectrum has been measured for a few of them, and the chemical shifts appear to be only slightly influenced by the nature of the second metal.
1.18.3 Structural features The presence of one or several nitrides is effective at modifying the metal cluster structure. Face bridging "naked" nitrides are not known but a nitrogen atom with a T-shaped coordination has been found in [ M o ~N)(O)(C0)4(C5H5)3]. ( Semiexposed nitrides have been found in the species with four or five metal atoms; they are invariably encapsulated into clusters of higher nuclearity. All the tetranuclear
1.18 Synthesis and Properties of Metal Curbonyl Clustrvs
339
species share the same butterfly arrangement of metals with the nitride bonding the wing-tips more strongly than the hinges. All the pentanuclear clusters have the square pyramidal structure and the most relevant variation within this group of clusters is the position of the nitrogen atom, which may be located either in the plane of the square face, as in [Fe5N(C0)14]-,r721 or slightly below, as in HFe5N(C0)14[’61and [ R U ~ N ( C O ) ~ ~ ] - . [ ~ ~ . ’ ~ ] Two distinct geometries appear with six metal atoms. Thus, [ C O ~ N ( C O ) ~ ~ ] - , [ ~ ~ ] [Fe6N(CO)151’- .[301 [FejRhN(CO)15 12-, KFeSIrN(C0)IS]’-, [Fe4Rh2N(CO) IS]-[^^^ and, probably, also [RugN(C0)16]- (no X-ray structure has been so far reported), are octahedral clusters encapsulating the nitrogen. A trigonal prism containing the interstitial atom is the alternative geometry found for the two anions From them the seven-metal species [Co6N(CO)15)- and [Rh6N(C0)15]-.[~*’ [Co6MN(CO)IS]’-‘681 and [ R ~ ~ M N ( C O ) ~ S( M ] ~=- Co, [ ~ ~Rh, ] Ir) have been obtained; their metal skeletons (besides the metal disorder described above) appear directly derived from those of the parent compounds by capping a square face of the trigonal prism. Two limiting skeletal situations may be recognized in the larger (po1y)nitrido species, one with relatively expanded structures and a second one with more “cornpact” geometries, often approaching the metallic state. This can be related to the influence of the interstitial nitrogen atom which may attain the full six-metal coordination with either a trigonal prismatic ( t p ) or an octahedral (oh) environment, conforming to the prototypes [M6N(CO)lj]- (M = Co, Rh) and [CogN(C0)13]-, respectively. ~ ~ ]5)~ the - [ ~nitrogen ] In [ColoN2(C0)19I4-[441 (Fig. 2) and [ C O ~ ~ N ~ ( C O )(Fig. “filled” tp sub-units are assembled. respectively, by edge and face sharing, so that even the resulting ‘*empty” polyhedra are more or less regular trigonal prisms. The related paramagnetic species [Co14N3(C0)26I2- and [Col4N3(C0)26l4-, have almost exactly the same structure as the parent trianion with minimal differences in some of the Co-Co bond distances. In the structure of the [ C O ~ ~ N ~ ( C O[481 ) M trianion ]~(Fig. 6), it is not possible to recognize any features of the parent Co14N3 cluster; however the two nitrogen atoms retain a tp environment within two adjacent ca-
Figure 5. The metal skeleton of I C O , ~ N I ( C O ) ~with ~ ] ’ the three nitrides in trigonal prismatic cages.
340
1 Molecular Clusters
Figure 6. The metal skeleton of [Col3N2(C0)24I3-with two trigonal prismatic nitride environments.
vities, which share one cobalt atom. Remarkably this is the only large structure with an isostructural, although not isoelectronic, carbido analogue, [ C O I ~ C ~ ( C O [741 )~~]~-. On the other hand, the "metallic" structures of [Ru10N(C0)24]-[311 (Fig. l ) , I H(Fig. ~ ] (8), ~ /whose ~ ) - metallic [Rh14N2(C0)25I2- (Fig. 7), and [ R ~ ~ ~ N ~ ( C O ) ~ [71 frameworks are slightly distorted fragments of the cubic close packed lattice (ccp), should be considered. In both the ruthenium and the giant 28-metal rhodium compounds, the interstitial nitrides are within oh cavities; in [Rh14N2(C0)25j2-;["] one nitrogen atom is in a semi-octahedral (or square pyramidal) environment while the second is pentaconnected in a seven-metal cavity. In other large species the nitride appears in a coordination geometry intermediate between the two limiting tp-oh structures, embedded in large irregular cavities.
1.18 Syntl1e.si.s and Propertie.r of Metal Curbony1 Clusters
34 1
Figure 8. The close packed metal array of
jRh~~NJ(COj41 Hx]J’5-: filled circles and hatched edges denotes the four nitrides and their octdhedral cavities.
This is the case for the two dodecanuclear species [RhlzNz(CO)z?H]’- and [RhlzN2(C0)24j2--(Fig. 9 and lo), which are chemically related and have two somewhat similar distorted cages. In both, one nitrogen is still hexacoordinated within a trigonal prism, while the second is located within an eight-metal square antiprismatic cavity with only five short Rh-N bond distances. Also in [Rhz3N4(CO)’p,l3-[ 5 6 1 (Fig. 3), all the nitrogen atoms are connected to five rhodium atoms only. and lodged in ill-defined environments, which can be alternatively considered as open semi-octahedral or trigonal prismatic cages. The structure of [PtRhloN(C0)21l3- (Fig. 4) is described as a two-layer arrangement of rhodium
Figure 9. The metal skeleton of [ R ~ ~ ~ N ~ ( C O I ~ I H ] ’
342
1 Molecular Clusters
W
atoms: a puckered square and a puckered hexagon, centered by the platinum.[”1 The nitrido ligand is connected to platinum (apparently with a strong interaction) and four rhodium atoms in a cavity half-way between a trigonal bipyramid and a square pyramid.
1.18.4 Spectroscopic data associated with the interstitial nitrides Vibrational duta - The analysis of the motion of interstitial atoms in clusters cages, especially for low-coordinated nitrides and carbides, is particularly relevant, because it should prove useful for the characterization of reactive surface species. The metal-nitrogen absorptions are located in the 900-650 cm-’ range, and can be unambiguously assigned by examining the isotopic shift in the spectra of the compounds labeled with 15N. This procedure must be adopted when the compound contains aromatic ring (for example in the counterion) which obscure the same region. The vibrational spectra for nitride ligands in ~ c t a h e d r a l , prismatic,[7s1 ~~~~~~] and butterfly1761metal cages are of increasing complexity, owing to the reduced symmetry of the nitrogen environment. The motion of interstitial atoms in square pyramidal clusters was analyzed only for carbon.[771 For the nitride in the octahedral cage of [ R u I o N ( C O ) ~ a~ ]single band was found at 682 cm-’ and definitively assigned after observing a shift in the l5N-enriched
1.18 Synthesis and Properties qf' Metal Curbonyl Clusters
343
compound.1311In the series of clusters [Fe6N(C0)15l3-, [FesIrN(C0)l5l2-, and [Fe4Rh2N(CO)l5]-, the vibrational spectra reflect the decrease in the symmetry of the clusters. Even in the homometallic [Fe6N(C0)15]~-anion, owing to the presence of three short bridged edges, the cage is distorted from the ideal Oh to D3 symmetry, and two bands are observed. In the heterometallic members, the effect of the heavy metals on the cage is much more relevant than the small perturbations induced by the carbonyls, and the number of v(M-N) bands are consistent with the (for [Fe5IrN(CO)l,I2-)and the CzI (for [Fe4Rh2N(CO),5IP)symmetry of the metal cages.[701In the spectra of the prismatic [M6C(CO),512-and [M6N(CO)15]- anions (M = Co and Rh) two bands were detected and assigned to axial and transverse vibrational modes, allowing two different force constants for each clusters to be calculated. For the nitride, force constants are always higher than for carbide.[7s1 Three v(M-N) bands are present in the spectra of butterfly ruthenium and osmium clusters. The highest frequency band, at about 860 crn-', was assigned to the vibration along the wing-tip-nitride vector.[761The assignment of the other two bands is less straightforward, and the spectra have been re-inter~reted.'~'] N M R duta The I4N nucleus (natural abundance 99.60/0,spin 1 ) is NMR active, but it is a quadrupolar nucleus. Therefore, when it is lodged in unsymmetrical environments, it gives rise to very broad NMR signals. For this reason, the routine I4N NMR spectroscopy of nitrides is impossible and the data are obtainable only for very symmetrical clusters. The 14N NMR spectra have been obtained for the prismatic [M6N(CO)15]- anion ( M = Co and Rh, D 3 h symmetry) and for [ R u I o N ( C O ) ~ ~(Td ] - , symmetry), whose spectrum consists of a singlet at 6 30 (ref~ ] chemical shift, when referenced to NH?, erenced to MeNO2, w l i 2 = 21 H z ) . [ ~This corresponds to 413 ppm and should be compared with the data in the following discussion, since the shifts of I4N and I5N are inter~hangeab1e.l~~~ The "N nucleus has very low natural abundance (0.37%) but is much more convenient for NMR spectroscopy since it has spin 1/2. The detection of the signals for enriched compounds is therefore a relatively easy task (2000-20000 scans, for samples of approximately 0.05 M). The majority of signals of the interstitial nitride are all confined in the 400-600 ppm range (referenced to NH?). Two very relevant exceptions are the prismatic compounds [ MbN(CO)151- whose chemical shifts were measured at 196 ppm (M = Co) and 108 ppm (M = Rh). The same anomalous high field shift was also found in the "C NMR of the corresponding carbides, and the reasons for such anomalies have been widely discussed, but not totally explained. They may be related to different interactions between interstitial ligand and metal atoms["] or to the compression of the metal cage.ls'] The relevant data are shown in Table 4. Unfortunately, these data are not homogeneous, since they were collected on loosely related compounds, which differ in the metal, the structure. the charge and the nature of the ligands. Therefore, they cannot be interpreted by a unifying model and the few trends, originally inferred from the very few known compounds, still wait to be confirmed by more experimental results: -
344
I Molecular Clusters
Table 4. The chemical shift of nitrides in homo- and hetero-metallic clusters (referred to NH3). d(ppm)
618 59 1 596 593 565 543
519 465 559 413
Ref
6(ppm)
Ref
196 108
Mixed-metal clusters [FeRu3N(CO)12]- Fe wing tip [FeRu3N(CO)&Fe hinge [HFeRu?N(CO)121 FeRu3N(CO)IzAuPPhi [FesM(CO)15]'- M = Rh, Ir [ Fe 4R h N CO )151-
559 520 53 1 533 514 470
i) when descending a group, the chemical shift of the nitrides in isostructural clusters are displaced by the heavier metals to higher fields (by about 100 ppm). ii) addition of a positively charged group, such as H+, AuPPh3' or NO+ (and, correspondingly, the reduction of the total negative charge) moves the chemical shift to lower 6 (by about 20-30 ppm)[15*641 iii) both effects are cooperative when the charge is reduced by substitution of iron atom with a more electron rich metal (such as Rh or Ir): the chemical shift is then reduced by 45 ppm.[701 iv) although cluster size and chemical shift of the nitride are not correlated, a compression of the metal cage (a smaller covalent radius of the interstitial atom) moves the I5N signal to higher ppm. This effect is common to boron, carbon and oxygen.["] v) The "N-lo3Rh coupling constant depends on the bond distance between the two atoms: 6 Hz in the prismatic [RhsN(C0)15]- (average Rh-N 2.13 A)and 8-9 Hz in the octahedral Fe-Rh clusters (2.02 A).[701 vi) The 2J('H-15N) coupling constant of 3.1 Hz was reported only for [HFegN(C0)15I2- anion, although several other compounds contain both ligand~."~]
Acknowledgements We are indebted to S. Martinengo for helpful discussions, to A. Sironi and M. Manassero for communicating unpublished structural data.
1.18 Syntliesis m d Properties of‘ Metal Curhoizyl Clusters
345
References [ 1 I P. Chini, Gtr::. Chirn. Itu/. 1979. 109, 225. [21 a ) K. Wade, .4dr. Oiory. C k w . Rtrrhheni.. 1976. 18. 67; b) D.M.P. Mingos, A S . May, in The Clzenii,my uf M e t d C’lirstcr Coinplews. ( D . F . Shriver. K.H. Kaesz, R.D. Adams Eds.). VCH Publishers 1990, 11-1 14 and refs therein.; c ) G. Ciani. A. Sironi, J. Orgunonicrul. Cherii. 1980. 197, 233 248 and refs therein. 131 A. Bandyopadhyay. M. Shang. C. So0 Jun, T.P. Fehlner, Inorcj. C%erii.1994. 33. 3611. 141 S. Martinengo. G. Ciani. A. Sironi, J. Orgrnio~nrt.C%rm 1988. 358, C23. [S] A. Ceriotti, A, Fait. G. Longoni. G. Piro. F. Demartin. M. Manassero. N. Masciocchi. M. Sansoni. J. Ani. C’/ieni. Soc. 1986, 108. 8091. 161 T. Chiara. R . Komoto, K. Kobayashi. H. Yamazaki. Y. Matsuura, 1nor.y. Clwnz., 1989. 28. 964. [71 A. Fumagalli, S. Martinengo, G. Bernasconi. G. Ciani, D.M. Proserpio, A. Sironi, J. Aiii. Chenz. Soc. 1997. 110. 1450. [ 8 ) E.G. Mednikov. N.K. Eremenko, Y.L. Slovokhotov, Y.T. Struchkov. J. Clzenz. Soc Clieni. Cornnzuri. 1987. 2 18. [9] G. Ciani. G.D’ Alfonso, M. Freni, P. Romiti. A. Sironi J. C / i m . Soc. Clleni. Conimurz. 1982; 705. [I01 A.J. Amoroso. L.H. Gade, B.F.G. Johnson. J. Lewis. P.R. Raithby. W.-T. Wong. Anqew. C ’ l i m i . hit. Ed Enijl, 1991, 30. 107. [ 1 1I R. Della Pergola. L. Garlaschelli. M. Manassero, N. Masciocchi. P. Zanello. Angeir.. C’I~CMI. 1/11, Ed. Eny/, 1993. 32. 1347. [I21 For the structure of the metal core see J.D. Roth. G.J. Lewis, L.K. Safford, X. Jiang, L.F. Dahl, M.J. Weaver; J. Ani. Clicni. Soc. 1992, 114. 6159. The arrangement of the 44 carbonyl ligands was recently determined by N. Masciocchi. A. Ceriotti, G. Longoni ‘personal communication). [ 131 P. Chini. V.G. Albano J. Orqmo~net.C/ieni. 1968, 15, 433. 1141 G. Ciani. S. Martinengo. J. Orgononwt.C/iun. 1986, 306. C49. [I51 W.L. Gladfelter, Ark. Or(qunonwt. Clieni. 1985. 24. 41. 1161 M. Tachikawa. J. Stein, E.L. Muetterties. R.G. Teller. M.A. Reno, E. Gebert, J.M. Williams, J. A m C/ieni. Soc, 1980. 102. 6649. 1171 al E. Fjare. W.L. Gladfelter: J . Am. Chcni. Soc. 1981. 103. 1572 bj E. Fjare. W.L. Gladfelter, I/lOl’<J. C/lel??.,1981. 20. 3533. [ 181 M.L. Blom, W.L. Gladfelter. Or(jcmoniL,tttNic.~~ 1985. 4. 45. 1191 M.A. Collins, B.F.G. Johnson. J. Lewis, J.M. Mace. J. Morris. M. McPartlin, W.J.H. Nelson. J. Puga, P.R. Raithby. J. Cliern. Soc., C/ieni. Comniii/z.. 1983. 689. 1201 D. Braga. B.F.G. Johnson. J. Lewis, J.M. Mace, M. McPartlin. J. Puga. W.J.H. Nelson, P.R. 1982. 1081. Raithby, K.H. Whitmire. J. C/ieni. Soc., (7ieni. Con~nurrz.. 1211 J.P. Attard. R.F.G. Johnson. J. Lewis. J.M. Mace. P.R. Raithby. J. C h i . Soc., Chrwi. Coniniun.. 1985. 1526. 1221 S. Martinengo, G. Ciani. A. Sironi. B.T. Heaton; J. Mason, J. Aiii. Clzem. Soc,. 1979, 101, 7095. [23] R . Bonfichi. G. Ciani. A. Sironi. S. Martinengo, J. Chern. Soc.. Dtrlton Trms. 1983. 253. [24] S. Martinengo. P. Chini. GK:. Chirn. Ztcrl. 1972, 102, 344. 1251 R.E. Stevens. P.C.C. Liu. W.L. Gladfelter. J . Or
346
I Molecular Clusters R. Della Pergola. C. Bandini, F. Demartin, E. Diana, L. Garlaschelli, P.L. Stanghellini, P. Zanello, J. Chem. Soc., Dalton Trans, 1996, 747. P.J. Bailey, G. Conole, B.F.G. Johnson, J. Lewis, M. McPartlin, A. Moule, H.R. Powell, D.A. Wilkinson, J. Chenz. Soc., Dalton Trans, 1995, 741. K.K.H. Lee, W.T. Wong. Inorg. Chem., 1996, 35, 5393. V.G. Albano, P. Chini, S. Martinengo, M. Sansoni, D. Strumolo J. Chem. Soc., Chem. Commun. 1974, 299. V.G. Albano, S. Martinengo, D. Strumolo, P. Chini, D. Braga, J. Chem. Suc., Dalton Trans. 1985, 35. V.G. Albano, M. Sansoni, P. Chini, and S. Martinengo, J. Chem. Soc., Dalton Trans., 1973, 651. V.G. Albano, D. Braga, S. Martinengo J. Chem. Soc., Dalton Trans. 1986, 981. V.G. Albano, D. Braga, S. Martinengo, J. Chem. Soc., Dalton Trans., 1981, 717. B.T. Heaton, L. Strona, S. Martinengo, J. Organomet. Chem., 1981, 215, 415. T. Blum, B.T. Heaton, J.A. Iggo, J. Sabounchei, A.K. Smith, J. Chem. Suc., Daltun Trans. 1994, 333; the reported IR is very similar to that of an elusive species we encountered many times and that we tentatively formulated as [Rh12N2(C0),,23]~-; see ref. 52. G. Ciani, N. Masciocchi, A. Sironi, A. Fumagalli, S. Martinengo, Inorg. Chem. 1992, 31, 331. S. Martinengo, G. Ciani, A. Sironi, J. Chem. Soc., Chem. Commun., 1984, 1577. G . Ciani, D.M. Proserpio, A. Sironi, S. Martinengo, A. Fumagalli, J. Chem. Soc., Dalton Trans. 1994, 471. S. Martinengo, A. Fumagalli, unpublished results. A. Fumagalli, S. Martinengo, M. Tasselli, G. Ciani, P. Macchi, A. Sironi, Inury. Chem. 1998, 37, 2826. P. Chini, J. Orgunomet. Chem. 1980, 200, 39 and refs. within. P. Chini, G. Longoni, V.G. Albano, Adv. Organomet. Chem. 1976, 14, 285. S. Martinengo, G. Ciani, A. Sironi, J. Organomet. Chem. 1988, 358, C23. S. Martinengo, A. Fumagalli, M. Tasselli, G. Ciani, P. Macchi, A. Sironi, work in progress and Thesis ofLLiurea in Chimica (M.T., a.a. 1996-97). S. Martinengo, G. Ciani, A. Sironi, J. Chem. Suc. Chem. Commun., 1986, 1742. S. Martinengo, G. Ciani, A. Sironi, N. Masciocchi, work in progress, personal communication. This monoanion is probably the conjugated hydride of [Rh12N2(C0)24I2-; in water, it is involved with the dianion in an equilibrium, which can be shifted in either directions as a function of pH. It is supposed to have the same metal skeleton of the dianion. A. Fumagalli, S. Martinengo, P. Ulivieri, work in progress and Thesis of Laurea in Chimica, (P.U., a.a. 1993-94). Postulated on the basis of analytical data and reactivity; A. Fumagalli, S. Martinengo, P. Ulivieri, work in progress and Thesis ofLaurea in Chimica, (P.U., a.a. 1993-94). A. Fumagalli, Muter. Chem. Phys. 1991, 29, 21 1. A. Fumagalli, Proceedings of the 5th Meeting on Syntheses and Methuddogies in Inorganic Syntheses, (Daolio, Tondello, Vigato Eds.) 1995, 5, 396. S. Martinengo, G. Ciani, A. Sironi, J. Chem. Soc., Chem. Commun. 1991, 26. S. Martinengo, G. Ciani, A. Sironi, J. Chem. Soc., Cliem. Commun. 1992, 1405. a) J.A. Hriljac, E.M. Holt, D.F. Shriver, Inorg. Chem., 1987, 26, 2943 b) G. Longoni, A. Ceriotti, R. Della Pergola, M. Manassero, M. Perego, G. Piro, M. Sansoni, Phil. Trans. R. Soc. Lond. A , 1982, 308,47. M. Tachikawa, R.L. Geerts, E.L. Muetterties, J. Organomet. Chem., 1981, 213, 11. S. Martinengo, G. Ciani, A. Sironi, J. Am. Chem. Soc. 1982, 104, 328. A. Fumagalli, S. Martinengo, P. Chini, D. Galli, B.T. Heaton, R. Della Pergola, Inorg. Chem. 1984,23, 2947. C.P. Gibson, L.F. Dahl, Organometallics, 1988, 7, 543.
I . 18 Sjntlzesis and Properties of Metul Curhonyl Clusters
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1621 L.J. Farrugia, D. Ellis, A.M. Senior, in The Synergy Betiiwn Dj~nuniicsund Reuctioity ut C/u.sttw und SurJirces, (L.J. Farrugia Ed.) Kluiver, Dordrecht, the Netherlands, 1995, p. 141. 1631 D.E. Fjare, W.L. Gladfelter, J. Am. Chern. Soc., 1984, 106, 4799. 1641 M.L. Blohm. W.L. Gladfelter, Inorg. Cliem., 1987, 26, 459. [65) M.L. Blohm. D.E. Fjare, W.L. Gladfelter, J. Am. Chem. Soc, 1986, 108, 2301. [66] D.E. Fjare, D.G. Keyes, W.L. Gladfelter, J. Organomef. Chem. 1983. 250, 383. 1671 S. Martinengo, G. Ciani, A. Sironi, J. Chenz. Soc., Chem. Corvimun. 1984, 1577. 168) S. Martinengo, A. Fumagalli. P. Ulivieri, A. Sironi, unpublished results and Thesis ojLuureci in Chiniica (P.U., a.a. 1993-94). [69] a) A. Cinquantini, P. Zanello, R. Della Pergola, L. Garlaschelli, S.Martinengo; J. Organoniei. Cheni., 1991, 4/2, 215; b'l R. Della Pergola, L. Garlaschelli, M. Manassero, N. Masciocchi; J. Orgurioriief. Chen?.. 1995. 488, 199. 1701 R. Della Pergola, A. Cinquantini, E. Diana, L. Garlaschelli, F. Laschi, P. Luzzini, M. Manassero, A. Repossi, M. Sansoni. P.L. Stanghellini. P. Zanello; Inorg. Chem., 1997, 36, 3761. [71] R . Della Pergola, M. Manassero. unpublished results and Thesis qf Luureu in Chirriicu (M.Branchini, a.a. 1996-97). 1721 a) R. Hourihane, T.R. Spalding. G. Ferguson, T. Deeney, P. Zanello, J. Chem. Soc., Dalton Trunis. 1993, 43; b) A. Gourdon, Y. Jeannin, J. Orgunoniet. Chem., 1985, 290, 199. 1731 M.L. Blohm. D.E. Fjare. W.L. Gladfelter, Inorg. Chem. 1983, 22, 1004. [74] V.G. Albano, D. Braga. A. Fumagalli, S. Martinengo, J . Cliem. Soc. Dulton Trans., 1985, 1137. [75] J.A. Creighton, R. Della Pergola, B.T. Heaton, S. Martinengo, L. Strona, D.A. Willis, J. Clieni. Soc., Cheni. Coninzun., 1982, 864. [76] C.E. Anson, J.P. Attard, B.F.G. Johnson, J. Lewis, J.M. Mace, D.B. Powell, J . Chem. Sue., Chem. Coinmun., 1986, 17 I 5. 1771 1.A. Oxton, D.B. Powell, R.J. Goudsmit, B.F.G. Johnson, J . Lewis, W.J.H. Nelson, J.N. Nicholls, M.J. Rosales, M.D. Vargas, K.H. Whitmire Inorg. Chim. Acta, 1982, 64, L259. [78] P.L. Stanghellini, M.J. Sailor, P. Kuznesof, K.H. Whitmire, J.A. Hriljac, J.W. Kolis, Y. Zheng, D.F. Shriver, Inorg. Chem., 1987,26, 2950. 1791 J. Mason, in Muhinuclear N M R (J.Mason Ed.); Plenum press, New York 1987, p. 335. [SO] a) M.J. Duer, D.J. Wales, Polyhrdron 1991, 15, 1749 b) M. Kaupp, J.Chem. Soc., Chem. Comniun.,1996, 1141. [81] J. Mason, J. Am. Chem. Soc, 1991, 113, 24.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.19 High Nuclearity Osmium - Gold Clusters Jack Lewis and Paul R. Raithby
1.19.1 Introduction Transition metal clusters have long been recognized as forming the link between conventional organometallic chemistry and the chemistry of the bulk metal."] In this context they have been used as catalysts themselves and as models for both homo- and heterogeneous catalysts,r21drawing on the fact that an array of metal atoms surrounded by carbonyls and organic ligands models the situation on a metal surface where adsorbed molecules can interact to form new molecular species. The advantage of metal clusters over mononuclear organometallic complexes is that they provide a number of different bonding sites for the coordination of organic groups, and that, in deltahedral clusters (clusters with triangulated faces) with five or more metal atoms, the steric and electronic properties of the different metal atoms may differ with respect to other metal centers in terms of both metal-metal connectivities and metal-ligand interaction^.^^] Also, using conventional spectroscopic and crystallographic techniques, it is much easier to characterize the reactive species involved in catalytic reactions on cluster surfaces as compared to the bulk metal surface. As a result, clusters provide useful models for studying metal-organic interactions in catalytic More recent developments in cluster chemistry have included the anchoring of clusters to silica or alumina surfaces via sol-gel processing, the products finding applications as catalyst precursors.[51Very recently, mixed-metal clusters have been incorporated onto the inner walls of mesoporous silica with a pore diameter of about 30 A,and have been subsequently converted into discrete nanoparticles by thermolysis, which have been shown to act as hydrogenation catalysts.[61This is a particularly important new use for mixed-metal cluster systems, since there are many well characterized mixed-metal cluster carbonyls (vide injia) that are readily available to act as precursors for such reactions.
1.I 9 High Nurlecrrity Osmium
~
Gold Clusters
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It seems that one of the future developments in cluster chemistry lies in the production of nanosized particles (1 nm = 10 A) with well defined stoichiometries, which can be used as catalysts or as catalyst precursors.[71 In this context, high nuclearity mixed-metal clusters are particularly useful because two or more metal atoms with different chemical properties can be combined in the same unit. The Cambridge group has spent the last few years designing rational synthetic routes to mixed-metal high nuclearity clusters of ruthenium and osmium with the coinage elements, which produce cluster cores of up to one nanometer in One of the reasons that it is advantageous to use preformed mixed-metal clusters is that the chemical and electronic properties of the clusters change as the number of metal atoms in the cluster core increases and, by tuning the properties of these precursors, it is possible to tailor the production of the nanoparticles themselves. Small clusters, with up to four metal atoms, behave very much as conventional organometallic complexes; the metal-metal bonding being described in terms of localized two-centre two-electron bonds. With clusters that contain five or more metal atoms the bonding is best described in terms of delocalized bonding schemes, and cluster cores based on an octahedron cannot be rationalized readily in terms of localized bonding. The chemical properties of the clusters also change as the nuclearity increases. Low nuclearity clusters undergo an extensive range of substitution or addition reactions under relatively mild conditions, whereas higher nuclearity clusters are more likely to undergo redox chemistry in competition with the substitution and addition processes.['o1Clusters with ten or more metal atoms can display a range of oxidation states without an apparent major change in core geometry, as the metal framework acts as an 'electron sink',["] whereas oxidation or reduction of lower nuclearity clusters almost always involves a framework rearrangement. As the nuclearity increases further, the clusters take on more metallic properties, and simple molecular orbital calculations show that clusters with 25 or more metal atoms have well developed electronic band structures.[']
1.19.2 Electron counting schemes and the rationalization of cluster geometries A number of electron counting schemes have been developed to rationalize or even predict the metal framework geometries adopted by metal cores in cluster complexes. The simplest of these is the Effective Atomic Number (E.A.N.) rule"21 which is based on the '1 8 electron rule', and assumes that each metal uses its five d, three p and one s valence orbitals for bonding, and that all metal-metal bonds are two centre - two electron bonds. Clusters which obey this rule are said to be 'electron precise'. However. often there may be a number of different structures, with the same electron count, which conform to these rules and there is no discrimination
350
I Molecular Clusters
between the possibilities. Also, this rule cannot rationalize structures where there are too many or too few electrons present to satisfy the two centre two electron bond criterion and is of limited use in rationalizing higher nuclearity cluster structures. The most widely applied counting scheme is the Polyhedral Skeletal Electron Pair Theory ( P.S.E.P.T.),[131which was developed from Wade’s Rules that had originally been used to rationalize the structures of the boron hydrides. In this method the metal polyhedra are characterized by a number of skeletal electron pairs which hold the framework together. For example, a closo metal polyhedron with n vertices is held together by n 1 electron pairs (a closo polyhedron is made up of entirely triangular faces), while a nido metal polyhedron, that is an n vertex polyhedron based on a closo polyhedron with one vertex missing, is held together by n + 2 electron pairs, and an arachno polyhedron with n vertices, is based on a closo polyhedron with two vertices missing, and is held together by n 3 electron pairs. A cluster with n metal vertices, which is held together by only n skeletal electron pairs, must adopt a polyhedral shape based on a closo polyhedron with n - 1 vertices, one face of which is capped by the remaining metal vertex. Examples of the application of P.S.E.P.T. are illustrated in Fig. 1. P.S.E.P.T. can be used to predict metal polyhedral structures for clusters if the molecular formula and hence the exact electron count is known, however, isomers are possible. The method generally only considers the most symmetric framework structure, e.g. for the structure of [OsgH~(C0)18] an octahedral metal framework would be predicted, but the structure is in fact a capped square based pyramid,[l4] that is a capped nido octahedron according to P.S.E.P.T.; both structures have seven skeletal electron pairs. Also, for ‘electron precise’ structures under the E.A.N. rule, the addition of an electron pair will generally result in the cleavage of a single metal-metal edge in the polyhedron, such that the metal framework retains all triangulated faces. For example, the ‘formal’ addition of an electron pair to the trigonal bipyramidal framework in [Os5(CO)161, and its derivatives, results in the formation of a cluster with an edge-bridged tetrahedral framework, rather than the square based pyramid, as would be suggested by P.S.E.P.T.[lolSimilarly, ‘on paper’, the bicapped tetrahedral [OSS(CO)~,] structure, can sequentially break three metal-metal edges, as three electron pairs are added, to open up and give the raft geometry observed in derivatives of [ o s 6 ( c o ) ~ ~ ] . [ 1 0 1 The P.S.E.P.T. prediction for the structure of [os6(co)21]would be an arachno dodecahedron. A third electron counting scheme, developed by Mingos from P.S.E.P.T., which can be used to rationalize metal framework geometries when the structure is known, is based on the premise that ‘the total electron count for a condensed polyhedron is equal to the sum of the characteristic electron counts for the parent polyhedra (A) and (B) minus the electron count of the atom, pair of atoms, or face of atoms common to both polyhedra’.[’’] This method is particularly useful for high nuclearity clusters, which are the topic of this discourse, and some examples of its use are shown in Fig. 2. However, as the nuclearity of the cluster core increases further and the band structure develops, ambiguities in this scheme become apparent. ~
+
+
72 valence electrons 6 electron pairs for cluster bonding
5 0 s atoms held by 6 pairs [(n+l)pars], therefore a cioso structure based on a trigonal bipyramid
(b) [Os,(Ct
86 valence electrons
7 electron pairs for cluster bonding 6 0 s atoms held by 7 pairs [(n i 1) pars], thercfore a closo structure bawd on a octahedron
84 valence electrons 6 electron pairs for cluster bonding
6 0 s atoms held by 6 pairs [(n)pairs], therefore a capped structure based on a trigonal bipyramid a capped trigonal bipyramid (or a bicapped tetrahedron)
98 valence electrons
7 electron pairs for cluster bonding 7 0 s atoms held by 7 pairs [(n)pairs], therefore a capped structure based on an octahedron a capped octahedron
352
1 Moleculur Clusters
48 e-
+
48 e-
Figure 2. Application of the Mingos condensed polyhedral counting scheme to (a) [Oss(CO)IgJ, (bj IOs7(COh I, and (4 I O s d C O h l * ~ .
1.19.3 Synthetic routes to higher nuclearity clusters Until recently, the favored route to higher nuclearity clusters has generally been via pyrolysis or thennolysis, and indeed the highest nuclearity, homonuclear osmium O ] ~isolated -, in reasonable cluster anion to have been prepared, [ O S ~ ~ ( C O ) ~was
I . 19 Hicjlh Nucleurity Osmiuni
Gold Clusters
353
yield by optimizing the thermolysis conditions for [Os1(CO)lo( MeCN )2].[161 However, these methods often lead to a range of products, each of which can be isolated only in low yield, as exemplified by the pyrolysis of [Osi(CO)12]where tetra-, penta-, hexa-, hepta- and octanuclear osmium clusters are all obtained in varying proportions, depending on the exact conditions employed.“ ’I A number of more selective routes, including photochemical activation, redox condensation and the reaction of activated clusters with neutral mono- or dimetal complexes. have proved more successful in generating higher nuclearity clusters.” A simple route which the Cambridge group has exploited extensively to generate mixed-metal clusters, in particular, is the ionic coupling between a preformed cluster anion and a monometal cation, in the so-called “capping” reaction.[”] In the absence of competing redox chemistry, which becomes dominant for clusters with nuclearities of ten or more, this is a particularly effective way of increasing the nuclearity of clusters by one or two metal units, or linking anions together uia cationic heterometal fragments. A range of cationic fragments have been used successfully and MeCN)3]’+, [Os(@-arene)these include [Ru(CO)3(MeCN)3]2+,[201 [Ru(v6-arene)( (MeCN)3J2+,[”] [R~(~~~-cyclopentadienej(MeCN)3]~,[~~~ [Rh(q5-C5Me5)(MeCN)3]2t,[231 [Cu(MeCN)4]+, LCu(PR3)]+ and [Au(PRi)]’ ( R = Me, Ph).[s.91 Once neutral mixed-metal clusters have been formed by this route, it is possible to generate new carbonyl dianions by reductively eliminating carbonyl ligands using Na/Hg amalgam or K/PhlCO, and treat these with additional cationic complexes in order to increase the cluster nuclearity further. For example, the reduction of [ O S ~ ( C O )with I X J K/Ph?CO affords the dianion [Oss(CO)17J2p in quantitative yield. Subsequent treatment of this dianion with [Os(q6-ChHs)( MeCN)3I2 gives reason~ H general ~)].‘~“~ able yields of the heptaosmium cluster [ O S ~ ( C O ) ~ ~ ( ~ ~ - CThis method for the synthesis of high nuclearity mixed-metal clusters forms the basis of the chemistry described in this article.
’
1.1 9.4 Characterization of the new clusters Once new high nuclearity carbonyl clusters have been prepared. it is necessary to characterize them fully in order to establish the precise nature of the product as well as the detailed geometry of the metal framework. The full range of spectroscopic and analytical techniques that are available to the cluster chemist are required to elucidate this far from simple problem. Many of the techniques are complementary, and the results from IR and NMR spectroscopies together with mass spectrometry data are all required to piece together the puzzle. Undoubtedly the most powerful technique that can be employed is single-crystal X-ray diffraction, since the results
354
I Molecular Clusters
obtained by this method give a full three-dimensional picture of the structure at the atomic level and, particularly with higher nuclearity clusters, it is not possible to establish the exact nature of the cluster products without the spectroscopic data being underpinned by cry~tallography.['~~ However, an X-ray structure does not necessarily tell the full story, and this should be remembered when reading this article, since, by necessity, much of the descriptive chemistry and arguments are based on the results of crystallographic studies. An X-ray structure does not always reflect what is occurring in solution, where the reaction chemistry takes place, as it is usually the thermodynamically stable product that crystallizes out. Also, the single crystal used in the structure determination may not be representative of the bulk sample, or worse still, it may not be possible to obtain a single crystal at all. Whether or not the complete structure of a new cluster is finally determined by an X-ray analysis, a useful preliminary spectroscopic technique for analyzing a carbony1 cluster is IR spectroscopy. This gives useful information about the symmetry of the carbonyl arrangement and the bonding mode of the carbonyls, shows whether there are edge or face bridging carbonyls present as well as terminal ones, and indicates whether isomers are present in solution.r2h1 Mass spectrometry gives quick results regarding the molecular mass of the cluster, and, for carbonyl clusters, often shows not only the molecular ion but also the sequential loss of a number of carbonyl ligands, which may be useful for confirming the formulation of the product. For high nuclearity clusters, and particularly the mixed-metal systems, an analysis of the isotope pattern obtained may give useful information regarding the number of atoms of a particular element present in the complex.[271For high nuclearity clusters the best mass spectroscopic results are obtained using Fast Atom Bombardment ( FAB) techniques rather than Electron Impact techniques ( EI ) where the cluster has a greater possibility of fragmenting or rearranging while moving through the spectrometer. The most useful technique for determining the detailed structure of clusters in solution is undoubtedly Nuclear Magnetic Resonance spectroscopy ( NMR). With mixed-metal clusters it is not only possible to probe nuclei such as 'H, 13C and 'IP but also metal nuclei such as lo'Rh and '95Pt.Taken together, the data can give very useful information regarding the metal environments. Variable temperature NMR studies give information on the dynamic processes that occur in solution.['*] The use of X-ray crystallography and mass spectrometry was of paramount importance in the development of cluster chemistry. Many of the complexes prepared in the 1940s and 1950s were polynuclear metal compounds but their identification was limited by the difficulties in obtaining accurate molecular masses and in the time taken to solve and refine an X-ray structure with the technology then available. The compound initially identified by Hieber as Rh4(CO)!]was later shown by Dahl, using X-ray crystallography, actually to be [Rh6(C0)16].The rapid determination of accurate molecular masses that became possible with the advent initially of E.I. mass spectrometry, for neutral compounds, and subsequently F.A.B. mass spec-
1.19 High Nucleurity Osniium
~
Gold Clusters
355
trometry, for ionic compounds, coupled with advances in separation techniques such as thin layer chromatography, lead to the rapid development of cluster chemistry in the subsequent decades.
1.19.5 Reactions of high nuclearity osmium cluster carbonyl anions towards monodentate and bidentate gold phosphine cations 1.19.5.1 Additions to hexanuclear osmium cluster anions Hexanuclear ruthenium and osmium clusters show two basic closed cluster core geometries. The first is the octahedron, which has a characteristic electron count of 86 electrons, and is held together by seven electron pairs according to P.S.E.P. theory. The other is the bicapped tetrahedron which has a characteristic electron count of 84 electrons, and is held together by six electron pairs according to P.S.E.P. theory; this would be conventionally described as a monocapped trigonal bipyramid according to P.S.E.P.T. A third hexanuclear metal core geometry that is closely related to the octahedron is the capped square based pyramid or the capped nidooctahedron according to P.S.E.P. theory. Like the octahedron it has a characteristic electron count of 86 electrons, and is held together by seven electron pairs.[291 The octahedral dianion [Osg(C0)18]~-is fully characterized, and is a suitable starting material for reactions with either mono- or dicationic species. The dianion [ O S ~ ( C O ) ~has ~ ]not ~ - been structurally characterized but, from spectroscopic data, is thought to retain the bicapped tetrahedral core of [Osg(C0)18],from which it is prepared by reduction with K / P ~ I C O .Further [ ~ ~ ~ evidence for the retention of the bicapped tetrahedral metal core in [O~ g (C 0 )1 7 ]comes ~from the structures of a number of derivatives which have this geometry (vide infra). The reaction of these two osmium dianions with two equivalents of the mono-cationic gold-containing fragment [ AuPR3]+ ( R = Me, Ph) will be described first. With [ O s g ( C 0 ) 1 ~ ] the ~ - , product is [Oss(CO)l7(AuPR3)2]in which the two [AuPR3lL fragments clip on to the existing cluster core,[3o1p3-capping two of the central tetrahedral faces as illustrated in Fig. 3. There are no Au. . .Au interactions and, if the ‘AuPR3’ units formally act as one electron donors, the cluster retains the electron count of 84. With [Osg(CO118]~-, even when an excess of [AuPR?]+is used, only the mono-anion [Osg(CO)Ig(AuPRj)1- is obtained,r301 presumably because the negative charge on the resultant mono-anion is sufficiently delocalized, and consequently the cluster is not sufficiently nucleophilic to attract the second equivalent of the cation to it. The structure of this mono-anion has not been determined but the
356
I Molecular Clusters
Q
=os
0 =Au
Figure 3. Diagramatic representation of the metal core geometry of [ O S ~ ( C O ) I ~ ( A U showing PR~)~] the positions of the p3-Au capping atoms.
metal framework is assumed to be an Osg octahedron with one face ,uu,-cappedby a ‘AuPR3’ unit. In order to encourage the coordination of a second cationic unit to the [ 0 s ~ ( C 01 8)1 ~ - dianion, the reaction was repeated using one equivalent of the dicationic reagent [Au2(dpprn)l2+(dppm = PhzPCH2PPh2). Only one product was obtained from the reaction, [Osg(CO)18Au2(dppm)],the structure of which is illustrated in Fig. 4. There has been a rearrangement of the octahedral Osg core found
Figure 4. The molecular structure of [Os6(CO)18A~2(dppm)].
1.19 High Nucleurity Osmium Gold Clusters ~
357
in the dianion to the electronically equivalent capped square based pyramidal framework. The ‘Au?(dppmj’ fragment adopts a p Z: p 2 bonding mode, a geometry which has been observed previously in the mixed ruthenium-gold clusters [ R L I ~ H ~ ( C O ) I Z A Uand ~ ([Ru6C(CO) ~ ~ ~ ~ ) 1]6[A~~ 2 ( d p p n ~ ) Jand , [ ~ ~caps ] the resulting Osq square face to form an O S ~ A trigonal U~ prism. The Au. . .Au separation is 2.8 114(13) A,and although this may be considered to be within ‘bonding’ distance, it may reflect the constraints of the ligand bite and hence is not necessarily indicative of a strong Au-Au interaction. Using this bidentate, chelating gold cation it is possible to prepare an octametal cluster with a formal electron count of 86 ec. When [ O S ~ ( C O )is~treated ~ ] ~ ~with one equivalent of [Au~(dppm)]’+or indeed [Au2(dppe)]’+ or [Au2(dppb)12+(dppe = Ph2PCHZCHZPPh2, dppb = Ph’P(CH2)q(x = m, e or b) is obtained in good yield. PPh?),one product [Os~(C0)~7Au21dppx)] The crystal structure of [Oss(CO)17Au2(dppm)] has been determined and the molecular structure is shown in Fig. 5. The Osg core retains the bicapped tetrahedral geometry of the parent dianion [ O s g ( C 0 ) 1 7 ] ~while ~ , the ‘AuZ(dppm)’ fragment p 2 : p 2caps a triangular face of the metal core. The Au-Au separation here is 2.7587(14) A. The electron count of 84 is consistent with the retention of the bicapped tetrahedral core, although here the two Au atoms, because of the steric requirements of the chelating ‘Au(dppm)’group, remain in close contact, unlike the
Figure 5. The molecular struc1. ture of [Osh(CO)17Auz(dppm)
358
I Molecular Clusters
situation previously described for [Oss(CO)17(AuPR3)2]where the ‘AuPR3’ remain well separated (Fig. 3). Thus, it is apparent that the addition of either mono- or digold cationic fragments to the cluster dianions [O~g(C0)17]~and [Os,(CO) occurs without significant changes to the osmium core geometries, other than to accommodate the requirements of the bidentate gold-containing ligands, and the osmium frameworks may be rationalized using P.S.E.P.T. in the same way as for the dianions themselves. However, it is possible to carry out reaction chemistry which interconverts the 84 e- bicapped tetrahedral osmium framework into the 86 e- octahedral framework. It has been stated that the reaction of [os6(co)18]2- with an excess of [AuPR3]+ only leads to the formation of the anion [ O S ~ ( C O ) I ~ ( A U P Rbut ~ ) ]the -, digold species [ O S ~ ( C O ) ~ ~ ( A Ucan P Rbe~ )obtained ~] by the careful carbonylation of [0sg(C0)l7(AuPR3)2].It has not been possible to obtain crystals of [ O S ~ ( C O ) ~ ~ ( A u P R ~ )but ~ ] IR and 31PNMR spectroscopic data indicates the presence of isoR~)~] mers in solution. The addition of a carbonyl group to [ O S ~ ( C O ) I ~ ( A U Pwill add two electrons and convert the bicapped tetrahedral osmium framework (84 e-) into the octahedral framework (86 e-). Then, assuming that the two ‘AuPR3’ groups adopt the ,u,-bonding mode, the 1,2, 1,3 or 1,5-isomers shown in Fig. 6a, 6b and 6c are possible. However, since the low temperature ,‘P NMR data suggests that the two phosphorus atoms are not equivalent, the two gold-containing units may adopt the p2:p3-bonding mode illustrated in Fig. 6d. This bonding mode for two ‘AuPR3’ fragments has been observed previously in the mixed-metal cluster [ R ~ ~ W C ( C O ) ~ ~ ( A U PThe E ~ ~cluster ) ~ ] .[Os6(CO)17Au2(dppm)] [~~] can also be converted into [Os6(CO)18Au2(dppm)],which has the capped square based pyramidal osmium core, by bubbling CO through a dichloromethane solution of the
(a) 1.2-isomer
(c) 1,5-1aomer
(b) 1,3-isomer
(d) p,p,-isomer
Figure 6. Diagramatic representation of some of the possible sites of co-ordination of the AuPR3 groups in [ O S ~ ( C O ) ~ R ( A U P R ~ ) ~ ] : (a) 1,2-p, :p3-isomer, (b) 1,3-p3:p,-isomer, (c) l,5-p3:p,-isomer, (d) pL2:pu,-isomer.
1.19 High Nucleurity Osmium
~
Gold Clusters
359
n
Figure 7. Structure of the [Os7(CO)?012-dianion.
17-carbonyl cluster. The process is reversible, and heating [Osg(CO)lsAu2(dppm)] results in the regeneration of [Osg(CO)17Au2(dppm)].
1.19.5.2 Additions to heptanuclear osmium cluster anions The parent heptanuclear osmium cluster, [Os,(CO),,], is prepared from the pyrolysis of [ O S ~ ( C O ) ~ ~ The ] . [ ’ crystal ” structure confirms that the metal framework is a capped octahedron consistent with electron count of 98 e- and the presence of seven skeletal electron pairs.[341The neutral carbonyl can be readily reduced to the dianion [OS~(CO)ZO]’with K/Ph?CO, and an X-ray analysis shows that the capped octahedral metal framework is retained (Fig. 7).[351The reaction of this dianion with two equivalents of [AuPR?]+( R = Et, Ph) leads to the formation of the neu80% These products have been fully tral clusters [ O S ~ ( C O ) ~ ~ ( A UinP R ~ ) ~yield. ] characterized by spectroscopic and crystallographic techniques, and the molecular structure of the triethylphosphine derivative is shown in Fig. 8 . The two ‘AuPEt3’ have clipped on to the existing monocapped octahedral osmium core in the p3bonding modes, and occupy adjacent faces 5 and 6 of the octahedron (assuming that the Os(C0)i unit caps face 1 of the central octahedron). The Au. . .Au separation is over 4 A so that it may be concluded that there is no direct interaction between these two metal atoms and, counting the ‘AuPEt3’ fragments as one electron donors, the 98 electron count for the cluster is retained and no significant change to the central osmium core structure is observed. With the bidentate gold cations [Au2(dppx)]’+ (x = m, e, b), the reaction with
360
I Moleculur Clusters
Figure 8. Molecular structure of [Os,iCOj*oiAuPEt~)?l.
[OS~(CO)~O initially ] ~ - affords the neutral complex [Os7(CO)2OAq(dppx)I, that from spectroscopic data is thought to have a capped octahedral osmium core similar to that found in [ O S ~ ( C O ) ~ O ( A U Pbut E ~on ~)~ standing ], in solution affords [0~7(C0)19Au2(dppx)] in which a carbonyl group has been lost from the initial product. An X-ray analysis of [Os7(CO),gAu2(dppm)] confirms the spectroscopic assignments and the presence of only 19 carbonyl ligands (Fig. 9). The loss of the two electron donating carbonyl ligand, and hence the formal decrease in the electron count to 96 e-, is accompanied by a change in the osmium core geometry from a structure based on an octahedron to one that is based on a bicapped tetrahedron. The osmium metal core consists of a bicapped tetrahedron with one edge of the central tetrahedron bridged by an Os(CO)3 unit similar to that observed for [ O S ~ H ~ ( C O ) ~The ~ ] .Os( [ ~ 1). ~ ]. .Os(4) separation is 3.705(3) A, and precludes a bonding interaction that would form a tricapped tetrahedron. The two Au atoms of the ‘AuZ(dppm)’ group p3-cap adjacent triangular Osj faces with a Au. . .Au separation is 2.832(2) A,which once again may be a reflection of the chelating ligand is bite. It is interesting to note that the count of 96 e- for [Os7(CO)1~Auz(dppm)] two electrons fewer than that required for the edge bridged bicapped tetrahedral geometry observed according to the Mingos Condensed Polyhedral counting scheme and may suggest that the cluster is somehow ‘unsaturated’ or that the ‘Au?(dppm)’ligand is contributing more than the formal count of two electrons to the osmium core. There may be some indication of ‘unsaturation’ in the osmium framework in that a number of the Os-0s contacts are among the shortest reported in osmium clusters. The atoms 0 4 2 ) and Os(5) in Fig. 9 have the highest metalmetal connectivity, with five Os-0s contacts and two Os-Au contacts. These two 0 s atoms, coordinated by two carbonyls, make short metal-metal contacts [Os(2)-Os(3) 2.685(2), Os(5)-Os(6) 2.704(2) A]. The edge bridging osmium atom, Os( l ) , also makes short Os-0s contacts to these two metals Os(2) and Os(5)
1.19 Hiqh Nuclearitjs Osrniuni
Gold C1u.ster.s
361
Figure 9. Molecular structure of [Os,(CO~loAuz!dppmil.
[Os(1)-0s(2) 2.691(2) and Os(1)-0s(5) 2.743(2) A] and the bridged edge [Os(2)-Os(5) 2.7239( 14) A] is also short, all of which may indicate some ‘unsaturation’ in this region of the framework. This feature of very short metal-metal distances in higher nuclearity clusters has been observed previously, and it has always been found that the shorter metalmetal interactions are associated with metals with the highest connectivities to other metals.[351This is not the case for the lower nuclearity clusters containing up to seven metal atoms, however, where higher coordination numbers are associated with longer metal-metal distances, as might be expected since, for mononuclear complexes, higher coordination numbers are associated with longer metal-ligand distances, simply on the grounds of reducing unfavorable ligand-ligand contacts. This trend for decreasing metal-metal distances as cluster nuclearity increases is consistent with the ‘bonding’ becoming more ‘metallic’ in character; distances between metal atoms in the bulk metal a generally shorter than those found in low nuclearity clusters although the ‘formal’ metal coordination number in the metal is as high as twelve. The presence of two Au atoms in non-equivalent environments in [Os7(CO)19Au?(dppm)] is inconsistent with the ” P N M R data, which gives only one signal
362
1 Molecular Clusters
even at low temperature, and this suggests that the ‘Auz(dppm)’ligand is fluxional on the NMR timescale if the structure is the same in solution.
1.19.5.3 Additions to octanuclear osmium cluster anions The bicapped octahedral octanuclear osmium cluster dianion [OSS( C0)22l2- can be prepared from the solid state pyrolysis of [OS~(CO)IO( NCMe)2].[371An X-ray analysis of the [N(PPh3)2]$ salt shows that the two Os(CO)3 caps occupy the 1 and 3 faces of the octahedron[381as shown in Fig. 10, and the structure of the dianion is consistent with the electron count of 110 e- and seven skeletal electron pairs. The reaction of [O~g(C0)22]~with two equivalents of the [AuPPh3]+ cation leads to the formation of two isomeric products with the formula [Os~(C0)22( A U P P ~ ~ ) The ~ ] . major [ ~ ~ ]isomer has been characterized by spectroscopy only, and is thought to retain the bicapped octahedral osmium framework of the parent dianion. The 3’PNMR spectrum exhibits two signals of equal intensity, which suggests that the two ‘AuPPh3’ units occupy non-equivalent sites on the OSSframework. Over a period of time, in solution, some of the major isomer converts into the minor isomer giving an equilibrium mixture. The initial major isomer is viewed as the kinetic product, which converts slowly into the thermodynamic product that initially was the minor product. The 31PNMR spectrum of this minor isomer exhibits only one signal, consistent with the ‘AuPPh3’ groups occupying equivalent sites. This equivalence of the ‘AuPPh3’ groups is confirmed by X-ray analysis, which also shows that there has been a rearrangement of the octanuclear osmium core (Fig. 11). When the structure was first reported the osmium framework was described as an Osg octahedron which shared a common edge with an Osq butterfly unit.[401The two ‘AuPPh3’ groups ,u,-cap the two sides of the butterfly to form a second distorted octahedron, but the Au. . .Au separation is close to 4 A so that it is felt that there is no direct Au. . .Au interaction. In the original description of the structure the contacts between Os(2) and Os(7) and Os(4) and Os(8) were not considered to be bonding, being slightly in excess of 3.1 A,but if these contacts are included, then the 0 s core framework can be considered as a bicapped octahedron, but the 1,5isomer rather than the 1,3-isomer found in [ O s ~ ( C 0 ) 2 2 ] ~The - . [ ~electron ~~ count of 110 e- is then at least consistent with the bicapped octahedral framework geometry observed. However, the structure of the O S ~ A ‘open’ U ~ octahedral unit is similar to
1.19 High Nuclearity Osmium
-
Gold Clusters
363
Figure I I . Diagramatic representation of the structure of the minor isomer of [Osg(COj22(AuPPhI)?]showing the metal core.
that found in [ O S ~ ( C O ) ~ ~ ( A U P P which ~ ~ Mis~viewed ) ~ J ,as’ ~ an~‘unsaturated’ ’ cluster that can readily form addition complexes with nucleophiles. This ‘open’ and in the minor isomer of Os4Auz unit found in [Os4(CO)l~(AuPPh~Me),] [Oss(C0)22(AuPPh3)2]is novel, and is not easily rationalized in terms of the commonly used electron counting schemes. However, as will be seen later, the observation of distorted Os4Auz octahedra in high nuclearity clusters is becoming common. This structure does illustrate the difficulty in assigning structure and bonding patterns from bond length data. It is clear that for metal-metal bonds in transition metal clusters. the bond-order-bond-strength relationship, which can be applied to complexes of the lighter elements, is not appropriate here, and a wide range of metal-metal distances, all corresponding to a bonding interaction, are observed. The metal-metal bonding interactions must lie on a fairly flat potential energy surface that partly reflects the delocalized nature of the bonding. The observations are also consistent with a bonding picture in which mixing between, say, the s and d , ~ orbitals occurs, resulting in a reduction in the energy separation of the valence orbitals. Two isomers are not observed in the analogous reaction with [ O s ~ ( C O ) 2 ~ and ]’p the digold cations [Auz(dppx)]’+ (x = m, e and b) but only one class of product is isolated in almost quantitative yield. The ” P N M R spectrum of this product shows two equal intensity signals, which indicates that the two phosphorus nuclei are in different environments and suggests that the two gold atoms are in inequivalent positions. This is confirmed by the X-ray structure of [Osg(CO)zzAuz(dppb)J(Fig. 12) which shows that the bicapped octahedral OSXarrangement in the parent diani0n[~’1[ O S ~ ( C O is ) ~retained, ~ ] ~ ~ and the two Au atoms adopt the ,u3:p,-bonding mode over a planar Osq face. The A u . . . A u separation is 2.975(1) A, which is
364
1 Moleculur Clusten
11.3,
Figure 12. Molecular structure of [Os~(CO)2~Au2(dppb)].
characteristic of the ligand bite. It is probable that this structure of the OsgAuz core is the one adopted by the kinetic isomer of [Osg(C0)22(AuPPh3j ~ ] . [ ~ ~ ]
1.19.5.4 Additions to nonanuclear cluster anions The only characterized nonaosmium dianion is [Osg(CO)24]2-,which can be prepared, in reasonable yield, from the vacuum pyrolysis of [Os3(C0)10(NCMe)2].r421 The structure of this anion has not been determined, but the electron count of 122 e- and the skeletal electron pair count of seven corresponds to a tricapped octahedral metal framework. The structure of the related monohydride [OsgH(CO)24]has been determined (Fig. 13) and has been shown to adopt this tricapped octahedral geometry.[421 The reaction of [Osg(C0)24l2- with two equivalents of the monogold cations } ) , to the formation of two [AuL]+ ( L = PPh3, PPhzMe, PCy3 {Cy = C ~ H I ~ leads new products, the neutral cluster [Osg(CO)24(AuL)2]and the anion [Osg(COj24-
1.19 High Nucleurity Osmium
~
Gold Clusters
365
Figure 13. Structure of the [ Osc,H(C0)24]- anion.
(AuL)IP.[431 Attempts to obtain crystals of the anionic complex failed but spectroscopic data are consistent with a structure based on a tricapped octahedral osmium core. Suitable crystals of the neutral cluster [Osg(C0)24(AuPCy3)2]were obtained and the molecular structure is illustrated in Fig. 14. An important structural rearrangement of the OSScore has occurred compared to that of the [ O S ~ ( C O ) ~ ~ ] ~ ~ dianion, although the new structure is still based on an octahedral structure consistent with the seven skeletal bonding electron pairs. The metal framework is reminiscent of that found in the minor, thermodynamic isomer of [O~x(CO)zz(AuPPhi)2],[~~] but in this case with an additional OsiCOj3 group capping a face of the Osg octahedron on the opposite side to the ‘open’ O S ~ A U unit. ~ As with [Os~(C0)22(A~PPh3)21,[~’] in [Osg(C0)24(AuPCy3)2] it is the interaction between the Os, capped octahedron and the O S ~ A ‘open’ U ~ octahedron that is of interest.[431It is tempting to consider that both the Osi2)-Os(3) [2.965(3) A] and Os(8)-Os(9) [3.391(3) A]interactions are bonding, and then the osmium core framework could be considered as a tricapped octahedron, consistent with the count of 122 e ~ - . However, the Os(S)-~Os(9)distance is very long to be considered to involve a bonding interaction, and the required count from the Mingos Condensed Polyhedral scheme would be 124 e-. Again the presence of the ‘open’ OSJAUZunit is concomitant with an ambiguity in the electron count. A clue to the solution of this ambiguity may be in the distortion of the ‘open’ O S ~ A octahedral U~ unit where the octahedron is becoming compressed and the trans axial metal atoms, Os(3) and
366
I Molecular Clusters
Figure 14. Molecular structure of IOs9(C0)24(AuPCyi)zI.
Os(9), are separated by only 3.224(3) A,and could be moving to within bonding distance.
1.19.5.5 Additions to decanuclear cluster anions The decaosmium carbido dianion [Osl&(CO)24J2- reacts with both mono- and bidentate gold phosphine cations, and the tetracapped octahedral core geometry, consistent with the electron count of 134 e- and seven skeletal electron pairs, is maintained in all the characterized examples. With [AuPPh31t, only the monoanion [OsloC(C0)24(AuPPh3)]- is obtained,[441as, presumably, the anionic charge on this monoanion is sufficiently delocalized, reducing the nucleophilicity, for the cluster to behave more like a neutral species. In the X-ray analysis of the anion the gold atom bridges an edge of one of the capping Os4 tetrahedra. In contrast, with the dication [Au2(dppm)I2+,the neutral cluster [0~1~C(C0)24Au2(dppm)] is formed.[451In the structure of this cluster the two gold atoms p 3 : p,-cap adjacent triangular faces of two of the capping Os4 tetrahedra with an Au...Au separation of 2.978(3) A (Fig. 15). An isostructural product, [RuloC(C0)24Au2(dppm) I, is obtained from the analogous reaction between [R~ loC (C 0 )2 4 ]~ and - [A~2(dppm)j~+.[~~] which is prepared by the vacuum pyThe non-carbido dianion [O~lo(CO)26]~-, rolysis of [ O S ~ ( C O ) I O ( N C M ~and ) ~ ]whose , [ ~ ~ ]structure is shown in Fig. 16, reacts
I . 19 High Nuclearity Osmium
Figure 15. Molecular structure of [Os1nCiC0)?4Au2idppm)].
Figure 16. Structure of the non-carbido [Oslo(COh(,]?- diaiiion.
-
Gold C1ustpr.s
361
368
1 Molecular Clusters
with excess [AuPPh3]+ to form the anion [ O S I O ( C ~ ) ~ ~ ( A U in P P90% ~ ~ ) yield. ]Again there is no evidence for the presence of a digold-substituted neutral species, presumably because of the delocalization of the anionic charge. The anion has been characterized spectroscopically, and it is assumed that the tricapped octahedral osmium core, with one of the caps capped by a further cap, observed in the dianion, is retained upon coordination of the ‘AuPPh3’ unit, consistent with the 134 electron count. With the bidentate gold cations [Au2(dppx)I2+(x = m, e, b), the reaction with [Osl0(CO)26]~-affords two products. These have been characterized as the neutral clusters [Os,o(C0)26Au2(dppx)land anionic products [Os,o(C0)26(Au(dppx)AuCl}]- . The clusters have been characterized spectroscopically, but it has not been possible to obtain crystals suitable for analysis to investigate the metal core geometries in detail. However, the IR spectrum in the carbonyl region shows a similar band pattern to that reported for [ O S ~ O ( C O ) ~ ~and ] ~ -it, appears [ ~ ~ ] that the core geometry of the dianion is retained in these products. In the neutral product the 31P NMR spectrum exhibits two signals consistent with the two Au atoms occupying inequivalent sites.
1.19.6 Synthesis of poly-aurated high nuclearity osmium carbonyl clusters 1.19.6.1 Hexaosmium cluster systems The cluster [Osg(C0)17Au2(dpprn)](Fig. 5 ) may be reduced with freshly prepared Na/Hg amalgam in tetrahydrofuran ( T H F ) , to give the anionic species [Osg(CO)17Au2(dppm)J2-, which is not isolated but is used in situ for reactions with further gold cations. The reaction with one equivalent of [Au2(dppm)I2+ affords two isomeric products, a major yellow isomer and a minor pink isomer. Mass spectrometric data are consistent with the formulation [Os6(CO)17Aq(dpprn)2] for both compounds, and the 31PNMR spectrum for the yellow isomer exhibits two doublets at -66.98 and -112.21 ppm, with J p p = 69.8 Hz, consistent with the nonequivalence of the gold atoms. The 31PNMR spectrum of the pink isomer exhibits only one signal at room temperature, which suggests that a dynamic process is occurring, and solubility problems prevent a detailed investigation being carried out at lower temperatures. Suitable crystals for an X-ray diffraction study were obtained for the major yellow isomer, and the molecular structure is shown in Fig. 17. The metal framework can be described as an Osg octahedron with two adjacent faces p3-capped asymmetrically by two Au atoms, one from each of the [Au2(dppm)]’+ units, while the remaining two Au atoms are in a terminal bonding
1.19 High Nuclarity Osniiuni - Gold Clusters
369
Figure 17. Molecular structure of the minor yellow isomer of [ Osb jC0)I ~ALIA,( dppm)?].
mode coordinated to Os( 1 ), which is already linked to the capping Au atoms. The Au-Au contacts lie in the range 2.677(3)-3.188(3) A,a similar range to that found in polygold homonuclear clusters.["71The shortest Au-Au distance is between the two terminally coordinated Au atoms, which have lower metal coordination numbers. The coordination of these Au atoms is somewhat similar to that observed U~ which ( ~ two ~ P Au ~ )atoms, ~ ] [ one ~ ~ from ] each of the in [ O S ~ H ~ ( C O ) ~ ~ A in [Au2(dppm)12+groups, are in the p,-bonding mode rather than in the p,-bonding mode as in (Oss(C0)17Au4(dppm)2]. Another significant feature of the structure is the change in the osmium core geometry from the bicapped tetrahedral arrangement in [Os~(CO)17Au2(dppm)] to the octahedral arrangement in [Osb(CO)17Au4(dppm)z], which is consistent with the increase in electron count from 84 e- to 86 e- on the formation of the tetragold cluster. Similarly, [ O ~ ~ ( C O ) ~ ~ A u ~ ( d p p also m ) ] ' -reacts with two equivalents of The IR spec[AuPPhi]+ to form two isomers of [Osh(C0)17Au2(dppm)(AuPPh3)2]. tra in the carbonyl region for these two isomers are similar in band pattern to those so that the structures may be observed for the isomers of [Os~(CO)~7Au4(dppm)2). similar. The " P NMR spectrum shows a large number of signals, and it was not possible to separate the isomers cleanly because of complex equilibria in solution. In contrast, [Oss(CO)~~Au?(dppm)] could not be readily reduced with Na/Hg amalgam.
370
I Moleculur Clusters
1.19.6.2 Heptaosmium cluster systems In a manner analogous to the hexaosmium systems, [0~7(CO)19Au~(dppm)] (Fig. 9) can be readily reduced with freshly prepared Na/Hg amalgam, in THF, to produce a highly reactive anionic species, which can be treated with one equivalent of [Au2(dpprn)l2 in situ. Two products are obtained in reasonable yield, a yellow product and a purple product, but on standing, the yellow product converts into the purple product irreversibly. The two products were characterized spectroscopically, and are isomers with the formula (O~~(CO)~~(Au2(dppm)}2]. The 31PNMR spectrum of the purple isomer exhibits two doublets at -83.29(d) and -96.59(d) ppm ( J p p = 62.25 Hz), indicating two non-equivalent sites for the gold atoms, while that of the yellow isomer exhibits four doublets, consistent with the presence of four non-equivalent gold atoms. From the formulation the electron count for these isomers is 98 e-, with each Au atom acting formally as a one electron donor, so that the osmium framework should either be based on a capped octahedron or on an edge bridged bicapped tetrahedron, the latter structure being found for the [0~7(CO)l~Au2(dppm)] precursor. Unfortunately, it has not been possible to obtain crystals of a suitable quality for an X-ray analysis to establish the core geometry. The cluster [O~-i(C0)20(AuPPh3)2] can also be reduced with Na/Hg amalgam and allowed to react with [AuPPh3]+ in situ, but a large number of products are obtained, and it has not been possible to obtain detailed characterizations of them. It is likely that cluster breakdown occurs during the reaction.
1.19.6.3 Octaosmium cluster systems The reduction of [Osg(C0)22(AuPPh3)2]with Na/Hg amalgam, and the subsequent reaction with an excess of [AuPPh3]+, affords one neutral product in reasonable yield, which has been fully characterized as [Osx(C0)20(AuPPh3)4]in which two carbonyl groups have been lost from the digold starting material as the two gold phosphine ligands have been added. The 3’P NMR spectrum displays a singlet at -64.03 ppm, consistent with the presence of equivalent gold sites on the cluster at room temperature. The X-ray analysis ( Fig. 18) reveals that the osmium core is that of a bicapped octahedron, with the two 0 s caps occupying the 1 and 5 faces. Two of the gold phosphine units ,uc,-capthe faces 3 and 7 of the octahedron, while the other two ‘AuPPh3’ groups cap the 0 s caps in a manner similar to that observed in the minor isomer of the [Osg(C0)22(AuPPh3)2]precursor (Fig. 11) to give the ‘open’ OS~AU octahedron.[411 ~ However, the description of the osmium core is open to some doubt since the Os-0s edges in the equatorial plane of the octahedron are elongated, lying in the range 3.149(2)-3.475(2) A. On the other hand, the Os(1)Os(2) and the Os(5)-Os(8) edges, which were long in the [Osx(C0)22(AuPPh3)2] precursor, now lie in the normal range for Os-0s interactions, at 2.95616) and
1.19 High Nucleurity Osmium
~
Gold Clusters
371
Figure 18. Molecular structure of [Os8(CO)~o(AuPPh~)4].
2.958(6) A,respectively. An alternative way of describing the metal framework is that the 0 s and Au atoms form a novel tubular framework as illustrated in the ‘endon’ view shown in Fig. 19. The lengthening of the equatorial 0 s - 0 s edges of the octahedron, 0s(3)-0s(4) and 0s(6)-0s(7) (average 3.20 A),is concomitant with an axial compression of the octahedron such that the 0s(2)-0s(5) distance is reduced to only 3.223(6) A (compared to a distance of ca. 4.1 A across an octahedron of edge length 2.9 A).This trend has been noted previously in the structures of some of the octa- and nonaosmium digold clusters. The electron count in this cluster is 108 ec, which does not correspond to that expected for a bicapped octahedron by using either P.S.E.P.T. or the Mingos Condensed Polyhedral approach, and the structure represents a new structural form for osmium-gold clusters. The structure may be viewed as ‘unsaturated’, and an electrochemical study shows that [Osg(C0)20(AuPPh3)4] undergoes two reversible one electron reductions, which suggests that there is no dramatic metal framework rearrangement when two additional electrons are added, to give an electron count consistent with a bicapped octahedral cluster geometry; there will, however, be significant changes in metal-metal distances. The cluster [Os8(C0)22Au2(dppm)] loses one carbonyl during the reduction
1 Moleculuv Clusters
Figure 19. The view along the ‘OsgAu4 tube’ in [Os~(CO)~o(AuPPh3)4] showing the core structure.
with Na/Hg amalgam, but forms only [Os~(C0)21Au2(dppm)] on reaction with [ A u (dppm)]+ ~ rather than a tetragold cluster. A crystallographic analysis of (Fig. 20) reveals that the osmium core consists of a tri[Os~(C0)21Au2(dppm)] capped tetrahedron with one of the caps capped again by the eighth 0 s atom. This core geometry is consistent with the electron count of 108 e- for the cluster, and the change in core geometry from the bicapped octahedral arrangement in the 110 electron starting cluster [Os~(C0)22Au2(dppm)] is consistent with the loss of two electrons. The two Au atoms are bound asymmetrically to the cluster. The Au(1) atom adopts an edge bridging ,u2-bonding mode, while the Au(2) atom exhibits an unusual p4-bonding mode lying over one face of the central Os4 tetrahedron and linking to one of the 0 s caps. The Au. . .Au separation is 2.826(4) A.
1.19.6.4 Nonaosmium cluster systems The reduction of [ O S ~ ( C O ) ~ ~ ( A Uand P P subsequent ~~)~] reaction with [AuPPh3]+ affords the tetragold cluster [ O S ~ ( C O ) ~ * ( A U Pwhich P ~ ~ ) has ~ ] , an electron count of 120 e- , two less than in the starting material. Therefore, there may be a structural change in the metal core from a structure based on a central Osg octahedron, as observed in [ O S ~ ( C O ) ~ ~ ( A U P(Fig. C Y ~14),1431 ) ~ ] to one based on fused tetrahedra but it has not been possible to obtain suitable crystals for an X-ray analysis to confirm this. The 31PNMR spectrum exhibits signals at -65.64, -65.69, -63.22,
I . I 9 Hi<& Nuclrcirity Osmium Gold Clusters -
373
Figure 20. Molecular structure of [OSX(CO)ZI Au2rdppm)l
-66.50, -67.80, and -77.88 ppm in the ratio 3 : 3 : 2 : 2 : 1 : 1 , which suggests that three isomers are present, but it has not been possible to separate and identify these products.
1.19.6.5 Decaosmium cluster systems As was discussed previously, in the case of decaosmium clusters, only monogold anionic derivatives of [AuPPh;]’ were obtained by the reaction of the parent dianionic cluster with an excess of the [AuPPh3]& One route that has been successful in introducing more than two gold phosphine groups on to an Oslo framework involves the use of the polygold cation [(AuPR~)30]’-.[4y1 The reaction of [ O S I ~ C ( C O with ) ~ ~(iAuPR3j30]BF4 ]~~ {PR3 = PCy3: PPh3, PMezPh) affords the neutral tetragold cluster [ O S ~ ~ C ( C O ) ~ ~ ( A U P R ~ ) ~ ] and the cationic cluster [ O S I O C ( C O ) ~ J A PR3)41+: U ~ ( oiri the [ O S I & ( C O ) ~ ~ ( A U P R ~ ) ) The cluster [OsloC(COjz4Au(AuPCy,)31 has been characterized crystallographically, and the structure is shown in Fig. 21. The tetracapped octahedral geometry of the parent Oslo dianion is retained, and the four Au atoms form a tetrahedral cluster that is linked to the 0 s core cia one Au atom that bridges an 0 s - 0 s edge of one of the tetrahedral caps. This 0 s - 0 s edge, at 3.025(2) A, is sig-
374
I Moleculur Clusters
Figure 21. Molecular structure of [Osl&(C0)24Au(AuPCy3)3].
nificantly longer than the other Os-0s edges in the tetrahedral Osq caps (mean ~ is 2.713(2) A, 2.783(2) A).The average Au-Au edge length in the A u tetrahedron which is shorter than the other Au-Au contacts discussed in this article. The previously described reduction route has also been used to introduce more gold-containing units into Oslo clusters. The carbido cluster dianion [O~loC(C0)24]~was further reduced with K/PhZCO to generate a highly reactive tetraanionic species. However, the reaction of the tetraanion in situ with excess [AuPPh3]+ afforded only [ O S ~ ~ C ( C O ) ~ ~ ( A rather U P P ~than ~ ) ]a~polygold cluster. When [Au2(dppm)I2+ was used instead of [AuPPh3]+ the cluster [Os1oC(CO)23{ AuZ(dppm)}21 was isolated and characterized spectroscopically, but which is assumed to have the tetracapped octahedral core of the parent dianion. The non-carbido dianion [O~lo(CO)26]~was readily reduced further with K/ Ph2C0, presumably to give a tetraanionic species, which was treated in situ with
1.19 High Nuclearity Osmium
~
Gold Clusters
375
Figure 22. Molecular structure of [Os,o(C0)24iAuPPhzMe)4]
[AuPPh2R]+ [R = Ph. Me]. The product was assigned the formula [Os10(CO)24(AuPP~~R from ) ~ ]spectroscopic studies, and the "P NMR spectrum showed only a singlet, which suggested that all the Au atoms were in equivalent environm e n t ~ . [These ~ ~ ' spectroscopic assignments were confirmed by X-ray analysis on the biphenyl methyl phosphine derivative [ O S ~ ~ ( C O ) ~ ~ ( A U P The P ~molecular ~M~)~].[~~~ structure is shown in Fig. 22. The osmium core framework may be described as two fused octahedra sharing a common edge with metal-metal contacts between their apices. All four Au atoms adopt the ,u3-bonding mode, pairs of Au atoms capping adjacent Osi faces at either end of the osmium core in a manner similar to that observed in [Os~(CO)~o(AuPPhi)4] (Fig. 19). The pairs of Au atoms are separated by 4.432(2) A.However, as in the octanuclear osmium cluster, it may be incorrect to describe the Oslo core structure as two edge-sharing octahedra because of the elongation of the Os-0s edges in the equatorial plane of the structure as viewed in Fig. 22; these Os-0s interactions lie in the range 3.286(2)-3.322(2) A.The axial compression of the Os6 octahedra is also observed so that the pairs of trans axial 0 s atoms, Os(2).. .0s(3a) and Os(3).. .Os(2a),are separated by ca. 3.3 A.The osmium core can then be described as a tubular cluster with two capping gold atoms at each
316
I Moleculur Clusters
end as shown in Fig. 23, and the length of the tube including the gold phosphine groups is in excess of 1 nm. The electron count for [Oslo(C0)24(AuPPh~Me)4] is 132 e-, which is not consistent with the observed geometry if P.S.E.P.T. is applied. Cyclic voltammetry studies show that the cluster undergoes two reversible one electron reductions, indicating that there is no change in core geometry with the uptake of two electrons, and as such the cluster may be described as ‘unsaturated’. This complex represents the second example of a new class of 0s-Au cluster which exhibits novel electronic properties. The reduction of [0~10(C0)26Au~(dppm)] can be achieved by use of Na/Hg amalgam, in THF, and subsequent reaction with an excess of the [Au2(dppm)12+ cation affords products, [Oslo(C0)25Au2(dppm)] and [Oslo(C0)25{ AUZ(dppm)}I], which have been characterized spectroscopically. The 3’P NMR spectrum of [Oslo(C0)25Au2(dpprn)]exhibits only one signal at -95.95 ppm, unlike the parent cluster [Oslo(CO)2,jAu2(dppm)],which may suggest that a fluxional process is occurring at room temperature. Since the electron count for [Oslo(C0)25Au2(dppm)] (132 e-) is two electron less than that in [Oslo(CO)26Au2(dppm)], it may well
1. I I) High Nucleuritj. Osn?ium Gold Clusters ~
377
be that a metal core rearrangement has occurred. The electron count for [Oslo(C0)25{Auz(dppm)j2] (134 e-) is the same as that for (Oslo(CO)~~Au2(dppm)] so it is assumed that the osmium core structure is similar.
1.19.7 Conclusion The spectroscopic and crystallographic studies on the new osmium-gold clusters described here show a number of interesting features. The gold phosphine cationic units tend to attach themselves to the periphery of the osmium cluster anion in the condensation process, and do not form part of the ] reaction chemistry central core as do some other cationic capping g r o ~ p s . [ l ” ~ ’The observed for the mixed osmium-gold clusters is largely centered on the osmium core and does not involve the gold centers directly. The positions of the gold fragments on the core are largely governed by steric factors, and the chelating bite requirements of the bidentate ‘Auz(dppx)’ fragments are also important in determining the geometry; the OsAu core geometries show differences between the cases where either one ‘Aua(dppx)’ unit or two ‘AuPR3’ units are co-ordinated to the osmium framework. With regard to Au-Au bonding, for the systems containing several ‘AuPR3’ fragments there are examples where these units are well separated, as in [Os,(CO)Z,,iAuPPh3)21(Fig. 8), and others, such E ~ ~ )the ~ ]units , [ ~are ~ ’within Au-Au bonding disas [ R U ~ W C ( C O ) L ~ ( A U P where tance. While the relatively short Au-Au distances between the two Au atoms in the ‘Auz(dppx)’groups may be a requirement of the ligand bite, in the higher nuclearity clusters with two ‘Auz(dppx)’groups [Osh(COjl7Au4(dppm)2](Fig. 17) has AU-ALI short contacts between Au atoms of the two ‘Auzidppxj’ ligands of 2.677(3) and 3.188(3) A.This suggests that, as in other systems,[”’ the tangential Au-Au bonding is relatively weak, but where steric factors permit there is an overall energy gain in forming tangential Au-Au bonds. As the nuclearity of the resultant clusters increases there is good circumstantial evidence that the delocalization of the bonding increases. Two major structural forms for the clusters can be picked out, those with osmium cores based on capped octahedra and those based on fused tetrahedra, the latter typically having an electron count of two electrons less than the former for related systems. The exceptions to this generalization are the two new tubular structures [Osg(C0)2~(AuPPh3)4] and [Oslo(C0)24(AuPPhzMe)4] where the electron counts do not correspond to the observed metal framework geometries. These represent a new class of metal cores with dimensions approaching nanometer size, and which have the potential to show new catalytic and electronic properties.
318
1 Molecular Clusters
Acknowledgements Finally, we would like to thank all the research students and post doctoral workers who have been associated with the research area over a number of years. Particular gratitude goes to Zareen Akhter, Muna Al-Mandhary, Scott Ingham and C.-K. Li who produced many of the ideas and carried out much of the synthetic work described in this article. We are also indebted to Nicholas Leadbeater and Gregory Shields for assistance with the preparation of this article. We are grateful to the E.P.S.R.C. for continued financial support and to Johnson Matthey for the loan of the metal salts.
References [I] E.L. Muetterties, T.N. Rhodin, E. Band, C.F. Brucker, W.R. Pretzer, Chem. Rev., 1979, 79, 91. [2] G. Suss-Fink, G. Meister, Adv. Organornet. Chem., 1993, 35, 41; W.L. Gladfelter, K.J. Roesselet, in ‘The Chemistry of Metal Cluster Complexes’, Eds. D.F. Shriver, H.D. Kaesz, R.D. Adams, VCH Publishers, Inc., New York, 1989; G.A. Somorjai, J. Phys. Chem., 1990, 94. 1031; P. Braunstein, J. Rost, in ‘Comprehensive Oryanometallic Chemistry’. Second Edition, Eds. E.W. Abel, F.G.A. Stone, G. Wilkinson, Pergamon: Oxford 1995, Vol. 10, p. 351; P. Braunstein, J. Rose, in ‘Catalysis by Di- and Polynuclear Metal Cluster Completes’, Eds. R.D. Adams, F.A. Cotton, Wiley-VCH, New York, 1997, 346. [3] See for example: P. Braunstein, C. de Miric de Bellefon, S.-E. Bouaoud, D. Grandjean, J.-F. Halet, J.-Y. Saillard, J. Am. Chem. Soc., 1991, 113, 5282; L.H. Gade, Anyew. Chem., Int. Ed. Enyl., 1993, 32, 24. 141 B.F.G. Johnson, L.H. Gade, J. Lewis, W.-T. Wong, Materials Chemistry and Physics, 1991, 2Y, 85. [5] P. Braunstein, D. Cauzzi, G. Predieri, A. Tiripicchio, J. Chem. Soc., Chem. Commun., 1995, 229; P. Baistrocchi, D. Cauzzi, M. Lanfranchi, G. Predieri, A. Tiripicchio, M. Tiripicchio Camellini, Inory. Chim. Actu., 1995, 235, 173. [6) D.S. Shephard, T. Maschmeyer, B.F.G. Johnson, J.M. Thomas, G. Sankar, D. Ozkaya, W. Zhou, R.D. Oldroyd, R.G. Bell, Angew. Chem., Int. Ed. Enyl., 1997, 36, 2242. [7] G. Schmid, J. Chem. Soc., Dalton Trans., 1998, 1077. [8] M.A. Beswick, J. Lewis, P.R. Raithby, M.C. Ramirez de Arellano, J. Chern. Soc., Dalton Trans., 1996, 4033; M.A. Beswick, J. Lewis, P.R. Raithby, M.C. Ramirez de Arellano, Angew. Chem., Int. Ed. Enyl., 1997, 36, 291; 2227. [9] Z. Akhter, S.L. Ingham, J. Lewis, P.R. Raithby, Angew. Clzern., Int. Ed. Engl., 1996, 35, 992. [I01 J. Lewis, P.R. Raithby, J. Orgunomet. Chem., 1995, 500, 227. [ I l l A.J. Amoroso, L.H. Gade, B.F.G. Johnson, J. Lewis, P.R. Raithby, W.-T. Wong, Angew. Chem., Int. Ed. Enyl., 1991, 30, 107. [ 121 K. Wade in ‘Trcmsition Metal Clusters’, Ed. B.F.G. Johnson, Wiley Tnterscience, Chichester, 1980; S.M. Owen, Polyhedron, 1988, 7, 253.
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Hi& Nuclearity Osmium - Gold Clusters
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1131 K. Wade, Adv. Iizory. Cherw. Rudiochem., 1976, 18, 1; D.M.P. Mingos, Nutiire Phys. Sci. (London), 1972, 236. 99. [I41 M. McPartlin, C.R. Eady, B.F.G. Johnson, J. Lewis, J. Chem. Soc., Chem. Commun., 1976, 883. (151 D.M.P. Mingos, J. Chem. Soc., C h m . Commun., 1983, 706. 1161 L.H. Gade, B.F.G. Johnson, J. Lewis, M. McPartlin, H.R. Powell, P.R. Raithby, W.-T. Wong, J. Chem. Soc.. Dulton Truns., 1994, 521. (171 C.R. Eady, B.F.G. Johnson, J. Lewis, J. Chwn. Soc.; DuEton Truns., 1975. 2606. [IS ] M.D. Vargas, J.N. Nicholls, Adr. Znorg. Clzem. Rudiochrn?., 1987, 30, 123. [191 P.R. Raithby, G.P. Shields, Polyhedron, 1998, 17, 2829. 120) G.N. Ward, Ph.D. Thesis, University of Cambridge. 1996. 1211 D. Braga, P.J. Dyson. F. Grepioni, B.F.G. Johnson, Chem. Rei).,1994, 94, 1585. 1221 R. Buntem, J. Lewis, C.A. Morewood. P.R. Raithby, M.C. Ramirez de Arellano, G.P. Shields, J. Chern. Soc.. Dulton Trctns., 1998. 1091. [23] J.E. Davies, S. Nahar, P.R. Raithby, G.P. Shields, J. Chem. Soc., Dalton Truns., 1997, 13. 1241 J. Lewis, C.-K. Li, C.A. Morewood, M.C. Ramirez de Arellano, P.R. Raithby, W.-T. Wong, J. Chem. Soc,.,Dolton Trrins.: 1994, 2159. [25] P.R. Raithby in ‘Truiisition Metul Chrstc~rChwnistry’, Ed. B.F.G. Johnson, Wiley Interscience, Chichester, 1980; D.H. Farrar. R.J. Gondsmit in ‘Metal Clusters’, Ed. M. Moskovits, Wiley, New York. 1986. 1261 I.A. Oxton, Rev. Inorg. Chern., 1982, 4, 1. 1271 J. Lewis, B.F.G. Johnson. Arc. Cheni. Res.; 1968, I , 245. 1281 B.T. Heaton, Phil. Trrms. R. Soc. London, 1982, A308. 95; E. Rosenberg. Polyheclron, 1989, 8. 383. 1291 D.M.P. Mingos. A.S. May in ‘The Chemistry of’Mrtul Cluster Complexes’,Eds. D.F. Shriver, H.D. Kaesz, R.D. Adams. VCH Publishers. Inc.. New York, 1989. [30] B.F.G. Johnson, J. Lewis, P.R. Raithby. M.D. Vargas, unpublished results. (311 S.S.D. Brown. I.D. Salter, A.J. Dent, G.F.M. Kitchen, A.G. Orpen. P.A. Bates, M.B. Hursthouse, J. Chem. Soc.. Dulton Truns.. 1989, 1227. 1321 P.J. Bailey, M.A. Beswick. J. Lewis. P.R. Raithby, M.C. Ramirez de Arellano, J. Oryanomet. Chem., 1993. 459, 293. 1331 S.R. Bunkhall, H.D. Holden, B.F.G. Johnson, J. Lewis, G.N. Pain, P.R. Raithby, M.J. Taylor, J. Chem. Soc.. Chem. Comniun., 1984, 25. [34] C.R. Eady, B.F.G. Johnson, J. Lewis, R. Mason, P.B. Hitchcock, K.M. Thomas, J. Chem. Soc., Chenz. Commun.. 1977, 385. 1351 A.J. Amoroso, B.F.G. Johnson, J. Lewis, C.-K. Li, C.A. Morewood, P.R. Raithby, M.D. Vargas, W.-T. Wong. J. Clust. Sci., 1995, 6 , 163. [36] E.J. Ditzel, H.D. Holden, B.F.G. Johnson, J. Lewis, A. Saunders, M.J. Taylor, J. Chem. Soc.. Chern. Conzmun.,1982, 1373. 1371 W.-T. Wong, Ph.D. Thesis, University of Cambridge, 1991. [38] P.F. Jackson. B.F.G. Johnson. J. Lewis, P.R. Raithby, J. Chem. Soc., Cheni. Commun.,1980, 60. [391 Z. Akhter, S.L. Ingham, J. Lewis, P.R. Raithby, J. Oryunomet. Clzem.,1994, 474, 165. [40] B.F.G. Johnson, J. Lewis. W.J.H. Nelson, P.R. Raithby, M.D. Vargas, J. Clzem. Soc.. Chem. Cornmun., 1983, 608. [41] C.M. Hay, B.F.G. Johnson, J. Lewis, R.S.C. McQueen. P.R. Raithby, R.M. Sorrel], M.J. Taylor. Orgunometullics, 1985, 4 . 202. 1421 A.J. Amoroso, B.F.G. Johnson, J. Lewis, P.R. Raithby, W.-T. Wong, J. Chem. Soc., Cliem. Cornnzun., 1991, 814. 1431 Z. Akhter, S.L. Ingham, J. Lewis. P.R. Raithby, J. Oryrinomet. Clwm., 1998, 550, 131.
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[44] B.F.G. Johnson, J. Lewis, W.J.H. Nelson, M.D. Vargas, D. Braga, K. Henrick, M. McPartlin, J. Chem. Sue., Dalton Trans., 1986, 975. 1451 A.J. Amoroso, M.A. Beswick, C.-K. Li, J. Lewis, P.R. Raithby, M.C. Ramirez de Arellano, J. Orgunornet. Chem., 1999, 573, 247. [46] A.J. Amoroso, B.F.G. Johnson, J. Lewis, P.R. Raithby, W.-T. Wong, Anyeiv. Chem., h t . Ed. Engl., 1991, 30, 1505. [471 D.M.P. Mingos, Gold B U N . , 1984, 17, 5; P.G. Jones, ibid., 1986, 19, 46; K.P. Hall, D.M.P. Mingos, Prog. hor g. Chem., 1984, 32, 237. [48j M.R.A. Al-Mandhary, J. Lewis, P.R. Raithby, J. Orgunornet. Chem., 1997, 536-537, 549. 1491 V. Dearing, S.R. Drake, B.F.G. Johnson, J. Lewis, M. McPartlin, H.R. Powell, J. Chem. Soe., Chem. Commun., 1988, 1331. [SO] Z. Akhter, S.L. Ingham, J. Lewis, P.R. Raithby, Anyew. Chem., Int. Ed. Engl., 1996, 35, 992. [ S I ] D.M.P. Mingos, Polyhedron, 1984, 3, 1289; H. Schmidbaur, Gold Bull., 1990, 23, 11.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.20 Novel Imido Rhodium Clusters: Synthesis and Perspectives Luis A . Oro, Miyuel A . Ciriano, Cristinu Tejel, You-Mu0Shi, and Juuier Modrego
1.20.1 Introduction Transition metal imidochemistry has experienced a remarkable growth in the last years because of the capability of the imido ligand to stabilize complexes in high oxidation states, and because of the properties conferred to the complexes by the presence of the M-N bond. These properties range from inertness to extreme reactivity. The imido (nitrene) ligand is particularly appropriate for the formation of a metal-N multiple bond with electron-poor high oxidation state metals."] Consequently, metal complexes that contain terminal imido ligands typically show electron do-d4counts and are concentrated on the early transition metals. Moving to the right in the periodic table, the first terminal imido complexes of ruthenium,[*] osmium[31 and iridiumF4' with d6 electron counts have recently been reported. Moreover, low-valent transition metal imido complexes of the late transition metals are rare or unknown, probably because the more electron rich metals have reduced ability to accept n electrons from the imido ligand. This allows the imido group to act as a ,uu,-bridgingligand, as observed in several clusters of osmium and ruthenium. which are surprisingly Indeed, the weakness of the M-N bond is attributed to the mismatch between the soft transition metal center and the hard imido ligand, which in turn, makes the low-valent transition metal imido complexes highly reactive molecules.[71On the other hand, imido-clusters of the group 8 metals are thought to be involved in catalytic processes of industrial interest such as the reduction and carbonylation of nitroaromatics to isocyanates, carbamates and ureas.[81
382
1 Molecular Clusters
1.20.2 The challenge of synthesizing rhodium imido complexes While some mono and d i n ~ c l e a r [ ~ "imidoiridium .~] complexes are known, there appears to be no fully substantiated claims for pure rhodium imido complexes. An early report from McGlinchey and Stone describes that the reaction of 2Hhexafluoropropyl azide with [RhCl(PPh3)3] affords [RhCl(NRF)(PPh3)2].[lo1Apart from this pioneering work, Peng and Sharp have reported the synthesis and reactivity of A-frame dppm complexes of rhodium showing an imido-amido tautomeric equilibrium." However, recent attempts by Wilkinson et al. to prepare the rhodium counterparts of the known imido cyclopentadienyliridium complexes have not been S U C C ~ S S ~ U In ~ . [the ~ ~ light ~ of these results, the synthesis of rhodium imido complexes represents a challenge for a preparative chemist with considerable development potential if the attempts were successful.
1.20.3 A synthetic strategy for rhodium triangular cores Our approach to the preparation of new imido clusters of rhodium consists of the reactions of equimolar quantities of [ { Rh(pu-Cl)(diolefin))2]with a solution, prepared in situ, by the addition of butyl lithium to para-toluidine in a 2 : 1 molar ratio in diethyl ether. At first glance, a hypothetical trinuclear cluster of formulation [Rh3(p-N-p-tolyl)2(diolefin)3]could be expected from these reactions on the basis of the formation the isoelectronic thiolate complexes [Rh3(pu-SPh)2(diolefin)3]+.[' 21 However, unexpectedly, the results of these "one-pot'' reactions were the formation of the novel tetranuclear clusters [Rh4(p-N-p-tolyl)2(diolefin)4]diolefin = l ,5cyclooctadiene (cod) (l),tetrafluorobenzobarrelene (tfb) (2), isolated as dark-red crystalline solids in good yields. The synthetic procedure is very critical. In our case, we believe that the imidolithium compound (Li2NR) is present in the solution of butyl lithium and purutoluidine, in diethyl ether, as reported for the dilithiated cc-naphtylamine.[131This is a noticeable difference in comparison to typical preparations of ruthenium, osmium, and iridium imido c o m p l e x e ~ , [ ~in . ~which , ~ ] a dichlorometal complex and the monolithium salt ( LiNHR) in a molar ratio 1 :2, appropriate for a bis-amido precursor. are used. In these cases a subsequent removal of amine, or a dehydrohalogenation step with LiNHR, is required to afford the products and free amine. Equation ( 1 ) summarizes our synthetic procedure:
2 [{Rh(p-Cl)(diolefin)}*]+ 2 LizN-p-tolyl +
+
[Rh4(p-N-p-tolyl)z(diolefin)4] 4 LiCl
(1)
1.20 Novel Irnido Rhodium Clusters: Synthesis and Perspectives
383
K
K
Figure 1. Molecular structure of [Rh4(pN-p-tolyl)~(cod)4] (1).
K
Complex 1 represents the first example of a p3-imido rhodium system characterized structurally (Fig. 1). The structure consists of a trirhodium triangle, face-capped on either side by two paru-tolylimido ligands through their nitrogen atoms, while one of the para-tolyl rings coordinates to a fourth isolated Rh(cod) fragment.['41 The trinuclear triangular core consists of two clearly bonded edges ( Rh-Rh: 2.765( 1) and 2.774(1) A) and one slightly longer (Rh-Rh: 3.108(1) A), apparently nonbonded edge. The coordination about each rhodium in the trimetallic core is essentially square-planar discarding the metal-metal interactions. The fourth rhodium atom is coordinated to the para-tolylimido ring in a q j-cyclohexadienyl fashion; the plane containing the ipso carbon makes an angle of 17.4(4)' to the ring plane, and the Rh-C,,,,, distance is 0.294 A longer than the average Rh-C distance. This peculiar q j-coordination is associated with a shortening of the CIpS0-Nbond (C-N: There1.355(5) A) and this indicates clear partial C-N double bond ~haracter."~' fore, complexes 1 and 2 should be regarded as triangular trirhodium clusters containing a fourth Rhidiolefin) moiety q5-coordinated to a para-tolyl ring, which is not involved in cluster bonding (Fig. 2). The electron count for these compounds requires some assumptions to be made about the contributions of the two types of imido ligand. While the simple imido ligand contributes four electrons to the cluster electron count, the para-tolylimido ligand that contains an additional cyclohexadienyl rhodium coordination can be considered overall as an eight electron donor (taking into account the coordination
384
1 Moleculur Clusters
Figure 2. A triangular trirhodium cluster containing a fourth Rh(dio1efin) moiety q5-coordinated to a puru-tolyl ring.
of the cyclohexadienyl ring and the presence of a C=N bond). Thus, the total electron count for the tetranuclear complexes 1 and 2 is 64e. Complexes 1 and 2 maintain the structure found for 1 in the solid state in solution, but decompose slowly. A useful fingerprint which reveals the y5-coordination of one para-tolylimido ligand is the shift of its resonances to higher field in the ' H and 13C{lH] NMR (by approximately 1 ppm and 20 ppm, respectively). Accordingly, the 13C{'H}NMR spectrum of the most soluble complex 1 shows the coupling of five carbons of the ring to the coordinated rhodium atom. The presence of two types of diolefin ligands in a 3 : 1 ratio is also observed in the ' H NMR, that coordinated to the trimetallic core and that bonded to the isolated rhodium atom, respectively. Complexes 1 and 2 react slowly with methanol to give the known methoxo complexes [{ Rh(p-OMe)(diolefin)}*]and para-toluidine. Mechanistically, this reaction might be initiated through the protonation of the imido ligand by methanol, which would indicate that the imido-N is still nucleophilic in character. However, the nucleophilic attack of methanol on one of the rhodium atoms in the trimetallic core followed by protonation of the imido ligand would give identical results. It should also be noted that mononuclear cationic species such as [Rh(cod)S,]BF4 (S solvent) fail to interact with any available electronic density in the uncoordinated para-tolylimido ring of the complex 1, and therefore, attempts to isolate pentanuclear complexes by coordination at the free phenyl group have proved unsuccessful.
1.20.4 Butterfly rhodium imido clusters It is noteworthy that when complex 2 is treated with carbon monoxide, in dichloromethane or toluene, at room temperature, the unexpected product with the
1.20 Nooel Iinido Rhodiunz Clurters: Sjnthesis atid Prrspectivrs
385
Figure 3. The planar butterfly structurc of [ Rh4(/r-N-/~-toIyl)z(tfo)(COl,]37 ,
formula [Rh4(pu-N-p-tolyI)2itfb)(CO)~] (3) is isolated in high yield as a dark-red crystalline solid. Complex 3 possesses an interesting structure ( Fig. 3 ) . which consists of four metal atoms in an essentially planar butterfly arrangement, where two rhodium triangles share a common edge." 61 The metal-metal separations lie in the range 2.758(2)-2.895(2) A. and are within the usual range for single rhodiumrhodium bonds. One of these triangles is bicapped by two para-tolylimido ligands forming a rhodium-nitrogen trigonal bipyramid. The second adjacent rhodium triangle is formed by the fourth rhodium atom, which bridges a rhodium-rhodium edge. Each of the rhodium atoms in the bicapped triangle possesses two terminal carbonyl ligands whilst the fourth rhodium completes a distorted trigonal bipyramidal coordination sphere bonding to a further terminal carbonyl and a chelating tfb ligand. Complex 3 can be considered as a 62electron system. consistent for a butterfly geometry." 71 and represents the first structurally characterized planar tetrarhodium cluster showing a raft-like structure. Other rare features include the coexistence of formally seven-, six- and pentacoordinated rhodium atoms, and the presence of carbonyl and diolefin groups on the same rhodium center. Starting from I , the reaction with carbon monoxide, is sequential depending upon the reaction conditions. The replacement of the ancillary ligands in the periphery of the trinuclear core of l is indicative of the inequivalence of the metals within this triad. Thus, reaction of 1 with carbon monoxide at room temperature. (4) in tetrahydrofuran leads to the formation of [Rh4(,u-N-p-tolyl)~(CO)~(cod)~]
386
1 Moleculur Clusters
-
co
co
A
-
- 2 cod
0
CH3
CH3
(11
(4)
cod
I CH3
(5)
Scheme 1. The formation of [Rh~(,u-N-p-tolyl)~(CO)~(cod)~] (4) (cod = cyclooctadiene).
(cod = cyclooctadiene) (Scheme l ) , which is isolated in high yield as a yellow microcrystalline solid. The spectroscopic data agree with a structure similar to 1 in which one Rh(cod) moiety appears y5-coordinated to a para-tolylimido ligand while the other remains as part of the trimetallic core. A further reaction with carbon monoxide in toluene (5), at 50 "C leads to the planar butterfly cluster [Rh4(pu-N-p-tolyl)2(cod)(C0)~] which is structurally similar to 3, and finally to the complete replacement of the diolefin ligands, resulting in a new planar butterfly cluster [Rh4(pu-N-p-tolyl)2(C0)g] (6) (Scheme 2). Careful control of the reaction and crystallization conditions is critical to ensure the isolation of either 5 or 6. The replacement of the last diolefin ligand by carbon monoxide in 3 and 5 requires prolonged heating under an atmosphere of carbon monoxide, and isolation of 6 by crystallization can only be carried out under an atmosphere of carbon monoxide. The opposite processes, i.e., the replacement of two carbonyl groups on the fourth rhodium atom by the diolefin ligand is accomplished easily, simply by placing complex 6 in the presence of cod or tfb under an inert atmosphere. An additional reaction of 5 is the slow release of the carbonyl group at the Rh(cod)(CO)vertex on standing in toluene solution, to give black (7). Complex 7 is also a 64-electron cluscrystals of [Rh4(p-N-p-tolyl)2(cod)(C0)6] ter having a structure similar to that described for 1, i e . , a trinuclear core of three Rh(CO)2 fragments face-capped by two para-tolylimido ligands with one of them y5-coordinated to the Rh(cod) moiety. Reaction of 7 with carbon monoxide gives 5 (or 6 depending on the reaction conditions) (Scheme 2) completing thus the stepwise carbonylation from 1 to 6.
1.20 Novel Imdo Rhoci‘iurn Clusters: S ~ m k s iand s Perspectives
co 4
oc I
1
387
*
+ co
\-co
’co
CH3
co co
1 , Ih
‘co
Scheme 2. Metal mobility of the Rh(cod) moiety V’-coordinated to a p-tolyl ring leading to the planar butterfly clusters [Rh4i,~c-N-p-tolyl’1~(cod)(CO~:] (5) and [Rh4(pN-p-tolyl)2(CO)g](6).
1.20.5 A “Rh3(,u-N-p-tolyl)z” core with tuneable donicity The implications of the reversible reaction 5 + 7 are quite remarkable. On addition of CO, a two-electron donor ligand, to complex 7 the fragment Rh(cod)(CO)migrates from the q5-cyclohexadienyl coordination site to the trimetallic core. This metal mobility involves the unprecedented formation of three metal-metal bonds in spite of the apparent paradox of increasing by one the number of ancillary ligands. In the opposite sense, the loss of a carbonyl ligand, usually associated with an increase in metal-metal interactions, results in the formation of the trimetallic core and the y5-cyclohexadienyl coordination site, breaking three metal-metal bonds. The solution to this puzzle is related with the role of the para-tolylimido ring as an electron reservoir. This provides a remarkable tuneable donicity that can be easily envisaged on analyzing the electron counts for complexes 5 and 7. Complex 5 is a 62e cluster while complex 7 is a 64e cluster. From this point of view, the formation
388
1 Molecular Clusters
of metal-metal bonds on going from 7 to 5 is not a surprising result, since it involves a decrease of the cluster electron count by 2e. However, the notable point is how and why this decrease can occur since the addition of the CO ligand to the cluster seems to increase the overall electron count by two electrons. The first question can be answered by imagining that the extra carbonyl ligand binds to the rhodium center coordinated to the arene ring. To avoid this metal center reaching a 20e count, the fragment Rh(cod)(CO)’ is released from the para-tolylimido ring. This de-coordination reduces the whole electron count. Moreover, the para-tolylimido ligand now becomes a 4e donor for the cluster electron count instead of being an eight electron donor overall. As the Rh(cod)(CO)+fragment migrates to the trimetallic core, the electron count is increased by two electrons because of the addition of the new carbonyl group and reduced by four because the de-coordination of the y5-cyclohexadienyl site, resulting in an overall a decrease of the electron count by 2e, i.e. from 64e for 7 to 62e for 5 . The opposite transformation 5 + 7 can be explained similarly. In this way, the ring in the para-tolylimido ligand acts as an electron reservoir that provides electron density when required. An additional point in this interpretation involves the coordination ability of the trimetallic core, as this metal triangle behaves as an anionic ligand in the sense of classical coordination chemistry in which two of the metal atoms act together as donor. As such, this unusual ligand provides two electrons and a coordination site for the [Rh(cod)(CO)+] fragment. Joining all pieces together, the proposal involved in this explanation is that the compounds 5, 6, and 7 contain a masked anion, the hypothetical cluster [Rh3(p-Np-toly1)2(C0)6]-, showing two coordination sites with tuneable donicity: the y5cyclohexadienyl coordination site and that provided by the two rhodium atoms of the open edged trimetallic core. The connection between these two sites should occur through the C,,,-N bond of one para-tolylimido ligand. Its partial double bond character for the complexes containing a q5-cyclohexadienyl coordination suggests a charge delocalization away from the nitrogen into the ring, while the negative charge in the anion should be delocalized in the trimetallic core, A related example of charge delocalization in the two phenyl rings of a y5-y5-biphenyl dianion has recently been reported.[’81This variability of the C-N bond order allows the charge transmission from the para-tolyl ring into the trimetallic core and vice versa, connecting both coordination sites electronically. Interestingly, the selective coordination of the two donor sites in a cluster by a metallic fragment is controlled by the addition of a two-electron ligand. The 12electron fragment Rh(cod)+ coordinates to the ys-cyclohexadienyl site to achieve electronic saturation. Following the introduction of a carbonyl group, however, the resulting 14-electron Rh(cod)(CO)+fragment binds two rhodium atoms of the trimetallic core. Indeed, the para-tolylimidoltrimetal core could be considered as a versatile “ligand”, which has two potential donor sites with distinct donicity that can coordinate a mononuclear fragment.
1.20 Novel Itnido Rhodium Clusters: Synthesis and Perspectives
389
CH3
\
CH3
(2)
(8) L2 = tfb; i = AuCI(PPh3) (9) L = co
(5)
Scheme 3. Formation of the yellow clusters [Rh7i/i-N-~-tolyl)z(tfbilAu( PPh3)l (8) and [Rh3(p-N-ptolyl)(CO)hAu(PPh3 )] (9).
1.20.6 Exploiting the ambivalent donor character of the “Rh3( p-N-p-tolyl)z” core: heterometallic clusters Related heterometallic planar butterfly clusters are easily obtained starting from 2. Thus, the reaction of 2 with AuCI( PPh3) in tetrahydrofuran, gives the yellow cluster [Rh3(p-N-p-tolyl)2(tfb)3Au( PPh,)] (8) in high yield (Scheme 3). A further yellow compound, structurally identical to 8 but containing carbonyl ligands, [ Rh3(p-N-ptolyl*(CO)sAu(PPh3)] (9) results from a transmetallation reaction of the planar ( 5 ) with AuCI( PPh3) in dichlorobutterfly cluster [ R ~ ( p - N - p - t o l y lCO),(cod)] )~i methane (Scheme 3). According to the spectroscopic data, the structures of 8 and 9 are similar to that found for complex 5, where the Rh(CO)(cod) vertex, which is not supported by bridging ligands, has been formally substituted by the AuPPh3 moiety. The formation of 8 from 2, involves the binding of the AuPPh3 fragment to the rhodium atoms at the open edge, making two new Au-Rh and a third Rh-Rh metalmetal bond. This causes a shift of the negative charge on the q5-cyclohexadienyl ring on to the trimetallic core, releasing the Rh(dio1efin) fragment, which is liberated as [{RhCl(diolefin)}2]. In contrast, the formation of 9 from 5 resembles a formal replacement of the Rh(CO)(cod) fragment by a Au(PPh3) unit. This method of synthesis of tetrametallic complexes is flexible enough to allow the construction of higher nuclearity clusters using the appropriate metal fragments. Thus, reaction
390
I MolecuEur Clusters
9 +
CH3
FO
1/2 PdCI,(L),
t
\
Scheme 4. Formation of the planar heptanuclear clusters [ { Rh3(,uu-N-p-tolyl)z(CO)6}~Pd(CO)(NCPh)](10) and [ ( R h ~ ( p N - p - t ~ l y l ) ~ ( C O ) ~(11) } ~ Hwhere g ] mercury or palladium link two trirhodium units.
of 5 with [PdC12(NCPh)z], in dichloromethane, gives the novel dark green hetero(NCPh)] (10). Complex 5 metallic cluster [(Rh~(pu-N-p-tolyl)~(C0)~}~Pd(CO) reacts also with HgI2 to give [{Rh3(p-N-p-tolyl)z(C0)6}2Hg] ( l l ) , where the mercury atom, a well-known "glue" for metal clusters,[' 91 links two trirhodium units (Scheme 4). The X-ray structure of 10 (Fig. 4) shows an unprecedented planar raft-like arrangement of the seven metal atoms, where two trirhodium units are linked by a palladium atom forming a central heterometallic bow-tie, which has opposite ends edge-bridged by further rhodium atoms.["] The central palladium atom links two identical trirhodium cores, each supported by two face-capping para-tolylimido ligands, through four unsupported Rh-Pd bonds. The metal-metal separations (2.796(1)-2.862( 1) A) are within metal-metal bonding range. A carbonyl and a benzonitrile group, in a trans arrangement, complete a pseudo-octahedral coordination around the Pd. Within the trirhodium core each Rh is bonded to two nitrogen atoms and two carbonyl groups, however the metal atoms adopt two distinct geometries since two metals are bound to the palladium atom.
1.20 Norel h i d o Rhodium Clusters: Synthesis and Perspectives
39 1
Figure 4. The X-ray structure of 10 showing an unprecedented planar raft-like arrangement of the seven metal atoms.
These reactions, described above, can be easily understood assuming that an anionic cluster, of formula [Rh3i,uu-N-p-tolyl)2( L)6]-, acts as a synthon which adds the Au(PPh3)+ fragment to a Rh-Rh edge. Thus, it behaves as an anionic ligand having two coordination sites, either the two metal-donor atoms in the trimetallic core or the arene ring. Bearing this in mind, the formation of the unusual palladium complex 10 represents the addition of two anionic ligands [Rh3(,u-N-p-tolyl)z(L)6Jp to the hypothetical palladium-carbonyl fragment [Pd(CO)(NCPh)12+, forming two Rh-Rh and four Rh-Pd bonds. This synthetic approach affords an outstanding potential for the comprehensive preparation of new heterometallic imido clusters of variable nuclearity.
1.20.7 Trinuclear anionic rhodium imido clusters Phosphorous-donor ligands react with the planar butterfly cluster 5 selectively at the rhodium center, which is unsupported by bridging ligands breaking off one wing tip of the butterfly and leaving a trirhodium triangle. Thus, the proposed anionic complex [Rh3(p-N-p-tolyl)2(C0)6Jp can be isolated as the yellow compound [Rh(CO)(dppm)~][Rh3(,~1-N-p-tolyI)~(C0)61 (12) by the addition of dppm to 5 , in CHlC12. In a similar way, addition of excess of P(OMe)3 to 5 gives the ionic com(13) (Scheme 5 ) leaving the trinupound [Rh(P{OMe}~)~~[Rh~(p-N-p-tolyl)~(CO)~] clear anion intact.
392
1 Moleculur Clusters
I
(13) L = C O , L ' = P(OMe)3, n = 5 (14) L~ = tfb, LA= CNBU', n = 4
Scheme 5. The formation of the anionic clusters [Rh~(p-N-p-tolyl)2L~]~ (13) and (14).
The X-ray structure of 12 (Fig. 5 ) shows that the anion has a trimetallic core of rhodium atoms, doubly capped by two para-tolylimido ligands.[201Meanwhile, the cation is the known mononuclear rhodium complex [Rh(CO)(dppm)2]+contaminated with another known cationic complex [Rh(02)(dppm)2lf.The metalmetal separations within the anion, in the range 2.840(2)-2.898(2) A,can be considered as suitable for metal-metal bonds. In addition, the isolation of diolefin pu,-imidoanionic complexes e.g. (Rhl(p-N-ptolyl)2(tfb)3]- requires the addition of ligands to form very stable cations such as [Rh(CNBu')4]+.In this way, reaction of 2 with excess of CNBu' leads to the rupture of the Rh-q5-cyclohexadienyl interaction yielding the anionic organoimido cluster [Rh(CNBu')4][Rh3(pu-N-p-tolyl)2(tfb)i] (14) (Scheme 5 ) . Complex 14 is isolated as a yellow crystalline solid in high yield, showing the expected spectroscopic features. Thus, the anion is detected in the negative FAB(-) mass spectrum and the para-tolylimido groups are equivalent in the 'H and I3C{'H} NMR spectra. Furthermore, the ortho carbons give rise to quartets in the 13C{'H) NMR spectrum because of coupling with three equivalent Rh nuclei while the @so carbons give a broad multiplet. Complex 14 is remarkably stable in spite of the low oxidation state of the metals and the high electron density (hard-base character) of the imido ligand. The major difference between the reactions of the isoelectronic carbon monoxide and tert-butylisocyanide with 2 comes, most probably, from the different stability of the hypothetical obtainable rhodium cations. The complex [Rh(CNB u ' ) ~ ] + ,a stable and well-known cation, allows the formation of 14, while the relatively low stability
1.20 N o i d Iniido Rhotliurn Clusters: Syrzthesi.~and Perspectives
393
Y Y
A
Figure 5. The X-my structure of' the anion [Rh?(pN-p-tolylj2[CO)5] showing a trimetallic core of rhodium atoms. doubly capped by two prrrcr-tolylimido ligands.
xi \.
I
of the cationic carbonylated rhodium species prevents the formation of a similar complex and therefore. the neutral complex 3 is obtained. The above mentioned reactions confirm our proposal regarding the key involvement of the anion [Rhl(pN-p-tolyl)z(L)6]- as a synthon in the reactions leading to heterometallic imido clusters.
1.20.8 The bonding in imido-rhodium clusters The new imido-rhodium clusters offer the possibility of giving a theoretical insight into the characteristics of the low-oxidation state metal imido-N bonding, to which properties such as stability and reactivity have been attributed. In a first approach we analyzed the electronic structure of the trimetallic anionic clusters [ Rh3(p-N-ptoly1)2(L)6]-, as the simplest class of compound, using [ R ~ ~ ( , L ~ - N H ) ~ (as CO a )~]model for the EHMO calculations. This anion can be regarded as resulting from the interaction of a trinuclear trirhodiuni cationic fragment [Rh3(C0)6l3+ with two ) The six frontier orbitals of the trinuclear cationic fragment imido ( N H l - ~ligands. [Rh3(C0)6l3+(Fig. 6) result from the linear combination of the lower unoccupied orbitals of three [ M(CO)?J+moieties in square-planar environments,[211which are empty d and hybrid sp orbitals. These frontier orbitals are appropriate to interact
394
I Molecular Clusters
Figure 6. The MO diagram for [Rh3(,uU-NH)2(CO)6]derived from the interaction of two NH2fragments with [Rh3(CO)s]’+.
with occupied orbitals of the model imido ligands (NH2-), which are basically two linear combinations of sp hybrids, of symmetries al’ and a2”, and two sets of degenerate e” and e’ orbitals coming from the two p orbitals contributed by each NH2- ligand. These interactions lead to a transfer of electronic density from the imido ligands to the trinuclear trirhodium core. containing the The equatorial plane of the trinuclear anion [Rh3(pU-NH)2(CO)6]-, metal atoms, is an electron rich zone suitable for acting as a sigma donor ligand towards unsaturated fragments. As an example, we have analyzed the interaction of this anion with a 14e T-shaped ML3 fragment modelled by [PdCl(CO)(NCH)]+. The bonding of the ML3 fragment occurs via a 0 donor-acceptor interaction (Fig. 7), where the trinuclear anion acts as a Lewis base with a negligible z-type overlap. The lack of n-interactions is in agreement with the observed free rotation of the trinuclear core around the Pd-Rhzvector (vide infru). If the actual p-tolylimido ligand is included in the calculations instead of the model (NH2-) for the anion [Rh3(p-NH)2(L)6Ip,some minor variations of the overlap of the N-donor ligand with the trinuclear [Rh3(C0)6l3+core are observed. These are a consequence of the delocalization of electron density from the N atoms
I .20 Noi:el Imido Rhodium Clusters: Synthesis und Perspectives
- O
395
C
Figure 7. The main G interaction between a ML3 fragment with the anion [Rhl(pNH)2(CO)h]-.
to the arene rings. Then, we undertook a study of the structural changes that occur on the trinuclear core and one of the p-tolyl rings, which lead to the q5-cyclohexadienyl coordination observed for the complex [Rh4(p-N-p-tolyl)2(cod)(C0)6] (7). The whole distortion process for the anion costs only about 2.5 kcal.mol-' at the EHT level. This process causes a decrease of symmetry of the complex, which induces a large orbital mixing obscuring the detailed analysis. As the energy involved in the distortions is small, the migration of the Rh fragment from the trinuclear core to the ring and rice ~jersushould be induced by the coordination needs of the mobile fragment and the tuneable donicity of the anionic cluster. The change in the donicity on migration of the fragment from the trinuclear site to the qs-cyclohexadienyl site occurs with a reduction of the overlap population between the trimetallic core and the distorted p-tolylimido ligand. This is due to the involvement of the nitrogen p-orbitals in the N-C,,,,<>double bond and reflects the changes in the formal electron counts for both types of clusters.
1.20.9 Fluxional behavior The butterfly clusters 3 and 5 show a broad averaged signal for the olefinic protons of the Rh(CO)(diolefin)vertex and equivalent para-tolyl rings at room temperature, which split into two well resolved multiplets and inequivalent puru-tolyl rings. respectively. on cooling the sample. The low-temperature ' H NMR spectra are consistent with the structure found in the solid state for 3. For example, one multiplet comes for two olefinic protons trans to the carbonyl group and the other for
396
I Molecular Clusters
those trans to the trimetallic core. A calculation of the activation energies (AG’) from the coalescence temperatures for 3 and 5 (ca. 13.4 and 12.3 kcal.mo1-’ respectively) is indicative of a low-energy process. This is intramolecular, and it could be described as the rotation of the diolefin ligand as a propeller if the rest of the molecule were considered rigid. However, the doublet signals for the carbonyl ligands in the I3C{‘H} spectra become broad at room temperature, which is indicative of the participation of at least the carbonyl group in the fluxional process. As the rhodium atom to which the diolefin and carbonyl groups are bonded, is pentacoordinated, the motion is probably related to the stereochemical non-rigidity associated with this coordination number. While the heteronuclear Rh-Au complex 8 is rigid on the NMR time scale, showing couplings of the phosphorous with the three rhodium nuclei, the Rh-Pd complex 11 is fluxional. The process makes the four paua-tolyl rings equivalent at room temperature, while the frozen structure corresponds to that found in the solid state, since two types of paru-tolyl rings are observed at low temperature. Dilution and variable temperature experiments along with a line-shape analysis applied to the interconverting AB systems indicate that the process is intramolecular with a low activation energy (AG’298 = 11.4 kcal.mol-' j. The thermodynamic data (AH’ = 9.24 kcal.mol-’, AS’ = -8.4 e.u.) confirm that it is a non-dissociative process, and can result from the rotation of the trimetallic cores [Rh3(pu-N-p-tolyl)2(C0j6]about the Pd-Rhz vectors.
1.20.10 Conclusions and outlook Neutral and anionic organoimidoimido clusters of rhodium( I ) with diolefin and carbonyl ligands are accessible and stable. The diolefin neutral compounds [Rh4(p-N-p-tolylj2(diolefin)4]are a convenient entry into this chemistry for which the synthetic route is an apparently simple replacement reaction, but the reproducibility of the results requires a careful technique. These tetranuclear compounds contain a trimetallic triangular cluster face capped on both sides by two para-tolylimido ligands and a further rhodium fragment coordinated to a para-tolyl ring in a v5-fashion, giving an overall valence electron count of 64e. Replacement of the diolefin ligands by carbon monoxide occurs in a stepwise fashion, starting at the trinuclear rhodium core. A further carbonylation at the fourth rhodium atom, not involved in the cluster, results in its migration from the arene ring to the trinuclear core to generate a planar butterfly cluster. This reversible transformation, controlled by the uptake and release of a carbonyl group, involves an unprecedented metal mobility and reveals the presence of two donor sites on the cluster. One is the open edge at the trimetallic core in which two metal atoms act as an anionic a-donor ligand, and the other is one of the pam-tolylimido rings,
1.20 Norel h i d o Rhodium Clu~ters:Synthesis a i d Perspectives
391
which acts as an electron reservoir when required. The C,,,,(,-N bond connects both sites electronically, and allows the transfer of the negative charge from the trimetallic core to the ring. Thus, a tuning of the variable donicity of the cluster associated with one or other site is controlled by a change in the order of this bond. The anionic clusters [Rh3(p-N-p-tolyl)2L6JP( L = CO, L2 = diolefin), hypothetically involved in these migrations, are isolated by addition of ligands froin the tetranuclear diolefin complexes and from the butterfly clusters, respectively. Moreover, metal electrophilic fragments abstract these anionic clusters from the homometallic tetranuclear compounds to build heterometallic planar butterfly clusters and higher nuclearity raft clusters. Thus, the idea of a cluster behaving as a 0electron donor ligand through two metal atoms is clearly open to exploitation, with the synthesis of complexes 8-12 demonstrating the potential of this methodology for the construction of both homo and heteronuclear metal clusters.
Acknowledgements The authors are grateful to Drs. Fernando J. Lahoz, and Andrew J. Edwards, for their invaluable resolution of the crystal structures included in this report, to Prof. Pierre Brauiistein (University Louis Pasteur, Strasbourgj for helpful comments, and Direccion General de Investigacion Cientifica y Tecnica ( DGICYT) for financial support (Projects PB95-221-C1 and PB94-1186).
References I I ] D.E. Wigley. PUI~J.Inor
398
I Moleculuv Clusters (a) A.A. Danopoulos, G. Wilkinson, T.K.N. Sweet, M.B. Hursthouse, J. Chem. Soc. Dalton Trans. 1996, 3771; (b) P.L. Holland, R.A. Andersen, R.G. Bergman, J. Am. Chem. Soc. 1996, 118, 1092. F. Ragaini, S. Cenini, J. Mol. Cat. 1996, 109, 1. D.A. Dobbs, R.G. Bergman, J. Am. Chem. Soc. 1993, 115, 3836. M.J. McGlinchey, F.G.A. Stone, J. Chem. Soc., Chem. Commun. 1970, 1265. Y.-W. Ge, F. Peng, P.R. Sharp, J. Am. Chern. Soc. 1990, 112, 2632. M A . Ciriano. J.J. Perez-Torrente, F.J. Lahoz, L.A. Oro, J. Chem. Soc. Dalton Trans. 1992, 1831. D.R. Armstrong, D. Barr, W. Clegg, S.R. Drake, R.J. Singer, R. Snaith, D. Stalke, D.S. Wright, Angeiv. Chem. 1991, 104, 1702; Anyew. Chem. Int. Ed. Engl. 1991, 30, 1707. C. Tejel. Y.-M. Shi, M.A. Ciriano, A.J. Edwards, F.J. Lahoz, L.A. Oro, Angew. Chem. 1996, 108, 707, Angeiv. Chem. Int. Ed. Engl. 1996. 35, 633. R. Uson, L.A. Oro, D. Carmona, M.A. Esteruelas, C. Foces-Foces, F.H. Cano, S. GarciaBlanco, A. Vazquez de Miguel, J. Organomet. Chem. 1984, 273, 11 I . C. Tejel. Y.-M. Shi, M.A. Ciriano, A.J. Edwards, F.J. Lahoz, L A . Oro, AngeM,. Chem. 1996, 108, 1814, Angew. Chem. Int. Ed. Engl. 1996, 35, 1516. D.M.P. Mingos, Ace. Chem. Res. 1984, 17, 31 1; E. Sappa, A. Tiripicchio, A.J. Carthy, G.E. Toogood, Prog. Inorg. Chem. 1987, 35, 437. M.D. Fryzuk, J.B. Love, S.J. Rettig, J. A m . Chem. Soc. 1997, 119, 9071. L. Gade, Angew. Chem. 1993, 105, 25. Angew. Chem. h t . Ed. Enyl. 1993,32, 24. C . Tejel. Y.-M. Shi, M.A. Ciriano, A.J. Edwards, F.J. Lahoz, J. Modrego, L.A. Oro, J. Am. Chem. Soc. 1997, 119, 6678. G.E. Herberich, U. Englert, L. Wesseman, P. Hofmann, Angew. Chem. 1991, 103, 329. Angeir.. Chem. Int. Ed. Engl. 1991,30, 313.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.21 Synthesis and Reactivity of Tripalladium Clusters Diinitvii Kovalu-Demevtzi
1.21.1 Introduction The synthesis, structure, and reaction chemistry of metal clusters has received intensive study, the primary motivation being their importance as models for chemisorption during homogeneous and heterogeneous catalytic processes occurring at metal surfaces."] The development of selective synthetic methodologies for the construction of molecular clusters by the assembly of smaller organometallic fragments is one of the important goals of cluster chemistry. This area has been reviewed by Puddephatt"] and Braunstein with special emphasis on the synthesis of small heteronuclear clusters.[31lmhof and Venanzi have published a review on the reactions of group 10 and 11 metal triangulo units as building blocks in cluster chemistry. The strategy of locking three metal atoms together in a triangle to give reactive, coordinatively unsaturated clusters was successful.[41For the group 10 metals Ni, Pd, and Pt, M ? triangulo clusters do not just act as building blocks for larger cluster compounds, but in addition they display a wide chemistry, which increases on descending the triad.[51 Palladium, in the form of both the metal and mononuclear compounds, is one of the best catalysts for a whole host of reactionsL6'and palladium compounds were found to exhibit potentially interesting biological activity."] Palladium cluster chemistry has been reviewed by Burrows and Mingos"] and is less well developed The unthan that of other metals, such as platinum, gold, and rutheni~rn.'~] saturated, 42 electron [M1(p~-CO)fp-dppm)3]'+clusters ( M = Pd, Pt; dppm = Ph2PCH2PPh2) have attracted a great deal of interest from Puddephatt and Manojlovic-Muir due to their rich thermal- and oxidative addition reactivities towards both organic and inorganic substrates, and for their mimicking properties of the (1 11) face of metal surfaces.12]The photochemical and photophysical properties and the host-guest chemistry of these clusters have also attracted attention by
400
I Moleculuv Clusters
Harvey et al. [ l o ] This review will focus on tripalladium clusters, their synthesis, structural and bonding aspects, reactivities and host-guest chemistry. Tripalladium complexes in which there is no significant metal-metal interaction have not been included. Some of these such as the triangular Pd3(pu-OAc)6,are used as homogeneous catalysts in a huge number of organic syntheses.[' 'I
1.21.2 Syntheses 1.21.2.1 Synthesis of cationic mixed-valence Pd3 clusters Dixon and co-workers reported the first structurally characterized triangulo Pd3 clusters with palladium in an oxidation state other than zero Pd3(4/3)."21 [Pd3(p-Cl)(p-PPhz)2(PR3)3][BF4], ( R = Et or Ph) was prepared from the reduction of Pd( 11) according to Eq. ( 1 ) : 3 [PdC1(PPh3)3][BF4]
THF/125 cc
[Pd?(~-C1)(pu-PPh2)2(PR3)3][BF4](1)
The cluster complex cation [Pd3(p3-X)(p3-CO)(p-dppm)3I2+ (dppm = Ph2PCH2PPh2)(his(dipheny1phosphino)methane);X = 02CCF3 is prepared by the reaction of Pd(OAc)2, dppm and CO in aqueous acetone, according to Eq. (2):
+ 3 dppm + 3 CO + 2 H 2 0 + 2 HX [Pd3(p3-X)(p3-CO)(p-dppm)3]2+ + 2X- + 2 C 0 2 + 6AcOH
3 Pd(0Ac)z +
(2)
The mechanism of this complicated reaction was recently studied by Puddephatt and Manojlovic-Muir. The initial reaction of Pd(OAc)2 with dppm in the presence of excess trifluoroacetic acid gives [Pd(02CCF3)2(dppm)].The reduction of the Pd( 11) complex, [Pd(02CCF3)2(dppm)], by CO/H20 is rapid and gives an equilibrium mixture of binuclear Pd( I ) complexes, [Pd2(02CCF3)2(p-dppm)2]and [Pd2(O2CCF3)2(pU-C0)(p-dppm)2], which are reduced more slowly to give the Kinetic data indicated that shortPd3(2/3) cluster [Pd3(p3-X)(p3-CO)(p-dppm)3]2+. lived Pd(0) complexes are formed as primary reduction products, but they were not formed in high enough concentration for direct detection to be possible. The reduction does not occur if X = halide is used rather than trifluoroacetate, which is attributed to the ability of CO to displace CF3COy but not halide from palladium, to give a cationic carbonyl which then undergoes nucleophilic attack by water to give CO2, HX and the Pd(0) intermediate (Scheme l).[2,131This mechanism of reduction by CO/H20 is often a key step in catalysis of the water gas shift reaction." 41
40 1
1.21 Sjwtliesis cind Reactivitj! (f Tripalludiunz Clusters
Ph2
I Ph
Ph,
p-p
'
D
/ I
+
3CO + H,O
\ ,F. 2CF,CO,H
-
+
CO,
-
P
Scheme 1. The mechanism of formation of the cluster (Pd?(/i,-X)(/,-CO)(/-dppm)31' , X = O2CCF3, C1. Br and I [2,13].
The cluster cation [Pd~(,~~,-Cl)(p~-CO)(p-dppm)~]Cl was also prepared from the reduction of Pd(1V) to Pd3(2/3) from CO and NaBH4 according to the reaction (3).l151 3 KZPdC16
+ 3 dppm + CO
tolucne/ethdnol
ethanol/NaBHd
The cluster [Pd3(pu-C1)( p-dppm)3ip3-PF3)][PF6],in the same fractional positive oxidation state Pd3(2/3),was prepared by disproportionation of [Pdz(p-dppm)zC12] in the presence of PF3 (Scheme 2) and fully characterized by Balch and co-workers." 61 Electrochemical reduction of [Pd(XylCN )4j2+ (Xyl = 2,6-Me~CbH3) and [Pd(XylCN ) 2 (p-dppm)]'+ produced [Pdi( XylCN )812+ and [Pd3( XylCN)6(dppm)Zl2'-, depending on the charge consumed and the reduction potential. Reaction of [Pd3(XylCN ) g I 2 ' ~ with PPh3 gave a linear axially-substituted [Pd3(XylCN)6(PPh3)2]2I-, Pd3(2/3) complex (Scheme 3). The structure of [Pd3(XylCN)6(yc-dppm)~]*'was shown by X-ray crystallography to have an isomorphous structure with the analogous platinum A-frame complex IPt,( XylCN )4(ji-dppm)212b . [ 1 7 1 This reaction provides evidence for the prediction
402
I Molrculur Clusters
Scheme 2. The synthetic route for the preparation of l P d 3 ( ~ - C 1 ) ( ~ 3 - P F ~ ) ~ ~ - d ~ ~ m ) 3 1 [ P F 6 1
made by Hoffman and Hoffmann for the existence of an A-frame complex containing a bridged ML2 moiety on the basis of the isolobal analogy between CH2 and d'O ML2 fragments."81 Similar linear complexes containing methyl isocyanide ligands [Pd3( CNMe)sI2$, [Pd3(CNMe)6(PPh3)212+,Pd3(2/3), were prepared from the combination of Pd( 11) and Pd(0) complexes according to the reactions (4) and (5).[l9]
+PPh,
I
1
I Pd
XyINC'
IyXN;
/CNXyl
1 7 N X y iI Pd
dXyl
Scheme 3. Electrochemical reactions of [Pd(CNXyl)4j2+,and transformation from a linear to an A-frame trinuclear cluster.
1.2 I Synthesis und Rracticity of Tripulludium Clusters
403
1.21.2.2 Synthesis of neutral mixed-valence tripalladium clusters Jones and co-workers reported the fully characterized neutral mixed valence cluster [Pd?(C1)(p-‘ Bu2 P)3 (COjz]. Pd3(4/3). Reaction of [PdCl(CO)ln with Li-‘Bu2P in THF at -78 “C produces a mixture of compounds (Eq. 6) from which the trinuclear phosphido-bridged complex was isolated in low yield.[201 THF/-l8 ”C
[PdCl(CO)],l+ Li-t- Bu2P : [ Pdi (CI)(pU-‘Bu2P)3 (CO)2]
(6)
The selective assembly of mono and dinuclear Pd fragments is used to form m u trul mixed-valence clusters. The reaction of CpPd(v3-C3HS) with secondary phosphines gives the monophosphido bridged, [Pd2(p-PCyz)(p-v3-C3Hg)( PCy2H)z], dinuclear complex. The n-ally1 dimer reacts with excess of PhEH ( E = S, Se) by protonation of the dinuclear unit and gives the mononuclear complex tmns[Pd(EPh)2(PCy?H)?]which reacted with the same n-ally1 dimer providing a synthetic route to the mixed-valence clusters [Pd3(pCI-PCy2)2(p-EPh) (PCy2H)2(EPh)] (Scheme 4). These triangulo complexes exhibit high stability both in the solid state and in solution, but are reactive toward weak proton donors.[”] PPh3)5] were used to give Fragmentation reactions of [ Pdj(p-SO2)2(pi-S02)2( the neutral [Pdi( p - S 0 2 ) 2(CNXyljzi PPh3)3] and the anionic tripalladium cluster [Pd3(p-Cl)(pu-S02)2( PPh3)3]--.[22,23a.81 Mingos and co-workers have synthesized the pentanuclear clusters [Pds(p-SOz)2(p3-S02)2( L)sJ (where L = PPh3, PMePhz, PMe’Ph, P(C6H40Me-pj3,P(C6H40Me-n2)3, AsPh3) from the reaction between [Pd?(dba)3J.CHC13 (dba = dibenzylideneacetone, PhCH=CHCOCH=CHPh) and tertiary phosphines and a r s i n e ~ . ~ [Pds(p-S02)2(p3-S02)2( ~~~’ PPh3)sJ undergoes a variety of substitution and fragmentation reactions to give smaller cluster compounds. Reaction with xylyl isocyanide gives the neutral tripalladium cluster while the addition of R+X-- ( R = NEt4, X = CI or Br); R = NEt?(CH2Ph), ( X = CI); R = PPh3(CH2Ph),(X = CI; R = NBu4, X = 1) gives the anionic cluster compounds R[Pd3(p-X)(pu-S02)1( PPh?)?](Scheme 5).r22*23a1
1.21.2.3 Synthesis of tripalladium Pd(0) clusters Trinuclear carbonyl-phosphine clusters can be prepared from palladium(0) compounds and CO, SO? or CNCy[2”’71 or from palladium( 11) compounds and various reducing agents[’] ( Eq. 7- 10):
404
1 Moleculur Clusters
3 [PdL2]+ 3 CO 3 Pd3('BuNC)6 Pd(vapour)
n-hexane
[Pd3(pu-C0)3L3] where L = P( ' B u ) ~ benzene
+ 2S02
+ 6CNCy
+
THF
3 [PdCl~(PPh3)2] 3 CO
(7)
[ P ~ ~ ( ~ - S ~ ~ ) ~ ( ' B. 2C6H6 U N C ) S ] (8)
[Pd3(pu-CNCy)3(CNCy)3]
PhN Hz / MeOH
(9)
[Pd3(p-C0)3(PPh3)4]
( 10)
Scheme 4. The two synthetic routes for the preparation of [Pdl(p-PCy2j2(pu-EPhj(PCy2H):(EPhj].
L I
PPh,
Scheme 5. Fragmentation reactions of [Pd~(p-SOz)2(pj-S02)2( L)5].
PPh,
1.21 Synthesis crm! Reactivity of Tripullodium Clusters
405
The reduction of Pd( 11) to Pd(0) in the presence of mild reductants is an important step in many catalytical reactions.
1.21.2.4 Synthesis of tripalladium clusters in the formal oxidation state of Pd( 11) The trinuclear cluster [Pd3(p-NPh)2(p-NHPh)(PEt3)3]Cl, containing bridging imido and amido ligands was prepared from Pd( 11)[24’according to the reaction ( 1 1):
+
[Li(THF),]2 [ P ~ N ~ N H C H I C H ~ N H [PdClI ] (PEt3)2]
A number of tripalladium compounds in oxidation state (11) have been structurally characterized. Triangular, linear or bent structures have been found.“ Most of these are aggregates in which there are no metal-metal interactions. The tripalladium mixed-valence Pd3(2/3) and Pd3(4/3j, Pd( 11) and Pd(0) clusters may be formed with a range of phosphines, isocyanides, or carbonyls as terminal ligands and carbonyls, nitrosyls, isocyanides, sulfur dioxide, phosphides, phosphines or diphosphines. halides, thiolates as doubly or triply bridging ligands. These clusters exhibit a wide range of steric and electronic environments.
1.21.3 Bonding and structure in tripalladium clusters The EHMO analyses for [Pd~(p3-CO)(p-dppm)3]2t[281 and the analogous triplatinum cluster [Pt3(p3-CO)( p-dppm)3]2i-[291 have been reported. The MO analyses for both clusters are essentially identical. The bonding in [Pd3(p3-CO)( ,~-dppm)3]~+ has been modeled by considering it to be formed from a planar, latitudinal, [Pd3(PH3)6]’+ unit and CO (Fig. 1). The HOMO, a ) , is mostly metal d orbitals in character, with some pz and some C=O n and o character. It is weakly metal-metal bonding in the Pd3 plane and is metal-carbon antibonding, this MO is doubly occupied and its electrons can be used to form a ‘dative’ bond. The HOMO-1 and HOMO-2, e, are almost entirely composed of in plane metal d and p orbitals and are formally metal-metal bonding. The LUMO, a?: is also strictly composed of metal in plane d orbitals (dxy,dx2py2)with some minor px and py contributions and some phosphorus p components. This MO is formally metal-metal and metalphosphorus antibonding and can be used as ‘acceptor’ orbital, giving Lewis-acid character to the Pd? framework. In terms of electron counting, these clusters have 42 electrons in the valence shell
406
I Molecular Clusters
e -
9.06 eV
P
- 10.87 eV
a2
p
LUMO a2
Figure 1. A simplified MO scheme of the frontier orbitals for the [Pd3(PH3)6j2+(C, point group). MO representations of the LUMO and HOMO according to references 10a, 38a,b.
and could reach coordinative saturation by adding one, two or three 2-electron donor ligands to give the 44e, 46e and 48 electron count clusters, respectively. The 42 electron clusters have Pd-Pd bond lengths between 2.576(1) and 2.719(6) A, while the 44 electron clusters have Pd-Pd bond lengths in the 2.734(4)-3.000(5) A range. The Pd-Pd bond length is also dependent on the size and the electronic properties of the bridging ligands. A large size of the bridging ligand leads to much larger Pd-Pd bond lengths. Also, in the clusters with two bridged bonds, the unbridged bond is longer than the two bridged bonds.[361The clusters can have three or two bridging ligands and three or four terminal ligands. The Pd-Pb(dppm)bond distances range from 2.294(2)-2.35( 1) A and the Pd-Sb(S02) from 2.152(3)-2.374(3) A.The Pd-Pt(phoshlnes) range from 2.267(5)-2.345(3) A, while PdPb(ph&,nes) from 2.22(2)-2.298 A [References from Table 11. Tripalladium clusters that have been characterized by X-ray crystallography are listed in Table 1, together with Pd-Pd and Pd-Xllgand bond lengths. Tripalladium compounds, of the non-cluster type, exhibit Pd. . .Pd separations > 3.00 or < 3.00 A.[24a.b1 Also, a large size of the bridging ligand leads to much larger Pd.. .Pd distances. Examples of tripalldium clusters are shown in Schemes 1-9.
1.21.5 Reactivity The availability of filled a1 and empty a2 orbitals in the frontier region makes the palladium triangle amphoteric. The metal triangle reacts with metal electrophiles,
1.21 Syntlzc~sisand Reactiuily of Tripulludiurn Clusters
401
such as AuPR; and main group nucleophiles such as PR3 and halide anions. The a-character of both orbitals enable them best to combine with totally symmetric acceptor or donor orbitals. The availability of the empty metal orbitals in the frontier region enables them to accommodate additional electrons and these orbitals make the substitution chemistry of 42-electron triangulo clusters easy, because the activation energy for the formation of the intermediate 44-e cluster is small.
1.21.5.1 Ligand substitution reactions Phosphine and halogen substitution reactions of the cluster [Pd3(pu-C1)(p-PPh2)2(PPh3)3]+ were studied by Dixon and co-workers (Scheme 6). The terminal tertiary phosphine ligands are labile and it is possible to substitute PPh3 with a less sterically demanding and stronger base PEt3, [Pd3(p-Cl)(p-PPh2)2(PEt3)3]+. Also, the bridging chloride is quite labile and simple exchange reactions using either potassium halides or silver trifluoromethyl sulfide facilitate the synthesis of a range of deX = C1, Br, I or SCF3 and R = PPh3 or rivatives [Pd3(pu-X)(p-PPh2)2(PR3)3]+, PEt3 .[’ Also, reaction with diphenylphoshine and p-toluidine leads to elimination of HCI and formation of [Pd3(p-PPh2)3(PR3)3][BF4]. An almost quantitative oxidation of the cluster [Pd3(p-X)(p-PPh2)2(PR3)3] with H202 in the presence of HCl leads to a stereospecific ring opening with oxidation of all the palladium atoms to Pd( I1 j. The product reacts with PR3 to yield trans-[PdClz(PR3)2] and [Pd2C12(p-PPh2)2(PR3)21.’ 2a1 The anionic tripalladium cluster (Pd3(p-S02)2(pu-C1)( PPh3)3]- reacts with NOBF4 to substitute one of the bridging ligands, Sol, for NO+.[331
Addition of PPh3 to the linear cluster [Pd3(CNMe)xl2+affords the substitution of CNMe and gives the linear cluster [Pd3(CNMe)h(PPh3)2I2+ where the three palladium atoms are colinear with the two phosphorous atoms of triphenylphosphine groups, forming a five atoms chain.[”] Isocyanide substitution reactions in linear tripalladium clusters with highly basic and bulky aromatic phosphines have been studied by Yamamoto. The trinuclear complex [Pd3(CNXyl)g]’+ reacts with phosphines, PPhz[C~H3(0Me)2]and PPh[CGH3(OMe)&, to give the corresponding [Pd3(CNXyl)6L2I2+ (Eq. 13); whereas with P[CGH3(OMe)& cleavage of metal-metal bonds occurred to give the binuclear complex [Pd~(CNXy1)4L21~+ [Pd3(CNXyl)g]’+ (where L
+2L
acetone
[Pd3(CNXy1)6L2I2+
= PPh2[C6H3(OMe)2] or
PPh[C~H3(0Me)2]2
(13)
Compound
2.5921 (5)
2.651(4)av
2.579( 1) 3.508( 1) 2.548( 1) 2.563( 1) 3.379(1) 2.576(4)2.599(4)
2.576( 1 ) 2.610( 1) 2.607( 1) 2.604( 1 ) 2.586( 1) 2.583( 1) 2.56 11(9) 2.625( 13) 2.604(2) 2.599( 1) 2.591(1) 2.599( 1) 2.6254(3) 2.6207(4)
Pd-Pd
Table 1. Trinuclear palladium clusters characterized by x-ray crystallography.
Pd~-Cb 2.072( l3)av Pd-Ct 1.989(8)
Pd-Ccob 2.175(11) 2.14 1( 12) 2.080( 10) 2.14(2) 2.25(2) 2.17(2) 2.188(8) 2.168(8) 2.153(9) 2.137(8) 2.192(8) 2.160(8) 2.174(4) 2.033(4) 2.061(3) Pd-Cc~t 2.059( 12) Pd-C, 1.98-2.04
Pd-C
Pd-CIb 2.63( 1)-2.75( 1) Pd-Pb 2.38( l)-2.47( 1) Pd-C, 2.004( 14)av
PdSb 2.297(2)
Pd-Clb 2.74 1(4)3.16 l(4) Pd-Clb 2.683(3)3.001(3) Pd-Ib 2.951( 1) 3.083(1)
Pd-Ob 2.77-2.92
Pd-X
19
27
16
36
2d
39
13a
Ref.
P
‘r
2
1.21 Synthesis [inn Reactiuity of Triplhdiimi Clirsters
1
-.
m m
409
410
1 Molecular Clusters
Scheme 6. Substitution and oxidation reactions of [Pd,(p(-CI)(pPPh2)2( PEt,)3 Ii.
The cluster [Pd3(p3-C0)(p-dppm)3l2+exhibits rich addition and oxidative addition reactivitie~.[~>~’ (Scheme 7) When isocyanide (xylyl isocyanide = 2,6,Me2CsH3N-C) reacts with [Pd3(,~~-CO)(,u-dppm)3]~+ the substitution product [Pd3(p3-CNXyl)(pu-dppm)3j2+ is obtained and a reversible reaction with a further equivalent gives [Pd3(p3-CNXyl)2(p-dppm)3I2+. No mixed carbonyl-isocyanide clusters were observed even in the presence of excess C0.[351Excess isocyanide leads to cleavage giving the binuclear palladium( I ) complex cation [Pd2(CNXyl)2(p-dppm)2I2’ and unidentified palladium(0) species. Addition of Pd(CNXyl)2 to this binuclear complex gives a trinuclear A-frame cluster complex [Pd2(CNXyl j2fpu-Pd(CNXyl)2} (p-dppm)2I2+, thus completing the cleavage and reassembly of trinuclear palladium clusters.[361
1.21.5.2 Oxidative addition reactions and electron-transfer reactions Reaction of the cationic cluster [Pd3(,~~-COj(p-dpprn)3]~+ with thiocyanate gives first the cluster [Pd3(p3-SCN)(p3-CO)(p-dppm)3]2+, and a subsequent rearrangement occurs to give the cluster [Pd3(p3-Sj(CN)(p-dppm)3]2+ through an electron
1.21 Synthesis and Recrctiuity OJ' Tripalludium Clusters
7
R N d d -bd-
I
I
41 1
CNR + Pd(dpprn)(CNR),
Scheme 7. Substitution, cleavage and assembly reactions of trinuclear cluster [Pd3(pu,-CO)(p-dppm)iI?+.
transfer reaction from the metal triangle Pd3 to thiocyanate (Scheme 8). The oxidative addition of thiocyanate has several interesting features. This reaction of thiocyanate to give sulfide and cyanide has not been observed in mononuclear complexes and so probably represents an example where activation by more than one metal center is necessary. It is suggested that the strength of the incipient Pd3(,uu,-S)unit stabilizes the transition state in which the strong C-S bond of thiocyanate is cleaved.[2d1 the oxidation state of each palladium is In the cluster [Pd3(,uu,-CO)(,u-dppm)3]'f, formally +2/3 and oxidation by two units leads to an average oxidation state of +4/3 for each palladium. In these products the three-fold symmetry is lost and they are identified as Pd( I)Pd(I)Pd(11) complexes, in which the Pd( 11) center is bound to the Pd(I)Pd(I)unit only by the two ,u-dppm ligands and by a third, introduced during the reaction.
Scheme 8. Oxidative addition reactions of [Pd~(p~-CO)(p-dppm)3j2'
412
1 Moleculur Clusters
1.21.5.3 Ligand addition reactions Pd3-Clusters us ‘electron acceptors’ - Reaction of [Pd3(p3-CO)(pu-dppm)3]*+ with CO gave the adducts LPd3 (p3-CO)(CO)( p-dppm)312++ 1Pd3(p3-CO)(p3-CO)( p-dppm)3l2+, it was proposed that fluxionality involving this equilibrium is rapid even at -90 oC.[2alWhen cyanide reacts with [Pd3(p3-CO)(p-dppm)3j2+the adduct [Pd3(CN)(p3-CO)(p-dppm)3]+ results, this complex is fluxional in solution with cyanide migrating rapidly around the Pd3 triangle even at -90 “C. These data establish that cyanide has a higher tendency than the iso-electronic carbonyl to act as a terminal ligand.[13b1The complex [Pd3(p3-CO)(pu-dppm)3]*+ also reacts with halides to give [Pd3(p3-X)(p3-CO)(p-dppm)3]+ (where X = C1, Br or I). The Pd3(p3-X) interaction is largely electrostatic in origin, although weak covalent character is indicated by the UV-Vis spectra. A p3-X ligand could be considered as a 6- or 2-electron donor and on that assumption, the complex [Pd3(p3-X)(p3-CO)(p-dppm)3]+ would have a coordinatively saturated 48- or unsaturated 44-electron configuration.[2a-c.sl Pd3-CZusters as ‘electron donors’- The anionic cluster [Pd3(pC1-C1)(p-SO2)Z( PPh3)3]-reacts with metal fragments such as Au(PPh3)+ or T1+ giving [Pd3Au(pu-C1)( ( p - S 0 2 ) (PPh3)4] and [Pd3T1(p-Cl)(pU-SO2)( PPh3)3],respectively. The chloride ligand may be abstracted by AgN03 leading via cluster aggregation to [Pds(p3-SO&(p-SO.&( PPh3)5] (Scheme 9).[221
1.21.5.4 Host-guest chemistry Host-Guest interactions have important implications in organic and bioinorganic chemistry and represent the basis of molecular recognition.[371In the field of organometallic chemistry, the concept of guest-host interactions is not as developed and seems to be nonexistent in cluster chemistry. Also, inorganic systems acting as host are rare. The X-ray structure of [Pd3(p3-CO)(p-dppm)3]*+reveal that the Pd3 metals are located at the bottom of a cavity described by six dppm phenyl groups fonning a
Scheme 9.
1.21 Syntliesis und Recictiuitj. of Tripulludiwn Clusters
4 I3
~-
IBrC1~
benzoate 11-toluate 4-aminobenzoate propionate acetate 4-dizo-N, N diethy laniline nitrobenzene benzonitrile pyridine DMF methanol nitroethane acetonitrile toluene p-xylene DMA benzene triphenylmethane water hexafluorophospha te thiethylamine
> 10000
28000" 25000" 3000d 10000 1000 9800 f 1000 3300 300 2600 i 200 730 & 30 (200 f 100)b
> 10000
*
1.75 i 0.15 1.35 & 0.20 0.30 i 0.01 0.090 f 0.005 0.024 f 0.001 0.17 f 0.05 0.042 & 0.005 0.08 0.02 0.07 0.01 0.07 0.01 < 0.07 < 0.07
3100 i 100
0.028 i 0.01 0.14 i 0.01 <0.01
1.65 i 0.02 0.56 i 0.01 0.13 f 0.04
0.95 f 0.01 0.80 f 0.01 0.33 f 0.01 0.21 0.01
<0.01
0.085 i 0.002
0.047
0.001
C
0.019 f 0.001 C
0.15 f 0.01 <0.01
0.065 <0.01
0.001
C
*
*
C
"The experimental uncertainty on the K II data in acetonitrile was large and difficult to evaluate, ref. lob. The large error was associated with the presence of a reaction. No bonding. All bindings were found reversible except for the halide salts (KBr and KI) for which a quasiirreversible process in these cases leads to the formation of [Pd3(X)(pu,-CO)(p-dppmj3]+ and [Pdi(X)((~,-CO)(~c-dPdmj~]+.
picket fence array of ca. 2-3 A opening.[2a1These systems exhibit a bifunctional binding cavity with the Pd32f center acting as a soft Lewis-acid center, and the phenyl groups acting as hydrophobic pocket. Puddephatt and co-workers were the first to demonstrate both spectroscopically and crystallographically that halide ions act as guest substrates with compound [Pd3(, ~ ~ - C O ) i p - d p p m ) 3 They ] ~ + . concluded < Br- < 1- .[2a--c1 that the binding ability varies as C 1 ~
414
I Molecular Clusters
The ground-state guest-host chemistry of [Pd3(p3-CO)(p-dppm)3]2+cluster has been described by Harvey and co-workers for a variety of The binding constants (K11) were measured spectroscopically using the BenesiHildebrand (B.-H.), Scatchard (Scat), and Scott (Scot) methods for about 20 different substrates. The stoichiometry of the association is found to be 1 : 1 where the K11 values range from 0.07 to 10000 M-'. The substrate-cluster associations are competitive and reversible for most of the systems studied. In some cases (nitro, cyano, and diazonium derivatives), very slow thermal reactions have been observed. The binding strength of a substrate (both organic and inorganic) into the cavity is found to be related to the charge, the ligand behavior or strength, the size, and the hydrophobic and agostic properties of the substrates, which is consistent with the bifunctional recognition properties of the cluster (metal cations and hydrophobic Harvey and Braunstein have compared the binding ability of halides cavity).[''"I between the unsaturated clusters [Pd3(p3-CO)(p-dppm)3]2+and [PdPtCo(p3-CO)z(CO)(CN'Bu)(pu-dppm)2]+ and found that the binding ability varies as C1- < Br- < I- and [Pd3( p3-CO)(p-dppm)312+>> [PdPtCo(p3-CO)2(CO)(CN'Bu)(pu-dppm)2]+. The findings for the [Pd3(,~~~-CO)(p-dpprn)~]*+ (larger cavity) strongly support the host-guest model where the cavity size influences the binding ability of ionic substrates.['Obl An increase in the cavity size of [Pd3(p,-CO)(p-dppmj3l2+was achieved by using dpam instead of dppm (As instead of P) in the [Pd3(pu,-CO)(p-dpam)3I2+ cluster host, which leads to greater binding constants with various neutral and anionic substrates.['0c1
References [ l] a) J. M Basset, R. Ugo, Aspects Homogeneous Cutal. 1977, 3; b) E. L. Muetterties, T. N. Rhodin, Band, C. F. Brucker, W.R. Pretzer, Chem. Reo. 1979, 79, 91. [2] a) R. J. Puddephatt, L. Manojlovic-Muir, K. W. Muir, Polyhedron, 1990, 23, 2767; b) L. Manojlovic-Muir, K. W. Muir, B. Lloyd, R. J. Puddephatt, J. Chem. Soc., Chem. Commun. 1983, 1336; c) L. Manojlovic-Muir, K. W. Muir, B. Lloyd, R. J. Puddephatt, J. Chem. Soc., Chem. Commun. 1985, 536; d ) G. Ferguson, B. Lloyd, L. Manojlovic-Muir, K. W. Muir, R. J. Puddephatt, Znorg. Chem. 1986, 25, 4190. [3] P. Braunstein, In Perspectives in Coordination Chemistry, A. F. Williams, C. Floriani, A. M. Merbach, Eds., Verlag, Helvetica Chimica Acta, Basel, Switzerland, 1992, p. 67. [4] D. Imhof, L. M. Venanzi, Chem. Soc. Rev. 1994, 185. [5] A. D. Burrows, D. M. P. Mingos, Coord. Chem. Rev. 1996, 154, 19 and references therein. [6] J. Tsuji, Organic Synthesis with Palludium Compound, Springer-Verlag, New York, 1980; R.F. Heck, Palladium Reagents in Organic Synthesis, Academic Press, London, 1985. [7] a) A. Papageorgiou, Z. Iakovidou, D. Mourelatos, E. Mioglou, L. Boutis, A. Kotsis, D. Kovala-Demertzi, A. Domopoulou, D. X. West, M. A. Demertzis, Anticuncer Res. 1997, 17, 247; b) D. Kovala-Demertzi, A. Domopoulou, M. A. Demertzis, A. Papageorgiou, D. X. West, Polyhedron, 1997, 16, 20, 3625; c) D. Kovala-Demertzi, A. Domopoulou, G. Valle, M. A. Demertzis, A. Papageorgiou, J. Znorg. Biochem., 1997, 68, 2, 147; d) A. G. Quiroga, J. M.
1.21 Sjntlirsis rind Reactivity of' Tripulludium Clusters
4 15
Perez. C. Alonso, C. Navarro-Ranninger. Appl. Or<janomef.Chenr. 1998, 12. I ; e) D. KovalaDementzi, N . A. Demertzis. V. Varagi. A. Papageorgiou. D. Mourelatos, E. Mioglou, Z. Iakondou, A. Kotsis. C%eriior/i~rc/p~pq.. 1998, 44. 421. [8] A. D. Burrows. D. M. P. Mingos Trcins. Met. Cl?enz. 1993, 18, 129. [9] a) K. P. Hall. D. M. P. Mingos. Prog. Inorg. C/iem. 1983, 32, 237; b) M. I. Bruce, Coorcl. Cliern. Rev. 1987, 76. I ; c ) D. M. P. Mingos and R. W. M. Wardle. Trans. Mct. Cliem. 1985, 10, 441. [ l o ] a) R. Provencher. K. T. Aye, M. Drouin. J. Gagnon, N. Boudreault, P. D. Harvey, I~iorlj. C/iern. 1994, 33, 3689; b, P. D. Harvey. K. Hierso, P. Braunstein, X. Morise, Inorg. Cllim. Acta. 1996, 250. 337; c) T. Zhang, M. Drouin. P. D. Harvey, J. Clzem. Soc.. Chem. Conimun. 1996, 877. 1111 a) G. W. Parshall, S. D. Ittel, Ilomogeneous ccitalysis. 2'Id ed., Wiley: New York, 1992: b) A. C. Skapski, M. L. Smart. J. Chem. Soc., C'lrrwi. Cornmiin. 1970. 658; c) F. A. Cotton, S. Han. Reo. Chin7. Miner. 1983, 496; d) F. A. Cotton. S. Han. Rev. Chiin. Miner. 1985, 277; e) D. H. R. Barton, J. Khamsi. N. Ozbalik; M. Ramesh, J. Sharma. Terraliedrivon, 1989. 30. 4661. [12] a) S. J. Cartwright, K . R. Dixon. A. D. Rattray, Znorg. Chem. 1980, 19, 1120; b) K. R. Dixon, A. D. Rattray. Inon). C/iern. 1978, 17, 1099; c) G. W. Bushnell, K. R. Dixon, P. M. Moroney, A. D. Rattray and C. Wan, J. C h n . Soc. Cheni. Comm. 1977, 709; d) D. E. Berry, C . B. Bushnell. K. R . Dixon. P. M. Moroney, C. Wan, Inor
416
I Molecular Clusters
[33] a) D. Fenske, H. Fleischer, H. Krautcheid, J. Magull, Z. Naturfbrsch. 1990, 45b; 127; b) R. L. Cowan, D. B. Pourreau, A. L. Rheingold, S. J. Geib, W. C. Trogler, Inorg. Chem. 1987. 26, 259; c) D. Kovala-Demertzi, N. Kourkoumelis, M. A. Demertzis; D. X. West; J. ValdesMartinez, S. Hernandez-Ortega, Eur. J. Inory). Chem. 1998, 861. 1341 Y. Yamamoto, F. Arima, J. Chem. Soc., Dalton Trans., 1996, 1815; c) E. M. Padilla, C. M. Jensen, Polyhedron, 1991, 10, 1, 89. [35] M. Rashidi, E. Kristof, J. J. Vittal, R. J. Puddephatt, Inorg. Cllem. 1994, 33, 1497. [36] M. Rhashidi, J. J. Vittal. R. J. Puddephatt, J. Cllem. Soc. Dalton Truns. 1994, 1283. [37] a) K. A. Connors, Bincling Constants: The Measurements of Molecular Complex Stability, Wiley-Interscience, New York; 1987; b) C. D. Gutsche, Calixarenes, Thomas Graham House, Royal Society of Chemistry, London, 1989; c) G. W. Gokel, Crown Ethers and Cryptunds, Thomas Graham House, Royal Society of Chemistry, London, 1991; d) J. F. Stoddart, R. Zarzycki, Cycludectrines, Thomas Graham House, Royal Society of Chemistry, London, 1992. 1381 P. D. Harvey, R. Provencher, Inorg. Chem. 1993, 32, 61; b) R. Provencher, P. D. Harvey, Inorg. Clzem. 1996, 35, 21 13. 1391 N . M. Boag, D. Boucher, J. A. Davies, R.W. Miller, A. A. Pinkerton, R. Syed, Organometallics, 1988, 7, 79 1 .
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.22 Platinate(11) Complexes as Building Blocks for Complexes with Pt-M (Donor-Acceptor) Bonds Juun Forniis am! Antonio Murtiii
1.22.1 Introduction In the early 1970s Shriver published a review on the basicity of transition metals and established that the metal centers in mainly carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, were basic in nature and, for this reason, adequate intermediates for the formation of metal-metal bonds of a donor-acceptor nature."] Since then metal carbonyls as well as cyclopentadienyl metal tricarbonyl complexes have been used in a large number of acidbase processes with the metal centers acting as a base.[" As far as platinum is concerned some polynuclear complexes with donor-acceptor metal-metal bonds (mainly Pt-Hg bondsL3])are known and some examples with Pt-Ag bonds are presented in Fig. 1.I4-'] The first two example^[^.^] are complexes in which only the metal centers seem to be involved in the donor-acceptor interaction, although in the second one a bridging ligand is supporting the Pt-Ag bond. In the other two c a ~ e s [ ~the . ~bonds ' seem to be the result of the interaction between the Pt-X bond and the silver center. In the course of our research, which focused on the synthesis of pentachloro or pentafluorophenyl-containing Pd or Pt complexes, we developed synthetic procedures for the preparation of square-planar Pt( 11) or Pd(I1) anionic complexes ( i e . palladate or platinate complexes) containing from one to four perhalophenyl groups per metal center.['] The study of the reactivity of these complexes revealed that because of their anionic nature, they have an excess of electron density around the metal center (probably located on the df orbital) and they can react with Lewis acids giving rise to neutral reaction products. When the Lewis acid is a metal complex or a metal salt. polynuclear derivatives with metal-metal bonds of a donoracceptor nature are obtained. Most of the work presented in this account has been carried out by reacting the platinate substrates with silver derivatives, which afford polynuclear complexes containing Pt + Ag bonds. Our first paper in this field ap-
418
I Molecular Clusters Q
Br\
/
f
A-N \
R'
Y
\
(I b)
(1a)
L
'/'\
\A u - - - - . p ~ \ R3P/Pt-Pt' L
Q
\
R,P--.-.-Au
\L
(id
(1 d)
Figure 1. Some examples of complexes containing donor-acceptor Pt 4 M bonds. In la the three Pt atoms of each Pti triangular cluster act as a donor atom; in lb, only one Pt atom is acting as donor and the Pt-Ag bond i s supported by a bridging ligand; finally, in lc and Id, the responsible for the Pt-M bonds seem to be not only the platinum center, but also the Pt-H or Pt-P bonds respectively.
peared in 1984 and since then around 60 complexes of different stoichiometries and with Pt + M bonds, mainly Pt + Ag bonds, have been prepared. More recently we have also prepared complexes in which the acceptor center M is Hg(II), Au(I), Pb( II), T1(I), and Sn(11).
1.22.2 Synthesis of Pt -+Ag complexes Some of the results that have been obtained in this field have already been r e v i e ~ e d ' ~and ~ ' ~before ~ presenting the more recent results some of the most noteworthy features relating to these Pt-Ag complexes will be discussed. To illustrate this part we have chosen the complexes obtained by reacting the tetuakispentafluorophenylplatinate or the cis-bis-pentafluorophenylbispentachlorophenylplatinate anions with tetrahydrothiophene silver perchlorate in 1 : 1 molar ratio (Eq. (1)).The resulting complexes which are presented in Figs. 2 and 3 , contain a donor-
X
=
F, C1; L = tht, PPh3
1.22 Plutirzcrte iI I ) Conzplc~srsC I S Building Blocks j b r Corrip1cw.s
Figure 2. Structure of the [(ChFj)4PtAg(tht)]anion. Pt-Ag 2.641(1) A, Ag-F(2) 2.738(9) A Ag-F(8) 2.712(12) A. Ag-F(I4) 2.704(101 A. Ag-Ft20) 2.722(14)
4 19
,,a, II
acceptor Pt + Ag bond, which is practically perpendicular to the square-planar coordination plane of the platinum atom.[''] The perhalophenyl groups are oriented in such a way that one of the ortho-halogen (fluorine or chlorine) atoms of these perhalophenyl groups makes a close contact with the silver atom. possibly donating some electron density to the silver center thus increasing the stability of the compound. This is a common feature in most of these c ~ m p l e x e s . [O1~ ~ ' Since these Pt + Ag interactions are donor-acceptor in character the resulting structures, and probably the strength of the interactions, are dependent on the other interactions between the silver center and the other donor atoms. Hence the acidity of the silver center is strongly dependent on the rest of interactions in which this metal is involved. Thus, while [NBu4][Pt(CsFs),(tht)] reacts with [Ag(OC103)(PPh~)] in a 1 : 1 molar ratio yielding the dinuclear complex
420
I Molecular Clusters
Figure 4. Structure of the complex I(thtj(ChF5j,PtA~(PPh3)1 (3). Pt-Ag 2.637( 1 j A, Ag.. .F(1) 2.763(8) A, Ag. . .F(6)2.757(7) A, Ag...F(11j 2.791(7) A; Pt--Ag-P 174.3".['21
[(tht)(C6F5)3PtAg( PPh3)] (3, Eq. (2)) the reaction of the same platinum salt with Ag(C104), under similar conditions, only takes place in a 2 : 1 molar ratio, (19) the resulting complex being the trinuclear [NBu4][{ (C6F5)3Pt(pU-tht)}2Ag] derivative (Eq. ( 3 ) ) .
The structure of the dinuclear compound which is presented in Fig. 4 indicates that the silver ion displays an almost linear coordination (Pt-Ag-P 174") to the platinum and phosphorus atoms.[121It is interesting to observe at this point that the silver center is only connected to the platinate substrate through the platinum atom, in spite of the fact that the tetrahydrothiophene ligand still has a pair of free electrons which could be used for the formation of a S-Ag bond and that there are known complexes with bridging tetrahydrothiophene." 31 The second reaction (Eq. ( 3 ) )with silver perchlorate affords a trinuclear derivative, can be easily understood, since the silver center has two coordination positions which, in principle, could be occupied by the platinum centers of two [Pt(C6F5)3(tht)lpmoieties.[141This explains why the reaction takes place in a 2 : 1 molar ratio. However, the X-ray study of these anionic complexes reveals some interesting features. There are two anions per asymmetric unit, which present slight but significant differences. Their structures are presented in Fig. 5 and there are several points about these structures, which are worth commenting on:
Figure 5. Structures of the two kinds ot diiioiis [((C6FS)IPti/c-tht)),Agl(19)in the unit cell (Sa dnd Sb) Sa Pt 1)-Ag(1) 2 783(1) A, Sl)-Agi 1 ) 2 778(21A, Ag(l)-I’(14) 2 717(6] A Sb Pt 2) Ag(2j 2 862(11 A. SI2)-Ag(2) 2 547(2 A, Ag(21 F1361 2 805(2) A ““
a) Firstly, both platinum fragments are “connected” to the silver center by the two potentially donor atoms Pt and S, i.e. the silver center is bonded to Pt and S so that it is four coordinated. b) Secondly, Pt-Ag and S-Ag distances are strongly related in both anions since the stronger Ag-S bonds are associated with the weaker Pt-Ag ones or vice versa It appears that the location of the silver atom is optimal for simultaneous interactions Ag distances seems to with Pt and S. However, the difference in Pt 4 Ag and S be the response for a flexible molecule to its crystal surrounding and to the packing forces that operate on it. Such flexibility of the molecules could be closely related to the nature of these donor-acceptor interactions. --f
A similar situation had been observed in the reaction of [NBu4][trans-Pt(C6F5)lClz] with [Ag(OC103)L]or Ag(C104).[’s1In the first process (Eq. (4)) the reaction has to be carried out in a 1 : 1 molar ratio (since a 1 : 2 molar ratio results in the
422
I Molecular Clusters
Figure 6. Structure of the [(ChFj)2Cl(,u-CI)PtAg( PPh3)I- (13) anion. Pt-Ag 2.796(2) A,Ag-CI(1) 2.473(5) A, Ag -P 2.350(6)
precipitation of AgCl) and gives the dinuclear [ N B ~ ~ ] [ ( C ~ F S ) ~ C ~ L)]. (~-C~)P~A The structure of one of these compounds ( L = PPh3, 13, Fig. 6) is the result of the simultaneous interaction of both Pt and C1 atoms with the [Ag(PPh3)]+ fragment. However, the reaction with Ag(C104) (Eq. ( 5 ) ) , which takes also place in a 1 : 1 molar ratio, results in the formation of an apparently more complicated tetranu(18, Fig. 7). This compound obclear compound [NBu4]2[(C6F5)2(pU-C1)zPtAg]z viously has to be the result of the dimerization of two dinuclear Pt-Ag fragments, initially formed by the interaction of tr~ns-[PtClz(C6F~)2]~with Ag+, in order to complete the coordination environment of the silver centers (Scheme 1). As in the
Figure 7. Structure of the anion [(CsFj)2(pCI)lPtAg]: (18). Pt-Ag 3.063(3) A, Pt-Ag’ 2.772(3) A,Ag-Cl(2) 2.408(8) A, Ag’-Cl(1) 2.724(8)
Scheme 1
R = C6F5
R
tht complex discussed before, the weaker C1. . .Ag interaction is associated with the stronger Pt-Ag one and c i r e W‘YSU.
As can be seen, some mononuclear platinate complexes act as bidentate ligands towards the silver center by using not only the platinum center but also another atom bonded to the platinum (Figs. 5 , 6, and 7). In addition, the use of dinuclear platinate substrates with the appropriate silver complexes allows the corresponding trinuclear derivatives, in which the anionic platinum complex acts as a bidentate chelating ligand, to be obtained. These complexes can thus be prepared according to Eq. (6).
X
= C1. ChFs; L = phosphines, tht, OEtz
In the case of the platinum complexes with bridging halide ligands. the coordination to the silver center results in the loss of the planarity of the platinum skeleton while in the derivative with bridging pentafluorophenyl ligands which is not planar, a decrease in the dihedral angle between the coordination planes of both platinum centers occurs in order to facilitate the coordination of both platinum atoms to the silver center (Fig. 8). The Pt-Ag bonds in these trinuclear complexes form an angle with the perpendicular to the platinum coordination plane in the range of 10150.[16.171
These trinuclear complexes, which contain two Pt-Ag bonds, are more stable than the complexes with only one Pt-Ag bond. So that, in most of the latter com-
424
1 Moleculur Clusters
Figure 8. Structure of the anion ( 10). [(CsFs)4(~-CsF5)2PtzAg(tht)lPt( l)-Ag 2.799( 1) A, Pt(2)-Ag 2.824( 1) A, Ag-S 2.423(3) A, Ag-F(30) 2.641(5) A, AggF(32) 2.716(6)
plexes the Pt-Ag bonds are broken by donor solvents and the I9F NMR spectra of some of them indicate that even in non-donor solvents, dissociative processes occur. The trinuclear complexes formed by dinuclear platinate substrates acting as chelating ligands are more stable and the Pt-Ag bonds are maintained not only in donor solvents but moreover, the complexes which contain diethyl ether bonded to the silver react with other neutral ligands: phosphines, isocyanides, or amines producing the substitution of the diethyl ether but not the cleavage of the Pt-Ag bonds. Figure 9 shows the I9F NMR spectrum of [NBu4][(C6Fj)4(1~-C6F5)2Pt?Ag(tht)] (10).At higher frequencies two signals corresponding to o-F atoms of the bridging C6Fj groups can be observed (Fig. 9bj, in keeping with the fact that one of the o-F atoms is in contact with the silver atoms and the other is not. On the other hand when one of the rneta-fluorine signals is irradiated the quartet is transformed into a ~ doublet because of the F-Ag coupling and from this doublet the ‘ J F - A coupling constant can be evaluated (62 Hz, Fig. 9c).“’] This fact has to be related to the rigidity of the “Pt2(p-C6F5)2” system. It is however rather surprising that [NBu4]2[Pt2(1~-C1)2(c6c15)4], a similar dinuclear derivative, reacts with Ag(C104) forming the hexanuclear compound 8 (Fig. 10) in which the diplatinate unit does not act as a chelating ligand but as a bridging one towards two silver atoms.[”] Other trinuclear complexes containing two Pt-Ag bonds have been prepared by reacting the anionic dinuclear platinum complexes, which contain bridging dppm and halide or hydroxo ligands,[”] with silver complexes. As we will see later, the formation of the Pt-Ag bonds results in important modifications of the structure and reactivity of the platinum skeleton. Thus, as can be seen in Scheme 2[’9,201the pentafluorophenyl dinuclear platinate complexes with dppm and halide or OHligands react with [Ag(OC103)(PPh3)] forming the expected trinuclear complexes. If the reactions are carried out with Ag(C104) the aquo complexes are crystallized.
1.22 Phtinrrte( I I ) Corizplc~se.sus Bcrikding Blocks for Con1pltxe.s
425
, -100
-110
I
I
-98
-99
-120
I
-100
-130
-140
I
I
-101
-98
-150
I
-99
-160
I
--loo
Fpm
I
-101
Figure 9. 19F NMR spectrum of the complex [ N B U ~ J [ ( C ~ F - , ~ ? I ~ ~ - C ~(10) F ~ in ) ?CDCII. P~ZA~(~~~)] 20 "C.
at
Such aquo complexes react with triphenylphosphine yielding the corresponding phosphino compound. It is noteworthy that while the dinuclear platinum complexes do not react with water to produce the hydroxo compound and the formation of this hydroxo compound requires the refuxing of the halide complexes with KOH or NBu40H for 24 hours. the silver derivatives decompose in solution with NBu40H and react with water under mild conditions to form the hydroxo complex. Similarly, while the dinuclear platinum complexes do not react, for instance, with tht to cleave the bridging Pt(p-X)Pt system, the aquo silver compound reacts with tetrahy-
426
1 Mokculuv Clusters
t
Figure 10. Structure of the anion II(CsC15h(~c-C1)2Pt2Agl~~ (8). Pt( 1)-Ag( 1) 2.766(4) A,Pt(3)-Ag( 1) 2.755(4) A,Pt(2)-Ag(2) 2.781(4) A, Pt(4)-Ag(2) 2.748(4) A,"*]
Q
A
P
P = PhZPCHzPPh, ;
X = CI, Br, I, OH
:
R = C,F5 ;
Q+ = N B u ~ 'N(PPh& .
Scheme 2
1.22 Plutinute ( I I ) Conzpleses NJ Building Blocks.for Comnplexes
427
e P
P
U
drothiophene forming AgX and the dinuclear platinum compound with dppm and tht as bridging ligands. In other words, it scems that the formation of the Pt-Ag bonds dramatically increases the reactivity of the Pt(p-X) Pt bridging system. In addition, the formation of Pt-Ag bonds results in important structural changes. Fig. 1 Ic shows the structure of the trinuclear derivative with the platinum atoms in nearly square pyramidal environmcnts, sharing an edge. The Pt-Ag bonds are practically perpendicular to the basal plane of each pyramid ( 83"). Figs. I I a and 11b also present the coordination skeletons of the platinum dinuclear derivatives. As can be seen, the coordination of the [Ag(PPhl)]+ cation produces a scissor effect and the platinum skeleton adopts a book-like structure in order to favor the formation of Pt Ag bond^."^^^^]
-
428
I Moleculur Clusters
1.22.3 Attempts to prepare Pd + Ag complexes We have also attempted to prepare similar complexes with Pd-Ag bonds. However, it must be pointed out that one of the greatest difficulties that we have found in the preparation of the Pt-Ag complexes described in Sec. 2 is the occurence of arylating problems, i. e. highly perarylated platinate substrates act on some occasions as arylating agents producing AgCsF5 and thus precluding the formation of the corresponding compounds with Pt-Ag bonds. Since palladium derivatives are more labile than the homologous platinum complexes, most of the attempts to synthesize polynuclear Pd-Ag complexes have failed. For instance, the reactions between [ N B u ~ I ~ ( P ~ ( and C ~ F[Ag(OC103)(tht)] ~)~] give (Eq. (7)) [NBud][Pd(C6F5)(tht)]and [Ag(C6Fg)],or the reaction between [NBu4][Pd(CsFj)3(tht)]and [Ag(OC103)(PPh3)] yields [Pd(C6Fj)2(PPh3)(tht)]and [Ag(C6Fj)] (Eq. (8)). In order to reduce the arylating capability of the substrates and thus increase the possibility of the synthesis of Pd-Ag complexes, we have prepared anionic substrates containing a lower number of C6F5 groups, such as INBu4][M(C6F5)2(acac)]and studied their reactions with silver complexes. However, the results obtained were strongly dependent on the metal (Pd or Pt).
Thus, the reaction between [NBu4][M(CgF5)2(acac)] and [Ag(OC103)(PPh3)1 affords the expected dinuclear compound 6 when M = Pt, (Eq. (9)) but when M is Pd, arylation of the silver center takes place and the dinuclear compound cannot be obtained (Eq.
However, when the acetylacetonato derivatives react with Ag[C104] (2 : 1 molar ratio) complexes of the same stoichiometry, although with very different structures,
1.22 Plcitiiiutel I I ) Coriiplt..vrs us Building Blocksfor Corvipleses
429
Figure 12. Structure of'the anion [{Pt(ChF?)2(acac))2Agl (6). P t ( l ) Ag2.681(1) A, Pt(2) ~ A 2.668(1) g A, A g , . .F(5)2.835(5'~A, Ag-F(l5) 2.842(5) A. Ag. . .F(10) 2.794(5) A, A g . . .F(20)3.027(5) A. Pt( 1 ) ~ - A gPt(2) 177.2".[2'1
are formed ( Eq. ( 1 I ) ) . When the metal is Pt, the resulting trinuclear anion contains the silver cation linearly bonded to the platinum centers of two [Pt(CsF5)z(acac)]fragments and four o-F-Ag contacts ( Fig. 121, whereas the palladium derivative does not show any Pd-Ag bonds and the silver atom is linearly bonded as well but to the C(2) donor atom of the acac ligands (Fig. 13).'211This fact illustrates the lower tendency of palladium to engage in the formation of metal-metal bonds.["]
As can be seen, in these trinuclear complexes there are still potential donor centers (the metal center in the palladium derivative or the C(2) atom in the platinum compound), so that one would think that complexes of higher nuclearity could be obtained if the reactions between the acac complexes and Ag(C104) were to be carried out with a higher proportion of silver. However, the results once again depend on the metal center (Pd or Pt).'z31For the palladium compound the reaction only takes place in a 2 : 1 molar ratio and the trinuclear derivative is again obtained ( Eq. ( 1 l ) ) , but when the platinum compound is used, the reaction takes place in a 1 : 1 molar ratio and results in the formation of a yellow tetranuclear compound (Eq. (12)). The structure of this compound (Fig. 14) displays some interesting features.
[NB u ~[Pt(C6F5)2 ] (acac)] -AeCIO,
-NBu4CI04
[NB u ~ [(Cs ] FS) (acac)Pt Ag (CH?Clz)]
( 12 )
430
I Molecular Clusters
Figure 13. Structure of the anion [{Pd(CsF5)2(acac)}2AgJ-. Ag-C(2) 2.237(7) A, P d , . .A& 3.31 l(1) A.12”
As expected, the silver center is bonded to both the platinum center and to the acetylacetonato ligand, although in this case the donor atom is not the C(2) atom but one of the oxygen atoms which, as a result, forms three bonds. This coordination mode of the acetylacetonato ligand is most unusual. Moreover, the silver center completes its coordination sphere with a dichloromethane molecule, which is a very unusual ligand. The Ag...Cl distance is similar to that found in other silver complexes with dichloromethane or 1,2-dichloroethane as ligand.[24,2 Finally each silver atom shows a short contact (2.651 A)with one of the ortho fluorine atoms of one of the CsF5 groups. We cannot offer a reasonable explanation for the special structural behavior of the acetylacetonato ligand in this complex, which coordinates to the silver center through the oxygen atom instead of the C(2) atom. However, it is noteworthy that the resulting structure displays very similar Pt-Ag and Ag-Ag (18, Fig. 7). distances to those in the tetranuclear anion [(C6F,)2(pU-Cl)2PtAg]i-
Figure 14. Structure of the complex [(Ch Fs )2 (acac)PtAg(CH2Clz112 ( 5 ) . Pt( 1 )-Ag’ 2.758( 1 ) A, 0(2)’-Ag’ 2.405(7) A, Ag‘ ’ . C I ( 1 ) 2.776(8) A, Ag’ . ,Ag 2.920(2) A, Ag‘ . . .F(1 ) 2.651(7) A, 0(2)’-Ag’- Pt(1) 131.7(2)”, 0(2)’-Ag’-CI(I) 124.1(2)”,CI(l)-Ag’-Pt( 1) 99.1(2)“. For clarity only i p o carbon atoms of the pentafluorophenyl groups are represented.[231
1.22.4 Structural types of Pt
+ Ag
complexes
As we commented at the beginning of this account, most of the Pt-M complexes prepared by reacting these basic anionic platinate substrates with metal complexes are, in fact, Pt-Ag containing derivatives and at this moment certain generalizations can be inferred from the reported complexes. First of all, three general types of Pt-Ag complexes have been obtained. Table 1 shows several examples of each type with the most relevant structural data. a) Type A complexes, which contain a Pt -Ag bond unsupported by any covalent bridge. They have a general structure as depicted in Scheme 3a and are usually formed by reacting the mononuclear platinate complexes with [Ag(OC103)( L)] (Eqs. ( 1 ) . ( 2 ) , (9)). In these complexes the Pt-Ag bond is almost perpendicular to the square-planar platinum environment. They show the shortest Pt-Ag distances and thus the strongest Pt-Ag bonds. This fact may be due to a better overlap of the filled platinum 5d,2 orbital with the orbital of the silver atom and to the fact that the silver center is only two coordinated. Complexes 5 and 7 (Table 1) have longer Pt-Ag bond distances probably because the silver center is tricoordinated. In complexes 3-6 the Pt-Ag vector is slightly inclined with respect to the platinum plane in such a way that the silver atom is nearer to the less bulky platinum ligands (tht or acac). Complexes 5 , 6 and 8, which contain two Pt-Ag bonds to the same Ag center can be structurally considered as complexes of type A in which L acts as a Pt donor ligand. b) Type B complexes. They contain two Pt-Ag bonds (to the same silver center) unsupported by any covalent bridging ligand. They display a general structure as depicted in Scheme 3b and have been obtained by reacting dinuclear platinate complexes with Ag(C104) or [Ag(OC103j[L)] (Eq. (6), Scheme 2). In these structures the two platinum square planar environments adopt an “open book” arrangement. The Pt-Ag vectors are also approximately perpendicular to the platinum square planes, with a deviation towards the bisecting plane of the “open book”, thus allowing shorter Pt-Ag distances. These distances are longer than those in type A complexes, probably due to the fact that the silver center is tricoordinated. The tendency of silver to reach tricoordination is reflected by the fact that in the reactions between the diplatinate substrates and Ag(C104), the solvent used in the reaction process coordinates to the silver atom, completing the tricoordinated environment (Eq. (6) and Scheme 2 ) . The formation of two Pt-Ag bonds with the same silver atom ( i e . the Pt-Ag-Pt angle) could facilitate the trigonal coordination around the silver center. It is noteworthy that the reaction between [NBu4]2(Eq. (6))results in the formation of the trinuclear [ P ~ ? ( / I - C ~ ) ? ( Cand ~ F Ag(C104) ~)~I
Complex 2.641 2.692 2.637 2.650 2.758 2.682 2.819 2.765, 2.755 2.782, 2.747 2.815, 2.804 2.199, 2.824 2.759 2.791, 2.831 2.797 2.781 2.945 2.917 2.855 2.771, 3.063 2.783, 2.862 2.790, 2.791
A A A A A A A A B B B B C C C C C C C C
Pt-Ag (A)
Type
Table 1. Some structural data for complexes containing Pt-Ag bonds.
10.4-12.2 3.1 18.5, 21.4 25.6, 20.1 15.0, 13.2 12.8, 14.4 13.9 6.8, 7.1 35.2 30.1 35.7 34.1 33.3 24.2, 37.6 23.9, 28.1 56.7, 58.5
9.8
6.9 9.3
0.7 6.5
Line (Pt-Agj-(plane Pt) (deg)
1211 [13a] 1181
2.66 (F) 2.80-2.84 2.69-2.63 2.78-2.87
2.95, 3.01 3.04 3.01 2.84 2.60, 2.70 2.69, 2.78
2.15, 2.83
-
2.65-2.67 2.64- 2.12
Q F [23]
51
5
2 0
G.
‘r
5 1121 [ll]
1111 [ll]
Ref
2.71-2.75 2.67, 2.73 (F) 2.83 (CI) 2.76-2.79 2.76 (F) 2.82 (CI) 2.18 (Cl)
o-X-Ag (A)
6 w
433
1.22 Plutinatej I I ) Cotnp1eve.s as Building Blocks.fbv Complexes L
I
Scheme 3
a
b
C
type B complex while a similar process carried out with the analogous pentachlorophenyl derivative results in the formation of [NBu4]2[(C6C15)4(p-C1)2Pt2Ag]2 (S), which is a hexanuclear compound (Fig. 10) in which the two platinum planes of each dinuclear platinate complexes are bent with respect to the other but the silver atoms are located outside the "open book", and each Ag center, which bridges two platinum atoms, is thus dicoordinated. The complex has a greater resemblance to type A complexes than to type B complexes. However, the angles between the Pt-Ag vectors and the best square planar Pt planes are around 20" with the silver atom nearer to the less bulky chloro bridging ligands (the average Ag-Pt-Cl angle being 75"). This should imply a smaller overlap between the Pt and Ag orbitals and hence weaker Pt-Ag bonds than in type A complexes with two coordinated silver, as can be seen in Table 1. c) Type C complexes. They contain Pt-Ag bonds supported by bridging ligands covalently bonded to Pt and Ag. They display the structure depicted in Scheme 3c with the ligand X (usually a halide ligand or the S atom of tht) bridging both metal centers. This arrangement causes the Pt-Ag vector to deviate severely from the line perpendicular to the Pt plane (see Table 1 ) The silver centers display, in all cases, coordination number three or higher and, in some cases, the formation of dimers (Scheme 1, Eq. ( 5 ) ) or polymers (Scheme 4, Eq. (13) is required to reach such x[NBu4]tmns-[PtClz(C6Cl5)L]
- t x AgCIO, -X
Scheme 4
NBujC104
[PtAgC1,(C6Cl5)Llx
(13)
434
I Molecular Clusters
coordination numbers.[261Both facts: the strong deviation of the Pt-Ag bond from the normal line to the platinum coordination plane and the high coordination number around the silver center should result in longer Pt-Ag distances compared to those in type A complexes (ca. 2.8-2.9 A).
1.22.5 0-X. - .Ag secondary interactions As previously mentioned, in most of the complexes the pentahalogenophenyl groups are oriented in such a way that one of the o-halogen atoms makes a close contact with the silver atom (see Figs. 2-5, 7, 8, 10, 12). Although this is a common feature not only for the Pt + Ag complexes but also for the other Pt + M complexes which will be reported later, we prefer to address to this question here, since there are many more complexes containing Pt + Ag bonds and more generalizations can be expected from these Pt + Ag complexes. At first sight, one could think that this type of contact could be the result of the necessary location of the perhalophenyl ligand bonded to the platinum center. However, it must be pointed out that this type of secondary interaction which, in our case, are obviously favored by the location of the ligand and the silver center have already been found in other types of complexes with very different structural situations. For instance, such interactions have been observed in complexes with halocarbon l i g a n d ~ , [ ~ ~ , ~ ~ ] and in some ionic derivatives in which the anion is a fluoro or chloro compound (i.e, BFq,[13b3291PF-6,[301 A SF;,[^^] AlCIi.. .[321). In our opinion they are weak X + Ag (donor-acceptor) interactions, which complete the electronic environment of the M atom. The experimental 0-X. . .Ag distances found in some of our complexes are shown in Table 1. Although, in principle, it would seem reasonable to consider that the strength of these 0-X. . .Ag contacts and thus the o-X. . .Ag distances (or vice versa) should depend on the acidity of the silver center (i.e. on the nature and number of ligands bonded to the silver center), we have not been able to find a relationship in the complexes we have prepared and studied by X-ray diffraction. In fact, we have observed that the 0-X. . .Ag distance is not necessarily the shortest possible distance taking into account the Pt-Ag distance, and the best orientation of the CsX5 ring. It would indeed seem that the Pt-Ag vector and the c6x5 ring adopt a position which depends on the structure of the whole molecule and which determines the o-X. . .Ag distance, probably regardless of the acidity of the silver fragment bonded PPh,)] to the platinum center. Thus, in the complexes [(tht)(CgF5)2(C6X5)PtAg( ( X = F (3), C1 (4)) and [{Pt(C6F5)2(acac)}2Ag]- (6) and [(CsF5)2(acac)PtAg(CH2Cl2)]2 ( 5 ) the Pt-Ag vector is slightly inclined with respect to the platinum plane in such a way that the silver atom is nearer to the less bulky platinum ligands
1.22 Plutinute f I I ) Complexes us Building Blocks,for Complexes
435
Figure 15. Structure of the complex [( PP~?)(C~C~~)CIP~(/L-CI)A~( PPh3)ll. Ag-CI( 1) 2.514(2) A, Ag-CI(1)' 3.023(2r A, Ag-Cl(2) 3.041(4)
(tht or acac). In most of the type A complexes the pentahalogenophenyl rings are rotated with respect to the platinum basal plane to such an extent that there is a final Ag.. .X contact distance in the range of 2.67 -2.84 A for fluorine and 2.82PPh3)] (3, 3.04 A for chlorine. For example, in the complex [(tht)(C6F5)4PtAg( Fig. 4) the Pt-Ag vector is slightly inclined, with respect to the best basal Pt plane (6.9", see Table l ) , toward the less bulky tht ligand and one of the CsF5 groups cis to it. The dihedral angle of this pentafluorophenyl ring with the Pt plane has a smaller value (62.6') than that of the other CsFs ligands (74.9' and 89.0"), and the overall result for the o-F. . .Ag distances is that they are quite similar for the three pentafluorophenyl groups (2.76, 2.79, and 2.76 A, respectively) Finally, we would like also to comment on the following complexes [(pph,)(CsC15)Cl(lu-C1)PtAg(PPh,)] (15) and [NBU4]2[(C6F~)2(lu~-C1)2PtAg]2 (18). Fig. 15 presents the structure of 15, which, in the solid state, is a dimer that displays three different types of Ag-Cl interactions. Cl(1) is bonded to the silver center through a conventional covalent bond, the Ag--Cl(1) distance being 2.514 A, and there are also two weak interactions Ag-Cl(1)' and Ag-Cl(2) with nearly equal distances. In the anion [(CgF5)2(p-Cl)2PtAg]z2-(18, Fig. 7), which is also a dimer, there are two different types of Ag-Cl interactions: Ag-Cl(2) (2.408 A) is a covalent bond while Ag'-Cl( 1) (2.724 A)seems to be a weak interaction similar to the Ag-Cl found for the o-CI. . .Ag contacts of other pentachlorophenyl derivatives, In these complexes it can be seen that the chlorine atoms make the same contacts with silver, regardless of the nature of the chlorine, that is, the Ag-Cl distances are the same whether the chlorine is part of a halocarbon ligand or forms part of an inorganic ligand (Cl).
436
I Molecular Clusters
Figure 16. Structure of the anion [Pb{Pt(CsF5)4)2l2-.Pt( l)-Pb 2.769(2) A, Pt(2)-Pb 2.793(2) A, F . . .Pb 2.761(2)-2.995(3) A, Pt( l)-PbbPt(2) 178.6(l)0.1371
1.22.6 Other Pt-M complexes Some reactions of these platinate substrates with other acidic metal centers M have also been recently carried out in order to synthesize polynuclear complexes with Pt-M bonds and we have succeeded in synthesizing polynuclear complexes with P~-AU,['~] Pt-Sn,[341Pt-T1,[351Pt-Hg,[361 or Pt-Pbr3'] bonds. In order to illustrate this point with the most recent results we will concentrate on the reactivity of several platinum substrates with lead(I1) salts. The complex [NBu4]2[Pt(C6Fj)4]reacts with Pb(N03)2 in methanol forming, after addition of water, the greenish-yellow trinuclear compound [ N B u ~{ Pb[( ] ~ PtC6F5)4]2} ( Eq. (14)),which displays, as can be seen in Fig. 16 a unique structure.[371The lead atom is linearly bonded to two Pt atoms of the square planar [Pt(C6Fj)4l2- anions. This is the first example of a complex with the central Pb(I1) atom linearly coordinated to two atoms through single covalent bonds and with the lone pair of the Pb(I1) stereochemically inactive. Moreover there are eight short o-F. . .Pb contacts (2.761 2.995 A),with one of the o-F atoms of each C6Fj group. The bulkiness of the [Pt(C6F5)4]2-could be responsible for the formation of this unusual linear Pb( 11) complex. -
1.22 Plrrtinutejll) Corriple.ues as Buildiiiy Blocksfor Complexes
437
Figure 17. Structure of the anion ((ChFs)~Pt(~-Pb!(y-CI)Pt(C6F5)I1-. Ptf1)-Pb 2.721(1) A, Pt(2)-Pb 2.729(1) A;Pt(l!-Pb-Pt(,) 85.3(1)", Pb. .F 2.819(6)-3.032(6)
Similarly, [ N B u ~ ] ~ ( P ~ ( C ~ (FX~=) ~C1, X ] Br, OH) react with Pb(C104)2 in dichloromethane, forming the trinuclear complexes. which contain two Pt-Pb angular bonds (Eq. (15)). The trinuclear anions (Fig. 17) are the result of the interaction of the dinuclear platinate compound [(C6F5)3Pt(p-X )Pt(C6F5)3] with Pb' +. The former acts as a bidentate chelating metalloligand with the Pt-Pb bonds almost perpendicular to the platinum basal planes and the platinum atoms slightly displaced towards the lead atom. In addition the lead center displays six short oF-Pb contacts, which can even be observed in solution (19FNMR).[3R1
'-
Finally, the dinuclear salt [NBu4J~[Pt:!(pu-C1)(C6F5)4] reacts with Pb(C104)2 form(Eq. (16). The structure of the ing the tetranuclear [NBu4][Pb(Pt((pC1-Cl)(Cr,F5)2}i]
438
1 Molecular Clusters
anion of this compound is presented in Fig. 18 and it contains three Pt-Pb bonds. The compound can be described as a [Pt3(,~-C1)3(CgFg)3]~~ unit with a six-membered, puckered Pt3C13 ring in which the three Pt atoms are interconnected by three chloro ligands. Such a unit acts as a tridentate metallo ligand towards the lead center. The Pt-Pb bonds form angles of approximately 8" with the perpendicular to each basal plane.[391A similar Pt-Sn compound had been obtained by reacting [NBu4]2[Pt2((,~-Cl)(CgF5)4] with SnC12.r341These Pt-Pb complexes imply donor-acceptor interactions between d8 and s2 centers and are, as was expected, luminescent.
1.22.7 Attempts to prepare complexes containing P t - + P tbonds We would like to finish this account by presenting some reactions which were designed in order to synthesize Pt + Pt donor-acceptor bonds. If, as we have seen before, the anionic platinate complexes behave as Lewis bases, a reasonable procedure for synthesizing complexes with Pt + Pt donor-acceptor bonds could be the reaction between platinate complexes of the type [Pt(C6F5)3(L-L)Ip ( L-L = being a bidentate neutral ligand acting in a monodentate fashion) and the cis[Pt(C6F5)2(THF)2] complex which, as is well known, contains two extremely labile THF ligand~.[~'] In such a reaction the platinate complexes should act as bidentate chelating ligands forming a Pt + Pt bond. We have studied the reaction presented in Scheme 5, using dppm and 1,s-
1.22 Plutinute(1I) C'orriplexes us Building Blocks j o y Complexes [ N B u 4 ] [ P t ( C ~ F 5 ) 3 ( L - L )+] cis-[Pt(C,F,),(THF),1
439
-
Scheme 5
naphtyridine as bidentate ligands, since both are excellent bridging ligands which can be found in a great variety of complexes supporting metal-metal bonds.[411In both cases, the expected dinuclear derivatives that are formed after the displacement complex are obtained. Howof both THF ligands from the cis-[PtiCsF~)z(THF)~] ever, in none of them are Pt + Pt bonds formed. Thus, in the dppm compound (Fig. 19, Eq. (17)) a r'-Pt-phenyl interaction is formed instead of the Pt + Pt bond.
+
[Pt(ChFs) (dppm)] cis-[M(CgXs), (THF),]
-[NBQ]
+
[Pt(C&s l3(p-dppm)M ( C ~ X S ) ~2]T H F
( 17 )
M = Pd, Pt X = F.CI
Figure 19. Structurc of the amon (Pt(ChFs)i(~-dppm)Pt(C6Fg)2]C(50)-Pt(2) 2 44(4)A, C(51) Pt(2) 2 42(3) A
[421
440
I Molecular Clusters
nr-
; ,\(, B
PP
-100
I
,
,
,
,
,
-110
Figure 20. I9F NMR spectrum of the complex "BU4I[Pt(C6F5)2(,uU-C6F5) ( ,u-napy)Pt(C6l75)2 1 in CDCli at 20 "C.
\if,, ,
-120
The Pt-C distances for such q2 interactions are similar to the shortest distances found in other y2-aryl-Pt complexes. The phenyl ring is nearly planar and the C-C distances are identical within experimental error, which indicates that the aromaticity of the ring is maintained, i.e. the q2-Pt-phenyl interaction is weak. As a result of this, treating this compound with neutral ligands such as PPh3, CO, p-toluidine produces the displacement of this q2-interaction.[421
On the other hand, the complex with l,S-naphtyridine, which is also dinuclear,
1.22 Plutinatei / I ) Cornplt._l-esus Building Blocks for Complexes
R
441
= C,F,
does not contain a Pt -Pt bond and although we have not so far been able to obtain suitable crystals for an X-ray study, the I9F NMR spectrum (Fig. 20) unambiguously reveals that the compound is dinuclear in nature and with a C ~ F S group acting as a bridging ligand (Fig. 21). This fact is very noteworthy since it indicates that the platinum center prefers to reach four-coordination by forming an electron deficient bridging system, in spite of the very low tendency of this group to participate in this type of bonds, instead of the formation of a Pt i Pt bond.[431 Both results point to the very low tendency of these Pt(I1) centers to form Pt i Pt bonds by this procedure. However, as chemistry is an experimental science, othcr strategic approaches can be used for the synthesis of polynuclear complexes with Pt i Pt bonds (Scheme 6).[441but that will be another story.
2 I N B ~ i l l ! C s F 5 )Pt!/i-PPh2lzPtCliCOil i
i-AgCIO, -AgCI -NBUaCIO,
l+co
co
Scheme 6
R'
co
co'
'R \
442
I Molecular Clusters
References [ l ] D. F. Shriver, Acc. Chem. Res. 1970, 3, 231. 12) H. Werner, Angew. Chem. Inf. Ed. Eng. 1983, 22, 927. 131 a) A. F. M. J. van der Ploeg, G. van Koten, K. Vrieze, A. L. Spek, Inorg. Chem. 1982, 21, 2014. b) A. F. M. J. van der Ploeg, G. van Koten, K. Vrieze, A. L. Spek, Organometallics 1982, 1, 1066. c) P. Braunstein, 0. Rosell, M. Seco, I. Torra, X. Solans, C. Miravitlles, Organometallies 1986, 6, 1113. d) R. A. T. Gould, L. H. Pignolet, Inorg. Chrm. 1994, 33, 40. e) M. E. Cicciolito, F. Giordano, A. Pannuzi, F. Ruffo, V. de Felice, J. Chem. Soc., Dalton Trans. 1993, 3421. f ) D. V. Toronto, A. L. Balch, Inorg. Chem. 1194, 33, 6132. [4] A. Albinati, K. H. Dahmen, A. Togni, L. Venanzi, Anyew. Chem. Int. Ed. Eng. 1985, 24,766. [5] A. F. M. J. van der Ploeg, G. van Koten, K. Vrieze, Inorg. Chem. 1982, 21, 2026. [6] a) A. Albinati, F. Demartin, L. M. Venanzi, M. K. Wolfer, Anyew. Chem. Int. Ed. Engl. 1988, 27, 563. b) A. Albinati, H. Lehner, L. M. Venanzi, M. K. Wolfer, Inorg. Chem. 1987,26, 3933. [7] R. Bender, P. Braunstein, A. Dedieu, Y. Dusausoy, Angew. Chem. Int. Ed. Engl. 1989,28, 923. [8] R. Uson, J. ForniCs, Adc. Organomet. Chem. 1988, 28, 219. [9] R. Uson, J. Fornies, M. Tomas, J. Organomet. Chem. 1988, 358, 529. [lo] R. Uson, J. ForniCs, Inorg. Chim.Aeta 1992, 198-200, 165. Ill] R. Uson, J. Fornies, M. Tomas, I. Ara, J. M. Casas, A. Martin, J. Chem. Soc., Dalton Trans. 1991, 2253. [12] F. A. Cotton, L. R. Falvello, R. Uson, J. Fornies, M. Tomas, J. M. Casas, I. Ara, Inorg. Chem. 1987,26, 1366. [I31 a) R. Uson, J. ForniCs, M. Tomis, I. Ara, J. Chem. Soc., Dalton Trans. 1990, 3151. b) B. Noren, A. Oskarsson, Acta. Chem. Scand., Ser. A 1984, 38, 478. 1141 R. Uson, J. Fornies, L. R. Falvello, M. Tomas, J. M. Casas, A. Martin, Inorg. Chem. 1993,32, 5212. [I51 R. Uson, J. Fornies, B. Menjon, F. A. Cotton, L. R. Falvello, M. Tomas, Inory. Clzem. 1985, 24, 4651. 1161 a) R. Uson, J. ForniCs, M. Tomas, J. M. Casas, F. A. Cotton, L. R. Falvello, Inorg. Chem. 1987, 26, 3482. b) R. U s h , J. ForniCs, M. Tomas, J. M. Casas, F. A. Cotton, L. R. Falvello, R. Llusar, Organometallics 1988, 7, 2279. [17] J. M. Casas, J. Fornies, A. Martin, B. Menjon, M. Tomis, Polyhedron 1996, 15, 3599. [18] R. Uson, J. Fornies, M. Tomas, J. M. Casas. Angew. Chem. Int. Ed. Eng. 1989,28, 748. [19] J. M. Casas, L. R. Falvello, J. ForniCs, A . Martin, M. Tomas, J. Chem. Soc., Dalton Trans. 1993, 1107. [20] J. M. Casas, L. R. Falvello, J. Fornies, A. Martin, Inorg. Chem. 1996, 35, 7867. [21] J. ForniCs, R. Navarro, M. Tomas, E. P. Urriolabeitia, Organometallics 1993, 12, 940. 1221 D. L. Kepert, K. Vrieze in Comprehensive Inorganic Chemistry, Vol. 4, (Eds.: J. C. Bailar, A. F. Trotman-Dickenson), Pergamon Press, Oxford, 1973, pp. 224. [23] J. Fornies, F. Martinez, R. Navarro, E. P. Urriolabeitia, Orgunometaliics 1996, / 5 , 1813. [24] M. R. Colsman, T. D. Newbound, L. S. Marshall, M. D. Noirot, N. M. Miller, G. P. Wulsberg, J. S. Trye, 0. P. Anderson, S. H. Strauss, J. Am. Chem. Soc. 1990, 112, 2349. [25] D. M. van Seegen, 0. P. Anderson, S. H. Strauss, Inorg. Chem. 1992, 31, 2987. [26] R. Uson, J. ForniCs, M. Tomas, I. Ara, Inorg. Chem. 1994, 33, 4023. [27] R. J. Kulawiec, R. H. Crabtree, Coord. Chem. Rev. 1990, 99, 89, and references given therein. J. P. Kiplinger, T. G. Richmond, C. E. Osterberg, Chem. Rev. 1994, 94, 373, and references given therein. [28] H . Plenio, Clzem. Rev. 1997, 97, 3363. 1291 a) F. W. B. Einstein, R. H. Jones, Xiaoheng Zhang, D. Sutton, Can. J. Chem, 1989, 67, 1832. b) R. W. Marshman, J. M. Shusta, S. R. Wilson, P. A. Shapley, Organometallics 1991, 10, 1671.
1.22 PlatinUte( I I ) Complexes as Building Blocks f o r Complexes
443
[30] a) H. H. Karsch, M. Schubert, Z. Nuturforsch., Teil B 1982, 37, 186. b) L. Carducci, G . Ciani, D. M. Proserpio, A. Sironi, Inorg. Chem. 1997, 36, 1736. [31] a) H. W. Roesky, E. Peymann, J. Schimkowiak, N. Noltemeyer, W. Pinkert, G. M. Sheldrick, J. Chem. Soc., Chem. Commun. 1983, 981. b) H. W. Roesky, T. Cries, P. G . Jones, K. L. Weber, G. M. Sheldrick. J. Chen?. Soc., Dulfon Trans. 1984, 1781. c) R. Maggiulli, R. Mews, W. D. Stohrer, M. Noltemeyer, Chem. Ber. 1990, 123, 29. [32] R. W. Tuner. E. L. Amma, J. Am. Chem., Soc. 1966, 88, 3243. [33] a) R. Uson, J. Fornies, M. Tomas, I. Ara, J. M. Casas, Inory. Chem. 1989, 28, 2388. b) R. Uson, J. Fornies, M. Tomis, I. Ara, Inorg. Chim. Acta 1991, 186, 67. [34] R. Uson, J. Fornies, M. Tomis, I. Uson. Angrw. Chem. In?. Ed. Eng. 1990, 29, 1449. [35] a) R. Uson, J. Fornies, M. Tomas, R. Garde, J. Am. Cheni. Soc. 1990, 29, 1449. b) R. Uson, J. Fornies, M. Tomas, R. Garde, R. Merino, h o r y . Chem. 1997, 36, 1383. c) 1. Ara, J. R. Berenguer, J. Fornies. J. Gomez, E. Lalinde, R. Merino, Inorg. Chem. 1997, 36, 6461. [36] R. Uson, J. Fornies, L. R. Falvello, I. Ara, 1. Uson, Inorg. Chim. Acta 1993, 212, 105. 1371 R. Uson, J. Fornies, L. R. Falvello, M. A. Uson, I. Uson, Inorg. Chem. 1992, 31, 3697. [38] J. M. Casas, J. Fornies, A. Martin, V. Orera, A. G . Orpen, A. Rueda, Inory. Clzem. 1995, 34, 6514. [39] I . Uson, Ph.D. Thesis. University of Zaragoza, 1992. 1401 a) J. M. Casas, J. Fornies, A. Martin, B. Menjon, M. Tomas, J. Chem. Soc., Dalton Truns. 1995, 2949. b) J . M. Casas, L. R. Falvello, J. ForniCs, A. Martin, Inorg. Chem. 1996, 35, 56, and references cited therein. [41] a) R. J. Puddephatt, Chem. So(.. Rro. 1983, 12: 99. b) C. Meally, F. Zanobini, J. Chem. Soc., Chem. Commun. 1982, 97. c) L. Saconni, C. Mealli, D. Gatteschi, Inorg. Chem. 1974, 13, 1985. d) A. Tiripicchio, M. T. Camellini, R. Uson, L. A. Oro, M. A. Ciriano, F. Viguri, J. Chrm. Soc., Dulton Trans. 1984, 125. [42] J. M. Casas, J. Fornies, F. Martinez, A. Rueda, M. Tomas, A. J. Welch, Inorg. Cheni., in print. 1431 1. Ard, J. M. Casas, J. ForniCs, A. Rueda, Inory. C/zem.1996, 35, 7345. [44] L. R. Falvello, J. Fornies, C. Fortuiio, A . Martin, A. P. Martinez-Sariiiena, Oryanometullics 1997, 16, 5849. [45] R. Uson, J. Fornies, M. Tomas, J. M. Casas, F. A. Cotton, L. R. Falvello, Inorg. Chem. 1986, 25. 4519.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.23 Complexes of Keggin-Type Monolacunary Heteropolytungstates: Synthesis and Characterisation Ana M. V. Cavaleiro, Julio D. Pedrosa de Jesus, Helena I. S. Nogueiva
1.23.1 Introduction The formation of isopolyanions and heteropolyanions is a very interesting and important aspect of the chemistry of vanadium, molybdenum and tungsten.",21 As the number of known compounds of this type is continuously increasing, the study of their properties and applications reveals interesting new p ~ s s i b i l i t i e s . [The ~,~~ development of new instrumentation and experimental methods has brought forward, in the last few decades, a considerable advance in the knowledge of structures and properties of polyoxometalates. Therefore, not only has current research interest in these species increased but also their areas of application (including analytical and clinical uses, catalysis, biomedical applications and solid state devices) have been enlarged.[3,41 The catalytic uses of polyoxometalates have been the subject of several recent reviews.[4p91 In particular, Keggin type heteropolyanions have shown a remarkable versatility, being used in several types of catalysis. One subgroup of those polyoxometalates are the complexes of the monolacunary Keggin type species, which have shown interesting properties as catalysts for oxidation reactions.[''] These compounds have been known since the late 1960s, but their use in oxidation catalysis dates from 1986, when some polyoxotungstates were presented as inorganic analogues of metallo-porphyrins." 1,121 The recent interest in these compounds prompted us to prepare this review where the synthesis, reactivity, and methods of characterisation of the complexes of the Keggin type heteropolytungstates will be discussed. A brief reference to the catalytic properties of some of these complexes, not considered in previous reviews, will also be presented.
1.23 Conipleses of' Keygin-Type Monolucunuiy Heteropolytungstates
445
0oxygen donor atoms Figure 1. Structure of the parent Keggin and of the monolacunary anion (XO4 tetrahedra not shown): a) r-[XM1?040]" : b) ~ - [ X M I , O ? ~ ] ( " + ~ ' - .
1.23.2 Keggin type heteropolyanions and related species Structurally, polyoxometalates are aggregates of MO, polyhedra (usually octahedra), sharing corners, edges or, more rarely, faces. The metal M is, generally, V, Mo, W, Nb, or Ta, with a large predominance of the first three. The general forrepresent most isopolyanions or heteropolymula [M,nOy]"- and [X,M,O,J"anions, respectively. The heteroatoms X may exist in several geometries. Keggin type polyoxoanions may still be divided in several groups. The parent Keggin anions have the general formula [XM12040ln-, where M = Mo, W, and X may be a non-metal or a semi-metal (e.g. P", Si", B"'), a transition or ap-block metal (e.g CoII, Fe'", Al'") or protons (in the metatungstate anion, [( H ~ ) W I ~ O ~ " ] ~ - ) . Approximately 20 elements have been found as X in this type of anions. Mixed transition metal anions with the formula [ X M I ~ - ~ M ; O ~(M, ~ ] ~M- ,I = Mo, W. V, and others) are also known. The structure of the x isomer of the parent Keggin anions is shown in Fig. la. This structure has overall Td symmetry. The coordination geometry of the heteroatom X is tetrahedral. Other isomers are possible but they will not be discussed in this review, as the most stable sc-isomers are generally used for catalytic studies. For this reason the prefix M - will be omitted in the rest of this text. Several monolacunary anions [XML 10391"- are known, either in salts with different cations or in complexes. These derivatives keep the original Keggin structure; the loss of one M atom with the attached oxygen creates a vacant position (Fig. lb). The lacunary anion, [XM11039ln-, may bind to metal cations as a penta- or tetradentate ligand. A large number of complexes are known, corresponding to formulas of the type [XMl, MI( L)O39]"- ( L = monodentate ligand, that may be absent) or [MI(XM11019)2]n, that can be isolated with a variety of counter-cations. When M ' is a first row transition metal (Mn", Co'', Fe"', Ni", etc.), complexes of
446
I Molecular Clusters
Figure 2. Structure of the complexes of the monolacunary anion (XO4 tetrahedra not shown): (a) [ X M I ~ M ' ( L ) O ~ ~ ] " - , (hatched octahedron represents that with M' in the center); (b) [M' (X MIIO J~)Z]' (geometry around M' not represented).
the first type, with metal to ligand stoichiometry of 1 : 1 and with L = H20, are generally formed. The structure of these anions may be visualised as that of a Keggin anion in which a MO group is substituted by a M'( H20) (Fig. 2a). Larger ions (CeIV,Nd"', etc. ) bind preferentially to two lacunary moieties, forming complexes with a metal coordination number of eight (Fig. 2b). Other lacunary anions, such as [XM9034ln-, and their complexes are k n o ~ n . [ ~ - - ~ ] They present a large number of possibilities in terms of structure, reactions and potential applications, but their study is outside the scope of this review.
1.23.3 Polyoxotungstates with formula [xwiiM(L)039In1.23.3.1 The 1 :1 complexes Many complexes represented by the formula [XW11M( L)039]"- are known.['-31 In this review we will consider those in which L, if present, is water or a related ligand, with the exception of the tungsten-containing mixed anions,
1.23 Complexes of' Keygin- Type Monolacunary Heteropol.~tuny.rtates
447
[XWI~L~M~O where ~ ~ ]M~ = - ,V? Nb, Ta, and Mo. For simplicity, the abbreviations XW12, XWll and X W I I M ( L )will be used to represent c(-[XW12040ln-, I[XW11039]~-and M-[XWIIM( L)039In-. The following metallic elements have been found in the lacuna: Ti'", V1", Crlll,V Mn". 1II.IV ~ ~ I I . 1 1 1 111 Nil1 CU" ZnI1 R 11.111.IV Rh"1 Pdll R v vI.vII > , > , > > . u > e . , ~ 1 1 1 1 ~ ~ 1 InllI 1 1 T111' G e l V Sn".lV Pb11,IV Sb111.V Th > , , , . e most studied anions are those where X = P, As, Si, Ge, B, but polyoxotungstates in which X is a metallic element, like Zn", Co", Fe"', Ti", have also been mentioned. Other 1 : 1 complexes have only been described in aqueous solution, as in the case of species formed with lanthanides," 31 Cd'land Hg1T.1141 The first reported transition metal substituted polyoxotungstates were those with X = Si, Hz, Co", Co"' and M = Co", Co"', Ga"1,['"171followed by other bimetallic anions, like those with X =Zn", Fe"', M =Zn", Ni11.[18,191 In the period 1967-1973 the number of compounds prepared increased considerably. The complexes with X = B, Si, Ge, P, As, and M being one of a variety of divalent and trivalent transition metals (Mn, Fe, Co, Ni, Cu, Zn, etc.) were then Complexes with p and s block metals were reported Selected references to other studies can be found in the reviews by Pope.11,21 Recent additions to the large number of known complexes are those of CrV, Mn", Sn", Pd", T c V , [ l 1.26-311
.
1
1
1.23.3.2 Synthesis and stability in solution Most of the known complexes of this type have been prepared in aqueous solution. In many cases they are obtained by reaction of the ligand, X W I I . with the metal cation, M. Two other methods are used: acidification of solutions containing W04*- and the appropriate reagents of X and M; reaction of XWl2 with base, in the presence of M.[231Chemical or electrochemical oxidation of M present in XWI1 M may be used, as for the preparation of the complexes where M = Co"', Mn111[231 or M = CrV, Mn'V,111327,281 respectively. The stability of these complexes in aqueous solutions depends upon the pH. The range of pH in which the complexes are stable varies with M and X.[221As a rule, all complexes release M in acid media, but to a different extent and at different rates (in certain cases, this only happens at very low pH). Decomposition of the heteropolyanions is observed in basic media. Non-aqueous solutions of the XWI1 M complexes may be prepared from tetraalkylammonium salts or by using phase transfer agents. The coordination of the organic solvent to M, replacing the water molecule, usually takes place with donor solvents. Dehydration of some (but not all) of the XWllM(H20) anions occurs after phase transfer for solvents like toluene with formation of species with a free
448
1 Molecular Clusters
coordination site on the metal M, to which donor molecules (pyridine, C1-, may coordinate.[323331
S02)
1.23.3.3 Structural characterisation 1.23.3.3.1 Electronic and vibrational spectra Electronic spectra of XW11 M( H20) anions in aqueous solution show characteristic oxygen to tungsten charge transfer bands at i= 245-256 nm for XW11,and 255270 nm for XW12 When M = transition metal, the visible absorption spectra, usually, show d-d bands due to the M06 moiety, with molar absortivity four to five times higher and positions corresponding to crystal field splitting smaller than the corresponding a q u o - i ~ n s [ Other ~ ~ - ~features ~~ may be observed, like the low energy charge transfer bands due to ( Mn+,W6+)+ ( M"+',W5+) transitions, which partially or totally obscure the d-d spectra, observed when M is a reducing ion like Fell [1,221
Diffuse reflectance spectra reported for a few compounds, generally, are similar to those obtained from aqueous solutions. Different spectra may be observed, as in the case of some anions with M=CoT1,in compounds with large cations, such as Et3NH+ and others.[241These distinct spectra may be due to a particular arrangement of the structural features in the solid, like the structure with chain-linked Keggin anions found for the compounds with X = P, Infrared absorption spectra of compounds with the parent Keggin anions [XW12040In-, with X = non-metal or semimetal, present a typical pattern where it is possible to identify the stretching vibrations of the different kinds of W-0 bonds: W=O (940-990 cm-I), W-0b-W ( o b = oxygen at a shared corner, 910- 870 cm-') and W-0,-W (0,= oxygen at a shared edge, 820-760 cm-'). Only the W=O stretch can be considered as a pure vibration, the stretches involving bridging oxygens include some bending character.[3s1The X-0 stretching vibrations may be also be observed: v(P-0) = 1080 cm-', v(Si-0) = -930 cm-': v(B-0) = -910 cm-1.r351The spectra of the corresponding lacunary anions, [XWII O ~ ~ ] ( " + ~ ) - , have more bands in the same region, due to their lower symmetry. However, the spectra of the XWllM anions are similar to those of the corresponding parent Keggin anion, as the symmetry is partially restored when the metal M occupies the lacuna (Fig. 3). Infrared and Raman spectra of the potassium salts of the anions with X = P , Si and M = lst row transition metal have been thoroughly studied.t 3 6 . 3 71 For the compounds with X = P, the v 3 of the PO4 group, observed at 1080 cm-' is split in two bands in in the spectra of several compounds with [PW12040]~-,[~~] the spectra of [ P W I ~ O ~and ~ ] ~of- some of its metal complexes (Fig. 3). The observed splitting varies from zero for some complexes to a value approximately equal to the one observed for the lacunar anion, e.g. when M = Sn" (Table 1) in salts
Figure 3. Infrared spectra of selected Keggin compounds ( KBr disk): a ) H ~ [ P W , ~ O ~nHlO; ,I]. bj K I I P W I I O ~nH10; ~ I . KslPWl,Co(H10)0-(9].nH20.
,
1200
1000
800
600
wavenumbedcm- 1
with the same counter-ion. In these cases the axial interaction of the metal M with the polyoxometalate is supposed to be weak, with the ligand binding in a tetradentate fashion.
1.23.3.3.2 NMR and EPR spectra NMR spectroscopy in solution has been used by different authors to study substituted lacunary polyoxotungstates, as in the examples in Table 2. Chemical shifts of the phosphorus nuclei in the "P NMR of [PW,,0191'- in the presence of different metal cations, in aqueous solution, have also been p ~ b l i s h e d . [ ~ ' . ~ ~ ~ Studies of "P or lsiW NMR spectra of paramagnetic heteropolyanions have been r e p ~ r t e d . ' " ~In~ the ~ ~ "'W NMR spectra, the tungsten atoms near to the paramagnetic ions are not observable, but broad 'I P signals are found. Solid state NMR have not been much uscd, so far, in the study of metal substituted lacunary polyoxotungstates and only a study of the coordination geometry of boron in polyoxometalates has been However, I'B NMR spectra of complexes like BWIICO"'and BWllZn or "Si spectra of SiW1lCoTTT show a single broad signal. that can be used to check the purity of the solids, as it is easily differentiated from those of the possible contaminants XWll and XWl2.
450
I Molecular Clusters
Table 1 . Splitting of v,, (P-0) in the infrared spectra of XWllM salts. Anion
Av K.+ salts
Av TBA+salts(")
Anion
Av K+ salts
Av TBAi-salts(")
a) TBA = tetrabutylammonium; b) F. Zonnevijlle, et al, Inorg Chem 1982, 21, 2742, 2751. c) C. Rocchiccioli-Deltcheff, et d,J. Chern. Res. ( S ) 1977, 46. d) G. S. Chorgade, et a/, J. Am. Chem. Soc. 1987, 109, 5134.
Table 2. Selected RMN solution studies of 1 : 1 complexes.
a ) G. S. Chorgade, eta/, J. Am. Chem. Soc. 1987,109, 5134. b) C. Brevard, et al, J. Am. Chem. Soc. 1983, 105, 7059. c) W. H. Knoth, et ul, Inorg. Chem. 1983, 22, 198. d) T. L. Jorris, et a/, J. Am. Chem. Sor. 1987, 109, 7402. e) G. M. Maksimov, et ul, R u ~ sJ. Inorg. Chem. 1987, 32, 551. f ) M. A. Fedotov, et a/, Rum. J. Inorg. Chem. 1987, 32, 362. g) C. Rong, M. T. Pope, J. Am. Chem. Soc. 1992, 114. 2932. h) Q. H. Yang, et al, Polyhedron 1997, 16, 3985.
EPR spectroscopy is a useful technique to probe the coordination geometry around the metal M or to determine the oxidation state of M."] Several studies have been performed, involving elements like iron,[421c ~ p p e r , [ ~manga~,~~] nese>[28.321 chromium.[' 1,271 In particular, the 6-coordinated pseudo-octahedral geometry of copper( 11) in XW11Cu" ( X = P, Si, B) was confirmed for K+ salts, but not for the Bu4Nf compounds ( X = P, B), where the geometry was better described as 5-coordinated square pyramida1.r43,441
1.23 Compleses of Keggin-Type Monolacunary Heteropolytungstates
45 1
1.23.3.3.3 Powder and single crystal X-ray diffraction studies The X-ray powder diffraction study published by Tourne and Tourni is the main reference for the metal-substituted Keggin anion^.[^^*^^] These authors studied the K+, Rb+ and NH4+ salts and verified that they crystallized in a small number of structural types, depending on the number of cations in the molecular formula. The water molecules of crystallization also have a small role, as they reinforce the crystal cohesion and influence the orientation of the anions. Loss of water leads to loss of crystallinity and may alter the lattice parameters. Potassium and ammonium salts of the metal-substituted polyoxotungstates are found to crystallize with structures belonging to symmetry groups of higher symmetry than those of the corresponding lacunary anions; in many cases they are isomorphous with the salts of the parent Keggin anions. This fact is related to the disorder in the occupation of the lacuna that is regularly observed for these types of compounds. Other isomorphous series are occasionally observed, like that of the compounds ( TBA)~H,[XWI1 M( H20)039].nH20, X = P, M" = Mn, Co, Ni, Cu, M"' = Fe, and X = B, M" = Mn, Cu, Zn (TBA = tetrabutylamm~nium).~~~~~~~ Not many single crystal structures have been published because, in the majority of the compounds, the occupation of the lacuna is disordered, as in the case of the reported examples. including Rb6H2[CoWI 1 Co(H20)039]. 13H20,['] Bal[BWl ICO(H20)019]. 26H20,[491 and K,[ZnWI 1Mn(OH)039]. 19H20.[281In the orthorhombic salt K7[GaW1lPb079].16H20. Pb(1I) is bound to 4 oxygen atoms in a pyramidal arrangement.[501 Recently, the crystal structures of two compounds with linked polyanions have been described: [bettf]s[PW~ I MnOi01 '2H20, (bettf = bis(ethy1enedithio)tetrathiaful~alene),[~'~ and [NEtlH]S[XWI1CoO?y]. 3H20, X = P, As.[341Both have chains of anions. connected through oxygen atoms.
1.23.3.3.4 Electrochemical properties The metal substituted polyoxotungstates are, like the lacunary and the parent Keggin anions, able to reversibly accept a certain number of electrons (up to six) on the tungsten atoms.['] Besides that, oxidation or reduction of the metal M may occur. Electrochemical techniques, like polarography" 7.46.5 '-'I and cyclic voltammetry,[27-29,3 1.46.5 1.5 3.5 6-59] h ave proved to be quite useful for the identification and characterisation of these anions. Figure 4 shows typical cyclic voltammograms for compounds of the type XWI I M, where X = P, Si, B and M = non reducible first row transition metal, in aqueous s o l ~ t i o n . [ ~ ~These * ~ ~ polyoxoanions , ~ ~ . ~ ~ , ~ afford ~ ~ voltammograms with 2-electron reversible or quasi-reversible waves (two for X = P , Si and one for X = B ) , that are pH dependent. The substituted lacunary anions are reduced at potentials which are more negative (are more difficult to reduce) than the corresponding lacunary anions or the parent anions under the same conditions.
452
I Molecular Clusters
(B)
1
-0m
-0.900
-0.200
-0.900
I
E (VOLT) Figure 4. Cyclic voltammograms of selected Keggin compounds (aqueous solution, pH 2.2): a) K ~ [ P W I I CH O2( 0 ) 0 ~ 9.]nH2O.; b) K6[SiW11NijH20)0~9] . nH2O; c) K ~ [ B W I I C O ( H ~ O .) O ? ~ ] nH2O.
1.23.3.3.5 Thermal stability The potassium salts of XWllM anions, where X = P, Si and B, and M","' = first row transition metal, are all heavily hydrated, loosing the water of crystallisation up to 200-220 "C. The anions may be stable at temperatures as high as 500 "C, depending on X and, slightly, on M. The relative stabilities are as follows: tungstosilicates > tungstophosphates > tungstoborates. Differential scanning calorimetry (DSC) and differential thermal analysis (IITA) studies of the potassium salts of H20)039]5-,[611 [SiW,lNi( H20)039]5-,[621and of a series of Al"' anions [PW~ICO ( with metallic X[631have been published. When X is a metal, the decomposition products may be pyrochlore or bronze phases, depending of the salts TBA salts follow a different decomposition course. The study of a series of these
1.23 Coinpleses of Keggin- Type Monolacunary Hetevopolytungstutes
453
salts has shown that the [PWl I M ( H20)039]"- anions decompose at about 300 "C giving the parent [PW12040]n-.[481A similar result has been found for some anions with X = Si.
1.23.4 Polyoxotungstates with the formula [M(XWii039)21n-
1.23.4.1 The 1: 2 complexes Almost all the lanthanides form 1 :2 complexes, that were indentified in aqueous solutions and isolated as solids with the formula Z,[M( XW11039)2] . yH2O ( Z = K f or tetra-alkylammonium cations; M = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho and Yb; X = P, Si, Ge, B, Ga, Ti; y = 22-36)."3,6s-7010 t hcr metal cations that are known to form 1 :2 complexes are M = Ca, Sr, Ba, Y, Th, U, Sc, Zr, Hf, Bi, and Ru; X = p. s i ) . [ 6 6 . 6 7 . 7 1 - 7 4 1
1.23.4.2 Synthesis and stability in solution The synthetic procedurcs generally used for the preparation of the 1 : 2 complexes are based on the reaction of the lacunary ligands with the appropriate cation. The preparation of compounds with the general formula K,[Ln"'( XWl1039)2] . yH2O ( X = P, Ln(II1) = Ce, Pr and Nd, n = 11; X = Si, Ln(II1) = Ce, Sm, Eu and Ho, n = 13; y = 22-28) and K,[Ce1V(XW11019)2]. yH20 ( X = P, n = 10, y = 25; ] comX = Si, n = 12, y = 28) was described by Peacock and W e a k l e ~ . " ~These pounds were isolated from an aqueous solution of Hn[XW12040] ( X = P, Si) and the appropriate lanthanide salt in the stoichiometric molar proportion; the pH was controlled by addition of a concentrated solution of potassium acetate. These lanthanide complexes are stable both in aqueous solution and as solids. Adaptations of this method were used for the preparation of most of the other known 1 :2 ~ o m p l e x e s . [ ~ ~ , ~ ~ , ~ ~ ~
1.23.4.3 Structural characterisation 1.23.4.3.1 Vibrational spectra The infrared spectra of the [M(XW11039)2ln- complexes show the characteristic W-0 stretching bands of the Keggin anions referred to in Sec.
454
I Molecular Clusters
1.23.3.3.1.[' Based on infrared spectroscopy, Peacock and Weakley['31 suggested that [PW11039l7- bonds to lanthanides only through the lacuna four exterior oxygen atoms. In the infrared spectrum of the lacunary anion, [PW11039]7-, the P-0 stretch appears as two distinct bands. In the lanthanide complexes [Ln(PW11039)2]"- the separation between these bands is similar to that observed for the ligand itself. This suggested that the interior oxygen atom (of the PO4 group) is not coordinated to the lanthanide cation. Other large cations, which are not capable of entering the cavity of the coordination vacancy of [PW11039]7-, form complexes where the ligand is tetracoordinated and their infrared spectra show splitting of the P-0 stretching bands similar to that of the free ligand.['3,65,66,751 The Raman spectra were also studied for a few [M(PW11039)2]~~complexes ( M = La, Ce, Zr, Hf, Bi and Sm).[66,671 The Raman spectra of solid K11[Sm(PW11039)2].20H20 and that of its aqueous solution show very similar profiles, suggesting that [Sm(PWI1 0 ~ 9 ) 2 ] ' ~ -retains its structural integrity in 3965366,68,69,723751
1.23.4.3.2 NMR spectra There are only a few studies on the 31PNMR spectra of [M(PW11039)2]"-. The observation of a single 31Presonance from aqueous solutions of [M(PW11039)2]'( M = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Th)[38 , 6 7 , 7 6 l shows the equivalence of the positions of the two P atoms in the structure of these complexes in solution. The free ligand [PW11039l7- shows one 3'P NMR peak at 10.5 ppm in aqueous solution. Large chemical shifts are reported for [M(PW11039)2In- when M is a lanthanide, varying from 217 (for Tm3+) to -223 ppm (for Tb3+).[761 170NMR spectra of [M(PW11039)21"-( M = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb), in aqueous solution, consist of a set of peaks corresponding to terminal W=O atoms (725-685 ppm) and a set of W-0-W bridging ~ ~ ]are situated near their position in the free liatom peaks (430-350 ~ p m ) . [They gand spectrum. Spectra of diamagnetic and paramagnetic complexes differ insignificantly except in the peaks for Ln-0-W bridging atoms. Peaks for terminal and bridging 0 atoms do not change position significantly along the lanthanide series. lS3WNMR spectra of [M(PW11039)2In- ( M = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Th) in aqueous solutions were also reported.r761The sets of peaks were more dependent on the lanthanide than were 31Pand I7O NMR spectra.
1.23.4.3.3 Single crystal X-ray diffraction studies The only X-ray crystal structure found in the literature for compounds with anions of the type [M(XW11O39)2ln-was that of Cs12[U(GeW11039)2] . 13-14H20.[771The two anions [GeW11039lS- form a distorted oxygen square antiprism around the
1.23 Complexes of Kegyin-Type Monolucunury Heteropolytunystates
455
central U atom. In the crystal the square facets of the antiprism differ from each other, making an angle of 11.6" with each other, and the sides of each square face differ by about lo'%. Most of the salts reported for this type of anion were strongly hydrated with some of the water molecules weakly bonded, and easily removed with an increase in temperature, causing fragmentation of the crystals.[771That is possibly the reason for there being few single crystal X-ray structural reports available.
1.23.4.3.4 Electrochemical properties The electrochemical behaviour of the heteropolytungstates [Ce"'( XWI1 0 3 9 ) 2 ] ' I ( X = P, Si, Ge) and [Ln'"(PW11039)2)~'-(L n = La, Gd and Tb) was studied by cyclic voltammetry in aqueous solution.[701Redox waves in the positive region were assigned to redox processes of the complexed ion. Waves in the positive potential range were observed in the cyclic voltammogram for the Ce heteropolytungstates, corresponding to the CeTV/CeTT1 redox couple in the complexes, but such waves were not observed for La, Gd or Tb compounds. Redox processes were assigned only to the cerium complexed ion. The redox potential of cerium( IV/III) couple may drop by almost 1 V upon complexation of cerium(II1) with the lacunary [PWI1 0 3 9 J 7 - . [ ' 3 . 7 0 1 Stabilisation of the oxidation state +4 of terbium has also been described for the complex [Tb"( PW11039)2]l0-, made from the oxidation of [Tb"'( PW11039)2]11with aqueous p e r s ~ l p h a t e . In [ ~these ~ ~ ~1~: 2~ complexes the central metal ion can accept electrons from the highly negatively charged anions and is shielded from the external environment^.^^^]
1.23.4.4 The 1 :2 complexes as oxidation catalysts The catalytic properties of the 1 : 2 complexes, [M"'(PW11039)2]'~-(M"' = Y, La, Ce, Pr, Sm, Tb, and Yb) and [M'V(PW1103g)2]1"-(MIv = Ce, Tb and Th), in oxiAlcohols were oxidised in dation catalysis, were investigated by Griffith et u1.[67,811 anions as catalysts, in the presgood yields, cu. 60-90'1/0, using [M(PW 11 039)2]"ence of hydrogen peroxide. These species also oxidised cyclic and linear alkenes to the respective epoxides, cu. 20-80'%, in biphasic systems using H202. Although these metal clusters are good catalysts, there was relatively little difference between yields and turnovers affected by [PWI1 0 3 9 1 7 - , [PW12040]~-or [M(PW11039)2]1'-.[671 It seems that, as happens with [LnW1"036]*~species,[x11the presence of a lanthanide centre conferred no additional reactivity. The presence of Ce"', CeIV or Th" even seems to have an inhibiting effect in the catalysis.[671 It was suggested[h71that the [M(PW11039)2]"- species are decomposed by H202, in aqueous solution, probably to give a mixture of species, such as [W2 0 3 ( 0 2 ) 4 (H20)2I2-, [po4iwo(02)2}413-, 1P03(OH){W0(02)2}2{W0(02)2(H20)}12and
456
I Moleculur Clusters
[P04{WO(02)2}2]3-,all of which will catalyse the epoxidation of alkenes and the oxidation of alcohol^.[^^^^^^ The formation of a 1 : 1 complex [MPW11039]"-, in the first stage of decomposition, was detected by 31PNMR spectroscopy.[671 The inefficiency of the cerium complexes is due to the fact that, on account of the higher strength of the CerV-O bond, they are less easily decomposed by H202 ([Ce"'( PW11039)2]11-is possibly oxidised to [Ce"( PW11039)2]10-by H202).[671
1.23.5 Concluding remarks This brief overview of the rich chemistry of the complexes of monolacunary Keggin heteropolyanions was aimed at collecting relevant information for those interested in further studies on these types of complexes and on their potential applications. To keep this review short, only the most common characterisation techniques used in the studies of these systems were described. Other potentially interesting techniques and properties that have recently been used in their study in our laboratory, such as fast atom bombardment mass spectrometry, were not considered. Although salts of cations other than alkaline metal or NH4+, namely organic or complex cations, have not been extensively studied, they may present new interesting structures and properties, which are well worth further study. Also, anions where X = metallic element have not been studied extensively, presenting interesting scope for research.
Acknowledgements This work would not have been possible without the support of the Centre of Inorganic and Materials Chemistry and of the Department of Chemistry of the University of Aveiro.
References [ l ] M. T. Pope, Heteropoly Isopoly Oxometulutes, Springer Verlag, Berlin 1983. [2] M. T. Pope, in Comprehensive Coordinution Chemistry, Vol. 3, (Ed.: G. Wilkinson) Pel-gamon Press, 1987. p. 1023. [3] M. T. Pope, A. Muller, Angew. Chern., Int. Ed. Engl. 1991, 30, 34.
1.23 ConqdeseJ of Keygin-Type Monolucunnry Heteropolytuny.rtutes
457
141 Pol~~oxo~netal~ite.~: /rani plutonic .solidy t o unti-rectroiiral mtiuify., ( Eds: M. T. Pope, A. Muller). Kluwer Academic Publ.. 1994. [ 5 ] W. P. Griffith; Trcms. Met. Client 1991, 16, 548. [6] Y . Ono, in Perspecriws in Cufal.v.sis,(Eds: J. M. Thomas. K. I. Zamaraev). Blackwell Scientific Publications, 1992, p. 431. [7] N. Mizuno, M. Misono, J. Mol. Ctilul. 1994. 86. 319. [8] 1. V. Kozhevnikov, C(ital. Rec. Sci. Eng. 1995. 37, 311. 191 T. Okuhara, N. Mizuno, M. Misono. Adouncrs in Catalj~.sis1996, 41, 113. [lo] C. L. Hill, C. M. Prosser-McCartha. Coord Cheni. Rer. 1995, 143, 407 and references therein. [ l 11 D. E. Katsoulis, M. T. Pope, J. Chern. Soc., Cliei??. Comniun. 1986. I 1 86. 1121 C. L. Hill, R. B. Brown, J. Am. (%ern. Soc. 1986, 108, 536. 1131 R. D. Peacock, T. J. R. Weakley, J. Chenz. Soc., A 1971. 1836. 1141 M. A. Fedotov, R. I. Maksimovskaya. G. M. Maksimov. K. I.. Matveev, Russ. J. Znorg. Clitw. 1987, 32, 362. [I51 L. C. W. Baker, T. P. McCutcheon, J. A m . Clzeni. Soc. 1956. 78, 4503. [I61 L. C. W. Baker, V. S . Baker, K. Eriks. M. T. Pope, M. Shibata. 0. W. Rollins, J. H. Fang, L. L. Koh, J. An?. C%cwi. Soc.. 1966. 88, 2329. 1171 L. C. W. Baker, J . S. Figgis. J. Am. Clzrwi. Soc. 1970, 92, 3794. 1181 R. Ripan, A. Duca. D. Stanescu, M. Puscasu, Z., Arzoug. Allgenz. Clzem. 1966, 347, 333. 1191 R. Ripan, M. Puscasu. Z. Anor(/. Allcgeni. Cliem. 1968, 358, 82. 1201 T. J. R. Weakley, S. A . Malik. J. Inor(/. Nucl. Clzeni. 1967, 29, 2935. [21] C. Tourne, Coinpt. Rend. Acod Sci., Sect. C 1968, 266, 702. [22] C . Tourne, G. Tourne, Bull. Soc. Chim.Fu. 1969, 1124. 1231 C. M. Tourne, G. F. Tourne. S. A. Malik, T. J. R. Weakley. J. Inorg. Nucl. Clzerrz. 1970, 32, 3875. 1241 T. J . R. Weakley, J. Clieni. Soc.. Dulton Trans. 1973. 341. 1251 G. Tourne, Bull. Soc. Chin?. Fr. 1982. 1-69. [26] A. M. Khenkin, C. L. Hill. J. An?. Clzeni. Soc. 1993, 115, 81 78. 1271 C. Rong, I-’. C. Anson. Inorg. Clwnz. 1994, 33, 1064. 1281 X . Zhang, M. T. Pope. M. R. Chance, G. B. Jameson, Polyliedron 1995. 14, 1381. 1291 G. S. Chorgade, M. T. Pope, J. Am. Clzern. Sor. 1987, 109, 5134. [30] G. M. Maksimov. R. I. Maksimovskaya, K. I . Matveev, Rus.~.J. Znorg. (%ern. 1987, 32, 551. [31] M. J. Abrams, C. E. Costello, S. N . Shaikh, J. Zubieta, Inorlj. Chitn. Acta 1991, 180, 9. 1321 D. E. Katsoulis, M. T. Pope, J. An7. Clieni. Soc. 1984, 106, 2737. [33] D. E. Katsoulis, V. S. Tausch, M. T. Popc. Znory. Cliern. 1987, 26, 215. 1341 H. T. Evans, T. J. R. Weakley, G. B. Jameson. J. Clzm. Soc., Dulton Trans. 1996, 2537. 135) C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R . Thouvenot, Znorg. Clleni. 1983, 22, 207. 1361 C. Rocchiccioli-Deltcheff, R. Thouvenot, C. R. Acuti. Sc. Pari.~,Sect. C 1974, 278, 857. 1371 C. Rocchiccioli-Deltcheff, R. Thouvenot, J. Cliern. Rex / S ] 1977, 46. [38] G. M. Maksimov, R. I. Maksimovskaya, 1. V. Kozhevnikov. Russ. J. Inorg. Chem. 1992, 37, 1180.
[39] R. I. Maksimovskaya. M. A. Fedotov, G. M. Maksimov. Russ. J. Inorg. (%eni. 1985, 30, 514. [40] T. L. Jorris, M. Kozik. N . Casan-Pastor. P. J. Domaille. R. G. Finke, W. K. Miller, L. C. W. Baker, J. Am. Clietn. Soc.. 1987, 109, 7402. 1411 A. F. Couto, M. C. Trovlo. J. Rocha. A. M. V. Cavaleiro. J. D. Pedrosa de Jesus, J. Chenz. Soc., Dalton Tucins. 1994, 2585. 1421 P. Rigny, L. Pinsky, J . M. Weulersse, Conipt. Rend Actid Sci., Sect. C 1973,276, 1223. 1431 G. Scholtz, R. Luck. R. Stosser. H. J. Lunk. F. Ritschl, J . Clieni. Soc., Fctraduy Trans. 1991, 87. 717. 1441 J. A . Gamclas. 1. S. Santos. C. M. Freire. B. Castro. A . M. V. Cavaleiro, Pulylzedron 1999, 18, 1163. 1451 C. Tourne, G. Tourne, Conipt. Rtwd A c d Sci..Sect. C 1968. 266, 1363.
458
I Molecular Clusters F. Zonnevijlle, C. M. TournC, G. F. Tourne, Inorg. Chem. 1982, 21, 2742. J. R. Galan-Mascaros, C. Gimenez-Saiz, S. Triki, C. J. Gomez-Garcia, E. Coronado, L. Ouahab, Angew. Chem. Int. Engl. Ed 1995, 34, 1460. J. A. Gamelas, F. A. Couto, M. C. Trovgo, A. M. V. Cavaleiro, J. A. S. Cavaleiro, J. D. Pedrosa de Jesus, Thermochim. Acta 1999,326, 165. T. J. R. Weakley, Acta Crystallogr., Sect. C 1984, 40, 16. G. F. Tourne, C. M. Tourne, A. Schouten, Actu Crystallogr., Sect. B. 1982, 38, 1414. A. Teze, P. Souchay, Rev. Chim. Min. 1970, 7, 539. J. M. Fruchart, G. Herve, J. P. Launay, R. Massart, J. Inorg. Nucl. Chem. 1976, 38, 1627. F. Zonnevijlle, C. M. Tourne, G. F. Tourne, Inorg Chem 1982,21, 2751. A. Schouten, B. Cros, Can. J. Chem. 1982, 60, 1368. A. Schouten, B. Cross, Polyhedron 1982, 1, 283. J. E. Toth, F. C. Anson, J. Electrounal. Clzem. 1988, 256, 361. M. Kozik, N. Casan-Pastor, C. F. Hammer, L. C. W. Baker, J. Am. Chem. Soc. 1988,110,7697. C. Rong, M. T. Pope, J. Am. Chem. Soc. 1992, 114, 2932. Q. H. Yang, D. F. Zhou, H. C. Dai, J. F. Liu, Y . Xing, Y. H. Lin, H. Q. Jia, Polyhedron 1997, 16, 3985. F. A. Couto, A. M. V. Cavaleiro, J. D. Pedrosa de Jesus, J. E. SimBo, Inorg. Chim. Acta 1998, 281, 225. A. Komura, M. Hayashi, H. Imanaga, Bull. Chem. Soc. Jpn. 1976, 49, 87. V. I. Spitsyn, I. D. Kolli, 0. L. Shakhnovskaya, G. A. Gordeeva, Russ. J. Inorg. Chem. 1986, 31, 508. E. Wang, Q. Wu, B. Zhang, R. Huang, Trans. Met. Chem. 1991, 16,478. Y. A. Moroz, V. I. Krivobok, M. N. Zayats, Russ. J. Inorg. Chem. 1986, 31, 986. L. G. Maksimova, T. A. Denisova, L. V. Kristallov, V. G. Kharchuk, N. A. Zhuravlev, V. L. Volkov, L. A. Petrov, Russ. J. Inorg. Chem. 1995, 40, 941. G. M. Maksimov, G. N . Kustova, K. I. Matveev, T. P. Lazarenko, Sou. J. Coord. Chem. 1990, 472. N. M. Gresley, W. P. Griffith, A. C. Laemmel, H. I. S. Nogueira, B. C. Parkin, J. Mol. Cat. 1997, 117, 185. J. Liu, W. Wang, Z. Zhu, E. Wang, Z. Wang, Transition Met. Chem. 1991, 16, 169. J. Liu, Z. Zu, B. Zhao, Z. Liu, Inorg. Chim. Actu 1989, 164, 179. N. Haraguchi, Y. Okaue, T. Isobe, Y. Matsuda, Inorg. Chem. 1994, 33, 1015. A. V. Botar, T. J. R . Weakley, Rev. Roum. Chim.1973, 18, 1155. M. Rusu, A. Botar, Stud. Univ. Babes-Bolyai Cheni. 1986, 31, 83. G . Marcu, M. Rusu, Rev. Roum. Chim. 1976,2I, 385. C . Tourne, G. TournC, Rev. Chim. Min. 1977, 14, 83. L. Shizhong, W. Enbo, H. Mingyue, W. Zuoping, Yingyongtluaxue 1993, 10, 71. M. A. Fedotov, B. Z. Pertsikov, D. K. Danovich, Polyhedron 1990, 9, 1249. C. M. Tourne, G. F. Tourne, M. C. Brianso, Acta Crystallogr. Sect B 1980, 36, 2012. W. S. You, Y . P. Gu, Chin. Chem. Lett. 1993, 4 , 369. A. S. Saprykin, V. P. Shilov, V. I. Spitsyn, N. N. Krot, Dokl. C'hem. 1976,226, 114. A. S. Saprykin, V. I. Spitsyn, N. N. Krot, Dokl. Phys. Chem. 1976, 231, 1038. W. P. Griffith, R. G. H. Moreea, H. I. S. Nogueira, Polyhedron 1996, 15, 3493. W. P. Griffith, B. C. Parkin, A. J. P. White, D. J. Williams, J. Chem. Soc. Dalton Trans. 1995, 3131. A. J. Bailey, W. P. Griffith, B. C. Parkin, J. Chem. Soc. Dalton Trans. 1995, 1833. A. C. Dengel, W. P. Griffith, B. C. Parkin, J. Chem. Soc. Dalton Trans. 1993, 2683. C. Venturello, R. D'Aloisio, J. C. J. Bart, M. Ricci, J. Mol. Cat. 1985, 32, 107. L. Salles, C. Aubry, R. Thouvenot, F. Robert, C. Doremieux-Morin, G. Chottard, H. Ledon, Y . Jeannin, J-M. Bregeault, Inorg. Chem. 1994, 33, 871. D. C. Duncan, R. C. Chambers, E. Hecht, C. L. Hill, J. Am. Chem. Soc. 1995, 117, 681.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.24 Reactivity of Diauracycles. A Way to Prepare Chains of Gold Atoms Mariuno Lagunu and Elenu Cerruda
1.24.1 Introduction The chemistry of dinuclear gold complexes containing two gold atoms held together by two identical or different bidentate hgands (diauracycles) has attracted a great deal of attention. One reason for the interest in these materials is the short intra gold-gold contacts they usually exhibit. Furthermore, some of these compounds display inter gold-gold contacts. The presence of different types of interactions within the same molecule has increased interest in this class of complexes. With two gold( I ) centers present as members of ring systems, the linear array of ligands around these metals induces steric constraints which are not easily accommodated in a ring with less than eight members. Thus, in metallocyclic chemistry of gold(1) the eight-membered ring size is the most abundant. Scheme 1 shows some examples of dinuclear gold(1) complexes most of which have been characterized by X-ray studies. They include [ A u ~ ( C H ~ P P ~ ~ C H ~1,) ~ ] [ " ~ ] [ A u ~ ( C ~ H ~ NTMS)2)-2)2][31 {C( 2, [ A u ~ ( C S H ~ P P ~3,~ [- A ~ u) ~Ph2PCH*PPh2)2](] [ ~ ' ( 5 , [ A U ~ ( C H ~ P P ~ ~ 6, S ) [~A] "u]{~(PPh3)CH(C104)2'514, [ A u ~Ph2PCHPPh2)2][61 ~ I~ ( C H ~ P M ~ ~ B H ~ P M ~9,~ C H ~ ) ~ ] [ ' COCH(PPh3))2]['] 7, [ A U ~ ( S ~ C N R ~8,) ~[ ]A[ U [Au?(CH2PR2CH2)(Ph?PCHPPh2)][' 10, [ A u ~PhzPCH2PPh2) ( { CH#O(NMe2)' ~ ] It is noteworthy that synCH2}]['2111, [Au2(PhZPCHIPPh2) { S ( C H ~ ) I S } ] [12. thetic pathways usually result in identical bidentate ligands bonded to the two gold atoms. The largest ring containing two-coordinate gold( I ) centers appears to be the 13. ~N Others H ~ } large ~ ] [ 16-, ' ~ ]18- or 16-membered [ A u ~ { H ~ N ( C H ~ ) ~ N H ( C H ~ ) 26 membered rings containing two trigonal gold( I ) atoms, two square planar gold (111) atoms or four gold( I ) two-coordinate centers respectively[15 'I have been described. Eight member ring diauracycles show structures which can be described as 'elongated cyclohexanes' if all the bonds involved are treated as single. As with cyclohexane, the saturated ring may adopt a chair, boat or twist conformation, and
460
+ cu I
I Molecular Clusters
N
f
a
50/" \a-$-a N
c
a
N
p*
c a
N
f
a
cy
a
3
a
1.24 Reucticity of‘Diuurucycl~s.A WUJS to Prepure Chuins of Cold Atonis
461
in fact all three of them have been detected in the structural chemistry of gold, the former conformation being the most common. In some cases the presence of two sp’ hybridized atoms produces an unsaturated ring resulting in the appearence of a planar structure (e.(1. the diphosphino-methanide 5 or the dithiocarbamate 8 derivatives), whilst the presence of only one of these atoms induces an ‘envelope’ structure (see some examples below). Irrespective of their ring conformations all of these eight-membered diauracycles show gold. . .gold transannular distances close to 3 A, which are likely to be a consequence of aurophilicity brought about by relativistic effects.“ The presence of these contacts in complexes with more than eight members in the ring (e.q. 3.128(1) A in 12 and other examples below) shows that this transannular interaction must result from a bonding interaction, however, the 16-membered ring complex 13 (Au.. .Au 4.54 A)is an exception.
1.24.2 Reactivity of [ A u ~ ( C H ~ P P ~ ~ C H ~ ) ~ ] By far, the most studied diauracycle is p, p’-his(dipheny1phosphinium)his(methy1ido)gold(I ) 1 which has been structurally characterized by Fackler et a/“’ and fully described by Schmidbaur et a/.[21This complex undergoes a two or four electron oxidative addition reaction to form cyclic gold( 11) or gold( 111) species respectively.[”- 211 Dimers containing both gold( 111) centers with halides in trans or cis disposition, or with different geometries at each center have been structurally characterized. Isomerization reactions between these various isomers have been reported. In all these reactions the bridging ylide ligand does not participate and it preserves the dimeric nature of the complexes by holding the two gold centers close together. In fact. only a few reactions have been described in which the ylide is an active component and some carbon-gold bonds are cleaved. These reactions are collected in Scheme 2: (a) the reaction with hydrogen chloride which symmetrically cleaves two gold-methylene bonds affording two monomeric gold-ylide fragments.[221 (b) Two different gold( I ) ylide dimer complexes containing different bridging ligands scramble when mixed in CHlCl,, leading to an equilibrium mixture of the three possible dimer~.[’~] (c) The phenyl-methyl analogue isomerizes between its cis and trans geometric forms, in CH’ClI. in the presence of catalytic amounts of weak Lewis acidsr241such as MeiSnC1, AuCI(PPh3) and SO>. This is a process that cleaves and reforms gold-carbon bonds. (d) The disproportionation of cyclic dinuclear gold( 11) complexes into non-cyclic mixed valence gold( I)/gold(111) derivatives.[2s1(e) The oxidative cleavage with bromide to form ring-opened dimeric gold( 111) complexes.[2h1 Recently[271 we showed that mononuclear ylide gold( I ) complexes like [Au(CH?PR3)21C104 have enough electron density at their gold centers to form non-supported metal-metal bonds when reacted with nearly ‘naked’ silver com-
462
I Molecular Clusters Ph2
rP\l
a)
Au
f
HCI
\,J
Au
PPh2Me
I
CI
Me
Me
I
I
G - p - p h-7Gsp-ph L
c)
Ph--P Me I
Scheme 2
Me-
Ph
I 2 4 ReuctiiTitjs of Dmiracjdes A W a y to Prepare Cliams of Gold A t o m
463
plexes. The X-ray structure of [Au2Ag2(CH2PPh3)4(C104)4] 14 shows the presence of four non-supported gold-silver donor bonds with lengths in the range 2.783( 1) to 2.760(2) A.The diauracycle complex 1 can be regarded as a double his-ylide gold( I ) and consequently, it must be expected to have an excess of electron density at its gold centers. Some examples that show the nucleophilicity of these gold centers are illustrated schematically in Scheme 3 . (a) Fackler et u1[24.281have elegantly shown in different examples that in fact complex 1, or its corresponding phenyl-methyl derivative, or( S O ~ ) ~ ] reacts with Lewis acids like SO2 to give [ A u ~ ( C H ~ P P ~ ~ C H ~15) ~ [Au2(CH2PMePhCH2)2(S02)2] 16, respectively. The X-ray structure of these complexes clearly show that these dinuclear gold compounds act as nucleophiles with Lewis acids and that such interactions may be enhanced by the formation of an increased Au-Au bonding interaction between the metal atoms. In fact, the gold-gold distances of 2.835(1) A 15 and 2.894(2) A 16 in the SO2 adducts are shorter than those in the starting diauracycles. 2.976(1) A[']and 3.002( 1) A[281respectively. (b) The addition of methyl iodide to [ A u ~ ( C H ~ P P ~ ~toC give H ~ )the ~ ]unsymmetrical digold( 11) complex [AuZ(CH2PPh?CH?)Z(I)(CH3)]was the first example reported concerning oxidative addition across two metal atoms with metal-metal bond formation.[21A great variety of other organic halides have been studied and an S N ~ process has been proposed['91 (process b, Scheme 3 ) . The ionic nature of the intermediate is connected to the nucleophilic character of these gold centers. (c) The reaction of [Auz(CHzPPh2CH2)2] 1 with organic acceptors, such as 7,7/8,8/tetracyanoquinodimethane ( TCNQ) or 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) led to the formation of the charge transfer complexes [AuzH ~ ) ~ ] (18, which pro(CH2PPh2CH2)2]2(TCNQ) 17 and [ A U ~ ( C H ? P P ~ ~ CDDQ)[291 vides additional evidence of the nucleophilic character of the gold centers in 1 and related complexes.
1.24.2.1 Reactions of [ A u ~ ( C H ~ P P ~ ~ C with H ~gold(II1) )~] or silver( I) complexes Short non-bonded A u ( I ) . . . A u ( I ) contacts of less than 3 A are well documented" 8-30.311-and numerous gold( 11) complexes containing Au( 11)-Au( 11) bonds, have been described." 9 , 3 ' 1 However, gold complexes displaying only a single unbridged gold-gold bond are scarce except in the case of the gold clusters. Complex 1 is an excellent candidate for the preparation of this class of complexes, because as stated above: a) complex 1 has electron density at the goldiI) centers. b) the ylide moiety holds together the metallic centers and prevents dimer cleavage, and c) the mononuclear ylide gold( I ) complex [ A u ( C H ~ P R ~ ) ~ forms ]C~O~ heteronuclear gold-silver complexes when reacted with silver centers[271or causes migration of ylide groups when reacted with [ A U ( C ~ F ~ ) ~ ( O E ~ ~ ) ] . [ ~ ~ ]
464
I Molecular Clusters
R2 = Ph2 15, PhMe 16
Ph2
Ph2
C)
CP\
Au
Au
I
I
+
*
Acceptor
(Acceptor),
\ P/ Ph2
CN
Acceptor =
Nc\~:~m~z< NC/
CN
TCNQ n = 0.5
17 Scheme 3
Scheme 4 illustrates the results obtained when complex 1 reacts with nearly ‘naked’ gold or silver metal centers.r331The reaction with 1 equivalent of [Au(R)3(OEt,)] ( R = C6F5 or C ~ F ~ Haffords Z ) the trinuclear complexes 19 and 20. The structure of 19 has been established by a single X-ray study. The initial diauracycle remains intact and one Au(CgF5)3 moiety is now bonded through a gold atom to one gold atom in the diauracycle, as shown in the Scheme 4. The two Au( I )
1.24 Reactiritj, o j Diuurrrtjdes A W q to Prepare Chains of Gold Atoms
465
Ph2 22
Ph, L = PPh3 24, tht 25
Scheme 4
centers, which were 2.977(1) A apart in li",are only 2.769( 1) A apart in 19. This distance is only slightly longer than the shortest known Au( I ) . . .Au(I ) bond and the longest Au( 11)-Au( 11) b o n d ~ . [ ' . ~ ' 0 . ~ne~ of - ~ the ~ ] gold (2.737(3)A[343) centers is bonded to the Au( 111) atom of the starting material [Au(CgF5)3(0Et2)], with a very short bond of 2.572( 1) A,similar to the shortest Au( 11)-Au( 11) bonds. The shortest known Au-Au bond of 2.553(1) is observed in the gold(I1) species [ A U ~ ( C H ~ P P ~ ~ SAlthough ) ~ C ~ ~the ] .assignment ~ ~ ~ I of the oxidation states to the gold atoms in 19 is to some extent a matter of debate, we believe that all gold centers remain unchanged from the starting materials: two gold( I ) in the cyclic dimer and one exo gold( I l l ) , which receives electron density from one of the gold( I ) centers. The similarities of the Au. . .Au distances across the ring of complexes 15, 16, and 19 support this hypothesis and, furthermore, theoretical calculationsi371de-
466
1 Molecular Clusters
scribe complex 19 as a chain of Au( I), Au( I), Au( 111) atoms. Reaction of complex 1 with two equivalents of [ A u ( C ~ F ~ ) ~ ( O affords E ~ ~ the ) ] tetranuclear complex 21. The reaction of complex 1 with silver derivatives such as [Ag(C6F5)]4, [Ag(OC103)(PPh3)] or [Ag(OC103)(tht)](tht = tetrahydrothiophene) affords gold ( 11) complexes [ A u ~CH2PPh2CH2)z ( (CbF5)2] 22, [ A u ~CH2PPh2CH2)2( ( PPh3)2]25, r e s p e c t i ~ e l y . [ ~In~ .these ~~] (C104)224, and [Au2(CH2PPh2CH2)2(tht)2](C104)2 cases, the donation of electron density from the gold to the silver centers is complete and an oxidation of the gold centers occurs, with a simultaneous reduction of the silver to the metallic silver, which is recovered from the reaction mixture. A garnet complex 23[40,411 could only be isolated when [Ag(OC103)(PPh3)] was used and the reaction performed at -20 "C. This compound could be described as the tetranuclear donor-acceptor complex represented in the Scheme 4. Complex 23 is considered to be an intermediate in the oxidation reaction as it is converted into 24 and metallic silver when dissolved in dichloromethane at room temperature. The structures of 24 and 25, established by single X-ray studies, show Au-Au distances across the eight-membered ring[39,401 of 2.579(3) A and 2.6463(7) A,respectively, in accordance with their formulation as gold(11) complexes. The triphenylphosphine derivative 24 shows a twisted conformation of the ring and the tetrahydrothiophene derivative into a chair conformation. Complex 22, which had previously been synthesized by Fackler et a1,[421exhibits a Au( 11)-Au( 11) distance of 2.677( 1) A.The factors influencing the Au( 11)-Au( 11) bond length are not clearly understood. The triphenylphosphine derivative 24 has a very short gold-gold distance despite the presence of the phosphine ligand which one would naively expect to lengthen the Au( 11)-Au( 11) distance due to the usually strong trans influence. This conflicts with the oxygen ligand, which has little affinity for gold but also produces short Au( 11)-Au( 11) bonds (<2.6 However, the trans influence may act through, rather than on, the Au( 11)-Au( 11) bond.[',451 The cationic gold(I1) complexes 24 and 25 were the first ever cationic gold(I1) complexes to the structurally characterized, although other cationic complexes had been reported, such as [ A u ~PPh2NHPPh2)2X2](C104)2[361 ( ( X = C1, Br or C6F5), [ A u (SPPh2CHPPh2Me)2C12]( ~ C104)2[461, [ A u(~C H ~ P P ~ Z C( XH)(CH2 ~ ) ~ PPh3)]X[47,481( X = Br or I), and [ A u (CH(C02Me)PPh2CH(C02Me))2L2](C104)2[491 ~ ( L = N, P or As ligands). A).[433441
1.24.2.2 Reactions of [ A u ~ ( C H ~ P P ~ ~ with C H ~mono ) ~ ] and binuclear gold( I) complexes In the last section reactions were presented in which the bridging ylide ligands do not appear to participate and the diauracycle remains intact. This is not the case when the starting bis-ylide reacts with gold( I ) derivatives, which are either cationic
1.24 Reucticity of Diuurucyles. A Wuy to Prepure Clzuins of Gold Atoms
467
such as [Au(PR3)2]C104 or anionic such as N B u ~ [ A u X ~(X ] = C1, Br), and ylide transfer reactions occur (Scheme 5). The reactions of the diauracycle 1 with the his-phosphine complexes [Au(PR3)2]C104 leads to the formation of dinuclear derivatives[501 [Au2(CH2PPh2CH2)(PR3)2JC104 26 (PR3 = PPh3 and PPh2Me). The reaction process is very slow at room temperature (between 12 and 24 h) but the reaction time can be halved by refluxing in chloroform. Initially, the reaction mixture turns black-red but the color fades away; similar processes occur in wet solvents. This seems to indicate that these processes should evolve through a non-dissociative pathway and possibly, through intermediates with gold-gold contacts. When the reaction of 1 is carried out with anionic complexes, such as NBu4[AuX2] (X = C1 or Br), ylide transfer reactions also occur affording dinuclear ~C Unlike H ~ ) the X , ~former procanionic d e r i v a t i ~ e s ’(~N ~ ’B U ~ ) ~ [ A U ~ ( C H ~ P P ~27. esses, the reactions are much faster being complete in only 15 minutes at room temperature. No color is observed in the reaction mixtures, which probably implies intermediates that do not have gold-gold contacts. Intermediates with halogens as bridges could be postulated as has been done in other examples of chelate ligand transfer reactions between gold center^.'^ ‘ s 2 ] Scheme 5 shows a tentative pathway for the processes required to form cationic derivatives [Au2(CH2PPh2CH2)(PR3)2]C104 26. Following the formation of an intermediate A containing a donor-acceptor bond, it evolves with partial ylide transfer to give the trinuclear gold compounds 28 which further react with a new molecule of [Au(PR3)?]C104. through the intermediate B, to afford the dinuclear derivatives 26. In accordance with this proposal, the triphenylphosphine complexes [Au3(CHZPPh?CH?)?(PR3)2]C104 28 have been isolated and the structure with TCNQ as counterrion established by X-ray studies. The gold-gold contacts of 3.391(1 ) and 3.544(1 ) A are considerably longer than those in the starting material and in other trinuclear gold complexes such as [ A u ~PPh2CH2PPh2)2C12]X ( ( X = C104[531or Cl[”’). Complexes 28 are intermediates in the reactions because they react with a new rnolccule of [Au(PR3)2]C104to give two molecules of the dinuclear 26 derivatives. Similar type of reactions can be performed by using dinuclear gold( I ) complexes instead of the mononuclear ones. Cationic or neutral derivative^[^^-^ ’I such as [ A u ~Ph2PCHiPPh2)2](C104)2, ( [ A u ~Ph2PNHPPh2)2](C104)2, ( [ A u ~PhzPC6H4N)zI ( (C104)2, [ A u ~ ( S ~ C P C Y ~ ) ~ ] ([ C A~UO~~( S ) ~~.C N R [~A) u~ zJ(,S ~ C O R ) or ~ ] [AM(SC5H4N)2J,react with the his-ylide 1 under very mild conditions (dichloromethane, room temperature). This is a new and very general way of synthesizing diauracycles 29-33 with two different ligands (Scheme 6) in very good yields. The X-ray structures of [Au2(CH2PPh?CHZ)(S2CNEt2)]31, [ A u ~ ( C H ~ P P ~ ~ C H ~ ) (SZCO’Pr)]32, and [ A u ~ ( C H ~ P P ~ ~ C H ~ ) ( S33C confirm ~ H ~ N the ) ] unsymmetrical nature of these diauracyclcs, showing an envelope conformation and intermolecular gold. . .gold contacts across the ring of 2.868( I ) , 2.881 (1) and 2.862(1 ) A, respec-
468
I Moleculur Clusters
26
27
+
L
20 A
I
2+
2
26 0
Scheme 5
tively. Only the dithiocarbamate derivative shows intra gold. . .gold contacts between two dimers of 2.984( 1) A.The two dimers are located almost perpendicular to each other in such a way as to form a chain of four atoms, with Au-Au-Au angles close to linearity[541(maximum deviation 6.5'). The reaction between the two symmetrical diauracycles should be through a donor-acceptor interaction in a similar way to that previously described for mononuclear gold(1) complexes. In fact no reaction occurs between 1 and [ A u ~Ph2PCH2PPh2)2C12] ( whose trico~rdinate[~] T-shaped gold( I ) centers cannot accept electron density from the former. An unsymmetrical tetranuclear complex C should be formed due to the donor-acceptor interaction. If this is the case, further interaction between the asterisk marked gold atoms could produce the unsymmetrical diaurdcycle (Scheme 7). However, only one tetranuclear complex similar to the intermediate C has been structurally characterized [Auq(C3S5)2( Ph2PCH2PPh2)2][58134 ( C ~ S= S 1.3-dithiole-2-thione-4,5-dithiolate, dmit). Complex 34 does not evolve dinuclear complexes on standing or heating in any solvent,
1.24 Reactivity of Diuuracyck~s.A Wuy to Prepare Chuins q f Gold A t o m
i
+
r az /+-O X
\a-2-0 N
c
a
c) 0
I" I z \ 2 g
/""
I"
$ II
X
: g 8
v!
469
470
1 Molecular Clusters
probably because the Au. . .Au distance between the two gold atoms, which must 5 Thus, we interact to give the unsymmetrical diauracycle, is too long ( ~ 4 . A). consider a concerted intermediate D to be the most likely. The reactions illustrated in Schemes 5 and 6 are more general[591and can be performed with other diauracycles such as [ A u ~ ( S ~ C N R [~A) ~u ~ ]S2COR)2] ,( and [ A u z ( S C ~ H ~ NThey ) ~ ] . react with mononuclear gold( I ) complex [Au(PPh3)2]C104 to give ring-opened dinuclear complexes [Auz(L-L)( PPh3)2]C104 (L-L = S2CNR2, S2COR and SC5H4N). These processes can occur in a similar way by nucleophilic attack of [ A u ~L-L)2] ( on [Au(PPh3)2]+ This proposal is also supported by the (pta = phosphatriazaarecent preparation[601of [ {Au(pta)3}2Au2{S2C2(CN)2}2] damantane), a tetranuclear chain complex with unsupported gold( 1)-gold(I ) interactions. In accordance with the proposed donor-acceptor interactions, the reaction of [Au~(dppm)2](C104)2 with [Au(PPh3)2]C104 does not progress as well as in the reaction of [Auz(L-L)2] (L-L = S2CNR2, S2COR and SC5H4N) with NBu4[AuC12]. The reactions of [Auz(S2CNR2)2] with dinuclear [Au2(dppm)2](C104)2or [Au2(dppe)2](C104)2affords unsymmetrical diauracyles [ A u ~ ( S ~ C N R P-P)]C104 ~)( (P-P = dppm or dppe).
1.24.2.3 Reactions of [ A u ~ ( C H ~ P P ~ ~ with C H protic ~ ) ~ ] acids The nucleophilic character of 1 exhibited in some of the previous reactions should be manifested in the reaction with protic acids. In fact, Fackler et a1 have shown that the isomerization of trans-[Au2(CH2PPhMeCH2)2]to a cis-trans mixture is acid catalyzed[241via a nucleophilic attack of the gold atoms on the Lewis acids (Scheme 2, process c). However, the use of stronger protic acids, such as hydrogen halides, resulted in rupture of the dimer to give two mononuclear units[221 [ A u ( C H ~ P P ~ ~ C H (Scheme ~ ) X ] 2, process a). It has been suggested that these processes involve a ring opened dinuclear intermediate [Au(CHzPPh2CH2)(CH2PPh2Me)XI.The reaction with a strong acid with a poorly coordinating anion such as HC104, in the presence of phosphorous ligands like triphenylphosphine, bis(dipheny1phosphino)methane (dppm) or his(dipheny1phosphino)ethane (dppe), occurs with loss of one molecule of [PPh2Me2]C104 and the formation of dinuclear [ A u ~ ( C H ~ P P ~ ~PR3)2]C104 C H ~ ) ( 26 and diauracycles [Au2(CHzPPhzCH2)(P-P)]Clod (P-P = dppm 29, or dppe 35)(Scheme 8). These processes can evolve through the same intermediate proposed in the reaction with hydrogen chloride. Other diauracycles such as [ A u ~C5H4PPh2-2)2] ( 3 give similar reactions with electrophiles such as methyl triflate, which may attack a gold- carbon bond of 3 and generate a cationic gold species which then rearranges to give a pentanuclear[6 derivative
[AU5(C5H4PPh2-2)41(SO3CF3). The reactions of 1 with weak protonic acids such as 2-thiol-pyridine or 2sulfanylbenzothiazole that can act as ligands either in the protonated or in the deprotonated form, do not follow the latter reaction pathway and unexpectedly1591
1.24 Reactiiity of Diuuracycles. A Way to Preparc. Chains of Gold Atonzs
471
Scheme 8
produce gold( 11) derivatives 36 and 37. The gold( 11) nature of 36 was established by X-ray crystallography, which identified a gold-gold bond distance of 2.669( 1) A. These processes are accelerated by bubbling oxygen through the solutions, whereas they are prevented by the use of deoxygenated solvents. The first step could be an approach of the sulfur donor ligand to give an adduct (similar to those reported for S02[24.281), which should be very easy to oxidize with atmospheric oxygen.
1.24.3 Mixed-valence linear chains of gold atoms The nucleophilic interaction between diaurdcycles and electrophilic gold centers can be extended to the use of gold( 11) complexes as starting materials. We s e l e ~ t e d [ ~ * , ~ ~ ] [Au~(CH~PP~~CH~) 22~ because ( C ~ F Swith ) ~ ]only gold-carbon bonds it cannot form any bridge by reaction with electrophilic centers. The reaction of 22 with (Au(CgF5)1(OEt2)]led, surprisingly, to the pentanuclear cationic complex [ { A ~ ~ ( C H ~ P P ~ ~ C H ~ ) ~ ( C , ~ F S ) } A U ( C38. ~F~ The )~] structure [ A U ( Cof~ Fthe ~)~] cation is shown in Scheme 9. Its backbone is a linear chain of five gold atoms, all of which have square planar geometry. Two diauracycle units [(C6F5)Au(CH2PPhzCH2)2Au]are bonded to a Au(C6Fs)z moiety, which lies nearly perpendicular to the two eight membered rings. The Au-Au distances of 2.7%( 1 ) and 2.640( 1) 8, are characteristic of metal-metal bonds, the former corresponding to the unsupported gold-gold bond. In accordance with extended Huckel calculation^[^^*^^^ the pentanuclear chain in 38 can be better described as Au( III)-Au( I)-Au( I)-Au( I)-Au( HI),so the central gold can be regarded as part of one [Au(CgF5)2]- unit which acts as an electron donor to the dinuclear gold cations [(C ~ F ~ ) A L I ( C H ~ P P ~ ~ C HComplexes ~)~AU]+. containing the organoaurates [AuR2]- ( R = C6F5 or 2,4,6-C6F3H2) have been de~ c r i b e d [ ~and ~ , cationic ~ ~ l dinuclear gold complexes have been postulated as intermediates in oxidative addition reactions[lgl (process b, Scheme 3 ) .
412
1 Molecular Clusters
AuRSOEt,
*
Au-Au-Au-Au-
(*)
2.755(1)
2.640(1) Au
Scheme 9
The use of these ideas in a synthetic strategy suggests the reaction of gold complexes able to act as electron donors with diauracycles in a 2 : 1 ratio would be successful. As nucleophiles we selected either the organoaurates NBu4[AuR2] ( R = CsF5 or 2,4,6-CsF3H2) or the diauracycle 1, whose nucleophilic character was well established. As diauracycles, in which the moiety [ R A u ( C H ~ P P ~ ~ C H ~ ) ~ A U ] + was required, we selected the c o m p l e x e ~ [[~R~A~u~( ~C]H ~ P P ~ ~ C Htht)]C104 ~)~AU( OC~O~)] ( R = CsF5 39 or 2,4,6-CsF3H2 40), and [ R A u ( C H ~ P P ~ ~ C H ~ ) ~ A (UR(= CsF5 41, 2,4,6-CgF3H2, 42 or CH3, 43), which had been reported recently. We selected the tetrahydrothiophene or perchlorate derivatives because both ligands are easily displaced in gold chemistry, although they are sufficient to stabilize the precursors. Scheme 10 collects the results of these reactions. Complexes 39 and 41 react with NBu4[Au(CsF5)2] affording 44, pentanuclear gold complexes similar to 38 with C104 as anion. The complex [ { ( C ~ F ~ H ~ ) A U ( C H ~ P P ~ ~ C H ~ ) ~ A U } ~ A U ( C104 45, containing the trifluorophenyl group, can be obtained using the starting trifluophenyl derivatives instead. When the diauracycle 1 is used as the electron donor, hexanuclear complexes [ { R A U ( C H ~ P P ~ ~ C H ~ ) ~ A U } ~ A U ~ ( C H ~ P P (C104)2( R = CsF5 46,2,4,6-CsF3H2 47 or CH3 48) were obtained. The X-ray structure of the trifluorophenyl derivative 47 is rcpresented in Scheme 10. The whole dication consists of three Au(CH2PPhzCH2)2Au diauracycles linked together through two unbridged gold-gold bonds, building up an almost linear six atom gold chain. The two external, symmetry-related, eight-membered rings display a boat conformation and are linked by another similar diauracycle with a chair conformation, in a nearly perpendicular disposition with respect to the others. The gold-gold distances in 38 and 47 are very similar despite the differences in the aryl group bonded to the terminal golds: 2.640( 1) A in 38 and 2.654(1 ) A in 47 in the outermost diaura-
Scheme 10
AU-
(A) 47 2.838(1) 2 737(1) 2.654(1) AUAlAu-Au-AU
R = c6F5 46,2,4,6-CeF3Hz 47 CH3 40
2
NEu~[AuR~]
Ph,
r
R-
R = &Ft, 41. 2.4,6-C6F3H2 42, Me 43
I
R
-
474
I Moleculur Clusters
cycle, and 2.755( 1) in 38 and 2.737 in 47 for the non-bridged metal-metal bonds. In accordance with these data the bonds in penta- Aug9+ and hexa-nuclear Auglof complexes should be explained as a donation of electron density from the central A U ] } + which were gold( 11) gold( I ) centers to two [ R A u ( C H ~ P P ~ ~ C H ~ ) ~fragments, in the starting materials. Extended Huckel calculations[621on complex 47 suggest a better description of it as a Au( 111)-Au( I)-Au( I)-Au( I)-Au( I)-Au( 111) unit, which is very similar to the description of the pentanuclear gold complexes 38 despite the slightly different preparative procedures. This could be interpreted in terms of interactions between the two golds of the external diauracycles after the formation of the chain complexes. In fact, other nucleophilic metal complexes can act as ligands in bis(ylide)gold(11) complexes affording the formation of metal-metal bonds without reduction of the gold precursor. That is the case in the formation[671 of [ A u ~ ( C H ~ P P ~ ~MeSi{MezSiN( C H ~ ) ~ ( p-t01)}3Sn)2] 49 by reaction of the triamidostannate [MeSi{ Me*SiN(p-tol)}3SnLi(Et20)] with [ A u ~ ( C H ~ P P ~ ~ C H ~ ) ~ C ~ ~ Complex 49 consists of a gold(I1) eight-membered diauracycle with a chair conformation, linked to two tripodal tris( amido)tin fragments. This generates a nearly linear Sn-Au-Au-Sn chain linked by covalent Sn-Au (2.680(1) A) and Au-Au (2.749(1) A)bonds.
Acknowledgements We thank the colleagues, postdoctoral researchers, and students who have contributed to the studies described in this article; their names are in the references. We also thank the Direction General de Investigacion Cientifica y Tecnica ( PB95-0 140) for finantial support.
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1.24 Reactivity qf Diauracycles. A Wrry to Prepare Chains of Gold Atoms
475
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416
I Molecular Clusters
[41] ' H NMR: 6 = 7.90 - 7.10 (m, 58 H, Ph), 1.42 (d, 2 J p - ~ = 11.7 Hz. 8H, CH2). 31P{'H} 526.4 g H z ~ J ~ -=K467.2 I ~ ~Hz, ~ PPh3). NMR: 6 = 32.72 (s, PPh2) and 12.26 (dd, 2J p- l~ ~=~~A [42] H. H. Murray, J. P. Facker Jr., L. C. Porter, D. A. Briggs, M. A. Guerra, R. J. Lagow, Inorg. Chem., 1987,26, 357. 1431 L. C. Porter, J. P. Facker Jr., Acta Crystallogr., Sect. C, 1986, 42, 1128. [44] L. C. Porter, J. P. Facker Jr., Acta Crystallogr., Sect. C, 1986, 42, 1646. [45] J. P. Facker Jr., J. D. Basil, Organometallic.s, 1982, I , 871. 1461 R. Uson, A. Laguna, M. Laguna, M. N. Fraile, I. Lazaro, M. C. Gimeno, P. G . Jones, C. Reihs, G. M. Sheldrick, J. Chem. Soc., Dalton Trans., 1990, 333. 1471 H. Schmidbaur, C . Hartmann, G. Reber, G. Miiller, Angew. Chem. Int. Ed. Engl. 1987, 26, 1146. [48] H. Schmidbaur, C. Hartmann, F. E. Wagner, Angew. Chem. Int. Ed. Engl. 1987,26, 1148. [49] J. Vicente, M-T. Chicote, 1. Saura-Llanas, J. Chem. Soc., Dalton Trans., 1990, 1941. [50] E. Cerrada, M. C. Gimeno, J. Jimenez, A. Laguna, M. Laguna, Organometallics, 1994, 13, 1470. [51] 1. J. B. Lin, J. M. Hwang, D. Feng, M. C. Cheng, Y. Wang, Inorg. Chem., 1994, 33, 3467. [52] M. Bardaji, A. Laguna, M. Laguna, J. Chem. Soc., Dalton Trans., 1995, 1255. 1531 R. Uson, A. Laguna, M. Laguna, E. Fernandez, M. D. Villacampa, P. G. Jones, G. M. Sheldrick, J. Chem. Soc., Dalton Truns., 1983, 1679. [54] M. Bardaji, N. G. Connelly, M. C. Gimeno, J. Jimenez, P. G. Jones, A. Laguna, M. Laguna, J . Chem. Soc., Dalton Trans., 1994, 1163. [55] M. Bardaji, N. G. Connelly, M. C. Gimeno, P. G. Jones, A. Laguna, M. Laguna, J. Chem. Soc., Dalton Trans., 1995, 2245. 1561 M. Bardaji, P. G. Jones, A. Laguna, M. Laguna, Orgunometallics, 1995, 14, 1310. [57] M. Bardaji, A. Laguna, M. Laguna, J. Organomet. Chem., 1995, 496, 245. [58] E. Cerrada, A. Laguna, M. Laguna, P. G. Jones, J. Chem. Soc., Dalton Trans., 1994, 1325. [59] M. Bardaji, E. Cerrada, P. G. Jones, A. Laguna, M. Laguna, J. Cheun. Soc., Dalton Trans., 1997, 2263. [60] J. P. Facker Jr., R. J. Staples, Z. Assefa, J. Chrm. Soc., Chem. Comm., 1994, 431. 1611 M. A. Bennett, L. L. Welling, A. C. Willis, Inorg. Chem., 1997, 36, 5670. 1621 L. F. Veiros, M. J. Calhorda, J. Organomet. Chem., 1996, 510, 71. [63] R. Uson, A. Laguna, M. Laguna, P. G. Jones, G. M. Sheldrick J. Chem. Soc., Chem. Comm., 1981, 1097. [64] R. Uson, A. Laguna, M. Laguna, B. R. Manzano, P. G. Jones, G. M. Sheldrick J. Chem. Soc., Dalton Trans., 1984, 285. 1651 A. Laguna, M. Laguna, J. Jimenez, F. J. Lahoz, E. Olmos, J. Organomet. Chem., 1992, 435, 71. 1661 A. Laguna, M. Laguna, J. Jimenez, F. J. Lahoz, E. Olmos, Organometallics, 1994, 13, 253. [67] B. Findeis, M. Contel, L. H. Gade, M. Laguna, M. C . Gimeno, I. J. Scowen, M. McPartlin, Inorg. Chem., 1997, 36, 2386.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.25 Aurophilicity at Chalcogenide Centers. Synthesis of Polynuclear Chalcogenido-centered Complexes with Gold-Gold Interactions Olga Crespo, M. Concepcibn Girneno and Antonio Laguna
1.25.1 Introduction The last few years have witnessed the development of a fascinating chemistry where ( phosphino)gold fragments coordinate around a central heteroatom."] The species formed are exciting not only from the experimental and structural but also from the theoretical standpoint. Thus, interesting hypercoordinated species[2-61of the type [ C ( A U P R ~ ) ~[C(AUPR?)&~ ]+, [ N ( A u P R ~ ) ~, [P(AuPR3)jI2+, ]~' and [P(AuPR3)6l3+ have been described, apart from other complexes with more conventional stoichiometry, and all have in common the presence of short gold-gold interactions of ca. 3 A.Usually, the chemistry of the first row elements of the p-block is known to follow classical rules of bonding. and only in cases of extreme electron-deficiency has the traditional electron count to be reconsidered to account for special types of molecular or solid state structures. Many of the heteroatom-centered complexes are electron deficient and the gold-gold interactions provide a significant contribution to their stability. However, although the chemistry of the carbon, nitrogen. phosphorus or arsenic centered complexes have developed rapidly, the corresponding chalcogen-centered derivatives is still growing. Similarly in these complexes the type of structure adopted is greatly influenced by the existence of gold-gold interactions, for this reason a great amount of work has dealt with the origin of this gold-gold attraction, called aurophilic attraction. It was not until the advent of routine structure determination by single-crystal X-ray diffraction, that a growing wealth of detailed structural information for gold complexes became available. Not only new oxidation states and coordination numbers were discovered for gold, but novel structural phenomena came to light, mainly regarded unexpected and unprecedented gold( I )-gold(I ) contacts absent with other elements and lacking in theoretical description, because closed-shell metal cations would normally be expected to repel one another. By the end of 1980s there was sufficient crystallographic evidence of attractions between gold( I ) cations
478
1 Molecular Clusters
to lead Schmidbaur to coin the expression “aurophilic attraction” or “aurophilicity”. The collection of these new observations coincided with a revised interest in gold on the part of theoretical chemist and the physicist in an attempt to reinterpret the chemistry of the heaviest elements in the Periodic Table on the basis of relativistic effects. These effects are especially important in the case of those elements with extremely high nuclear charges, since they modify significantly the properties of their valence electrons. In the case of g0ld,[’3~]according to its position in the Periodic Table, the relativistic effects are at maximum, and it was natural to inquire as to whether the aurophilic attraction is also a relativistic effect. Many theoretical studies have been carried out on several complexes, e.y. Gorling et al. have studied[’] the main-group-element-centered octahedral complexes [X(AuPH,),]”+ ( X = B, n = 1; C, n = 2; N, n = 3 ) and the conclusion is that the A u . . .Au bonding is achieved through 6s/5d hybridization. This is a consequence of the relativistic modification of the gold valence atomic orbital energies, which brings the 5d and 6s orbitals into close energetic proximity. However, Pyykko et al. have presented theoretical evidence[”- 12] that these attractions are pure correlation effects strengthened by relativistic effects. In a later study of octahedral gold complexes, Haberlein et al. concluded that some treatment of correlation effects is needed and that relativity and the interaction of gold with the phosphine ligands combine to produce an effective gold electronic configuration closer to 5d9 than 5dI0.“ 31 Kaltsoyannis in his review on relativistic effects in inorganic and organometallic chemistry[’41has concluded that the origin of the aurophilic attraction has not yet been uniquely and unambiguously established. It is not clear if this is due to intrinsic differences in the Au. . .Au interactions in the different molecules under investigation, or to philosophically different theoretical methods used to study them. Here we present the advances made in the area of chalcogen-centered derivatives. Sulfur is the element which has been studied in more detail but it is worth mentioning that the species [O(AuPR3)3]+has found the greatest synthetic applicability, not only in other element-centered complexes but in many organometallic derivatives. We will study each element separately.
1.25.2 Oxygen-centered complexes The chemistry of oxygen-centered complexes has been limited until very recently to the tri(go1d)oxonium cations [O(AuPR3)3]+, in spite of the fact that the salts of [O(AuPPh3)3]+were the first structurally reported examples of this interesting type of derivatives.[’51Although the AuL’ moiety is a “soft” Lewis acid and does not usually combine well with oxygen containing anions, Nesmeyanov et al. found that tris(triphenylphosphinego1d)oxonium salts could be easily prepared and that they were highly reactive. These salts were obtained in high yield by reaction of water
1.25 Aurophilicitj, (it Chulcogenide Centers [Aux ( pPh3)I
Scheme 1
i, ii or iii
479
> [C(AUPPh
0 AgBF4, OH- (X = Cl), I/) H20, HBF4 (X = CH3COO); H20, CHC13 + NaBF4 or KMn04 (X = CI, Br)
(X = CFBCOO), iii) Ag20
with coordinatively unsaturated AuPPh3+ cations in an alkaline or acid media. Another convenient method of synthesis, with even higher yield, was the treatment of [AuCl(PPh3)] with silver( I ) oxide in the presence of NaBFq (Scheme 1). These methods for preparing the oxonium salts have been used, sometimes with slight modifications, to synthesize this type of complex with a wide range of phosphine ligands," 6-201 e.g. PR3 = PPh3, PMePh2, PMezPh, PEtPh2, P'PrPh2, P(p-C1Ph)3, P(o-Tol)3, P(0Et)Phz.P(OMe)3; PEt3; P'Bu3, PMe3, P'Pr3, P(NMe2)3. Recently a new preparative way has been reportedr2'] for PR3 = PPh3 or P( p-MeOPh)3 that consists of the reaction of the acetylacetonate complexes, [Au(acac)PR3],with the ammonium salts (NH2R'z)OTf ( R ' = Et, Ph) followed by hydrolysis, although the yield is very low compared with the other methods. The tendency of the AuPR3+ cations to form oxonium species in the reactions that would be expected to lead to the hydrate, [Au(OH2)(PR3)]+, hydroxide [Au(OH)(PR3)] or oxide [O(AuPR3)2] suggest the existence of structural factors enhancing the stability of the [O(AuPR3)3]+ion compared to the above complexes, which are not known in the monomeric state. Such a factor could easily be the existence of inter- and intramolecular interactions of gold atoms. The solid state structure of these complexes depends on the size of the phosphine ligand used. Thus, for example, for more sterically demanding phosphines such as P(o-To1)3,["] PiPr3[l9]or P( NMe,)3[201the structure consists of a trigonal OAu3 pyramid (1) with the oxygen atom in the apical position. The coordination geometry of the gold atoms is nearly linear and there are intramolecular Au. . .Au contacts with an average value of 3.086, 3.198 or 3.176 A, respectively. For phosphine ligands such as PPh3,'' 5 1 PMePh2[16]or P( p-MeOPh)3[211 the structure also is based on trigonal pyramidal [O(AuPR3)3]+cations, which undergo intermolecular aggregation to form centrosymmetric dimers (2).The intermolecular interactions between the cations cause the gold atoms to form a rectangular Auq sub-unit, with the six gold atoms of the dimer forming a six-membered ring with a chair conformation. For the smallest possible tertiary phosphine PMe3'' the monomeric unit, which is also an OAu3 pyramid, are aggregated through shared edges to give a tetrahedron A u sub-unit ~ (3). A characteristic that all the structurally determined [O(AuPR3)3]+cations have in common is that the geometry of the monomeric unit shows only small distortions with the size of the phosphine. Some studies in solution have been carried out on these trigold oxonium cations," 6 ] and the complexes show rapid exchange processes with chloro complexes
480
I Moleculur Clusters
l+
L
1
2
3
leading to equilibrium mixtures (Scheme 2). The first step could be the dissociation of C1- from [AuCI(PR3)] to produce AuPR3+, then exchange could occur through an [O(AuPR3)4I2+intermediate. The exchange processes appear to slowed down with increasing steric size, suggesting associative pathways. The dimerization of tri(go1d)oxonium cations has been studied theoretically[221by means of the all-electron scalar relativistic linear combination of Gaussian-type orbitals density functional ( LCGTO-DF) theory, examining the model compounds AuPH3)3]+. For [OAu3]+ the tetrahedral dimerization was calcu[OAu3]+ and [0( lated to be slightly favorable, while the addition of phosphine ligands leads to a preference for a rectangular dimer structure. Thus, the experimentally observed rectangular dimer structure of [0(A~PPh3)3]2~+ is claimed to be determined by steric intermonomer repulsion, and for [O(AuPMe3)3]22+ which dimerizes tetrahedrally, crystal packing effects should play a decisive role because the intermonomer ligand repulsion differs only in a minor way between the rectangular and the tetrahedral structures. Recently the hyper-aurated oxygen-centered complexes [O(AuPR3)4]*+(4)have been reported by Schmidbaur’s and have been synthesized in high yield by reaction of the tri(go1d)oxonium salts with the aurating agent [Au(BF4)(PR3)] ( R = Ph, o-Tol). The complex with R = o-To1 was structurally characterized and the central oxygen atom has an imposed, by crystal symmetry, tetrahedral geometry coordinated by four gold atoms. The gold-gold distances (between the edges of the O A q tetrahedron) are 3.3590(4) A which, although longer than those found in re-
I L
Scheme 2
1.25 Aurophilicity at Cliulcoyenido Centers
4
48 1
5
lated complexes with a square pyramidal geometry, are still much shorter than the sum of the van der Waals radii for Au( 1). Then the unprecedented stability of the [O(AuPR3)4I2’ unit has been attributed to significant bonding between the closedshell gold cations. Heteronuclear p4-oxide derivatives, [{O(AuPPh3)2{Rh(dien)}}l]( BF4)2 (5) (dien = I ,5-cyclooctadiene, or norbornadiene, have been prepared by reaction of [O(AuPPh3)3)BF4 with [ R h ( , ~ - C l) ( d ie n ) ] 2 . The [ ~ ~ ’ complex with dien = norbornadiene (nbd) has been structurally characterized and shows an oxygen atom with an unusual trigonal pyramidal geometry. The dication consists of a planar [(nbd)Rh(p-O)Rh(nbd)]array in which two AuPPh3+ groups are coordinated to each oxygen atom. The Au-Au distance, 3.008(1 j A,is typical of this type of complexes, whereas the Rh-Au distances (3.020(1 ) and 2.984( 1) A)are similar to those observed in compounds where long Au-Rh bonds are found.
1.25.3 Sulfur-centered complexes Complexes of the type [S(AuPEtl)ZI or [ S ( A U P P ~ ~ ) Iwere ] C I synthesized by Kowala and Swan‘251a long time ago by reaction of [AuBriPEt3)]with Na2S or by treatment of [AuCI(PPhl)] with H2S in ethanolic pyridine, respectively. However the species [S(AuPPhl)l]was claimed by the authors to be unstable because it could not be obtained by reaction with Na2S or to be an intermediate in the synthesis of [S(AuPPh3)3]Cl.Other reports deal with the preparation[26p2s1 and crystal struct u r e r 2 ~ - 3 1 1 of the doubly or triply bridging sulfido ligand complexes, however, these were not well established procedures and in some cases the sulfur-centered gold complexes were obtained as by-products. With the resurgence of the chemistry of sulfur-centered derivatives easier methods have been developed for the synthesis of the doubly and triply bridging species and hyper-aurated compounds have also emerged.
482
I Moleculur Clusters
1.25.3.1 Double bridging sulfido ligands We have developed a high yield synthesis for the species [S(AuPR3)2]that involves the reaction of [AuCl(PR3)] with Li2S in ethanol; the desired products precipitate in ethanol and are obtained in almost quantitative yield.[321These complexes are excellent starting materials for the synthesis of highly aurated homoleptic sulfurcentered derivatives. Following the symmetry rules applied to the frontier orbitals, the proton, H-, carbocations R+ and AuPR3+ fragments can be classified as isolobal; then the molecules SH2, SR2 and [S(AuPR3)2]should be isolobal. However, their behavior is very different because no gold complexes with the SH2 are known; usually the SR2 molecules are used as weakly coordinating ligands and are easily displaced from gold in substitution reactions, whereas the sulfur atom in [S(AuPR3)2]acts as a good Lewis base. This is a common feature in this type of complexes as, for example, the dications OH42+,NH52+ and CH62+ are unknown and predicted to be intrinsically unstable, but the corresponding gold complexes have been isolated. Other bridging sulfido gold species have been obtained with diphosphines such (p-P-P) as dppf ( 1,l '-bis(diphenylphosphin~)ferrocene)[~ 31 or C6H4(CH2PPh2)2 ( 1,4-bis[(diphenylpho~phino)methyl]benzene)[~~~ by reaction of [Au2C12(p-P-P)] with Li2S or NazS, respectively. The structure of these complexes consists of two AuPR3+ or one Au2(P-P)z2+ unit bridged by a sulfido ligand. The main features are short gold-gold distances and an acute Au-S-Au angle. However the differences in the values depending on the ligand are remarkable. For PPh3 and C6H4(CH2PPh2)2 are very similar, with Au-Au distances of 3.018(1) and 3.146(1) A and Au-S-Au angles of 88.7( 1) and 86.7( l)', respectively, whereas for dppf the Au-Au distance is considerably shorter, 2.882( 1) A,and has a narrower Au-S-Au angle, 77.57(9)".
1.25.3.2 Triply bridging sulfido ligands For the synthesis of the tri(go1d)sulfonium cations easier methods have also been reported (Scheme 3).
Scheme 3
1.25 Aurophilicity ut Cliulcoyenide Centers
483
Homoleptic species with different phosphine ligands such as PPh3, PMe3, P'Pr3 and PMePhl have been prepared in high yield['7.32.351 through one of these methods. Recently the complex [S{Au(mbpa))3]Cl[mbpa = methyl 4,6-O-benzilydene-3deoxy-3-(diphenylphosphino)-cc-D-altropyranoside] has been synthesized[361in low yield from [AuCl(mbpa)]and L-cysteine. The crystal structures of the homoleptic species resemble those of the tri(go1d)oxonium cations. Furthermore, it becomes more and more obvious that steric and electronic effects play a decisive role in the strength of the Au. . .Au interaction and in the supramolecular aggregation of the sulfur-centered cations. Again the basic structural framework consists of a trigonal pyramidal SAu3 with the sulfur atom in the apical position and short gold-gold contacts in the base of the pyramid. However, the size of the phosphine influences the further aggregation of the molecules. For the bulky tris(isopropy1:iphosphine or mbpa the structure is monomeric and the asymmetric unit contains 1/3 of the cations because thcy have threefold crystallographic symmetry. The Au-Au distances are almost identical in both cations, 3.253(1) and 3.251(1) A,and also the Au-S-Au angles are close to 90°, which are indicative of steric repulsion. There is, however, a difference for the complex with the phosphine mbpa because each chloride anion links to three [S{Au(mbpa)}3]+cations through hydrogen bonds between the O H group of the altropyranose ring to form an infinite two-dimensional net structure. The structure for phosphine ligands such as PPh3 and PMePhl are dimers related by a center of symmetry and with little differences between the intra- and inter-molecular goldgold distances within the dimeric unit. However, the nature of the anion may also play a significant role because the cation [S(AuPPh3)3]+is a monomer as the tetrafluoroborate salt, but a dimer with the hexafluorophosphate counterion. The complex [S(AuPMe3)3]BF4 crystallizes with two independent formula units (A and B), which are both forming pairs (dimers A? and B2) through a crystallographic center of inversion. These dimers are aggregated further into strings of dimers (-A-A-B-B-) with short gold-gold contacts (A-B) as links (6). Pseudopotential ab initio calculations on SAY' and S(AuPH3)3- have been carried and they reproduce closely the experimental Au-S-Au angles found in the monomeric [S(AuPR3)3]+ cations (PR3 = PPh3, P'Pr3) if both correlation and relativistic effects are included.
12+
6
484
I Molecular Clusters
i) [Au(C~Fg)(tht)], ii) 1/2 [Au(tht)2]ClOq,hi) [Au(CH2PPh3)(tht)]Cl04
or [Au(OC103)(PR3)],iv) 1/2 [Au2(0Tf)2(p-dppf)]
Scheme 4
Another homoleptic compound with a triply bridging sulfido ligand is ( N E ~ ~ ) ~ [ S { A U ( C ~ F obtained ~ ) } , ] , by bubbling H2S through a solution of
N E ~ ~ [ A U ( C ~ F S )The C ~ ]structure . [ ~ ~ ] corresponds to a distorted trigonal pyramidal geometry for the sulfur atom. There are dissimilar Au-Au distances and Au-S-Au angles; two of them are close to those found in other complexes with a p,-S ligand whereas the other ones, 3.4772(9) A and 97.65(13)", lie outside of the normal ranges. The complex [S(Au2dppf)] serves as a building block for preparing polynuclear sulfur-centered complexes.[331The reactions carried out are summarized in Scheme 4. The only compound structurally characterized is [ { S(Au2dppf ) } 2 (puAu2dppf)](OTf)2 and has a S2Aug core with short intramolecular gold-gold interactions, which vary from 2.905(2) to 3.272(2) A. It is worth mentioning that the 31P{'H} NMR spectrum at -55 "C exhibits three singlets arising from three different phosphorus environments (in each S A U ~ P unit) ~ instead of only the two types of phosphorus expected and this comes from the difference strength of the gold-gold contacts, thus, these interactions can be detected in solution at low temperature. The crystal structure of [ { S(AuPPh3)2}2Au][SnMe3C12]has
1.25 Auuophilicity ut C/iulcoqmide Centers
485
been reported["' and the atomic arrangement is similar to that drawn for [ { S(Au2dppf )}2Au]C104 with short gold-gold distances between the central gold atom and those of each S(AuL)z unit. Mixed valence gold( 1)-gold(I l l ) derivatives (7 or 8) have been preparedr391 ) ]U ( C ~ F ~ ) ~ ( O E ~ ~ ) ~ ] O T ~ . by treatment of [S(Au?dppf)]with [ A u ( C ~ F S ) I ( O Eor~ ~[ A The crystal structures show short gold( 1)-gold(111) contacts of 3.404(1) A in [S(Auzdppf){Au(CgF5)7)]and 3.2195(8) and 3.3661(10) A in [{S(Auzdppf)}2{Au(ChF5)2}]0Tf. DFT calculations, including correlation and relativistic corrections, show that the Au( I)-Au( 111) distances are indicative of weak interactions. Extended Huckel calculations suggest that such interaction is similar in origin to the well-studied Au( I)-Au( I ) weak interactions and that it may occur in some real complexes.
7
8
Taking into account the isolobal relationship between the H+, AuPPh3+ and AgPPh?+ the anionic species NBu ~ [SM ( P P ~ ~ ) { A U ( C ~ F( M ~ )= ? }Au, ~ ] Ag) have been prepared by reaction of NBu4[S(H ) {Au(CgF5)3]2]with MPPhl+ cations in the presence of NazC03.[401Both complexes are isostructural and have a trigonal pyramidal geometry at the sulfur atom, although with no short gold-gold or goldsilver interactions.
1.25.3.3 Quadruply bridging sulfido ligands The discovery that [S(AuPPh3)2]or [S(AuPPh3)3]+ can add additional AuPPh3+ units led to the preparation of the unprecedented tetra(go1d)sulfonium (2+) salts [S(AuPPh3)4I2+(9).["']These cations have a square pyramidal structure, clearly at variance with any of the classical rules of structure and bonding which demand a sulfur-centered tetrahedral complex similar to [O{AuP(o-T01)3}~]~+. There are short metal-metal distances between adjacent Au atoms 2.883(2)-2.938(2) A and longer ones (ca. 3.4 A) between opposite Au atoms in the base of the pyramid. For the salt with the anion OTf the cations are paired across symmetry centers (lo),with short intermolecular S-Au distances; however with the anion C104- the cations are monomeric cations.
486
I Molecular Clusters
\
9
10
PPh 3
There is precedent for this non-classical pyramidal structure in the analogous tetra(go1d)arsonium cations [As(A~PPh3)41+,[~~' which one would expect to be tetrahedral as required for standard arsonium salts AsR4+. The calculations made with ab initio or density functional methods, including relativistic effects, show that the system is lower in energy with close contacts between the gold atoms, which is very easy in a square pyramid and impossible in a tetrahedron encapsulating a large sulfur or arsenic Other quadruply bridging species have been synthesized by treatment of [S(Au2dppf)]and two equivalents of [Au(OC103)(PR3)] (PR3 = PPh3, PMePh2).[331 (11) reveals a new type of The structure of [S(Auzdppf)(AuPMePh2)2](Cl04)2 structural framework, because it can be regarded as a trigonal bipyramid with one of the apical positions occupied by the lone pair of electrons of the sulfur atom and the other by a gold atom. There are intramolecular gold-gold contacts, the shortest being between the equatorial gold atoms and the gold atom perpendicular to them. The intermolecular gold-gold contacts lead to the formation of dimers (12).
11
The mixed-valence gold( 1)-gold(111) derivatives [S(AuPPh3)2{Au(CgF5)3121 (13) and [S(Auzdppf){Au(CgF5)3}2](14) have also been synthesized by reaction of two equivalents of [Au(CgF5)3(OEtz)]with [S(AuPPh3)2]or [S(Au~dppf)] and structurally characterized.[441Now the geometry is distorted tetrahedral, which does not allow short gold-gold interactions, however for [S(Auzdppf){Au(CgF5)3}2]there is a short gold( I)-gold( I ) distance of 2.9561(7) A and the gold( 1)-gold( 111) distances are very dissimilar, at 3.533(1) and 2.915(1) A, which indicates again a certain degree of gold( 1)-gold(111) interaction.
1.25 Aurophilicity ut Chulco~genideCenters
481
14
13
Finally an heteronuclear p4-sulfido complex (15) has been reported, [ { S(AuPPh3)3}zAg]( BF1)3, obtained by treatment of [S(AuPPh3)3]BF4 with AgBF4 (molar ratio 2 : l).[451The structure consists of two SAu3Ag square pyramids but more distorted than in the corresponding SAu4 unit. The SAu3 units are related by symmetry and the silver atom is disordered over two positions in the crystal. There are close contacts between the silver atom and the sulfur and three out of the six gold atoms.
PPh 3 15
1.25.3.4 Quintuply and sextuply bridging sulfido ligands The complex [S(AuPPh3)1]can react with three or four equivalents of [Au(OTf)(PPh3)j to give the penta or hexa(go1d)sulfonium cations [S(AuPPh3)5]'+ (16) and Unfortunately these complexes could not be characterized [S(AuPPh3)6I4+( 17).[321
16
17
488
I Molecular Clusters
by X-ray crystallography and only analytical, conductivity and NMR data are available. They are highly charged and unstable in solution where they give metallic gold and lower nuclearity products. However, we believe that these species exist in solution and their solid state structure might be similar to those reported for other main-group element-centered complexes.
1.25.4 Selenium-centered complexes At the present time the chemistry of gold-selenonium cations has scarcely been developed. We are currently working in this area and most of the results we present here have not been published yet. The only previously reported selenium-centered PF6,[301 S~FG[~~'). complexes are [Se(AuPPh3)~1[~~] and [Se(AuPPh3)3IX ( X = Cl,[271
1.25.4.1 Double bridging selenido ligands The complex [Se(AuPPh3)2] has been obtained in a two step reaction from [AuC1(PPh3)] and selenourea followed by treatment with aqueous Na2C03 (Scheme 5 ) . This derivative is isostructural with the analogous sulfur complex and has a Au-Au distance of 3.051(1) A and an Au-Se-Au angle of 79.1(1)". By a similar procedure we have prepared the compound [Se(Auzdppf )] for which the crystal structure has also been determinedr471and shows a gold-gold bonding interaction of 2.9353(8) A and a very narrow Au-Se-Au, 74.90(3)".
1.25.4.2 Triply bridging selenido ligands The methods used to synthesize [Se(AuPPh3)3]+ are very similar to those described for the analogous sulfur complex (Scheme 6). Similarly, the structure consists of SeAu3 pyramids and there are two independent molecules in the asymmetric unit that are also paired across a center of inversion. The intra- and inter-molecular
--,u(PPh3){SeC(NH&}]CI
-W
MeOH Scheme 5
1.25 Aurophikity iit Chakogenide Centers [Au(SbF6)(PPh3)] + [Se(AuPPh3)2]
[AuCI(PPh3)]
+
489
\
Se(SiMe&
w
[Se(AuPPh3)3]+
+ Se(SnMe3)~ [Au(PFG)(PP~~)] Scheme 6
gold-gold distances are of the same order as those found in the related oxonium or sulfonium cations. We have studied the reactivity of [Se(Auzdppf )] towards other metallic fragments. The reactions that lead to several complexes with a p,-selenide ligand are shown in Scheme 7. The mixed-valence gold( I)-gold(111) derivative [Se(Au?dppf){Au(CgF5)3}]has been structurally characterized and the overall features resemble those seen for sulfur. Again there is one Au( I)-Au( 111) distance much shorter than the other one, indicating that gold( I)-gold(111) interactions are possible.
l+
Scheme 7
490
I Moleculur Clusters
1.25.4.3 Quadruply bridging selenido ligands Complexes with a p4-selenido ligand have also been obtained, although structural characterization by X-ray diffraction studies have not yet been carried out. The homoleptic species [Se(Au2dppf )2](OTf)2 (18) and [Se(AuPPh3)4](OTf)2 have been prepared and the NMR data are consistent with their formulation. It has been observed in this type of complexes that coordination of a further AuPPh3+ group to a main-group element is concommitant with a highfield displacement of the 31Psignal and this can be unambiguously observed in the latter complexes. Since the selenium atom is similar in size to arsenic we think that the most probable geometry will be the square pyramidal as found in the p4-As or p4-S derivatives. The mixed gold( I jgold( 111j species [Se(Auzdppf){Au(C6F5)3}2] (19) and [Se(AuPPh3)2{Au(CsFs)3>2] are easily obtained from [Se(Auzdppfj] or [Se(AuPPh3)2] and two equivalents of [ Au(ChF5)3(0Et?)]and we suppose they may have the tetrahedral geometry found for the sulfur derivatives.
r-
18
19
1.25.5 Tellurium-centered complexes The preparative methods to access to poly(go1d)telluronium salts are similar to those of the corresponding sulfonium and selenonium salts. The reaction of [O(AuPPh3)]BF4 with (‘BuMe2Si)zTe in dichloromethane at -78 “ C leads to the trinuclear complex [Te(AuPPh3)3]BF4, however only in 33%0 yield.[481With the ligand PMe3 only decomposition products were obtained, so it seems that the tellurium complexes are more unstable than the analogous oxygen, sulfur or selenium derivatives. The structure of [Te(AuPPh3)3]BF4consists of two independent cations in the asymmetric unit, each one with a trigonal pyramidal geometry and associated in centrosymmetric dimers through short intercationic contacts. The intramolecular Au-Au distances are very dissimilar and range from 3.074(1) to 3.515(1) A, which indicates a more distorted geometry for this salt compared with the oxonium, sulfonium, or selenonium salts. The reaction of (‘BuMe2Si)zTe with four equivalents
1.25 Aurophilicity ut Ckdcoyenide Centers
49 1
of [Au(BF4)(PPh?)] gives [Te(AuPPh3)4](BF4)2, which is also unstable and could not be crystallized. The authors also propose a square pyramidal geometry for this data.
References [ I ] H. Schmidbaur. Gold Bull. 1990, 23, 11 -21; Interdis. Sci. Rer. 1992, 17. 213-220; Pure & App/. C/zrm. 1993, 65. 691 698; Clieni. SOCRev. 1995, 391-400. [2] F. Scherbaum. A. Grohmann. G. Muller, H. Schmidbaur. An
492
1 Moleculur Clusters F. Canales, M. C. Gimeno, A. Laguna, P. G. Jones, J. Am. Chem. Soc. 1996,118,4839-4845. S. Hofreiter, M. Paul, H. Schmidbaur, Chem. Ber. 1995, 128, 901-905. K. Angermaier, H. Schmidbaur, Chenz. Ber. 1994, 127, 2387-2391. J. C. Shi, B. S. Kang, T. C. W. Mak, J. Chem. Soc., Dulton Trans. 1997, 2171-2175. P. Pyykko, K. Angermaier, B. Assmann, H. Schmidbaur, J. Chem. Soc., Chem. Commun. 1995, 1889-1890. J. Vicente, M. T. Chicote, P. Gonzilez-Herrero, C. Griinwald, Oryunometallics 1997, 16, 3381-3387. M. J. Calhorda, F. Canales, M. C. Gimeno, J. Jimenez, P. G. Jones, A. Laguna, L. F. Veiros, Organometullics 1997, 17, 383773844. F. Canales, S. Canales, 0. Crespo, M. C. Gimeno, P. G. Jones, A. Laguna, OrganometuNics, 1988, 17, 1617-1621. F. Canales, M. C. Gimeno, P. G. Jones, A. Laguna, Anyew. Chem. Int. Ed. Engl. 1994, 33, 169-710. E. Zeller, H. Beruda, A. Kolb, P. Bissinger, J. Riede, H. Schmidbaur, Nature 1991, 352, 141143. J. Li, P. Pyykko, Inory. Chem. 1993,32, 2630-2634. F. Canales, M. C. Gimeno, A. Laguna, P. G. Jones, Organometallics 1996, 15, 3412-341 5. A. Sladek, H. Schmidbaur, Z. Nuturjorsch. 1997,52b, 301-303. P. G. Jones, C. Thone, Chem. Ber. 1991, 124,2125-2129. S. Canales, 0. Crespo, M. C. Gimeno, P. G. Jones, A. Laguna, unpublished results. K. Angermaier, H. Schmidbaur, Z. Nuturforsch. 1996, Sib, 879-882.
Metal Clusters in Chemistry Volume 1: Molecular Metal Clusters Edited by P. Braunstein, L. A. Or0 & P. R. Raithby Copyright OWILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
1.26 Au( I). - -Au(I) and Au( I). - -Ag(I) Loose Clusters Jose Vicente, Maricc- Teresa Chicote, Isabel Saura- Llamas, Maria-Cristina Lagunas, M. Curmen Ramirez de Arellano, Pablo Gonzulez-Hevrero, Maria-Dolores Abrisyueta, and Rita Guerrero
1.26.1 Introduction Short A u ( I ) . . . A u ( I )contacts (2.7 to 3.5 A)occur in many gold(1) complexes." 'I This tendency to form aggregates was termed aurophilicity by Schmidbaur.r61Although the nature of the aurophilic interaction has not yet been clearly established, it has been suggested that it is caused by correlation effects, strengthened by relativistic and estimated to be of similar energy to hydrogen bonds (ca. 7 kcal"olF').['' As the latter are important for determining protein structure and function, it would be interesting to consider if these aurophilic interactions can play a similar role, under biological conditions, to that of hydrogen bonds and thus be important to understand the therapeutic action of some gold( I ) complexes against arthritis. Additionally, these interactions are partially responsible for interesting photophysical properties of many of these compounds,"' 12] and for the stability of Schmidbaur's hypercoordinated compounds [(AuL)6(p6-C)I2+, [ (AuL)j( p5-C)]+, I ( A ~ L (p5-N )s )I2+, I(AuL)6(p6-N)I '-, [(AuL)s( ~ 5 - pI*+,) I(AUL)~(P~-CR)I', [(AuLh(p4-CR2)]+,[ ( A u L ) ~ ( ~ ~ - P Rand ) J +[,( A U L ) ~ ( ~ ~ - B P(RR~=) H, ] + alkyl. aryl; L = tertiary phosphine).f'.'3,141We have recently reported the first functionalized member of the carbon family: [ ( A U P P ~ ~ ) ~ ( , U ~ - C S ( O ) 'IM ~ ~ ) ~ ? + . [ ' These weak bonds are different from the typical metal-metal bonds which gold( I ) forms with other metal centers (1, Scheme ]).[I6]with other gold atoms in conventional low valent gold clusters['71(2, Scheme 1 ) or in gold( 11) complexes (3, Scheme l).[l8] For this reason these compounds are known as loose Similar interactions between other d'' ions [Au(I)/Ag(I),f1y-2s1Au( I)/Cu(I),[261 A g ( l ) / C ~ ( l ) , ' ~Ag(I)/Ag(I),[2'1 '] C U ( I ) / C U ( I ) ] [ 2~9*a " and Cu( I)/Hg(II)[29d1are rare. We have proposed the term numi~miphilirity(from the Latin numisma = money, coin) to account for this common feature of the three coinage metals.["' We will also refer to some compounds with short Au( I ) . . .Ag(I ) contacts.
494
1 Molecubr Clusters L Ph
\ /
(OC),Mn-AUPPh,
'
Me02CHC
1
[ LiK
Ph
]*+
I Cl-AuMeOZCHC, I
P 'CHC02Me
I AU-CI ,CHC02Me I
L
X
4
L
\
5
/p\
LAA U~ '-L 2
Ph Ph 3
L
Scheme 1
Loose clusters can be classified as assisted or unassisted clusters depending on the existence of a bridging ligand between the aurophilically connected gold atoms (4 and 5 , Scheme I). We will classify as mixed clusters those compounds in which both assisted and unassisted aurophilic interactions coexist (6, Scheme 1). Furthermore, we will refer mainly to complexes, with three or more gold atoms although we will also allude to some dinuclear gold( I ) complexes, which are important to show the significance of the aurophilicity, as this phenomenon does not depend on the number of gold atoms involved.
1.26.2 Assisted loose clusters Typical examples of this family of compounds are the hypercoordinated complexes mentioned above and triaurated oxonium (4,Scheme 1; n = + I , E = 0, L = PR3, R = ~ - t o l y l , [ Pr1[311), ~~] sulfonium (4,Scheme 1; n = $1, E = S , L = PR3, R = Ph,[321 ammonium (4, Scheme 1; n = +1, E = C ~ H I ~ N H ( C H ~ ) ~ N , [ ~ ~ I Bu'N, CsHllN,1351L = PPh3) and hydrazonium (4, Scheme 1; n = +1, E = Ph2N2, L = PPh3)[361complexes. We have recently reported the synthesis of the first organometallic sulfide complex (4,Scheme 1; n = -2, E = S , L = C6F5).[371When less bulky ligands are used dimers are formed through Au( I). . .Au(I ) contacts (see below under Mixed Loose Clusters). Recently, tetra-aurated oxonium (7, Scheme 2; n = 2, E = 0, L = PPh?)["], sulfonium (8, Scheme 2; n = 2, E = S , L = PPh?)["' and arsonium (8, Scheme 2; n = I , E = As, L = PPhj)[401salts have been reported. The oxonium salt is tetrahedral, like the homologous compounds where E = C (7, Scheme 2; n = 0, E = C, L = P C Y ~ ) [or ~ ' E] = N (7, Scheme 2; n = 1 , E = N, L = PPhl).[421The others have square-pyramidal structures, probably because the
1.26 Ail( I ) . . .Au( I ) und A u j l ) .. , A g ( I ) Loose Clusters
8
7
495
10
9
Scheme 2
small oxygen atom accommodates the six possible Au( I ). . .Au(I ) contacts in the tetrahedral environment of the AuPR3 groups, while the larger sulfur or arsenic atoms would accommodate fewer contacts even if a distortion of this geometry were allowed. With a square-pyramidal structure, complexes 8 exhibit four short Au( I ) . . .Au(I ) contacts. Similar reasoning could be used to explain the trigonalbipyramidal structure of [(AuL)5(p5-E)]”~ when E = N (9, Scheme 2)[431or the square-pyramidal arrangement when E = P (10, Scheme 2).[441 Some dithiolatodigold( I ) complexes can coordinate one or two additional AuL’ groups giving tri- (11: Scheme 3, L = PPh3)[4s1or even tetra-aurated complexes or [ A u ~ ( ~ ? - C ~ S ~ ) ( , L ~ ~ -The P P complex ~~CH~PP~~)]. like 12 (Scheme 3)[4h [ A u {~Ag2 { S(CH?)?S)? j :I2- (13, Scheme 3) is a beautiful example of numismophilic Ag. . .Ag and Ag. . .Au interactions.[”’ The crystal structures of complexes [Au4(p2S2CMe)4],[”] [Auq {p2-(NPh)?N)4],[s21 (14, Scheme 4) each show a square of gold atoms bridged by the ligands while in [AuqC12{( , U , - P ~ ~ P C H ? ) ~ A Sthe P ~ four }~]~+ gold atoms form a chain (15, Scheme 4).[531A triangle of gold atoms is formed in [AunC1(p3-(PPh2)3CH)2J2’ (16, Scheme 4)Is4]or in [Au3I31p3-(PPh2)3CH)],[”] while the three gold atoms in [ A u ~ C(p3-PPh(CH2PPh2)2}2]+, ~Z [Au3C12(p2121- form a chain.[”’ PPh2CH2PPh2)2]+[ss3s61 and [Au(AuPEt3)2{p2-S2C=C(CN)2 or Finally, the pentanuclear complexes [ A~~(quinoline-2-thiolate)~(dppm)2]~+ [Au(AuPPh3)q(p3-NBu‘)~lt have a chain of four gold atoms with the fifth bridging two of them (17. Scheme 4)ISs1or, respectively, two Au3 triangles sharing a gold
c
11
Scheme 3
12
13
496
1 Moleculur Clusters
Scheme 4
atom.[591 Although the presence of a bridging ligand could raise some doubts about whether the aurophilic interaction is simply a consequence of the architecture of the bridge, the structures of some compounds show the importance of this interaction. Thus, the ylidediphosphine MesP=C( PPh2)Z (Scheme 5 ) has a syn-anti arrangement of the lone pairs (minimizing their mutual repulsion) both in solution and in the crystalline state. However, the complex [(AuC1)2( p 2 - (PPh2)2C=PMe3}]displays a syn-syn orientation; the Au( I). . .Au(I ) interaction acting as a conformationdetermining force (Scheme 5).['] We have found similar behavior in complex 18 (Scheme 6) with respect to its derivatives, the [ A u ~ ] and ~ + [Au2Ag13' loose clusters 19 and 20. The crystal structure of the precursor 18 shows a short Au. . .Au contact and the pyridine nitrogen lone pair in the anti position with respect to the [Au~]*+ moiety. When 18 reacts with [AuCl(PPh3)] and T103SCF3 or with AgN03, the [AuPPh3]+ or [Ag(02NO)(OC103)]-moieties coordinate to nitrogen while the pyridine ring rotates to allow triangular Au( I). . .Au(I). . .Au( I ) or Au( I ) . . .Ag(I). . . Au(1) short contacts (19 and 20, Scheme 6).[L9,203601 Complex 18 is a member of a family of ylide complexes we have synthesized by reacting phosphonium salts with [Au(acac)(PPh3)I or [Au(acac)$ (acac = acetylacetonate) Thus, by reacting [( Ph3PCH2)2C0I2+with [Au(acac)L] (1 : 4), tri and tetranuclear complexes with three or five short Au. . .Au contacts can be obtained depending on the size of the phosphine ligand L (see Scheme 7).[lS1 Using the same synthetic method, but starting from the sulfoxonium salt [Me3SOIf, one of the carbon atoms can be mono- (21) or tri-aurated (22). The hypercoordinated complex 23 can be isolated by reacting 22 with AgC104 and [AuCl(PPh3)] (see Scheme S).[l5I The stepwise metallation of the same methyl group .[18p20960p651
+ 2 [AuCI(CO)]
\ QPPh2
Scheme 5
Ph,P-+uCI
Me,P< \
PhzP-
AuCI
497
1.26 A u ( I ) . .. A u ( I ) und A u j I ) . .Ay( I ) Loose Clusters
Q
Scheme 6
is a remarkable feature of these acid-base reactions, attributable to the aurophilicity. If this tendency were not so important, one might have expected that, after the first AuPPh: group had replaced a hydrogen atom in a particular methyl group, further replacements would occur on non-metallated methyl groups as their hydrogen atoms should be somewhat more acidic according to the weak +I effect of the AuPPh: moiety."8,621The same would be expected on steric grounds. In addition, in the absence of aurophilic interactions, coordination of the fourth AuPPh3 group to the oxygen atom would be expected. All these ylide complexes show short Au( I ) . . .Au(I ) contacts, particularly among those gold atoms attached to the same carbon atom (2.7999(7)-3.175(2) A) and, consequently. narrow Au-C-Au angles (80.2(3)-97.2(6)"), far from the expected tetrahedral sp' angle." 5,19,60,61.63.641 These features, which clearly support the +
0 P
'PPh
\
L = PPh,
Scheme 7
498
1 Molecular Clusters 2+
-I
22
23 L = PPh,
I
)
+ [Au(acac)PPh,] - Hacac; ii) + AgCIO4 + (AuCI(PPh,)] - AgCl
Scheme 8
existence of the aurophilic interaction, are also observed even in complex 18 (Scheme 6) with the bulky L = PCy3 (Cy = C ~ H ~Au(1). I , . .Au(I), 2.9757(9) A; Au-C-Au, 90.4(3)"). To account for these data, we postulated that the bonding in these species could be described as a hybrid between the resonance forms (a), a diaurated phosphonium salt, and (b), a diaurated ylide, in which a three-center twoelectron bond is responsible for the bonding interaction in the CAu2 moiety, and the remaining p(ci-C) orbital forms a pn(ci-C) + dn(P) bond (see Scheme 9). In fact, the P-C bond distances [1.753(11)-1.783(14) A] are intermediate between the standard C(sp3)-PR3bond distance in phosphonium salts [ 1.787(4)[661-1 .893(8) A[631] and that found in the related carbonyl-stabilised ylide Ph3P=C(C6F4CN-4)C02Et [1.722(3) A].[671In an attempt to confirm the multiple bond character of the P-CAu2 bond in such species, we carried out a variable temperature ~ PTo3)R}]C104 'H-NMR spectroscopic study on a series of complexes [ ( A u L ){p-C( [To = CsH4Me-4, L = PPh3, R = C(O)NMeZ, py-2, C(O)Ph, C(O)CsH40Me-4,
+
Au
\I
Scheme 9
t
1.26 Au(I),'.Au(I) u n d A u ( I ) . . , A g ( I )Loose Clusters
499
Scheme 10
C(O)C6H4N02-4, COlMe, CN; L = P(CsH40Me-4)3, PCy3, PMe3, R = py-2; Scheme 91. We found the rotation of the PTo3 around the aC-P bond to be restricted at room or lower temperatures in all these complexes. However, according to the data obtained, the phenomenon can be mainly attributed to steric effects, although the occurence of minor electronic effects cannot be ruled out.["]
1.26.3 Unassisted loose clusters Many neutral gold( I ) complexes form chain-like polymers with short Au( I). . .Au(I ) contacts unsupported by any ligand. This is the case for [AuX(L)] (24, Scheme 10) [X = halide or cyanide, L = isocyanide,[681tetrahydrothio~ h e n e , [ C13P,[io1 ~~] 1,4,7-trithiacycl0nonane,[~~~ P(o-t0lyl)H2,['~]PPh(Me)H,[731 PMei,[741Ph2C=NH].[751 In [AuCl(CO)], an aurophilic square-planar coordination of each gold atom leads to the formation of a puckered plane of gold atoms thereby achieving a pseudo-octahedral coordination.[761 In (AuCl(CNCH2CO2Me)la sheet structure is formed in which each gold atom has long A u ( I ) . . . A u ( I ) contacts (3.430(1)- 3.55311) A) to three others leading to a quasi-hexagonal array of gold atoms (25, Scheme 10).[h81Similarly. [ A u C N ( C N M ~ ) ]shows [ ~ ~ ] a layer structure in which each gold atom has contacts with six neighbors. However, the distances are slightly above the upper limit (3.524(4)-3.724(3) A) expected for Au.. .Au contacts. Some neutral complexes aggregate forming oligomers. Thus, complexes [AuX(L)] [X = halogen, L = PR3,[78-801isocyanide;r681 L = PR3, X = RS,[s1,821RSe;'831 X = PhCz, L = C ~ H , N H Z , [ *etc.185,R61 ~] are dimers while [AuCl(piperidine)] (26, Scheme 10) is a tetramer based on a square of gold atoms.[871Recently, we have reported [Au(C-CSiMe3)(CNBut)](27, Scheme 10) which is also a tetramer but with a central molecule connected to three others by Au( I ) . . .Au(I ) contacts. The repulsion between the bulky SiMei and But groups is avoided by rotation of the
500
1 Molecular Clusters
Scheme 11
peripheral molecules by 121°.[s81In some complexes with bridging ligands interrather than intra-molecular Au( I). . .Au(I ) contacts exist. These interactions give a two-dimensional framework in [(AuC1)4(p4-L)]( L = a tetraphosphine; 28, Scheme 1 1)[lZ1and [(AuC1)2{p2-PhS(CH2)2SPh}],[891 a chain-like polymer in [(AuC1)4{p3-P{(CH2)2PPh2}3][901and [(AuSC6H4Me-4)2{p2-Ph2P(CH2),PPh2}] (n = 4, 5),[911or a dimer in [(AUPP~~)~(,LL~-N=C=N)].[~~] Close Au( I ) . . .Au(I ) contacts are more scarce for charged gold( I ) complexes than for neutral ones. Thus, contrary to a theoretical prediction,[931[( PPh3)2N][Au(SH)2] is not associated.[941The same behavior occurs in other anionic comC-CCH20H,[961 plexes such as, [( PPh3)2N][Au(X)(Y)]with X = Y = P(0)Ph2,[951 [~~] [ A u ( S ~ O ~ , )forms ~]~or X = C-CH, Y = C&(N02)3-2,4,6, C , ~ F S . However, pairs with a Au(I)...Au(I) distance of 3.24 A in spite of its triple negative ~harge.[~~.~~l Cationic complexes seem to be more prone to establish short Au( I ) . . .Au(I ) contacts than anionic ones. Thus, [Au(pyridine-2-thione)2]+ shows a linear penwhile [Au(NH2But)(PMe3)]' tameric structure through Au( I ) . . .Au(I ) contacts,[1001 is dimeric.[861The complex [Au(NH3)2]Br (29, Scheme 11) consists of dimers formed through aurophilic and NH. . .Br hydrogen bonds. These dimers also interact with each other inter-molecularly through additional hydrogen bonds to give a three-dimensional network.["'] The Au( I ) . . .Au(I ) distance (3.4 A) is longer than the Ag(I)...Ag(I) distance in [Ag(NH3)2]N03 (3.1 A) in spite of the recently determined shorter covalent radii of Au( I ) (1.25 A) than A&(I ) (1.33 A).[1o21 The greater aurophilicity of cationic over anionic gold(I ) complexes is also supported by i) our recent observation that [(AuC,F,)3(pu,-S)l2- does not add A u C ~ For~ (n-2)+ ,1103,1041 do AuPPh; units,[371while the cationic complexes [(AuPR3),(pu,-S)] and ii) complexes [AuL~][AuX~] with L = pyridine and X = C1, Br, I are tetrameric and display one cation-cation and two cation-anion interactions: [AuX2]- . . . [AuL# . . .[AuL2]+. . . [ A U X ~ ] - . [ ' A ~ ~similar , ~ ~ ~structure ] is found for X = C1 and L = Ph2C=NH although contacts of 3.6 A are found between the Au( 1)/Au(3)and Au(2)/Au(4)pairs of gold atoms (30, Scheme 1 l).[751 For L = PPhzMe and X = C1 1
1.26 A u ( I ) . . . A u ( I ) unrlAu(I)...Ay(I) Loose Clusters
501
X
33 L
32
0
!t
Scheme 12
and SiPhl,['"71 or L = [NC(CH?)*]?Pand X = CN,['08] Au.. Au contacts give cation-anion dimers. Finally, for X = I and L = tetrahydrothiophene or tetrahydroselenophene, polymers with alternate anions and cations are f ~ r m e d . [ ~ ~ . " ~ ] The complex [Au(CNR)*]+ shows a polymeric meandering string with long A u ( I ) . . . A u ( I )contacts (3.6 A) when R = Me but isolated molecules occur for larger groups ( R = Ph, mesityl).[l'O1The only polymeric complex formed through short cation-cation interactions (3.17 A)is our recently reported [Au(NH=CMeZ)+ (CF3SOj) complex (31. Scheme 12).["1 However. outer sphere coordination of the anion through hydrogen bonds helps to attain polymerization. Therefore, it seems that aurophilic interactions can only compensate for a limited number of equally charged gold( I ) complexes unless an additional interaction occurs. A combination of aurophilic and hydrogen bonding also exists in the complex [Au?C13L4] ( L = pyrrolidine) where the unit [AuCl(L)]. . .[AuL?]+. . .[AuCI(L)] forms a chain through additional [AuCl(L)]. . .[AuCl(L)] interactions. In addition, one chloride per Au? unit is hydrogen bonded to the NH groups of four ligands (32, Scheme 1 2 ) . [ 1 1 1In 1 the above two cases, and in that of the complex [AU(NHI)Z]BI(29, Scheme 1 the hydrogen bonds support the aurophilic interaction. On the contrary, the structures of complexes [Au(CNR)(S C ~ H ~ C O Z H ) ] mesityl) show ( But, dimers formed through hydrogen bonds, which are also connected through Au. . .Au contacts to give chains.['l2I (33. Scheme 12) is a rare The complex [{Au(CH?PPh~j~)~{Ag(OClO~j?)?] example of an unassisted heteronuclear loose cluster showing a square of alternating gold and silver atoms connected through Au. . .Ag numismophilic inter)? ~ (p2-AgL)~] )~ ( L = tetrahydrothiophene, benzene) actions,[221 whereas [ { A u \ C ~ F shows the same square of atoms but additional connections through Au.. .Au contacts give a polymer (34, Scheme 12).[23"1We have recently determined the
502
1 Molecular Clusters \
E+
Scheme 13
crystal structure of another derivative ( L = acetone) and shown that the Au. . .Ag interactions are supported by CsFs bridging groups (see below).
1.26.4 Mixed loose clusters Some assisted loose clusters of gold( I ) form aggregates through additional aurophilic interactions. The most typical family of these species is [(AuL)3(p3-E)]+ where L is a phosphine and E is 0, S or Se. As indicated above, depending on the steric requirements of L and the nature of E, the pyramidal monomers can be non-associated (see above, assisted loose clusters) or form dimers establishing two (35,Scheme 13; E = 0, L = PPh3, PMePh2;[301E = S, L = p M e P h ~ ; [ E~ ~=]Se, L = PPh311l 3 ] ) or four additional unsupported Au( I). . .Au(I ) contacts forming a A u ~tetrahedral core (36, Scheme 13; E = 0, L = PMe3).[1141A string is formed when Au( I ) . . .Au( I ) contacts are established among pyramidal dimers ( E = S, L = PMe3).[331 Some dinuclear complexes [ A u ~p-L)] ( form dimers through two new aurophilic interactions (37, Scheme 13).[30,33,82~83,86,115~1171 In [Au2(p-L)(p-L')] complexes, one or two additional unsupported Au( I). . .Au(I ) contacts lead to dimers (38, Scheme 14, e.g., L = dithiocarbamato, L' = methylenediphenylphosphinomethylene;[1181L = bis(diphenylphosphino)methane,L' = 2-dimethylamino-2-oxy-
Scheme 14
1.26 Au(I),..Au(I) andAu(I)...Ag(I) Loose Clusters
CH,OH
503
42
Scheme 15
2-thiapr0pane-l,3-diyl)["'~) or polymers (39, Scheme 14; L = L' = S2PR2, where R = Ph,[1201or OPr').[1211In the ionic complex [ A U L ~ ] ~ [ A U ~ ( S ~ C = C ( C N ) ~ ] , the two [AuL1]+ cations are in contact with the anion (40, Scheme 14; L = phosphatriazaadamantane) .[481 The tetranuclear thiolate complexes [(AuPPhl)4{p4-S(CH2)5 S} and [ (AuPPhl)4{p4-SCH2CH(CH20H)S}I2+ polymerise or dimerise, respectively, through unsupported Au( I). . .Au(I ) contacts, giving rise to squares of gold atoms (41 and 42, Scheme 15).[1221 A fine example of the aurophilic attraction acting against steric forces is the adduct [(AuL)5(p5-N)l2'.2[AuCI(PMe3)] (43, Scheme 16) in which each [AuCl(PMe?)]molecule interacts with two of the equatorial gold atoms of the
1 2+ 43
Scheme 16
44
'
'
504
I Molecular Clusters
When crystals of the complex [Au3{RN=C(OMe)}3] ( R = Me, 44, Scheme 16) are irradiated with long-wavelength ultraviolet light and then put in contact with a liquid, a remarkable luminescence occurs. In addition to the short intramolecular Au( I). . .Au(I ) contacts (3.3 A), the molecules in 44 are stacked through intermolecular Au( I). . .Au( I ) interactions (3.35 A) to form a trigonal prismatic array. This solvent-stimulated luminescence, described by Balch et ~ 1 . , [ ' ~ . ''I is attributed to the extended supramolecular aggregation as a related species ( R = PhCH2) whose crystal structure reveals individual molecules with no additional Auj 1). . .Au(I ) contacts does not show the luminescent properties of 44. The structure of the complex [(AuAsPh~)2(,u2-mesityl)~Ag]C10~ shows the Au . . . Ag . . . Au atoms forming an angle of 180°, and the mesityl ligand bridging each pair of Au...Ag atoms. In addition, short Au...Au contacts lead to a polymeric chain.[25a1 Similarly, the structure of [ {Au(CgF5)2}2{p2-AgL}2](45, Scheme 16, L = acetone) shows squares of alternating Au and Ag atoms bonded through additional Au. . .Au contacts. The aryl groups bridge each pair of Au and Ag atoms in the squares.[971This is remarkably different with respect to those derivatives with L = tetrahydrothiophene and benzene (34, Scheme 12).[23a1
1.26.5 Aurophilic coordination According to the structural data described above, in assisted loose clusters the number of gold atoms in short contact with any other (its aurophilic coordination number) is one, two (linear and angular), three (pyramidal and T shaped) or four (pyramidal). Sometimes, the geometry is not imposed by the nature of the bridging ligand(s) but by aurophilicity. In unassisted loose clusters the gold atoms are in contact with one or two others, with the exceptions of the three-coordination in our tetramer the polymeric complex [AuCl(CNCH2C02Me)] (25, Scheme [Au(C-CSiMe3)(CNBut)] (27, Scheme 10),rs81 and the tetracoordination in [AuC~(CO)].[~~~
1.26.6 Conclusions Self-assembly of Au( I ) complexes through weak Au( I). . .Au(I ) interactions is now recognized as an interesting feature of heavy transition metal chemistry whose wider implications are slowly being understood. The utility of these compounds as chemical and photochemical switches or as energy storage devices and photochemical sensors is one of the possible fruits of this research. Although a com-
1.26 A u ( I ) . . . A u / I j a n d A u ( I ) . . . A y ( I )Loose Clusters
505
plete understanding of aurophilicity (which would allows us to predict the nature of the ligands that permit short Au( I ) . . .Au( I ) contacts) still remains some way off, it is already clear that the steric demand of the substituents on the ligands plays a crucial role. Hopefully, the extension of this family of loose clusters to all possible combinations of the three coinage metals will offer exciting new discoveries.
Acknowledgements We are indebted to Prof. Peter G. Jones for many years of fruitful collaboration in gold chemistry. We thank DGES (Spain) for financial support.
References [ I ] H. Schmidbaur, Gold Bid/. 1990. 23, 1 I . [2] P. G. Jones. Gold Birll. 1981. 14. 102. 131 P. G. Joncs. Gold Birll. 1981, 14, 159. 141 P. G. Jones. Gold Bid. 1983, 16, 114. 151 P. G. Jones, Gold Bull. 1986, I Y >46. 161 F. Sherbaum. A. Grohmann, B. Huber. C. Kruger. H. Schmidbaur. Anyeit.. C/7e/n.. fnt. Ed Engl. 1988. 27. 1544. 171 P. Pyykko. N . Runeberg, F. Mendizabal, Clienz.-Ezrr.J. 1997. 3, 1451. 181 P. Pyykko. F. Mendizabal, Clie~n.-Ezw.J. 1997, 3. 1458. 191 H. Schniidbaur, W. Graf. G. Miiller, Anyen. C/itvn., I n t . Ed. Emgl. 1988, 27, 417. 1101 J. C. Vickery, M. M. Olmstcad, E. Y . Fung, A . L. Balch. An6qeit.. Clitw., fnt. Ed. Emgl. 1997. 36. 1179. [ I I ] L. H. Gade, Angoi.. Clinn.. f n t . Ed Etiql. 1997. 36, 1171. [I21 B. C. Tzeng. K. K. Cheung, C. M. Che. C/zeni. Conmzrn. 1996. 1681. [ I 31 H. Schmidbaur. fntm/isciplin. Sci. Rer. 1992, 17. 213. 1141 H. Schmidbaur, Clien?.Soc. Rei..1995, 391. [IS] J. Vicente, M. T. Chicote. R. Guerrero. P. G. Jones. J. A n ! . C/wn. Soc~.1996. 118, 699. [16] E. C. Coffey. J. Lewis, R . S. Nyholm. J. C l i m . Soc. 1964, 1741. [ 171 E. Zeller, H. Beruda. H. Schniidbaur. fnor
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1.26 Au(I)...Au(I) and Au(I)...Ag(I) Loose Clusters
507
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[99] H. Ruben, A. Zalkin, M. 0. Faltens, D. H. Templeton, Inorg. Chem. 1974, 13, 1836. [loo] R. Uson, A. Laguna, M. Laguna, J. Jimenez, M. P. Gomez, A. Sainz, P. G. Jones, J. Chem. Soc., Dulton Truns. 1990, 3457. [ 1011 D. M. P. Mingos, J. Yau, S. Menzer, D. J. Williams, J. Chem. Soc., Dalton Trans. 1995, 319. 11021 A. Bayler, A. Schier, G. A. Bowmaker, H. Schmidbaur, J. Am. C k m . Soc. 1996, 118, 7006. [I031 F. Canales, M. C. Gimeno, A. Laguna, P. G. Jones, J. Am. Chem. Soc. 1996, 118,4839. [I041 F. Canales, M. C. Gimeno, A. Laguna, P. G. Jones, Orgunometullics 1996, 15, 3412. [I051 H.-N. Adams, W. Hiller, J. Strahle, 2. Anorg. Allg. Chem. 1982, 485, 81. [I061 W. Conzelmann, W. Hiller, J. Strahle, G. M. Sheldrick, Z. Anorg. Allg. Chem. 1984, 512, 169. [lo71 J. Meyer, H. Piana, H. Wagner, U. Schubert, Chem. Ber. 1990, 123, 791. [lOS] M. S. Hussain, A. R. Alarfaj, M. N. Akhtar, A. A. Isab, Polyhedron 1996, 15; 2781. 11091 S. Ahrland, B. Noren, A. Oskarsson, Inorg. Chem. 1985, 24, 1330. [ 1101 W. Schneider, A. Sladek, A. Bauer, K. Angermaier, H. Schmidbaur, Z. Nuturforsch. B 1997, 52, 53. [ l l l ] P. G. Jones, B. Ahrens, Chem. Ber. 1997, 130, 1813. [I 121 W. Schneider, A. Bauer, H. Schmidbaur, Organomrtallics 1996, 15, 5445. [113] C. Lensch, P. G. Jones, G. M. Sheldrick, Z. Nuturforsch. B 1982, 37, 944. 11141 K. Angermaier, H. Schmidbaur, Inorg. Chem. 1994, 33, 2069. [115] P. G. Jones, G. M. Sheldrick, E. Hiidicke, Actu Crystallogr. B 1980, 36, 2777. [I 161 S. Wang, J. P. Fackler Jr., Inorg. Chem. 1990,29, 4404. [117] A. Sladek, H. Schmidbaur, Chem. Ber. 1995, 128, 907. [I 181 M. Bardaji, N. G. Connelly, M. C. Gimeno, J. Jimenez, P. G. Jones, A. Laguna, M. Laguna, J. Chem. Soc., Dalton Trans. 1994, 1163. [1191 I. J. B. Lin, C. W. Liu, L. K . Liu, Y. S. Wen, Orgunometullics 1992, 11, 1447. [120] G. Siasios, E. R. T. Tiekink, Z. Kristallogr. 1995, 210, 698. 11211 S. L. Lawton, W. J. Rohrbauh, G. T. K okotdo, Inorg. Chem. 1972, 11, 2227. [122] A. Sladek, H. Schmidbaur, Inorg. Chem. 1996, 35, 3268. 11231 K. Angermaier, H. Schmidbaur, Inorg. Chem. 1995,34, 3120.
2 Metal Clusters in Catalysis
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.1 Metal Clusters in Catalysis - An Overview Richard J. Puddephutt
2.1.1 Introduction Transition metal clusters are intriguing compounds for many reasons, several of which can be traced back to their forming a natural interface between simple metal complexes and such materials as metal colloids, particles, and surfaces.['-91 This relationship naturally leads to questions about such fundamental issues as structure and bonding in clusters and the transition from simple molecular orbital treatments to more delocalized band structures,['I especially as very large clusters such as [Cu146Se73(PPh3)30] have been characterized crystallographically and giant clusters have been characterized by the array such as [Pd~6~(1,1O-phenanthroline),j0](0Ac)~~~ of modern physical methods used for materials for which single crystals cannot be grown.[''-' 2 ] It also stimulates questions about potential applications in materials science and in catalysis, the latter being of primary interest here. Simple organometallic and coordination complexes have found many important commercial applications in homogeneous catalysis, where they often have high reactivity and selectivity. Their properties are readily tailored by ligand design chiral catalysis, size selectivity, branched or linear products can all be achieved by clever manipulation of the steric and electronic properties of the supporting ligand~."~] The heterogeneous catalysts are less easily manipulated but have the strong commercial advantages of higher thermal stability, enabling high temperature processes, and ease of separation of products from the catalyst.['4] The colloidal or bulk surface materials can also catalyze reactions that require cooperative effects between several metal atoms; this is not possible using simple homogeneous metal complex catalysts. The promise of homogeneous cluster catalysis is, then, the best of both worlds, that is to have the advantage of cooperative effects between several metal atoms while also maintaining the ability to control reactivity and selectivity by appropriate ligand design. Another approach is to prepare particularly small and well-defined homonuclear or heteronuclear metal particles on an ~
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oxide support by adsorption and then decomposition of appropriate metal cluster complexes; these materials might then act as more reactive or selective heterogeneous catalysts than similar catalysts prepared from mononuclear precursors. In addition, cluster complexes can act as valuable models for the structures and reactions of reagents and fragments that are intended to be bonded to metal surfaces during heterogeneous catalysis. Great progress has been made in these fields but there have been many problems to overcome along the Subsequent chapters describe some of the exciting recent advances in detail; this chapter gives a brief overview.
2.1.2 Clusters as models for heterogeneous catalysis The validity of using metal clusters as models of surface catalysis was firmly established by a series of reviews by Muetterties,"] and has been reinforced strongly over the succeeding years.['-9] In the simplest example, the presence of well characterized carbonyl ligands with terminal, edge- or face-bridging, semi-bridging or four-electron bonding modes is established with certainty in many cluster complexes by X-ray structure determinations; spectroscopic properties, such as the carbonyl stretching frequencies, can then be used as a benchmark to assist in assigning bonding modes of surface-bound carbonyl groups. This method has been extended to a very wide variety of organic functional groups bound to clusters and surfaces and has been very useful in elucidating structures of surface-bound fragments, such as alkenes, alkynes, vinylidene, ethylidyne, etc., which are involved in many surface catalytic processes. Some key functional groups known to occur on both clusters and surfaces are illustrated in Fig. 1. This is essentially a structural cluster-surface analogy; it was the first to be established and has had a major influence in the field. In larger clusters, interstitial atoms or larger fragments can be incorporated in the center rather than as external ligand~.['-~I These can model aspects of bulk materials or thin films. The range of interstitial atoms known is large, the smallest being hydride and perhaps the most common being carbide. C2 fragments can also be formed as interstitial groups in larger clusters (Fig. 2). The chemistry of the interstitial carbide clusters is particularly rich and is the subject of Section 2.11 by Johnson who, with Lewis, has been a pioneer in the field. Coordinatively unsaturated clusters can mimic some aspects of the reactivity of metal surfaces in addition to the structure. This area has developed more recently than the above because most of the early clusters, for example of the iron group elements, tended to be coordinatively saturated in contrast to the situation on a clean metal surface. There were, however, some early examples of coordinatively unsaturated clusters, or their equivalents, and one classic example is the 46electron cluster [ O S ~ H ~ ( C101 O )which readily adds ligands L to give 48-electron [ O S ~ H ~ ( C O ) (Eq. ~ O L1). ] It has now been shown that many such reactions can occur
2. I Metal Clusters in Catalysis
terminal carbonyl
pcarbonyl
~
An Overview
p,-carbonyl
p,-hydride
Me C
Fieure 1. Common functional
M-
M
M-
601
H /
M
M-
M
M-
M
readily not only in solution but also in the solid state, where the analogy with surface catalysis is stronger and where the greater rigidity of the lattice can prevent further reaction or isomerization of the initial products of kinetic control. Section 2.9 describes in detail the state of the art in this important area of cluster chemistry. The later transition metal clusters, especially of the nickel group, often have several centers of coordinative unsaturation and so they can add more than one ligand or add ligands that donate more than two electrons, as illustrated in Eqs. (2)-(4).[15,161
Q
608
2 Metal Clusters in Catalysis
Figure 2. Skeletal structures of some interstitial clusters: (a) [Ru5C(C0)15]; (b) [Rh6C(C0)l5l2-; (c) [ C O S C ( C O ) I (4 ~ ] ~[RhuSb(C0)27l3-. ~;
In this chemistry, it is natural to focus on models for the catalytic reactions that are most important economically or which are most poorly understood because of difficulties of direct study. One which best fits these criteria is the catalytic hydrotreatment of petroleum feedstocks, which is used to remove sulfur and other heteroatoms, which interfere with subsequent catalytic reactions such as petroleum reforming, from the hydrocarbons. Molybdenum sulfide is the most common metal sulfide used in this catalysis, and hydrogen activation and C-S bond hydrogenolysis are known to be key reactions occurring at the catalyst surface but details are difficult to ~ b t a i n . [ ~ Study * ' ~ ] of model binuclear and cluster complexes has elucidated mechanisms of several of the key reactions and Section 2.6 describes important recent advances in this field, with the focus being on models for hydrodesulfurization catalysts.
2.1.3 Stabilization of metal clusters for catalysis Mechanisms of even simple reactions can rarely be proved, but are usually deduced by a process of elimination. It is inherently difficult to determine the mechanisms of reaction of cluster complexes because of the great number of possible mechanisms that must be eliminated to leave only one viable route. In catalysis, the problem is much greater because complexes involved in the actual catalytic cycle are often present at concentrations too low for direct detection by spectroscopic methods. The cluster that is added might not be involved in the productive catalytic steps, but might be merely a precursor or a resting state. It is probably fair to say that we do not yet know beyond reasonable doubt the mechanism of any cluster-catalyzed reaction, and this of course presents a challenge for future research because mechanistic understanding is at the heart of the logical development of homogeneous catalysis. Having said this, much progress has been made and there are now reasonable proposals for many cluster-catalyzed reactions.[g] A major problem in defining cluster catalysis has been to prove that catalysis does not occur after either fragmentation to mononuclear complexes, followed by
2.1 Metal Clusters in Catalysis
-
An Overview
609
homogeneous catalysis by simple organometallics, or association to colloidal or larger metal particles, followed by heterogeneous catalysis. Thus, it is well known that, under the conditions commonly used for catalytic carbonylation reactions, even 'stable' clusters like [Ru3(C0)12]react by addition of CO and fragmentation by cleavage of metal-metal bonds to give lower carbonyl derivatives, in this case to give [Ru(CO)s],which are known to be active homogeneous catalysts. Similarly, clusters often undergo thermal ligand loss, and aggregation often follows, either to give higher clusters or metal particles, which are known to be active heterogeneous catalysts. It is not simple to exclude these processes. Typically, a combination of qualitative tests in conjunction with detailed kinetic studies is needed to define true cluster catalysis and there is no single definitive test that can be used.['] Tests for heterogeneous catalysis include light scattering to detect small particles in suspension and the observation of inhibition by metallic mercury, which forms inactive amalgams with transition metal particle^;[^.^^] kinetic or product selectivity studies are usually the bases of tests for fragmentation. The carbonylation of methanol to acetic acid or reduction of CO to ethylene glycol by [Ru~(CO)IO]-Iare examples of catalysis that occurs after cluster fragmentation to mononuclear ruthenium c~mplexes;~''~ catalytic hydrogenation of arenes is commonly thought to occur after decomposition to metal particles.["] A very valuable method for avoiding or reducing the above problems is to use anchoring ligands to prevent cluster fragmentation, association or decomposition. Normally, simple bridging ligands such as carbonyl or hydride are not sufficiently rigidly bonded to serve this purpose, so specifically designed ligands are used. Edgebridging ligands are typified by units L-X-L, where L is usually a group 14, 15 or 16 atom donor and X is a single atom bridging group (C-, N-, or 0-based). These ligands then yield stable five-membered chelate rings XL2M2, where the M-M bond forms part of the cluster. Some examples with the ligand Ph2PCHzPPh2 are shown in Eqs. (2)-(4) and illustrate that it is necessary to bridge more than one edge of a cluster to maintain the nuclearity, especially in reactions where metal-metal bonds are cleaved. A detailed account of the use of edge-bridging ligands is given in Section 2.3 by Lahuerta and Estevan, who show that dirhodium( 11) units are effectively stabilized by a combination of bridging acetate and ortho-metalated phosphine ligands, and that these stabilized compounds are promising catalysts for decomposition of diazo compounds. Similar dirhodium( 11) complexes have proved to be active in cyclopropanation, C-H insertion, hydrosilation, and several other useful processes. Face-bridging ligands can effectively lock three or more metal atoms together and so are even more effective than edge-bridging ligands at maintaining cluster nuclearity during stoichiometric or catalytic reactions. Eq. ( 5 ) illustrates how a bridging PhP ligand can lead to retention of cluster nuclearity during ligand addition with cleavage of one or more metal-metal bonds.[20]The cluster [ C O ~ ( C O ) I & , - P Pis ~ )a~catalyst ] for hydroformylation of alkenes and it seems that the bridging PhP ligands are effective at maintaining cluster nuclearity during these reactions.[21]Many other face-bridging ligands are known, and some of the most exciting results in cluster catalysis are described in Section 2.5 on the hydrogenation
610
2 Metal Clusters in Catalysis
of alkynes by ruthenium clusters containing the face-bridging ligand 2-amido-6methylpyridine; a simplified catalytic cycle is shown in Eq. (6).
Yh
A potential problem with these stabilizing bridging ligands is that they are often bulky and some catalytic reactions are then blocked because larger reagents cannot find access to the metal centers. An elegant solution is to use an interstitial atom which can bind to six or more transition metal atoms. This is an important component of Section 2.1 1, in which clusters containing interstitial carbide are used as examples.
2.1.4 Homogeneous catalysis with cluster complexes It is convenient to divide this material according to whether the cluster complex is homo- or heterometallic in nature, although many of the principles apply to both cases.
2.1.4.1 Homometallic clusters for homogeneous catalysis The emphasis here is to find reactions for which clusters can give unique catalytic reactions because of the involvement of several metal atoms in the bond-activation processes or in which clusters are more active than simple complexes. Several such systems have been discovered and r e ~ i e w e d . [Thus, ~ ~ ~ ~for ] example, the catalytic
2.1 Metal Clusters in Catalysis
-
A n Overview
61 1
synthesis of cyclic polythioether macrocycles can be achieved from thietanes (CH2)3S by use of binuclear complexes such as [Rez(CO)g(MeCN)] or cluster complexes such as [Os4(CO)ll(pu-H)4{ S(CH2)3}].[91 Trimers or tetramers are usually formed, depending on the catalyst used, and several intermediates have been isolated. Multielectron processes are particularly attractive for cluster catalysis. The catalytic chemistry of nitrobenzene and other nitroarenes falls into this class, and conversion of PhN02 to PhN=C=O by carbonylation, or to PhNH2 by hydrogenation, can be achieved by use of several cluster catalysts, mostly based on ruthenium c a r b ~ n y l . [Now, ~ , ~ ~by~ combination with other reagents, a wide variety of organonitrogen compounds, including carbamates, ureas and nitrogen heterocycles, can be synthesized; Section 2.4 reports the most recent results in this field. In particular, it is shown that co-catalysts, which can be neutral ligands or halide salts, are needed for high activity and partial mechanisms have been elucidated by detection of reaction intermediates. Heterometallic clusters such as [Rh40s(CO)1 5 1 ~ -can be particularly active catalysts for these reactions. Binuclear and cluster complexes containing metal-metal multiple bonds have special properties in bond-activation reactions because they are coordinatively unsaturated, can often undergo multielectron redox reactions and can readily be supported on oxide materials for heterogeneous catalysis. Reactions known to be catalyzed by binuclear complexes include hydrogenation of alkenes, dienes, and alkynes, hydrodesulfurization, alkene metathesis and ROMP (ring opening metathesis polymerization of cyclic alkenes), and polymerization of unsaturated compound^.[^^^^] Catalysis using the anions [W2X9I3-, [Re2X8I2-, and [Re3X12l3- can be compared with those of binuclear complexes containing single metal-metal bonds in dirhodium( 11) complexes, as described in Section 2.3, and of clusters containing single metalkmetal bonds Section 2.5. Another frontier in catalysis follows the synthesis and study of nanostructured metal particles, which give a natural interface between homogeneous and heterogeneous catalytic material^.[*.^.^','^^^^^ The very large soluble clusters, which can be neutral (e.g. [AuS5C16( PPh3)12]), anionic (e.g. [Pt3s(C0)44l2-), or cationic (e.g. [Pd561( hen)^^] lSo+), form a natural overlap with colloidal particles and there is much current activity in developing catalytic applications in this Section 2.12 reviews progress in this general field. The nanometallic units can be homo- or heterometallic, can be homogeneous or heterogeneous, and can be free or supported on oxide materials. This field therefore presents a truly interdisciplinary challenge.
2.1.4.2 Heterometallic clusters for homogeneous catalysis The presence of two or more different metal atoms in a cluster opens new possibilities in catalysis, because cooperative effects between the different metals can take many forms and can lead to enhanced stability, reactivity, or selectivity. For example, the PtAu cluster complex [Pt(AuL)8I2+ reversibly adds hydrogen to
612
2 Metal Clusters in Cutulysis CO groups omitted Ph Ph PhCH=CHPh
Ph
3Ph
Ph-
Ph
Scheme 1
give [PtH2(AuL)gl2+ and acts as a very active catalyst (an order of magnitude greater than for simple platinum complexes) for exchange between H2 and Dz, to give HD, and between H2 and D20.[261 Similarly, the layered cluster [P~~Ru~H~(C PhCCPh)] O ) ~ O ( is a good catalyst for hydrogenation of the alkyne PhCCPh (Scheme l).[271Of course, these types of cluster can also be supported on other materials or decomposed to heterometallic particles to give additional catalytic materials. Section 2.2 gives an overview of this field including catalysts for carbonylation, oxidation, skeletal rearrangements and hydrodesulfurization. A major continuing challenge is to understand the action of the different metals in each catalytic reaction, to enable the design of clusters with the optimum properties.
2.1.5 Heterogeneous catalysis involving cluster complexes Many heterogeneously catalyzed reactions are performed using metal particles dispersed on oxide supports, commonly on alumina or silica but also many others, including transition metal and rare earth metal oxides. Metal clusters have interesting reactions with or at the surfaces of such oxide materials.[28]It has recently been shown that the oxide surface is actually a good medium for the synthesis of
2. I Metal Clusters in Cutalysis
-
An Overview
6 13
metal clusters. The product commonly depends on the acidic or basic properties of the oxide, with neutral clusters favored on acidic supports like silica and anionic clusters favored on basic supports like magnesia. Metal carbonyl clusters can also be prepared similarly inside the cavities of zeolites or mesoporous oxide materials, where size- and shape-selective catalysis may occur. Section 2.10 shows how a wide range of osmium clusters from [Os3(CO)12]to [ O S ~ & ( C O ) ~can ~ ] ~be- synthesized advantageously by the oxide surface technique. Some of the anionic clusters formed on oxide surfaces can be useful in catalysis, the clusters can react with protic reagents and coordinated ligands can undergo reversible hydration-dehydration reactions as described here in Section 2.8. As in catalysis by supported metal particles, it is now clear that the supports can play an active part in cluster reactivity and catalysis. Heterometallic cluster complexes can also be decomposed to metal particles on the surface of metal oxides and these can have enhanced catalytic properties based on their controlled stoichiometry. This was first demonstrated by the carbonylation of organic nitro As another example, the bimetallic catalyst Pt-Re-Al203 is used in petroleum reforming. It is commonly formed by reduction of a mixture of the simple complex ions [PtC14I2- and [Re04]- on alumina but this probably gives individual particles of Pt-Re alloy with varying composition. Decomposition of preformed cluster complexes such as [Pt(C0)2{ Re(C0)5}2] on alumina can give particles with well-defined stoichiometry, and studies of more complex Pt-Re clusters give insights into how the metal alloy particles are anchored to the oxide support.19]By using clusters with the metal atoms in different ratios, such as in the series of Pt-Co clusters [PtCo2(C0)8(CyNC)2],[Pt2Co2(CO)8(PPh3)2], and [Pt3Co2(C0)7(PEt3)3] (Scheme 2), metal particles with varying Pt/Co ratios can be prepared and their catalytic properties compared.[30]The formation of heterogeneous, heterometallic catalysts by techniques of this kind is described in the review by Braunstein and Rose (Section 2.2). When metal particles are formed on an oxide support, they can still be modified in a controlled way by using surface organometallic chemistry. Section 2.7 shows how Bu3Sn, BuZSn, or BuSn fragments can be grafted onto metal particles such as Ni, Rh, or Pt on silica. Pyrolysis of these materials can then cause dealkylation of tin to give, initially, naked tin adatoms and then the corresponding Sn-M alloy particle. Each of these three states (grafted organotin, tin adatom, tin alloy) can lead to modified catalysts with different properties in hydrogenation or isomerization of alkenes or in dehydrogenation of alcohols or alkanes.
2.1.6 Conclusion The field of catalysis by use of metal clusters is undergoing an impressive renaissance. The interdisciplinary aspects of large clusters and colloids and of free and
614
2 Metal Clusters in Catalysis
L
I
(0C)sRe -Pt-Re(C0)s
I
L L = co
oxide-supported nanoparticles continue to offer new opportunities. More controlled synthetic methods for both homo- and heterometallic clusters are being developed thus yielding a much greater variety of clusters for use in catalysis. As a result, more new catalytic processes are being discovered. Mechanistic studies and the insights obtained from model cluster complexes are bringing a deeper level of understanding of catalysis by both homometallic and heterometallic clusters and its relation to surface catalysis. Subsequent chapters show that European chemists are leading the charge across these frontiers of clusters in catalysis.
References [ I ] E.L. Muetterties, Bull. SOC.Chim.Belg., 84, 959 (1975); 85, 451 (1976). [2] B.F.G. Johnson, ed., Transition Metal Clusters, Wiley, Chichester, 1980. [3] F. A. Cotton and R. A. Walton, Multiple Bonds Between Metal Atoms, Wiley, New York, 1982. [4] M. Moskovits, ed., Metal Clusters, Wiley, New York, 1986. [5] D.F. Shriver, H.D. Kaesz and R.D. Adams, eds., Chemistry of Metal Cluster Complexes, VCH, New York, 1990. [6] J.P. Fackler Jr., ed., Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum Press, New York, 1990. [7] D.M.P. Mingos and D.J. Wales, Introduction to Cluster Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 1990.
2.1 Metal Clusters in Catalysis
~
An Overview!
6 15
[ 81 G. Schmid, ed., Clusters and Colloids, from Theory to Applications, VCH, Weinheim. 1994. [9] R.D. Adams and F.A. Cotton, eds., Catalysis by Di- and Polynuclear Metal Cluster Complexes, Wiley-VCH, New York. 1998. [ 101 H. Krautscheid, D. Fenske. G. Baum and M. Semmelmann, Angew. Cliem., Int. Ed. Engl., 32, 1303 (1993). [ I 11 N.M. Vargaftik, 1.1. Moiseev, D.I. Kochubey and K.I. Zamaraev, Faruday Discuss. Chem. Soc., 92, 13 (1991) [12] P.R. Raithby, Platinum Metals Rev., 42, 146 (1998) [ 131 R.H. Crabtree, The Organometallic Chemistry Transition Metals, Wiley-Interscience, New York, 1988. [ 141 J.H. Sinfelt, Bimetallic Catalysts: Discoveries, Concepts and Applications, Exxon, New York, 1983. [I51 R.J. Puddephatt, Lj. Manojlovic-Muir and K.W. Muir, Polyhedron, 9, 2767 (1990). [I61 J. Xiao and R.J. Puddephatt, Coord. Clzem. Rev., 143, 457 (1995). [17] H. Topsoe, B.S. Clausen and F.E. Massoth, Hydrotreating Catalysis Science and Technology, Springer, Berlin, 1996. [18] Y. Lin and R.G. Finke, Inorg. Clzem.,33, 4891 (1994). [19] B.P. Dombek, J. A m . Cliem. Soc., 103, 6508 (1981). J.F. Knifton, Chem. Eny. News, 43 (May 5 , 1986). [20] G. Huttner, J. Schneider, H.D. Muller, G. Mohr, J. von. Seyerl, and L. Wohlfahrt, Angew. Chem., Int. Ed. Engl., 18, 76 (1979); J. Schneider and G. Huttner, Chem. Ber. 116, 917 (1983). [21] C.U. Pittman, G.M. Wilemon, W.D. Wilson and R.C. Ryan, Angew. Chem., 92, 494 (1980). [22] G. Suss-Fink and G. Meister, Adu. Organomet. Cliem., 35, 41 (1993). [23] M. H. Chisholm, J. Chem. Soc. Dulton, 1781 (1996). [24] K.S. Weddle, J.D. Aiken and R.G. Finke. J. Am. Chem. Soc., 120, 5653 (1998). [25] L.N. Lewis, Clzem. Rev., 93, 2693 (1993). 1261 L.I. Rubinstein and L.H. Pignolet, Inorg. Clzem.,35, 6755 (1996). I Am. . Chem. Soc., 116, 9103 [27] R.D. Adams, T.S. Barnard, Z. Li, W. Wu and J.H. Yamamoto, . (1994). [28] B.C. Gates, Chem. Rev., 95, 511 (1995). A. Zhao and B. C. Gates, J. Am. Chem. Soc., 118, 2458 (1996). [29] (a) P. Braunstein, R. Bender and J. Kervennal, Organomrtullics, 1, 1236 (1982); (b) P. Braunstein, J. Kervennal and J.-L. Richert, Angeiv. Chem. Znt. Ed. Engl. 24, 768 (1985); (c) P. Braunstein, R. Devenish, P. Gallezot, B. T. Heaton, C. J. Humphreys, J. Kervennal, S. Mulley and M. Ries, Angew. Chem. Int. Ed. Engl. 27, 927 (1988). [30] G. Maire, 0 . Zahraa. F. Garin, C. Crouzet, S. Aeiyach, P. Legare and P. Braunstein, J. Chim. Phys., 78, 951 (1981). ~
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.2 Heterometallic Clusters in Catalysis Pierre Braunstein and Jacky Ros6
2.2.1 Introduction Cluster chemists have used great skill and imagination in the synthesis and structural characterization of novel and complex molecules.[1p4]Although the synthesis of molecules with complex architectures or unusual metal combinations often remains a challenge, the considerable progress made in the development of new and efficient synthetic methodologies has opened new areas for the investigation of the reactivity and catalytic applications of metal clusters. Thus, the increasing diversity and availability of mixed-metal clusters associated with the exact knowledge of their molecular structure and stereochemistry have stimulated considerable academic and industrial research interest in their catalytic properties. It has been known for a long time that bimetallic catalytic systems, even those obtained by simple mixing of the individual components, often have unique catalytic properties.[5p8]Recent reviews have detailed the various reactions catalyzed by homometallic~9p19~ and heteromolecular cluster compounds. Our intentions here are to encourage research in this field, stimulate a wider use of well-defined, heterometallic clusters in catalysis, help identify the catalytic properties of bimetallic couples (or trimetallic systems when available) and provide a list of the bimetallic combinations that are useful for the catalysis of a given reaction (see Table 1). If the current limitations are the unavailability of suitable bimetallic molecular precursors, then synthetic chemists should find here encouragement to fill the gap! The importance of heterometallic clusters for homogeneous and supported molecular catalysis is based on the reasonable expectations that two or more adjacent metal centers might enable ligand mobility, thus promoting reactions between different partners and cooperative reactivity, while the intrinsic polarity of heterometallic bonds can provide bior multifunctional activation and direct the selectivity of substrate-cluster interactions. Metal-specific electronic and steric interactions are expected to lead to increased selectivity in the overall chemical transformation.[24]Thus, new, more
co.,
Hydrogenation of CO and
Coal liquefaction
Co-Rh, Rh-Ir, Rh-Pt, Rh-Cu, Rh-Ag, Rh-Au, Rh-Zn
Fe-Co, Ru-Co, Ru-Rh, Ru-Cu, Ru-Ag, Ru-AU
Mn-Rh, Mn-Pd, Re-Rh
Co-Rh, Co-Pt, Rh-Au, Ir-Pt
Fe-Ru, Fe-Ru-Co, Fe-Co, Fe-Rh, Fe-Pd, Ru-Co, Ru-Rh, Ru-Ir, Ru-Ni, Ru-Pt, Ru-Cu, Ru-Au, 0s-Ni
Mn-Fe, Mn-Ru, Re-Rh, Re-Pt
Mo-Co, Mo-Pt, W-Pt
Fe-Ru, Fe-Pt, Ru-Os, Ru-Pt, 0s-Rh, 0s-AU
Cr-Fe, Cr-Pd, Mo-Fe, Mo-Fe-Co, Mo-Ru-Co, Mo-Co, Mo-Co-Ni, Mo-Rh, Mo-Pd, Mo-Pt, W-Fe, W-Fe-Co, W-Ru-Co, W-Ni, W-Pd, W-Pt
Rh-Ir
Fe-Co, Fe-Rh, Ru-Co
Co-Pt, Rh-Pt
Ta-Ir
Mo-Pd
V-Fe, Ta-Ir
Hydrogenation/isomerization .~ of carbon-carbon multiple bonds
Co-Rh
Fe-Ru, Fe-Os, Fe-Co, Fe-Rh, Fe-Ir, Fe-Pd, Fe-Pt. Ru-Os, Ru-Co, Ru-Rh, 0s-Rh, 0s-Ni
Mn-Fe, Mn-Ru, Mn-Co, Re-0s
Cr-Ru, Cr-Pt, Mo-Fe, Mo-Ru, Mo-Os, Mo-Co, Mo-Rh, Mo-Ni, W-Os, W-Rh, W-Ir, W-Pt
Mo-CO
Pt-Au
Fe-Ru, Fe-Co, Ru-Os, Ru-Co, Ru-Ni, Ru-Ag, 0s-Ni, 0s-Ni-Cu
Pt-Au
Pt-AU
Heterogeneous catalysis
H2-D2 equilibration
Supported catalysis
Homogeneous catalysis
Catalyzed reaction
Table 1. Heterometallic couples of molecular complexes used in homogeneous> supported or heterogeneous catalysis.
Carbonylation and hydrocarbonylation reactions
Hydroformylation reactions
Water-gas shift reaction
Hydrogenation of ketones and aldehydes
Hydrogenation of oxygen
Catalyzed reaction
Table 1 (continued)
Co-Pd
Mo-Fe
Co-Rh, Co-Pd, Co-Pt
Fe-Co, Fe-Rh, Fe-Ni, Fe-Pd, Fe-Cu, Fe-Hg, Ru-Co, Ru-CO-Au, Ru-Rh, 0s-CO
Mn-Pd
Ru-CO
Zr-Rh
Co-Rh, Co-Ni, Co-Pt, Rh-Zn, Ir-Cu
Fe-Ru, Fe-Co, Fe-Rh, Ru-Co, Ru-Rh
Fe-Rh, Fe-Ir, Fe-Pd, Fe-Pt, Ru-Co
Co-Rh, Co-Pd, Rh-Ir
Cr-Ru, Cr-Pd, Mo-Fe-Co, Mo-Ru, Mo-Co, Mo-Co-Ni, W-Ru, W-Rh, W-Pd
Co-Rh
Mo-Rh
Fe-Rh, Ru-Os, Ru-Co
Zr-Rh
Co-Rh, Co-Ir
Fe-Ru, Fe-Co, Fe-Ir, Ru-Co, Ru-Rh
Co-Cu, Co-Zn
0s-Ni
Fe-Ru, Ru-Rh, Ru-Ir
Pt-Au
Heterogeneous catalysis
Mo-CO
Supported catalysis
Cr-Ru, Mo-Ru, W-Ru
Homogeneous catalysis
Ta-Ir
Silylation reactions
Olefin polymerization and copolymerization
Olefin oligomerization
Norbornadiene dimerization
V-Cr Cr-Mo
Zr-Fe Mo-Pt, W-Pt
Co-Pt
Cr-Rh, Mo-Ru, Mo-Pd. W-Pd
Co-Pt, Co-Zn, Co-Cd, Co-Hg
Fe-Co. Fe-Pt
Cr-Mo, Cr-W, Mo-Rh
Olefin metathesis
Ru-Ni, 0s-Ni
Ru-Ni, 0s-Ni, 0s-Ni-Cn
Fe -Pd
Mo-Pd
Mo-Pd
Heterogeneous catalysis
Fe-Ru
Mo-Fe
V-Fe
Co-Rh, Co-Ni
Fe-Co, Fe-Rh
Supported catalysis
C Z H Self-homologation ~
Synthesis of ammonia and amines
Mo-Pd
Dehydrogenation/ dehydration of alcohols
Mo-Fe-Co, Mo-Co. Mo-Co-Ni, Mo-Pd, Mo-Pt. W-CO, W Pd, W-Pt
Fe-Ru. Ru-Rh, 0s-Rh
Homogeneous catalysis
Reductive carbonylation of organic nitro derivatives
NO Reduction
Catalyzed reaction
~
Table 1 (continued)
5'
Mn-Co
V-Rh Cr-Mn, Cr-Co
Mo-Ni
Cr-0s Pd-Cu
Oxidation reactions
Mo-Co, W-Rh, W-Ni
Mo-Pd
Cycloaddition of alkynes and CO2 or carboxylic acids
Hydrodesulfurization
Mo-Pd
Cyclization of alkynoic acids
Fe-Rh
Fe-Pt
Synthesis of poly(ferroceny1silanes)
Supported catalysis
Hydrocarbon dehydrogenation and skeletal rearrangements
Homogeneous catalysis
Catalyzed reaction
Table 1 (continued)
Ru-CO
Mo-Fe, Mo-Co
Co-Rh, Co-Pt, Rh-Ir, Rh-Pt, Ir-Pt
Fe-Ru, Fe-Co, Fe-Pt, Ru-Ni, Ru-Pt
Re-Os, Re-Ir, Re-Pt
Cr-Pd, Mo-Ir, Mo-Pt, W-Ir, W-Pd
Pt-Au
Fe-Rh
Heterogeneous catalysis
8 G.
B
9 u
ru
0
N
m
2.2 Heterometullic Clusters in Catalysis
62 1
active and/or more selective homogeneous or heterogenized (by adsorption, or attachment of the clusters on to the pores of solid micro- or mesoporous materials by simple impregnation methods or by ‘ship-in-bottle’ synthesis inside the cavities) catalysts have become a ~ a i l a b l e . [ ~Since ~ - ~ the ~ ] exact nature of the active species is usually unknown, we shall refer to the molecular bimetallic complexes as catalysts throughout the text, although they are better viewed as catalyst precursors. There is a considerable potential in the use of heterometallic clusters as precursors to heterogeneous catalysts.[201The metal core of these clusters constitutes a well defined unit which can be viewed as a ‘molecular microalloy’. Stripping off the ancillary ligands is expected to provide catalytic materials with improved control of both particle size and composition. Relatively little is, however, known at present about the actual shapes and structures of the metallic particles resulting from the thermal decomposition of bulk or supported heterometallic cluster compounds and, particularly, about the extent to which they ‘keep the memory’ of their well-defined molecular precursors. The better performances of mixed-metal cluster-derived (MMCD) catalysts are often explained by the strong interactions between the metals and by the greater dispersion of the particles on the support, two features generally not encountered with the conventionally prepared catalyst. The study of similarities and differences between ligands on clusters and adsorbates on surfaces is being much investigated in the context of the cluster-surface analogy.128-331Thus, knowledge of the precise structural and spectroscopic properties of atoms or ligands bound to the metal frameworks enables conceptual and comparative models to be established and evaluated for understanding the coordination of these species to metal surfaces. Between large molecular clusters and small metal particles, one finds the regime of colloidal metals. This field of research has made considerable progress over the last few years, in particular owing to new synthetic developments for mixed-metal systems and to the use of numerous high-performance analytical methods. Colloidal dispersion of bimetallic clusters are generally prepared by reduction of the metal halides in the presence of a protecting agent, e.g. ammonium cations or poly(Nvinyl-2-pyrrolidone). The structures of nanoclusters, their change as a function of nuclearity, metal, and (for bimetallic systems) composition, segregation phenomena, and their relevance to catalytic properties raise important issues that are germane to those discussed for colloids. Increasing work is being performed in this rapidly developing area.’ 7,22,34-431 Here we intend to review the catalytic properties of well-defined and characterized heterometallic metal-metal bonded cluster compounds of the transition metals (including group 11 and 12 elements) associated with a given bimetallic couple. In each case, we will examine their use in homogeneous catalysis, for the preparation of heterogenized (or anchored) catalytic and as precursors to new types of heterogeneous catalysts. Selected recent examples involving heterodi- or polynuclear complexes, sometimes even without a metal-metal bond, will also be i n c l ~ d e d . lBecause ~~] we shall only consider transition metal systems, other bimetallic couples, e.g. Pt-Sn, Ru-Sn or Ti-A1 for which synergistic effects
622
2 Metal Clusters in Catalysis
have also been discovered, will not be discussed Metal couples will be examined in the order of the earliest transition metal they contain. The groups of the periodic table are presented in succession and within each group the metals are arranged by period (e.g. Cr-M will appear before Mo-M and W-M for M in groups 2 7 ) . For each bimetallic couple, we present the various types of catalysis investigated (e.g. homogeneous, supported, heterogeneous), the different catalytic reactions performed, usually in the sequence corresponding to the classification of Table 1, i e. H2-D2 equilibration, hydrogenation reactions, ammonia synthesis, water-gas shift reaction, hydroformylation and carbonylation reactions, dehydration of alcohols, silylation reactions, olefin metathesis, olefin dimerization and oligomerization, oxidation reactions, hydrocarbon skeletal rearrangements, and hydrodesulfurization catalysis. The heterogeneous, mixed-metal cluster-derived (MMCD) catalysts will be designated by the composition of the metal core of the precursor cluster in square brackets (e.g. [Mo~Pdz]).
2.2.2 Catalytic properties of heterometallic metal-metal bonded cluster compounds of transition metals Zr-Fe Dinuclear and trinuclear ferrocenyl derivatives of the type [ CpFe(pC5H4SiMe2C5H3R'R2)ZrCpC12]( R ' = R 2 = H; R ' = Me, R 2 = H; R ' = R2 = Me; R ' = Ph, R 2 = H) and [{CpFe(pc-CsH4SiMe2CsH4)}ZrC12] are very active olefin polymerization catalysts and the former type is active in the copolymerization of ethene and propylene or the terpolymerization of ethylene, propylene and ethylidene-2-norbornene.[' '1
Zr-Rh The phosphido-bridged complex [Cp2Zr(p-PPh2)~RhH( CO)(PPh3)] is a homogeneous catalyst for the hydroformylation of l-he~ene.['~I Complexes l a and l b
s s, la
(0C)Rh//\hhp)
I
Ph2P\
\
YPh2
x = zrcp2
1b X = Zr(q-C5H4-'Bu), IC
X = -CH,CH2-
2.2 Heteronzetallic Clusters in Cutalysis
623
+
also catalyze the hydroformylation of I-hexene at 353 K and 5 atm CO Hz. Their enhanced activity compared with the analogous complex l c was attributed to an increase of electron density on rhodium from the Zr.Is7- 5 9 1 Homogeneous carbonylation of ethylene to acrolein was investigated in the presence of another early-late bimetallic complex, [ A S P ~ ~ ] [ C ~ * ~ Z ~ ( ~ - S ) ~ R ~ ( C O ) ~ ] . V-Cr
A heterogeneous [VCr] catalyst prepared from [VCrCp3(C0)3] on SiO2 was used as an ethylene polymerization catalyst.r6'1
V-Fe Cubane clusters that contain a VFe& core and catecholate or multicarboxylate ligands homogeneously catalyze the reduction of hydrazine to ammonia and that of acetylene to ethylene.162p6s1 V-Rh
Selective oxidation of propene to acetone was achieved with SiOz-grafted catalysts prepared from [{q3-C4H7)2Rh}2(V401z)]2- (2) and [(Cp*Rh)4V6019] (Scheme 1).[26,661
+ 02
/
--( 360 - 410 K
--(
-7H
H '
H
?
?
?
Scheme 1. Suggested grafting mode of complex 2 to silica surface (adapted from ref. 66)
624
2 Metal Clusters in Catalysis
Ta-Ir
The complex [CpzTa(pU-CH2)21r(C0)2] catalyzes the hydrogenation of ethylene, the isomerization of higher olefins, and the hydrosilylation of ethylene.[67]Although the specific role of the tantalum remains unclear, it is a required component. Under hydrogenation conditions the related complex [Cp2Ta(p-CHz)zIrCp*( H)] was found to decompose to a small extent to a very active heterogeneous ethylene hydrogenation catalyst.l6*1 Cr-Mo
Reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes led to very active bimetallic, heterogeneous olefin metathesis catalysts. Surface metal ions might be involved in bonding interactions with the organometallic complex, possibly leading to heterometallic species on inorganic oxides.[69p7 '] Polymerization of ethylene (at 358 K) or 1-octene (at 296 K) was studied with [CrMo(OAc)4.2H20]as the catalyst and comparisons were made with the corresponding homometallic catalysts [ C ~ ~ ( O A C ) ~ . and ~ H ~[ M O ]o ~ ( O A C ) ~ . ~ H ~ O ] on silica. The presence of Lewis acid cocatalysts such as aluminum or tin alkyls increases the reaction rate, drastically reduces the induction period, and favors the formation of isomers and oligomers. Comparison of the corresponding [CrMo] and [Crz] [ M o ~ heterogeneous ] catalysts indicated a cooperative effect of the two centers. It is believed that the Mo(V1) centers in the [CrMo] catalyst interact with the neighboring coordinatively unsaturated Cr( II).[72]
+
Cr-W
A very active heterogeneous olefin metathesis catalyst was prepared by reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne c o m p l e x e ~ [ ~or ~ -with ~ ~ I Schrock-type carbyne complexes [L3W-C( Bu')] ( L = C1, O( Bu'), n e ~ p e n t y l ) . [Surface ~~] species of the type shown in Scheme 2
Si
Si Scheme 2
2.2 Heterometallic Clusters in Catalysis
625
could account for the increased activity of this catalyst compared with the homogeneous system. Note that the olefin polymerization activity of the reduced Philipps catalyst is lost in this new bimetallic catalyst.[70]
Cr-Mn Synergistic effects have been discussed for the oxidation of cyclohexene catalyzed by Cr-Mn complexes immobilized on ~ilica.1~~1
Cr-Fe The anion [ HCrFe( CO)9]- catalyzes the isomerization of 1-hexene and allylbenzene into internal olefins in the presence of light. Allylbenzene was converted into cisand trans-propenylben~ene.[~~I
Cr-Ru The dinuclear complex [(OC)4Cr(p-PPhl)zRu(CO)3] was used as a homogeneous catalyst for the hydrogenation of cyclohexanone and the hydroformylation of A heterogeneous catalyst derived from [Cr2Ru3C(CO)1 6 1 ~ - was less active for hydrogenation of CO to hydrocarbons and oxygenates than [RuCoz], [RuCo3], and [ R u ~ C OMMCD ~] catalysts see below), although the selectivities for oxygenates were in the range l2-20%.[”
\
Cr-0s The selective catalytic oxidation of primary and secondary alcohols was performed using 0x0-bridged heterobimetallic complexes of the type [Os(N)Rz(CrO4)]( R = Me, CH2SiMe3).1781
Cr-Co Oxidation of cyclohexene was investigated with Cr-Co complexes immobilized on
Cr-Rh The indenyl-bridged complex trans-[Cr(C0)3(p-heptamethylindenyl)Rh(CO)z] (with no metal-metal bond) is a very efficient homogeneous catalyst precursor for the cyclotrimerization of DMAD.[79]
626
2 Metal Clusters in Catalysis
3a 3b 3~ 3d 3e 3f
R3P-
0%
M=Cr; R = M e M =Cr; R = Et M=Mo;R=Et M=Mo; R = P h M = W ; R = Et M=W; R=Ph
'i' I Cr-Pd The planar cluster [Cr2Pd2Cp2(C0)6(PEt3)2] (3b) was used as a homogeneous catalyst for selective hydrogenation of 1,5-COD; isomerization to 1,3-COD was also observed.[''] Homogeneous hydroformylation of styrene was catalyzed by [(OC)4Cr(pPPh~)2Pd(PPh3)1.[~~' A heterogeneous [Cr2Pd2] catalyst derived from [CrzPd2Cp2(C0)6(PMe3)2] (3a) on alumina was used to isomerize 2-methylpentane in methylcyclopentane. It makes the same contribution of the cyclic mechanism as a conventional Cr-Pd catalyst containing 6.6% Pd.['lI
Cr-Pt Compared with conventional monometallic or bimetallic heterogeneous catalysts, greater activity and methanol selectivity (>90%) was observed in CO2 hydrogenation with [CrPt] catalysts prepared from [HCrPt(p-PPh2)(p-CO)(CO)4(PPh3)] on SiO2. The monometallic system Cr/SiO2 is almost inactive and Pt/SiO2 has very low activity, CO being the major product. Addition of chromium to the latter catalyst by conventional impregnation methods increases activity, although the selectivity in MeOH remains modest: 15.5% at 473 K and 12.8% at 573 K. In contrast, the [CrPt] catalyst has activities 5.5-6.5 times higher than Pt/Si02.[821 Mo-Fe
The [HMoFe(CO)g]- anion is a homogeneous photocatalyst or catalyst precursor in the isomerization of 1-hexene and allylbenzene into internal olefins in the presence of light. Allylbenzene has been converted into cis- and trans-pr~penylbenzene.[~ 51 The arsenic-bridged complex [Cp(OC)3Mo(p-AsMez)Fe(CO)4] also has catalytic activity for alkene hydrogenation and is~merization.['~]The catalytic activity
2.2 Heterometullic Clusters in Cuta[vsis
621
of the anionic clusters [MoFeS4(SCN)2(0Me)2l2- and [Mo2FeSgO(OMe)2I3for acetylene reduction to ethylene increases with the percentage of Fe in the comp l e ~ . [ Anionic '~~ clusters with an MOzFe& core catalyze the reduction of acetylene by KBH4, although less effectively than those with a Mo2Fe4C02S8 core.[85] Cubane clusters with a MoFe3S4 core also catalyze the reduction of acetylene to ethylene and of hydrazine or cis-dimethyldiazene to ammonia or methylamine, respective1y.ls61Such clusters are actively investigated owing to their structural relationship with the nitrogenase cofactor.[62p65,87p911 Their electrocatalytic effect for the reduction of nitrates and nitrites was studied by using a cluster-modified glassy carbon electrode.'921Homogeneous carbonylation of ethanol to ethyl propionate was catalyzed by [MoFe(,~-Ph2Ppy)(CO)~].~~~l A supported bimetallic catalyst derived from the sulfido cluster [Mo2Fe2S2Cp2(C0)g] (4) on MgO had high selectivity for C2 products in CO m e t h a n a t i ~ n . [When ~ ~ ] adsorbed on MgO, the selectivity of this [M02Fez] catalyst was greater than 95 mol % C2H4 and C2H6 (ca 1 :2) compared with more than 95 mol YOCH4 when adsorbed on y-AlzO3. The suggestion that the cluster is not fragmenting and re-aggregating into larger crystallites was based on the fact that the selectivities of the [Mo~Fez] catalyst differ from those of Mo/A1203, MoS2 or Fe/A12 O3.[9 6] Environmental problems have emphasized the importance of catalysts capable of removing sulfur, and hydrodesulfurization (HDS) has become a central issue in coal and petroleum refining.[971With the same [Mo2Fe2] catalyst on y-Al203, SiOz, TiO2, and MgO, activities for removal of sulfur from thiophene were comparable with those of commercial HDS catalysts while showing decreased H2 consumption (less butane f ~ r m a t i o n ) . [ ~ ~ , ~ ~ . ~ ' ~ 3
CP 4
Mo-Fe-Co
Acetylene reduction has been homogeneously catalyzed by [Et4NI3 [Mo2Fe4CoZSs(SPh)6(0Me3JCH3CN which is more efficient than clusters containing only Fe or Mo.[" Homogeneous hydrogenation of 2-pentene and styrThe ~ (catalytic CO)~] activity .~~~ of~ enes was catalyzed by [ H M o F ~ C O ( , U ~ - C M ~ ) C
1
628
2 Metal Clusters in Catalysis
[MoFeCo(p3-S)(p-dppe)(Cp')(CO)6] in hydroformylation has been associated with the fact that the bidentate dppe ligand bridges the Fe-Co bond.['001Asymmetric hydrosilation of acetophenone was attempted with the optically active cluster (+)-5 but photoracemization proceeds faster than the hydrosilation reaction." Mo-RU The complex [ ( O C ) ~ M O ( ~ - P P ~ ~ ) ~ Rwas U ( found C O ) ~to] be an active catalyst for the homogeneous hydrogenation of cyclohexanone (more active than the corresponding Cr-Ru and W-Ru complexes) and the hydroformylation of styrene.[76] The heterogeneous catalyst derived from [Mo2Ru3C(C0)16I2- on SiO2 was less active for hydrogenation of CO to hydrocarbons and oxygenates than [RuCoz], [RuCo3], and [ R u ~ C Ocatalysts, ~] although the selectivities for oxygenates were in the range l2-2O~b.[~ 71 Phenylacetylene is trimerized at 37 1 K by head-to-tail coupling around the mixed-metal cluster [Mo2Ru(pu,-S)Cp2(CO)7].It is believed that the dimolybdenum unit serves as the alkyne oligomerization site but the chemistry overall is dependent on the entire cluster functioning as a nit.['^^.^^^]
Mo-Ru-CO The tetrahedral cluster [MoRuCo(p3-S)Cp(CO)s](6) catalyzes the hydrogenation of 2 - ~ e n t e n e [ ~and ~ I [ HMoRuCo(p3-CMe)Cp(CO)s] catalyzes the conversion of various fumaric esters to the corresponding maleic esters, with high turnover numbers." 041
5 M=Fe 6 M=Ru
Mo-0s
A comparison of catalytic properties for CO reduction was performed for aluminasupported catalysts derived from the tetrahedral clusters [HMoOs3Cp(CO)12] and [HWOs3Cp(C0)12]and from [ O S ~ ( C O ) ~ No ~ ] .difference [ ' ~ ~ ~ was observed and this was explained by the fragmentation of the surface-bound clusters during thermal activation or by poisoning of the group 6 metal atom by carbon originating from the cyclopentadienyl ligands.
2.2 Heterometallic Clusters in Catalysis
629
Mo-CO Cyclohexene hydrogenation was catalyzed by Mo-Co clusters of different structural types." 06] The mixed-metal clusters 7-9 have been tested as homogeneous catalysts for the hydroformylation of 1-pentene and styrene.['07,''*I Isomerization of 1-pentene competes with hydroformylation to hexanal and 2-methylpentanal. Hydroformylation of styrene was achieved under mild conditions with moderate to high branched-to-normal selectivity when cluster 8 was employed. These clusters could be recovered in high yield (>90%) after catalysis.['0s1 Photoinitiated hydrosilation of acetophenone with triethylsilane occurred in the presence of the clusters 7-9.['011 A supported bimetallic catalyst derived from the sulfido cluster [ M02C02(p4S)(p3-S)2Cp2(CO)4](10) has been studied in CO methanation. It was suggested that the clusters do not fragment and re-aggregate into larger c r y s t a l l i t e ~ . [ ~ ~ ~ ~ ~ ~ Me
7 L=CO
9
8 L = P(OMe)3
CP 10
A heterogeneous catalyst derived from the mixed carboxylates [Moz{ Co3(CO)9CC02}4]selectively hydrogenated crotonaldehyde to crotyl alcohol, in contrast to conventional catalysts selective for the hydrogenation of the C=C double b ~ n d . [ ' ' ~ ~ ~ ' ~ ~ Hydrodesulfurization ( HDS) has been extensively studied with this metal c ~ u p l e . [ ~"-" ~ - ~31 ~ HDS ,' of thiophene and thiophenol by [Mo2Co~(p4-S)(pu,-
630
2 Metal Clusters in Catalysis
S)2Cpi(CO)4] (analogous to 10) was performed and although it does not seem to be catalytic, it provides an interesting insight into the possible role of sulfidesubstituted bimetallic clusters in a homogeneous form of this important reaction.[114]When placed on an alumina support, the catalytic activity of this cluster was similar to that found for heterogeneous Mo/Co/S catalysts.[941Extensive studies with [ M o ~ C Ocatalysts ~] derived from 10 have been performed on y-Al203, Si02, Ti02, and Mg0.[97,112,1151 It was found that the MMCD catalyst was more efficient in producing the active site for HDS than conventionally prepared CoMoS catalysts and that its activity for removal of sulfur from thiophene was comparable with those of commercial HDS catalysts, and its consumption of hydrogen was lower (less butane f ~ r m a t i o n ) . ~ ' ~Other ~ ' ~ ] Mo-Co clusters were also found to have synergistic effects in HDS catalysis.['061 The cubane cluster [ M O ~ C O ~ S ~ ( ~ - C ~ H ~reacts E ~ ) ~with ( C OPhSH ) ~ ] under CO pressure to give PhSSPh and PhSC(O)Ph, also indicative of catalytic HDS From a series of thiocubane clusters, [ M o ~ C O ~ S ~ C ~ was ~ ( Cfound O ) ~ to ] be the best precursor to dispersed bimetallic catalysts for liquefaction of a subbituminous coal." "1 Mo-Co-Ni The cluster [MoCoNi(pu,-CMe)Cp2(CO)~] (11) was found to isomerize 1-pentene, a competition reaction observed during hydroformylation catalysis, and hydrogenate 2-pentene and Hydroformylation of styrene was also investigated.[1 0 7 %1081 Photoinitiated catalytic hydrosilation of acetophenone with triethylsilane occurred in the presence of ll.[lO1l 0791081
11
Mo-Rh
The complex [ M o R ~ C ~ ( ~ - C O ) ~ (PPh3)2] C O ) ( in toluene solution catalyzes the hydrogenation of cyclohexene to cyclohexane at ambient temperature and atmospheric pressure of hydrogen.["'] With the [MoRh] catalyst derived from the same
2.2 Heterometallic Clusters in Cutulysis
63 1
bimetallic complex on alumina or silica, selective hydrogenation of CO led to dimethyl ether or methanol, respectively, whereas for both oxide supports methanol was the main oxygenate in hydrogenation of C02. The performance of the MMCD catalyst was related to the greater dispersion of the Rh atoms, separated from each other by the Mo atoms.['201The selectivity for alcohols in CO hydrogenation with the [Mo?Rh] catalyst derived from [MozRhCp3(CO)s] on Si02 is higher than with conventional catalysts and this was explained by Mo-promoted COi n s e r t i ~ n . [ ' ~ ' - 'By ~ ~ ]comparison with the related [ WzRh] catalyst, it was noted that molybdenum has a greater promotional effect than The [Mo2RhCp3(CO)s]cluster on Si02 was also used as precursor to a heterogeneous catalyst for the hydroformylation of ethylene and propene. Molybdenum increases the rate of both hydrogenation and hydroformylation of olefins. It was suggested that propanal, the primary product of the hydroformylation, was hydrogenated to propanol on active bimetallic center^.^'^^.'^^^ After CO photoreduction and thermal treatment the silica-supported cubane-type clusters [(Cp*Rh)2Mo309(0Me)4]and [(Cp*Rh)4Mo4016]have significant catalytic activity in propene metathesis.[261 Mo-Ir
The heterogeneous catalysts prepared from [MoIr3Cp(C0)11] (12) and [ M O ~ I ~ ~ C P ~ ( C(13) O ) Ion O ]alumina were studied for n-butane hydrogenolysis, a structure-sensitive reaction." 261 From comparisons made with catalysts derived from homometallic clusters or their mixtures, it was deduced that the properties of the MMCD catalysts originated from bimetallic interactions maintained in the activated materials.
12
13
Mo-Ni
Heterogeneous, alumina-supported bimetallic catalysts derived from the sulfido clusters [ M o ~ N ~ ~ Sand ~ C[PM~o] ~ N ~ ~ S ~ C P ~have (CO been ) ~ ]studied in CO metha-
632
2 Metal Clusters in Cutulysis
nation.[951The cubane-typed cluster [Mo3Ni&Cl( H20)9]3+ was incorporated into various zeolites by ion-exchange and the resulting material had catalytic activity in the HDS of benzothi~phene.[~~~.'~~] Mo-Pd
The hydrogenation of 1,5,9-~yclododecatriene was performed using [MoPd(pPh2Ppy)(C0)3C12] as a homogeneous catalyst or as a resin-immobilized catalyst. The former had higher selectivity for cyclododecene but the conversions were similar.['291The planar clusters [ M o ~ P ~ ~ C P ~ (PR3)2] C O ) ~(3c, ( 3d) proved to be only moderately active hydrogenation catalysts for 1,5-~yclooctadieneand double-bond isomerization competed effectively with hydrogenation. They also enabled hydrogenation of phenylacetylene to a mixture of styrene and ethylbenzene. Styrene was the major product, suggesting that these clusters might lead to efficient and selective catalysts for reduction of alkynes to alkenes.["] The photocatalyzed hydrosilation of 1-pentene was also performed with these clusters, as was butadiene oligomerization, giving low molecular weight polymers and a mixture of 4-vinylcyclohexene, 1,5-~yclooctadiene,and cyclododecatriene.[80] a tcn, M+s
/I S-Pd? ,M-S
I
"CI
13'
/S-/7MO--tacn
tacn 14
The cuboidal cluster [Mo3PdS4(tacn)3Cl][PF6]3(14) is an efficient catalyst for the stereoselective addition of carboxylic acids to electron-deficient alkynes [' 301 and the cyclization of alkynoic acids to enol lactones (Eq. l).[13'] 14
// / - X C O O H NEt3 in CH3CN * tacn = 1,4,7-triazacycIononane
Qo (l)
When [Mo2Pd2Cp2(C0)6(PPh3)2] (3d) was used as precursor to [Mo2Pd2] MMCD catalysts, selective catalytic reduction of NO was achieved,['32]as was the catalytic carbonylation of organic nitro derivatives into isocyanates (Eq. 2).L133,1341 Ar-NO2
+ 3 CO + Ar-N=C=O + 2 CO
(2)
2.2 Heterornetullic Clusters in Catalysis
633
There is a considerable academic and industrial interest in this reaction, which represents a route to aromatic isocyanates avoiding phosgene, classically used to convert amino derivatives into this important class of chemicals. The [MolPdz] catalyst was more selective for phenylisocyanate than were conventional catalysts prepared by mixing the individual components. The bimetallic, octanuclear cluster Naz[Pd4 { MoCp(C0)3}4] has been found to catalyze the dehydration of alcohols such as MeOH, EtOH, and Me2CHOH, to give carbene ligands. trans-Stilbene was formed from PhCH20H." 351 Mo-Pt
Mono- and dihydrogenation of terminal alkynesl ' 361 and polymerization of transNBD1'371 have been catalyzed by the chain molecule [Pt{MoCp(CO)3}2(CNCy)2]. The planar clusters [ M o ~ P ~ ~ C P ~ PR3)2] ( C O ) ~(15) ( were used as homogeneous catalysts for the hydrogenation of 1,5-~yclooctadiene and the hydrogenation of phenylacetylene to a mixture of styrene (major) and ethylbenzene. The MMCD catalyst prepared from the related complex trans[Pt{MoCp(CO)3}2(NCPh)2] supported on MgO was characterized by various spectroscopic techniques, including EXAFS,1'31' and tested for hydrogenation of toluene at 1 atm and 333 K.['391 The photocatalyzed hydrosilation of 1-pentene was also performed in the presence of the tetranuclear clusters 15 although they had little activity. The predominant reaction was olefin isomerization.'sOl The inorganic cluster [Mo6Pt024I8- on MgO was used as precursor to a catalyst having much greater activity in the dehydrogenation of butane, isobutane, and propane than conventionally prepared bimetallic Mo-Pt or monometallic Pt/MgO and Mo/MgO catalysts. The selectivity to the corresponding alkene was typically above 9 7 % ~ ~ ' ~ ~ ~
15a 15b 17a 17b
M=Mo; R = E t M=Mo; R = P h M=W; R=Et M=W: R=Ph
634
2 Metal Clusters in Catalysis
W-Fe The [HWFe(C0)9]- anion catalyzes the isomerization of 1-hexene and of allylbenzene into internal olefins in the presence of light.[75] W-Fe-Co The cluster [ WFeCo( p3-PMe)Cp(CO)8]catalyzes the hydrogenation of 2-pentene and styrenes.[991 W-Ru The complex [(0C)4W(pu-PPh2)Ru( CO)3] was studied as a homogeneous catalyst for the hydrogenation of cyclohexanone and the hydroformylation of styrene. For the latter reaction, it was more active than the corresponding Cr-Ru or Mo-Ru catalyst~.l~~] W-Ru-CO The cluster [HWRuCo(p3-CMe)Cp(CO)8]catalyzes the conversion of fumaric esters to the corresponding maleic esters with high turnover
w-0s The catalytic properties of alumina-supported [ HWOs3Cp(CO)121 for CO reduction have been compared with those of related heterogeneous [Moos31 catalysts (see above).[ O 51 w-co Photoinitiated hydrosilation of acetophenone with triethylsilane occurred in the presence of the complex [ W C O ~ ( ~ ~ - C H ) C ~ ( C O ) ~ ] . [ ~ ~ ' ~ W-Rh The [WzRh] catalyst prepared from [W2RhCp3(C0)5] on Si02 was active in the hydrogenation of C0.[124] Homogeneous hydroformylation of alkenes and alkynes has been performed with [(OC)4W(pU-PPh2)RhH(CO)( PPh3)]. High selectivities for the branched chain aldehyde were observed in the reaction with styrene.[141]Tungsten-rhodium systems might also become interesting for homogeneous HDS catalysis.[142]
2.2 Heterometullic Clusters in Cutulysis
635
W-lr The tetrahedral clusters [ WIr3Cp(CO), I ] and [ W2Ir2Cp2(CO)lo] and the corresponding homometallic complexes [ Ir,(CO) 121 and [ W2Cp2(CO)6] were adsorbed on to y-Al203 from cyclohexane solutions.['431The supported compounds were then subjected to temperature-programmed decomposition in a stream of hydrogen. Most of the coordinated CO was hydrogenated to CH4 rather than being released intact; the extent of hydrogenation decreased as the W/Ir ratio was increased. Like their Mo-Ir analogs, these clusters were examined for n-butane hydrogenolysis. The selectivity of the [ WIr3I MMCD catalyst for ethane production was 2 70%, compared with < 50% for the [ W2Ir2] ~ a t a 1 y s t . l ' ~ ~ ~
W-Ni Very little seems to have been reported on the use of W-Ni clusters in catalysis. Cage-type clusters were, however, found to be very active in homogeneous cyclohexene hydrogenation and HDS reactions. The results indicated the promoting role of Ni.llohl
W-Pd The dinuclear complex [(OC)4W(p-PPh2)2Pd(PPh3)l (16) was used as a homogeneous catalyst for the hydroformylation of styrene."61 PR3)2] (3e, 3f ) catalyze the hydrogenation The planar clusters [ W~Pd2Cp2(C0)6( and isomerization of 1,5-~yclooctadieneand the hydrogenation of phenylacetylene to a mixture of styrene (major) and ethylbenzene. These clusters also catalyze the photoinitiated hydrosilation of 1-pentene and butadiene oligomerization.lsol CO)~( The heterogeneous [ W2Pd2I catalyst, prepared from [ W ~ P ~ ~ C P ~ (PPh3)2] (3f ), has completely different properties for the isomerization of 2-methylpentane in methylcyclopentane from those of the corresponding [Cr2Pd2] cluster-derived catalyst it selectively afforded 3-methylpentane.['441 -
Ph2 16
636
2 Metal Clusters in Catalysis
W-Pt The planar clusters [ W2Pt2Cp2(C0)6(PR3)2] (17) catalyze the homogeneous hydrogenation of 1,5-~yclooctadieneand of phenylacetylene to styrene (major) and ethylbenzene, and are active for the photocatalyzed hydrosilation of 1-pentene.["I A heterogeneous catalyst prepared from trans-[Pt{ WCp(CO)3}2(NCPh)2] supported on MgO was tested for toluene hydrogenation at 1 atm and 333 K. Strong W-Pt interactions seem to reduce the catalytic activity compared with catalysts containing the same metals but lacking bimetallic interactions (see also M o - P ~ ) . " ~ ~ ] Compared with conventionally prepared mono- or bimetallic catalysts, higher activity and methanol selectivity (>goo/,) was observed in C02 hydrogenation with [ WPt] catalysts, prepared from [HWPt(pu-PPh2)(p-CO)(C0)4( PPh3)] on Si02 (see also above under Cr-Pt). The importance of having two different metals in the catalytic system was clearly Trinuclear chain complexes trans- Pt{WCp(C0)3}2(CNR)2]( R = Cy, Bu') catalyze the polymerization of NBD.['37
I
Mn-Fe The arsenic-bridged complex [(OC)5Mn(pu-AsMe2)Fe(C0)4] has catalytic activity in the hydrogenation and isomerization of 1-0ctene.['~1 Addition of manganese to conventional iron- or cobalt-based Fischer-Tropsch catalysts generally leads to increased formation of light olefins whereas the catalyst activity decreases. An [MnFez] MMCD catalyst was prepared from the carbonyl cluster [Et4N] [MnFe2(CO)l2]on A1203, Si02, MgO, Ti02, and ZrO2. The decrease in CO methanation and the increased yield of olefins and higher hydrocarbons when using the MMCD catalyst, compared with conventional catalysts, was taken as an indication that the manganese-iron interaction still occurs after the thermal decomposition of the supported cluster.['451Turney et al. studied the properties of silica-supported catalysts derived from the potassium salt K[MnFe2(CO)121. At elevated temperatures CO/H2 conversion led to aromatics (mainly toluene and xylenes) with a maximum of ca 20% at 673 K.['461Depending, in particular, on the support, hydrogenation of CO over A1203-, MgO- or Zr02 Al203-supported [MnFez] catalysts also prepared from this cluster occurred with variable selectivity, although oxygenates were the major products.['471Carbon-supported Mn-Fe and K-Mn-Fe catalysts were employed for the selective synthesis of C2-C4 olefins from CO and H2.[1481 The effect of varying the Mn/Fe ratio and the amount of potassium was studied by using the mixed-metal clusters K[MnFe(CO)9], [Et4N][MnFe(CO)g], [MnzFe(C0)14], and [Et4N][MnFez(CO)l2] as precursors. Highly dispersed Mn-Fe catalysts were obtained with selectivities to C2-C4 olefins as high as 85-90% w / w , with the balance being methane.['48]
+
2.2 Heterometullic Clusters in Cutulysis
637
Mn-Ru The catalytic activity of [MnRu(C0)6{PhC=C(H)C(H)=N(Pr')}] for the homogeneous hydrogenation of styrene was explained by the reversible hapticity change of the Mn-coordinated azadienyl The MMCD catalyst derived from [MnRu3C(CO)13]- on silica was less active for hydrogenation of CO to hydrocarbons and oxygenates than [RuCoz], [RuCo3], and [ R u ~ C Ocatalysts, ~] although selectivities for oxygenates were in the range 1220%J Mn-Co The performance in CO hydrogenation of MMCD catalyst prepared from [MnCo(CO)g] on alumina was compared with those of conventional catalysts and of catalysts prepared by successive or simultaneous impregnation of [Mnz(CO)101 and [Co2(CO)8].The activity at 473 K and 1 atm was strongly enhanced with only small changes in selectivity; only n-alkanes were formed with a maximum at Cg. Two main reasons were invoked to explain the different behavior of the MMCD catalysts: i) higher temperatures are required with conventional catalysts to reduce the salts and this leads to larger crystallites and inhomogeneous samples, in contrast to the situation with zerovalent precursors; and ii) the counter ions, e.g., chlorides, present when using conventional metallic salts are difficult to eliminate completely and the oxidation state of manganese is not as well known as when using zerovalent precursor complexes.['501
Mn-Co complexes immobilized on silica have also been used for the oxidation of cyclohexene.[ 41 Mn-Rh
A salt formulated as [Mn{Rhl2(CO)30}]was reported to catalyze CO hydrogenation.["'' Mn-Pd The complex [ MnPdBr(pu-dppm)2(C0)3](18a) catalyzes the formation of ethyl formate from a C02/H2 mixture (1 : 1, 12 atm) in the presence of ethanol and triethylamine at 403 K. The corresponding iodo complex 18b has been tested in methanol homologation. In the presence of aqueous HI, methanol reacted with
2 Metal Clusters in Catalysis
638
I IM B)C O(
t
Pd-X
t
18a X = B r 18b X = l
CO/H2 (1 : 1, 14 atm) at 403 K to afford Me20, AcOH, AcOMe, and MeCH(OMe)2 (55% molar selectivity).['521 Re-0s
A MgO-supported [ReOs3] catalyst for CO hydrogenation was prepared from [H3ReOs3(C0)13]. The bimetallic particles were stable under catalytic conditions and Re was found to prevent formation of the otherwise observed [ O S I O C ( C O ) ~ ~The ] ~ ~same . [ ' ~precursor ~~ on y-Al203 leads to an active MMCD catalyst for n-butane hydrogenolysis.[' 541 Re-Rh
The hexahydride complex [( HCy2P)ReH5(pu-PCy2)2RhH( PCy2H)I was shown to be an active, long-lived homogeneous catalyst for hydrogenation of alkynes, alkenes, and d i e n e ~ . [ ' ~ ~ ] A salt formulated as [Re2{Rh12(C0)30}3]was reported to catalyze CO hydrogenation.[' 561 Re-Ir
The isomeric clusters 1,4-[Re&(CO)l~{p3-Re(C0)3} {p3-Ir(CO)2}]2- (19a) and 1,3-[ResIrC(C0)17{p3-Re(C0)3}2I2- (19b) with different countercations, Et4N+ or PPN+ were supported on y-A1203 and used to prepare MMCD catalysts.['57] The largest nanoparticles, derived from the Et4N-containing precursors, comprised an Ir core surrounded by a three-dimensional cluster of up to 28 metal atoms. Smaller nanoparticles were obtained when the catalysts were derived from the PPNcontaining precursors. The rate of ethane hydrogenolysis increased with average metal cluster size.
2.2 Heterometallic Clusters in Catalysis
639
1
2-
0 0 19a
19b
Re-Pt The complex [Cp(OC)2HRePtH(PPh3)2] in benzene catalyzes the hydrogenation of ethylene at room temperature and 0.6 atm of H2.[1581 The turnover frequency for methane formation by hydrogenolysis of cyclopentane with the [RezPt(CO)12]-derivedcatalyst supported on ?-A1203 is over 100 times higher when compared with a mixture of monometallic complexes and this was ascribed to direct contact between the Re and Pt atoms on the surface.['s91The same complex on ?-A1203 showed that the MMCD catalyst was more resistant to deactivation during catalytic dehydrogenation of methylcyclohexane than conventional catalysts and this was explained by the role of Re in stabilizing the dispersion of the Pt.['60]Sulfidation of various RePt3 molecular clusters has been investigated as a model for reactivity of metal surfaces and sulfidation of bimetallic Re-Pt/ A1203 catalysts used in petroleum refining.['61*'621 Fe-Ru The homogeneous hydrogenation of 1,3-cis- and 1,3-trans-pentadiene, 1- and 2pentyne, or diphenylacetylene and the isomerization of cis-stilbene have been examined with the clusters [FeRu?(C0)12],[FezRu(C0)12], and [H2FeRu3(C0)13]as catalyst p r e c ~ r s o r s . [ ' ~ ~Their ~ ' ~ ~anchoring 1 on '/-A1203 leads to a decrease in activity for pentyne hydrogenation but to increased activity for pentadiene hydrogenation. The tetrahedral cluster [ H ~ F ~ R U ~ ( Creadily O ) ~ ~catalyzed ] the isomerization of 1-hexene to give a mixture of cis- and trans-2-hexenes with a turnover number of approximately 4500 mol of hexene mol-I cluster x h.['651Homogeneous hydrogenation-isomerization of 1,3- and 1,4-~yclohexadienewas catalyzed by [Fe2Ru(CO)12]and [ H ~ F ~ R u ~ ( C but O ) Ithere ~ ] was evidence of catalysis by metal species rather than intact The cluster [H2FeRu3(CO)n] was used to prepare highly dispersed metal particles on GLC-grade Chromosorb to catalyze the hydrogenation of dienes and aromatic hydrocarbons and the disproportionation of monoenes.l'67'
640
2 Metal Clusters in Catalysis
In CO hydrogenation, the [FezRu] catalyst derived from [FezRu(C0)12] on y A1203 was found to be ca 50 times more active and ca 6.2 times more selective towards C 2 4 5 hydrocarbons than the [Fe3] catalyst derived from [Fe3(C0)12]. In contrast, conventional Fe-Ru catalysts, prepared by impregnation of an aqueous solution of Fe( N03)3 and RuC13, produced predominantly CH4 under identical conditions.['68] ZrO2-supported catalysts prepared from [FeRu2(CO)12] and [FezRu(CO)121 also had much higher activity than monometallic or conventional catalyst^.['^^^'^^] When supported on A1203, A1203/KOH or MgO the [FeRu3] catalyst derived from [H2FeRu3(CO)131 was active for methanation and FischerTropsch reactions involving CO and C02.[1711 The activity and selectivity in CO hydrogenation of highly dispersed MMCD catalysts prepared from [FezRu(CO)121 and [H2FeRu3(C0)13] have also been compared with those of catalysts derived from a mixture of the homonuclear clusters [Fe3(CO)12] and [ R u ~ ( C O ) ~The Z]. catalyst [2Fe3 + R u ~ had ] the same activity as [Fe2Ru], in agreement with their composition, whereas the catalyst [FeRu3], with the highest amount of Ru, was the most active.['44]With [ F e R q ] on y-alumina, silica, or Na-Y zeolite the best yields were found with alumina as a support.[' 72,1731 Highly dispersed Fe-Ru bimetallic crystallites were obtained on amorphous carbon black when using [FeRuz(CO)121, [FezRu(C0)12], or [ H ~ F ~ R U ~ ( CasOprecursors. )~~] Reduction of these clusters on carbon seems to be more facile and complete than on many oxide supports. The data showed that ruthenium is more active than iron for CO hydrogenation but is less active than iron in forming C02. There was a gradual change in selectivity of the Fe-Ru catalysts when their intermetallic composition was varied (methane formation increased with Ru content).[' 741 The sequentially impregnated [Fe3 + R u ~ ] catalyst produced a higher olefin/paraffin ratio than any of the bimetallic Fe-Ru clusters examined.['75]The [Fe3Ru3]catalyst derived from [Fe3Ru3C(C0)16I2- was less active in CO hydrogenation than [RuCo2], RuCo31, and [ R u ~ C Ocatalysts ~] but had significant selectivity for oxygenate^.[^^,'^^ The complex [(OC)3Fe(pPPh2)2Ru(CO)3] has moderate activity in the hydrogenation of cycl~hexanone.[~~] The clusters [FeRu2(C0)12], [Fe2Ru(CO)12],their phosphine and phosphite derivatives, and [H2FeRu3(C0)13] are more active water gas shift reaction (WGSR) 7-1801 The results show a catalyst precursors than the monometallic prec~rsors.['~ clear decrease in turnover frequency as the iron content of the cluster decreases. Monosubstitution by phosphine or phosphite enhances the activity of clusters with a high iron content by stabilizing the parent cluster. Disubstitution of [FezRu(C0)12] seems to reduce the stability of the cluster under basic WGSR conditions.'' 79] The bimetallic complex [(OC)3Fe(pu-PPh&Ru(C0)3] is also an active catalyst for the homogeneous hydroformylation of styrene (393 K, 20 atm, CO/H2 = 1). A synergistic effect between iron and ruthenium was observed and the intact complex was recovered at the end of the reaction.[76] The [FeRu2] catalyst prepared from [FeRu2(CO)lz] on ?-A1203 was the most active [Fe3-,Ru,] (x = 0-3) catalyst for C2H4 self-homologation. Increasing
\
2.2 Heterometallic Clusters in Catalysis
641
selectivity (C3 : Cq ratio) with increasing Fe content was observed for the MMCD catalysts in contrast with conventional catalysts. The [FeRu*]catalyst was more than one order of magnitude more active in ethane hydrogenolysis than was the [FezRu] catalyst." 6 8 ] Active sites of different structure and arrangement are required for ethane hydrogenolysis compared with those active in ethylene self-homologation.
Fe-Ru-Co The cluster [HFeRuCo(p3-PMe)(CO)9] 20) has been used as a catalyst for the hydrogenation of styrenes and 2-pentene.r99,
Me
20
Fe-0s Thermal activation under argon at ca 400 K of [HzFeOs3(CO)13]physisorbed on silica cleaved the heteronuclear Fe-0s bonds and metallic particles were formed (ca 16A) at temperatures greater than 523 K. As expected from the breakdown of the mixed-metal cluster, the MMCD catalyst had activity and selectivity patterns in ] Fischer-Tropsch catalysis intermediate between those of the [ Fe3] and [ O S ~homometallic systems.['"] The [FeOs3] catalyst on y-AlzO3 was found to be two orders of magnitude less active at 543 K in CO hydrogenation than the [ O S ~ Rcatalyst, ~] but had a high selectivity for ether formation.['s21
Fe-Co The photocatalytic isomerization of 1-pentene has been studied in the presence of [F~~COC~(CO)~].~"~] The cluster [HFeCo3(CO)lz] has been used to prepare highly dispersed metal particles on GLC-grade Chromosorb. These were tested for the hydrogenation of dienes, aromatic hydrocarbons, and alkynes." 'I The efficiency of tetranuclear clusters for the homogeneous proton-induced reduction of CO to CH4 increases in the order [Co4(CO)12]< [FeCo3(C0)12]< [Ru3Co(C0)13]-< [Fe3Co(CO)13]- < [Ru4(C0)13l2- < [Fe4(C0)13]2-.['841An-
642
2 Metal Clusters in Catalysis
choring the cluster [HFeCo3(C0)12]on a silica gel matrix bearing amino functions followed by decarbonylation in a stream of hydrogen at atmospheric pressure and 473 K led to highly active catalysts in Fischer-Tropsch synthesis. At atmospheric pressure and 513 K, 20% conversion of synthesis gas was observed and an unusually narrow product distribution had a maximum at C6."' 51 Hydrogenation of CO at atmospheric pressure and 493-553 K was also performed with a highly active [FeCo] catalyst derived from [FeCoCp(C0)6] on Carbon-supported catalysts derived from K[FeCo(CO)g], [HFeCo3(C0)12], K[FeCos(CO) 121, K[Fe3Co(CO) 131, and [Et4N][Fe3Co(CO)131 have also been studied in CO hydrogenation. Although addition of potassium was found to reduce catalytic activity markedly, it greatly enhanced olefin selectivity.[ls8l Methanation and Fischer-Tropsch reactions involving CO and C02 have been catalyzed by heterogeneous catalysts derived from [HFeCo3(C0)12]on y-A1203/KOH, pA1203, or M ~ O . [ ~ ~ ~ I The cluster [FeCo2(p3-PPh)(C0)9](21) has been tested as a catalyst for the homogeneous hydroformylation of 1-pentene and ~ t y r e n e . [ ' ~ ~ ~ ' ~ ' l
Ph P
21
Isomerization of 1-pentene was found to compete strongly with hydroformylation to hexanal and 2-methylpentanal. A synergistic effect was observed when [ Et4N][FeCo3(CO)121 was used as homogeneous catalyst in the hydroformylation of cyclohexene to yield cyclohexanecarbaldehyde. In comparison, [Et4N][Fe3Co(CO) I 31 had little activity."891 The cluster [( P ~ C H ~ ) M ~ ~ N ] [ F ~ Chad O ~good ( C Ocata)~~] lytic activity and a selectivity of CCI. 100% for the hydroformylation of terminal, inwas shown to ternal and cyclic ole fin^.^'^^-'^^] The cluster [Fe2C02(p4-PPh)2(CO)11] persist during hydroformylation of terminal olefins at 403 K.[l9 3 . 1941 The hydrido cluster [HFeCo3(C0)12] was evaluated as a catalyst for the WGSR.[1791 Salts of [FeC03(C0)12]-['~~~'"~ or [Et4N][Fe&o( CO) 1 3][19631971 promoted with methyl iodide have proven to be catalysts for methanol homologation when the reaction was performed at relatively high pressures and moderate temperatures. It was possible to obtain acetaldehyde in 80% selectivity with methanol conversions
2.2 Heterometullic. Clusters in Catalysis
643
of 7 5 % ~For optimum yield of ethanol long reaction times at high temperature are required.['951Higher selectivity for ethanol at lower conversion of methanol was obtained with [Et4N][Fe3Co(C0)13]at 40 atm CO, 80 atm H2, and 453 K.11961 Homogeneous photoinitiated hydrosilation of acetophenone has been catalyzed by 21.11011 The anionic cluster [Me4N][FeCo3(C0)12] had greater catalytic activity than [ HFeCo3(CO)l2] or [Co4(CO) 121 for the stereospecific dimerization of norbornadiene to 'Binor-S'.119y1 Finally, Fe-Co alloys prepared by sonochemistry had very high catalytic activity for dehydrogenation and hydrogenolysis of cyclohexane.[200]
"
Binor-S"
Fe-Rh Under 10 atm hydrogen, isomerization of 1-heptene into cis- and trans-2-heptene ( R = H, Ph). Formation of 14% was catalyzed by [HF~~R~(CO)II(,U~-~'-C=CHR)] heptane was also observed. Isomerization of 2-heptene into 3-heptene subsequently takes place at a lower rate, indicating that fragmentation of the cluster occurs.J2o11 By comparison, [Ph4P][Fe3Rh3(p4-q3-MeC=C=CH2)(p-C0)3(CO)lo] was far less active toward the isomerization of alkenes.[2021These are two examples of mixed iron-rhodium systems in which the known hydrogenation ability of rhodium is reduced by the presence of iron, thus favoring isomerization over hydrogenation reactions. Catalysts of type [Fe2Rh4(CO)l6l2-/Na-Y zeolite were investigated for CO hydrogenation and for h y d r o f o r m y l a t i ~ n . ~Promotion ~ ~ ~ ~ ~ ~ ~by l iron in CO hydrogenation and olefin hydroformylation might be associated with the heteronuclear adjacent Fe-Rh sites in zeolite supercages, which enhance CO migratory insertion into Rh-H and Rh-alkyl bonds.[205*2061 MMCD catalysts derived from SiO2-supPorted [Me4N]2[FeRh4(co)I 51, [TMBA][FeRhdco)I 61, [ T M B A ] ~ [ F Q R ~ ~ (161, CO) and [Fe3Rh~C(C0)14] on SiO2 had higher activity and selectivity for ethanol formation from CO HZthan catalysts prepared from homometallic metal c 1 ~ s t e r s . [ ~ ~ ~ , ~ ~ ~ ~ Hydrocarbon formation was considerably suppressed, possibly because of the siteblocking of the Fe-Rh ensembles.[207]The performances of MMCD Fe-Rh catalysts are generally found to be better than those of catalysts prepared from monometallic salts.[20y1
+
644
2 Metal Clusters in Catalysis
Hydroformylation of ethylene has been catalyzed by [FeRh{p-P(Bu‘)~} (p-dppm)(C0)4].[2101In the hydroformylation of 1-pentene, the cluster [HFe3Rh(C0)11{p4-y2-C=CH( Ph)}] had the same activity as [Rh4(CO)12]whereas in olefin isomerization reactions the mixed-metal system had specific activity.[201] The nitrido clusters [Fe4Rh2N(C0)15]- and [FesRhN(C0)15I2- are homogeneous catalysts for the hydroformylation of 1-pentene with a selectivity of 35-65% for nhexanal.[’”] The cluster [Ph4P][FesRhC(C0)16 has been used as catalyst precursor for the hydroformylation of 1-pentene)”J and under catalytic conditions (60 atm CO H2 at 373 K), it was transformed into [Fe4RhzC(CO)16] and [Ph4P][Fe4RhC(CO)14].The Fe4Rh2C cluster had greater activity than the FesRhC cluster, and the activity of the Fe4RhC cluster was similar to that of Fe5RhC, (22). The although it was transformed during catalysis into [Ph4P][Fe3Rh3C(CO)15] latter cluster was also shown to be a catalyst for the hydroformylation of 1-pentene, with an activity similar to that of [Fe4RhlC(CO)16].When supported on silica, the [Fe4Rh] and [FesRh] catalysts derived from the corresponding carbido clusters had high activity and selectivity for propene hydroformylation and CO hydrogenation.[’ 31
+
SiOz-supported MMCD olefin hydroformylation catalysts have also been prepared from [Me4N]2[FeRh4(CO)151, [TMBA [ FeRhs(C0) 161, [TMBA]* [ Fe2Rb(CO) 161, and [ FesRhzC(CO)141.[” 7 , 2 0 4,2 A promoter effect of iron was observed and Mossbauer data suggested that Fe atoms were mostly in the Fe3+ state even after H2 reduction at 673 K. Most of the Fe atoms are highly dispersed and help anchor Rh atoms, forming Rh-Fe3+-0 bimetallic sites in which the Fe3+ ions are stabilized and not reduced by H2 even at 673 K.[2141 Carbonylation of ethanol with 92.4% conversion and 52% selectivity for ethyl propionate was performed with [FeRh(p-Ph2Ppy)2(CO)4C1].[931
’,’
1
2.2 Heterometullic Clusters in Cutulysis
645
Carbonylation of nitrobenzene to phenylcarbamate was catalyzed by [PPN]2[FeRh4(CO)15] in the presence of methanol. Addition of bipyridine greatly enhances the reaction rate and selectivity.[2i61 Hydrosilation of PhC-CH was catalyzed by [Me4N][Fe5RhC(C0)16]and the activity was assigned to the Rh atom.12i71 Cycloaddition of C02 and propyne to 4,6-dimethyl-2-pyrone was achieved with an [FezRh4]catalyst derived from [ T M B A ] ~ [ F ~ ~ R ~ ~on ( Cmetal O)I~] Catalysts of type [Fe2Rh4(CO)l6I2-/Na-Y zeolite have been investigated for CO alkane hydrogen01ysis.l~~~~ Fe-Ir Clusters such as [ E ~ ~ N I ~ [ F ~ ~ I and ~ ~ ([Et4N]2[Fe2Ir4(CO)16] CO)I~I on MgO are effiThe cient heterogeneous catalysts for MeOH synthesis from CO + H2.[220*2211 MMCD silica-supported [FeIr4] catalyst, prepared from [TMBA]2[FeIr4(CO)151, was much more selective than Fe-Rh MMCD catalysts for methanol formation in the reaction of CO with H2; it was also more active than a catalyst prepared by mixing homometallic iron and iridium c l ~ s t e r s . ~ ~ ~ ~ , ~ ~ ~ ] Naz[FeIr4(CO)151 had moderate activity as a homogeneous catalyst precursor in the WGSR, indicative of a weak synergistic effect between the two metals. Using [ Ir4(CO)12]alone produced only a weakly active system.[”91 The heterogeneous catalyst derived from [ T M B A ] ~ [ F ~ I ~ ~ ( on C OSi02 ) I ~ ]had better alcohol selectivity in ethylene hydroformylation than [ 11-41on Si02.[208,2221 Fe-Ni Ethanol carbonylation to ethyl propionate has been catalyzed by [FeNi(pPh2Ppy)(C0)3(NCS)2] with a conversion of 95.2% and a selectivity of 47.7%.[93,’291 Fe-Pd The complex [(OC)4Fe(p-PPh2)Pd(p-C1)]2 is a selective catalyst for the isomerization of l-octene to 2-octene and the hydrogenation of l-hexyne in the presence of l-hexene. At 448 K, under 100 atm H2, 93% of a sample of l-hexyne in benzene was reduced to hexene and only 3% to hexane. This is unexpected because palladium is usually an excellent catalyst for the hydrogenation of olefins. It also catalyzes the carbonylation of 1-octene under mild conditions (348 K, 50 atm). The total yield of esters was ten times greater than with [PdC12(PPh3)2]as a catalyst. This bimetallic complex was also an effective catalyst for the carbonylation of 1,5-~yclooctadiene.[~~ 31 Silica-supported catalysts, prepared from [TMBA]2[Fe4Pd(CO) 161 and [TMBA]3[HFe6Pd6(C0)24](23) have different catalytic properties in CO reduction. The former is more active and less selective for MeOH than [FesPds], giving mostly
646
2 Metal Clusters in Cutulysis
Metal core of [HFe6Pd6(C0)24]3-(23)
Fe(C0)3N0
24
25
methane and CT hydrocarbons whereas the [FesPds] catalyst is very selective for MeOH synthesis (79%), behavior reminiscent of that of a palladium-only catalyst.I20 7920832221 Heterogeneous [FesPds] catalysts on Si02 had enhanced alcohol selectivity in ethylene hydroformylation compared to PdC12.[222] The [ Fe4PdI catalyst led to an even higher selectivity in alcohol, because of its higher Fe Propylene hydroformylation was also catalyzed by these bimetallic systems. (24) and [FezPd2(CO)s(NO)2(p-dppm)2] The clusters [FePd2(C0)4(pu-dppm)2] (25) have been used as precursors to silica-supported heterogeneous catalysts for the transformation of o-nitrophenol to b e n z o x a z o l - 2 - 0 n e . [(Eq. ~ ~ ~3). ~~~~~
The results clearly indicate that both MMCD catalysts have significantly improved properties compared to the corresponding conventional catalysts under comparable
2.2 Heterornetallic Clusters in Catalysis
641
conditions. Using silica instead of alumina as a support gave much better results with these Fe/Pd catalysts. Preliminary analytical electron microscopy studies indicate that the metal particles of the MMCD catalysts are initially all bimetallic but metal segregation occurs during r e a ~ t i o n . 1 ~ ~ ~ 1 Fe-Pt With phosphine-functionalized poly(styrene-divinylbenzene) as a support, it was possible to generate the polymer-bound cluster [FezPt(CO)s(Ph2P P)2] [P = poly(styrene-divinylbenzene)] which catalyzed ethylene hydrogenation under mild conditions.[226] The [Fe3Pt3] heterogeneous catalyst prepared from [ T M B A ] ~ [ F ~ ~ P ~51~ ( C O ) I (26) was very selective for CO hydrogenation in methanol whereas the [TMBA]2[Fe4Pt(CO) ~hl-derived [Fe4Pt] catalyst, also silica-supported, was more active but yielded predominantly methane.1207,208,2141 These catalysts were less active in propylene hydroformylation than iron-palladium systems.[208] The [ 21-platinasilaferrocenophane 27 functions as a precatalyst for the ringopening polymerization of the silicon-bridged complex [ Fe(q-C~H4)2SiMe2]to yield the poly(ferrocenylsi1ane) [ {Fe(q-CsH4)2SiMe2}],.[2271
-
I
Fe &SiMe2
26
27
High catalytic activity with stereospecific dimerization of NBD to the 'headto-head' dimer 'Binor-S' was obtained with the chain complex trans[Pt{Fe(C0)3N0}2(CNR)2]( R = Cy, Bu') when a Lewis acid was present whereas in its absence, the 'exo-trans-exo' isomer was selectively obtained (Scheme 3). Related Mo-Pt-Mo and W-Pt-W complexes resulted in polymerization of norb~rnadiene.['~~] The demethylation of methylcyclopentane was studied with the catalyst [ FezPt] derived from the chain complex trans-[Pt (Fe(CO)jN0}2 {CN(Bu')}~]on y-Alz03. Its properties when lightly loaded with platinum (e.g. 0.3% Pt) are similar to those of highly dispersed Pt catalysts (0.2% Pt/A1203).[14492281
648
2 Metal Clusters in Catalysis
i : Catalyst
NBD
"
Bin0 r-S"
"exo-trans-exo" Scheme 3. Dimers of NBD.
Fe-Cu, Fe-Hg Ethanol carbonylation to ethyl propionate has been catalyzed by [ FeCu(pPhzPpy)(CO)3Cl] and [FeHg(p-Ph2Ppy)(CO),(SCN)2]with conversions of 9 1.4% and 94.4% and selectivities of 24% and 26.8%, respectively.[931 Ru-0s The alumina-supported cluster [Al]+[HRuOs3(CO)131- was catalytically active for 1-butene isomerization. After catalysis, [ A ~ ] + [ H ~ R U O S ~ ( Cwas O ) ~the ~ ] -only detectable metal carbonyl species.[2291This supported catalyst decomposed during ethylene hydrogenation at 340 K to give catalytically active metal particles. A heterogeneous, alumina-supported [RuOs3] catalyst prepared from [HzRuOs3(C0)13]was found to be slightly active in the formation of hydrocarbons and dimethyl ether from CO H2.[2301 Polymeric supports with R3N+ functional groups were used to attach the anionic cluster [ H ~ R U O S ~ ( C ~The ) ~ resulting ~]-. catalyst was active in the isomerization and hydroformylation of l - h e ~ e n e . [ ~Its ~ ' Iactivity was greater than that of corresponding homometallic systems, suggesting a synergistic enhancement.
+
Ru-CO
The clusters [RuCo2(p3-S)(C0)9], [RuCo2(p3-PR)(C0)9] ( R = Me, Ph), and [HRu2Co(pu,-PMe)( CO)g] have been used for the catalytic hydrogenation of 2pentene, styrene, and a-substituted styrenes.[991Hydrogenation and isomerization of 1-hexene was catalyzed by [RuCo2(p3-S)(C0)9]and [RUCO~(,LL~-S~)(CO)~].[~~~~ The cluster [HRuCo3(CO)12] has been used to prepare highly dispersed metal
2.2 Heterometallic Clusters in Catalysis
649
particles on GLC-grade Chromosorb for the hydrogenation of monoenes, dienes, and [Ru3Co(C0)13]- has been invesaromatic hydrocarbons, and alkynes, tigated for the proton-induced homogeneous reduction of CO to CH4.L1841 The bimetallic clusters [RuCoz(C0)1 I ] , [HRuCo3(C0)12],and [ H ~ R u ~ C ~ ( C O ) I ~ ] on silica had low activity in CO hydrogenation but greater selectivity for oxygenated products than the corresponding monometallic clusters.[2331A catalyst derived from [HRuCo3(CO)12]on ?-A1203 or MgO has been studied in methanation and Fischer-Tropsch reactions involving CO or COz.“ 711 A comparative study in CO hydrogenation of MMCD catalysts prepared from [HRuCo3(C0)12], [Ru2C02(C0)13],[ H ~ R u ~ C O ~ ( Cand O ) ~[ H ~~ ] ,R u ~ C O ( C Oon) I silica ~ ] showed that Xiao et al. have found those with a 1 : 1 ratio of Ru/Co had the lowest that [RuCoz], [RuCo3], and [ R u ~ C Ocatalysts ~] on silica were more active both for hydrocarbons and for oxygenates than the [Fe3Ru3] catalyst derived from [ Fe3Ru3C(CO)16]2761 0 t her studies have reported the properties of these cataIt was suggested lysts in CO hydrogenation and Fischer-Tropsch that hydrogenation of CO with the [RuCo3] catalyst involved a carbene mechan i ~ m . [The ~ ~use ~ ]of CO-rich syngas showed that the [RuCo3] catalysts led to more oxygenates (Scheme 4).1771 [16731711
0
v (CO) = 1680 cm-’
N
CHBOH
c, ,c=o Go2+
Ruor
?
v (CO) = 1680 cm-’
‘HZ
H,
v (CO) = 1584 cm-’
C
Hads
I
HZ
cr i’
/I\
+
H
I
1
HZ CH&HO, C ~ H S O H
Scheme 4. Proposed mechanism for methanol and c 2 - C ~alcohol formation from CO hydrogenation on bimetallic Ru-Co adjacent sites (adapted from ref. 77).
650
2 Metal Clusters in Catalysis
With y-alumina, silica and Na-Y zeolite as supports, catalysts derived from
[ P P N ] [ R U ~ C O ( C O and ) ~ other ~ ] [Ru-Co ~ ~ ~ ~clusters[2381 ~ ~ ~ ] were studied in the hydrogenation of CO to methane. The formation of C02 suggested the occurrence of the WGSR and the possibility, at low CO/H2 ratios (1 : 4),of there being some C02 methanation. Under these conditions the best yields were obtained with alumina as support. The catalytic activity of Ru-Co clusters as homogeneous catalysts for the WGSR has also been r e p ~ r t e d . [ ” ~Only , ~ ~ ~[ H I ~Ru~CO(CO initiated ) ~ ~ ] a catalytic system. Under basic conditions, [RuCo2(C0)11] and [HRuCo3(CO)12] were almost inactive. The cluster [H3Ru3Co(C0)12]readily decomposes under a CO atmosphere to yield [Ru3(C0)12];this seems to be responsible for the catalytic activity of this system. The activity of [HRuC03(C0)12] was investigated in the homogeneous hydroformylation of alkenes in the presence of triphenylphosphine and tris(dipheny1phosphin~)methane.[~~~.~~~I The surface Co/Ru ratio in a bimetallic catalyst supported on polymer ligands containing N, P, or S coordinating atoms was found to have a more important role in hydroformylation than the total Co/Ru ratio.[241,242] Carbon-supported heterogeneous catalysts were studied for the selective hydroformylation of ethylene and propene, using [HRuC03(C0)12],[H3Ru3Co(C0)12I7or as) precursors. Higher rates and selectivities for n-alcohol [ E ~ ~ N ] [ R u ~ C O131 (CO production were observed with the cobalt-rich MMCD c a t a l y s t ~ . [ ~ ~ ~ 1 The clusters [Et4N][RuCo3(C0)12]and [ E ~ ~ N ] [ R u ~ C O ( C have O ) ~been ~ ] used as homogeneous catalyst precursors in methyl acetate homologation. The presence of the two metals considerably improved the yield of ethyl Methanol homologation with catalysts employing both ruthenium and cobalt are preferred for ethanol production, presumably because they readily hydrogenate acetaldehyde, thereby eliminating the undesired by-products. Thus, high ethanol selectivity at high methanol conversion was observed with [CpRu(PPh3)2Co(CO 4]. A mixture of [CpRu(PPh3)2CI] and [Co~(C0)g] had the same activity.[7,8,245-247 Improved catalytic activity has been observed with [HRuCo3(C0)12]and [cation][RuC03(CO)~~] promoted by methyl iodide, compared with those for [Co2(CO)g]or [ R u ~ ( C O ) ~ ~ ] The anionic cluster [ R u ~ C O CO)13]( resulted in a lower yield alone.“ of ethanol under the same condition^.^'^^*^^^] Although consistent with consecutive transformations, the synergism resulting from a mixed-metal system has been identified for Ru-Co carbonyl clusters. High activity and good selectivity were also observed with [Et4N][RuCo3(pU,-y2-C2Ph2)( CO) 101 .[2491 The role of ruthenium as promoter metal in the hydrocarbonylation of MeOH catalyzed by cobalt on active carbon has been investigated with MMCD catalysts obtained from [HRuCo3(CO)12], [ R u ~ C O ~ ( C O )and I ~ ] , [HRu3Co(C0)13]. The close neighborhood of promoter metal and cobalt centers seems to be a condition for cooperative carbonylation and hydrogenation to occur. Hydrocarbonylation, affording acetaldehyde and its dimethylacetal, carbonylation, leading to methyl acetate, and methane formation were observed as the primary reactions. The reduction steps are facilitated by the presence of ruthenium.[250] 97,2409246~2481
2.2 Heterometullic Clusters in Cutulysis
65 1
The thiophene desulfurization activities of [RuCol] and [ R u ~ C Oheterogeneous ] catalysts derived from carbon- and y-alumina-supported [RuCo2(p3-S)(CO)9] and [HRu2Co(p3-S)(C0)9],respectively, were lower at 673 K than those of catalysts containing only Ru or Co. At 423 K however, this trend was reversed.[251]
R u-Co-A u High activity and good selectivity were observed in methanol homologation with as homogeneous [Au(PPh3)2][RuCo3(CO)12] and [RuCo3(p3-AuPPh3)(C0)12]
Ru-Rh [H*Ru*Rh2(CO)121 had very high activity and selectivity in the hydrogenation of ~ ~ ~ 1 there is no direct metalthe C=C double bond of 2 - ~ y c l o h e x e n o n e . ~Although metal bond in [H(OC)(PPh3)2Ru(p-bim)Rh(COD)](bim = 2,2'-bi-imidazolato), electronic communication through the bridging ligand is believed to account for their higher catalytic activity in hydrogenation of cyclohexene and in hydrogen transfer from propan-2-01 to cyclohexanone, styrene, or benzylideneacetophenone than the corresponding mononuclear ~ o m p l e x e s . 3' ~* 2s41 With the complex [H(OC)(PPh~)2Ru(pu-Cl)(p-pz)Rh(diolefin)] ( pz = pyrazolate; diolefin = COD or TFB ( TFB = tetrafluorobenzobarrelene)) reduction of cyclohexanone by hydrogenation transfer from propan-2-01 was more efficient than with the corresponding mononuclear c ~ m p l e x e s . '1 ~ Synthesis gas has been converted to ethylene glycol and its monoalkyl ether derivatives by use of Ru(acac)3-Rh(acac)3 systems from which the cluster [RuRhz(CO)l2] was isolated.[256] A [Ru3Rh3] catalyst prepared from [ E ~ ~ N ] [ R u ~ R ~ ~ Con( C SiOz O ) has ~ ~ ]been used in CO hydrogenation.[771A pyridine solution of the Ru-Rh mixed-metal clusters catalyzes the WGSR.1'791 The complex [RuRh(p-dpprn)2Cl(CO)3]was tested for the homogeneous hydroformylation of pentene under mild conditions.i2 '] Some hydrogenation was noted in the hydroformylation of I-hexene when using [ P P N ] [ R U R ~ S ( C Oas )I~] catalyst.[2581 The synergistic effect of ruthenium and rhodium in methanol homologation was observed at 100 atm synthesis gas pressure, whereas ruthenium or rhodium chloride alone is inactive for ethanol No enhancement of ethanol production was observed with the mixed-metal compounds [HRuRh3(CO)121, [HRuRh3(CO)la(PPh3)2],[ H ~ R U ~ R ~ ~ ( and C O [PPN][RuRh5(CO)16] )~~], as catalyst precursors. This is consistent with the cluster decomposition found to occur in all the experiments.[2591 Carbonylation of nitrobenzene to phenylcarbamate was catalyzed by [PPN]2[RuRh4(CO)l S ] in the presence of methanol. Addition of bipyridine greatly enhances reaction rate and selectivity.[21 6 ]
652
2 Metal Clusters in Catalysis
Ru-Ir Electronic communication through the bi-imidazolato ligand might account for the in the hydrogenahigher catalytic activity of [H(OC)(PPh3)2Ru(pu-bim)1r(COD)] tion of cyclohexene and in hydrogen transfer from propan-2-01 to cyclohexanone, styrene, or benzylideneacetophenone compared to the corresponding mononuclear c o m p l e x e ~ . [Similarly, ~ ~ ~ ~ ~reduction ~~I of cyclohexanone by hydrogenation transfer from propan-2-01 was more efficient with [H(OC)(PPh3)2Ru(p-Cl)(p-pz)Ir(diolefin) (diolefin = COD or TFB) than with the corresponding mononuclear complexes.[255
1
Ru-Ni The tetrahedral cluster [Ru3Ni(p-H)3Cp(CO)9] is active in the isomerization of 1-pentene and fairly active in the isomerization of 1,4-pentadiene to cis-1,3pentadiene, in the absence of H2.[2601It is also a selective hydrogenation catalyst for linear dienes such as cis-1,3-pentadiene. The conjugated cyclic diene 1,3cyclohexadiene is selectively hydrogenated to cyclohexene.[260,26Introduction of a phosphine ligand in this tetrahedral cluster generally leads to increased activity for homogeneous isomerization of dienes rather than hydrogenation.[2621Analogous 6 molecules such as cis-l,6hexadiene, 2,4-hexadiene, behavior was observed for c 1,5-hexadiene, trans-3-hexene, and 3-hexyne. Conjugated or non-conjugated dienes are converted to the corresponding monoenes with high efficiency and selectivity.[263,2641 The [Ru3Ni] catalyst derived from this cluster on Chromosorb P is more active in the hydrogenation of benzene or toluene than the [Os3Ni] catalyst prepared from the isostructural cluster [ O S ~ N ~ ( ~ - H ) ~ C ~ It ( Cis Oalso ) ~active ] . [ ~ in ~~~ the hydrogenation of dienes and the dehydration of a l ~ o h o l s . [ ~ ~ ~ , ~ ~ ~ ] When supported on y-alumina (modified with 30% w/w K as KOH) this [Ru3Ni] catalyst had activity in ammonia synthesis.[267]It was again more effective than the corresponding [Os3Ni] catalyst (prepared from the analogous [Os3Ni(p3H)Cp(CO),]) but the monometallic ruthenium catalyst derived from [ R u ~ ( C O ) ~ ~ ] was the most active.1' 1' This [ R q N i ] MMCD catalyst transforms toluene to cyclohexane via demethylation of the intermediate methylcyclohexane. Cracking of the latter to n-hexane was also observed to a limited extent. Phenylacetylene is mainly hydrogenated but some hydrogenolysis to cyclohexane and n-hexane was also Ru-Pt
The bioctahedral cluster [Ru6Pt3(p3-H)(p-H)(p3-C2Ph2)(C0)20l (28) was recently reported to exhibit a high catalytic activity for the 100% selective hydrogenation of C2Ph2 to cis-stilbene at 1 atm. and 323 K. This is significantly higher than for either pure platinum or pure ruthenium clusters, suggesting the existence of cooperativity effects.[26 9 , 2 O1 This cluster contains discrete triangular layers of pure platinum and pure ruthenium which may synergistically interact.
2.2 Heterometullic Clusters in Cutulysis
653
Metal core of 28
With a phosphine-functionalized poly(styrene-divinylbenzene) support, it was possible to generate the polymer-bound cluster (RuPtz(CO)5(Ph2P- P)3] [P = poly(styrene-divinylbenzene)] which catalyzes ethylene hydrogenation under mild conditions.i226] A complex formulated as [Ru3Pt(CO)12Py3] was impregnated on to inorganic oxides or carbon to produce a very active and selective catalyst for hydrocarbon conversion. The properties of the heterogeneous catalyst [ R u ~ Pwere ~ ] governed by the high hydrogenolysis activity of ruthenium, and would make it more useful for hydrogenation reactions. [271,2721 Ru-CU
Isomerization of 1-pentene was studied in the presence of the complexes [H3Ru4 {CU(PPh3 11(CO)I 21 and [H2R~4 {CU(PPh3)1 2 ( CO)I 21." 31 An homogeneous process for converting CO and H2 to methanol (CO/H2 ratio 1 : 1, THF, 548 K, 1200 atm) has been investigated in the presence of [Ru6(CuL)2C(CO)16]( L = ~rganonitrile).[~~~.~~~] Ru-Ag
The large cluster [ R U ~ O A ~ ~ C ~ ( (29) C O was ) ~ adsorbed ~ C ~ ] ~on ~ to the inner walls of the mesoporous silica MCM-41 and thermally converted into discrete nanoparticles that catalyze the hydrogenation of l - h e ~ e n e . [ ~ ~ ~ ]
Metal core of 29
654
2 Metal Clusters in Catalysis
The cluster [Ru6(AgL)2C(C0)16] ( L = organonitrile) has been used for the homogeneous conversion of CO and H2 to methanol (CO/H2 ratio 1 : 1, THF, 548 K, 1200 atm).[274,2753 RU-AU The isomerization of 1-hexene to cis- and trans-2-hexene was performed with [RuAu(p-H)2(CO)(PPh3)4](PF6).[273,277,2781 Isomerization of 1-pentene was studied in the presence of [ H ~ R u ~ { APPh3)}(C0)12] u( and [ H ~ R{Au( u ~ PPh3)}2(C0)12].[2731 These two clusters are clearly more active for l-pentene isomerization than the parent [H4Ru4(C0)12],in marked contrast to the behavior of the ruthenium-copper analogs. Homogeneous conversion of CO and H2 to methanol (CO/Hz ratio 1 : 1, THF, 548 K, 1200 atm) has been investigated in the presence of [ R u ~ ( A u L ) ~ C ( C ~ ) ~ ~ ] As for the related Ru-Cu and Ru-Ag clusters used for ( L = ~rganonitrile).[~~~~'~~] this reaction (see above), no hydrocarbon was formed, indicating the absence of ruthenium metal; the latter is known to catalyze the formation of hydrocarbons from synthesis gas.
os-co High activity for methanol homologation has been achieved with [Et4N][OsCo3(CO)12],but selectivity in ethanol was much lower than that observed with Ru-Co systems.['961 0s-Rh The cluster [HzOssRh(acac)(CO) 101 was supported on poly(styrene-divinylbenzene), giving [H20~3Rh(acac)(CO)lo( PhzP P)]. It was tested in the hydrogenation of ethylene and the isomerization of l-butene. Although the cluster was initially anchored intact to the support, it fragmented to give catalytically active species.[279]In another study, this cluster was physisorbed on y-Al203 and after treatment with CO H2 and catalysis, break-up of the supported cluster gave mononuclear rhodium complexes and triosmium clusters." 8 2 ] A comparative study of the alumina-supported catalysts prepared from [HzFeOs3(CO)13], [ HzOs3Rh(acac)(CO) 101, and [ Rh4(CO)12] was performed and each catalyst was found to be active in the conversion of CO+H2. The major product observed in each experiment was methane and the hydrocarbon products were formed in approximately a Schulz-Flory-Anderson distribution. The heterogeneous [Os3Rh] catalyst was two orders of magnitude more active at 543 K than the [FeOss] catalyst, but showed a lower selectivity for ether formation.[ls2I Carbonylation of nitrobenzene to phenylcarbamate has been catalyzed by [PPN]2[0sRh4(C0)15]in the presence of methanol. Addition of bipyridine greatly enhances reaction rate and selectivity.[' 6]
-
+
'
2.2 Heterometullic Clusters in Cutulysis
655
Os-Ni Isomerization and hydrogenation of mono- and dienes in the presence of [Os3Ni(pH)3Cp(CO)9] has been i n v e ~ t i g a t e d . ~Selective ~ ~ ~ , ~homogeneous ~~] hydrogenation of linear dienes such as 1,3-cis-pentadiene is more effective than with the ruthenium analogue.[2611 Phosphine derivatives of this cluster were also The cluster has low activity and selectivity in the hydrogenation of t-butylalkynes or t-butylalkenes,12801 but is more effective in the selective hydrogenation of 3,3-dimethylbutThe same applies to 1-pentyne compared with 1-pentene, the former being easily hydrogenated to 1-pentene. During the hydrogenation reactions of t-butylalkynes or t-butyl-alkenes, the more active [0~3Ni3Cp3(C0)9] (30) decomThe hydrogenation of 1,3-cis- and 1,3-transposed to [0~3Ni(p-H)~Cp(CO)g].[~*~l pentadiene has been investigated using [Os3Ni(p-H)3Cp(CO)9]supported on Chromosorb-P. A gas-chromatographic column was used as a catalytic reactor.f2' '1 This cluster was also supported on y-Al207 and the derived heterogeneous catalyst [Os3Ni]was shown to be very efficient in the hydrogenation of acetylene to ethylene and ethane. With this catalyst, ethylene, propylene and benzene are converted to ethane, propane, and cyclohexane, respectively, at room temperature.12821 The y-A1203-supported clusters [0~3Ni(p-H)~Cp(C0)9] and [ O S ~ N ~ ~ C P ~ ( C O ) ~ ] (30) have been used for the heterogeneous hydrogenation of benzene, acetylene, CO, and C02.L2s31 When comparing the influence of these precursors with [Ni2Cp2(C0)2],[H20~3(C0)10], and [Os3(CO)12]on the hydrogenating properties of the corresponding y-AlzO3-supported catalyst, it was found that [Os3Ni] was the best catalyst for CO methanation. It gave good conversions (0.83 mole of CO converted per g. atom of metals) and high selectivity for CH4 (96-100Y0) at temperatures above 523 K, with small amounts of CO2 and CZ hydrocarbons as byproducts.[2821For C02 methanation, [Os3Ni] was found to give yields >90% at temperatures between 523 and 623 K.1282,2831It is more effective than the [ H ~ O S ~ ] , [Os?],or [Niz] catalysts in terms of conversion and selectivity. A heterogeneous hydrogenation catalyst enabling the reduction of acetone to propane was prepared from the cluster [Os3Ni(pU-H)7Cp(CO)g] supported on Chromosorb-P and thermally treated under H2.I2 84j A multi-step reaction pattern was proposed, which involved first hydrogenation of acetone to isopropanol, dehydration of the latter on the support with formation of propylene, followed by hydrogenation of the propylene to propane. Catalytic dehydration of alcohols was observed with this An alumina-supported [Os3Ni] catalyst prepared from [0~3Ni(p-H)3Cp(C0)9] was active in ammonia ~ y n t h e s i s . ' ~ ~ ~ , ~ ~ ~ ] Os-Ni-Cu Comparison of the hydrogenation of 1,3-cis- and 1,4-cis-pentadiene with heteropgeneous catalysts derived from the trimetallic cluster [Os3Ni(pu-CuPPh3)(
656
2 Metal Clusters in Catalysis
H)2Cp(C0)9] (31), [Os3Ni(p-H)3Cp(CO)9],and [Ru3Ni(pU-H)3Cp(C0)9] showed that the first was more selective for m o n o e n e ~ . [ ~ ~ ~ ] The [Os3NiCu]MMCD catalyst on Chromosorb-P has been used for the dehydration of alcohols. Formaldehyde and acetaldehyde were obtained from methanol and ethanol, respectively. The behavior of this catalyst was significantly different from that of catalysts derived from [Ru3Ni(p-H)3Cp(C0)9] and [Os3Ni(pH)3Cp(C0)9].‘2655’2661
0s-AU While studying the catalytic properties of [HOs3Au(CO)lo( Ph2P-SIL)] and [ClOs3Au(CO)lo( Ph2P-SIL)], prepared by anchoring the PPh3 clusters on to phosphine-functionalized silica, Knozinger et al. found no catalytic activity for 1-butene isomerization or ethylene hydrogenation below 383 K.[” 51 The former system was more active for 1-butene isomerization at 383 K; the corresponding homonuclear system [HOs3(C0)9(Ph2P-SIL)], however, was ca 10 times more active.[226]The sysstability of the ‘ClOs3Au’ system was much lower than that of the ‘HOS~AU’ tem. The functionalized poly(styrene-divinylbenzene) support was also used to tether the cluster ‘ClOs3Au’. This produced an active catalyst for ethylene hydrogenation at 1 atm and temperatures below 373 K.[2861This catalyst was found to be significantly more stable than [ClOs3Au(CO)lo(Ph2P-SIL)].[z851 Co-Rh The importance of the composition of the metal core in the low pressure hydrogenation of styrene was indicated by the observation that the initial hydrogenation rate with [Co2Rh2(CO)12]is roughly double that of [Co3Rh(C0)12]. This is in agreement with the inactivity of [Co4(CO)121 alone. Addition of trimethylphosphite was found to enhance the rate of hydrogenation.[287]
2.2 Heterometullic Clusters in Catalysis
657
The complex Co[Rh12(C0)30]catalyzes the synthesis of organic compounds from CO and H2 [ 1 5 1 3 1 5 6 1whereas salts of the dianion [Co3Rhg(C0)3ol2- catalyze the conversion of CO and H2 to oxygenated compounds such as methanol, ethylene glycol, glycerol, and 1,2-propylene glyco1.[288,2891 The [Co2Rh2] and [Co3Rh] catalysts derived from [Co2Rh2(C0)12]and [Co3Rh(C0)12] on silica were used in CO H2 reactions and found to have a higher selectivity for oxygenates than monometallic systems.12341 ZrO2 has also been used as a Under basic WGSR conditions, [CozRh2(C0)12]and [Co3Rh(C0)12]produced a highly active catalytic system. The cobalt carbonyls are totally inactive.[1791 There is some evidence of the involvement of the coordinatively unsaturated bimetallic compound [CoRh(CO)7] as an active species in hydroformylation r e a c t i o n ~ . [ ~A~ synergistic ~ , ~ ~ ~ ] effect was observed in the hydroformylation of pentafluor~styrene.[~~~l Excellent regioselectivities were reported for the hydroformylation step of the hydroformylation-amidocarbonylation of trifluoropropene using a [Co2(CO)s]/[Rhs(CO)16]system in which [CoRh(CO)-i] could have been generated. A mixed-metal [Co,Rh,( CO),] species has previously been postulated as an active catalyst in the hydrocarbonylation of diketene.[294-296] Hydroformylation of 1-hexene and 3,3-dirnethylbut-l-ene catalyzed by [Co2Rh2(CO)12]and of 1-hexene by [Co3Rh(CO)l2] have been r e p ~ r t e d . [ ~In~the ~ , hydroformylation ~~~] of cyclohexene under mild conditions (323 K, 1 atm CO H2), [Co2Rh2(C0)12] was found to have more activity than [Rh4(C0)12] and addition of P(OPh)3 enhanced that Homogeneous catalytic hydroformylation of terminal olefins was reported for the dinuclear complexes [(OC)LCo(p-H){pP( Bu')~}R~(CO){HP( Bui)}] ( L = CO, HP( B u ~ ) ~ A ) .cobalt-rhodium [ ~ ~ ~ ~ system, promoted by triphenylphosphine, enables selective hydroformylation of dicyclopentadiene under relatively mild In the homogeneous hydroformylation of formaldehyde, [Co2Rh2(CO)121 afforded, unexpectedly, both glycolaldehyde and ethylene glycol with an overall molar selectivity of ca 50%.[3001 Hydroformylation of N-allylacetamide afforded pyrrolidine B with 2 98% selectivity when [Co2Rh2(C0)12]was used as a catalyst (Eq. 4). With rhodium catalysts, 2-formylpyrrolidine A was the major product. (The hemiamidal C was shown to be the precursor of A and B.) Synergistic effects of the mixed-metal system were invoked to account for the selectivity observed.[3011
+
+
H
I
catalyst
ph'
Amine-functionalized resins have been used to support Co-Rh clusters such
658
2 Metal Clusters in Cutulysis
as [Co4-,RhY(C0)l2] (x = 0-2) and studied in olefin hydroformylation reactions.[ 2 s 8 , 3 02-30 s ] and a two step process for producing isobutene from propylene and synthesis gas was described. The alcohol mixture obtained in the first step was dehydrated on aluminum oxide.‘3061A [Co3Rh(CO)lo(PhzP-SIL)z] catalyst was used for 1-hexene hydroformylation, leading to 94.2‘1/0conversion and 97.7%) selectivity for aldehydes (normal/branched ratio = 2. l).[3071 When [C02Rhz(C0)12] was supported on alumina, silica, or magnesium silicate, it catalyzed 1-hexene hydroformylation to C7-aldehydes with good yields. The best results for the production of C7-alcohols (yields > 90%) were obtained with alumina as support in the presence of NEt3 at 50 bar (CO/Hz = 1) and 373 K.[2s81 Atmospheric hydroformylation of ethylene and propylene was investigated with SiOz-supported [CozRh2(C0)12]- and [Co3Rh(CO)lz -derived catalysts which had excellent activity in the formation of oxygenates.[308 The activity of the [Co3Rh] catalyst for ethylene hydroformylation was about 20 times that of an [Rh4(CO)12]/ ] Th and selectivity of these SiOz-derived monometallic ~ a t a l y s t . 1 ~2 ~ ~ -e~ activity ’ heterogeneous catalysts deposited on ZnO in the vapor-phase hydroformylation of ethylene and propene were studied as a function of the composition of the clusters.[3131Performances decreased in the order [Rh4] > [CozRhz] > [Co3Rh] > [ C O ~Precursors ]. with greater cobalt content produced higher proportions of linear aldehyde. Supported bimetallic Co-Rh particles on ZnO might behave as ‘highly dispersed alloys’ of compositions similar to those of the corresponding precursor clusters. Other results on these [CozRhz] and [Co3Rh] catalysts supported on carbon have provided further evidence of the greater activity of the MMCD catalysts in the gas-phase hydroformylation of ethylene and propene.l3 31 The [Co2Rh2] catalyst on various supports was studied for hexene hydroformylation and found to give 97% yield of the C7 High activities for MeOH homologation were also observed with [CozRh2(C0)1z]and [Co3Rh(C0)12]but selectivities for ethanol were always lower than for the RuCo3 systems already described.[’96,197.245-2481 [Co2Rh2(C0)12] and [C03Rh(C0)12] catalyze the hydrosilation of isoprene, cyclohexanone, cyclohexenone, and 1-alkynes, the regioselective inter- or intramolecular (Eq. 5 ) silylformylation of alkynes and 1-alkynals, and the silylcarbocyclization of enynes, diynes, and alkynals. These synthetically important reactions have been reviewed recently.[23]
1
n=0,1
Hydrocarbon skeletal rearrangement has been catalyzed by an aluminasupported [CozRh2(CO)1z]-derived Hydrogenolysis of methylcyclo-
2.2 Heterometullic Clusters in Catalysis
659
pentane with this catalyst led to remarkable specificity for hydrogenolysis of the methyl-substituted five-membered ring to non-cyclic Cg isomers.[3 6]
Co-Ir Under basic WGSR conditions, the cluster [Co2Ir2(CO)12]was only a weakly active catalyst precursor and no synergistic effect was observed between cobalt and iridium.[ 791
'
Co-Ni The cluster [Co2Ni(pu,-CMe)Cp(CO)g] has been used to catalyze the homogeneous hydroformylation of 1-pentene to hexanal and 2-methylpentanal. Hydroformylation of styrene was achieved under mild conditions with moderate to high branched-to-normal selectivity. This cluster could be recovered in high yield (>90%) after catalysis." 08]It also catalyzes the hydrosilation of acetophenone.l'O'i Co-Pd The catalytic hydroformylation of propylene under mild conditions (313-373 K, 1 atm CO) occurred with greater activity with an anchored heteronuclear cobaltpalladium complex than with individual homometallic cobalt and palladium complexes, under similar conditions.[3171 A possible reason for the synergistic effect observed when using cobalt-palladium complexes could be the simultaneous formation of reactive Pd-H and Co-C bonds.13''] When [CozPd(CO)7(dppe)]was used as a homogeneous catalyst, the rate of methanol carbonylation to acetaldehyde was higher than when [ C o z ( C 0 ) ~or ] [Coq(CO)12] were used, but the reduction of acetaldehyde to ethanol was not e n h a n ~ e d . " ~ Pd-Co ~ , ~ ~ ' intermediates ~ were suggested in carbonylation reactions, including in the carbonylation of aryl iodides in the presence of HSiEt3.13'91The role of palladium as promoter in the hydrocarbonylation of MeOH catalyzed by cobalt on active carbon has been investigated with a [Co2Pd] catalysts derived from [ C ~ ~ P d ( C O ) ~ ( d p p eThe ) ] . [close ~ ~ ~ ]neighborhood of promoter metal and cobalt centers seems to be a condition for cooperative carbonylation and hydrogenation to occur (Scheme 5). Hydrocarbonylation, affording acetaldehyde and its dimethylacetal, carbonylation, leading to methyl acetate, and methane formation were observed as the primary reactions. The reduction steps are facilitated by the presence of palladium. Co-Pt
The cobalt-platinum butterfly cluster [CozPtz(CO)g(PPh3)2] (32b) catalytic properties in the hydrogenation and isomerization of
has 1,3-
660
2 Metal Clusters in Catalysis
CH3CH0,
f
-4c-$
HI
co
-69-
active carbon-supported M-sites (M =Co, Pd), not necessarily mononuclear
Scheme 5. Catalytic cycle for methanol hydrocarbonylation (adapted from ref. 250)
butadiene." 36,3201 Hydrogenation of 1-hexyne occurred with the linear complex trans-[Pt{Co(C0)4}2(CNCy)2]and the triangular clusters [Co2Pt(CO);r(dppe)]and [Co2Pt(CO)7(dpae)] as catalyst precursors.["] Selective hydrogenation of diphenylacetylene to cis-stilbene in relatively good yield was observed with 32b and [Co2Pt2(CO)g(AsPh3)2] 32c). Extensive transformation and rearrangement was found with this ~ a t a l y s t ~ ' "An ~ ] active catalyst for olefin hydrogenation was proIts slightly duced by immobilizing 32b on a phosphine-functionalized improved stability compared with that of its molecular precursor under homogeneous conditions is of interest because under these conditions, the mixed-metal cluster rapidly transforms into the homometallic cluster [Pt5(C0)6(PPh3)4].[1361 Hydroformylation of 1-pentene has been examined in the presence of the linear complex trans-[Pt {Co(CO)4}2(CNCy)2],the triangular cluster [Co~Pt(CO)7(dppe)], and 32b; these were quite active catalysts within the 353-375 K range but not at 333-338 K.[801It is interesting that [CozPt(CO)7(dppe)]is an active hydroformylation catalyst at 353 K in contrast to its dpae analogue, because the chelating ligand is not bonded to the active cobalt atoms. The activity of 32b for hydroformylation of 1-hexene and 1,3-butadiene was found to be higher than that of the corresponding mononuclear Co-P and Pt-P complexes.[3221 This was related to the electron distribution within the mixed-metal cluster.[320] The cluster [Co2Pt(CO)7(dppe)] catalyzes MeOH homologation and gave a higher yield of ethanol than the corresponding [CozPd(CO)7(dppe)]c a t a l y ~ t . [ ' ~ ~ ~ ~ ~ * 1 High catalytic activity with stereospecific dimerization of NBD to 'Binor-S' was
2.2 Heterometullic Clusters in Catalysis
66 1
observed with the chain complexes truns-[Pt{Co(C0)4}2(CNR)2] ( R = Cy, Bu'), the butterfly clusters [ C O ~ P ~ ~ ( C O(32), ) ~ Lor~ ]the trigonal bipyramidal cluster [ C O ~ P ~ ~ ( C O(33), ) ~ Lin~ ]the presence of a Lewis acid.[137]The catalytic cyclooligomerization of 1,3-butadiene by 32b was also The three heteropolymetallic complexes with varying Co/Pt ratios trans[PtI Co(CO)4} 2 ( CNCY)2 ], [co2P b (C0)8( PPh3)2 ] (32b) and [Co2Pt3(CO)9( PEt3)3] (33a) have been used to prepare heterogeneous bimetallic catalysts for the hydro~ ~ ~catalysts , ~ ~ ~ ~ [Co2Pt2] and [Co2Pt3] had genolysis of m e t h y l ~ y c l o p e n t a n e . [The greater selectivity for demethylation of methylcyclopentane (Cg + C5 Cl ) than the [Co2Pt] catalyst. These effects were tentatively attributed to the different nature of the ligands bound to the molecular precursors ( phosphines compared to isonitriles) rather than to the change in the Co/Pt ratio. Thus, these ligands might have been difficult to eliminate completely during thermal activation of the catalysts, and this could have led to modifications in the composition and/or structure of the final catalyst.
+
(COIL 32a L = PEt3 32b L=PPh3 3 2 ~L=AsPh3
33a L=PEt3 33b L=PPhs
co-cu Controlled pyrolysis of [CUZ { C O ~ ( C O ) ~ C C Oled ~ J to ~ ] a catalytic material of high surface area for the selective hydrogenation of crotonaldehyde to crotyl alcoho1.[109.1101
Co-Zn Selective hydrogenation of crotonaldehyde to crotyl alcohol was observed with a catalytic material of high surface area obtained by controlled pyrolysis of
[Z~~O{CO~(CO)~CCO~}~].[~~~,~~~~
662
2 Metal Clusters in Catalysis
‘Binor-S’ (Scheme 3 ) was formed in quantitative yield by dimerization of norbornadiene with the bifunctional transition metal catalyst [Zn{Co(CO)4}2],with or without a Lewis acid c ~ c a t a l y s t . [A~ ~possible ~] transition state was suggested in which two substrate molecules could come sufficiently close for bond formation, giving rise to ‘Binor-S’. The complex [ C O ~ { , U - Z ~ C O ( C O ) ~ }is~ (also C Oan ) ~ efficient ] catalyst for the stereospecific dimerization of norbornadiene. Its induction period was much shorter than that of [Zn{C0(C0)4}2].[~~~] Co-Cd
The trinuclear complex [Cd{ Co(CO)4}2] dimerizes norbornadiene to a mixture of dimers, including ‘Binor-S’ and thus behaves differently from the mercury analog [Hg{Co(CO)4}2](see
The chain complex [Hg{Co(C0)4}2]dimerizes norbornadiene to a mixture of four dimers but no ‘Binor-S’ was formed. In the presence of Lewis acids, however, this catalyst gives ‘Binor-S’ exclusively.[32 Rh-Ir Salts formulated as [Ir2{Rh12(C0)30}]and [Ir2{Rh12(C0)30}3][156,3281 and Al, Ga, Ir, Se, Y, or Re salts of anions such as [Rh6Ir6(C0)30I2- or [Rh&-3(CO)30l2-catalyze the reaction between CO and H2. Oxygenated compounds such as methanol, ethylene glycol, glycerol, and 1,2-propylene glycol are p r o d ~ c e d . ~Catalysts ~~~,~~~] of the type [Rh6-,Irx(CO)16]/NaY (x = 1-3 have been tested in CO hydrogenation and hydroformylation.~203.204.207.329~330 The activity in butane hydrogenolysis of Rh-Ir heterogeneous catalysts prepared from [Rh6-,IrX(C0)16] (x= 0-6) in zeolites was suppressed by increasing the Ir ~ontent.[”~I This was explained in terms of ensemble size effects, owing to breakup of the Rh ensemble sites with an inactive Ir atom. The cluster-derived catalyst consisted of bimetallic particles < 10 A in diameter, uniformly distributed inside the NaY zeolites with a metal composition similar to that of their molecular precursors. For the conversion of butane to ethane, maximum selectivity toward central C-C bond scission was observed for cluster-derived [ R h ~ l r ][Rh&], , and [Rh3Ir3]catalysts, which were better than the homometallic cluster-derived catalysts.
1
Rh-Pt Active catalysts for aromatic ring hydrogenation reactions of toluene, phenol, and anisole were prepared from [Rh~Pt(C0)15]-on an Amberlite anion-exchange r e ~ i n . ‘11 This ~ ~ catalyst did not work for aniline or nitrobenzene. Hydrogenation of CO catalyzed by [ P P N ] [ R ~ s P ~ ( C Oyielded ) I ~ ] ethylene glycol
2.2 Heterornetallic Clusters in Catalysis
663
and methanol as the favored product^.'^ 321 In contrast, the catalytic properties of [PPN]z[Rh4Pt(C0)12]are greatly reduced. This low activity was correlated with the enhanced amount of [PPN]+ present rather than with the increased Pt/Rh ratio. Because platinum-only catalysis yields methanol and not ethylene glycol,[3331 the authors suggested that mixed molecular clusters of unknown character were probably involved in the A metal salt formulated as Pt[Rh12(C0)30]catalyzes the synthesis of organic compounds from CO and H2.[ls1I Although this bimetallic couple is currently studied for post-combustion catalysts in automobile converters, there seems to be no reported example of MMCD catalyst for this reaction. Rh-Cu, Rh-Ag, Rh-Au The dinuclear complex [RhAu{HC( PPh2)3}(COD)(PPh,)]( BF4)l has been tested in olefin hydrogenation.[3341 Salts formulated as [M2{Rh12(C0)30}]( M is Cu, Ag, Au) have been used as catalysts in the reaction of CO and H2 to oxygenates such as methanol, ethylene glycol, glycerol, and 1,2-propylene glycol. The clusters were recovered from the filtrate by recrystallizati~n.[~~~] Rh-Zn A salt formulated as [Zn{Rhl2(CO)30}]has been reported to catalyze CO hydrogenation.lt5'1 Metal ion cooperativity has been observed with the Rh-Zn complex 34 which catalyzes hydroformylation of functionalized terminal olefins with no induction period. It is possible than Zn( 11) functions as an internal acceptor which removes the chloride ion from the Rh( I ) center during formation of the active species.i33s1
34
Ir-Pt
[ Ir*Ptl(C0)7(PPh3)3] has high activity as a homogeneous catalyst for the hydrogenation of cyclohexene, 2-methylcyclohexene,2-butenal, and 2-cyclohexenone. Only the olefinic bond was hydrogenated in the last two c o m p o ~ n d s . [ ~ ~ ~ 1
664
2 Metal Clusters in Catalysis
The complexes formulated as [ Ir2Pt(C0)7Py2] and [ IrsPt(CO)15Py2] were impregnated on to inorganic oxides to produce a very active and selective catalyst for hydrocarbon conversion. The conversion of n-heptane into methane and the isomeric xylenes by the cluster-derived [ Ir,Pt] heterogeneous catalysts was maintained at a significantly higher level than that of a standard catalyst prepared from H2MC16 ( M = Ir or Pt) acid solutions. The selectivity for toluene of the [IrsPt] catalyst was found to be 5-7% higher than that of the [IrzPt] catalyst. The 30% lower coking rate of [ Ir,Pt] compared with a conventional catalyst was found to be another advantage of the MMCD catalyst. For naphtha reforming, the [ IrzPt] and [Ir6Pt] catalysts have been found to be, at least, 1.6 and 2.4 times, respectively, more active than conventional catalysts and the [ Ir,Pt] catalyst 1.8 times more active than the [ IrzPt] catalyst. It was suggested that unique heterometallic sites on the cluster-derived catalysts performed the aromatization reactions more efficiently than those present on conventional Ir-Pt ~ a t a l y s t s . [ ~ ~ ~ , ~ ~ ~ ] Ir-Cu Hydroformylation of styrene at 353 K in 80 atm CO/Hz (1 : 1) has been performed with [ IrCu(CO)Cl(p-Ph2Ppy)2]BFqas a homogeneous catalyst.[337] Ni-Pt Although synergistic effects of Ni and Pt supported on SiOz have recently been observed in catalytic methanol decomposition,[33 no catalysis by the mixed-metal cluster seems to have been reported. Pd-Cu The p4-0x0 cluster [Pd6Cu4C11204(HMPA)4] catalyzes the oxidation of alkenes to ketones by 0 2 . [ 3 3 9 ] Oxidation of cyclohexene to a mixture of cyclohexanone and cyclohexenone (88 : 12) was achieved in 3600/0yield by use of the polymeric complex [C1CuL4(pu-C1)PdC12.PdC1z],( L = pyrrolidin-2-one) as catalyst.[340] Pt-Au Many platinum-gold cluster compounds in the form of solid microcrystals perform fast and clean H2 activation as evidenced by Hz-Dz equilibration. This property has been observed with the clusters [Pt(AuPPhs),( PPh,)]( N03)z (35), [HPt(AuPPh3)7(PPh,)]( N03)z (36), and [Pt(AuPPh3)8](N03)2 (37) for which rates of H2-Dz equilibration are significantly higher (turnover rates ca 12-170 s-') than under homogeneous conditions (turnover rates ca 2-5 sP1). It was suggested that Pt-Au bonds function as sites for H2 activation, allowing the formation of bridging hydride~.[~~l-~~~]
2.2 Heterometullic Clusters in Catalysis
’L
Au
L
665
L‘
35
36
L‘
L
L
37
The 16-electron cluster [Pt(AuPPh?)s](NO?)* (37) in the form of solid microcrystals slowly catalyzes the hydrogenation of ethylene to ethane. Intermediate formation of a hydrido cluster was p o s t ~ l a t e d . [ ~Wh ~ en ~ , ~supported ~~] on silica, cluster 35 remained intact and was a good catalyst for Hz-Dz equilibration, hydrogenation of ethylene, and CO oxidation.[3461Detailed discussion of the cluster catalysis of the Hz-Dz equilibration reaction and, more generally, of catalysis by mixed-metal clusters containing gold-phosphine groups have recently been p~blished.~~~~,~~~] As a microcrystalline powder, 37 catalyzes the hydrogenation of oxygen at 303 K and 1 atm. The rate of this reaction is similar to that observed for Hz-Dz equilibrati011.l~~~’
2.2.3 Conclusion We have attempted here to review the various bimetallic couples derived from mixed-metal cluster compounds that have been used as precursors to homogeneous, supported, or heterogeneous catalysts. They are finding applications in an increasing number of catalytic reactions, as is apparent from Table 1. Occasionally a given complex has been used for more than one type of catalysis, which provides a useful qualitative comparison between different approaches. In general, however, it is very difficult or impossible to draw more quantitative conclusions because valuable comparisons can only be made for systems studied under strictly analogous conditions, which is only rarely possible, partly because of the diversity of precursors and types of catalysis studied and also because of the increasing number of research groups engaged in this field. In many instances, a synergistic effect has been found by comparing catalyst performances with those of the individual components. Much remains to be learned about the stoichiometric and catalytic reactivity of mixed-metal cluster compounds and particularly about the site-selectivity induced The study of new catalysts prepared by by the presence of different
666
2 Metal Clusters in Catalysis
immobilizing mixed-metal clusters on solid supports such as organic polymers or inorganic oxides, functionalized or not (with e. g. oxygen-donating ligands, the only ones available on the surface of oxides, or with groups having a greater affinity for low oxidation-state metal carbonyls), and on to the pores of well-defined micro- and mesoporous materials has advanced greatly in recent years. The original motivation for producing such systems was to overcome the limitations and difficulties encountered in both homogeneous catalysis, (i.e., catalyst recovery and cluster fragmentation), and in heterogeneous catalysis, (i. e., limited selectivity tuning frequently because of ill-defined active sites, reduced effectiveness of components of a multimetallic catalyst, and severe reaction conditions). At the same time, it was hoped that the advantages inherent in homogeneous catalysis (i.e., molecular understanding of the mechanisms and catalytic cycles, mild reaction conditions, high efficiency of the metal atoms, and easier tuning of the electronic and steric properties), could be combined with those characterizing heterogeneous catalysis, (i. e., higher stability of the catalyst, applicability to a wide range of reactions, technological versatility, and ready separation from the reaction products). Supported mixed-metal cluster-derived (MMCD) heterogeneous catalysts still constitute a new class of catalytic material. Thermal treatment of the supported molecular cluster is performed to remove the ligands and this generates metal particles that often have unique catalytic properties. When the molecular precursors are in a low oxidation state, preferably carbonyl complexes, the comparison is most meaningful because lower activation temperatures are required. Furthermore, comparing the properties of an MMCD catalyst with those of a conventional catalyst prepared from inorganic salts of the metals, e.g. chlorides or nitrates, is perhaps more informative economically if one wishes to replace ‘old’ but currently used catalysts with new ones. It is generally observed that the use of well-defined bimetallic molecular precursors leads to metal particles of much better controlled size and composition than those obtained from mixtures of monometallic salts. Despite all the results available, much remains to be done in performing the critical tests enabling clear-cut evaluation of MMCD catalysts. This is largely because of the number of parameters involved in such studies, e.g. the nature of the support, of the method of impregnation used, of the thermal decomposition and activation procedures of the catalysts, which make comparisons between results from different research groups very difficult and sometimes meaningless. We expect that ‘heterometal-sensitive reactions’, i. e. reactions which require at least two different metals to proceed with high activity and selectivity, should be particularly worthy of study because results from these will be very informative, even if only qualitatively, about the specificity of MMCD catalysts. Studying cluster-derived catalysts should, in general, help our understanding of fundamental aspects of catalytic chemistry. The potential of heterometallic complexes in catalysis is considerable as many bimetallic associations remain poorly investigated or are absent from this review, although mixtures of homometallic complexes or, more recently, bimetallic colloids have been used and are of industrial relevance.
2.2 Heteronietullic Clusters in Cutulysis
661
Acknowledgments We are grateful to the CNRS for financial support.
List of abbreviations acac Ar bim Bu COD CP CP’ CP* CY DMAD dpae dPPe dPPm Et Me MMCD NBD Ph PY PZ Ph2 PPY PPN tacn TFB TMBA TOF [MXM’YI
acetylacetonato aromatic 2,2’-bi-imidazolato butyl cyclooctadiene q j-cyclopentadienyl ( q-CjHj) q5-methylcyclopentadienyl(q-CgH4Me) q5-pentamethylcyclopentadienyl(q-CjMej) cyclohexyl dimethylacetylenedicarboxylate 1,2-bis(dipheny1arsenio)ethane 1 , 2 4 4dipheny1phosphino)ethane bis(dipheny1phosphino)methane ethyl methyl mixed-metal cluster-derived catalyst norbornadiene phenyl pyridine pyrazolate (2-dipheny1phosphino)pyridine bis(tripheny1phosphine)iminium 1,4,7-triazacyclononane tetrafluoro benzobarrelene trimethyl( benzy1)ammonium turnover frequency heterogeneous catalyst prepared from a molecular cluster of metal core composition MxM’y
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2 M e t a l Clusters in Catalysis
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[283] G. Albanesi, R. Bernardi, P. Moggi, G. Predieri, E. Sappa, Gazz. Chim. Itul. 1986, 116, 385. [284] M. Castiglioni, R. Giordano, E. Sappa, J. Mol. Catal. 1986, 37, 287. [285] M. Wolf, H. Knozinger, B. Tesche, J. Mol. Catal. 1984,25, 273. [286] R. Pierantozzi, K. J. McQuade, B. C. Gates, in Proceedings of the 7th International Congress on Catalysis, Tokyo 1981, Part B, (Elsevier, Amsterdam, 1981), pp. 941. [287] D. Labroue, R. Poilblanc, J. Mol. Catal. 1977, 2, 329. [288] E. S. Brown, U.S. Patent 3 929 969 (1975), Chem. Abstr., 1976, 84, 127 210. [289] E. S. Brown, U.S. Patent 3 989 799 (1976) Chem. Abstr., 1977, 86, 96 646. [290] M. Ichikawa, CHEMTECH 1982, 674. [291] I. Ojima, M. Okabe, K. Kato, H. B. Kwon, I. T. Horvath, J. Am. Chem. Soc. 1988,110, 150. [292] M. Garland, Organometallics 1993, I 2 , 535. [293] I. Ojima, J. Mol. Catal. 1986, 37, 25. [294] F. H. Spindler, G. Bor, U. K. Dietler, P. Pino, J. Organornet. Chem. 1981, 213, 303. [295] F. H. Spindler, Dissertation E. T. H. (Zurich) No. 7012 1982. [296] P. Pino, D. von Bezard, Ger. Offen. 2 807 251 (1978), Chem. Abstr., 1979, 90, 6235. [297] A. Ceriotti, L. Garlaschelli, G. Longoni, M. C. Malastesta, D. Strumolo, A. Fumagalli, S. Martinengo, J. Mol. Catal. 1984, 24, 309. [298] B. Walther, H. C. Boetcher, M. Scheer, G. Fischer, D. Fenske, G. Suss-Fink, J. Organomet. Chem. 1992, 437, 307. [299] L. Garlaschelli, M. Marchionna, M. C. Iapalucci, G. Longoni, J. Mol. Catal. 1991, 68, 7. [300] M. Marchionna, G. Longoni, J. Mol. Catal. 1986, 35, 107. [301] I. Ojima, Z. Zhang, J. Org. Chem. 1988, 53, 4425. [302] G. E. Hartwell, P. E. Garrou, U.S. Pat. 4 144 191 (1979), Chem. Abstr., 1979, 90, 203 478 (Dow Chemical Co.) [303] P. E. Garrou, U.S. Pat. 4 374 999 (1983), Chem. Abstr., 1983, 98, 197 608. [304] L. Alvila, T. A. Pakkanen, 0. Krause, M. Joutsimo, Eur. Patent 253 827 B1 (1991), Chem. Abstr., 1987, 107, 178 629 [305] L. Alvila, 0. Krause, T. A. Pakkanen, M. Joutsimo, U.S. Patent 4 652 539 (1987), Chem. Abstr., 1986, 105, 99 542. [306] L. Alvila, T. A. Pakkanen, 0. Krause, M. Joutsimo, U.S. Patent 4 777 320(1988), Chem. Abstr., 1988, 108, 77 617. [307] G. Su, Y. Chen, S. Yang, S. Luo, W. Xie, Z . Yang, in China-Japan-US. Symposium Heterogeneous Catalysis and Related Energy Problems (Chinese Academy of Science, Dalian Institute Chemical Physics, Dalian, Peoples Republic of China, 1982). [308] L. Huang, Y. Xu, G. Piao, A. Liu, W. Zhang, Catal. Lett. 1994,23, 87. [309] L. Huang, Bull. Soc. Chim. Fr. 1996, I33, 359. [310] L. Huang, A. Liu, Y. Xu, J. Mol. Catal. A: Chem. 1997, 124, 57. [311] L. Huang, J. Mol. Cutal. A: Chem. 1997, I25, 47. [312] L. Huang, Y. Xu, Catal. Lett. 1996, 40, 203. [313] M. Ichikawa, J. Catal. 1979, 59, 67. [314] L. Alvila, Dissertation, University of Joensuu, Finland, 1992. [315] M. Ichikawa, Adu. Catal. 1992, 38, 283. [316] J. R. Anderson, D. E. Mainwaring, J. Catal. 1974, 35, 162. [317] B. L. Moroz, V. A. Semikolenov, V. A. Likholobov, Y. I. Yermakov, J. Chem. Soc., Chem. Commun. 1982, 1286. [318] B. L. Moroz, 0. N. Shumilo, E. Paukstis, V. A. Likholobov, N. N. Bulgakov, E. N. Yurchenko, Y. I. Ermakov, in Homogeneous Heterogeneous Catalysis, Proceedings of the 5th International Symposium on Relations between Homogeneous and Heterogeneous Catalysis, ( Eds.: Y. I. Ermakov, V. A. Likholobov), VNU Science Press, Utrecht, 1986. [319] Y. Misumi, Y. Ishii, M. Hidai, J. Chem. Soc., Dalton Trans. 1995, 3489. [320] D. Li, W. Zhai, Z . Chen, Y. Sun, X. Zhoa, Z . Wang, Huaxue Xuebao 1986, 44, 990.
2.2 Heteroinetallic Clusters in Ccita1y.si.s
611
[321] B. C. Gates. in 'NSF-CNRS International Seminar on the Relationship Betitwn Mctml Cluster Conipounds, Surface Science and Cutalj~sis'(Asilomar, 1979). [322] W. Zhai, D. Li, Y. Ma, Z. Zhao, Youji Huaxue 1983, 3, 180. [323] G. Maire, in Studies In Sutjiace Science and Cutalysis, Vol. 2Y: Metal Clusters in Catalysis, (Eds.: B. C. Gates, L. Guczi, H. Knozinger), Elsevier Science Publishers, Amsterdam, 1986, pp. 509. [324] G. Maire, 0. Zahraa, F. Garin. C. Crouzet, S. Aeiyach, P. Legare, P. Braunstein, J. Chim. Phys. 1981, 78, 951. [3251 G. N. Schrauzer, R. K. Y. Ho, G. Schlesinger, Tetruhedron Lett. 1970, 543. [326] J. M. Burlitch, S. E. Hayes, J. T. Lemley, Orynnometallics 1985, 4, 167. [327] G. N . Schrauzer, B. N. Bastian. G. A. Fosselius. J. Am. Chem. Soc. 1966, 88, 4890. [328] W. E. Walker, E. S. Brown, R. L. Pruett, U.S. Patent 3 878 292 (1975), Clzem. Ahsir.; 1975, 83, 45 426 (Union Carbide Co.). [329] A. Fukuoka. L. F. Rao, N. Kosugi, H. Kuroda, M. Ichikawa, Appl. Cutal. 1989, 50, 295. [330] L. F. Rao, A. Fukuoka. M. Ichikawa, Shokzihai 1989, 31, 64. 13311 S. Bhaduri, K. R. Sharma, J. C'lirm. Soc., Dalton Truns. 1984, 2309. [332] M. Roper, M. Schieren. A. Fumagalli. J. Mol. Cutal. 1986, 34, 173. [333] W. Keim, M. Berger. J. Schlupp, J. Cutal. 1980, 61. 359. [334] H. El-Amouri, A. A. Bahsoun, J. Fischer, J. A. Osborn, M. T. Youinou, 0ryanonietullic.s 1991, 10, 3582. [335] J. R. Lockemeyer? A. L. Rheingold, J. E. Bulkowski. Oryunometallics 1993, 12, 256. [336] S. Bhaduri. K. R. Sharma, W. Clegg, G. M. Sheldrick, D. Stalke, J. Chrm. Soc., Dalton Truns. 1984, 2851. [337] G. Francio, R . Scopelliti, C. G . Arena, G. Bruno, D. Drommi, F. Faraone, Orqunometallics 1998, 17. 338. [338] Y. Matsumura, K . Kagawa, Y. Usami, M. Kawazoe, H. Sakurai, M. Haruta, J. Chem. Soc., Chem. Coniriiun. 1997, 657. [339] T. Hosokawa, M. Takano, S.-I. Murahashi, J. Am. Cheni. Soc. 1996, 118, 3990. [340] T. Hosokawa, M. Takano, S. Murahashi, H. Ozaki, Y. Kitagawa, K. Sakaguchi, Y. Katsube, J. Chenz. Soc., Chem. Comniun. 1994, 1433. [341] M. A. Aubart, L. H. Pignolet, J. Am. Cliem. Soc. 1992, 114, 7901. [342] M. A. Aubart, B. D. Chandler, R. A. T. Gould, D. A. Krogstad, M. F. J. Schoondergang, L. H. Pignolet, Inory. Cheni. 1994, 33, 3724. [343] T. G. M. M. Kappen, J. J. Bow, P. P. J. Schlebos, A. M. Roelofsen, J. G. M. van der Linden, J. J. Steggerda, M. A. Aubart. D. A. Krogstad, M. F. J. Schoondergang, L. H. Pignolet, Inory. Cheni. 1993, 32, 1074. (3441 L. H. Pignolet, M. A. Aubart, K. L. Craighead, R. A. T. Gould, D. A. Krogstad, J. S. Wiley, Coord. Chenz. Rea. 1995, 143, 2 19. [345] Y. Yuan, K. Asakura, H. Wan. K. Tsai, Y. Iwasawa, Chern. Lett. 1996, 129. [346] Y. Yuan, K. Asakura, H. Wan. K. Tsai, Y. Iwasawa, J. Mol. Catal. A: Chem., 1997. 122. 147. [347] L. Pignolet, in Cutri1y.si.s b)! Di- und Polynuclear Metal Cluster Complexes: Catalysis by Mixed-Metal Clusters Containing Gold Phosphine Groupings, (Eds.: R . D. Adams, F. A. Cotton), Wiley-VCH, New-York, 1998, pp. 95.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.3 ortho-Metalated Dinuclear Rhodium( 11) Compounds. Synthesis, Structure and Catalytic Applications Pascual Lahuerta and Francisco Estevan
Since F.A. Cotton et al. reported in 1984 that the reaction of rhodium acetate and triphenylphosphine, yields R ~ ~ ( A c O ) ~ [ ( C ~ H ~ ) P P ~rhodium( ~ ] ~ ( A11) C OcomH)~, pounds with ortho-metalated phosphines bridging the metal atoms have become a new class of compounds. The above described reaction is of general application for phosphines containing at least one aryl group. The displacement of acetate ligands by ortho-metalated phosphines entails merely the transfer of protons from the ortho positions of the phenyl rings to the acetate anions. In some cases all the intermediates in this particular reaction have been detected or isolated. In the final products the phosphine ligands are mutually cis, adopting a head-to-tail or head-tohead configuration. The compounds with head-to-tail configuration are promising catalysts for a-diazo compound decomposition reactions.
2.3.1 Introduction Intramolecular coordination compounds have received a great deal of interest since the first example of such a complex was reported."] Although many compounds with only one metal atom involved in the metallocycle (I) have been described, intramolecular coordination compounds containing metal-metal bonds (11) are relatively rare."]
C
L
\ / M
L
C
M-
M
I
2.3 ortho-Metuluted Dinucleur Rkodiun?( I I ) Compounds
679
In 1984 F.A. Cotton et a]. reported that the reaction of rhodium acetate and triphenylphosphine in pure acetic acid as solvent, yields R ~ ~ ( A C O ) ~ [ ( C ~ H ~ ) P P(~ l).131 ~ ]N~o( net A ~ redox O H ) ~process occurs during the reaction and formally the displacement of two acetate ligands by two orthometalated phosphines involves merely the transfer of protons from the ovtho positions of the phenyl rings to the acetate anions. The phosphine ligands are mutually cis and bridge the two rhodium atoms adopting a head-to-tail (H-T) configuration.
Me I
PPh,
1 AcOH
I
Me
Me
L= AcOH
1
A different synthetic route yielded rhodium( 11) compounds with the general for(2) (pz = pyrazolyl and substituted mula RhzBr(p2-pz)2[p2-(C6F4)PPh2](CO)L2 pyrazolyl groups; L2 = (C0)z or q2-P(o-BrCsF4)Ph2), which also have bridging arylphosphine anions. These were prepared in low or moderate yield by thermal reaction of [Rh(p2-pz)(C0)2]2with PCBr [PCBr = P(o-BrCsF4)PhzI in refluxing toluene.[4] This metalation reaction involves two center-two electron oxidative addition with cleavage of the C-Br bond of the phosphine.
’I’
R h __ R h
I PPhh , P e +
’co F4
2
680
2 Metal Clusters in Catalysis
2.3.2 The chemical reaction The reaction of rhodium acetate and the phosphine P(o-BrCsF4)Phz was studied under different conditions. This phosphine was potentially capable of reacting by either or both of two reaction routes, involving C-H or C-Br cleavage. No products resulting from oxidative addition were ever detected. In non-protic solvents, such as toluene, compounds 3 and 4, containing only one ortho-metalated PCBr ligand, were isolated. The structure for 3 was proposed on the basis of spectroscopic data. The molecular structure of 4 has two cisoid acetate ligands bridging a dinuclear rhodium( 11) unit.l5I One acetate has been completely displaced by an orthometalated phosphine, which acts as a three-atom-bridging bidentate ligand.
Me 3
Me
F4
4
In the fourth bridging site the acetate displacement is incomplete. The previously bridging acetate is shifted to a mode of ligation that is intermediate between chelating and monodentate (Rh-O,, = 2.04(2) A;Rh-O,, = 2.43(3) A.The equatorial site on the other Rh atom is now occupied by a phosphine, which is of appropriate dimensions and properly oriented to introduce the o-Br atom of the C6F4Br group into the second axial site. The use of acetic acid as solvent promotes the second ortho-metalation step. Thus after 1 h of reaction of rhodium acetate and P(o-BrC6F4)Phz, two new products, 5 and 6 were in addition to the previously described compounds 3 and 4. This was the first direct evidence that acetic acid catalyzes the ortho-metalation reaction. Both 5 and 6 are bimetalated Rh( 11) dimers. Compound 6 has a close structural resemblance to 1, with two metalated ligands. In this case the PCBr ligands assume an q 3 ,p-mode of ligation. In 5 one of the axial positions was found to be occupied by a molecule of water. 31P NMR spectroscopy of the reaction mixture shows that this site is initially occupied by a molecule of acetic acid which is displaced by water during the subsequent chromatography.
2.3 ortho-Metalnted Dinuclenv Rhodium ( I I ) Compounds
68 1
,Me H
Me
Me
6
5
On the basis of the isolation of the intermediate compound 4 a reaction pathway was proposed (Scheme 1). Initially there is coordination of one phosphine with the formation of adduct 111. This phosphine rearranges from axial to equatorial coordination with partial displacement of one acetate group. Proton transfer in this species IV yields the monometalated compound V, which further reacts with one additional phosphine to form the bimetalated compound. Reaction intermediates IV and V were isolated to study individual reaction steps of the metalation process. The selective formation of the monometalated species V required adduct 111 to be the only species in solution. However, I11 was characterized only when P is P(o-MeOC&)Phz.[’] In the solid state two units with the formula Rh2(02CCH3)4P(o-MeOC6H4)Ph2(7) are associated, forming a centrosymmetric dimer of dimers. One oxygen atom from one acetate in one Rh2P unit, axially coordinates one Rh atom on another Rh2P fragment. Osmometric measurements confirm that in CHzCl2 solution, 7 is in the monomeric form. It was found that photochemical irradiation of 7 in CHC13 at room temperature
111
Scheme 1
IV
V
682
2 Metal Clusters in Catulysis
P = P(o-MeOCGH4)Ph2
I
Me
7
affords the equatorial compound Rh2(p-02CCH3)3(q1-O2CCH3)[q2-(CgH4)P(oMeOC6H4)Ph](HzO) (8) in high yield.''] In solution this compound slowly evolves
11.7 ppm
I
Me
Me
Me
8
9
(3) 7
to form the metalated product 9. This reaction, which is faster in the presence of acid, can be easily monitored by 31PNMR spectroscopy, because the chemical shift of phosphorus is quite different for all the compounds involved in the process. A second structurally characterized equatorial compound[9] is Rhz(p02CCF3)3(q'-02CCF3)[~2-(C6H4)P(o-C1C6H4)Ph]( H20) (10).Compounds 8 and 10, of structural type IV, have high chemical shift doublets in the 31PNMR spectrum, at 38.2 ppm (compound 8) and 41.3 ppm (compound lo), diagnostic of the presence of a non-metalated P ligand in an equatorial coordination site. A common and very particular structural feature in these compounds is the presence of one monodentate carboxylate in an equatorial position and one chelating phosphine occupying an equatorial (P) and an axial ( M e 0 or C1) coordination site. In both structures one phenyl ring is oriented to the partially displaced carboxylate group in an arrangement that makes the transfer of one proton from the phenyl ring to the carboxylate very favorable. The chelating coordination of the phosphine seems to
2.3 ortho-Metcrhted Dinuclem Rkodium(II) Compounu's
683
be responsible for the relative stability of these intermediates, which in solution undergo slow metalation reaction to form species of type V. Triphenylphosphine and other non-functionalized phosphines give very reactive equatorial intermediates that can barely be detected when the progress of the reaction is monitored by 31P NMR spectroscopy. Monometalated species of type V have been obtained with several phosphines with yields in the range 60-900/;1 and have been structurally characterized.["] Stepwise exchange reactions of CH3COO- groups by CD3COO- groups are observed )] ~.( in CDC13-CH3COOD mixfor R ~ z ( ~ z C C H ~ ) ~ [ ( C ~HHO~Z)CPCPH~~~(11) tures.l"l The first step involves a fast exchange of the acetate group trans to the metalated phosphine and exchange of the two axial molecules of acetic acid. In a second and slower step the exchange of the other two acetate groups occurs. The intramolecular hydrogen bonding between axial and equatorial ligands, and the longer Rh-0 bond distances observed for the acetate group trans to the metalated phosphine, might be the main factors responsible for the lability of the trans acetate group.
The kinetic data for the slow exchange process, which was studied by ' H NMR spectroscopy,'"' follow a two-term rate law, v = [kl k2[CD3COOD]''2] [Rh2]. The first-order constant kl at 298 K is (2.08 k 0.02) x lop6 s-' ( A H z = 98 f 5 kJmol-I; ASi = 45 k 20 J K - ' mol-I), and the second-order rate constant at the same temperature, k ~ is, (3.83 f 0.01) x 1 0 - ' s ~ ' MP1I2 ( A H $ = 103 - 5 kJmol-I; AS$ = -15 15 JK-'mol-'). Electrophilic attack at one oxygen atom of the bridging acetate group by a proton (or protonated acetic acid) was suggested as the first and rate-determining step of the exchange process.
+
+
+
684
2 Metal Clusters in Catalysis
One interesting observation that emerged from these experiments is that, under the usual reaction conditions, the metalation is a reversible process. Thus, Rh2(02CCH3)3[(C6D4)P(c6D~)2]( H02CCH3)2 in CH?COOH, undergoes D-H exchange at the ortho positions of the phenyl ring (90% exchange after 3 h reflux).["]
e
Me
Me
Me
(5)
This can only be explained by electrophilic attack at the rhodium-carbon bond by acetic acid, which causes protonation of the aromatic ortho carbon atom, followed by a cyclometalation at one of the ortho C-H bonds. Thus, the metalation is an equilibrium reaction IV + V rather than an irreversible process. If a stronger acid, CF3COOH, is used the D-H exchange is observed even at room temperature.['Oel This I V + V equilibrium seems to be general, but species IV can only be detected if the compound has an ortho functionalized phosphine able to act as a bidentate ligand. As an example of this behavior, Rh2(p-O2CCF3)3(ljl102CCF3)[lj12-(C6H4)P(o-C1C6H4)Ph]( H20) (lo),is easily obtained by reaction of the monometalated Rh2 p-02CCF3)3[pU-( C6H4)P(o-C1C6H4)Ph](CF3C02H)2 (14) with trifluoroacetic acid.[' Additionally 12 reacts in boiling acetic acid to produce 13, an isomer of 12 in which it is now the ortho-chloro-containing phenyl ring that is metalated." 21 This isomerization can be explained by the proposed V + IV equilibrium. In the case of the compound Rh2(p2-02CCH3)3[(C6H4)PMePh](CH3C02H)2 (15) H-D exchange was observed not only at the ortho hydrogen atoms of the phenyl rings but also at the methyl group of the pho~phine.['~I It has been observed that, as is shown in Scheme 2, two pathways must be operating in the second metalation reaction, as bimetalated compounds with head-to-tail (pathway A) and head-to-head (pathway B) configurations are obtained depending on the reaction conditions. The first evidence of that behavior was obtained from the reaction of the monometalated compound Rhz(02CCH3)3[(C6H4)P(o-ClC6H4)Ph]( CH3C02H)2 (12) with different triarylphosphines PR3 ( R = ( P - X C ~ H ~X) ,= Me, H, Cl), to produce bimetalated com-
\
2.3 ortho-Metaluted Dinuclour- Rhorliurn( I I ) Cmipounds
685
I
Me 12
f
+CF3C02H
+CF3C02H L
1
-CF3C02H I
I
CF3
CF3
14
10
pounds that have mixed sets of ph~sphines.l'~] When a stoichiometric amount of phosphine was used, the products had structures of type VIIa, whereas if the reaction was run in the presence of excess phosphine, only compounds of type VIIb were formed. Under the latter conditions the metalation is considerably faster. Further experiments confirmed that the rate of the second metalation reaction, and the structure of the reaction products, mainly depends on the nature and amount of the incoming p h ~ s p h i n e . ' ' Three ~~ bimetalated compounds with the head-tohead structure have been characterized by X-ray method^.['^^'^] There is spectroscopic evidence for the formation of two adducts, 11.Pa and l l . P b , of structural types VIa and VIb (Scheme 2) in the reaction of the monometalated compound (11) with a stoichiometric amount of triphenylphosphine. Low-temperature 3 ' P NMR spectra confirmed that these two adducts are in equilibrium and that the [ll.Pa]/[ll.Pb] ratio increases as the temperature increases from -60 to +25 "C. If excess phosphine is used the bis adduct, 11.P2, is also detected. Similar equilibria occur with other monometalated compounds.[' 71 These two pathways A and B proceed via equatorial intermediates VIa and VIb.
686
2 Metal Clusters in Catalysis
Pathway A
VIIa
Via
V Pathway B
V.Ph
VIb
Scheme 2
Species of type VIb have only occasionally been detected spectroscopically.llobI Compound 4, resulting from the reaction of rhodium acetate and P(o-BrCsF4)Ph2, was the first intermediate of type VIa to be structurally ~haracterized.'~] This type of intermediate is quite stable when the phosphine can have both axial and equatorial coordination. If the equatorial phosphine is monodentate they can only be prepared by photochemical methods and these intermediates are moderately stable in the solid state for several weeks, but in chloroform solution at room temperature they evolve slowly (tip > 24 h) to yield the bimetalated compounds VIIa. In the absence of acid the VIa 4 VIIa reactions are very slow, with observed rate constants two or three orders of magnitude smaller than those obtained in the presence of protic acid at the same temperature.['*] All the compounds studied to date have metalated triphenylphosphines, and the equatorial ligands were P(XC6H4)3, ( X = H, p-CH3, p-C1 , m-CH3, m-Cl). The reaction is first order in the concentration of the monometalated species VIa. For the acid-catalyzed experiments, the experimental data were fitted to a limiting rate equation of type kobs= k[H']/(Ke [H']) where k is the limiting first-order rate constant and K, is the equilibrium constant for the acid-base equilibria before the rate-limiting step. The enthalpies of activation AH$ are in a narrow range of values around 70 kJ mol-' for the acid-catalyzed reaction and about 15-20 kJ mol-' higher for the thermal reaction. The entropies of activation are negative (AS1 = -60 to -100 J K-' mol-').
+
2.3 ortho-Metuluted Dinucleur Rhodiun?( I I ) Coqmunds
687
The mechanism proposed on the basis of these data included a highly ordered transition state, responsible for the very negative values of the activation entropy. According to the observed value for the deuterium kinetic isotopic effect, the C-H bond breaking process must not be very important in this transition state. For the acid catalysed reaction a fast protonation equilibrium of species VIa, followed by the rate-determining step was proposed. VIa + H’
Ke e [VIa.H’]
k
Va
(7)
The K , values for this equilibrium are low, indicating that the proposed protonated species must exist at low concentration. The nature of the protonated species is not completely clear, but it was suggested that it involves protonation of the chelating acetate group. There is no experimental evidence to support an oxidative addition mechanism followed by reductive elimination.
2.3.3 Selectivity of the metalation reaction There are several reports of metalation reactions with phosphines having more than one site for C-H activation. Thus, the reaction of rhodium acetate and the phosphines P(o-XChH4)Ph2, ( X = C1, Br), initially produced only compounds metalated at the unsubstituted phenyl rings. The crystal structure of the bimetalated compound with P ( Y M - C H ~ C confirmed ~ H ~ ) ~ that the metalation occurs selectively at the carbon atom trans to the methyl group.” 91 Tocher et al. studied the reaction of rhodium tetracarboxylates with tertiary phosphines having both alkyl and aryl substituents.lZo1These authors reported that the reaction of rhodium acetate with PMePh2, in the presence of excess pivalic acid, is diastereoselective, giving only one of the three possible isomers of Rhz{,u202C(CH3)3}2[(C,jH4)PMePh]2,( 16) the one with structure IX (Scheme 3).[’Oa1 Further studies confirmed that the same reaction in the absence of pivalic acid, yields Rh2(,uz-02CCH3)2[(C6H4)PMePh]2 (17) selectively as isomer VIII.1211 This was an example of how the nature of the leaving carboxylate group can affect the stereoselectivity of this particular reaction. The reaction with PMe2Ph only gave the product resulting from activation of aromatic C-H bonds.[20b1 There are two examples reported of rhodium( 11) compounds metalated at an alkyl group of a phosphine.lZz1 One compound with the formula (18) was obtained by serenRh2(,uu,-02CCH3)2[,u2-(CH2)PPh2][,u2-(C,jH4)PPh2]PPh3
688
2 Metal Clusters in Catalysis
R
R
I
Ph
Ph
R
I
I
I
Ph
I
I
ph'pwp'ph ' R-pYYp-R R
Ph
-'p-
('3
X
IX
VIlI
R = Me, C6F5
Scheme 3
dipity from the reaction of Rh2(pU,-02CCH3)3[(C6H4)PMePh] (15) with two moles of triphenylphosphine. The structure of 18 consists of discrete dinuclear rhodium units [Rh-Rh = 2.532(2) A] with two phosphines in an H-H configuration.[22a,bl The relevant feature of this structure is that the PMePhz ligand is metalated at the methyl group, forming a four-membered Rh-P-C-Rh metallocycle. Compound 18 is also obtained in low yield in the reaction of 15 with one mole of triphenylphosphine, which gives 17 as the main product (Scheme 4). The reaction of 17 with one mole of phosphine also gives 18 as a minor compound. A detailed study of the reaction suggests that 15 + 17 + 18 is not be the main reaction pathway from 15 to 18. The other compound containing a metalated aliphatic carbon is Rh2(C5H5)(H)Cl( Pri2PCH(CH3)CHCCH2Ph)(P( Pri3), (19). It was formed by a novel C-C coupling of a methyl group of the phosphine to a coordinated vinylidene
17
H +2PPh,
PPh3 H25
I
Me
18
Scheme 4
R.= iPr
Ph
19
2.3 ortho-Metuluted Dinuclear Rhodium ( I I ) Compounds
689
The reaction of rhodium acetate and P(C6F5)Ph2 is relatively slow. Mixtures of mono and bimetalated compounds are always obtained, even after heating for long periods in mixtures of toluene-acetic acid. The three possible isomers are detected in solution by j'P NMR spectroscopy. The isomer having a different configuration at each P atom (isomer IX, Scheme 3 ) is the dominant product (75%). The other two isomers are in fractions of 20%+ (isomer X ) and < 5% (isomer VIII). The two major isomers, IX and X, have been characterized by X-ray crystallography.[23] Rhodium( 11) compounds with ortho-metalated diphosphines have been also rep ~ r t e d . [ ' ~ "The , ~ ~ two ] P atoms coordinate two equatorial positions instead of having the axial-equatorial coordination observed for dinuclear platinum metalated compounds.[2e1
2.3.4 Chiral ortho-metalated compounds The bimetalated compounds of type VIIa are chiral. The reaction of triphenylphosphine with rhodium acetate under achiral conditions leads to a racemic mixture of products 1 and 1'. The resolution of this mixture has been achieved by exchanging the bridging acetates for chiral carboxylates with subsequent separation of the resulting diastereoisomers. The readily available and inexpensive tosylated proline has been used for this The resulting diastereoisomers 20 and 20' were separated by standard column chromatography on SiO2. Treating these pure diastereoisomers with trifluoroacetic acid leads again to the complete exchange of the carboxylate yielding 21 and 21'. Finally, crystals were grown for 21 and 21' with trifluoroacetic acid and pyridine, respectively, in the axial positions and the structures determined by X-ray diffraction. The absolute configuration was assigned to the chiral centers.
1' 20' 21 I
690
2 Metul Clusters in Cutulysis
2.3.5 NMR spectroscopy The presence of several magnetically active nuclei, Io3Rh, 31P,I3C , ' H and occasionally I9F, makes NMR spectroscopy a very useful technique for the characterization of these metalated compounds. In particular, 3'P NMR has been an extremely useful technique. A correlation has been observed between the chemical shift and the coordination of the phosphine in the dinuclear unit. The values of the different homo and heteronuclear coupling constants Jp-p and JRh-p in conjunction with the chemical shift values usually enable structural identification of the species present in solution on the basis of spectroscopic data alone. The reactions of formation of metalated compounds can be monitored easily, by following the change in the 31PNMR signals with time. Some of the relevant correlations are represented in Scheme 5. The I3CNMR spectra of the metalated compounds show characteristic low-field, low-intensity resonances in the 160-165 ppm chemical shift range, assigned to carbon bonded to rhodium. The values of the 'JRh-C coupling constants are around 20-25 Hz. In compound 18 the resonance due to the metalated carbon is shifted to higher field, and appears at -5.2 ppm.[22b1 The number and relative intensity of the proton resonances from the carboxylate groups, especially in acetate derivatives, are useful for determining the symmetry
I
50
I
40
30
20
10
II
-10
'
'
lI
I
Gppm
- 40 C
150 Hz
Scheme 5
Rh-
Rh
- 50
2.3 ortho- Mctulatcd Dinucleur Rhodium( II) Compounds
69 1
and stoichiometry of the molecule. '03Rh NMR has been used for several dinuclear rhodium( 11) compounds. The chemical shift values are in the range 2300-2500 ppm for bimetalated compounds. In monometalated compounds the rhodium nuclei attached to P appears at 5450-5550 ppm whereas the rhodium attached to C appears R ~ 20-25 H z . [ ~The ~ ] chemical shifts at 3550--3650 ppm. The values for ' J R ~ - are were measured relative to E('O'Rh) = 3.16 MHz.12']
2.3.6 Crystal data Except for compounds of type VIb the crystal structure has been determined for at least one compound of each of the structural types 111 to VII included in the Schemes 1 and 2. Crystal structures have been determined for several monometalated compounds.['"] Two features of these structures merit comment. Using 11 as an example,^'""] the Rh-0 bond trans to the carbon atom, 2.218(4)& is longer than that trans to the P atom, 2.163(3) A, and both bonds are longer than the other four R h - 0 bonds involving acetate ligands (in the range 2.025(4)2.073(4)8,). The OH groups of the axially coordinated acetic acid molecules are linked to the oxygen atoms of the acetate ligands trans to the phosphine, through intramolecular hydrogen bonds (0-0distances 2.586(6) and 2.705(5)8,). The Rh-Rh bond distance increases with the number of metalated ligands and P~~)]~~' ranges from 2.414( 1) 8, for {Rh2(02CCH3)4P}Z, [P = P ( O - M ~ O C ~ H ~ ) to 2.770(3) A for Rh2C12(dmpm)2[(ChH4)PPh2]2CH~Cl~[281 (dmpm = bis(dimethy1phosphin0)methane). Thus this distance is shorter for monometalated compounds 2.426( 1)-2.476( 1) A than for bimetalated 2.493( 1)-2.770(3) A. Typical values for Rh-C bonds are 1.97-2.09 8, and for Rh-P bonds 2.20-2.33 A. The longest values (dmpm = bis(dimethy1correspond to Rh2C12(dmpm)~[(C~H4)PPh2]2CH2C121281
Me 11
2 Metal Clusters in Catalysis
692
R = Ph
Me2P-
PMe2
22
23
phosphinomethane) (22). Distances are within these ranges for compounds with the formula Rhz(p2-C12)[(C6H4)PPh2]2(PR3)z (23).lz9I
2.3.7 ortho-Metalated dinuclear compounds in catalysis Rhodium( 11) compounds have become the premier choice in catalytic transformation of a-diazo compounds to induce cyclopropanation, aliphatic carbonhydrogen bond insertion, heteroatom-hydrogen bond insertion, aromatic substitution, and ylide formation. It is accepted that these catalytic reactions occur via a metallocarbene intermediate.13'1 The structure suggested for this species is shown in Scheme 6. Rho-
insertion
Cyclopropanat ion
Arom. Substitution
Dimerization R
Scheme 6
2.3 ortho- Metalcited Dinucleur Rhodiuni ( I / ) Compounds
693
dium( 11) compounds have been shown to be very efficient at generating transient electrophilic metal carbenes from x-diazo compounds with capability of inserting into unactivated C-H bonds.13 Results have demonstrated that in intramolecular reactions the formation of the five-membered ring is a favored process for diazo comp~unds.[~~l The behavior of bimetalated rhodium( 11) compounds with head-to-tail configuration (type VIIa) in catalytic transformation of a-diazo compounds has been also 3.341 Results have confirmed that by changing the carboxylate groups and ~tudied.1~ the substituents on the metalated and non-metalated rings of the phosphine the activity and the regio and chemoselectivity of these compounds can be substantially modified. Thus, in the competition between C-H insertion reaction and aromatic substitut i ~ n l (Table ~ ~ ] 1) metalated rhodium( 11) compounds with basic phosphines, e.g. triphenylphosphine, and acetate ligands have poor chemoselectivity and reactivity. This value compares well with that reported for rhodium( 11) c a p r ~ l a c t a m a t e . [ ~ ~ l The selectivity is increased to some extent if the basicity of the phosphine is reduced by introducing electron-withdrawing substituents into the non-metalated phenyl ring, as for pentafluorophenyldiphenyl phosphine. However, the most effective way of increasing both reactivity and chemoselectivity is found to be the use of a basic phosphine, e.g. triphenylphosphine, and high electrophilic carboxylate groups, e.g. perfluorobutyrate. Table 1. C-H insertion vs aromatic substitution
&WN2
0
Catalyst
+
Rh(ll) CH2C12
Ph
Ref.
35 35 35 33 33 33 33 33
Yield (%)
96 96 64 61 82 92 93 95
Relative ratio C-H insertion
aromatic substitution
65 0 70 50 80 100 100 100
35 100 30 50 20 0 0 0
694
2 Metal Clusters in Catalysis
Table 2. Tertiary C-H insertion vs secondary aliphatic C-H insertion.
Catalyst
Ref.
Yield (YO) Relative ratio tertiary C-H insertion
secondary C-H insertion
Competing C-H insertion reactions mediated by this type of catalyst have been also studied.[341In this type of reaction the complex with triphenylphosphine and acetate was shown to be both reactive and very selective, yielding only the tertiary C-H insertion product (Table 2). When a catalyst with the less basic phosphine PPh2(C6F5) was used, this slightly reduced both reactivity and selectivity. More electrophilic catalysts with heptafluorobutyrates as carboxylates were less selective. We also tested the reactivity and selectivity of mono metalated complexes (Type V ). These catalysts, although reactive, were in general less selective than bimetalated compounds with head-to-tail configuration. Finally, preliminary studies of reactivity of doubly metalated compounds with the head-to-head configuration (Type VIIb) showed that these compounds are not promising catalysts for C-H insertion
References [ l ] (a) Kleiman J. P. and Dubeck M., J. Am. Chem. Sor. 1963,85, 1544. (b) Omae 1 Coord. Chem. Rev. 1980,32, 235. (c) Ryabov A. D. Chem. Rev. 1990, 90, 403. [2] (a) Barder T. J., Tetrick S. M., Walton R. A,, Cotton F. A. and Powell G. L., J. Am. Chem. Sor. 1984, 106, 1323. (b) Bennett M. A., Bhargava S. K., Griffiths K. D. and Robertson G. B., Angew. Chem. Int. Ed. Eny. 1987, 26, 260. (c) Bennett M. A., Barghava S. K., Ditzel E. J., Robertson G. B. and Willis A. C., J. Chem. SOC.Chem. Commun. 1987, 1613 (d) Arnold D. P.,
2.3 ortho-Metaluted Dinuclrar Rhodium ( I I ) Conpounds
695
Bennett M. A,, Bilton M. and Robertson G . B., J. Chem. Soc. Chem. Commun. 1982, 115 (e) Arnold D. P., Bennett M. A,, McLaughlin G. M., Robertson G. B. and Whittaker M. J., J. Chem. Soc. Chem. Commun. 1983, 32 ( f ) Arnold D. P., Bennett M. A., McLaughlin G. M. and Robertson C. B., J. Cheni. Soc. Chem. Crimmun. 1983, 34. [3] (a) Chakravarty A. R.. Cotton F. A. and Tocher D. A., J. Chem. Soc. Chem. Commun. 1984, 501. (b) Chakravarty A. R., Cotton F. A,, Tocher D. A. and Tocher J. H., Organometullics 1985. 4, 8. [4] (a) Barcelo F.. Lahuerta P., Ubeda M. A , , Foces-Foces C., Cano F. H. and Martinez-Ripoll M., J. Chem. Soc. Chem. Cornmun. 1985. 43. (b) Barcelo F., Lahuerta P., Ubeda M . A., FocesFoces C.. Cano F. H. and Martinez-Ripoll M., 0rganornetullic.s 1988, 7, 584 [ 5 ] Barcelo F., Cotton F. A,. Lahuerta P.. Llusar R., Sanau M., Schwotzer W. and Ubeda M. A,, 0nqunometullic.s 1986. 5, 808. [6] Barcelb F., Cotton F. A.. Lahuerta P., Sanau M.. Schwotzer W. and Ubeda, M. A., Organonwtul1ic.s 1987. 6 , 1105. [7] Alarcon C. J., Lahuerta P.. Peris E., Ubeda M. A., Aguirre A., Garcia-Granda S. and GomezBeltdn F., h u r y . Chiin. Actu. 1997, 254, 177. [8] Alarcon C. J., Estevan F.. Lahuerta P., Ubeda M.A.. Gonzalez, G. and Martinez M., Inory. Chim Actu. 1998, 278, 61. [9] Lahuerta P., Peris E., Ubeda M. A,. Garcia-Granda S.. Gomez-Beltrin F. and Diaz. M. R., J. Oryjanomet. Chrm. 1993, 455, C 10. [lo] (a) Lahuerta P., Payli J., Peris E., Pellinghelli M. A. and Tiripicchio A,, J. Orgunornet. Chenz. 1989, 373, C5. (b) Lahuerta P.. Payi J.. Solans X. and Ubeda M. A., Inorg. Chem. 1992: 31, 385. (c) Lahuerta P.. Pay& J., Garcia-Granda S., Gomez-Beltran F. and Anillo A,, J. Orcqanornet. Chem. 1993, 443, C14. (d) Lahuerta P.. Latorre J., Peris E., Sanau M. and GarciaGranda S., J. Oryanonzet. Chem. 1993. 456. 279. (e) Lahuerta P., Latorre J., Peris E., Sanau M., Ubeda M. A. and Garcia-Granda S., J. Organornet. Chenz. 1993, 445, C10. ( f ) GarciaGranda S.. Lahuerta P., Latorre J.. Martinez M., Peris E., Sanau M. and Ubeda M. A,, J. Chenz.Soc. Dnlton. Truns. 1994. 539. (g) Garcia-Granda S., Diaz M. R., Gomez-Beltran F., Peris E. and Lahuerta P., Actu. Crystallocqr. Sect. C. 1994, 50, 691. [ 111 Lahuerta P. and Peris E.. Znoryq. C/iem. 1992. 31, 4547. [12] Gonzilez G.; Martinez M., Estevan F., Garcia-Bernabe A.; Lahuerta P., Peris E., Ubeda M. A., Diaz M. R.. Garcia-Granda S. and Tejerina B., New J. Ci7em. 1996, 20, 83. [ 131 Estevan F., Lahuerta P. and Peris E. Unpublished results. 1141 Cotton F. A., Barcelo F., Lahuerta P., Llusar R., Payi J. and Ubeda M. A., Inorg. Chem. 1988,27. 1010. [ 151 Lahuerta P., Payi P.. Peris E.. Aguirre A,, Garcia-Granda S. and Gomez-Beltran F., Inorcj. Chim. Ac,ta. 1992, 192. 43. [I61 (a) Lahuerta P., Payi J.. Pellinghelli M. A. and Tiripicchio A,, Inorg. Chem. 1992, 31, 1224 (b) Estevan F., Lahuerta P., Latorre J., Peris E., Aguirre A,. Garcia-Grande S., Gomez-Beltrin F. and Salvado M. A,, J. Chem. Soc. Dulton. Trans. 1993, 1681. [I71 Borrachero M. V., Estevan F.. Lahuerta P.. Payi J. and Peris E., Po/.vhedron 1993, 12. 1715. [ 181 Lahuerta P., Peris E.. Sanau M., Gonzalez G . and Martinez M.. J. Chem. Soc. Dalton. Truns. 1994. 545. [I91 Lahuerta P., Martinez-Maiiez R.. Paya J., Peris E. and Diaz, W., Inory. Chim. Actu. 1990, 173, 99. [20] (a) Morrison E. C. and Tocher D. A,, Inorq. C/7it77. Actu. 1989. 157, 139. (b) Morrison E. C. and Tocher D. A,, J. Oryunotner. Chen7. 1991, 408. 105. [21] Estevan F. and Lahuerta P. Unpublished results. [22] (a) Borrachero M. V.. Estevan F., Garcia-Granda S . , Lahuerta P., Latorre J., Peris E. and Sanau M.. J. Chnn. Soc. Clzern. Coninnin. 1993, 1864. (b) Estevan F., Garcia-Granda S., Lahuerta P., Latorre J., Peris E. and Sanau M.. Inory. Chin?. Actu. 1995.229, 365. (c) Werner H., Wolf J.. Muller G . and Kruger C., Angeii.. Cliern. Int. Ed. Engl. 1984, 23, 431.
696
2 Metul Clusters in Cutalysis
[23] Estevan F., Lahuerta P. and Pereira I. Unpublished results. [24] (a) Bruno G., De Munno G., Tresoldi G., Lo Schiavo S. and Piraino P., Inorg. Chem. 1992, 31, 1538. (b) Tresoldi G., De Munno G., Nicolo F., Lo Schiavo S. and Piraino P., Inorg. Chem. 1996, 35, 1377. [25] Taber D. F., Malcolm S. C., Bieger K., Lahuerta P., Sanau M., Stirba S. E., Perez-Prieto J. and Monge A,, J. Am. Chem. Soc. 1999, 121, 860. [26] Estevan F. Unpublished results. [27] Goodfellow R. J. and Garth Kidd R. N M R and the Periodic Table, Eds Harris R. K. and Mann B. E., Academic Press London 1978, p 245. [28] Cotton F. A,, Dunbar K. R. and Verbruggen M. G., J. Am. Chem. Soc. 1987, 109, 5498. [29] Cotton F. A,, Dunbar K. R. and Eagle C. T., Inorg. Chem. 1987,26, 4127. [30] Doyle M. P., Chem. Rev. 1986, 86. [31] For reviews: (a) Padwa A. and Austin D., Angew. Chem. Int. Ed. Engl. 1994, 33, 1797. (b) McKervey M. A. and Ye T., Chem. Rev. 1994, 94, 1091. (c) McKervey M. A. and Doyle M. P., J. Chem. Soc. Chem. Commzm.1997, 983. (d) Doyle M. P. In Homogeneous Transition Metal Catalysts in Organic Synthesis, Moser, W. R., Slocum, D. W., Eds.; ACS Advanced Chemistry Seies 230; American Chemical Society: Washington, DC, 1992; Chapter 30. (e) Taber D. F., Comprehensioe Organic Synthesis, Eds.; Pergamon Press: Oxford, 1991; Vo1.3 pp 1045. (f) Taber D. F., Meth0d.s of Organic Chemistry, Houben-Weyl, Helmchen G., Hoffman W., Mulzer J. and Schaumann E. Eds.; George Thieme Verlag: Stuttgart, 1995, 1127. (g) Wulff W. D., Comprehensive OrGganic' Synthesis, Trost B. M . and Fleming I. Eds.; Pergamon Press; New York, 1990 Vol. 5. [32] For relevant examples in Rh (11) mediated C-H insertion, see: (a) Doyle M. P., Dyatkin A. B., Roos G. H. P., Cafias F., Pierson D. A., Van Basten A,, Muller P. and Polleux P., J. Am. Chem. Soc. 1994,116,4507. (b) Wang P. and Adams J., J. Am. Chem. Soc. 1994, 116, 3296. (c) Doyle M. P., Westrum L. J., Wolthuis W. N. E., See M. M., Boone W. P., Bagheri V. and Pearson M. M., J. Am. Chem. Soc. 1993, 115, 958. (d) Taber D. F., Amedio Jr J. C. and Raman K., J. Org. Chem. 1988, 53, 2984. [33] Estevan F., Lahuerta P., Pkrez-Prieto J., Sanai M., Stiriba S. E. and Ubeda M. A,, Organometallic's 1997, 17, 880. [34] Estevan F., Lahuerta P., Perez-Prieto J., Pereira I. and Stiriba S. E., Organometnllics 1998, 17, 3442. [35] Padwa A,, Austin D. J., Price A. T., Semones M. A,, Doyle M. P., Protopopova M. N., Winchester W. R. and Tran A,, J. Am. Chem. Soc. 1993, 115, 8669.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.4 The Role of Co-catalysts in the Reductive Carbonylation of Aromatic Nitro Derivatives Catalyzed by Transition Metal Carbonyl Clusters Seryio Cenini and Fahio Rayaini
2.4.1 Introduction The application of carbon monoxide as a reductant in organic synthesis is confined to relatively few examples. In the last 25 years, the catalytic reductive carbonylation of organic nitro compounds has become a very intense field of research. This is because a series of industrially important compounds can be obtained from nitro compounds and carbon monoxide in a single step.['] Very important chemicals such as isocyanates (Eq. l ) , carbamates (Eq. 2) and ureas (Eq. 3) can be obtained by this methodology:"'
+ 3 CO ArNCO + 2 COz ArN02 + ROH + 3 CO + ArNHCOOR + 2 CO2 ArN02 + ArNH? + 3 CO ArNHC(0)NHAr + 2 CO2 ArN02
+
4
These reactions avoid the use of the poisonous, energy-intensive, and corrosive phosgene, now currently employed in the industrial production of these chemicals (Eqs. 4-6): ArN02 H',ArNHz -H20
ArNCO + ROH
4
ArNCO + ArNHz
coc12
ArNCO + 2 HC1
(4)
ArNHCOOR
+ ArNHC(0)NHAr
The key compounds are the isocyanates. However, their catalytic synthesis is still not appealing because of the insufficient selectivity of the reactions, mainly because of polymerization of the resulting isocyanates, and by the unsatisfactory turnovers
698
2 Metal Clusters in Catalysis
shown even by the most promising catalysts. These problems can be partially circumvented by conducting the reaction in the presence of an alcohol or an amine, so that the more stable carbamates or ureas are obtained as products (Eqs. 2 and 3). Carbamates are also suitable raw materials for the synthesis of aromatic isocyanates by thermal cracking (Eq. 7): ArNHCOOR
5 ArNCO + ROH
(7)
This approach has found widespread acceptance and most of the catalytic systems developed in recent years afford carbamates (Eq. 2) or ureas (Eq. 3 ) . It is worth mentioning that ureas can also react with alcohols to afford carbamates (Eq. 8) and they seem to be formed as intermediates during many catalytic reactions, even when the final product is the carbarnate."' ArNHC(0)NHAr + ROH
i
ArNHCOOR
+ ArNH2
(8)
Transition metal carbonyl derivatives are the catalysts of choice for these catalytic reactions. However, they are not particularly active when used as such and they must be activated by the appropriate co-catalysts. The co-catalysts most commonly used by us in the synthesis of carbamates are neutral ligands, especially chelating ones, tetraalkyl ammonium halides and alkali metal halides. Their role will be discussed in the following paragraphs. Similarly, transition metal clusters also require the proper co-catalysts for efficient catalysis of the carbonylation of aromatic nitro compounds having an appropriate group in the ortho position, which is able to trap the reactive intermediate. Such groups include the nitrene or nitroso groups, formed by deoxygenation of the nitro group by carbon monoxide; this led to the synthesis of nitrogen-containing heterocycles (Eq. 9): catalyst +
-
co
co2
(9) H
(X = Y = CH for example, giving the indole nucleus)
In this report we will discuss only transition metal carbonyl clusters as catalysts, or catalyst precursors. However, the reader must be aware that the family of compounds able to catalyze reactions 1-3 is mainly composed of mononuclear carbonyl derivatives."] Moreover, even when the starting compound is a cluster, the true active species is often mononuclear (or at least of lower nuclearity) and only little, indirect evidence supports the participation of the cluster in the main catalytic cycle.
2.4 Co-catalysts in the Reductive Carbonylation of Aromatic Nitro Derivatives
699
2.4.2 Synthesis of carbarnates and ureas 2.4.2.1 Neutral ligands as co-catalysts The ruthenium carbonyl cluster Ru3(C0)12 is a poor catalyst for the reductive carbonylation of nitrobenzene.[21Its activity in the catalytic synthesis of carbamates (Eq. 2) can be significantly increased by adding neutral, chelating ligands such as DPPM (bis(diphenylphosphino)methane), DPPE ( 1,2-bis(diphenyIphosphino)ethane), DPPP (1,3-bis(diphenylphosphino)propane)and Bipy (2,2’-bip~ridine).[~I Preformed complexes obtained by reaction of Ru3(CO)12 with the chelating diphosphines such as Ru3(CO) lo( p-DPPM), Ru3(CO)lo( p-DPPE), Ru3(CO)l,( p-DPPP), and mixtures of Ru3(CO)ll(q1-DPPP)with { R U ~ ( C O ) ~ ~ } ~ ( ~ -are D Palso P P active ) catalysts. The addition of the mononuclear triphenylphosphine to Ru3(CO)12 gave a much less active catalytic system. The highest turnover frequency is reached with Bipy as co-catalyst, which also showed the best selectivity. It is known that the purple cluster Ru3(p-C0)2(CO)s(Bipy) is obtained by reaction of Ru3(C0)12 with Bipy in cyclohexane under reflux.14] However, we consider this complex useless as a catalyst because, under experimental conditions similar to those used by us, evidence has been provided for the formation of mononuclear compounds as the active species.[5]Moreover, it is well known that the reaction of Ru3(C0)12 with a nonrigid alkyldiazabutadiene derivative, Pr’N=C(Me)C(Me)=NPr ‘( Pr‘-DAB), affords a mononuclear complex, Ru(C0)3(Pr’-DAB).[61We have also recently reported that the same reaction conducted with a rigid a, a’-diimine ligand such as Ar-BIAN ( R = Me) affords the analogous mononuclear compound Ru(C0)3(To~-BIAN).[’~ This species is also observed under CO pressure, together with Ru3(C0)12 in equilibrium with Ru(C0)5. All these facts point to the formation of mononuclear species as the active catalysts.
I00
2 Metal Clusters in Catalysis
With chelating phosphines as ligands, it is worth mentioning that a compound such as Ru(C0)3(DPPE) is an active catalyst for reaction 2.[81When a chelating phosphine is added to Ru3(C0)12 the promotion efficiency follows the order DPPE > DPPP > DPPM.131The same order is found if preformed mononuclear complexes of the kind Ru(C0)3(P-P) are used,['] supporting the fundamental role of these species. It seems that the role of the phosphine cannot be one of increasing the stability of Ru3(C0)12 against fragmentation by bridging two ruthenium atoms in the cluster. In fact, use of Ru3(CO)lo(pu-DPPE)as a catalyst precursor gave a lower conversion than did the separate addition of Ru3(C0)12 and three moles of DPPE. Finally, the independently synthesized imido cluster Ru3(C0)8(p3-NPh)(pDPPE) was more resistant towards carbonylation than the corresponding cluster containing CO in place of the phosphine,'' 1' an observation which further supports the idea that cluster species are not involved in this catalytic reaction. We have observed that Rh6(C0)16 in toluene, with or without methanol, is essentially inactive in the carbonylation of nitr~benzene.'~] However, in the presence of methanol and Bipy, the conversion was high, with good selectivity in the formation of the carbamate. DPPE seemed to be a less efficient and selective co-catalyst than Bipy. Use of Rh4(C0)12 as catalyst afforded similar results.["] However, as we have recently discussed in detail,"] the main function of the ligand is to induce a disproportionation reaction of the rhodium clusters, with chelating ligands having higher efficiency than non-chelating ligands. Under reducing conditions, as are used in the catalytic experiments, the carbonyl anion [Rh(C0)4]- is generated. This anion is one of the most active catalysts for the carbonylation of nitrobenzene to methyl phenylcarbamate in the presence of bases.['21 We also tested the use of mixed-metal clusters of general formula [PPN]z(MRk(CO)15] (PPN = (PPh3)2N+, M = Fe, Ru, 0 s ) as catalyst precurs o r ~ . [ 'In ~ contrast to other rhodium compounds, these clusters are active even in the absence of any added base, although the addition of Bipy still markedly enhanced their activity. Concerning the role of the second metal, the conversion was virtually indistinguishable for all of the three clusters, but the selectivity increased in the order Ru > 0 s > Fe. However, when we tested the homometallic cluster [PPN][Rh5(CO)I 51, higher activity and selectivity was found and the mononuclear [PPN][Rh(C0)4]gave even better results. Thus we concluded that it is the rhodium component of the cluster, and not their bimetallic nature, that is responsible for the catalytic activity, with the second metal playing only a limited role.
2.4.2.2 Halide anions as co-catalysts Some years ago, research in our group showed that Ru3(C0)12, when activated by a tetraalkylammonium halide, is a very active catalyst for the carbonylation of nitroarenes in the presence of alcohols to afford the corresponding carbamates.L2 Because it was already known that Ru3(C0)12 reacts with nitrobenzene to afford a
2.4 Co-catalysts in the Reductive Carbonylation of Aromatic Nitro Derivatives
701
ArNHC02Me
Y
/
I
MeO' Scheme 1
mixture of the two imido clusters Ru3(CO)10(p3-NPh)and Ru3(C0)9(p3-NPh)2and because the monoimido cluster could also be used as a catalyst precursor, a catalytic cycle was tentatively proposed, which included its intermediate formation (Scheme 1). It was also tentatively proposed the chloride attacks the imido cluster generating the equivalent of an acyl chloride, which might then react more easily with methanol. Protonation of the imido nitrogen and reductive elimination would yield the final product, regenerating R u ~ ( C O ) I Z . In subsequent years, research in Geoffroy's group supported some of the intermediate steps proposed and added additional details.['4p16]Among the most relevant discoveries, it was found that the bis imido iron cluster Fe3(C0)9(p3-NPh)2 reacts with methoxide to afford an isolable methoxycarbonyl compound. Reaction of this second cluster with methanol slowly afforded the carbamate[l4](Scheme 2). The reaction is analogous to part of the pathway proposed in Scheme 1, although evidence was also provided that the site of protonation of the cluster by methanol is the acyl oxygen atom, rather than the imido nitrogen, and that a nitrene-carbene reductive elimination then occurs, followed by tautomerization of the organic product to the final carbamate. Other studies, again conducted by Geoffroy's group['51,also led to the discovery of interesting promotion effects of halides. It was found that the addition of [PPN][X] (PPN'- = (PPh3j2N+, X = C1, Br, I ) strongly accelerates the reaction between Ru3(C0)12 and nitrosobenzene, to afford an imido cluster in which the
702
2 Metal Clusters in Catalysis Ph
Ph
I
1H+
Ph NHC(0)OMe Scheme 2
halide ion is bound in a terminal position. The same clusters can be obtained by reaction of halides with the preformed imido cluster. Irrespective of the synthetic route employed, the imido-halide clusters react with CO under very mild conditions to afford phenyl isocyanate. Interestingly, the order of efficiency of the halides (C1 > Br > I) in this reaction is the same as found in our catalytic reactions (Scheme 3). The promoting role of halides in the isocyanate-producing reaction was ascribed to their ability to assume a bridging position in the cluster, which is likely to weaken one of the N-Ru bonds, so rendering the imido fragment more likely to attack a - CO?
Ph
I Ph
0
Scheme 3
I
0
1-
coordinated CO. The occurencc of such reactions was indeed confirmed by later studies on a related osmium clusteri ' I . for which several intermediates could be isolated. Some model reactions on related trinuclear ruthenium clusters. especially related to the reduction reactions of nitrobenzene t o aniline. have also been reported by Bhaduri and coworkers1 and several papers have also been published by different groups on relatcd reactions of ruthenium and osmium clusters containing imido. aniido or isocyanate fragments. although the latter were not intended as models for cat a 1y t ic rcac t i oils. Although in all of Geoffi-oy's papers it was always remarked that no firm cvidence existed that the reactions reported were of any relevance to the actual catalytic cycle, this prudent statement is not always found in other authors' papers and seems anyuay to have becn overlooked by many readers. so that the involvement of iniido clusters as intermediates in the catalytic carbonylation reactions of nitroarenes catalyzed by R u ~ ( C O ) ~seeins 2 now to be taken for granted by many researchers in the field. However. some results apparently in contrast with tlie above mentioned scenario have been reported in a series of patents by Grate and coworkers.ly,'X1 It was reported that the addition of aniline strongly accelerates the carbonylation reaction. If no aniline is added. a s in our early experiments. some is generated by reduction of nitrobenzene. the necessary hydrogen atoms coming from the dehydrogenation of tlie alcohol present in the reaction mixture.l').'X.'"I If n o alcohol was added. but in the prcsence of added aniline. diphcnylurea was obtained at comparable initial rates. I f an alcohol is present. the urea is then alcoholyzed to afford one equivalent of carbarnate and one of aniline. which I-e-enters the catalytic cycle. llrea alcoholysis WIS independentl>, tested and is a spontaneous reaction at high temperature. which requires n o metal catalyst. We I-ecall that a pathway in which aniline is an interincdiate would be analogous to the one later investigated by Gladfelter and his gi-o~ipfor the related carbonylation reactions catalyzed by Ru(CO)i(DPPE).I8l In this case. a key intermediate is a bis-carbalkoxy complex. At this point. the ti-aditionally accepted mechanism required some re-investigation to gain some insight into the real processes involved. In recent work,l'"l wc unequivocally showed that aniline is :in intermediate during the reaction. Thus any mechanistic picture not implying the intermediate reduction of the nitro compound to aniline. including all of the prcviously suggested reactions. cannot be of any relevance to the actual catalytic cycle. Particularly relevant is the problem of the possible cluster nature of the active t o be in equilibrium with R L I ( C O )under ~ high CO catalyst. R L I ~ ( C O is ) ~known L pressure (1 Eq. 10):
+
R L I ~ ( C O ) , ,3 CO e 3 RLI(CO),
( 10)
By use of a high-prcssure IR cell. we observed that under o ~ i rconditions the
704
2 Metal Clusters in Catalysis
equilibrium is essentially completely shifted towards Ru(C0)5. A typical test to identify the real cluster nature of the active catalyst in systems of this kind is to examine the turnover frequency as a function of the total metal concentration.[2 As is clear from general considerations and from the data reported, the cluster + monomer equilibrium is further shifted to the right at lower ruthenium concentrations. It follows that a decrease in turnover frequency as the metal concentration increases indicates that the lower nuclearity species is the active one, whereas an increase in conversion in the same series indicates that a cluster compound is active as catalyst. As we are in a region where almost all of the ruthenium is present as Ru(CO)~,a very small variation should be observed if the mononuclear compound were active (the concentration of Ru(C0)5 will vary very little in percentage), whereas a marked increase in activity should be observed if the cluster were so. From our results it is clear that Ru(C0)5 is the real catalyst and the cluster is inactive or, at best, has a much lower activity. Because the cluster is also present in a relatively small amount under the catalytic conditions, its contribution to the overall reaction is surely negligible. We next analyzed the role of the chloride anion. We have already mentioned that chloride has been reported to accelerate the reaction of nitrosoarenes with Ru3(C0)12 and also to promote the formation of phenyl isocyanate from a trinuclear imido-halide cluster." 51 However, neither of these effects seems relevant to the effect observed in catalysis. In fact, apart from the problem of the cluster nature of the compounds involved, nitrosoarenes are always much more reactive than their nitro counterparts and the reaction of a free nitroso intermediate should be very fast relative to the other steps in the catalytic cycle, so that its further acceleration should not be influential. The promotion effect on the isocyanate-producing step, on the other hand, cannot be relevant to the catalytic cycle because it has been shown that the corresponding imido clusters are not involved. So we must look for other effects. We observed two different effects. One effect is that the chloride anion substantially increases the rate of interconversion of Ru3(C0)12 and Ru(C0)5, which would otherwise be slow even at high temperature. Thus the presence of chloride in the reaction mixture enables more rapid formation of the active Ru(C0)s moiety under the reaction conditions. The importance of a slow conversion of Ru3(C0)12 into Ru(CO)5 and its acceleration by chloride goes well beyond the current mechanistic study, because it might be relevant to many of the catalytic reactions in which the trinuclear cluster is used as a precatalyst. A second effect is to be found in the acceleration of the initial activation of the nitroarene. We could observe this effect on the reactivity of both Ru3(C0)12 and Ru(C0)5. The effect is derived from the formation of a chloride adduct, which for Ru3(C0)12 is [ R u ~ ( C O ) ~ ~ ( C but ~ )is] -not isolable for Ru(C0)5. This adduct is more easily oxidizable than the starting neutral compounds. As the initial activation of the nitroarene is always an electron transfer from the metal to the nitro group, this increase in oxidizability renders the reaction much easier. Several experiments were done to assess which of the two effects of chloride identified is that responsible for the acceleration of the catalytic reaction. It turned out that only the first effect is kinetically relevant to the acceleration of the
2.4 Co-catalysts in the Reductive Curbonylution of Aromatic Nitro Derivatives
705
catalytic reactions, except, perhaps, when the nitroarene concentration is very low. This is interesting, because this effect is on a part of the reaction which is formally outside the main catalytic cycle. The reason for the higher efficiency of chloride with respect to bromide and iodide cannot now be ascribed to its different tendency to assume a bridging position in a cluster or to any labilization effect.[”] The most reasonable explanation is that it is simply more nucleophilic than the other two halides in the reaction medium. It is well known that the order of nucleophilicity of the halides depends on the solvent. In protic solvents iodide is more nucleophilic than bromide and chloride because bromide and chloride form increasingly stronger hydrogen-bonds. However, the exact reverse is true in aprotic solvents, with chloride being the strongest n u c l e ~ p h i l e . [Evidently, ~~~ a solution containing 10-20% methanol in toluene is ‘sufficiently aprotic’ that the latter order of nucleophilicity predominates. It should be noted that hydrogen-bonds should be formed less easily at the high temperatures employed in the catalytic reactions, a factor which might by relevant. We have thus identified the reason why chloride increases the rate of the carbonylation reaction. However, during our catalytic studies’’] it was also noted that an increase in the selectivity to carbamate was observed upon addition of the co-catalyst and that the identity of the countercation also played an important role. The best countercations were Et4N+ and Bu4N+ whereas PPN+(PPN+ = (PPh3),N+) and Me4N+ gave inferior results and Et3NH+ was by far the worst. The tetraethylammonium salt was chosen for most subsequent experiments. Several authors have then used this salt in related reaction^.^'^"^^ without questioning the reason for its higher efficiency, which had not been revealed. The lower activity of [Et3NH][Cl]as a co-catalyst has been shown to be due to its capacity to form a hydrogen-bond between chloride and the nitrogen-bonded hydrogen atom, preventing the former from acting as a nucleophile. The reason why PPN+ and Me4N+ result in lower conversions and have even lower selectivities than Et4N+ and Bu4N+ is because the latter have hydrogen atoms in the position p to the nitrogen atom, whereas the others do not. Hydrogen atoms in the p position enable Hofmann degradation (Eq. 11):
It was indeed possible to detect the presence of triethylamine at the end of the reactions. With regard to the role of the triethylamine formed, several pieces of evidence, and data from the literature, make it clear that the carbonylation reaction proceeds through the intermediate formation of diphenylurea, which is only later alcoholyzed to carbamate, regenerating the aniline necessary for the reaction to proceed. Such alcoholysis is analogous to transesterification and is expected to be base-catalyzed. Indeed, a series of experiments on urea alcoholysis under typical catalytic reaction conditions showed that triethylamine does indeed accelerate urea alcoholysis.
706
2 Metal Clusters in Catalysis
Scheme 4
The reason for the greater activity of the tetraethyl- and tetrabutylammonium salts is thus to be found in their ability to decompose during the catalytic reaction, affording the right species at the right moment. Indeed, at the beginning of the reaction the countercation has no active role, but must leave chloride completely free, so that it can catalyze the transformation of Ru3(C0)12 into Ru(C0)j. All of the countercations employed fulfil this requirement, except for [Et3NH][C1],which is indeed the least effective. As the reaction proceeds, chloride becomes completely useless, but diphenylurea starts to accumulate and needs to be alcoholyzed. At this point, the presence of a base is beneficial and tetraethyl- and tetrabutylammonium can provide it, whereas tetramethylammonium and PPN cannot. A general view of the catalytic cycle is shown in Scheme 4.['O]
2.4.3 Synthesis of heterocycles In the following section the synthesis of several types of heterocyclic compound will be discussed. It should be noted that there have been almost no real mechanistic
2.4 Co-cutulysts in the Reductive Curhonylution
of'Aromutic Nitro Derivutives
707
studies in this field and the mechanistic proposals must be considered as mainly speculative. We will first consider the synthesis of carbazole (2) from 2-nitrobiphenyl (l),in view of the information that can be gained from this reaction on the mechanism of the carbonylation of organic nitro compounds having an unsaturated substituent in the ortho position. At 220 "C and 50 atm CO, Ru3(C0)12 is a catalyst in CH3CN for the conversion of 1 to 2 (Eq. 12).12412-Aminobiphenyl (3) is the main byproduct:
RU3(C0)12
+2co
I
(I)
H +
2
co2
(12)
The selectivity of the synthesis of 2 was rather poor (ca 32%). Because it is known that aromatic nitro compounds react with Ru3(C0)12 to give imido complexes in which the nitrogen atom is coordinated to all of the three ruthenium atoms of the cluster skeleton, the imido complex 4, a possible intermediate in the reaction, was synthesized (Eq. 13) and its crystal structure determined:[241
NO
+
2
co2 co +
(13)
The ortho-substituted phenyl ring in compound 4 is free to rotate around the carbon-carbon bond and thus can assume the proper arrangement enabling insertion of nitrogen into an aromatic C-H bond to give 2. In fact, by treating 4 with CO (50 atm) at 220 "C in CH3CN, carbazole (2) was obtained, albeit in low yield (Eq. 14). The amine 3 and Ru3(C0)12 were the other products:
708
2 Metal Clusters in Catalysis
The stability of the intermediate aryl nitrene triply bridging in the ruthenium cluster is probably responsible for the poor yield of the heterocyclic product. Force-field calculations have shown that when there is a spacer between the two phenyl rings, such as in (2-nitrophenyl) phenyl thioether and 2-nitrobenzophenone, the distances between the nitrene nitrogen and the ortho carbon, where cyclization should occur in the hypothetical imido clusters, are much longer than in compound 4.[251This explains why these substrates afforded the corresponding amines, with only trace amounts of the desired six-membered heterocycles being observable, when subjected to reaction conditions the same as those used for Eq. ( K!).[’~] 6,261 and others[”] provided evidence that halide anions proGeoffroy et al.11591 mote the breaking of one Ru-N bond in triply bridged imido clusters such as 4, affording a doubly-bridged nitrene species. This should increase the reactivity of the intermediate and should favor the intramolecular insertion, which occurs within the coordination sphere of the metal. In agreement with this assumption, the addition of catalytic amounts of dry sodium halides in the carbonylation of 2-nitrobiphenyl (1) strongly affects the rate of the reaction and the product distribution (Eq. 15):[28]
In the presence of halides, yields of heterocyclization products 2 and 5 were ca. 75-85%, with preference for 2 in the order C1- > Br- 2 F- >> I-, whereas the selectivity towards 5 followed the reverse order, 1- > Br- 2 F- > C1-. The amount of o-aminobiphenyl(3), probably derived by protonation of the intermediate imido complex, was strongly reduced. It could be that even in this reaction the X- anions promote the formation of a doubly-bridged nitrene (A), which then undergoes aromatic C-H insertion or, after carbonylation, affords compound 5 (Scheme 5). (See, however, the discussion on the effect of the chloride anion on the reactivity of R u ~ ( C O ) Ireported Z in the previous section.) Clarification of the mechanism of this and of the following intramolecular cycli-
2.4 Co-catalysts in the Reductive Carhonylation of Aromatic Nitro Derivatives
1-
R I
R
709
1-
Ru
Scheme 5
zation reactions requires a deep mechanistic study. Anyhow, if Scheme 5 is still to be considered valid, the different reactivities of species A and B depend on the nature of X-, cyclization from the former being favored for X- = C1- and cyclization from the latter being prevalent for X- = I-. It is possible that iodine makes the nitrene species more nucleophilic, thus favoring attack to carbon monoxide, which, on the other hand, is activated by the interaction with the cation (see below). It could also be, as for Ru3(C0)12, that chloride affords a doubly-bridged derivative, whereas iodine, under the catalytic conditions, affords a triply-bridged derivative,126]strongly modifying the electronic and steric situation in the cluster. However, preformed complexes [PPN][Ru3(p-X)(CO)lo]( X = C1, Br, I ) react rapidly with PhNO under ambient conditions to yield [PPN][Ru3(q1-X)(p3NPh)(CO)g]['51.The halide promoting ability is C1 E Br > I. Reaction of the latter species with CO releases PhNCO, and the starting materials are recovered. The order of the promoting ability of the halides is that previously noted for the catalytic carbonylation of nitrobenzene in the presence of Ru3(C0)12 to give ethyl phenylcarbamate,L2]but it is the reverse of that observed in the carbonylation of 1, where the formation of the carbonylated product 5 is favored by iodide (Scheme 5). Thus what is observed under mild conditions with PhNO need not be the path followed even in the catalytic reactions of substituted nitroarenes. An interesting aspect, never observed previously in the catalytic carbonylation of nitrobenzene, is the role of the cation in driving the reaction towards cyclization products.[28]Small cations promote heterocyclization reactions, ca. 85%. When the size of the cation was increased, the amount of carbazole decreased. With KCI and
710
2 Metal Clusters in Cutulysis
(2.2.2.)cryptand, no heterocycles were formed. When the carbonylation of 1 was performed in the presence of NaBF4 as co-catalyst, 100'Yn conversion was observed, with formation of 2 and 5 in 48% and 32% yield respectively. Here the role of the anion is minimized by the low coordinating ability of BF4-. It is known that alkali cations polarize the coordinated CO group, interacting with the oxygen lone pair. Thus the small and highly polarizing Lif drives the reactive intermediate towards B (Scheme 5 ) thus leading to high yields of compound 5. The effect of the other cations ( Naf and K+) is no less important, because they increase the amount of heterocyclization products and inhibit the formation of the amine. Several nitrogen-containing heterocycles can be obtained by carbonylation, at high temperature and pressure, of organic nitro compounds with, in the ortho position, an unsaturated group able to interact with the presumed intermediate nitrene species.['] When Ru3(C0)12 is used as a catalyst, an intermediate imido complex similar to 4 can be suggested on the basis of what has been discussed. However, repeated attempts to synthesize such a complex by reaction of Ru3(C0)12 with /3substituted o-nitrostyrenes (the precursors of indoles 6-Eq. 16)[291have so far been unsuccessful.
The participation of cluster species in these catalytic syntheses of heterocycles can find some support in a chemometric optimization of the Ru3(CO) I2-catalyzed The results obtained suggest cyclization of 2-nitrostilbene to 2-nitrophenylind0le.~~~~ the existence of two different mechanisms, one based on a Ru(C0)s-catalyzed process (favored at low concentrations of the catalyst) and another based on a Ru3(CO)12-catalyzed process (favored at high catalyst concentrations). This is related to the well known equilibrium between monomeric and cluster species under CO pressure (previous section) The Ru(CO)s-catalyzed process is first-order with respect to the substrate and electron transfer from the complex to the nitro compound is probably the rate-determining step. The Ru3(CO)l*-catalyzed process seems to be zero-order with respect to the substrate and possibly dissociation of CO is rate-determining. When Ru3(CO)12 is used alone in these catalytic cyclization reactions, the conditions are rather severe."] Milder conditions can be used when Ru3(C0)12 is activated by a rigid cc-diimine ligand such as Tol-BIAN. For example, the carbon-
7 11
2.4 Co-catalysts in the Reductive Carbonylation of' Aromatic Nitro Derivatives
ylation of 2'-nitrochalcones to give a mixture of 2-substituted 4-quinolones (7) and 2,3-dihydro-2-substituted4-quinolones (8) in ethanol-water can be conducted at 170 "C and 30 atm CO (Eq. 17):[311
R
(7)
I
H
R uj ( C 0 )12/Tol-BIAN b
CO. EtOH/H20, 170 "C
+
(17)
a5LR HI
(8)
We have already pointed out that under these conditions, the true catalytic species is Ru(C0)3(Tol-BIAN).['] In this complex, the metal atom should be much more basic than in Ru(C0)5 or R u ~ ( C O )thus ~ ~ ,facilitating electron transfer from the metal to the nitro group. Such a process is now emerging as a common feature of the interaction of organic nitro compounds with low oxidation-state transition metal complexes.['1 A similar situation was found in the carbonylation of 2-nitrochalc-4-ones catalyzed by Ru3(C0)12-Tol-BIAN, which gave a mixture of the corresponding acyl indoles (9) and quinolines (10) (Eq. 18):13"
In discussing the effect of the co-catalysts in the reductive carbonylation of
712
2 Metal Clusters in Catalysis
organic nitro compounds catalyzed by R u ~ ( C O ) Da, useful example is the synthesis of benzotriazoles (11) from the corresponding o-nitrophenylazo derivatives ( Eq. 19):[33]
+
2 co2
Even under drastic conditions, R U ~ ( C O )alone , ~ gives compound 11 only with difficulty. Both rate and selectivity are markedly enhanced by addition of a large amount of a base such as triethylamine; the molar ratio should be no lower than two relative to the azo derivative. The mechanism of this reaction is peculiar because we could show that the amine works as an oxygen scavenger[341and the metal complex apparently only catalyses the transfer of the N-bound oxygen atom to CO, with formation of CO2, so that the reaction becomes catalytic with regard to the amine (Scheme 6).
H?
Scheme 6
,R1
2.4 Co-cutulysts in the Reductive Curbonylution qf Aromatic Nitro Derivutives
713
The reaction can be repeated many times by adding fresh azo derivative to the reaction vessel. Thus the amine acts as a catalyst, which is not the case for other deoxygenating agents such as P(OEt)3, the corresponding oxide of which could not be reduced by CO.
2.4.4 Conclusions It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(C0)4]- species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. As far as participation of cluster species in the catalytic cycles is concerned, all the evidence gained from the synthesis of carbamates is in favor of their disaggregation to mononuclear species. The situation is less clear for the catalytic synthesis of nitrogen-containing heterocycles, mainly because no complete and clarifying mechanistic study has yet been performed. It could be that, in these cases, the concomitant presence of the nitro function and of an unsaturated group in the ortho position enables the preservation of the cluster structure. However, no model compounds are known, with the exception of the imido cluster Ru3(p3-NC6H4-oC6H5)2(C0)9(4), and greater efforts are required to elucidate this point.
References [ I ] S. Cenini, F. Ragaini Catalytic Reductive Curhonylution of Organic Nitro Compounds; Kluwer Academic Publishers. Dordrecht (The Netherlands). 1997. [2] (a) S. Cenini, M. Pizzotti, C. Crotti, F. Porta, G. La Monica J. Clzem. Sot.> Chern. Comrnun. 1984. 1286. (b) S. Cenini, C. Crotti, M. Pizzotti, F. Porta J. Org. Clzem. 1988, 53, 1243. [3] S. Cenini, M. Pizzotti, C. Crotti, F. Ragaini, F. Porta J. Mol. Cutal. 1985, ZY, 77. [4] M.I. Bruce, M.G. Humphrey, M.R. Snow, E.R.T. Tiekink. R.C. Wallis J. Orgunomet. Chern. 1986, 314, 31 1. [ 5 ] E. Alessio, G. Clauti, G. Mestroni J. Mol. Cutal. 1988, 4Y, 59. [6] M.J.A. Kraakman, K. Vrize, H. Kooijman, A.L. Spek Organometallics 1992, 11, 3760.
714
2 Metal Clusters in Catalysis
[7] S. Cenini, F. Ragaini, S. Tollari, D. Paone J. Am. Chem. Soc. 1996, 118, 11964. [8] S.J. Skoog, J.P. Campbell, W.L. Gladfelter Oryanometallics 1994, 13, 4137 and references therein. [9] J.H. Grate, D.R. Hamm, D.H. Valentine US Patent 4,603,216, 1986. [ 101 M. Pizzotti, F. Porta, S. Cenini, F. Demartin J. Organornet. Chem. 1988, 356, 105. [ I l l S. Cenini, F. Ragaini, M. Pizzotti, F. Porta, G. Mestroni, E. Alessio J. Mol. Catal. 1991, 64, 179. [I21 (a) F. Ragaini, S. Cenini, F. Demartin J. Chem. Soc., Chem. Commun. 1992, 1467. (b) F. Ragaini, S. Cenini, F. Demartin Oryunometallics 1994, 13, 1178. [I31 F. Ragaini, S. Cenini, A. Fumagalli, C. Crotti J. Organornet. Chem. 1992, 428, 401. [I41 G.D. Williams, R.R. Whittle, G.L. Geoffroy, A.L. Rheingold J. Am. Chem. Soc. 1987, 109, 3936. [15] S.-H. Han, J.-S. Song, P.D. Macklin, S.T. Nguyen, G.L. Geoffroy, A.L. Rheingold Organometallics 1989, 8, 2127. [16] D.L. Ramage, G.L. Geoffroy, A.L. Rheingold, B.S. Haggerty Organometallics 1992, 11, 1242. [I71 S. Bhaduri, H. Khwaja, N. Sapre, K. Sharma, A. Basu, P.G. Jones, G. Carpenter J. Chem. Soc., Dalton Trans. 1990, 1313 and references therein. [IS] J.H. Grate, D.R. Hamm, D.H. Valentine. US Patent 4,600,793, 1986, US Patent 4,629,804, 1986, US Patent 4,705,883, 1987. [19] C.-H. Liu, C.-H. Cheng J. Organornet. Chem. 1991, 420, 119. [20] F. Ragaini, S. Cenini Manuscript in preparation. [21] R.M. LaineJ. Mol. Catal. 1982, 14, 137. [22] Halides are known to labilize CO ligands in trinuclear ruthenium clusters. For a survey of these reactions see: G. Lavigne in The Chemistry of Metal Cluster Complexes, D.F. Shriver, H.D. Kaesz and R.D. Adams (eds), VCH, New York, 1990, p. 201. [23] For one example in organometallic chemistry see: D. Forster. Znorg. Chem. 1972, 11, 1686. [24] C. Crotti, S. Cenini, A. Bassoli, B. Rindone, F. Demartin J. Mol. Catal. 1991, 70, 175. [25] A. Bassoli, S. Cenini, F. Farina, M. Orlandi, B. Rindone J. Mol. Cutal. 1994, 89, 121. [26] S.-H. Han, G.L. Geoffroy, B.D. Dombek, A.L. Rheingold Znorg. Chem. 1988, 27, 4355. [27] S. Rivormanana, G. Lavigne, N. Lugan, J.-J. Bonnet Organometallics 1991, 10, 2285. [28] M. Pizzotti, S. Cenini, S. Quici, S. Tollari J. Chem. Soc., Perkin Trans. 2 1994, 913. [29] C. Crotti, S. Cenini, B. Rindone, S. Tollari, F. Demartin J. Chem. Soc., Chem. Commun. 1986, 784. [30] C. Crotti, S. Cenini, R. Todeschini, S. Tollari J. Chem. Soc., Furaday Trans. 1991, 87, 2811. [31] S. Tollari, S. Cenini, F. Ragaini, L. Cassar J. Chem. Soc., Chem. Commun. 1994, 1741. [32] S. Cenini, E. Bettettini, M. Fedele, S. Tollari J. Mol. Catal. A 1996, 111, 37. [33] M. Pizzotti, S. Cenini, R. Psaro, S. Costanzi J. Mol. Catul. 1990, 63, 299. [34] (a) M. Pizzotti, F. Ragaini, S. Cenini Gazz. Chim. Ztal. 1993, 123, 683. (b) M. Pizzotti, F. Ragaini, S. Cenini, S. Costanzi Ztal. Pat. Appl. MI 91A 000917, 1991.
FULL PAPER DOI: 10.1002/ejoc.200800036
Facile Entry to 4,5,6,7-Tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones from Amines and Amino Acids V. Sai Sudhir,[a] R. B. Nasir Baig,[a] and Srinivasan Chandrasekaran*[a][‡] Dedicated to Professor E. J. Corey on the occasion of his 80th birthday
Keywords: Triazoles / Alkynes / Cycloaddition / Nitrogen heterocycles / Amino acids A practical and high-yielding regioselective synthesis of several enantiopure 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones is described starting from primary amines and α-amino acid derivatives in a three-step reaction sequence by employing a constrained intramolecular “click”
reaction as the key step. The method obviates chromatographic purification of products.
Introduction
the emergence of combinatorial chemistry and high-speed parallel synthesis, the development of synthetic procedures that can be used to generate a library of compounds, especially the synthesis of target heterocycles in an enantiomerically pure form, is challenging. Ring-constrained intramolecular Huisgen cycloaddition has been used by many research groups to synthesize several bicyclic, as well as polycyclic, triazole-fused nitrogen-containing heterocycles, especially triazole-fused piperazine, pyrazine, and pyrazinone[7] derivatives and also in the synthesis of triazolefused oxygen-rich heterocycles.[8] Literature reports on 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones are scarce[7a–7e] and these scaffolds remain relatively under-explored. Rohr et al. studied the herbicidal activity of a few triazole-fused pyrazinones.[7d] Recently, Pokorski et al. synthesized a 1,5-triazole-substituted amino acid (Tzl) by hydrolysis of triazole-fused tetrahydropyrazinone derived from bromoacetamide of propargylamine.[7e] Their focus was to study its effects upon the secondary structure of short peptoids. The significant biological profiles of 1,2,3-triazoles coupled with our interest in the development of short and versatile routes to a variety of novel heterocyclic structures, as well as the paucity of the literature for the synthesis of 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones, stimulated us to develop a synthetic protocol that would generate a library of enantiopure, triazole-fused bicyclic compounds from easily available starting materials. Herein, we report a simple synthesis method involving a three-step protocol starting from primary amines, that is, sequential alkylation, acylation and one-pot substitution/cycloaddition (Scheme 1) for the preparation of triazole-fused tetrahydropyrazinones.
Huisgen 1,3-dipolar cycloaddition of an azide and an alkyne to afford the 1,2,3-triazole ring system has been widely used in organic synthesis.[1] Several compounds containing the 1,2,3-triazole structural motif possess a broad spectrum of biological properties like anti-HIV,[2a] antibacterial,[2b] antiallergic,[2c] herbicidal, fungicidal,[2d] and antihistamine activity.[3] With careful choice of appropriate building blocks, “click chemistry” can provide derivatives or mimics of traditional drugs, pharmacophores, and natural products.[4] Its ability lies in the generation of novel structures that might not necessarily resemble known pharmacophores. The CuI-catalyzed formation of 1,2,3-triazoles from azides and terminal acetylenes is a particularly powerful linking reaction because of its high degree of dependability and complete specificity, which generates regioselectively the 1,4-substituted ring system; this system is extensively used in an intermolecular[5] fashion for the synthesis of various conjugates[6a–6h] and in an intramolecular fashion for the synthesis of triazole-containing small peptides.[6i–6k] Ideally, an intramolecular reaction should provide a powerful method for the synthesis of structurally diversified analogs difficult to obtain by an intermolecular version, particularly to generate fused polycyclic triazole derivatives that should be of interest to medicinal chemists. Because of [a] Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India Fax: +91-80-2360-2423 E-mail:
[email protected] [‡] Honorary Professor, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India Supporting information for this article is available on the WWW under http://www.eurjoc.org/ or from the author. Eur. J. Org. Chem. 2008, 2423–2429
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2423
FULL PAPER
V. Sai Sudhir, R. B. Nasir Baig, S. Chandrasekaran
Scheme 1. Synthesis of bicyclic triazole 3a from benzylamine.
Results and Discussion
Table 1. Ring-constrained cycloaddition of chloroacetylated alkynes 2b–e derived from alkylamines.[a]
Our initial efforts were devoted to the synthesis of compound 3a from benzylamine. N-benzyl propargylamine (1a) was synthesized from benzylamine and propargyl bromide in DMF by employing LiOH·H2O as the base. This was then treated with chloroacetyl chloride in the presence of triethylamine to give chloroacetylated alkyne 2a in excellent yield (Scheme 1). The reaction of 2a with an excess amount of sodium azide (5 equiv.) in DMF at 100 °C for 1 h resulted in the formation of azide 2a⬘. Without isolating azide 2a⬘, the reaction mixture was further heated for 4 h, where the latter underwent constrained intramolecular cycloaddition with the alkyne moiety to furnish regioselectively the desired bicyclic compound 3a in 97 % yield as the only product. This result encouraged us to synthesize other chloroacetylated alkynes derived from primary amines by employing the same set of reaction conditions as those described for the synthesis of 2a. Accordingly, compounds 2b– e were synthesized from 1-phenylethylamine, 2-phenylethylamine, (S)-α-naphthylethylamine, and 2-furfurylamine, respectively. These compounds existed as rotameric mixtures,[9] as exemplified by their 1H and 13C NMR spectra. Compounds 2b–e were then treated with sodium azide under identical conditions as those described earlier, and the results are summarized in Table 1. The reactions proceeded smoothly (3–6 h) to afford 1,2,3-triazole-fused 4,5,6,7-tetrahydropyrazin-6-ones 3b–e as the only product in excellent yields.
[a] Conditions: NaN3 (5 equiv.), DMF, 100 °C.
Characteristic resonances were observed around δ = 127 and 129 ppm in the 13C NMR spectra for compounds 3a– e, which correspond to the olefinic carbon atoms of the tri-
Scheme 2. General scheme for the synthesis of enantiopure fused triazoles from -amino acids. 2424
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Eur. J. Org. Chem. 2008, 2423–2429
Facile Entry to 4,5,6,7-Tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones
azole ring. We felt that this methodology could then be extended to synthesize enantiopure 4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones from chloroacetylated alkynes derived from α-amino acids. In this context, we synthesized chloroacetylated alkynes 5a–g from the naturally abundant -amino acids alanine, phenylalanine, valine, methionine, leucine, phenylglycine and tryptophan, respectively, according to the general scheme (Scheme 2). LiOH·H2O mediated N-alkylation of amino acid esters with propargyl bromide in DMF was performed by using a literature procedure[10] to furnish N-propargylated amino acids 4a–g in good yield (70–80 %). These N-propargyl amino acid esters were then treated with chloroacetyl chloride (1.2 equiv.) and triethylamine (2 equiv.) at 0 °C in CH2Cl2 to afford compounds 5a–g in excellent yield (80–90 %). All chloroacetylated alkynes 5a–g existed as rotTable 2. Intramolecular cycloaddition of chloroacetylated alkynes 5a–g derived from amino acids.[a]
americ mixtures, and the rotamer ratio varied with each substrate. Enantiopure chloroacetylated alkynes 5a–g were then heated in the presence with an excess amount of sodium azide (5 equiv.) in DMF at 100 °C. At this temperature, the azides formed in situ underwent constrained intramolecular cycloaddition in a facile manner (3–6 h) to furnish optically pure triazole-fused bicyclic compounds 6a–g as the sole products in excellent yield (92–97 %) (Table 2). The methodology was then extended to suitably protected -lysine-derived chloroacetylated alkynes 7a and 7b synthesized from H-Lys(Z)-OMe·HCl and Z-Lys-OEt, respectively. They underwent facile cycloaddition to furnish 8a and 8b, respectively, in excellent yield (Table 3). These two compounds can be viewed as novel, protected amino acids incorporating a heterocyclic backbone. In general, cycloaddition of chloroacetylated alkynes containing a bulkier side chain (2d and 5g) was faster than the less bulky substrates. Table 3. Novel protected amino acids derived from -lysine.[a]
[a] Conditions: NaN3 (5 equiv.), DMF, 100 °C.
[a] Conditions: NaN3 (5 equiv.), DMF, 100 °C. Eur. J. Org. Chem. 2008, 2423–2429
This may be attributed to the spatial proximity of the azide and alkyne substituents enforced by the bulky side chain. After successfully synthesizing amino acid derived fused triazoles 6a–g, 8a, and 8b, we decided to extend the scope of this methodology by varying the alkyl, as well as the acyl components, which may furnish other triazolefused heterocycles. Accordingly, compound 1a was treated with chloromethane sulfonyl chloride in the presence of triethylamine to furnish chlorosulfonamide derivative 7c in moderate yield (60 %). Compound 7c was then treated with an excess amount of sodium azide in DMF at 100 °C. Because the cycloaddition was sluggish under these conditions, the temperature was increased to 120 °C and the cycloaddition took place to furnish 8c in moderate yield (35 %) in 9 h. It is worth noting that 8c has a substructure hitherto unknown in the literature. Similarly, alkylating tryptophan methyl ester with 1-bromo-2-butyne followed by chloroacetylation in CH2Cl2 furnished compound 7d in good yield (70 % over 2 steps). When 7d was treated with NaN3 (DMF, 100 °C, 9 h) compound 8d could be isolated in moderate yield (50 %) (Table 4).
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V. Sai Sudhir, R. B. Nasir Baig, S. Chandrasekaran
Table 4. Cycloaddition of a few other substrates.[a]
[a] Conditions: NaN3 (5 equiv.), DMF, 100 °C.
Conclusions We disclosed an access to achiral, as well as enantiopure, 1,2,3-triazole-fused 4,5,6,7-tetrahydropyrazin-6-ones in excellent yields from several primary amines and amino acids in a three-step protocol (alkylation, acylation, one-pot substitution/cycloaddition) involving constrained intramolecular cycloaddition as the key step. The methodology in general obviates chromatographic purification of products.
Experimental Section Physical Properties and Spectral Measurements: All glassware was oven dried before starting a reaction. Propargyl bromide was purchased from Sigma–Aldrich and used as such. Solvents like DMF, CH2Cl2, and MeOH were purified as mentioned in “Purification of Laboratory Chemicals” by Perrin & Armarego, Pergamon Press, Third Edition, 1988. Analytical TLC was performed on commercial plates coated with silica gel GF254 (0.25 mm). Silica gel (230– 400 mesh) was used for column chromatography. Melting points are uncorrected. 1H and 13C NMR spectra were recorded with 300 MHz (300 MHz, 1H and 75 MHz, 13C) or 400 MHz (400 MHz, 1 H and 100 MHz, 13C) spectrometers. Chemical shifts are reported in parts per million downfield from the internal reference tetramethylsilane for 1H and CDCl3 for 13C NMR. The following abbreviations explain the multiplicity: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br. s = broad singlet, and dd = doublet of doublets. IR spectra were recorded with an FTIR spectrometer. High-resolution mass spectra (HRMS) were recorded with a Micromass Q-TOF mass spectrometer. All propargylamines were synthesized according to a literature procedure.[10] General Procedure for the Synthesis of Chloroacetylated Alkynes: To a solution of propargylamine (1 mmol) in CH2Cl2 (10 mL) at 0 °C was added triethylamine (2 mmol) followed by chloroacetyl chloride (1.2 mmol) under an atmosphere of nitrogen. After 0.5 h, the reaction mixture was washed with water (2 ⫻ 10 mL) followed by brine (10 mL) and dried with anhydrous Na2SO4. The filtrate was concentrated, and the crude product was purified by flash chromatography on silica gel (230–400 mesh) by using ethyl acetate and petroleum ether to obtain the corresponding chloroacetylated alkynes. 2426
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N-Benzyl-N-(2-propynyl)-2-chloroacetamide (2a): Rf = 0.5 (hexane/ EtOAc, 7:3). Yield: 229 mg (94 %). Pale-yellow oil. IR (neat): ν˜ = 3292, 2120, 1661, 1448, 1212, 956, 795, 699 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.38–7.24 (m, 5 H), 4.73, 4.71 (2 s, 2 H), 4.25–4.0 (m, 4 H), 2.37, 2.26 (2 t, J = 2.4 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3, major rotamer): δ = 166.3, 135.7, 128.8, 128, 126.7, 77.4, 73.3, 50.3, 41.1, 36.3 ppm. HRMS: calcd. for C12H12ClNO [M + Na]+ 244.0505; found 244.0501. N-(1-Phenylethyl)-N-(2-propynyl)-2-chloroacetamide (2b): Rf = 0.40 (hexane/EtOAc, 7:3). Yield: 232 mg (90 %). Pale-yellow oil. IR (neat): ν˜ = 3293, 2980, 2119, 1658, 1448, 1413, 1173, 699 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.36–7.27 (m, 5 H), 6.0, 5.24 (2 br. s, 1 H), 4.36–4.21 (m, 2 H), 3.93 (d, J = 18.9 Hz, 1 H), 3.69–3.49 (m, 1 H), 2.28–2.15 (m, 1 H), 1.89–1.42 (m, 3 H) ppm. 13C NMR (100 MHz, CDCl3, major rotamer): δ = 166.7, 139.2, 128.6, 127.8, 127.3, 79.4, 72.8, 52.1, 41.7, 32.4, 16.1 ppm. HRMS: calcd. for C13H14ClNO [M + Na]+ 258.0662; found 258.0662. N-Phenylethyl-N-(2-propynyl)-2-chloroacetamide (2c): Rf = 0.40 (hexanes/EtOAc, 7:3). Yield: 219 mg (85 %). Pale-yellow oil. IR (neat): ν˜ = 3292, 2944, 2120, 1659, 1454, 1426, 1137, 701 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.36–7.12 (m, 5 H), 4.26, 3.93 (2 d, J = 2.1 Hz, 2 H), 4.14 (s, 1 H), 3.73–3.65 (m, 3 H), 2.98–2.89 (m, 2 H), 2.35–2.30 (m, 1 H) ppm. 13C NMR (100 MHz, CDCl3, major rotamer): δ = 166.2, 137.7, 128.9, 128.7, 126.9, 78.2, 72.6, 49.3, 40.5, 34.7, 33.6 ppm. HRMS: calcd. for C13H14ClNO [M + Na]+258.0662; found 258.0659. N-[(1S)-1-(1-Naphthyl)ethyl]-N-(2-propynyl)-2-chloroacetamide (2d): Rf = 0.40 (hexanes/EtOAc, 6:4). Yield: 265 mg (86 %). Pale-yellow solid. M.p. 101 °C. IR (neat): ν˜ = 3292, 2979, 2119, 1654, 1446, 1414, 1177, 805, 782 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.90–7.83 (m, 3 H), 7.62–7.46 (m, 4 H), 6.58 (q, J = 6.3 Hz, 1 H), 4.29 (d, J = 12.6 Hz, 1 H), 4.23 (d, J = 12.9 Hz, 1 H), 3.71 (dd, J1 = 19.5 Hz, J2 = 2.1 Hz, 1 H), 3.56 (dd, J1 = 19.2 Hz, J2 = 2.1 Hz, 1 H), 2.11 (t, J = 2.4 Hz, 1 H), 1.75 (d, J = 6.9 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3, major rotamer): δ = 166.2, 134.5, 133.8, 131.9, 129.3, 128.6, 126.9, 126.1, 125.2, 124.8, 123.6, 79.4, 72.5, 49.8, 41.9, 31.9, 16.6 ppm. [α]23 D = –88.3 (c = 1.0, CHCl3). HRMS: calcd. for C17H16ClNO [M + Na]+ 308.0818; found 308.0832. N-(2-Furylmethyl)-N-(2-propynyl)-2-chloroacetamide (2e): Rf = 0.40 (hexanes/EtOAc, 7:3). Yield: 196 mg (84 %). Pale-yellow oil. IR (neat): ν˜ = 3292, 2121, 1662, 1451, 1353, 1198, 1012, 748 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.4 (s, 1 H), 6.33 (s, 2 H), 4.67 (s, 2 H), 4.28–4.12 (m, 4 H), 2.34, 2.27 (2 br. s, 1 H) ppm. 13C NMR (75 MHz, CDCl3, major rotamer): δ = 166.0, 148.6, 143.0, 110.4, 109.2, 77.8, 72.6, 43.4, 41.1, 34.5 ppm. HRMS: calcd. for C10H10ClNO2 [M + Na]+ 234.0298; found 234.0294. Methyl (2S)-2-[(2-Chloroacetyl)(2-propynyl)amino]propanoate (5a): Rf = 0.30 (hexanes/EtOAc, 7:3). Yield: 204 mg (85 %). Pale-yellow oil. IR (neat): ν˜ = 3287, 2953, 2120, 1742, 1666, 1454, 1222, 1197, 801, 676 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 5.04 (q, J = 7.2 Hz, 1 H), 4.34–4.08 (m, 4 H), 3.76, 3.72 (2 s, 3 H), 2.43 (t, J = 2.7 Hz, 1 H), 1.65, 1.54 (2 d, J = 7.5 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3, major rotamer): δ = 171.4, 166.4, 78.3, 73.4, 53.4, 52.2, 41.3, 35.2, 14.6 ppm. [α]23 D = –57.1 (c = 1.0, CHCl3). HRMS: calcd. for C9H12ClNO3 [M + Na]+ 240.0403; found 240.0406. Methyl (2S)-2-[(2-Chloroacetyl)(2-propynyl)amino]-3phenylpropanoate: (5b): Rf = 0.40 (hexanes/EtOAc, 7:3). Yield: 272 mg (86 %). Pale-yellow oil. IR (neat): ν˜ = 3286, 2952, 2122, 1742, 1666, 1450,
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Facile Entry to 4,5,6,7-Tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones 1222, 700 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.29–7.21 (m, 5 H), 4.90 (dd, J1 = 6.0 Hz, J2 = 9.6 Hz, 1 H), 4.23–4.05 (m, 3 H), 3.85–3.80 (m, 1 H), 3.74–3.72 (m, 3 H), 3.4 (dd, J1 = 5.7 Hz, J2 = 14.4 Hz, 1 H), 3.19 (dd, J1 = 9.6 Hz, J2 = 14.4 Hz, 1 H), 2.3 (t, J = 2.1 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3, major rotamer): δ = 170.3, 166.5, 136.7, 129.0, 128.5, 126.8, 77.4, 73.7, 60.1, 52.3, 41.1, 36.9, 34.7 ppm. [α]23 D = –111.3 (c = 1.0, CHCl3) HRMS: calcd. for C15H16ClNO3 [M + Na]+ 316.0716; found 316.0708. Methyl (2S)-2-[(2-Chloroacetyl)(2-propynyl)amino]-3-methylbutanoate: (5c): Rf = 0.70 (hexanes/EtOAc, 7:3). Yield: 241 mg (90 %). Pale-yellow oil. IR (neat): ν˜ = 3288, 2967, 2122, 1740, 1670, 1445, 1212, 1170, 1008, 668 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 4.91 (d, J = 10.2 Hz, 1 H), 4.44–3.98 (m, 4 H), 3.75, 3.72 (2 s, 3 H), 2.49–2.19 (m, 2 H), 1.07–0.95 (m, 6 H) ppm. 13 C NMR (100 MHz, CDCl3, major rotamer): δ = 170.9, 167.4, 78.5, 73.4, 61.6, 51.9, 41.4, 33.9, 28.0, 19.4, 18.9 ppm. [α]22 D = –141.4 (c = 1.0, CHCl3). HRMS: calcd. for C11H16ClNO3 [M + Na]+ 268.0716; found 268.0713. Methyl (2S)-2-[(2-Chloroacetyl)(2-propynyl)amino]-4-(methylsulfanyl)butanoate (5d): Rf = 0.40 (hexanes/EtOAc, 7:3). Yield: 246 mg (82 %). Pale-yellow oil. IR (neat): ν˜ = 3276, 2953, 2120, 1739, 1666, 1436, 1220, 1198, 798 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 5.08, 4.79 (2 dd, J1 = 9.3 Hz, J2 = 4.8 Hz, 1 H), 4.40–4.07 (m, 4 H), 3.79–3.73 (m, 3 H), 2.76–2.12 (m, 8 H) ppm. 13 C NMR (100 MHz, CDCl3, major rotamer): δ = 170.7, 167.1, 78.1, 73.9, 57.4, 52.4, 41.3, 36.3, 30.7, 28.5, 15.3 ppm. [α]23 D = 63.4 (c = 1.0, CHCl3). HRMS: calcd. for C11H16ClNO3S [M + Na]+ 300.0437; found 300.0431. Methyl (2S)-2-[(2-Chloroacetyl)(2-propynyl)amino]-4-methylpentanoate: (5e): Rf = 0.40 (hexanes/EtOAc, 7:3). Yield: 234 mg (83 %). Pale-yellow oil. IR (neat): ν˜ = 3372, 2958, 2872, 2120, 1741, 1668, 1446, 1178, 672 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 5.3 (dd, J1 = 9.9 Hz, J2 = 4.8 Hz, 1 H), 4.38–4.07 (m, 4 H), 3.71 (s, 3 H), 2.40 (t, J = 2.7 Hz, 1 H), 1.86–1.65 (m, 3 H), 1.01– 0.93 (m, 6 H) ppm. 13C NMR (75 MHz, CDCl3, major rotamer): δ = 171.8, 167.2, 78.5, 73.4, 55.3, 52.2, 41.4, 37.8, 34.4, 24.8, 22.8, 21.5 ppm. [α]23 D = –40.6 (c = 2.0, CHCl3). HRMS: calcd. for C12H18ClNO3 [M + Na]+ 282.0873; found 282.0865. Methyl (2S)-2-[(2-Chloroacetyl)(2-propynyl)amino]-2-phenylethanoate: (5f): Rf = 0.40 (hexanes/EtOAc, 7:3). Yield: 241 mg (80 %). Pale-yellow oil. IR (neat): ν˜ = 3289, 2954, 2122, 1747, 1668, 1446, 1173, 1003, 698 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.40–7.30 (m, 5 H), 6.28 (s, 1 H), 4.36 (s, 2 H), 4.14– 3.97 (m, 2 H), 3.78 (s, 3 H), 2.15 (s, 1 H) ppm. 13C NMR (100 MHz, CDCl3, major rotamer): δ = 170.3, 167.5, 132.9, 129.3, 129.1 129.0, 78.3, 72.6, 60.9, 52.5, 41.6, 34.7 ppm. [α]23 D = 140.3 (c = 1.0, CHCl3). HRMS: calcd. for C15H18ClNO3 [M + Na]+ 302.0560; found 302.0561. Methyl (2S)-2-[(2-Chloroacetyl)(2-propynyl)amino]-3-(1H-3-indolyl)propanoate (5g): Rf = 0.30 (hexanes/EtOAc, 6:4). Yield: 291 mg (82 %). Light-brown oil. IR (neat): ν˜ = 3292, 2952, 2122, 1738, 1731, 1660, 1456, 1342, 1246, 1045, 746 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 8.30, 8.19 (2 br. s, 1 H), 7.62 (d, J = 7.2 Hz, 1 H), 7.40–7.45 (m, 4 H), 5.06, 4.72 (2 dd, J1 = 9.6 Hz, J2 = 6 Hz, 1 H), 4.34–3.35 (m, 9 H), 2.20 (t, J = 2.4 Hz, 1 H) ppm. 13 C NMR (100 MHz, CDCl3, major rotamer): δ = 170.7, 166.7, 136.2, 127.3, 122.9, 122.1, 119.6, 118.4, 111.2, 110.8, 77.3, 73.5, 59.1, 52.4, 41.4, 36.8, 24.7 ppm. [α]23 D = –162 (c = 1.0, CHCl3). HRMS: calcd. for C17H17ClN2O3 [M + Na]+ 355.0825; found 355.0818. Eur. J. Org. Chem. 2008, 2423–2429
Methyl (2S)-6-[(Benzyloxy)carbonyl]amino-2-[(2-chloroacetyl)(2propynyl)amino]hexanoate (7a): Rf = 0.40 (hexanes/EtOAc, 6:4). Yield: 345 mg (80 %). Colorless oil. IR (neat): ν˜ = 3292, 2951, 2120, 1714, 1667, 1531, 1454, 1248, 1018, 698 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.36–7.32 (m, 5 H), 5.14–5.08 (m, 3 H), 4.87 (br. s, 1 H), 4.41–4.10 (m, 4 H), 3.74–3.68 (m, 3 H), 3.21–3.15 (m, 2 H), 2.38 (t, J = 2.4 Hz, 1 H), 2.09–1.77 (m, 2 H), 1.58–1.34 (m, 4 H) ppm. 13C NMR (75 MHz, CDCl3, major rotamer): δ = 171.1, 167.4, 156.3, 136.6, 128.5, 128.4, 128.0, 78.3, 73.7, 66.5, 57.0, 52.3, 41.3, 40.6, 34.8, 29.3, 28.5, 23.2 ppm. [α]22 D = –27.3 (c = 1.0, CHCl3). HRMS: calcd. for C20H25ClN2O5 [M + Na]+ 431.1350; found 431.1352. Ethyl (2S)-2-[(Benzyloxy)carbonyl]amino-6-[(2-chloroacetyl)(2-propynyl)amino]hexanoate (7b): Rf = 0.30 (hexanes/EtOAc, 6:4). Yield: 356 mg (80 %). Colorless oil. IR (neat): ν˜ = 3297, 2943, 2120, 1719, 1659, 1533, 1454, 1251, 1026, 698 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 7.36 (br. s, 5 H), 5.38 (d, J = 7.8 Hz, 1 H), 5.11 (s, 2 H), 4.38–4.08 (m, 7 H), 3.42 (br. s, 2 H), 2.34, 2.23 (2 br. s, 1 H), 1.86–1.58 (m, 4 H), 1.41–1.34 (m, 2 H), 1.27 (t, J = 6.9 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3, major rotamer): δ = 172.1, 166.4, 155.8, 136.3, 128.5, 128.1 (2 C), 78.0, 66.9, 61.4, 53.7, 47.4, 41.2, 37.6, 34.7, 32.5, 27.9, 22.2, 14.1 ppm. [α]22 D = +11.3 (c = 2.0, CHCl3). HRMS: calcd. for C21H27ClN2O5 [M + Na]+ 445.1506; found 445.1505. N-Benzyl-N-(2-propynyl)chloromethanesulfonamide (7c): Rf = 0.50 (hexanes/EtOAc, 6:4). Yield: 168 mg (60 %). Colorless oil. IR (neat): ν˜ = 3285, 3017, 2121, 1495, 1356, 1244, 1161, 1064, 899 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.38–7.34 (m, 5 H), 4.67 (s, 2 H), 4.66 (s, 2 H), 3.94 (d, J = 2.7 Hz, 2 H), 2.45 (t, J = 2.7 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 134.7, 128.8, 128.5, 128.3, 76.6, 74.4, 55.7, 51.6, 35.7 ppm. HRMS: calcd. for C11H12ClNO2S [M + Na]+ 280.0175; found 280.0164. Methyl (2S)-2-[2-Butynyl(2-chloroacetyl)amino]-3-(1H-3-indolyl)propanoate (7d): Rf = 0.40 (hexanes/EtOAc, 6:4). Yield: 295 mg (80 %). Light-brown oil. IR (neat): ν˜ = 3404, 2231, 1735, 1652, 1457, 1433, 1346, 1215, 1170, 744 cm–1. 1H NMR (300 MHz, CDCl3, mixture of rotamers): δ = 8.19 (s, 1 H), 7.63 (d, J = 7.5 Hz, 1 H), 7.35 (d, J = 8.1 Hz, 1 H), 7.20–7.05 (m, 3 H), 5.14 (dd, J1 = 9.3 Hz, J2 = 5.7 Hz, 1 H), 4.24–3.59 (m, 7 H), 3.52 (dd, J1 = 15.3 Hz, J2 = 6 Hz, 1 H), 3.38 (dd, J1 = 15.3 Hz, J2 = 9.6 Hz, 1 H), 1.80, 1.72 (2 br. s, 1 H), 1.57 (t, J = 2.4 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3, major rotamer): δ = 170.9, 166.8, 136.2, 127.4, 122.8, 122, 119.5, 118.5, 111.1 (2 C), 81.6, 73.0, 58.6, 52.2, 41.6, 37.0, 24.7, 3.23 ppm. [α]23 D = –56.7 (c = 1.0, CHCl3). HRMS: calcd. for C18H19ClN2O3 [M + Na]+ 369.0982; found 369.0992. General Procedure for the Synthesis of Triazole-Fused Tetrahydropyrazin-6-ones: To a solution of chloroacetylated alkyne (1 mmol) in DMF (5 mL) was added NaN3 (5 mmol). The mixture was heated to 100 °C for appropriate time. After the reaction was complete (monitored by TLC) DMF was evaporated under vacuum, and the compound was extracted from the reaction mixture with CH2Cl2 (25 mL). The combined organic layer was then dried with anhydrous Na2SO4, filtered, and concentrated to obtain the corresponding triazole-fused tetrahydropyrazin-6-ones. 5-Benzyl-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-one (3a): Rf = 0.30 (hexanes/EtOAc, 4:6). Yield: 222 mg (97 %). White solid. M.p. 147 °C. IR (neat): ν˜ = 2936, 1659, 1489, 1349, 1255, 991, 836 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.51 (s, 1 H), 7.37– 7.28 (m, 5 H), 5.11 (s, 2 H), 4.77 (s, 2 H), 4.57 (s, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 161.7, 134.8, 129.1, 129, 128.4, 128.3, 127.2, 50.2, 48.6, 41.4 ppm. HRMS: calcd. for C12H12N4O [M + H]+ 229.1089; found 229.1085.
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FULL PAPER
V. Sai Sudhir, R. B. Nasir Baig, S. Chandrasekaran
5-(1-Phenylethyl)-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6one: (3b): Rf = 0.30 (hexanes/EtOAc, 4:6). Yield: 254 mg (96 %). Colorless viscous oil. IR (neat): ν˜ = 2980, 1656, 1472, 1346, 990, 701 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.48 (s, 1 H), 7.37– 7.31 (m, 5 H), 6.21 (q, J = 6.9 Hz, 1 H), 5.12 (s, 1 H), 5.11 (s, 1 H), 4.51 (d, J = 16.2 Hz, 1 H), 4.14 (d, J = 16.2 Hz, 1 H), 1.64 (d, J = 7.2 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 161.6, 137.9, 129.1, 128.7, 128.1, 127.5, 127.3, 50.8, 48.7, 36.3, 14.9 ppm. HRMS: calcd. for C 1 3 H 1 4 N 4 O [M + Na] + 265.1065; found 265.1066. 5-Phenethyl-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-one (3c): Rf = 0.40 (hexanes/EtOAc, 4:6). Yield: 252 mg (95 %). White solid. M.p. 158 °C. IR (neat): ν˜ = 2925, 1652, 1489, 1347, 1169, 991, 836, 750, 701 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.52 (s, 1 H), 7.33–7.20 (m, 5 H), 5.08 (s, 2 H), 4.43 (s, 2 H), 3.8 (t, J = 7.2 Hz, 2 H), 2.98 (t, J = 7.5 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 161.7, 137.9, 129.0, 128.9, 128.7, 127.2, 126.9, 49.4, 48.7, 43.1, 33.4 ppm. HRMS: calcd. for C13H14N4O [M + Na]+ 265.1065; found 265.1059. 5-[(1S)-1-(1-Naphthyl)ethyl]-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-one (3d): Rf = 0.30 (hexanes/EtOAc, 5:5). Yield: 311 mg (99 %). White solid. M.p. 94 °C. IR (neat): ν˜ = 3052, 2980, 1651, 144, 1346, 784 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.96–7.85 (m, 3 H), 7.66–7.46 (m, 4 H), 7.37 (s, 1 H), 6.79 (q, J = 6.9 Hz, 1 H), 5.19 (d, J = 17.7 Hz, 1 H), 5.09 (d, J = 18 Hz, 1 H), 4.39 (d, J = 16.2 Hz, 1 H), 3.67 (d, J = 16.5 Hz, 1 H), 1.75 (d, J = 6.9 Hz, 3 H) ppm. 13 C NMR (100 MHz, CDCl 3 ): δ = 161.1, 133.7, 133, 131.4, 129.4, 129.1, 128.8, 127.4, 127.1, 126.1, 125.2, 124.7, 122.7, 48.5, 48.2, 36.6, 15.3 ppm. [α]22 D = –121.8 (c = 2.0, CHCl3). HRMS: calcd. for C17H16N4O [M + Na]+ 315.1220; found 315.1230. 5-(2-Furylmethyl)-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6one (3e): Rf = 0.40 (hexanes/EtOAc, 5:5). Yield: 229 mg (95 %). Colorless oil. IR (neat): ν˜ = 3511, 2986, 1665, 1485, 1424, 1347, 1011, 751 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.91 (s, 1 H), 7.59 (s, 1 H), 6.40 (d, J = 3.3 Hz, 1 H), 6.36 (t, J = 2.1 Hz, 1 H), 5.1 (s, 2 H), 4.77 (s, 2 H), 4.72 (s, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 161.4, 148.4, 142.8, 128.9, 127.2, 110.4, 109.8, 48.3, 42.5, 41.7 ppm. HRMS: calcd. for C 1 0 H 1 0 N 4 O 2 [M + Na] + 241.0701; found 241.0706. Methyl (2S)-2-(6-Oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)propanoate (6a): Rf = 0.30 (hexanes/EtOAc, 4:6). Yield: 216 mg (96 %). Colorless gummy liquid. IR (neat): ν˜ = 2953, 1742, 1692, 1666, 1556, 1350, 1201, 991, 735 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.65 (s, 1 H), 5.31 (q, J = 7.5 Hz, 1 H), 5.15 (s, 2 H), 4.79 (d, J = 15.3 Hz, 1 H), 4.71 (d, J = 15.6 Hz, 1 H), 3.76 (s, 3 H), 1.57 (d, J = 7.2 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 170.9, 162.5, 129.3, 127.4, 52.6, 52.0, 48.7, 38.6, 14.1 ppm. [α]22 D = –4 (c = 1.0, CHCl3). HRMS: calcd. for C9H12N4O3 [M + Na]+ 225.0988; found 225.0981. Methyl (2S)-2-(6-Oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)-3-phenylpropanoate (6b): Rf = 0.30 (hexanes/EtOAc, 4:6). Yield: 286 mg (95 %). Colorless gummy liquid. IR (neat): ν˜ = 2953, 1741, 1666, 1474, 1435, 1200, 703 cm –1 . 1 H NMR (300 MHz, CDCl 3 ): δ = 7.51 (s, 1 H), 7.27–7.14 (m, 5 H), 5.28 (dd, J 1 = 10.8 Hz, J2 = 5.7 Hz, 1 H), 5.05 (d, J = 18 Hz, 1 H), 4.91 (d, J = 18 Hz, 1 H), 4.59 (d, J = 16 Hz, 1 H), 4.49 (d, J = 16 Hz, 1 H), 3.76 (s, 3 H), 3.49 (dd, J1 = 14.7 Hz, J2 = 5.4 Hz, 1 H), 3.2 (dd, J1 = 14.7 Hz, J2 = 11.1 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 169.8, 162.5, 135.6, 128.9, 128.7, 128.5, 128.3, 127.2, 58.3, 52.6, 48.5, 40.3, 34.0 ppm. [α]23 D = –5.8 (c = 1.0, CHCl3). HRMS: calcd. for C15H16N4O3 [M + H]+ 301.1300; found 301.1302. 2428
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Methyl (2S)-3-Methyl-2-(6-oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5a]pyrazin-5-yl)butanoate (6c): R f = 0.50 (hexanes/EtOAc, 4:6). Yield: 240 mg (95 %). Colorless gummy liquid. IR (neat): ν˜ = 2966, 1740, 1691, 1555, 1210, 992, 767 cm –1 . 1 H NMR (300 MHz, CDCl3): δ = 7.65 (s, 1 H), 5.16 (s, 2 H), 5.03 (d, J = 10.8 Hz, 1 H), 4.97 (d, J = 16.2 Hz, 1 H), 4.66 (d, J = 16.2 Hz, 1 H), 3.75 (s, 3 H), 2.39–2.27 (m, 1 H), 1.07 (d, J = 6.6 Hz, 3 H), 0.91 (d, J = 6.3 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 170.6, 162.9, 129.3, 127.7, 61.2, 52.3, 48.9, 38.3, 27.2, 19.5, 18.9 ppm. [α]22 D = –17.8 (c = 1.0, CHCl3). HRMS: calcd. for C11H16N4O3 [M + H]+ 253.1300; found 253.1301. Methyl (2S)-4-(Methylsulfanyl)-2-(6-oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)butanoate (6d): Rf = 0.30 (hexanes/ EtOAc, 1:1). Yield: 288 mg (94 %). Colorless gummy liquid. IR (neat): ν˜ = 3474, 2921, 1740, 1665, 1475, 1430, 1199, 991 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.65 (s, 1 H), 5.18–5.14 (m, 3 H), 4.84 (d, J = 15.6 Hz, 1 H), 4.72 (d, J = 15.9 Hz, 1 H), 3.77 (s, 3 H), 2.60–2.40 (m, 3 H), 2.27–2.10 (m, 4 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 170.0, 162.9, 129.2, 127.4, 56.5, 52.6, 48.7, 40.2, 30.5, 27.3, 15.3 ppm. [α]23 D = –2 (c = 1.0, CHCl3). HRMS: calcd. for C11H16N4O3S [M + Na]+ 307.0841; found 307.0836. Methyl (2S)-4-Methyl-2-(6-oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5a]pyrazin-5-yl)pentanoate (6e): R f = 0.40 (hexanes/EtOAc, 4:6). Yield: 253 mg (95 %). Colorless gummy liquid. IR (neat): ν˜ = 3475, 2957, 2875, 1740, 1667, 1471, 1349, 1185, 990 cm –1 . 1 H NMR (300 MHz, CDCl3): δ = 7.64 (s, 1 H), 5.42 (dd, J1 = 10.2 Hz, J2 = 5.7 Hz, 1 H), 5.15 (s, 2 H), 4.81 (d, J = 15.6 Hz, 1 H), 4.6 (d, J = 15.6 Hz, 1 H), 3.75 (s, 3 H), 1.94–1.77 (m, 2 H), 1.51–1.40 (m, 1 H), 0.96 (t, J = 6.9 Hz, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 171.1, 163, 129.3, 127.5, 54.1, 52.5, 48.8, 38.3, 36.6, 24.9, 23, 21.2 ppm. [α]22 D = –9.2 (c = 1.0, CHCl3). HRMS: calcd. for C12H18N4O3 [M + H]+ 267.1457; found 267.1456. Methyl (2S)-2-(6-Oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)-2-phenylethanoate (6f): Rf = 0.30 (hexanes/EtOAc, 5:5). Yield: 284 mg (92 %). Pale-yellow gummy liquid. IR (neat): ν˜ = 2926, 1740, 1647, 1441, 1347, 1201, 986, 698 cm –1 . 1 H NMR (300 MHz, CDCl3): δ = 7.45 (s, 1 H), 7.46–7.44 (m, 3 H), 7.30– 7.26 (m, 2 H), 6.53 (s, 1 H), 5.18 (s, 2 H), 4.84 (d, J = 15 Hz, 1 H), 4.12 (d, J = 15 Hz, 1 H), 3.82 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 169.9, 162.9, 131.8, 129.4, 129.3 (2 C), 129.2, 127.6, 59.9, 52.7, 48.7, 38.9 ppm. [α]23 D = 9.5 (c = 1.0, CHCl3). HRMS: calcd. for C14H14N4O3 [M + H]+ 309.0964; found 309.0964. Methyl (2S)-3-(1H-3-Indolyl)-2-(6-oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)propanoate (6g): Rf = 0.30 (hexanes/EtOAc, 5:5). Yield: 351 mg (97 %). Pale-yellow gummy liquid. IR (neat): ν˜ = 3397, 2950, 1740, 1664, 1432, 1349, 1099, 833, 744 cm–1. 1H NMR (300 MHz, CDCl3): δ = 8.56 (s, 1 H), 7.55 (d, J = 7.8 Hz, 1 H), 7.37 (s, 1 H), 7.33–7.06 (m, 4 H), 6.95 (d, J = 2.1 Hz, 1 H), 5.31 (dd, J1 = 11.1 Hz, J2 = 5.1 Hz, 1 H), 5.04 (d, J = 17.7 Hz, 1 H), 4.92 (d, J = 18 Hz, 1 H), 4.52 (d, J = 15.6 Hz, 1 H), 4.43 (d, J = 15.9 Hz, 1 H), 3.78 (s, 3 H), 3.57 (dd, J1 = 15.9 Hz, J2 = 5.1 Hz, 1 H), 3.42 (dd, J1 = 15.3 Hz, J2 = 10.8 Hz, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 170.4, 162.8, 136.3, 129.1, 127.6, 126.9, 122.5 (2 C), 119.8, 117.9, 111.7, 109.9, 58.1, 52.8, 48.8, 40.5, 24.2 ppm. [α]23 D = –29.4 (c = 1.0, CHCl3). HRMS: calcd. for C17H17N5O3 [M + Na]+ 362.1229; found 362.1223. Methyl (2S)-6-[(Benzyloxy)carbonyl]amino-2-(6-oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)hexanoate (8a): R f = 0.40 (hexanes/EtOAc, 4:6). Yield: 420 mg (96 %). Colorless gummy liquid. IR (neat): ν˜ = 3338, 2938, 2867, 1715, 1666, 1534, 1457, 1251, 743, 699 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.63 (s, 1 H), 7.34–7.32 (m, 5 H), 5.27 (dd, J1 = 10.5 Hz, J2 = 5.1 Hz, 1 H), 5.13
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Facile Entry to 4,5,6,7-Tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-6-ones (br. s, 2 H), 5.05 (s, 2 H), 4.84 (br. s, 1 H), 4.75 (d, J = 15.9 Hz, 1 H), 4.59 (d, J = 16.2 Hz, 1 H), 3.74 (s, 3 H), 3.23–3.15 (m, 2 H), 2.21–2.05 (m, 1 H), 1.96–1.25 (m, 5 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 170.6, 163.2, 156.5, 136.5, 129.4, 128.5, 128.1, 128, 127.5, 66.7, 55.9, 52.6, 48.8, 40.3, 38.6, 29.4, 27.5, 23.0 ppm. [α]22 D = 2.8 (c = 1.0, CHCl3). HRMS: calcd. for C20H25N5O5 [M + Na]+ 438.1753; found 438.1752. Ethyl (2S)-2-[(Benzyloxy)carbonyl]amino-6-(6-oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)hexanoate (8b): R f = 0.40 (hexanes/EtOAc, 4:6). Yield: 425 mg (94 %). Colorless gummy liquid. IR (neat): ν˜ = 3313, 2939, 1720, 1661, 1532, 1346, 1256, 1213, 1026, 737 cm–1. 1H NMR (300 MHz, CDCl3): δ = 7.61 (s, 1 H), 7.30 (br. s, 5 H), 5.51 (d, J = 8.1 Hz, 1 H), 5.07 (s, 2 H), 5.04 (s, 2 H), 4.63 (s, 2 H), 4.34 (dd, J1 = 12.6 Hz, J2 = 7.5 Hz, 1 H), 4.19 (q, J = 6.9 Hz, 2 H), 3.55 (t, J = 6.3 Hz, 2 H), 2.04–1.56 (m, 4 H), 1.42–1.39 (m, 2 H), 1.26 (t, J = 6.9 Hz, 3 H) ppm. 13 C NMR (75 MHz, CDCl3 ): δ = 172.1, 161.7, 155.9, 136.2, 129.1, 128.4, 128.1, 127.9, 127.2, 66.9, 61.5, 53.5, 48.5, 46.8, 42.1, 32.1, 26.0, 22.2, 14.1 ppm. [α]23 D = 6.3 (c = 1.0, CHCl3). HRMS: calcd. for C21H27N5O3 [M + Na]+ 452.1910; found 452.1905. 5-Benzyl-4,5,6,7-tetrahydro-6λ6-[1,2,3]triazolo[5,1-d][1,2,5]thiadiazine-6,6-dione (8c): Rf = 0.40 (hexanes/EtOAc, 5:5). Yield: 100 mg (35 %). White solid. M.p. 110 °C. IR (neat): ν˜ = 2993, 2941, 1455, 1371, 1302, 1188, 1161, 903, 736 cm –1 . 1 H NMR (300 MHz, CDCl3): δ = 7.56 (s, 1 H), 7.55–7.27 (m, 5 H), 5.53 (s, 2 H), 4.54 (s, 2 H), 4.38 (s, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 133.2, 130.2, 129.2, 128.9, 128.6, 126.8, 61.6, 52.8, 42.6 ppm. HRMS: calcd. for C 11 H 12 N 4 O 2 S [M + Na] + 287.0579; found 287.0754. Methyl (2S)-3-(1H-3-Indolyl)-2-(3-methyl-6-oxo-4,5,6,7-tetrahydro[1,2,3]triazolo[1,5-a]pyrazin-5-yl)propanoate (8d): Rf = 0.40 (hexanes/EtOAc, 5:5). Yield: 188 mg (50 %). Pale-yellow gummy liquid. IR (neat): ν˜ = 3336, 2928, 1741, 1665, 1435, 1344, 1234, 745 cm–1. 1H NMR (300 MHz, CDCl3): δ = 8.23 (s, 1 H), 7.56 (d, J = 7.8 Hz, 1 H), 7.35 (d, J = 8.1 Hz, 1 H), 7.26 (s, 1 H), 7.22–7.09 (m, 2 H), 6.97 (d, J = 2.1 Hz, 1 H), 5.34 (dd, J1 = 10.8 Hz, J2 = 5.4 Hz, 1 H), 5.04 (d, J = 17.7 Hz, 1 H), 4.93 (d, J = 18 Hz, 1 H), 4.41 (d, J = 15.9 Hz, 1 H), 4.32 (d, J = 15.3 Hz, 1 H), 3.81 (s, 3 H), 3.58 (dd, J1 = 15.9 Hz, J2 = 5.7 Hz, 1 H), 3.46 (dd, J1 = 15.3 Hz, J2 = 10.5 Hz, 1 H), 2.09 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 170.3, 162.9, 138.0, 136.2, 126.9, 124.1, 122.6, 122.2, 119.9, 117.9, 111.5, 110.1, 58.1, 52.7, 48.9, 40.2, 24.2, 9.8 ppm. [α]23 D = 4.4 (c = 1.0, CHCl 3 ). HRMS: calcd. for C 18 H 19 N 5 O 3 [M + Na] + 376.1386; found 376.1381. Supporting Information (see also the footnote on the first page of this article): 1H and 13C NMR spectra for all new compounds.
Acknowledgments V. S. S. thanks the Council of Scientific and Industrial Research, New Delhi for a senior research fellowship and S. C. N. thanks Department of Science and Technology, New Delhi for the JC Bose National Fellowship. [1] R. Huisgen in 1,3-Dipolar Cycloaddition Chemistry (Ed.: A. Padwa), Wiley, New York, 1984, pp. 1–176.
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[2] a) R. Alvarez, S. Velazquez, A. San-Felix, S. Aquaro, E. De Clercq, C.-F. Perno, A. Karlsson, J. Balzarini, M. J. Camarasa, J. Med. Chem. 1994, 37, 4185–4194; b) M. J. Genin, D. A. Allwine, D. J. Anderson, M. R. Barbachyn, D. E. Emmert, S. A. Garmon, D. R. Graber, K. C. Grega, J. B. Hester, D. K. Hutchinson, J. Morris, R. J. Reischer, C. W. Ford, G. E. Zurenko, J. C. Hamel, R. D. Schaadt, D. Stapert, B. H. Yagi, J. Med. Chem. 2000, 43, 953–970; c) D. R. Buckle, C. J. M. Rockell, H. Smith, B. A. Spicer, J. Med. Chem. 1986, 29, 2269–2277; d) H.Wamhoff, in Comprehensive Heterocyclic Chemistry (Ed.: A. R. Katritzky, C. W. Rees), Pergamon, Oxford, 1984, vol. 5, pp. 669–732. [3] D. R. Buckle, C. J. M. Rockell, H. Smith, B. A. Spicer, J. Med. Chem. 1986, 29, 2262–2267. [4] a) H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. Int. Ed. 2001, 40, 2004–2021; b) G. W. Bemis, M. A. Murcko, J. Med. Chem. 1996, 39, 2887–2893. [5] a) V. V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless, Angew. Chem. Int. Ed. 2002, 41, 2596–2599; b) S. Kamijo, T. Jin, Z. Huo, Y. Yamamoto, J. Org. Chem. 2004, 69, 2386–2393; c) T. Jin, S. Kamijo, Y. Yamamoto, Eur. J. Org. Chem. 2004, 3789–3791; d) S. Kamijo, T. Jin, Y. Yamamoto, Tetrahedron Lett. 2004, 45, 689–691. [6] a) Q. Wang, T. R. Chan, R. Hilgraf, V. V. Fokin, K. B. Sharpless, M. G. Finn, J. Am. Chem. Soc. 2003, 125, 3192– 3193; b) A. J. Link, D. A. Tirrell, J. Am. Chem. Soc. 2003, 125, 11164–11165; c) A. E. Speers, B. F. Cravatt, Chem. Biol. 2004, 11, 535–546; d) A. E. Speers, G. C. Adam, B. F. Cravatt, J. Am. Chem. Soc. 2003, 125, 4686–4687; e) L. V. Lee, M. L. Mitchell, S.-J. Huang, V. V. Fokin, K. B. Sharpless, C.-H. Wong, J. Am. Chem. Soc. 2003, 125, 9588–9589; f) J. M. Casas-Solvas, A. Vargas-Berenguel, L. F. Capitan-Vallvey, S.-G. Francisco, Org. Lett. 2004, 6, 3687–3690; g) B. H. M. Kuijpers, S. Groothuys, A. R. Keereweer, P. J. L. M. Quaedflieg, R. H. Blaauw, F. L. Van Delft, F. P. J. T. Rutjes, Org. Lett. 2004, 6, 3123–3126; h) S. Hotha, S. Kashyap, J. Org. Chem. 2006, 71, 364–367; i) Y. Angell, K. Burgess, J. Org. Chem. 2005, 70, 9595–9598; j) R. A. Turner, A. G. Oliver, R. S. Lokey, Org. Lett. 2007, 9, 5011– 5014; k) T. S. Hu, R. Tannert, H. D. Arndt, H. Waldmann, Chem. Commun. 2007, 3942–3944. [7] a) I. Arkitopoulou-Zanze, V. Gracias, S. W. Djuric, Tetrahedron Lett. 2004, 45, 8439–8441; b) F. Couty, F. Durrat, D. Prim, Tetrahedron Lett. 2004, 45, 3725–3728; c) D. K. Mohapatra, P. K. Maity, R. G. Gonnade, M. S. Chorgade, M. K. Gurjar, Synlett 2007, 12, 1893–1896; d) W. Rohr, A. Fischer, A. Bad, A.-G. Sodafabr, Pesticide Chemistry: Proceedings of the Second International IUPAC Congress (Ed.: A. S. Tahori), Gordon and Breach, New York, 1972, vol. 5, pp. 177–188; e) J. K. Pokorski, L. M. Miller Jenkins, H. Feng, S. R. Durell, Y. Bal, D. H. Appella, Org. Lett. 2007, 9, 2381–2383; f) I. Kumar, C. V. Rode, Chem. Lett. 2007, 36, 592–593; g) H. Yanai, T. Taguchi, Tetrahedron Lett. 2005, 46, 8639–8643. [8] a) S. Hotha, R. I. Anegundi, A. A. Natu, Tetrahedron Lett. 2005, 46, 4585–4588; b) S. Chandrasekhar, C. L. Rao, C. Nagesh, C. R. Reddy, B. Sridhar, Tetrahedron Lett. 2007, 48, 5869–5872; c) H. Yanai, T. Taguchi, Tetrahedron Lett. 2005, 46, 8639–8643. [9] G. Gerona-Navvoro, M. A. Bonache, R. Herranz, M. T. Garcia-Lopez, R. Gonzalez-Muniz, J. Org. Chem. 2001, 66, 3538– 3547. [10] J. H. Cho, B. M. Kim, Tetrahedron Lett. 2002, 43, 1273–1276. Received: January 12, 2008 Published Online: March 26, 2008
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Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.5 Homogeneous Catalysis with Ruthenium Carbonyl Cluster Complexes: Hydrogenation of Alkynes Juvier A . Cuheza
2.5.1 Introduction Although industry uses heterogeneous catalysts for a large majority of its chemical transformations, the nature of the elemental processes that take place on the surface of a solid catalyst during a particular heterogeneous reaction remains obscure."' This is because the tools we use to study surface chemistry are still very limited in number and in efficiency."' Molecular clusters have been considered for more than 20 years as models that help rationalize the reactions occurring on the active surface of heterogeneous catalysts.l'*2]In fact, like metal surfaces, they contain several metal atoms bonded to each other and they can bind ligands through both polycentric and direct ligandmetal interactions. Moreover, as they are molecular coordination compounds, they are normally soluble in organic solvents, and we nowadays have powerful, readily available, spectroscopic tools ( IR, NMR, etc.) for characterizing molecular species in solution. The intermediacy of transition metal clusters in catalytic transformations of organic chemicals under homogeneous conditions has been observed in many occasions and the subject has been reviewed several time^.[^-^^] The number of well documented catalytic mechanisms involving polynuclear species is, however, still small, despite the recognition that the knowledge of a catalytic mechanism is important in tuning the activity of the catalyst and the selectivity of the chemical process. Furthermore, the determination of mechanisms of homogeneous catalytic reactions in which metal cluster complexes are involved might shed light on fundamental steps of heterogeneously catalyzed reactions. Polynuclear transition-metal carbonyl derivatives are among the most accessible cluster compounds. This class of compounds also has the advantage that their re-
7 16
2 Metal Clusters in Catalysis
actions are very easily monitored by IR spectroscopy. When working with carbonyl cluster complexes, however, it is very common to observe metal-metal bond cleavage (for example, [ R u ~ ( C O ) ~isZa] useful starting material for the preparation of many bi- and mononuclear ruthenium carbonyl complexes[' 'I) and this is a major problem encountered in trying to establish mechanisms of catalytic reactions in which such clusters are involved. It should be noted that the recovery of the intact catalyst precursor at the end of a catalytic reaction does not necessarily imply its involvement as catalytic species. In this context, Lainel'] has proposed a method based on kinetic data for distinguishing between cluster-catalyzed reactions and reactions in which the clusters are precursors for mono- or binuclear derivatives which are the actual catalytic species. In general, when cluster fragmentation is not desired, the integrity of the metal framework has often, but not always, been preserved by using bridging ligands or by working under very mild conditions. In order to characterize catalytic species, the use of spectroscopic techniques sometimes induces errors, because the catalytic species might be present in very small concentrations and might have very short life-times. On the other hand, the preparation of catalytic intermediates by stepwise reaction of the catalytic precursor with the reagents involved in the catalytic reaction is essential to establish mechanisms, but this is not easily achieved because the catalytic intermediates are often very unstable. The determination of the mechanism of a cluster-promoted catalytic reaction is, therefore, difficult, requiring an adequate combination of kinetic, spectroscopic and reactivity studies.[61This contribution reviews the homogeneous hydrogenation of alkynes promoted by ruthenium carbonyl cluster compounds, emphasizing the mechanistic aspects of such reactions. Ruthenium complexes are among the most active complexes promoting hydrogenation reactions,[16.'1' and alkynes are interesting compounds for use as model substrates for hydrogenation reactions because, as alkenes are the initial hydrogenation products, one can study, not only the activity of the catalyst, but also its selectivity towards the hydrogenation of triple compared with double
2.5.2 Survey of catalyst precursors Table 1 lists alkyne hydrogenation reactions which use ruthenium carbonyl cluster complexes as catalyst precursors. The contents of the table are discussed in the following paragraphs, which are classified according to the type of metal cluster complex used as catalyst precursor.
2-Pentyne
PhzCz PhzC2 PhrCz PhlC2 Ph2C2 I-Pentyne
5
6
I
8
9
10
11
1-Pentyne
4
100 "C, 50 atm 80 "C, 1 atm
120 "C, 1 atm
120"C, 1 atm
120 "C. 1 atm
120°C. 1 atm
8 0 T , 1 atm
8 0 T , 1 atm
120 "C, 1 atm
80 " C , 1 atm
S O T , 1 atm
Z-2-pentene, E-2-pentene, 1-Pentene, Pentane Z-2-pentene, E-2-pentene, 1-Pentene, Pentane Z-Stilbene, E-Stilbene Z-2-pentene, E-2-pentene, 1-Pentene, Pentane Z-2-Pentene, E-2-Pentene, 1-Pentene. Pentane Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene, E-Stilbene Z-2-Pentene, E-2-Pentene, 1-Pentene, Pentane 1-Pentyne
2-Pentyne
Conditions
Products
Substrate
Ph2C2
Catalyst precursor
3
Entry
Table 1. Homogeneous hydrogenation of alkynes promoted by homonuclear ruthenium carbonyl cluster complexes. Ref.
23
22
21
20
19
18
17
16
15
14
13
12
Entry
Catalyst precursor
Table 1 (continued) Substrate 80 " C , 1 atm
Z-2-Pentene, E-2-Pentene, 1-Pentene, Pentane Z-2-Pentene, E-2-Pentene, 1-Pentene, Pentane Z-2-Pentene, E-2-Pentene, 1-Pentene, Pentane BU'CH=CH2, Bu'CH~CH~ Z-Stilbene, E-Stilbene Bu'CH=CH2, Bu'CH~CH~ Z-Stilbene, E-Stilbene Bu'CH=CH2, Bu'CH~CH~ Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene, PhC2H4Ph Z-Stilbene, E-Stilbene
60 "C. 0.4-0.8 atm
120 " C , 1 atm
120"C, 1 atm
120°C, 1 atm
120"C, 1 atm
120"C, 1 atm
120 "C, 1 atm
120 " C , 1 atm
120"C, 1 atm
8 0 ° C 1 atm
80"C, 1 atm
Conditions
Products
Ref.
31
30
29
28
21
25 26
24 Z-Stilbene, E-Stilbene, Styrene Z-PhCH-CHMe, E-PhCH=CHMe Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene Z-Stilbene, E-Stilbene Z-Stilbene, E-S t il bene 60 "C, 0.4-0.8 atm
80 "C, 0.3-0.6 atm
60 "C, 0.2-0.6 atm
60 "C, 0.3-0.7 atm
60 "C, 0.4-0.8 atm
100 "C, 15 atm 65 "C, 40 atm
80 "C, 0.7 atm
720
2 Metal Clusters in Catalysis
Figure 1. The structures of compounds [ R u ~ ( C O ) I(l), ~ ] [R~(p-H)4(C0)12] (2) and [Ru&H)2(C0)13](3).
2.5.2.1 Clusters containing only hydride and carbon monoxide ligands Valle and coworkers have studied the hydrogenation of 1- and 2-pentyne under mild conditions using [Ru3(C0)12](1) and [Ru4(p-H)4(C0)12](2) (Fig. 1) as catalyst precursors (Table 1, entries 1, 2, 4 and 5).[18,191 Both complexes give similar hydrogenation results. This is not surprising, because [ R u ~CO)12] ( gives [Ru4(p-H)4(CO)12] when treated with hydrogen.[”] 1-Pentyne is hydrogenated to 1-pentene, but the hydrogenation is accompanied by isomerization of 1-pentene to Z-2-pentene and E-Zpentene. Pentenes are converted to pentane only after all 1-pentyne has been hydrogenated. 2-Pentyne is hydrogenated to Z-2-pentene, but the latter is subsequently isomerized to E-2-pentene. Again, pentane appears only when the alkyne has been consumed. Under the same conditions, the initial rates of hydrogenation of 2-pentyne are ca 1.5 faster than those of the hydrogenation of 1-pentyne. No catalytic intermediates have been isolated or characterized in these reactions. Sappa and coworkers have studied the hydrogenation of diphenylacetylene promoted by 1, 2 and [ R u ~ ( , u - H ) ~ ( C O (3) ) ~under ~ ] mild conditions (Table 1, entries 3 , 6 and 7).[”] Whereas 1 and 2 have similar catalytic activity, 3 is somewhat less active. Under the reaction conditions used, the three complexes give a mixture of Zand E-stilbene as hydrogenation products without forming any diphenylethane. Several cluster complexes have been isolated from the catalytic solutions (Fig. 2): cluster 1 gives a mixture of 2, [Ru3(p-H)2(p3-Ph2C2)(C0)9] (4) and [Ru3(p3Ph4C4)(CO)8](5 ); cluster 2 gives a mixture of 2, 4, [Ru3(p3-Ph2C2)2(CO)s](6) and (7);and cluster 3 gives a mixture of 2,4, 5 , 6 , 7 and [ R ~ q ( p ~ [Ru4(p4-Ph2C2)(CO)~2] Ph2C2)2(C0)11](8 ). Apart from cluster 4, which was subsequently found to be catalytically active for the hydrogenation of diphenylacetylene, all the other cluster compounds formed in these reactions (complexes 5-8) are rather stable byproducts which react very slowly with hydrogen to give small amounts of hydrogenation products. Although no kinetic studies have been conducted, it is suggested that the
2.5 Homogeneous Catalysis icith Ruthenium Carbony1 Cluster Complexes
121
Ph
Ph
\
Ph
(7)
trinuclear dihydridoalkyne cluster 4 is a common catalytic intermediate in all these reactions, and probably the only isolated species involved in the catalytic cycle.[21,221 In fact, although it has been found that complex 7 promotes a slow hydrogenation this cluster forms a mixture of 2, 4 and 5 of diphenylacetylene (Table 1, entry 9),[231 when it is treated with hydrogen.[21] The complex Na[Ru3(p-H)(CO)11](9) is the only anionic ruthenium cluster reported to promote an alkyne hydrogenation reaction (Table 1, entry 10). It has been found to reduce diphenylacetylene selectively to Z-stilbene, in dimethylformamide under moderate pressure, but no further details have been
2.5.2.2 Clusters containing P-donor ligands The catalytic hydrogenation of 1-pentyne and 2-pentyne in the presence of phosphine- and phosphite-substituted derivatives of compound 2 has been studied under mild conditions by Valle and coworkers (Table 1, entries 11-14).[25] As occurs
722
2 Metal Clusters in Catalysis
with pure 2, 2-pentyne is always hydrogenated faster than 1-pentyne. The initial rates of hydrogenation of both 1- and 2-pentyne with the monosubstituted derivatives [ R u ~ ( , u - H ) ~ ( P R ~ ) ( C[R O= ) ~Bun, ~ ] (lo), Ph (ll),OEt (12), OPh (13)] decrease in the sequence 10 = 11 > 12 = 13, indicating that the rate is favored by basic ligands. It is interesting to note that an increase in the substitution of the catalyst precursor, i.e. using the di- and trisubstituted complexes [Ru~(,uH ) ~ ( P R ~ ) , ( C O ) I ~[n- ,= ] 2, R = Ph (14), OEt (15);n = 3, R = Ph (16), OEt (17)],is reflected in an increase of the initial rate of hydrogenation of 1-pentyne; however, the opposite trend is observed for 2-pentyne. The specificity of all these substituted catalyst precursors, towards the formation of 2-2-pentene and 1-pentene from 2and 1-pentyne, respectively, is similar but it is better than that observed with the unsubstituted compound 2. No catalytic intermediates have been isolated or characterized in these reactions. Sappa and coworkers have studied the behavior of the diphenylphosphineand diphenylphosphido-substituted derivatives [Ru3(PPh2H),(C0)12-,] [ H = 1 (18), 2 (191, 3 (20)1, [RU~(,U-H)(,U-PP~~)(CO)~O-,I [n = 0 (21), 1 (22)]and [Ru~(,u-H)~-,(,uPPh2)2+n(C0)8-n][n = 0 (23), 1 (24)] (Fig. 3) in the homogeneous hydrogenation of tert-butylacetylene (Table 1, entries 15, 17, 19).[26,27] All the clusters 18-24 give similar catalytic results, involving a 30-40% conversion of the substrate to
..
1\
(21)
2.5 Homogeneous Cutulysis with Ruthenium Curbonyl Cluster Complexes
723
3,3-dimethyl-l-butene after 90 min. Small amounts (3-70/1) of 2,2-dimethylbutane are also formed. The rather similar activities and selectivities observed for these complexes suggest that a common catalytic species is probably formed in solution. When treated with the alkyne at high temperature, all these clusters form a similar product mixture which contains variable amounts of 23 and 24 accompanied by the phosphalkyne derivatives 25 and 26. The fact that compounds 25 and 26 give 3,3dimethyl-1 -butene when treated with hydrogen and that the activity of the catalytic solutions decreases when the amounts of 25 and 26 are progressively reduced, induced the authors to propose 25 and 26 as catalytic intermediate^.^^^] Clusters 19-22 and 24 have also been tested as catalyst precursors for the hydrogenation of diphenylacetylene (Table 1, entries 16, 18, 20).[281 Z-Stilbene (kinetic product) and E-stilbene (thermodynamic product) are formed with higher conversions (70-1000/1 after 90 min) than in the case of terminal alkynes.[”] As with tertbutylacetylene, similar activities and selectivities are observed for the five cluster complexes, suggesting the intermediacy of common catalytic species in solution. Compound 27 (which arises from the reactions of 21 or 22 with diphenylacetylene) and compound 28 (which arises from the reaction of 24 with the same alkyne) (Fig. 4) have been proposed as catalytic intermediates in these hydrogenation reactions.i2*1
(27) Ph
724
2 Metal Clusters in Catalysis
The tetranuclear phosphinidene cluster [Ru4(pu,-PPh)(CO)t3] (29) (Fig. 4) is also a catalyst precursor for the homogeneous hydrogenation of diphenylacetylene under mild conditions (Table 1, entry 21).[29]No catalytic intermediates have been isolated or characterized in this reaction, although it is known that 29 reacts (30) and with diphenylacetylene with hydrogen to give [Ru4(p-H)2(pu1-PPh)(CO)121 to give Ru4(p4-PPhPh2C2)(C0)t2](31) and [Ru4(p4-Ph2C2)(p4-PPh)(CO)~1] (32) (Fig. 4).1301
2.5.2.3 Clusters containing cyclopentadienyl ligands The trinuclear cluster [Ru3(Cp)2(p3-Ph2C2)(C0)5] (33) (Fig. 5 ) is a good catalyst precursor for the hydrogenation of diphenylacetylene, but only modest for the isomerization of 2-stilbene (Table 1, entry 22).[3113-Hexyne is hydrogenated with more difficulty than diphenylacetylene. Although cluster catalysis might occur, cluster fragmentation to give the catalytically inactive metallacyclic derivative [Ru2(Cp)2(pU-Ph4C4)(C0)] (34) (Fig. 5 ) is observed during the catalytic runs. Because compounds 4 and 33 are efficient catalyst precursors for the hydrogenation of diphenylacetylene and both contain an alkyne bound parallel to a metalmetal bond, E. Sappa and coworkers have made the hypothesis that clusters with an alkyne bound parallel to one edge of a triangular metal array could act as catalyst precursors or as intermediates in the hydrogenation of a1kynes.L' 1,22,311 In fact, the unsaturated binuclear complex [Ru2(Cp)2(pU-Ph2C2)(CO)] (35),which has the alkyne bonded in a perpendicular mode to the metal-metal bond (Fig. 5), is a poor hydrogenation ~ata1yst.L~ '1
2.5.2.4 Clusters containing edge-bridging N-donor ligands The edge-bridged carbonyl cluster compound [ R u ~p-H)( ( p-dmdab)(CO)s] (36) (Hdmdab = 3,5-dimethyl-1,2-diaminobenzene) (Fig. 6)[321has proved to be an ex-
2.5 Homogeneous Cutulysis with Ruthenium Curhonyl Cluster Complexes
725
Figure 6. The structures of compounds [Ru,(p-H)(p-dmdab)(CO)g](36) and JRu&-dmdab)(pPhC=CHPh)(C0)5](37).
cellent catalyst precursor for the homogeneous hydrogenation of diphenylacetylene under very mild conditions: turnover frequency = 1.3 min-' at 60 "C and 0.838 atm Hz (Table 1, entry 23). A kinetic analysis of this catalytic reaction has shown that it is first-order in catalyst precursor concentration, first-order in hydrogen pressure and zero-order in substrate concentration.["] No reaction is observed between 36 and hydrogen under conditions similar to those used in the catalytic reaction; however, 36 reacts readily with diphenylacetylene to give a mixture of products from which the binuclear derivative [Ru2(p-drndab)(pu-PhC=CHPh)(CO)~] (37) (Fig. 6) could be isolated and characterized. Complex 37 is observed spectroscopically in the catalytic solutions; however, no hydrogenation of diphenylacetylene occurs when this complex is used as catalyst precursor.[331 Although it is not possible to propose a mechanism for the catalytic hydrogenation reaction promoted by complex 36, the following data suggest that the catalyst is a mononuclear species: (a) 36 does not react with hydrogen but reacts with diphenylacetylene to give a catalytically inactive binuclear compound (37) and a presumably mononuclear species; and (b) the catalytic reaction is first-order in the concentration of 36 added, and this concentration should be equal to that of the mononuclear species which arises from the quantitative reaction of 36 with diphenylacetylene. These results raise doubts about the possible intermediacy of polynuclear catalytic species when edge-bridged trinuclear carbonyl cluster complexes are used as catalyst precursors.
2.5.2.5 Clusters containing face-bridging N-donor ligands The cluster complex [R~~(p-H)(p~-ampy)(CO)~][~~~ (38) ( Hampy = 2-amino6-methylpyridine) (Fig. 7) is an efficient catalyst precursor for the selective
126
2 Metal Clusters in Catalysis
Figure 7. The structures of compounds [Ru3(pH)(p3-ampy)(CO)9](38), [Ru3(p3-ampy)(,u(40) and [Ru6(~-H)6(p,-ampY)2(co)141 (41). phC=cHPh)(Co)~]
homogeneous hydrogenation of alkynes. Diphenylacetylene is hydrogenated at pressures below 1 atm, whereas terminal alkynes require higher pressures (Table I, entries 24, 25).[351cis-trans Isomerization of stilbene is also observed. The complex [Ru3(p-H)(p3-anipy)(CO)g](39) (Hanipy = 2-anilinopyridine), which is structurally similar to 1, has been reported to promote the hydrogenation of l-phenyl-l-pr~pyne.'~~] In the hydrogenation of diphenylacetylene (turnover frequency 27.1 h-', in toluene at 80 "C and 0.663 atm H2), IR monitoring of the reaction shows the alkenyl derivative [ R u ~p3-ampy)(p-PhC=CHPh)(CO)sJ ( (40) as the only detectable carbony1 In separate experiments, it was confirmed that 38 reacts readily whereas the reaction of 38 with hydrogen ocwith diphenylacetylene to give 40,[351 curs only at high temperatures (above 100 "C), giving the hexanuclear hexahydride [Ru&~-H)&i~-ampy)2(CO)~4] (41) (Fig. 7).[371Several kinetic studies of this hydrogenation reaction repeatedly resulted in reaction orders between 0.3 and 0.4 This value most probably arises from CO diswith respect to the precursor 38.[38] sociation from 38, rather than from cluster fragmentation, because in the kinetic
2.5 Homogeneous Cutulysis with Ruthenium Carbonyl Cluster Complexes
100
0
diphenylacetylene
A
cis-stilbene trans-stilbene
0
80
.
0
$?
c
0
127
A
0
.O 60 fn
0
I
a,
40
n
0
n
0 0
20
n o
n
n
0
' j
A
0
n 0 0
0
0
50
t / m i n 100
150
Figure 8. Progress of the hydrogenation of diphenylacetylene promoted by cluster 40 (toluene, 0.838 atm HI, [substrate] = 0.205 M, [40] = 3.07 x lo-' M, 60 "c).
experiments we were unable to control the CO pressure derived from CO dissociation from complex 38. A similar kinetic situation was reported for the hydrogenation of 3,3-dimethylbut-l -ene promoted by [PPN][Ru,(p-NCO)(CO),,] ([PPN]+= [(Ph3P)2N]+).In this reaction, in which there is a CO dissociation step, no cluster fragmentation is proposed.'391 All these data suggest that 40 is a catalytic intermediate in the hydrogenation of diphenylacetylene promoted by compound 38 and that the catalytic species are trinuclear ruthenium carbonyl clusters. To test this hypothesis, detailed kinetic and chemical studies relevant to the hydrogenation of diphenylacetylene using the alkenyl-bridged complex 40 as catalyst precursor have been conducted. Complex 40 has been found to be a more efficient catalyst precursor for the homogeneous hydrogenation of diphenylacetylene than complex 38 (turnover frequency 38.8 hk', in toluene at 60 "C and P( H2) = 0.838 atm), rendering a mixture of Z- and E-stilbene as the kinetic and thermodynamic products, respectively (Fig. 8) (Table 1 , entry 27). No hydrogenation is observed at room temperature except when very low [substrate]/[catalyst] ratios are used.[401A kinetic analysis of this reaction has revealed that, over a wide range of concentrations of 40 and hydrogen pressures, the rate is first-order in each of these two components, but the ratedependence on substrate concentration is not linear, because it is negative-order at low [Ph2C2]/[40]ratios and approximately zero-order at high [Ph2C2]/[40]ratios,'401 suggesting the existence of two different catalytic cycles, each working in different ranges of [Ph?C2]/[40]ratios. At room temperature, complex 40 reacts with hydrogen (1 atm), in the absence of diphenylacetylene, to give a mixture of 38, 41, Z- and E-stilbene. Although a very slow catalytic hydrogenation reaction can be observed at low [Ph2C2]/[40]ratios, no reaction is observed when more diphenylacetylene is present in solution.[401These
728
2 Metal Clusters in Catalysis
uR‘
Figure 9. Mechanism of the hydrogenation of diphenylacetylene promoted by complex 40, at low [Ph~C2]/[40]ratios. All cluster intermediates contain eight CO ligands.
results, and the negative rate-dependence on substrate concentration observed in the catalytic hydrogenation reaction for low [Ph2C2]/[40]ratios, suggest that, at low substrate concentrations, complex 40 releases diphenylacetylene (Kl K2, Fig. 9) before the rate-determining step k3 (which is likely to be the oxidative addition of hydrogen because the reaction is first-order in hydrogen pressure) to give an unsaturated species B which is responsible for the hydrogenation of the alkyne and for the subsequent formation of compounds 38 and 41 when all the substrate has been consumed.‘40]This mechanism (Fig. 9), which corresponds to the rate law:
+
is also compatible with the fact that 40 is the only complex observed during the catalysis, because the equilibria defined by K1 and K2 should be largely displaced to
2.5 Homogeneous Cutcilq'sis tvith Ruthenium Curbon.yl Cluster Complexes
129
Figure 10. Mechanism of the hydrogenation of diphenylacetylene promoted by complex 40, at high IPhzCl]/[401 ratios. All cluster intermediates contain eight CO ligands.
the left ( A and B have never been observed in solutions of complex 40). Unfortunately, the nature of the fast steps that lead from C to B remains unknown. Fig. 10 displays a mechanism for the hydrogenation of diphenylacetylene promoted by complex 40 at high [Php22]/[40] ratios. This mechanism, which follows the rate law:
is compatible with the kinetic data obtained at high [Ph2C2]/[40]ratios, being also supported by the following experimental results. The sequential addition of a proton and a hydride to a metal complex may be considered equivalent to an oxidative addition of hydrogen. With this in mind, the ( BF4] (42) cationic hydrido derivative [ R u ~p-H)(,~~,-ampy)(,u-PhC=CHPh)(C0)~][ (Fig. 11) was made and subsequently treated with [PPN][BH4].This latter reaction was found to lead to Z- and E-stilbene and to a mixture of cluster decomposition products. However, the cluster 40 is reformed when it is sequentially treated with HBF4 and [PPN][BH4] in the presence of diphenylacetylene. Z- and E-stilbene are It should be noted that complex 42 is cooralso produced in this dinatively saturated and that a vacant site is needed before its reaction with [BH4]-.
730
2 Metal Clusters in Catalysis
Ph Figure 11. The structures of compounds [R~~(p-H)(p~-arnpy)(p-PhC=CHPh)(CO)~]~ (42) and [Ru&-ampy) { p-C(O)PhC=CHPh}(CO)s] (43).
That 40 and 42 contain the same number of CO ligands supports the hypothesis that the vacant sites needed for the reactions of 40 with hydrogen (at high [Ph2C2]/ [40] ratios) and of 42 with [BH4]- arise from bridging to terminal transformations of the alkenyl groups in 40 and 42 instead of from CO dissociation processes. The reaction of complex 40 with CO gives the acyl derivative [Ru3(p3-ampy){pThis reaction explains why the rate of C(O)PhC=CHPh}(CO)g] (43) (Fig. 1 l).[40b1 the catalytic hydrogenation reaction decreases when CO is added into the system. Moreover, the mild conditions required for this reaction (1 atm, room temperature) also imply that the bridging-to-terminal transformation of the alkenyl group shown in Fig. 10 (K4) is a low-energy process. In general, both catalytic cycles (Figs. 9 and 10) should participate in the hydrogenation reaction, with individual contributions depending on the particular [Ph2C2]/[40]ratio. Therefore, the general rate law for the hydrogenation of diphenylacetylene promoted by complex 40 takes the form:
where k, = KlK2k3 and kb = K4k5. This general rate law agrees well with all experimental kinetic data, at any [Ph2C2]/[40]ratio. The hexanuclear derivative [Ru6(pL-H)~(p3-ampy)2(C0)14] (41) (Fig. 7) also promotes the hydrogenation of diphenylacetylene under mild conditions (Table 1, entry 28).14’1 A kinetic analysis of the catalytic reaction, which is first-order in the concentration of 41, suggests that the catalytic species are hexanuclear, but no catalytic intermediates have been characterized. The cationic complex [Ru3(p-H)(p3-ampy)(p-PhC=CHPh)(CO)8][BF4] (42) (Fig. 11) is a good catalyst for the hydrogenation of diphenylacetylene under mild con-
2.5 Homogeneous Catalysis with Ruthenium Curbonyl Cluster Complexes
-1 +
- . -
7 31
i k 7
Figure 12. Mechanism for the hydrogenation of diphenylacetylene promoted by the cationic complex 42. All cluster intermediates contain eight CO ligands.
ditions (Table 1, entry 29).[421A kinetic analysis of this catalytic reaction, which represents the first catalytic reaction promoted by a cationic carbonyl cluster complex, has revealed that the reaction is first-order with regard to both the concentration of 42 and the hydrogen pressure and zero-order with regard to the alkyne. These data confirm that the incorporation of the substrate into the catalytic cycle should occur after the rate-limiting step, which should be the oxidative addition of hydrogen or a subsequent reaction. The first-order dependence on cluster concentration indicates that each trinuclear cluster 42 produces only one catalytic species (which might be itself). Fig. 12 shows a possible mechanism for this hydrogenation reaction. This mechanism, which corresponds to the rate law: u = -d[PhzCz]/dt = K&7[42][H2]
is in excellent agreement with the experimental kinetic data. This mechanism is analogous to that proposed for the neutral complex 40 (Fig. 10); however, the rates obtained using 42 as catalyst precursor are not affected by the addition of HBF40Et2 to the catalytic solutions, supporting the absence of a deprotonation
132
2 Metal Clusters in Catalysis
Figure 13. The structures of compounds [Ru&~-H)(p~-ampy)( PPh3)(CO)x](44) and [ R ~ ~ ( P - H ) ( , u ~ amPY)(PCY3)(C0)xl(45).
step in the catalytic process, because 42 is formed quantitatively when 40 is treated with HBF4. OEtz and 40 is more efficient than 42 as catalyst precursor for the hydrogenation of diphenyla~etylene.[~'IThat 42 is the only carbonyl complex observed in solution during the catalytic runs also supports the proposed mechanism, because the equilibrium constant & should have a very small value (species A has never been observed in solutions of complex 42), and if k7 is the rate-limiting step, all of the other intermediates in the catalytic cycle should have very short lives.
2.5.2.6 Clusters containing face-bridging N-donor ligands and phosphine ligands The cluster complexes [Ru3(p-H)(p3-ampy)(PR~)(CO)B] [R = Ph (44),[433441 Cy (45)1"']1] can be easily made by treating complex 38 with the corresponding phosphine ligand. Interestingly, as a consequence of the different basicity of the phosphine, both compounds have different structures (Fig. 13). These facts prompted us to undertake a comparative study of the catalytic activity of these two complexes in the hydrogenation of diphenylacetylene as a model reaction. Both compounds promote this reaction, but the rates are slow: turnover frequencies 10.9 h-' for 44[441 and 8.6 h-' for 45L4'] (toluene, 80 "C, 0.663 atm H2). Again, Z- and E-stilbene are the kinetic and thermodynamic products, respectively (Table 1, entry 30). Kinetic analyses of these reactions have revealed first-order dependencies on hydrogen pressure and on catalyst precursor and zero-order dependence on substrate concentration, for both catalyst precursors. A positive dependence on substrate concentration can be observed at very low [substrate]/[prec~rsor] ratios when complex 44 is used as precursor.[441 To obtain information about the nature of the metallic species involved in the catalytic processes, the reactions of the catalytic precursors 44 and 45 with diphenylacetylene and hydrogen have been studied. Neither 44 nor 45 reacts
2.5 Homogeneous Catalysis with Ruthenium Carbonyl Cluster Complexes
733
44
H2
45
(47)
(49)
Figure 14. Sequential reactions of compounds 44 and 45 with diphenylacetylene and hydrogen.
with hydrogen (1 atm, 80 "C), indicating that, under catalytic conditions, they react first with the substrate and then with hydrogen. Both 44 and 45 react readily with diphenylacetylene to give the alkenyl-bridged derivatives [ Ru3(p3-ampy)(pPhC=CHPh)(PR3)(CO),] [R = Ph (46), Cy (47)]. Subsequent reaction of these compounds with hydrogen gives the dihydrides [Ru3(p-H)2(p3-ampy)(pPhC=CHPh)( PR3)(CO)h] [R = Ph (48), Cy (49)] (Fig. 14).144,45]All these complexes (44, 46 and 48, or 45, 47 and 49) can be observed by NMR spectroscopy in the catalytic solutions, indicating that they are in equilibrium. Moreover, the observation of 48 and 49 implies that their transformations should be the ratedetermining steps of their corresponding catalytic processes. The coupling of a hydride with the alkenyl ligand in 48 or 49 cannot, however, be possible in one elemental reaction, because the c( carbon atom of the alkenyl ligand is not cis to any of the hydrides; the rate-determining step (klo in Fig. 15) must, therefore, be an isomerization reaction which would place a hydride and the alkenyl ct carbon atom in a cis arrangement (compound X in Fig. 15), before the reductive elimination of stilbene. From all these data, the mechanism depicted in Fig. 15 has been proposed. This mechanism is also supported by the available kinetic data. Therefore, despite the different basicity of the PCy3 and PPh3 ligands and despite
734
2 Metal Clusters in Catalysis
R
R
Figure 15. Mechanism for the hydrogenation of diphenylacetylene promoted by clusters 44 or 45.
the different structures of their complexes, compounds 44 and 45 behave similarly when used as catalyst precursors for the hydrogenation of diphenylacetylene. That 44 gives faster reaction rates than 45 has to be related to the lower basicity of the PPh3 compared with that of the PCy3 ligand. A basic ancillary ligand increases the electron density of the metal atoms, enhancing the retrodonating component of the metal-CO bonds; therefore, because KS and K9 involve the release of carbon monoxide from the corresponding cluster complexes, the transformation of 46 into 48 should be easier than that of complex 47 into 49. Unfortunately, the extent to which klo is influenced by the basicity of the phosphine ligands remains unknown. It is curious that the phosphine-disubstituted cluster compound [ R u ~p-H)(pu,( ampy)(PPh3)2(C0)7] (50)[461has proved to be a poor catalyst precursor for the homogeneous hydrogenation of diphenylacetylene. It does, however, react with hydrogen and diphenylacetylene to give [Ru3(p-Ph)(p3-ampy)(p-PPh2)2(CO)6](51) and PPh3)(CO)5](52), respectively ( Fig. 16), [ R u ~Ph)(p,-ampy)(p-PhC=CHPh)(p-PPhz)( ( which are rare examples of ruthenium clusters containing Vl-phenyl l i g a n d ~ . ' ~ ~ ] Because the triphenylphosphine-a,n-alkenyl derivative [ Ru3(p,-ampy)( pPhC=CHPh)( PPh3)(C0)7] (46) is a catalytic intermediate in the hydrogenation of
2.5 Homogeneous Cutulysis with Ruthenium Cuvhonyl Cluster Complexes
735
Figure 16. Reactions of compound 50 with hydrogen and diphenylacetylene.
diphenylacetylene promoted by cluster 44 (see above), we thought it of interest to study the hydrogenation catalytic activity of the bis(tripheny1phosphine)-o,nalkenyl derivative [Ru3(p3-ampy)(p-PhC=CHPh)( PPh3)2(C0)6] (53), a compound which cannot be made by direct reaction of [Ru3(p-H)(p3-ampy)( PPh3)2(C0)7] (50) with diphenylacetylene, but can be readily prepared from 40 or 46 and triphenylpho~phine.'~~] The catalytic hydrogenation of diphenylacetylene promoted by cluster 53 is very slow under mild conditions (Table 1, entry 31).148]The low rate, the occurrence of activation periods, and the deactivation of the catalyst after long reaction times (ca 500 min), have prevented a kinetic analysis of the reaction. Nevertheless, the PPh3)z(C0)5] observation of the dihydride [Ru3(p-H)z(p3-ampy)(p-PhC=CHPh)( (54) (Fig. 17) in the catalytic solutions suggests that the catalytic hydrogenation of diphenylacetylene promoted by complex 53 follows a similar mechanism to that described above for complex 44 (Fig. 15). The slower reaction rate and the activation period are probably because the activation energy for the release of CO from 53 (to create the necessary vacant site for the subsequent reaction with hydrogen to
736
2 Metal Clusters in Catalysis
Figure 17. The structures of compounds [Ru3(,u3-ampy)(,u-PhC=CHPh)( PPh3)2(C0)6] (53) and
[RU~(,~-H)~(~~-~~PY)(,~-P~C=CHP~)(PP~~)~(CO)~I (54).
give 54) is rather high because 53 contains only six CO ligands (it is well known that triruthenium carbonyl cluster complexes with fewer than six CO ligands are difficult to prepare by CO substitution reactions).
2.5.2.7 Heteronuclear clusters containing ruthenium Very few heteronuclear carbonyl clusters containing ruthenium have been tested as catalyst precursors for the homogeneous hydrogenation of alkynes. The iron-ruthenium clusters [RuFe2(C0)12] (55), [Ru;?Fe(C0)12] (56) and [Ru3Fe(p-H)2(CO)13](57) promote the hydrogenation of diphenylacetylene to a mixture of Z- and E-stilbene.[21]Their hydrogenation activity is slower than that found for the homonuclear ruthenium analogues, decreasing as the Ru/Fe ratio decreases in the precursors. These results, coupled with the isolation of the tri(4) from the catalytic solutions, ruthenium complex [Ru3(p-H)2(pu,-Ph2C2)(CO)g] suggest that these iron-ruthenium precursors undergo cluster fragmentation to give a common ruthenium catalytic intermediate which most probably is cluster 4.L2'1 R. D. Adams and coworkers have found that the mixed-metal cluster [Ru6Pt3(p3H)(p-H)3(C0)21](58) and its alkyne-substituted derivative [RugPt3(p3-H)(p-H)(pPh2C2)(C0)20](59) (Fig. 18) are effective catalyst precursors for the hydrogenation of diphenylacetylene to Z-stilbene at 50 "C and 1 atm h y d r ~ g e n . [ ~ ~In- ~particu'] lar, compound 59 has been found to be more active toward the hydrogenation of diphenylacetylene than a variety of homonuclear platinum and ruthenium carbonyl clusters, tested under identical reaction conditions. This unusual high activity is attributed to a synergistic interplay of the molecular activations occurring at the platinum and ruthenium metallic layers. A kinetic study of this reaction has resulted in the experimental rate expression:
2.5 Homogeneous Cutulysis with Ruthenium Curhonyl Cluster Complexes
737
Ph
Figure 18. The structures of compounds [Ru~Pt3(p3-H)(p-H)3(C0)2~] (58) and [Ru6Pt3(p3-H)(pH)(p-Ph2Cz)(CO)zo](59). Carbonyl ligands have been omitted for clarity.
v
-d[Ph2C*]/dt
= k[59][H2][PhzC2]/[CO]( 1
+ k'[Ph2C2])
which is in accordance with a mechanism that begins with dissociation of a CO ligand from the cluster, followed by hydrogen activation and alkyne addition steps. The latter is believed to induce the transfer of the hydrogen ligands to the coordinated alkyne in the slow step of the reaction. It has also been reported that [ Ru6Pt3(p3-H)(p-H)(p-To12C2)\CO)20] (59) (To1 = p-tolyl) promotes the hydrogenation of di-p-tolylacetylene. ' * ]
2.5.3 Concluding comments The results discussed in this contribution, although far from having wide application, demonstrate that internal and terminal alkynes can be effectively and selectively hydrogenated to alkenes under mild homogeneous conditions by use of a variety of ruthenium carbonyl clusters as catalyst precursors. Z-Alkenes are isomerized to E-alkenes during the hydrogenation reactions, although the best precursors for alkyne hydrogenation are generally poorly effective in alkene isomerization reactions. It is also generally true that internal alkynes are hydrogenated faster than terminal alkynes. The intermediacy of polynuclear cluster species in catalytic cycles has been unequivocally proved in few occasions only. These occasions are those that use catalyst precursors derived from the face-bridged compound [ Ru3(p-H)(pu,ampy)(CO)o] (38) (Table 1, entries 24, 25, 27, 29-31) and from the heteronuclear (S8),149'11 cases in which the study of the catacluster [Ru6Pt3(p3-H)(p-H)3(C0)21] lytic activity has been accompanied by the identification of intermediate cluster
738
2 Metal Clusters in Catalysis
species and by complete kinetic studies. In other cases, kinetic and chemical studies have confirmed that the catalyst precursor undergoes fragmentation to species of lower nuclearity under the reaction conditions, as occurs with the edge-bridged precursor [Ru3(p-H)(p3-dmdab)(CO)g] (36) (Table 1, entry 23). Usually, however, the lack of kinetic studies has prevented the verification of the nuclearity of the catalytic species, although polynuclear intermediates have been tentatively postulated; for example [Ru3(p-H)2(p3-Ph2C2)(CO)9] (4) has been postulated as the catalytic intermediate in the hydrogenation of diphenylacetylene when tri- and tetranuclear ruthenium carbonyl clusters such as [Ru3(C0)12], [Ru(p-H)4(C0)12]or [Ru&-H)2(C0)13] are used as catalyst precursors, but nothing is known about the catalytic cycle. It seems clear that face-bridging ligands are far more efficient than edge-bridging ligands at maintaining cluster integrity during catalytic reactions. The few studies made with catalytic precursors containing phosphine ligands have revealed that the presence of such ligands in the clusters generally results in slower catalytic rates and sometimes in ligand degradation via P-C or C-H bondactivation processes. Current data do not, therefore, warrant recommendation of the use of phosphine-substituted clusters as catalyst precursors for hydrogenation reactions. When catalytic intermediates have been characterized, at least three metal atoms are directly involved in the reactions. The observed types of coordination and activation of the substrates in the clusters might have mechanistic implications in heterogeneously catalyzed reactions in which the substrates are bound to a metal surface. Therefore, from these comments, it can be concluded that the results surveyed in this review can only be considered as preliminary in a field where much more work is needed: (a) many more ruthenium carbonyl clusters, containing different types of ligands, have to be tested as catalyst precursors; (b) selectivity studies involving the hydrogenation of organic substrates containing several unsaturated groups have not yet been conducted; and (c) mechanistic studies with new and known catalyst precursors are essential for ideal control of the activity and selectivity of a particular catalytic process.
References [ I ] See, for example: a) Handbook of Heterogeneous Catalysis (Eds.: G. Ertl, H. Knozinger, J. Weitkamp), VCH, Weinheim, 1997; b) Surface Organometallic Chemistry: Molecular Approaches to Surflce Catalysis (Eds.: J. M. Basset, B. C. Gates, J. P. Candy, A. Choplin, M. Leconte, F. Quignard, C. Santini), Kluwer Acad. Publ., Dordrecht, 1988; c) I. M. Campbell, Catalysis at Surfaces, Chapman and Hall, London, 1988.
2.5 Homogeneous Cutalysis with Ruthenium Curhonyl Cluster Complexes
739
[2] See, for example: a) J. Lewis, B.F.G. Johnson, Pure Appl. Cliem. 1975, 44. 43; b) E. L. Muetterties, Bull. Soc. Chinz. Belg. 1975, 84. 959; c ) E. L. Muetterties, ihid 1976, 85,451; d) E. L. Muetterties. Science 1977, 196, 839. [3] G. Siiss-Fink. G. Meister, Adc. Orgunornet. Chem 1993, 35, 41. [4] L. N. Lewis, Chem. Rev. 1993, 93, 2693. [51 R. M. Laine, J. Mol. Cutal. 1982, 14, 137. [6) W. L. Gladfelter, K. J. Roesselet, in The Chemistry qf Metal Cluster Cowplexes (Eds.: D. F. Shriver, H. D. Kaesz, R. D. Adams), VCH. New York, 1990, p. 329. [7] E. L. Muetterties, H. J. Krause, Angew. Chem. Znt. Ed Engl. 1983, 22, 135. [8] J. A. Cabeza, J. M. Fernindez-Colinas, A. Llamazares, Synlett 1995, 579. [9] R. Whyman, in Transition Metal Clusters (Ed.: B. F. G . Johnson), Wiley, New York, 1980, p. 545. [lo] L. Marko, A. Vizi-Orosz, in Metal Clusters in Cuta1j:u.y (Eds: B. C. Gates, L. Guczi, H. Knozinger), Elsevier, Amsterdam, 1986, p. 89. 1111 G. Suss-Fink, F. Neumann, in The Chemistry of’the MetalLCarbon Bond. Vol. 5 (Ed.: F. R. Hartley). Wiley, New York, 1989, p. 23 1. 1121 R. D. Adams, Ace. Chem. Res. 1983, 16, 67. [ 131 P. Braunstein. Nouv. J. Chem. 1986, 10. 366. [I41 a) P. Braunstein, J. Rose, in Chemical Bonds: Better Wuys to Make Them and Break Them (Ed.: I. Bernal), Elsevier, Amsterdam, 1989, p. 3; b) P. Braunstein, J. Rose, in Comprehensive Organometallic Chemistry, Second Edition (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson) Pergamon, Oxford, 1995, Vol. 10. Chapter 7, p. 35 1. 1151 See. for example: M. 1. Bruce, in Comprehensive Organometallic Chemistry, Vol. 4 (Eds.: G. Wilkinson, F. G. A. Stone, E. W. Abel) Pergamon, Oxford, 1984, p. 843. [ 161 H. Brunner, in Applied Homogeneous Catalysis with Oryunometullic Compounds, Vol. I (Eds.: B. Cornyls, W. A. Herrmann), VCH, Weinheim, 1996, p. 201. [ 171 See, for example: P. A. Chaloner, M. A. Esteruelas, F. Joo, L. A. Oro, Homogeneous Hydrogenation, Kluwer Acad. Publ., Dordrecht, 1994. [ 181 P. Michelin-Lausarot; G. A. Vaglio, M. Valle, J. Oryanon~et.Chem. 1984, 275, 233. [ 191 P. Michelin-Lausarot, G. A. Vaglio, M. Valle, Inory. Chim. Aetu 1977, 25, L107. [20] a) S. A. R. Knox, W. J. Koepke. M. A. Andrews, H. D. Kaesz, J. Am. Chem. Soc. 1975, 97, 3942; b) M. I. Bruce: M. I . Williams, Znorg. Synth. 1990, 28, 221. [21] R. Giordano, E. Sappa, J. Organomet. Chem. 1993. 448. 157. [22] D. Cauzzi, R. Giordano, E. Sappa, A. Tiripicchio, M. Tiripicchio-Camellini, J. Cluster Sci. 1993. 4, 279. [23] M. Castiglioni, R. Giordano, E. Sappa, J. Oryanomet. Chem. 1983, 258, 217. [24] G . Suss-Fink, unpublished results cited in reference 3, p. 63. [25] P. Michelin-Lausarot, G. A. Vaglio, M. Valle, Inory. Chim. Acta 1979, 36, 213. [26] M. Castiglioni, R. Giordano, E. Sappa, J. Ovyanomet. Chem. 1988, 342; 97. [27] M. Castiglioni, R. Giordano, E. Sappa, J. Organornet. Cliem. 1989, 362, 399. 1281 M. Castiglioni, R. Giordano, E. Sappa, J. Oryanomet. Cliem. 1989, 369, 419. [29] M. Castiglioni, R. Giordano, E. Sappa, J. Organornet. Cliem. 1991, 407, 377. [30] J. Lunniss, S. A. MacLaughlin; N. J. Taylor, A. J. Carty, E. Sappa, Organornetallies 1985, 4, 2066. [31] R. Giordano, E. Sappa, S. A. R. Knox, J. Cluster 5’c.i. 1996, 7. 179. [32] J. A. Cabezd. V. Riera, M. A. Pellinghelli, A. Tiripicchio, J. Orqanomet. Chem. 1989, 376, C23. [33] J . A. Cabeza. J. M. Fernandez-Colinas. A. Llamazares. V. Riera, Organometullics 1992, 11, 4355. [34] P. L. Andreu, J. A. Cabeza, V. Riera, Y. Jeannin, D. Miguel, J. C%em.Soc.. Dalton Trans. 1990. 2201. 1351 J. A. Cabeza, J. M. Fernandez-Colinas, A. Llamazares, V. Rierd, J. Mol. Cutal. 1992, 71, L7.
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2 Metal Clusters in Catalysis
[36] N. Lugan, F. Laurent, G. Lavigne, T. P. Newcomb, E. W. Liimatta, J. J. Bonnet, J. Am. Chem. Soc. 1990,112, 8607. [37] J. A. Cabeza, J. M. Fernandez-Colinas, S. Garcia-Granda, A. Llamazares, F. Lopez-Ortiz, V. Riera, J. F. van der Maelen, Organometallics 1994, 13, 426. [38] J. A. Cabeza, J. M. Fernandez-Colinas, unpublished results. [39] J. L. Zuffa, M. L. Blohm, W. L. Gladfelter, J. Am. Chem. Soc. 1986, 108, 552. [40] a) J. A. Cabeza, J. M. Fernandez-Colinas, A. Llamazares, V. Riera, S. Garcia-Granda, J. F. van der Maelen, Organometallics 1994, 13, 4352; b) ibid 1995, 14, 3120. [41] J. A. Cabeza, J. M. Fernandez-Colinas, A. Llamazares, V. Riera, J. Organomrt. Chem. 1995, 494, 169. [42] J. A. Cabeza, I. del Rio, J. M. Fernandez-Colinas, V. Riera, Organometallics 1996, 15, 449. [43] P. L. Andreu, J. A. Cabeza, V. Riera, C. Bois, Y. Jeannin, J. Chem. Soc., Dalton Trans. 1990, 3347. [44] J. A. Cabeza, J. M. Fernandez-Colinas, A. Llamazares, V. Riera, Organometallics 1993, 12, 4141. [45] S. Alvarez, P. Briard, J. A. Cabeza, I. del Rio, J. M. Fernandez-Colinas, F. Mulla, L. Ouahab, V. Riera, Organometallics 1994, 13, 4360. [46] P. L. Andreu, J. A. Cabeza, M. A. Pellinghelli, V. Riera, A. Tiripicchio, Znorg. Chem. 1991, 30, 4616. [47] P. Briard, J. A. Cabeza, A. Llamazares, L. Ouahab, V. Riera, Organometallics 1993, 12, 1006. [48] J. A. Cabeza, A. Llamazares, V. Riera, P. Briard, L. Ouahab, J. Organornet. Chem. 1994, 480, 205. [49] R. D. Adams, Z. Li, P. Swepston, W. Wu, J. Yamamoto, J. Am. Chem. Soc. 1992, 114, 10657. [SO] R. D. Adams, T. S. Barnard, Z. Li, W. Wu, J. Yamamoto, Organometallics 1994, 13, 2357. [51] R. D. Adams, T. S. Barnard, Z. Li, W. Wu, J. Yamamoto, J. Am. Chem. Soc. 1994,116,9103.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.6 Metal Clusters as Models for Hydrodesulfurization Catalysts Michael Brorson, Jason D. King, Keranio Kiriakidou, Fabio Prestopino and Ebbe Nordlander
2.6.1 Introduction Hydrodesulfurization ( HDS) is the catalytic process by which sulfur is removed from organosulfur compounds present in crude oil and petroleum distillates. By treatment with hydrogen gas under pressure at elevated temperatures hydrogen sulfide is formed.''] In terms of product tonnage it is one of the most important industrial chemical reactions. Because of its widespread use and its economic and environmental importance, there is considerable research into the catalytic mechanisms of the commercial HDS process and into the development of new, more efficient, catalysts. In this context, the possibility of homogeneous HDS catalysis is being explored as a potential substitute for the heterogeneous Biodesulfurization, i.e. microbial attack on organosulfur compounds, is also the subject of active research.[*] A commercial heterogeneous HDS catalyst consists of small non-stoichiometric metal sulfide crystallites located on a support surface. It is difficult to obtain detailed information about the active site(s) of such a catalyst under the relatively extreme process conditions employed (cf. Section 2.6.3). A detailed mechanistic understanding of the chemical processes occurring is even more difficult. The use of model systems that mimic some aspects of HDS processes is therefore widespread and has led to significant advances in HDS research. Surface scientists have used ultra high vacuum (UHV) methods to study adsorption and cleavage processes of organosulfur compounds on single-crystal metal and metal chalcogenide surfaces that might be used as models for HDS catalyst^.^'] To study the molecular processes of HDS by other methods than those used in surface science, molecular metal compounds have been developed and studied. Several reviews have been published on the use of transition metal complexes in HDS This review aims to summarize some of the important results that
742
2 Metal Clusters in Catalysis
have been obtained in investigations where metal clusters are used, or might be thought of as models for hydrodesulfurization catalysts. For the purpose of this review, dimetallic compounds and polymetallic complexes without direct metal-metal bonds will be considered as clusters.
2.6.2 Industrial hydrodesulfurization Hydrodesulfurization is one of the many catalytic hydrogenation processes, collectively termed hydrotreatment, performed in a modern oil refinery.".' 1' Particular oil streams (characterized by, for example, their boiling point range and/or their origin in the refinery flow-chart) are treated with 10-140 atm hydrogen gas at 300400 "C in the presence of different kinds of heterogeneous CoMo-, NiMo- and NiW-containing catalysts. Several hydrogenation processes occur. Aromatic and olefinic hydrocarbons become hydrogenated and organic compounds that contain sulfur and/or nitrogen are converted into the corresponding heteroatom-free molecules and H2S and NH3 are liberated. These latter processes are denoted HDS and HDN (hydrodenitrogenation). The HDS process is usually simple abstraction of sulfur from the hydrocarbon skeleton. Cracking rarely occurs. Thus HDS of alkyl sulfides gives alkanes. Sometimes, however, sulfur abstraction is preceded or succeeded by hydrogenation. HDS of thiophene gives, for example, butadiene, butene and/or butane. The main purpose of the HDS ( H D N ) process is to remove sulfur (nitrogen) from fuel so that SO2 (NO,) emissions to the atmosphere are avoided or reduced when the fuel is combusted. The H2S produced by the HDS process is typically converted into elemental sulfur by the Claus process (2H2S SO2 + 3s 2H20). Hydrotreatment also serves the additional important purpose of preventing sulfur and nitrogen from reaching and poisoning noble metal catalysts and zeolite-containing cracking catalysts, respectively, used down-stream in the refinery operation. The types of organosulfur compound present in petroleum feedstocks are alkyl and aryl thiols (RSH), thioethers (RSR'), disulfides (RSSR'), and thiophenic compounds (Fig. 1). The ease with which sulfur is abstracted depends very much on the nature of the sulfur-containing molecule; aliphatic compounds (thiols, thioethers) are usually desulfurized much more easily than heteroaromatic (e.g. thiophenes, benzothiophenes, dibenzothiophenes). Among the latter, reactivity decreases in the order thiophene > benzothiophene > dibenzothiophene. The presence of aliphatic substituent groups can sometimes alter reactivity. The sterically hindered compound 4,6-dimethyldibenzothiopheneis, for example, very difficult to desulfurize.
+
+
2.6 Metal Clusters us Models f o r Hydrodesulfiurizution Cutulysts
RSH
RSR
743
RSSR
Figure 1. Organosulfur compounds present in petroleum feedstocks are thiols, thioethers, disulfides, thiophenes, benzothiophenes and dibenzothiophenes.
2.6.3 The active phases in heterogeneous HDS catalysts Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m2 g-') alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes; whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m3 volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. The nature of industrial HDS catalysts is complex and the system is difficult to characterize. The catalysts contain several phases and a small fraction of active sites. Much effort has been devoted to elucidating the nature of the active sulfide phases. It is generally accepted that these phases consist of small MoS2 crystallites located on the oxidic support surface. Bulk MoS2 consists of stacked S-Mo-S layers with only weak van der Waals S-S interactions between the layers. The crystallites present on the support surface are just a few layers high. The role played by the promoter atoms (Co or Ni) has been an issue of much controversy and several structural models have been proposed. Mossbauer emission spectra of sulfided CoMo catalysts show a dominating spectral component that does not correspond to any of the known bulk Co compounds (e.g. Co9Ss or CoA1204) and which is attributed to the so-called Co-Mo-S phase." ']This phase consists of MoS2 crystal'] lites that have Co coordinated to sulfur atoms located at the crystallite The use of Ni- or Co-promoted molybdenum-based catalysts in refinery opera-
744
U
2 Metal Clusters in Catalysis
1 0l8
s \
4
u E
a
rl
i
-..
B 4
10"
> 0
w 0 'u 0
s rl a
U
1Ole
rl
B
IV
v
VI
VII VIIIVIIIVIII IB
Periodic p o s i t i o n
Figure 2. The HDS activities of (unsupported) high surface-area metal-sulfides show a systematic variation across the transition periods.['*] For the second and third transition periods, the metals in the middle of the periods form sulfides with very high activity. Promotion of MoS2 with Co increases its activity dramatically (not shown).
tions is based on economics. Other metal sulfides, particularly those of the precious metals of the second and third transition periods, are also active HDS catalysts but would be prohibitively expensive to use on an industrial scale. Systematic studies"'] have shown that the maximum activity of high-surface area, bulk sulfides is obtained for the group 8 metals ( R u and 0 s ) (Fig. 2). MoS2 is not very active, but promotion with Co or Ni brings its activity up to a level similar to that of the precious metal sulfides.
2.6.4 The mechanism(s) of HDS catalysis The mechanism of HDS is a matter of great uncertainty and therefore subject to considerable research effort. According to a popular model, sulfide vacancies are
2.6 Metal Clusters us Moc1el.sfor Hydrodesulfurizution Cutulysts
145
present in steady-state concentrations on the metal sulfide crystallite faces or edges (cf. Ref. 10 and references therein). These vacancies abstract sulfur from organosulfur species and are constantly being regenerated somewhere on the crystallite when metal-sulfide sulfur reacts with H? to give HlS. Several investigators have studied the mechanism( s) of desulfurization of sulfurcontaining hydrocarbons, in particular thiols, on single-crystal metal surfaces under ultra-high vacuum.i131By systematic studies of the interactions of thiols with metal surfaces, particularly Mo( 1 lo), Friend and coworkers have found that the cleavage of the sulfur-hydrogen bond to form a metal-bound thiolate is, in general, rapid whereas the cleavage of the carbon-sulfur bond seems to be rate-limiting. On Mo( 1 lo), the temperature required for hydrogenolysis of thiols is correlated to the C-S bond strength. Both homolytic and heterolytic C-S bond cleavage have been detected.[',' 31 Several mechanisms for HDS of thiophene based on reactor studies have been presented. It has not yet been established, however, whether thiophene undergoes initial C-S bond cleavage to give butadiene that is hydrogenated to a mixture of C4 products, or the thiophene is initially hydrogenated to di- or tetrahydrothiophenes that subsequently yield the C4 products.[' 0,141 Insights into the mechanism of HDS have come from spectroscopic and thermal desorption studies of thiophene chemisorbed on single crystals of metals and metal sulfides. These surface studies indicate that the geometry of chemisorbed thiophene depends on the metal, temperature and surface concentration. Under conditions of high coverage it is found that thiophene binds to the metals through sulfur at an angle of 130". Recent studies on molecular complexes and on surfaces show that the plane of the thiophene ring is tilted with respect to the M-S vector. At low coverages, thiophene bonds to surfaces in a o-bonded mode. Of the thiophene adsorption modes that have been proposed, the most commonly cited are depicted in Fig. 3. Although advances have been made regarding possible and probable modes of absorption of sulfur-containing organic molecules, very little is known about the mechanism(s) of C-S bond cleavage and hydrogenation. The study of molecular models might be of importance for the elucidation of these mechanisms. Metal clusters offer advantages as models for catalyst surfaces because: i) it is possible to study the substrate-metal interaction(s) in great detail by analytical techniques not available to surface scientists, e.g. NMR spectroscopy and X-ray crystallography; and ii) it is possible to study substrates which have such relatively complex structures that it is currently very hard or impossible to study them by conventional surface scientific methods. Thus, although the use of metal clusters as surface models cannot replace surface science, it might complement surface science in many important aspects.
146
2 Metal Clusters in Catalysis
S-bound
M
q4(S)-y2-bound
q2-bound
q4-bound
$-bound
M q4(S)-~3-bo~nd
ring-opened
Figure 3. Documented coordination modes of thiophene and/or its derivatives (cf. ref. 3 and references therein).
2.6.5 Model clusters and model reactions In this section, specific examples of the use of metal clusters in modeling HDS processes, e.g. adsorption of sulfur-containing organic compounds and desulfurization, will be reviewed. The metal cluster complexes used to model HDS events can be divided into two categories: i) organometallic, usually low-valent (oxidation state < 2) transition metal clusters, usually containing carbonyl ligands; and ii) high-valent (oxidation state 2 2) transition metal clusters; these usually incorporate chalcogenides in the cluster core. Both categories usually contain direct metal-metal bonds. The former type of cluster has been used mainly to study (possible) adsorption modes of thiols, thioethers, and thiophenes but desulfurization of thiophenes has also been effected by metal carbonyl clusters. In terms of metal oxidation states, coordination numbers and coordination geometries, the latter type of metal cluster shows greater resem-
2.6 Metal Clusters as Models jbr Hydrodesulfurization Catalysts
741
blance to commercial HDS catalysts than the low-valent clusters. Furthermore, the reactivities of these high-valent clusters seem to be more closely related to those of the HDS catalysts. Occasionally actual desulfurization activity has been observed.
2.6.5.1 Low-valent transition metal clusters The use of transition metal carbonyl clusters as models for catalyst surfaces is based on the cluster-surface analogy proposed and investigated by Muetterties and coIn this context, the cluster is viewed as a small fragment of a surface. It is, of course, realized that the metal atoms of the clusters do not have properties which are similar to those of bulk metals. It is, however, believed that the binding of molecules on surfaces and metal clusters is similar, because the coordination of a molecule on a surface is essentially a localized phenomenon. An overview of reactions of thiols, thiophenes, and thioethers with low-valent metal clusters, and their possible relevance to HDS adsorption and desulfurization phenomena, is given below. A plethora of reactions of transition metal carbonyl clusters with sulfurcontaining hydrocarbons has been reported in the chemical literature. Because it is not possible to give a comprehensive review of this chemistry here, we have limited the overview to some of the more well-studied reactions and clusters, with an emphasis on the chemistry of di- and trinuclear clusters of the iron and cobalt triads. Much of the chemistry described here has not been developed with the specific purpose of modelling HDS processes; it is, nevertheless, relevant.
Reactions with thiols When thiols are reacted with dinuclear complexes, S-H bond cleavage often occurs without S-C bond scission to give products containing bridging thiolate 1igands.l' 71 Similar products can be obtained from diorganodisulfides, the reaction of which with mono- and dinuclear complexes often results in S-S bond cleavage without S-C bond rupture." 1' Occasionally the S-C bond is also broken and sulfide bridges are found in the products. Representative reactions with thiols and disulfides, leading to complexes 1-6, are illustrated in Schemes 1 and 2, respectively. Conversion of such sulfide bridges to p-SH ligands on thermal reaction with hydrogen has been demonstrated, e.g. in the formation of [Cp'Mo(S)(SH)]2 from [Cp'MoS?]z (Cp' = methylcyclopentadienyl).~'91 A related reaction["] is the reduction of polymeric ( C P Z M O Z S to ~ ) ,[CpMo(S)(SH)]2 ~ by LiEt3BH (Cp = cyclopentadienyl). The product complexes have been shown to catalyze the reduction of elemental sulfur to hydrogen sulfide in the presence of hydrogen. These studies provide some support for an HDS mechanism of the type proposed by Rauchfuss'"] in which reversible formation of SH groups from metal-bound sulfide is necessary for both hydrogenation of a coordinated organic sulfur species and removal as hydrogen sulfide of the sulfur atom expelled from the catalyst surface.
148
2 Metal Clusters in Catalysis R
R'SH R=Me
(OC)&pMo-
R'SH
Me0
R'= Et (la), 'Pr (lb)
R=C02Me
Me0
R'='BU (2)
R'= Et (3a), 'Pr (3b)
Scheme 1
It is well known that trinuclear carbonyl clusters of the iron triad (Fe, Ru, 0s) can readily cleave aryl groups from phosphines and other ligands.[22]The cleavage of carbon-sulfur bonds in CS2 and related ligands by triosmium clusters has been demonstrated,[231and it has also been found that such cleavage of alkyl or aryl sulfide molecules is Two coordination modes are known for thiolates bound to trinuclear carbonyl metal clusters (Fig. 4): the bridging mode, in which the sulfur atom bridges two metal atoms, thus acting as a formal three-electron donor, and the triply bridged ( p 3 )arrangement, in which the sulfur atom acts as a five-electron donor. These coordination modes can be expected to be found when thiols are adsorbed on a metal surface and/or an active phase of an HDS catalyst. The first studies on the interactions between trinuclear iron carbonyl clusters and thiols were reported by DeBeer and H a i n e ~ . ' ~Secondary ~] alkanethiols RSH [R = Pri,Bus] were reacted with [Fe3(C0)12] under reflux in benzene with the aim of synthesizing [Fe(C0)3(SR)]*.In addition to the desired product, new tri- and tetranuclear sulfido-bridged derivatives of [ Fe3(C0)12] were isolated. Initially, [Fe3(CO)&-H)( p3-SR)] was obtained. The corresponding reaction involving Bu'SH similarly afforded [Fe3(CO)~(p-H)(p3-SBuL)] (7). Prolonged heating of a benzene solution of this thiol and [Fe3(CO)12] yielded four
2.6 Metal Clustevs us Model.s,for Hydrodesuljiuvizution Cutulysts
749
6
Scheme 2
products: [Fe(C0)3(SBu')]2, [Fe3(C0)9(p3-SBu')2], [Fe4(C0)l2(S)(SBuf)2],and [Fe3(CO)gS2].[241 The molecular structures of the above mentioned compounds have been r e p ~ r t e d . ' ~ The first investigation of reactions of [ R u ~ ( C O ) ,and ~ ] [Os3(CO)12]with thiols dates back to 1967.[27]Some similarity in their chemical reactivity to that of 53261
R
Figure 4. Coordination modes of thiolates bound to trinuclear transition metal carbonyl clusters.
2 Metal Clusters in Catalysis
750
'Bu
I
7
[Fe3(C0)12]was expected, but whereas reactions of the iron cluster normally lead to cleavage of the trimeric unit, the ruthenium analog seems to give stable trinuclear species. This was correlated with an increase in the stability of metal-metal bonds on descending the transition metal triad.1281 For instance, when R = Et, Bu" or in the case of osmium, Ph, the reaction in benzene or toluene under reflux ~ ~ ~ structure of affords [ M ~ ( C O ) ~ O ( ~ - H ) ( ~( -MS = R )Ru, ] 0 s ) ~ o m p 1 e x e s . lThe [Os,(CO)lo(p-H)(p-SEt)],synthesized in octane under reflux, has been reported.[30] In each reaction, oxidative addition of the thiol to the cluster via S-H bond cleavage is observed. The mechanism by which this process occurs is unknown but one proposal features initial formation of the cyclic product, [M3(C0)12(RSH)] (8), in which S and H are bound to two different metals (Sa, 8b). The acyclic species [ M ~ ( C O ) I ~ ( S R ) (might H ) ] then be produced before loss of two carbon monoxide molecules to give the product [M3(CO)~ & L - S R ) ( ~ - H ) ] . [ ~ ~ ] -
R
\
S-H
M = Ru @a),0 s (8b)
The mechanism of addition of thiols to the labilized cluster [ O S ~ ( C O )MeCN)] II( has been elucidated. The kinetics to form [ O S ~ ( C O ) I O ( ~ - H ) ( ~(9) - S Rclusters )] of reactions of the hindered thiols ortho-, meta- and para-thiocresol, thiophenol, benzyl mercaptan, 2-naphthalenethiol, cyclopentanethiol, and cyclohexanethiol with [Os3(CO)I 1 ( MeCN)] have been studied by UV-visible spectrophotometry and
2.6 Metirl Clusters us Model.s,fbr Hq'drode.su~uri3utionCirtolq'sts
151
NMR spectroscopy. It has been possible to detect, isolate, and identify the intermediate species [ O S ~ ( C O ) I ~ ( R S H in) this ] ; cluster, the thiol is bound via the sulfur atom and also through an agostic interaction between the thiol hydrogen and the metal (Scheme 3). The kinetic measurements are consistent with the mechanistic scheme depicted. It is possible to trap and preserve the intermediate at low temperature (-60 "C) for a long time (hours, occasionally days). NMR studies on I3Cenriched samples of these complexes support the proposed structure for the inter'] mediate ~pecies.1~ The nature of the product formed from the reaction of a thiol with [ R u ~ ( C O is) ~ ~ ] dependent upon the sulfur s u b s t i t ~ e n t .When l ~ ~ ~ R is a primary alkyl group such as Et or Bun, the decacarbonyl complex [Ru3(CO)lo(pu-H)(p-SR)] is p r o d ~ c e d . l ~ It ~ l has been found, however, that when R is Bu', the SBu' ligand caps the trimetallic B U ' ) ~ . Isimilar ~ ~ ] behavior observed for the face in [ R U ~ ( C O ) ~ ( ~ - H ) ( ~ ~ - SAlthough analogous iron system has been attributed to steric influences,i241the variation in the basicity of sulfur brought about by changing R cannot be ignored.'"] The is converted into [Ru~(CO)lo(p-H)(p-SBu')] complex [Ru3(C0)~(p-H)(p3-SBu')] under 15 atm CO, and subsequent decarbonylation to regenerate [Ru3(C0)9(pH)(p3-SBu')],can be effected either by melting (97 "C) the solid complex or heating briefly in boiling h e ~ a n e . 1 ~ This ~ 1 facility for interchange between p and p3 modes of sulfur-ligand coordination is also observed for the isopropyl analogs of the But derivatives described a b o ~ e . 1 ~ ~ 1 Another interesting example of the reactivity of thiolate-bridged trinuclear carbonyl clusters is represented by the thermal degradation of the complexes [OS~(CO)I~(~-H)(~-SR)].["~ When R = CH2Ph, reductive elimination of the aryl substituent as toluene occurs with concomitant generation of sulfido-osmium carbony1 cluster compounds such as [Os3(CO)9(p3-S)];an analogous process occurs when R = CgH51361or CgFs. For the benzylthiolate derivative only, a second, competing reaction occurs in which C-S bond homolysis leads to elimination of dibenzyl and formation of hydride-containing sulfido-osmium carbonyl cluster^.'^ 51 These reactions demonstrate that stoichiometric desulfurization of aryl thiols to arenes can be effected by trinuclear osmium carbonyl clusters. Triply bridging thiolate complexes of osmium carbonyl clusters are less common than for iron. The [ O S ~ ( C O ) & L - H ) ( ~ ~complexes -SR)] ( R = Me, Et) are unstable but can be conveniently trapped as ethylene adducts; subsequent regeneration of the parent complexes is facile. The X-ray structure of one such n-coordinated ethylene has been Very few examadduct, [OS~(CO)~(~-C~H~)(~-H)(~~-SM~)], ples of Ru clusters with p3-SR coordination ( R = Bu', Pr') have been reported in the literature.[32]
Reactions with thiophenes Although thiophene in general does not readily coordinate to transition metals, even to the electronically softer ones, compounds with thiophene coordinated in
752
2 Metal Clusters in Catalysis
2.6 Metal Clusters as Model.s,for Hyd~ode,sulfurizLltionCatalysts
153
various ways have nevertheless been reported.l3’’ One of the very first reports of the interaction of thiophene with organometallic reagents described the desulfurization of the heterocycle by a metal cluster; Stone and coworkers discovered that thiophene reacts with [Fe3(CO)12]to give the ferrole [Fe2(C4H4)(C0)6](loa) (Scheme 4).1381 The product can be described as an Fe(C0)3 fragment coordinated by a fivemembered ferracycle. The ring-inserted Fe(C0)3 moiety can be viewed as having been substituted for the sulfur atom of the thiophene. Further ~ t u d i e s [ ~ dem~,~~] onstrated that methylthiophenes and benzothiophene react with tri-iron dodecacarbonyl to give thiaferroles (11, 12) by regiospecific insertion of an Fe(CO), group into the less hindered C-S bond. A parallel reaction for dibenzothiophene does not occur. On heating, the thiaferroles are easily converted to the corresponding ferroles by extrusion of the sulfur atom; the facility of these conversions for both 2-methylthiaferrole and 2,5-dimethylthiaferrole demonstrates that M protons are not involved in desulfurization. Benzothiaferrole does not readily desulfurize to benzoferrole on heating.[401 Aime and coworkers[41]have shown that the ferrole [Fe2(C,H,)(CO)6] reacts with hydrogen to give butadiene and but-l-ene as the organic products. Although the benzathioferrole does not convert thermally to a benzoferrole, hydrogenation does bring about desulfurization from which ethylbenzene and 2-ethylbenzene thiol are the major organic pr0ducts.[~’1 This was the first example of metal-assisted opening and hydrogenation of benzothiophene. Metal fragment insertion into one of the rings of 2,2/-bithiophene occurs on reaction with [Fe3(CO)12],1421 thus yielding the thiaferrole derivative [Fez(pC4H3S-C4H3S)(C0)6] (13a), which has been characterized by X-ray diffraction, and the ferrole derivative [Fe2(p-C4H3-C4H3S)(CO)(j](14a) (Scheme 4). The iron atom is inserted regiospecifically into the less hindered S-C bond, as for the methyland benzothiaferroles. Analogous products have been obtained where the reaction is repeated with 2,2/ : 5/,2”-terthiophene (13b, 14b). Reaction of [Ru 3 (C012] with t h i o ~ h e n e ‘proceeds ~~] via C-S bond cleavage, as for the iron c o m p ~ u n d ) ” ~to~yield ~ ~ the sulfur-free ferrole-type complex [ R u ~p-( C4H4)(C0)6] (15a), and the tetranuclear cluster [Ru4(p3-S)(p-C4H4)(C0)~1] (16a). Analogous products are also obtained from the reaction of 2-methylthi0phene‘~~I and [Ru3(C0)12](15b, 16b) (Scheme 5). In this instance the crystal structure of the tetranuclear cluster has been determined, and shown to contain separated S and C4H4 ligands. This cluster constitutes the first example of desulfurization of a thiophene in which both the extruded sulfur and the hydrocarbon residue remain coordinated within the same complex. With 2-methylthiophene cleavage of the C-H bond rather than the C-S bond can occur to generate the exo and endo isomers of [Ru3(pu-H)(p-C4H2MeS)(CO)lo] (17, 18); the exo isomer is the major isomer, as found in the equivalent reaction with the osmium compound (vide infra). In the reaction of [ R u ~ ( C O )and ~ ~ ]b en z~ th io p h e n e ,l~ insertion ~] of the metal cluster fragment into the less hindered C-S bond occurs to give the ruthenium
754
2 Metal Clusters in Cutulysis I (I)
e h
2 [r
(I)
(I)
+
4
h
m
v
z II
d
m
g
-
i
4
U
2.6 Metal Clusters as Modelsf o r Hl'drodesulfuri=utjOIl Cutalysts
755
156
2 Metul Clusters in Catalysis
analogs of benzothiaferrole and benzoferrole but, in addition, the desulfurized trinuclear ruthenaindenyl complex [RU3(C&j)(C0)6](19) is produced (Scheme 6). Similarly, regiospecific insertion into the less hindered C-S bond occurs in the reaction of 2,2/-bithiophene with [Ru3(C0)12], which thus leads to [ R U ~ ( C ~ H & ) ( C O ) ~ ] (2O),1"'] (Scheme 6). In neither reaction does C-H bond cleavage occur to yield thienyl-bridged clusters similar to the exo and endo isomers observed in the reaction with 2-methylthiophene. In contrast, when thiophenes react with triosmium carbonyl clusters it is C-H bond activation that occurs in preference to C-S bond cleavage. The direct reaction of [ O S ~ ( C O ) or ~ ~ the ] , bis-acetonitrile complex [Os3(CO)lo(NCMe)2], with thiophene'4s-46]results in the oxidative addition of a C-H bond to give the exo- and endo isomers of the p, y2-thienyl hydrido cluster [Os3(pU-H)(p-C4H3S)(CO)~~] (21a, 22a) (Scheme 7). These isomers are in rapid equilibrium at room temperature. The (23a) also results from his reaction, precomplex [Os3(p-H)2(p3-C4H2S)(C0)9] sumably by additional thermal C-H bond activation and decarbonylation of the thienyl complex. This cluster contains a triply bridging cyclic alkyne ligand related to p3-benzyne.["'I The related reaction of [OS~(CO)IO( NCMe)2] with 2-methylthi0phene[~~*~'] results in the formation of [ O S ~ ( ~ - H ) ( ~ - C ~ H ~ M ~(21b, S ) ( C22b); O ) , proton ~] NMR indicates that this thienyl cluster also exists as exo (major) and endo (minor) isomers. The thienyl compounds that are formed with thiophene and 2-methylthiophene can be compared with the reported fury1 complex [Os3(p-H)(pC4H30)(C0)10], which is formed in a similar way from f ~ r a n . [ " ~ * ~ ' 1 Analogous reactions with 2,2'-bithiophene and 2,2' : 5',2"-terthiophene give the oxidative addition products [Os3(p-H)(pU-C4H2S-C4H3S)(CO)~~] (21c, 22c) and [ O S ~ ( , U - H ) ( ~ - C ~ H ~ S - C ~ S)(CO) H ~ S - 101 C ~(21d, H ~ 22d ), respectively (Scheme 7).[421 In these examples more than one thiophene ring was expected to react to give ligand bridges between di- or tri-nuclear metal systems, but instead it was found that the chemistry is closely related to that of thiophene itself with C-H bond cleavage occurring to afford exo and endo thienyl isomers. Subsequent thermolysis effects further C-H bond cleavage and decarbonylation to give triply-bridged complexes analogous to those described above for the thiophene derivative (23c, 23d). In contrast with the reactions already discussed, reaction of osmium carbonyl clusters with the substituted thiophenes 2-formylthiophene and 2-methylaldiminethiophene proceeds via oxidative addition of a C-H bond from the substituent group to generate complexes 24 and 25. In the reaction of 2-methylaldiminethiophene, initial imine coordination at one metal center might induce reactivity of the heterocyclic ring at another metal ~ e n t e r . 1 ~With ~ 1 2-formylthiophene, the bridging acyl complex [Os3(pU-H)(p-COC4H3S)(CO)lo] (24), is the major product (26), is also obbut a second product, [0s3(pu-H)(SCH=CHC=CCHO)(CO)lo] tained.'"] In this second product, the thiophene is metalated at a ring site (Scheme 8). On heating, [Os3(pu-H)(p-C0C4H3S)(CO)1o] (24) readily undergoes double deS ) ((Scheme C O ) ~ ] 8). carbonylation to give the cluster [ O S ~ ( ~ - H ) ~ ( , U - C ~ H ~(27)
2.6 Metal Clusters us Models for Hydrodesulfurization Catalysts
t
20 1
..
Po,,
Scheme 6
757
158
2 Metal Clusters in Catalysis
E = Se (28a), Te (28b)
R
L
Refluxing cyclohexane L'XO
(21a-d)
R
23a-c Scheme 7
0s
-
( W 3
2.6 Metal Clusters as Models f o r HydrodesuIJurization Catalysts
+ d
N
t-
N
m
759
2 Metal Clusters in Cutalysis
760 F
S
0,:
(OC)40s-
24
-
25
Although C-S bond cleavage reactions between thiophenes and osmium clusters have not been observed, selenophene and tellurophene undergo ring opening reactions with [OS~(CO)IO( NCMe)2] to give complexes 28a and 28b (Scheme 7).[461 It is likely that the sulfur-extrusion reactions of iron and ruthenium carbonyl clusters with thiophenes proceed via ring-opened intermediates of this type. Pursuing the idea of employing functionalized thiophenes to introduce the heterocycle into the coordination sphere of a cluster, the known ligand diphenyl-2-thienylphosphine has been reacted with [ R u ~ ( C O )to~ ~ yield ] the cluster [Ru3(p-H)(pUPh2PC4H2S)(C0)9] (29), which is cyclometalated at the thiophene ring, and a PhzPC4H3S)I (30) (Scheme 9).[5 small amount of [Ru3(pU-H)(p-Ph2PC4H2S)(CO)8( Thermal treatment of [Ru3(pu-H)(p-Ph2PC4H2S)(C0)9] (29) with [Ru3(CO)12]leads to the formation of two tetranuclear clusters - [Ru4(p4-PPh)(p4-C4H2S)(CO) I I] (31), by elimination of benzene, and the known cluster [Ru4(p,-PPh)(p4C ~ H ~ ) ( C O(32) ) ~ Iby ] elimination of thiophene. Although these two compounds are stoichiometrically equivalent, their crystal structures show that they adopt different geometries (Scheme 9). Angelici and coworkers have demonstrated that coordination of thiophenes to an IrCp* fragment (Cp* = pentamethylcyclopentadienyl) in the y4 mode leads to enhanced reactivity of the heterocycle as a donor. The monoiridium complex [Cp*Ir(y4-2,5-Me2C4H2S)](33) reacts with triply-bonded [MoCp(C0)2]2 to afford a cluster (34) in which the Mo-Mo bond is bridged by the thiophene sulfur.[52]The iridium moiety remains attached to the thiophene in the same fashion as in the starting material. Reaction of the same iridium precursor with [Fe2(C0)9] or A [Fe2(Co)8I2- also gives a product (35)with a y2, q2-Ir, p2-S(Fe2) thi~phene.”~] different reaction occurs when the same monoiridium complex reacts with [Fe3(C0)12].[~~] In the heterometallic product (36), the iridium fragment substitutes for sulfur in the same way as Fe(C0)3 to yield a ferrole but in the present case, the displaced sulfur atom remains within the coordination sphere, bridging the Fe-Ir bond and coordinated additionally to an Fe(C0)4 group. Carbonylation at room temperature replaces the bridging sulfide with a bridging carbonyl ligand, thus effecting complete desulfurization of the thiophene and yielding a heterometallic ferrole-type complex (37) (Scheme 10).
2.6 Metul Clusters us Models,for Hydrodesulfurization Cutulysts
Ph2P
0 -co
PO),
31
76 1
*
29
32
Scheme 9
This tendency for the IrCp* fragment to insert into the C-S bond of the q4coordinated thiophene has been exploited in the synthesis of new heterobimetallic complexes (38, 39) in which the six-membered iridathiacyclic ligand coordinates an iron or molybdenum center in the q6 mode (Scheme 11).1541 Clearly this type of complex might be suggested as a precursor to the iridacyclic complexes identified above in which the sulfur atom has been extruded from the ring or completely expelled from the complex. However. when the rhodium analog [RhCp*(q4-TMT)] ( T M T = tetramethylthiophene) is reacted with [Fel(C0)12]. the sulfur atom is again displaced by an Fe(C0)l group to give the five-membered ferracycle which coordinates the Rh atom in the q5 mode (40. Scheme l l ) . l s s l The complex Cp*Rh[(q4 : q'-CqMe&)Fe(C0)4] (41) has been identified as an intermediate in this reaction.' 6l This q4 coordination mode of thiophene has not been reported with cobalt but a dicobalt complex which resembles a thiaferrole has been synthesized by Jones et
'
162
2 Metal Clusters in Catalysis cy
6
0 a
,
d 0
P
=
u r n
g-
m
0
2.6 Metal Clusters as Models,for Hl'dror~esu~urizationCutulysts
iL"
aa
r;"
a
+
763
164
2 Metal Clusters in Cutalysis
0 ycoy
*
&?
cp*co'
cocp*
42
Scheme 12
al.1571Reaction of the mononuclear diethylene complex Cp*Co(CzH4)2 with thiophene yields (Cp*Co)2(pu-C4H4S)(42, Scheme 12). Extending work on the y4 mode to ruthenium, the complexes (C"e6)Ru(r4-C4R'R''2MeS) (43a-d ) have been prepared. These species can be reacted with [CpRu(MeCN)3]PF6 to afford diruthenium complexes (44a-d ) in which the thiophene has been opened to yield a thiapentadienyl ligand, coordinated in the y4 mode to one metal center and in the r3 mode to the other (Scheme 13).[581This work suggests a pathway by which the sulfur atom could be extruded when thiaferrole-type complexes are converted to ferrole-type products. Desulfurization of thiophene by iridium complexes has been further investigated by Jones and coworkers'591using the diiridium complex [Cp*IrH3]2(45).Thermal reaction of this bimetallic compound with thiophene in the presence of tert-butylethylene leads directly to a product (46) in which both of the C-S bonds have been broken, leaving sulfide and butadiene ligands bridging the two iridium centers (Scheme 14). When d4-thiophene is used in this reaction, a d4-butadiene ligand is
+
b
Ru C6Me6
R'=R"=Me; R=H (43a) R'=Me; R"=H; R=H (43b) R'=H; R"=Me; R=H (43c) R'=R"=Me; R=Me (43d) Scheme 13
44a-d
2.6 Metul Clusters us Models for Hy~~rtrod~sulfurization Cutulysts
+
+
a
0
Q
0
a
0
765
766
2 Metal Clusters in Catalysis
H2
120°C-
H
SH
0+
"Rh"
/
20°C
H2
70°C-
50
0+
Scheme 15
formed but no d3 or d5 product, indicating that the butadiene ligand is formed by transfer of two hydride ligands to the C4D4 residue. Reaction of the butadiene complex, 46, with hydrogen produces butane. Carbonylation of the product liberates the butadiene from the complex leaving the sulfide coordinated (complex 47); this can be contrasted with carbonylation of the iron-iridium complex 36 shown in Scheme 10 where CO displaces the sulfide but leaves the C4 organic residue coordinated. This reactivity of thiophenes with group 9 metal hydride complexes has been exploited in the synthesis of a mononuclear 2-vinylthiophenolate complex (49) from [( MeC(CH2PPh2)3)RhH3](48) and benzothiophene.[601Whereas hydrogenation of this ring-opened complex leads to 2-ethylbenzene thiol, reaction with [W(C0)5(THF)] leads to a bimetallic Rh-W complex with a bridging 2-ethylbenzenethiolate ligand (50), which is subsequently desulfurized to ethylbenzene on hydrogenation (Scheme 15). In contrast, hydrogenation of the ring-opened benzothiophene complex 51, shown in Scheme 16, leads to a thiolate-bridged dimanganese cluster (52) without subsequent desulfurization.[6 1 It was found very recently[62]that [{(Cp*)Ru}3(,~-H)~(p~-H),1 (53) can, in two
'
2.6 M e t d Clusters (is M o d d ~ f o r Hyu'ro~~i.sulJiri--aiion Cuta fysts
767
+
52 Scheme 16
non-catalytic reaction steps, desulfurize benzothiophene to ethylbenzene, and in the process form the cluster 54 in almost quantitative yield. In the first reaction step, carbon-sulfur bond breaking occurs under very mild conditions (50 "C, 12 h). The ,uu,-sulfido,uu,-alkylidynecluster formed is then, in a second step, under slightly more severe conditions (80 "C, 7 days, p(H2) = 7.2 atm) hydrogenated to form 54 and ethylbenzene in 84% yield. Also the very refractory
53
54
768
2 Metal Clusters in Catalysis
Co-MoIA1203 400°C
2,3-DHT
-
H2S
+
/=/
THT
Scheme 17
molecule dibenzothiophene (cf. Section 2.6.2) is desulfurized by 53 under very mild conditions (110 "C, 8 days). Here no external source of hydrogen is necessary as the products are 54 and biphenyl.
Reactions with dihydrothiophenes Catalytic reactor of 2,3-dihydrothiophene (2,3-DHT) have shown it to undergo desulfurization; this supports the proposal that it might be an intermediate in the HDS of t h i ~ p h e n e [ ~(Scheme ~ . ~ ~ ]17). Because there is also substantial conversion of this partly hydrogenated thiophene to tetrahydrothiophene (THT), it is of interest to investigate the reactivity of 2,3-DHT with metal hydride complexes to see whether hydrogen is transferred from the metal to the unsaturated heterocycle. Insertion of 2,3-DHT into the hydride-bridged cluster [ H ~ O S ~ ( C O ) ~ yields P P ~a ~ ] metalathiacyclopropane complex[66]and thus models a plausible process by which 2,3-DHT could be hydrogenated at two metal centers on a catalyst surface. In contrast, when 2,5-DHT reacts with [Ru3(C0)12],C-H bond activation occurs and hydrogen is transferred from the heterocycle to the cluster framework.[67]The product complex, [Ru3(p-H)(C0)9(p3-1-4-q4-DHT)], has the cyclometalated organosulfur residue bound via both the sulfur atom and the olefin.
2.6 Metal Clusters us Models,for Hydrodesulfurization Cutulysts
769
Di-iron nonacarbonyl reacts with 2,Sdihydrothiophene to give the mononuclear complex [Fe(CO)4(C4H6S)]in which the heterocycle donates via the sulfur atom.[681 Subsequent thermolysis brings about C-S bond cleavage giving butadiene and an insoluble black material presumed to be FeS. Reaction of 2,5-dihydrothiophene with [Rez(C0)9(MeCN)] leads to a dirhenium product which features S-bound t h i 0 ~ h e n e . I ~This ' ~ decomposes readily at 110 "C and butadiene is detected among the decomposition products. Because 2,Sdihydrothiophene does not decompose when heated at 120 "C over a period of three days, it can be concluded that the S-coordination of dihydrothiophene promotes decomposition with liberation of butadiene.
Reactions with thioethers Adams and c o ~ o r k e r s l ~have ~ 1 studied the coordination chemistry and reactivity of cyclic thioethers - particularly thiirane, thietane and their derivatives - bound to transition metal clusters. Thiirane readily undergoes (stoichiometric) desulfurization to yield sulfide-containing metal cluster^.^^^^^^^ The larger cyclic thioethers, e.g. thietane and pentamethylene sulfide, coordinate in both m ~ n o d e n t a t e i ~and ~] modes. Thietane derivatives are known to bridge metal-metal bonds and to function as bridges between metals that are not directly bonded to each It has been shown that clusters can function as catalysts in ring-opening oligomerization of thietanes, including cyclooligomerization to form thiacrown ethers. The catalysis is initiated by a nucleophilic attack by thietane itself on a thietane-containing c l ~ s t e r . [ ~ ~ . ~ ~ I DuBois and coworkers have studied metal-metal bonded dinuclear cyclopentadienylmolybdenum complexes which contain p - S , p-SH, p-SR, p-Sz, and/or p thioether ligands in different corn bin at ion^.[^^,^^^ One such complex (55) reversibly binds MezS according to Scheme 18, yielding complex 56 and thereby mimicking the binding of an organosulfur molecule to a 'sulfide vacancy' (cf. Section 2.6.4).[77]
55 Scheme 18
56
710
2 Metal Clusters in Catalysis
57
+
58 Scheme 19
is seen when A rare example of real (stoichiometric) desulfuri~ation[~~] - the labile Me2S ligand is eliminated and the thioethers are desulfurized by breaking of both C-S bonds (Scheme 19). Complex 58 is produced.
[(C~MO)~(S~CH~)(,LL-SM~)(,LL-SM~~)]+ (57) is reacted with cyclic thioethers
2.6.5.2 Transition metal sulfido clusters This section is concerned with transition metal clusters that, in addition to metal atoms, contain sulfur atoms in the cluster core (rather than just in the peripheral ligands). Metal-metal bonds often supplement sulfur-atom bridges in stabilizing the structures encountered. Generally, such clusters are likely to resemble the HDS active phases of heterogeneous metal-sulfide catalysts to some extent, e.g. in terms of coordination sphere and metallic oxidation states. Because of the large number of molecular metal-sulfide clusters now known, we shall focus on homometallic clusters of Mo( W) and heterobimetallic clusters of Mo( W)-Co( Ni) (next two sections), i.e. molecular clusters containing the elements that are relevant for industrial
2.6 Metul Clusters us Models jbr Hydrodesulfurtution Cutulysts
77 1
(heterogeneous) HDS. Only to the extent that HDS-related chemistry has been conducted shall clusters of other elements be discussed. Homometallic Mo( W ) clusters MoS2 and WS2 phases unpromoted by cobalt or nickel have HDS activity similar to that of CoMoS, NiMoS, and NiWS phases but at a much lower level. Several homometallic molecular cluster cores e ~ i s t , ~ ~e.y. * , ’the ~ ~cubane [Mo4S4Ini- (59) ( n = 4,5 or 6): the incomplete cubane [M1”3S4l4+ ( M = Mo(60a),W(60b)), the spiro dicubane IMo7Sg18+ and the partly disulfide-bridged incomplete cubane
I,
,,’
-S 7-; “ O - J
I
,S-I7Mo---
--yJ-S
,-
’
I
59
M = Mo(6Oa),W(60b)
Although all these homometallic molecular clusters have metal oxidation states in the interval +3 to +4, i.e. very close to those of heterogeneous catalysts, their current chemistry is often not directly relevant for HDS research. A few examples to the contrary are provided below. The incomplete cubane [Mo3S4l4+forms the core of the complex [Mo3(p3-S)(pS)3(SCH2CH2S)3I2-,which might be regarded[*’] as a molecular representation of the ‘sulfide vacancy’ model (cf. Section 2.6.4). The cluster complex reacts with three CH it ~ S ) ~ ] ~ - , equivalents of elemental sulfur to form [ M O ~ ( , U ~ - S ) ( , L L - S ~ ) ~ ( S C H ~i.e. binds S without change in external charge or metal oxidation state. Treatment of the disulfide cluster with [SCH2CH2SI2-, PPh3 or CN- in CH3CN quantitatively regenerates the starting material. In another reaction of [Mo3S4I4+,its aqua complex [Mo3S4(H20)9I4+ reacts[”I with acetylene at room temperature in 1 M HC1 to yield complex 61, according to Scheme 20. Here a bond is formed between a carbon atom within a hydrocarbon molecule and a sulfur atom of a metal-sulfide cluster. The adduct can be viewed as representing the transition state or intermediate presumed to be formed when an organosulfur molecule reacts with a sulfide vacancy of a heterogeneous catalyst (cf. Section 2.6.4) or a cluster. However, whereas the reaction depicted in Scheme 20 is the first step of a sulfurization process (C-S bond formation), the direction of the conversion would have to be reversed to make it the second step of a desulfurization process (C-S bond cleavage).
772
2 Metal Clusters in Catalysis
61 Scheme 20
Boorman and coworkers have demonstrated that the ditungsten complex [Cl3W(p-SEt2)3WC13]is susceptible to attack by the anions SR-, SeR-, Br-, and H-. This results in cleavage of a C-S bond in the bridging ligand and formation of the complex anion [C13W(pu-SEt)(p-SEt2)2WC13]which now features a bridging ethanethiolate ligand. With the related tetrahydrothiophene complex [Cl3W(pTHT)3WC13], the same anions again result in C-S bond cleavage leading to the (62a-d) where X- is the ring-opened products [C13W(pU-THT)2(S(CH2)4X)WC13]attacking anion. In these reactions, it seems that S-coordination labilizes the C-S bond, thus promoting nucleophilic attack with concomitant C-S bond cleavage. When the attacking nucleophile is hydride, the reactions may model those occurring in the hydrodesulfurization process.I81'
X = SR(62a), SeR(62b), Br(62c), H(62d)
Heterobimetallic Mo(W)-Co( Ni) clusters This section is concerned with molecular clusters that contain the same elements as the active CoMoS/NiMoS/NiWS phases in heterogeneous HDS catalysts. One
2.6 Metul Clusters us Models for Hydrodesuljiurizution Cutulysts
773
particular Co-Mo-S cluster can effect non-catalytic desulfurization of a wide range of sulfur-containing organic compounds.
Co2M02Sn clusters in solution with [Fe2(CO),], [ C O ~ ( C O or )~] Reaction of [(Cp)2Mo2(pU-S)2(p-SH)2] [Ni(C0)4]gives[s31the tetranuclear bimetallic clusters [ C ~ ~ M O ~ F ~ ~ ( C O ) ~ ( ~ , - S ) ~ ( , U S)2 I (63), [ C P ~ M Q (CO)4( C ~ ~ ~ 3 - 2s(~14-S)] ) (64) and [ C ~ 2 M oNi2 2 (CO)2(~3 -S)4] (65), respectively. Whereas the iron-molybdenum and nickel-molybdenum clusters are electron-precise, the cobalt-molybdenum cluster is electron deficient and contains the unusual p4-S ligand.
65
66
Under 15 atm H2 at temperatures between 110 and 150 “C, [(Cp’)2M02C02(CO)4(p3-S)2(,u4-S)] (64) was foundls4] to have the property of abstracting sulfur from thiophene to effect almost quantitativels5] conversion to [(Cp’)2M02C02(C0)2(p3-S)4](66) with liberation of a mixture of saturated and unsaturated C2, C3, and C4 hydrocarbons. If the same reaction is performed in the absence of H2, the same cluster product is obtained along with a black, insoluble solid. In neither reaction does thermal decomposition of the reactant cluster occur. (64) vis-aThe sulfur-abstracting capacity of [(Cp’)2M02C02(C0)4(p3-S)2(p4-S)] vis sulfur-containing molecules other than thiophene has been studied. For example, whereas sulfur abstraction from thiophene produces a mixture of hydrocarbons cracked to various extents (but notably no butadiene, which is a product of HDS of thiophene over heterogeneous catalysts), a completely clean reaction is seen for thiols (RSH) which are desulfurized to RH.[s51Thiophenol ( PhSH) is desulfurized to give [(Cp’)2M02C02(C0)2(p3-S)4](66) in 80%) yield and benzene as the sole
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2 Metal Clusters in Catalysis
organic Substituted thiophenes were found to be less reactive than the parent compound; whereas thiophene is fully desulfurized, the conversion ratios are substantially smaller for 2-methylthiophene (30%), 3-methylthiophene (30%) and 2,5-dimethylthiophene (20%). When conditions similar to those described above are employed, benzothiophene, PhSMe and PhSCH=CH2 were not desulfurized in the presence of [(Cp’)2M02C02(CO)4(p3-S)2(p4-S)] (64).[851Sulfur to is abstracted from the molecule COS by [(Cp’)2M02C02(C0)~(p3-S)~(p4-S)] give [(Cp’)2M02C02(C0)2(p3-S)4]in 50% yield.[851In what begins to resemble a catalytic cycle, [(Cp’)2M02C02(C0)2(p3-S)4] can be converted back to [(Cp’)2M02C02(C0)4(p3-S)2(p4-S)] by means of CO (70 atm CO at 150 “C) with COS as the only gaseous reaction product.[841The yield of cluster is, however, only 20%. Back conversion by means of (pure) H2 does not occur. To gain an insight into the mechanism by which [(Cp’)2M02C02(C0)4(p3-S)2(p4S)] desulfurizes sulfur-containing organic compounds, its reaction with para-toluenethiolate ( MeC6H4SP) was studied at -60 “C by solution NMR spectroscopy.[s6] The thiolate ion was found initially to bind to a cobalt atom to form a red adduct. When the temperature of the solution was increased to -24 “C the red adduct was transformed to a green compound where the thiolate bridges the Co-Mo bond. Desulfurization of the bound thiolate occurred when the bridged adduct was heated under reflux in CD3CN and the only organic product was toluene-dl . It was not, however, [(Cp’)2M02C02(C0)2(p3-S)4]but the radical anion [(Cp’)2Mo2Co2(CO)2(p3-S)4lPthat was formed and crystallographically characterized.[s61Formation of the radical cluster anion and toluene-dl indicates homolytic cleavage of the C-S b ~ n d . [ ~ ~ , ~ ’ ] The fact that the Curtis clusters are capable of cleaving carbon-sulfur bonds at temperatures several hundred degrees below those employed in the commercial heterogeneous process has been taken to suggest that it is removal of sulfur from the catalyst surface rather than C-S bond activation that is the rate-determining step in the heterogeneous system.[85]
M’Mo3S4 clusters Mainly through the work of Shibahara and coworker^,[^^,^^^ several cubane-like derivatives of [Mo3S4I4+ and, to a lesser extent, [W3S4I4+ have been prepared by introducing a heterometal (Cr, Fe, Co, Ni, Cu, Ga, Mo, Pd, In, Sn, Sb, W, Hg, T1, Pb, or Bi) into the incomplete cubane [M3S4I4+(M= Mo, W). Two types of cluster core can be formed, monocubanes [M’M3S4In+(67) and spiro dicubanes [M&-M’M3S4Im+(68),the latter containing a central heterometal with an S g coordination sphere. In both types, three coordination bonds extend from each M. In the monocubanes one or three exogenous ligands are associated with M’. The monocubanes sometimes form dimers, at least in the solid phase. Whereas the average oxidation states of the metals in the Curtis-type clusters [(Cp’)2M02C02(C0)4(p3-S)2(p4-S)] (64) and [(Cp’)2M02C02(C0)2(p3-S)4](66) are +2 and +2.5, respectively, the
2.6 Metul Clusters us Models jor Hy~~rvodt~sulfuri,.ution Catulysts
67
115
68
Shibahara-type clusters contain metals with average oxidation states of approximately f3. The chemistry of these clusters is almost exclusively aqueous. N o HDS-related chemistry has apparently been performed with these clusters, but it is notable that [ N ~ M ~ S ~ \ H ~ O( )M, O ~ + W ) takes up CO at atmospheric pressure at the Ni = ]Mo, site.190,91Carbon and nitrogen r n ~ n o x i d e ' ~are ~ . ~often ~ ] used as probe molecules in characterizing supported MoS2 and WS2 catalysts; CO (or NO) is only absorbed by the active phases of these catalysts, and it is believed that the absorption sites are identical to the active sites. Regardless of the exact absorption site, correlations between activity and CO (or NO) absorption have often been obtained. Analogously, solutions of [NiMo3 &I4+, [ NiMo2 W3 S4I4+, [ NiMoW2S4I4+, and [NiWiS4I4+react with ethylene to form complexes which contain a v2-C2H4 ligand coordinated to Ni.[951 The cluster [ C O M O ~ S ~ ( C ~ ' ) ~ (which C O ) ]contains , Mo-coordinated Cp' and Co-coordinated CO, seems to be a thermodynamic sink for the and [(Cp')2MozCoz(CO)2(pc,decomposition of [(Cp')2M02C02(CO)4(p3-S)2(p4-S)] S)4]. It is formed in 15-30% yield when, for example, a toluene solution of [ ( C ~ ' ) ~ M O ~ C O ~ ( C O ) ~ ( , Dis~reacted - S ) ~ (for ~ ~3-hS at ) ] 150 "C with 200 psig H2 and 50 psig C0.[961It seems that the reactivity of the CO ligand has not been studied. Linear, trinuclear [M'(MS4)2I2- clusters (M' = Co, Ni; M
=
Mo, W)
From the reaction of divalent, first-row hexaaqua transition metal ions and MSd2( M = Mo, W), [Co(MS4)2I2- and [Ni(MS4),I2- can be isolated, typically as quaternary phosphonium salts. The [MS4I2- ions act as bidentate ligands to the divalent metal ion, but the coordination geometries are different - tetrahedral for Co and square planar for NLi9'] These compounds model, in a structurally well-defined way, sulfide-bridged Co-Mo and Ni-Mo interactions and may serve as references for such interactions in CoMoS and NiMoS phases.
Other metals The numerous homometallic and heterobimetallic sulfide clusters that exist seldom have chemical reactivity directly relevant for HDS reactions. For ruthenium,
776
2 Metal Clusters in Catalysis
however, a rare thiophene cluster compound has been prepared. Reaction of [(cymene)RuCl2]2 with TMT gives [( TMT)RuCl2I2,which, on further reaction with (Me3Si)2S, the trinuclear cluster [Ru3(,u3-S)(p-C1)3( TMT)3]+ (69). The oxidation state of ruthenium here is +2, i.e. d6 electron configuration.
69
Complete and incomplete cubane-type sulfide clusters (e.g. [RuqS4(RCgH4)4In+, [ R u ~ S ~ ( C P * )[Rh4S4(Cp*)4]) ~]+, that correspond to the very active, unpromoted sulfides of Ru and Rh have been extensively studied by Rauchfuss and coworker~.[~~~~~~]
2.6.6 Chevrel phases: heterogeneous HDS catalysts containing molecular clusters The so-called Chevrel phases constitute a class of ternary molybdenum sulfides, often electric superconductors, with the general formula M,Mo&. The cation M can be any of nearly 40 metallic elements. For small M, x can be varied continuously within certain limits (e.g. 1.6 < x < 4 for Cu,Mo&). For large M, x is always very close to 1 (e.g. in PbMosSg). The cation-free phase Moss8 is also known.[102]All types contain as building blocks the Mo6S8 cluster unit in which six molybdenum atoms form an irregular octahedron with eight sulfides coordinated to its faces. Mo-Mo distances range from 2.65 to 2.80A (cf. 2.72A of metallic Mo). The oxidation state of molybdenum is, depending on M and x,between +2 and 12.66, i.e. much lower than that (+4)of Mo in commercial, heterogeneous, MoS2based catalysts. The thiophene HDS activities (at 400 "C, 1 atm H2) of a number of Chevrel phases were foundi'02-104]to be comparable with those of unsupported MoS2 and
2.6 Metal Clusters us Models for Hydrodesulfurizution Cutnlysts
777
unsupported cobalt-promoted MoS2. Large cation phases ( M = Ho, Pb, Sn) had higher activities than small cation phases, e.g. those containing the traditional promoter atoms Co and Ni. No signs of degradation of the Chevrel phases under catalytic reaction conditions were apparent by X-ray diffraction studies.'' 03j X-ray photoelectron spectroscopy indicated that Mo" had been formed on the surface of ~~1 the small cation phases but not on the more active large cation p h a ~ e s . 1 ~The hydrogenation activity of the Chevrel materials towards 1-butene was found to be substantially lower than that of the traditional MoS2-based catalysts." 0 2 - 04] A detailed study has been made of thiophene HDS reaction mechanisms.[' 051
2.6.7 Summary The use of transition metal carbonyl clusters and chalcogenide clusters as models for hydrodesulfurization catalysts has provided information on both possible mechanistic pathways in HDS processes and the nature of commercial HDS catalysts. Low-valent transition metal carbonyl clusters are usually used to investigate possible binding modes of sulfur-containing hydrocarbons at ( polynuclear) metal sites. Addition of thiols to clusters usually leads to the formation of bridging thiolate moieties; desulfurization of thiols to form sulfide-containing clusters has also been observed. It has been shown that the addition of thiols to certain trinuclear clusters results in the initial formation of metastable intermediates which contain thiols that are bound via agostic metal-H-S-R interactions. Metals in dinuclear complexes may insert into thiophene rings (in the s( position with respect to the heteroatom) to form metallacycles. Subsequent sulfur extrusion, yielding a desulfurized metallacycle has been detected in several cases. Coordination of cyclic thioethers with some ring strain to transition metal carbonyl clusters leads to increased susceptibility of these thioethers to nucleophilic attack. The group of transition metal sulfide clusters can be viewed as constituting discrete molecular fragments of the metal sulfide phases present in heterogeneous catalysts. However, with a few very notable exceptions, it has so far been difficult to identify sulfide cluster chemistry that directly reflects the mechanisms of HDS. The main exception is the desulfurization of e.g. thiophene by the sulfide vacancy cluster [(Cp')2Mo2Co2(C0)4(pU3-S)2(p4-S)], a reaction which closely mimics the mechanism envisaged for the heterogeneous metal sulfide catalysts. The financial and environmental importance of hydrodesulfurization reactions assures continued interest in HDS research. The knowledge of the mechanisms in commercial HDS reactions is still considerably limited; determination of the intimate mechanisms of these reactions constitutes a formidable challenge. The use of metal clusters as models for HDS catalysts is continuously developing and we might therefore expect that several new model systems will be investigated in the future.
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2 Metal Clusters in Catalysis
Acknowledgments Funding for research on hydrodesulfurization provided by the European Union (TMR Network MECATSYN - Metal Clusters in Catalysis and Organic Synthesis), the Swedish Engineering Science Research Council (TFR) and the Swedish Natural Science Research Council ( NFR) is gratefully acknowledged.
List of abbreviations CP CP’ CP* DHT HDN HDS NMR THT TMT XPS
Cyclopentadienyl Methylcyclopentadienyl 1,2,3,4,5-pentamethylcyclopentadienyl Dihydrothiophene H ydrodenitrogenation H ydrodesulfurization Nuclear Magnetic Resonance Tetrahydrothiophene Tetramethylthiophene X-ray photoelectron spectroscopy
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[48] D. Himmelreich, G. Miiller, J. Organomet. Chem. 1985, 297, 341. [49] A.J. Arce, Y. De Sanctis, L. Hernandez, M. Marquez, A.J. Deeming, J. Organomet. Chem. 1992, 436, 35 1. [50] (a) A.J. Arce, Y. De Sanctis, A.J. Deeming, J. Organomet. Chem. 1986, 311, 371. (b) A.J. Deeming, A.J. Arce, Y. De Sanctis, M.W. Day, K.I. Hardcastle, Organometallics, 1998, 8 , 1408. [51] A.J. Arce, Y. De Sanctis, A.J. Deeming, S.N. Jayasuriya, Organometallics 1996, 15, 786. [52] J. Chen, R.J. Angelici, Organometallics 1990, 9, 879. [53] J. Chen, L.M. Daniels, R.J. Angelici, J. Am. Chem. Soc. 1991, 113, 2544. [54] J. Chen, V.G. Young, R.J. Angelici, J. Am. Chem. Soc. 1995, 117, 6362. [55] A.E. Ogilvy, A.E. Skaugset, T.B. Rauchfuss, Organometallics 1989, 8, 2739. 1561 S. Luo, A.E. Ogilvy, T.B. Rauchfuss, Organometallics 1991, 10, 1002. [57] W.D. Jones, R.M. Chin, Orgunometallics 1992, 11, 2698. [58] K.M. Koczaja Dailey, T.B. Rauchfuss, A.L. Rheingold, G.P.A. Yap, J. Am. Chem. Soc. 1995,117, 6396. [59] W.D. Jones, R.M. Chin, J. Am. Chem. Soc. 1994, 116, 198. [60] C. Bianchini, M.V. Jiminez, C. Mealli, A. Meli, S. Moneti, V. Patinec, F. Vizza, Angew. Chem. Int. Ed. Engl. 1996, 35, 1706. [61] C.A. Dullaghan, S. Sun, G.B. Carpenter, B. Weldon, D.A. Sweigart, Angew. Chem. Int. Ed. Engl. 1996, 35, 212. [62] K. Matsubdra, R. Okamura, M. Tanaka, H. Suzuki, J. Am. Chem. Soc. 1998, 120, 1108. [63] (a) R.J. Angelici, Ace. Chem. Res. 1988, 21, 387; (b) E.J. Markel, N.N. Sauer, R.J. Angelici, G.L. Schrader, J. Catal. 1989, 116, 11. [64] (a) P. Desikan, C.H. Amberg, Can. J. Chem. 1964, 42, 843; (b) M. Zdrazil, Collect. Czech. Chem. Commun. 1975, 40, 3491; (c) J. Devaneux, J. Marvin, J. Catal. 1981, 69, 202. [65] (a) D.A. Lesch, J.W. Richardson Jr., R.A. Jacobson, R.J. Angelici, J. Am. Chem. Soc. 1984, 106, 2901; (b) S.C. Huckett, N.N. Sauer, R.J. Angelici, Organometallics 1987, 6, 591. [66] G.N. Glavee, L.M. Daniels, R.J. Angelici, Organometallics 1989, 8, 1856. [67] M.-G. Choi, L.M. Daniels, R.J. Angelici, Inorg. Chem. 1991, 30, 3647. [68] N.N. Sauer, E.J. Merkel, G.L Schrader, R.J. Angelici, J. Catal. 1989, 17, 295. [69] R.D. A d d m and S.B. Falloon, Chem Rev., 1995, 95, 2587. [70] R.D. Adams, Polyhedron, 1985, 4, 2003. [71] R.D. Adams, J.E. Babin and M. Tasi, Inorg. Chem., 1986, 25, 4514. [72] K. Kiriakidou, E. Kretzschmar, F. Prestopino, M. Monari and E. Nordlander, unpublished results. [73] R.D. AdamsandM.P. Pompeo, J. Am. Chem. Soc., 1991, 113, 1619. [74] R.D. Adams and S.B. Falloon, Organometallics, 1995, 14, 1798. [75] M.R. DuBois, D.L. DuBois, M.C. Vanderveer, R.C. Haltiwanger, h o r g . Chem. 1981, 20, 3064. [76] M. McKenna, L.L. Wright, D.J. Miller, L. Tanner, R.C. Haltiwanger, M.R. DuBois, J. Am. Chem. Soc. 1983, 105, 5329. [77] (a) J. Gabay, S. Dietz, P. Bernatis, M. Raskowski DuBois, Organometallics 1993, 12, 3630; (b) D.S. Tucker, S. Dietz, K.G. Parker, V. Carperos, J. Gabay, B. Noll, M. Raskowski DuBois, Organometallics 1995, 14, 4325. [78] J.R. Dilworth in Molybdenum: An Outline of its Chemistry and Uses, Studies in Inorganic Chemistry, Vol. 19 (Eds: E.R. Braithwaite, J. Haber), Elsevier, Amsterdam 1994, Chapter 4. [79] C.D. Garner in Comprehensive Coordination Chemistry, Vol. 3 (Eds. G. Wilkinson, R.D. Gillard, J.A. McCleverty), Pergamon Press, Oxford, 1987, Chapters 36.3 and 36.6. [80] T.R. Halbert, K. McGauley, W.-H. Pan, R.S. Czernuszewicz, E.I. Stiefel, J. Am. Chem. Soc. 1984, 106, 1849. [81] T. Shibahara, G. Sakane, S. Mochida, J. Am. Chem. Soc. 1993, 115, 10408. [82] P.M. Boorman, X. Gao, J.F. Fait, M. Parvez, Inorg. Chem. 1991. 30, 3886.
2.6 Metirl Clusters us Models fbv Hq’dvo~ksulfcri3utioii Cutulysts
78 1
[83] M.D. Curtis, P.D. Williams, Inorg. Chenz. 1983, 22; 2661. [84] U. Riaz, 0 . Curnow, M.D. Curtis, J. Am. Chem. Soc. 1991, 113, 1416. [85] U. Riaz, O.J. Curnow, M.D. Curtis, J. Am. Clzem. Sot.. 1994, 116, 4357. [86] S.H. Druker, M.D. Curtis, J. Am. Cliem Soc. 1995, I f 7, 6366. [87] M.D. Curtis, S.H. Druker, J. Am. C/wrn. Soc. 1997. 119, 1027. [88] T. Shibahara. A&. Inorg. Clzeni. 1991, 37, 143. [89] T. Shibahara. S. Kobayashi. N . Tsuji. G. Sakane, M. Fukuhara, Inorg. Chem. 1997, 36, 1702. 1901 T. Shibahara, S. Mochida, G. Sakane, Cliem. Letters. 1993, 89. [91] I. Schmidt, J. Hyldtoft. J. Hjortkjm, C.J.H. Jacobsen, Actu Chem. Scund. 1996, 50, 871. [92] M. Breysse. J. Bachelier, J.P. Bonnelle, M. Cattenot, D. Cornet, T. Decamp, J.C. Duchet. R. Durand, P. Engelhard, R. Frety, C. Cachet, P. Geneste, J. Grimblot, C. Guegucn, S. Kasztelan, M. Lacroix, J.C. Lavalley, C. Leclercq, C. Moreau, L. de Mourgues, J.L. Olive, E. Payen, J.L. Portefaix; H. Toulhoat, H. Vrinat, Bull. Soc. Chirn. Bely. 1987, 96, 829. [93] L. Portela, P. Grange, B. Delmon, Cutul. Rev. Sci. Eny. 1995, 37, 699. [94] N.-Y. Topsac, H. Topsae, J. Cutal. 1983, 84, 386. [95] T. Shibahara, G. Sakane, M. Maeyama, H. Kobashi. T. Yamamoto, T. Watase, Inorg. Chim Actu 1996, 251, 207. [96] M.D. Curtis, U. Riaz, O.J. Curnow, J.W. Kampf. 0rgnnonietallic.s 1995, 14,5337. [97] A. Miiller. E. Diemann in Comprehcnsiw Coordination Clzemistry, Vol. 2 (Eds: G. Wilkinson, R.D. Gillard, J.A. McCleverty), Pergamon Press, Oxford, 1987. Chapters 16.3. [98] J.R. Lockemeyer, T.B. Rauchfuss, A.L. Rheingold, S.R. Wilson, J. Ani. Chenz. Soc. 1989, l f l , 8828. 1991 E.J. Houser, T.B. Rauchfuss, S.R. Wilson, Inorg. Cliem. 1993, 32, 4069. [lo01 E.J. Houser, H. Krautscheid, T.B. Rauchfuss, S.R. Wilson, J. C h n . Soc., C/iem. Commun. 1994, 1283. [ 1011 A.E. Skaugset. T.B. Rauchfuss. S.R. Wilson, Orgunometallics 1990, Y, 2875. [I021 K.F. McCarty, G.L. Schrader, Inn‘. Eng. C h m . Prod. Res. Dec. 1984, 23, 519. [ 1031 K.F. McCarty, G.L. Schrader in Proc~edingsof the 8th Internntional Congress on Cutu Berlin, 2-6 July, 1984. pp. IV-427. [lo41 K.F. McCarty, J.W. Anderegg, G.L. Schrader, J. Cutnl. 1985, 93, 375. [I051 J.W. Benson, G.L. Schrader, R.J. Angelici, J. Mol. Cutal. A ; Chemical 1995, 96, 283. -
FULL PAPER DOI: 10.1002/ejic.200701263
Topochemical Synthesis of Micron-Platelet (Na0.5K0.5)NbO3 Particles Lihong Li,[a] Jun Chen,[a] Jinxia Deng,[a] Ranbo Yu,[a] Lijie Qiao,[b] Guirong Liu,[a] and Xianran Xing*[a,c] Keywords: Template synthesis / Niobium / Layered compounds / Ceramics / Salt effect Micron-scale platelet (Na, K)NbO3 particles were synthesized from the platelet precursor K4Nb6O17 in a KCl medium using a topochemical method . The salt took part in the reaction and affected the composition of the products. Stoichiometric (Na0.5K0.5)NbO3 could be accurately synthesized by controlling the amount of the reactant Na2CO3. X-ray diffraction analysis revealed that the crystallographic {010} plane of
K4Nb6O17 was converted into the pseudo-cubic {001} plane of (Na0.5K0.5)NbO3. The polycrystalline Na0.5K0.5NbO3 particles exhibited a plate-like shape with a high aspect ratio, and were suitable for preparing textured ceramics by the template grain growth process. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
Introduction
choice to synthesize texture (Na0.5K0.5)NbO3-based ceramics. Many efforts have been made to modify the shape of (Na, K)NbO3, such as hydrothermal and molten salt synthesis (MSS), but the platelet (Na, K)NbO3 particles are still not obtained.[8,9] The nanofingers were obtained together with cubes by hydrothermal methods and cubes alone were formed by molten salt methods. However, the composition of (Na0.5K0.5)NbO3 with a good piezoelectric performance has not been obtained both by hydrothermal and MSS routes. In these processes, the activity of Na+ is higher than that of K+, and Na+ enters into the A-site of (Na, K)NbO3 more easily, leading to difficulty in controlling the composition of (Na, K)NbO3.[8,9] Therefore, it is still a challenge to synthesize the accurate composition (Na0.5K0.5)NbO3 with a micrometer-size plate-like shape, which is suitable to be used as templates in fabricating textured ceramics. The topochemical method has successfully synthesized anisotropic particles such as SrTiO3,[10] Na0.5Bi0.5TiO3,[11] BaTiO3,[12] NaNbO3,[6] KNbO3,[13] but usually involves low temperatures and long periods of chemical modification. It is well known that the MSS is a fast and simple way to prepare complex oxides.[14,15] When the MSS route is introduced to the reaction system the topochemical synthesis is a simple and large-scale approach with relatively low temperatures and short times.[16] At present, we describe a large-scale method to synthesize the platelet (Na0.5K0.5)NbO3 particles derived from K4Nb6O17 by a topochemical micro-crystal conversion method in molten salt KCl. The structure and morphology of (Na, K)NbO3 were characterized by X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) methods. The elemental analyses were conducted by inductively coupled plasma (ICP) spectrometry. The piezoelectric property was determined, and the possible reaction mechanism in the molten salts was investigated.
Alkaline niobates have attracted great interest in recent years in the study of lead-free piezoelectric ceramics because of their considerable electrical properties and environmental friendliness. (Na, K)NbO3 (NKN) exhibits optimized piezoelectric properties in morphotropic phase boundaries (MPBs) at around 50 % K separating two orthorhombic phases or a monoclinic phase and an orthorhombic phase.[1–3] However, it is known that the piezoelectric performance of lead-free ceramics is still inferior to that of some lead-compositional ceramics.[4] One of the most effective ways to improve the piezoelectric properties of the leadfree ceramics is to fabricate ceramics with a more uniform grain orientation, i.e. textured ceramics by the (Reactive) Template Grain Growth [(R)TGG] method to mimic the properties of single crystals with the same composition. The synthesis of anisotropic seeds with a proper scale in the range of 5–50 µm is a key procedure for the preparation of textured ceramics.[5] Recently, it was reported that (Li,Ta,Sb)-modified (Na0.5K0.5)NbO3 materials with the plate-like seeds of NaNbO3 exhibited a high d33 value by the (R)TGG method.[6] Since the (R)TGG method requires anisometric particles with a lattice match of the desired final composition,[7] platelet (Na0.5K0.5)NbO3 particles is a promising [a] Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China Fax: +86-10-62332525 E-mail:
[email protected] [b] Key Laboratory of Environmental Fracture (Ministry of Education), University of Science and Technology Beijing, Beijing 100083, China [c] State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China 2186
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Results and Discussion The K4Nb6O17 sample prepared by the molten salt synthesis at 1060 °C for 2 h was characterized by XRD methods. The K4Nb6O17 particles were dispersed in ethanol and then cast on glass substrates. In this process the largest developed plane of the crystalline particles is easily aligned with the glass plane. Figure 1 shows the oriented particulate layer XRD pattern of casting K4Nb6O17 particles, and is compared with that of the noncasting sample. All of the XRD peaks of the noncasting sample can be assigned to the perovskite-type K4Nb6O17 phase (JCPDS 76–977). From XRD patterns of the casting particles the predominant diffraction intensity of the (040) peak is clearly seen, which indicates that the surfaces of the precursor are parallel to (0k0). This phenomenon is in agreement with ref.[17] Well-developed plate-like layered crystals of K4Nb6O17 are clearly seen in the insert of Figure 1. K4Nb6O17 particles are rectangular platelets with a width of 3–10 µm, a length of 10–20 µm, and a thickness of 0.5–2 µm, which are precursors for the following synthesis of (K0.5Na0.5)NbO3.
Figure 2. (a) XRD patterns of plate-like (Na, K)NbO3 particles synthesized by the TMC method from K4Nb6O17 precursor particles heated at 850 °C for 2 h with different values of x(Na2CO3); (b) Rietveld refinement profiles for NKN2. Data were refined in the space group Amm2. Figure 1. XRD pattern of K4Nb6O17 precursor particles prepared by molten salt synthesized at 1060 °C for 2 h cast on a glass substrate, and without casting. The insert is a SEM micrograph.
(Na, K)NbO3 particles were synthesized from a topochemical micro-crystal conversion (TMC) reaction by mixing the platelet precursor K4Nb6O17 with different amounts of Na2CO3 (x = 2, 2.5, 3, 5, and 6) soaked in KCl melts at 850 °C for 2 h, and the sample was marked as NKNx. The reaction involving the (K0.5Na0.5)NbO3 formation was firstly assumed to be as shown in Equation (1). When no excess Na2CO3 was added (x = 2), corresponding to Equation (1), the impurity K4Nb6O17 was detected; see Figure 2, (a). When more Na2CO3 was added (x ⬎ 2) pure perovskite (Na, K)NbO3 phases were obtained. A similar phenomenon was observed for the synthesis of KNbO3.[13]
Although the amount of Na2CO3 at x = 2.5, mixed into the starting reactants, was in excess according to Equation (1), Na0.5K0.5NbO3 was not obtained. The Na/K ratio of the synthesized sample (NKN2.5) measured by ICP is 0.266:0.730 (see Table 1). When the addition of Na2CO3 was increased to x = 5, i.e. a compound with an accurate composition of Na/K = 1, Na0.5K0.5NbO3, was achieved. When more Na2CO3 was added, such as x = 6, the Na/K ratio was higher than 1. This means that these reactions do not follow Equation (1). From the XRD analysis and the Na/K ratio it is obvious that the KCl salt takes part in the reaction. Therefore, Equation (1) has to be modified; see reaction (2).
K4Nb6O17 + x Na2CO3 + Nb2O5 씮 8 (K0.5Na0.5)NbO3 + (x – 2) Na2CO3 + 2 CO2앖 Eur. J. Inorg. Chem. 2008, 2186–2190
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(2)
Table 1. Na/K/Nb ratio and the possible formula of (Na, K)NbO3 synthesized by the TMC method at 850 °C for 2 h with different values of x (amounts of Na2CO3). Sample
Na [atom-%]
NKN2.5 13.4 (⫾0.1) NKN3 17.1 (⫾0.1) NKN5 24.8 (⫾0.1) NKN6 25.8 (⫾0.1)
K
Nb
[atom-%]
[atom-%]
36.5 (⫾0.4) 33.0 (⫾0.4) 25.1 (⫾0.3) 24.0 (⫾0.3)
50.1 (⫾0.3) 49.9 (⫾0.3) 50.0 (⫾0.3) 50.2 (⫾0.3)
Na/K ratio[a] Formula
0.266:0.730
Na0.27K0.73NbO3
0.341:0.660
Na0.34K0.66NbO3
0.496:0.503
Na0.5K0.5NbO3
0.516:0.480
Na0.52K0.48NbO3
[a] Measured by ICP.
In reaction (2) y is the amount of KCl salt that takes part in the reaction and z is the amount of K+ from K4Nb6O17 and is replaced by Na+. The K+ ions compete with Na+ ions to occupy the A-sites of (Na, K)NbO3. Because of the large amount of K+ in the KCl melts part of the K+ ions from the melts tend to enter into the A-sites in the cases where less Na2CO3 is added (x ⬍ 5). While when the amount of Na2CO3 is largely in excess (x ⬎ 5) active Na+ ions are more likely to occupy the A-sites to form (Na, K)NbO3. XRD patterns of NKN indicate the phase evolution with x; see Figure 2, (a). The XRD Rietveld method was used to refine the structure and it was found that NKN2.5 is orthorhombic (Amm2), compared with the monoclinic symmetry; see Figure 2, (b) that was reported in ref.[3] Table 2 lists the crystal structure and the lattice parameters of (Na, K)NbO3 indexed from the XRD patterns. NKN2.5, NKN3, and NKN5 were indexed in an orthorhombic symmetry (space group Amm2), while NKN6 was indexed in an orthorhombic symmetry (space group Bmmb). From the lattice parameter changes of the orthorhombic 1 symmetry it can be found that the substitution of some of the Na for K results in decreasing lattice constants, since the ionic radius of K+ (1.33) is much larger than that of Na+ (0.93). A phase transition of (Na, K)NbO3 occurs from an orthorhombic phase (JCPDS 71–946) to an orthorhombic phase (JCPDS 73–882) with an increase in the Na content. It is
Table 2. The crystal system and lattice parameters of (Na, K)NbO3. Sample NKN2.5 NKN3 NKN5 NKN6 2188
Symmetry orthorhombic orthorhombic orthorhombic orthorhombic
1 1 1 2
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Lattice parameters a [Å] b [Å]
c [Å]
3.943(3) 3.942(5) 3.930(0) 7.836(0)
5.673(3) 5.672(3) 5.633(6) 7.906(8)
5.643(5) 5.640(3) 5.603(3) 7.919(5)
likely that there is a morphotropic phase boundary at approximately 50 % K between NKN5 and NKN6. In order to identify the largest developed planes of the (Na, K)NbO3 particles synthesized by the TMC method, (Na0.5K0.5)NbO3 particles were also dispersed in ethanol and then cast on glass substrates. Figure 3 shows the oriented particulate layer XRD pattern of casting (Na0.5K0.5) NbO3 particles and compares it with that of the noncasting particles. The XRD pattern of NKN5 was reindexed in pseudo-cubic perovskite notation. The larger peaks of (011), (100), (022), and (200) are accordingly reindexed as the {001} plane in the pseudo-cubic form, which indicates that the {001} plane from the pseudo-cubic (Na0.5K0.5)NbO3 platelets is derived from the {010} plane of the single crystalline K4Nb6O17 particles (see Figure 1 and Figure 3).
Figure 3. XRD pattern of plate-like NKN5 particles synthesized by the TMC method at 850 °C for 2 h cast on a glass substrate, and without casting.
The NKN5 particles exhibit a plate-like shape with a width of 5–10 µm, a length of 10–20 µm, and a thickness of 0.5–2 µm (Figure 4, a). The particles have a polycrystalline structure that is built up of many small grains, and inherits the K4Nb6O17 precursor shape (Figure 4, b). High aspect ratio NKN5 platelets are suitable templates to obtain textured ceramics, especially sodium-potassium niobate systems, by the template grain growth process. The yield of the anisotropic NKN5 particles, using the topochemical synthesis method, could be explained from the crystal structures shown in Figure 5. It can be seen from the structure of K4Nb6O17 that the edge-sharing NbO6 octahedra are only along the [001] direction. In the KNbO3 crystal the NbO6 octahedron units connect by sharing corners along the [100], [010], and [001] directions. When the plane of {010} K4Nb6O17 converts to the plane of {001} NKN5, Na atoms need to diffuse inside the K4Nb6O17 crys-
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ture exhibits a minimum with composition suggests that there may be two structurally similar orthorhombic phases involved near NKN5.
Figure 4. SEM micrographs of (a) plate-like (Na, K)NbO3 particles synthesized by the TMC method from K4Nb6O17 precursor particles of x = 5 at 850 °C for 2 h; (b) a typical enlarged platelet particle.
tal, since there is a deficiency of Na in the A-sites of K4Nb6O17 compared with that of (Na, K)NbO3. The edgesharing NbO6 octahedra of K4Nb6O17 along the [001] direction must be separated and rotate, and then they reconnect by corner-sharing. It is assumed that the process involves the bond breaking, rebonding and new generation of bonds, rotation of NbO6 octahedra along the [001] direction, and the diffusion of Na and K atoms. Therefore, NKN5 inherits the K4Nb6O17 precursor shape and has a polycrystalline structure with small grains.
Figure 6. DSC pattern of plate-like (Na, K)NbO3 particles synthesized by the TMC method from K4Nb6O17 precursor particles at 850 °C for 2 h.
The ceramic of the as-prepared platelet NKN5 particles was sintered by the normal treatment under pressureless conditions in air. Its piezoelectric constant d33 is 89 pC/N, close to the d33 (86 pC/N) of the Na0.5K0.5NbO3 ceramic reported in the literature.[13]
Conclusions
Figure 5. Schematic crystal structure along the [100] direction. (a) Layer structure of K4Nb6O17 and (b) perovskite structure of NKN5.
The DSC curves of plate-like (Na, K)NbO3 particles were shown in Figure 6. Each curve possesses two peaks, one from a cubic-tetragonal transition [Curie temperature (Tc)] at around 400 °C, and another from a second phase transition at around 200 °C. The phase transition temperatures of Na0.5K0.5NbO3 are 193 °C and 401 °C, respectively. Tc decreased with an increase in the Na content, while the temperature change of the phase transition was nonlinear, which is in good agreement with the (Na, K)NbO3 synthesized by the solid-state method.[2] The temperature of the cubic-tetragonal transitions (Curie temperatures) decreased, which is in good agreement with that in ref.[18] It was reported that no great change (from 420 °C to 390 °C) was observed in the Curie temperature for the complete range of solid solutions. The tetragonal-orthorhombic transition temperature changed through a minimum for (Na0.5K0.5)NbO3 (NKN5). The fact that the lower transition temperaEur. J. Inorg. Chem. 2008, 2186–2190
Plate-like (Na, K)NbO3 particles were synthesized from platelet precursor K4Nb6O17 using a topochemical microcrystal conversion reaction. Molten salt KCl took part in the reaction and affected the composition of the products. By controlling the amount of reactant Na2CO3 the (Na0.5K0.5)NbO3 was accurately synthesized. The polycrystalline Na0.5K0.5NbO3 particles exhibit a plate-like shape with a width of 5–10 µm, a length of 10–20 µm, and a thickness of 0.5–2 µm. The crystallographic {010} plane of K4Nb6O17 was converted into the pseudo-cubic {001} plane of (Na0.5K0.5)NbO3 (NKN5). The phase transition temperatures of Na0.5K0.5NbO3 are 193 °C and 401 °C, respectively. Using normal sintering, the piezoelectric constant d33 of the nontextured platelet (Na0.5K0.5)NbO3 ceramic is 89 pC/N. The high aspect ratio Na0.5K0.5NbO3 platelets are still potential seeds for the (R)TGG method.
Experimental Section (Na0.5K0.5)NbO3 was synthesized from K4Nb6O17 by the topochemical micro-crystal conversion method. The starting materials were K2CO3 [analytical reagent (A.R.) ⬎ 99.0 %], Nb2O5 (A.R. ⬎ 99.9 %), Na2CO3(A.R. ⬎ 99.8 %), and KCl (A.R. ⬎ 99.5 %). Firstly, plate-like K4Nb6O17 precursor particles were prepared by molten salt synthesis. K2CO3, Nb2O5, and KCl were mixed in ethanol (A.R. ⬎ 99.7 %) according to a molar ratio of 2:3:15. The mixture was dried at 120 °C for 2 h. The dry mixture was heated at 1060 °C
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L. Li, J. Chen, J. Deng, R. Yu, L. Qiao, G. Liu, X. Xing
for 2 h. After the product was washed several times with hot deionized water to remove the KCl salt, the plate-like K4Nb6O17 particles were obtained. Using plate-like K4Nb6O17 as precursor particles, the topochemical micro-crystal conversion from K4Nb6O17 to (Na0.5K0.5)NbO3 was carried out at 850 °C for 2 h in an equal weight of molten salt KCl. The ratio of reactant K4Nb6O17/ Na2CO3/Nb2O5 is 1:x:1, where x was selected as 2, 2.5, 3, 5, and 6. The molten salts and the remaining Na2CO3 were removed from the products by several washings of hot deionized water. The assynthesized powders were finally dried at 120 °C for 4 h. Without any special treatment, the as-prepared powders were pressed into disks. The disks were sintered at 1100 °C under pressureless conditions in air for 2 h. The ceramic pellets were polished and coated with silver paste on both sides. Polarization was carried out in a silicon oil bath at 130 °C under applied fields of Ep = 3.5 kV/mm for 20 min. The specimens were cooled to room temperature in the silicon oil bath, and then aged for 24 h in air for the measurement of the piezoelectric properties. The structure of samples was characterized by X-ray diffraction patterns (model M21XVHF22). The microstructure of the samples was observed using a scanning electron microscope (model CAMBRIDGE S-360). The composition of powders was determined by inductively coupled plasma (ICP, INC profile) spectrometry. The thermal behaviors of the precursor powders were studied by differential thermal analysis (DSC, Model TA-Q200) in air, at a heating rate of 20 °C/min. The piezoelectric coefficient d33 of the samples was measured using a quasi-static d33/d31 meter (model ZJ6A).
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20731001, 20571009, 50725415) and Funds of Ministry of Education of China, Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).
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[1] E. Hollenstein, M. Davis, D. Damjanovic, N. Setter, Appl. Phys. Lett. 2005, 87, 182905. [2] L. Egerton, D. M. Dillon, J. Am. Ceram. Soc. 1959, 42, 438– 442. [3] M. Ahtee, A. M. Glazer, Acta Crystallogr. 1976, 32, 434. [4] E. M. Sabolsky, S. Trolier-McKinstry, G. L. Messing, J. Appl. Phys. 2003, 93, 4072–4080. [5] L. Zhao, F. Gao, C. Zhang, M. Zhao, C. Tian, J. Cryst. Growth 2005, 276, 446–452. [6] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Nature 2004, 432, 84–87. [7] G. L. Messing, S. Trolier-Mckinstry, E. M. Sabolsky, C. Duran, S. Kwon, B. Brahmaroutu, P. Park, H. Yilmaz, P. W. Rehrig, K. B. Eitel, E. Suvaci, M. Seabaugh, K. S. Oh, Crit. Rev. Solid State 2004, 29, 45–96. [8] C. Sun, X. Xing, J. Chen, J. Deng, L. Li, R. Yu, L. Qiao, G. Liu, Eur. J. Inorg. Chem. 2007, 13, 1884. [9] J. T. Zeng, K. W. Kwok, H. L. C. Chan, Mater. Lett. 2007, 61, 409–411. [10] Y. Saito, H. Takao, Jpn. J. Appl. Phys. 2006, 45, 7377–7381. [11] J. T. Zeng, K. W. Kwok, W. K. Tam, H. Y. Tian, X. P. Jiang, H. L. W. Chan, J. Am. Ceram. Soc. 2006, 89, 3850–3853. [12] D. Liu, Y. Yan, H. Zhou, J. Am. Ceram. Soc. 2007, 90, 1323– 1326. [13] Y. Saito, H. Takao, J. Eur. Ceram. Soc. 2007, 27, 4085–4092. [14] Z. Cai, X. Xing, R. Yu, X. Sun, G. Liu, Inorg. Chem. 2007, 46, 7423–7427. [15] X. Xing, C. Zhang, L. Qiao, G. Liu, J. Meng, J. Am. Ceram. Soc. 2006, 89, 1150–1152. [16] J. Chen, X. Xing, A. Watson, W. Wang, R. Yu, J. Deng, L. Yan, C. Sun, X. Chen, Chem. Mater. 2007, 19, 3598–3600. [17] K. Teshima, K. Horita, T. Suzuki, N. Ishizawa, S. Oshi, Chem. Mater. 2006, 18, 3693–3697. [18] G. Shirane, R. Newnham, R. Pepinsky, Phys. Rev. 1954, 96, 581–588. Received: November 26, 2007 Published Online: March 26, 2008
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2186–2190
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.7 Synthesis with Supported Metal Particles by Use of Surface Organometallic Chemistry: Characterization and some Applications in Catalysis Fr6dkric Lefehvre, Jean-Pierre Candy, and Jean-Marie Basset
2.7.1 Introduction Numerous reasons, such as production cost, environmental protection, and easy product recovery have led the chemical industry to search for catalysts with high chemo-, regio-, and stereoselectivities. For this purpose, supported or unsupported metallic catalysts were often modified by various kinds of dopants. A classical example is the addition of a metal to another metal to obtain improved catalytic properties. The main objective is to prepare well defined systems to obtain structureactivity relationship. Such a relationship is necessary for progress in the field of heterogeneous catalysis. Various routes can be used to prepare such compounds but it seems increasingly clear that surface organometallic chemistry on metals is a new way of obtaining such well controlled bimetallic catalysts.['] First of all, by careful control of the reaction parameters, the organometallic complex can react selectively with the metal and not with the chemical functionalities present on the surface of the support, e.g. silica. Even if there is a reaction between the organometallic complex and the oxide, it usually proceeds at a temperature higher than that of the reaction with the metal. For example, tetrabutyl tin reacts with the hydroxyl groups of silica at 150 'C['] but, surprisingly it also reacts at room temperature with a reduced metal particle supported on this oxide.[31Secondly, surface organometallic chemistry leads to the formation of well defined species (see below) enabling better understanding of the catalytic results. Here we will present some results on the surface organometallic chemistry on metals. We first describe the various structures achievable and then give some catalytic applications of these systems, emphasizing the role of the structure of the supported organometallic fragments or adsorbed metallic atoms derived from them.
2.7 Synthesis Icith Supported Metal Purticles
783
2.7.2 Synthesis and characterization of bimetallic catalysts by reaction of organometallics with supported metal particles We will only present here the synthesis and characterization of bimetallic catalysts containing tin, because these are known to lead to highly selective catalysts. The complex most often used for these reactions is tetrabutyl tin Sn(n-C4H9)4 but several other precursors can be used.[41The reaction is usually performed at low temperature, typically 50 "C, to avoid reaction of the tin complex with the hydroxyl groups of the support, as shown by solid-state I3C CP-MAS NMR.131Indeed, it has been shown that tetrabutyl tin reacts with the hydroxyl groups of silica, silicaalumina, or alumina with formation of a surface grafted =M-Sn(n-C4H9)3 fragment 1). and evolution of b ~ t a n e ' ~(Scheme .~]
Scheme 1
Typically, small particles of the desired metal supported on a given oxide (silica, carbon, alumina) are obtained by the following procedure: i) calcination of the support in air to remove undesirable physisorbed species like hydrocarbons (for silica or alumina) or to increase the number of functional sites (carbon); ii) deposition of a salt of the desired metal on the support by chemical exchange (see, for example, the work of Benesi et a1.161); iii) treatment of the resulting solid under oxygen and/or vacuum at 350-500 "C, depending on the reaction conditions in which the catalyst will be used; iv) reduction under a flow of dihydrogen at the same temperature but below 500 "C, to prevent sintering of the metal particles; and v) storage of the catalyst under argon and, just before use, re-reduction under a flow of dihydrogen at the same temperature as above to ensure complete reduction of the surface to the metallic state. After characterization of the metal by electron microscopy and chemisorption methods such as hydrogen, oxygen, and carbon monoxide adsorption, the reaction with tetrabutyl tin was performed and followed by G C and volumetry. The reaction
784
2 Metal Clusters in Catalysis
0 '
H*-A
C-H
+
I
H
/ S b
Scheme 2
was usually performed without solvent but some experiments have been carried out with Sn(n-C4H9)4 in heptane. Whatever the method and the metal, the reaction can be subdivided into several steps, quite similar to those reported for the reaction with the surfaces of oxides.
i) Firstly there is physisorption of tetrabutyl tin on the surface of the support. This is well established for silica['] by IR spectroscopy which shows a strong shift to lower wavenumbers of the v(0-H) band of silica from 3760 cm-' to ca. 3700 cm-'. This physisorption can be described as resulting from van der Waals interactions between the hydroxyl groups of the support and the butyl chains (Scheme 2). ii) The next step is a migration of the physisorbed complex from the support to the metal surface.['] Because the Sn-C bond is weaker than the C-C and C-H bonds (Sn-C <240 kJmol-', C-C = 610 kJmol-', C-H 340 kJmol-'), and with rhodium as the metal because the Rh-H, Rh-C, and Rh-Sn bonds have quite similar enthalpies of formation, it should be cleaved more easily. This explains why its hydrogenolysis occurs at the reaction temperature whereas the C-H and C-C bonds do not react. The mechanism of this hydrogenolysis is not yet well understood but by analogy with studies on hydrogenolysis of heteroleptic tin complexes['] one can reasonably suggest that the first step of the reaction is pentacoordination of the tin atom followed by the cleavage of the Sn-C bond with formation of a metal alkyl which is finally hydrogenated into butane (Scheme 3). Obviously, depending on the metal and the reaction conditions, the metal-grafted tributyl tin complex can be further dehydrogenated, leading to dibutyl, monobutyl, and even totally de-alkylated tin species. In the totally de-alkylated tin species the tin atom can be considered as an adatom on the metal surface. Thermal treatment can cause this adatom to migrate into the outer layers of the metal particle, leading to the formation of a surface alloy. As an outline, we can say that three types of
785
2.7 Sjnthcsis with Supported Metal Purticles
Yu
H
H
BuHSn\ "'IlBu Bu
f
-
I
M -
M-M-
I
-M-M-M-
BuH
materials can be obtained by reaction of the organometallic compound with the metal surface: i) a material in which the organometallic fragment is only partly decomposed on the metal particles but remains linked to it; ii) a material in which the organometallic compound has lost all its ligands and is present on the metallic surface as a kind of naked adatom; and iii) a solid in which the adatoms have been incorporated into the particle leading to the formation of a surface alloy. Naturally the above situations can coexist on the same solid, resulting in a complex situation. By judicious choice of reaction conditions it is, however, possible to obtain for a given metal a single and well defined environment for tin, enabling it to be fully characterized by physicochemical methods and above all by EXAFS. The 'monografted' organotin species MSnBu3 was obtained by reaction of SnBu4 with a Ni/SiOz catalyst. Its characterization by EXAFS gave one Sn-Ni bond at
786
2 M e t d Clusters in Catalysis
Monografted tin species
Example, with Ni:
I
R=~ - B u
2.68 A
Scheme 4
0.268 nm and three Sn-C bonds at 0.217 nm in the first coordination sphere of tin17](Scheme 4). The ‘digrafted’ organotin species MzSnBu2 was prepared on rhodium particles supported on silica.[31EXAFS studies of this catalyst revealed two Sn-Rh bonds at 0.262 nm and two Sn-C bonds at 0.212 nm (Scheme 5). The ‘trigrafted’ tin surface complex M3SnBu was isolated on a Pt/SiO2 catalyst.[’] EXAFS measurements performed at the Sn K edge indicated that tin was surrounded by three platinum atoms at 0.268 nm and one carbon atom at 0.211 nm (Scheme 6). Naked tin adatoms were also obtained on a Pt/SiOz catalyst but, to remove all the butyl ligands, the solid was further heated at 300 “C. In agreement with the gaseous evolution of a total of four butane equivalents per tin, EXAFS data indicated that tin was only surrounded by four platinum atoms at the same distance of 0.276 nm (Scheme 7). This result clearly indicated that tin was located ‘on’ the metal surface and not in the bulk. For example, in a bulk Pt3Sn alloy tin is sur-
Digrafted tin species 2.12A
Example with Rh R=~ - B u Scheme 5
I
2.62 A
2.7 Synthesis with Suppurled Metal Puvticles
787
Trigrafted tin species R
I
Scheme 6
Example with Pt R = n-BU
Tin adatoms on metal 2.76 A
M --
Example with Pt
Scheme 7
rounded by 12 platinum atoms whereas in a surface alloy on bulk platinum it is surrounded by six platinum atoms Note that such tin adatoms were always obtained when low amounts of tin were deposited on the metal. This is probably because tetrabutyl tin coordinates first on the metal atoms of the faces which are the most hydrogenating. This will be very important in catalysis (see later). Further heating of the above sample under dihydrogen at 500°C results in an increase of the number of platinum atoms surrounding tin up to ca 5. This can be explained by a migration of the tin atom into the first platinum layer (Scheme 8). Depending on the desired application, the catalyst will behave differently, each type of tin compound on the metal modifying its properties differently.
Surface alloy 2.78 A
/
-. / / , Sn
Scheme 8
SnlPt, = 0.9
/
788
2 Metal Clusters in Cutalysis
2.7.3 Applications in catalysis We have seen above that three types of bimetallic catalysts could be obtained by reaction of tetrabutyltin with a metallic surface, depending on the coordination sphere of the tin (presence of remaining organometallic fragments, naked adatoms or surface alloy). We will give here some examples of the properties of each of these solids.
2.7.3.1 Ligand effect: Hydrogenation of a, /?-unsaturated aldehydes'' 'I Citral is an a, P-unsaturated aldehyde containing three different kinds of unsaturation - a non-conjugated trisubstituted double bond, a conjugated trisubstituted double bond, and a conjugated aldehyde function which can be in either the cis or the trans position to the alkyl chain. Hydrogenation of this molecule can then lead to numerous products. For example, the first hydrogenation step could lead to 3,7-dimethyl-2,6-octenol (geraniol or nerol, depending on the stereochemistry of the double bond), 3,7-dimethyl-6-octenal (citronellal), and 3,7-dimethyl-2-octenal (Scheme 9). Further hydrogenations can then occur leading finally to 3,7-dimethyloctanol. When the hydrogenation of citral is performed with a supported metal, for example Rh/SiOz under classic conditions (liquid phase, rhodium dispersion
Rh-S nlSi0 2
3,7-dimethyl-2,6-octenol (geraniol + nerol) sel. > 95 %
Scheme 9
citronellal sel. = 75 %
2.7 Synthesis with Supported M e t d Particles
789
80%, citral/Rhs = 200, P(H2) = 80 bar, T = 340 K) the catalytic activity is very high but most of the above products are obtained and the reaction is totally nonselective, even if the major product is citronellal. The situation is totally different when the catalyst is Rh/SiOz modified by reaction with SnBu4 (Sn/Rhs = 0.95). The catalytic activity decreases slightly but the selectivity is completely different and the major product is now 3,7-dimethyl-2,6-octenol, with a selectivity of ca 96% (Scheme 9). A detailed physicochemical study of the catalyst was performed. It showed that tin was present as two species: a small amount of tin adatoms, corresponding to tin complexes having reacted with the most hydrogenating sites and poisoning them (see below), and a large amount of digrafted dibutyl organotin species. Compared with the same system but with only the tin adatom species (see below), both the catalytic activity and the selectivity to (geraniol nerol) have increased. The enhancement of the activity should result from an electronic effect, in which coordination and activation of the aldehydic function of citral is enhanced by the presence of butyltin moieties. Simple molecular modelling studies show that after grafting the SnBuz fragments onto the metal particle there is significant steric hindrance because of the butyl ligands. This steric hindrance could prevent the coordination of citral by the internal double bond and allow only coordination of the carbonyl group, rendering the reaction highly selective. This effect of the butyl ligands was also observed by changing the organometallic complex grafted onto the metal.L4l With GeBu4, which also leads to the formation of a digrafted surface species, the same effect was observed, whereas with PbBu4 all butyl groups were lost and the catalytic activity became insignificant.
+
2.7.3.2 Adatom effects Several adatoms effects were observed, with various reactions. This effect can usually be depicted as poisoning of undesirable sites, resulting simultaneously in a decrease of the global catalytic activity and in a significant increase of the selectivities for the desired products. We describe here three examples, the hydrogenation of x , P-unsaturated aldehydes (the same reaction as above but now in the presence of a very small amount of tin), the isomerization of 3-carene into 2-carene, and the dehydrogenation of butan-2-01 into methyl ethyl ketone.
Hydrogenation of a, P-unsaturated aldehydes As described above, the hydrogenation of citral by unmodified rhodium supported on silica is non-selective. It leads to numerous products whereas the addition of tin to a Rh/SiOz catalyst (atomic ratio of tin/rhodium = 1) results in a totally selective reaction for 3,7-dimethyl-2,6-octenol (geraniol nerol). A study of the catalytic properties as a function of the amount of tin per surface rhodium has shown that
+
790
2 Metal Clusters in Catalysis
Scheme 10
there was another interesting feature of this system. When small amounts of tin are added (Sn/Rhs = 0.15), the catalytic activity is reduced by a factor of three. This kind of poisoning is, however, highly selective because it does not affect the sites responsible for the hydrogenation into citronellal" 'I (Scheme 9). This results in the high selectivity of the reaction toward this product (more than 75%). It has been shown by physicochemical characterization that the tin complex has been fully dealkylated, probably because it is chemisorbed first on the most 'hydrogenolysis' sites, resulting in poisoning by adatoms.
Isomerization of 3-carene into 2-carene[l21 (+)-3-carene (3,7,7-trimethy1-[4,1,O]-bicyclohept-3-ene)is a monoterpene present in natural compounds such as oils of turpentine. Its industrial applications are, unfortunately, limited (it is only used as solvent in coatings). In contrast, (+)-2-carene is potentially more interesting for the production of fine chemicals, because the double bond is conjugated with the strained C-C bonds of the cyclopropyl moiety. It would be of interest to transform (+)-3-carene into (+)-2-carene (Scheme lo), even if thermodynamically, the equilibrium between the two isomers corresponds to nearly equal amounts of the two carenes (60% of 3-carene and 40% of 2-carene at 120 "C). Metals (Raney nickel or nickel on silica, palladium on carbon, etc.) can easily catalyze this reaction but the selectivity is low, because of the hydrogenation of the two isomers into carane. Addition of very small amounts of tetrabutyl tin can completely transform the performance of these catalysts by poisoning the hydrogenation sites. For example, when a Nio/Si02 catalyst is used, the best result is a yield of 2-carene of 300/0and at least 30% of the carenes have been transformed into byproducts. Addition of 0.04 mol tetrabutyl tin mol-' surface nickel increases of the yield of 2-carene to 37% and reduces the amount of byproducts to less than 10%. As above, tin is now present as adatoms on the most hydrogenating sites (most probably those situated on the faces rather than on the corners and edges).
Dehydrogenation of butan-2-01 into methyl ethyl ketone" 31 A similar effect was observed during the dehydrogenation reaction of butan-2-01 into methyl ethyl ketone on Raney nickel (Scheme 11). Raney nickel is a very efficient catalyst for this reaction and leads to methyl ethyl ketone with a selectivity of ca 90'%,.This result is good but for industrial applications higher selectivities are required. This can be achieved by poisoning some sites by reaction with tetrabutyl tin (the best results are obtained with an Sn/Ni ratio of 0.02). Indeed, the reaction
2.7 Synthesis with Supported Metal Purticles
y Scheme 11
NIRaney-Sn)
OH
'rf\
191 +
Hp
0
occurs first with the sites responsible for the side reactions these are then selectively poisoned by the resulting tin adatoms. The consequence is a slight decrease in catalytic activity and an increase of the selectivity to methyl ethyl ketone which can reach 99%. This catalyst, developed by IFP, has been used commercially in Japan for several years. -
2.7.3.3 The site isolation effect by alloys Here we present two examples of such effects, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. Usually the effect of tin, present as a surface alloy, is to dilute the active sites, thus reducing the yield of competitive reactions.
Dehydrogenation of isobutane into isobutene The dehydrogenation of isobutane into isobutene proceeds at high temperature (ca 550 "C) and low hydrogen pressure (1 bar). Under these conditions, the catalyst is very active (turnover frequency 5 s-') and highly selective (93%), and the byproducts arise as a result of the hydrogenolysis properties of the metal. This catalyst was modified by addition of large amounts of tin (Sn/Pts between 0.5 and l ) , by reaction with tetrabutyl tin. To ensure complete hydrogenolysis the solid was heated at 300 "C and then reduced under hydrogen at 500 "C. EXAFS analysis showed that after such treatment all the tin on the surface was present as a surface alloy on the metal particles.['] This modified catalyst can be considered as totally selective for isobutene (for example, when the Sn/Pts ratio is equal to 0.85, the selectivity to isobutene is 99.5")). This increased selectivity for isobutene can be simply explained by the 'site isolation' effect.[14p171 It is now generally accepted that the coke formation and the hydrogenolysis reactions occur on large platinum surfaces, because more than one platinum atom is involved in the reaction m e c h a n i ~ m . ~T' YPi~.~~~ cally, the mechanism involves g-H abstraction then formation of a metallacycle and the cleavage of the C-C bonds. The presence of tin atoms regularly distributed on the platinum surface increases the distance between adjacent platinum atoms, as do copper atoms on a nickel surface[201and tin atoms on rhodium, platinum, or nickel surfaces,17. 14.1 7.2 11 and so avoids the hydrogenolysis reaction, leading to a more selective catalyst. Indeed, the formation of isobutene from isobutane involves only one platinum atom, the reaction passing through a P-H elimination (Scheme 12). Another consequence of the suppression of the hydrogenolysis reactions is an increase of the turnover number measured at the pseudostationary part of the curve
792
2 Metal Clusters in Cutulysis
Sn-Pt-Sn-Pt
Sn-Pt-Sn-Pt
Sn-
H
Pt -Sn-
Pt
H
H
Scheme 12
giving the conversion as a function of time. Because hydrogenolysis reactions are suppressed, the coke formation, responsible of the deactivation of the catalyst, is strongly reduced, with the result that the tin-modified catalysts are more active than the unmodified catalysts.
Selective hydrogenolysis of esters and acids to aldehydes and alcohols Hydrogenolysis of esters to aldehydes or alcohols is difficult to achieve either by homogeneous or heterogeneous catalysis. Indeed high temperatures and high pressures are required to achieve the reaction, leading to non-selective hydrogenolysis with formation of acids, alcohols, and hydrocarbons. Bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on silica and prepared by reaction of the M metal with tetrabutyl tin have catalytic properties quite different from those of the monometallic catalysts. Indeed, they are very selective for the hydrogenolysis of ethyl acetate into ethanol.[221For example whereas the selectivity to ethanol is 12% with Ru/SiO2, it increases to 90% for a Ru-Sn/SiOz catalyst with a Sn/Ru ratio of 2.5.[231In addition, the reaction proceeds at lower temperatures than with the classical catalysts (550 K instead of temperatures higher than 700 K). The reaction mechanism proposed for these alloys" is depicted in Scheme 13. The first step is the coordination of the ester to the alloy, most probably to the tin atoms, via its oxygen atoms. The second step is breakage of the RC(0)-OR' bond with formation of aldehyde which is further hydrogenated into the corresponding alcohol. A quite similar reaction is observed with acids instead of esters. Indeed, the organic acids can undergo hydrogenolysis leading, as above, to aldehydes and alcohols (Scheme 14). Typically, in the hydrogenolysis of acetic acid, high selectivities towards ethanol ( 2 87%) are achieved with the Ru-Sn alloy, compared with that of the pure metal, for which the major product is methane. It is also possible to increase the selectivity for the corresponding aldehyde, for example by reducing the ratio of hydrogen to 7 3 2 1 3 2 3 3 2 4 1
2.7 Synthesis with Supported Mrtul Particles
Yo--
793
0
Sn-
i
M-
0
'k
Sn-
M-
I
Sn
I-
Sn
Sn
M-
Sn
Scheme 13
Ru-SnlSiO, O H '
525 K
*
+ H2O
P(H,) = 50 bars
-OH Scheme 14
acetic acid in the reactants, as might be expected if we assume that this product is a primary product of the reaction. These results have been patented by Rh6nePoulenc which claims selectivity higher than 85'% and conversion higher than 95% for the hydrogenolysis of nonanoic and trifluoroacetic The mechanism proposed from the results of kinetic experiments is depicted below (Scheme 15). This mechanism is comparable to that proposed for the hydrogenolysis of esters. Note also that during the reaction esters are formed which can also undergo the hydrogenolysis reaction, by the mechanism depicted above.
2.7.4 Conclusion These examples have shown that supported group 8 metals modified by reaction with organometallic compounds are very useful catalysts with numerous applica-
794
HZ
2 Metal Clusters in Cutalysis
I
CH3
\
KC-OH
/H
I ,\sn 'O Sn-Ru-Sn sn+
'f CHSCHO
Sn+ Scheme 15
tions in the fine chemical and petrochemical industries. Depending on the reaction conditions, the metal, and the metal-to-tin ratio, various structures can be obtained: organometallic tin complexes with one, two or three alkyl chains linked to the surface, naked tin adatoms on the metal surface, or surface alloy. Each structure has specific properties. The presence of alkyl ligands on tin introduces steric constraints which, apparently, prevent the coordination of citral by its internal double bond and allow only the hydrogenation of the C=O double bond. Tin adatoms have numerous applications because they selectively poison highly active sites responsible for the side-reactions (see for example the isomerization of 3-carene into 2-carene, the hydrogenation of unsaturated aldehydes, and the dehydrogenation of 2-butanol into methyl ethyl ketone). The amount of tin is often very low typically the Sn-tosurface-metal-atom ratio is < 0.1. After treatment at high temperature, these adatoms penetrate the metal surface layers leading to the formation of surface alloys which also have properties quite different from those of the starting metal. Indeed, because of the site-isolation effect of the metal atoms by tin, reactions occurring on more than one metal atom are now avoided. Two examples of such effects have been given the dehydrogenation of isobutane to isobutene and the hydrogenation of esters and acids to aldehydes and alcohols, reactions in which the hydrogenolysis processes are suppressed by addition of tin. These results show that some correlations can now reasonably be made between the structure of the catalyst and its ~
~
2.7 Synthesis with Supported Metal Particles
195
catalytic properties. This should lead, in the near future, to more rational design of catalysts and to the application of metals modified by surface organometallic chemistry to a larger number of reactions.
References Candy, J. P.; Didillon, B. D.; Smith, E. L.; Shay, T. B.; Basset, J. M. J. Mol. Catal. 1994,86, 179. Nedez, C.; Theolier, A,; Lefebvre, F.; Choplin, A,; Basset, J. M.; Joly, J. F. J. Am. Chem. Soc. 1993, 115, 722. Didillon, B.; Houtman, C.; Shay. T.; Candy, J. P.; Basset, J. M. J. Am. Chem. Soc. 1993, 115, 9380. Didillon, B.; Candy, J. P.; Le Peltier, F.; Ferretti, 0. A,; Basset, J. M. Studies in SurJhce Scicnce and Catalysis. Heterogeneous Ca talysis and Fine Chemical III. 1993, 78, 147. NCdez, C.; Lefebvre, F.; Choplin, A,; Basset, J. M.; Benazzi, E. J. Am. Chem. Soc. 1994, 116, 3039. Benesis, A. H.; Curtis, R. M.; Studer, H. P. J. Catal. 1968, 10, 328. Lesage, P.; Clause, 0.;Moral, P.; Didillon, B.; Candy, J. P.; Basset, J. M. J. Catal. 1995, 155, 238. Cordonnier, M A . , PhD Thesis 1994, Lyon. Humblot, F.; Candy, J. P.; Didillon, B.; Clause, 0.;Corker, J.; Bayard, F.; Basset, J. M. J. Am. Chem. SOC.1998, 120, 131. Meitzner, G.; Via, G. H.; Lytle, F. W.; Fung, S. C.; Sinfelt, J. H. J. Phys. Chem. 1988, 92, 2925. Didillon, B.; El Mansour, A,; Candy, J. P.; Bournonville, J. P.; Basset, J. M. Studies in Surface Science and Catalysis, Heterogeneous Catalysis and Fine Chemicals II 1991, 59, 137. Lesage, P.; Candy, J. P.; Hirigoyen, C.; Humblot, F.; Leconte, M.; Basset, J. M. J. Mol. Cat. A: Chem. 1996, 112, 303. Bournonville, J. P.; Snappe, R.; Miquel, J.; Martino, G. European Patent 81-400985 810619. Ichikawa, M.; Lang, A. J.; Shriver, D. T.; Sachtler, W. M. H. J. Am. Chem. Soc. 1985, 107, 7216. Sinfelt, J. H. J. Catal. 1973, 23, 91. Sinfelt, J. H. Bimetallic catalysts: Discoueries, Concepts, and Applications; Wiley: New York, 1983, pp 130. El Mansour, A,; Candy, J. P.; Bournonville, J. P.; Ferretti, 0. A,; Basset, J. M. Angew. Chem. Int. Ed. Engl. 1989, 28, 347. Barbier, J. Appl. Cutal. 1986,23, 225. Fan, Y.; Xu, Z.; Zang, J.; Lin, L. Stud. SurJ Sci. Catal. 1991, 683. Martin, G. A,; Dalmon, J. A. J. Catul. 1980, 66, 214. Ferretti. 0. A,; Bournonville, J. P.; Mabilon, G.; Martino, G.; Candy, J. P.; Basset, J.-M. J. Mol. Cat. 1991, 67, 283. Basset, J. M.; Candy, J. P.; Louessard, P.; Ferretti, 0. A,; Bournonville, J. P. Wiss. Zeitschr. THLM 1990, 32, 657. Louessard, P.; Candy, J. P.; Bournonville, J. P.; Basset, J. M. In Structure and Reactivity of Surfaces; C. Morterra, A. Zecchina and G. Costa, Ed.; Elsevier: Amsterdam, 1989; Vol. 48; pp 591. Agnelli, M.; Candy, J. P.; Basset, J. M.; Bournonville, J. P.; Ferretti, 0. A. J. Catal. 1990, 121, 236. Ferrero, R.M.; Jacquot, R. French patent 91-13147 91 1024.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.8 Activation of Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters. Homogeneous and Surface-mediated Reactions Stefan0 Deabate, Philip J. King and Enrico Sappa
2.8.1 Introduction Metal carbonyl clusters are beautiful ‘objects’ whose structures and chemistry have attracted the attention of scientists all around the world. Depending on the interest of research groups, different approaches to their chemistry have been adopted.“] In the synthetic approach clusters are used in stoichiometric and catalytic homogeneous organic reactions;F2]clusters are also used as ‘templates’ for trapping intermediates not easy to detect in organic syntheses[31and as biological markers.i41In the surface approach clusters with increasing numbers of metal atoms (up to 500) stabilized by protective layers of ligands form colloidal systems with catalytic properties, or nano-scale powders with interesting physical properties.[5]Smaller polymetallic aggregates are ‘soluble models’ for the reactivity of small molecules chemisorbed on metal surfaces[61providing useful information about the mechanisms of heterogeneous catalysis. Finally, clusters are precursors of very small metal particles supported on inorganic oxides giving rise to a new class of polyfunctional heterogeneous catalysts.[’] Between these two extremes, there was space enough for a new approach, ‘surface organometallic chemistry’.[’] This interdisciplinary research field resulting from the overlap of organometallic chemistry and surface science deals with organic or organometallic reactions promoted by surfaces (usually inorganic oxides). The surfaces can be chosen to provide acidic, basic, or redox properties; according to the nature of the surface material and to the reaction conditions, mono- or polynuclear species are obtained upon chemisorption of carbonyl clusters.[9] These can be ‘grafted’ onto the surface either by acid-base interaction with the oxide element (e.g. aluminum), or by interaction with the hydroxyl groups always present on surfaces; examples are shown in Fig. 1. ‘Molecular models’ for surface-cluster interactions have been obtained[”] and new interactions between cluster metals and ‘silica’ have been observed.[’‘1
797
2.8 Curbon Monoxide, Water, and Alcohols on Metal Curbonyl Clusters
r: 0
k
‘-E
c d 1 I L
L
198
2 Metal Clusters in Catalysis
Several surface-mediated reactions leading to metal carbonyl clusters start from hydrated metal chlorides on wet silica;[*]it is also known that the presence of water can result in the formation of cluster anions at the expense of coordinated carbonyls.[’*] Hydridic cluster anions can be active in the water-gas shift reaction ( WGSR), the Fischer-Tropsch and other reactions, as discussed below.[’3] The following surface-mediated processes could, therefore, be expected of clusters on oxides: i) CO activation, including electrophilic or nucleophilic attack; ii) loss of CO and formation of cluster anions, which can undergo further reactions (e.g. condensation with metal carbonyl fragments) or which are active in catalytic processes; iii) hydration-dehydration reactions, including splitting of water into its components; and iv) protonation or deprotonation reactions, involving either the cluster or the coordinated ligands.
2.8.1.1 The aim of this work The surface organometallic reactions mentioned above do not require solvents, the inorganic support itself acting like a solvent. During our investigations on the synthesis and reactivity of clusters in organic solvents, however, we realized that silica or alumina, used as chromatographic materials, could promote some reactions; these are, indeed, the same supports used in surface organometallic chemistry and in addition - might contain moisture. We also established that impurities in the reactants (e.g. methanol used as ‘stabilizer’)could give unexpected products in reactions promoted by inorganic surfaces. A glance at the literature provided other examples of ‘homogeneous’ reactions of water or alcohols with metal carbonyls which are presumably promoted by surfaces; we thus decided to write a short review on the subject. This task was hard for several reasons. i) Although in most of the reactions examined the products were purified by column chromatography or by TLC, it is difficult to understand - especially from the earliest literature reports if surface materials have played a role. The reactivity of surface materials was underestimated before the recent growth of surface organometallic chemistry. ii) For the same reasons the reactivity of water towards metal carbonyl clusters has not yet been fully explored. iii) Last, but not least, examples different from each other, and therefore difficult to rationalize, are available in the literature. ~
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters
799
We have tried, despite this, to do our best and hope that our work will stimulate further research in this field. There is increasing evidence indicating that organometallics and water do mix. For example, water-soluble iridium( I ) derivatives have been reported; hydrogenbonding interactions play a role in the behavior of the complexes.['4] Cyanation of a-ketoalkynes catalyzed by anionic Ni(0) cyano complexes in water has also been rep0rted.1'~' Syntheses of water-soluble clusters have been achieved; for example, and related the 0x0-capped trinuclear cation [Ru3(C6H6)3(p-C1)(p3-O)(p-H)2]+ complexes have been obtained via low-pressure hydrogenation of the hydrolysis mixture of R U ~ ( C ~ H ~in)water.["j] ~CI~ Several important homogeneous catalytic reactions (e.g. hydroformylations) have been accomplished in water by use of water-soluble catalysts;[' 71 in some instances water can act as a solvent and as a reactant for hydroformylation.["] In addition, formation of aluminoxanes by partial hydrolysis of alkylaluminum halides results in very high activity bimetallic AI/Ti or Al/Zr metallocene catalysts for ethene polymerization which would be otherwise inactive.[''] Polymerization of aryl diiodides and acetylene gas has recently been achieved in water with palladium catalysts.[201 Finally, nickel-containing enzymes, such as carbon monoxide dehydrogenase (CODH) and acetyl-CoA synthase, operate in water with reaction mechanisms comparable with those of the WGSR or of the Monsanto methanol-to-acetic-acid process.i2'1
2.8.2 The role of surfaces in CO and water activation Some reactions in solution may be compared with those on surfaces; for example, the formation of cluster anions by surface organometallic chemistry mentioned above should be compared with the formation of cluster anions in CH30H-KOH solution, as discussed below.
2.8.2.1 Activation of cluster-bound carbonyls These can be activated by surfaces or by chemical procedures under homogeneous conditions.
'Chemical' activation of cluster-bound carbonyls CO have been activated by reacting osmium carbido-carbonyl clusters with alcohols to form coordinated C02R groups under mild conditions; no evidence of surface effects was established.[22]A comparable reaction is methoxide attack on
800
2 Metal Clusters in Catalysis
THF
PhC(O)CI
Figure 2. Reaction of [Fe(C0)4I2-with benzoyl ch10ride.l~~~
coordinated carbonyls in the clusters H z O S ~ ( C O ) ~ ( O ~(EER=) S, P, As; R = 0, OH, Ph).[231 Although chemical activation of coordinated carbonyls is mostly used for synthetic purposes, some examples relevant for comparison with the reactivity on surfaces should be mentioned. CO activation can be achieved in several ways; the best known are the use of Me3N0,[241of Na-benzophenone ketylLz5]or of [PPNIX ( PPN = bis(tripheny1phosphino)iminium cation [N(PPh3)2]+; X = halogen) salts.[26]Metal complexes may also be effective; these usually work via electrontransfer catalysis or radical mechanisms.[27] ‘Side reactions’ can, however, occur. For example, Na-benzophenone ketyl can The P~).[~~*~~ release COPh fragments forming the acyl cluster H R U ~ ( C O ) ~ ( ~ ~ - O C reaction mechanism is unclear. A comparable reaction could be that of the anion [Fe(C0)4l2- with benzoyl chloride, leading to Fe2(C0)6(C2Ph20), Fig. 2.[30*1This complex is further mentioned below (Section 2.8.3, Scheme 14). The use of Me3NO to induce substitution of dppm (bis(diphenylphosphin0)methane) for CO molecules on dinuclear iron complexes led to insertion of CO into C-C bonds of alkyne-derived metal la cycle^.[^^] Similar behavior was observed when [PPNICl salts were used to favor the formation of alkyne-substituted triruthenium dppm-containing clusters.[32*]This behavior should be compared with the insertion of CO into allenylidene and phosphido-bridging ligands occurring when dppm coordinates to binuclear ruthenium complexes as shown in Fig. 3.[33*1This reaction is a nucleophilic attack of the coordinated allenylidene and phosphido groups on a coordinated CO (see Section 2.8.2.2). The formation of Me3NO can also be exploited in reductive carbonylation reactions for example, in the synthesis of benzotriazole photostabilizers from ~
2.8 Carbon Monoxide,, Water, and Alcohols on Metal Carbonyl Clusters
801
Figure 3. Structural changes induced by the coordination of dppm to a phosphido-bridged allenyl diruthenium complex.
ortho-nitrophenylazo compounds Me3N acts as a deoxygenating agent for the nitro group and forms Me3N0.[341 Finally, CO activation leading to the ketenyl phosphinidyne cluster Ru3(CO).i(dppm){p3-PC(=C=O)Bu‘}2occurs when Ru3(CO)lo(dppm) is reacted with Bu‘CP in refluxing THF.135*1
Formation of cluster anions on surfaces or in solution On surfaces Formation of cluster anions on surfaces has been discussed in Section 8.2.1; further examples on hydrated alumina or silica or on hydroxylated magnesia can be found e l ~ e w h e r e . [ ~ Organic ~ . ~ ’ ~ and inorganic iodides can also favor the formation of An example of oxidative addition of a vinyl group favored by the release of cluster-bound CO upon reaction with the silica hydroxyl groups, via a WGSR mechanism, is apparent from the TLC plates during the work-up of solutions containing Ru3(CO)10L2 ( L = diphenylvinylphosphine or divinylphenylphosphine), the red color of the disubstituted complexes changes into yellow and the final products are the hydridic clusters HRu3(C0)8(L)(HCCHPPh2)[39a1or (Fig. 4). The reaction was followed HRu3(CO)8(L) {PhP(CHCH)(CH=CH2))139b1 by collecting the top (yellow), medium (orange) and bottom (red) fractions of the very broad band observed on the TLC plates; 3’P NMR of the eluted fractions showed the disappearance of the signals typical of Ru3(CO)loL2 and the growth of the signals of the products. Another surface-mediated reaction is shown in Scheme 1. To the best of our knowledge, hydrido-acetylide iron clusters have not been synthesized by ‘direct’ methods (e.g. oxidative addition of alkynes); HFe3(CO)gCzSiMe3 has, however,
802
2 Metal Clusters in Catalysis
H
Ph I ,c-
H
'
Ru (CO),
Figure 4. Structure of HRu3(CO)s(Ph2PCH=CH2)( PhZPC2H2) obtained by oxidative addition of diphenylvinylphosphine to Ru,(C0)12 on TLC plates.
been obtained by vaporizing nickel atoms into a methylcyclohexane solution of Fe(C0)5 and bis(trimethylsily1)acetyleneat - 120 "C. The cluster is formed during the chromatographic work-up of the reaction mixture on wet alumina.[401The role of nickel in the reaction is unclear; one could suspect that it favors CO activation as discussed above.[271 Cationic-anionic pairs of clusters are uncommon; examples are [Ni(PMe3)2(C5Hs)]+[Fe2Ni(C0)6(C2Ph2)(C5H5)]-,[41 "I [{(biPY)Pd}2(1.-H)(P-
AI,03H20 SURFACE
(CQ;
Scheme 1. Formation of an hydrido-acetylide triiron cluster mediated by alumina and water.
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters
803
’
CO)I[H3,0s4(CO~,21[42*1 and [RU~(CO)~CP( NPri2)2)3] [Ru6(CO)ls(C) {P(NPr‘2)2}].[43 The role of water and surfaces in these syntheses has not been clarified. In solution Several cluster syntheses require the formation of anions (and/or the presence of water).[441Basic methanolic (CH30H-OHp) solutions were used; these are the homogeneous counterpart of the formation of anions on surfaces. This is seen, for example, in syntheses of Fe3(C0)121451which require the formation of [ H F ~ ~ ( C O ) I IAlso ] - . one of the preparative methods leading to H4Ruq(C0)12 and H ~ R uCO) ~ ( 1 3 starting from Ru3(CO) 1 2[461 occurs in water-methanol solution; this is further discussed below. Reactions of Ru3(C0)12 in alkaline solution with chalcogen compounds led to = S, Se, Te) derivatives;[471 the synthesis of H Z R U ~ ( C O )was ~S H ~ R u ~ ( C O )( X ~X also achieved by protonation of HRu3(CO)loSEt with concentrated H2SO4; cluster cations are formed first, then dilution with water leads to the neutral Ru3(C0)12 reacts with C2Ph2 in alkaline solution[49a1 to form H2Ru3(C0)9(C2Ph2), with a ‘parallel’ a l k ~ n e ; ’ ~ ~other ’ ] derivatives, obtained upon acidification, were H4Ru4(CO)12, H ~ R u ~ ( C O and ) ~ ~ the , metallacyclic Ru3(CO)s(C2Ph2). We have recently hydrogenated 3-hexyne in the presence of the clusters Ru3(C0)12, H4Ruq(C0)12, and H ~ R u ~ ( C Odeposited )I~ on inert s u p p o r t ~ ~or~ on ~ ” oxides l such as A1203, Si02, ZnO and Mg0,[50h1 under solid-gas conditions; in all experiments the active intermediate H ~ R u ~ ( C O ) ~ ( was C~E~~) formed. After the reactions, we could isolate Ru3(CO)g(C2Et2)2, which is a byproduct, inactive in hydrogenation. We therefore hypothesize that both in CH30H-KOH solution and on surfaces, hydridic species such as [ H R U ~ ( C O11-) I are formed, which then react with alkynes forming the catalytic species and/or byproducts. Formation and conversion of [ H R u ~ ( C O ) Ion I ] ~hydroxylated magnesia that mimic reactions in solutions have been recently reported.[511It has also been shown that H ~ R u ~ ( C Oobtained ) ~ ~ , by the method of Keister,[521 reacts readily with acetylene forming Ru3(C0)9(p-CO)(C2H2)with a ‘parallel’ a l k ~ n e . ’Other ~ ~ ] reactions involving iron carbonyls which occur in the presence of CH30H-KOH are discussed in Section 2.8.3 (Scheme 3). Finally, activation of ( C P ) ~ M ~ ( C O ) ~ ({Cp ~ - P=) cyclopentadienyl; ~ M = Mo, W} upon reaction with M’OH (M’ = Na, K) in water-THF followed by acid( ~ -been P H reported.[541 ~) ification with HBF4 to give ( C P ) ~ M ~ ( C O ) ~ ( ~ - H )has Cluster anions (e.g. H R u ~ ( C O ) ~are ~ - )homogeneous catalysts for the deoxygenating carbonylation of nitrobenzene or for alkene hydroformylation.[2dlHydridic anions can transfer hydrogen to other ruthenium species forming ‘Ru(CH0)’ intermediates of interest in the Fischer-Tropsch process.1551 ‘Proton-induced reduction of CO’ on [Fe4(CO)13Ip and its butterfly derivatives leads to -COH l i g a n d ~ [ ~ ~ I which, upon protonation, loose water and give carbido clusters; these can be further
804
2 Metal Clusters in Catalysis
Figure 5. 'Modeling' the Fischer-Tropsch reaction on tetranuclear iron clusters. Formation of oxygenated products is not shown.
protonated and release methane, or may react with methoxide to give oxygenated products. A full Fischer-Tropsch sequence leading from coordinated CO to CH4 and water has therefore been obtained, Fig. 5.[571 The complex Co3(C0)9(,u3-COH)is formed upon acidification of [CO~(CO)IO]and is considered a possible intermediate in the reduction of CO with molecular hydrogen; it is also obtained by reacting HCo(C0)4 with C O ~ ( C O ) ~ . [ ~ ~ ] Hydridic cluster anions are also important in the homogeneous WGSR catal y ~ i s . [ The ~ ~ ] mechanism of WGSR reactions occurring on Ru3(C0)12 with an acid cocatalyst (CF3COOH) has been fully elucidated.[60]The radical anion [Fe3(C0)1I ] ~ ,obtained in the phase-transfer-catalyzed reaction of Fe3(CO)12 with OH-, catalyzes the reduction of nitrobenzene to aniline.[61]
2.8.2.2 Nucleophilic-electrophilic attack at coordinated carbonyls Formation of cluster anions on surfaces or in solution involves nucleophilic or electrophilic attack of water, hydroxyl groups, or alcohols on coordinated carbonyl groups; this behavior is relevant both to the WGSR reaction and to the formation of carbene-substituted complexes and related derivatives. Examples in organic solvents are: i) the attack of alkynes at CO bound to the acetylide cluster HRu3(C0)9(C2Buf) to form the carbenic complexes R U ~ ( C O ) ~ ( C I ~ H ~ O C O (two ) ( C ~isomers) ~H~O)
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carhonyl Clusters
805
1
and finally Ru3(CO ~ ( C , ~ H ~ O ) ( C , ~(two H~O isomers) O) containing a coordinated heterocycle;[62 ii) the attack of Me1 at a CO of FeCo2(CO)s(C2Me2)2followed by protonation and fragmentation to form FeCo(C0)6(CMeCMeCMeO) with a ferrole-like structure;[63]and iii) the electrophilic attack of H+ or Me+ on the acetyl anion [Fe3(CO)9(MeCO)]to form the neutral species Fe3(CO)9(MeCOX) ( E = H, Me) followed by C-0 bond cleavage and formation of the ethylidyne-methoxo Fe3(CO)g(CMe)(OE);this, in the presence of E+ ( E = Me, H) releases Ez0 and yields [Fe3(C0)9(CMe)]+, which reacts further with Me+ forming a coordinated ,u3-COMeligand,[22,641 Fig. 6. Finally, unexpected P-0 bond formation occurs during the reaction of PPh2C1 with [HFe3(CO)9(MeCzPh)]-; the metallacyclic Fe2(CO)6(PhCCMeCHOPPhz) is formed by shift of the cluster-bound hydride to the carbon and attack of PPh2 on the oxygen of a coordinated CO (Fig. 7).1651
2.8.3 Reactions leading to clusters containing water or water components 2.8.3.1 Clusters containing water or the fragments of water Clusters containing water Clusters substituted with water or water fragments are known. The 58-electron anion [Re4(p-H)3(,u3-H)(CO) 121- (Fig. 8) reacts reversibly with a water molecule, which is probably coordinated as a bridging ligand. This complex is unstable above 270 K and gives a derivative with a triply bridging OH, presumably via a tetrahydridic intermediate substituted with a water molecule coordinated to only one metal Substitution of water molecules by ClF or Br- has been reported for the chalcogenide clusters [Mo3Y7(H20)6I4+ (Y = S, Se).[671Finally, dirhodium cyclopentadienyl derivatives with -OH bridges have been reported.16*]An unusual fivecoordinated hydrido-hydroxo mononuclear derivative of osmium is obtained upon reaction of HOsCl(CO)(PPr'3)2 with KOH in methanol.l"1 Hydrolysis of the organometallic aqua ion [Re(C0)3(H 2 0 ) 3 ] + leads to [Re3(Cq)9r/12-OH)3(,u3-OH)]-and to [Re(CO)3(OH)]4 (as a DMF or OPPh3 adduct). 7 0 Methanol can also react with H4Re4(C0)12 in chloroform, forming H ~ R ~ ~ ( C O ) I ~ ( MH3Re3(CO)9(,u3-OMeH) ~OH)~, and (NEt4)[H3Re3(C0)9(~3OMe)]; this is a rare example of a fifth-order reaction.17']
806
2 Metal Clusters in Catalysis
Me
I
\
E = Hf,Mei
0 \Me
Figure 6. Reaction sequence after electrophilic attack at a p(,-COMe ligand coordinated to a triiron anion.
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters
I
807
MeEC-Ph
Figure 7. P-0 and C-C bond formation upon attack on a CO coordinated to a triiron alkyne anion.
The dynamic equilibrium between the dihydride trans-Ru( H)z(dppm)z and the H2)]+(OR)- occurs in the presence of hydrido-dihydrogen complex [(dppm)~HRu( phenol or hexafluoroisopropyl Crystalline salts [(CsHs)Cr]+[(CHDO),]- or [(CsH,Me)Cr]+[(CHDO)2](CHDO = 1,3-~yclohexadienone)with paramagnetic cations have been obtained upon reaction of Cr(Ar)2 complexes with 0 2 , formation of the dioxygen radical-ion and reaction of the latter with H(CHD0) and with water. The crystals are held together by hydrogen-bonds between the supramolecular [(CHD0)2]- anions. The hydrated crystalline species ( C6H6)2Cr][OH].3H20and [(CsH6)2Cr][(CHDO)].3H20 have been ~ h a r a c t e r i z e d . ~ ~ ~ Examples of 0x0 clusters obtained from metal alkoxides have been reported and the analogy between [W4C1(0)(OPri)g]and [W4(0)(OPri)l~] and tetrahedral carbonyl clusters has been discussed.[74]Trirhenium neopentoxide clusters give interesting reactions with ethylene and a l k y n e ~ . ' ~ ~ ]
I
808
2 Metal Clusters in Catalysis
1-
Figure 8. Coordination and splitting of water on a trirhenium hydridic cluster.
Clusters containing the fragments of water OsO4 reacts in methanol with CO under moderate pressures and temperatures to give Os3(CO)12 and small amounts of H(pu-OH)Os3(CO)10 and H(pO M ~ ) O S ~ ( C O ) IThese O . [ ~ products ~] and the reaction sequence are given in Scheme 2.[77a1 It is likely that these derivatives are obtained upon splitting of methanol or of water on chromatographic materials; no clear evidence for this behavior was, however, observed. Improved yields of the products were obtained by use of O S ~ ( C O ) ~ O ( C ~ H X ) (C6Hs = 1,3-CHD; CHD = cyclohexadiene, acts as a good 'leaving Modest yields of the ruthenium homologs with OR groups ( R = Me, Et, Pri, Bun) could be obtained from Ru3(CO 12 and the corresponding alcohols in reactions catalyzed by diiron c o m p l e x e ~The ~ ~ derivatives ~*~ H(SR)Os3(CO)lo have also been ~ynthesized.~~ 91 Chemisorption of M3(C0)12 ( M = Ru, 0 s ) on silica or alumina lead to surfaceanchored HM3(CO) lo(OH) species, which were characterized by EXAFS;[801the anchored hydrido cluster HOs3(CO)lo(OSi-) can be displaced from the surface with HF, forming HOs3(CO)lo(OH) in much higher yields than those reported for the above 'homogeneous' reactions."'] There are other examples of the synthesis of clusters in the presence of water; Stone obtained FeRuz(C0)12, FezRu(CO)12, H~FeRus(C0)13,and some
2.8 Carbon Monoxide, Water, and Alcohols on Metal Curbonyl Clusters
809
H
co MeOH
'A
OS~(CO),,(85%)+ red solution
0
I
R = H, CH,
red solid
yellow sdution
270 alm
Scheme 2. Synthetic pathways and structure of Os,(CO)l,(pH)(p-OR) clusters.
H ~ R u ~ ( C Oby) ~reacting ~ RuC13, [Ru(C0)3C12]2 (or Ru3(C0)12) with excess Fe(C0)5 in hydrocarbons; the origin of the hydridic hydrogen can be explained by the presence of water or hydroxyl groups in the chromatographic materials used.[821 The reaction of [Ru(CO)3C12]2and [Fe(C0)4l2- in water, followed by acidification with H3P04 and extraction with hexane, yields Fe~Ru3(C0)12,HZFeRu3(C0)13, and the new H ~ F ~ ~ R u ~ ( C OFrom ) ~ Z these . [ ' ~syntheses ~ compounds of general formula Ru4(CO)l&l,(OH),.(OR), ( R = Me, Et; x y z = 4) were isolated and characterized by IR and NMR and by mass spectrometry.[46b1 The structure of the complex Rq(CO)l~(p-C1)2(p3-OEt)2, related to the above compound, was determined by X-ray diffractometry; it was obtained by reacting Ru3(C0)12 with [PPNICl under reflux in EtOH, followed by We obtained good yields of H4Ru(C0)12, H2Ru4(C0)13, FezRu(C0)12, HzFeRu3(C0)13 and FeRu2(C0)12 by reacting Fe(C0)5 with Ru3(C0)12in heptane and adding some water before heating the solution under r e f l ~ x . [When ~ ~ ] D20 was used, deuterated H ~ R u ~ ( C Oand )I~ H2FeRu3(C0)13 were obtained.[861This might indicate that the role of water is that of forming hydridic anions by CO activation. It is known that cluster anions can undergo condensation with metal carbonyls or carbonylmetalate anions forming either cluster anions18 or heterometallic neutral clusters.ls81
+ +
810
2 Metal Clusters in Catalysis
R=H,Ph
Figure 9. Addition of water at a coordinated alkylidene on a Ru-Pt frame.
2.8.3.2 Surface-mediated splitting of water into its components (hydration and dehydration reactions) The allenyl-substituted ruthenium-platinum derivatives shown in Fig. 9[891react with water during chromatography on alumina giving metal-alkylidene hydrides; separate addition of the three fragments forming water to the allenyl ligand apparently occurs. Formation of allenylidene derivatives comparable with the above is observed in the reaction of (Cp)2Mo2(C0)4 with lithium isopropenylacetylide; protonation of the reaction mixture in an alumina column leads to products with coordinated This process should be allenylidene or isopropenyl-alkyne ligands, Fig. compared with the behavior of coordinated propargyl alcohols (Section 2.8.4). Finally, nucleophilic addition of alcohols at the CI carbon of allenyls coordinated on binuclear, phosphido-bridged, iron complexes followed by insertion of a carbonyl, leads to coordinated unsaturated esters[’’] (Fig. 11). Water may also split in two, instead of three, separate fragments; an example is the synthesis of the butterfly complexes R w ( C 0 )10 C=CHPr’)(p3-OR)(PPh2) ( R = H, Et) obtained in wet THF or in THF-EtOH.[92* The reaction of Fe3(C0)12 with C2Et2 in hydrocarbons, or that of Fe(C0)5 with C2Et2 in CH30H-KOH lead to maleoyliron, to hydroxyferrole derivatives, and to Fe2(C0)6[(C2Et2)COO] (Scheme 3); the origin of ‘sawhorse’ Fe2(CO)6{[C(OH)]2(C2Et2)} containing C OH) groups inserted into Fe-C(a1kyne) bonds, was not further investigated. 93*1 The ‘non-sawhorse’ complexes Fe2(C0)6{C4(OH)2R2} ( R = H, Me) had been obtained by reacting acetylenes with [HFe(CO)4]- in aqueous s o l ~ t i o n . [ ~ ~ l The origin of Fe2(C0)6[(C2Et2)COO] has instead been the object of further investigations. It was first established that C02 was not responsible for its formati~n.‘’~] Osella and coworkers subsequently obtained the C2Ph2 and HC2Bu‘ homologs and showed (by using H2”0] that water was needed for the formation of
I
\
2.8 Carbon Monoxide, Water, und Alcohols on Metal Curbonyl Clusters [Cp(CO)1Mo~4fo(CO)3CpJ
I
8 11
CH, =C(Me)C=CLI
Me I
/
C==CH2
\
Li
Me
H'/alumina
1
H'ialumina
Me \ C=CH,
Figure 10. Formation of ene-yne and/or alkylidene ligands on dimolybdenum frames by acidic treatment on alumina.
'%
/
Me
k
these cornplexe~.[~~1 In their opinion moisture in solvents was responsible for the behavior observed. The reaction pathway of Scheme 4 was proposed on the basis of intermediates reported in the literature. Finally, the same complex was obtained by reacting hex-1-en-3-yne with Fe3(CO)12; experimental TLC evidence showed that it was formed by splitting of water into its three components.[y61This reaction should be compared with the dehydration of 1-phenyl-2-propyn-1-01 HC2C( H)Ph(OH) on Fe3(CO)12 (Section
812
2 Metal Clusters in Cutulysis
R = H ,R = Et R = 1'11, R' = M e R = H, R ' = Me R = 1-1. R = 'Pr
Figure 11. Attack of alcohols and CO insertion on binuclear phosphido-bridged iron-allenyl derivatives.
2.8.4, Fig. 23) to give an allenyl complex;[97]this reacts with CH30H forming a binuclear structure comparable with that of Fe2(C0)6(C2Et2COO).[981 Other examples involving hex-3-yne are the reactions of this alkyne with Co2(CO)8/Fe(CO)~or with HFeCo3(C0)12. These lead to tri- and tetranuclear clusters and to dicobalt complexes with oxygen incorporated M to the acetylenic triple bond (e.g. C O ~ ( C O ) ~ [ E ~ C ~ C H ( O Hand )CH CO ~ ]~ ( C O ) ~ [ E ~ C ~ C O(Fig. CH~]$ 12); Fe(C0)s-mediated dehydroxylation of C O ~ ( C OEtC2CH(OH)CH3] )~ led to the formation of the complex FeCo(C0)6[EtC2CHCH3].99*1 Worthy of note is the dehydroxylating ability of Fe(CO)S;this could explain the easy dehydration of propargyl alcohols on Fe3(C0)12 (Section 2.8.4).
/
2.8.3.3 Clusters with ligands which could come from water The reaction of tungsten carbynes with mono- or diiron carbonyls and Me3NO under oxygen-free nitrogen yields, among other products, trinuclear clusters with an oxygen bound to the tungsten atom. Diethyl ether instead of THF had been used as a solvent[lOOa*l and this led the authors to suspect the presence of water or peroxides in the solvent; further experiments showed that oxygen from air can substitute c o . [100b,c*] Another example of an 0x0 complex obtained from dioxygen in organic solvents The cluster Ru3(p3-O)(p3-CO)(CO)~(dppm)2 was is Cp*W(0)Re*(CCR)(C0)6.[1011 also obtained by reacting Ru3(CO)g(dppm)2 with atmospheric oxygen in organic solvents[' 02a] and its chemistry has been H2Ru3(p3-O)(CO)~dppm)2 is an efficient catalyst precursor for homogeneous olefin hydrogenation. '02'] 0 x 0
1
2.8 Curhon Monoxide, Water, and Alcohols on Metul Curhonyl Clusters
-
-
N
7 .
3 N
4
N
L.
0
F
e-
,L --
e
?
N
I I
8 13
814
2 Metal Clusters in Catalysis k2(C0)9 + E t C d E t
- 2co
+ "0"
Et
L Scheme 4. Proposed reaction pathway for the formation of Fe2(C0)6{(C2Et2)C00}.[951
clusters can also be obtained by C-0 bond cleavage of carbon monoxide; these are relevant to the cluster-metal-surface analogy.['03] [ H O S ~ ( C O ) ~reacts ~ ] - with atmospheric oxygen to give OS~(CO)~[C(=O)O~] which reacts with Oss(C0)18 eliminating C02 and forming the carbonylate-linked
2.8 Curhon Monoxide, Water, und Alcohols on Metal Curhonyl Clusters
I
CHR(0H)
8 15
R=Me
Et I
C
Fe:CO), M
Figure 12. Reaction of a heterometallic iron-cobalt cluster with hex-3-yne and water.
Me \
complex o~~(co)~(oco)o~~(co) 17.'104' Another complex containing separate c and 0 fragments (from splitting of a CO ligand) iS ~ u ~ ( ~ ) ( ~ ) ( ~ ~ ) ~ ~ which eliminates COz under thermal conditions, to form a carbido cluster.[lo5*l Some reactions involve water instead of oxygen. The 'hydrolysis' of an amino-
816
2 Metal Clusters in Catalysis
phosphinidene ligand to form a coordinated P=O group and an amine, Fig. 13a, Reaction mechanisms have occurs on the alumina used for been proposed.['06b]Oxidation of phosphorus coordinated to three metal centers can, however, also be obtained under mild conditions with atmospheric oxygen in organic solvents (Fig. 13b).'107] In the reaction sequence illustrated in Fig. 14 a water-derived 0x0 complex has The 0x0 ligand originates from been isolated and structurally water and not from Me3NO; it has been shown that the complex is not formed in rigorously anhydrous solvents, and that the parent cluster reacts with water giving the 0x0 derivative. Finally, in some clusters the origin of the bridging COH or OH groups is unclear; for example the square-pyramidal H2Rug(C0)14(p4-COH) is obtained by reacting [PPN][RU~(CO)~B with H ~[(] MeC6H4-4-CHMe2)RuC12]2or with W(C0)4(MeCN)2 in CH2C12. The possible role of water was not taken into account, despite the use of TLC for the purification of the products. It was suggested that protonation of the cluster forms the carbides H ~ R U ~ ( C O )orI ~Ru5(C0)15C C and The reverse reaction has not been investigated; however, the formation of carbides first, followed by hydration, could explain the results observed. Another example is the heptanuclear cluster (CpMo)4C03(p3-OH)(p3-CPh)(p6C)(p-C0)3(C0)3,obtained from [CpMo(C0)3]2 and C O ~ ( C O ) ~ ( Cunder ~ P ~reflux ~) in o-xylene.["O*l Last, but not least, is the formation of alkyne-substituted butterfly clusters starting from precursors bearing coordinated phosphinoalkynes; the origin of the alkyne hydrogen is unclear. Examples are RuCo3(pu-C0)2(C0)7( PPh2)(HC2Bu' obtained from (p-cymene)RuC12(PPh2C2Bu') and C02(C0)8 in THFf'") and (Cp)NiFe3(CO).i(PPh2)(HC2Pr') obtained from [CpNi(CO)]z, PPh2C2Pr' and Fe3(C0)12 in hydrocarbon solvents.['121
2.8.4 Hydration and dehydration reactions of cluster-bound propargyl alcohols Propargyl alcohol, HCzCCH2OH is an important intermediate in the synthesis of 1,4-butandiol, furan, and THF from acetylene["3] (Fig. 15). HCzCMez(0H) is used in the synthesis of aryl- and diaryl-acetylenes" 14] or of carbamates.[' 51 Dehydration-hydration reactions of cluster-bound propargyl alcohols will be discussed in this section, after a short glance at their behavior on mono- and binuclear complexes.
8 17
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carhonyl Clusters
0 I
Ti m
.-Do
c
3
E L-
p:
3
m k
0
c
0 .-c
m
9 0
Y
".f
-
'v
" c
8 18
2 Metal Clusters in Catalysis
F
xv,
a r?
G o_
f
2.8 Carbon Monoxide, Water, and Alcohols on Metal Curbonyl Clusters
I
819
HCEC-H HCHO
Figure 15. Reactions of acetylene with HZ and CO leading to propargyl alcohols and to furan and THF.
2.8.4.1 Mononuclear complexes Dehydration of propargyl alcohols occurs commonly on mononuclear ruthenium complexes.["6] Water is formed from the terminal alkynic hydrogen and by the alcoholic OH; this is the more common dehydration process (it is denoted Route A). These reactions afford organic intermediates leading to cumulene complexes useful for the synthesis of doped polyacetylenes or of non-linear optic materials.'' ' 6 , 1 71 Dehydration of rhodium-coordinated propargyl alcohols' ' ' 71 leads to free or coordinated cumulenes and can be catalyzed by alumina and chloride ions, Fig. 16. Surface-catalyzed isomerization of mononuclear alkynyl tungsten complexes to ally1 derivatives has also been reported; it occurs on silica gel.'' 1 8 ' The literature contains several examples of deprotonation of dipropargyl alcohols on mononuclear ruthenium complexes leading to dimeric derivatives linked by five
820
2 Metal Clusters in Catalysis
Figure 16. Formation of cumulenes by dehydration of propargyl alcohols coordinated to a metallic center.
to six carbon atom chains; these can be induced by KOH can also promote this type of reaction.“ ‘1 Reverse processes, that is reactions of propargyl or allenylidene ligands coordinated to mononuclear centers with water, methanol, amines, and other ligands are known. Tungsten[’221and platinum[’231complexes give a series of reactions (some of which are mediated by surfaces) comparable with those discussed for clusters. Hydration of the cumulene ligand in the complex [CpRu(PPh3)2(=C=C=CH2)]+ (Fig. 17) occurs even in the presence of traces of water (atmospheric moisture); the reaction was explained as nucleophilic attack of water on the cationic center followed by loss of a proton.[’24]It is unclear, however, if the reaction is mediated by surface materials. A similar complex, PtBr{C( H)=C=CMez}( PPh3)2 is oxidized to the alkynic trans-PtBr { CzCMe2(00H)}(PPh3)2 by atmospheric oxygen under sunlight.[ 251
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters
821
Figure 17. Hydration of a cumulene coordinated to ruthenium.
Finally, insertion of alkynes into dihydrogen mononuclear cationic rhenium complexes, stabilized by the triphos (= 1,1,1-tris(diphenylphosphinomethy1)ethane) ligand, leads to vinylidene derivatives which react with water or ethanol eliminating silanols or methane,[lz6]Fig. 18. Solvents were acetone or THF; no chromatography was used.
2.8.4.2 Bimetallic complexes Protonation of Co2(C0)6{HC2R(Me)(OH)} complexes with HF.SbF5 or HBF4.Et20 at low temperature results in the formation of stable carbonium ion salts. Loss of water occurs; this comes from the alcoholic OH and one of the Me groups and a vinylacetylene ligand is formed (this is denoted Route B).['271 Dicobalt 'metal-substituted' propargyl derivatives undergo an unusual protonation reaction in the presence of CF3COOH (Fig. 19a).[89]Although the mechanism of the reactions is not well understood, exchange of metal fragments on dicobalt derivatives seems to be a common process[991(Fig. 12). Protonation of alkynes or allenyls coordinated to dimolybdenum derivatives results in loss of methanol and formation of cations with coordinated propargyls (Fig. 19b);[1281 again, these reactions should be compared with the behavior of dicobalt complexes.[991
Reactions of cobalt and nickel derivatives Co2(C0)x reacts with propargyl alcohols giving benzenic cyclotrimers and dehydrated oligomers.L2",b1 By contrast, (CDT)Ni (CDT = cyclododecatriene) reacts with alkynes to give homoleptic trinuclear clusters.[1291No dehydration occurs during the formation of these complexes; it is worthy of note that diols do not react with nickel. Instead, cyclopentadienyl tricobalt clusters substituted with the cyclic furyne
2 Metal Clusters in Catalysis
822
lf
l+
I
1-
L
H-C=C-SiMe,
H'
It
c
H '
RNC'H
I
I
H3
l+
l+
I
EtoH 1 --
H"'\SiMe3
I I
+ H20 - SiMe30H
l+
Figure 18. Reaction of triphos-dihydrogen complexes of rhenium with alkynes, water, and alcohols.
ligand have been obtained by dehydration of coordinated butyndiol in decalin under r e f l ~ x [ '(Fig. ~ ~ ] 20).
2.8.4.3 Reactions of propargyl alcohols with metal carbonyl clusters of the iron triad Different reactivity trends on M~(C0)12clusters ( M = Fe, Ru, 0 s ) have been observed, depending on the nature of the cluster metals. Two dehydration routes have been observed for cluster-bound propargyl alcohols the less common is similar to the main process observed for mononuclear complexes and involves loss of the terminal alkynic hydrogen and of the alcoholic OH (Route A). The more common, when CH3 groups are available, involves loss of one methyl hydrogen and of the adjacent OH (Route B). The former process leads to allenylidene and the second to '1 Examples are given in Fig. 21. ene-yne (or vinylacetylide) derivati~es.''~ ~
CHIOMe
/
N
\
l+
Figure 19. (a) Protonation of 'metallo-substituted' dicobalt propargyl derivatives. (b) Loss of methanol from dimolybdenum propagyl-alkyne or -allenyl derivatives.
I .
Me
824
2 Metal Clusters in Catalysis OH /
OH \
Figure 20. Dehydration cyclization reaction of butyndiol to give a coordinated furyne.
Cluster-bound vinylacetylides may react with alcohols, forming allenylidene complexes; for example Cp*WRez(C0)9 { CzC(=CH2)Me} reacts with methanol giving an open allenylidene cluster,['321Fig. 22. Interestingly the proton of CH30H forms the C=C=CMe2 ligand, whereas the CH30 bridges the open Re-Re edge.
Reactions of triiron dodecacarbonyl The dehydration trends discussed above are well exemplified by the reactions of Fe3(C0)12 with HCzC( Me)2(0H) which lead to the (main product) pentagonal bipyramidal Fe3(C0)6(p-C0)2{ [HCzC(Me)z(OH)]HC2C(Me)(=CHz)} cluster in which one of the two alkyne ligands has lost water (via route B), and to small yields of the allenylidene Fe3(C0)9(p-CO)tC=C=CMe2) upon loss of the terminal hydrogen and of the OH (Route A).[1331 Although it might be expected that in the absence of methyl substituents only dehydration Route A could occur, Fe3(C0)9(pL-CO)(C=C=CPh2) is obtained in small yields only from reaction of Fe3(C0),2 and HCzC(OH)PPh*.['341 Allenylidene triiron clusters such as that shown in Fig. 21b were obtained upon C-C bond cleavage in c ~ m u l e n e s . [ 'The ~ ~ ] first complex of this type was, however, obtained by reacting Fe(CO)5 and [C2C(Bu')20I2- in the presence of Me3N0, through chemical CO activation and metal fragment condensation.[' 361 The reaction of 1-phenyl-2-propyn-1-01 (HC2C( H)(OH)Ph) with Fe3(CO) 12 leads to the allenylidene Fe3(C0)9(p-C0){ C=C=C( H)Ph} and to Fez(CO)6[Ph(H)CCCH(OMe)0].1971The latter is formed upon addition of methanol to the triiron allenylidene complex.['371In Fig. 23 the proposed pathway of formation of the binuclear derivative is compared with the hydration of coordinated hex-1-en-3-yne (Schemes 3 and 4). Propargyl alcohols can react with triiron dodecacarbonyl in unexpected ways; one example is the 'deoxygenation' of HC2C(Me)(OH)Ph to give a binuclear derivative['38*](Fig. 24). This reaction can be compared with the WGSR reaction. A similar structure was obtained through a complex reaction pathway involving ( R = Me, protonation and loss of MeOH from Fe2(C0)6[p-C(OR)](pu-CR'=CR'H) Et), Fig. 25.[139*]
2.8 Curhon Monoxide, Water, and Alcohols on Metal Curbonyl Clusters
825
Figure 21. Some examples of dehydration routes A and B. (a) The main process occurring for hydrido-acetylide triruthenium derivatives.[14'l (b) Dehydration occurring on Fe3(C0)l2;l1331 the proposed intermediate is given in square brackets. (c) Possible intermediate for dehydration route B.11471
826
2 Metal Clusters in Catalysis
Figure 22. Reaction of methanol with a coordinated ene-yne ligand on a trirhenium cluster.
Contrasting with the behavior of M3(CO)12 ( M = Ru, 0 s ) discussed below, Fe3(CO)12 dehydrates alkynediols without C-C bond cleavage forming dinuclear butatriene complexes and allenylidene-ket~nes.[~~~~
Reactions of M3(C0)12 carbonyls (M = Ru, 0s) In contrast with the easy dehydration observed for the iron compound, the M3(C0)12 clusters with M = Ru or 0 s form hydrido-acetylide derivatives which undergo dehydration only under acidic conditions ( protonation reactions). Protonation of 'unsubstituted' clusters generally results in the formation of bridging hydride l i g a n d ~ . " ~Protonation ~] of cluster-bound ligands can follow different mechanisms; for acetylide-substituted clusters, e.g. HRu3(C O ) ~ ( C ~ B U ' ) , [ ' ~ ~ ] ' ~give ~ , a~ ~ ~ ] protonation occurs either at the cluster or at the acetylide c( c a r b ~ n ~ to cationic dihydride. In some instances, e.g. [Fe3(C0)9(C20R)IP,C-C bond cleavage and formation of bis-methylidyne complexes This process has not been observed for Ru or 0 s deri~atives.['~'1 As previously pointed out, the reaction of the propargyl alcohols HC2CRR' (OH) ( R = Me, R' = El, Ph) with Ru3(C0)12 results in oxidative addition and formation of the acetylide complexes HRu3(C0)9 { C2CRR'( OH)}; protonation, with CF3COOH, of the complex with R = C(Me)(OH)Ph) in benzene solution affords H R U ~ ( C O ) ~ { C ~ C ( = C Hupon ~ ) Ploss ~ ) of water via Route B (Fig. 21a).[146] A possible intermediate for dehydration Route A (leading to allenylidene complexes) was instead obtained upon protonation of H O S ~ ( C O ) ~ ( C ~ C P ~ ~ the (OH)); product, (H)(OH)Os3(C0)9C=C=CPh2) is characterized by a bridging OH on an open cluster edge (Fig. 21c).0147'1 Coordination-dehydration reactions of propargyl alcohols have been followed in detail on triosmium clusters; an example is given in Fig. 26.[1481Protonation of triosmium clusters substituted with 'parallel' alkynes leads to propargyl cationic derivatives which, in turn, can isomerize to allenylidene complexes, Fig. 27.[149*] Hydrido-acetylide derivatives can also be obtained upon cleavage of a C-C bond of a diol, as in the example in Fig. 28. Worth noting is the formation of a ketone
2.8 Curhon Monoxide, Wuter, and Alcohols on Metal Curbonyl Clusters
827
n HC=C-C-Ph
I
1
OH
1
HzO
1
1
+ CH;OH
1
t
H.O
Figure 23. Comparison of the reactions of hex-3-yne and 1 -phenyl-2-propyn-l-ol with Fe3(C0)12. Formation of ferrole derivatives upon splitting of water or of methanol.
2 Metal Clusters in Catalysis
828
Me I HC=C--C--OH
I
+ OC--ML,
---D
[HCC(H)C(Me)Phl-ML,
+ C02
Ph
Figure 24. Deoxygenation of HCzC(Me)(OH)Ph on Fe3(CO)12.
and the shift of the OH hydrogen to the cluster, to form the hydride substituent.“’’] This behavior occurs when there are no hydrogen atoms ct to the triple carboncarbon bond; with HOCH2C2CH20H 1,4-dihydroxybut-2-yne) formation of allenyl and ally1 derivatives is observed,[’” Fig. 29.
(I
New hydration-dehydration reactions of propargyl alcohols coordinated to triruthenium dodecacarbonyl A systematic study has been performed on the protonation reactions of the hydrides HRu~(CO)~{C~R(OH)R’} (Fig. 21a) in organic solvents;[’521the yields of dehydrated products increase either with time or on increasing the acid concentration, thus indicating direct electrophilic attack of Hf on the alcoholic OH in a process comparable with the protonation mechanisms reported for alcohols on s~rfaces.[’’~] Very low yields of dehydrated products were, however, obtained on the silica of the TLC plates; the same occurred when hydration reactions of the H R U ~ ( C O ) ~ ( C ~ C ( = C Hderivatives ~)R} were attempted either on TLC plates or in a slurry (suspension) of silica and water in hydrocarbon solvents. This indicates that ‘surface materials’ play a minor role in these reactions. In contrast, the reactions of R u ~ ( C O with ) ~ ~ propargyl alcohols in CH30HKOH solutions lead to the hydrido-allenylidene complexes HRu3(C0)9(HCCCRR’) (Fig. 30) not previously observed in the ‘direct’ synthetic route starting from R u ~CO)l2 ( and propargyl alcohols in hydrocarbons; these complexes are formed upon loss of OH with a reaction mechanism still unknown. Finally, the chiral cluster HRu3(C0)9 {C2CMe(OH)Ph} reacts with dppm-
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters
k
H
H
H I
Figure 25. Formation of a diiron allylic complex upon protonation and loss of alcohol.
829
HC=CCMe20H
'
Me !,O .H
HC S C M e 2 0 H
Me rI -OH
I
0
+
Figure 26. Coordination-dehydration clusters.'48
routes for propargyl alcohols coordinated to triosmium
2.8 Carbon Monoxide, Wuter, and Alcohols on Metal Curhonyl Clusters
i -;
+
I
I
a
c,
k h h e
t,
L.
0
83 1
832
2 Metal Clusters in Catalysis
(H0)RR'C -C-C-CRK'(
OH)
,CRR(OH)
+ (CO),
R\
Figure 28. Splitting of a C-C (sigma) bond in diols to form hydrido-acetylide clusters and a ketone.
MejN0.2HzO forming HRu3(CO),(dppm) {C2CMe(OH)Ph} whose structure and fluxionality have been studied. The substitution reaction occurs without dehydration of the coordinated propargyl alcohol. Protonation of the complex leads to HRu3(C0)7 {C2C(=CH,)Ph} (Fig. 31). Neither reorientation of the acetylide nor formation of parallel alkynes is observed in this reaction.['52]
2.8.5 Concluding remarks The examples discussed above indicate that although - at present several homogeneous and surface-mediated reactions of water or alcohols with metal carbonyl clusters are known, much more work is still needed to gain a full knowledge of these reactions and for exploiting this reactivity for synthetic or catalytic purposes. In recent times an increasing number of reports on this subject have appeared. New examples will hopefully appear in the near future and the analogies between surface organometallic chemistry and 'surface-mediated homogeneous reactions' (occurring in solvents) will be further clarified. We could collect literature evidence for the following types of reactivity: ~
i) electrophilic or nucleophilic attack at cluster-bound carbonyls under homogeneous conditions or on surfaces; ii) homogeneous or surface-mediated formation of (hydridic) cluster anions;
2.8 Carbon Monoxide, Water, and Alcohols on Metal Curbonyl Clusters
HOH,C-CS--CH,OH
I
iii) splitting of water into its components promoted by surface materials; iv) hydrolysis and condensation reactions; and v) hydration-dehydration reactions under acidic conditions.
833
834
2 Metal Clusters in Catalysis
Figure 30. Isomeric, neutral, triruthenium allenylidene derivatives obtained from reactions with propargyl alcohols in basic methanolic solution. The terminal C-H has been omitted for clarity.
'\
Me
Figure 31. Dehydration (Route A, see text) of a dppm-substituted chiral cluster.
2.8 Carbon Monoxide, Wrrtev, and Alcohols on Metal Carhonyl Clusters
835
Last but not least, the interaction of organometallic complexes with water in the presence of inorganic oxides (alumina or silica, the main components of the Earth's crust) could also help to understand the speciation and reactivity of these complexes in the environment.
Acknowledgments Financial support of this work came from MURST (Rome) and from the University of Torino (scambi culturali). A fellowship (to P. J. K.) in the European Scientific Exchange Programme ( Royal Society of Chemistry-Accademia dei Lincei) is acknowledged.
List of abbreviations Bu" n-butyl Bu' = isobutyl Bur = t-butyl. CDT = cyclododecatriene CHD = cyclohexadiene (e.g. 1,3-CHD, 1,4-CHD) CHDO = 1,3-~yclohexadienone COD = cyclo-octadiene COT = cyclo-octatetraene DMF = dimethylformamide dppm = bis(dipheny1phosphino)methane Et = ethyl ETC = Electron Transfer Catalysis Me = methyl NLO = Non Linear Optics Ph = phenyl PPN = bis(tripeny1phosphino)iminium cation [N(PPh3)2]$ Pr" = n-propyl Pr' = iso-propyl WGSR = Water Gas Shift Reaction THF = tetrahydrofuran T.1.c. = thin layer chromatography. To1 = tolyl Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane.
836
2 Metal Clusters in Catalysis
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[71] S. R. Wang and C. P. Cheng, J. Chem. Soc., Chem. Commun., 1993,470. [72] J. A. Ayllon, C. Gervaux, S. Sabo-Etienne and B. Chaudret, Organometallics, 1997, 16, 2000. [73] D. Braga, A. L. Costa, F. Grepioni, L. Scaccianoce and E. Tagliavini, Organometallics, 1997, 16, 2070. [74] M. H. Chisholm, K. Folting, C. E. Hammond, J. C. Huffman and J. D. Martin, Angew. Chem. Int. Ed. Engl., 1989, 28, 1368. [75] W-W. Zhuang and D. M. Hoffman, Organometallics, 1997, 16, 3102. [76] B. F. G. Johnson, J. Lewis and P. A. Kilty, J. Chem. Soc., Chem. Commun., 1968, 180. [77] (a) B. G . F. Johnson, J. Lewis and P. A. Kilty, J. Chem. Soc., 1968, A , 2859. (b). E. G. Bryan, B. F. G. Johnson and J. Lewis, J. Chem. Soc., Dalton Trans., 1977, 1328. [78] S. Aime, M. Botta, R. Gobetto, D. Osella and F. Padovan, J. Chem. Soc., Dalton Trans., 1987, 253. [79] G. R. Crooks, B. F. G. Johnson, J. Lewis and 1. G. Williams, J. Chem. Soc., 1969, A , 797. [SO] V. A. Alexiev, N. Binsted, J. Evans, G. N. Greaves and R. J. Price, J. Chem. Soc,., Chem. Commun., 1987, 395. [81] C. Dossi, A. Fusi, M. Pizzotti and R. Psaro, Organometallics, 1990, 9, 1994. [82] D. B. W. Yawney and F. G. A. Stone, J. Chem. Soc., 1969, A , 502. [83] T. Venalainen and T. A. Pakkanen, J. Orgunornet. Chem., 1986, 316, 183. [84] B. F. G. Johnson, J. Lewis, J. M. Mace, P. R. Raithby and M. D. Vargas, J. Orgunomet. Chem., 1987,321, 409. [85] R. Giordano and E. Sappa, J. Organornet. Chem., 1993,448, 157. [86] R. Gobetto, E. Sappa and F. Verre, unpublished. [87] See for example: (a) P. A. Dawson, B. F. G. Johnson, J. Lewis, D. A. Kaner and P. R. Raithby, J. Chem. Soc., Chem. Commun., 1980, 961. (b) P. F. Jackson, B. F. G. Johnson, J. Lewis, M. McPartlin and W. J. H. Nelson, J. Chem. Soc., Chem. Commun., 1979, 735. (c) S. Kennedy, J. J. Alexander and S. G. Shore, J. Organomet. Chem., 1981,219, 385. [88] G. L. Geoffroy and W. L. Gladfelter, J. Am. Chem. Soc., 1977, 99, 7565. [89] (a) A. Wojciki and C. E. Schuchart, Coord. Chem. Rev., 1990,105, 35. (b) C. H. Young and A. Wojcicki, J. Organornet. Chem., 1990, 390, 351. (c) D. Nucciarone, N. J. Taylor and A. J. Carty, Organometallics, 1996, 5, 1179. The similarity of reactions between propargyl and allenyl derivatives is of interest for the reactivity of propargyl alcohols, discussed in Sec. 2.8.4. [90] S. F. T. Froom, M. Green, R. J. Mercer, K. R. Nagle, A. G. Orpen and S. J. Schwiegk, J. Chem. Soc., Chem. Commun., 1986, 1666. [91] S. Doherty, M. R. J. Elsegood, W. Clegg and D. Mampe, Organometallics, 1997, 16, 1186. [92] A. J. Carty, S. A. MacLaughlin and N. J. Taylor, J. Chem. Soc., Chem. Commun., 1981, 476. [93] S. Aime, L. Milone, E. Sappa, A. Tiripicchio and A. M. Manotti Lanfredi, J. Chem. Soc., Dalton Truns., 1979, 1664. [94] (a) I. Wender, R. A. Friedel, R. Markby and H. W. Sternberg, J. Am. Chem. Soc., 1955, 77, 4946. (b) H. W. Sternberg, R. A. Friedel, R. Markby and I. Wender, J. Am. Chem. Soc., 1956, 78,3621 and 6206. (c) H. D. Kaesz, R. B. King, T. A. Manuel, L. D. Nichols and F. G . A. Stone, J. Am. Chem. Soc., 1960, 82, 4749. For a discussion on the sawhorse/non-sawhorse isomerism see: (d) F. A. Cotton and J. M. Troup, J. Am. Chem. Soc., 1974,96, 1233. (e) W. P. Fehlhammer and M. Stolzenberg, in G. Wilkinson, F. G. A. Stone and E. W. Abel (eds), Comprehensive Orgunometallic Chemistry, 1 st Ed., Vol. 4, p. 548, Pergamon Press, Oxford 1982. [95] L. Milone, D. Osella, M. Ravera, P. L. Stanghellini and E. Stein, Gazz. Chim. Ital., 1992, 122, 451. [96] G. Gervasio and E. Sappa, J. Orgunomet. Chem., 1995, 498, 73. [97] G. Gervasio, D. Marabello and E. Sappa, J. Chem. Soc., Dalton Trans., 1977, 1851. [98] ‘Lactone’ derivatives with formulae C O ~ ( C O ) ~ ( ~ - CRC2R’ O ) ( COO), with the ligands bound to the metals only through a carbon atom, are obtained upon regioselective carbonylation of coordinated alkynes: (a) G. Palyi, G. Varadi, A. Visi-Orosz and L. Marko, J. Organomet.
2.8 Carbon Monoxide, Water, and Alcohols on Metal Carbonyl Clusters
841
Chem., 1975, 90, 85. (b) D. J. S. Guthrie, I. U. Khand, G. R. Knox, J. Kollmeier, P. L. Pauson and W. E. Watts, J. Orgunomet. Chem., 1975, 90, 93. (c) G. Varadi, I. Vecsei, I. Otvos, G . Palyi and L. Marko, J. Organomet. Chem., 1979, 182, 415. (d) G. Palyi, G. Varadi and I. T. Horvath, J. Mol. Cat., 1981, 13, 61. (a) S. Aime, L. Milone and D. Osella, J. Chem. Soc., Chem. Commun, 1979, 704. (b) S. Aime, L. Milone, D. Osella, A. Tiripicchio and A. M. Manotti Lanfredi, Inorg. Chem., 1982, 21, 501. (c) S. Aime, D. Osella, L. Milone, A. M. Manotti Lanfredi and A. Tiripicchio, Inorg. Chim. Acta, 1983, 71, 141. (d) S. Aime, D. Osella, L. Milone and A. Tiripicchio, Polyhedron, 1983, 2, 77. (a) L. Busetto, J. C. Jeffery, R. M. Mills, F. G. A. Stone, M. J. Went and P. Woodward, J. Chem. Soc., Dalton Trans., 1983, 101. (b) G . A. Carriedo, J. C. Jeffery and F. G. A. Stone, J. Chem. Soc., Dalton Trans., 1984, 1597. (c) L. Busetto, J. C. Jeffery, R. M. Mills, F. G. A. Stone, M. J. Went and P. Woodward, J. Chem. Soc., Dalton Trans., 1983, 101. Y. Chi, H-L. Wu, C-C. Chen. C-J. Su, S-H. Peng and G-S. Lee, Organometallics, 1997, 16, 2434. (a) G. Lavigne, N. Lugan and J.-J. Bonnet, New. J. Chem., 1981, 5, 423. (b) A. Colombie, J.-J. Bonnet. P. Fompeyrine, G. Lavigne and S. Sunshine, Organometallics, 1986, 5, 1154. (c) C. Bergounhou, P. Fompeyrine, G. Commenges and J.-J. Bonnet, J. Mol. Cat., 1988, 48, 285. See for example: (a) M. H. Chisholm, C. E. Hammond, V. J. Johnston, W. E. Streib and J. C. Huffman, J. Am. Chem. Soc., 1992,114, 7056. (b) J. T. Park, M-K. Chung, K. M. Chun, S. S. Yun and S. Kim, Oryanometallics, 1992, ! I , 3313. (c) B. C. Gates, Angew. Chem. Int. Ed. Engl., 1993, 32, 228. See also: (d) P. Blenkiron, A. J. Carty, S-M. Peng, G-H. Lee, C-J. Su, C-W. Shiu and Y. Chi, Oryanometullics, 1997, 16, 519. C. J. Cathey, B. F. G. Johnson, J. Lewis, P. R. Raithby and W. T. Wong, J. Organornet. Chem., 1993, 450, C 12. P. J. Dyson, B. F. G. Johnson, C. M. Martin, D. Reed, D. Braga and F. Grepioni, J. Chem. Soc., Dalton Trans., 1995, 909. (a) J. F. Corrigan, S. Doherty, N. J. Taylor and A. J. Carty, J. Am. Chem. Soc., 1994, 116, 9799. (b) W. Wang and A. J. Carty, New J. Chem., 1997, 21, 773. J. E. Davies, M. C. Klunduk, M. J. Mays, P. R . Raithby, G. P. Shields and P. K. Tompkin, J. Chem. Soc., Dalton Trans., 1997, 715. J. T. Park, J-H. Chung, H. Song, K. Lee, J-H. Lee, J-R. Park and I-H. Suh, J. Organomet. Chem., 1996,526, 215. J. R. Gdlsworthy, C. E. Housecroft, R. L. Ostrander and A. L. Rheingold, J. Organomet. Chem., 1995, 492, 21 1. A. D. Shaposhnikova, M. V. Drab, G. L. Kamalov, A. A. Pasynskii, I. L. Eremenko, S. E. Nefedov, Yu. T. Struchov and A. I. Yanovsky, J. Organomet. Chem., 1992, 429, 109. D. F. Jones, P. H. Dixneuf, A. Benoit and J.-Y. Le Marouille, J. Chem. Soc., Chem. Commun., 1982, 1217. E. Sappa, D. Belletti, A. Tiripicchio and M. Tiripicchio Camellini, J. Organomet. Chem., 1989,339,419. G . L. Castiglioni, C. Fumagalli and A. Vaccari, Chim. e Ind. (Milan), 1996, 78, 575, and references therein. [ I 141 A. Carpita. A. Lessi and R. Rossi, Synthesis, 1984, 571. [ I 151 T.-J. Kim, K.-H. Kwon. S.-C. Kwon, J.-0. Baeg, S.-C. Shim and D. H. Lee, J. Organomet. .~ Chem., 1990,389, 205. [I161 See, for example: (a) J. P. Selegue, J. Am. Chem. Soc., 1983, 105, 5921. (b) C. Bruneau and P. H. Dixneuf, J. Chem. Soc., Chem. Commun., 1997, 507. (c) B. M. Trost, Chem. Ber., 1996, 129, 1313-1322. (d) M. A. Esteruelas, L. A. Oro and J. Schrickel, Organometallics, 1997, 16, 796. (e) M. P. Gamasa, J. Gimeno, C. Gonzales-Bernardo, J. Borge and S. Garcia-Granda, Organometullics, 1997, 16, 2483.
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[ I 171 (a) R. W. Lass, P. Steinert, J. Wolfand H. Werner, Chem. Eur. J., 1996, 1, 19. (b) H. Werner, R. Wiedemann, N. Mahr, P. Steinert and J. Wolf, Chem. Eur. J., 1996,2, 561. (c) H. Werner, J. Chem. Soc., Chem. Commun., 1997, 903. [I 181 J. Ipaktschi, F. Mirzaei, G. J. Demuth-Eberle, J. Beck and M. Serafin, Orgunometallics, 1997, 16, 3965. [ 1191 G. Jia, H. P. Xia, W. F. Wu and W. S. Ng, Organometallics, 1996, 15, 3634. [120] (a) A. J. Hodge, S. L. Ingham, A. K. Kakkar, M. S. Khan, J. Lewis, N. J. Long, D. G. Parker and P. R. Raithby, J. Organomet. Chem., 1995, 488, 205. (b) V. Cadierno, M. P. Gamasa, .I. Gimeno, E. Lastra, J. Borge and S. Garcia Granda, Organometallics, 1994, 13, 745. H. P. Xia, W. F. Wu, W. S. Ng, I. D. Williams and G. Jia, Orgunometallics, 1997, 16, 2940. (a) R-S. Keng, Y-C Lin, Organometullics, 1990, 9, 289. (b) J. Pu, T.-S. Peng, A. M. Sharif and J. A. Gladysz, Organometallics, 1992, 11, 3232. (c) M.-C. Cheng, R.-S. Keng, Y.-C. Lin, Y. Wang, M. C. Cheng and G.-H. Lee, J. Chem. SOC.Chem. Commun., 1990, 1138. (a) J.-T. Chen, T.-M. Huang, M.-C. Cheng, Y.-C. Lin and Y. Wang, Organometallics, 1992, 11, 1761. (b) T.-M. Huang, R.-H. Hsu, C.-S. Yang, J.-T. Chen, G.-H. Lee and Y. Wang, Organometallics, 1994, 13, 3657. M. I. Bruce, P. Hinterding, E. R. T. Tiekink, B. W. Skelton and A. H. White, J. Organornet. Chem., 1993, 450, 209. See also: M. I. Bruce, D. N. Duffy, M. G. Humphrey and A. G. Swincer, J. Orgunornet. Chem., 1985, 282, 383. J. M. A. Wouters, K. Vrieze, C. J. Elsevier, M. C. Zoutberg and K. Goubitz, Organometallics, 1994, 13, 1510. C. Bianchini, A. Marchi, L. Marvelli, M. Peruzzini, A. Romerosa and R. Rossi, Organometallics, 1996, 15, 3804. R. E. Connor and K. M. Nicholas, J. Organomet. Chem., 1977,125, C 45. A. Meyer, D. J. McCabe and M. D. Curtis, Organometallics, 1987, 6, 1491. T. Klettke, D. Walther, A. Schmidt, H. Gorls, W. Imhof and W. Gunther, Chem. Ber., 1996, 129, 1451. W. D. King, C. E. Barnes and J. A. Orvis, Organometallics, 1997, 16, 2152. See for example; (a) H. El Amouri and M. Gruselle, Chem. Rev.,1996, 96, 1077. (b) S. Doherty, J. F. Corrigan, A. J. Carty and E. Sappa, Adv. Organomet. Chem., 1995, 37, 39. J.-J. Peng, K.-M. Horng, P.-S. Cheng, Y. Chi, S.-M. Peng and G.-H. Lee, Organometallics, 1994, 13, 2365. E. Sappa, G. Predieri, A. Tiripicchio and F. Ugozzoli, Gazz. Chim. Ital., 1995, 125, 51. P. J. King, E. Sappa, unpublished results. M. Iyoda, Y. Kuwatani and M. Oda, J. Chem. Soc., Chem. Commun., 1992, 399. H. Berke, U. Grossmann, G. Huttner and I. Zsolnai, Chem. Ber., 1984, 117, 3432. Methanol was present as stabilizer in the Fe3(C0)12used in the reaction. G. Gervasio and E. Sappa, Organometallics, 1993, 12, 1458. R. Yanez, J. Ros, X. Solans, M. Font-Altaba and R. Mathieu, Organometallics, 1990, 9, 543. R. Victor, J. Organomet. Chem., 1977, 127, C25. See for example: M. Green, R. M. Mills, G. N. Pain, F. G. A. Stone and P. Woodward, J. Chem. Soc., Dalton Trans., 1982, 1321. S. Aime, G. Gervasio, L. Milone, E. Sappa and M. Franchini-Angela, Znorg. Chim. Acta., 1978,26, 223, and references therein. C. Barner-Thorsen, E. Rosenberg, G. Saatjian, S. Aime, L. Milone and D. Osella, Inorg. Chem., 1981,20, 1592. J. A. HriljacandD. F. Shriver, J. Am. Chem. Soc., 1987, 109, 6010. Protonation of dihydridic methylidyne-substituted triosmium clusters containing a 4,4'bipyridine ligand (,u3-CNC6H4C6H4N)leads to trihydridic dicationic derivatives soluble in water because of the formation of a N-H bond on the ligand. W-Y. Wong and W-T. Wong, J. Chem. Soc., Dalton Trans., 1995, 3995.
2.8 Curhon Monoxide, Wuter. und Alcohols on Metul Curhnnyl Clusters
843
[ 1461 S. Ermer, R. Karpelus. S. Miura, E. Rosenberg, A. Tiripicchio and A. M. Manotti Lanfredi, J. Organornet. Chem.. 1980, 187, 81. [I471 S. Aime, A. J. Deeming. M. B. Hursthouse and J. D. J. Backer-Dirks, J. Chen?. Soc.. Dulton Truns., 1982, 1625. [ 1481 S. Aime and A. J. Deeming, J. Chen?. Soc., Dalton Truns., 1981, 828. [I491 V. V. Krivykh, 0. A. Kizds, E. V. Vorontsov, F. M. Dolgushin. A. I. Yanovsky, Yu. T. Struchov and A. A. Koridze, J. Orgunornet. Chern., 1996, 508, 39. [I501 S. Aime. L. Milone and A. J. Deeming, J. Chern. Soc.. Chcn?. Cornnzun., 1980, 1168. [ 1511 S. Aime. A. Tiripicchio. M. Tiripicchio Camellini and A. J. Deeming, Inorg Chenz., 1981, 20, 2021. [152] G. Gervasio, R. Gobetto, P. J. King, D. Marabello and E. Sappd, Polyhedron, 1998. 17, 2937. [I531 K. Tanabe, M. Misono, Y. Ono and H. Hattori, in B. Delmon and J. T. Yates (Ed), 'New Solid Acids and B~i.se.s',Studies in Surface Science and Catalysis, Vol. 51, Elsevier, Tokyo. 1989.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.9 Solid-gas Reactions Involving Metal Carbonyl Clusters Silvio Aime, Walter Dastru, Roberto Gobetto, and Alessandra Vide
2.9.1 Introduction As early as 1918, H.W. Kohlshutter"] noted that solid-state reactivity differs from traditional solution reactivity essentially in that the former occurs within the constraining influence of the three-dimensional (if bulk) or two-dimensional (if surface) periodic environment of the crystal which can control both the kinetics and the nature of the products. The advantages of a solid-state approach compared to routine solution studies are manifold: i) the possibility of avoiding the use of solvent has positive environmental implications; ii) topochemical control might address the formation of products that differ from those obtained in solution, affording new synthetic opportunities; iii) in the solid-gas reactions a single product is often obtained, instead of a plethora of compounds as found in solution; and iv) in fortunate cases it is possible to 'freeze' reaction intermediates; the possibility of characterizing such species can be of paramount importance in the elucidation of reaction pathways. A better understanding of stoichiometric transformations of solids might lead to the development of catalytic processes in the solid state, or might provide ideas for a crystal engineering approach and useful insights into the design of novel sensor devices for gases. In recent years an increasing number of applications of solid-state reactions in many different fields of applied chemistry has been developed, mainly for pharmaceutical or macromolecular purposes.['] Also in organometallic chem-
2.9 Solid-gas Reactions lnvoluing Metal Carhonyl Clusters
845
istry, compounds which react thermally in the solid state have been reported in processes such as ligand exchange, racemization, isomerization, polymerization, and oxidative addition Recently a systematic approach has been developed for the design of organometallic complexes capable of reacting with organic and inorganic gaseous reactants. For example, a large number of reactions involving mononuclear organometallic complexes containing Ni, Co, Rh, and Ir with gaseous molecules such as 1 ~It~has also been hydrogen, nitrogen, and carbon monoxide has been r e p 0 r t e d . 1 '~ found that Ir complexes loose hydrides reversibly and undergo interconversion between classical and non-classical dihydrogen in the solid state." 21 Furthermore, the H2 ligand in solid M(C0)3(PR3)2(H2) complexes ( M = Mo, W; R3 = Cy3, Pri3, Cyz-Pr') exchanges with D2 to give HD,[13]and solid Ir(COD)(PPh3)2PW12040 reacts with D2 to give highly deuterated cyclooctene and c y c l ~ o c t a n e . ~ ' ~ ~ ~ ~ ~ Few studies have dealt with the use of transition metal clusters as solid reactants for solid-gas reactions. This seems surprising if one considers that polymetallic systems offer the advantages related to the exceptional variety of bonding modes for different ligands, as has been well demonstrated in recent decades with many examples. Furthermore, molecular metal clusters were proposed as models for metal surfaces nearly 20 years ago." 6] Their solid-state reactivity might, then, provide interesting insights into the development of the cluster-surface analogy. This contribution is a report on progress in the authors' laboratory in the field of solid-gas reactions involving trinuclear metal carbonyl clusters of osmium as solid substrates.
2.9.2 Solid-gas reaction pathways By using reactive solid substrates one might expect that the atoms or molecules of the surface would react rapidly with a gaseous reactant, but obviously this effect extends only over a very limited depth. Two possible pathways can be proposed to account for the completion of a reaction between a bulk solid and the gaseous reactant: i) if the crystalline solid is loosely packed and contains channels, the small gaseous molecules can diffuse and flood the whole solid; and ii) if the crystal does not contain channels, the gaseous molecules react only at the surface of the solid but disruption of the crystalline order can enable further substrate molecules to interact with the gaseous reactant which penetrate the interlayer spaces. Lattice expansion is, then, responsible for the diffusion of the gas inside the solid substrate.
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2 Metal Clusters in Catalysis
Thus a solid reaction can proceed if two criteria are satisfied:
i) the surface reaction is spontaneous; and ii) the compound formed separates from the reactant substrate molecules. In principle one expects solid-gas reactions to proceed faster in solids with a high surface area. Promising routes for enhanced reactivities are then represented by the dispersion of the solid substrate on an inert support with a high surface area. Hightemperature conditions can also improve gas permeability into the crystal structure, thus enabling the reactivity of the inner layers. The use of thermal activation might, nevertheless, have several disadvantages for example partial decomposition of the substrate, the presence of undesired secondary products, or the greater reactivity of intermediates that cannot be characterized. For these reasons it is preferable to avoid high temperature conditions and to use very reactive, finely dispersed organometallic materials. In solid-gas reactions the material crystallinity of the substrate is usually lost, thus precluding single-crystal X-ray diffraction studies; high-resolution solid-state NMR and IR spectroscopies are the techniques of choice for product characterization. -
2.9.3 Solid-gas reactions involving unsaturated transition metal clusters 2.9.3.1 Reactions of (,u-H)~OS~(CO)~~ with CO, NH3, H2S and HCI On the basis of these considerations, good candidates for solid-gas reactions can be found in the class of stable coordinatively unsaturated species. Among the transition metal carbonyl clusters the best known unsaturated system is represented by the 46-electron (pU-H)20s3(C0)10 derivative (1). Several studies have been reported over approximately 25 years showing the high reactivity of this species in solution with a variety of ligands. In particular it has been shown that ( ~ - H ) ~ O S ~ (reCO)~~ acts instantaneously at room temperature with a number of Lewis bases (L) to yield the coordinatively saturated ( ~ - H ) ~ O S ~ ( C Oadducts'l ) I O L 7p191 as a result of ligand addition to the metal cluster according to the equation:
According to the nature of L three structural isomers of H ( ~ - H ) O S ~ ( C O )(Ia, I O LIb and Ic) have been characterized (Scheme 1).
2.9 Solid-gas Relictions Involuing Metal Curhonyl Clusters
la
lh
841
lc
Scheme 1
All three structural forms contain one terminal and one bridging hydride and ten terminal carbonyl ligands. The entering ligand occupies an equatorial site (Ia) when L is a phosphine or a phosphite whereas it coordinates axially (Ib or Ic) when L contains a nitrogen as the donor atom (nitriles, amines). The occurrence of transformation (1) is accompanied by a net color change from the intense violet of (p-H)20~3(CO)10 to the pale yellow of the saturated adducts. The first set of solid-gas reactions involving (p-H)20s3(CO)lo we investigated were those with CO, NH3 and H2S.I2'' The reaction with CO proceeds smoothly at 80 "C under a pressure of < 1 atm to afford a yellow material identified as H(pu-H)Os3(CO)11by IR and solid state ' H NMR spectroscopy with magic angle spinning (MAS). The latter technique was applicable because the 'H nuclei in H ( ~ - H ) O S ~ ( C O(Fig. ) I I 1) are sufficiently intramolecularly dilute to result in reduced dipole-dipole interactions. In fact, by rotating the sample at high speeds (>8 kHz) two relatively sharp signals have been observed at -20.1 and - 10.5 ppm. These values are very similar to the H chemical shifts found in solution for H(pu-H)0s3(CO)1 I. If the reaction is performed with 13C0 and the I3C NMR solid-state spectrum of the product is recorded under cross-polarization magic-angle-spinning conditions (CPMAS), only broad adsorption resonances are detected, because of extensive overlap of the eleven different carbonyl resonances expected for this derivative. It was, nevertheless, clear that all the carbonyl ligands are 13C-enriched,indicating that CO scrambling occurs at some stage of the reaction. Upon prolonged heating of a solid sample of H ( ~ - H ) O S ~ ( C Ounder ) I I vacuum, a net disproportion takes place to yield (p-H)20s3(CO)lo and Os3(CO)12. Some (p-H)4Osq(CO)n is also formed. Although from these experiments there is no evidence of the reaction mechanism, the results show that extensive rearrangements such as those required for H2 elimination and formation of cluster fragments and recombination are also possible in the solid state. The reaction of solid (pu-H)20s3(CO)lo with NH3 affords the adduct H(p-H)Os3(CO)lo(NH3), an interesting model for the chemisorption of ammonia on a metal surface. The reaction occurs in 2 h at room temperature in the presence of 1 atm NH3 gas. This compound has also been characterized by solid-state IR (carbonyl pattern similar to that of H(p-H)Os3(CO)11 and presence of stretching
'
848
2 Metal Clusters in Catalysis
I
I
Figure 1. ' H MAS NMR spectrum (300 MHz) of H(P-H)O~-I(CO)I I obtained by reacting solid
and bending absorptions for the coordinated NH3) and NMR spectroscopies. The 'H MAS NMR spectrum of the product contains three peaks, at 0.2 ppm (NH3), -10.5 ppm (terminal hydride) and -16.5 ppm (bridging hydride), in the relative intensity ratio of 3 : 1 : 1 (Fig. 2). In this system some of the dipole-dipole interactions are substantially reduced by the rapid intramolecular rotation of the NH3 ligand around the C3-axis of the metal-nitrogen bond. In fact the static solid-state spectrum of the compound shows just a broad line (half height width of about 2.1 kHz) centered at the same chemical shift as that found for the NH3 protons in the MAS spectrum, but the hydride ligand resonances cannot be detected because the mutual dipolar interaction, not moderated by any intramolecular motion, results in much broader lines. Interestingly, H(pu-H)Os3(CO)lo(NH3) is stable for a long time in the presence of NH3 but it decomposes when dissolved in chlorinated solvents yielding 1 and free NH3. The compound seemed, however, to be sufficiently stable in toluene solution to enable complete characterization in solution by 'H and 13CNMR spectroscopy. By comparing the 13CNMR spectra obtained in solution (at -80 "C) (Fig. 3 ) and in the solid state (room temperature, CPMAS) and by measuring the nuclear Overhauser effect between the NH3 and the hydride resonances it was possible to establish that in solution and in the solid state H(pu-H)Os3(CO)lo( NH3) adopts
2.9 Solid-gas Reactions Involving Metal Carhonyl Clusters
849
NH3
Jk lerminal hydride
Figure 2. IH MAS NMR spectrum (300 MHz)
of H(pH)Os3(CO)lo(NH3) obtained by reacting solid ( , ~ - H ) ~ O S ~ ( Cwith O)IO NH3.
-10
0
10
bridging llydride
-20 ppm
0
I
0 0,
Figure 3. I3C {'HI NMR (400 MHz, CD2C12, 233 K) solution spectrum of a I3C-enriched sample of H(P-H)OS~(CO)I~(NH~) obtained by reacting solid ( ~ - H ) ~ O S ~ ( Cwith O ) I NH3 O 1 ' 1 ' 1 ' 1 ~ 1 (* denotes impurities). 190 188 186 184 182
'
1 180
1 1 178
' 1 176
PPm 1 1 1 1 174 172
850
2 Metal Clusters in Catulysis
structure Ib. Further evidence of this structure is apparent from the solid state 13C CPMAS NMR spectrum, which shows clear enhancement of the resonance assigned to carbonyl g relative to those assigned to carbonyls j and a. came to the conclusion that the large dominance of isomer Ib over We Ic is due to the stabilization effects of intramolecular hydrogen-bonds involving metal hydrides as proton acceptors from N-H bonds. In solution, the occurrence of this type of interaction has been unambiguously demonstrated by evaluating the contribution to the relaxation of HT and HB arising from the N-H moiety. When solid (p-H)20s3(CO)lo is treated with H2S (1 atm) at 80 "C the violet starting material is completely converted into a yellow solid in 5 h. Spectroscopic characterization of the final mixture ( ' H MAS and IR) strongly suggests the formation of H(p-H)Os3(CO)lo(H2S) and (p-H)20s3(CO)~(p3-S). In particular the presence in the ' H MAS spectrum of two pairs of absorptions centered at -9.6 and -17.1, and at -13.8 and -15.9 (Fig. 4)can be associated with the formation in the solid state of the cis and anti isomers of H(pu-H)Os3(C0)lo(H2S). When the solid mixture is dissolved in chlorinated solvents at low temperature the ' H NMR spectrum shows the presence of (p-H)20s3(C0)9(p3-S)and (pL-H)(p-SH)Os3(CO)lo. Thus the solid-state reaction enables observation of reaction intermediates which are not detectable in solution, leading to a better understanding of the reaction pathway. It is probable that the reaction passes through the formation of the coordinatively saturated H(p-H)Os3(CO)lo(H2S) (cis and anti isomers), then elimination of H2 takes place with the formation of (p-H)(pu-SH)0s3(CO)lo.Eventually, (p-H)(p-SH)Os3(CO),oloses CO to afford (p-H)20s3(C0)9(p3-S). (p-H)20~3(C0)10 also reacts in the solid state with HCl. In this case an interesting differencein the reaction pathway between solid and solution chemistry is observed. In fact, if the reaction is conducted in CD2C12 solution at room temperature H
I
I
-6
'
l
-8
'
l ' l ' l ' l -10 -12 -14 -16
'
l -18
'
l
-20
'
PPm l ' l -22 -24
Figure 4. 'H MAS NMR spectrum (300 MHz) of H(P-H)Os3(CO)lo(H2S) obtained by reacting solid (p-H)20~3(C0)10 with H2S.
2.9 Solid-gas Reactions Involving Metal Curhonyl Clusters
85 1
Scheme 2
the H(pu-H)zOs3(CO)~oCl species is produced (Scheme 2) ( ' H NMR: 6 = -9.46, -16.54, -19.24; I3C NMR: ten terminal CO resonances), whereas in the solid state (room temperature, 5 days) a mixture of H ( ~ - H ) ~ O S ~ ( C Oand )~~CI (p-H)(p-C1)0~3(CO)lo is formed. If the reaction is performed at higher temperature the quantity of (p-H)(p-Cl)Os3(CO)l0increases. One thus concludes that the H(pu-H)20s3(CO) 10Cl species is an intermediate which evolves into the final product. These observations suggest that the extremely rich chemistry of (p-H)20s3(CO)lo in solution can be further widened by exploring its reactivity in the solid state.
2.9.3.2 Selective incorporation of 13C0 in (,u-H)OS~(CO)~( p3-q2-4-Me-CgHsN) Another interesting example of an unsaturated 46-electron cluster is ( p H)Os3(C0)9(p3-v2-4-Me-CgH5N), that can be obtained by thermolysis of the decaspecies (Scheme 3). carbonyl (p-H)Os3(CO)lo(p3-v2-4-Me-C9H5N) The C(8) atom of the quinoline ring is involved in a three-center two-electron bond and the cluster is highly reactive towards Lewis bases such as CO, amines, and phosphines.[221 (p-H)Os3(CO)g(p3-v2-4-Me-C9H5N) reacts with CO in the solid state at 90 "C yielding the decacarbonyl derivative. By using "CO as reactant it was shown[22] that selective enrichment occurs at the radial position cis to the hydride (whose resonance was assigned on the basis of solution data) on the C-bound 0 s (Fig. 5). Equilibration of the I3C-enriched ligand at the coordination sites is a rather slow process. In solution at room temperature it takes approximately 2 days. In the solid state, in the absence of any intramolecular rearrangement, the site-selectivity of the added I3CO ligand is maintained even at high temperature.
2.9.4 Solid-gas reactions involving lightly stabilized transition metal clusters The observation that it is possible to generate coordinatively unsaturated, highly reactive organometallic species 'in situ' by the use of saturated derivatives contain-
852
2 Metal Clusters in Catalysis
'R
\LH\ -
-0s
/
IF/-. I
I\
R=H.A=H R = C b ,A' =ti R = H. R =CH3
Scheme 3
ing good leaving groups, such as nitrile or olefin ligands, opens new possibilities for the solid-gas reaction approach. The reactivity of the 'lightly stabilized' Os3(CO)11L ( L = NCCH3, C2H4) clusters with gaseous reactants such as CO, NH3, and H2 has been investigated in When O S ~ ( C O ) ~ ~ ( N C C orHO~S) ~ ( C O ) ~ ~ (isC reacted ~ H ~ ) with CO (1 atm) at SO "C the pale yellow powder of Os3(CO)12 formed is easily characterized by 13C solid-state NMR. The minor line broadening observed for the 13C resonances of Os3(CO)12 obtained from these solid-gas reactions, compared with the published spectrum of the recrystallized complex,[241appears to be associated with the powdery nature (microcrystals) of the product. According to a dissociative reaction pathway one would expect preferential coordination of the incoming CO ligand at the same site as that occupied by the leaving group. Interestingly, the 13C MAS spectrum (Fig. 6 ) shows that 13C0 is equally distributed over axial and equatorial sites when both 0s3(CO), 1 (NCCH3)
2.9 Solid-gas Reactions Involving Metal Carbonyl Clusters
853
Ii
\ I
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195
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I
185
I
I
180
I
I
175
I
I
170
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I
165
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I
160
I
I
155
Figure 5. I3C CPMAS NMR spectrum (67.8 MHz) of a 13C-enriched sample of (p-H)Os3(CO) p3-q2-4-Me-CgH~N)obtained by reacting (p-H)Os3(CO)g(p3-q2-4-Me-CgH~ N) with I3CO in the solid state.
(axially substituted) or Os3(CO)11 (C2H4) (equatorially substituted) are employed as substrates. This observation implies that a fluxional step must occur at some stage of the reaction. Fast axial-equatorial CO exchange in solid Os3(CO)12 can be ruled out because no broadening of the resonances in its I3C MAS spectrum is observed at high temperatures. Furthermore a slow exchange process does not seem to be responsible for the observed equilibration of the 13C0 enrichment, as shown by results obtained from shorter reaction times. It was then suggested that the vacant coordination site in the unsaturated Osj(CO)11 intermediate can exchange between axial and equatorial positions. The reaction with 13C0then yields the isoenergetic ax- and eq-( '3CO)Os3(CO)11in a 1 : 1 ratio according to Scheme 4. Axially vacant Os3(C0)11was observed by Wrighton et al.[251as the product of a photoreaction of Os3(CO)12 in hydrocarbon glass at T < 110 K; rearrangement of the vacant coordination site has been proposed to explain the experimental reactivity. C HO~ S) ~ ( C O ) I I ( C with ~ H ~NH3 ) The analogous reactions of O S ~ ( C O ) ~ ~ ( N Cand
854
2 Metal Clusters in Catalysis
0
~~~~~~~~~~
Figure 6. I3C MAS NMR spectrum (67.8 MHzl of a I3C-enriched samule of
afford only the axial O S ~ ( C O ) ~ I ( Nisomer, H ~ ) as determined by 13Cand I5N NMR spectroscopy (Fig. 7). By assuming that both reactions proceed through the same fluxional intermediate Os3(CO), 1, the formation of the axially substituted NH3 derivative only might be explained in terms of two possible mechanisms (Scheme 5): i) Os3(CO)ll reacts with ammonia only when the vacant coordination site is in an axial position (kinetically controlled reaction); or ii) both the ax and eq isomers are formed, but the less stable equatorial isomer reacts back or rearranges quickly to give the axial isomer (thermodynamically controlled reaction). The reaction of solid O S ~ ( C O ) I O ( N C Cwith H ~ ) ~H2 or D2 (1 atm) at 80 "C affords (y-H)20~3(C0)10or (y-D)20s3(CO)lo in 8 h. Interestingly when solid Os3(CO)lo(NCCH3)2 is reacted with HD a mixture of (y-H)20s3(CO)lo, (y-H) (y-D)Os3(CO)lo,and (y-D)20~3(C0)10 is obtained in the relative ratio 1:2:1. Quantification of the three isotopomers was possible by integrating the relative areas of the I3C resonances assigned to a pair of radial carbonyls in the solution spectrum,
2.9 Solid-gas Reactions Involving Metal Carbonyl Clusters
855
Me
Scheme 4
which displays a noticeable isotopic shift (176.42 ppm in (p-H)20~3(C0)10, 176.28 ppm in (p-H) (p-D)Os3(CO)lo,and 176.11 ppm in (p-D)20s3(CO)lo)(Fig. 8). The quantification of the relative amounts of the three isotopomers from the I3C spectrum does not exactly correspond to the computed values of the integrals of the resonances, because signal enhancement is observed for (p-H)20~3( CO)10 and (p-H)(p-D)0~3(CO)lo owing to the nuclear Overhauser effect. Because (p-H)20~3(C0)10 does not react in the solid state with D2 or HD at high temperature, we can rule out the possibility that H/D isotopic exchange takes place in (p-H)(p-D)0~3(CO)lo via the addition of a second HD molecule. The results are in agreement with the suggestion that this exchange occurs on the surface of the cluster after the formation of the elusive (y2-HD)20s3(CO)1oadduct from OSJ(CO)~~(CH~C asNshown ) ~ , in Scheme 6.
2.9.5 Conclusions Despite the wealth of informations contained in the thousands of publications dealing with cluster reactivity, very little is known about the factors controlling their solid-state reactivity. The awareness that observation of stable organometallic molecules whose structures might resemble those of reactive intermediates on cata-
2 Metal Clusters in Catalysis
856
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PPm I I I -700 -800
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Figure 7. {'H}-13C(CD2C12, 183 K), and ISN CPMAS NMR spectra of a I3C and I5N enriched sample of Os3(CO)1I(NH3) obtained by reacting solid O S ~I3CO)ll( ( NCCH3) with 'jNH3.
lytically active metal centers will prompt more studies in this field in the near future. The interaction of simple ligands such as hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide, nitrogen oxides, etc. with organometal carbonyl clusters might expand our view of how many simple chemical transformations take place on a metal surface and we can obtain models for ligand mobility and for understanding heterogeneous catalytic reactions such as hydrogenation, hydroformylation, carbonylation, oxidation, etc. Furthermore, the possibility of generating highly reactive organometallic species 'in situ' by using labile leaving groups opens novel interesting synthetic strategies. Finally the knowledge, at the molecular level, of the interaction of a gas with an highly dispersed coordinatively unsaturated species can offer new insights for
2.9 Solid-gas Reactions Involving Metal Carbonyl Clusters
I
H
851
Fluxional Transition State
Scheme 5
Figure 8. { IH)-l3CNMR spectrum (400 MHz, CDC13, 298 K) of a I3C enriched mixture of (p(-H)zOs3(CO) 10, ( P - H ) ( ~ - D ) O ~ ~ ( Cand O)IO, ( , u - D ) ~ O S ~ ( Cobtained O ) ~ ~ by reacting solid O S ~ ( C O ) I O ( N C C H ~I ) ~ I 184 with HD.
1
L PPm ~~
1 182
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858
2 Metal Clusters in Catalysis
C
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Scheme 6
potential applications of solid-gas reactions as devices for gas sensors. For such use there is a need for fast reactions under mild conditions and for devices able to quantify the gas concentration in terms of spectrochemical response.
References [ l ] H.W. Kohlshutter, Z. Anorg. Allgem. Chem. 1918, 105, 121. [2] J.M. Thomas, S. E. Moro and J. P. Desvergne, Adv. Phys. Org. Chem. 1977, 15, 63 and references therein. [3] H.E. LeMay Jr., in Comprehensive Coordination Chemistry; G. Wilkinson, Ed. Pergamon Press, New York, 1986, Vol. 7, p. 463 and references therein. [4] C . Bianchini, M. Peruzzini and F. Zanobini, Organometallics 1991, 10, 3415. [5] C . Bianchini, E. Farnetti, M. Graziani, J. Kaspar and F. Vizza, J. Am. Chem. Soc. 1993, 115, 1753. [6] C . Bianchini, C . Mealli, M. Peruzzini and F. Zanobini, J. Am. Chem. Soc. 1992, 114, 5905. [7] C . Bianchini, M. Peruzzini, A. Vacca and F. Zanobini, Organometallics 1991, 10, 3697. [8] C . Bianchini, C. Mealli, M. Peruzzini and F. Zanobini, J. Am. Chem. SOC.1987, 109, 5548. [9] C . Bianchini, F. Zanobini, S. Aime, R. Gobetto, R. Psaro and L. Sordelli, Organometallics 1993, 12,4751. [ 101 C.S. Chin, B. Lee, S. Kim, Organometallics 1993, 12, 1462. [Ill B.R. Flynn and L. Vaska, J. Chem. Soc. Chem. Commun.1974, 703. [12] A.R. Siedle and R.A. Newmark, J. Am. Chem. SOC.1989, 111, 2058.
2.9 Solid-gas Reactions Involving Metal Carbonyl Clusters
859
[I31 G.J. Kubas, C.J. Unkefer, B.I. Swanson and E. Fukushima, J. Am. Chem. SOC.1986, 108, 7000. [14] A.R. Siedle, R.A. Newmark, M.R. Sahyun, P.A. Lyon, S.L. Hunt and R.P. Skarjune, J. Am. Chem. SOC.1989, 111, 8346. [ 151 A.R. Siedle, R.A. Newmark, K.A. Brown-Wensley, R.P. Skarjune and L.C. Haddad, Organometallics 1988, 7, 2078. [I61 E.L. Muetterties, T.N. Rhodin, E. Band, C.F. Brucker and W.R. Pretzer, Chem. Rev. 1979, 79, 91. [17] A.J. Deeming and S. Hasso, J. Organomet. Chem., 1975, 88, C21. [18] A.J. Deeming and S. Hasso, J. Organomet. Chem., 1976, 114, 313. [19] J.B. Keister and J. R. Shapley, Znorg. Chem., 1982, 21, 3304. [20] S. Aime, W. Dastrh, R. Gobetto and A.J. Arce, Organometallics, 1994, 13, 4232. [21] S. Aime, R. Gobetto and E. Valls, Organometallics, 1997, 16, 5140. [22] E. Arcia, D.S. Kolwaite, E. Rosenberg, K. Hardcastle, J. Ciurash, R. Duque, R. Gobetto, L. Milone, D. Osella, M. Botta, W. Dastru, A. Viale and 1. Fiedler, Organometallics, 1998, 17, 415. [23] S. Aime, W. Dastrh, R. Gobetto, J. Krause and E. Sappa, Organometallics, 1995, 14, 3224. [24] S. Aime, M. Botta, R. Gobetto, D. Osella and L. Milone, Znorg. Chim. Acta 1988, 146, 151. [25] J.G. Bentsen and M.S. Wrighton, J. Am. Chem. SOC.1987, 109, 4518.
SHORT COMMUNICATION DOI: 10.1002/ejic.200800054
A Phenanthroline Heteroleptic Ruthenium Complex and Its Application to Dye-Sensitised Solar Cells Anna Reynal,[a] Amparo Forneli,[a] Eugenia Martinez-Ferrero,[a] Antonio Sanchez-Diaz,[a] Anton Vidal-Ferran,[a,b] and Emilio Palomares*[a,b] Keywords: Dyes / Solar cells / Ruthenium / Heteroleptic complexes / Charge recombination / Electron transfer We report here the synthesis and characterization of a new heteroleptic ruthenium(II) complex and its applications as efficient light-harvesting sensitizer in functional dye-sensitized solar cells. The relation between the interfacial charge-trans-
fer processes that govern the device performance and the cell efficiency under illumination are also discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
Introduction
device stability but also its performance under illumination at full sun light (100 mW/cm2 1.5 AM G). The same strategy has been applied by Zakeeruddin and co-workers with K77 dye [cis-{4,4⬘-bis[2-(4-tert-butoxyphenyl)ethenyl]-2,2⬘bipyridyl}bis(2,2⬘-bipyridyl-4,4⬘-dicarboxylic acid)bis(isothiocyanato)ruthenium(II)] which, in combination with a non-volatile electrolyte, exhibits a unique performance by combining high efficiency and stability.[6] Thelakkat and coworkers[7] have also reported the use of tris(phenylamine)substituted bipyridines in RuII complexes as efficient sensitizers for solid-state DSSC when using an organic hole conductor as a solid electrolyte. In this communication we would like to report the synthesis and characterization of a new heteroleptic ruthenium(II) complex with one of the bipyridine ligands replaced by a more conjugated ligand such as the 5,6-dimethyl-1,10phenanthroline. Scheme 1 illustrates the molecular structure of the ruthenium(II) heteroleptic complex. Moreover, we have also carried out a study of the interfacial charge-transfer kinetics of the molecule when anchored onto the surface of nanocrystalline TiO2 semiconductor particles.
Bis(bipyridine)ruthenium complexes have been widely studied as efficient light-harvesting molecules when adsorbed onto the surface of mesoporous semiconductor thin films, which are used as working electrodes on dye-sensitised solar cells (DSSC).[1] Despite the low molecular extinction coefficient and the lack of absorbance in the near infrared region of the solar spectrum when compared to, for example, phthalocyanines, they have the best light-toenergy conversion efficiencies to date. During the last 10 years the molecule known as N719 [chemical name: bis(tetrabutylammonium) cis-bis(isothiocyanato)bis(2,2⬘bipyridyl-4,4⬘-dicarboxylato)ruthenium(II)] has been the paradigm of a molecular dye because of its high solar-toelectricity efficiency achieved when used as sensitizer in DSSC.[2] Hence, an interesting challenge for many researchers has been the design and synthesis of new ruthenium(II) complexes with enhanced properties such as slow back-electron transfer from the photo-injected electrons at the mesoporous semiconductor either with the oxidized dye or the electrolyte[3] as well as the dye long-term stability under device operation.[4] To this end, a successful strategy has been the design and synthesis of ruthenium(II) heteroleptic compounds where one of the 4,4⬘-dicarboxy-2,2⬘-bipyridines has been replaced by a more appropriate ligand. As an example Nazeruddin et al.[5] showed that the presence of a bipyridine on the ruthenium(II) complex bearing long alkyl chains {common name: Z907, chemical name: cis-[bis(2,2⬘bipyridyl-4,4⬘-dicarboxylic acid)(4,4⬘-dinonyl-2,2⬘-bipyridyl)bis(isothiocyanato)ruthenium(II)]} improves not only the [a] Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans 16, 43007 Tarragona, Spain Fax: +34-977-920-241 E-mail:
[email protected] [b] Institució Catalana per a la Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain Eur. J. Inorg. Chem. 2008, 1955–1958
Scheme 1. Molecular structure of AR25 [chemical name: cisbis(2,2⬘-bipyridyl-4,4⬘-dicarboxylic acid)(5,6-dimethyl-1,10-phenanthroline)bis(isothiocyanato)ruthenium(II)].
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1955
SHORT COMMUNICATION
E. Palomares et al.
Results and Discussion The AR25 shows a typical UV/Vis spectrum with a metal-to-ligand charge-transfer band (MLCT) centred at λ = 518 nm with a molecular extinction coefficient of 6578 –1 cm1. Figure 1 illustrates the absorption spectra of the complex in solution and adsorbed onto a transparent mesoporous TiO2 film. As can be seen, the spectra do not show any significant shift, which can be understood as a low or negligible presence of dye molecular aggregates. Moreover, in solution, after excitation at the maximum of the MLCT band, a broad emission band can be observed with a maximum at λem = 746 nm. The large Stokes shift is due to the nature of the excited state, which – as reported before for other ruthenium(II) complexes[8] – is a triplet energy state. It is worthy to note that the AR25 emission is strongly quenched when its molecules are anchored to nanocrystalline TiO2 particles (Figure 2). Hence, we can conclude that the electron-injection process from the excited state into the semiconductor conduction band (CB) is responsible for the immediate disappearance of luminescence upon light excitation. Furthermore, the excited-state emission lifetime for the complex is strongly shortened when anchored to the mesoporous TiO2 film. The emission lifetime for AR25 in solution (we used dimethylformamide, DMF, as solvent) gives in our hands a decay that was fitted to two kinetic components, τ1 = 9.57 ns (18.9 %) and τ2 = 56.67 ns (81.1 %) whereas for the AR25/TiO2 samples τ1 = 2.23 ns (48.5 %) and τ2 = 12.61 ns (51.5 %).
Figure 2. Emission decay kinetics measured using time-correlated single-photon counting under normal conditions for AR25 in DMF (1 ⫻ 10–4 ) and adsorbed onto a 4 µm thick transparent TiO2 film. Dashed lines correspond to the adjusted fit decay. The excitation wavelength was λex = 405 nm, and the emission was monitored at λem = 745 nm.
Once the electrochemical and the emission properties of the ruthenium complex were measured, we turned to the light-induced charge-transfer kinetics between the sensitizer and the semiconductor nanocrystalline TiO2 particles. As we have reported before,[10] we have utilized laser-transient absorption spectroscopy (L-TAS) to investigate the electron-recombination dynamics. Figure 3 shows typical decay kinetics for the AR25/TiO2 samples. We assigned the tran-
Figure 1. UV/Vis spectra of AR25 in DMF (1 ⫻ 10–4 , ----) and adsorbed onto a 4 µm thick transparent TiO2 film (–).
The electrochemical properties of the ruthenium(II) complex AR25 were also analyzed using cyclic voltammetry (CV) in dry DMF as solvent with 0.1 tetrabutylammonium hexafluorophosphate as electrolyte. The CV allowed the observation of a quasi-reversible couple at 0.79 V vs. SCE assigned to the RuII/III redox couple.[9] 1956
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Figure 3. Transient-absorption decay kinetics for a 4 µm thick transparent mesoporous film sensitized with AR25. The solid line corresponds to the fitting to a stretched exponential function: ∆O. D. = exp[–(t/τ)α]. The excitation wavelength was λex = 535 nm and the probe wavelength was λpr = 800 nm.
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1955–1958
A Phenanthroline Heteroleptic Ruthenium Complex
sient-decay signal to the recombination of the photo-injected electrons, upon laser excitation, into the semiconductor conduction band and the oxidized dye species. The recorded excited spectra show a broad absorption band with a maximum centred at 800 nm, which has previously been assigned to the cation species of similar ruthenium(II) complexes such as the above-mentioned N719 molcule.[11] The measurement of the electron-recombination lifetime at the half maximum of the signal is 35 µs. Finally, we carried out measurements on complete functional devices. We measured the incident photon-to-current conversion efficiency (IPCE) spectra for devices sensitized with AR25 using as electrolyte a solution containing the redox couple iodine/iodide (see Experimental Section). Figure 4 illustrates the IPCE spectra for an AR25/DSSC.
Figure 5. Photocurrent vs. voltage curve (curves I–V) for a 1 cm2 AR25 DSSC (circles) and N719 DSSC (squares). Measurements were performed at 1 sun (full signal) and dark conditions (empty signal).
Conclusions
Figure 4. IPCE spectra for DSSC sensitized using AR25 (full circles) and the homoleptic dye N719 (empty squares).
In our hands, the IPCE spectrum for AR25 devices showed higher intensity at the maximum absorption wavelength at λ = 550 nm when irradiated with simulated sunlight. The corresponding photocurrent vs. voltage characteristic curves were also measured to give an overall efficiency of 2.6 % under irradiation at 1 sun (100 mW/cm2) with simulated 1.5 AM G solar spectrum (Figure 5). Under the same conditions, we examined DSSC sensitised with the N719 dye, and the overall efficiency was 3.6 %. The main difference between both devices was in the open-circuit voltage (Voc). While for the former ruthenium(II) complex a Voc = 0.69 V was obtained, for the latter a Voc = 0.78 V was observed. We also noted that the devices made using AR25 as sensitizer usually showed lower fill factors when compared to devices made using the N719 complex (38.5 % and 47.6 %, respectively). We believe that the higher recombination on AR25 devices limits the photocurrent and the overall performance of the solar cell. Further work focussed on the control of such wasteful reactions is being carried out. Eur. J. Inorg. Chem. 2008, 1955–1958
We presented a new ruthenium(II) heteroleptic complex which shows the appropriated redox electrochemistry to be used as sensitizer in dye-sensitised solar cells. Furthermore, we have characterized the charge-transfer processes occurring at the interface between the AR25 dye and the nanocrystalline TiO2 nanoparticles showing that the photo-induced electron injection is particularly efficient despite the low-lying π*-level character of the phenanthroline moiety as coordinating ligand,[12] and the electron recombination processes is at least one order of magnitude slower than the regeneration reactions, which normally occur on the nanosecond time scale; this makes feasible the possibility to optimize the devices and achieve higher light-to-energy efficiencies in the same order of magnitude than the most popular dye, N719. The high photocurrent observed using AR25 makes the dye an interesting candidate for “molecular cocktails” where several dyes with light absorptions in different regions of the solar spectrum are combined to achieve the desired panchromatic sensitization of DSSC.
Experimental Section Synthesis of AR25: The synthesis of cis-bis(2,2⬘-bipyridyl-4,4⬘-dicarboxylic acid)(5,6-dimethyl-1,10-phenanthroline)bis(isothiocyanato)ruthenium(II) [Ru(dcbpy)(dmphen)(NCS)2] (AR25). The synthesis of AR25 was carried out according to that reported by Kasuga et al.,[13] but by adding 5,6-dimethyl-1,10-phenanthroline (81.6 mg, 0.4 mmol) instead of 1,10-phenanthroline. Yield: 56.6 %. 1 H NMR (400 MHz, [D7]DMF): δ = 9.75 (d, J = 5.94 Hz, 1 H), 9.68 (d, J = 5.24 Hz, 1 H), 1.26 (d, J = 1.26 Hz, 1 H), 9.06 (d, J = 1.39 Hz, 1 H), 9.04 (d, J = 8.87 Hz, 1 H), 8.66 (d, J = 8.56 Hz, 1 H), 8.44 (dd, J = 5.80, 1.65 Hz, 1 H), 8.35 (dd, J = 8.56, 5.25 Hz, 1 H), 8.11 (d, J = 5.25 Hz, 1 H), 8.11 (d, J = 5.25 Hz, 1 H), 7.92 (d, J = 5.95 Hz, 1 H), 7.63 (dd, J = 8.45, 5.38 Hz, 1 H), 7.56 (dd,
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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SHORT COMMUNICATION J = 5.76, 1.34 Hz, 1 H), 2.92 (s, 3 H), 2.81 (s, 3 H) ppm. ESIMS: m/z = 670 [M + H]. FTIR: ν˜ = 2080 [ν(N=C=S)as],1709 [ν(C=O)], 858 [ν(N=C=S)sym] cm–1. C28H20N6O4RuS2 (670): calcd. C 50.2, H 3.1, N 12.5; found C 53.3, H 5.29, N 11.1. Optical, Electrochemical and Spectroscopical Measurements: The UV/Vis and fluorescence spectra were recorded using a 1 cm pathlength quartz cell with a Shimadzu UV spectrophotometer 1700 and an Aminco–Bowman series 2 luminescence spectrometer with temperature controller. The electrochemical data was obtained employing a conventional three-electrode cell connected to a CH Instruments 660c potentiostat-galvanostat. For the cyclic voltammetry, we used a platinum working electrode, a calomel reference electrode (SCE) and a platinum wire as auxiliary electrode. The picoseconds to microseconds emission lifetime measurements were carried out with a Lifespec© picosecond fluorescence lifetime spectrometer from Edinburgh Instruments©. As excitation source, the diode laser with 405 nm nominal wavelength was used. The instrument response measurement at the HWHM (half width a high maximum) was below 350 ps. Laser transient-absorption measurements (TAS) were carried out with a home-built system as reported before.[10] 1H NMR spectra, NOESY and COSY experiments were measured with a Bruker 400 MHz spectrometer. A Waters LCT Premier liquid chromatograph coupled to a time-of-flight mass spectrometer (HPLC/MS-TOF) with electrospray ionization (ESI) was used to measure mass spectra. The FTIR spectra were obtained with an FTIR ThermoNicolet 5700 spectrometer. Nanoparticle Synthesis and Film Preparation: The nanocrystalline TiO2 particles were synthesized as reported before.[14] In brief: Titanium isopropoxide (40 mL, 0.13 mol) was added to glacial acetic acid (9.12 g) under argon while stirring. The reaction mixture was cooled in an ice-bath, and 0.1 nitric acid (240 mL) was added while vigorously stirring. The mixture was heated in an oil bath at 80 °C during 8 h. and, after cooling, was filtered through a 0.45 µm syringe filter. The resulting product was diluted to 5 wt.-% of TiO2 by adding water and then autoclaved at 220 °C for 12 h. The aqueous phase was removed by centrifugation, and the solid nanoparticles were isolated and rinsed twice with ethanol. An ultrasonic horn was used to break the aggregates, and the solvent was removed under vacuum. The solid nanoparticles were diluted to 15 wt.-% in TiO2, using ethyl cellulose and terpineol, and the paste was homogenized by using a ball mill. Device Preparation and Characterization: DSSCs were made using 4 µm thin films consisting of 20 nm TiO2 nanoparticles deposited onto a conducting glass substrate (Hafford Glass Inc., 15 Ω/cm2 resistance) by using the well-known doctor-blade technique. The active area was 0.152 cm2. The as-prepared electrodes were gradually heated under airflow at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min. Then, they were submerged into 40 ⫻ 10–3 TiCl4 aqueous solution at 70 °C for 15 min, washed with ethanol, heated again at 500 °C for 30 min,
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E. Palomares et al. and cooled to 50 °C before soaking the films in a 5 ⫻ 10–4 AR25 solution in acetonitrile/tert-butyl alcohol (1:1) overnight. The counter electrodes were prepared by spreading a solution of H6PtCl in ethanol onto a conducting glass substrate with a small hole to allow the introduction of the liquid electrolyte, under vacuum. The liquid electrolyte was prepared by using 0.6 DMPII (1-propyl2,3-dimethylimidazolium iodide), 0.05 I22, and 0.1 LiI in acetonitrile/valeronitrile (85:15). The photovoltaic measurements were carried out with a 150 W xenon lamp from Oriel Instruments with the appropriated set of filters for the correct simulation of the 1.5 AM G solar spectrum. The incident light power was calibrated by using a silicon photodiode previously calibrated at 1000 W/m2 at 1.5 AM G.
Acknowledgments E. P. would like to thank the Spanish Ministerio de Educación y Ciencia (MEC) for the funding through the CONSOLIDERHOPE-CSD-0007-2007 and the CTQ2007-60746/BQU project. E. M. F. also thanks the MEC for her Juan de la Cierva Fellowship.
[1] N. Robertson, Angew. Chem. Int. Ed. 2006, 45, 2338–2345. [2] M. Gratzel, Prog. Photovolt. 2006, 14, 429–442. [3] J. R. Durrant, S. Haque, E. Palomares, Coord. Chem. Rev. 2004, 248, 1247–1257. [4] M. K. Nazeeruddin, C. Klein, P. Liska, M. Gratzel, Coord. Chem. Rev. 2005, 249, 1460–1467. [5] C. Klein, M. K. Nazeeruddin, D. D. Censo, P. Liska, M. Gratzel, Inorg. Chem. 2004, 43, 4216–4226. [6] D. Kuang, C. Klein, S. Ito, J. E. Moser, R. Humphry-Baker, N. Evans, F. Duriaux, C. Grätzel, S. M. Zakeeruddin, M. Grätzel, Adv. Mater. 2007, 19, 1133–1137. [7] C. S. Karthikeyan, H. Wietasch, M. Thelakkat, Adv. Mater. 2007, 19, 1091–1095. [8] J. R. Lakowicz in Principles of Fluorescence Spectroscopy, 2nd ed., Kluwer Academic/Plenum Publishers, New York, 1999. [9] M. Yanagida, L. P. Singh, K. Sayama, K. Hara, R. Katoh, A. Islam, H. Sugihara, H. Arakawa, M. K. Nazeeruddin, M. Gratzel, J. Chem. Soc., Dalton Trans. 2000, 2817–2822. [10] S. Tatay, S. Haque, B. C. O’Regan, J. R. Durrant, W. Verhees, J. Kroon, A. Vidal-Ferran, P. Gaviña, J. Mater. Chem. 2007, 17, 3037–3044. [11] S. Haque, E. Palomares, B. Cho, A. Green, N. Hirata, D. Klug, J. R. Durrant, J. Am. Chem. Soc. 2005, 127, 3456–3462. [12] D. J. Stufkens, A. Vlcek Jr, Coord. Chem. Rev. 1998, 177, 127– 179. [13] N. Onozawa-Komatsuzaki, O. Kitao, M. Yanagida, Y. Himeda, H. Sugihara, K. Kasuga, New J. Chem. 2006, 30, 689–697. [14] S. Hore, E. Palomares, H. Smit, N. J. Bakker, P. Comte, P. Liska, K. R. Thampi, J. M. Kroon, A. Hinsch, J. R. Durrant, J. Mat. Chem. 2005, 15, 412–418. Received: January 16, 2008 Published Online: March 26, 2008
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1955–1958
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.10 The Surface of Inorganic Oxides: an Unusual Reaction Medium for the High-yield and Selective Synthesis of Various Osmium Carbonyl Clusters Elena Cariati, Elena Lucenti, Dominique Roberto, and Renato Ugo
2.10.1 Introduction The practice of conducting reactions in solution might reflect tradition rather than actually being advantageous.['] For instance, it was clearly shown that one can perform many synthetically useful organic reactions on solids12p61 (notably alumina,[2p41 silica gel,[2-41zeolites,[5]~ l a y s [ ~ and , ~ ]polymer^[^^^]) often with higher rates, higher yields, and/or better selectivity than when using solution methods."] In particular, the surface-mediated synthesis of metal carbonyl complexes and clusters, whereby the surface of an inorganic oxide plays the role of the solvent in conventional syntheses, is a new and promising area. In many cases, selectivities and yields are so high and reaction conditions so mild that surface-mediated organometallic syntheses can be recommended over conventional syntheses in s ~ l u t i o n . [ ~ - ' ~ ] Surface-mediated organometallic syntheses are controlled by: i) the nature and loading of the metal salt or organometallic precursor adsorbed on the solid; ii) the nature of the inorganic oxide; iii) the physical and chemical properties of the surface as such or after addition of specific reactants (e.g. alkali or acid); iv) the nature and composition of the gaseous phase; and v) the temperature, pressure and reaction time. The products synthesized on the surface can usually be recovered simply by extraction or sometimes by sublimation. In few cases only are the organometallic moieties covalently linked to surface oxy groups, requiring selective cleavage of the covalent bond (e.g. M-OS, S = Si, Al, or Mg) between the organometallic species and the support at the end of the reaction. As a general trend, strongly basic sur-
2.10 The Surface of Inorganic Oxides
861
faces such as MgO favor the formation of anionic metal carbonyl clusters['-"' whereas uncharged carbonyl complexes and clusters are generated on the surface of more neutral supports such as Si02.[12,13,171 It became apparent recently, however, that anionic metal carbonyl clusters can also be prepared on the surface of silica in the presence of adequate amounts of a specific base such as Na2C03 or ~ ~ c ~ ~ . [ ~ ~ - ~ ~ l This review will cover the preparation of various osmium carbonyl clusters which led to very convenient surface-mediated syntheses of both neutral and anionic osmium carbonyl cl~sters.['~'~~'~~~~~'~~ A 1arge part of this work has been performed in our laboratorie~.['~~'~~~~,'~1
2.10.2 Synthesis of neutral osmium carbonyl clusters Various
neutral
osmium
carbonyl
clusters
(i.e.
[OS~(CO)~~],['~]
[H~OS~(CO)~~],['~,'~,~~] [HOs3(CO)loY]where Y = a three-electron donor['7]) have been generated, in high yields and under mild conditions, on the surface of silica treated['5] or ~ n t r e a t e d [ ' ~ .with ' ~ I a small amount of an alkali metal carbonate (e.g. Na2C03 or K2C03).
2.10.2.1 Synthesis of [H40sq(C0)12] From [OS~(CO)IZ] (Scheme 1) Treatment of silica-supported [Os3(CO)12](2% ( w / w ) of 0 s relative to Si02) with (68% yield), which is recovered H2 (1 atm) at 100 "C for 5 days gives [H40~4(C0)12] from the surface by extraction with CH2C12.["I The particularly mild pressure required for this synthesis, compared with the high pressure (120 atm H2) required in inert solvents (e.g. octane) to obtain similar yields[l8I,is explained by the easy activation of physisorbed [Os3(CO)12]by interaction with surface silanol groups to give the silica-anchored species [HOs3(CO)loOSi-], a reactive and labile intermediate.' 91 This silica-mediated synthesis has the disadvantage of requiring a very long reaction time because of the need to work at relatively low temperatures (100 "C) to avoid parallel sublimation of [ O S ~ ( C O ) ~ ~This ] . [ ' ~observation ] led to a more convenient silica-mediated synthesis which involves two steps: i) formation of silica-anchored [HOs3(CO)loOSi=] (98% yield) by heating an octane solution of [ O S ~ ( C O )under I ~ ] reflux with silica;[20] ii) treatment of [HOs3(CO)loOSi-] with HZ (1 atm) at 150 "C for 24 h to generate [H40~4(C0)12] in 94% yield.['71
862
2 Metal Clusters in Catalysis
150°C, 24h
I
40°C, 7h
I
90.C. 6h
Scheme 1. Best syntheses of various osmium clusters on the surface of sili~a.['~.'~I
From [Os(CO)3C12]2 or OsC13 (Scheme 2) It was important to use a less expensive species than [Os3(CO)12](e.g. OsC13 or [Os(CO)3C12]2)as starting material for the synthesis of [H40s4(C0)12].Treatment of silica-supported [Os(CO)3C12]2 with 1 atm H2 (100-200 "C) does not afford [H40~4(C0)12]because of the easy sublimation of the starting dimer at temperatures higher than 100 "C and the difficulty of removing chloro ligands from the osmium coordination sphere when working at relatively low temperatures.'' 51 After addition of weak bases, such as alkali metal carbonates, however, removal of chloro ligands is easy.['4p161When silica-supported [Os(CO)3C12]2 (2-15% (w/w) of 0 s relative to Si02) is stirred with a slurry of Na2C03 in CH2C12 (molar ratio NazCO3/Os = 2: l), dried, and heated under H2 (1 atm) at 150 "C for 72 h, [H40~4(C0)12] is obtained in 70-83% yields.[15]Similar yields are obtained by use of silica-bound [Os(CO)3C12(HOSi-)I as the starting material (this is easily obtained in situ by reductive carbonylation (1 atm CO, 180 "C) of silica-supported O S C ~ ~ [ ~ ~ ] ) . An alternative and very convenient way of preparing [H40~4(C0)12]directly from OsC13, via the anionic cluster [H30~4(C0)12]-,involves three steps:[l5I
i) formation of silica-bound [Os(CO)3C12(HOSi=)];r'31 ii) addition of K2CO3 (molar ratio K2C03/0s = 10: 1) followed by reductive carbonylation (1 atm CO, 150 "C, 24 h) to give silica-supported K[H30~4(C0)12]; and iii) extraction with CH2C12 acidified with H2S04. With this attractive surface-mediated synthesis, high total yields (76%) of [H40~4(C0)12] can be obtained.[15]
2.10 The Surface of Inorganic Oxides
863
Scheme 2. Best syntheses of various osmium clusters on the surface of silica added with alkali metal carbonates.' 4, 5,341
' '
2.10.2.2 Synthesis of [Os3(CO)12] from [Os(CO)3C12]2 or OsCI3 (Scheme 2) The reductive carbonylation of silica-supported [Os(CO)3C12]2 to [Os3(CO)l2] does not occur readily because of the rapid sublimation of chlorocarbonyl osmium complexes ([Os(CO)qC12],[Os(CO)3C12]2)and clusters ([HOs3(CO)&1], [HOs3(CO)11Cl]) when working at relatively high temperatures (130-250 "C) and to the difficulty of removing chloro ligands from osmium when working at lower temperatures.['s1However, when silica-supported [Os(CO)3C12]2(2-15% (w/w)of 0 s relative to Si02) is stirred with a slurry of Na2C03 in CH2C12 (molar ratio NazCO3/Os = 2: l), dried, and heated with CO ( 1 atm) at 200 "C for 3 days, [Os3(CO)12]is obtained in 76-82% yields. Similar yields are obtained by starting from Os(CO)3C12(HOSi-)],[' easily generated in situ from silica-supported OsCl3 31 These silica-mediated syntheses are very attractive because, in solution, the best route to [Os3(CO)l2]reported so far is reductive carbonylation under high pressure (75 atm CO) of OsO4 dissolved in methanol (125 0C).[211
1'
2.10.2.3 Syntheses of [HOs3(CO)loY] (Y = OH, OR, C1, Br, 02CR) from [Os3(CO)12] (Scheme 1) The facile and quantitative activation of [Os3(CO)12]by the silica surface, via reaction with surface silanol groups to give [HOs3(CO)loOSi-], provides a new con-
864
2 Metal Clusters in Catalysis
venient general route to the selective, high-yield synthesis of various osmium carbonyl clusters of the type [HOs3(CO)loY]( Y = a three-electron donor such as OH, OMe, OBu, OPh, C1, Br, 02CCF3, 02CCH3). These silica-mediated syntheses compare favorably with more conventional syntheses in solution, which often require specific activation of [Os3(CO)12],because: i) unlike the intermediates usually used in solution, [HOs3(CO)loOSi-] is easily obtained in one step and in nearly quantitative yield starting from [ Os3(CO)1 2 1 ; ' ~ ' ~and ii) the conversion of [HOs3(CO)loOSi-] into [HOs3(CO)loY]occurs in a one-pot synthesis usually in high yields and under mild reaction condition^.['^]
Synthesis of [HOs3(C0)loOH] The synthesis of [HOs3(CO)loOH] was reported to occur in solution by direct reaction of [Os3(CO)12] with NaBH4[221or by hydrolysis of the reactive species [HOs3(CO)I O ( O C H = C H ~ ) ] , [[Os3( ~ ~ ]CO)lo(cyc1ohexa-1,3-diene)][24], or [HOS~(CO)~O( NCHNMe2)][251 (total yields starting from [Os3(CO)12]= 9-33%). [HOs3(CO)loOH] can be obtained in fair yields (total yield starting from [Os3(CO)12]= 56%) by treatment of [HOs3(CO)loOSi-] with aqueous H F which dissolves silica.[261 Better yields can be obtained by using milder reaction conditions. In fact, when [HOs3(CO)loOSi-] is stirred at 95 "C under N2 for 5 h, in a biphasic water-toluene system, [HOs3(CO)loOH] is obtained in almost quantitative yield = 91%).[17] (total yield from [OS~(CO)IZ]
Synthesis of [HOS~(CO)~O(OR)] (R =Me, Bu, Ph) In solution, [HOs3(CO)loOR] derivatives have been prepared by direct reaction of [ O S ~ ( C O ) Iwith ~ ] alcohols at elevated temperatures[271or by a multi-step process involving as intermediates [Os3(CO lo(~yclohexa-l,3-diene)],[~~~ [OS~(CO)IO (cy~looctene)2],[~~~ [Os3(C0)10(CH3CN)2])z81 or [HOs3(CO)lo(NCHNMe2)]J251 Total yields from [Os3(CO)12]were never good (10-48% for R = Me, Ph).[24,25,271 After stirring a slurry of [HOs3(CO)loOSi-] for 20 h in n-butanol, under N2, at reflux temperature (1 18 "C), [HOs3(CO)loOBu]is obtained in 87% yield.[I7]In contrast, [HOs3(CO)loOSi=]is quite unreactive when treated with methanol under reflux because of the rather low reaction temperature (65 "C). However addition of a few drops of HBF4.Et20 catalyzes the exchange reaction affording [HOs3(CO)loOMe]in 54% yield after 24 h.[I7]When a slurry of [HOs3(CO)loOSi-], phenol (molar ratio PhOH/Os3 = 100 : 1) and heptane is stirred at 98 "C under N2 for 5 h, [HOs3(CO)loOPh]is obtained in 66% yield.[171 [HOs3(CO)loOMe]and [HOs3(CO)loOPh]can be prepared in better total yields (82-87%) by use of a three-step process starting from [Os3(CO)12]:
2.10 The Surface of Inorganic Oxides
865
i) formation of [HOs3(CO)loOSi-]; ii) hydrolysis to [HOs3(CO)loOH];and iii) reaction of [HOs3(CO)loOH]with methanol (10 min at 65 "C) or phenol dissolved in heptane (45 min at 98 "C) in the presence of a few drops of HBF4.Et20."71
Syntheses of [HOg(CO)loYJ(Y = C1, Br, 02CR) Treatment of [HOs3(CO)loOSir]with a mixture of aqueous HY ( Y = C1, Br) and CH2C12, under N2 at 40 "C for 7 h, affords [HOs3(CO)loY] in excellent yields ( Y = C1, yield = 87%; Y = Br, yield = 89%). Similarly, [HOs3(CO)loOSi=]reacts with an excess of CF3C02H or CH3C02H in toluene, under N2 at 90 "C for 6 h, affording [HOs3(CO)lo(02CR)](R = CF3, yield = 72%; R = CH3, yield = 56%).[l7I [HOs3(CO)loOH], prepared by hydrolysis of [HOs3(CO)loOSi-], reacts readily with CF3C02H or CH3C02H in heptane (R = CF3, at 25 "C for a few minutes; R = CH3, at 78 "C for 5 h) to give [HOs3(CO)10(02CR)]in excellent total yields These synfrom [Os3(CO)12](R = CF3, yield = 91%; R = CH3, yield = 820/0).[171 theses are more convenient than those previously described in solution (total yields from [ Os3(CO)121 = 30-55%).[' 3-2 In this type of silica-mediated synthesis there is the limitation that the formation of [HOs3(CO)loOSi-] is controlled by the number of specific silanol groups available on the silica so that only loadings of the surface up to 4% ( w / w ) Os/SiO2 have been achieved.[32]Amounts of 200-300 mg of final cluster can be easily obtained by use of approximately 10 g of silica, which can be recycled after work-up and completion of the reaction.['71 '3'
99301
2.10.3 Synthesis of anionic osmium carbonyl clusters Surface-mediated syntheses of anionic osmium carbonyl clusters from neutral complexes, clusters, or salts require a surface of adequate basicity as the reaction medium. This has been achieved by two different approaches. The f i r ~ t [ * ,is~ the ,~~] use of an intrinsically strongly basic surface such as magnesia. The basicity of this surface can be easily varied by adjusting the temperature of the treatment. Increasing the temperature leads to gradual decarbonation and dehydroxylation of the surface with creation of very basic 02-centers, and therefore to increased surface basicity.[33]Similar thermal behavior is observed with the less basic alumina surface.[33l The second approach entails increasing the basicity of the surface of silica, a more neutral support, by treatment with an alkali metal ~ a r b o n a t e . [ l ~ - In ' ~ ,this ~~] latter case, the basicity of the surface is influenced by:
866
2 Metal Clusters in Catalysis
i) the nature and amount of the alkali metal carbonate; ii) the temperature; and iii) the manner in which the alkali metal carbonate is deposited on silica. The surface basicity is affected not only by the amount of alkali metal carbonate added but also by its - there is clear evidence that silica-supported K2CO3 behaves as a stronger base than silica-supported Na2C03 (see Section 2.10.3.1), probably because of low solvation of the alkali metal carbonate on the silica surface and, therefore, stronger ion-pair interaction of CO32- with the relatively small Na+ cation. Therefore, because of a reduction of the amount of physisorbed water which should dissolve and solvate a certain amount of the added alkali metal carbonate, an increase of the temperature (e.g. up to 150 "C) increases surface basicity. Finally, a high alkali metal carbonate loading (e.g. 36% (w/w) Na2C03 relative to silica)[l6Ileads to a rather inhomogeneous dispersion on the silica surface when a slurry in CH2C12 is used for the deposition of the alkali metal carbonate. Use of an aqueous solution instead of a CH2C12 slurry for the impregnation leads to a more homogeneous dispersion of the base and consequently to increased surface basicity for a given alkali metal carbonate l ~ a d i n g . [ ' ~ , ~ ~ ] Various anionic osmium clusters (i.e. and [Osl&(CO)24]2-[8,9,15-3sl 1 ( H ~ O S ~ ( C O ) ~ ~[OS5C(CO) ] ~ ~ , [ ]4]2--,[8,9,15,37' ~~,~~I have been synthesized, usually in high yields and under mild conditions, both on the surface of and on the surface of silica treated with alkali metal carbonated 4, 5,341 starting from H2 OsC16,[ OsCl3,[ 4, 51 [Os(C0)3C12]2,['4J 5' [OS3(C0)]2],[8.9,34,37' or [ H40s4(CO)12].[34-36] The cluster [H3Os4(CO)l2]- has also been prepared by deprotonation of [H40s4(C0)12]on the surface of a l ~ m i n a , [ ~ ~ . ~ ~ ] Zn0,[35]and La203.[351The final anionic osmium clusters can be recovered from the surface, by a process depending on the nature of the solid support.[40]When an anionic osmium carbonyl cluster is formed on an intrinsically basic surface such as magnesia or alumina, it interacts with surface A13+ or Mg2+ counterions and therefore has to be extracted from the surface by ion exchange (for example with [( Ph3P)2N]Cl dissolved in CH2C12, acetone or 2-propano1).[8,9*33.35-40] In contrast, on the surface of silica treated with an alkali metal carbonate the anionic osmium carbonyl cluster interacts with a free alkali metal cation (e.g. K+ or Na+) and therefore can be easily and directly extracted, as K+ or Naf salt, with a solvent, e.g. acetone or acetonitrile, of polarity sufficient to dissolve the organometallic salt.[14s15,34,401 's9,
2.10.3.1 Surface-mediated syntheses of [H30~4(C0)12] - and [H20S4(C0)1212-
In solution, the anionic cluster [H30s4(C0)12]- can be synthesized by deprotonation of [H40~4(C0)12] with KOH in methanol (75% yield)[18]or by heating a solu-
2.10 The Surface of Inorganic Oxides
861
tion of [Os3(CO)12]and KOH in butanol under reflux (45% yield).[411 The related cluster [ H ~ O S ~ ( C Ois) best I ~ ]prepared ~~ by heating a solution of [ O S ~ ( C O and )~~] NaBH4 in dioxane under reflux (39% yield).[42]
When [H40~4(C0)12] is adsorbed, from its hexane suspension under reflux, on to yA1203 pretreated at 400 "C, the resulting species is [H30~4(C0)12]-,ionically bound to the surface. The anionic cluster could be extracted from the surface by ion exSimilarly, when a solution of change with [( Ph3P)zNICI dissolved in CH2C12.1333391 [H40s4(C0)12]in CH2C12 is treated with MgO, ZnO or La203 and dehydrated at 200 "C in vacuo, deprotonation occurs. Extraction of the surface species with [( Ph3P)2N]Cl dissolved in CH2Clz at room temperature produces excellent yields of [H30~4(C0)12].'35' When a suspension of [H40sq(C0)12] in hexane is stirred under N2 for 1.5 h with highly dehydroxylated MgO (pretreated in vacuo at 800 "C) adsorbed [H30s4( CO)12]-, the predominant organometallic surface species, is formed together with some adsorbed [H20s4(C0)12I2-. A mixture of these two anions can be removed from the surface by cation metathesis with [( Ph3P)2N]Cl dissolved in CH2C12 or acetone. The formation of the dianion by deprotonation confirms the strong basicity of the surface of highly dehydroxylated Mg0.[33,361 Interestingly, when a slurry of silica, [H40~4(C0)12],K2C03 (molar ratio K2CO3/Os = 10 : 1) and CH2C12 is stirred at room temperature for 3 days and then evaporated to dryness, some deprotonation to [H30s4(C0)12IPoccurs, as shown by extraction of silica with CH3CN which affords a mixture of [H40~4(C0)12] and K[H30s4(C0)12]. When the same silica powder is heated for 3 h under CO at 150 "C, deprotonation is complete; extraction affords K[H30~4(C0)12] in quantitative yield. In contrast, when [H40s4(C0)12] supported on silica treated with NazCO3 (molar ratio NazCO3/Os = 10: 1) is heated at 150 "C under CO for 24 h the deprotonation reaction proceeds to a very limited extent. Extraction with CH2C12 affords [H40~4(C0)12] (92% yield) and further extraction with CH3CN gives some K[H30~4(C0)12](8% yield). These results show that silica-supported K2CO3 behaves as a stronger base than silica-supported Na2C03
From
[oS3(co)i21
Adsorption of [Os3(CO)12]on MgO, pretreated at 400 "C, followed by heating at 150 'C under CO (1 atm) for 2 h gives mainly [H30s4(C0)12]-, with a small quantity of [ O S ~ C ( C O (Scheme ) ~ ~ ] ~ 3).[81 ~ When silica-supported [Os3(CO)12]is treated with CO (1 atm) for 24 h at 150 "C in the presence of KzCO3 (molar ratio K2CO3/ 0 s = 10: l), K[H30~4(C0)12]is obtained in excellent yields (95-100%) (Scheme 2).[341
868
2 Metal Clusters in Catalysis
I
I
I
'
I
1 atm, 15O"C, 2h 11 atm, 275"C, 12h
CO+H2
1 atm, 275OC, 5h
Scheme 3. Best syntheses of various osmium clusters on the surface of magnesia pretreated at 400 oC.'s]
From H20sC16 (Scheme 3) Treatment of adsorbed H20sC16 on MgO, pretreated at 400 "C, for 12 h with a flow of CO + H2 (equimolar) at 275 "C and under 11 atm affords H3Os4(CO)12]which can be extracted with [( Ph3P)2N]Cldissolved in CH2C12.[s,9 However, during the reaction, 42% of the initial 0 s content is lost, presumably as volatile neutral carbonyls.['I
I
From OsCl3 and [Os(CO)&1~]2(Scheme 2) When [Os(CO)3C12]2supported on silica (2-15% (w/w)0 s relative to Si02) in the presence of K2CO3 (molar ratio K2CO3/Os = 10-20: 1) is heated at 150 "C for 24 h under 1 atm CO, K[H30~4(C0)12]is selectively formed on the surface. Extraction with CH3CN affords this anionic cluster in 91% yield.['4] Similar yields are obtained starting from silica-supported OsC13 using a two-step route: i) in situ formation of [ O S ( C O ) ~ C HOSi-)I ~ ~ ( by reductive carbonylation (1 atm CO, 180 " C )of silica-supported O~Cl3['~1; and ii) addition of K2C03 (molar ratio K2C03/0s = 10-20: 1) followed by further reduction (1 atm CO, 150 "C, 24 h).[l41
By working at 200 "C instead of 150 "C, under the same reaction conditions, both silica-supported [Os(C0)3Cl2]2 and [Os(CO)3C12(HOSi-)I are easily converted to K2[H20~4(C0)12] in the presence of excess K2C03. After 48 h under CO, extraction with CH3CN affords this dianion in 92% yield.[l5]These high-yield silica-mediated
2.10 The Surface of Inorganic Oxides
869
syntheses of K[H30s4(C0)12] and K2[H20~4(C0)12]are much more convenient than the best known syntheses in s o l ~ t i o n [ ' ~ ,which ~ ' ~ ~ use ~ 1 the more expensive [H~OS~(CO)I~ or] [[' ~ O]S ~ ( C O ) I ~clusters ] [ ~ ~ as ~ ~starting ~] materials and which afford, usually under more drastic conditions, lower yields of the final products.
2.10.3.2 Surface-mediated syntheses of [OsloC(C0 ) 2 4 ] [Os10C(C0)24]~-is usually prepared by reaction of [Os3(CO)12]with Na in tetraglyme at 230 "C (63% yield)[431or by pyrolysis of [Os3(CO)ll(CsHsN)]at 250 "C for 64 h (65% yield).[441
From H2OsCbj (Scheme 3) Treatment of adsorbed H20sC16 on MgO, pretreated at 400 "C, for 5 h with a flow which can be of CO H2 (equimolar) at 275 "C and 1 atm affords [Os10C(C0)24]~recovered by extraction with [( Ph3P)2N]Cl dissolved in acetone (65% There are no apparent osmium carbonyl byproducts, but some [Osl0C(C0)24]~-is retained by the MgO after extraction (perhaps entrapped in the solid matrix), therefore reducing the yield.18]
+
From OsC13 or [Os(CO)3C12]2 (Scheme 2) Treatment of silica-supported [Os(CO)3C12]2or [Os(CO)3C12(HOSi-)I, obtained by reductive carbonylation (1 atm CO, 180 "C) of silica-supported O S C ~ ~ , with ['~I a slurry of Na2C03 (molar ratio NazCO3/Os = 10 : 1; 2- 15% (w/w)of 0 s relative to Si02) in CH2C12 followed by evaporation of the solvent and reaction with H2 (1 atm) at 200 "C for 24 h, affords Na2[0~10C(C0)24]. Extraction with CH3CN gives this cluster in 81% yield. This silica-mediated synthesis is an attractive and convenient way to prepare [ O S ~ & ( C O ) ~in ~ ]high ~ - yield and under mild conditions.[15]
'
2.10.3.3 Surface-mediated syntheses of [Os5C(CO)14] In solution, [ O S S C ( C O ) ~ ~ is]obtained ~in low yields (ca 37%) from [Os3(CO)12]by a multi-step process involving pyrolysis of the latter cluster to give [Os~C(C0)15] followed by reaction with Na2C03 in methanol.[45]
From [Os3(CO)12](Scheme 3) Adsorption of [Os3(CO)l2]on to MgO, pretreated at 400 "C, followed by treatment Extracfor 4 h with a flow of CO (1 atm) at 275 " C , generates [Os~C(C0)14]~-. tion with (Ph3P)2N]Cl dissolved in acetone affords this anionic cluster in 65% yield.I8,9,37
\
870
2 Metal Clusters in Catalysis
From [Os(CO)3C12]2 (Scheme 2) Treatment of silica-supported [Os(CO)3C12]2(2-5% ( w / w ) of 0 s relative to Si02) with a slurry of K2C03 (molar ratio K2C03/0s = 20: 1) in CH2C12, followed by evaporation of the solvent and reaction with CO (1 atm) at 265 "C for 24 h affords K ~ [ O S ~ C ( C Owhich ) I ~ ] can be extracted with CH3CN (74% yield)." 51 This silicamediated synthesis is much more efficient than the best known synthesis in ~olution.~~~]
2.10.4 The understanding of the process of nucleation of surface osmium( 11) carbonyl species to osmium carbonyl clusters 2.10.4.1 On magnesia surfaces[']
+
When the reductive carbonylation (1 atm CO or CO H2) of H20sC16 supported on MgO is performed at 200-250 "C the only surface carbonyl species detected are osmium(I1) di- and tricarbonyl species of the type [Os(CO),{OMg}2] (x = 2 or 3). These surface intermediates are quite stable under CO or CO + H2 and are reduced to give anionic osmium clusters only at temperatures approaching 275 "C. The reduction of the osmium(11) subcarbonyl species and the initiation of cluster growth were found to coincide with the loss of chemically bound water from the MgO surface, which becomes significant only at temperatures exceeding 250 "C. It was suggested that the reductive carbonylation of Os(11) carbonyl species on MgO in the presence of CO alone could be initiated by nucleophilic attack of adsorbed water, or surface hydroxyl groups, to generate the reactive species [HOs(C0)4]-. This anionic species could also be obtained by reduction of [Os(C0)3{OMg}2]at 275 "C with CO + Hz:
co +
[HOs(CO)J
+ 2{Mg-OH} + 3{Mg(HC03)}
The cluster growth, which can occur via attack of the nucleophile [HOS(CO)~]on Os(11) subcarbonyl surface species, depends on the nature and pressure of the gaseous phase (Scheme 4). Treatment of [Os(CO),{OMg}~](x = 2 or 3) under 1 atm of CO or equimolar CO H2 at 275 "C affords [O~l&(C0)24]~whereas
+
2.10 The Surface of Inorganic Oxides
871
H2°sC16/Mg0400
I
200-250°C CO or 1 atm CO+H,
Scheme 4. Process of the nucleation of osmium( 11) carbonyl species to osmium carbonyl clusters on the surface of MgO.[*]
54(CO)1z l - M @
1
275"C, CO or equimolar 4h CO+H,,, 1 atm
CO+H, (1:3 molar ratio) 275OC, 10 atm, > 2 days
[ o ~ , 0 c ( c o ) 2 , 1 2 - ~ g4 o
[H30s4(C0)12]- is the predominant product obtained by working under 11 atm equimolar CO H2. Treatment of [H30~4(C0)12]-,supported on MgO, with 10 atm CO H2 (1 : 3 molar ratio) at 275 "C generates [ O S I ~ C ( C O ) ~ but ~]~-, very slowly ( > 2 days). It seems that a high pressure of H2 inhibits the growth of high-nuclearity clusters. This observation is confirmed by the report that [H30~4(C0)12]-,prepared by adsorption of [Os3(CO)12]on MgO and treatment with 1 atm CO at 150 "C, can be rapidly (4 h) converted to [ O S + ~ ( C O ) ~under ~ ] ~ -1 atm CO at 275 "C. Therefore the reactivity of the key intermediate [H30s4(C0)12]toward the formation of high-nuclearity clusters is very sensitive to the presence and pressure of H2 in the gaseous phase.[8]
+
+
2.10.4.2 On silica surfaces with added alkali metal carbonates We recently obtained evidence for a step-by-step process (Scheme 5) which explains the origin of the high selectivity observed in the controlled reduction of silicasupported [Os(CO)3C12]2to generate different neutral or anionic osmium carbonyl clusters, in the presence of alkali metal carbonates (Schemes 1 and 2).[34] When a slurry of CH2C12, M2CO3 (M = Na or K; molar ratio M/Os = 4-40 : 1) and silica-supported [Os(CO)3C12]2or [Os(CO)3C12(HOSi-)I is stirred at room temperature, reactive dehalogenated surface Os( 11) carbonyl species are produced. The nature of these depends on the basicity of the treated silica surface. If the basicity is low (for instance, molar ratio NazCO3/Os = 2 : l), it seems that neutral surface species such as [Os(C0)3(0R)2In(R = H or Si-) are formed. Increasing the surface basicity (molar ratio Na2C03 or K2C03/0s = 10-20: 1) leads to the formation of anionic {[OS(CO)~(OR)~],(OR)>entities (R = H or Si=; m > 1) up to the less reactive species [OS(CO)J(OH)~]-. The low reactivity of this last species explains why low yields of carbonyl clusters are obtained when a strong base, such as an alkali metal hydroxide, is used in the reductive carbonylation of silicasupported [Os(CO)3C12]2instead of an alkali metal ~ a r b o n a t e . [ ' ~ * ~ ~ ]
812
2 Metal Clusters in Catalysis
[OS(CO)~C~~II /SiO2
M,CO, (M=Na, K) M,CO,:O~=2-20:1
*
M"Os(CO) (OR),I,(OR)I/SiO,
@=h,si=;m>l)
CO or H,
very high basicity
$. 150-275'C
HOs3(CO)lo(OSi=)and/or HOs,(CO),,(OH)/SiO, neutral or weak basicity
high basicity H, , 150°C
150-2000c neutral or weak basicity, H, , 1 5 0 T Os,(CO),, N O , H40s4(CO),, /SiO, high basicity CO , 150°C high basicity co l 5 O S [H30s4(CO),,]-/SO,
very high basicity
I
'
[OS,,C(CO),~]~/Sio,
[H,0s4(C0)l,]2- /SiO, I
CO 265-275°C some H, is produced by water-gas s& [0s~c(c0)1412- /si02
very high basicity H, 200°C
I
Scheme 5. Process of the nucleation of osmium( 11) carbonyl species to osmium carbonyl clusters on the surface of S i 0 ~ . ~ ' 4 ~ 1 5 ~ 3 4 ]
In agreement with the suggestion that [Os(CO)3(OSi-)2], and/or silica-supported [Os(C0)3(0H)2Incould be the active surface species generated in situ, the selectivity of the reduction (CO or H2) of both [Os(CO),(OSi-)2], ( x = 2.7; 2% (w/w) Os/SiO2; prepared by thermal treatment in air of [ H O S ~ ( C O ) ~ ~ ( O S ~ - and )]~""~) silica-supported [Os(C0)3(0H)2In(2% (w/w)of 0 s relative to Si02; prepared by impregnation of silica with an aqueous solution of [ O S ( C O ) ~ ( O H ) ~in] ~the [~~~) presence of alkali metal carbonates is similar, under similar reaction conditions, to the selectivity observed when starting from silica-supported [Os(CO)3C12]2 in the presence of alkali metal carbonates.['4,' 51 Unfortunately, one cannot easily distinguish between [Os(C0)3(OSi=)2],,and silica-supported [Os(C0)3(0H)2In on the basis of either their infrared spectra, which are very similar, or of extraction experiments, because silica-supported [Os(CO)3(OH& although weakly supported on the silica surface, cannot be extracted with non-protic donor solvents such as acetone or CH3CN, because of its low solubility. Since we have evidence from the behavior of some osmium silanolate carbonyl complexes that the 0s-OSi bond is very susceptible to hydrolysis,[48]we can suggest that any surface-silanolate species generated by reaction of the silanol groups with [Os(CO)3C12]2could easily hydrolyse, because of the presence of water under our reaction conditions, to generate [Os(C0)3(0H)2In.With high osmium loadings (15% (w/w)of 0 s relative to Si02) some [Os(C0)3(0H)2Inshould always be formed, because there
2.10 The Surface of Inorganic Oxides
873
would be insufficient surface silanol groups to convert all [ O S ( C O ) ~ Cto~ ~ ] ~ [Os(C0)3(OSi=)2In. As suggested for the MgO surface,[81it is conceivable that the reactive silicasupported dehalogenated Os(11) carbonyl species initially formed are first converted to [HOs(C0)4]-. Cluster growth could result from redox condensation of this reactive anion with unreacted [Os(C0)3(0R)2In (R = H and/or Si=) surface species, by analogy with the suggested condensation of [Rh(CO 4]- and [Rh(CO)* (OAl)(HOAl)] to give [Rh6(C0)16]on the surface of A1203.149The first condensation product seems to be the surface cluster [HOS~(CO)~O(OR)] (R = H and/or Si=). The facile equilibrium between [HOs3(CO)lo(OSi-)] and [HOS~(CO)IO(OH)] on the silica does not enable discrimination between them. Both species are converted, with similar selectivities, to [Os3(CO)12], [H40s4(C0)12], [H30~4(C0)12]-,[Os~C(C0)14]~-, and [Osl0C(C0)24]~-by reduction with CO or H2 in the presence of alkali metal carbonates.i501This is indirect evidence that either [HOs3(CO)lo(OSi=)]or physisorbed [HOS~(CO)IO(OH)], or both, could act as intermediates in the silica-mediated synthesis of various osmium carbonyl clusters starting from [Os(CO)3C12]2 in the presence of alkali metal carbonates. This hypothesis is also suggested by the detection of traces of both trinuclear clusters during the silica-mediated syntheses of [Os3(CO)12] and [H30~4(C0)12]-from .I151 [OS(CO)~C1~]~ It is known that [HOsj(CO)lo(OR)](R = H and/or Si=) species are readily converted to silica-supported [Os3(CO)12]or [H40s4(C0)12]by working in the presence of CO or H2, respectively, when the surface basicity is low. By increasing the basicity, further transformation of [Os3(CO)12]into [H30~4(C0)12]-or deprotonato [H30s4(C0)12]- or [H20s4(C0)12I2- can occur, depending tion of [H40~4(C0)12] on the basicity of the surface and on the reaction conditions (Scheme 5). The latter anion is favored by strong surface basicity (molar ratio K2CO3/Os = 10: 1 and temperatures above 200 "C); the former is favored by relatively mild surface basicity (molar ratio NazCO3/Os = 10: 1) and, strangely enough, by the presence of a large amount of H2 in the gas phase ([H30~4(C0)12]does not deprotonate to [H20s4(C0)12I2- when treated at 200 "C under H2 even in the presence of a 10: 1 molar ratio K ~ C O ~ / O S )The . [ ~ reactivities ~I of these two anions are different, which leads to specific pathways of condensation on the silica surface. The nuclearity of [H3Os4(C0)12]- is increased, resulting in the formation of [ O S & ( C O ) ~ ~ ] ~ - , by increasing the temperature under either CO or H2. The nuclearity of [H20s4(C0)12I2-, which is stable under CO even at high temperatures, is increased at 200 "C in the presence of relatively low amounts of H2, leading to the formation of [ O S ~ C ( C O ) ~ ~The ] ~ - reactivity . [ ~ ~ ] of [H30~4(C0)12]-therefore suggests that it might be a key intermediate in silica-mediated syntheses of [O~loC(C0)24]~starting from [ O S ( C O ) ~ C ~ ~(Scheme ] ~ [ ' ~ , 5). ~ ~At I 200 "C under H2, in the presence of low surface basicity (molar ratio Na2C03/0s = 2: l), [H40sq(C0)12] is the major product. When the reduction is performed in the presence of greater basicity (molar
i
874
2 Metal Clusters in Catalysis
ratio NazCO3/Os = 10 : l), deprotonation of [H40~4(C0)12]to [H30~4(CO)i2]must occur, followed by condensation to [O~loC(C0)24]~-.The cluster [ O S ~ C ( C O ) ~is,~on ] ~ the - other hand, generated in high yield by reductive carbonylation (1 atm CO) of silica-supported [Os(CO)3C12]2 at 275 "C in the presence of K2C03(molar ratio K2C03/0s = 10 : 1). Under these conditions, both H2 and C02 are produced by the water gas shift reaction, as confirmed by gas chromatographic analysis of the gaseous phase at the end of the reaction. The quantity of H2 produced is probably low enough to enable further deprotonation of the intermediate [H30~4(C0)12]-to [ H ~ O S ~ ( C O but ) ~ ~it] is ~ -high enough to favor further thermal condensation of this latter intermediate to [Os5C(CO)14]2-.[34) Similarly, by working on the surface of MgO under CO (1 atm) at 275 "C, the condensation of the intermediate [H30s4(C0)12]- to [ O S ~ C ( C O ) M ] (Scheme ~ - [ ~ ] 3) could be due to the positive role of some H2, produced by the water gas shift reaction catalyzed by [ H ~ O S ~ ( C O ) ~ ~on] - the , [ ~ ~deprotonated ] species [H20s4(C0)12I2- generated in situ by the very basic surface of MgO. In the presence of excess H2 (for example with a mixture of 10 atm of CO : H2, 1 : 3 molar ratio), [H30~4(C0)12]-is stabilized and is, therefore, only slowly thermally converted into [Osl0C(C0)24]~-but not into [Os~C(C0)14]~(Scheme 4).['1 In fact the quantity of H2 in the gas phase is probably too high and therefore deprotonation of [H30~4(C0)12]-to [H20s4(C0)1212-,the intermediate species for further condensation to [ O S ~ C ( C O ) ~is~ ]hindered. ~-, In summary, the basicity of the surface of the inorganic oxide stabilizes specific intermediates such as [H30sq(CO)121- and [H20s4(C0)1212-,which are characterized by a rather different surface reactivity. Their stability and further condensation to [Os10C(C0)24]~or [ O S ~ C ( C O ) ~are ~]~controlled not only by surface basicity and temperature, but also by the amount of H2 in the gas phase.
2.10.5 Conclusion The use of the surface of inorganic oxides as reaction media enables selective and high-yield syntheses of a variety of neutral and anionic osmium carbonyl clusters starting from simple and readily available materials. The procedures are simple and straightforward. Because the use of a solid as a reaction medium is not limited, as in solution, by boiling points, it is possible to work at atmospheric pressure even at relatively high temperatures. From a synthetic standpoint yields and pressure of the gaseous phase are usually better and lower, respectively, than those of related syntheses in solution. In addition the possibility of usually achieving high yields and selectivities even working with high metal loadings of the inorganic oxide surface gives to this synthetic method the potential of being used for the preparation of large amounts of osmium clusters by use of only a few grams of the inorganic oxide.
2.10 The Surface of Inorganic Oxides
875
References [ I ] G. W. Kabalka, R. M. Pagni, Tetrahedron 1997,53, 7999-8064. [2] Preparative Chemistry Using Supported Reagents (Ed.: P. Lazlo), Academic Press, San Diego, 1987. [3] Solid Supports and Catalysis in Organic Synthesis (Ed.: K. Smith), Ellis Hanvood, Chichester, 1992. [4] J. H. Clark, Catalysis of Organic Reactions by Supported Inorganic Reagents, VCH, New York, 1994. [5] Y. Izumi, K. Vrube, M. Onaka, Zeolite, Clay and Heteropoly Acid in Organic Reactions, VCH, Weinheim, 1992. [6] M. Balogh, P. Lazlo, Organic Reactions Using Clay, Springer, Berlin, 1993. [7] J. J. 111 Bergmeister, B. E. Hanson, J. Organomet. Chem. 1988,352, 367-372. [8] H. H. Lamb, A. S. Fung, P. A. Tooley, J. Puga, R. Krause, M. J. Kelley, B. C. Gates, J. Am. Chem. SOC. 1989,111, 8367-8373. [9] B. C. Gates, J. Mol. Catal. 1994,86, 95-108. [lo] S. Kawi, Z . Xu, B. C. Gates, Inorg. Chem. 1994,33, 503-509. [ l l ] Z . Xu, S. Kawi, A. L. Rheingold, B. C. Gates, Inorg. Chem. 1994,33, 4415-4417. [I21 C. Dossi, R. Psaro, D. Roberto, R. Ugo, G. M. Zanderighi, Inorg. Chem. 1990,29, 43684373. [13] D. Roberto, R. Psaro, R. Ugo, Organometullics 1993,12, 2292-2296. [ 141 D. Roberto, E. Cariati, R. Psaro, R. Ugo, Organometallics 1994,13, 734-737. [15] D. Roberto, E. Cariati, R. Ugo, R. Psaro, Inorq. Chem. 1996,35, 2311-2313. [ 161 D. Roberto, E. Cariati, E. Lucenti, M. Respini, R. Ugo, Organometallics 1997,16,4531-4539. [ 171 D. Roberto, E. Lucenti, C. Roveda, R. Ugo, Organometallics 1997,16, 5974-5980. [I81 B. F. G. Johnson, J. Lewis, P. R. Raithby, G. M. Sheldrick, K. Wong, M. McPartlin, J. Chem. Soc., Dalton Trans. 1978,673-676. [I91 D. Roberto, M. Pizzotti, R. Ugo, Gazz. Chim. ItaI. 1995,125, 133-135. [20] R. Barth, B. C. Gates, H. Knozinger, J. Hulse, J. Catal. 1983,8, 147-159. [21] B. F. G. Johnson, J. Lewis, Inorg. Synth. 1972,13, 92-95. [22] B. F. G. Johnson, J. Lewis, P. A. Kilty, J. Chem. Soc. ( A ) , 1968,2859-2864. Dalton Trans. [23] A. J. Arce, A. J. Deeming, S. Donovan-Mtunzi, S. E. Kabir, J. Chem. SOC. 1985,2479-2482. [24] E. G. Bryan, B. F. G. Johnson, J. Lewis, J. Chem. SOC. Dalton Trans. 1977, 1328-1330. [25] J. Banford, M. J. Mays, P. R. Raithby, J. Chem. SOC.Dalton Trans 1985, 1355-1360. [26] C. Dossi, A. Fusi, M. Pizzotti, R. Psaro, Organometallics 1990,9, 1994-1995. [27] K. A. Azam, A. J. Deeming, R. E. Kimber, P. R. Shukla, J. Chem. SOC. Dalton Trans. 1976, 1853-1858. [28] M. Tachikawa, J. R. Shapley, J. Organomet. Chem.1977,124, C19-C22. [29] B. F. G. Johnson, J. Lewis, D. A. Pippard, J. Chem. Soc. Dalton Trans. 1981,407-412. [30] A. J. Deeming, S. Hasso, J. Organomet. Chem. 1976,114, 313-324. [31] T. B. Shay, L.Y. Hsu, J.-M. Basset, S. G. Shore, J. Mol.Catal. 1994,86, 479-489. [32] T. H. Walter, G. R. Frauenhoff, J. R. Shapley, E. Oldfield, Inorg. Chem. 1991,30, 4732-4739. [33] H. H. Lamb, B. C. Gates, H. Knozinger, Angew. Chem. Znt. Ed. Enyl. 1988,27, 112771144, [34] E. Cariati, P. Recanati, D. Roberto, R. Ugo, Organometallics 1998,17, 1266-1277. [35] L. D’Ornelas, A. Choplin, J.-M. Basset, J. Puga, R. A. Sanchez-Delgado, Inorg. Chem. 1986, 25,4315-4316. [36] H. H. Lamb, L.C. Hasselbring, C. Dybowski, B. C. Gates, J. Mol. Catal. 1989,56, 36-49. [37] A. S. Fung, P. A. Tooley, M. J. Kelley, B. C. Gates, J. Chem. SOC.Chem. Commun. 1988, 371-372.
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[38] H. H. Lamb, T. R. Krause, B. C. Gates, J. Chem. SOC.Chem. Commun. 1986, 821-823. [39] T. R. Krause, M. E. Davies, J. Lieto, B. C. Gates, J. Catal. 1985, 94, 195. [40] D. Roberto, E. Cariati, M. Pizzotti, R. Psaro, J. Mol. Catal. A: Chemical 1996, 111, 97-108. [41] B. F. G. Johnson, J. Lewis, W. J. H. Nelson, M. D. Vargas, D. Braga, K. Henrick, M. McPartlin, J. Chem. Soc., Dalton Trans. 1984, 2151-2161. [42] B. F. G. Johnson, J. Lewis, P. R. Raithby, G. M. Sheldrick, G. Suss, J. Organomet. Chem. 1978,162, 179-187. [43] C. M. T. Hayward, J. R. Shapley, Inorg. Chem. 1982,21, 3816-3820. [44] P. F. Jackson, B. F. G. Johnson, J. Lewis, W. J. H. Nelson, M. McPartlin, J. Chem. Soc., Dalton Trans. 1982, 2099-2107. [45] B. F. G. Johnson, J. Lewis, W. J. H. Nelson, J. N. Nicholls, J. Puga, P. R. Raithby, M. J. Rosales, M. Schroder, M. D. Vargas, J. Chem. SOC.,Dalton Trans. 1983, 2447-2457. [46] R. Psaro, R. Ugo, G. M. Zanderighi, B. Besson, A. K. Smith, J.-M. Basset, J. Organomet. Chem. 1981,213, 215-247; C. Dossi, A. Fusi, R. Psaro, R. Ugo, R. Zanoni, in Structure and Reactivity of Surfaces, (Eds.: C. Morterra, A. Zecchina, G. Costa), Elsevier, Amsterdam, 1989, p. 375. [47] E. Cariati, E. Lucenti, M. Pizzotti, D. Roberto, R. Ugo, Organometallics 1996,15,4122-4124. [48] M. Pizzotti, D. Roberto, R. Ugo, unpublished results. [49] J.-M. Basset, A. Theolier, D. Commereuc, Y. Chauvin, J. Organomet. Chem. 1985,279, 147158. [50] E. Cariati, D. Roberto, R. Ugo, Gazz. Chim. Ital. 1996,126, 339-343. [51] M. Lenarda, R. Ganzerla, M. Graziani, R. Spogliarich, J. Organomet. Chem. 1985,290, 213218.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.11 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters Brian F. G. Johnson and Caroline M. Martin
2.1 1.1 Introduction Metal triangles, tetrahedra, and octahedra form the basic building blocks of transition metal carbonyl clusters. The smaller clusters with between three and six metal atoms often adopt these pseudo-spherical deltahedral geometries, but as the nuclearity of the cluster increases, condensed structures, built up from the smaller polyhedra by vertex-, edge-, or face-sharing, tend to be favored in preference to the larger spherical deltahedra based on the Platonic and Archimedean solids. In general, this is in contrast to the structures found in borane chemistry. The pattern, based on the condensation of cluster fragments, continues as the nuclearity of the cluster increases further, leading ultimately to the close-packed structures observed in the bulk metallic lattice. In particular, the condensation of tetrahedral and octahedral units lead to the familiar hexagonal close-packed and cubic-close packed arrangements observed for the great majority of bulk metals. For example, the cluster anions [H3Rh13(C0)24l2- and [Rh14(C0)25I4- can be regarded as fragments of hexagonal close-packed and body-centered cubic lattices,['-2]respectively (Fig. I), while the series of clusters based on four, ten, and twenty metal atoms, [H40s4(C0)12],[ O S I O C ( C O ) ~, ~and [Os2o(CO)40]~-follow the cubic close-packing growth sequence precisely.[3 The structural similarities between these clusters and a 'piece' of metal are obvious, and the same tetrahedral and octahedral sites present in a close-packed metallic lattice are also found in molecular transition metal clusters. Solid-state metal alloys are industrially important materials; the simplest can be regarded as a metallic lattice containing individual main-group atoms in a set of interstitial holes. Likewise, there are numerous examples of discrete molecular clusters containing either electron-deficient, electron-precise, or electron-rich atoms in their interstitial cavities.
r-
878
2 Metal Clusters in Catalysis
Figure 1. The metal skeletal frameworks of (a) [H3Rhl3(C0)24I2-, (b) [Rh14(C0)25I4-, and (c) [ O S ~ O C ( C O ) ~showing ~ ] ~ - , their similarities to a fragment of a HCP, BCC and CCP lattice, respectively.
Since the initial structural determination of [FejC(CO)ls] by Dahl in 1962,L4] which demonstrated the possibility of forming transition metal clusters incorporating main-group interstitial atoms, many examples have been characterized. The description of a heteroatom as ‘interstitial’ is well-defined when it is completely encapsulated within a polyhedron of metal atoms as, for example, in the octahedral cluster [Ru&(CO) 17].[~] There is, however, a large number of clusters containing ‘naked’ main-group atoms in a variety of coordination modes which cannot be classified as readily. For example, the carbon atom in [RujC(CO)lj]lies slightly below the basal plane of a square pyramidal cluster framework where it
2.11 The Role of Interstitial Atoms in Transition Metul Curbonyl Clusters
879
Figure 2. The M,E, framework geometries of (a) [Ru5C(C0)15]and (b) [C06Ge2(C0)20]
could be described as a surface atom.[6]It is, however, also bound to the fifth, apical, ruthenium atom and is partially enclosed by the metal cage, hence, it could also be described as occupying an interstitial site (Fig. 2a). Atoms of this type are generally termed 'semi-interstitial'. Alternatively, in the cluster [CosGez(C0)20]the four cobalt atoms form a square which is bicapped by quadruply bridging germanium atoms, thus generating an irregular Co4Ge2 octahedral core.[71An additional Co(CO)4 group bonds to each germanium atom, resulting in five Ge-Co bonds and the germanium atom adopting a distorted square-pyramidal configuration. Although the Ge atoms in this species are bonded solely to metal atoms they are surface atoms with respect to the cluster and are, therefore, not considered to be interstitial (Fig. 2b). A fascinating and extensive range of transition metal carbonyl clusters containing interstitial and semi-interstitial main-group atoms now exists, and although this review is not intended to be a comprehensive survey of the area, it should hopefully illustrate the typical sites of such atoms and how their encapsulation affects the surrounding metal polyhedral framework. Mechanistic information as to the origin of such atoms is provided, together with a few reasons as to why this area of chemistry has been of considerable interest during the past thirty years.
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2.11.2 Types of interstitial atom Many main-group atoms have been found to occupy interstitial positions in transition metal carbonyl clusters and there are now examples of H, B, C, N, 0, Si, P, s, Ge, As, Sn, and Sb atoms being encapsulated, either fully or partially, within a metal skeleton. In general, the size limitations associated with the internal cavity of the cluster determine the type of atom which can occupy the interstitial site. Although the radius of the cavity is determined primarily by the nuclearity and geometry of the cluster, the size of the metal atoms relative to that of the interstitial atom is a critical factor, and clusters with larger cavities are observed as the ratio of the covalent radius of the main-group atom to that of the transition metal atoms increases. Table 1 summarizes the closed polyhedral frameworks (Fig. 3) usually observed for metal clusters with nuclearities between four and twelve, together with the relative dimensions of their internal cavities. A comparison is made between this cavity size and the steric demands of the main-group atoms known to occupy such positions in an attempt to illustrate the extent to which such atoms can expand or contract to meet the steric requirements of the cavity. A general trend emerges that the first-row main-group interstitial atoms, H, B, C, and N, tend to occupy octahedral and trigonal prismatic holes, second-row atoms such as Si, P, and S show a preference for square-antiprismatic cavities, and the heavier main-group interstitial atoms e.g. Ge, Sn and Sb require even larger cavities such as the icosahedron. As would be expected, there are exceptions to these rules, for example phosphorus has been located in a trigonal prismatic cavity in the cluster [ O S S P ( C O ) ~ ~ ] - [ ~ ] and arsenic in a square antiprismatic cavity in [RhloA~(C0)22]~-,[~1 even though ‘theoretically’it seems unlikely. The aim of this section is to highlight the different interstitial sites in which main-group atoms have been located, and to illustrate the effects they have on the polyhedral cluster core.
2.11.2.1 Three-coordinate interstitial sites The lowest coordination number observed for a ‘semi-interstitial’main-group atom in a cluster is three. For example, the complex [MO~N(CO)~(O)(CP)~] contains a nitrogen atom occupying an unusual site in the plane of three molybdenum atoms.[lolTwo metal-metal bonds hold the Mo3 unit in an open triangular arrangement and result in a very exposed nitrogen atom with ‘T’-shaped coordination (Fig. 4).
2. I 1 The Role of Interstitial Atoms in Transition Metal Curbonyl Clusters
88 1
Table 1. The closed polyhedral frameworks usually observed for metal clusters with nuclearities between four and twelve, and the relative dimensions of their internal cavities.
Md Mg
Polyhedron
Radius Cavity radius Radius of interstitial Example of cavity* for r = 1.32At atom ( A)$
Tetrahedron Octahedron
0. 225r 0.414r
0.30 0.55
Trigonal Prism
0.52%
0.70
~
H = 0.37 B = 0.90 C = 0.77 N = 0.75 ~
~
P = 1.10 Mg
Square Antiprism
0. 645r
0.85
-
Si = 1.18 -
M12
Icosahedron
0.902r
Cubeoctahedron 1.OOr Anticubeoctahedron 1.OOr
1.19
1.32 1.32
s = 1.02 AS = 1.20 Ge = 1.22 Sn = 1.40 Sb = 1.40 Pt = 1.39 ~
Rh = 1.34
* Regular polyhedral edge = 2r.
t Average value for the metallic radius of an element from the Fe, Co, and Ni triads. $All covalent and metallic radii taken from Chemistry of the Elements, Greenwood and Earnshaw, Pergamon Press, 1984.
2.11.2.2 Four-coordinate interstitial sites There are two basic structural motifs in which a tetranuclear cluster could accommodate an interstitial atom - fully encapsulated within a tetrahedral cavity or partially encapsulated between the wings of a butterfly framework.
The tetrahedral cavity Whether it is possible for a tetrahedral cluster to accommodate an H atom in its interstitial cavity without causing substantial 'swelling' and hence destabilization of the cluster is an intriguing question. Although the occupation of tetrahedral holes is a well-documented phenomenon in binary metal hydrides where the metallic lattice
882
2 Metal Clusters in Cutulysis
Tetrahedron
Icosahedron
Octahedron
Trigonal prism
Cubeoctahedron
Square antiprism
Anticubeoctahedron
Figure 3. The main closo-polyhedral geometries observed in transition metal cluster chemistry which incorporate interstitial-atoms.
itself provides considerable stabilization, discrete molecular clusters have no such stabilizing counterpart and whether they are likely to exist is, therefore, a matter of some debate. An interstitial H atom located in a tetrahedral environment was first reported in 1982, in the tetracapped octahedral cluster [HOs&(C0)24]-.11 Evidence came from X-ray studies which showed complete coverage of the metal core by carbonyl ligands very similar to that of [Osl0C(C0)24]~-from which it was prepared. It was therefore assumed that the hydride was in an interstitial site, and because the octahedral cavity was already occupied by a carbido-atom it seemed reasonable to assume that the hydride was sited within a tetrahedral cap. The lS7Ossatellite pattern associated with the hydride signal in the 'H NMR spectrum was entirely as would be expected for a tetrahedrally coordinated hydride, which helped to confirm this proposal.
Figure 4. The Mo3N cluster framework of [MO3N(C0)4(0)(CP)31.
2. I I The Role of Interstitiul Atoms in Trunsition Metal Curbonyl Clusters
883
A similar situation was later observed for the non-carbido cluster [ H ~ O S ~ ~ ( C O )for ~ ~which ] ~ ~it, was ~ ' ~proposed I that three hydrides were situated in tetrahedral capping sites and the fourth occupies the octahedral cavity filled '-. Significant lengthening of several by carbon in the isoelectronic [Os10C(C0)24] metal-metal distances was observed for this cluster; although this could be attributed to the cluster expansion produced by internal hydrides, results from variabletemperature 'H and I3C NMR studies could not be explained in terms of such a model. More recently neutron diffraction studies have established, beyond doubt, that all four hydride ligands are located on the cluster surface, two in p3- and two in p~-sites.['31 This study clearly demonstrates that negative evidence for surface location of H ligands in large clusters is unreliable and that neutron diffraction is essential for unambiguous characterization. As far as we are aware there are no examples of the location of an H atom, by neutron diffraction, within a tetrahedral cavity, and so the debate continues.
The butterfly cavity For tetranuclear clusters to incorporate main-group elements interstitially, they must adopt a more open skeletal arrangement. The M4 butterfly cluster framework can be derived from the tetrahedron by cleaving a metal-metal edge; alternatively, it can be viewed as an arachno-octahedron (Scheme 1). Butterfly clusters containing semi-interstitial B, C, N, and 0 atoms have been observed and they all adopt similar structures in which the heteroatom interacts with all four metal atoms and is located in a rather exposed position midway between the wing-tip atoms of the M4 skeleton. There are numerous p4-carbide- and nitride-containing clusters; examples from ] [M4N(CO)12]-( M = Fe, the iron-triad include [M4C(C0)13]( M = Fe, R U ) ' ' ~and Ru, and Os)." 1' By comparison, however, cluster chemistry containing 0x0 atoms is far less well developed; the mixed metal butterfly cluster, [Fe3Mn(p4-O)(C0)12]-,['61 is a rare example (Fig. 5a). As far as we are aware there are no examples in which an 0x0-atom is fully encapsulated within a cluster polyhedron. Although the rarity of such clusters seems surprising given the vast chemistry associated with the car-
Tetrahedron (6Ue)
Scheme 1
Ruttertly (62e)
Octahedron (86e)
884
2 Metal Clusters in Catalysis
Figure 5. The core skeletal geometries of (a) [Fe3MnO(CO)$ and (b) [HRUBH~(CO)U].
bid0 and nitrido clusters, the apparently facile loss of the 0x0 atom as C02 might, in part, offer an explanation (such a ready decomposition route is not accessible for nitrido and carbido clusters). The M4B clusters, [ H M ~ B H ~ ( C O()M I ~=] Fe, Ru, Os), are slightly unusual in that they do not contain a ‘naked’ boron atom, but retain two B-H bridging interactions ( Fig. 5b).[l7lInspection of the electronic structure and geometrical parameters (dihedral angles and the height of the boron atom above the Mwingtip-Mwingtip axis) of their tetrametallic frameworks, reveals that the boron atom is buried within the cluster core and as such behaves as an interstitial atom. Thus, despite being involved in B-H bonding interactions, all three of the valence electrons are used in cluster bonding. Butterfly clusters of this type can also be regarded as models for the chemisorption of heteroatoms on the step-site of a metal surface, and it is apparent that when such atoms are coordinated within the cavity of a butterfly framework they often have unusual chemical reactivities (see later).
2.11.2.3 Five-coordinate interstitial sites As for tetranuclear clusters, pentanuclear transition metal clusters are only capable of partially encapsulating a main-group heteroatom. The closo-pentanuclearcluster, the trigonal bipyramid, has an interstitial cavity of similar dimension to that of the tetrahedron, and must therefore ‘open-up’ if it is to accommodate a semi-interstitial atom. The most commonly observed M5E clusters ( E = main-group atom) adopt square based pyramidal structures, which can be derived from the trigonal bipyramid by M-M bond cleavage or can, alternatively, be viewed as nido-octahedral species (Scheme 2).
2.11 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters
Trigonal Bipyramid (72e)
Square Based Pyramid (74e)
885
Octahedron (86e)
Bridged Butterfly (76e)
Scheme 2
Carbon and nitrogen atoms have been found to occupy such 'semi-interstitial' positions in the clusters [M5C(CO)15]( M = Fe, Ru, or and [M5N(C0)14], ~ * ]heteroatoms in each cluster lie in a slightly exposed ( M = Fe or R u ) . [ ~ ~ The position just below the center of the basal plane of a square pyramidal cluster framework. They are, however, within bonding contact of all five metal atoms, and are hence partially enclosed by the metal cage. A gold( I ) derivative of the pentaruthenium boride cluster [ R U ~ B ( C O ) I ~ ]has -['~] also been isolated; it has a similar structure to the analogous carbide and nitride clusters, but the additional gold( I) fragment caps a basal triangle defined by two ruthenium atoms and the boron atom (Fig. 6a). Finally, the hydrogen ligands in the are also thought to be series of clusters [H5-nRh13(C0)24]n- ( n = 1, 2, 3 , or 4)i201 located in the basal plane of a square pyramid of metal atoms and are therefore considered to be five-coordinate. These hydrogen atoms have not been located directly but their positions have been inferred from solid-state bond-length considerations. 'H and 13C NMR studies have revealed that, in solution, the hydrides migrate freely within their cluster frameworks. An alternative, pentanuclear geometry which has been shown to accommodate heteroatoms as semi-interstitial ligands is the 'bridged butterfly' structure. This skeletal framework might be derived from the square-based pyramid by M-M bond cleavage, which in practice can be performed by nucleophilic addition. Clusters with this geometry have been found incorporating boron, as in [Fe5B(CO),5I2- and [HoS$(C0)16~211(Fig. 6b), or carbon, e.g. [Fe5C(C0)12Br2I2- and [M5C(CO)16] ( M = Ru, 0 s ) . 221
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2 Metal Clusters in Catalysis
Figure 6. The core skeletal geometries of (a) [Ru5B(CO)ls(AuPPh?)]and (b) [HOssB(Co)16].
2.1 1.2.4 Six-coordinate interstitial sites Hexanuclear closo-polyhedral clusters include the compact octahedral and the less compact trigonal prismatic cages. These two cages are related in having two M3-triangles either mutually staggered or eclipsed, and both are capable of fully encapsulating a range of main-group heteroatoms, in particular B, C, and N.
The octahedral cavity Because the radius of an octahedral cavity is 0.414 times the metallic radius of the metal atoms on the surface of the cluster, only the first-row main-group atoms can be incorporated in this site. There are several octahedral carbido- and nitridoclusters, but examples are also known that incorporate hydrogen and boron. A series of related clusters include [Ru~H(CO)IS]-, [Ru6B(CO)17]-, [Ru&(C0)17], and [RugN(CO)16]-, all of which contain a fully encapsulated interstitial heteroatom lodged in the center of the octahedral cavity. The first example of a six-coordinate interstitial hydride was [Ru6H(CO)l ~ ] - ; [ ~ ~ ] the unexpected location was originally deduced indirectly from the close-packed arrangement of the carbonyl ligands which left no space on the cluster surface for an H atom. This structural assignment has since been vindicated by a neutron diffraction study. Similarly, neutron diffraction studies of the hexacobalt cluster
2. I I The Role of Interstitial Atoms in Transition Metal Curbonyl Clusters
887
Scheme 3
[C06H(C0)15]-[241 have revealed that the lone H atom is located at the center of the C06 octahedron. NMR studies have shown that these H atoms are extremely mobile and can quite easily enter and leave their metal cages. For example, protonation of the octahedral [ R U ~ ( C O ) I dianion ~ ] ~ - yields [Ru6H(CO)1gIpin which the hydride immediately occupies a position at the center of the octahedron. However, further protonation to the neutral cluster [ H ~ R u ~ ( C O )results I ~ ] , in the interstitial hydride atom leaving the central cavity and taking up a bridging site on the surface of the octahedron (Scheme 3). In a similar fashion, the hydrido cobalt cluster is prepared by protoat low temperature, but if dissolved in water or methanol the nating [c06(co)15]2monoanion is unstable with regard to loss of the proton, illustrating, again, how migration of the proton through the metal skeleton seems to be quite facile. Single-crystal neutron diffraction has also revealed that the H atoms in the two large nickel clusters [Nil2H(C0)2ll3- and [ N ~ I ~ H ~ ( C Oare ) ~ Ilocated ] ~ - in octahedral cavities.[251In contrast with the near perfect centering of the H atom in [CogH(C0)15]- and [Ru6H(CO)IS]-, however, those in the Nil2 clusters are displaced from the centers of the octahedral holes toward one of the non-equivalent faces. Hence, the interstitial H atoms might be considered as pseudo-triply bridging, capping a triangular face of an octahedron from within.
The trigonal prismatic cavity The trigonal prism has a slightly larger internal cavity than the octahedron (0.528 times the metallic radius) and although the majority of examples incorporate first~ h] -g,C ( C 0 ) 1 5 ] ~and ~, row main-group interstitial atoms e.g. [ H ~ R u ~ B ( C O ) ~[ R [Co6N(CO)I5]-, the anion [os6P(co)1g]- provides a rare example of how this cluster geometry can also encapsulate heavier main-group atoms. Inspection of the octahedral and trigonal prismatic clusters [Co6N(CO)131- and [Co6N(CO)~ 5 ] - , [ ~ ‘ - ]respectively, provides evidence that an interstitial atom can, to some extent, ‘expand’ or ‘contract’ to meet the size requirements of the metal cavity in which it is encapsulated. For example, the trigonal prismatic cluster anion [CogN(CO)15]- loses two carbonyl ligands on heating to form the octahedral species [CogN(C0)13]- with, as a consequence, the apparent radius of the interstitial
888
2 Metal Clusters in Catalysis
nitrogen decreasing from 0.67 A in the larger trigonal prismatic cavity to 0.54 A in the octahedral cavity. If these values are compared with the covalent radius of nitrogen (0.75 A), which is similar to the nitride radius found for the larger, secondrow transition metal cluster [Rh6N(C0)15I2- (0.74A) then the extent of this contraction can be appreciated. The metallic cluster core is also quite flexible and ‘swelling’ of the polyhedron is often observed, in the form of lengthened M-M bonds, upon encapsulation of the interstitial atom. The hexacobalt and hexaosmium cluster anions, [co6P(co)16]- and [oS6P(co)18]- are interesting examples which illustrate the effects of introducing a larger, second-row, main-group element into a six-coordinate interstitial cavity. They also enable a comparison of the differences between the cavity sizes for first and third-row transition metals to be made. The six metal atoms of the hexacobalt anion [co6P(co)~6]-‘271 form an open network of four, linked, edge-sharing triangles which wrap around a phosphidoligand. The P atom bonds to all six metal atoms and is encapsulated within the cluster framework in such a way that it can be considered ‘semi-interstitial’ (Fig. 7a). Theoretical values suggest (using rp/rc, = 0.88, with the metallic radius of Co = 1.25 A) that a phosphide-atom cannot occupy the cavity of a C06 octahedron (0.414) or trigonal prism (0.528), without dramatically distorting the cluster framework, and although cluster polyhedra are known to be slightly elastic (enabling them to accommodate interstitial atoms), there are certain limits which cannot be exceeded. Therefore, the C06 open array can be rationalized by considering it as an
Figure 7. The M6P skeletal frameworks of (a) [co6P(co)16]and (b) [osgP(co)18]-.
2.11 The Role of Interstitial Atoms in Transition Metal Curbonyl Clusters
889
octahedron with three broken edges, or a trigonal prism with one broken edge and distortion of the remaining square face. The trigonal prismatic framework occurs in [oS6P(co)181- and fully encapsulates an interstitial phosphido atom (Fig. 7b).[81There is some variation in the 0s-0s bond distances; the three interbasal distances are significantly longer [mean 3.140(3) A; a distance usually considered to be too long for a formal metal-metal bond] than those within the basal triangles of the prism [mean 2.932(5)A]. It is tempting to attribute this bond lengthening to the size requirements of the interstitial phosphorus atom, although this argument should be approached with caution since this effect is also observed in other trigonal prismatic clusters, even those such as [Pts(C0)12l2- which do not have an interstitial atom.[28] It is therefore apparent that most first-row main-group atoms are of an appropriate covalent radius to be fully encapsulated within either an octahedral or trigonal prismatic cavity. The same is true on moving to higher nuclearity clusters e.g. heptanuclear clusters usually adopt monocapped-octahedra1 ([RUTH( C0)20]and [Re7C(C0)21]3-)129]or monocapped-trigonal prismatic ( [HFe7B(C0)20]2-,1211 [Rh6MN(C0)15I2- ( M = Co, Rh, Ir)[301)arrangements of metal atoms, rather than frameworks based on the pentagonal-bipyramid. Similarly, clusters such as [Re&(C0)24j2- adopt bicapped octahedral and [HOs1&(C0)24]and [RuloN(C0)241- adopt tetracapped octahedral metal Although it is also apparent that the interstitial carbido- and nitrido-atoms behave similarly in these clusters, as the nuclearity of the cluster increases further, distinct differences start to emerge. For example, interstitial carbides generally lie in regular cavities with coordination numbers of six or eight which retain the compactness of the metal atom fragment (as in the [ H N i & j C0)42l5( anion[331) whereas the coordination numbers tend to be lower (five in particular) for interstitial nitrides, which show a preference for unusual and less regular cavities which reduce the overall compactness of the cluster. Hence, the chemistry of large nitrido clusters reveals a variety of unusual geometries made possible by the stabilizing effects of the interstitial atoms. These observations are best illustrated by the series of rhodium cluster anions [PtRhloN(C0)21I3-, [HRhn(N)2(CO)23j3-, [Rh14(N)2(C0)25l2- and [Rh23N4(C0)38]3-,[341 in which the nitrogen atoms are often located close to surface square faces in trigonal prismatic, square-pyramidal, or larger irregular cavities. However, despite the possibility of a higher metallic coordination number, they are invariably found to be five-coordinate species.
2.11.2.5 Eight-coordinate interstitial sites As we proceed to the heavier second-row main-group atoms, we find that larger cavities are required for the atom to be fully encapsulated within the cluster polyhedron. There is an obvious preference for the square antiprismatic arrangement of
890
2 Metal Clusters in Catalysis
metal atoms over the cubic (or triangular dodecahedral) arrangement. The reasons for this preference are not obvious, especially when the larger cavity size associated with the cube (0.732~compared with 0.645, where r = metallic radius) could reduce the cluster expansion which is usually observed upon accommodation of the interstitial atom. However, it seems that the presence of four extra metal-metal bonds in the resulting structure are likely to provide the required driving force for its formation.
The square antiprismatic cavity The cluster dianions [CosC(C0)1~]~(Fig. Sa) and [Ni&(CO)16l2- provide rare examples of a first-row main-group atom occupying a square antiprismatic cluster cavity,i351 and also illustrate the capacity of carbon to become eight-coordinate. However, on progressing to the heavier main-group elements such as Si, P, S and As we find a propensity for clusters based on the square-antiprism. The paramagnetic anion [Co9Si(CO)21]- is the first example of a fully encapsulated silicon atom.[36]The nine cobalt atoms define a capped square-antiprism with the silicon atom occupying the central cavity of the Cog cage. A silicon atom ( r = 1.12A) is considerably larger than the cavity formed by a regular squareantiprism of cobalt atoms (ca. 0.85w), and therefore the cluster undergoes significant cage expansion to accommodate the Si atom interstitially. This is apparent from markedly lengthened Co-Co interactions when compared, for example, with [cO&(co)18]2-. Because of this it is not surprising that similar clusters containing interstitial germanium atoms ( Y = 1.22A) are unknown. The clusters [RU8P(CO)19(p-q1: q6-CH2C6H5)],[371 [RhgP(CO)21l2- (Fig. Sb), and [RhloP(C0)22]3-,[381 provide examples of P-atoms encapsulated in un-, monoand bi-capped square antiprismatic geometries, respectively. The replacement of phosphorus by arsenic in the cluster [Rh1oAs(C0)22l3- (Fig. SC),[~]shows the capacity of the basic square antiprismatic cluster polyhedron to accommodate interstitial atoms of this size. Elongation of the interplanar Rh-Rh contacts results, however, suggesting that the increase in the steric demands of the central atom is accommodated, at least in part, by expansion of the metal polyhedron. Finally, sulfur can also be associated with the square antiprism. For example, the structure of [Rhl0S(C0)22] consists of a bicapped square-antiprismatic metal skeleton,[391incorporating a sulfur atom in its interstitial cavity, and in the high nuclearity trianionic cluster, [Rh17Sz(C0)32]3-,[401 each sulfur atom is bound to nine rhodium atoms. Sixteen rhodium atoms define the skeleton of the cluster and these are distributed at the corners of four stacked parallel squares, which are staggered by 45". An S-Rh-S group is located in the cluster cavity such that the central Rh atom is coordinated to the eight Rh atoms of the two internal squares (in addition to being bonded to the two sulfur atoms), and the two sulfur atoms lie in the cavities between the internal and external square planes resulting in a sulfur coordination number of nine (Fig. 9). The arrangement of metal atoms found in
2.11 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters
891
Figure 8. The M,E framework geometries of (a) [Co~C(C0)18]~-, (b) [Rh9P(C0)21I2- and (c) [RhioAs(C0)22I3-.
892
2 Metal Clusters in Catalysis
Figure 9. The core skeletal geometry of [Rh17&(C0)32]~-.
this cluster can also be regarded as a fragment of a tetragonally distorted bodycentered cubic metallic lattice.
2.11.2.6 Ten-coordinate interstitial sites A pentagonal antiprismatic array of nickel atoms is centered by an interstitial germanium atom in the dianionic cluster [NiloGe(C0)20]~(Fig. This is a very unusual and, as far as we are aware, unique example of a closo-decanuclear transition metal cluster. Its metallic skeleton is thought to arise because of the steric requirements of the bulky germanium atom, which as stressed before, is too large to fit inside a square antiprismatic cavity.
2.11.2.7 Twelve-coordinate interstitial sites Finally, if even larger main-group elements, e.g. Sn and Sb, are to be fully encapsulated within a cluster polyhedron, it follows that even larger cavities are required. Steric effects suggest that because the fourth-row main-group elements have covalent radii of a similar size to those of most platinum-group transition metals,
2.11 The Role of Interstitial Atoms in Transition Metal Curbonyl Clusters
893
clusters based on the icosahedron, the anti-cubeoctahedron, and the cubeoctahedron should be possible. These latter two polyhedra forms are well-documented for transition metal clusters, being present, for example, in [ H ~ R ~ I ~ ( C Oand )~~]~[Rh14(C0)25]4-,11’21 and being characteristic fragments of hexagonal close-packed and body-centered cubic metallic lattices, respectively. The icosahedral geometry, however, is more unusual in carbonyl clusters, even though non-centered icosahedral clusters are common for boranes and carboranes and centered icosahedra are common in gold and silver-phosphine chemistry. The first example of a transition metal carbonyl cluster with icosahedral geometry was [Rh12Sb(C0)27]3-,[42] made possible by the stabilizing effect of the central antimony atom. The structure of the anion consists in twelve rhodium atoms situated at the corners of a distorted icosahedron with a naked antimony atom in the cavity of the polyhedron (Fig. 11). Germanium and tin atoms can also expand their coordination to twelve by occupying interstitial sites in the icosahedral clusters [Ni12Ge(C0)22]~-and [Ni12Sn(C0)22]2-.1411 Both structures are very similar, consisting of a slightly distorted icosahedron of nickel atoms with a Ge or Sn atom located in the interstitial cavity. The distortion is more pronounced in the tin derivative, probably because of the accommodation of the bulkier Sn atom.
2.11.3 Mechanistic insights into interstitial atom formation One general synthetic route to transition metal carbonyl clusters incorporating interstitial main-group atoms involves cluster build-up by the simple thermolysis of
Figure 11. The metallic framework of [Rhi2sb(CO)nl3-.
894
2 Metal Clusters in Catalysis
low-nuclearity carbonyl clusters in the presence of a heteroatom source. Metalmetal bonds replace the expelled carbonyl ligands, and higher nuclearity clusters with various polyhedral structures are formed. In certain situations it seems that the heteroatoms (with their adaptable coordination numbers) serve as focal points for the aggregation of metal atoms and hence, assist and control the formation of metal-metal bonds as the cluster is forming. The next section considers several examples which have helped elucidate the mechanisms by which interstitial-atom formation occurs.
2.11.3.1 Interstitial hydrogen atoms Hydrido-clusters are usually prepared by protonation of anions. For example, the reaction of [Ru~(CO)IZ] with aqueous KOH in THF solution results in cluster expansion to the octahedral dianion [Ru6(CO)1812-, protonation of which gives the monoanion [ R u ~ H ( C O ) I ~(in ] - which the single H atom resides inside the Rug cavity), or the neutral dihydrido-cluster [H2Rug(C0)18](in which the hydride ligands cap opposite faces of the octahedral metal cage), depending on the reaction conditions.
2.11.3.2 Interstitial boron atoms Most metallaboride clusters are synthesized from metal-rich metallaboranes ( Lewis base adducts of BH3 are often used as the source of the B atom); several different methods are used to transform B-H bonds into B-M interactions and hence isolate a ‘naked’ boron atom. Two recent review articles have been devoted to this s~bject.[~~,~~I The transformations place the ‘denuded’ boron atom in a metallic environment which is quite unlike that found in a borane cluster, but bears similarities to that found in solid-state metal borides. This can be illustrated by the tetraruthenaborane cluster [HRuqBH2(C0)12],which may be prepared by several routes including the reaction of [BH3.THF] with either [ H ~ R U ~ ( C Oor) I[ ~ R ]u ~ ( C O ) IThe ~ ] . boron atom in [HRwBH2(C0)12]resides in a semi-interstitial butterfly site but is still associated with two bridging hydrogen atoms, and one of the simplest ways of stripping the boron of these hydrogen atoms is by deprotonation (Scheme 4). The loss of a boron-bound proton results in a concomitant increase in the amount of direct metal-boron bonding, and although relatively straightforward, these deprotonation steps are important in activating the semi-interstitial boron atom towards further reaction. Alternatively, replacement of these protons with [AuPR3]+ cations can lead to compounds in which the boron atom is essentially boridic in nature, i.e. it interacts with metal atoms only. For example, the reaction of [HRu~BH(CO)IZ]with [PPh3AuC1]leads to [ H R U ~ B H ( C O ) ~ ~ A and U P P[HRwB(CO)12(AuPPh3)2]; ~~] in the latter compound the boron atom is within bonding distance of all six metal
2.11 The Role o j Interstitial Atoms in Transition Metal Carbonyl Clusters
+ -
PI P
5
i,
+
+ f-
895
896
2 Metal Clusters in Catalysis
A+
A Cluster
+
Expansion
Scheme 5
atoms but is non-octahedral; the two gold atoms are skewed across the open face of the R w B butterfly core. A variety of methods are used for the synthesis of fully interstitial borides; most involve a ‘building-block’ approach starting with an [M3B], [M3B2], or [M4B] core. For example, the [M4B] butterfly framework in [HRu4BH(C0)12]- can also be regarded as an arachno-octahedron, and as such it should be possible to close the cage up by adding two metal vertices, and at the same time completely denude the boron atom of its B-H interactions. This has been demonstrated by reaction with [ {Rh(C0)2C1}2]to yield the closo-octahedral anion [RuqRh2B(C0)16]-. This anion undergoes a cis -+ trans isomerization during the reaction, illustrating the stabilizing effect of the interstitial boron atom on the metal array that surrounds it. Another example is the reaction of [Ru3(CO)10(MeCN)2] with [ R U ~ B ~ H ~ ( C O ) ~ ] to yield the octahedral and trigonal prismatic boride anions [Ru6B(CO)171- and [H~Ru~B(CO)IX]-, respectively. This is a systematic approach which involves ‘sandwiching’ together [ R u ~and ] [ R u ~ Bunits ] (Scheme 5).
2.11.3.3 Interstitial carbon atoms The formation of carbido-atoms in transition metal clusters is quite often achieved by pyrolysis or thermolysis of low-nuclearity carbonyl complexes. For example, the vacuum pyrolysis of [Ru3(C0)12] produces octahedral [Ru&( CO)17] in 65% yield, and the thermolysis of a solution containing [H2Re(CO)4]- yields salts of [ H ~ R ~ ~ C ( C O )[Re7C(C0)21I3-, I~]~-, and [ResC(C0)24l2-, all of which are based on octahedral or capped-octahedral frameworks with a carbide atom occupying their central cavities. Since the initial discovery of [FesC(CO)1 5 1 , there ~ ~ ~ has been a widespread interest in carbido-clusters and much effort has been devoted to elucidating the origin of the carbido atom. It is now generally considered that the carbide atom is derived from one of two different sources. i) Work on iron-triad carbido-clusters with isotopically enriched 13C0has enabled workers to confirm that the source of the isolated C atom is via the thermally
2.11 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters
897
induced cleavage of a coordinated carbonyl ligand. Carbon dioxide is often detected in the gaseous products of the reaction, suggesting that the cluster unit brings about carbonyl disproportionation (2 CO + 'C' COz), with the elimination of C02 and the formation of a carbido atom.[44]Alternatively, hydrido clusters have been shown to lose the elements of water, e.g. in the reaction of [ H ~ R u ~ ( C Oto) Iyield ~ ] [RugC(C0)17]and H20.
+
ii) In most cobalt-triad carbido-clusters, halomethanes such as CHC13 or CC14, used as solvents in the synthesis, have been found to act as an external source of carbon. This has been confirmed in the synthesis of [Rh6'3C(C0)15]2- from "CCl4 and the [Rh(C0)4]- anion.[45] The relevance of the former discoveries to the dissociative adsorption of CO on metal surfaces and the subsequent reactivity of the resulting surface carbido species were immediately recognized as pivotal processes in hydrocarbon formation reactions such as the Fischer-Tropsch synthesis.[461This led to further studies to try to elucidate the precise mechanism by which C-0 bond cleavage actually occurs, and it is now generally accepted that both metallic surfaces and molecular clusters activate CO in a similar fashion - by interaction of both the C and 0 with several metal atoms simultaneously. This di-hupto coordination of CO leads to considerable elongation and weakening of the C-0 bond (as evidenced by IR spectroscopy), hence facilitating bond cleavage. Clusters containing di-hupto carbonyl ligands are still quite rare, however, those that have been isolated often prove to be important intermediates in the formation of carbido-clusters. One example, [R~6(p~-r~-C0)2(CO),3(1;1~-C6H3Me3)], formed from the thermolysis of [ R u ~ ( C O )in~ ~ heptane-mesitylene, ] can be used to illustrate this point. Thermolysis in mesitylene results in conversion to the octahedral carbido-cluster [ R U ~ C ( C O ) I ~ ( ~ ~ - Cand ~ Hto ~M [HRu6(p4-q2-CO)(CO)13(p2~~)] r' : r6-C6H3Me2CH2)]in equal amounts, together with C02 (Scheme 6).[471The mechanism of carbide formation seems to be intermolecular in this instance, and involves the cleavage of one of the q2-C0 ligands of [Ru6(p4-q2-C0)2(CO)13(r6CsH3Mes)l via nucleophilic attack of its oxygen on a terminal carbonyl of a second
A
Mesitylene
*
+
+
898
2 Metal Clusters in Catalysis
cluster molecule. Elimination of C02 generates a carbide coordinated to the first cluster, which then seems to undergo rearrangement to encapsulate the newly formed carbido-atom and so produce [RU6C(C0)14(q6-C6H3Me3)]. Another interesting reaction is the thermolysis of [Ru3(C0)12] with [2.2]paracyclophane; this has been shown to yield several cluster species, two of which have proved valuable intermediates in elucidating the mechanism of carbide formation in the octahedral cluster [ R u ~ C ( C O ) I ~ ( ~ ~ -(Scheme C ~ ~ H 7). I~)] The open hexaruthenium cluster, [R~6C(C0)15(p3-C16H16-p2-o)],[~~~ is a key intermediate compound in the route to the carbido-cluster; its pyrolysis yields [Ru6C(CO)14(p3-C16H16)]and C02 quantitatively according to the equation:
This complex is a rare example of a molecular system containing both carbido and 0x0 ligands of a carbonyl group (which has been cleaved during the cluster build-up process) trapped within a metal cluster framework (Fig. 12). It is easy to envisage an intramolecular mechanism for its conversion to [Ru&(C0)14(p3C16H16)] whereby the lone pair on the oxygen attacks a terminal carbonyl, generating CO2 which is rapidly expelled from the cluster core. This is followed by rearrangement of the metal framework to produce the octahedrally encapsulated carbido atom. It has also seen shown that the non-carbido bi-capped octahedral cluster [H4R~s(C0)18(r~-C16H16)][~~~ is converted quantitatively into the octahedral carbido-cluster [ R U ~ C ( C O ) I ~ ( ~ U and ~ - C[Ru3(C0)12] ~ ~ H ~ ~ ) ]upon reaction with CO. Clearly during this degradative carbonylation reaction the metal core of the precursor compound not only undergoes a reduction in nuclearity, but must also endure a substantial polyhedral rearrangement to accommodate the carbido-atom. Although the mechanism by which this process occurs is not fully understood, the cluster [H2Rus(p6-q2-CO)(CO) 19(116-C16H16)],[501 also produced (albeit in low yield) from the thermolysis of [Ru3(C0)12]with [2.2]paracyclophane, might be an intermediate formed during this process (even though there is no direct evidence of this) (Scheme 7). This cluster has an unusually open metallic framework and provides a unique di-hapto coordination mode for carbon monoxide in which it interacts with six metal atoms and acts as a six-electron donor (Fig. 13). The C-0 bond length of 1.378(11)A is possibly the longest recorded for a carbon monoxide ligand by crystallographic techniques, and as such should be considerably weakened and hence, very susceptible to cleavage. Finally, carbido-atoms have also been shown to arise from the scission of acetylide ligands. For example, the reaction of [WRu2(C0)s(qs-Cp)C-CPh] with [Ru3(C0)12]in heptane under reflux results in the square pyramidal and octahedral mixed-metal clusters, [WRuC(CO)12( q5-Cp)(p-CPh)] and [WRu5C(CO) 14(q 5 Cp)(p-CPh)], respectively (Scheme 8).[511The structures of both products suggesl
899
2.11 The Role of Interstitial Atoms in Transition Metal Curbonyl Clusters
8+
I-
-
900
2 Metal Clusters in Catalysis
Figure 12. The core skeletal geometry of [Ru6C(CO)15(~3-Ci6H16-~2-o)].
2.11 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters
901
Scheme 8
that the C-CPh ligand in the precursor has been cleaved to give an edge-bridging CPh unit and an interstitial curbido-atom.
2.11.3.4 Interstitial nitrogen atoms In the synthesis of nitrido-clusters use is often made of the nitrosonium ion (NO+) or of a preformed metal nitrosyl complex. Alternatively, the azide ion (N3-) has also proved to be an efficient source of interstitial nitrogen atoms. For example, the tetrahedral cluster [H3R~q(C0)12]-reacts with NOBF41 to give the nitrido butterfly { ” ~reaction is thought to proclusters [HRwN(C0)12]and [ H ~ R U ~ N ( C O ) I I ]This ceed via the insertion of NO+ into a Ru-Ru bond and then reduction of the coordinated NO to nitrogen. The ruthenium nitrosyl intermediate seems to be unstable at room temperature and forms a nitride cluster by loss of either H20 or C02; this mechanism has been substantiated by the analogous reaction of [H30s4( CO)121which yields the nitrosyl cluster [H30s4(pu-NO)(CO)12] (as the major product) and ~].[~~] the anion [HOs4(C0)13]- rethe nitrido cluster [ H O S ~ N ( C O ) ~ Alternatively, acts with NO+ to give [HOsq(C0)13(NO)] which loses C02 on heating to form the nitride cluster [HOs4N(CO)12](Scheme 9). During these reactions a coordinated nitrosyl ligand is deoxygenated to a nitride atom with the elimination of C02 or H20 and, as in carbide formation, it is thought that this reaction may proceed via a di-hupto nitrosyl intermediate which activates the N-0 bond towards cleavage. The formation of [Ru6N(CO)16]- uses the azide ion (N3-) instead of NO+ as a source of the nitrogen atom,[541and the reaction of [Ru3(C0)12]and [PPN][N3] has been found to generate the isocyanato clusters [ R u ~NCO)(CO)l1]( and [Ru3(pu-NCO)(CO) 101- rapidly at room temperature. The clusters contain a terminal and a bridging isocyanate ligand, respectively. When these clusters are heated with one equivalent of [ R u ~ ( C O ) ~ the ~ ]hexanuclear , nitrido cluster [ R U ~ N ( C O ) I ~ ] is isolated in 82% yield (Scheme 10). This reaction involves the dissociation of a coordinated isocyanate (NCO) into a coordinated nitrido atom and carbon monoxide; such a decomposition of NCO might be promoted by polynuclear species via the successive weakening of the N-C bond as the N goes from terminal
902
2 Metal Clusters in Catalysis
1
+
g
+
g
+
l2
2.11 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters
I
i
v
P
v
1"
d
I
I
+
s]
903
904
2 Metal Clusters in Catalysis
to bridging, to face-capping. [Ru6N(CO)16]- has also been converted quantitatively to [Ru5N(CO)14]-and [Ru3(C0)12]within minutes under a carbon monoxide atmosphere. Almost all high nuclearity nitrido-carbonyl clusters are obtained from the build-up of smaller preformed nitrido-cl~sters.[~~] For example, reaction of [Rh6N(C0)15]with [PtRb(C0)14I2- results in the formation of [PtRh1oN(C0)21l3- whereas the thermolysis of K[Rh6N(C0)15] under different conditions yields a number of products including the anions [HRh12(N)2(C0)23l3-, [Rh14(N)2(C0)25l2- and [Rh23(N)4(C0)38l3-. Finally, gentle pyrolysis of [CogN(C0)15]- leads to rearrangement of the metal framework to form the octahedral species [C06N(C0)13]-, although pyrolysis under more drastic conditions (heating in diglyme at 140-150 "C) affords the high-nuclearity [Co14N3(C0)26l3-trianionic species.[551
2.11.3.5 Interstitial oxygen atoms Exposure of a solution of the parent dianion [Fe3(CO)11I2- to air results in the formation of the 0x0-dianionic cluster [ F ~ ~ ( , L Q - O ) ( C Oin ) ~almost ] ~ - quantitative yield, with the concomitant loss of C02. If the PPN salt of [Fe3(p3-O)(C0),l2- is then treated with one equivalent of [Mn(C0)3(CH3CN)3][PFs]an ionic-coupling, cluster build-up, reaction occurs uia loss of the labile acetonitrile ligands to give the butterfly 0x0 cluster P P N [ F ~ ~ M ~ ( , u ~ - O ) (Scheme ( C O ) ~ ~1]1).[16]
2.1 1.3.6 Interstitial phosphorus (arsenic and antimony) atoms Transition metal phosphido clusters usually result from pyrolysis reactions in the presence of an external source of the phosphorus atom. PCl3, white phosphorus, PH3, and, more commonly, PPh3 have all been used in this respect, and it has been shown that the thermal decomposition of triphenylphosphine by successive loss of phenyl radicals can result in the formation of 'naked' phosphorus atoms in interstitial cavities. This phosphorus metalation is thought to prevail in cluster chemistry because of the presence of several transition metal atoms neighboring the arylphosphine coordination site which aid the P-C bond cleavage process.
Scheme 11
2. I 1 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters
905
The reaction of [Rh(acac)(C0)2]and PPh3 in the presence of cesium benzoate at high temperature and pressure results in the formation of [Rh9P(C0)21I2- and [RhloP(CO)22]3-,[381 the phosphorus atom being derived from triphenylphosphine. This reaction is difficult to follow and seems to proceed in a non-specific, nonstoichiometric manner, probably because the harsh conditions employed in the reaction do not allow for the isolation of intermediate species. The trigonal prismatic cluster [os6P(co)18]- has, however, been prepared in a far more systematic fashion and some key intermediates have been isolated; these have helped elucidate the mechanism of interstitial phosphido atom formation.[56] Reaction of the activated cluster [Os3(CO)11(MeCN)] with PH3 affords the substituted product [Os3(CO),,( PH3)] in high yield. Treatment with sodium carbonate in methanol, followed by acidification with trifluoroacetic acid, causes metalation of a P-H bond to yield the hydrido phosphido triosmium cluster [HOs3(CO)lo(pPH2)] which, if reacted with a further equivalent of [Os3(CO)11(MeCN)], forms the This latter cluster contains two hexaosmium cluster [(p2-H)20s6(Co)21(~u3-PH)]. independent, perpendicular, Os3 cluster units linked by a p3-phosphinidine ligand which bridges an 0s-0s edge of one cluster and is terminally coordinated to the other. The formation of this cluster suggests the occurrence of oxidative addition of a second P-H bond to an unsaturated center formed by the loss of an acetonitrile ligand from [Os3(CO)ll(MeCN)]; this results in the linking p3-PH group and an additional hydride ligand. Thermolysis of [ ( ~ - H ) ~ O S ~ ( C O ) ~ ~leads ( ~ ~to - PfurH)] ther P-H bond activation, loss of CO and H2, and the shrinking of the metal framework (to accommodate the decrease in cluster valence electrons), affording the ) ( Cthe O )phosphorus atom effecinterstitial phosphido cluster [(~ - H ) O S ~ ( ~ ~ - P181; tively pulling together two Os3 triangles to form a trigonal prismatic cage. This cluster readily deprotonates in the presence of excess [PPh3Me]Br or [PPNICl to yield the [os6(~6-P)(co)18]anion. (Scheme 12). Just as the thermal decomposition of PPh3 at high temperatures and pressures led to interstitial phosphido atoms, the thermal decomposition of triphenylarsine and triphenylantimony can lead to interstitial ‘As’ and ‘Sb’ atoms, respectively (the lower strength of the As-C and Sb-C bonds relative to the P-C bonds should make the cleavage process easier). This is illustrated by the reaction of [Rh(acac)(C0)2] with AsPh3 and SbPh3 in the presence of alkali carboxylates at high temperature and pressure. Reaction with AsPh3 results in the isolation of the [RhloAs(C0)22I3trianion,[’] which reacts with further [Rh(C0)4]- to yield [Rh9As(C0)21I2-;reaction with SbPh3 results in the selective formation of [Rh&3b(C0)27]3-.[421
2.11.3.7 Interstitial sulfur atoms HzS, S02, S and CS2 have all been used as sources of sulfido atoms to introduce atomic sulfur into transition metal clusters. For example, the reaction of a solution of [Rh(CO)z(acac)]and alkali carboxylates with H2S or SO2 at high tem-
2 Metal Clusters in Catalysis
2.11 The Role of Interstitial Atoms in Transition Metul Curbonyl Clusters
907
peratures (140-160 "C) and pressures (300 atm CO/H2) results in the isolation of [ R ~ I ~ S ~ ( C O ) Cluster ~ ~ I ~build-up - . [ ~ ~ is~ thought to occur as a result of the multicoordination and donor capacity of the sulfide ligand to link the metal fragments before subsequent reorganization of the metal-metal bonding. Alternatively, a cluster which already contains a sulfur-based ligand can be used as the starting material, as illustrated in the preparation of [Rh1oS(C0)22]~-by the slow decomposition of [Rh6(C0)14(SCN)2I2-in THF.[39]
2.1 1.4 Why the interest? Interest in the synthesis of transition metal carbonyl clusters incorporating interstitial main-group atoms is because they provide a conceptual bridge between organometallic chemistry and the areas of inorganic solid-state and surface chemistry. In addition to serving as useful models for either solid-state binary alloys or for the chemisorption of heteroatoms on the step-site of a metal surface, these discrete molecular clusters are often markedly more stable, especially toward the temperatures and pressures required for catalytic reactions.
2.11.4.1 Binary phases Solid-state metal alloys are usually opaque, very hard, refractory materials with metallic luster, high thermal and electrical conductivity, and high thermal stability and general chemical inertness.1571 They are, therefore, of huge industrial importance. There is significant interest in the fact that the structural motifs present in these extended materials sometimes replicate those found in the discrete cluster compounds. For example, the structures of the refractory carbides NbC or WC can be related to the discrete octahedral [RugC(C0)17] or trigonal prismatic [Rh&(C0)15I2- clusters - in NbC each carbon is octahedrally surrounded by six metal atoms, in WC by a trigonal prism,[58]and the trigonal prismatic geometry of [H2Ru,jB(CO)lg]replicates a structural unit observed in R u ~ B ~ . ' ~ ~ ' Although there are close structural ties between molecular clusters and interstitial alloys, this structural analogy is usually restricted to the stereochemistry of the interstitial atom. The ever-increasing number of high nuclearity, multi-heteroatom molecular clusters which are being isolated serve as models for an extended fragment of a metallic alloy. For example, the [HNi&6(CO)42l5- penta-anion contains a truncated octahedral Ni&6 moiety, stabilized by a shell of carbonyl ligands, which is closely related to the structure of a fragment of the Cr2& 1 a t t i ~ e . l ~ ~ ~ Clusters like these suggest the possibility of a molecular approach to new M-C binary phases. Indeed, one of the most interesting aspects of multiatomic clusters is the gradual transition from the molecular to the sub-microcrystalline metallic state.
908
2 Metal Clusters in Catalysis
2.11.4.2 The surface-cluster analogy The chemistry of metal carbonyl clusters containing exposed or semi-exposed maingroup elements has received attention because of the belief that such molecular clusters might serve as models for the chemisorption and subsequent reactivity of the corresponding species on a metal surface.[601In particular, the M4 butterfly clusters containing p4-carbido and -nitrido ligands have played a pivotal role in the development of the area, because of their obvious similarities to stepped-sites on metal surfaces. Attempts to use metal carbonyl clusters as models for heterogeneous catalysts have met with varying degrees of success. A good example involves the proton-induced reduction of a carbonyl ligand in the conversion of [HFe4(CO)13Ip to [HFe4(C0)12(q2-CH)](Scheme 13).r6’]This has provided a homogeneous analogy with the activation of CO on a Ni surface, whereby the CO is cleaved, producing an active surface carbide which is subsequently reduced,[621and hence has, played an important role in elucidating many aspects of the Fischer-Tropsch reaction. Nitrido clusters have also been proposed as models of intermediate species in many important catalytic reactions such as the Haber process and the reduction of NO, pollutants. Several surface reactions involving NO or N2 are known to occur via an adsorbed nitrogen atom; in the Haber process the mechanism is believed to occur via formation of the metal nitride followed by successive addition of hydrogen atoms to the nitrogen, giving ammonia.[63]
4
Scheme 13
2.1 1 The Role of Interstitial Atoms in Transition Metal Curbonyl Clusters
909
2.11.4.3 Enhanced cluster stability The presence of an interstitial atom has often been shown to confer considerable stability on a metal cluster. The reasons for this stabilizing influence are thought to be both thermodynamic and kinetic in origin. Firstly, the interstitial atom can stabilize the cluster bonding network because its orbitals overlap strongly with the radial molecular orbitals of the peripheral atoms. Secondly, the interstitial atom can donate valence electrons from within the cluster core, thus reducing the number of ligands required on the surface of the cluster. This, in turn, prevents the destabilization which would presumably result from the steric demands imposed by additional surface ligands. In general, carbonyl clusters containing main-group interstitial atoms are more thermally robust than corresponding clusters with no interstitial atoms. For example, cluster degradationof [H3Rhl3(CO)24l2-to [Rh5(CO)15]-and [Rh(C0)4]- occurs readily at low temperatures and pressures, whereas, [RhgP(C0)21l2- is stable under 600-800 atm CO/H2 and at temperatures up to 230 0C,[381 and [Rh12Sb(C0)27l3-is stable under 500 atm CO/H2 at 150 0C.[421 Interstitial atoms also stabilize unusual ‘open’ cluster geometries, and often enable the cluster polyhedron to be flexible in structural transformations that result from the addition and removal of electrons. For example, when nucleophiles such as MeCN add to [Ru5C(C0)15] a Ru-Ru bond in the square-pyramidal metal framework is broken and the cluster opens to give a bridged ‘butterfly’ configuration. This metal-metal bond reforms upon subsequent loss of the ligand and the cluster reverts to the square-based pyramidal geometry (Scheme 14a).[64]Alternatively, the trigonal prismatic cobalt cluster [CosN(C0)15]- can be converted to the octahedral form, [CogN(C0)13]-, by heating in THF.[26]The collapse of the trigonal prism to an octahedral cluster is consistent with a change in the total electron count from 90 to 86 as two CO ligands are expelled; this reaction is reversible, and under a CO atmosphere the starting anion is readily regenerated (Scheme 14b). Although the interstitial atoms in these clusters play no direct r61e in the cluster rearrangement, they do donate electrons to the cluster without occupying coordination sites on the periphery; this might, as a result of steric constraints, prevent formation of metal-metal bonds. In fact, the interstitial atom might be considered to hold the cluster together while the metal skeleton reorganizes itself.
2.11.5 Concluding remarks It is clear that in the past few years a broad and diverse chemistry of interstitial cluster compounds has developed. Synthetic routes have been established and some clues to the functions of the encapsulated atoms have emerged. The stereochemical
910
2 Metal Clusters in Catalysis
+ MeCN
P
- MeCN
A, THF, - 2CO
\
+ 2co
Scheme 14
demands of the interstitial atom upon the metallic cluster clearly define the size and geometry of the cavity, and in this sense they differ from macromolecular materials in which the demands of the infinite structure govern the nature of the interstitial sites. Nevertheless, interstitial clusters provide considerable insight into the nature of the interstitial process and the mechanisms by which such atoms are produced. Their utility as precursors to new materials remains to be proven but offers considerable potential for the future.
References [l] V. G. Albano, A. Ceriotti, P. Chini, G. Ciani, S. Martinengo, W. M. Anker, J. Chem. SOC. Chem. Commun. 1975, 859. [2] S. Martinengo, G. Ciani, A. Sironi, P. Chini, J. Am. Chem. SOC.,1978, 100, 7096. [3] A. Amoroso, L. H. Gade, B. F. G. Johnson, J. Lewis, P. R. Raithby, W. Wong, Angew. Chem. Znt. Ed. Engl. 1991, 30, 107.
2.11 The Role of Interstitial Atoms in Transition Metal Carbonyl Clusters
91 1
[4] E. H. Braye, L. F. Dahl, W. Hubel, D. L. Wampler, J. Am. Chem. SOC.1962, 84, 4633. [5] A. Sirigu, M. Bianchi, E. Beneditti, J. Chem. SOC.Chem. Commun. 1969, 596. [6] D. H. Farrar, P. F. Jackson, B. F. G. Johnson, J. Lewis, J. N. Nicholls, M. McPartlin, J. Chem. SOC. Chem. Commun. 1981,415. [7] S. P. Foster, K. M. Mackay, B. K. Nicholson, Znorg. Chem. 1985,24, 909. [8] S. B. Colbran, C. M. Hay, B. F. G. Johnson, F. J. Lahoz, J. Lewis, P. R. Raithby, J. Chem. SOC. Chem. Commun. 1986, 1766. [9] J. L. Vidal, Inorg. Chem. 1981, 20, 243. [lo] N. D. Feasey, S. A. R. Knox, A. G. Orpen, J. Chem. SOC.Chem. Commun. 1982, 75. [Ill P. F. Jackson, B. F. G. Johnson, J. Lewis, M. McPartlin, W. J. H. Nelson, J. Chem. Soc. Chem. Commun. 1982, 49. [ 121 D. Braga, J. Lewis, B. F. G. Johnson, M. McPartlin, W. J. H. Nelson, M. D. Vargas, J. Chem. SOC.Chem. Commun. 1983, 241. [13] A. Bashall, L. H. Gade, J. Lewis, B. F. G. Johnson, G. J. McIntyre, M. McPartlin, Angew. Chem. Int. Ed. Engl. 1991, 30, 1164. [14] (a) J. S. Bradley, G. B. Ansell, E. W. Hill, J. Am. Chem. SOC. 1979, 101, 7417; b) A. G. Cowie, B. F. G. Johnson, J. Lewis, P. R. Raithby, J. Organomet. Chem. 1986,306, C63. [15] a) D. E. Fjare, W. L. Gladfelter, Znorg. Chem. 1981,20, 3533; b) D. E. Fjare, W. L. Gladfelter, Organometallics 1985, 4, 45. [I61 C. K. Schauer. D. F. Schriver, Angew. Chem. Znt. Ed. Engl. 1987,26,255. [17] For example K. S. Wong, W. R. Scheidt, T. P. Fehlner, J. Am. Chem. SOC. 1982,104, 1111. [I81 M. Tachikawa, J. Stein, E. L. Muetterties, R. G. Teller, M. A. Beno, E. Gerbert, J. M. Williams, J. Am. Chem. Soc. 1980, 102, 6648. [19] C. E. Housecroft, D. M. Matthews, A. L. Rheingold, Organometallics 1992, 11, 2959. [20] G. Ciani, A. Sironi, S. Martinengo, J. Chem. Soc. Dalton Trans. 1981, 519. [21] C. E. Housecroft, Coord. Chem. Rev. 1995, 143, 297. [22] B. F. G. Johnson, J. Lewis, W. J. H. Nelson, J. N. Nicholls, J. Puga, P. R. Raithby, M. Rosales, M. Schroder, M. D. Vargas, J. Chem. Soc. Dalton Trans. 1983, 2447. [23] P. F. Jackson, B. F. G. Johnson, J. Lewis, P. R. Raithby, M. McPartlin, W. J. H. Nelson, K. D. Rouse, J. Allibon, S. A. Mason, J. Chem. SOC.Chem. Comrnun. 1980,295. [24] a) D. W. Hart, R. G. Teller, C. Wei, R. Bau, G. Longoni, S. Campanella, P. Chini, T. F. Koetzle, J. Am. Chem. SOC.1981,103, 1458; b) D. W. Hart, R. G. Teller, C. Wei, R. Bau, G. Longoni, S. Campanella, P. Chini, T. F. Koetzle, Angew. Chem. Int. Ed. Engl. 1979, 18, 80. [25] R. W. Broach, L. F. Dahl, G. Longoni, P. Chini, A. J. Schultz, J. M. Williams, Adu. Chem. Ser. 1978, 167, 93. [26] a) S. Martinengo, G. Ciani, A. Sironi, B. T. Heaton, J. Mason, J. Am. Chem. Soc. 1979, 101, 7095; b) G. Ciani, S. Martinengo, J. Organomet. Chem. 1986, 306, C49. [27] P. Chini, G. Ciani, S. Martinengo, A. Sironi, L. Longhetti, B. T. Heaton, J. Chem. Soc. Chem. Commun. 1979, 188. [28] J. C. Calabres, L. F. Dahl, P. Chini, G. Longoni, S. Martinengo, J. Am. Chem. Soc. 1974, 96, 2614. [29] C. E. Housecroft, A. L. Rheingold, X. Song, Inorg. Chem. 1992, 31, 4023. [30] S. Martinengo, G. Ciani, A. Sironi, J. Chem. SOC. Chem. Commun. 1984, 1577. [31] C. Ciani, G. D’Alfonso, M. Freni, P. Romiti, A. Sironi, J. Chem. SOC.Chem. Commun. 1982, 339. [32] P. J. Bailey, G. Conole, B. F. G. Johnson, J. Lewis, M. McPartlin, A. Moule, H. R. Powell, D. A. Wilkinson, J. Chem. SOC.Dalton Trans. 1995, 741. [33] A. Ceriotti, A. Fait, G. Longoni, G.Piro, J. Am. Chem. SOC.1986, 108, 8091. [34] a) S. Martinengo, G. Ciani, A. Sironi, J. Am. Chem. SOC.1982, 104, 328; b) S. Martinengo, G. Ciani, A. Sironi, J. Chem. SOC.Chem. Comrnun. 1986, 1742; c) S. Martinengo, G. Ciani, A. Sironi, J. Chem. SOC. Chem. Commun. 1991, 26; d) S. Martinengo, G. Ciani, A. Sironi, J. Chem. SOC.Chem. Commun. 1992. 1405.
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2 Metal Clusters in Catalysis
[35] V. G. Albano, P. Chini, G. Ciani, S. Martinengo, D. Strumolo, M. Sansoni, J. Chem. SOC. Dalton Trans. 1978, 463. [36] K. M. Mackay, B. K. Nicholson, W. T. Robinson, A. W. Sims, J. Chem. SOC.Chem. Commun. 1984, 1276. [37] L. M. Bullock, J. S. Field, R. J. Haines, E. Minshall, D. N. Smit, G. M. Sheldrick, J. Organomet. Chem. 1986,310, C47. [38] a) J. L. Vidal, W. E. Walker, R. L. Pruett, R. C. Schoening, Znorg. Chem. 1979,18, 129; b) J. L. Vidal, W. E. Walker, R. C. Schoening, Inorg. Chern. 1981,20, 238. [39] G. Ciani, L. Garlaschelli. A. Sironi, J. Chem. SOC.Chem. Commun. 1981, 563. [40] J. L. Vidal, R. A. Fiato, L. A. Cosby, R. L. Pruett, Inorg. Chem. 1978, 17, 2574. (411 A. Ceriotti, F. Demartin, B. T. Heaton, P. Ingallina, G. Longoni, M. Manassero, M. Marchionna, N. Masciocchi, J. Chem. SOC.Chem. Commun. 1989, 786. [42] J. L. Vidal, J. Organomet. Chem. 1981, 213, 351. [43] C. E. Housecroft, Chem. SOC.Rev. 1995, 215. [44] (a) C. R. Eady, B. F. G. Johnson, J. Lewis, J. Organomet. Chem. 1972, 39, 329; (b) B. F. G. Johnson, J. Lewis, K. Wong, M. McPartlin, J. Organomet. Chem. 1980, 185, C17. [45] V. G. Albano, P. Chini, S. Martinengo, D. J. A. McCaffrey, D. Strumolo, B. T. Heaton, J. Am. Chem. SOC.1974, 96, 8106. [46] (a) W. A. Herrmann, Angew. Chem. Int. Ed. Engl. 1982,21, 117; (b) E.L. Muetterties, J. Stein, Chem. Rev. 1979, 79, 479. [47] (a) C. E. Anson, P. J. Bailey, G. Conole, B. F. G. Johnson, J. Lewis, M.. McPartlin, H. R. Powell, J. Chem. SOC.Chem. Commun. 1989, 442; (b) P. J. Bailey, M. J. Duer, B. F. G. Johnson, J. Lewis, G. Conole, M. McPartlin, H. R. Powell, C. E. Anson, J. Organomet. Chem. 1990,383,441. [48] P. J. Dyson, B. F. G. Johnson, C. M. Martin, D. Reed, D. Braga, F. Grepioni, J. Chem. SOC. Dalton Trans. 1995, 41 13. [49] D. Braga, F. Grepioni, P. J. Dyson, B. F. G. Johnson, C. M. Martin, J. Chem. SOC.Dalton Trans. 1995, 909. [50] C. M. Martin, P. J. Dyson, S. L. Ingham, B. F. G. Johnson, A. J. Blake, J. Chem. SOC.Dalton Trans. 1995, 2741. [51] S. J. Chiang, T. Chi, P-C. Su, S-M. Peng, G-H. Lee, J. Am. Chem. SOC.1994, 116, 11181. [52] M. A. Collins, B. F. G. Johnson, J. Lewis, J. M. Mace, J. Morris, M. McPartlin, W. J. H. Nelson, J. Puga, P. R. Raithby, J. Chem. SOC.Chem. Commun. 1983, 689. [53] D. Braga, B. F. G. Johnson, J. Lewis, J. M. Mace, M. McPartlin, J. Puga, W. J. H. Nelson, P. R. Raithby, K. H. Whitmire, J. Chem. SOC.Chem. Commun. 1982, 1081. [54] M. L. Blohm, D. E. Fjare, W. L. Gladfelter, Znorg. Chem. 1983,22, 1004. [55] S. Martinengo, G. Ciani, A. Sironi, J. Organomet. Chem. 1988, 358, C23. [56] S. B. Colbran, F. J. Lahoz, P. R. Raithby, J. Lewis, B. F. G. Johnson, C. J. Cardin, J. Chem. SOC.Dalton Trans. 1988, 173. [57] N. N. Greenwood, A. Earnshaw, Chemistry ofthe Elements, Pergamon Press, 1984. [58] R. Hoffmann, S. D. Wijeyesekera, S. Sung, Pure & Appl. Chem. 1986, 58, 481. 1591 For example N. N. Greenwood, R. V. Parish, P. Thornton, Quart. Rev. Chem. SOC.1966, 20, 441. [60] E. L. Muetterties, T. N. Rhodin, E. Band, C. Brucker, H. Pretzer, Chem. Rev. 1979, 79, 91. [61] E. M. Holt, K. H. Whitmire, D. F. Shriver, J. Organomet. Chem. 1981, 213, 125. [62] S. V. Ho, P. Harnot, J. Catal. 1980, 64, 272. [63] P. H. Emmett in E. Drauglis, R. I. Jaffe (Eds.): The Physical Basisfor Heterogeneous Catalysis, Plenum Press, New York 1975, p. 3. [64] J. Lewis, B. F. G. Johnson, J. N. Nicholls, I. A. Oxton, P. R. Raithby, M. J. Rosales, J. Chem. SOC.Chem. Commun. 1982, 289.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
2.12 Potential Applications of Nanostructured Metal Colloids Helmut Bonnemann und Werner Brijoux
2.12.1 Introduction In this contribution the term 'nanostructured metal colloids' is used to denote small aggregates of zero-valent metals (size ca 1-50 nm), stabilized by organic materials (ligands, surfactants, polymers) to give redispersible metal sols. Since the seminal studies of T ~ r k e v i c h " ~on ~ 1the mechanism of the nucleation, growth, and ripening of colloidal metals, a plethora of synthetic methods has been developed for the reproducible and size-selective preparation of nanostructured metal sols. Depending on whether lipophilic or hydrophilic protecting groups are applied for the stabilization, the resulting metal colloids are soluble in organic media (organosols) or water (hydrosols). Via the co-reduction of different metal salts, alloyed bi- and multi-metallic colloids with either homogeneous or gradient particle structure became accessible. For the synthetic details the reader is referred to comprehensive reviews.[4p6]Recently, Pd/Pt bimetallic colloids (size 1.5-5.5 nm) with a controllable core/shell structure,['] and an elegant preparation method for layered bimetallic colloids, e.g. gold-plated palladium particles (size 20-50 nm),r81 have been reported. In catalysis, nanostructured metal colloids are often considered as 'dissolved surfaces' with highly unsaturated atoms. The incentives for the application of these materials as precursors for the manufacture of metal-colloid catalysts['] emerge from the enhanced activity, good selectivity (controllable via the colloidal modifiers), synergistic effects in bimetallic particles, and remarkable long-term stability characteristic of a number of model catalysts. Additional potential for catalytic applications exists in colloidal metals dispersed in mesoporous environments," '1 micelles,l'2,13 ] and biornembrane~.''~] Another field of application is electrocatalysts for fuel cells. The fuel cell is an electrochemical reactor. From recent papers on fuel cell t e ~ h n o l o g y l l ~it ~ 1 be deduced that the application of .- ~can CO-tolerant high-performance electrocatalysts based on colloidal bimetallic par31 might significantly accelerate the process of commerticles sized below 2 nm[21p2
914
2 Metal Clusters in Catalysis
cialization, especially in the automotive sector. The long-term prospects of applications include the fabrication of a supported and insulated quantum wire. The feasibility of forming one-dimensional configurations of 15 nm gold colloids and 1.4 nm gold clusters via template methods of synthesis has recently been demon~ t r a t e d . [ Extraordinarily ~~] high values of coercive force and magnetization in nanostructured colloids of the magnetic metals furnish possibilities of producing magnetic fluids, magnetic polymeric films, and coatings using these material^.^^^'^^] Some further keywords from the long list of possible future applications of nanostructured metal colloids are nanometal pigments for magneto-optical data storage, magnetic lubricant fluids for mechanical low friction bearings, and nanomaterials for the magnetic cell separation in biological samples. The following paragraphs try to summarize briefly some potential applications expected to enter industrial practice within the next few years.
-
2.12.2 Catalysis 2.12.2.1 Chemical applications The manufacture of heterogeneous catalysts from pre-prepared nanometal colloids as precursors via the so-called ‘precursor concept’[g1has attracted industrial interest.I2’] An obvious advantage of the new mode of preparation compared with the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors can be tailored for special applications independently of the support. In addition, the metal particle surface can be modified by lipophilic or hydrophilic protective shells, and covered with intermediate layers, e.g. of oxide. The addition of dopants to the precursor is also possible. The second step of the manufacture of the catalyst consists in the simple adsorption of the pre-prepared particles by dipping the supports into organic or aqueous precursor solutions at ambient temperature. This has been demonstrated, e.g., for charcoal, various oxidic support materials, even low-surface materials such as quartz, sapphire, and highly oriented pyrolytic graphite. A subsequent calcination step is not required (see Fig. 1). In the course of a government-funded project[281Degussa has clearly demonstrated the industrial feasibility of the new catalyst preparation. As a result so-called egg-shell catalysts were obtained which contain the active metal particles on the support surface within a narrow range of <250 nm. Metal colloid catalysts of this type have been applied successfully for the hydrogenation of carbon-carbon double bonds, organic carbonyl groups, unsaturated C-N bonds, and the reduction of N-0 By use of Pd colloids (e.g. 1.4 and 3.8 nm) as the precursors Reetz et al.[291designed so-called ‘cortex-catalysts’, in which the active metal forms an extremely thin (< 10 nm) shell on the supports (e.g. A1203). Compared with
2.12 Potential Applications of Nunostructured Metal Colloids
9 15
Figure 1. The precursor concept.
a conventionally prepared catalyst of the same metal-loading (5% Pd on A1203) the new ‘cortex catalysts’ were reported to be three times more active in olefin hydrogenation. Antonietti and coworkers[’2.’31 have developed a generally applicable metal colloid synthesis in microemulsions, especially nanostructured noble metals in amphiphilic block copolymers. The resulting metal-containing micelles were shown to be very stable and effective hydrogenation catalysts without significant change of the colloidal properties such as size and polydispersity. Depending on the strength of the reducing agent, the morphology of the metal core can be switched between ‘cherry’ and ‘raspberry’ architecture’. Block copolymer stabilized Pd-‘raspberry’ colloids have an extraordinary large metal surface. Because of the very efficient stabilizer no additional support is needed for catalytic applications in solution. First tests in olefin hydrogenation have demonstrated that this type of colloid catalyst combines the advantages of homogeneous and heterogeneous catalysis, i.e. the high selectivity and reactivity of homogeneous catalysts with the processability and stability of the heterogeneous systems.[30] In his pioneering contributions Moiseev has shown that “giant cationic palla~ bipyridine), characdium clusters”, e.g. Pd561 L ~ ~ ( O A C( L) I=~phenanthroline, terized by use of high-resolution TEM, SAXS, EXAFS, IR and magnetic susceptibility data, catalyze, under mild conditions (293-363 K, 1 bar), the oxidative acetoxylation of ethylene into vinyl acetate, propylene into ally1 acetate, and toluene into benzyl acetate. The oxidation of primary aliphatic alcohols to esters, and the conversion of aldehydes into acetals were also studied.1311 According to Toshima et a1.[32,331 the stabilization of nanostructured particles in a solvent can be brought about also by a polymeric coat. Polymer-protected colloidal dispersions of noble-metal clusters can be prepared by reducing the corresponding noble-metal ions, under reflux, in alcoholic solution containing a coordinating polymer, e.g. poly(N-vinyl-2-pyrrolidone).The resulting nanostructured metal particles are quite stable and composed of fine particles (ca. 1-3 nm average
916
2 Metal Clusters in Catalysis Activity [Nml/(g min)]
300 250
0,2% Ti, oxgenated
200 150
100 50
0
Industrial catalyst 5% Rh on charcoal
Colloidal catalyst 5% Rh on charcoal
Figure 2. Activity of Rh/C catalysts in the butyronitrile hydrogenation test.
diameter) with narrow size d i s t r i b ~ t i o n . '51~ ~ These . ~ materials have been successfully evaluated as catalysts, e. g. for olefin hydrogenation,[36]nitrile and photo-induced electron etc. In a recent perspective article G. Schmid et al. summarized some general phenomena and the great catalytic potential of ligand-stabilized transition-metal clusters and colloids.[391The catalytic properties of large ligand-stabilized palladium clusters has been described.[401 By using surfactant-stabilized rhodium (5% on charcoal) in the butyronitrile hydrogenation test, we were able to demonstrate that the activity of supported metal colloids was superior to that of conventional salt-impregnation catalysts with the same metal loading[41](see Fig. 2). Further significant enhancement of the activity was achieved by doping the noble metal precursor with 0.2% colloidal Ti(0).[42] The doping of noble metal catalyst precursors is not restricted to the early transition metal series. T o ~ h i m a ' ~ has ~ ]reported significant promotion of catalytic activity in the hydrogenation of acrylic acid by the addition of neodymium ions to palladium particles. Recently, Liu and coworkers have studied the influence of metal ions on the hydrogenation of o-chloronitrobenzene over platinum colloidal clusters,[441and the metal complex effect on the catalytic performance of metal
2.12 Potential Applications of Nanostructured Metal Colloids
9 17
clusters in liquid medium.[451The catalytic properties of platinum clusters for the hydrogenation of o-chloronitrobenzene to o-chloroaniline were noticeably affected by the metallic cations added. The most favorable influence on activity and selectivity was obtained when Ni2+ ions were used as the modifier. The selectivity for ochloroaniline was increased from 44.7 to 66.3% and the activity of the catalyst was nearly doubled. The highest activity and selectivity (82.9%) to o-chloroaniline was obtained when the molar ratio of Ni2+/Pt was 8.[441The same authors also studied the metal complex effect on the activity and selectivity of platinum clusters in the homogeneous liquid-phase hydrogenation of inna am aldehyde.[^^] With Na3FeF6 as the additive the conversion was promoted from 65.5 to 78.0% and the selectivity to cinnamic alcohol was increased from 17.7 to 75.0%. On addition of Ni(bipy)3Clz to the colloidal platinum catalyst the conversion was enhanced to 82.1% to give hydrocinnamaldehyde with 97.3% selectivity. Selectivity control in the hydrogenation of cinnamic acid to cinnamic alcohol can also be achieved by doping Rh colloid catalysts with Sn. A Rh colloid/C catalyst doped with Sn (Rh/Sn ratio = 1.5 : 1) is 86% Alloy-like nanometal colloids have played an important role in the study of the mutual influence of two different metals on catalytic properties. After the pioneering studies of Sinfelt,[47-491who for the first time introduced effective bimetallic catalysts into bulk industrial processes, bimetallic nanostructured particles as catalyst precursors are currently of strong scientific interest. In recent years several new preparation methods - based mainly on the controlled co-reduction of two different metal ions have been developed for bimetallic colloids by different research g r o ~ p s . [ ~51' - Further ~ synthetic refinements led to the possibility of varying the structure of the bimetallic particles from a homogeneous alloy over gradient core/shell metal d i ~ t r i b u t i o n s [ ' ' to ~ >~fi~nally, ~~~~ a truly layered particle structure, consisting of, e.g., a gold core plated by palladium or vice versa.[8]Bimetallic particles with a gradient metal distribution or a layered structure seem to be the most interesting catalytically. When the catalytic hydrogenation of crotonic acid to butanoic acid was used as a test reaction, a clear synergistic effect of Pt and Rh[60] was observed for bimetallic colloidal precursors ( Pt20Rh80) with a gradient core/shell structure with increasing Rh concentration from the core towards the surface of the particle[661(see Fig. 3). A similar effect was reported for a Pt80Pd20 colloid catalyst in the partial hydrogenation of 1,3-~yclooctadiene.[~~] Because of their similar atomic radii and crystal structures, gold and palladium are miscible in any ratio. Electronically, however, gold-palladium alloys behave differently from the pure metals. The combination of the partially filled d band of Pd and the completely filled d band of Au on the one hand and the different electronegativities on the other hand, gives a novel electronic structure, which might be responsible for the frequently observed unique catalytic behavior of gold-palladium al10ys.l~1-80] In contrast with alloy-like systems, bimetallic particles with layered structures are fascinating prospects for the design of new catalysts. By using the classical seed growth method[811 Schmid et al."] recently synthesized layered bi~
918 5000
2 Metal Clusters in Catalysis Activity [Nrnllg min]
5000
Rhl Pt-colloid on C (curve 1)
4000
4000
3000
3000
2000
2000
1000
1000
100 mol % Pt
70130
50150
20180
100
rnol % Rh
Figure 3. Comparison of the activity of alloyed and mixed Rh/Pt/C catalysts in the crotonic acid test.
metallic Au/Pd and Pd/Au colloids in the size range 20-56 nm. (In accordance with the author’s nomenclature the core metal is always quoted first and the cover metal second). For example, a hydrosol of gold is pre-prepared by reduction of its cation with sodium citrate. In a second reduction step, palladium is grown on the gold core with hydroxylamine as reducing agent. The outer metal is stabilized by trisulfonated triphenylphosphane and sodium sulfanilate and the resulting bimetallic hydrosols consisting of more than 90% metal can be isolated in the solid state. Redispersion in water is possible at high concentration. Stabilized and non-stabilized Au/Pd and Pd/Au systems on a Ti02 support have been used as heterogeneous catalysts for the hydrogenation of hex-2-yne to cis-hex-2-ene. Both the palladium-plated gold seeds and the gold-plated palladium particles were considerably more active than the pure metals. The ligand shell has no effect on catalytic behavior. Except for the longer lifetime of the ligand-stabilized colloid catalysts, protected and unprotected colloids were reported to behave very similarly. The influence of the electronegativity of the colloidal core metals (Au compared with Pd) on the activity and selectivity of the surface Pt in hydrosilation reactions is described in a recent paper by the same research group. Au and Pd are more electronegative and electropositive, respectively, than Pt. The results clearly prove that layered bimetallic precursors in various core/ shell compositions are valuable tools for optimizing the activity and selectivity, and for improving catalyst After the absorption of the dissolved colloidal precursor on solid supports such as
2.12 Potential Applications of Nunostructured Metal Colloids
9 19
Activity in [Nml/g rnin]
250 *Colloidal PCVC catalyst -t Industrial PCVC catalyst
0
I
0
20000
40000
I
I
60000
80000
100000
TON
Figure 4. Lifetime of Pd/C catalysts in the cyclooctene hydrogenation test.
charcoal, glassy carbon, zeolites, TiOz, A1203, CaCO3, SiO2, single-crystal oxides, metal loading), according to or highly oriented pyrolytic graphite ( 1 and 5% (w/w) chemisorption measurements, a residual amount of the stabilizing agent is still present at the surface of the immobilized particles. Irrespective of the chemical nature of the stabilizer used well defined ligand molecules,[39]surfactants of the various type^,[^%^^] organic ‘envelopes’ such as polymer^[^^.*^] - several researchers have reported a marked influence of the protective shell on the catalytic performance of the heterogeneous metal colloid catalyst formed. Specifically, the lifetimes of the colloid catalysts are longer than those of conventional precipitation systems. Whereas the activity of a conventional Pd/C catalyst tested in the hydrogenation of cyclooctene to cyclooctane expires completely after the performance of 38 x lo3 catalytic cycles per Pd atom, the Pd colloid/C catalyst still had residual activity after 96 x lo3 catalytic turnovers[851(Fig. 4). The improved durability of colloidal catalysts was confirmed for Pd/C catalysts used for the oxidation of glucose using molecular oxygen. After recycling the catalyst 25 times, the reduction in activity was found to be much less for the colloidal than for the conventional Pd/C system.‘86]On the basis of the chemisorption results it seems reasonable to assume that the catalytically active nanometal particles are protected by a ‘coat’ which, although permeable to small molecules such as H2 or 0 2 , prevents direct contact with poisons. An impressive example of the selectivity control brought about by even subtle changes in the chain length of alkyl-substituted phenanthrolines used as the colloid stabilizers was recently reported by Schmid for the semihydrogenation of hex-2-yne to ci~-hex-2-ene.[~~] A mixture of seven- and eight-shell palladium clusters on TiOz, ~
920
2 Metal Clusters in Catalysis
protected by phenanthroline, catalyzes the semihydrogenation of hex-2-yne to cishex-2-ene highly selectively giving a maximum of ca 93% cis-hex-2-ene. Similar results were obtained with 3-n-decylphenanthroline as the stabilizing ligand. In contrast, on substitution of the phenanthroline with n-butyl- or n-heptyl groups the activity drops dramatically and the consecutive isomerization or total hydrogenation of cis-hex-2-ene is completely suppressed. Because of the small electronic differences between the n-heptyl and n-decyl substituents, geometric reasons are obviously responsible for the unexpected findings. The strong influence of ligand geometry on catalytic hydrogenation reactions has been illustrated by the same authors by use of further examples. These impressive results demonstrate how relatively small changes in the ligand geometry result in dramatically different catalytic behaviour, especially with respect to the selectivity. Use of the concept of rational ligand control on the basis of molecular modeling promises spectacular results in heterogeneous catalysis similar to those previously obtained in homogeneous metal complex catalysis. Apart from these academic model catalysts a more practical field of application for nanostructured metal colloids in heterogeneous catalysis resides in the manufacture of fine chemicals.[88]Bimetallic, even multimetallic precursors on various supports, eventually promoted by dopants, have been successfully tested as highly active, selective catalysts of remarkable durability. For example, surfactant-stabilized Pd-Pt-charcoal catalysts, promoted by bismuth have been shown to be superior catalysts for the carbohydrate oxidation reaction shown in Eq. ( 1):[891
- HoIii COONa
Pd-Pt-BiIC
+
112
o2
+ NaOH CH, OH D-(+)-glucose
+
(1)
H20
PH 9,5
H20
CH, OH D-gluconic acid Na-salt
Compared with industrial heterogeneous Pd/Pt catalysts charcoal-supported Pdss/Ptlz-NOctCl alloy particles have excellent activity and high selectivity in the oxidation of glucose to gluconic acid. According to TEM, XRD/DFA, XPS, XANES, and EXAFS analysis the chemical co-reduction of PdC12 and PtC12 in the appropriate ratio with NOctBEt3H yielded the alloyed Pd/Pt colloids in organic solvents. They are effectively screened from coagulation and poisoning by the lipophilic NOct4Cl surfactant layer. TEM showed particle sizes to be in the range 1.5-3 nm. In 1996 approximately 17000 tons sodium D-gluconate was produced by the enzymatic air-oxidation of D-glucose via biotechnology.
2.12 Potential Applications o j Nanostructured Metal Colloids
92 1
Another example of the application of supported nanometal colloids in fine chemicals catalysis is the cis-selective partial hydrogenation of 3-hexyn-1-01 to leaf alcohol, a valuable fragrance; in 1996 the amount produced was 400 tons, including ester^[^^*^'] (see Eq. 2).
H O d - - - E d
2"'&=// +( Pd
"5 ) ,
"
O
w
(2)
The performance of heterogeneous catalysts based on surfactant-stabilized palladium colloids was compared with that of conventional Pd/C and Lindlar catalysts for the partial hydrogenation of 3-hexyn-1-01 under optimized reaction conditions. The selectivity can be influenced by the protective shell and by the support and various promoters. The zwitterionic surfactant sulfobetaine-12 (SB12; N,N-dimethyldodecylammoniopropanesulfonate)seems to be the most suitable protective shell for highly selective palladium-colloid catalysts under the surfactants tested. The preferred support is CaC03; Pd( SB12) colloids supported on CaC03 have the highest selectivities and activities of all the catalysts tested. The best selectivity (98.1%) towards the desired cis-3-hexen-1-01 (leaf alcohol) was obtained with a lead-acetate-promoted palladium colloid supported on CaC03. This catalyst was shown to have slightly (0.5%) better selectivity and twice the activity of a conventional Lindlar catalyst. Pre-prepared Pt hydrosols stabilized by surfactants can be used as precursors for heterogeneous hydrogenation catalysts active in the selective high-pressure transformation of 3,4-dichloronitrobenzeneto the corresponding aniline ( Fig. 5).19'1 The catalytic performance of the new systems was evaluated in batch and continuous tests and the results were compared with those obtained from conventional Pt/C systems. All aspects of performance of the hydrosols matched those of the optimized Pt/C catalyst used in the industrial process. The additional potential of the new type of heterogeneous catalyst lies in the possibility of optimizing the properties of the catalyst precursor for special applications via the addition of doping agents or 'poisons' (such as sulfur) to the colloidal nanometal. Further 'fine tuning' might be achieved by using the synergistic effect of bimetallic precursors (e.g. Pt/Cu). Finally, the surfactant used for the stabilization of the colloidal metal precursors acts as an effective modifier of the colloidal metal surface. Beller et al.[92]have shown for the first time that palladium colloids are effective catalysts for the olefination of aryl bromides (Heck reaction). Reetz et al.[93]have studied Suzuki and Heck reactions catalyzed by preformed palladium clusters and palladium/nickel bimetallic clusters and further progress was achieved by Reetz and L ~ h m e r 'using ~ ~ ] propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. In addition, the use of nanostructured titanium clusters in McMurry-type coupling reactions has been demonstrated by Reetz et
922
2 Metal Clusters in Catalysis NH2
I
-
NH 2
I
dechlonnation
CI CI
CI
CI
azo-type byproducts
/
CI
0'
CI
Figure 5. Reaction scheme for the hydrogenation of 3,4-dichloronitrobenzene.
al.[95]The potential applications resulting from the strong redox capacity of nanostructured iron oxide particles, available from preformed Fe(0) colloids (size 3 nm) via controlled air oxidation,[96p98] have not yet been fully exploited (Fig. 6). The liquid phase hydrogenation of benzene on carrier-fixed ruthenium colloid catalysts suspended in an aqueous solution of sodium hydroxide proceeds with 59% cyclohexene selectivity at 50% benzene conversion. The catalysts are prepared by adsorbing a hydrophilic stabilized ruthenium metal colloid on lanthanum oxide.[99] Protection of metal colloids with chiral molecules can lead to a new type of enantioselective catalyst combining good selectivity control with extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in the form of platinum sols stabilized by the alkaloid dihydrocinchonidine[lo0] (Fig. 7). This concept, leading to this new type of catalyst, is an extension to colloidal metals of the well-known heterogeneous enantioselective catalyst.[lol]Platinum sols stabilized by dihydrocinchonidine (Fig. 7) can be synthesized in different particle sizes by reduction of platinum tetrachloride with formic acid in the presence of different amounts of alkaloid. The resulting nanoparticles are enantioselective in the hydrogenation of ethyl 2-oxopropionate (ethyl pyruvate); optical yields are 75-80%
2.12 Potential Applications of Nanostructured Metal Colloids
923
Figure 6. Oxidation of the NR4+-stabilized Fe colloid.
X: AcO, CI
X-
Figure 7. Schematic representation of the cinchona-stabilized platinum colloid ( X = AcO, Cl).
924
2 Metal Clusters in Catalysis
ee (Eq. 3 ) . Turnover frequencies (ca 1 s-l) and enantiomeric excess were both found to be independent of particle size. To evaluate the catalytic characteristics of colloidal platinum, the efficiency in the reaction shown in Eq. (3) of Pt nanoparticles in 'quasi-homogeneous' phase was compared with that of supported colloids of the same charge and with that of a conventional heterogeneous platinum catalyst. 0
Dihydrocinchonidin-
I1
/-./o+
/I
(3)
stabilized Pt-colloid 11
0
0
H2
The advantage of the 'quasi homogeneous' colloidal system compared with the conventional catalyst resides in a threefold turnover frequency."011 Bradley et al. have recently reported['02]that the presence of HCl in as-prepared Pt sols modified by cinchona alkaloids has a marked effect on rate and reproducibility, and that removal of HCl by dialysis gives catalysts with exceptional reproducibility and higher reaction rates. The reliability of these colloidal catalysts will enable their use as a reliable test system both for precise rate studies, in the absence of possible support effects, by variation of the reaction conditions which affect catalyst performance in this reaction, and also for screening of alternative modifiers to cinchona alkaloids. A kinetic probe of the effect of a stabilizing polymer on a colloidal catalyst - accelerated enantioselective hydrogenation of ethyl pyruvate catalyzed by poly(vinylpyrro1idone)-stabilizedPt colloids - has shown that the presence of a stabilizing polymer in the colloidal catalyst does not hinder access of the modifier molecule to the colloidal metal surface, but that the polymer can, by adsorption at the metal surface, reduce the number of surface-modified sites available for the faster enantioselective hydr~genation."~~] The scope of nanostructured metal colloids with chiral stabilizers includes the enantioselective transformation of specific prochiral substrates into valuable fine chemicals of, e.g., pharmaceutical relevance.
2.12.2.2 Fuel cells The fuel cell is an electrochemical reactor, the required output of which is the energy released rather than the reaction product. The main fields of its application include transport systems, stationary power generation, and combined heat and power sources. What emerges from the specialist literature is that, despite the innovations and improvements made in recent years, there is still much room for further developments.['"201 Central to the success of fuel-cell technology are the catalyst systems. This is where bi- and multi-metallic nanostructured colloids, especially those of small particle size (1-2.5 nm), scattering as nearly perfect 'single crystals', are important. They offer improved efficiency and tolerance against CO-
2.12 Potential Applications of Nanostructured Metal Colloids
925
contamination of the feed.['*] A brief description of the six main types of fuel cell currently under research and development is given elsewhere." 51 Most electrocatalysts for fuel cells rely on platinum. With regard to colloidal metals as catalyst precursors, this interim state-of-the-art report focuses on recent developments in phosphoric acid fuel cells (PAFC) and proton-exchange membrane fuel cells (PEMFC). PAFC operate at 190-210 "C in orthophosphoric acid as the electrolyte; the anode catalyst is Pt and the cathode catalyst is Pt/Cr/Co.["] PEMFC work at 50-125 "C, with a solid proton conducting polymer as the electrolyte. The catalyst Pt/Ru is recommended for the anode, because of the CO tolerance required; the cathode catalyst is based on Pt.[18] For use as PAFC catalysts a trimetallic colloidal (3.8 nm) precursor of composition Pt50C0&-20 was prepared by co-reduction of the corresponding metal salts.['041XRD examination showed that the particles consist of a trimetallic alloy with an ordered fcc structure.['05] In a standard half-cell test the electrocatalytic performance was compared with that of an industrial standard catalyst manufactured by co-precipitation and subsequent annealing to 900 "C, giving 5.7 nm trimetallic crystallites.[' 06] Preliminary results have shown that the trimetallic colloid catalysts have advantages in respect of long-term stability essential for PAFC catalysts. The decay of the potential after 22 h was found to be less than 10 mV. Further work is in progress.['07] Pt/Ru alloys are currently the most promising anode catalysts for low-temperature (80 "C) polymer-membrane fuel cells (PEMFC), whether direct methanol fuel cells (DMFC) or Hz/air fuel cells operated with CO-contaminated hydrogen (H2PEMFC). To achieve the ultimate dispersion state of the metals, truly bimetallic Pt/Ru precursors of particle size less than 2 nm are desired. The electrocatalytic activity of a bimetallic P ~ ~ O / R U ~ ~ - colloid, N O C ~prepared ~ C ~ by the co-reduction method,i54]toward the oxidation of CO and a CO-H2 gas mixture (simulated reformer gas) was measured The particle-size distribution with a mean diameter of 1.7 nm was determined by high-resolution transmission electron microscopy, and the formation of stoichiometrically alloyed particles was verified by point-resolved energy-dispersive X-ray analysis. The CO-stripping voltammetry of the glassy carbon-supported P ~ ~ o / R u ~ o - N O colloid C ~ ~ C Iwas found to be in excellent agreement with CO-stripping voltammetry data measured on wellcharacterized bulk alloy electrodes. The activity of the colloid toward the continuous oxidation of 2% CO in H2 was assessed in a rotating disk electrode configuration at 25 "C in 0.5 M H2SO4, leading to the conclusion that these Pt/Ru colloids are very promising precursors of high-surface-area fuel-cell catalysts. Further structural information on the precursor was obtained by use of in-situ XRD via Debye function analysis. XANES data provided further evidence of the bimetallic character of the particles. To elucidate the catalytic function of the alloyed Ru, the CO oxidation of a Pt/Ru catalyst was studied by in situ XRD. At 280 "C surface oxide species are formed; these slowly coalesce to Ru02 particles. After re-reduction the catalyst consists of a pure hcp ruthenium phase and larger alloy particles ~
926
2 Metal Clusters in Catalysis
enriched with platinum.[231In terms of operating an PEMFC, the improved CO oxidation activity of colloidal Ptso/Ruso precursors ( < 2 nm) results in higher CO tolerance; i.e., higher CO concentrations in the H2-feed can be tolerated without a significant reduction in
2.12.3 Miscellaneous Classical nanometal colloid synthesis via simple nucleation-and-growth processes in situ yields rather low metal concentrations (generally 0.1-100 mg L-’). These restrictions probably make monometallic colloids too expensive for large-scale industrial applications. In contrast, there is a good chance after a recent field study[281 of practical realization of bi- and trimetallic nanostructured colloids, eventually promoted by additional dopants, as catalyst precursors - because truly alloyed and monodispersed nanosized active components (with controlled ‘cherry’ or ‘raspberry’ architecture and gradient or layered particle structures) on supports are virtually inaccessible by conventional methods. The production of organic fine chemicals by redox catalysis and rapidly developing fuel-cell technology are presently regarded as the main field of potential applications for the new types of industrial catalyst. A promising strategy for future developments are reactions in vesicles, gels, and microemulsions,where growth processes in nanostructured matrices are limited by the size of the structures themselves. In fact, metal colloid syntheses in microemulsions and surfactant micelles have been shown to be successful for microparticle synthesis. It also enables the direct synthesis of aqueous systems and of stable inverse systems, i. e. metal colloids dispersed in organic solvents.[l2I Aqueous dispersions of PVP-protected ( PVP = poly(2-vinylpyrrolidone)) Pd, Pt, Rh, Pd/Pt, and Pd/Rh colloids, anchored on to the surface of water-insoluble, crosslinked PVP have been successfully used for the catalytic hydrogenation of liposomes as models of biological membrane^."^] Hydrogenation of biological membranes might influence many important functions of the cells and might give diagnostic information about their physical state and the role of the biomembranes in certain important physiological processes of the living cells, for example cold acclimatization and heat-shock tolerance. Quantum size effects related to the dimensionality of a system in the nanometer range suggests a plethora of future applications using novel material properties.[39] Whereas three-dimensional systems have an infinite extent in all three directions, in layered systems, for example atomic monolayers and thin films, the dimensionality is two, i.e. they are characterized by a limited number of layers. Consequently, a one-dimensional material is represented by wires on an atomic or molecular scale and may be realized in fibers or polymers. Zero-dimensional particles are reduced in all directions to such an extent that the properties of the original bulk system cannot ~
~
2.12 Potential Applications of Nunostructured Metal Colloids
921
be maintained and, consequently, dramatic changes of physical properties are implied. The application of this basic knowledge to nanometallic systems has resulted in the possibility of changing the properties of bulk metals by reducing the size of a metal particle in one, two, or even three dimensions. Zero-dimensional metal particles might, however, still comprise thousands or hundreds of atoms. The decisive difference from a three-dimensionally oriented material is the loss of its characteristic magnetic domain size, scattering length of conduction electrons, and the de Broglie wavelength of the electron; even changes in melting point might result from size reduction. The first steps toward fabrication of a supported and insulated quantum wire has recently been made by Schmid et al., who produced one-dimensional configurations of gold clusters and colloids in alumina nanot~bes.[’~] The pore channels were filled by vacuum induction (colloids only), electrophoresis (clusters only), or immersion (clusters, which were then converted into colloids by heating). Rudimentary wires consisting of colloids and clusters were successfully formed. In both instances the diameter of the pore channel exceeded that of the clusters or colloids. The wires thus formed fitted to the pore channel by forming helical secondary structures. Quantum dots can communicate via single electrons. So these results are not merely of theoretical interest but have high potential for future application in nanoelectronics. A special field of technological interest stems from the high spin density of nanostructured magnetic metals of the Fe, Co, Ni series. For example, Co nanoparticles (1-1 0 nm) have superparamagnetic properties which could be exploited in the manufacture of magnetic controlling devices. The use of cobalt nanoparticles in block copolymer micelles for the production of stable magnetic fluids was recently disc~ssed.[’~1 The superparamagnetic properties of nanometer-sized Ni particles have also been studied.” 08] Although chemical synthesis has made nanostructured ferromagnetic metal powders and colloids including alloys readily accessible,[261 the possible technical applications discussed, e.g. magneto-optical data storage and fluid magnetic lubricants, have not yet been exploited practically. The object of this survey has been to summarize the prospects for the nanostructured metal colloids emerging from current basic research. It remains to be seen how the practical applications of these materials develop over the next few years. -
List of abbreviations DMFC PAFC PEMFC TEM XRD/DFA
Direct methanol fuel cell Phosphoric acid fuel cell Proton-exchange membrane fuel cell Transmission Electron Microscopy X-ray Diffraction/Debye Function Analysis
-
928 XPS XANES EXAFS SAXS fcc hCP
2 Metal Clusters in Catalysis
X-ray Photoelectron Spectroscopy X-ray absorption near-edge structure spectroscopy X-ray absorption fine structure spectroscopy Small angle x-ray scattering face centered cubic hexagonal close packing
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[66] H. Bonnemann, W. Brijoux, J. Richter, R. Becker, J. Hormes, J. Rothe, Z. Naturforsch. 1995.506. , , 333-338. [67] L. E. Aleandri, H. Bonnemann, D. J. Jones, J. Richter, J. Roziere, J. Muter. Chem. 1995, 5, 749-752. [68] N. Toshima in: E. Pelizzetti (Ed.), ‘Fine Particles Science and Technology’,Kluwer Academic Publishers, 1996, p. 371-383. [69] M. Harada, K. Asakura, N. Toshima, J. Phys. Chem. 1994, 98, 2653-2662. [70] N. Toshima, Macromol. Symp. 1996, 105, 11 1-1 18. [71] B. J. Joice, J. J. Rooney, P. B. Wells, G. R. Wilson, Discuss. Faraday Soc. 1966, 41, 223. [72] J. Schwank, Gold Bull. 1985, 18, 2. [73] W. Juszczyk, Z. Karpinski, D. Lomot, J. Pielaszek, J. W. Sobczak, J. Catal. 1995, 151, 67. [74] A. 0. Cinneide, J. K. A. Clarke, J. Catal. 1972, 26, 233. [75] S. H. Inami, W. Wise, J. Catal. 1972, 26, 92. [76] C. Visser, I G. P. Zuidwijk, V. Ponec, J. Catal. 1974, 35, 407. [77] N. Toshima, J. Macromol. Sci. 1990, A 27, 1225. [78] A. 0. Cinneide, F. G. Gault, J. Catal. 1975, 37, 311. [79] N. Toshima, H. Harada, Y. Yamazaki, K. Asakura, J. Phys. Chem. 1992, 96, 9927. [80] H. Liu, C. Mao, S. Meng, J. Mol. Catal. 1992, 74, 275. [81] J. B. Michel, J. T. Schwartz in: Catalyst Preparation Science IV, (B. Delmon, P. Grange, P. A. Jacobs, G. Poncelet), Elsevier, New York 1987, p. 669-687. [82] G. Schmid, H. West, H. Mehles, A. Lehnert, Inorg. Chem. 1997, 36, 891-895. [83] N. Toshima, T. Takahashi, H. Hirai, J. Macromol. Sci.-Chem. 1988, ,425 (5-7), 669-686. [84] M. Ohtati, M. Komiyama, H. Hirai, N. Toshima, Macromolecules 1991,24 (20), 5567-5572. [85] H. Bonnemann, R. Brinkmann, P. Neiteler, Applied Organomet. Chem. 1994, 8, 361. [86] H. Bonnemann, W. Brijoux in: W. Moser (Ed.), Advanced Catalysts and Nanostructured Materials, Chapter 7, Academic Press, 1996, p. 186. [87] G. Schmid, V. Maihack, F. Lantermann, St. Peschel, J. Chem. Soc., Dalton Trans. 1996, 591594. [88] H. Bonnemann, W. Brijoux, A. Schulze Tilling, K. Siepen, Topics in Catalysis 1997, 4, 217227. [89] H. Bonnemann, W. Brijoux, R. Brinkmann, A. Schulze Tilling, T. Schilling, B. Tesche, K. Seevogel; R. Franke, J. Hormes, G. Kohl, J. Pollmann, J. Rothe, W. Vogel, Znorg. Chim. Acta 1998, 270, 95-1 10. [90] H. Bonnemann, W. Brijoux, K. Siepen, J. Hormes, R. Franke, J. Pollmann, J. Rothe, Appl. Organomet. Chem. 1997, 11, 783-796. [91] H. Bonnemann, W. Wittholt, J. D. Jentsch, A. Schulze Tilling, New J. Chem. 1998, 22, 713717. [92] M. Beller, H. Fischer , K. Kiihlein, C.-P. Reizinger , W. A. Herrmann, J. Organomet. Chem. 1996, 520, 257-259. [93] M. T. Reetz, R. Breinbauer, K. Wanninger, Tetrahedron Lett. 1996, 37, 4499-4502. [94] M. T. Reetz, G. Lohmer, Chem. Commun. 1996, 1921-1922. [95] M. T. Reetz, S. A. Quaker, C. Merk, Chem. Ber. 1996, 129, 741-743. [96] H. Bonnemann, W. Brijoux in: W. Moser (Ed.), Advanced Catalysts and Nunostructured Materials, Chapter 7, Academic Press, 1996, p. 172. [97] H. Bonnemann , G. Braun, W. Brijoux, R. Brinkmann, A. Schulze Tilling, K. Seevogel, K. Siepen, J. Organomet. Chem. 1996, 520, 147. [98] M. T. Reetz, S. A. Quaker, M. Winter, J. A. Becker, R. Schafer, U. Stimming, A. Marmann, R. Vogel, T. Konno, Angew. Chem. Int. Ed. Engl. 1996,35, 2092. [99] H. Bonnemann, P. Britz, H. Ehwald, Chem. Technik 1997,49, 189-192. [loo] H. Bonnemann, G. A. Braun, Angew. Chem. Znt. Engl. 1996,35, 1992-1995. [loll H. Bonnemann, G. A. Braun, Chem. Eur. J. 1997,3, 1200-1202. [lo21 J. U. Kohler, J. S. Bradley, Catalysis Letters 1997, 45,203-208.
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1031 J. U. Kohler, J. S. Bradley, Lnnymuir, 1998, 14, 2730-2735. 1041 W. Wittholt, PhD Thesis, RWTH Aachen, 1997. 1051 F. J. Luczak, D. A. Landsman, USP4,447,506, 1984. 1061 A. Freund, J. Lang, T. Lehmann, K. A. Starz, Cutalysis Toduy 1996,27, 279-283. 1071 H. Bonnemann, W. Wittholt, A. Freund, E. Auer, in preparation. 1081 Y. Volokitin, J. Sinzig, G. Schmid, H. Bonnemann, L. J. de Jongh, J. Phys. D: At., Mol. Clusters 1997, 40, 136--139.
3 Dynamics and Physical Properties
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.1 Dynamics and Physical Properties An Overview John R. Shapley
The articles included in this section represent contributions towards an understanding of the basic structural and electronic properties of metal carbonyl clusters. They illustrate the tremendous progress that has been made in formulating a broad view of these molecules as well as highlight some of the significant challenges yet remaining. Although the results presented here focus on the properties of clusters as individual molecular entities, the concepts are also relevant to understanding the r61e of clusters as catalytic reaction centers and as models for metal surface reactions. Much progress has been made by adopting a global approach to the properties of metal clusters, but the limitations of this approach are also becoming apparent. The paradigm established by the Wade-Mingos rules allows for a ready rationalization of cluster framework structure with electron count, but it does not provides us with a means of evaluating the relative energies of isomeric structures with the same number of electrons, Similarly, the Johnson Ligand Polyhedral Model offers a simple view of the arrangement of ligands in the cluster coordination shell, but it does not help in trying to assess how close in energy alternative ligand arrangements might be, particularly if the ligands are of different types. Thus, neither of these approaches are reliable tools for the purpose of evaluating possible mechanisms of cluster rearrangements, which may involve changes in the cluster framework, the distribution of ligands, or some combination of both. There is also a growing body of evidence that the intermolecular environment of a cluster can influence its ligand arrangement, i.e., different solid state structures can be shown for different crystal forms, counter cations, etc. When metal-metal bonds get sufficiently weak, even the framework structure can be affected by intermolecular forces. The first set of articles in this section are at the forefront of new developments in dealing with aspects of transition metal carbonyl cluster rearrangements. Sironi discusses how ligand-ligand interactions are augmented by ligand-metal and chargecharge interactions in determining the overall energetics of the ligand stereochemistry. Heaton’s NMR studies on higher nuclearity rhodium clusters illustrate the level of detail regarding structure and dynamics that can be garnered by careful
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3 Dynamics and Physical Properties
use of modern NMR methods. Farrugia and Orpen link atom displacement parameters measured by X-ray diffraction with carbonyl fluxionality, and they show the utility of solution EXAFS measurkments for monitoring cluster framework isomerization. A promising new development is presented by Hughes and Wade, who show how relating a set of framework metal-metal distances to a total framework energy allows one to predict as well as to rationalize stability differences between closely related componds. It will be particularly exciting to see how this concept is extended to handle heterometallic bonds, which then would allow enthalpy comparisons of isomeric heterometallic clusters. These developments should provide a solid basis for understanding the general question of skeletal isomerism in transition metal clusters, examples of which are reviewed by Rossell, Seco, and Segales. An important frontier in cluster chemistry is the effect of electron count on reactivity. Are clusters merely passive electron reservoirs or does the number of electrons have a critical influence on reactions other than electron transfer? Conversely, how does the number of metal atoms, the relative ratio of heterometals, and the specific ligand set relate to the ability of clusters to enter into redox reactions? These questions are of fundamental interest, but the answers also may have practical consequences, for example, in the development of metal nanoparticles or colloids to act as electrocatalysts. In a final group of articles Longoni and coworkers review the ability of homoleptic carbonyl clusters to act as ‘electron-sinks’, Zanello and Fabrizi de Biani examine the effect of heterometallic interactions on cluster redox aptitude, and Ignaczak and Gomes report on modelling of electrode interactions with metal clusters.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters Angelo Sironi
3.2.1 Introduction In the last 30 years cluster chemistry has become a major new area of inorganic chemistry. The ‘basic’ rules are now well understood, are described in dedicated textbooks,[’] and are normally part of any serious inorganic chemistry curriculum. However, even in the ‘old’ field of metal carbonyl clusters, there are still questions which elude simple laws or rules and are difficult to answer in a general way. For instance, despite a substantial knowledge of the relationships between the shape of the metal cage and the stoichiometry of the cluster (the so called electron counting rules),[’] a simple working theory of ligand stereochemistry is lacking. Accordingly, we can predict with paper and pencil the stoichiometry of a metal carbonyl cluster (given the shape of the metal cage) but not necessarily its ligand stereochemistry indeed, fluxional processes, the presence in solution of two or more isomers,[31and the tendency of different crystalline states (like from different solvents) to promote different stereo isomer^,[^] are well documented. This behavior, and the similarity of the energies spanned by the few low-energy stereoisomers suggests the presence, in the potential energy surfaces (PES) of metal carbonyl clusters, of a few shallow minima and low energy saddles interconnected by flat valleys (even if many high-energy regions, mainly related to conformations with unreasonable bond distances or ligand distributions, are also present). This suggests that ligand stereochemistry depends on many cooperative or opposing weak interactions. Unfortunately, these weak forces cannot be easily linked with a localized type of bonding because different carbonyl coordination modes have similar donor properties (if carbon is the only donor atom)[51and many cluster orbitals are available for metal-ligand bonds. Significantly, this variety of possible matches between carbonyl and metal orbitals, earlier suggested by Cotton in the form of a smooth continuum of possible conformations,[61 has been demonstrated by Crabtree and Lavinc7Iby constructing, by the structure correlation method,[*] the
938
3 Dynamics and Physical Properties
'reaction' pathway for the terminal/p,-bridge/terminal carbonyl exchange between two iron atoms. Moreover, by following the same approach Orpen has recently mapped the CO exchange process on a triangular M3 face showing that the terminal/ pu,-bridge/p3-bridgepath is more likely to be followed than the terminal/p3-bridge path, which is, however, not ruled out.['] Despite all the difficulties inherent in the rationalization of ligand behavior, immediately after the appearance in the 1960s of the first structurally characterized 'binary metal carbonyl clusters' ( BMCCs), favorable packing of the CO ligands about the cluster core['o1 and charge equalization over all the metal atoms[''] were soon recognized as prerequisites for any reasonable ligand stereochemistry; eventually, it was the proper coupling of these two 'physical' principles that led to the development of the model for BMCCs ligand stereochemistry discussed here.
3.2.2 Favorable ligand packing 3.2.2.1 The ligand polyhedral model Favorable packing of the CO ligands about the cluster core was and later (serni)quantitati~ely['~~ investigated by Johnson et al. who, while searching for the best fit between the metal cluster and the optimum ligand polyhedron, eventually derived what is known as the Ligand Polyhedral Model ( LPM).[l4I According to the LPM: i) the geometry of the ligand shell in BMCCs is determined not by major bonding forces, but rather by interactions between the ligands themselves; ii) the number and type of CO ligands depend upon possible arrangements of one polyhedron or polygon (the metal cage) within the other (the CO envelope); iii) the relative stability of the CO polyhedra can be determined by a points on a sphere (POS) model using a repulsive potential acting between the oxygen atoms alone; and iv) the fit between the metal and the ligand polyhedra can be inspected by simple symmetry and/or geometrical considerations, within the assumption that a CO ligand is always well represented by a sphere of 3.0w radius centered on the mid-point of the C-0 bond (whatever the metal and the coordination mode). For example, in the M3(C0)12 ( M = Fe, Ru, 0 s ) family, LPM states that whereas a triangle of Fe atoms (with Fe-Fe bond lengths close to 2.6 A)fits into the hole of an icosahedron (made by twelve spheres of 3.0A radius) those of Ru or 0 s atoms (with M-M bond lengths close to 2.85 A) cannot; these larger triangles will, however, fit into an anticubeoctahedron which, given its larger radius ratio (1.00
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
939
Figure 1. Molecular conformations of the three most significant M3(C0)12 ( M = Fe, Ru and 0 s ) stereoisomers, minimized with various symmetry constraints: a) C2, b) D3,c) D3h. Steric energies (in kcalmole-I), relative to the most stable stereoisomer, are reported for Fe derivatives. The numbers in parentheses refer to Ru derivatives and can also be considered informative for the 0 s derivatives given the almost identical size of Ru3(C0)12 and O S J ( C O ) I ~ .
compared with 0.95), has a larger interstitial cavity. Placement of an M3 triangle within an anticubeoctahedron will eventually lead to the observed all-terminal structure of idealized D3h symmetry (c in Fig. 1); on the contrary, given that an icosahedron is a better fit than an anticubeoctahedron for the packing of 12 spheres, the Fe3 triangle can be accommodated by an icosahedron of COs, in agreement with the observed bridged structure of idealized C, symmetry (a in Fig. 1). Despite the apparent success, the above considerations are flawed because one of the possible stereochemical choices has been overlooked - polyhedral expansion has Z an icosahedral ligand envelope with not been considered; could not R u ~ ( C O ) Ihave looser carbonyl contacts and a larger internal cavity? The a priori dismissal of this possibility (which follows from radius ratio considerations) implies the hidden assumption that COs behave like sticky rigid spheres, which can neither interpenetrate nor become detached, i.e. that polyhedral interconversions (between polyhedra with equal edges) are always energetically favored relative to polyhedral expansions of any size. Such a rigid assumption is definitely unjustified when one further considers that: i) the radius ratio and POS concepts are grounded on contrasting hypotheses - the former implies polyhedra with a common edge length inscribed into spheres of diflerent size whereas the latter implies polyhedra inscribed in a common sphere with diflerent edge lengths; ii) terminal and bridging carbonyl ligands have intrinsically different distances from the cluster center and a POS model can hardly cope with the complexity of a (CO), moiety. In [ C o ~ ( C 0 ) 1 4 for ] ~ ~instance, , the oxygen atoms of the six terminal and eight bridging COs belong to two spheres of different radius, 4.62 and 3.66 A,respectively!
940
3 Dynamics and Physical Properties
The intrinsic weakness of the radius ratio approach can be further recognized by considering the M4(CO)12 ( M = Co, Rh, Ir) family; here the very same LPM arguments account for the change in structure (or polyhedral form) on going from C04(CO)12 (icosahedron) to Ir4(CO)12 (cubeoctahedron) but fail to explain why Rh4(C0)12 has the Co4(C0)12 stereochemistry although Rh and Ir have similar metallic radii.
3.2.2.2 The equal potential surface (EPS) model Molecular mechanics (MM) is a major tool for dealing with structures and thermodynamic properties of large molecules and is now widely used throughout the Periodic Table," particularly when a straightforward evaluation of steric effects is needed. As beautifully outlined by Lauher,[l6] an MM treatment of BMCCs requires the introduction of an equal potential surface (EPS - the bottom of the potential well associated with M-C bond stretches) on which the carbon atoms of the CO ligands are free to float (maintaining the C-0 vector normal to the surface). Lauher's theoretical efforts materialized in a formalism which did not explicitly consider atom connectivities and was difficult to implement in existing MM programs. We showed later that an EPS model can be defined even in the presence of a connectivity pattern, provided that it is periodically redetermined (on the basis of actual geometries) until convergence is reached;"'] this technique has enabled us to implement the EPS algorithm in Allinger's MM3 program with the full retention of all its features.[l8I In the local connectivity approach the nature of M/C and C/C interactions is variable, when a carbonyl floats over the EPS, previous M-C bonds might become 1,3 M . . . C non-bonded interactions (and vice versa) and some previously 1,3 C . . . C non-bonded interactions might be converted into 1,4 interactions (and vice versa). Thus, to ensure continuity of the energy function over the EPS, the van der Waals parameters for the M . . . C non-bonded interactions are set to zero (for a smooth interchange of the M/C bonding and non-bonding terms), the 1,3 C . . . C non-bonded interactions are explicitly considered (for a smooth 1,3/1,4 inter~hange),"~] and C-M-C bending and any torsional term are omitted (but could be partially restored, when the connectivity pattern is stable, to address specific 'electronic effects'). It can be shown that valence forces maintain the carbonyl ligands on the EPS without a substantial contribution to the steric energy (because of the lack of bending and torsional terms) whereas the major contribution to the computed steric energies ES arises from the non-bonding interactions. This enables not only modeling of BMCCs stereochemistries according to LPM principles, i.e. as determined by the packing of ligands around the surface of the metal core, rather than by the sum of the structures about the individual metal atoms, but also quantitative evaluation of their relative stabilities. In the M3(C0)12 family ( M = Fe, Ru, 0 s ) steric energies support the same stereochemistry (b in Fig. 1) for the whole family, and suggest a flatter potential energy
3.2 The Liyand Stereochemistry of Transition Metal Carbonyl Clusters
941
hypersurface for the Ru and 0 s derivatives (compared with that for Fe) and cannot differentiate between the Ru and 0 s derivatives (despite their different dynamic behavior).['6*201 Similar results have been obtained for the M4(C0)12 family ( M = Co, Rh, Ir) where the most stable stereoisomer, whatever the metal, has an all terminal COs stereochemistry of T symmetry and has not been observed experimentally.['61 In other words: intramolecular steric interactions alone fail to account for the observed stereochemical behavior. However, when a particular isomer is assumed to be correct, for reasons lying outside the actual MM parametrization, MM is a powerful tool for predicting the principal stereochemical features of the given isomer. For Ru3(C0)12, for instance, MM can reproduce the outward bending of the axial M-C-0 angles (1 74.7" compared with an experimentally measuredL2'I value of 173.0"), the substantial linearity of the equatorial ones (178.8' compared with 178.9") and the values of the C,,-M-C,, (173.6" compared with 178.3"), C,,-M-C,, (108.6" compared with 104.1"), and M-M-C,, (95.7" compared with 97.9') angles. Indeed, as suggested by Lauher, simple overlap arguments can partially explain the stereochemistries of the Ru and 0 s derivatives. In addition, we believe that the failure to describe such a simple system correctly results from the lack of parametrization of the stereochemical preferences of lighter transition metals for bridged structures. In fact, as suggested by Evans,[221the formation of structures containing bridging carbonyls is favored, owing to the greater number of M-L o bonds, for the lighter elements, because of the more contracted nature of nd orbitals and the greater ( n l)p - nd mixing for n = 3. As the more contracted 3d orbitals are transformed into the less contracted 5d orbitals, TC overlap becomes more effective. This stabilizes terminal carbonyls relative to bridging carbonyls because the (metal)tz, - n*(carbonyl) interactions, which are responsible for the M + CO back-donation, arise from TC overlap for terminal carbonyls but from o overlap for bridging carbonyls. In contrast, on steric grounds only, we would expect a greater occurrence of bridging carbonyls for second and third transition metal clusters, because bridging carbonyls, despite the longer M-C interactions, are intrinsically closer to the cluster center (see above); i.e. bridged isomers, which usually have higher steric energies than the non-bridged isomers, should be more disfavored in smaller systems where the energy spread between different isomers is larger. The 'bias' toward all terminal structures, introduced by considering 'only' the nonbonded intramolecular interactions, is further confirmed by MM computations on the M4(C0)12( M = Co, Rh, Ir) family,['61for which the global minimum seems to correspond to an all-terminal COs stereoisomer of T symmetry.
+
3.2.3 Stereochemical regularities in BMCCs Because intramolecular steric interactions do not seem to be the most important term in determining BMCC stereogeometries[ we must reconsider the whole question,
942
3 Dynamics and Physical Properties
by looking at the known stereochemical properties of BMCCs, to discover ‘new’ interactions or electronic effects that can be incorporated in the force field. In particular we recognize that: i) although there is no reason to attribute greater stability to bridging carbonyl ligands than to terminal ligands (the few thermochemical data available suggest similar binding energies for the three coordination modes),[231bridging and terminal carbonyls have different z-acidities and hence have a different effect on the adjacent metals - the greater the electron density on the metals the larger the number of bridging carbonyls (the ratio of bridging to terminal carbonyls increases markedly as the negative charge of the cluster increases, and the replacement of CO by better o-donor ligands can have a similar effect);[241 ii) the occurence of bridging carbonyls is more widespread in clusters of the lighter transition metals (compared with those of the second and third rows); iii) ,u,-CO ligands are less common than p2-C0 in clusters of the heavier transition metals; iv) equidistribution of the volume around the individual metal centers does not always imply analogous equidistribution of the valence electrons and effective distribution of the charges; v) local ligand geometries are often (but not always) suggested by the effective atomic number (EAN) for the metal vertex; vi) for some metals local ligand stereogeometry is obviously anisotropic (platinum, for instance, often has strong (short) in-plane and weak (long) out-ofplane interactions); vii) intermolecular interactions, i.e. the crystal lattice (in the solid state) but also the solvent (in solution), affect the ligand stereochemistry rather unpredictably; viii) lengths of CO-bridged M-M bonds are systematically shorter than those of unbridged ix) M-CO interactions trans to a M-M bond are shorter than those trans to a M-CO bond; x) M-M-C angles are affected by long-range d -+ TC* interactions;[261 xi) the presence of an interstitial atom can promote the ligand stereochemistry with the best inward-pointing cluster orbitals (for maximization of interstitialatom-cage interactions);[271and xii) the presence of an interstitial atom can severely distort the metal cage, sometimes with rather subtle effects.[281 Close analysis of points (i)-(xii) suggests that several factors should be considered for further parametrization: a) the local electron book-keeping and the distribution of local formal charges (i-iv); b) the tendency for a definite local coordination geometry on selected metal centers (v,vi);[’91
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
943
c) the intermolecular interactions (vii); and d) the dependence of the ‘natural’ values of M-C, M-M, and M-M-C on actual local geometries (viii-~ii).[~O] If all these stereochemical features were equally significant, only their simultaneous parametrization could eventually afford a definitive picture of metal carbonyl cluster stereochemistry. We must, however, realize that the most important improvement to the force field will arise from the parametrization of the electron flow correlated to the floating of the carbonyl ligands on the EPS. Indeed, in a conventional MM study the connectivity of the atoms is precisely defined (and is not allowed to change during the minimization) and, as a consequence, the number of valence electrons of each atom is also strictly controlled. In contrast, within the EPS formalism, allowing for variable connectivity of the metals, we lose the control of the local number of valence electrons on each metal center. This is the major pitfall of the EPS approach and we believe it is chemically sound to allow jor a variable M-CO connectivity only f a new component of the force field will compensate for the tendency of homogeneous spreading of valence electrons, which, sometimes, is not ensured by the best steric distribution of the ligands, particularly when dealing with mixed metal clusters.
3.2.4 Charge equalization Charge equalization over all the metal atoms was qualitatively investigated by Cotton while trying to account for the presence of semibridging carbonyl groups[311 in terms of their r61e in mitigating charge imbalance between two (or more) metal atoms.r61In his opinion, charge imbalance must be avoided because of Pauling’s electroneutrality principle and the r61e of charge equalization in determining the ligand stereochemistry around inherently different metal atoms (carrying opposite formal charges) arises from the ability of the metals to perturb adjacent CO groups through d + TC* interactions. In [ F ~ C O ( C O ) ~ ] -for , [ ~instance, ~] the attainment of 18-electron configurations on the two metal atoms leads to highly polarized structures both for the stereoisomer with only terminal COs (implying a full formal negative charge on the Fe atom) and for that with a completely symmetric CO bridge (implying a full formal negative charge on the Co atom). Thus the semibridging CO group (which is principally bonded to the Co atom) acts to mitigate this polarization by accepting more or less (depending on which reference isomer is considered) electron density from a filled Fe d orbital (Fig. 2). A complementary view is to assume that the two electrons donated by the semibridging CO group are (somehow) partitioned between the Fe (6) and Co (2 - 6) atoms, for 6 = 0.5 the charge would be equally spread over the metal atoms. In line with this complementary but fully equivalent point of view we have pro-
944
3 Dynamics and Physical Properties
Figure 2. (a) The structure of [ F ~ C O ( C O ) ~Without ]-. the semibridging CO, the attainment of 18electron configurations on both metal atoms leads to a highly polarized structure (with a negative charge on the Fe atom). It should be noted that a structure with a completely symmetric CO bridge is also unsatisfactory in terms of charge distribution, because it places a full formal negative charge on the Co atom. (b) According to Cotton, the semibridging CO group acts to mitigate the charge imbalance by electron-density transfer from a filled Fe A orbital to an empty n" orbital of the semibridging CO group (which is principally bonded to the Co atom). A complementary view is to assume that the two electrons donated by the semibridging CO group are (somehow) partitioned between the Fe (6) and Co (26) atoms, for 6 = 0.5 the charge would be equally spread over the metal atoms.
that the electrons donated by CO ligands to their adjacent metals should be partitioned according to a bond valence scheme.[341This has enabled: i) quantification of the electron flux associated with the bending of CO ligands; ii) assignment of the proper local formal charge to each metal atom; and iii) inclusion in our MM force field for metal carbonyl clusters of a force accounting for the tendencies of the formal charges to spread, and of the metal atoms to fulfil their 'ideal' EAN.
3.2.4.1 The formal local charge equalization approach We briefly report here a few definitions concerning global and local electron counting procedures, remembering also that the number of cluster valence electrons (CVE) strongly determines the stoichiometry and the cluster shape, while the correct local valence electrons ( L V E ) distribution influences the ligand conformation. For a BMCC: CVE = E + Ec - Q
where E is the number of electrons derived from the metal, EC is the number of electrons donated by the carbonyl ligands and Q is the charge of the cluster. Similarly: LVEj
Ej
1
+ E ~ M+ Ejc
-
Qj
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
945
where EJ is the number of electrons from metal j , E J is~ the number of electrons obtained from the M,-M bonds, EJc is the number of electrons donated by the MJ-C bonds (computed according to a bond valence partition)[331and Q, the local charge. Note that to obtain an average LVE number of 18 electrons, E J =~ anJM/2, where n,M is the number of MJ-M bonds and a is the formal number of electrons shared in an M-M bond, which is exactly two electrons only when the cluster follows the EAN rule [a = (18N - CVE)/nM;where N is the number of metals and n M the number of M-M bonds, as determined from the connectivity]. Eventually, given that the average L V E number is 18e and that E,M and E,c can be computed for any given stereochemistry, the above equation can be used for defining the formal local charge Q,:
which is the charge a metal would have in order to reach the average L V E count (18). Obviously, there is correlation between LVEj and Q; and a local deficit of valence electrons (LVE; < 18) appears as a negative local charge (if the cluster is neutral). This implies that the tendency towards charge equalization and that to equidistribution of the valence electrons will appear as a unique efect. Formal local charges are not ‘true’ charges, i.e. they do not interact through Coulomb’s law, but rather they are only a mathematical device for taking into account the local valence electron distribution, thus enabling translation of Pauling’s electroneutrality principle into the language of MM by assuming, as a new component of the force field, the following energy expression: Elc
= xj&c (Qj
-
Qav)
where El, is the energy associated with a particular local charge distribution, Kl, is a force constant and Qav is the average charge per metal atom. Elc is a non-negative quantity with a minimum (at zero) for a totally delocalized (Qj = Qav, for all j ) formal local charge.
3.2.4.2 [FeCo(CO)8]The structure of [ F ~ C O ( C O ) S ] -with , [ ~ ~idealized ] C, symmetry, is closely related to that of the lowest energy M2(CO)s isomer (which has D2d symmetry) but does not correspond to a minimum of the PES computed from steric forces alone (Lauher, to force the calculation toward the experimental stereogeometry, had to constrain the Fe-Co-C angle of the semibridging carbonyl group to its experimental value).[’61 As a matter of fact, the sterically favored D2d structure, when the metals contribute a different number of valence electrons, is highly polarized and distortion must occur to quench charge polarization partially (Fig. 2).
946
3 Dynamics and Physical Properties
The addition to the force field of the local charge component, El, makes the absolute minimum of the PES close to the experimental structure. This is, however, not enough to reproduce correctly the solid state structures of C O ~ ( C O ) ~ , [ ~ ~ I [ R U ~ ( C O ) ~ ] ,and ~ - [[~R~U] F ~ ( C O ) ~ ] ~ This - . [ ~ can ~ I . be considered, rather than as a pitfall of the method, as an indication that packing effects and/or the tendency of certain metals to adopt a definite local ligand stereogeometry can be relevant in determining the actual geometry of a given carbonyl cluster.
3.2.4.3
[M2(C0)10]
Most M2(CO)loderivatives (vide infru) are built up from two staggered M(CO)5 fragments and have an idealized D 4 d symmetry. This is the low-energy conformation and the only relevant stereochemical feature to be discussed here is the value of the M-M-C,, angles. The bending of the equatorial CO ligands towards the adjacent metal atom has been rationalized by Bau et al. in terms of long range d 4 n* interactions.[261They have clearly shown that such bending cannot be due to steric repulsion alone. In our computations, because we allow M-C,, to be substantially shorter than M-C,, (as is found experimentally), we partially account (on steric grounds) for the bending but we cannot yet handle the small attractions due to long range d 4 7t* interactions.[381Accordingly, for [Cr2(CO)loI2- we predict M-M-C,, angles (87.0") that are larger than those found experimentally (average value 85.5").[391It is, however, rewarding to analyze CrOs(CO)lo, for which the different number of electrons contributed by the two metal atoms determines an uneven distribution of the formal local charge. In fact, current opinions of CrOs(C0)lo assume the presence of a dative (semipolar) ( C O ) 5 0-+ ~ Cr(C0)s bond which determines the presence of a formal charge of 1 and - 1 on the 0 s and Cr atoms, respectively. The structure of CrOs(C0)lo has not yet been determined; however, Pomeroy and coworkers have extensively studied complexes with unbridged dative bonds between osmium and group 6 elements. They have invariably found that there is inward leaning the of equatorial COs in the donor half of the molecule (M-Os-C,, < 90") whereas such inward leaning of the equatorial carbonyls is not present in the acceptor half of the molecule (Os-M-C,, zz 90").[401This feature is very well modeled by our computations even if we do account for the charge transfer only, and not for the net attraction as a result of long range d -+z * interactions.[261Accordingly, we compute Cr-Os-C,, and Os-Cr-C,, angles of 85.7" and 88.5", respectively. These values are similar to those found in (Me3P)(C0)40sCr(CO)s with average values of 84.1" and 89.2", respectively.[411
+
3.2.4.4 Labeling problems Although the positioning of metals differing by one atomic number is often difficult with single-crystal X-ray techniques, a difference of one electron between two sites
3.2 The Ligand Stereochemistry o j Transition Metal Carbonyl Clusters
947
might be enough to induce significant perturbations of the overall cluster geometry; this could eventually enable discrimination between the two metal atoms to be made on stereochemical grounds only. Indeed, back in 1974 the Fe/Co labels for [FeCo(Co)8]- were assigned on the basis of stereochemical considerations![321We believe that comparison both of the local electron book-keeping and the local stereogeometry of the pertinent atoms can lead, in general, to the correct labeling of metal centers even if they are not distinguishable on the basis of X-ray diffraction data. For instance, when dealing with two d i e r e n t metals with similar local stereogeometry, the presence of a semibridging carbonyl ligand is particularly informative because the electron-poorer metal, to fulfil its ideal EAN and for better spreading of its formal local charge, is expected to be involved in the shortest M-C interactions. On this basis the original rationalization of the metal disorder in [Fe&oRhC(CO) 161 seems to be wrong (see Fig. 3 and its caption).[421 Further insight into this problem might be gained by considering [RuRh3(CO)lz]where, with reference to the parent cluster [Rh4(CO)12], the (formal) substitution of a d9 atom by an isoelectronic (cis)- anion should be (formally) followed by a slight,
P
Figure 3. View of [Fe4CoRhC(CO)l~]. X-ray diffraction clearly shows that the Rh atom is disordered over the M1 and M2 vertices but it cannot be used to determine the location of c o because Co and Fe have similar diffraction powers. Lopatin et al. suggested that the Co atom was disordered over the M3 and M5 vertices.[421This implies that the four vertices (MI, M2, M3, and M5) are equivalent electronically (similar d s / d 9 character) and, given that the closer local minimum on the ‘steric’ PES has two symmetric pc,-CO’s,it would require that the p,-CO’s bridging the M2-M3 and MlLM5 edges were symmetric. They are however, markedly asymmetric [M1-C3 2.29( l), M5-C3 1,94(1), M2-C6 2.19( I), M3-C6 1.99(1) A]. Accordingly, we suggest localization of the Rh/Co disorder over the cis M1/M2 positions (Rh/Fe disorder is not distinguishable from Rh/Co disorder in a conventional X-ray diffraction experiment) which, by locating the two ‘electron-rich’ metal atoms (Co and Rh) on the positions bearing the longer M-(pc,C) interactions, offers a straightforward explanation of the CO’s asymmetry in terms of charge delocalization (the closer local minimum on the ‘steric + local charge’ PES has, indeed, two markedly asymmetric p,-CO’s).
948
3 Dynamics and Physical Properties
Figure 4. (a) A view of the [RuRh3(pU-C0)5(CO)7] anion. Its uneven ligand distribution, in terms of occupied oolumes but not of local valence electrons, is better observed in (b) by the ligand polyhedron, obtained by joining the oxygen atoms closer than 5 A, which does not resemble the icosahedron of the parent [Rh12(CO)12]and shows that the CO ligands, grouping around the apical Ru atom (which carries five COs), leave a hole below the basal face. A similar situation, which clearly tells us that intra-molecular steric interactions are not alone in driving the ligand stereochemistry, and, despite the different ligand stereochemistry, even also occurs for [R~2Rh2(p-C0)5(CO)7]2-[~’~ for [NiRu3(p-CO),(CO),] 2-.[671 ~
but visible, reorganization of the ligand envelope to delocalize charge which would, otherwise, accumulate on the d8 atom. Such (electronically driven) reorganization should primarily involve terminal + semibridging + bridging interconversion of some CO ligands but a ‘second order’ (sterically driven) reorganization of the terminal CO’s is also expected. Indeed, the unusual [RuRh3(C0)12]- stereochemistry (see Fig. 4)[431is related to that of [Rh4(C0)12]by bending two (originally) terminal COs toward the apical metal atom with slight rearrangement of all the other COs (which maintain their original connectivity) and enables unique labeling of the apical atom as Ru. Starting from the [Rh(CO)12]stereochemistry, however, the substitution could have occurred either at a basal or at an apical position. In the former hypothesis, we expect the ,u2-C0s to strengthen their interactions with the d8 atom (at the expense of those with the d 9 atoms), as is experimentally confirmed by the structures of [FeIr3(C0)12]-r441and [ R U I ~ ~ ( C O ) ~ ~ In] the - . [ latter ~ ~ I hypothesis, the apical d8 atom, which would originally lack bridging COs, is expected to ‘attract’ some CO’s from the basal moiety establishing a semibridging interaction with them; this actually occurs in [RuRh3(C0)12]- and in [ O S R ~ ~ ( C O ) ~ ~Significantly, ]-.[~~] the explanation of the different behavior of [MIr3(C0)12]- ( M = Fe, Ru) and [MRh3(CO)12]- ( M = Ru, 0 s ) does not lie in charge equalization but because third-row transition elements prefer the apical position (as far as [Rh4(C0)12]stereochemistry is concerned).r461This effect has not been explicitly parametrized, thus MM computations cannot yet predict a priori the different stereochemical choices of say [RuIr3(CO)1Z]- and [RuRh3(C0)12]-. However, once a particular stereoisomer is selected MM, computations enable most of its stereochemical features to be reproduced and, eventually, the correct atom labels to be as-
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
949
signed. This approach has also been applied to the labeling of the anions [ Ru2Rh2(CO)1212-, [Ru2Rh2(CO)12( pH)]-, and [ Ru2Rh2(CO)12( p3-AuPPh3)]-
3.2.5 Metal digand carbonyl clusters The chemistry of carbonyl clusters has long since passed the point where carbonyls are the only ligands present and only new metal geometries are of primary importance. Over the years, plenty of new mixed-ligand ‘organometallic’ clusters have been synthesized as part of a systematic study of the reactivity of organic fragments toward clusters. An area within this field which has been extensively studied, both for the potential catalytic implications and for a number of theoretical features, is that concerning the interaction of n-ligands with clusters.[481 The presence of a mixed-ligand envelope, in general, and the polyhapticity of nligands, in particular, push LPM to the limit and the technique cannot easily be applied to such systems. MM could, in principle, deal with such complex systems, provided that a suitable force field for n-ligands bonded to vertices, edges, or faces of metal carbonyl clusters is developed. This has been possible within the dummy atoms approach,[491i.e. attributing the connectivity of a group of selected atoms (for instance, an allylic moiety, a cyclopentadienyl group or a cluster face) as a whole to a ‘dummy’ atom, located at the centroid of the group, which will carry all the valence forces acting on the original group. So far we have rationalized the stereochemistry of M3L3(C0)3 ( M = Co, Rh, Ir; L = Cp, Cp*, Ind) clusters and we were also able to deal with face-capping arenes (by representing both Cg and M3 moieties with dummy atoms) and to account for the fluxional behavior of Co3Cp3(Arene) clusters in solution.[501
3.2.5.1 [(Ind)31r3(p2-C0)31 In dealing with the general aspects of the ligand stereochemistry of metal carbonyl clusters, the rationalization of the [( Ind)31r3(p2-C0)3]stereochemistry, because of the anomalous presence (for a third-row transition metal cluster) of three edgebridging rather then of three terminal carbonyls, is particularly significant. This behavior has been related to the presence of an v 5 i q3 distortion of the three indeny1 ligands and naively assigned to ‘electronic factors’ on the base of EH computations on (allyl)3Ir3(C0)3[and (allyl)31r3(C0)3].51However, even in the presence of marked allylic distortions, to infer from (allyl)3Ir3(C0)3 much about (Ind)3Ir3(C0)3 sounds somewhat doubtful because the two derivatives are not isoelectronic. In contrast, MM computations offers a good ‘steric’ reason for the observed stereochemical choice which, essentially, depends on the preference of the (CO)3 moiety (on one side of the metal triangle) to be staggered (e.g. A in Fig. 5)
950
3 Dynamics and Physical Properties
Figure 5. Molecular conformations of the three most significant isomers of Ir3L3(C0)3 ( L = Cp, Ind). Steric energies (in kcalmol-’), relative to the most stable stereoisomer, are reported for Ind derivatives (the numbers in parentheses refer to Cp derivatives).
rather than eclipsed (C in Fig. 5) relative to the (Ind)3 moiety (on the other side of the metal triangle). Similar arguments are responsible for the greater energy (relative to A) of B in Fig. 5, which also lacks bridging COs; moreover, A and B belong to different regions of the PES[”] and cannot easily interconvert once formed, and their (possible) existence depends on the reaction conditions. The bias of third-row transition metals toward non-bridged structures should promote stereoisomers B and C for the iridium derivatives. Interestingly, B has been observed only for Ir3Cp3(CO)3 (and not for the Co and Rh analogs) but C has never been detected (being strongly destabilized on steric grounds compared with A). ‘Electronic’ factors dominate Ir3Cp3(CO)3 stereochemistry but cannot outweigh steric factors in Ir3Ind3(C0)3; hence, according to the data in Fig. 5, the energetic bias because of ‘electronic’ factors should be greater than 1.8 but lower that 9.8 kcalmol-’ and, reasonably, much closer to former than to the latter value.
3.2.5.2 [MM’(C0)4Cpz] The stereochemical behavior of the [MM’(C0)4Cp*]derivatives ( d 8 / d sM = M’ = Fe;r521d 7 / d 9M/M’ = and d6/d” M/M’ = C I - / N ~ [ ~is~particularly ]) relevant to understanding the subtle interplay between steric and electronic factors. Judging from their molecular formula (and the EAN rule), the three species should differ either in the connectivity pattern or in the nature of the M-M’ bond (note the dative bond in the Mn/Rh derivative),” ‘I their reference unbridged structures being: Cp(C0)zFe-Fe(CO)zCp, Cp(CO)3Mn+Rh(CO)Cp and Cp(C0)3Cr-Ni(CO)Cp. Their crystal structures show, however, that the three compounds share the same connectivity pattern: trans-Cp(CO)M(p-C0)2M’(C0)Cp but, on varying the relative number of the electrons contributed by the metals, there is increasing polarization of the two p,-bridging COs towards the electron poor metal. This bridging/
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
95 1
Figure 6. The structures of trun.~-MM’(CO).+Cp2 complexes: (a) d 8 / d 8Fe/Fe ( C l h ) ; (b) d 7 / d 9Mn/ Rh (Cs); (c) d6/d1’ Cr/Ni (Cs). Steric forces favor the most symmetric C2h conformation whereas the spreading of the local charges favors the maximization of the M-CO connectivity with the most electron-demanding metal. The competition between these two effects is eviolent since the metal atoms are not formally neutral (the local charges on M/M’ for the three derivatives in their minimum energy conformations are OjO, -0.19/0.19 and -0.46/0.46, respectively). The actual stereochemistry is a compromise between steric and local charge effects and, according to the agreement between experimental and modeled geometries (1.94(1.92)/1.94(1.92), 1.85(1.87)/2.24(2.17) and 1.88( 1.88)/2.45(2.43) A, respectively, for calculated and (observed) M-Cp,/M‘-Cp, bond distances in the three derivatives). their blending seems to be reasonably well parametrized.
semibridging transformation is perfectly reproduced by use of our MM computations only when the tendency for charge equalization is taken into account. Moreover, when the steric energy of the three minimized structures is partitioned into its components it clearly emerges that steric forces favor the most symmetric C 2 h conformation whereas the spreading of the local charges favors maximizing the M-CO connectivity with the ‘electron poor’ metal. For the diiron derivative both effects favor the C 2 h conformation. When the two metals differ in their electronic requirements, however, the need for a similar local charge forces the asymmetrization of the CO ligands, eventually requiring, for the Cr/Ni derivative, an all-terminal stereoisomer, Cp(C0)3Cr-Ni(C0)Cp. In the latter conformation, however, the need for similar steric distributions about the two metals pushes the geometry toward that with two (semi)bridging COs (Fig. 6).
3.2.6 Intermolecular interactions Because there are many discrepancies between ‘theoretical’ (ideally referring to the gas phase) and experimental stereochemistries (mostly determined in the solid state) of BMCCs, we have developed a strong interest in ascertaining the r61e of the crystal lattice (i. e. of packing interactions) in addressing the actual stereochemistry of such flexible molecules. Accordingly, we implemented in our local version of Allinger’s MM3 program the possibility of optimizing the conformation of a molecule within the field of its crystal 1 a t t i ~ e . IThis ~ ~ ’ has enabled a study of the solid state dynamics offlexible molecules to be performed, including an evaluation of the
952
3 Dynamics and Physical Properties
effects of a (rigid) host crystal lattice on a (flexible) guest molecule, and matching the performances of Kitaigorodsky's atom-atom pairwise potential method[571on rigid molecules.[58s91 Our model of the crystal comprises the ensemble of a reference molecule (RM) and its surrounding molecules (SMs), i.e. by a cluster of molecules partially obeying (being finite) the space group symmetry of the crystal. The steric energy (E,) per RM in the crystal, will be: Es = Eintra
+ Einter
where ,Tintrais the conventional (intramolecular) steric energy of the RM and Einter (known as the potential packing energy, PPE, when dealing with rigid molecules) is the sum of all non-bonded interactions, van der Waals and Coulombic, between the RM and SMs atoms. E, can be minimized essentially by use of two different assumptions, depending on whether or not the crystal lattice is periodically updated on the basis of the actual RM conformation. The former assumption makes it possible to reach the closest minimum on the (crystal) potential energy surface and/or to study correlated motions i.e. plastic deformations of the whole crystal lattice. Alternatively, the latter assumption enables the study of uncorrelated processes occurring (randomly) to individual molecules within the (fixed) crystal lattice field.
3.2.6.1 Solid-state structure of [Cr2(CO)10][2,2,2-Crypt-M]2 (M = Na, K) The staggered conformation of the [Cr2(CO)1ol2-anion has been supported by Xray diffraction analysis of a number of different Very recently, however, the [2,2,2-Crypt-K]+cation has been shown to promote an eclipsed conformation of the [Cr2(CO)loI2-anions in the solid a feature which is even more unusual because the closely related [2,2,2-Crypt-Na]+ cation does not. An accurate computer simulation of these two systems is hampered by the need to have an estimate of the charge distribution in the two salts (dealing with ionic compounds) and of the Cr-Cr stretch terms (given that M-M bond lengths are highly sensitive to their environment).[601Even a rough simulation offers a simple explanation of the observed pseudopolymorphism, however, by showing that the [Cr2(CO) anion has a more favorable conformation in the K+ salt whereas the [2,2,2-Crypt-M]+cation has a lower energy conformation in the Naf salt. More importantly, our computations show that the overall conformation and the distortions of the [Cr2(CO)loI2anion from its idealized symmetry are uniquely determined by the given packing mode; i.e. the pockets left for the anions by the packing (of the cations and the other anions) are suitable for containing only the actual anion conformation with the observed distortion from its idealized symmetry ( D 4 h or D 4 d ) . Indeed, allowing the [ C ~ ~ ( C O ) I anion( O ] ~ - s) to reach the closest energy minimum (keeping all the atoms of the cations, except the hydrogens, in their experimental locations and taking the
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
953
Table 1. Experimental (and computed) Cr-Cr-Ceq bond angles in [Cr2(CO)lo][2,2,2-Crypt-M]2 ( M = Na, K). For M = K, there are two crystallographically different [Cr2(C0)10]~anions (each lying on a center of symmetry) with eclipsed conformations. In contrast, for M = Na, there is a single anion (lying about a twofold axis) with the more usual staggered conformation. The computed values refer to the closest energy minimum reached by thc [Crz(CO),ol2- anion(s) keeping all the atoms of the cations, except the hydrogens, in their experimental locations and taking the reference Cr-Cr bond lengths to be equal to the experimental values. The last line reports the Cr-Cr-Ceq bond angles for the isolated molecules whose energies have been minimized within the constraints of D 4 h and D4d symmetry, respectively (fixing the reference Cr-Cr bond lengths to be equal to the experimental values). Eclipsed (Cr-Cr Crl '-Crl-C,,
c1 c2 c3 c4
84.7 (86.5) 90.0 (91.7) 90.3 (90.7) 84.3 (84.0)
Average Vacuum
87.3 (88.2)
=
3.090 A) Cr2'-Cr2-Ce, 83.0 (83.4) 87.9 (89.1) 92.8 (95.0) 85.7 (86.3) 87.3 (88.4)
D4h
(89.3)
Staggered (Cr-Cr Cr1'-Cr 1-C,,
=
2.976 8)
82.3 (82.9) 85.6 (87.4) 84.9 (87.0) 85.7 (87.5)
84.6 (86.2) (87.8)
D4d
reference Cr-Cr bond lengths as being equal to the experimental values) there were no conformational changes (the barrier for a 90" Cr(C0)S rotation in the solid state exceeded 20 kcalmol-' with a maximum for a 45" rotation in both cases) and the computed spread of Cr-Cr-Ceq and Cr-C-0 bond angles was similar to those measured experimentally (see Table 1 and its caption).
3.2.6.2 Solid-state dynamics of Fe3(C0)12 X-ray diffraction shows that at room temperature Fe3(C0)12 is orientationally disordered about a crystallographic center of symmetry;[611variable-temperature magic-angle-spinning (MAS) 13C NMR shows that it undergoes a dynamic process (in the solid state), having an estimated activation energy of ca. 10 kcalmol-', Whether or not these two observations are related which is frozen out at 180 K.162c1 is still matter of debate which, more generally, concerns the interpretation of the dynamics of Fe3(C0)12 and its derivatives in different media.[631 Johnson achieved a sort of little gestaltic revolution by suggesting that in solution the Fe3 triangle migrates within an essentially undisturbed icosahedron of carbonyl l i g a n d ~ .This ~ ~ ~hypothesis ] was later used by Hanson et al. to rationalize their solid state MAS NMR data;r62a'c1 however, the LPM evolved along a different path and presently it regards fluxionality, in general, as a concerted motion consisting of a low-energy libration of the metal cage within the ligand envelope and (if needed) of
954
3 Dynamics and Physical Properties
0
aeg.
Figure 7. Energy profiles for rotation of the Fe3 triangle about it pseudo threefold axis (I, solid line) and libration of the Fe3 triangle about it pseudo twofold axis (11, crosses, the dashed line is the polynomial fit to 11). Inset: iron atom atomic displacement parametersc6'b1 in Fe3(CO),z projected along the pseudo twofold axis (a) and the pseudo threefold axis (b).
a higher-energy ligand polyhedral interconversion involving some complementary geometry of the ligand envelope. In particular, as far as the solid state behavior of Fe3(C0)12 is concerned, the low-energy process (seen by CP MAS NMR) was correlated with a libration of the Fe3 triangle about its pseudo-twofold axis, as suggested by the anisotropy of the Fe mean-square displacement parameters obtained from X-ray diffraction (inset in Fig. 7); the possibility of 60" jumps about its pseudothreefold axis was discarded because it would imply a 'breathing' motion of the ligand polyhedron, which should be a much higher-energy process in the solid To check the relevance of the two different dynamic processes we have modeled them under the 'external' constraints of a rigid crystalline environment. According to our results (Fig. 7), the libration motion around the pseudo twofold axis (11) is the softer dynamic mode (so soft that large librations about the equilibrium structure are still possible at 100 K) but in-plane 60" jumps of the Fe3 triangle ( I ) are also allowed at room temperature (computed E # ca. 12 kcalmol-'). Remembering that NMR and X-ray diffraction (XRD) afford complementary information on solid-state dynamics (roughly speaking, NMR measures the depth whereas XRD sees the shape at the bottom of a potential well)1651 and that it is not necessarily true that the flattest well is the shallowest (compare for instance I and 11,
3.2 The Liyand Stereochemistry of Transition Metal Curhonyl Clusters
955
in Fig. 7), the anisotropic displacement parameters can be used to trace the softest reaction coordinate (11) but cannot be used to discard I as a concurrent dynamic process. According to LPM, a libration of f 15" about the pseudo twofold axis (11) is enough to average the local molecular conformation, because it corresponds to a D3 H C2 H CzVH C2 H 0 3 isomerization;[h2',f as highlighted by the marked asymmetry of reaction path 11, however, the intermolecular environment cannot be averaged. More importantly, as shown by very recent XRD studies on the effect of temperature on the solid state molecular structure of Fe3(C0)12,[61c,d1 such large libration still persists at 160 and even at 100 K; thus, it cannot be the dynamic process that is frozen out at 180 K.[62d1 In contrast, in-plane 60" jumps of the Fe3 triangle within the ligand envelope effectively average both molecular and intermolecular environments of the CO ligands by creating the pseudo center of symmetry and enabling all the carbonyls to 'see' all the metals on the NMR time-scale. An obvious implication of the rotational freedom of the Fe3 triangle about its pseudo threefold axis (at room temperature) is that the observed solid-state disorder has a dynamic rather than static nature and this has been recently demonstrated by Farr~gia.~~~"]
3.2.7 Conclusions As outlined in the introduction, much experimental evidence suggests that the ligand stereochemistry of metal carbonyl clusters depends on many, cooperative or independent, weak interactions the blending of which is difficult to evaluate a priori. Accordingly, a simple and working theory for the stereochemical behavior of BMCCs has not yet been devised and is unlikely to appear in the future. In comparison with main-group compounds, although metal carbonyl clusters obey electron-counting rules corresponding to the octet rule it will never be possible to treat them with a predictive structural theory equivalent to the valence shell electron-pair repulsion theory. LPM is simple and very attractive as a descriptive tool for cluster fluxionality but gives a distorted picture of the physical factors affecting BMCCs ligand stereochemistry. In contrast, MM is more successful but is not amenable to a paper and pencil approach. MM modeling of the 'soft' PES of metal carbonyl clusters has been made possible by a local connectivity approach which, freeing metal-to-ligand connectivity while tightening cluster-to-ligand connectivity, has enabled the fluxionality of ligands to be taken into account; it has been shown theoretically that intramolecular steric interactions alone fail to account for the ligand stereochemistry of transition metal carbonyl clusters. At present, in our BMCCs model, and perhaps in the real
956
3 Dynamics and Physical Properties
world,[661the overall ligand stereochemistry of the metal cluster is controlled by three different factors: i) valence forces, which are responsible for the cluster-to-ligand bonding (local or otherwise) and for explicitly considered stereochemical preferences ii) van der Waals (and Coulombic) interactions, which promote a ‘uniform’ distribution of ligands about the metal cage and are largely responsible for effects of the (intermolecular) ‘environment’ on ligand stereochemistry; and iii) local charge interactions, which fulfil local electron book-keeping and favor a better spread of the total charge on the cluster. One peculiar aspect of molecular mechanics is an ability to learn from its own errors. Indeed, the failure to predict the actual stereochemistry of some BMCCs has enabled a posteriori recognition of the presence of a specific electronic effects and has been heuristically relevant to our understanding of the factors affecting BMCCs ligand stereochemistry. For instance, we recognize that the bias of third-row transition metals toward non-bridged structures can outweigh the actual energetic balance when a few stereoisomers have similar energies and different connectivities (tentatively, within 3 kcal mol-’ ). Unfortunately, this ‘knowledge’ cannot easily be incorporated into the force field because of the lack of sound thermodynamic observations. Even in the presence of this bias, however, steric energies can be used safely to justify small distortions around a given geometry, to label ‘indistinguishable’ metal atoms in mixed metal clusters, to exclude a particularly crowded stereoisomer, and to check the feasibility of a given dynamic pathway even within the constraints of a selected environment.
References [l] a) D. M. P. Mingos, D. J. Wales, Introduction to Cluster Chemistry, Prentice-Hall, London, 1990; b) G. Gonzales-Moraga, Cluster Chemistry, Springer-Verlag, Berlin, 1993. [2] a) D. M. P. Mingos, Nature, 1972, 236, 99; b) K. Wade, Chem. Britain, 1975, 11, 177; c) K. Wade, Adv. Znorg. Chem. Radiochem., 1976, 18, 1; d) J. W. Lauher, J. Am. Chem. SOC.,1978, 100, 5305; e) G. Ciani, A. Sironi, J. Organomet. Chem., 1980, 197, 233; f ) D. M. P. Mingos, Acc. Chem. Rex, 1984, 17, 311; g) B. K. Teo, Inorg. Chem., 1984,23, 1251; h) B. K. Teo, G. Longoni, F. R. K. Chung, Znorg. Chem., 1984,23, 1257. [3] B. F. G. Johnson, R. E. Benfield, in Transition Metal Clusters, B. F. G. Johnson Ed., Wiley, New York, 1980, p. 471. 141 a) [Ir6(CO)16]L. Garlaschelli, S. Martinengo, P. L. Bellon, F. Demartin, M. Manassero, M. Y. Chiang, c . Wei, R.Bau, J. Am. Chem. Soc., 1984, 106, 6664; b) [Ru6C(CO)16]*-(C2” - [AsPh4]+ salt) B. F. G. Johnson, J. Lewis, s. W. Sankey, K. Wong, M. McPartlin, W. J. H. Nelson, J. Organomet. Chem., 1980,191, C3; (C, - [NEtd]+ salt) - J. S. Bradley, G. B. Ansell, E. W. Hill, J. Organomet. Chem., 1980,184, C33; G.B. Ansell, J. S. Bradley, Acta Cryst., 1980,B36,726; c) [ H ~ R u ~ ( C OM. ) ~McPartlin, ~] W. J. H. Nelson, J. Chem. SOC.Dalton Trans., 1986, 1557
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
957
and refs. therein; d) [Rhll(CO)23j3- A. Fumagalli, S. Martinengo, G. Ciani, A. Sironi, B.T. Heaton, J. Chem. Soc. Dalton Trans., 1988, 163; e) [Fed(C0)13]*- ([Fe(Py)#+ salt) R. J. Doedens, L. F. Dahl, J. Am. Chem. SOC.,1966,88,4847; ([PPN]+ salt) - G. van Buskirk, C. B. Knobler, H. D. Kaesz, Oryanornetallics, 1985, 4, 149; f ) [Ru&(C0)17] D. Braga, F. Grepioni, P. J. Dyson, B. F. G. Johnson, P. Frediani, M. Bianchi, F. Piacenti, J. Chem. Soc. Dalton Trans., 1992, 2565 and refs. therein; g) [FesN(C0)14]- R. Hourihane, T. R. Spalding, G. Ferguson, T. Deeney, P. Zanello, J. Chem. Soc. Dalton Trans., 1993, 43; h) [Cr2(CO)loI2- ([2,2,2Crypt-Na]+ salt) H. Borrmann, A. M. Pirani, G. J. Schrobilgen, Acta. Cryst., 1997, C53, 19 ([2,2,2-Crypt-K]+ salt) H. Borrmann, A. M. Pirani, G. J. Schrobilgen, Acta. Cryst., 1997, C53, 1007 and refs. therein; i) [Pt2M02Cp;(CO),PR3] P. Braunstein, C. de MCric de Bellefon, S.-E. Bouaond, D. Grandjean, J.-F. Halet, J.-Y. Saillard, J. Am. Chem. SOC.1991, 113, 5282; j ) see also [Pt3(pc-PPh2)3Ph(PPh3)2] R. Bender, P. Braunstein, A. Dedieu, P. D. Ellis, B. Huggins, P. Harvey, E. Sappa, A. Tripicchio, Inory. Chem. 1996, 35, 1223; (j). [5] C. P. Honvitz, D. F. Shriver, A h . Oryanomet. Chem., 1984, 23, 219. [6] F. A. Cotton, Proy. Inory. Chem., 1976, 21, 1. [7] R. H. Crabtree, M. Lavin, Inory. Chem., 1986, 25, 805. [8] H. B. Biirgi, J. D. Dunitz, Ace. Chem. Res., 1983, 16, 153. [9] A. G. Orpen, Chem. SOC.Rev., 1993, 191. [lo] C. H. Wei, L. F. Dahl, J. Am. Chem. Soc., 1969, 91, 1351. [I I ] a) M. R. Churchill, M. V. Veidis, J. Chem. Soc. A , 1971, 2170 and 2995; b) F. A. Cotton, J. M. Troup, J. Am. Chem. Soc., 1974, 96, 1233. [12] B. F. G. Johnson, J. Chem. SOC.Chem. Commun., 1976, 211. [I31 a ) R. E. Benfield, B. F. G. Johnson, J. Chem. Soc. Dalton Trans., 1980, 1743; b) B. F. G. Johnson, R. E. Benfield, Topics in Stereochemistry, 1981, 12, 253. [I41 B. F. G. Johnson, Y. V. Roberts, Polyhedron, 1993, 12, 977. [ 151 A. K. Rappt, C. J. Casewit, Molecular Mechanics across Chemistry, University Science Books, Sausalito (CA), 1997. [I61 J. W. Lauher, J. Am. Chem. Soc., 1986, 108, 1521. [I71 A. Sironi, Inory. Chem., 1992, 31, 2467. [IS] a) N. L. Allinger, Y. H . Yuh, J.-H. Lii, J. Am. Chem. Soc., 1989, 111, 8551; b) J.-H. Lii, N. L. Allinger, J. Am. Chem. Soc., 1989, I l l , 8566; c) J.-H. Lii, N. L. Allinger, J. Am. Chem. Soc., 1989, 111, 8576. [I91 a) T. W. Hambley, C. J. Hawkins, J. A. Palmer, M. R. Snow, Aust. J. Chem., 1981, 34, 45; b) D. M. Ferguson, D. J. Raber, J. Comp. Chem., 1990, 11, 1061. [20] A. Sironi, Inory. Chem., 1996, 35, 1725. [21] M. R. Churchill, F. J. Hollander, J. P. Hutchinson, Inory. Chem., 1977, 16, 2655. [22] D. G. Evans, J. Chem. Soc. Chem. Commun., 1993, 675. [23] J. A. Connor, in ‘Transition Metal Clusters’ B. F. G. Johnson, Ed., Wiley, New York, 1980,345. [24] P. Chini, G. Longoni, V. G. Albano, Ada Oryanomet. Chem., 1976, 14, 285. [25] [Fe4(C0)13I2- for instance has been characterized either as the hexapyridineiron( 11) salt or as The stereochemistry of the anions in the two different the PPN+ salt (PPN = [(Ph3P)2N]+).4e salts differs mainly in the degree of semibridging of three equatorial CO ligands in the basal plane. In the hexapyridineiron( 11) salt, where the COs are strongly semibridging, the (bridged) Fe-Fe bonds are 2.50A long while in the PPN+ salt, where the COs are only weakly semibridging, the corresponding Fe-Fe bonds are 2.55 %, long. [26] R. Bau, S. W. Kirtley, T. N. Sorrell, S . Winarko, J. Am. Chem. Soc., 1974, 96, 988. [27] A. Sironi, J. Chem. Soc. Dalton, 1993, 173. [28] See the discussion about the stereochemistry of [Co2Rh4C(C0)13I2- in ref. I . [29] The tendency for a definite stereogeometry about a selected metal atom can be partially restored by considering explicitly the proper C-M-C bending interactions. The geometric preferences expressed by such bending interactions will smoothly add to those of the EPS even if some care is needed because of the strong correlation between bending and 1.3 interactions. In
958
3 Dynamics and Physical Properties
fact, within the local connectivity approach, the shape of the EPS is controlled by a careful use of M-C and M-C-0 interactions while the local geometry on each metal center is controlled by the 1,3 interactions only. [30] Long range d + x* interactions (x) could be taken into account, for instance, by a careful use of charge-charge (Coulombic) interactions. [3I] Cotton recognized the existence of two broad classes of unsymmetrical bridging carbonyl groups: Compensating sets which are distributed in a cyclic fashion over two or more equivalent metal atoms and semibridging carbonyl groups which connect nonequivalent metal atoms. [32] H. B. Chin, M. B. Smith, R. D. Wilson, R. Bau, J. Am. Cliem. Soc., 1974,96, 5285. [33] A. Sironi, Inorg. Chem., 1995,34, 1432. [34] I. D. Brown, in Structure and Bonding in Crystals, M. O’Keeffe, A. Navrotsky, Eds. Academic Press, New York, 1981,2,I; L. Pauling, J. Am. Chem. Soc., 1929,51, 1010. [35] G. G. Summer, H. P. Klug, L. E. Alexander, Acta Cryst., 1964, 17, 732; P. C. Leung, C. Coppens, Acta Cryst., 1983,B39, 535. [36] L.-H. Hsy, N. Bhattacharyya, S . G. Shore, Organometallics, 1985,4, 1483. [37] N. Bhattacharyya, T. J. Coffy, W. Quintana, T. A. Salupo, J. C. Bricker. T. B. Shay, M. Payne, S . G. Shore, Organometallics, 1990,9,2368. [38] Since local charges might originate from long range d -+ x* interactions, we implicitly handle the ‘asymmetric’ part of d -+ x* interactions. However, the concerted bending of a group of CO ligands (a compensating set in Cotton’s terminology) does not afford any charge separation (as in [Crz(C0)10]~-) and is not accounted for by the local charge approach. [39] 1. B. Handy, J. K. Ruff, L. F. Dahl, J. Am. Chem. Soc., 1970,92,7312; E. Hey-Hawkins, H. G. von Schnering, Chem. Ber., 1991,124, 1167. 1401 J. A. Shipley, R. J. Batchelor, F. W. B. Einstein, R. K. Pomeroy, Organometallics, 1991,10, 3620 and references therein. [41] H. B. Davis, F. W. B. Einstein, P. G. Glavina, T. Jones, R. K. Pomeroy, P. Rushman, Organometallics, 1989,8, 1030. [42] V. E. Lopatin, S. P. Gubin, N. M. Mikova, M. TS. Tsybenov, Y . L. Slovokhotov, Y . T. Struchkov, J. Organomet. Chern., 1985,292,275. [43] A. Fumagalli, M. Bianchi, M. C. Malatesta, G. Ciani, M. Moret, A. Sironi, Inorg. Cliem. 1998,37,1324. [44] R. Della Pergola, L. Garlaschelli, F. Demartin, M. Manassero, N. Masciocchi, M. Sansoni, J. Cliem. Soc., Dalton. Trans., 1990, 127. [45] A. Fumagalli, F. Demartin, A. Sironi, J. Organometal. Cliem., 1985,279,C33. [46] A. Fumagalli, S. Martinengo, G. Ciani, M. Moret, A. Sironi, Inorg. Cliem., 1992,31, 2900. [47] A. Fumagalli, D. Italia, M. C. Malatesta, G. Ciani, M. Moret, A. Sironi, Inorg. Cliem., 1996, 35, 1765. (481 B. F. G. Johnson, J. Organornet. Cliem., 1994,475,31. [49] T.N. Doman, C. R. Landis, B. Bosnich, J. Am. Chem. Soc., 1992,111, 7264. [50] P. Mercandelli, A. Sironi, J. Am. Chem. Soc., 1996,118, 11548. (511 D. Braga, F. Grepioni, H. Wadepohl, S. Gebert, M. J. Calhorda, L. F. Veiros, Organometallics, 1995,14,5350. [52] A. Mitschler, B. Rees, M. S . Lehmann, J. Am. Chem. Sac., 1978,100, 3390. [53] M. L. Aldridge, M. Green, J. A. K. Howard, G. N. Pain, S. J. Porter, F. G. A. Stone, P. Woodward, J. Chem. Soc. Dalton Trans., 1982,1333. [54] T. Madach, K. Fischer, H. Vahrenkamp, Chem. Ber., 1980,113,3235. [55] R. D. Barr, T. B. Marder, A. G. Orpen, D. Williams, J. Cliem. Soc. Chem. Commun., 1984, 112. (561 P. Mercandelli, M. Moret, A. Sironi, Inorg. Cliern. 1998,37,2563. [57] A. J. Pertsin, A. I. Kitaigorodsky, The Atom-Atom Potential Method, Springer-Verlag, Berlin, 1987. [ 5 8 ] A. Gavezzotti, M. Simonetta, Chem. Rev., 1982,82,1.
3.2 The Ligand Stereochemistry of Transition Metal Carbonyl Clusters
959
[59] D. Braga, Chem. Rev., 1992, 92, 633. [60] A. Martin, A. G. Orpen, J. Am. Chem. Soc., 1996, 118, 1464. [61] a) C. H. Wei, L. F. Dahl, J. Am. Chem. Soc., 1969, 91, 1351; b) F. A. Cotton, J. M. Troup, J. Am. Chem. SOC.,1974, 96,4155; c) D. Braga, L. J. Farrugia, F. Grepioni, B. F. G. Johnson, J. Oryanomet. Chem., 1994, 464, C39; d) D. Braga, L. J. Farrugia, F. Grepioni, B. F. G. Johnson, J. Chem. Sac. Dalton. Trans., 1994, 2911. [62] a ) H. Dorn, B. E. Hanson, E. Motell, Inory. Clzim. Acta, 1981, 54, L71; b) J. W. Gleeson, R. W. Vaughan, J. Chem. Phys., 1983, 78, 5384; c) B. E. Hanson, E. C. Lizic, J. T. Petty, G. A. Iannacone, Inory. Chem., 1986, 25, 4062; d) T. H. Walter, L. Reven, E. Oldfield, J. Phys. Chem., 1989,93, 1320; e) C. E. Anson, R. E. Benfield, A. W. Bott, B. F. G. Johnson, D. Braga, E. A. Marselia, J. Chem. SOC.Chem. Commun., 1988, 889; f ) D. Braga, C. E. Anson, A. Bott, B. F. G. Johnson, E. A. Marselia, J. Chem. SOC.Dalton. Trans., 1990, 3517. [63] a) B. E. Mann, J. Chem. Soc., Dalton. Trans., 1997, 1457; b) B. F. G. Johnson, J. Chem. SOC., Dalton. Trans., 1997, 1473; c ) L. J. Farrugia, J. Chem. Soc., Dalton. Trans., 1997, 1783. [64] B. F. G. Johnson, J. Chem. SOC.Chem. Commun., 1976, 703. [65] a) R . E. Benfield, D. Braga, B. F. G. Johnson, Polyhedron, 1988, 7, 2549; b) D. Braga, Clzem. Rev. 1992, 92, 633. [66] ‘The hope of gaining insight into the nature of intramolecular forces from force field calculations is however hampered by the strong correlation between parameters. Therefore, we must not look at a M M calculation and ask ‘what interactions are really occurring in the molecule’; rather the question must be ‘what interactions are really occurring in our model of the molecule” Burkert, U.; Allinger, N. L. ‘Molecular Mechanics’; American Chemical Society: Washington, DC, 1982; ACS Monogr. No. f77. [67] E. Brivio, A. Ceriotti, R. Della Pergola, L. Garlaschelli, M. Manassero, M. Sansoni, J. Cluster Sc., 1995, 6, 271.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.3 Multinuclear NMR Studies on Homo- and Heteromet a l k Rhodium Clusters Containing 6 or More Metal Atoms Brian T. Heaton, Jonathan A. Iggo, Ivan S. Podkorytov, Daniel J. SmawJield, and Sergey P. Tunik
3.3.1 Introduction The variety and size of homo- and heterometallic transition metal carbonyl clusters have increased enormously over the last 30 years. Of all the transition metals, '03Rh is unique in being 100% abundant with I = 1/2 and it is timely to review the NMR developments and studies on hexa- and higher-nuclearity Rh-containing clusters. These have enabled: i) structures in solution to be established - very useful when X-ray-quality crystals cannot be obtained or the cluster is stable at low temperatures only; ii) detailed pathways of ligand and metal migrations to be established; iii) H-site occupancies in Rh clusters to be established; and iv) information on the bonding of interstitial atoms, e.g. C, N etc., within various metal frameworks to be obtained. This review endeavors to cover all solution NMR studies performed until December 1997 on homo- and heterometallic rhodium-containing clusters with hexa- and higher nuclearity.
3.3.2 NMR measurements
'
Early measurements relied on direct H, I3C, and 31Pvariable-temperature measurements, where appropriate, to establish structures; peak coalescence provided information on exchanging groups. More unambiguous assignments were later made possible by the construction of probes enabling 'H-{ lo3Rh}INDOR,"' '3C-{X}
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
961
(X =Io3Rh,[’] 31 Pr3])and direct 103Rh[21 NMR measurements which enabled connectivities to be established. Because of the sensitivity of 6( lo3Rh)to variations in solvent, cation, concentration of any compound chosen as an external reference, all Io3Rh chemical shifts are referenced to 3.16 MHz (= 0 ppm) at such a magnetic field that the protons of SiMe4 in CHC13 resonate at exactly 100 MHz; highfrequency shifts are positive. 13Cmeasurements are usually performed on samples containing Cr(acac)3 (ca. 1 mg) as relaxing agent, to overcome long CO TI values, and samples are usually ca. 30Y0 enriched with 13C0. This is optimum for enhancement of signal-to-noise (S/N) without complicating the spectra by the introduction of “J( 13C-’3C’)( n = 2, 3). Nevertheless, other long-range couplings to CO can be useful in assisting with assignments and it has recently been shown that there is a strong stereochemical dependence of 3J( 13C-31P)in substituted Rh6 clusters.[41 In the 1970s direct rhodium NMR measurements required the use of ca. 1 g cluster in 15 mm NMR tubes in high field (360 MHz!) spectrometers; it is now much more convenient to obtain lo3RhNMR data using 2D methods. Using l3C{Io3Rh} inverse-detected heteronuclear multiple quantum coherence ( HMQC)[’] it is possible to obtain high resolution rhodium data in a few hours. Furthermore, coupling to other spin-active nuclei is retained which further assists with the assignment of the spectra. The 13C{103Rh}HMQC spectrum of the terminal region of [Rh6(CO),,(P(pFC6H4)3}],shown in Fig. 1, enables easy assignment of both the Io3Rhand 13C NMR spectra; coupling to the phosphorus ligand is evident in the Io3Rh dimension. Interpretation of the HMQC spectra of phosphine-substituted derivatives in which P-P’ coupling occurs is slightly complicated by the appearance
Figure 1. 13C-{ Io3Rh} HMQC spectrum of [Rhs(C0)15{p(P-&H4)3 } ] (see Fig. 5a for labelling scheme).
T
3
9
@ e l
, I---. _
PPm
180
962
3 Dynamics and Physical Properties
of P-P' coupling in the rhodium dimension, although it is usually obvious when this occurs. A more serious difficulty is that the detector nucleus might now couple to several I spins, e.g. the 13Cnucleus of a p3-C0 will be coupled to three lo3Rhspins. Multiple quantum transitions can occur in which the assembly of metal spins acts as a unit. Multiple quantum transitions can displace the coherence from the true chemical shift, generate additional coherences and modulate the intensity of coherence as a function of the mixing time. In effect, this means coherences may or may not be observed and, if observed, the chemical shift of the insensitive nucleus might or might not be in the 'right place'. This is a particular problem with p3-COs and can result in total absence of correlations with these ligands unless care is taken in setting up the experiment. The values of 6(CO) in solution cover quite a wide range and depend on: i) site occupancy with 6(p3-CO)> 6(p2-CO) > b(COtemina1); ii) the position of the metal in the periodic table with the chemical shifts for COs associated with 1st row > 2nd row > 3rd row (e.g. for [M(C0)4]-6 = 214.0, 206.3, and 182 ppm, respectively, for M = C O , [ ~Rh,[71 ] and Irr8])and rationalized recently using density functional theory;['] iii) charge delocalization with increased negative charge producing a low field shift.['.lol The variation in 'J(l3C0-lo3Rh) for different types of CO is p 3 CO < p2-CO < COteminaland there are very few examples of 2J(13C-103Rh). When solid-state structures contain asymmetric p2- or p,-COs and the static structure can be realized in solution, increasing bond length is reflected by decreasing values of 'J(13CO-'03Rh)for the bridging COs. For COs undergoing fast intra-exchange, the observed value of d(C0) is the sum of the weighted mean of the individual chemical shifts found for the static structure and the time-averaged value of ' J ( l3C0-lo3Rh)' is ca. 80 Hz/n where n is the number of atoms over which the CO-migration occurs and 80 Hz is the average value for a terminal CO. Traditionally, variable temperature NMR studies have been used to establish the fluxional processes in carbonyl compounds. Such studies give a macroscopic (i.e. whole molecule) view of the dynamic process and for simple, low nuclearity systems it is possible to establish more or less precisely the mechanism of fluxionality. For higher nuclearity systems, several independent or interdependent movements can occur with similar activation energies and, from variable-temperature measurements, it can be difficult to determine unambiguously the ligand migrations involved in each process. 2D EXSY NMR spectroscopy, in principle, enables easy access to the microscopic detail (i. e. the individual ligand movements) of the fluxional process, assuming a firm assignment of the 13C0NMR resonances is available. The 13C EXSY spectrum of [Rh6(CO)lS(NCMe)] shown in Fig. 2 contains crosspeaks which clearly indicate exchange between the face-bridging CO ( C 3 0 )and the terminal COs ( C 7 0 and C'O) localized on the RhDRhcRhcj-face (see Fig. 5a for
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
963
3
Figure 2. I3C EXSY spectrum of[Rhs(CO)ls(NCMe)]. * = [Rh6(C0)16]present as impurity (see Fig. 5a for labelling scheme).
v 236 2 3 2 2 2 8 224 220 (P P m )
labeling scheme); the relative intensities of the cross peaks contain, moreover, rate information and show that separate exchanges involving C 3 0 with C 9 0 and C 3 0 with both C 7 0 and C 8 0 are occurring rather than a concerted process involving all three terminal carbonyls. The range of chemical shifts is much wider for I7O than for 13Cand, as a result, provides a better opportunity for obtaining instantaneous structures for fluxional clusters than do I3C NMR measurements. Although C170 has been used for other clusters,["] there are no published reports of work on analogous rhodiumcontaining clusters.
3.3.3 Hexanuclear homo- and heterometallic clusters containing Rh 3.3.3.1 [Rh6(CO),6] and related homo- and heterometallic derivatives (see Table 1) The structure of [Rh6(C0)16]['~] is shown in Fig. 3 and, although it is quite insoluble, I3C NMR spectra could be obtained at +70 0C[121which showed that the structure is retained in solution.
964
3 Dynamics and Physical Properties
Table 1. References to solution NMR measurements on, and X-ray structures of, [Rh6(C0)16] and related homo- and heterometallic derivatives. Cluster
NMR. 1 3 c
12 12 12 14 15 15 15 17 18 19
X-ray 31 P
Io3Rh
195Pt
-
19
17 18 20
Figure 3. The structure of [Rh6(P3-C0)4(C0)12]( M = Rh) and [MRh5(p3-CO)4(C0)1$ ( M = Fe, Ru, 0 s ) ; the carbonyl and metal labelling refer to the symmetry and assignments of the l3C/Io3Rh NMR spectra when M # Rh. Structures have been constructed from diffraction coordinates except in some cases when the structure is “schematically” represented.
=H
= Rh
= Fe, Ru, Co, Ni, Pt, Au = CO, B, C, N, P, S. =
pco
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
965
Progressive reduction of [Rh6(CO)16]gives [Rh6(C0)15I2- and [Rh6(C0)14I4-. There is a progressive linear increase in the weighted average of 6( 13CO)for these clusters with increasing average negative charge per carbonyl group.['21 The I3C NMR spectra of both Rh6(C0)16 and [Rh6(C0)14I4-are consistent with their solidstate structures but the I3C NMR spectrum of [Rh6(C0)15l2- consists of a symmetrical septet (6(I3CO) 209.2 ppm, 1J(13CO-*03Rh) 13.9 Hz) even at -70 "C, because all the COs become equivalent as a result of rapid migration around the Rh6-0ctahedron."~~The structure of [Rh6(cO)1,l2- has not been determined but, in view of the IR spectral similarity, it is assumed to be isostructural with [co6(co)15]2-,[211 and, compared with the other isoelectronic Rh6 clusters, this small structural change in the CO distribution results in a profound effect on the activation energy (Ea) for CO fluxionality. As for CO inter-exchange processes, it is difficult to predict the magnitude of Ea for CO fluxional processes but detailed pathways for these intra-exchange processes can often be obtained. The analogous cluster, [C03Rh3(CO)16],[141 which was originally formulated as [ C O ~ R ~ ~ ( C Ohas ) ~ been ~ ] , [prepared ~~I by heating [Co2Rh2(C0)12](see Eq. 1) and shown by 13C NMR to consist of all three possible isomers (mer,fac-A,fac-B, see Fig. 4) in the ratio 3 : 1 : 1 respectively. Detailed assignment of the 13Cspectrum was not undertaken these assignments were based on the progressive shift to low field of the face-bridging carbonyl resonances with increasing Co-bonding. ~
+
~ [ C O ~ R ~ ~ ( -+ C 2[Co3Rh3(CO)i6] O)I~] 4C0
(1)
All members of the isoelectronic triad [MRh5(C0)16]- ( M = Fe,['59'61Ru,['~I 0 ~ "are ~ known ~ ) and have structures related to [Rh6(CO)16].The metal sites have variable mixed metal occupancy, however, which makes direct comparison of 'J(13CO-103Rh)with d( Rh-CO) difficult. Nevertheless, Table 2 shows that there is
Figure 4. Schematic representation of the isomers of [CojRhj(p3-C0)4(C0)12]: (a)fac-A, (b)fac-B, (c) rner.
180.1 (70) 180.1 (70) 180.1 (70) 180.1 (70) 231.5 (24) 231.5 (24) -426 -426
211.0 186.8 (70.2) 186.1 (67.1) 184.7 (70.2) 256.8 ( 1 2.2) 233.5 (25.9, 21.4) -400.6 -501.7
[FeRhdCO)161-
-
-
-
-
J ( 13C-IssPt). 7 J ( 'O'Rh '95 Pt j .
1
[OsRhj(CO)I b ] 177.7 188.2 (67.9) 182.6 (71.4) 184.8 (69.7) 248.7 ( C L I 9.5) 239.2 (27.6)
~~
194.8 186.3 (68) 184.7 (71) 183.8 (70) 255.6 (14.5) 237.8 (30,25)
[RuRhj(CO) I h ]
' Values of 6( 13CO)and 6( Io3Rh)with ' J ( liC-l"'Rh) in parentheses.
C"O CbO C'O CdO C'O C'O RhA RhB
[Rh6(CO)i6] 212.4 189.7 (73.2) 189.7 (73.2) 212.4 260.8 (19.8) 260.8 (19.8) -408.9 -408.9
trarzs-[Fe2Rh4(CO)16]
'-
192.5 (2629") 189.8 (71.4) 188.4 (74.2) 183.3 (69.5) 235.8 ((n14, 791') 237.7 (32.8. 24.7) -243 (24') -495 (72')
[ PtRhi(C0)151-
Table 2. Comparative "C and "'Rh NMR data* for the structurally related clusters, [Rh6(CO)lh].[MRhj(CO),6]- ( M = Fe. Ru, 0 s ) . ~ r r m s - [ F e z R h ~ ( C O ).land ~ ] ~[PtRhS(CO)lj]-. For labelling scheme. see Fig. 3.
s, r,
-%
-.
:
'b
=:
2z
y-._
2
2
4
-.5
3
2-
b '-c
cy
m
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
967
a significant variation in ' J ( I3CO- lo3Rh)for the face-bridging COs, reflecting the different charge distributions in this closely related series. All these clusters are static at room temperature. The preparation of [Fe2Rh(C0)16l2- is rather complicated and gives a mixture of trans and cis isomers in the ratio 3 : 1; both the 13C and lo3Rh spectra are completely in accord with trans geometry but, because of the low abundance, the spectra of the cis isomer have not been assigned. Both are static at room temperature. Related to the above clusters is [PtRh~(C0)15]-['~] which has the structure shown in Fig. 3 except that the M(C0)z group is replaced by a Pt(C0) group with the CO vector being aligned with the center of the PtRh, octahedron. At low temperatures the NMR spectra are consistent with the static structure but this cluster is fluxional at higher temperatures and the exact pathway remains to be elucidated. [Fe3Rh3(CO)17l3- adopts an unusual structure which consists essentially of an Fe2Rh3 trigonal bipyramid with the apical positions occupied by Fe and Rh and the unique apical Rh is attached to a dangling Fe(CO)3 group via two bridging C O S . [ ~ ~ I It is surprising that this cluster does not rearrange to the octahedral analog, [Fe3Rh3(p3-C0)4(CO),~l3-, but there is no preparative/spectroscopic evidence for this and no NMR data have been reported.
3.3.3.2 Substituted derivatives of [R h6(c0)161 Monodentate ligand derivatives [Rh6(CO)l6_,Ljn- (n = 0, L = 2-electron donor, x = 1, 2, 4; n = 1, L = H, halide or pseudo halide, x = 1). The pattern of substitution products containing monodentate ligands is shown in Fig. 5; data presently available suggest this pattern minimizes steric interactions and is relatively independent of cone angle. There are no reports of examples containing more than four neutral 2-electron P-donors and only one example of a Rh6 cluster containing two anionic ligands; no examples of substituted heterometallic analogs have so far been reported. Monosubstituted derivatives
The structure of these derivatives is shown in Fig. 5a; Table 3 summarizes the NMR measurements which have so far been reported. All the data in solution are consistent with the solid-state structure being retained and all are static except for [Rh,j(C0)15L] ( L = monodentate triarylphosphine). For L = PPh3[41it has been shown that there is exchange of the unique terminal C 4 0 with the adjacent p3-COs (C'O and C"0) with a concomitant oscillation of PPh3 on RhA (Fig. 6) and other arylphosphines (e.g. P(pXC,jH4)3 X = CF3, F, C1, OMe) induce a similar CO fluxionality around the same pathway. Other ligands L, however, (e.g. NCMe, P(OPh)3, PBu"3) induce a dzfleerent process as described above in the EXSY discussion. This other pathway involves localized exchanges around the RhcRhcRhD
Figure 5. The structure of [Rhh(jij-CO)d(CO)I? ,L,]”- (x = 1 . . .4): (a) s = I , n = 0. L = neutral 2-electron donor; n = I , L = H , halide or psrudohalide; (b) s = 2, n = 0, L = neutral 2-clcclron donor; (c) z = 4. = 0, L = neutral 2-clcctron donor.
face (see Fig. 5a for numbering scheme) and is of slightly higher energy than that found for arylphosphines. These different pathways do not seem to be related to electronic or steric differences between the ligands. The only distinguishing feature for these two groups of ligands is the presence or absence of aryl substituents on phosphorus. The possible existence of an ‘aryl effect’, that cannot be attributed to
Table 3. References to N M R data and X-ray structural information for [Rhh(CO)ISL]’’- ( P I L = neutral 2-electron donor ligand; P I = I L = H. halide or pseudo halide).
=
0.
~
11
0 0 0 0 0 0 0 I 1 1 1 1
L
X-ray
NMR
4 4 4 4 4 24 4 26 21 28 28 28
~
~
28 28 28
29 ~
~
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
969
8
r-
id
970
3 Dynamics and Physical Properties
the usual Tolman steric and electronic parameters, has been proposed on the basis of statistical analysis of data for a range of iron and chromium phosphine complexe~.[~'] It might be that the different CO fluxional pathways observed in these substituted derivatives of Rhs(C0)16 are also a result of the 'aryl effect' and are clearly of interest in connection with the controversial debate on the mechanism of CO fluxionality in smaller c l ~ s t e r s . ' ~ ~ ~ ~ ~ ] Mono-anionic ligand derivatives [Rh6(C0)15L]- ( L = H, halide or pseudo halide) also have the structure shown in Fig. 5a; NMR measurements (Table 3) show that their structure in solution is consistent with their static structure and, apart from [HRhs(C0)15]-, vide infro., are non-fluxional at room temperature. [HRh6(C0)15]- is only stable at low temperatures and was one of the first clusters to be completely characterized by multinuclear NMR studies.[261The preparative r o ~ t e s [ ~are ~ , shown ~ ~ I in Eq. (2) and it should be noted that the structure of [HRh6(C0)15]- is different from that of the cobalt analog which contains an interstitial h ~ d r i d e ; [at ~ ~room ] temperature loss of H2 occurs with subsequent dimerization (Eq. 2).
Bis-substituted derivatives The stereochemically most favorable structure for [Rh6(CO)14L2]is shown in Fig. 5b and, when L = P(OPh)3, is the structure adopted in the solid state[361and in soluThere are, however, four other isomers which are theoretically possible and there is chromatographic and spectroscopic evidence for two of these isomers when L = P(OPh)3. 31PCOSY measurements and simulations[361suggest that these are the next two least sterically hindered isomers. Both of these isomers, on heating, are transformed into the thermodynamically more stable isomer shown in Fig. 5b. When L = py or NCMe, separation into individual isomers was impossible because of their rapid intercon~ersion.[~~~
Tris-substituted derivatives No structural/NMR data have been reported for the species [Rh6(C0)13L3].This is probably because of the number of stereochemically similar isomers and the problems associated with their separation.
Tetrakis-substituted derivatives The structure which minimizes steric interactions for [Rh6(C0)1&4] is shown in Fig. 5c and is found by X-ray analysis when L = P(OPh)3.[371Multinuclear ( 13C, 31P,'03Rh) NMR data show that the solid-state structure is retained in solution at room
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
971
Figure 7. The structure of [ R ~ & Q - C O ) ~ ( C O ) ~ ~ - ~ . ( L(xL= ) , ]1-3): (a) x = 1, LL = dppm; ( b ) x = 2; ( c ) x= 3, LL = dppm; ( d ) x = 1, LL = cis-CHz=CMe-CMe=CH2.
It has been suggested that steric effects are responsible for not allowing [Rh6(CO)12L4]to be retained for the bulkiest ligands, and for there being no further reaction with P(OPh)3 to give higher-substituted derivatives.
Bidentate ligand derivatives [Rh,j(C0)16_, (LL),] (LL = bidentate, 4-electron donor; x = 1-3) Most work in this area has been performed with bidentate phosphines, Ph2P(CH2),PPh2, and the most stereochemically stable isomers for increasing values of x are shown in Fig. 7a-c. Table 4 summarizes currently available NMR measurements and X-ray crystallographic data together with data on [Rh6(C0)14(C~S-CH~=CM~-CM~=CH and ~ ) ][Rh6(C0)~4(1,3-cyclohe~adiene)],[~~~ [~~] which both adopt the structure shown in Fig. 7d with the diene replacing two COs on the same metal. All the clusters shown in Table 4 are stereochemically rigid. For structures containing chelating diphosphines it is, however, necessary, for accurate spectral simulations, to allow for second-order effects and introduce longer-range c o ~ p l i n g s . 8l[ ~ , ~
912
3 Dynamics and Physical Properties
Table 4. References to NMR data and X-ray structural information for [Rh6(C0)16-2r(LL),] (x = 1-3; LL = neutral, 4 electron donor). X
LL
NMR 'H
1 1 1 1 3
dPPm dPPe cis-CHz=CMe-CMe=CH2 1,3-~yclohexadiene dPPm
-
-
38 39 -
X-ray 13c
3lP
4 4 4
4 4
Io3Rh 4 -
-
-
-
-
-
28
28
28
25 24 38 39 40
3.3.3.3 Hexanuclear, homo- and heterometallic rhodiumcontaining clusters with an interstitial atom (B, C, N) Since the first reported example of such a species, [RhgC(C0)15]2-,[71the encapsulation of a p-block element within a rhodium-containing polyhedron is now quite a familiar phenomenon. For rhodium-containing clusters there are several examples of interstitial p-block elements (B, C, N ) within an octahedral and/or trigonal prismatic skeleton and these are often interconvertable, sometimes quantitatively (Eq. 3).[411For hexanuclear clusters, however, only Osg polyhedra have so far been found to accommodate the more sterically demanding p h o s p h o r ~ s . [ ~ ~ , ~ ~ ~ 80 "C(-2CO)
[RhgC(CO)1512-
25"C(+2CO)
[RhgC(co)1312-
(3)
Hexanuclear Rh-containing clusters with an interstitial B The hexanuclear rhodium-containing clusters with an interstitial B all have 86 electrons giving rise to an octahedral metal framework (see Table 5 ) and are related to their homometallic counterpart, [Fegc(C0)1g]2-.[441 It should, however, be noted that although there are no reports of trigonal prismatic rhodium-containing hexanuclear clusters with an interstitial B, the 90-electron trigonal prismatic anion [ H ~ R u ~ B ( C O ) Iand ~ ] - the octahedral anion [RugB(C0)17]- have both been obtained from the same r e a c t i ~ n . [ ~The ~ , ~boron ~] atom can, however, occupy a trigonal prismatic site in a rhodium-containing polyhedron, as found for [RugRh3B2(CO)23]- (see Section 3.3.6).[481 X-ray analysis shows that trans-[M4Rh2B(CO)lg]- (M = Fe,[471Ru[~'])adopts the structure shown in Fig. 8; all the carbonyls are terminal, three COs are associated with each Fe and the two Rh(C0)2 groups are orthogonal to each other. As a result of this symmetrical environment, the IlB resonance is well resolved - at low
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
973
Table 5. Octahedral, hexanuclear, rhodium-containing clusters with an interstitial B. Cluster truns-[Fe4RhzB(CO),6]cis-[Fe4RhzB(CO)161trans-[Ru4RhlB( CO)161truns-[HRwRhzB(CO)161 mer-[HzFe3Rh3 B(CO)151 a
'
6( H)" -
-
-17.8 s -
6( I ' B)db
X-ray
+211.1 t (25.8)[471
I471
+205 t (23.3)14'1 +193.4 t (26)I4*1 +191.4 t (26);481 +199.7 q (24.1)[491
-
1481 -
1491
s = singlet, t = triplet, q = quartet.
"B-Io3Rh) ( in Hz and the chemical shift reference is Figures in parentheses are values of '.I BF3OEt2.
field because of the interstitial site occupancy and the well-resolved coupling to lo3Rh enables the number of Rh atoms to be deduced. There are, however, intriguing differences between this closely related pair of clusters. i) There is evidence for the cis isomer when M = Fe but not when M = Ru; the cis isomer is less thermodynamically stable than the trans isomer and kinetic studies show that the transformation isfirst order in CO. 31PNMR measurements show that addition of PMe2Ph to cis-[Fe4RhzB(CO)16]- also promotes cis -+ trans isomerization and results in addition to both Fe and Rh; the isomerization process is more rapid than ligand substitution by p M e ~ P h . [These ~ ~ ] results suggest that cis i trans isomerization involves an associative mechanism via a looser 88-electron, more open metal framework rather than via a trigonal
Figure 8. The structure of [M4RhzB(C0)16] ( M = Fe, Ru).
974
3 Dynamics and Physical Properties
a
ii) NMR measurements show that protonation of trans-[Fe4Rh2B(CO)l6]~results in gross rearrangement and formation of mer-[H~Fe3Rh3B(CO) 15][491 whereas both the anion and protonated neutral cluster, trans-[HxR~4Rh2B(C0)16](1 (x = 0, 1) are obtained from the same reaction pot.r481The high-field singlet for [HRu4Rh~B(C0)16] suggests Ru-H-Ru occupancy but disorder and lack of sufficient spectroscopic data prevented the complete structural assignment of m u - [H2Fe3Rh3B(CO)15]. -
F
Hexanuclear Rh-containing clusters with an interstitial C The Rh6-trigonal prism and octahedron can both encapsulate C, as exemplified by the formulations shown in Eq. (3) and their structures (see Fig. 9).[41,501 The preparative route to [Rh&(C0)15l2- is shown in Eq. (4); NMR studies with isotopic labeling of the halocarbon were the first to prove unambiguously the source of the carbide.l7]The remarkable transformation shown in Eq. (4) occurs almost quantitatively. The I3C and lo3Rh NMR spectra of [Rh6C(C0)15]2-[71are completely in accord with the solid-state structure whereas [Rh&(C0)13I2- is fluxional even at low temperature.[511Thus, seven COs are involved in exchange around the Rh4 square-plane with the T-shaped arrangement of the two apical transRh(2,4)(CO)3-groups7which involve a terminal and two p-COs (see Fig. 9b), remaining static. This cyclical CO movement regenerates the original structure and is of low energy. It was, in fact, possible to show by detailed X-ray analysis that there is an extensive vibrational motion of the carbonyl ligands around the same R h square plane in the solid state and this behavior in the solid state mirrors exactly the migration found in 6[Rh(C0)4]-
+ 13CC14+ [Rh6'3C(C0)15]2-+ 4C1- + 9CO
(4)
It is now convenient to discuss separately the NMR studies of derivatives
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
975
Table 6. References to NMR and X-ray studies on [Rh6(CO)l5CI2- and related derivatives. Cluster
X-ray
NMR
‘H -
-
-
-
26
M = AU M = Ag
I 53 26 54
-
-
-
-
-
31P
1 3 c
55 55 55 55 55 55
of the trigonal prismatic cluster [Rh6C(CO)!512[Rh6C(CO)I3l2-.
-
53 -
54 54 -
-
-
~
Io3Rh 26 53 26 54 54 55 55 55 55 55 55
50 53 -
-
55 -
-
-
and the octahedral cluster
Derivatives of [Rh6C(CO)1512NMR studies and X-ray structural studies of these derivatives are summarized in Table 6. Progressive carbonyl substitution by PPh3 occurs in [Rh&(CO) l5I2- at room temperature under CO but only the mono-substituted derivative has been spectroscopically/structurally characteri~ed.[’~] [Rh6C(CO) 1 512- undergoes electrophilic attack by a variety of metal fragments and by H+; the adducts consist of selective capping of the trigonal face to give either the mono- or bis-substituted adduct; those studied by NMR are shown in Table 6. Addition of [Rh(CO)2(MeCN)2If, however, leads to the formation of a complicated series of reactions, which are highly dependent on reaction conditions, giving rise to various higher nuclearity clusters, vide infl.a.[561 It is worth mentioning the adducts formed on progressive addition of Ag+ to [Rh6C(C0)15l2-; NMR studies showed oligomers of increasing molecular weight are formed as a result of Ag+ becoming sandwiched and bridging trigonal faces of [Rh6C(CO)l5I2- units, followed by progressive breakdown of these oligomers through end-capping of the trigonal faces, resulting in the eventual formation of [{Rh6C(CO)15)Ag2].r’’1 It is, of course, not possible to determine from NMR whether the {Rh6C(CO)l5} units are eclipsed or staggered in the oligomers, but it was possible to obtain an X-ray structure of [{Rh6C(C0)15)2Ag]~which showed that they are ~ t a g g e r e d51. ~ ~
916
3 Dynamics and Physical Properties
The formation of [HRh&(C0)15]- is shown in Eq. ( 5 ) . It is only stable at low temperature and multinuclear NMR provides unambiguous evidence of H+ having added to the trigonal face. Warming to room temperature results in loss of H2 and dimerization to give [Rh12C2(C0)24l2-or [Rh12C2(C0)23l4- (see Section 3.3.9); NMR measurements indicate the conversion to be almost quantitative.
Derivatives of [Rh6C(CO) 1312NMR studies performed on [Rh6C(C0)13I2- and its derivatives are shown in Table 7. Introduction of a mono- or bidentate phosphine into [Rh6C(Co)13l2- occurs in the Rh4 square plane around which the COs migrate in [Rh&(C0)13I2-, vide supra; in both instances, the P-donor is trans to a non-CO-bridged metal-metal bond but the sites occupied by PPh3 and dppe are not the same (see Fig. 10). NMR Table 7. References to NMR and X-ray studies on [Rh6C(Co)13l2- and related derivatives. Cluster
NMR 'H
X-ray 13C
31P
lo3Rh 41 53
58
61 62
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
977
Figure 10. The structure of mono- and bidentate phosphine substituted derivatives of [Rh6C(Co),3]*-:(a) [Rh6C(C0)12(PPh3)]*-;(b) [Rh,jC(CO)II(dppe)j2-. In both examples the R b square plane is viewed from the same direction, which makes the apical Rh(pC0)2(CO) groups above and below this plane coincident, making the two structures pseudo-mirror images.
studies suggest that the LL site occupancy is independent of ring size (LL = PhzP(CH2).PPh2, n = 1L4).[571 Phosphine substitution has a profound effect on CO fluxionality:
i) in [Rh&(C0)13]~-seven COs migrate around a R h square plane, even at -70 "C; ii) in [Rh6C(C0)12(PPh3)]2-all the COs are fluxional at 25 "C with partial fluxionality and complicated exchange processes at lower temperatures; and iii) in [Rh6C(C0)1l(dppe)l2- COs are completely static at room temperature. [HRh&(C0)13]- is formed by addition of acid to [Rh6C(CO)13]2-at low temperature. It has been completely characterized by NMR, and H occupies the same edge of the octahedron as dppe (see Fig. lob). This cluster provided the first example of complete H and CO migration around the metal skeleton periphery but, above -20 "C rearranges quantitatively to give either [Rh&2(C0)24l2- or [Rhl2C2(C0)23I4-(see Section 3.3.9). There was no evidence for the formation of [H2Rh6C(C0)13].This should be contrasted with the formation of the mono- and bis-[Au(PPh3)]+ adducts (see Table 7). It is well known that H+ and [Au(PPh3)]+ are isolobal but it was surprising to find that in both [Rh&(C0)13 {Au(PPh3)}]-[54] and [Rh&(CO)13 {Au(PPh3)}2][581there is an extremely facile migration of the
978
3 Dynamics and Physical Properties
Au( PPh3) group(s), together with complete CO migration around the periphery of the Rhh octahedron, even at -80 "C. It is of interest that I3C NMR spectra provide evidence of the unsaturated 84-electron cluster [Rh&(C0)12{Au(PPh3)}]-.r541 Two heterometallic octahedral carbides have been reported (see Table 7); although both have been structurally characterized by X-ray analysis few spectroscopic data and reactions have been reported. The structure of [Fe3Rh3C(C0)15lpis particularly interesting because it consists of two staggered metal triangles containing the Rh3(CO)3(pu-C0)3-groupand the Fe,(CO)s-triangle with all terminal cos.[60' Various octahedral Ru5Rh carbides have been prepared by addition of a variety of cationic rhodium complexes to the square-pyramidal cluster [Ru5C(CO)1 4 1 ~ (see Table 7); X-ray and 'H NMR data are consistent with the organic fragment on Rh in the starting material remaining with Rh in the cluster. Protonation of [ R u ~ R ~ C ( C O ) I ~ ( C O D gives ) ] - [HRu5RhC(CO)14(COD)] (6('H) -18.6 ppm, 'J('H-lo3Rh) 1.5 Hz.) and, because of the small coupling constant and by analogy with the X-ray structure of [ R u ~ R ~ C ( C O ) I ~ ( C O D )PPh,)}], { A U ( it is suggested that the hydride is on the Ru3-face.
Hexanuclear Rh-containing clusters with an interstitial N Structural and NMR studies on hexa-nuclear Rh-containing clusters with an interstitial nitride are summarized in Table 8. was first reported in 1979[641and I5N-labeling conclusively [Rh6N(CO) proved this formula rather than the alternative [Rh&(CO)14(NO)]-. It is isostructural and isoelectronic with [Rh&(C0)15]~-(see Fig. 9a) but, despite this similarity, behaves somewhat differently chemically. Thus, [Rh6N(CO)l5]- loses three COs on heating to give an unsaturated, 84-electron, symmetrical octahedral structure [Rh6N(CO)12]p[54] in which each Rh is equivalent and bonded to one terminal and two bridging COs (see Fig. 11).
Table 8. References to NMR and X-ray studies on trigonal prismatic and octahedral rhodiumcontaining clusters with an interstitial N. Cluster
NMR
'H
X-ray 1 3 c
15N
31P
Io3Rh
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
979
Figure 11. Proposed structure of [Rh6N(p-C0)6(C0)6]
As found for the trigonal prismatic carbide analog, NMR provides good evidence that electrophiles add to the trigonal face, e.g. [Rh6N(CO)lj{Au(PPh,)}], but, surprisingly, [Rh6N(C0)15]- does not add a H+, even in excess of strong acids.[541 The isoelectronic hydride [HRh6N(CO)1 4 1 ~ - has, however, been isolated from the reaction shown in Eq. (6); X-ray analysis shows that it has the same structure as that shown in Fig. 9a except that one intra-triangular bridging CO has been replaced by a bridging hydride.[661Consistent with this formulation, the 'H NMR has a high-field resonance (6('H) -10.7 ppm, 'J('H-Io3Rh) 34.6 Hz) which is unchanged from +25 to -60 "C.
+
[Rh6N(C0)15]- 30H-
[HRh6N(C0)14I2-+ C032-+ H20
4
(6)
Low-field "N resonances are consistent with complete encapsulation of N in both [FejRhN(C0)15l2- and cis-[Fe4Rh2N(CO)ls]-; this has been confirmed for both species by X-ray studies (see Fig. 12).[671
3.3.4 Heptanuclear Rh-containing clusters The structure of the two capped octahedral clusters [MRh6(C0)16In- ( n = 3, M = Rh;[681n = 2, M = Ni[691)are shown in Fig. 13. Both are static in solution at low temperature and their 13C and Io3Rh NMR spectra have been completely a s ~ i g n e d . [ ~Despite , ~ ~ ] their overall structural similarity, however, they have quite different CO fluxional processes. These are described below and for M = Ni are related to the asymmetries present for the ,u2- and ,u3-COs in the solid-state struc-
980
3 Dynamics and Physical Properties
a
b
Figure 12. The structures of (a) [Fe~RhN(p-C0)3(C0)12]~and (b) C ~ S - [ F ~ ~ R ~ ~ N ( ~ - C O ) ~ ( C O ) I ~ ] - .
tures; for M = Rh the refinement of the X-ray data was not so complete and bondlength data of comparable accuracy are not available. For [NiRh6(C0)16l2- (see Fig. 13), there are three CO-exchange processes of increasing energy.
Figure 13. The structure of [MRh&3-CO)3(p-CO)6(C0)7]n- ( n = 3, M = Rh; n = 2, M = Ni)
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
981
i) Localized rotation of CbO, CdO and CfO on RhB as a result of CdO and CfO being more strongly bound to RhB. The analogous exchange process was not found for [Rh7(C0)16l3- as a result of CdO being more strongly bonded to the capping atom and thus less able to swing into a terminal position on RhB. ii) Localized exchange of CcO and CeO around the (Rh& triangular face. This pathway is the same as observed for [Rh7(C0)16I3- but, in this case, only this exchange was observed. iii) Exchange of all the COs. This closely related pair of clusters illustrates how profoundly different CO fluxional processes can result. The mechanism of these exchanges can be adequately explained through localized rotations on a particular metal atom, or through exchange of tenninal/bridge COs around a particular face, and reflect the asymmetries found in the solid state. An isoelectronic cluster [PtRh,j(C0)16]*-- has been structurally characterizedc7 but it is not isostructural with those shown in Fig. 13. Thus, there is considerable asymmetry in the capping {Pt(C0)4} region and, because of its instability both under CO and N2, NMR data are not available. Reference was made above to electrophilic attack on the trigonal face of [Rh6C(CO)15]2-. It has been shown that condensation of the analogous cluster [Rh6N(C0)15]- with nucleophiles, [M(C0)4]- ( M = Co, Rh, Ir), results in the formation of [Rh6MN(C0)15I2( M = Co, Rh, Ir) which contains a metallic array consisting of a square-face-capped trigonal prism.1721For M = Ir, X-ray studies show that Ir is mostly localized in the capping position whereas when M = Co, Co is within the trigonal prism. Unfortunately, no NMR studies of this system have been reported; they would be of interest to quantify the different possible isomers, their stability, and possible interconversions. has recently been isolated from An unusual cluster, uiz. [HO~4Rh3(CO)1~(nbd)2], the reaction of [ H O S ~ ( C O ) ~and ~ ] -[Rh(nbd)C1]2;[731 the structure consists of an Oscapped Os3Rh3 octahedron; ‘H NMR studies suggest that the H is face-bridging the Os,,,Rh2 face.[731
3.3.5 Octanuclear Rh-containing clusters This nuclearity is still rather uncommon for Rh-containing clusters. RhsC(C0)19 has been obtained by oxidation of [Rh6C(CO)*512- and structurally characterized by X-ray analysis (see Fig. 14);[741no NMR data are presently available. The mono-capped octahedral cluster [Re7C(C0)21I3- has been likened to Cp-; it adds to a variety of 12-electron [ML,] fragments, including [Rh(C0)2-xLx]+ (x = 0, 1, L = P-donor; x = 2, L = COD) to produce a trans-bicapped octahedral
982
3 Dynamics and Physical Properties
f r a m e ~ o r k . ' ~ ~Localized ,'~] CO scrambling of the terminal COs about each atom of the C3" framework of [Re7C(C0)21l3- accounts for the three observed 13C resonances of relative intensity 3:9:9[773781 and similar behavior is found for the Re7Rh-~lusters.[~~I Comparative NMR studies suggest [Re7C(C0)21I3- is a weaker electron donor than Cp- and reaction of [Re&(C0)21 {Rh(C0)2}l2- with P-donors occurs exclusively on Rh.
3.3.6 Nonanuclear Rh-containing clusters The preparation of both clusters [Rh9-,Pt,(CO)19](3-x)- (x = 0 , l ) has been reported and shown to result from similar routes (Eq. 7). Although these redox reactions are complicated, both products have been isolated and shown to be isostructural (see Fig. 15).[79,801
It should be noted that, consistent with the preference of Pt for sites of higher metallic connectivity as exemplified by [RhloPtN(CO)21]3- ,[791 [RhsPt2(CO)22]3- ,r801 [ Rhl1Pt2(C0)24l3- ,[8 and [Rhl2Pt( CO)24]4-,[811 (see Sections 3.3.8 and 3.3. lo), Pt
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
983
Figure 15. The structure of
[Rhg-.yPt.y(~~-CO)~(p-CO)g(CO)~](3-~y)( X = 0, 1).
occupies one of the vertices of the central triangle and there is a mirror plane through M, Rh3, and RhS. I3C NMR studies on [RhsPt(CO)19I2- show that, at room temperature, there is complete CO fluxionality over the entire RhsPt framework giving rise to a first-order pattern which consists of a triplet (‘J(’3C-103Rh)’8.7 Hz) of septets (‘J( 13CO-103Rh)’ 10.5 Hz) with associated satellites from ‘J(13CO-195Pt)’, 125.7 Hz; if the metallic skeleton remains intact, the septet pattern could arise from accidental degeneracy of the couplings to the non-equivalent rhodium atoms on the outer layers or could arise from rotation of the triangles about the pseudo-3-fold axis as found in [Pt9(C0)18]2-.[821 At -100 “C, partial fluxionality still occurs but, as these spectra were measured on low-field spectrometers (Bruker WP80 and Varian XL 200), it would be worth re-measuring the spectra on higher-field spectrometers to obtain detailed pathways for the CO-migration. It would also be of interest to correlate this behavior with solid-state structural data which show that both the p,- and p3-COs bridge more asymmetrically than in the analogous cluster, [Rh9(C0)lsl3-,for which, unfortunately, NMR data are not yet available. One of the few substituted higher nuclearity clusters results from the reaction NCMe)z}]. of [Rh9(CO)19]3pwith [Cu(NCMe)4]+, viz. [Rhg(C0)19(NCMe){pu-Cu( X-ray analysis[831shows that the Rhg unit consists of two condensed, face-sharing octahedra, as found in the starting material, with one of the outer faces containing both the terminal NCMe and the bridging Cu( NCMe)z units. No NMR data have yet been reported. Clusters containing a more close-packed metal framework, viz. mono-capped square antiprismatic, are exemplified by [RhsE(C0)21l2- ( E = P,IS4] As). The structure is schematically shown in Fig. 16 and it is strictly related to the trans-
984
3 Dynamics and Physical Properties
Figure 16. The structure of [Rh9E(p-C0)12(C0)9I2- ( E = P, AS)
bicapped square antiprismatic frameworks exemplified by [RhloE(CO)22]n- ( n = 3, E = P,[851As;r861y1 = 2, E = S[871)(see Fig. 17 and Section 3.3.7); all contain an interstitial p-block element and the solid-state structures indicate that all the bridging carbonyls are significantly asymmetric. For [RhgP(C0)21l3-, the presence of an interstitial P proved particularly useful for NMR measurements.r881At low temperature all the multinuclear NMR spectra (I3C, 31P, and Io3Rh)are completely in accord with the solid-state structure, with the value of ' J ( I3C-lo3Rh) for the asymmetric bridging COs exactly mirroring the distortions found in the solid state. Increasing temperature induces the onset of both CO and Rh fluxionality, however, so that all the COs and all the Rhs become equivalent; both these exchanges occur with a similar activation energy. It is proposed that the mechanism for Rh-migration involves a symmetrical tricapped trigonal prismatic intermediate and the intra-exchange of COs is facile because of the asymmetries found in the static structure. The determination of the formula of [ R L Q R ~ ~ B ~ ( C Orelies ) ~ ~ heavily ]upon the isotopic splitting pattern found in the mass spectrum, because it was difficult to distinguish between Rh and Ru by X-ray analysis and, although there was a low field IIB resonance, it was broad and so the number of Rhs could not be ascertained.r481The structure (Fig. 18) nevertheless provides an interesting addition to the differently condensed trigonal prismatic metallic cores found in the analogous mono- and di-carbide series of clusters. An unusual paramagnetic cluster [HxRh~Ni6(C0)21l3-(x = 1 or 3) has been
3.3 Homo- and Heterometullic Rhodium Clusters Containing 2 6 Metal Atoms
Figure 17. The structure of [RhloE(/~-CO)lz(CO)lo]"( n = 3, E E = S).
985
P, AS; n = 2,
1
isolated from an equimolar reaction of [Ni,j(C0)12]~-and [Rh*(C0)4C12].X-ray analysis[891shows the presence of a trigonal bipyramidal Rhs-unit penetrating a Ni6-trigonal prism. Magnetic moments and ESR measurements are consistent with one unpaired electron and it is suggested that H occupies one of the two tetrahedral Rh4 cavities; attempts at deprotonation were unsuccessful and oxidation/reduction reactions to prepare the diamagnetic analogs have not yet been reported.
986
3 Dynamics and Physical Properties
[RhdCO)i913-
a
[Rhio(C0)21I2-
b
C
Figure 19. Stepwise cluster growth as illustrated by the metal framework relationship of [Rh9(CO)191~-,[ R ~ I o ( C OI*-, ) Z I and [Rhll(C0)23I3-.
3.3.7 Decanuclear Rh-containing clusters Controlled cluster growth as depicted in Fig. 19 has been realized from the reactions shown in Eq. (8). The structure of [Rh10(C0)21]~has been established by X-ray crystallography[g01as a fragment of hexagonal close packing. The COs are located in terminal (7) edge-bridging (12) and face-bridging (2) positions and range from being symmetrically bonded to very asymmetrically bonded. No NMR data are currently available.
As;["] n = 2, The trans-bicapped structure [Rhl0E(C0)22]~-( n = 3, E = E = S[871)is shown in Fig. 17, vide supra. As found for the Rhg analogs, NMR measurements show that, at low temperature, the solid-state structure (including the distortions) is retained in solution but at higher temperatures there is complete randomization of both the Rhs and COs about the central interstitial
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
987
Figure 20. The carbonyl distribution around the MI^ polyhedron found for the anions of: (a) Milpolyhedron based on face-to-face condensed octahedra sharing a common edge coincident with the Mold axis; (b) (NM~~)~[R~II(P-CO)I~(CO)I Il.Me2CO; (c) (NMe4)3[Rh1 I(P-CO)I~(CO)IO].C~HSM~; (d) (NEt4)3[hRhiI (P-CO)I i(C0)iiI.
3.3.8 Undecanuclear Rh-containing clusters Controlled cluster growth has been shown in Eq. (8) and in Fig. 19. Related to the structures in Fig. 19 are those of [Rhll(C0)23]3-r921 and [Pt2Rh~(CO)22]~-[~~] (Fig. 20); in both of these the metal framework consists of three octahedra which each share a pair of adjacent triangular faces. The Rhll cluster occurs in two isomeric forms, which arise from different carbonyl site occupancies as a result of different crystal packings (Fig. 20b and c); both structures are related to that of the RhyPt2 cluster (Fig. 20d). Because of the similar energies for different CO-site occupancies (Fig. 20b and c), it is not surprising that both of these isomers have identical NMR spectra in solu-
988
3 Dynamics and Physical Properties
tion; at low temperatures there is complete CO fluxionality around the intact Rhll polyhedron and the solid-state structures can be considered as two frozen steps that occur in the CO-exchange process. These structures provide a rare example of isomers of clusters which contain carbonyl groups only.[921 An unusual mixed metal nitride [Rh1oPtN(C0)21l3- has been structurally characterizedL7’]but no NMR data have been reported; the metal geometry is unusual and the nitride occupies an unusual cavity in which it is connected to five metal atoms (one Pt and four Rhs).
3.3.9 Dodecanuclear Rh-containing clusters Clusters containing twelve metal atoms have a wide variety of metal frameworks potentially available, as illustrated in Fig. 21. The X-ray structure of [H2Rh12(C0)25] (Fig. 21a) consists of three face-sharing octahedral units and is obviously related to [Rh~(C0)19]~in Fig. 19; although it was too insoluble for NMR measurements, it has been suggested from bond length variations that there is one H in the central interstitial octahedral site, with the other H either in one of the remaining octahedral sites or in a ,~i~-site.[’~] In basic solvents, deprotonation occurs to give [HRhl~(C0)25]-which has a septet (‘J(’H-Io3Rh) 14.45 Hz) at high field
a
b
Figure 21. Various metal polyhedra adopted by clusters containing 12 Rhs: (a) (H2Rh12(p3-C0)3. (P(-co)9 (co)I 31; (b) [Rhl2(p3-co)8 ( p - C 0 ) 2( c 0 ) 2 0 1 .
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
989
Figure 22. The structures of (a) [Rh12C2(p-C0)8(CO)1612- and (b) [Rh&2( p - c O ) l ~CO) ( 131 4-.
(- 16.6 ppm) consistent with H occupancy of the central interstitial octahedral site.l9 31 [Rhl2(C0)30I2- (Fig. 21b) is the first product from reduction of [Rh6(C0)16]; NMR show that the solid[951and solution structures are identical. The distorted icosahedral arrangement found for [Rh12Sb(C0)27]3-[961 starts to approach a close-packing arrangement. In solution, however, it was not possible to obtain limiting low-temperature spectra and at room temperature the COs and Rhs are fluxional, probably via the well known icosahedral tf cube-octahedral pathway.[881 Other clusters containing twelve rhodium atoms are all based on dicarbide derivatives in which the carbide occupies a Rh6 trigonal prismatic site. Fig. 22 shows and [Rhl2C2(CO)23ln-( n = 3, the closely related structures of [Rh12C2(C0)24]2-r971 4).[981NMR measurements show that either [Rh12C2(C0)24I2-or [Rhl2C2(C0)23I4is formed on warming [HRh6C(C0)13]- and that there is independent CO migration around the outer and middle metal layers at room temperature.[531 Section 3.3.3 described how [Au(PPh3)]+ added to the trigonal face(s) of [RhsC(CO)15l2-. Analogous reactions with [Rh,2C2(C0)24I2- have been described, but these products are much more complicated. The structure of the metal skeleton in the first-formed intermediate, [Rh12Cz(C0)23{Au(PPh,)}]- is unique (Fig. 23) and consists of three shared trigonal prisms (two containing the carbide atoms).[991 In solution, however, 31PNMR measurements indicate that the Au( PPh3) group becomes bonded to a Rh3 face.[991[Rhl2C2(C0)23{Au(PPh,)}]- reacts further with (n = 18 or Au(PPh3)Cl resulting in the formation of [R~IoC~(CO)~{AU(PP~~)}~] 20)" Ool through chloride abstraction of [Rh(CO)2C12]- but their insolubility prevented detailed NMR The preparation and structure of another
990
3 Dynamics and Physical Properties
Figure 23. The metal skeleton in [RhizCz(p-CO)io(CO)13 PPh3)11which consists of two rectangular, face-shared Rhs-trigonal prisms.
Rhlz-dicarbide [Rh&~(C0)25]has been described""] NMR measurements have been described.
but this is insoluble and no
3.3.10 Tridecanuclear Rh-containing clusters [H,Rh13(C0)24](5-")- (x = 1-4) was the first close-packed metal cluster to be isolated and structurally characterized; the metal skeleton is based on a hexagonal close-packed metal framework with each of the outer rhodium atoms associated with a terminal CO and there are twelve bridging COs (see Fig. 24). All the above clusters are interconvertable by protonation/deprotonation reactions and are isostructural,[' 02-' 051 (see Fig. 24). Detailed NMR studies["] have shown that all of the clusters (x = 1-4) behave similarly in solution, uiz.: i) all of the COs, except the three bridging COs in the hexagonal plane, are involved in exchange at room temperature; and ii) H migrates interstitially.[l,'O1
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
991
Figure 24. (a) The neutron diffraction structure [H2Rh13(p3-C0)12(CO)121);each H is coplanar with a Rh-square-face. H-occupancy is easiest when there are fewer p-COs on the square face; for [H3Rhl3(p2-C0)l2(C0),2l2NMR and X-ray studies are consistent with the additional H occupying the symmetry related square-face B, which contains two p-COs on the lower half of the cluster. (b) and (c) Schematic representation of the structure and the CO-exchange pathways found for [H,Rhl3(p-C0)12(CO)12](~-\)~ (x = 1-4). Each of the outer Rhs is associated with one terminal CO and two pC0s which allows the cyclical CO-exchange shown.
For [H3Rh13(C0)24l2-it was possible to stop the above migrations at low temperatures and confirm the H-site occupancies as being in the unique square-faces A and the two symmetry-related square-faces, B (Fig. 24).['01 These sites had been suggested from X-ray studies and preferential H-occupancy occurs in square-faces associated with the smaller number of bridging COs. Neutron diffraction studies on [H2Rhl3(CO)24l3-have recently shown that sites A and B (Fig. 24) are occupied by H and both Hs are coplanar with the Rh4 square faces.[lo6]It was impossible in solution to obtain the limiting low-temperature spectra for [H2Rh13(C0)24l3- but, consistent with the neutron diffraction data, there are two high-field H resonances from the two non-equivalent Hs in the solid-state ' H NMR spectrum and it seems that H migration in the solid state is much more difficult than in solution.['071 Although no NMR data have yet been reported, the structures of an isoelectronic series of clusters, [Rh13-,Pt,(C0)24](5-")- (x = 1, 2) have been reported;[811when x = 1, Pt uniquely occupies the interstitial site in the hexagonal close-packed framework and when x = 2, the second Pt is disordered. Surprisingly, the carbonyl distribution in these Pt/Rh clusters is different from that found for the Rh13 analogs. It should also be noted that in these and all other higher nuclearity clusters containing Rh and/or Pt, which have a close-packed metal framework, the terminal and p-COs are located near to idealized positions such that the CO vector is centered on the center of an M3-triangle, an M4-tetrahedroq or an M6-octahedron. This results in very non-closed packed CO-polyhedra and previous ideas about CO fluxionality for smaller clusters[321are certainly not sustainable for these higher nuclearity clusters.
'
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3 Dynamics and Physical Properties
3.3.11 Tetradecanuclear Rh-containing clusters The formation of [H,Rh14(C0)25](4-X)-(x = 0, 1) is shown in Eq. (9); the X-ray structures of both clusters (x = 0, 1) are similar (Fig. 25) and the metal framework is part of a body-centered cubic
Detailed multinuclear ('H, 13C, lo3Rh) NMR studies have been conducted on [H,Rh14(C0)25](4-x)-(x = 1, 0)[10~111~1121 and show that at low temperature there is: i) facile H-migration (when x = 1) within the intact metal skeleton; and ii) for both x = 0 and 1, rapid exchange of the terminal and p-COs around the (Rh~)4-square-face(Fig. 25) with the other. COs being static and distributed (including asymmetries) exactly as found in the solid state.r'08-1'11 At room temperature, all the COs are fluxional in both clusters. Thus, H migration within the intact Rh14 cluster occurs with a lower Ea than complete CO migration on the outside of the cluster; they therefore occur independently of each other. This should be contrasted with [H3Rhl3(C0)24I2- which seems to have very similar values of Ea for both interstitial H migration and CO migration."'] It should also be noted that both the Rh13- and Rhl4-metal frameworks remain intact whereas less well-packed polyhedral metal skeletons containing a main group interstitial atom (see Sections 3.3.6 and 3.3.7) are fluxional.
Figure 25. The structure of [HxRhi&-C0)i6(CO)~](4-X)~ (X = 0, 1).
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
993
Figure 26. The structure of [ { R ~ ~ C ( C O ) ~ ~ R ~ } ~ ( , U - C O ) ~ ] ~ .
A closely related cluster [Rh14(C0)26]~~ has been structurally but, apart from a VT 13C NMR study which suggested CO f l u ~ i o n a l i t y , [ 'no ~~~ well-defined NMR studies have been reported. The structure of the cluster [Rh14C2(C0)33]2p,which results from addition of [Rh(C0)2(MeCN)2]+to [Rh6C(C0)15]2p,[561 is interesting (Fig. 26). IR measurements suggest the involvement of Rh7/Rhs intermediates but no NMR data on this sequence of reactions has yet been reported.
3.3.12 Higher nuclearity Rh-containing clusters ( 215 metals) For those higher nuclearity clusters which have been structurally characterized (Table 9), there have been few detailed NMR studies and the clusters in Table 9 will be alluded to only when NMR studies have been reported.
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3 Dynamics and Physical Properties
Table 9. References to higher nuclearity Rh-containing clusters ( 2 15 metals) which have been structurally characterized by X-ray crystallography. Cluster
Reference
108 114 115 116 117 13 I18 119 119
120
[Rh15(C0)27l3- can be prepared as shown in Eq. (10); it fragments as shown in Eq. (11). Ill-defined VT 13C NMR spectra of [Rhls(CO)27l3- have been reported[105.1 2 1,1221 but a detailed understanding awaits elucidation.
The structure of [Rh17S2(C0)32I3- is shown in Fig. 27.I1l7]VT 13C NMR measurements indicate that at 5 4 0 "C the cluster is whereas at +180 "C there is exchange of one set of terminal and p-COs but, as the relative intensities of the terminallp-COs associated with the inner and outer layers are the same, it is difficult without l3C-( lo3Rh) measurements to be sure which of these two sets of groups is exchanging.[1231The direct lo3Rh NMR spectrum of [Rh17S2(C0)32I3- has been reported and is consistent with the static structure; there are three resonances of relative intensity 1 : 8 :8 from the interstitial and inner/outer Rhs, respectively.[1241 In view, however, of the errors in 6( lo3Rh)made by this group on other clusters, the absolute values of the rhodium chemical shifts must be treated with caution, although the resonance from the interstitial Rh is obviously at low field. There is spectroscopic and crystallographic evidence that [HxRh22(CO)35]5 - and [H,+1Rh22(C0)35I4- co-crystallize.[1191 It was difficult to obtain well resolved NMR spectra because of insolubility but VT 'H NMR spectra suggested the presence of a high-field 'H resonance (-18.5 ppm) and a broad multiplet in the 13CNMR spectrum at 212 ppm. These spectra are, however, rather ill-defined and attempts to
3.3 Homo- and Heterometallic Rhodium Clusters Containing 2 6 Metal Atoms
995
obtain better I3C spectra by direct enrichment with I3CO produced evidence for cluster fragmentation." 191 Low-temperature 'H NMR studies on [Pt4Rhl8(C0)35I4- failed to detect a hydride resonance and the absence of a hydride(s) in this formulation is consistent with both space-filling models, which exclude H-coordination to the metal surface, and the lack of reaction with Na2C03.['201
3.3.13 Concluding remarks Modern developments in NMR measurements enable the efficient acquisition of data required for the unambiguous clarification of structural and rearrangement pathways for Rh-containing clusters. There is now quite a large structural data base available for comparative purposes but several significant advances can be expected over the next few years as a result of improved instrumentation and techniques. i) Rationalization of sites of L substitution in homo- and heterometallic higher nuclearity carbonyl clusters. There are presently very few known examples of substituted clusters containing 2 7 metals. ii) Although preparative and X-ray studies of the resultant products have made a valuable contribution to our present knowledge, much remains to be done
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3 Dytzornics und Physical Properties
to gain a better understanding of controlled cluster growth/fragmentation cia redox reactions of homo- and heterometallic clusters. iii) The effect of systematically varying the cone angle/basicity of L upon the COfluxionality. iv) Comparative solution/solid state NMR studies. v) A better understanding of the bonding of interstitial main group atoms within different homo- and heterometallic frameworks. Initial density functional calculations of I3C chemical-shift tensors for interstitial carbides in homometallic clusters[1251are in excellent agreement with available experimental data.[* However, apart from the chemical shift tensors for the interstitial P in [RhgP(C0)21I3- and [RhloP(C0)22]2~,"271 there are few other accurate experimental data available to enable extension of these ideas, e.g. to interstitial B, N , and to carbides in heterometallic frameworks.
Acknowledgements We would like to thank INTAS (Grant RFBR 95-242), which enabled this collaboration to develop and EPSRC, for financial support for NMR instrumentation and a studentship (DJS). BTH thanks the Leverhulme Foundation for the award of a Research Fellowship and also thanks Professor T. Eguchi, Osaka University, for his kind hospitality during the writing of this review. We also thank Dr. C. Jacob for assistance with the production of the Figures.
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3 Dynamics and Physical Properties
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Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.4 Structure and Dynamics in Metal Carbonyl Clusters: NMR, EXAFS and Crystallographic Studies Louis J. Farruyia and A. Guy Orpen
3.4.1 Introduction The history of transition metal carbonyl clusters stretches back to the beginning of the 20th century, with the reports of the synthesis of the polymetallic compounds C O ~ ( C O ) S ,Fe2(C0)9,r21 [~] and Fe3(C0)12.[31During the late 1920s and 1930s, pioneering work was performed by Hieber and coworkers[41on iron polynuclear carbonyls, a remarkable feat given that the characterization techniques taken for granted these days, such as X-ray diffraction or NMR, either remained to be discovered or were generally unavailable. The first carbonyl-bridged metal-metal bond was discovered in 1939 in the compound Fez(C0)q by use of photographic X-ray technique^,^^] but later X-ray studies on archetypal carbonyl clusters such as and IrL9])were seriously hampered Fe3(C0)12[~] and M4(C0)12 (M = Co,['] Rh,[7b381 by problems of disorder or twinning. As we shall see in this chapter, a detailed examination of the disorder, and its temperature dependence, in several metal carbony1 clusters provides clues as to the dynamic behavior of these species. The structures of metal carbonyl clusters posed several problems. As more and more complex structures were determined by X-ray diffraction, it became clear that there was an underlying relationship between the metal core skeletal structure, and the overall valence electron count.["] More difficult to rationalize however, were the finer points of structural detail, such as why bridging carbonyls are present in Fe3(C0)12,[~] but not in the closely related species M?(C0)12 (M = Ru,[I11 Os"21) and in M4(CO)12(M = Co,['] Rh"b,81) but not in Ir4(C0)12.[91This problem was tackled by Johnson and in a series of papers which elaborated the ligand polyhedral model." 41 This idealized (and essentially geometric) model considers metal carbonyl clusters as polyhedra of metal atoms surrounded by polyhedra of ligand atoms. The different structures of the basic carbonyl clusters mentioned above is considered to arise from different geometrical ways of interconnecting these two polyhedra. The ligand polyhedral model" 3,141 was also
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3 Dynamics and Physical Properties
used to explain the dynamic behavior of cluster compounds, in a completely general fashion. The dynamic behavior, or fluxionality, of metal carbonyl clusters (and indeed of organometallic molecules in general) was first recognized with the advent of nuclear magnetic resonance spectroscopy, especially I3C NMR." An early example of ligand fluxionality in clusters was provided by Fe3( CO)12 .[' 61 Occasionally the metal skeletal structure is also dynamic, e. y. in the capped square anti-prismatic cluster anion [Rh9(p9-P)(C0)21l2-,which was shown by lo3Rh NMR to have equivalent rhodium environments at ambient temperature^."^' This dynamic aspect of the structure of metal ensembles is not limited to metal cluster compounds, but is also commonly seen in 'naked' metal clusters.['*] For fluxional molecules in general, one or more structures lie so close in energy to the ground-state structure that they are easily accessible thermally. These excited-state structures provide the pathway for the atomic site permutations which are manifest in the NMR spectra. The complete identification of these excited state geometries (i.e. the determination of the exact fluxional pathway) is a very difficult problem from an experimental perspective, and will probably only be solved by high-level quantum calculations. It is worth bearing in mind that dynamic NMR experiments usually only provide direct information on the rates of the nuclear permutations, not on the pathways that the nuclei take. It is common to assume that the path of least motion is the actual one, but this might not always be so. For Fe3(C0)12 (see below) there has been considerable controversy in the literature regarding this matter. One very commonly observed exchange process in clusters is the so-called tripodal rotation of M(CO)3 groups about their local C, axis. It is often assumed that the fragment rotates 'intact' in a concerted fashion but, in theory at least, other processes such as random pair-wise exchange between CO ligands could also account for the observed exchange. A careful study"91 of this localized exchange process in (1) has shown that the three the ally1 cluster Ru3(p-H)(p3-y3-CMe.CH.CMe)(CO)~ chemically distinct CO ligands in the two chemically equivalent Ru(CO)? groups undergo mutual exchange, with experimentally identical exchange rate constants. In this instance at least, a concerted process seems most likely. Moreover, recent
1
3.4 Structure and Dynamics in Metal Carhonyl Clusters
1003
spin-relaxation studies by Bain and Cramer[”’ on tripodal rotation in R u ~ ( C O ) ~ . (,u-PPh2)(p-v2-C=CBu‘)also strongly suggest a concerted mechanism.
3.4.2 Dynamic NMR spectroscopy of clusters By far the largest number of dynamic NMR studies on clusters have been underfew studies in the solid state.r2‘I taken in s o l ~ t i o n [ ’b1~with ~ ~ ’comparatively ~~~~ The exchange of nuclei between chemically different sites in a molecule often occurs at a frequency comparable with the dijjkrence between the resonance frequencies of these different sites, i. e. the timescales of NMR spectroscopy and fluxionality are similar. For this reason NMR spectroscopy is the technique par excellence for investigating dynamic processes. Nevertheless, as we shall see, dynamic (i. e. variabletemperature) X-ray crystallography can provide information which is not available from the NMR experiment. If the exchange rates become sufficiently fast, exchange broadening on the much shorter IR timescale can occur,[221though this is a much rarer phenomenon. The most widely used DNM R technique is still variable temperature line-shape analysis, although this method has its drawbacks.[231A more useful method of investigating fluxional exchange involves magnetization transfer,[241whereby nonequilibrium magnetization is induced in one site and is then carried through to other chemical sites by the exchange process(es). The one-dimensional form of this experiment involves direct selective excitation of a single resonance, followed by analysis of the time-dependent intensities of other signals. Alternatively, excitation of all relevant nuclei can be performed by the 2D EXSY technique.[251Magnetization transfer experiments are the methods of choice in complex exchanging systems, because the individual site-to-site exchange rate constants can be obtained directly, without recourse to any preconceived mechanism. A clear example of the incisiveness of magnetization transfer experiments comes from a study of the well known alkynyl cluster Ru3(,u-H)(C=CBut)(C0)9(2) This
2
1004
3 Dynamics and Physical Properties
was first investigated by Milone et using band-shape analysis, and it was concluded that there were three exchange processes at work. The lowest energy process involved tripodal rotation in the unique Ru(C0)3 group, and the broadening and collapse of the remaining resonances was attributed to a second tripodal rotation of the other two (equivalent) Ru(CO)3 groups. Finally an uncharacterized higher-energy process led to total scrambling of the CO ligands over the metal framework. By use of a 2H-substituted sample of 2, Rosenberg et aZ.[271observed a kinetic deuterium isotope effect on the exchange rate for the second tripodal exchange process; they concluded that some mobility of the hydride was required for this process, either via an opening of the hydride bridge, or through the formation of a ,u,-hydride. This was an intriguing result, particularly because the same authors[271observed no kinetic deuterium isotope effect for the similar tripodal rotation in Os3(,uU-H)2(CO)lo. Some time later it was demonstrated by Predieri et U Z . [ ~ ~ ' and Chi and coworker~['~] that alkynyl rotation could easily occur about metal triangular faces. In using both 1D and 2D view of this, the fluxional behavior of 2 was rein~estigated~~'] magnetization-transfer techniques. The I3C{'H} EXSY spectrum at 272 K (Fig. 1) immediately showed there to be cross peaks between all five signals, indicating one or more processes which would lead to the previously observed complete CO scrambling. It turned out to be difficult to obtain accurate rate constants from this EXSY experiment, because the wide variation in rate constants led to no suitable mixing time. Careful analysis of the I D magnetization transfer data, however, revealed three independent exchange modes, see Fig. 2. The lowest energy mode MI is the previously reported tripodal rotation,[261but the next higher energy process M2 is rotation of the alkynyl ligand about the R u ~triangle, coupled with a concerted movement of the hydride. This results in a degenerate exchange process. The activation barrier for process M2 was shown to be the same as that previously reported[261for the second 'tripodal rotation' in 2. The kinetic deuterium isotope is related to process M2 and is entirely reaeffect reported by Rosenberg et sonable, because Ru-H bonds are made and broken in this process. A third process M3 was also observed, the tripodal rotation of the pair of equivalent Ru(CO), groups. The operation of these three processes will lead to the observed complete CO scrambling, without any intermetallic CO migration, although the possibility of intermetallic scrambling as a much higher energy process cannot be precluded. This example is not intended as a criticism of the earlier work, but rather as an illustration of the inadequacies of band-shape analysis, even in relatively simple systems. This also highlighted the fluxional behavior of the phosphine derivative Ru3(,uu-H)(C=CBu')(CO)8( PMe2Ph) (3). This compound exists as three interconverting isomers in solution; the major isomer corresponds to the solid statestructure, which has the phosphine ligand in an equatorial position on the unique Ru atom. The lowest-energy fluxional process in the major isomer involves a tripodal rotation of the Ru(C0)2(PMe2Ph) moiety, which results in the exchange of enantiomeric isomers. This tripodal rotation can be followed either by the exchange
3.4 Structure and Dynamics in Metul Curbonyl Clusters
1005
T 273 K t,, 0.6s
0
0
B
O
J '
z o b 0
1 4 0
:&.o
1d.o
14.0 PPX
1d.0
181.0
18A.O
ISl.0'
ppH
Figure 1. I3C{'H} EXSY spectrum of R U ~ ( ~ - H ) ( C - C B ~ ' ) ( at CO 273 ) ~K in the carbonyl region.
of non-equivalent carbonyls or by the exchange of the diastereotopic methyl groups on the phosphine. Both experimental rate constants were measured, and are identical within experimental error, which strongly indicated a concerted process. Interestingly, the activation barrier for tripodal rotation of the Ru(C0)2L group in 3 is significantly lower than for the Ru(C0)3 group in 2. This is somewhat counterintuitive, but a similar effect is observed with quite bulky phosphine ligands such as PPh3,[26b1 and leads to the conclusion that any inertial contribution to the activation barrier is not significant. Indeed it is difficult to draw any hard and fast conclusions about the significance of these results, because any relative reduction in the activa-
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3 Dynamics and Physical Properties
d '
Figure 2. Proposed fluxional mechanisms in RU&L-H)(C-CBU')(CO)~.
tion barrier might result either from a reduction of the energy of the transition state/ intermediates or an increase of the energy of the ground state. It is not clear which of these is the primary effect, or even whether both occur. As stated above, relatively little information on the nature of the transition state/ intermediates can be gleaned directly from dynamic NMR studies. There is, however, one experiment which can provide some direct information. A measurement of the activation volume AVJ of the fluxional processes can be obtained from the pressure-dependence of the site-to-site exchange rate constants. Various models for the transition state can then be examined, and the differences between the reactant and transition state molar volumes compared with the measured results. Roulet and coworkers[3 have shown that the cluster Ir4(C0)9(p3-1,3,5-trithiane)consists of two isomers in solution, one with three bridging CO ligands around the basal plane, the other with an all-terminal CO arrangement. The magnitude of the activation volume for isomer interconversion strongly implies a symmetric transition state with three semi-bridging carbonyl groups. Keister and M e r b a ~ h [have ~ ~ ]concluded, from the measured activation volumes for hydride exchange in O S ~H)(p-H)(CO) ( 10-
3.4 Structure und Dynumics in Metal Curhonyl Clusters
1007
(PPh3) and Ru3(p-H)2(p3-CHC02Me)(CO)9,that transition states with terminal hydrides are the most likely. It should be emphasized, however, that this method depends critically on an estimate of the partial molar volumes of both reactant and transition state. Given the well known variability in metal-metal bond lengths,[331 and by implication in the volume of the metal cluster itself, it is clear that some caution needs to exercised in the interpretation of these results. This is particularly so when ligands with such small steric footprints as hydrides are considered.
3.4.3 General mechanisms of ligand exchange Despite the above mentioned problems in obtaining detailed experimental information about the pathways of nuclear permutations, several general empirical models for the fluxional behavior in clusters have been proposed. Several general classes of fluxional exchange have been observed in clusters, e. g. :
i) ii) iii) iv)
localized ligand exchange at individual metal centers; localized ligand exchange about two or more metal atoms in a larger cluster; global exchange of ligands over the whole cluster; and metal framework mobility.
Class (iv) is quite common with metals of the late and post-transition series, e.g. the Group 11 metals,[341and mercury,[351but is apparently much rarer for the central transition In part this could be because of the lack of suitable NMRactive metal isotopes to act as ‘sentinels’ for such exchange processes. It is also worth noting that for (iii), the apparent global exchange of ligands might be illusory, and merely the result of the higher time-averaged symmetry of the molecule because of the exchange of other ligands. This was shown to be true for the carbony1 ligands in cluster 2. Of the localized exchange processes, perhaps the most common is the above mentioned tripodal rotation, where fac-M( CO)3 groups undergo an apparent rotation about their threefold axis. More interesting, however, are the processes which lead to complete scrambling of carbonyl ligands. Two general mechanisms have been proposed to explain this phenomenon. An early suggestion by Cotton[36b1was the so-called ‘merry-go-round’ mechanism, in which concerted migration of carbonyls between terminal and bridging sites occurs about either one or more metalmetal bonds. This was originally proposed to explain the fluxional behavior of C O ~ ( C O )but ~ ~ this , ‘merry-go-round’ model might also be extended to explain similar motions around a metal-metal edge, e.g. in Fe3(C0)12,[61about square , [ ~even ’ ~ hexagonal planes as in metal planes, r.y. in [ R h & ~ ~ - C ) ( C 0 ) 1 3 ] ~ -or [Rh,3(p-H),(C0)24]np5.[381 This process involves the physical migration of ligands from one metal center to another, and consecutive applications of such a fluxional mode can often lead to complete CO scrambling.
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3 Dynamics and Physicul Properties
Analysis of the anisotropic thermal parameters (adps) from an accurate X-ray diffraction on [ R ~ ~ ( , u ~ - C ) ( C O has ) ~ , ]shown ~that the greatest thermal motion of the cdrbonyl ligands occurs in one of the square-metal planes, which concurs with the earlier 13C NMR delineation[371of a 'merry-go-round' process about one of these metal planes. This study has also led to the suggestion that the examination of the adps of the light atoms might provide further direct information about the fluxional pathway. Unfortunately, for many structural determinations of carbonyl clusters, the light atom adps are too poorly determined, and/or too subject to the vagaries of other systematic errors to be of much diagnostic utility. The second general fluxional mechanism is the ligand polyhedral m ~ d e l [ ' ~ ,of '~] Johnson and coworkers, which was originally proposed in 1976.[401The ligand polyhedral model (LPM) considers metal carbonyl clusters as consisting of two concentric polytopes, the central metal skeleton and the peripheral ligand. Fluxional behavior has been described in terms of the generalized motions which occur between the (relatively rigid) metal skeleton and this ligand polytope. Cases have been considered where the ligand polytope itself remains relatively rigid, and also cases where there is a change in the geometry of the ligand polytope, such as icosahedral*cubeoctahedral or icosahedralHanticubeoctahedra1. The LPM describes the relative motion of the two polytopes and in solution phase, where molecules are tumbling rapidly, it is only the relative atomic motions which may be investigated by NMR techniques. The question of whether a metal core rotates within a fixed ligand polytope, or vice-versa, cannot be answered, and the distinction is essentially meaningless. In the solid state, however, this is no longer true, as will be shown below. The LPM approach to fluxionality has not only been applied to homoleptic carbonyl clusters, but also to simple ligand-substituted derivative~.['~] For such substituted derivatives, it is worth remembering that ligands which have steric and/ or electronic requirements different from those of CO might increase the energies of certain pathways (e.g. those involving bridging intermediates) to such an extent that they become unobserved. For instance, it does not seem that phosphine ligands migrate easily between metal atoms, apart from very rare instances such as in Ir2Rh2(C0)11(PPh3).[411
3.4.4 Fluxionality in the archetypal clusters M3(C0)12 M = Fe, Ru, 0 s The metal clusters of the iron triad M3(C0)12, 4 M = Fe, 5 M = Ru, and 6 M = Os, despite being some of the simplest carbonyl clusters known, are fascinating molecules, which still present interesting structural and dynamic problems. Because of their simplicity, they have been, and indeed remain, archetypal molecules for discussions on fluxional mechanisms. The two heavier congeners 5 and 6 have a clearer and less complicated history. In the solid state they are isostructural, and possess
3.4 Structure and Dynamics in Metal Curbonyl Clusters
1009
Q
0
Figure 3. The structure of Ru3(C0)9( PMe3)3.
very similar distorted anti-cubeoctahedral arrangements of CO ligands surrounding an ordered M3 triangle." ',12] The molecular structure approximates to D3h, although cluster 5 shows a small distortion towards a 0 3 structure, which is much more pronounced in its phosphine and phosphite derivatives Ru3(CO)lzPn( PR3), ( n = 2-4),[421as exemplified by R U ~ ( C O ) ~ ( P M(see ~ ~Fig. ) ~ 3[) ~ . The ~ ~0 ]3 structure can be viewed as arising from the more symmetric D3h structure by skewing the ML4 groups about their twofold axes in the same direction. Molecular mechanical calculations[431indicate that the 0 3 form is of the lowest energy for clusters of this type, presumably because steric interactions are at a minimum. Irrespective of whether cluster 5 adopts the D3h or 0 3 structure in solution, there are only two sets of chemically distinct CO ligands, the axial and equatorial sites. The inter-site exchange barrier in 5 is very low Aime and coworkers[441,in a recent elegant study using T I relaxation measurements, have shown it to be approximately 20 kJmol-I. The axial-equatorial exchange barrier in the heavier congener 6 is considerably higher, approximately 70 kJ rno1-I .[451 By using a sample of 6 endemonstrated unequivocally that this riched in 1870s,Koridze and exchange occurs by intermetallic CO migration. Solid-state I3C MAS studies of both 5 and 6 show no evidence for fluxional motion in the solid phase.[471 For Fe3(C0)12 (4) the situation is considerably more complex and considerably more interesting. The oft-repeated saga of the numerous structural and spectroscopic studies on 4 is well known and does not bear repeating here the interested reader is directed elsewhere[6d1for a recent resume. In a nutshell, cluster 4 in the solid phase['] at room temperature has a disordered structure. The carbonyl ligand polytope corresponds to a distorted icosahedron, in contrast with the anticubeoctahedron found for 5 and 6, and the metal triangle is statistically disordered over two sites, as required by the crystallographic inversion center in the space group P21/n. This is the well known 'Star of David' disorder, which has been observed in a number of other related c l ~ s t e r s .It[ arises ~ ~ ~ because ~ ~ ~ the distorted ~
~
1010
3 Dynamics and Physical Properties
icosahedral CO cage is, to a first approximation, invariant to inversion, so that there is little energy difference in terms of packing[491whether the Fe3 triangle adopts one orientation, or the other. The problem of the disorder in the roomtemperature molecular structure of 4 was resolved more than twenty years ago by Cotton and Troup,[6C1 after earlier studies by Wei and DahF"] and Corradini and Paiaro.[6a1This work showed the molecule to have two asymmetric carbonyl bridges, with a molecular symmetry of CZbut very close to C2". Despite many studies, the nature of Fe3(C0)12 in solution remains less than certain. It is clear beyond reasonable doubt however, that several isomers must be present in solution. The IR spectrum[501is very simple, and not consistent with any one structure (although the possibility of exchange on the IR t i m e ~ c a l e [has ~ ~ Ibeen mooted), and an EXAFS (see Section 3.4.5, below) has led to the same conclusion. The most favored structures are the CZor C2" bridged forms (as in the solid) and unbridged D3 or D 3 h forms, although less symmetrical structures might also be present. Although Fe3(C0)12 is highly fluxional in solution, very little can be directly ascertained about the exchange mechanism, because the slow-exchange 13C NMR spectrum has not been obtained.[161 In the solid phase, however, the available experimental NMR evidence is much more interesting. Hanson and have reported that at low temperature the 13C MAS NMR spectrum of 4 is consistent with the molecular structure, in that bridging and terminal environments can be distinguished. On warming to room temperature, there is a pair-wise coalescence to give six signals, one of which may be assigned to the average of bridging and terminal CO resonances. They proposed[531 a mechanism whereby the Fe3 triangle rotates about the pseudo-C3 axis in 60" jumps, a proposal consistent with the observed crystallographic disorder. In a later broad-line 13C NMR study Aime and G o b e t t ~ concluded ~~~l that, at elevated temperatures, the COs experienced "an averaging process with partial loss of the axial pattern". They suggested large-amplitude motions of the CO ligands, although the ' ~ ~also ~ lead to axial-bridging exchange. process described by Hanson et ~ 1 . would Variable-temperature Mossbauer studies by Long and coworkers[5 provide little evidence for solid-state dynamic behavior, at least on the Mossbauer timescale. Whereas the exchange barrier for complete carbonyl scrambling in Fe3(C0)12 in solution is too low to enable a slow-exchange NMR spectrum to be obtained, several phosphine, phosphite and isocyanide derivatives have been shown to have a considerably higher barrier.[561It seems reasonable to assume (indeed it is generally assumed) that broadly similar exchange mechanisms operate in the substituted and unsubstituted clusters. Three proposals for general mechanisms of fluxionality in these species have arisen from these studies. The ligand polyhedral model['41 has been specifically applied to the Fe3(C0)12 problem in a number of papers,[571giving a detailed proposal for the exchange process in solution and in the solid state. The lowest-energy process is suggested to be a 'libration' of the Fe3 triangle about the molecular C2 axis, with the CO ligands remaining relatively rigid and maintaining their antipodal relationships. If this 'libration' is of sufficient amplitude, a change in
3.4 Structure and Dynamics in Metal Carbony1 Clusters
101 1
molecular structure from C2 to D3 occurs, assuming that some ‘breathing’ motion of the carbonyl polytope is allowed. The 0 3 form might then collapse back to the C2 form but with the bridging carbonyls situated about a different Fe-Fe edge, 3 structural type is thus a key intermediate in leading to total CO scrambling. The 0 the LPM approach to cluster fluxionality in M ~ ( C O ) Iclusters Z and, as stated above, molecular mechanical calculations[431indicate that this geometry is very favorable for these molecules. The anisotropic displacement parameters of the two Fe atoms associated with the bridging CO ligands show their greatest amplitude normal to the metal triangular plane, and this has been cited[57a,b1 as confirmatory evidence for this proposal. The reader should be aware that in disordered systems, the adps of even the heavier elements might be suspect, and not too great a reliance should be placed on this conclusion. In the solid crystalline phase, because of packing effects, it is considered that the carbonyl polytope is relatively immobile, and that only a minor ‘breathing’ motion is allowed. The iron triangle is thus rotating or ‘librating’ inside the carbony1 polyhedron. In solution there is the additional possibility is that the ligand polyhedron can interconvert from an icosahedron to the anticubeoctahedral complementary Mann[56.5 has proposed a second mechanism, the ‘concerted bridge-opening bridge-closing mechanism’, applicable to Fe3(CO)12 and its derivatives both in solution and in the solid phase. In a recent Dalton Perspective, Mann[”] has elaborated his mechanism using a Biirgi-Dunitz approach for a number of crystal structures of Fe3(C0)12 and derivatives, and a ‘movie’ of this mechanism is available on the World Wide Web.[591The Mann process also involves a motion of the Fei triangle relative to the icosahedral carbonyl polytope, the axis of relative motion being one of the fivefold axes of the CO icosahedron (parallel to an Fe-Fe bond), whereas in the Johnson approach it is the molecular twofold axis. In Fe3(C0),2, according to Mann[561“both the C, libration and concerted bridgeopening bridge-closing mechanisms produce the same exchange. They only differ in the pathway followed”. Thus the two mechanisms are probably indistinguishable by any NMR experiment. Lentz and Marschall[601have offered yet a third proposal for the solution fluxionality of Fe3(CO)I 1 (CNCF3) and derivatives Fe3(CO)Io(CNCF3)(PR?), involving rotation of the Fe3 triangle within the icosahedral ligand polytope, about a different fivefold axis. All three approaches are broadly similar, but there has been considerable controversy in the l i t e r a t ~ r e ~ ~ ~ , ~ about this matter. For instance, both Johnson[57d1 and Mann[621consider the Lentz and Marschall mechanism[601to be consistent with their own proposals. Both the Johnson and Mann mechanisms result in the pair of bridging carbonyls moving from one edge to another. Figure 4 shows a stereo-view of the superposition of two molecules of Fe3(C0)12, where the bridging carbonyls have been set on different Fe-Fe edges, and the metal atom positions have been fitted by least squares. The metal atom positions in the two molecules are very close, and only one position is shown. What is clear from this graphical representation is:
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3 Dynamics and Physical Properties
Figure 4. Superposition stereo view of two molecules of Fej(C0)12 with the bridging carbonyls on adjacent edges.
i) there are two carbonyls which have very similar positions in the two orientations, lying trans to one another through one of the Fe-Fe bonds not involved in the bridge; and ii) there is a 'least motion' pathway which relates two sets of five CO positions, shown as a dashed line in Fig. 4. The 'least-motion' pathway is extremely similar to the pathway proposed by Mann,[j6]and it has the great advantage that the carbonyl ligands always remain within bonding distance of the metal atoms. Taking this simple geometrical analysis one step further, by calculating the mid points of each pair of C and 0 atoms along this path, it is possible to generate a 'mid-way' structure (potentially a transition state or intermediate in the mechanistic pathway). This structure is shown in Fig. 5. While it closely resembles the 0 3 type structure proposed as the intermediate by J o h n ~ o n , [ and ~ ~ ,experimentally ~~] observed, for example in Fe3(C0)9 {P(OPh)3}3[631 and numerous phosphine and phosphite derivatives of R u ~ ( C O ) ~ there ~ , [ ~are ~ ]two minor, but possibly pertinent differences: i) The two equatorial carbonyls, roughly colinear with the Fel-Fe3 bond, are much closer to the metal plane than the remaining equatorial CO ligands. Whereas 026 lies 0.33 A below the plane and 0 2 4 lies 0.33 above it, the remaining equatorial oxygen atoms are all l.0w out of the plane. In the 'classic' 0 3 type strucPMe3)3,[42b1 all equatorial ligands lie similar distances out ture of, e.g., Ru~(CO),( of the metal plane. ii) Four of the carbonyls (C05, C O l l , C017 and C023), associated with the incipiently broken and re-formed pair of bridging carbonyls are distinctly bent, with Fe-C-0 angles 157", all others being essentially linear. In the 'classic' 0 3 type, the axial CO ligands have M-C-0 angles generally > 170".
-
-
3.4 Structure and Dynamics in Metal Curbony1 Clusters
Figure 5. ‘Mid-way’ structure taken from Fig. 4 (see text).
1013
W
This geometrical ‘mid-way’ structure is remarkably similar to the experimentally determined structure of F ~ R U ~ ( C O ) as ~ ~shown , [ ~ ~in I Fig. 6. The two carbonyl ligands C011 and C021 lie 0.3 A from the trimetal plane, whereas the remaining equatorial 0 atoms are at a mean distance of l.OA from this plane. In addition, both pseudo-axial carbonyls on the Fe atom C033 and C 0 3 4 are more bent (the average Fe-C-0 angle is 169.1’) than the Ru-C-0 angles. The structure of FeRuz(CO)I* has previously been described as of the 0 3 type,[641but the slight distortions from the ‘classic’ 0 3 type might be of considerable relevance in this context. Mann himself has proposed[561that the geometry found in Fe3(CO)lo{ 1,2-(M ~ ~ A S ) ~ C ~ isHthe ~ }half-way , [ ~ ~ Ipoint in his mechanism. The ligating atom disposition is very similar to that in Fig. 6, and both structures lend great plausibility to the Mann viewpoint. As outlined in a recent Dalton Perspective,[231there is new experimental evidence on the solid state structure of Fe3(C0)12 which further stirs up these rather murky waters. It has now been shown that Fe3(C0)12 undergoes a phase transition at approximately 210 K to give a second, disordered monoclinic phase. This phase is considerably more complex than the room-temperature phase, indeed it can be viewed as a superstructure of that phase. There are five crystallographically independent molecules. One molecule is essentially ordered at 100 K, while the remaining molecules all show, to a lesser or greater extent, a ‘Star of David’ type disorder. The ordered molecule has symmetrical CO bridges, and is very close to C2” symmetry. The reversibility of this phase change demonstrates beyond doubt that the disorder seen at room temperature is dynamic in origin. The solid-state NMR results of Hanson et ul. ,[531 would seem to imply that the reorientation of the Fe3 triangle inside the pseudo-icosahedral carbonyl polytope is rapid (on the NMR
-
1014
3 Dynamics and Physical Properties
Figure 6. ORTEP view of FeRuz( CO)12.
timescale) at room temperature in the crystalline material. Unfortunately, these new results on Fe3(C0)12 do not shed any further light on the pathway (or mechanism) whereby the triangle moves from one site to the other. Some evidence for possible pathways is afforded by the variable-temperature structures of the closely related molecules FezM(CO)12 7 M = and 8 M = O S , [ ~and ~ ] for F ~ R U ~ ( C O ) ,The ~ . [structure ~~] of 8 at room temperature was first reported by Churchill and F e t t i ~ ~ g e r There . ' ~ ~ ] are two independent molecules in the asymmetric unit, which are similar but not identical, and which have the Fe3(C0)12 structure with a pair of slightly asymmetric bridging carbonyls spanning the Fe-Fe bond. Both molecules show 'Star of David' disorder of the metal atoms, but in a ratio of 88 : 12 rather than 50 : 50 as seen for Fe3(CO),2. The threefold symmetry of the triangle is now broken by the presence of the 0 s atom. The second position is related to the primary image by a pseudo-inversion center or twofold axis, so that the 0 s atom is effectively rotated by 180". On cooling to 223 K this secondary image disappears for both molecules, and the structure becomes perfectly ordered. The change is reversible, clearly showing the disorder to be dynamic in nature. On warming to 373 K a phase-change occurred, giving a unit cell very similar in dimensions to Fe3(C0)12at room temperature. This indicates the onset of a further dynamic process, but unfortunately the crystal proved unstable at this temperature, and no data could be collected to confirm this. Variable-temperature 13C MAS NMR data on crystalline solid 8 were also obtained;r661they showed that the most likely mechanism was a staged process involving 60" jumps. The ruthenium analog 7 is isomorphous and isostructural with 8, and has almost identical behavior.[641The high-temperature phase change occurs at a slightly lower temperature, between 3 13 and 323 K. This centrosymmetric phase is isomorphous
3.4 Structure und Dynunzics in Metal Curbonyl Clusters
1015
Figure 7. Superimposition stereo view of the two independent molecules of FezRu(C0) 12.
and isostructural with the room-temperature phase of Fe3(C0)12, and 50 : 50 disorder of the Fe2Ru triangle sited at a crystallographic inversion center is found. Conversely, reducing the temperature of the non-centrosymmetric phase to 223 K results in a perfectly ordered structure, as observed for the osmium analog. The excellent quality of the crystal sample of 7 has yielded perhaps the most accurate determination to date of the Fe3(C0)12 structural archetype. The two independent molecules are similar but not quite identical. In one of the independent molecules, the bridging carbonyls are only marginally asymmetric, with bridge asymmetries of 0.073 and 0.085 A (e.s.d. 0.006 A), whereas in the other molecule the bridge asymmetry is more marked (bridge asymmetries of 0.207 and 0.180 A). A clear comparison between the two molecules is afforded by the superimposition diagram of the two molecules, shown in Fig. 7. The metal atoms have been fitted by least squares, and the misfit is minimal. If the set of vectors joining the related oxygen atoms in the two molecules (termed here the displacement coordinate) is considered as a whole, it can be seen that the direction of these vectors, and their magnitudes ( 0.1-0.3 A) are reasonably consistent with the oxygen atom anisotropic displacement parameters (adps), which have RMS displacements ranging from 0.15 to 0.35 A. This strongly implies that the experimentally determined adps of the two molecules (which are chemically identical, and only differ in the packing potentials they experience) actually represent true thermal motion, rather than a repository of systematic and non-systematic errors. This can be seen more clearly in a PEANUT[681stereo-view, in which the display shows the diference between the experimental adps and the calculated adps because of the rigid-body motion of the molecule as a whole.[691Fig. 8 shows the PEANUT plot for one of the independent molecules, the plot for the other molecule is very similar. Because the residual motion of the metal atoms is seen to be rather small,
-
1016
3 Dynamics and Physical Properties
Figure 8. PEANUT view of difference anisotropic displacement parameters for F qRu(C0)12
this picture gives, to a first approximation at least, a graphical display of the motion of the carbonyl polytope relative to the metal framework. Since both the displacement coordinate, and the pattern of the PEANUT difference adps is symmetric about the molecular C2 axis ( i e . is of A2 irreducible representation in point group C&),this result would suggest that the lowest energy soft mode of 7 also follows this symmetry. This in turn is in line with the J o h n ~ o n [ ’ ~ proposal ,’~] of a C2 ‘libration’ of the carbonyl polytope about the metal core, because it is expected that soft fluxional modes follow the symmetries of the normal modes.[701 , ~ [proved ~ ~ I more interesting than expected. The The structure of F ~ R u ~ ( C O ) has cluster also undergoes a phase transition, from a centrosymmetric disordered phase at temperatures above 228 K, to a non-centrosymmetric phase below this temperature. The non-centrosymmetric phase is disordered in the metal atom positions immediately below the phase-transition temperature, but becomes perfectly ordered at 173 K. As discussed above, the cluster has approximate C, symmetry, but the overall geometry is that of the 0 3 type structure with some small distortions. The molecular structure of 9 at room temperature is similar to that at 173 K, but there is considerable metal atom disorder, which is shown in Fig. 9. The FeRu2 triangle resides on a site of 2 / m crystallographic symmetry, so the disorder is more complex than in the previously discussed systems. There are two major ‘Star of David’ components at -82% populations, and a second set of four positions containing the remaining metal atom density. The two Ru atoms in these secondary images lie considerably out of the plane of the main ‘Star of David’ component, although the Fe atoms are coincident with the sites labelled Rul . It is proposed that
3.4 Structure and Dynamics in Metal Carhonyl Clusters
101I
Fela
Rulb Flu3
Ru3c
1 Rula
Figure 9. ORTEP view of the disordered metal atom positions of FeRur(C0)lz. Only two of the four minor components are shown.
these secondary images represent a pathway for the migration of the metal triangle from one of the 'Star of David' components to the other (i.e. an overall eflkctive rotation of 180"). Such a motion is necessary to convert the low-temperature ordered non-centrosymmetric phase to the disordered centrosymmetric room-temperature phase. In all these phase transformations, the most significant atomic motions are suffered by the metal atoms. There is a small complementary motion of the carbonyl ligand polytope which is also required, but the atomic displacements involved are generally ca 0.58L or less. The crystal lattice provides an absolute frame of reference, making it is possible to state categorically that the metal atoms must move. Because it is not possible to distinguish one oxygen atom, or one carbon atom, from another, it is impossible to determine, from the crystallographic evidence alone, whether the light atoms also move significantly, i.e. whether the whole molecule rotates intuct within the lattice. The close intermolecular packing[491indicates this is unlikely, and solid-state NMR data for FeZOs(CO)lz also strongly suggestr661 that the intact rotation does not occur. Bruce and have, moreover, demonstrated unequivocally that a similar 'Star of David' metal atom disorder in Ru3(CO)11(CNBur)is dynamic in origin, and it has been shown further that at 100 K this disorder disappears completely.[721Because in this instance the ligating C and the N atoms of the isonitrile ligand move by only -0.5-0.9& the metal atom triangle must be migrating inside a relatively fixed ligand polytope. In summary, it has now been conclusively demonstrated that the metal triangle disorder observed in the solid state structures of Fej(CO)IZ, FeZM(C0)lZ (M = Ru, Os), FeRuz(CO)IZ, and R u ~ ( C O ~ ) I(C NB U ') arises from dynamic phenomena. This is most likely to occur via effective rotation of the metal triangle inside a relatively rigid metal framework. Although there is undoubtedly some breathing motion of the ligand polytope, it does not seem that there is substantial light atom motion in
1018
3 Dynamics and Physicul Properties
the solid. The barrier to this metal atom migration is rather small, and it is likely that rapid reorientation is taking place in the crystalline phase at room temperature. Although the exact mechanism has yet to be proved, there is a substantial body of evidence to suggest that this effective rotation can occur by out-of-plane motions of the metal triangle. The intermediate molecular geometry is almost certainly that of the 0 3 type, or something very close to it.
3.4.5 X-ray absorption spectroscopy studies of metal carbonyl clusters The characterization of structure and dynamics in metal carbonyl cluster chemistry has understandably been based around single-crystal X-ray diffraction and NMR spectroscopic studies, respectively. As has been shown in the earlier part of this chapter, these sources of data can be used in conjunction to provide a coherent picture of the possible molecular configurations of a given species and the processes by which they interconvert, at least in favorable cases. In general however, it is not possible to determine the geometric structures of these species in solution, nor to obtain direct experimental evidence to confirm that the same structure( s) obtain in solution as in the solid state. Fortunately the rebirth and exploitation of X-ray absorption spectroscopy ( XAS) and in particular the application of synchrotron X-ray sources and improved methods of data analysis to EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy has provided exactly this sort of experimental evidence. In this section we provide selective coverage of such work as relates to the structure and dynamics of metal carbonyl clusters. As became evident following the pioneering work of Lytle et al. ,[73-761 XAS is a unique tool for probing the local environment of X-ray absorbing atoms, and providing element-specific information on bonded and non-bonded distances, coordination numbers, and Debye-Waller factors, as well as information about the identity of the neighboring atoms. The potential of this technique for the study of cluster complexes and related systems was noted early and it has been applied with success over the past two decades. The applications of EXAFS spectroscopy to metal carbonyl cluster chemistry can be subdivided into a number of broad and overlapping areas: i) determination of the structures of cluster complexes in the solid state; ii) determination of the structures of cluster complexes in solution; iii) determination of the structures of clusters attached to supports (such as metal oxides or polymers);
3.4 Structure and Dynamics in Metal Carhonyl Clusters
1019
iv) study of dispersed metal particles on supports derived from cluster complexes; and v) study of the flexibility and rearrangement of the geometry of metal cluster complexes in solution. Although the topic of this chapter is the last of these areas, it is worth noting the impressive body of work that has accumulated in the other areas because they lay the foundations for this last subject. A b initio determination of the structures of cluster complexes in the solid state by XAS methods is often a substitute for X-ray crystallographic methods, but provides only limited information in comparison. Nevertheless as an adjunct to studies of types (ii)-(v) it is invaluable because in those areas single-crystal studies are, of course, not possible. Several studies have the potential of EXAFS as a probe of molecular structure for metal cluster complexes. For example, Evans and co-workers demonstrated[78a1the potential of EXAFS spectroscopy for the determination of cluster skeleton geometry in high nuclearity clusters such as the dianion [o~loc(Co)2412-. In area (ii) the main focus has been to test the hypothesis that the solution structure is the same as that in the crystalline solid, or to confirm that it is not. Thus an important study by Evans and ~ o - w o r k e r saddressed [~~~ the thorny problem of the structure of Fe3(C0)12 in petroleum and frozen CH2C12 solutions, using to advantage a key technical development in the field the treatment of multiple-scattering processes in EXAFS studies of linear and near-linear systems such as found when terminal (and bridging) carbonyl ligands bound to the absorbing element.[791With care the average M-C-0 angle can be determined in these circumstances and this is used to distinguish between alternative possible structures. As a consequence of these studies[521the carbony1 ligands in Fe3(C0)12 were assigned as occupying primarily terminal sites in petroleum but a substantial proportion are in bridging sites in frozen CH2C12 solution. Clearly this type of study is intimately linked with those of type (v). The ability to conduct EXAFS studies on complexes in non-crystalline phases and in particular on active catalyst systems in situ has been the driving force for much of the development and application of the technique to the analysis of cluster geometry. In the earlier phase of this sort of work, i.e. studies of type (iii), the objective was to identify the cluster species present on the support and the nature of the cluster-support interaction. Three general modes of attachment of cluster complexes to supports have been explored: -
i) those in which support atoms (usually oxygen) are bonded directly to metal atom(s) of the cluster complex;[801 ii) those in which the support is functionalized so as to bear pendant groups able to bind to metal atoms;["] iii) entrapment of the cluster inside the pores of a zeolite.[821
1020
3 Dynamics and Physical Properties
In each of these it has been shown that in conjunction with other techniques (notably IR spectroscopy) EXAFS spectroscopy can give valuable information about both cluster-support interactions and cluster geometry. Often the objective of tethering the cluster to a support is to activate it by thermal or chemical treatment and to exploit its reactivity as a catalyst for reactions of small (gaseous) molecules.[s31A host of studies of this sort have incorporated EXAFS spectroscopy as an invaluable adjunct to the more traditional techniques of heterogeneous catalysis. The field has been reviewed by Gates,[841K ~ n i n g s b e r g e r ,and ~~~] their co-workers who have been among the leaders in the field. Although homogeneous catalysis studies of metal-cluster species are rather rare, XAS has proved its worth in that field also, albeit primarily for mononuclear complexes.[* In our own work there have been two main strands the determination of solutionphase structures of heteronuclear clusters and the study of cluster reactions in which the metal framework is assembled or rearranges. In this work we have sought to obtain enhanced structural data as a result of the presence of more than one X-ray absorbing element (metal) in the cluster framework. Furthermore the presence of platinum or palladium in many of these systems leads to much structural variability. In all of fields (i)-(iv) the capacity of XAS to explore atomic environments in an element-specific manner enables additional detail to be extracted for heteronuclear clusters. This is especially so for those clusters in which the metal atoms have substantially different atomic numbers (e.g. Ru-Pt species) because the absorption edges of the different metals do not interfere strongly with one another. -
3.4.5.1 Cluster structures in solid and solution We have studied a range of heteronuclear cluster compounds containing Ru3Pt, Os3Pt, Ru3Pt2, FezOs, and MozPtz cores in solid and solution phases to establish, or confirm, the geometry of the solid species and, occasionally, to monitor such lo( PR3)] changes that occur upon dissolution. For Ru3Pt(,u-H)(p3-COMe)(CO) spectra measured in both solid and THF solution, and both Ru K and Pt LIII absorption edges,[861show that the complex adopts essentially identical tetrahedral structures with two short Pt-Ru distances and one long one. For Fe;?Os(C0)12 EXAFS spectra measured (Fe and 0 s edges) in THF and in the solid state at five different temperatures confirm that solid and solution-phase structures are, again, essentially identical.r661The complex PtzM02(C0)6(PCy3)2Cp2 is known to occur as two in the solid state:
i) tetrahedral, with each metal atom bonded to the other three; and ii) planar, with the four metal atoms in a -Pt-Mo-Pt-Mosquare. Pt
Llll
edge EXAFS spectra measured on each isomer confirmed their structures
3.4 Structure and Dynamics in Metal Carhonyl Clusters
1021
in the solid state. Those for the tetrahedral species in THF solution indicated that it retains its geometry. Finally, small differences between the solid and solution phase EXAFS Pt LIII edge spectra are observed for the spiked-triangular species (X = N, NO) which have two long and complexes HRu3Pt(p4-X)(CO)IO(PPr13) one short Pt-Ru distances; this is indicative of slight lengthening of the Pt-Ru bonds in solution. In general, therefore, there is little evidence from these studies of potentially flexible complexes of substantial variation of cluster geometry on dissolution.
3.4.5.2 Cluster reaction studies In two studies we have investigated the transformation of the spiked triangular cluster Ru3Pt(p-H)(p4-CCBu')(CO)g(dppe)into the butterfly cluster Ru3Pt(p4-q2C=CHBu')(CO)g(dppe) (Eq. 1) and the formation of the tri-nuclear cluster complex WCo2(CO)&-CCsHdMe-4)Cp, from W ( C O ) ~ ( C ~ H ~ M ~and - ~ ) C oP~ ( C 0 ) sin THF (Eq. 2).
These reactions have desirable properties for such studies in being quantitative (as measured by NMR spectroscopy) and having a unique metal atom from which to view the progress of the reaction (Pt or W respectively). Therefore Pt and W LIII-
3 Dynamics and Physical Properties
1022
4 2
0 -2 -4
7
I
I
9
11
k
I
I
13
15
17
Figure 10. Platinum Llll edge EXAFS [k3x(k)] for the reaction shown by Eq. (1) measured over 12 h.
edge EXAFS spectra are the probes of choice for the reactions shown in Eqs. (1) and (2). The spectra and their sine transforms show the effects of the reaction on the environment of the unique metal during the reaction. The reactions were monitored in situ, by transmission EXAFS (and QuEXAFS) methods over periods of up to 12 h and a variety of procedures was developed to meet two principal objectives. i) The extraction of relative concentrations, and hence of rate-constant information, for a reaction containing just two EXAFS-sensitive species X and Y, i. e. of the sort X + Y. Thus from a series of EXAFS spectra measured as the reaction proceeds, the relative concentrations of each species ( X and Y ) were determined, and hence the rate constant of the reaction was determined. ii) The extraction of the individual component spectra from a spectrum (or spectra) of mixtures of species X and Y obtained during the course of reactions (as above). For both reactions geometries derived from XAFS spectra for the isolated starting materials and products are in good agreement with the available crystallographic data.[87,881 In each case for both sine transform and k-EXAFS spectra of the reaction mixtures, isosbestic points were present indicative of an equilibrium involving only two species. Two alternative approaches to the extraction of the concentration and rate-constant information were investigated use of the k-space data (i.e. EXAFS spectra) and use of r-space data (i.e. the sine transforms). For reaction A the k-space method proved more reliable and robust, notably for the part of the spectrum with 8 < k < 11 k' (see Fig. 10). In contrast, for reaction B the r-space method was more useful in the region 2.3 < Y < 2.7 A (where the W-Co -
-
3.4 Structure and Dynamics in Metal Curbonyl Clusters
1023
4
2
0
2
4
Figure 11. Tungsten L T , ~ edge EXAFS Fourier sine transforms for the reaction shown by Eq. (2).
-'
5
contacts develop in the course of the reaction, see Fig. 11). Rate constants for the reactions were calculated by iterative least squares fits to the affected regions of the k3-weighted EXAFS spectra (for Eq. 1) or the sine transform (for Eq. 2) yielding values of 0.004(1) min- 1 and 0.0029(5) min-', respectively.[881 The extraction of individual component spectra from the spectrum of mixtures is less straightforward. Thus the computed spectrum of the product in Eq. (1) was well reproduced (see Fig. 12) in general although the features in the region used for fit6
4
2
0
2
4
Figure 12. Platinum Llll edge EXAFS spectra as calculated (0) and observed (+) for the product in Eq. (1).
5
7
9
11
kiAI)
1 3
1 5
7
1024
3 Dynamics and Physical Properties
ting and extraction of concentration and rate-constant data are highly sensitive to the details of the procedure adopted and hence susceptible to distortions. The sensitivity is perhaps to be expected given that subtractions of spectra are being used to extract both concentration and computed spectrum information. The experimental data must be of exceptionally high quality to provide accurate deconvolution of the individual spectra. It is clear that structure determination of cluster species in solution is a valuable adjunct to crystallographic methods in this field, providing data on solution-phase geometries which are not otherwise available. Rate-constant information can be successfully extracted from EXAFS data by either k-space and r-space methods. The merit of the r-space method is partly the noise removal inherent in the Fourier transformation of the spectra. In contrast, the k-space method probably places more stringent requirements on data quality although Fourier filtering provides some amelioration of the situation. Spectra of mixtures have been deconvoluted although the resultant computed spectrum is highly sensitive to the details of the procedure adopted and to data quality.
References [ l ] L. Mond, H. Hirtz and M. D. Coop, J. Chem. Soc., 1910, 798. [2] J. Dewar and H. 0. Jones, Proc. R. Soc. London, Ser. A,, 1905, 76, 558. [3] J. Dewar and H. 0. Jones, Proc. R. Soc. London, Ser. A,, 1907, 79, 66. [4] W. Hieber, Adv. Organomet. Chem., 1970, 8, 1. [5] H. M. Powell and R. V. G. Ewens, J. Chem. Soc., 1939,286. [6] (a) P. Corradini and G. Paiaro, Ric. Sci. 1966, 36, 365 (b) C. H. Wei and L. F. Dahl, J. Am. Chem. Soc., 1969, 91, 1351. (c) F. A. Cotton and J. M. Troup, J. Am. Chem. Soc., 1974, 96, 4155. (d) D. Braga, F. Grepioni, L. J. Farrugia and B. F. G. Johnson, J. Chem. Soc., Dalton Trans., 1994, 29 11 [7] (a) P. Corradini, J Chem. Phys. 1959, 31, 1676 (b) C. H. Wei, Znorg. Chem. 1969, 8, 2384. (c) F. H. CarrC, F. A. Cotton and B. A. Frenz, Znorg. Chem. 1976, 15, 380. [8] C. H. Wei, G.R. Wilkes and L. F. Dahl, J. Am. Chem. Soc. 1967 89,4792. [9] M. R. Churchill and J. P. Hutchinson, Znorg. Chem. 1978, 17, 3528. [lo] For a comparative review of electron counting in clusters see: S. M. Owen, Polyhedron, 1988, 7, 253. [ I l l M. R. Churchill, F. J. Hollander and J. P. Hutchinson, Inorg. Chem. 1977, 16, 2655. [12] M. R. Churchill, B. G. DeBoer, Inorg. Chem. 1977, 16, 878. [13] (a) R. E. Benfield and B. F. G. Johnson, J. Chem. Soc. Dalton Trans. 1978, 1554. (b) R. E. Benfield and B. F. G. Johnson, J. Chem. Soc. Dalton Trans. 1980,1743 [14] B. F. G. Johnson and Y . V. Roberts, Polyhedron, 1993, 12, 977 and refs therein. [15] (a) E. L. Muetteries and E. Band, Chem. Rev., 1978, 78, 639. (b) B. F. G. Johnson and R. E Benfield in Transition Metal Clusters, ed. B. F. G. Johnson, J. Wiley, Chichester, 1980, Ch. 7. [16] F. A. Cotton and D. L. Hunter, Znorg. Chim. Acta, 1974, 11, L9. [I71 (a) J. L. Vidal, R. C. Schoening, R. L. Pruett and W. E. Walker, Inorg. Chem., 1979, 18 129. (b) 0. A. Gansow, D. S. Gill, F. J. Bennis, J. R. Hutchison, J. L. Vidal and R. C. Schoening, J. Am. Chem. Soc., 1980, 102, 2449.
3.4 Structure und Dynamics in Metal Curhonyl Clusters
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[ 181 J. Jellinek and Z. B. GiivenG, in The Svnergy Betwwen Dynamics and Reactivity at Clusters and Surfaces, ed. L. J. Farrugia, NATO AS1 Series C, Vol465, Kluwer, 1995, p 217. [I91 G. E. Hawkes, L. Y. Lian, E. W. Randall and K. D. Sales, J. M a p . Reson, 1985, 65, 173. [20] (a) A. D. Bain and J. A. Cramer, J. Mayn. Reson., 1993, 103, 217. (b) A. D. Bain and J. A. Cramer, J. Magn. Reson., 1996, 118, 21. [21] (a) L. J. Farrugia, in Comprehensive Oryanometallic Chemistry II 1995, Ed. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon, Oxford, Vol 10, Ch. 4. (b) G. Schmid, G. Longoni and D. Fenske, in Clusters and Colloids, From Theory to Applications Ed. G. Schmid, VCH, Weinheim, 1994, pp 99-104. (c) D. Braga, Chem. Rev. 1992,92, 633. [22] (a) J. J. Turner, F.-W. Grevels, S. M. Howdle, J. Jacke, M. T. Haward and W. E. Klotzbucher, J. Am. Chem. Soc., 1991, 113, 8347. (b) H. L. Strauss, J. Am. Chenz. Soc., 1992, 114, 905. (c) J. J. Turner, C. M. Gordon and S. M. Howdle, J. Phys. Chem., 1995, 99, 17532. (d) V. J. Johnson, F. W. B. Einstein and R. K. Pomeroy, Organometallics, 1988, 7, 1867. [23] L. J. Farrugia, J. Chem. Soc., Dalton Trans. 1997, 1783. [24] S. Forsen and R. A. Hoffman, J. Chem. Phys., 1963, 39, 2892. [25] (a). J. Jeener, B. H. Meier, P. Bachman and R. R. Ernst, J Chem. Plzys., 1979, 71, 4546. (b) C. L. Perrin and T. J. Dwyer, Chem. Rev., 1990, 90, 935. (c) E. W. Abel, T. P. J. Coston, K. G. Orrell. V. Sik and D. Stephenson, J. M a p Reson., 1986, 70, 34. (d) R. Willem, Proy. Nucl. M a p Reson. Sprctrosc. 1987, 20, 1. (e) K. G. Orrell, V. Sik and D. Stephenson, Proy. N M R Spectrosc., 1990, 22, 141. [26] (a) S. Aime, 0. Gambino, L. Milone, E. Sappa and E. Rosenberg, Inorg. Clzim. Acta 1975, 15, 53. (b) C. Jangala, E. Rosenberg, D. Skinner, S. Aime and L. Milone, Inorg. Chem. 1980,19,1571. [27] E. Rosenberg, E. V. Anslyn, C. Barner-Thorsen, S. Aime, D. Osella, R. Gobetto and L. Milone, Organometallics 1984, 3, 1790. [28] (a) G . Predieri, A. Tiripicchio, C. Vignali, E. Sappa J. Organomet. Chem. 1988, 342, C33. (b) E. Sappa, G. Predieri, A. Tiripicchio, A. Vignali, J. Oryunomet. Chem. 1984, 378, 109. [29] (a) Y. Chi, B.-J. Liu, G.-H. Lee and S.-H. Peng, Polyhedron, 1989, 8, 2003. (b) D.-K. Hwang, Y. Chi, S.-M. Peng and G.-H. Lee, Organometallics, 1990, 9 2709. [30] L. J. Farrugia and S. E. Rae, Organometallic.s, 1992, 11, 196. [31] A. Orlandi, U. Frey, G. Suardi, A. E. Merbach and R. Roulet, Inory. Chem., 1992, 31, 1304. [32] J. B. Keister, U. Frey, D. Zbinden and A. E. Merbach, Oryanometallics, 1991, 10, 1497. [33] V. G. Albano and D. Braga, in Accurate Molecular Structures: Their Determination and Importance, eds, A. Domenicano and I. Hargittai, Oxford University Press, 1992, pp 530-553. [34] (a) K. P. Hall and D. M. P. Mingos, Prog. Inorg. Chem. 1984, 32, 237. (b) I. D. Salter in Comprehensive Oryanometallic Chemistry I I 1995, Ed. E. W. Abel, F. G. A. Stone and G . Wilkinson. Pergamon, Oxford, Vol 10, Ch. 5. [35] (a) A. Bianchini and L. J. Farrugia Organometallics 1992, 11, 540. (b) E. Rosenberg, K. I. Hardcastle, M. W. Day, R. Gobetto, S. Hajela and R. Muftikian Oryanometallics, 1991, 10, 203. (c) P. J. Bailey, L. H. Gade, B. F. G. Johnson and J. Lewis Chem. Ber. 1992, 125, 2019. [36] (a) P. Braunstein, C. de Meric de Bellefon, S.-E. Bouaond, D. Grandjean, J.-F. Halet and J.-Y. Saillard, J. Am. Chem. Soc., 1991, 113, 5282. (b) F. A. Cotton, Inorg. Chem., 1966, 5, 1083. [37] B. T. Heaton, L. Strona and S. Martinengo, J. Organomet. Chem., 1981, 215, 415. [38] C. Allevi, B. T. Heaton, C. Seregni, L. Strona, R. J. Goodfellow, P. Chini and S. Martinengo, J. Chem. Soc., Dalton Trans., 1986, 1375. [39] D. Braga and B. T. Heaton, J. Chem. Soc., Chem. Commun. 1987, 608. [40] B. F. G. Johnson, J. Chem. Soc., Chem Commun., 1976, 211. [41] G. Laurenczy, G. Bondietti, A. E. Merbach, B. Moullet and R. Roulet, Helu. Chim. Acta 1994, 77, 547 [42] (a) M. I. Bruce, J . G. Matisons, R. C. Wallis, R. M. Patrick, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1983, 2365. (b) M. I. Bruce, J. G. Matisons, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1983, 2375. ( c ) M. I. Bruce, M. J. Liddell, C. A. Hughes, J. M. Patrick, B. W. Skelton and A. H. White, J. Oryanomet. Chem., 1988, 347, 181.
1026
3 Dynamics and Physical Properties
[43](a) J. W. Lauher, J. Am. Chem. Soc., 1986,108, 1521. (b) A. Sironi, Inorg. Chem. 1996,35, 1725. [44]S.Aime, W. Dastru, R. Gobetto, J. Krause and L. Milone, Organometallics, 1995,14, 4435. [45](a) S. Aime, 0. Gambino, L. Milone, E. Sappa and E. Rosenberg, Inorg Chim Acta, 1975,15, 53. (b) A. Forster, B. F. G. Johnson, J. Lewis, T. W. Matheson, B. H. Robinson and W. G. Jackson, J. Chem. Soc., Chem. Commun., 1974,1042. [46]A. A. Koridze, 0. A. Kizds. N. M. Astakhova, P. V. Petrovskii and Y . K. Grishin, J. Chem. Soc., Chem. Commun., 1981,853. [47](a) S. Aime, M. Botta, R. Gobetto, D. Osella and L. Milone, Inorg. Chim. Acta., 1988, 146, 151. (b) T.H.Walter, L. Reven and E. Oldfield J. Phys. Chem., 1989,93, 1320. [48](a) R. F. Alex, F. W. B. Einstein, R. H. Jones and R. K. Pomeroy, Znorg. Chem. 1987, 26, 3175.(b) J. Pursiainen, T. A. Pakkanen, M. Ahlgren and J. Valkonen, Acta Crystallogr., Sect C., 1993,49, 1142. [49]D. Braga, F. Grepioni, E. Tedesco, M. J. Calhorda and P. E. M. Lopes J. Chem. Soc., Dalton Trans. 1995,3297. [50]See for example: S. Dobbs, S. Nunziante-Cesaro and M. Maltese, Inorg. Chim. Actu, 1986, 113, 167 and refs therein. [51]F.-W. Grevels, J. Jacke and K. Seevogel, J. Mol. Structure, 1988,174, 107. [52]B. Binsted, J. Evans, G. N. Greaves and R. J. Prince, J. Chem. Soc. Chem. Cornmun., 1987,1330. [53] 13. E. Hanson, E. C. Lisic, J. T. Petty and G. A. Iannaconne, Inorg. Chem., 1986,25,4062. [54]S . Aime and R. Gobetto, J. Cluster. Sci., 1993,4, 1. [55] (a) F. Grandjean, G. J. Long, C. G. Benson and U. Rosso, Inorg. Chern. 1988,27, 1524,(b) F. Grandjean and G. J. Long, Inorg. Chem. 1996,35,4532. [56]B. E.Mann, J. Chem. Soc., Dalton Trans. 1997, 1457 and refs therein. [57](a) B. F.G. Johnson and A. Bott, J. Chem. Soc., Dalton Trans., 1990,2437.(b) D. Braga, C. E. Anson, A. Bott, B. F. G. Johnson and E. Marseglia, J. Chem. Soc., Dalton Trans, 1990,3517. (c) B. F.G. Johnson, Y. V. Roberts and E. Parisini, J. Chem. Soc., Dalton Trans., 1992,2573. (c) B. F. G. Johnson, E. Parisini and Y. V. Roberts, Orgunometallics, 1993, 12, 233. [58]H. Adams, N. A. Bailey, G. W. Bentley and B. E. Mann, J. Chem. Soc., Dalton Trans., 1989, 1831. [59]http://www.rsc.org/is/journals/current/dalton/fe3carb.htm [60]D. Lentz and R. Marschall, Organornetallics, 1991,10,1487. [61]B. F. G. Johnson J. Chem. Soc., Dalton Trans., 1997,1473 [62]B. E. Mann, Organometallics 1992,11, 481. [63]H. Adams, X.Chen and B. E. Mann, J. Chem. Soc., Dalton Trans. 1996,2159. [64]D.Braga, L. J. Farrugia, A. L. Gillon, F. Grepioni and E. Tedesco, Organornetullics, 1996,15, 4684. [65]A. Bino, F. A. Cotton, P. Lahuerta, P. Puebla and R. Uson, Inorg. Chem. 1980,19, 2357. [66]L. J. Farrugia, A. M. Senior, D. Bragd, F. Grepioni, A. G. Orpen and J. G. Crossley, J. Chem. Soc., Dalton Trans., 1996,631. [67]M. R. Churchill and J. C. Fettinger, Organometallics, 1990,9, 446. [68]W. Hummel, J. Hauser and H.-B Burgi, J. Mol. Graphics 1990,8,214. [69]W. Hummel, A. Raselli and H.-B Biirgi, Acta Cryst. 1990,B46, 683 and refs therein. [70]B. F. G. Johnson and A. Rodgers, in The Chemistry of Metal Cluster Complexes Eds D. F. Shriver, H. D. Kaesz and R. D. Adams, VCH, Weinheim. 1990,p 303. [71]M. I. Bruce, G. N. Pain, C. A. Hughes, J. M. Patrick, B. W. Skelton and A. H. White, J. Organomet. Chem. 1986,307,343. [72]L. J. Farrugia, C. Rosenhahn and S. Whitworth, J. Clust. Sci., 1998,9, 505. [73]D. E. Sayers, E. A. Stern and F. W. Lytle, Phys. Rev. Lett., 1971,27, 1204. [74]B. K. Teo and D. C. Joy (Eds.), EXAFS Spectroscopy Techniques and Applications, Plenum, New York, 1981.
3.4 Structure und Dynuinics in Metal Curhonyl Clusters
1027
[75] B. K. Teo, EXAFS: Busic Principles And Data Anal-vsis, Springer-Verlag, Berlin, 1986. [76] D. C. Koningsberger and R. Prim (Eds.), A'-Ray Absorption, Wiley, London, 1988. [77] (a) T.E. Wolff, J.M. Beng, K.O. Hodgson, R.B. Frankel and R.H. Holm, J. Am. Chem. Soc., 1979. 101, 4140. (b) F. W. Lytle, G.H. Via and J.H. Sinfelt, in Synchrotron Rudiution Reseurch, H. Winich and S. Doniach (Eds.) Plenum New York, 1980. (c) R. Psaro. R. Ugo, G.M. Zanderighi, B. Besson; A.K. Smith and J.M. Basset, J Orgunomet Chem., 1981, 213, 215. [78] (a) S.L. Cook. J. Evans, G.N. Greaves, B.F.G. Johnson, J. Lewis, P.R. Raithby, P.B. Wells, P. Worthington. J. Cheni. Soc., Cheni. Commun., 1983, 777. (b) F.B.M. Vanzon, P.S. Kirlin, B.C. Gates, and D.C. Koningsberger, J. Phys. Chem., 1989, 93, 2218. (c) M. C. Fairbanks, R. E. Benfield, R. J. Newport and G. Schmid, Solid State Communications 1990, 73, 431. [79] (a) S.J. Gurman, N. Binsted and I. Ross, J. Phys. C, 1986, 19, 1845, (b) A. Filipponi, A. Dicicco, R. Zanoni, M. Bellatreccia, V. Sessa, C. Dossi, R. Psaro, Chem. Phys. Lett., 1991, 184, 48 5 [SO] (a) S.L. Cook, J. Evans, and G.N. Greaves, J. Chem. Soc., Clzrm. Cornmun, 1983, 1287. (b) V.D. Alexiev; N. Binsted, J. Evans, G.N. Greaves and R.J. Price, J. Chem. Soc., Cheni. Commun: 1987, 395. (c) N. Binsted, J. Evans, G.N. Greaves, and R.J. Price, Oryanometallics, 1989, 8, 613. [81] V.D. Alexiev, N. Binsted, S.L. Cook. J. Evans, R.J. Price, N.J. Clayden, C.M. Dobson, D.J. Smith and G.N. Greaves, J. Chrm. Soc., Dalton Trans., 1988, 2649. 1821 (a) S. Kawi, J.R. Chang and B.C. Gates, J. Am. Chem. Soc., 1993, 115,4830. (b) W.A. Weber and B.C. Gates, J. Phys. Chem. B, 1997, 101: 10423. [83] (a) N. Binsted, S.L. Cook, J. Evans and G.N. Greaves, J. Chem. Soc., Chem. Commun, 1985, 1103. (b) A S . Fung, M.J. Kelley, D.C. Koningsberger and B.C. Gates, J. Am. Chern. Soc., 1997, 119, 5877. (c) J.R. Chang, D.C. Koningsberger and B.C. Gates, J. Am. Chem. Soc., 1992, 114, 6460. [84] B.C. Gates, Chem. Rei.. 1995, 95, 511. [85] J. Evans. Cliem. Soc. Rev. 1997, 11. [86] D. Ellis, L.J. Farrugia, P. Wiegeleben, J.G. Crossley, A.G. Orpen, P.N. Waller, Organometullics, 1995. 14, 481. [87] A.J. Dent, L.J. Farrugia. A.G. Orpen and S.E. Stratford, J. Chem. Soc., Chem. Commun., 1992, 1456. [88] D. Ellis. L. J. Farrugia. J. G. Crossley, A. G . Orpen and P. N. Waller, unpublished results.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.5 Reversible Skeletal Rearrangements in Transition Metal Clusters Paul J. Dyson
3.5.1 Introduction The possibility that clusters may have catalytic properties was proposed shortly after their initial discovery and it did not take long for chemists to establish that As with clusters can be used to catalyze a range of organic transformation~.[~-~] mononuclear inorganic and organometallic catalysts, clusters can undergo ligand dissociation as the first step in the catalytic process. Unlike mononuclear species, however, clusters can also attain a state of coordinative unsaturation via metalmetal bond scission, thus providing an alternative mechanism for substrate association in catalytic processes. In fact, many clusters are thought to undergo complete fragmentation, with the mononuclear fragments acting as the active catalytic species. In other processes it has been established that the cluster remains intact throughout the reaction. This chapter explores the different types of reversible polyhedral rearrangement that clusters undergo, because they might be related to rearrangements which take place in cluster catalysis. Polyhedral rearrangements are quite varied, many take place in solution spontaneously on the NMR timescale, and the theoretical arguments behind some of the more commonly observed rearrangements have been investigated and this area is well documented e l s e ~ h e r e [and ~ ~ is ~ ]not discussed here. This article is primarily concerned with reversible skeletal rearrangements that require changes in the ligand shell, such as the addition of a ligand, to cause the cleavage of a metal-metal bond. A subsequent ligand elimination (not necessarily the same ligand as that introduced) returns the cluster to its original polyhedron. With low nuclearity clusters, in which the metal-metal bonds are regarded as localized two-center-two-electron bonds, the addition of a two electron-donor ligand results in cleavage of one metal-metal bond, however, higher nuclearity clusters, which require a delocalized bonding model to account for their shape, often undergo far more complicated rearrangements. The types of low-energy poly-
3.5 Reversible Skeletal Rearrangements in Transition Metal Clusters
1029
hedral transformation which occur in solution on the NMR timescale are described briefly, before more detailed discussion of some chemically induced transformations.
3.5.2 Stereochemical non-rigid behavior of metal cluster polyhedra Cluster skeleton rearrangements which can be studied by variable temperature NMR spectroscopy are of comparatively low energy and are often characterized by the presence of weak metal-metal interactions within the cluster core or a lowenergy pathway between two polyhedra with similar relative energies. Although these low energy skeletal transformations are not the main subject of this article, some key examples will be briefly described. The polyhedral motion can often be quite subtle, for example, the wings of the tetraplatinum butterfly cluster, Pt4(C0)5L4 ( L = PEt3, PMe3, PMezPh, PPh2Me, or PEtzBu) apparently undergo a flapping motion.r6a1Cluster core isomerization from planar to tetrahedral has been investigated for Pt2Mo2 clusters.[6b1The heteronuclear cluster PtOs3(CO)9(p3-S)2(PMe2Ph)z can be viewed as a p2-platinum bis-phosphine fragment bonded to an open triangle of 0 s atoms.['] The platinum fragment migrates from one 0 s - 0 s edge to the other as shown in Scheme 1. The migration of certain metal fragments over otherwise rigid metal units is quite common; in particular, Cu, Ag, Au, Pt, and Hg units are frequently found to undergo site exchange.['] In the heteronuclear tetrahedral clusters, H ~ F ~ O S ~ R U ~ - ~ ( C O ) ~ ~ ( n = 0, 1, and 2) the cluster polyhedron undergoes a breathing mechanism, which enables the metals to interchange positions.['] The stacked platinum clusters, [Ptn(C0)2,l2- ( n = 6, 9, 12, 15, and 18) consist of trigonal prismatic stacked triangular Pt3 units. The metal-metal bonds within the triangular units are significantly shorter than those between the layers. Variable temperature 195PtNMR indicates that the metal polyhedra undergo interconversion between prismatic and antiprismatic structures via the rotation of the triangular units relative to each other."'] Several high-nuclearity rhodium clusters are observed to undergo polyhedral rearrangements on the NMR timescale at ambient temperatures and some gold clus-
Scheme 1
1030
3 Dynamics and Physical Properties
ters undergo polyhedral rearrangements that are even too fast to be detected on the NMR timescale. Examples of fluxional rhodium clusters include [Rhl2Sb(C0)27l2-, [RhloE(C0)22In- ( E = P or As, n = 3; E = S, n = 2), and [RhgE(C0)21l2- ( E = P or In general, the low-temperature NMR spectra of these clusters agree with the structures observed in the solid state by single-crystal X-ray diffraction, but at higher temperatures all the Rh atoms are usually found to be in one chemical environment. Mechanisms have been proposed to explain these changes and the diamond-square-diamond (or inverse diamond-square-diamond) mechanisms are considered the most likely. The gold clusters which undergo fluxional polyhedral rearrangements have centered pseudospherical polyhedra in which the bonding between the central Au atom and the peripheral atoms is stronger than that connecting the peripheral Skeletal isomers of [Au9(P(C6H40Me-p)3}][N03]3 have been observed in the solid state; one structure might be derived from an icosahedron with the other based on a centered crown geometry of Au atoms.['51What is clear is that the energy difference between the two structures is small and the structures rapidly interconvert in solution. Although skeletal isomers are seldom observed in the solid state, spectroscopic evidence in solution suggests that average structures or facile intramolecular skeletal rearrangements frequently take place.
3.5.3 Polyhedral rearrangements in trinuclear clusters The transformation observed in the cluster skeleton upon addition of CO to the trirhodium cluster Rh3(CO)3(p-PBur)3is unusual in that the overall structure does not change, instead the Rh-Rh bonds lengthen." The cluster Rh3(C0)3(pu-PBu')3 reacts with CO (1 atm) in hexane or THF to afford Rh3(CO)j(pu-PBu')3in high yield. Comparison of the Rh-Rh bond lengths of these clusters reveals that the former cluster contains three typical Rh-Rh bonds (mean 2.65 A) whereas in the latter cluster one bond is 2.78A and the other two are longer (mean 3.02A). Although these distances have lengthened considerably they are still within the upper limits that are generally considered to constitute a Rh-Rh single bond. In Rh3(C0)3(pu-PBu')3,however, each Rh atom formally has 16 valence electrons and for this to remain constant in the product, Rh3(C0)j(p-PBuf)3, two Rh-Rh bonds must be broken. If the bonds are considered to remain intact then two of the Rh atoms have formal valence-electron counts of 18 which is still entirely feasible. The reaction is easily reversed and solutions of R ~ ~ ( C O ) S ( ~ - P Blose U ' ) CO ~ under vacuum with the regeneration of Rh3(CO)3(p-PBut)3.Over prolonged periods of time crystals of Rh3(CO)j(p-PBur)3 also eliminate CO to regenerate the precursor compound. Although in this example it is not clear whether any bonds are actually broken,
3.5 Reversible Skeletal Rearrangements in Transition Metul Clusters
1031
Scheme 2
two of the metal-metal bonds may be broken and reformed in the phosphinidyne, diironmanganese clusters CpMnFe2(CO)s(,u3-PR)( R = Me, Et, Bu", Ph, or MeOCgH4).[171The bonds susceptible to scission are the Mn-Fe bonds which undergo sequential cleavage by reaction with CO (Scheme 2). The reaction of CpMnFe2(C0)8(p3-PR) under a blanket of CO affords CpMnFe2(CO)9(,u3-PR)in which one CO ligand has been added. The addition of a second CO ligand requires more rigorous conditions (viz. 10 atm) affording CpMnFez( CO)lo(,u3-PR).These reactions are reversible with the loss of CO achieved by heating at 80 "C under an inert atmosphere. A similar process takes place when CpMnFez(CO)g(,u3-PR) is reacted with two electron nucleophiles including phosphines, phosphites, arsines and stibines. A reversible rearrangement that has important implications in heterogeneous catalyzed carbonyl insertion reactions into methylene bonds has been established for the triangular, methylene-bridged cluster O S ~CO) ( 11 (p-CH2).["] This cluster reacts with two equivalents of CO in dichloromethane under ambient conditions to afford O S ~ ( C O ) ~ ~ ( , ~ - P ~ Cwhich H ~ Ccontains O), a ketene ligand (Scheme 3). This compound is not particularly stable and slowly decomposes to yield Os3(CO)12. Alternatively, heating O S ~ ( C O ) I ~ ( , ~ - ~ ~ -in C chloroform H ~ C O ) at 60 "C for 30 min under reduced pressure results in the regeneration of the methylene compound, OS~(CO)~,(~-C although H ~ ) , in only 10% yield.
Scheme 3
1032
3 Dynamics and Physical Properties
3.5.4 Polyhedral rearrangements in tetranuclear clusters The tetrahedral butterfly rearrangement is quite common in cluster chemistry but few examples are reversible. One example of a reversible process involves the interconversion of Pd4(C0)5(PBun3)4to Pd4(C0)6(PBun3)4(Scheme 4).[19]What is perhaps surprising about this reaction is that Pd4(C0)5(P B u ” ~has ) ~ a butterfly geometry whereas the cluster with the additional CO ligand has a tetrahedral palladium skeleton. This is clearly in contrast with the usual observation that tetrahedral clusters have two fewer valence electrons than butterfly clusters ( i e . 60 and 62 cluster valence electrons (CVE), respectively). This anomaly is a consequence of the capacity of the palladium atom to form stable compounds with 14 and 16 valence electrons and as such Pd4(C0)5(PBun3)4and Pd4(C0)6(PBun3)4do not conform to the usual total electron counts for compounds that obey the EAN rule but have 54 and 56 electrons, respectively. --f
Scheme 4
Experimental and theoretical aspects of the cluster core isomerization from planar to tetrahedral have been investigated for a family of Pt2Mo2 clusters of the type [ P ~ ~ M o ~ ( ? & H ~ M ~ ) ( C O ) ~The (PR steric ~ ) ~properties ]. of the phosphine ligands were found to control the cluster geometry.[6b1The tetrahedral osmium cluster, [ H ~ O S ~ ( C O ) ~is~ Ireadily ]-, protonated to afford the butterfly cluster, H ~ O S ~ ( C O ) ~ ~(Scheme ( , U - I ) 5).[’01 in the course of this reaction the bonding mode
Scheme 5
3.5 Reversible Skeletal Rearrangements in Transition Metal Clusters
1033 ,CNR
D,
RNC
W Scheme 6
of the iodine changes from a terminal site to a bridging position between the wingtip atoms of the butterfly. This change in bonding mode is accompanied by an increase from 1 to 3 of the number of electrons the I atom donates to cluster bonding; this is required for the butterfly cluster to have the expected total electron count of 62. Deprotonation of H ~ O S ~ ( C O ) ~ Z with ( ~ -aI suitable ) base regenerates the tetrahedral 60-electron anion [ H20s4 CO) 12I]-. The heteronuclear tetrahedral cluster H20~3Pt( CO) lo( PCy3) undergoes a transformation involving a butterfly intermediate.r2'' Reaction of HzOs3Pt(CO)lo( PCy3) with isonitrile ligands, CNR ( R = Cy and Bu'), yields the product HzOs3Pt(CO)IO(PCy3)(CNR) as a result of ligand addition (Scheme 6). This cluster has a butterfly skeleton and exists in solution as three isomers which differ in the location of the ligands. Heating HzOs3Pt(CO)lo(PCy3)(CNR) in hexane causes the cluster to eliminate CO thereby generating the substituted tetrahedral cluster H20s3Pt(C0)9(PCy3)(CNR). This reaction supports the mechanism for substitution reactions in clusters which involve metal-metal bond cleavage as the primary step in the reaction. This point is developed further in Section 3.5.5. ( M = Mo and W ) The tetrahedral clusters, CpMFeCo2(p3-S)(p-AsMe2)(CO)s undergo addition with two equivalents of CO under ambient conditions to afford CpMFeCoz(p3-S)(p-AsMe2)(CO)lo (Scheme 7).[221This latter cluster has a spiked triangular geometry of metal atoms with the Fe atom adopting the spike position,
Scheme 7
1034
3 Dynamics and Physical Properties Cp*W(C0)3(C=CH20Me)+ Os3(CO),o(NCMe)2
Scheme 8
Cp*WOs~(CO),,(C~CH,OMe)
bonding directly to one of the Co atoms and bridged to the other by the AsMez ligand. Presumably the conversion of the tetrahedron to the spiked triangle proceeds via a butterfly intermediate. Two carbonyl ligands are lost when CpMFeCo2(p3-S)(p-AsMe2)(CO)lo is heated in benzene for 15 h in an evacuated vessel, regenerating the precursor cluster in 70% yield. A triosmium-tungsten butterfly cluster undergoes a polyhedral rearrangement in which the hinge-hinge bond is broken and a new one is formed between the wing-tip atoms (the diamond-square-diamond mechanism). The acetylide complex Cp*W(C0)3(C-CCHzOMe) reacts with the activated cluster O S ~ ( C O )NCMe)2 ~O( to yield two butterfly isomers of formula Cp*WOs3(CO)1I(C-CCH2OMe) (Scheme 8).[231These isomers differ in that in one the hinge bond of the butterfly is formed between the W atom and one of the 0 s atoms whereas in the other isomer the two 0 s atoms form the hinge bond. The thermolysis of either of these isomers (in pure form) gives a mixture of both compounds in approximately equal quantities. The spiked triangular cluster HRu3Pt(dppe)(CO)g(p4-C-CBu')exists in equilibrium with the butterfly cluster, Ru3Pt(dppe)(C0)&~~-C=CHBu').[~~] This process also involves the migration of an H atom from an Ru-Ru edge to the p carbon of the alkynyl ligand (Scheme 9). Experiments have shown the hydride migration occurs via an intramolecular migration.
-
h
Scheme 9
3.5 Reversible Skeletal Rearrangements in Transition Metal Clusters Ph
Ph
Ph
Ph
I
1035
I
The reaction of R u ~ ( C O )with I ~ PH2Ph affords a number of products including the rectangular cluster Ru4(,u4-PPh)2(CO)11(which can also be viewed as a pseudooctahedron formed by the four ruthenium and two P atoms).[251The iron analog has also been preparedLz6] and both are coordinatively unsaturated in that although a rectangular cluster should have 64 CVE these clusters have only 62 CVE. 1 with CO affords a simple addition product, The reaction of Fe4(p4-PPh)2(CO)~ Fe4(p4-PPh)2(C0)12, with the central cluster unit remaining intact. In contrast, Ru4(p4-PPh)2(CO)1I reacts reversibly with two equivalents of CO to afford Ru4(p3-PPh)2(CO)13(Scheme 10). The hypothetical intermediate cluster, assumed to be similar to the iron product, was not observed. In Ru4(p3-PPh)2(C0)13one Ru-Ru has undergone scission and the PPh groups both cap three metal atoms rather than four. A similar rearrangement has been observed when the square cluster 0s4(p4-S)(p4-HC2C02Me)(C0)1l takes up CO to form Os4(,u3-S)(p4-HC2CO~Me)(CO)12 (Scheme 1 l).[271In this cluster the four metal atoms are connected by three metalmetal bonds. The same square cluster also reacts with hydrogen yielding the spiked triangular cluster, H20s4(p3-S)(p4-HC2C02Me)(CO) 1 I , which also reacts reversibly with CO (although at a much slower rate) to afford H20s4(p3-S)(p3-HC2C02Me)(CO)12. In this compound one 0 s-0 s bond has been cleaved.r281Regeneration of 1 and H20s4(p3-S)(p4-HC2C02Me)(CO)11 is both Os4(pu,-S)(p4-HC2C02Me)(CO)1 achieved by thermal elimination of one carbonyl ligand on heating the cluster at 125 "C; the square cluster is the main product obtained from the thermolysis.
3.5.5 Square pyramidal-bridged butterfly interconversions The facile interconversion between square pyramid and the pseudotrigonal bipyramid geometry (bridged butterfly) dominates the chemistry of RugC(C0)15 and its derivatives. The square-pyramidal ruthenium cluster, Ru5C(CO) 15, is isolated in quantitative yield from the degradative carbonylation of RusC(C0)17, which yields both the pentamer and R U ( C O ) ~ . [ ' ~ ]
1036
3 Dynamics and Physical Properties
Scheme 11
Dissolving Ru5C(CO)15 in acetonitrile results in an instantaneous color change from red to orange. The subsequent removal of the solvent regenerates the initial colour. The orange compound is Ru5C(C0)15(NCMe). In this cluster the metal core has a bridged butterfly geometry and the acetonitrile ligand coordinates the bridging Ru atom. This process, illustrated in Scheme 12, has been found to occur under ambient conditions with a number of small nucleophiles including CO (10 atm). The kinetics of the reaction of RugC(CO)15 with 21 P-donor nucleophiles, L, has been studied.[301The products from these reactions are not addition products but substitution products of formula Ru5C(C0)14L. It was found that two different mechanisms are in operation during the substitution process, depending upon the L
-
+L -L L = MeCN. CO
-
3.5 Reversible Skeletul Rearrangements in Trunsition Metal Clusters
1037
Scheme 13
size of the ligand employed. With smaller nucleophiles (6'I 133") the reactions occur viu two well defined steps, initial adduct formation involving the bridged butterfly intermediate followed by CO dissociation to form the product (Scheme 13). With larger nucleophiles ( Q 2 136") the reaction involves a second-order, onestep process with no spectral evidence for adduct formation; this compares well with phosphine substitution reactions in many other cluster systems. It is also worth noting that substitution with small nucleophiles is much more facile than comparable nucleophile dependent reactions of other carbonyl clusters. The polyhedral opening of the square-pyramidal RusC core can take place at lower pressures of CO when a benzene ligand is coordinated to one of the basal Ru atoms (1 atm compared with 10 atm for the homoleptic cluster). Presumably the less efficient benzene ligand renders the Ru atom to which it is coordinated more susceptible to nucleophilic attack. The benzene cluster in question, Ru~C(C0)12(l;l-CgH6),is prepared in a two-step reaction from Ru~C(C0)15.[~First, cyclohexadiene replaces two carbonyl ligands (removed by oxidation to CO2 by use of Me3NO) on adjacent metal atoms of one of the basal edges yielding Ru~C(C0)13(p2-CgH8).In the second step a further aliquot of Me3NO is added in the absence of any potential donors; this affords both Ru5C(CO)12(p3-CgHg)and RusC(CO)12(q-CgH6). These two compounds are readily distinguished by H NMR spectroscopy the protons on the face-capping ring have a singlet resonance at 6 4.12 ppm whereas those on the terminal ring give rise to a singlet at 6 5.93 ppm. The
'
~
3 Dynamics and Physical Properties
1038
face-capping isomer Ru5C(CO)12( p3-C6H6) undergoes irreversible conversion to R U ~ C ( C O ) I ~ ( ~ on - Cheating ~ H ~ ) and this isomerization has been monitored by use of variable-temperature H NMR spectroscopy, which indicates that the benzene ligand remains associated with the cluster during migration.[321 The face-capping benzene cluster Ru5C(CO)12( p3-C6H6) has not been observed to react with CO under ambient conditions whereas the isomer with the terminal undergoes ~ H ~ ) , an addition reaction with CO to give benzene, R U ~ C ( C O ) I ~ ( ~ - C Ru5C(CO)13(q-CgH6),which has a bridged butterfly ge0metry.1~l ] On standing, freshly prepared samples of RusC(C0)13(q-C6Hfj)readily evolve carbon monoxide H ~ ) . RusC(C0)13to regenerate the initial compound R u s C ( C O ) ~ ~ ( ~ - CIf,~however, (q-C6H6) is prepared and crystallized from solution under a CO atmosphere, and then redissolved in dichloromethane, a third, new isomer of RusC(CO)12(q-CgH6) is isolated (Scheme 14). Characterization of this new isomer is based entirely on spectroscopic data and it is proposed that the benzene ligand bonds to the apical Ru atom of the square pyramid. The isomer in which the benzene bonds to the basal Ru atom can be regenerated from this new isomer by thermolysis; these isomers are assigned I and 11, as shown in Scheme 14.
'
H
H
H
+ co
- co
__L
z
isomerisation
Scheme 14
/Aco
3.5 Reversible Skeletul Reurrunyrments in Transition Mrtul Clusters H
1039
H
The formation of I1 is not easily explained and might involve cleavage of a second Ru-Ru bond or migration of the ligands over the bridged butterfly polyhedron before elimination of CO and reformation of the square-pyramidal polyhedron. Spectroscopic evidence suggests that the isomerization of I1 to I proceeds via a polyhedral rearrangement of the Ru5C core (Scheme 15). The process involves cleavage of edge (a) ( Ruapex-Rubasal) to generate the intermediate with a bridgedbutterfly structure and then the formation of the new edge (a’) to regenerate the (new) square-pyramidal structure. This pseudorotation has the effect of transferring the benzene from the apical position in I1 to the basal position in I. The reverse isomerization, viz. I + 11, occurs under photolytic conditions when I is embedded in a polymer film.[331A pseudorotation involving Ru-Ru bond cleavage is believed to bring about the rearrangement. The apparent conflict between the thermal and photolytic processes is probably a result of heterolytic and homolytic Ru-Ru bond fission brought about by the different initiation techniques. An appreciation of the square pyramidal-bridged butterfly rearrangement facilitates an understanding of certain reactions which might seem somewhat unusual at first glance. For example, the bis-cyclohexa-1,3-diene cluster Ru5C(CO)1 1(q4-C6H8)2 reacts with CO under ambient conditions to afford R u ~ C ( C O ) I ~ ( L ~ ~In- C ~ H ~ ) . [ ~ ~ ] this reaction, one of the cyclohexa-1,3-diene ligands is displaced, one carbonyl group added, and the remaining cyclohexa-l,3-diene undergoes dehydrogenation to give the face-capping benzene ligand found in the product. The proposed mechanism by which Ru5C(CO)12(p3-C6H6)is formed involves the initial addition of CO bringing about a change in the geometry of the cluster core from square pyramidal to the bridged butterfly arrangement yielding ‘RusC(C0)12(q4-C6H8)2’.Subsequent ejection of one of the q4-C6H8 ligands not only causes the closure of the metal polyhedron, but results in the cluster being coordinatively unsaturated, hence creating the driving force for the aromatization of the ring as it is transformed from a four-electron donor to a six-electron donor ligand.
1040
3 Dynamics and Physical Properties
3.5.6 Octahedral-trigonal prism interconversions The interconversion between octahedral and trigonal prismatic is of relatively high energy because an octahedron has 12 metal-metal bonds and a trigonal prism has only nine metal-metal bonds and, therefore, three bonds must be broken or formed. Such processes are quite rare and examples of reversible processes are limited to a few carbide clusters of the Group 9 metals. The cobalt cluster [ c o ~ c ( c o ) ~ ~is] obtained 2in high yield by reaction of the alkylidyne species [Co3(CO)&Cl] with three equivalents of [ C O ( C O ) ~ ] - . This [~~' cluster has 90 valence electrons and accordingly has a trigonal prismatic cobalt in THF for 2 h under an inert polyhedron. The thermolysis of [Co~C(C0)15]~atmosphere affords the 86-valence-electron cluster [co6c(Co)1~]2-in 80% yield (Scheme 16).[361The metal polyhedron in this cluster adopts an octahedral geometry in accordance with a cluster with 86 CVE. The process is readily reversed with regeneration of [Co~C(C0)15]~on reaction with CO. An identical process takes place for the rhodium analogs [Rh&(C0)15I2- and [Rh6C(C0)13]2-.[371
[ M d X O )I 51'M = Co, Rh
Scheme 16
The cobalt cluster [Co&( CO)1512- also undergoes a reversible polyhedral rearrangement to the radical anion [co6c(co)14]-.[381This cluster has a valenceelectron count of 87 and has a distorted octahedral structure in which one Co-Co bond is very long, 2.91 A, in comparison with the remaining 11 bonds (mean 2.63 A).[391 This distortion is attributed to the presence of the additional electron. It is produced in about 80% yield by oxidation of [Co~C(C0)15]~with either FeCl3 or iodine at room temperature. The reverse process can be induced with NaOH in methanol under an atmosphere of CO. The octahedral-trigonal prismatic rearrangement corresponds to a rotation of two triangular units by 60" relative to one another and three bonds must be broken (or formed). It is possible that the carbide atom in the above clusters enables this
3.5 Reversible Skeletal Rearrangements in Transition Metal Clusters
1041
H
H
Scheme 17
transformation to occur by conferring stability to the metal core and preventing fragmentation. Ligand isomerization in a R Q C ~pseudooctahedral cluster has been attributed to an octahedral-trigonal prismatic rearrangement.[401 The cluster in question, Ru4(p4-C6Hs)(C0)9(1;1-CgH6), exists in two isomeric forms, which have the same central polyhedron comprising four Ru atoms in a butterfly arrangement with a cyclohexyne ligand bonded between the wings of the butterfly forming z interactions with the wing-tip atoms and n bonds with the hinge atoms. This cluster can also be viewed as a RuqC2 pseudooctahedron. These clusters are derived from the reaction of Ru4(p4-C6H~)(C0)12with cyclohexa-1,3-diene (Scheme 17). At high temperature, i. e. in octane under reflux, the isomer of R U ~ ( ~ U , - C ~ H ~ ) ( C O ) ~ ( ~ - C ~ H ~ ) in which the benzene coordinates to a hinge atom, I, is obtained, whereas at ambient temperatures, using Me3N0, an isomer in which the benzene is coordinated to a wing-tip atom of the butterfly, I1 is produced. If I is left in a solution of dichloromethane at room temperature it slowly isomerizes to 11. Comparison with the benzene migration in the pentaruthenium cluster (see above) suggests it is not unreasonable to conclude that the isomerization at ambient temperature takes place via the migration of the benzene over the surface of the cluster. However, isomer I can be regenerated from I1 by heating at 125 "C for 2 h. Clearly a different mechanism is required to account for this tramformation and, because higher energies are required, a polyhedral rearrangement has been proposed, which involves a trigonal prismatic intermediate (Scheme 18). This example differs from the others described in this section, because a polyhedral rearrangement is not actually observed. It does, however, indicate that ligand isomerization should not automatically be invoked to account for ligand migrations but might also arise from a polyhedral rearrangement. A further example of this type is described in Section 3.5.7.
1042
3 Dynamics and Physical Properties
n H
Scheme 18
3.5.7 Octahedral-capped polyhedral interconversions The high-temperature vacuum pyrolysis of O S ~ ( C O )or , ~ Os3(C0)lo( NCMe)2 affords several different products; depending upon the conditions employed Os6(CO)1g can be isolated in high yield.[411This cluster has a monocapped trigonal bipyramidal geometry, or alternatively, the Os6 core can be viewed as a bicapped tetrahedral geometry and this latter name is usually used because of the aesthetic appeal of the three fused tetrahedra which it implies. The thennolysis of O S ~ ( C O ) ~ ~ and sodium in diglyme affords the dianionic octahedral cluster [oS6(co)1g]2in high yield.[421It has been found that these clusters undergo interconversion either electrochemically[431 or by use of suitable oxidants and reductants such as iodineiodide (Scheme 19).[42,441 The reduction of Oss(CO)1g to [os6(co)lS]2-is pseudofirst order and the structural rearrangement takes place during the first electrontransfer step. The reverse reaction is less straightforward and it would seem that the structural rearrangement takes place after the oxidation process is complete.[451 This facile interconversion is believed to influence the substitution chemistry of O S ~ ( C O ) The ~ S . reaction of Os6(CO)1g with two equivalents of Me3NO in acetoni-
Scheme 19
3.5 Reversibke Skeletal Rearrangements in Transition Metal Clusters
1043
trile affords the bis-substituted product, o s ~ ( C 016() N C M ~ ) Z . [Although ~~] this cluster has not been fully characterized, it is believed to retain a bicapped tetrahedral polyhedron. The in situ reaction of Os6(CO)16(NCMe)? with trimethylphosphite affords the bis-phosphite complex Osg(CO)l6{P(OMe)3$2.Two isomers of this compound have been identified, one with the P(OMe)3 ligands on the capping atoms, I, the other with the P(OMe)3 ligands on trans osmium atoms of the trigonal prism sub-fragment, I1 (Scheme 20). On standing in solution, isomer I1 converts to I and it is suggested that mechanistically this rearrangement takes place via a polyhedral interchange involving a single-edge cleavage and a monocapped square-pyramidal intermediate. The reversible rearrangement between octahedral and monocapped square pyramid has been found to take place in a hexaruthenium cluster[471and a pentarutheni~m-platinum[~~] cluster. The reaction of the octahedral carbide cluster Ru6C(C0)17 with Me3NO in the presence of but-2-yne (C2Me2) affords, in the first instance, the mono-substituted cluster Ru6C(CO)1s(p3-C2Me2).This cluster reacts with further but-2-yne to give the bis-substituted, monocapped square pyramid (Scheme 21).[471Heating Ru6C(C0)14cluster, RugC(C0)14(p-C2Me2)(p3-C2Me2) (p-C2Me2)(p3-C2Me2)in heptane for 1 h, or treatment with additional Me3NO in an inert solvent, results in the expulsion of one CO ligand and brings about the closure of the polyhedron with the regeneration of an octahedron in Ru6C(C o )l 3 ( p3-C2Me2)2. It is particularly surprising that a polyhedral rearrangement of this nature takes place in the Ru6C cluster as the carbide atom is considered to confer stability to the cluster unit, preventing degradation or a polyhedral rearrangement. Relatively minor modifications to the Ru6C skeleton have been observed in Ru6C(CO)15( ~ - C 4 P h q ) [and ~ ~ ]RugC(C0)15(, ~ - d p p f ) [(both ~ ~ ] prepared from RugC(C0)17 under ambient conditions) in which one and two Ru-Ru edges are opened, respectively, to distances beyond that which is generally considered to constitute a Ru-Ru bond. Their frameworks can, however, still be viewed as pseudooctahedra in which the sterically demanding ligands bring about the skeletal distortions. The heteronuclear octahedral cluster RusPtC(CO)16 undergoes a similar reaction with alkynes to RusC(C0)17 with the formation of the monocapped square pyraThe synthesis of this cluster is midal cluster R~5PtC(C0)13(p-C2Ph2)(p~-C2Ph2).[~*] somewhat different from that of the related homonuclear cluster and involves UV irradiation of the cluster and alkyne. When RusPtC(C0)I~(p-C2Ph2)(pu,-CzPh2) is treated with CO the alkyne ligands are displaced and the starting material, R U ~ P ~ C ( C Ois )regenerated ~~, (Scheme 22). It is possible that this transformation involves either a concerted rearrangement or complete dissociation of the Pt atom before migration from one part of the cluster to the other. The valence-electron count for Ru6C(CO)~4(p-C2Me2)(pu,-C2Mez)and RusPtC(CO)13(p-C2Ph2)(p3-C2Ph2)is 88 (two more than expected for clusters with P~~)(~~-C~P~~ this geometry). The excess electrons in R U ~ P ~ C ( C O ) I ~ ( ~ - C ~have been accounted for from a study of the X-ray structure which shows that two bonds
1044
3 Dynamics and Physical Properties
3.5 Reversible Skeletal Rearrangements in Transition Metul Clusters
1045
Me
Me
Me
A or Me3N0
are considerably longer than the others because of the presence of these additional electrons. A similar effect is not, however, observed in the hexaruthenium cluster and theoretical studies are currently in progress in an attempt to understand this intriguing compound.
3.5.8 Polyhedral rearrangements of high-nuclearity
osmium clusters Thermolysis of Oss(CO)18in butanol for 53 h affords several clusters, including the ] - yield." 'I A single-crystal X-ray diffraction analysis of anion [ H O S ~ ( C O ) ~in~30%
1046
3 Dynamics and Physical Properties
-
hv, C2Phz L
CO (1 atrn), 68°C
Scheme 22
this cluster reveals an unusual osmium skeleton comprising four fused tetrahedra (Scheme 23). This structure is somewhat surprising given that the related compound octahedral arrangement of metal atoms.rs21The re[ O S ~ ( C O ) ~has ~ ]a~bicapped action of [ H O S X ( C O ) ~with ~ ] - iodine (I+) in dichloromethane results in the formation of the neutral cluster H O S ~ ( C O ) ~ ~ InI this . [ ~ cluster ~] the polyhedron found in the precursor has undergone a considerable transformation. The structure can be viewed as a bicapped tetrahedron with two edge-bridging 0 s atoms. Reaction of HOss(CO)221 with tetrabutylammonium iodide results in the regeneration of the anionic cluster [HOsg(C0)22]-. Clearly, there is not simple relationship between the structures of these two clusters. The decanuclear cluster [ O S ~ O C ( C O )can ~ ~ ]be~ -prepared in 65% yield by vacuum pyrolysis of Os3(CO)11(NC5H5).r531Its structure is based on a tetrahedron formed by three layers of close-packed 0 s atoms in the sequence 1 : 3 : 6. An alternative view of this cluster is as a tetracapped octahedron with the carbide atom located inside the octahedral cavity. Reaction of [Osl0C(C0)24]~with iodine initially affords the monoanion [OSIOC(CO)~~I]-, and subsequently, the neutral cluster [Os~oC(C0)~41~] (Scheme 24).[s41In [Os1oC(CO)241]-one 0s-0s bond of a capping atom has cleaved with the concomitant insertion of an I atom into the metal-metal
3.5 Reversible Skeletal Rearrangements in Transition Metal Clusters
1047
1048
3 Dynamics und Physicul Properties
bond. In [ O S ~ ~ C ( C O a) ~second ~ I ~ ] 0s-0s bond cleavage and insertion has taken place. Treatment of the iodo clusters with 1- eliminates the coordinated I atoms regenerating the dianivn [Osl0C(C0)24]~-.
3.5.9 Polyhedral rearrangements of heteronuclear group 8-platinum clusters The reaction of os6(co)17(NCMe) with two equivalents of Pt(q4-C8H12)2 at ambient temperature affords several products including OS6Ptz(CO)17(r4-CsH~2)2 in 20% ~ i e l d .511 ~In a similar reaction involving the bis-acetonitrile cluster Osg(C0)16(NCMe)2, the related cluster 0s6Pt2(CO)16(q4-CsH12)2is isolated in 15% yield.[551The metal polyhedra formed by these clusters are shown in Scheme 25. The 0 s atoms in 0~6Pt2(CO)~~(q~-C8H12)2 adopt an octahedral geometry and the two Pt atoms cap two of the faces. The cluster skeleton of Oss~t2(CO)16(r~-CsH12)2 consists of two osmium tetrahedra fused along an edge with the two Pt atoms capping the faces of one of the tetrahedral units. Using the PSEPT an electron count of 110 would be predicted for this cluster; the cluster has 108 CVE only, however. Crystallographic analysis of this cluster shows that one of the 0s-0s edges is abnormally short (2.647 A) and this has been assigned as a double bond, which would account for the cluster having two electrons fewer than predicted. The unsaturated nature of Os6Pt2(CO)16(r4-CsH12)2is demonstrated by its reactivity. In solution, Os6Pt2(CO)16(r4-CsH12)2reacts immediately with CO at room temperature to form Os6Pt2(CO)1,(r4-C8H12)2in good yield. This process can be reversed by heating OssPt2(C0)17(r4-C8H,2)2in octane for a few minutes, resulting in the regeneration Of OSfjPt2(co)16(r4-CgH12)2.
3.5 Reversible Skeletul Reurrangements in Trunsition Metal Clusters
1049
- +- 44 CC OO
Scheme 26
Thermolysis of the heteronuclear cluster RuqPt2(CO)18 with diphenylacetylene in heptdne affords several products including RusPt3(CO)14(,u3-C2Ph2)3 in moderate yield.[561This cluster has an unusual metal core comprising a tricapped octahedron with a bond formed between two of the capping atoms. Reaction of this cluster with CO under ambient conditions produces three compounds including the carbonylated species RusPt3(CO)18(p3-C2Ph2)3in 20% yield (Scheme 26). Uptake of four equivalents of CO has been accompanied by a polyhedral rearrangement to a cluster with a central platinum triangle in which two edges are fused to a tetrahedral unit made up of four of the Ru atoms; the third edge forms a type of butterfly arrangement. When heated to 68 "C this compound eliminates four molecules of CO to regenerate the precursor cluster in nearly quantitative yield.
3.5.10 Concluding remarks Some reversible polyhedral rearrangements have been described. Although most are of academic curiosity, others provide insight into the substitution mechanisms and methods by which ligand rearrangements might occur. This is clearly evident in certain examples and merely proposed (based on surrounding evidence) in others. of CO react with the cluster via nucleoIn O S ~ ( C O ) I I ( ~ - C two H ~ ) moles , philic attack at both a metal atom and at the methylene, affording Os3(CO)12(p-v2-CH2CO).['*I This process is clearly of catalytic significance even though it is not catalytic in itself. Several reactions are, however, catalyzed by molecular clusters which are believed to involve reversible polyhedral rearrangement^.'^] For example, in the hydroformylation reaction of cyclohexene the tetrahedral rhodium
1050
3 Dynamics and Physical Properties
catalyst Rh4(C0)12 is thought to cleave an Rh-Rh bond upon oxidative addition of H2.[571Reformation of the bond takes place several steps later when the product is finally eliminated from the cluster.
Acknowledgements I would like to thank the Royal Society for providing me with a University Research Fellowship and Daren Bryce for his help preparing this Chapter.
References [ l ] E. L. Muetterties, Bull. Soc. Chim. Belg., 1976, 85, 451. [2] R. Whyman, in ‘Transition Metal Clusters’, Ed. B. F. G. Johnson, Wiley, Chichester, 1980, 545. [3] (a) P. Braunstein and J. Rose, in Comprehensive Oryanometallic Chemistry II, Eds. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon, Oxford 1995, vol. 10, p. 351; (b) G. Suss-Fink and G. Meister, Ado. Organornet. Chem., 1993, 35, 41. [4] D. M. P. Mingos and D. J. Wales, ‘Introduction to Cluster Chemistry’, Prentice-Hall, New Jersey, 1990, 218. [ 5 ] D. J. Wales, D. M. P. Mingos and L. Zhenyang, Inorg. Chem., 1989, 28, 2764. [6] (a) G. Douglas, K. W. Muir and I. B. R. Lloyd, J. Chem. Soc., Clzem. Commun., 1988, 149; (b) P. Braunstein, C. de Meric de Bellefon, S.-E. Bouaond, D. Grandjean, J.-F. Halet and J.-Y. Saillard, J. Am. Clzem. Soc. 1991, 113, 5282. [7] R. D. Adams and S. Wang, Inorg. Chem., 1985,24,4447. [8] I. D. Slater, Adv. Organomet. Chem., 1989, 29, 249. [9] G. L. Geoffroy, Acc. Chem. Rex, 1980, 13, 469. [lo] C. Brown, B. T. Heaton, A. D. C. Towl, P. Chini, A. Fumagalli and G. Longoni, J. Organomet. Chern., 1979, 181, 233. [ l l ] J. L. Vidal, W. E. Walker, R. L. Pruett and R. C. Schoening, Inorg. Chem., 1979, 18, 129. [12] L. Garlaschelli, A. Fumagalli, S. Martinengo, B. T. Heaton, D. 0. Smith and L. Strona, J. Chem. Soc., Dulton Trans., 1982, 2265. [I31 B. T. Heaton, L. Strona, J. L. Vidal and R. D. Pergola, J. Chem. Soc., Dalton Trans., 1983, 1941. [I41 K. P. Hall and D. M. P. Mingos, Prog. Inory. Chem., 1984, 32, 237. [I51 C. E. Briant, K. P. Hall and D. M. P. Mingos, J. Chem. Soc., Chem. Commun., 1984, 290. [I61 R. A. Jones and T. C. Wright, Inorg. Chem., 1986, 25,4058. [I71 (a) G. Huttner, J. Schneider, H.-D. Miiller, G. Mohr, J. v. Seyerl and L. Wohlfahrt, Anyew. Clzem., Int. Ed. Enyl., 1979, 18, 76: (b) J. Schneider and G. Huttner, Chem. Ber., 1983, 116, 917. [IS] E. D. Morrison, G. R. Steinmetz, G. L. Geoffroy, W. C. Fultz and A. L. Rheingold, J. Am. Chem. Soc., 1983, 105,4104.
3.5 Reversible Skelrtul Reurrunyement,s in Transition Metul Clusters
1051
[I91 E. G. Mednikov, N. K. Eremenko, S. P. Gubin, Y. L. Slovokhotov and Y. T. Struchkov, J. Organomet. Chem.. 1982, 239, 401. [20] J. Puga, A. Arce, R. A. Sanchez-Delgado, J. Ascanio, A. Andriollo, D. Bragd and F. Grepioni, J. Chem. Soc., Dulton Truns., 1988, 913. [21] P. Ewing and L. J. Farrugia, Organometallics, 1988, 7, 871. [22] F. Richter and H. Vahrenkanmp, Organometallics, 1982, 1 , 756. [23] P.-C. Su, S.-J. Chiang, L.-L. Chang, Y. Chi, S.-M. Peng and G.-H. Lee, Orgunometallic.s, 1995. 14, 4844. [24] P. Ewing and L. J. Farrugia, Organometallics, 1989, 8, 1246. [25] J. S. Field, R. J. Haines, D. N. Smit, K. Natarajan, 0. Scheidsteger and G. Huttner, J. Orgunomet. Chem.: 1982, 240, C23. [26] H. Vahrenkamp and D. Walters, J. Organomet. Chem., 1982, 224, C17. [27] R. D. Adams and S. Wang, J. Am. Chem. Soc., 1987, 109, 924. [28] R. D. Adams and S. Wang, Organonzetullics, 1986, 5, 1272. 1291 B. F. G . Johnson, J. Lewis, J. N. Nicholls, J. Puga, P. R. Raithby, M. McPartlin and W. Clegg, J. Chem. Soc., Dalton Trans., 1983, 277. [30] D. H. Farrar, A. J. Pot; and Y. Zheng, J. Am. Chem. Soc., 1994, 116, 6252. [31] (a) P. J. Bailey, D. Braga, P. J . Dyson, F. Grepioni, B. F. G. Johnson, J. Lewis and P. Sabatino, J. Chem. Soc., Chem. Commun., 1992, 177: (b) D. Braga, F. Grepioni, P. Sabatino, P. J. Dyson, B. F. G. Johnson, J. Lewis, P. J. Bailey, P. R. Raithby and D. Stalke, J. Chem. Soc., Dalton Trans., 1993. 985. [32] P. J. Dyson, B. F. G. Johnson and D. Braga, Inorg. Chin?. Acfu., 1994, 33, 3218. [33] D. B. Brown, P. J. Dyson, B. F. G. Johnson and D. Parker, J. Orgunornet. Chem., 1995, 491, 189. [34] D. Bragd, P. Sabatino, P. J. Dyson, A. J. Blake and B. F. G. Johnson, J. Chem. Soc., Dalton Trans., 1994. 393. [35] V. G. Albano, P. Chini, S. Martinengo, M. Sansoni and D. Strumolo, J. Chem. Soc.. Chem. Commun., 1974, 299. [36] V. G. Albano, D. Braga and S. Martinengo, J. Chern. Soc., Dalton Trans., 1986. 981. [37] V. G. Albano. D. Braga and S. Martinengo, J. Chent Soc., Dalton Truns., 1981. 717. [38] S. Martinengo, D. Strumolo, P. Chini, V. G. Albano and D. Braga, J. Chem. Soc., Dalton Trans., 1985, 35. [39] P. Chini, G. Ciani, M. Sansoni, D. Strumolo, B. T. Heaton and S. Martinego. J. Am. Chem. Soc., 1976, 98, 5027. [40] D. Braga, F. Grepioni, J. J. Byrne, C. M. Martin, B. F. G. Johnson and A. J. Blake, J. Chem. Soc.. Dalton Trans., 1995, 1555. [41] C. R. Eady, B. F. G. Johnson and J. Lewis, J. Chem. Soc., Dulton Trans., 1975, 2606. [42] C.-M. T. Hayward and J. R. Shapley, Inorg. Chem., 1982, 21, 3816. [43] B. Tulyathan and W. E. Geiger, J . Am. Chem. Soc., 1985, 107, 5960. [44] C. R. Eady, B. F. G. Johnson and J. Lewis, J. Chem. Soc., Chem. Commun., 1976, 302. [45] G. R. John, B. F. G. Johnson, J . Lewis and A. L. Mann, J. Orgunornet. Clzern., 1979, 171, c9. [46] B. F. G. Johnson, R. A. Kamarudin, F. J. Lahoz, J. Lewis and P. R. Raithby, J. Chem. Soc., Dalton Trans.. 1988, 1205. [47] R. L. Mallors, A. J. Blake, P. J. Dyson, B. F. G. Johnson and S. Parsons, Orgunometullics, 1997, 16. 1668. [48] R. D. Adams and W. Wu. Organornetallics, 1993, 12, 1238. [49] P. J. Dyson, S. L. Ingham, B. F. G. Johnson, J. E. McGrady, D. M. P. Mingos and A. J. Blake, J. Chem. Soc., Dalton Trans., 1995, 2749. [SO] A. J. Blake, B. F. G. Johnson, S. Parsons, D. Reed and D. S. Shephard, Organometallic.s, 1995. 14, 4199.
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3 Dynamics and Physical Properties
[51] B. F. G. Johnson, J. Lewis, W. J. H. Nelson, M. D. Vargas, D. Braga, K. Hendrick and M. McPartlin, J. Chem. Soc., Dalton Trans., 1984, 2151. [52] P. F. Jackson, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Chem. Soc., Chem. Commun., 1980, 60. [53] P. F. Jackson, B. F. G. Johnson, J. Lewis, W. J. H. Nelson and M. McPartlin, J. Chem. SOC., Dalton Trans., 1982, 2099. [54] D. H. Farrar, P. G. Jackson, B. F. G. Johnson, J. Lewis, W. J. H. Nelson, M. D. Vargas and M. McPartlin, J. Chem. Soc., Chem. Commun., 1981, 1009. [ 5 5 ] C. Couture and D. H. Farrar, J. Chem. Soc., Dalton Trans., 1986, 1395. [56] R. D. Adams and W. Wu, Organometullics. 1993, 12, 1248. [57] N. Rosas, C. Marquez, H. Hernandez and R. Gomez, J. Mol. Catal., 1988, 48, 59.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.6 Skeletal Isomerism in Transition Metal Clusters Oriol Rossell, Miquel Seco, and Gloria Seyulis
3.6.1 Introduction Several reports dealing with metal-cluster isomerism and possible mechanisms of polyhedral rearrangements have recently been published. Although work in this field was first undertaken by Braunstein in 1991,['] no further studies have subsequently appeared. Given the increasing interest in cluster isomerism, the purpose of this article is to provide an updated systematic classification to facilitate the study of this area. Three types of isomerism have been described for metal clusters skeletal, positional, and ligand isomerism. Skeletal isomers were initially defined as those compounds with identical stoichiometry but with different skeletal geometry in the solid state.['] Later, the concept of skeletal isomers was extended to include compounds which have similar, though not necessarily identical, ligands."] For example, two phosphine ligands and a chelating diphosphine group are seen as being equivalent and, consequently, complexes A and B (Fig. 1) are skeletal isomers.[31Interestingly skeletal isomers occasionally share the same electron count, although more usually the change in geometry is the result of a change in the total electron count of the metal clusters. Positional or permutational isomerism occurs in compounds which have the same metal skeletal framework but differ in the occupation of the vertices of the polyh e d r ~ n , [for ~ I example, complexes C and D (Fig. 2)."l Ligand isomerism is observed in compounds that have the same stoichiometry and metal core, but in which two or more ligands formally interchange coordination sites, for example compounds E and F (Fig. 3).r61 Note that for positional and ligand isomerism neither the core geometry nor the electron count changes on rearrangement from one isomer to another. Although this article focuses on skeletal isomerism and extends the concept to those compounds with different geometries in solution, it deals only with examples ~
1054
3 Dynamics and Physical Properties Ph2P-
I
PPh2
1
Figure 1. Skeletal isomerism.
for which all the isomers have been clearly characterized. The compounds are discussed in an order based on the number of metal atoms present. Schemes 1-9 show the metal frameworks that have been identified in examples of skeletal isomerism; each particular pair of skeletal isomers is connected with a double arrow. In the diagrams the metal-metal bonded atoms are linked with a straight line whereas those with no direct metal-metal interaction are depicted with a wavy line.
3.6.2 Trinuclear clusters Clusters containing three metal atoms are the simplest. The most frequently reported geometry is triangular, characterized by a total electron count (TEC) of 48.
C
D
Figure 2. Positional (or permutational) isomerism.
[PtOsj(CO),,,(p-q’-dpprn) { Si(OMc131(p-H)I
E
F
Figure 3. Ligand isomerism.
3.6 Skeletal Isomerism in Transition Metul Clusters
1055
Scheme 1
The elements of the nickel triad, however, form planar triangular clusters with 4244 electrons because of the greater tendency of the low oxidation states of these metals to form 16-electron complexes. In addition to the triangular metal skeleton (1, Scheme l), a bent or a linear metal chain containing two metal-metal bonds (2) and a simple dinuclear metal-metal bonded fragment, with the third metal atom having no direct metal-metal interaction (3), have been observed. Formally, a type1 cluster will become type 2 simply by addition of two electrons to the total electron count. This can be achieved by varying the electron contribution of one of the ligands of the molecule by altering its mode of coordination to the metal atoms. Several examples of type-1 and type-2 structures have been reported in the literature. One is the triangular bent-chain isomerism observed in [Fe3(C0)9. (p3-q2-N2Et2)]."] Thermolysis of this triangular 48e cluster in solution results in N-N cleavage to form the nitrene-bridged 50e cluster [Fq(CO)9(p3-NEt)2]with a bent chain skeletal framework (2). This behavior is also observed for the triangular cluster [Fe3(C0)9. {p3-P(C6H2Me3)C(Ph)C(Ph)}], which upon thermal activation rearranges to [Fe3(C0)9{p3-P(C6H2Me3)}{p3-C(Ph)C(Ph)}] with an open Fe3 chain.[81 Temperature and solvent polarity are responsible for the skeletal isomerism observed in the cluster [Os3(CO)lo(Pr'-DAB)] (Pr'-DAB = 1,4-diisopropyl-l,4diaza-l,3-butadiene).[" Thus the 48e triangular cluster [Os3(CO)lo(Pr'-DAB)] (1) is converted, by heating under reflux in n-hexane, to the 50e skeletal isomer with an open metal chain (2). In the former compound the Pr'-DAB ligand contributes four electrons while in the latter it contributes six. Two non-interconvertible metallacyclopentadienyl cluster isomers [ WRe2(p-H). CP*(CO)~ {CHCH(C6Hg)}]were obtained by heating a solution of the vinylacetylide complex [WRe2Cp*(C0)9{C=C(C6H9))] to reflux in toluene, under hydrogen."'] Structural analysis revealed that one isomer has a triangular backbone with a metal-metal double bond, which supposes a total electron number of 46, whereas the other has a bent chain of metal atoms (TEC 50). Skeletal isomerism has also been detected in the trinuclear platinum cluster [Pt3(p-PPh2)$h( PPh3)2].['' I The framework obtained depends on the solvent used for crystallization-dark red [Pt3(p-PPh2)3Ph(PPh3)2].2CH2C12has a triangular core
1056
3 Dynamics and Physical Properties
(1) whereas bright red [Pts(p-PPh2)3Ph(PPh3)2]has a bent Pt3 chain (2). The flexibility of the bridging PPh2 units enables very large variations in metal-metal separations and MO calculations indicate that the energy difference between the two isomers is very small. Several examples of skeletal isomers belonging to structures 1 and 3 have also been reported. Here structure 3 can be described by cleavage of two metal-metal bonds of the starting cluster 1. The triangular compound [Re2(pu-AuPPh3). ( pu-PPh2CH2PPh2)( p-CzPh)(CO)6] and the rhenium-rhenium bonded complex have this type of isomerism.[l2I [Re2(p-H)(pPPh2CH(AuPPh3)PPh2)(pu-C2Ph)(CO)6] Another example is the orange cluster [Mn2(p-AuPPh3)(p-PCyH)(CO)g] (1) and the yellow compound [Mn&-H) {pu,-PCy(AuPPh3)}(CO)g](3). Similar isomer formation is also observed for compounds containing other AuPR3 fragments ( R = Cy, Ph, p-CsH40Me, p-C6H4F). Remarkably, in solution the isomers are in dynamic equilibrium, the displacement of which depends on the PR3 group and on the solvent used." 31 When the triangular cluster [Ru3(p3-PhC2CCPh)(p-dppm)(p-CO)(C0)~] is heated under reflux in xylene it isomerizes to [ R u {p3-CPhCHCC(CgH4-2)}. ~ (p-dppm)(CO)g], the crystal structure of which reveals that one of the ruthenium atoms is not bonded to the others (3). This implies substantial reorganization of the organic ligand coordination to the metal atoms, including metalation of one phenyl ring.[' 41
3.6.3 Tetranuclear clusters The butterfly metal core (type 4)is the most frequently found geometry in tetranuclear clusters with skeletal isomerism (Scheme 2). It is worth noting that for metals
5
7
Scheme 2
4
8
6
10
11
12
13
9
3.6 Skeletal Isomerism in Transition M e t d Clusters
1057
other than those of the nickel triad the TEC for a tetrahedral structure is 60 whereas for the remaining geometries the TEC is obtained by adding two electrons for each metal-metal bond lost. Thus, a butterfly is characterized by 62, a planar square or a spiked triangle by 64, and a chain by 66 electrons. Other less common examples of tetranuclear species have geometries 10-13. Several examples are known with structures 4 and 5. The cluster compounds [WM3Cp(CO)l2(H)] ( M = Ru, 0 s ) have been reported to occur as an equilibrium between species with butterfly and tetrahedral metal skeletons.[' This different geometry is because of the presence in the former of one carbonyl ligand acting as a four-electron donor. In general, the less voluminous WRu3 core tends to adopt the butterfly geometry because this creates more space for the CO ligands. For the same reason, substitution of the Cp ligand for a bulky CsMes (Cp*) group promotes displacement of the equilibrium to the butterfly form. Further studies have shown the occurrence of this type of isomerism in the cluster [ M o R u ~ ( C O ) ~ ~ H C ~ * ] . [ ' ~ ] Similarly, the low-temperature I3C and ' H NMR spectra of [HFe4(CO)13]- indicate the presence of two distinct metal frameworks a butterfly (4)and a tetrahedral core (5).['7JThe first contains a dihapto CO ligand bridging the wingtips whereas the hydride ligand bridges the hinge. The cluster [Ru3Au(H)(CO)g(PPh3)(p-PPh2)2]occurs as an equilibrium between (4) and the tetrahedral the butterfly species [Ru3(p-AuPPh3)(pu-H)(p-PPh2)2(C0)g] [Ru3(p3-AuPPh3)(p-H)(p-PPh2)2(CO)~] (5).[18] This is, in fact, an interesting example in which an Ru3 triangle is bridged by the AuPPh3 fragments in two different bonding forms - edge bridging and ,+-capping, respectively. Particularly interesting are the alkylidyne-alkyne complexes [W0s3(CO) 10. Cp(p3-q2-C2R2)(p3-CR)] ( R = Tol, Ph) in which the four metal atoms are present in a spiked triangular arrangement (6).['91 Thermal coupling of the alkyne and alkylidine ligands gives an allyl complex with a tetrahedral metal framework [WOs3(CO)~~Cp(p3-q3-C3R~ToI)] (5); this cluster, in which the allyl ligand is 71coordinated to one 0 s atom, can undergo a thermal rearrangement to give a ligand isomer with the allyl group 71-coordinated to the W center. Prolonged thermolysis of the latter induces scission of an allyl C-C bond to produce an alkylidyne-alkyne when R = Tol. This molecule complex [WOs3(CO)loCp(p3-q2-C2R2)(p3-CTol)] contains a butterfly metal core (4). The orange hydrido alkynyl complex [Ru3Pt(p-H){p4-q2-C-C(But)}(CO)9(dppe)] has a spiked triangular metal arrangement ( 6 ) whereas its red vinylidene isomer [Ru3Pt{p4-q2-C=C( H)(Bu')}(CO)9(dppe)] has a butterfly framework (4).[201Both complexes readily interconvert in solution, with 4 being the thermodynamically favored product. The hydrido alkynyl to vinylidene transformation involves the formal loss of two electrons donated by the ligands. Two examples involving structures 4 and 7 have been reported. In the butterfly form of cluster [Mo2Fe2Cp2(C0)8(p3-S)2](4),the p3-S ligands are situated in the two external faces of the two MozFe wings, but in the planar structure (7), the sulfido ligands lie on opposite sides of the Mo2Fe2 plane, and the molecule contains an inversion center. These clusters are very stable and non-interconvertible.r2 -
1058
3 Dynamics and Physical Properties
The cluster [WOs3Cp*(C0)7(p3-CPh){C( Me)C(Me)CC(Tol)C(Tol)}] has a planar triangulated rhomboidal metal arrangement (7).Thermolysis of this compound in xylene under reflux induces rearrangement of the cluster core to give a skeletal isomer with a butterfly arrangement of metal atoms and with the W atom occupying a wingtip position (4).[”1 Reaction of the cluster complex [Os4(CO)lz(p3-S)]with PhC-CH yields two noninterconvertible skeletal isomers yellow [Osq(CO)12(p3-S) {p4-C=C(Ph)H}] and black [Os4(C0)12{p,-q3-SC(Ph)=CH}].[231 The first is a puckered rhombus of four osmium atoms (9) with a triply bridging sulfido ligand and a quadruply bridging phenylvinylidene ligand; the second isomer has a butterfly metal arrangement (4) with a quadruply bridging SC(Ph)=CH thiolato ligand. When solutions of this isomer are heated to reflux an open-chain isomer, [ O ~ ~ ( C O ) I ~ ( ~ ~ - S ) ((8), ~ - is HC~P~)] obtained.[241 Skeletal isomerism involving geometries of types 5 and 6 has been observed for the pair of clusters [WOs3(C0)11(p-H)2(p3-SMe)Cp] and [ W O S ~ ( C O ) I ~ ( ~ - H ) ~ . (p-SMe)Cp]. The former has spiked-triangular geometry (6) with a triply bridging thiolato ligand between two osmium and the tungsten atoms whereas the second adopts a tetrahedral metal core arrangement (5)in which the thiolato group bridges an Os-0s bond.[251 Complexes of formula [Pt2M02Cp2(CO)&2] ( L = PR3) are a good example of planar triangulated rhombohedral (7)/tetrahedral isomerism (5).[’] Their solutions contain both, interconvertible, isomers in a ratio which depends on the solvent used, the temperature, and the steric and electronic properties of the phosphine ligands. Solvents with a small dielectric constant and bulky and basic phosphine ligands favor the tetrahedral isomer, 5, whereas more polar solvents and small or less basic phosphine ligands lead to the planar triangulated rhombohedral isomer, 7. The two isomeric clusters have identical TEC of 58 but MO calculations suggest the platinum atoms have the ‘buffer’ capacity to change the electron count from 16 to 18 between the two isomers. If, moreover, L2 is a diphosphine, the arrangement is dramatically changed to a spiked triangular structure (6), which demonstrates the role of the accessory ligands in determining the final configuration of the metal backbone of the cluster.[261 When [Re(AuPPh3)3H2(MezPhP)3], which contains a planar ReAu3 arrangement (7), is dissolved it partially isomerizes to a species containing a tetrahedral metal framework (5).[271 Reaction of [RezHs(PMezPh)4] with [PPh3AuOBu‘] gives a mixture of two black isomers of [RezHs(AuPPh3)2(PMe2Ph)4].[281Spectroscopic analysis suggests that one is type 6 with an Au-Au bond and no Re-Re bond and, as shown by an X-ray diffraction analysis, the structure of the second isomer has a planar rhomboidal arrangement with a Re-Re bond and the Au atoms symmetrically bridging the two Re atoms (7). The only example of isomerism between structures 6 and 9 are the clusters [Os4(CO)15(L)].[z91 Whereas for L = CO, PF3 the metal core consists of a puckered-
3.6 Skeletal Isomerism in Transition Metal Clusters
1059
square arrangement (9), for L = P(OCH2)3CMe, PMe3, and CNBu' a metal spiked triangle is found (6);it is concluded that the structure adopted by a particular cluster is dictated mainly by the electronic properties of L rather than by the size of the incoming ligand. Deprotonation of the dimetallic complex [Mn2(p-H)(p-PCyH)(CO)g],followed by reaction with ClAuPPh3 results in the isomers [Mn2(p-AuPPh3){p3PCy(AuPPh3)f(CO)g] (10) and [Mn2(AuPPh3)2(p4-PCy)(CO)g] (11).[301 The latter, which is thermodynamically preferred in solution, was identified by single-crystal X-ray analysis whereas the structure of the former was suggested on the basis of 31P NMR data. It has been found that the pu,-bridgedisomer is favored by electronwithdrawing R groups on the phosphine and also by steric hindrance of these ligands. The ratio 10 : 11 is reduced by increasing the solvent polarity, irrespective of the nature of the R groups. The reaction of [Rul(CO)l2]with 3,5-trrt-butyl-l,2-benzoquinone yields two isomers of [Ruq(C0)8{p3-02C6H2(Bu')}~],that differ in the arrangement of metal atoms (types 12 and 13); all contain p3-semiquinone ligands coordinated via terminal and bridging oxygen atoms and a z - y 6 - C ~ring.[311The two isomers are not interconvertible under these reaction conditions. Finally, it is interesting to note that there are examples in which the metal skeletons of the isomers obtained are only slightly different (a dihedral angle is closed or some metal-metal distances are e l ~ n g a t e d ) . [ ~ These ~ , ~ ~compounds ] are, therefore, best described as deformation isomers.
3.6.4 Pentanuclear clusters Only six examples of skeletal isomerism have been reported for metal clusters with this nuclearity; the arrangements of their metal atoms are shown in Scheme 3. In four of the examples one of the structures is a trigonal bipyramid (14), characterized by a TEC of 72. In solution, the trigonal bipyramidal anionic clusters [ Fe4M(CO)13]-, where M = AuPR3, HgMe, CuPPh3, HgMo(C0)3Cp, or HgFe(CO)zCp, are in equilibrium with a bridged butterfly form that contains a z-CO ligand (15). The acceptor strength[341and the size[351of the M group have been found to make this latter form more stable than the trigonal bipyramid. Interestingly, both structures can be obtained in the solid state by changing the cation (for M = AuPPh3) or the rate of crystallization (for M = HgMe).["] The trigonal bipyramidal cluster [ W O ~ ~ C ~ ( C O ) I ~ ( , U - O ) ( ~(14) ~-C isomerCH~)] (16) on standing izes to the bridged tetrahedron [WOs4Cp(C0)~2(,u3-O)(p3-CCH3)] in solution at room temperature for several At 190 "C the reaction is reversed. The driving force for this rearrangement seems to be the adoption of an
1060
3 Dynamics and Physical Properties
21
22
18
Scheme 3
appropriate molecular geometry that can accommodate the 74 valence electrons of the complex. The opening of an 0s-0s bond is accompanied by a transformation of the 0x0 ligand from edge-bridging to face-bridging. The same metal cores, 14 and 16, are in equilibrium in solution for clusters [ M R u ~ C ~ * ( C O ) , ~ ( ~ (, M - H= ) ]Mo, W).[381Structure 16, which contains a pu,-y2CO ligand and therefore two more valence electrons, is the predominant species in solution and has been characterized by X-ray diffraction in the solid state for M = Mo. The Mo atom occupies the bridging position. Interestingly, a small change in a ligand can change the preferred isomeric form. Thus, when a Cp ligand is used instead of Cp*, the molecular structure of [WRu4Cp(CO)&i3-H)]assumes a trigonal bipyramidal skeleton with the W atom in an equatorial position (14). It seems that the Cp* ligand favors the edge-bridged-tetrahedral arrangement 16 because it leads to greater steric repulsion than is provided by the Cp ligand, as suggested by the fact that the Cp*-containing metal fragment resides at the less congested position. The addition of P(OMe)3 to freshly prepared [Os4Ru(p-H)2(CO)12(r6-CgH6)] affords a major purple isomer, [Os4Ru(p-H)2(CO)12(r6-C6H6){P(OMe)3}] (16), and a minor orange isomer, [Os4Ru(p-H)3(CO)12(p,-r6-C6H5){ P(OMe)3}] ( 17).[391 When the purple product is dissolved in dichloromethane it is converted into the orange compound over several hours. Although the former has not been structurally characterized, spectroscopic evidence suggests it consists of an edge-bridged tetrahedron; this is in accord with the structure of the analogous compound [ O S ~ H ~ ( C{P(OMe)3}]. O)~~ The crystal structure of the more stable form was shown to contain an Os4 tetrahedron with a coordinated Ru atom spike (17). This struc-
3.6 Skeletal Isomerism in Transition Metal Clusters
1061
tural rearrangement is accompanied by the loss of one H atom from the arene to the metal framework, the coordination of one carbon of the ring to an 0 s - 0 s edge, and a transfer of a CO ligand to the Ru atom. The replacement of two monophosphine ligands with a diphosphine is enough to change the trigonal bipyramidal R u ~ A ucore ~ of [ R u {AuPPh3}2(p3-S)(C0)9] ~ (14) to a square pyramid skeleton (18) in the [Ru3Au2(p3-S)(CO)s(dpprn)]clusterr31and is best envisaged as being interthe geometry of [Ru3Au2(p3-S)(CO)s(dppm)] mediate between a trigonal bipyramid and a square pyramid. In these and other Ru3Auz-containing clusters the two gold fragments exchange in solution and an intramolecular core rearrangement through a restricted Berry pseudo-rotation mechanism has been p r o p o ~ e d . [ ~ * ~ ~ ] A variation in the coordination by ligands of a metal core provides our next example. Two non-interconvertible isomeric compounds, [ R u {p4-C2PPh2Ru( ~ C0)z. (i7-C3H5)(p-X)}(p-PPhz)(CO)lo](19) and [Rus {,u5-CCC(0)CH2CH=CH2}(p-PPh2)2. (p-X)}(CO)ll](20), are obtained from the reaction of [Ru5(p5-C2PPh2)(p-PPh2). (CO)13]with ally1 chloride or bromide.[411The former has a butterfly Ru4 core with the fifth Ru atom attached through the ligands whereas the latter has a spiked square skeleton. As in those species where the change of one phosphine for another causes rearrangement of the metal framework, there are several examples in which changing a metal fragment for an isolobal fragment produces a different skeleton. Thus the coordination of the isolobal fragments AuPRT, H+, and Hgm’ to the [Fe4C(C0)12I2- anion give three different skeletons - 21, 15 (the H atoms are considered as part of the cluster framework), and 22 in Scheme 3, depending on the . ~ ~shall ~ ~be~denoted ~ ~ ‘isolobal skeletal coordination site of these g r o ~ p s These isomers’.
3.6.5 Hexanuclear clusters Several geometries have been reported for skeletal isomers of clusters containing a nuclearity of six (Schemes 4 and 5). Most of the examples of hexanuclear skeletal isomerism are heterometallic clusters that contain one or two group-1 1 metal fragments. The M( PR3) moieties ( M = Cu, Ag, Au) can adopt a bonding mode either bridging a metal-metal bond or capping a triangular metal face. It has been shown experimentally that the energy difference between these two bonding modes can be very The structures of species in which there is more than one MPR3 might or might not contain the group-1 1 metals in close contact; the energy is very similar for both. In the following examples it can be seen that the simple change of one phosphine ligand for another can induce the adoption of a different skeletal structure.
1062
3 Dynamics and Physical Properties
23
25
34
24
26
35
Scheme 4
The [Ru4M2(p3-H)2(CO)12L2]( M = Cu, Ag, Au; L2 = (PR3)2 or diphosphine) clusters can be obtained with capped-trigonal bipyramidal geometry with a M-M bond (23) or as a bridged-trigonal bipyramid with no M-M contact (24) by choosing the appropriate phosphine and Although the larger phosphines favor structure 24, with gold the substitution of two monophosphines for a diphosphine produces a third isomer a capped-square pyramid [Ru4Au*(p-H)(p3-H)(C0)12. (dppm)] (25).["'] In solution, fluxional processes attributable to metal core rearrangements are observed for these clusters.r401 More striking is [RusC(AuPEt,)(CO)13(NO)] for which two different molecules, bridged-square pyramid (26) and capped-square pyramid (25), are found in the same crystal.r481In 25 the AuPEt3 caps a triangular face, the nitrosyl ligand being on the opposite side of the molecule. In form 26, however, the gold fragment bridges an edge and the nitrosyl ligand is bonded to another iron atom of the same face. This might indicate that the bonding mode of the gold atom is determined by the types of ligand present on the face of the pentanuclear precursor attacked by the [AuPEt3]+ cation. For [HFe4B(AuPR3)2(CO)121 two distinct isomers have also been characterized. They are in equilibrium in solution with a ratio related to the cone angle of the AuPR3 fragments.[491For bulky phosphines type 27 (Scheme 5) is the main isomer, whereas for PMe3 60% of isomer 28 is observed. It is noteworthy that the solid state structure of [HFe4B(AuPEt3)2(CO)12],28, corresponds to the minor isomer in solution, thus suggesting the importance of crystal packing forces in altering structural preference. Analogous ruthenium clusters have not been so widely studied but ~
1063
3.6 Skelrtul Isomerism in Trunsition Metal Clusters
I
9 Q CI N
4
m
1064
3 Dynamics and Physical Properties
also have different isomeric
Thus spectral data indicate that [Ru4B-
H{AuP(~-M~-C~H~)~}~(CO)~~] occurs in solution as forms 28 and 29. When the
phosphine ligand is PPh,, however, the cluster adopts structure 27. Similarly, [ Fe4C(AuPEt3)2(CO)121 (30) and [ Fe4C{Auz(dppm)}(CO)121 (31) also have different geometry in the solid ~ t a t e . [ ~ The ~ , ~ Fe4C '] core is nearly the same in both compounds but the coordination of the gold fragments is very different, although in both cases a similar gold-gold interaction is present. In solution fluxional thus the two phosbehavior is observed for cluster [Fe4C{Auz(dppm)}(C0)12]; phorous atoms are equivalent and structure 30 would seem to be an intermediate in this exchange. The NMR spectrum suggests, moreover, that [Fe4C{AuPPh3}2. This pro(CO)12] exists in a third isomeric form with no Au-Au contact (32).[531 posed structure is consistent with that observed for [RU~C(AUPP~~)~(CO)~~].[~~] A fourth skeleton, an isolobal isomer, is observed for [Fe4CH2(C0)12];this has a slightly different structure (29) in which an H atom bridges a C-Fe(wingtip) bond.[551 The influence of the bite of a diphosphine on the structure is clearly demonstrated The six-metal framework by [Fe2Aw(CO)s(dppm)]and [FezA~4(CO)g(dppe)].[~~I of the dppm cluster consists of a butterfly of gold atoms with two Fe(C0)4 units spanning opposite edges (27). In [FezAu4(CO)s(dppe)],however, no six-metal core is found and the diphosphine ligands link two well-separated FeAu2 units (33). Capped and bridged square pyramidal geometries are also observed for the cluster [MO~RU~(CO)~~C~~(,~~-S)].[~'~ In this example the change in the coordination mode to a terminal position is the cause of mode of a CO ligand from a (,u4-r2) the rearrangement from the bridged-square pyramid (88 valence electrons, 34 in Scheme 4) to the capped-square pyramid (86 electrons, 25). This isomerization takes place on heating the cluster to 80 "C. As in the former the metal atom of the bridge is a ruthenium whereas in the latter the capping metal is molybdenum, a mechanism with interchanging capping metals is proposed. For Os6 clusters the metal core rearrangement is accompanied by a change of the ligand positions but the number of metal-metal bonds is retained. Thus, the bridged-square pyramidal cluster [Os6(CO)16(,u4-S)(p,-S)](34) partially isomerizes upon heating to 125 "C to give a basket skeleton (also with ten Os-0s bonds) in which the two sulfur atoms are in a ,u, mode (35).I5*] The yellow and red isomers of [O~~(CO)I~(CHNM~~)(,~~-S)(,~~-S)(,~-H characterized by X-ray analysis both contain seven Os-0s bonds; the former is a spiked bow-tie cluster (36, see Scheme 5) and the latter a 2-atom-spiked butterfly cluster (37).[591 36 is partially converted to 37 by heating under reflux in octane in a CO atmosphere. was described Thermolysis of [ { Ru~(,u-H)(p-C~CBut)(CO)g}2(p-PPh2CCPPh2)] in a very recent paper.[601A condensation reaction between a C2CBu' and dppa ligands occurs leading to two non-interconvertible Rug isomeric clusters. One has a two butterfly edge-condensed skeleton (90e, 27) whereas the other is a bridged square pyramid with the bridged-edge broken (38). This latter geometry also has a 90 valence-electron count if one considers that two electrons are needed to break
3.6 Skeletal Isomerism in Transition Metal Clusters
1065
one bond of an 88e bridged square pyramid. The ligand coordination is, however, distinct and whereas in the former only one C-PPh2 bond of the dppa ligand is broken, in the latter both PPh2 fragments are coordinated to ruthenium atoms only. Although not strictly skeletal isomers, it is noteworthy that two conformational PF6)2 are obtained in isomers of the cluster [Rh2(p-AuPPh3)(pu-C1)2(CNCsH9)4]2( the solid state; these easily recognizable by their color.r611For penta to heptanuclear chain clusters of type [Pt3M,(p3-CR)Lx] ( M = Mo or W, R = Me or C6H4Me-4, and L = (p-CMe), CO, cod, Cp or Cp*) several diastereoisomers have been observed.[621
3.6.6 Hepta- and higher nuclearity clusters The pairs of skeletal isomers found for seven-metal-atom clusters are depicted in Scheme 6. The [OsgPt(CO)17(p3-NCMe)(CgH12)]cluster (39) is converted to [OssPt(CO)17(p4-NCMe)(CsH12)](40) by standing in dichloromethane for several This can be explained if we consider that the acetonitrile ligand provides four electrons to the metal atoms in the former compound (39) and six electrons to the more open structure 40; the Pt atom is a 16-electron square-planar center in the former but acts as an 18-electron trigonal bipyramidal center in the latter. This is another good example of the electronic 'buffer' capacity of platinum mentioned above. Interestingly, this isomerization process does not occur when the COD ligand is substituted by the better a-donor ligands P(OMe)3 or dppe, suggesting that the electronic unsaturation of the 16-electron Pt center is responsible for the rearrangement.
39
40
43
Scheme 6
41
44
\
42
45
1066
3 Dynamics and Physical Properties
All the other heptanuclear examples contain gold atoms. When a mixture of cisand trans-[Rh2Ru4B(CO)ls] is reacted with ClAuPR3 a mixture of cis-[RhzRQB. {pu-AuPR3)(C0)16] (41) and trans-[Rh2Ru4B(p3-AuPR3)(CO)16] (42) is obtained.[641 The former isomerizes to the latter on standing in dichloromethane. Interestingly, the reaction of the analogous trans-[Ir2R~B(CO)l,j] with the gold compound results (41) only. This process is exin the formation of ~~.Y-[I~~RU~B{~-AUPR~)(CO)~~] plained by the preference of the gold atom to form Ir-Au bonds rather than Ru-Au bonds. Changing two monophosphine for a diphosphine ligand converts the bicapped trigonal bipyramid, [ H R u ~ A uCO) ~ ( 12( PPh3)3] (43), to a bicapped-square pyramid, [ H R U ~ A U ~ ( C O ) ~ ~ ( P P ~(44).[651 ~)(~PP~)] A major rearrangement occurs when this type of substitution is made in MSCAu2 clusters ( M = R U , [ ~Fe[671) ~] the bridged octahedron with no Au-Au bond in [ M ~ C ( A U P R ~ ) ~ ( (41) C O is ) ~converted ~] to a double-capped-square pyramid with a Au-Au interaction in [M5CAu2(CO)14(diphosphine)](45). In this species the bite of the diphosphine ligands is obviously too small to enable the gold atoms to adopt structure 41. The importance of the steric constraints of the diphosphine ligand is also seen when comparing the crystal structures of [Fe5CAu2(CO)14(dppm)]and [RusCAu2(C0)14 (dppe)]. Although both clusters adopt structure 45, in the dppm derivative the bonding of the gold atoms to the ruthenium edge and face is highly asymmetric whereas in the dppe derivative the gold atoms can bond to each symmetrically. The importance of the phosphine attached to the gold atom is also seen in the metal clusters R u ~ A u ~Thus, . although the two gold fragments bridge two opposite edges of the Rug octahedron in the solid-state structure of [ R u ~ C ( A U P M ~ P ~ ~ ) (46, ~ ( CScheme O ) ~ ~ ]7), in solution this structure coexists with -
48
Scheme 7
1067
3.6 Skeletal Lromerism in Transition Metal Clusters
another isomeric form.[6x1It has been proposed that this isomer has the structure [ R U ~ W C ( A U P E ~ ~ ) ~in ( Cwhich O ) I ~one ] gold atom caps a Ru3 face and the second caps a RuzAu face (47). A third metal core does, however, occur in the dppm analog [RugCAu2(CO)ls(dppm)](48). As can be seen in Scheme 7, both gold atoms bridge adjacent Ru-Ru edges. The global structure can be envisaged as a Rug octahedron fused with a R U ~ A Usquare Z pyramid.[691 In solution the 1,4-bicapped octahedral cluster [Re,jC(CO)lg{p3-Re(C0)3}. {p3-Ir(C0)2}] (49) slowly becomes converted to 1,3-bicapped octahedral [Re51rC(C0)17{p3-Re(C0)3}2] (50) in which the Ir atom now occupies a vertex of the octahedron.[701Isomerization pathways via diamond-square-diamond (DSD) processes have been proposed, but unfortunately labeling experiments to provide mechanistic evidence failed because of carbonyl scrambling in at least one intermediate. One force driving this rearrangement might be the increased number of Re-Ir bonds. This assumption is made because data from bimetallic surfaces indicate that heterometallic Re-Ir bonds are stronger than homometallic Ir-Ir bonds. The crystal structure of [ N E ~ ~ ] ~ [ A u ~ F ~has ~ (aCunit O ) cell ~ ~ ]containing two different anions, both consisting of an inner rectangle of gold atoms and four edgebridging Fe(C0)4 groups, but which differ on the edges of the gold rectangle.[711 Whereas in one molecule the rectangle is nearly a square (51) the other has two long edges suggesting the absence of any significant bonding interaction (52). The weakening of these Au-Au contacts in 52 compared to 51 is compensated by some tightening of the Au-Fe bonds. These two isomers could also be designed as deformation isomers. The next compounds to be discussed constitute a second example of a different isomer being obtained in the solid state as a result of a change in the counter-anion employed. Thus, [Au9{ P(CsH40Me-p)3}s][BF4]3has centered-crown geometry (53, Scheme 8) in contrast with [ A U{P(CgH40Me-p)3}8][PFg]3 ~ which has D2h skeletal geometry derived from an icosahedron (54).[”] When nitrate is used as counteranion, the two modifications co-crystallize and can be easily separated on the basis of their color: golden brown and green.[21In solution the isomers either adopt a common structure or are made equivalent by a rapid rearrangement process. In the solid state isomer 54 is, moreover, converted into isomer 53 at high pressure.[731An
53 Scheme 8
55
56
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3 Dynamics and Physical Properties
57 59
60
Scheme 9
energetically favorable change in the intermolecular forces governing the lattice structure seems to be the driving force of this transition. [Os7H2(C0)20] and [Os7(AuPEt3)2(C0)20] are further examples of isolobal skeletal isomerism arising from different Os7E2 frameworks ( E = H, Au) even though H and AuPEt3 are isolobal fragments. The X-ray characterized isomer of [Os7H2(C0)20]has a bridged-capped-bipyramid Os7 core with two triply bridging hydrides (55)""' whereas the gold analog consists of a capped octahedron of osmium atoms capped by two AuPEt3 units (56).[751It is noteworthy that in both compounds H and AuPEt3 adopt a p, coordination mode. An isomeric form of [ O S ~ H ~ ( C Ois, ) ~moreover, O] obtained by protonation of [Os7(C0)20]~-rather than by decarbonylation of [ O S ~ H ~ ( C OAlthough ) ~ ~ ] . this isomer could not be analyzed crystallographically it might be isostructural with [ O S ~ ( A U P E ~ ~ ) ~as( Csug~)~O] gested by recent X-ray characterization of the d i a n i ~ n . ~ ~ The decanuclear cluster [OsgAuz(CO)22(PPh3)2]is obtained as a mixture of two isomers.[761Isomer 57 is converted to 58 by standing in solution (Scheme 9). When the related [OsgAu2(C0)22(diphosphine)] is synthesized, however, the chelating phosphine ligand does not allow rearrangement to 58, and 57 could be structurally characterized for diphosphine = dppb. It is interesting to note that, in contrast with other gold-containing examples, in this species not only does the skeletal isomerism arise because of a change in the gold-fragment bonding position or mode, but that also the osmium core rearranges from a 1,3-bicapped octahedron to a 1,2-bicapped octahedron on going from the kinetically (57) to the thermodynamically (58) favored product. produces a The replacement of one H by AuPR3 in [Pt3Rug(C0)21(p3-H)(pU-H)3] cluster with the same geometry, with the gold fragment capping a Ru3 face. When two H atoms are substituted, however, an isolobal skeletal isomer is obtained, [Pt3Ru6(CO)21(p3-AuPR3)2(p3-H)2]. Thus, instead of binding in a p and p, fashion to an Ru-Ru edge and to a Ru3 face (59), the gold fragments coordinate in a p,mode to Ru2Pt faces (60), while the two remaining H cap the Ru3
3.6 Skeletal Isomerism in Transition Metal Clusters
1069
3.6.7 Conclusions From all these results we can conclude that in most cases skeletal isomers are obtained under the same reaction conditions. When an equilibrium mixture is possible, one or other isomer can be obtained in the solid state by choosing the appropriate counter-anion, solvent, or rate of crystallization. A small change in a ligand, usually a phosphine, can also induce the precipitation of one isomer rather than the other. The different bonding mode of ligands to a metal core, usually accompanied by a change in the electron contribution to the cluster electron count, seems to be one of the major causes of skeletal isomerism. These ligands are mainly organic, sulfurdonor, or carbonyl ligands. Some other causes of skeletal isomerism are the bonding versatility of AuPR3, the unsaturation of 16e Pt centers, or the mere presence of an heterometal. The increasing number of metal clusters containing heterometal atoms and/or organic fragments suggest that skeletal isomerism will develop considerably in the near future.
References [ l ] Braunstein P., de Meric de Bellefon C., Bouaoud S. E., Grandjean D., Halet J. F. and Saillard J. Y. (1991) J. Am. Chem. SOC.113: 5282. [2] Briant C. E., Hall K. P. and Mingos D. M. (1984) J. Chem. SOC.Chem. Commun. 290. [3] (a) Bruce M. I., Shawkataly 0. B. and Nicholson B. K. (1985) J. Organomet. Chem. 286: 427; (b) Brown S. S. D., Hudson S., Salter I. D. and McPartlin M. (1987) J. Chem. SOC.Dalton Trans. 1967. [4] See for example: a) Bantel H., Powell A. K. and Vahrenkamp H. (1990) Chem. Ber. 123: 677; (b) Briickner P. G., Peters G. W. and Preetz W. (1994) Z. anorg. allg. Chem. 620: 1669; (c) Khattar R., Puga J., Fehlner T. P. and Rheingold A. L. (1989) J. Am. Chem. SOC.111: 1877; (d) Chen C. C., Chi Y., Peng S. M. and Lee G . H. (1993) J. Chem. SOC.Dalton Trans. 1823; (e) Fjare D. E. and Gladfelter W. L. (1984) J. Am. Chem. SOC.106: 4799. [5] Adams R. D., Cortopassi J. E., Aust J. and Myrick M. (1993) J. Am. Chem. SOC.115: 8877. [6] Braunstein P., J. Kervennal J. and Richert J. L. (1985) Angew. Chem. Tnt. Ed. Engl. 24: 768. [7] Wucherer E. J., Tasi M., Hansert B., Powell A. K., Garland M. T., Halet J. F., Saillard J. Y. and Vahrenkamp H. (1989) Inorg. Chem. 28: 3564. [8] Knoll K., Huttner G. and Zsolnai L. (1986) J. Organomet. Chem. 312: C57. [9] Zoet R., Jastrzebski J. T. B. H, van Koten G., Mahabiersing T., Vrieze K., Heijdenrijk D. and Stam C. H. (1988) Organometallics 7: 2108. [lo] Peng J. J., Horng K. M.. Cheng P. S., Chi Y., Peng S. M. and Lee G. H. (1994) Organometallics 13: 2365.
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[I I] Bender R., Braunstein P., Tiripicchio A. and Tiripicchio-Camellini M. (1985) Angew Chem. Int. Ed. Engl. 24: 861; Bender R., Braunstein P., Dedieu A., Ellis P. D., Huggins B., Harvey P. D., Sappa E. and Tiripicchio A. (1996) Inorg. Chem. 35: 1223. 1121 Bruce M. I., Low P. J., Skelton B. W. and White A. H. (1993) J. Chem. Soc. Dalton Trans. 3145. (131 Haupt H. J., Schwefer M., Egold H. and Florke U. (1995) Inorg. Chem. 34: 5461. [I41 Bruce M. I., Zaitseva N. N., Skelton B. W. and White A. H. (1997) J. Organomet. Chem. 5367: 93. [15] Chi Y., Wu F. J., Liu B. J., Wang C. C. and Wang S. L. (1989) J. Chem. SOC.Chem. Commun. 873. [16] Chi Y., Su C. J., Farrugia L. J., Peng S. M. and Lee G . H. (1994) Organometallics 13: 4167. [17] Honvitz C. P. and Shriver D. F. (1984) Organometallics 3: 756. [I81 Jungbluth H., Stoeckli-Evans H. and Suss-Fink G. (1990) J. Organomet. Chem. 391: 109. [I91 Park J. T., Woo B. W., Chung J. H., Shim S. C., Lee J. H., Lim S. S. and Suh I. H. (1994) Organometallics 13: 3384. [20] Ewing P. and Farrugia L. J. (1989) Organometallics 8: 1246. [21] Curtis M. D., Williams P. D. and Butler W. M. (1988) Inorg. Chem. 27: 2853; Braunstein P., Jud J. M., Tiripicchio A,, Tiripicchio-Camellini M. and Sappa E. (1982) Angew. Chem. Int. Ed. Engl. 21: 307. [22] Chi Y., Hsu H. F., Peng S. M. and Lee G. H. (1991) J. Chem. SOC.Chem. Commun. 1019. [23] Adams R. D. and Wang S. (1985) Organometallics 4: 1902. [24] Adams R. D. and Wang S. (1987) J. Am. Chem. Soc. 109: 924. [25] Tsay C. W., Tu W. C., Chi Y., Peng S. M. and Lee G. H. (1994) J. Chin. Chem. Soc. (Taipei) 41: 621. [26] Bender R., Braunstein P., Dusausoy Y. and Protas J. (1979) J. Organomet. Chem. 172: C51; Braunstein P., Jud J. M., Dusausoy Y. and Fischer J. (1983) Organometallics 2: 180. [27] Sutherland B. R., Folting K., Streib W. E., Ho D. M., Huffman J. C. and Caulton K. G. (1987) J. Am. Chem. Soc. 109: 3489. [28] Sutherland B. R., Ho D. M., Huffman J. C. and Caulton K. G. (1987) Angew. Chem. Int. Ed. Eng. 26: 135. [29] Einstein F. W. B., Johnston V. J. and Pomeroy R. K. (1990) Organometallics 9: 2754. [30] Haupt H. J., M. Schwefer M. and Fliirke (1995) Inorg. Chem. 34: 292. [31] Churchill M. R., Lake C. H., Paw W. and Keister J. B. (1994) Organometallics 13: 8; Paw W., Keister J. B., Lake C. H. and Churchill M. R. (1995) Organometallics 14: 767. [32] Bruce M. I. and Nicholson B. K. (1983) J. Organomet. Chem. 250: 627. [33] Moiseev I. I(1995) J. Organomet. Chem. 488: 183. [34] Horwitz C. P. and Shriver D. F. (1985) J. Am. Chem. SOC.107: 8147. [35] Wang J., Sabat M., Horwitz C. P. and Shriver D. F. (1988) Inorg. Chem. 27: 552. [36] Honvitz C. P., Holt E. M., Brock C. P. and Shriver D. F. (1985) J. Am. Chem. Soc. 107: 8136. [37] Gong J. H., Hwang D. K., Tsay C. W., Chi Y., Peng S. M. and Lee G. H. (1994) Organometallics 13: 1720. 1381 Su C. J., Chi Y., Peng S. M. and Lee G. H. (1995) Organometallics 14: 4286. [39] Al-Mandhary M. R. A.. Lewis J. and Raithby P. R. (1997) J. Organomet. Chem. 530: 247. [40] Orpen A. G. and Salter I. D. (1991) Organometallics 10: 11 1. [41] Adams C. J., Bruce M. I., Liddell M. J. and Nicholson B. K. (1991) J. Organomet. Chem. 420: 105. 1421 (a) Bogdan P. L., Horwitz C. P. and Shriver D. F. (1986) J. Chem. Soc. Chem. Commun. 553; (b) Rossell O., Seco M., Segalks G., Alvarez S., Pellinghelli M. A,, Tiripicchio A. and de Montauzon D. (1997) Organometallics 16: 236. [43] Holt E. M., Whitmire K. H. and Shriver D. F. (1981) J. Organomet. Chem. 213: 125. [44] Rossell O., Seco M., Gomez-Sal P., Martin A,, Reina R. and Riba 0. (1997) Organometallics, 16: 5113.
3.6 Skelctul Isomerism in Trunsition Metul Clusters
I07 1
[45] Salter I. D. (1989) Adv Organomet. Chem. 27: 249. [46] (a) McCarthy P. J., Salter I. D. and Adatia T. (1995) J. Organomet. Chem. 485: 191; (b) Brown S. S. D., Salter I. D. and Toupet L. (1988) J. Chem. Soc. Dalton Trans. 757. [47] Bates P. A,, Brown S. S. D., Dent A. J., Hursthouse M. B., Kitchen G. F. M., Orpen A. G., Salter I. D. and Sik V. (1986) J. Chem. Soc. Chem. Commun. 600. [48] Henrick K.. Johnson B. F. G., Lewis J., Mace J., McPartlin M. and Morris J. (1985) J. Chem. Soc. Chem. Commun. 1617. [49] Housecroft C. E., Shongwe M. S. and Rheingold A. L. (1989) Organometallics 8: 2651. [50] Draper S. M., Housecroft C. E.. Rees J. E., Shongwe M. S., Haggerty B. S. and Rheingold A. L. (1992) Organometallics 11: 2356. [51] Johnson B. F. G., Kaner D. A., Lewis J., Raithby P. R. and Rosales M. J. (1982) J. Organomet. Chem. 231: C59. [52] Rossell O., Seco M.. SegalCs G., Johnson B. F. G.. Dyson P. J. and Ingham S. L. (1996) Organometallics 15: 884. [53] For this compound two very different phosphorous atoms are observed in the NMR spectrum of a thf solution: 54.6 (lP, s) and 38.0 (lP, s) ppm. [54] Cowie A. G., Johnson B. F. G., Lewis J., Raithby P. R. (1984) J. Chem. Soc. Chem. Commun. 1710. [55] Tachikawa M. and Muetterties E. L. (1980) J. Am. Chem. Soc. 102: 4541. [56] Briant C. E.. Hall K. P. and Mingos D. M. P. (1983) J. Chem. Soc. Chem. Commun. 843. [57] Adams R. D., Babin J. E. and Tasi M. (1988) Inorg. Chem. 27: 2618. [58] (a) A d a m R. D., Horvath I. T. and Yang L. W. (1983) J. Am. Chem. Soc. 105: 1533; (b) Adams R. D., Chen G., Pompeo M. P. and Sun S. (1990) Polyhedron 9: 2385. [59] A d a m R. D., Babin J. E. and Kim H. S. (1986) Inorg. Chem. 25: 4319. [60] Bruce M. I.. Humphrey P. A.; Skelton B. W. and White A. H. (1997) J. Chem. SOC.Dalton Trans. 1485. [61] Bray K. L., Drickamer H. G., Mingos D. M. P., Watson M. J. and Shapley J. R. (1991) Inorg. Chem. 30: 864. [62] Davis S. J., Elliott G. P.. Howard J. A. K., Nunn C. M. and Stone F. G. A. (1987) J. Chem. Soc. Dalton Trans. 2 177. [63] Couture C. and Farrar D. H. (1987) J. Chem. Soc. Dalton Trans. 2245. [64] Galsworthy J. R., Hattersley A. D., Housecroft C. E., Rheingold A. L. and Waller A. (1995) J. Chem. Soc. Dalton Trans. 549. [65] a)Bateman L. W., Green M., Howard J. A. K., Mead K. A,, Mills R. M., Salter I. D., Stone F. G. A. and Woodward P. D. (1982) J. Chem. Soc. Chem. Commun. 773; (b) Bruce M. I. and Nicholson B. K. (1983) J. Organomet. Chem. 252: 243; (c) Howard J. A. K., Salter I. D. and Stone F. G. A. (1984) Polyhedron 3: 567; (d) Adatia T., McPartlin M. and Salter I. D. (1988) J. Chem. Soc. Dalton Trans. 751. [66] Amoroso A. J., Edwards A. J., Johnson B. F. G., Lewis J., Al-Mandhary M. R., Raithby P. R., Saharan V. P. and Wong W. T. (1993) J. Organomet. Chem. 443: C11. [67] (a) Johnson B. F. G., Kaner D. A,, Lewis J. and Rosales M. J. (1982) J. Organomet. Chem. 238: C73; (b) Rossell O., Seco M., Segales G., Pellinghelli M. A., Tiripicchio A,, (1998) J. Organomet. Chem. 571: 123. [68] Bunkhall S. R., Holden H. D., Johnson B. F. G., Lewis J., Pain G . N., Raithby P. R. and Taylor M. J. (1984) J. Chem. Soc. Chem. Commun. 25. [69] Bailey P. J., Beswick M. A,, Lewis J., Raithby P. R. and Ramirez de Arellano M. C. (1993) J. Organomet. Chem. 459: 293. [70] Ma L., Wilson S. R. and Shapley J. R. (1994) J. Am. Chem. SOC.116: 787. [71] Albano V. G., Calderoni F., lapalucci M. C., Longoni G . and Monari M. (1995) J. Chem. Soc. Chem. Commun. 433. [72] (a) Hall K. P.. Theobald B. R. C., Gilmour D. I., Mingos D. M. P. and Welch A. J. (1982) J. Chem. Soc. Chem. Commun. 528; (b) Bellon P. L., Cariati F., Manassero M., Naldini L.,
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and Sansomi M. (1971) J. Chem. Soc. Chem. Commun. 1423; (c) Smits J. M. M., Beurskens P. T., Bour J. J. and Vollenbroek F. A. (1983) J. Cryst. Spec. Res. 13: 365. [73] Coffer J. L., Drickamer H. G. and Shapley J. R. (1990) Inorg. Chem. 29: 3900. [74] Ditzel E. J., Holden H. D., Johnson B. F. G., Lewis J., Saunders A. and Taylor M. J. (1982) J. Chem. SOC.Chem. Commun. 1373. [75] Amoroso A. J., Johnson B. F. G., Lewis J., Li C. K., Morewood C. A,, Raithby P. R., Vargas M. D. and Wong W. T. (1995) J. Cluster. Sci. 6: 163. [76] Akhter Z., Ingham S. L., Lewis J. and Raithby P. R. (1994) J. Organomet. Chem. 474: 165. [77] Adams R. D., Barnard T. S. and Cortopassi J. E. (1995) Organometallics 14: 2232.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.7 Bond Length-Bond Enthalpy Patterns in Metal Carbonyl Cluster Chemistry Andrew K. Hughes and Ken Wade
3.7.1 Introduction The dramatic development of metal-cluster chemistry over the past three decades has led to the isolation and characterization of large numbers of molecular metal clusters, notably neutral or anionic metal carbonyls, but including many systems with a wide range of other ligands.[',21Our knowledge and understanding of such systems has benefited enormously from recent improvements in the speed and precision with which their structures can be determined by single-crystal X-ray diffraction studies, sometimes helped by neutron diffraction to locate hydrogen ligands. Structural databases are now conveniently accessible by electronic means to individuals searching them from their own offices.r31Much effort and some ingenuity has been spent showing how the ever more intricate structures revealed by diffraction experiments conform to patterns. Thus cluster structures can, arguably, be best regarded as assemblies of close-packed atoms, as fragments of bulk metal,[41 as analogs of borane c l ~ s t e r s , [ ~as, ~multicapped ] octahedra or other polyhedra,r71 or as assemblies of metal atoms held together by classical electron-pair bonds.[*] Meanwhile we are approaching the stage at which developments in theoretical chemistry, and our ability to calculate with confidence the distribution of the electronic glue that holds metal clusters together, will enable us to refine our ideas about the metal-metal bonding in clusters, and ultimately to calculate their thermodynamic stabilities.['] Despite these developments in experimental and theoretical methods of probing metal-cluster chemistry, the thermodynamic stabilities of metal clusters remain experimentally relatively uncharted, and theoretically difficult to calculate with any precision. Calorimetric studies on clusters, e. g., determinations of their enthalpies of combustion, have been far too few to provide an adequately broad experimental database, and tend to date from the early days of cluster chemistry.["] The purpose of this survey is to outline a method by which, working from the limited calorimetric data available, one can use the now comprehensive
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database of metal cluster structures to gain some feeling for the relative stabilities of metal clusters, and the strengths of their metal-metal and metal-ligand bonds. The trends in stabilities revealed by our approach provide insight into how effectively various metals use their valence-shell electrons for metal-metal and metal-ligand bonding, and provide some insight into how ligands, and the other metal atoms in the cluster, compete for the (limited) binding capacity of sets of metal atoms. We note the relevance of our discussion of cluster chemistry to reactions at metal surfaces, and draw attention to aspects of cluster chemistry that would benefit from further or more precise experimental or theoretical study.
3.7.2 The method 3.7.2.1 Basic assumptions Our approach is unashamedly simplistic, and is based on the twin assumptions that: i) the metal-metal bonds in clusters resemble those in the bulk metal, varying analogously in strength with length, so enabling the strength of cluster metalmetal bonds to be calculated from their lengths; and ii) the enthalpies of disruption of metal carbonyl clusters into gaseous metal atoms and carbon monoxide can be expressed as the sum of enthalpies of the metalmetal and metal-ligand bonds broken:
As used by us, our method was first outlined in a short series of papers published some twenty years ago," '-l4] although the basic premise-that bonds of a particular type become systematically weaker as they lengthen-was recognized by Pauling in his classic studies in the twenties and thirties,[l5I and is a familiar feature in organic chemistry['61and in the covalent chemistry of the non-metallic elements in general. Although the approach we wish to follow measures bond strength in terms of bond enthalpy (in kJ mol-') there are other possible measures of bond strength. One such measure is the fractional valence between two atoms, and the bond-valence method has been extensively used in studies of extended lattices, especially metal oxides." 'I Approaches using a thermodynamic measure of bond strength have been applied less widely to bonds between metal atoms, or between metal and non-metal atoms,r181in part because of the paucity of calorimetric data, and also because it
3.7 Bond Length-Bond Enthulpy Patterns in Metal Carbonyl Cluster Chemistry
107.5
was felt that the polarity of the latter types of bonds to metal atoms might affect the transferability of data. It was also considered that the relative weakness and polarizability of metal-metal bonds might enable them to accommodate significant changes in length without corresponding changes in strength. Whilst noting these concerns expressed by others, we consider that the trends revealed by our approach amply vindicate its use.
3.7.2.2 Bulk metals: rates at which metal-metal bonds change in strength with length As the strength of the bonds in bulk metals, and the rates at which such bonds vary in strength with length, form the basis for our treatment of the metal-metal bonds in clusters, it is convenient to start our discussion by considering bulk metals. Firstly, in referring to metal-metal ‘bonds’, we should stress that we are not referring to classical electron-pair bonds, and are not, as a basis, making any assumptions about the numbers of electrons available for bonding in bulk metals, or about their distribution. Rather, we are assuming that all the pairwise links between neighboring atoms are bonding, and we deduce their strength from the enthalpy change when all such links are broken, as when the bulk metal is vaporized to form a monatomic gas. For a close-packed metal lattice, whether hexagonal or face-centered cubic, in which each atom in the bulk metal is surrounded by twelve neighboring atoms, one needs to break on average six two-center bonds between neighboring atoms to free one metal atom:
Some metals crystallize in both close-packed and body-centered cubic forms that differ little in their atomization enthalpy. For body-centered cubic lattices, in which each atom is surrounded by only eight nearest neighbors, it has often been assumed that the bond-energy term appropriate for these nearest neighbor links is one quarter of the atomization enthalpy of the metal, because four such links have to be broken on average to free one metal atom. This leads to the (absurd) conclusion that these links are 50% stronger that those in the close-packed lattice, even though they are only marginally (typically 1.7 to 1 .S%) shorter than the latter. This problem is readily resolved if one treats the immediate coordination sphere of an atom in a body-centered cubic lattice as containing a total of fourteen atoms, the eight nearest neighbors at a distance d~,,,, and six more at a distance 2dt,,,/J3, i.e . , 1.1 S.5dbCc, through the faces of the cube defined by the other eight (see Fig. 1). Disruption of the lattice into gaseous metal atoms therefore requires, on average,
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3 Dynamics and Physical Properties
Figure 1. The body-centered cubic lattice, drawn to emphasize the coordination of each atom by eight atoms at the corners of a cube (distance d b ) , and six next-nearest neighbors at the corners of an octahedron (distance a pm, a = unit cell length = 2dbCC/J3).
cleavage of four of the shorter bonds and three of the longer bonds per metal atom:
for a body-centered cubic lattice, where E’(M-M),,, and E”(M-M),, are the bond-enthalpy terms for the shorter and longer bonds, respectively, to the fourteen neighboring atoms. If, moreover, the enthalpies of vaporization of the close-packed and body-centered cubic lattices are the same (which is approximately true), then:
For those transition metals known to crystallize in both close-packed and bodycentered cubic forms, the distances to nearest neighbor atoms, dcpand dbcc, bear a remarkably consistent ratio to each other, dcp/dbcc being 1.0177 for Ti, 1.0172 for Cr, and 1.0171 for Fe. The corresponding ratio for alkali and alkaline earth metals is very similar, ca 1.018. We infer from the consistency of this ratio that the bonds in these metals are varying in strength with length at comparable rates which can be calculated from the ratio d,p/dbcc. Of the many empirical relationships that have been found to link the energies of bonds to their lengths, the most generally applicable, particularly when wide variations in length need to be accommodated (e.g., as between carbon-carbon single and triple bonds, although note that our method makes no reliance on the concept of bond order), is the following:
E(X-Y) = A[d(X-Y)lPk
3.7 Bond Length-Bond Enthulpy Patterns in Metal Curbonyl Cluster Chemistry
1077
where A and k are constants characteristic of the bond type. Plots of log E(X-Y) against log d(X-Y) give good straight lines, of slope -k, for wide ranges of compounds; for carbon-oxygen bonds, k 5; for carbon-carbon bonds, k 3.3; and for metal-oxygen bonds (where k can be recognized from the bond valence method), k lies in the range 2-7, depending on the metal. If we use an equation of the form:
-
E(M-M)
-
= A[d(M-M)lpk
for the metal-metal bonds in bulk metals capable of crystallizing in both closepacked and body-centered cubic forms of the same enthalpy of vaporization, then we can write:
Taking dcp= 1.017dbcc,we can solve this to obtain k = 4.6, and we have used this figure to deduce the strength of the metal-metal bonds in our treatment of the metal carbonyl clusters discussed below. A corollary of our treatment that is useful when dealing with metals that crystallize only in a body-centered cubic form is that one can calculate E(M-M) for the bonds to the eight nearest neighbors in such a structure by dividing the vaporization enthalpy, AHvaporlzation, by 5.55 (not 4 as commonly assumed), and the energies of the six bonds to the next nearest neighbors by dividing AHvaporiration by 10.76. Whether a particular metal is known to crystallize in only a close-packed or body-centered cubic form, or in both, we are therefore able to calculate, from its vaporization enthalpy, the bond enthalpy term E(M-M) for bonds of a known length d, and so, using the equation E(M-M) = Adp4.6,we can calculate the value of A for that metal. Values of length-energy correlation constants, A , calculated thus are listed in Table 1, with other relevant data. From these we can, in turn, calculate the enthalpies of all of the metal-metal bonds in (homonuclear) cluster compounds of these metals, if their structures, and so their bond lengths, have been determined.
3.7.3 Applications to metal carbonyls with known heats of formation If the enthalpy of formation of a particular metal carbonyl cluster compound M,(CO),, has been measured, typically by calorimetric determination of the enthalpy of combustion, one can calculate the enthalpy of disruption of that cluster in
1078
3 Dynamics and Physical Properties
Table 1. The idealized structures (bcc, hcp or fcc), enthalpies of atomization, AfH M(g) kJmol-', metal-metal bond lengths, d( M-M) pm, bond enthalpies of the individual M-M bonds [E(M-M) = AfH/6 or AfH/5.55 as appropriate], and resulting length-energy correlation constants A (= Eld-") x for the metals of the Ti through Ni groups. Ni V Symbol Ti Mn (a) Fe ( 4 c o (4 Cr (4 fcc bcc bcc Structure hcp bcc bcc hCP 280.7 416.3 424.6 429.7 514.2 396.6 AfH M(g) 469.9 249.16 266.79 (6) 248.23 250.61 262.24 249.80 d( M-M) 289.56 75.1 71.4 71.5 51.3 92.7 71.6 E(M-M) 78.3 0.766 1.239 0.780 0.774 0.756 A 1.651 0.742 Zr hCP 608.8 317.9 101.5 3.288
Nb bcc 725.9 285.84 130.2 2.587
Mo hcc 658.1 272.51 118.4 1.888
Tc hCP 677.8 270.3 115.8 1.780
Ru hCP 642.6 265.02 108.5 1.522
Rh fcc 556.9 269.01 92.9 1.396
Pd fcc 378.2 275.1 1 62.1 1.034
Hf hCP 619.2 312.73 (a) 103.2 3.100
Ta bcc 782.0 286 141.7 2.823
W bcc 849.3 274.09 153.8 2.519
Re hCP 769.8 274.1 129.3 2.188
0s hCP 790.7 267.54 131.6 1.929
Ir fcc 665.2 271.4 110.9 1.735
Pt fcc 565.3 274.9 94.6 1.571
All data on structure, enthalpy of atomization (in kcal mol-') and metal-metal bond lengths were taken from CRC Handbook of Chemistry and Physics, ed. D. R. Lide, 72"d edn, CRC Press, Boca Raton. For the metals with bcc structure the metal-metal bond length quoted is the %coordinate distance; for the fcc and hcp metals the distance quoted is the 12-coordinate distance. For metals which have more than one phase the table indicates the phase for which data have been taken. Manganese has a large number of phases, none of which has an idealized structure, data for this element should be treated with caution.
the gas phase into gaseous metal atoms and carbon monoxide molecules:
That disruption enthalpy, can be assumed to be equal to the sum the of bond enthalpies of the metal-metal and metal-ligand bonds broken: AHdisrupt = CE(M-M)
+ X:E(M-CO)
3.7 Bond Length-Bond Enthulpy Patterns in Metal Curhonyl Cluster Chemistry
1079
Table 2. Bond length, d, and bond enthalpy, AH, data for metals and metal carbonyls, including calculated metal-ligand bond enthalpies. Experimental (kJmo1-I)
AHdisrupt
d(M-M) (pm)
E( M-M) (kJ mol-I)
E( M-CO) (kJmol-I)
Percentage of AHdlsruptaccounted
for by CE( M-M) 417 (4) 585 (8) 1 I73 (25) 1676 (29)
248 252 256 268 (2 off)
75 70 65 52
-
117 123 I26
0 6 10
651 (8) 2414 (29)
265 285
109 78
182
10
790 (8) 2690 (29)
268 288
132 94
20 1
11
428 (2) 2121 (29)
25 1 252 249
71 70 74
136 140
6 21
557 2648 (29) 3874 (29)
269 273 278
93 86 80
178 182
20 25
665 (8) 3051 (29)
271 268
111 117
196
23
I I60 (12)
Data are taken from C.E. Housecroft, K. Wade and B.C. Smith, J. Chem. Soc., Chem. Commun., 1978, 766; see the original for references.
The total enthalpy change associated with the cleavage of all the metal-metal bonds in the cluster, CE(M-M), can be found by summing the individual bond enthalpy terms calculated as above from their lengths. Hence XE(M-CO) for that compound can be determined. Division of CE(M-CO) by y , the number of carbony1 ligands, gives the average value of E(M-CO) for that compound M,(CO),.. Values are listed in Table 2 which is adapted from elsewhere.['21 It should be stressed that the values of E(M-CO) given in Table 2 relate to the average enthalpy change between the coordinated and free molecular states of carbon monoxide, and cannot be regarded as measures of the metal-carbon bond enthalpies in these compounds. Carbon monoxide is a very versatile, accommodating, n-acidic ligand that can form strong bonds to one or more metal atoms not simply by use of the formal lone pair of electrons on a free carbon monoxide molecule, but by taking up electron density from metal atoms in n* (carbon-oxygen antibonding) orbitals that weaken (and lengthen) the carbon-oxygen bonds of coordinated carbon monoxide relative to the free molecule. If one could estimate precisely by how
1080
3 Dynamics and Physicul Properties
much the carbon-oxygen bond in a carbonyl complex had been weakened relative to the free molecule, one could add that amount to the values of E(M-CO) in Table 2 to estimate the average strength of the metal-carbon bonds in these species. We shall return to this point later. Examination of the data in Table 2 shows that for all the metals listed the metalligand bond enthalpies E ( M-CO) are significantly greater than the metal-metal bond enthalpies E(M-M), and that the total disruption enthalpy associated with metal-metal bond cleavage in such systems accounts for only a small percentage of the overall disruption energy-some 6% for dinuclear clusters, 10%0for trinuclear clusters, 20% for tetranuclear (tetrahedral) clusters, and still only 20% for hexanuclear (octahedral) Rh6(C0)16. The data also show a slight, but apparently significant, increase in the strength of the metal-ligand bonds with increasing nuclearity of the clusters MJCO),, i.e., as the ratio of carbonyl ligands to metal atoms, y / x , decreases, and so also the competition for an adequate share of the capacity of the metal atoms to bind such ligands. The values of E(M-CO) for the cluster species M,(CO), are fully in line with the values obtained for mononuclear carbonyls M(CO),, where there is no metal-metal bonding to complicate the interpretation of thermochemical data, and also with measurements of the enthalpies of adsorption of carbon monoxide by metal surfaces.['91The significantly greater strength of the metal-ligand bonds compared with the metal-metal bonds is also consistent with experimental observations of the realignment of the positions of metal atoms on the surfaces that result from exposure of metal surfaces to ligand vapor, observations of considerable significance for heterogeneous catalytic systems.
3.7.4 Applications to neutral osmium carbonyl clusters, OSX(CO), We saw in the previous section that realistic estimates of the strength of the metalligand bonds in metal carbonyl clusters M,(CO), of known heats of formation could be made by assigning bond enthalpies to the cluster metal-metal bonds that reflected their length, using the equation E = Adp4.6 appropriate for the metal concerned. Here, we illustrate what other insight into the structures and bonding of metal carbonyl clusters can be gained by using our approach, by considering some systems of known structure that have not been thermochemically characterized. We use osmium carbonyl clusters1201 to illustrate our discussion because their chemistry has been particularly well documented, thanks in large part to the synthetic and structural work of Johnson, Lewis, McPartlin, Raithby, Einstein, Pomeroy and their co-workers. Nine neutral osmium carbonyl clusters, Os,(CO),, are known to us to have been structurally characterized, only one of which, O S ~ ( C O )has ~ ~been , the subject of a
3.7 Bond Length-Bond Enthalpy Putterns in Metal Carbonyl Cluster Chemistry
1081
calorimetric study to determine its enthalpy of formation. They fall into three formula categories: i) O S ~ ( C O ) and I Z os4(co)16 have a ligand/metal ratio y/x of 4 : 1, and are cyclic oligomers of o s ( c o ) 4 units; ii) Osg(CO)18 and Os7(CO)21 have a ligand/metal ratio y/x of 3 : 1, and are compact aggregates of Os(CO)3 units (capped closo systems in PSEPT terminology); and iii) Five other osmium carbonyl clusters of formula Os4(C0)14, Os4(CO)15, osg(co)l6, Osj(CO)18, and Osj(CO)19 have intermediate values of y/x between 3 and 4, and consist of mixed aggregates of both Os(CO)3 and Os(CO), units. In common with metal carbonyl clusters in general, these compounds have structures that, for a given nuclearity, become progressively more open (fewer metal-metal bonds) as the ligand/metal ratio increases. Indeed, irrespective of their nuclearity, these compounds can be shown quite simply to form metal-metal bonds of decreasing strength per metal atom as the ligand/metal ratio y/x increases. The metal-metal bonds get progressively longer, and they get fewer in number, as y/x increases. When for individual compounds 0 s y( CO),, the enthalpies of individual bonds are calculated from their lengths by use of the equation E = 1.928 x l0I3 d-46, the bond enthalpies are summed to give a total metal-metal bond enthalpy, CE(0s-Os), and this sum divided by the nuclearity, x,to give an average metal-metal bond enthalpy per metal atom, ZE(0s-Os)/x, the resulting values fall on a smooth curve when plotted against y/x, as illustrated in Fig. 2. What this curve shows is that the capacity of osmium atoms to bond to other osmium atoms, , progreswhich is zero when y/x = 5, i.e. for the mononuclear OS ( C O ) ~increases sively as the number of ligand molecules per metal atom is reduced. Our data from the known osmium carbonyls is necessarily limited to the range 3 < y/x < 4, but we confidently predict that any neutral osmium carbonyl clusters prepared in future with values of y/x < 3 will have sufficiently compact structures to generate CE(0s-Os)/x values that fall on the same smooth curve, which can be extrapolated back to a value of ca 791 kJmol-' when y/x = 0, the enthalpy of vaporization of the bulk metal. The slightly different values of CE(0s-Os)/x for pairs of compounds with identical y/x ratio (3.0 or 4.0) incidentally reflect differences in the efficiencies with which these [Os(CO)3], and [Os(CO)4], oligomers form metal-metal bonds, which in turn might reflect differences with nuclearity between non-bonded repulsions between ligands. For example, the twelve carbonyl groups in the triangular cluster Os3(CO)12, for which CE(0s-Os)/x = 94 kJ mol-I, seem to experience less crowding than the sixteen carbonyl groups in the (flattened) butterfly-shaped cluster Os4(CO)16, for which C E ( 0 s - O s ) / x = 87 kJmol-'. The metal-metal bonds in the latter, longer than in the former, apparently stretch to enable bonding to the potentially more crowded larger number of ligands. The compound Os3(C0)12 is
1082
3 Dynamics and Physical Properties
8 200
++ 50
Figure 2. The relationship between the 0s-0s bond enthalpy per metal atom in binary Os,(CO), clusters and the number of carbonyl ligands per metal atom, y / x .
known to be the more stable oligomer [Os(CO),],, suggesting that the metal-ligand bonds in the two oligomers are of similar strength. The difference between Os~(C0)18 and Os7(CO)21, as far as the metal-metal bonding is concerned, is less pronounced (X:E(Os-Os)/x is 215 kJmol-' for the former, 218 kJmol-' for the latter). Despite the larger number of ligands these higher nuclearity clusters accommodate, their metal cores are appreciably larger, so non-bonding interactions between ligands are of lesser importance. We have so far considered only the metal-metal bonding in these neutral osmium carbonyl clusters. The one calorimetric study performed on such compounds, that ~ ( -1651 ~) f 30 kJmol-I, and an enthalpy led to a value of AfH for O S ~ ( C O ) , of of disruption of this compound into gaseous metal atoms and carbon monoxide molecules of 2695 30 kJmol-', leads to an average value of 201 kJmol-' for its metal-ligand bond enthalpy term, E( 0s-CO). Further thermochemical studies on other osmium carbonyl clusters are urgently needed to provide a sound experimental basis on which to estimate other heats of formation. Nevertheless, meaningful estimates can be made of the enthalpies of disruption of the other known osmium carbonyl clusters into gaseous metal atoms and carbon monoxide if it is assumed that the enthalpy of the metal-ligand bonds in these clusters Os,(CO), vary little from compound to compound, although (following the results obtained from the thermochemical studies of iron, cobalt and rhodium clusters) increasing slightly as the ratio of ligand to metal y/x,increases.
3.7 Bond Lenyth-Bond Enthalpy Patterns in Metal Carbonyl Cluster Chemistry
1083
Reference to Table 2 indicates the scale of increase expected for metal-ligand bond enthalpy terms as the ratio of ligand to metal decreases. Taking E(0s-CO) for Os(CO)4 units as 201 kJmol-I, as found for Os?(CO)12,we estimate that a realistic units will be some 4%)higher, i.e. 209 kJmol-I. value of E(0s-CO) for OS(CO)~ Using these values, one can estimate values of the enthalpy of disruption of all the known osmium carbonyls Os,(CO),. into gaseous metal atoms and carbon monoxide molecules:
where I is the number of Os(CO)?(as distinct from Os(CO)4)units ( z = 4x - y ) . In turn, estimates can be made of the enthalpies of formation of these compounds (see Table 3 ) . Detailed discussion of the implications of these estimated AfH" values are inappropriate here and can be found elsewhere.r201 One other aspect of the structures of these neutral osmium carbonyls worth discussing is the detailed geometry of their carbonyl ligands, and in particular the metal-carbon and carbon-oxygen distances (virtually all of the carbonyl ligands in the neutral osmium carbonyls occupy terminal sites on individual metal atoms). If these terminally-attached carbonyl ligands are indeed coordinated more strongly in OS(CO)~ than in Os(CO)4groups, then even though the difference is slight, it should in principle be apparent from a shortening of the metal-carbon distances and lengthening of the carbon-oxygen distances. The more strongly a carbonyl ligand binds to a metal, the shorter the metal-carbon bond becomes as it acquires more n character. When carbon monoxide functions as a n-acidic ligand, however, it does so by use of its n* carbon-oxygen antibonding orbitals, so metal-carbon bond shortening will only be achieved at the cost of carbon-oxygen bond lengthening, albeit difficult to detect and measure precisely because of the high bond-order, great strength, and short length of the carbon-oxygen bonds, for which a change in length of only 1 pm corresponds to a bond enthalpy change of some 40 kJ mol-l. Despite these problems, and the limited precision with which the metal-carbon and carbon-oxygen bond distances in metal carbonyl clusters have been measured by X-ray diffraction, we illustrate in Figs 3 and 4 that compelling X-ray evidence is already available for the variations in metal-carbon and carbon-oxygen distances we expect for compounds M,(CO), as the ratio y/x varies. The mean metal-carbon distances in osmium carbonyl clusters Os,(CO), decrease by approximately 3 pm, from ca 193 to 190 pm, as y/x is reduced from 4 to 3, while the mean carbonoxygen distances in the coordinated ligands lengthen over the same y/x range by just over 1 pm. Interpreting the published structural data is complicated by variations in the refinement methods used in different structural determinations, the carbon atom appearing to 'slide' along the metal - oxygen vector as the refinement method is changed.[211 For example, isotropic refinement of all relevant carbon and
Formula
89 85 89 99 95 100.5 98 101 100 102
89 113 134 149 153 167.5 168 186 186 204 3 4 6 6 8 10 12 11 13 14
17 20 23 48 59 1
266 339 536 596 763 1005 1179 1114 1300 1430 6 5 6 6 7 8 9 7 8 7
16 15.33 15 15 14.8 14.67 14.57 14.33 14.29 14
-
7
I -
78 110 114 116 113 125 120
39.3 20 1 243 257 260 354 355
78 1205 1943 2314 2603 601 1 7108
11
1
5 7 7
17 14.33 13.75 13.56 13.4 12.35 12.1
106 108 109
101
93 99 96
116 138 143 152 191 215 218
464 690 860 608 955 1290 1526
5 7 9 6 9 12 14
7 8 9 6 6 6 7
15.5 15.2 15.0 15.0 14.4 14.0 14.0
87 94 91
87 94 109
349 283 543
4 3 6
8 6 9
16.0 16.0 15.6
ZE(os-os)/sIb
CE(0s-OS)/X
SIb
S,
No. electrons per osmium ZE(0s-0s)
44 68 89 99 109 I26 131 159 162.5 204
-
-
16 172 278 331 372
66 86 96 101 159 215 218
44 47 60
CE(os-os)/sp
Table 3. The neutral binary osmium carbonyls, [Os,(CO),], studied, listing electron numbers, structural types and metal-metal bond energies.=
3 6 6 6 9 11 17 20 23
299 575 600 614 935 1161 1864 2178 2513 100 144 150 153.5 187 193.5 233 242 25 1 102 104 105 110 109 109
100
96
100
50 96 100 102 156 166 266 31 1 359
All thermodynamic data are in kJ mol-I. x is the number of metal atoms, S l b is the number of localized bonds required using the 18-electron rule, and S, is the number of skeletal electron pairs required by PSEPT.
14.4 14.33 13.75 13.56 13.4
15 15
16 15
3 Dynamics and Physical Properties
1086 193
3
/
./ -.
192
g . Q
191
189
188
1
'
0
0
-I 28
_ 1 -------
3
32
34
36
38
4
42
CO 0 s ratio
Figure 3. Plot of d(0s-C) as a function of the number of CO ligands per osmium atom (see text for details of the two sets of data).
oxygen atoms leads to shorter 0s-C and longer C--0 distances than are found when both carbon and oxygen atoms are refined anisotropically. Thus in Fig. 3 the data effectively fall on two parallel straight lines. The upper line, with data points (all atoms anisotropic) together with shown as closed circles, represents O S ~C(0 ) l ~ O S ~ ( C O Os4(CO)14, )~~, and OSS(CO)~S, for all of which the metal and oxygen atoms were refined anisotropically and only the carbon atoms isotropically. The lower line, shown as open circles, represents structures for which only the metal atoms were refined anisotropically, both carbon and oxygen being refined isotropically, generally because too few reflections were observed to enable refinement of more parameters, or because the high absorption associated with these clusters made the parameters associated with anisotropic refinement unreliable. Only the (more reliable) former type are shown in Fig. 4. It is to be hoped that in future, as methods of structure determination acquire greater precision, more accurate determinations of metal-carbon and carbonoxygen distances in polynuclear metal carbonyl complexes will enable such trends as are illustrated in Figs. 3 and 4 to be documented more precisely and for further systems to be explored. It should then be possible to assign bond energy terms, E(M-C) and E(C-0), to the metal-carbon and carbon-oxygen bonds in metal carbonyl complexes such that the metal-ligand bond enthalpies we discuss here represent the energy needed to cleave the metal-carbon bond offset in part by the gain in carbon-oxygen bond energy experienced by the ligand when it is no longer coordinated:
E(M-CO) where E(C-O),,
= E(M-C)
-
[E(C-O)f,.,,
-
E(C-O),,,,,]
represents the bond energy term for a free, gaseous carbon
3.7 Bond Length-Bond Enthalpy Putterns in Metal Curhonyl Cluster Chemistry
1087
1155,
115
I
I
S 1145 2 0
1135
4 28
3
32
34
36
38
4
42
CO 0 s ratio
Figure 4. Plot of d(C-0) as a function of the number of CO ligands per osmium atom (see text for details of the data points included).
monoxide molecule (1076.5 & 0.4 kJmol-') and E(C-O)boundis the (lower) bond enthalpy term attributable to the longer carbon-oxygen bond in the coordinated ligand. In principle, E(C-O),,,,, can be deduced from the vibrational frequencies, IJ(C-O),of carbonyl complexes,[221though the exercise is rarely straightforward. Variations in v(C-0) with L in mononuclear MoL3(CO)3 compounds are known.[231An attempt has already been made to partition the metal-ligand bond enthalpy between metal-carbon and carbon-oxygen components in a discussion of the dinuclear iron carbonyl Fe*(CO)g, which has a structure with six terminal and three bridging ligands (CO)3Fe(pz-C0)3Fe(CO)3, and in which the carbonoxygen bonds of the bridging ligands are longer (1 17.6(5) pm) than those of the and terminal ligands (1 15.6(4)pm). The assumptions necessary in the treatmer~t,"~] the limited precision of the data restricted the outcome to the conclusion that the carbonyl ligands were probably bound equally strongly, whether in terminal or bridging sites. This finding was consistent with the general behavior of carbonyl ligands on metal clusters, where their z-acidic nature enables them to bind equally effectively, whether bridging or terminal, and indeed there seems to be merit in the view that the ligand sheath in many metal carbonyl clusters defines a polyhedron of a suitable shape to minimize non-bonding repulsive interactions between the ligand molecules, whereas the metal polyhedron inside the ligand polyhedron adopts an orientation that best suits the space available, apparently irrespective of whether this requires all carbonyl ligands to be terminally bonded or for some to play bridging roles. This view, moreover, enables ligand fluxionality to be viewed in terms of the rotation or rocking of the metal polyhedron within the ligand polyhedron. A terminal mode of bonding of carbonyl ligands seems, however, to be
1088
3 Dynumics and Physical Properties
stronger for some metals like osmium. The tendency to accommodate bridging ligands is more pronounced in anionic metal carbonyl systems. The dinuclear iron carbonyl Fe2(CO)9, incidentally, has no osmium counterpart Os2(CO)9, perhaps because one or more of the ligands therein would need to play a bridging role (an unsymmetrical, polar unbridged structure (CO)4OsOs(C0)s would be extremely unlikely). If Os2(CO)9could be prepared, reference to Fig. 2 suggests that it would be expected to contain a metal-metal bond of enthalpy 81 kJmol-’ and length 297pm. Other predictions about further osmium carbonyls yet to be prepared are to be found elsewhere.[”]
3.7.5 Applications to osmium carbonyl anions [OsX(CO),J2-and neutral and anionic osmium carbonyl hydrides [Os,(CO),H,] ‘When we extend our discussion to the themiochemistry of osmium carbonyl anions and neutral and anionic osmium carbonyl hydrides, further complications arise. Such systems have not been subjected to calorimetric study; discussion of anionic systems requires counter-cations to be specified, some knowledge of cluster electron affinities, and lattice energies to be calculated if experimentally meaningful enthalpies of formation are to be predicted, and hydride clusters require assumptions to be made about the strength of binding of their hydride ligands. It would be inappropriate to venture far into such speculative territory here. Nevertheless, simply confining our attention to the overall strength of the metal-metal bonding in these systems, calculated from the collective strength of their 2-center (nearest-neighbor) bonds, and expressed as an average metal-metal bond enthalpy per metal atom, XE(0s-Os)/x, much can be learnt about such systems.[241Our discussion of the neutral carbonyls Os,( CO), revealed a clear relationship (Fig. 2) between XE(0s-Os)/x and y / x , the ratio of ligand to metal atoms. This relationship is intelligible in that the more ligands a set of metal atoms needs to accommodate, the more metal orbitals are required for metal-ligand bonding, and the fewer are available for metal-metal bonding. Because ligands in general vary in the number of electrons they contribute towards metal-ligand bonding, and so in the metal orbital demands they make, when we are dealing with anionic systems [OS,(CO),]~or neutral or anionic carbonyl hydrides [OS~(CO),H,]~-, we should expect the metal-metal bonding to reflect the numbers of electrons provided by the ligands and anionic charges, rather than just the number of carbonyl ligands. Accordingly, having calculated XE(0s-Os)/x for these species, we plot in Fig. 5 their values against the number of electrons provided by the valence-shell electrons of the metal atoms plus the carbonyl and hydride ligands and anionic charges, expressed
3.7 Bond Length-Bond Enthalpy Patterns in Metal Carhonyl Cluster Chemistry
1089
400
350
-'
300
-
x neutral binary clusters
?*
h ?
Z 250
- t
~
E
7
+K
Y
5 200
Q
$ 150 W 100 50
0 cluster anions +neutral hydride clusters -anionic hydride clusters
-
~
&
++
+ +
xx
~
4
0
o i 12
x
~
13
14
16
15
17
18
Number of metal plus ligand electrons
Figure 5. Plot of CE(0s-0s) per osmium atom against number of metal-plus-ligand electrons per osmium atom.
+ + +
as metal-plus-ligand electrons per osmium, (xv 2y z c ) / x (for osmium the number of valence electrons v = 8). Plotted in this way, the limit, corresponding to Os(CO)5, has 18 electrons per osmium. As expected, the data fall on effectively the same smooth curve as for the neutral carbonyls (Fig. 2), showing that, whether electrons are provided by a carbonyl ligand, anionic charge or a hydride ligand, it is their total electron number that is important the more ligand electrons per metal atom the system needs to accommodate, the less metal-metal bonding is possible. It is interesting that the points in Fig. 5 show no systematic variation with the source of the extra electrons. It might have been expected that the values of CE(0s-Os)/x for anions [OS,~(CO),]*would be consistently higher than those for neutral carbonyl clusters, because their extra (anionic) electrons were not distracted by ligand nuclei (as are the lone-pair electrons formally donated by carbonyl ligands). Although there is a hint of such an effect in the cluster of data points for anions containing fewer than 14 metal-plus-ligand electrons per metal atom, this cannot be regarded as significant. It is, however, worth noting that although the mono-protonation of a general osmium carbonyl cluster [Os,( CO),H,] '-to generate [Os,(CO),,H,,,]("-')- does not alter the total number of ligand electrons as we have chosen to count them, the additional proton will typically convert a pair of formally two-center-two-electron (2c2e) 0 s - 0 s bonding electrons into three-center-two~
1090
3 Dynamics and Physical Properties
electron (3c2e) Os-H-0s bonding electrons, reducing the total metal-metal bond enthalpy. Thus, in Fig. 5 , for clusters with the same number of electrons per metal atom, the neutral osmium carbonyl hydride clusters represent the data points with the lowest values of CE(0s-Os)/x. The curves in Figs. 2 and 5 increase in slope as the number of electrons per metal atom decreases, showing that as more electrons become available for metal-metal bonding, so they are used more efficiently. The metal-metal distances in small clusters are typically 10-15 % longer than those in the bulk metal; those in larger clusters are nearer to those in the bulk metal. This is consistent with the idea that the bonding in clusters becomes more 'metallic' as cluster size increases. The shapes of the osmium clusters considered here are illustrated in Fig. 6. They can be rationalized in various ways. Some have the number of bonding contacts polyhedron edges - that are compatible with descriptions that assign a bonding pair of electrons to each such link without violating the 18-electron rule whereby each metal atom has nine electron pairs in its valence shell. Many have structures that can be rationalized by PSEPT polyhedron skeletal electron pair theory - placing their metal atoms on some or all of the vertices of the deltahedron compatible with the number of skeletal electron pairs they contain. A few are best treated as fragments of bulk metal clothed in ligands, the number of which might not be simple to rationalize in terms of the electron numbers needed for metal-metal bonding. Because PSEPT and 2c2e bonding schemes are commonly applied, however, it might be useful at this point to explore briefly the varying efficiency with which the electrons formally available are actually used for metal-metal bonding, whether as skeletal electron pairs as counted in PSEPT, or as localized 2c2e bonds in classical bond schemes. The reader interested in a detailed discussion of these issues is directed elsewhere.[241 Table 3 lists a representative selection of data,[241showing the numbers of electrons per metal atom, ( x v 2 y z c)/x, the total metal-metal bond enthalpy, CE(0s-Os), and that enthalpy divided by the number of metal atoms ( x ) ,by the number of localized bonds required using the 18-electron rule (Sib), and by the number of skeletal electron pairs required by PSEPT (Sp).We have already shown the variation of CE(0s-Os)/x with ( x v 2 y z c ) / x in Fig. 5. The variation in CE(Os-Os)/Slb has not been plotted, but for each category of compound considered (neutral or anionic carbonyls or carbonyl hydrides) the value of CE(Os-0s) / S ] b increases with cluster nuclearity, though by no means smoothly, from values below 90 kJ mol-' to ca. 110 kJ mol-' for neutral carbonyls and anionic carbonyl hydrides. Apart from an anomalously low value for [OS~(CO)S]~(78 kJmol-I), the values of CE(Os-Os)/S]b for carbonyl anions [OsX(CO),]'- vary only from 1 10 to 120 kJmol-' for nuclearities x = 6 to x = 20; for neutral carbonyl hydrides the variation is from ca. 90 to 100 kJ mol-' . These variations with cluster nuclearity and with cluster type underline the dangers associated with the assignment of bond enthalpy terms to notional 2c2e metal-metal bonds in such clusters. Use of localized 2c2e bond schemes is anyway inappropriate for several of these systems. They have -
-
+ + +
+ + +
i -.3
I
i
6
.A '
-4
.i$ "
.--i .'
1092
3 Dynamics and Physical Properties
some merit for simple systems, for example Os3(CO)12 and Os4(CO)16 and their anionic or hydride counterparts, which from the isolobal relationship of Os(CO)4 to CH2 are analogs of cyclopropane and cyclobutane, respectively. For an anion such as [0ss(CO)l~]~-, however, a regular octahedral structure with twelve equivalent metal-metal bonds is difficult to reconcile with a total electron number (43 pairs) compatible with only eleven 2c2e Os-0s bonds. The variations in metal-metal bond enthalpy with the numbers of skeletal electron pairs, S,, counted as by PSEPT, shows the substantial variations with cluster type (closo, nido,arachno, etc.) that are expected. As the number of skeletal electron pairs exceeds the number of skeletal atoms to an increasing extent going from closo to nido to arachno to hypho, so their bonding role diminishes as the structures become progressively more open, and some of the skeletal electrons effectively acquire skeletal lone pair character (most easily seen by analogy with arachno boranes such as B4H10 or BsHll, for which the electrons used to bond exo-terminal hydrogen atoms are counted among the skeletal electron pairs to enable understanding of their shapes, though playing no skeletal bonding role). Although, as noted earlier, our calculated values of C E ( 0 s - 0 s ) for various types of osmium clusters cannot usually be converted into enthalpies of disruption or formation, it is nevertheless possible to explore systematic changes in composition that shed light on relative stabilities of compounds. For example, consistent changes in CE(0s-0s) are found to accompany formal addition of Os(CO)2, Os(CO)3, or OS(CO)~ units to osmium carbonyl clusters, whether neutral or anionic, carbonyl or carbonyl hydrides. Formal addition of Os(CO):! units, which effectively have three vacant orbitals available for use in skeletal bonding, generally increases CE(Os-0s) by some 310-340 (average 326) kJmol-', as in the comparison of Os3(CO)12 (283 kJmol-I) with Os4(CO)14(608 kJ mol-I). Addition of an Os(CO)3 unit, which can provide two vacant orbitals (or a pair of electrons and three orbitals) for skeletal bonding use, increases Z E ( 0 s - 0 ~ )by some 190-230 (average 209) kJ mol-I, whereas formal addition of an OS(CO)~ unit, with only one vacant orbital (or two electrons and two orbitals) increases CE(0s-0s) by only ca 70-75 kJ mol-' . Considering reactions of osmium carbonyls of more direct relevance to their experimental behavior, one finds that addition of an extra carbonyl ligand to a particular cluster (as in conversion of Os4(CO)14 into Os4(CO)15)causes loss of ca 128 kJ mol-I of metal-metal bond enthalpy, which should make such reactions exothermic if E(0s-CO) for the metal-ligand bond formed has an expected value in the region of 200 kJmol-', although the thermodynamic viability of carbonyl addition will be influenced by entropic factors.[251Structural data are available for three pairs of osmium carbonyl clusters that differ by two hydrogen ligands Os5(CO)ls and O S ~ ( C O ) ~OS~(CO)IS ~H~; and O S ~ ( C ~ ) I &and I ~ ;Os7(cO)21 and O S ~ ( C O ) ~If~ oxidative H~. addition of dihydrogen to the metal carbonyl for each of these three pairs is a roughly thermoneutral process, as experimental behavior suggests, it can be deduced that the metal-hydrogen bonds in the products have enthalpies E(0s-H) of approximately 306 f 25 kJ mol-' . The hydrogen ligands in osmium carbonyl hydride clusters, whether neutral or -
3.7 Bond Length-Bond Enthalpy Patterns in Metal Curbonyl Cluster Chemistry
1093
anionic, have rarely been shown unambiguously to occupy specific sites. Their positions have usually been inferred from gaps in the carbonyl ligand coordination sphere, or from potential energy calculations,i261and these are usually over 0 s -0 s polyhedron edge bonds that are presumed to be bridged by p2-H ligands. Indeed, in localized bond terms, one can regard 0s-H-0s sites as 3c2e links resulting from protonation of 2c2e 0 s - 0 s polyhedron edge bonds. Detailed considerationi241of the data now available for neutral and anionic osmium carbonyl hydrides suggests that reasonable values for the enthalpies of their osmium-hydrogen bonds are E(Os-H) = 264 If: 10 kJmol-' and E(0s-H-0s) = 324 & 10 kJmol-'.
3.7.6 Applications to rhenium carbonyl clusters Although less fully documented than osmium cluster chemistry, rhenium cluster chemistry has been subjected to many structural studies, including those on approximately 20 neutral or anionic carbonyls, particularly carbonyl hydrides [Re,(CO),H,]'- of nuclearities x = 2 to 6 (Fig. 7). In addition, some ten or more rhenium carbonyl carbides [Re,(CO),H,C]'- have been shown to contain a core carbon atom, usually occupying a central octahedral site. These systems offer scope not only to explore for rhenium the trends we have already shown for osmium, but also to study the effect on metal-metal distances (and so enthalpies) of such core carbon atoms, which formally donate all four of their valence shell electrons to the cluster bonding. To our knowledge only one rhenium carbonyl cluster compound, ReZ(CO)lo, has been subjected to calorimetric study to determine its enthalpy of formation.['Oa1 The equation generating bond enthalpies E( Re-Re) (kJ mol-' ) from bond distances d (pm) that fits the known enthalpy of vaporization of rhenium metal (770 kJmol-') is E = 2.188 x lOI3d-46. By use of this equation we have calculated CE(Re-Re) for all the systems that have been structurally characterized,[271and in Fig. 8 we plot CE(Re-Re)/x against the number of electrons per metal atom provided by the metal ( v = 7 for Re) plus the ligands, core carbon and/or anionic charge (xv 2y I? c i ) / x , where i is the number of electrons supplied by the interstitial atom, four for carbon. As with osmium, the metal-metal bond enthalpies for the neutral and anionic carbonyls and carbonyl hydrides [Re,( CO), H,] '- generate points that fall on or near a smooth curve that can be extrapolated to C E / x = 770 kJmol-' when the ligands etc. supply no extra electrons (i.e., bulk metal), and to ZE/x = 0 kJmol-' when the metal plus ligands supply eighteen electrons per metal atom (corresponding to Re(CO)5H or [Re(CO)5]-). Interestingly, the points for systems containing core carbon atoms do not lie on this curve. For all of the core carbon systems studied for rhenium, the value of Z E / x exceeds that expected from the electron count by some 30 kJmol-', i.e. it is approximately 25% higher than the value ex-
+ + + +
Figure 7. The structures of rhenium carbonyl clusters: . , Re(C0)d; A , Re(C0)3. Lines are drawn to indicate connectivity, and do not imply the presence of single bonds. Hydride and bridging carbonyl ligands are drawn explicitly; dashed lines are drawn to face-bridging hydrides.
3.7 Bond Length-Bond Enthulpy Patterns in Metul Curhonyl Cluster Chemistry
1095
pected, and indeed found when core carbon atoms are not present. It is as if the core carbon atoms were providing no electrons to the cluster bonding, rather than all four valence shell electrons. However, we do not interpret this difference between the core carbon clusters and the other rhenium clusters as suggesting that we should disregard the four electrons each carbon atom contributes. Rather, we need to consider the origin of the shortening of the metal-metal bonds in the carbide clusters that is the reason why they are calculated to have stronger bonds than their analogs with no core carbon. The origin is twofold. Firstly, all the hexanuclear rhenium carbonyl clusters that do not contain core carbon atoms have many hydride ligands, believed to occupy ,LQ or ,u3 bridging sites that cause their metal-metal links to be longer than their unbridged counterparts (and incidentally render detailed discussion less fruitful than for osmium clusters); secondly, the core carbon atoms apparently pull the rhenium atoms closer together than they would otherwise be. The mean Re-Re distance in [Re6C(C0)19l2- is 300.6 pm, implying a rhenium atom radius of 150.3 pm and a radius for the core carbon atoms of 150.3 x 0.414 = 62 pm, rather smaller than the single-bond radius of carbon (77 pm). The I3C NMR chemical shifts of some corecarbon metal clusters have been correlated with effective carbon radii and interpreted in terms of a compression of the interstitial carbon atoms,[281although DFT calculations have been interpreted differentl~.'~'] We suggest that our data support a model in which a cationic carbon, C"+, of radius 1 6 2 pm, is pulling the metal atoms towards itself, a situation which implies that the metal-carbon bond enthalpy is significantly greater than the metal-metal bond enthalpy, a point to which we shall return. Fig. 8 shows a significant trend in the CE(Re-Re)/x values for the five rhenium carbonyl clusters which have 16 valence-shell electrons per rhenium atom, these being the oligomers of [Re(C0)4H], ( n = 2,3,4) and two anions derived by deprotonation. By application of the isolobality of Re(C0)4H and CH2 these clusters are analogs of ethylene, cyclopropane, and cyclobutane. Ring strain and bond protonation effects on bond lengths are evident here. For the neutral examples, the complex with the strongest metal-metal bonding is the trimer ( n = 3), a result borne out by the relative thermal stabilities of these clusters. Another feature of the bond enthalpy data for rhenium clusters is that they show a very wide range of values of CE(Re-Re)/&, the enthalpy per notional 2c2e metal-metal bond present the value varies from 45 to 85 kJmol-', underlining the unsatisfactory nature of associating bond enthalpies with skeletal bond pairs. -
3.7.7 Applications to rhodium carbonyl clusters The neutral and anionic carbonyls and carbonyl hydrides of rhodium that have been prepared and subjected to crystallographic structural study include a greater
3 Dynamics and Physical Properties
1096
--
0
140
\
\
0
120-
x core carbide clusters 0-
c
d
o non-carbide clusters ,
7
2
x
0
0
100
0
0
, \
0
'\
80
0
%
W
60
40 20
0 0 0
I
0 13 5
, Q
,
0 0-
14
14 5
15
15 5
16
16.5
17
17 5
la
Metal plus ligand electrons per Re
Figure 8. Plot of CE(Re-Re) per rhenium atom against the number of metal ligand electrons per rhenium atom.
proportion of high nuclearity systems, mostly anions [Rh,( CO),HZ]'-and including some containing a central metal atom coordinated only by other metal atoms.[21As with rhenium, many rhodium carbonyl clusters are known that contain core carbon atoms, and examples of other centered clusters containing core nitrogen, phosphorus, arsenic, antimony, and sulfur atoms have been structurally characterized.[301 Although we do not have the space here to discuss their structures and bond energies in any detail, we can report that, as with osmium and rhenium systems, calculation of their metal-metal bond enthalpies, in this instance by use of the equation E(Rh-Rh) = 1.396 x 1013d-4.6, generates estimated total metal-metal bond enthalpies, CE(Rh-Rh), which when plotted as bond enthalpy per metal atom, XE(Rh-Rh)/x, against the number of electrons provided by the metal atoms plus ligands, anionic charges, and core atoms (Fig. 9) shows the now familiar slightly curved plot that implies yet again progressively more effective use of electrons for metal-metal bonding as the number of electrons provided by the ligands etc. decreases. The carbonyls and carbonyl hydrides, whether neutral or anionic, have values of CE(Rh-Rh)/x that lie within 5 kJ mol-' (generally less) of the values corresponding to the curve that best fits the data. Values of CE( Rh-Rh)/x calculated for species containing core main group atoms of the 1st period (C and N ) generally fall below
3.7 Bond Length-Bond Enthalpy Patterns in Metal Carbonyl Cluster Chemistry 300 280
1097
, ~
0
260 0
0Q
+
-
z
220
c
A
7
0
clusters without core atoms
+ clusters with core C or N
Y
$ 200
A clusters with core (not C or N)
5
g 180
~
W G1
160
~
140 Q
t 100
o
~
12
12.5
13
13.5
14
14.5
15
15.5
16
Number of ligand electrons per Rh atom
Figure 9. Plot of ZE( Rh-Rh) per rhodium atom against the number of metal-plus-ligand electrons per rhodium atom.
that curve, though on average less than 10 kJ mol-' lower than the 'ideal' values, as if the metal-metal distances were greater than the electron numbers might have led one to expect. It might be that in these rhodium systems the core atoms serve to shoulder the metal atoms apart to some extent, rather than attract them inwards as we suspected was happening in rhenium carbonyl carbide systems. Species containing the larger 2nd and 3rd period main group atoms have significantly lower values of CE(Rh-Rh)/x, reflecting the larger size of these main group interstitial atoms, requiring longer Rh-Rh distances in order to accommodate them.
3.7.8 Core carbon atoms: the relevance of metal carbides We noted in our discussion of rhenium and rhodium carbonyl clusters the importance of systems containing core atoms, particularly core carbon atoms that often feature in metal carbonyl clusters prepared by thermal decomposition of other clusters, when they result from reactions between pairs of carbonyl ligands that involve transfer of one oxygen atom from one carbon atom to the other, leading to
1098
3 Dynamics and Physical Properties
the elimination of the second carbon atom as carbon dioxide: Mx(C0),+2
+
MxC(CO),
+ co2
These core or interstitial carbon atoms typically occupy octahedral sites in the metal cluster, although they are also found in ‘open’ (octahedral fragment) sites such as square pyramidal or even butterfly-shaped arrangements of metal atoms. Other closed cavities that have been shown to contain core carbon atoms include trigonal prismatic arrays of six metal atoms (e.g. [Rh6(CO),5Cl2-), or even D2d dodecahedra1 (slightly distorted square antiprismatic) arrangements of eight metal atoms (e.g. [Rh12(C0)24(C)2I2-). It was evident from our calculations of CE(Re-Re)/x for rhenium carbonyl clusters that core carbon atoms can affect CE(M-M)/x in a manner not simply reflecting the number of electrons they contribute for bonding, apparently by causing the metal atoms to draw closer together than if there had been no core atom present. A lesser and reverse effect was noted for rhodium clusters, in that systems containing core carbon or nitrogen atoms have slightly lower values of CE(Rh-Rh)/x than might have been expected judging from rhodium clusters with no core atoms. Three factors seem to be operating in systems containing core atoms: i) the core atom, because of its location, can be assumed to be contributing all of its valence-shell electrons to the cluster bonding; ii) the core atom can be expected to have an effective radius that might cause the surrounding metal atoms to become closer to or further apart from each other, so that it might bond effectively to all of the surrounding metal atoms; and iii) the bonding between the metal and core atoms, such as carbon, is likely to be much stronger than that between the metal atoms. In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry; the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC,; and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers.
3.7 Bond Length-Bond Enthalpy Putterns in Metal Curbonyl Clustrr Chrmistry
1099
That metal carbides contain metal atoms at similar distances d( M-M) to those in the bulk metal itself, and are metallic conductors of electricity, suggests that we can treat the metal-metal bonding therein in the same way as for bulk metals or metal clusters, by assigning enthalpies to neighboring metal-metal bonding contacts. We can ignore carbon-carbon interactions as non-bonding because of their length (equal to the metal-metal distances in the rock-salt MC carbide structures known for metals of the titanium, vanadium, and chromium subgroups), and so regard their enthalpies of disruption into metal atoms and carbon atoms as the sums of two terms representing the total energies of their metal-metal and metal-carbon bonds:
When the enthalpy of formation of a metal carbide is known and the structure has been established we can calculate CE(M-M), subtract it from the calculated disruption enthalpy, AHdlsrupt, and so deduce CE( M-C). These calculations are easiest to perform for those carbides which have simple regular structures, such as the rock-salt lattices adopted by monocarbides, MC, of the titanium and vanadium groups, and the high-temperature W2C phase adopted by the M2C carbides of the vanadium group and molybdenum and tungsten. Values of CE(M-M) and CE(M-C) for these binary carbides are listed in Table 4.[311There are several points to note from the data. Firstly the CE(M-C) values, which in a molecular context represent the sum of the energies of six M-C bonds to an octahedrally coordinated carbon atom, are in the range of 1000 to 1280 kJmolF'. The second, related feature, is that (except for W2C) the M-C bonds contribute more to the enthalpy of disruption than do the M-M bonds, ranging up to nearly 75% of the enthalpy of disruption of the rock-salt MC carbides. This observation is consistent with the hardness and refractory nature of these carbides, and was first noted by Pauling in a similar analysis of the bonding of the iron carbide, Fe3C, cementite.[321It is worth emphasizing that the mechanical properties of the early transition metal carbides result from their strong M-C bonds and not the weaker M-M bonds. In examining the relationship between the MC and M2C carbides, we note that the total metal-metal bond enthalpy per mole of metal, CE(M-M)/x, increases from MC to M2C, indicating shorter, stronger metal-metal bonds in the M2C carbides. The total metal-carbon bond enthalpy, CE(M-C), also increases from MC to M2C, indicating shorter, stronger metal-carbon bonds in the M2C carbides. Both of these observations are accounted for by considering how the MC and M2C lattices can be constructed from close-packed metal lattices by expanding the lattice of the bulk metal, increasing the metal-metal distance, to generate octahedral sites large enough to accommodate carbon atoms. Larger expansion of the metal lattice is required for the stoichiometry MC than for M2C. Trigonal prismatic carbon sites are found in WC.
Nb2C Ta2C Mo~C W2C
v2c
TIC ZrC HfC VC0,gg NbC TaC
306.0 332.2 328.1 295.2 316.1 315.1 290.2, 283.6 312.7, 307.2 310.6, 305.4 301.1, 295.2 299.3, 292.8
M-M distance in MC, (pm) 364.4 497.3 496.4 322.6 491.8 544.5 736.9 1077.7 1210.5 940.4 1296.6
ZE( M-M) in MC (kJmol-')
-
-184 -203 -209.5 -101.9 140.6 -148.1 147.2 -195.5 -197.5 -53.1 -26.4
ArH of MC (kJmol-I) 1370.6 1528.5 1545.2 1236.8 1613.2 1646.7 1892.3 2364.0 2478.2 2086.0 2444.1
AH,jlsrupt (kJmol-I)
1006.2 1031.2 1048.8 914.2 1121.4 1102.2 1155.4 1286.3 1267.7 1145.6 1144.5
ZE( M-C) (kJmol-')
Table 4. Calculated contributions to the enthalpy of disruption of selected metal carbides.
216.4 234.9 232.0 208.75 223.5 222.8 202.9 219.2 217.8 210.8 209.3
M-C distance (pm)
73.4 67.5 67.9 73.9 69.5 66.9 61.1 54.4 51.2 54.9 46.9
Percentage bonding contrib. by E(M-C)
8
2
s9
2
k
3.
;s
"a
24
rsi
2E'
3
b
LJ
0 0
L
e
3.7 Bond Length-Bond Enthalpy Patterns in Metal Carhonyl Cluster Chemistry
1101
These observations on the structurally simple carbides of the early transition metals show how the strength of binding of core carbon atoms in molecular metal carbonyl clusters can in principle be estimated by comparison with metal carbides for which structural and theoretical data are available, and leads us to hope that examination of the wider body of transition metal carbides will provide relationships between the length and strength of bonds between metal atoms and octahedrally coordinated carbon atoms that can be applied to specific molecular metal carbonyl clusters containing core carbon atoms.
3.7.9 Concluding remarks In this survey, we have demonstrated the following. i) The strength, E(M-M), of the metal-metal bonds in molecular metal carbony1 cluster compounds can be assessed by comparison with those in the bulk metals, by use of a relationship E(M-M) = A[d(M-M)]-46 to make allowance for variations in bond distances d( M-M). ii) When the total energy, CE(M-M), needed to break the metal-metal bonds in metal carbonyl clusters is subtracted from the enthalpy of disruption, AHdlsrupt, of these clusters into gaseous metal atoms and carbon monoxide molecules, realistic estimates can be made of the strength of binding of the ligands, E(M-CO), which is typically much greater than E(M-M). iii) The strength of binding of the carbonyl ligands apparently increases slightly, but we believe significantly, as the number of carbonyl ligands per metal atom decreases, an effect evident from metal-carbon and carbon-oxygen distances. iv) The total enthalpy of the metal-metal bonds per metal atom, CE(M-M)/x, increases smoothly as the total number of valence electrons per metal atoms decreases from 18, when ZE(M-M)/x = 0, to a maximum of v, the number of valence shell electrons characteristic of the metal itself, when CE(M-M)/x = the enthalpy of atomization of the bulk metal. v) The variation of ZE(M-M)/x with cluster valence shell electron numbers referred to in (iv) applies broadly to mixed ligand systems containing hydride and carbonyl ligands, and also to anionic systems, [M,(CO), HI]‘+, although minor departures from ideality arise from steric effects or the tendency of hydrogen ligands in bridging sites to lengthen metal-metal bonds. vi) Variations in the total energy of the metal-metal bonds, CE(M-M), with the number of 2c2e metal-metal bonds expected using localized bond schemes for these metal clusters show that the bond enthalpy per ‘bonding’ electron pair itself varies considerably from system to system, suggesting that assignment of bond enthalpies to notional bonding electron pairs is an unsatisfactory way of treating such systems.
1102
3 Dynamics and Physical Properties
vii) Core carbon atoms in metal carbonyl carbide clusters, M,(CO),C, etc., can (apparently through radius ratio effects) cause the metal-metal bonds in such clusters to have lengths different from those expected from the electron count. viii) Estimates of the strength of the bonds to core carbon atoms (which are generally much greater than those between metal atoms) can be made by consideration of the bonding in binary metal carbides, MC and M2C, as the sum of the metal-metal and metal-carbon bond enthalpy terms, CE(M-M) CE(M-C) AH,jisrupt. ix) The effects noted above, demonstrated here for molecular cluster carbonyls formed by the metals osmium, rhenium and rhodium, and for binary carbides of the titanium, vanadium, and chromium subgroups, are expected to be generally true for similar compounds of the other transition metals.
+
Acknowledgments We acknowledge the contributions made to this work by our co-workers, Catherine Housecroft, Marion O’Neill, and Barry Smith in the early phase of this work, and Karen Peat, Kate Hillary, Lyndsey Rabbitt, and Andrew Johnson more recently. We thank EPSRC and Kvaerner Process Technology for a CASE studentship to ALJ. Structural data have been retrieved from the Cambridge Structural Database,[31and the Inorganic Crystal Structure Database using the EPSRC Chemical Database Service at D a r e ~ b u r y . ~ ~ ~ ]
References [ 11 (a) M. P. Cifuentes and M. G. Humphrey, in Comprehensive Organometallic Chemistry II, eds. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon Press, Oxford, 1995, vol 7, ch 16; (b) B. F. G. Johnson and J. Lewis, Adv. Znorg. Chem. Radiochem., 1981, 24, 225. [2] C. E. Barnes, in Comprehensive Organometallic Chemistry 11, eds. E. W. Abel, F. G. A. Stone and G. Wilkinson, Pergamon Press, Oxford, 1995, vol 8, ch 4 [3] F. H. Allen and 0. Kennard, Chem. Des. Autom. News, 1993, 8, 1; 31. [4] (a) E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker and W. R. Pretzer, Chem. Rev., 1979, 79, 91; (b) M. R. Albert and J. T. Yates, The Surface Scientists Guide to Organometallic Chemistry, American Chemical Society, Washington, 1987. [ 5 ] K. Wade, Ado. Znorg. Chem. Radiochem., 1976, 18, 1. [6] C. E. Housecroft and K. Wade, Gazz. Chim. Ital., 1980, 110, 87. [7] (a) M. McPartlin, Polyhedron, 1984, 3, 1279; (b) M. McPartlin and D. M. P. Mingos, Polyhedron, 1984, 3, 1321. [8] S. M. Owen, Polyhedron, 1988, 7, 253.
3.7 Bond Length-Bond Entliaby Putterns in Metal Curhonyl Cluster Chemistry
1 103
[9] (a) D. J. Wales, L. J. Munro and J. P. K. Doyle, J. C/iein. Soc., Dalton Trans., 1996, 61 I ; (b) A . Berces and T. Ziegler, Topics in Curr. Chetn., 1996, 182, 41; (c) E. Folga and T. Ziegler, J. Am. Chem. Soc., 1993, 115, 5169. [lo] (a) J. A. Connor, H. A. Skinner and Y. Virmani, F u r ~ ~ d uSynzp. y Cliem. Soc., 1973, 8, 18; (b) J. A. Connor. Top. Curr. Chem., 1977, 71, 71; (c) H. A. Skinner, J. C/iem. Thermodyn., 1978, 10. 309; (d) G. Pilcher and H. A. Skinner, in The Chemistry o f t h e Metal-Carbon Bond, eds. F. R. Hartley and S. Patai, Wiley, Chichester, 1982, vol 1; (e) S. P. Nolan, Bonding Energetics uf Orgunonierullic Compounds, in Encyclopediu of Inorgunic Chemistry, ed. R. B. King, Wiley, 1994; ( f ) J. A. Martinho Simoes and J. L. Beauchamp, Clzetn. Rev., 1990, 90, 629. [ I 11 K. Wade, Znorg. Nucl. Cheni. Lrtters, 1978, 14, 71. [I21 C. E. Housecroft, K. Wade and B. C. Smith, J. C/iem. Sue., Chem. Comrnun., 1978, 766. [13] C . E. Housecroft, K. Wade and B. C. Smith, J. Orgunomet. Chem., 1979, 170, C1. 1141 C. E. Housecroft, M. E. O’Neill, K. Wade and B. C. Smith, J. Oryanomet. Chem., 1981, 213, 35. [ I S ] L. Pauling, The Nature oj‘tlze Chetnicul Bond, 3rd Ed., 1960, Cornell University Press. [I61 J. March, Adrunced Organic Chernistrj’, 3rd Ed., 1985, Wiley, pp. 23. [I71 (a) I. D. Brown and R. D. Shannon, Actu Cryst., 1973, A29, 266; (b) 0. Sulpecki and I. D. Brown, Actu Cryst., 1982. B38. 1078; (c) I. D. Brown and D. Altermatt, Acta Cryst., 1985, B41, 244; (d) N. E. Brese and M. O’Keeffe, Actci Ciyst., 1991. B47, 192; (e) M. O’Keeffe and N. E. Brese. Actu Cryst., 1992, B48, 152; ( f ) 1. D. Brown, Actu Cryst., 1992, B48, 553. [ I S ] For some extensions of the Bond Valence method to thermodynamic measures of bond strength see: (a) J. Ziolkowski, J. Solid Stute Chem., 1985, 57 269; (b) M. O’Keeffe and J. A. Stuart, Itiorg. Clzrnt, 1983, 22, 177. [I91 (a) D. Brennan and F. H. Hayes, Phil. Trans. Roy. Soc., 1965, 258A, 347; (b) R . W. Joyner and M. W. Roberts, C/7em. Phys. Lett., 1974, 29, 447; (c) W. Biemolt and A. P. J. Jansen, J. Computational Cliern., 1994, 10, 1053. [20] A. K. Hughes, K. L. Peat and K. Wade, J. Chem. Soc.. Dalton Trans., 1996, 4639. [21] D. Braga and T. F. Koetzle, J. Chem. Soc., Chem. Commurz., 1987, 144. [22] S. F. A. Kettle, E. Diana, R. Rossetti and P. L. Stanghellini, J. Am. Clzem. Soc., 1997, 119, 8228. [23] C. Elschenbroich and A. Salzer, Orgunometullics u concise introduction, VCH, Weinheim, 1989, pp 230. 1241 A. K. Hughes, K. L. Peat and K. Wade, J. Clzem. Sue., Dulton Trans., 1997, 2139. [25] M. E. Minas de Piedade and J. A. Martinho Simoes, J. Orgunomet. Clzem., 1996, 518, 167. 1261 (a) A. G. Orpen, J. Chem. Soc., Dalton Trans., 1980, 2509; (b) G. Ciani, D. Gusto, M. Manassero and A. Albinati, J. Chem. Soc., Dalton Truns., 1976, 1943. [27] K. M. Hillary, A. K. Hughes, K. L. Peat and K. Wade, Polyhedron, 1998, 17, 2803. [28] J. Mason, J. Am. Cliem. Soc., 1991, 113, 24. [29] M. Kaupp, J. Chem. Soc., Chetn. Cornmun., 1996, 1141. 1301 J. N. Nicholls, Polyhedron, 1984, 3, 1307. [31] A. K. Hughes and K. Wade, manuscript in preparation. [32] L. Pauling, J. Am. Chem. Soc., 1947, 69. 542. [33] R. F. McMeeking and D. J. Parkin, J. CIiem. In$ Comput. Sci.,1996, 36, 746. ~
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.8 Bimetallic Effects on the Redox Activity of Transition-metal Carbonyl Clusters* Piero Zanello and Fabrizia Fabrizi de Biani
3.8.1 Introduction Most of the uncommon and appealing chemical and physicochemical properties which characterize heterometallic molecules stem from the synergistic effects of their polar metal-metal bonds. The spectacularly better catalytic activity of bimetallic compared with monometallic systems is well known.['~21 As an extreme case one could also extend the concept to the phenomenon of the superconductivity in ceramic oxocuprates, for example YBa2Cu3O-ipX,even if here the nonstoichiometry is probably more important than the heterometallic composition. In previous articles we have reviewed the electrochemical behavior of homonuclear[31 and heteronu~lear[~] metal-carbonyl clusters and the structural consequences which accompany their redox changes; in this article we wish to underline the differences in redox propensity (i.e. the tendency to add or lose electrons) caused by simple changes of the metal in isostructural, and possibly isoelectronic, metal-carbonyl clusters. Accounting for these differences requires much, so far almost unexplored, theoretical work.
3.8.2 Trinuclear clusters 3.8.2.1 Triangular complexes [M3(C0)12] (M = Fe, Ru) As illustrated in Fig. 1, all the complexes of formula [M3(C0)12]( M = Fe, Ru) are characterized by an almost equilateral M3 triangle, although there are some differences in the disposition of their carbonyl groups - in [Fe3(C0)12]and [Fe2Ru(CO)12] ten are terminal and two bridge the same edge; in [Ru~(CO)IZ] and [FeRu2(CO)lz] all are terminal.[51
.
3.8 Bimetallic Efsects on Transition-metal Curhonyl Clusters
1 105
Figure 1. Schematic diagram of: (a) [Fe3(CO)12]bridged Fe-Fe = 2.56 , unbridged Fe-Fe = 2.68 A; (b) [Fe2Ru(CO),2] (bridged) Fe-Fe = 2.58A, Fe-Ru = 2.73A; (c) [FeRuz(CO)ll]. Fe-Ru = 2.77A, Ru-Ru = 2.80A; (d) [Ru3(CO)l2].Ru-Ru = 2.85A.
According to normal electron counting these complexes are isoelectronic; all have 48 cluster valence electrons (CVE). In dichloromethane solution, [ Fe3(CO) 121 undergoes two sequential one-electron reductions, Fig. The first step [F~~(CO)I~]/[F~~(CO)~~Iis chemically reveris complicated by slow sible; whereas the second step [Fe3(CO)l2]-/[Fe3(C0)12l2decomposition of the dianion. In contrast, in dichloromethane solution [ R u ~CO) ( 121 undergoes an irreversible, single-step, two-electron Although a detailed electrochemical investigation has not appeared, it has been reported that [Fe2Ru(C0)12] and [FeRu2(C0)12], upon electrolysis at low temperature, afford the corresponding short-lived mono anion^,^^^ i.e. the mixed-metal complexes seem to exhibit intermediate behavior (single one-electron additions leading, however, to quite unstable anions) with respect to their homonuclear precursors. Table 1 lists the redox potentials of the redox changes cited above. 2.r3371
1106
3 Dynamics and Physical Properties
Volt vs AgIAgC1 Figure 2. Cyclic voltammetric response recorded at a platinum electrode in a CH2C12 solution of [Fe3(CO),,]. Scan rate 0.2 Vs-I.
Monocapped triangular complexes Chalcogen-capped complexes [M3(C0)9(pu,-X)]"(A4= Fe, Ru, Co; X = S, Se)
A series of sulfur-capped triangular homo- and heteronuclear complexes is known with the general formula [M3(C0)9(p3-S)ln(where n is the charge - either positive or negative). The triangular pyramidal geometry of [Fe3(CO)9(p,-S)]2-, [Co3(CO)9(p3-S)],and [FeCo2(C0)9(p3-S)]is shown in Fig. 3.r10-'21. The selenium-capped analogs [Co,(CO)9(p3-Se)]and [FeCo2(CO)9(p3-Se)]have similar geometries." 31 [ F ~ ~ ( C O ) ~ ( P ~ [Fe2CO(CO)9(P3-S)]-, -S)]~~, [FeCo2(Co)9(P3-S)], [RuCo2(CO)9' (p3-S)], and their selenium analogs have 48-CVE, whereas [Co3(CO)9(p3-X)] ( X = S, Se) are 49-CVE species. In the tricobalt complex the metal-metal bonds are significantly longer than in the 48-electron derivatives, and the metal-sulfur dis-
Table 1. Formal electrode potentials (in V, relative to the SCE) and concomitant CVE variations, for the redox changes of [M3(C0)12](M = Fe, Ru) in CH2C12 solution. Comp1ex
a
E"'0,-
E"'-,2-
CVE changes
Complicated by slow chemical reactions. Complicated by rapid chemical reactions.
3.8 Bimetallic Efects on Trunsition-metal Curhonyl Clusters
Figure 3. Schematic diagram of: (a) [Fe~(CO)y(p,-S)]'-,Fc-Fe = 2.57w, Fe-S (b) [ C O , ( C ~ ) S ( ~ ~ Co- S ) Co ] , = 2.64A. Co-S = 2.14A; (c) [FeCoz(CO)y(p,-S)],M-M M-S = 2.16A.
1 107
=
2.19A;
= 2.55A,
tances being are almost identical; this suggests that the electron in excess resides in a metal-metal antibonding orbital. The main redox changes observed for these complexes in 1,2-dichloroethane are: [ ' 4l
i) the homonuclear complexes [ F ~ ~ ( C O ) & L ~ - S and ) ] ' [Co3(CO)9(p3-X)] ( X = S, Se) undergo one-electron oxidations to their, probably stable, congeners [Fe3(C0)9(p3-S)I- and [ C O ~ ( C O ) ~ ( P ~ - X ) ] + ; ii) the heteronuclear complexes [FezCo(CO)~(p3-S)]-, [FeCoz(CO)9(p3-X)],and [RuCo2(C0)9(p3-X)]( X = S, Se) undergo one-electron reductions to the corresponding congeners [ FezCo( CO)9(p3-S)]'-, (FeCoz(CO)9(p3-X)]-, and [Ru CO~(CO)~(P~-X)]-. Table 2 summarizes the electrode potentials of the electron transfers cited. At first glance it is not easy to discern an unequivocal trend in redox propensity after changing the metal atom, in that the substitution of one iron atom for one cobalt atom in [Fe3(CO)9(p3-S)]'- results in the electron-transfer capacity changing from oxidation to reduction. Nevertheless, if, as indicated, one takes into consid-
1108
3 Dynamics and Physical Properties
Table 2. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of [Mj(CO)g(pj-X)]"(M = Fe, Rn, Co; X = S, Se) in 1,2-C2H4C12 solution. Complex
EO'OX
P'Red
EPa
-0.40 +0.57'
-
-
-
-1.71 -0.65 -0.58 -0.80 -0.73
-
-
-0.70 -0.77
+0.44 f0.42 -
-
+0.44 +0.42
-
CVE changesb 48/47 48/49 48/49 48/49 48/49 48/49 49/48 49/48 48/49 48/49
Irreversible two-electron reduction. CVE variations of chemically reversible redox changes. 'Complicated by slow chemical reactions. See text.
eration the isoelectronic 48-CVE species (i.e. the sequence [Co3(CO)9(p3-X)]+, [FeCoz(CO)9(p3-S)],[F~~CO(CO)&~-S)]-) it can be deduced that the higher the content of cobalt atoms relative to iron atoms, the easier is the reduction to the corresponding 49-CVE derivatives. The substitution of each cobalt atom for an iron atom involves an unexpectedly high cathodic shift of about 1 V and so it is not unreasonable to believe that the 48/49-CVE change [Fe3(CO)9(p3-S)l2-/ [Fe3(C0)9(p3-S)l3- might occur beyond the cathodic window of the solvent. Finally, minor effects result both from substituting iron atoms for ruthenium atoms and from changing the capping chalcogen atom.
Phosphinidene-capped complexes [M3 (CO)g(p,-PPh)]" ( M = Fe, Ru, Co) As shown in Fig. 4, which refers to the molecular structure of the 49-CVE [Co3(CO)g(p3-PPh)]and the 48-CVE [Co2Fe(CO)9(p3-PPh)] clusters,[''] the family [M3(C0)9(p3-PPh)]is geometrically related to the chalcogen-capped M3 clusters. As for the pair [Co3(CO),(p3-S)]/[FeCo2(CO)9(p3-S)], for these species also the metal-metal bond distances testify that in the tricobalt species the extra electron is located in a metal-metal antibonding orbital. As expected, in non-aqueous solution, [Co3(C0)9(p3-PPh)] undergoes a chemically reversible one-electron oxidation to the corresponding 48-CVE monocation, whereas the mixed-metal 48-CVE species [Co2M(C0)9(p3-PPh)]( M = Fe, Ru) and [CoFe2(CO)9(p3-PPh)]-undergo a chemically reversible one-electron reduction to the corresponding 49-CVE congeners.[141The relevant redox potentials are summarized in Table 3 . Here also, limiting our attention to the 48/49-CVE change, it is clearly evident that the progressive substitution of cobalt atoms for iron or ruthenium atoms makes
3.8 Bimetallic Effects on Transition-metal Carbonyl Clusters
1 109
Figure 4. Schematic diagram of: (a) [Co3(C0)y(p3-PPh)].Co-Co = 2.71 A; Co-P = 2.13A; (b) [Co2Fe(CO)y(p3-PPh)].Co-Co = 2.63A, Co-Fe = 2.62A; Co-P = 2.12A; Fe-P = 2.16A.
the reduction increasingly difficult by about 1 V, which evidently cannot be simply attributed to the different electrostatic effects arising from the addition of an electron to a monocation, to a neutral species, or to a monoanion, respectively.
Alkyne-capped complexes [M3 ( C O )9 ( RC= CR)] ( M = Fe, Co) Fig. 5 , which shows the molecular structures of [Fe3(CO)9(PhC=CPh)]"61 and [FeCoz(CO)9(EtC-CEt)]" 71, illustrates the most important structural features of alkynyl clusters with an M3C2 core. In the formally saturated 48-CVE [FeCo2. (C0)9(EtCXEt)] complex, the alkyne is parallel to one metal-metal edge, thus forming a square-based pyramid. In the unsaturated 46-CVE [ Fe3(CO)9(PhC-CPh)] complex the alkyne is perpendicular to one metal-metal edge, thus forming a trigonal bipyramid."']. Fig. 6, which illustrates the electrochemical behavior of [Fe3(CO),(PhC-CPh)], shows that the 46-CVE derivatives undergo two sequential, chemically reversible,
Table 3. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of [M3(CO)y(p3-PPh)]"(M = Fe, Ru, Co) in 1,2-C2H4C12 solution.
a
CVE variations of chemically reversible redox changes. bComplicated by slow chemical reactions.
11 10
3 Dynamics and Physical Properties
Figure 5. Schematic diagram of: (a) [Fe3(C0)9(PhCzPh)], Fel-Fe2-FeI-Fe3 = 2.49A, Fe2-Fe3 = 2.58A, CIO-CIl = 1.41A; (b) [FeCo2(CO)g(PhCzPh)]Co-Co = 2.58A, Co-Fe = 2.48A; ClO-Cll = 1.37A.
one-electron additions. There is spectroscopic evidence that in the electrogenerated 48-CVE dianions [ Fe3(C0)9(RC-CR)] 2 - the alkyne undergoes reorientation from the perpendicular orientation to being purullel to one metal-metal edge. In contrast, the saturated heterometallic 48-CVE complex [FeCoz(CO)9. ( EtC-CEt)] undergoes an irreversible reduction step."'] It is surprising that the electrogenerated 48-CVE dianions [Fe3(C0)9(RC-CR)] 2 - can be reversibly oxidized to their 46-CVE parents, whereas the 48-CVE [FeCoZ(C0)9(EtC-CEt] does not support electron removal without destruction of its framework. The electrode potentials of the redox changes of these alkynyl clusters are reported in Table 4.
3.8 Bimetullic Effects on Trunsition-metal Carhonyl Clusters
111 1
Table 4. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of [M3(CO)g(PhC=CPh)](M = Fe, Co) in CH2C12 solution. Complex
E"'",
[Fe3(C0)9(PhC2 Ph)] [FedCO)s(MeCzMe)] [FedC0)9(EtC2Et)] [FeCo2(CO)9(EtCz Et)]
-0.36 -0.47 -0.49 -0.86h
a
~
E?,2-
Main CVE changes"
-0.60 -0.65 -0.67
46/48 46/48 46/48
-
-
CVE variations of chemically reversible redox changes. Complicated by fast chemical reactions.
The inductive effects of the alkyne substituents on redox potentials indicate that there is electronic communication between the trimetallic core and the alkynecapping unit.
Bicapped triangular complexes
Carbonyl-capped complexes [M3 (q5-C5R5)3( p 3 - C 0 ) z ] ( M = Co, Ni;
R = H, M e )
As typical examples of the geometry of the family [M3(q5-CsH5)3(p3-C0)2] ( M = Co, Ni), Fig. 7 shows the X-ray structure of [ C O ~ ( ~ ~ - C ~ H S ) ~ ( , D ~ - C O ) ~ ] , [ [ CoNiz(q5-C5H s )(~ p 3-CO)2], and [Ni3(q s-CSH s )(p~3-CO)2].[' It is apparent that two triply bridging carbonyl groups cap the metallic triangles on opposite sides. The X-ray structures of the permethylated analogs [ C O ~ ( ~ ' - C S M ~ ~ ) ? ( ~ ~ - C O ) ~ ] , [CoNi2(qs-CsH~)2(qS-C~Me~)( , D ~ - C O ) ~ ]and , [ ~ [Ni3(q5-C5MeS)3(, ~ , - C 0 ) 2 ]31[ ~have also been determined. As illustrated in Fig. 8, in non-aqueous solution the 46-CVE [Co3(q5-C5H5)3. (p3-CO)2] undergoes a one-electron reduction to the probably stable 47-CVE monoanion [CO~(~~-CSHS)~(~,-CO)~]-,[*~~ whereas the 49-CVE [Ni?(q5-CsHs)3. (p3-CO)2] reversibly undergoes either a one-electron oxidation to the 48-CVE monocation [Ni3(q5-CsH5)3(p3-C0)2]+ or a one-electron reduction to the 50-CVE monoanion [Ni3(q s-C5 H5)3( p ,-CO)2] .[241 The X-ray structure of the monoanion [Ni3(q5-CsH5)3(p,-CO)z]- has been determined.[231It is substantially similar to that of the neutral precursor, except that the Ni-Ni distances are slightly longer (2.42A),thus testifying to the antibonding character of the LUMO of [Ni3(q5-CsHs)3(p3-C0)2]. Finally, the 48-CVE mixed metal [CoNi2(qS-C5H5)3(p3-C0)2] undergoes a chemically reversible one-electron reduction.[2 Because the electrochemistry of the different complexes has been investigated in different solvents and using different (and not always reported) reference electrodes, to give a brief (although approximate) account of the large differences in redox potentials (particularly for the 48/49-CVE changes), Table 5 simply summarizes the formal electrode potentials of the unmethylated complexes. -
1 112
3 Dynamics and Physical Properties
1
&
b
C
Figure 7. Schematic diagram of: (a) [Co3(qS-C~H5)3(,uU,-CO)2] CO-CO = 2.39A, Co-C(c0) = 1.97A, Co-C(cpj = 1.70A; (b) [CoNi2(qs-C5Hs)3(p3-CO)z] M-M = 2.36A, M-C(c0) = 1.93A, M-C(cpj = 1.73A; (c) [Ni3(q5-CsH5)3(p3-C0)2]Ni-Ni = 2.39A, Ni-C(co) = 1.93A, Ni-C(cp) = 1.76 A.
3.8.3 Tetranuclear clusters 3.8.3.1 Tetrahedral complexes [M~(CO)I$ (M = Fe, Co, Rh) Several 60-CVE homo- and heteronuclear complexes of general formula [M4-,M’,y. (C0)12In- (M, M’ = Fe, Ru, Co, Rh) are known. Their tetrahedral geometry is here exemplified in Fig. 9, which refers to [C04(C0)12][~~] and [ C O ~ R U ( C O ) ~ ~re]-,[~~] spectively. It is apparent that in both species each metal-metal edge is p-bridged by a carbonyl group lying in the basal plane. Fig. 10 shows that in dichloroethane solution [Co4(C0)12]undergoes either an
3.8 Bimetallic Effects on Transition-metal Carhonyl Clusters
1113
a -1.5
-1.0
-0 7
-1.7
Volt vs SCE
I 1,
/e--
i
/
Figure 8. Cyclic voltammograms recorded at a platinum electrode on: (a) a THF solution of [Coi(q5-CsH~)3(,u3-CO)2] scan rate 0.01 V s - l ; (b) a CHlC12 solution of [Ni3(q5-C5H~)3(,uu,-CO)2] scan rate 0.2 Vs-I.
'.---
i
'\
ti \j
I
I
I
I
1.0
0.5
0.0
-0.5
I -1.0
-1.5
b
initial one-electron reduction to the relatively short-lived monoanion [ C O CO) ~ ( 121followed by a second irreversible one-electron reduction, or an irreversible fourelectron oxidation.[271 The behavior of [Rh4(C0)12] and [Co2Rh2(C0)12]is quite similar to that of [Coq(CO)12]except that their monoanions [Rh4(C0)12]- and [Co2Rh~(C0)12]are completely unstable.[271 Table 5. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of [M3($-C5H5)3(,u3-C0)2]( M = Co, Ni). Complex
CVE changes 46/41 E"'
a
1.2-Dimethoxyethane
Solvent 48/49 E"'
49/50 E"'
1114
3 Dynamics and Physical Properties
: l
l b
a
Figure 9. Schematic diagram of: (a) [CO~(,U-CO)~(CO)~] Co-Co (CO)g]- CO-CO= 2.53 A, Ru-CO = 2.63 A.
= 2.49&
(b) [Co3Ru(pc-C0)3.
The cathodic behavior of the isoelectronic complexes [Co3M(C0)12]- ( M = Fe, Ru) is slightly different in that they undergo an irreversible, single-step two-electron reduction.[281 Table 6 summarizes the electrochemical characteristics of this class of tetrahedral clusters. Except for complexes [CojM(C0)12]- ( M = Fe, Ru), which seem unable to attain the 61-CVE state, the most significant difference among the other complexes is the increasing instability of the 61-CVE configuration as the number of rhodium atoms increases; there are also significant variations among their electrode potentials.
2.0
1.0
0.0
Volt vs SCE
-1.0
-2.0
Figure 10. Cyclic voltammogram recorded at a platinum electrode on a 1,2-CzH4C12 solution of [ c O ~ ( C O ) ~ ~ I scan , rate 0.1 V s-' .
3.8 Bimetallic Effects on Transition-metal Curhonyl Clusters
1 1 15
Table 6. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the series [M4-,M:(C0)12l1' ( M , M ' = Fe, Ru, Co. Rh) in 1,2-C2H4C12 solution. Complex
CVE changes 60156 EP
6016 1 E"'
6 1/62 EP
Complicated by slow chemical reactions. bComplicated by fast chemical reactions.
[ M ~ ( C ~ ) ~ { H C ( P P ~(M Z )=~ CO, } ] Rh) Closely related to the family [M4(CO),2In is the 60-CVE series [M4(C0)9. {HC(PPh2)3}] ( M = Co, Rh). As illustrated in Fig. 11 for [Co4(CO)9. { HC( PPh2)3}],[291 the tripodal ligand 1, 1,1-tris(dipheny1phosphino)methane caps one face of the Co4 tetrahedron. From Fig. 12 it is evident that, with regard to the electron-transfer capacity of the fully carbonylated series [M4(CO)12]", the presence of the basal polyphosphine gives stability to the redox congeners of [M4(C0)9{HC(PPh2)3}]. On the cyclic voltammetric timescale, the Co4 complex undergoes either a reversible one-electron oxidation or a one-electron r e d u ~ t i o n , [ ~ 'the * ~ ~Co2Rhz ' complex undergoes a reversible one-electron reduction,[271and the Rh4 complex, under CO, also undergoes a reversible one-electron reduction.[271
11 16
3 Dynamics und Physical Properties
Volt its AglAgCl
I 1.5
1.0
05
0.0
-0.5
-1.0
-1.5
b -2.0
Volt vs SCE Figure 12. Cyclic voltammograms recorded at a platinum electrode for: (a) [ C O ~ ( C O ) ~ . {HC(PPh2)3}] in CH2C12 solution; (b) [Co2Rh2(C0)9{HC(PPh2)3}] in 1,2-C2H4C12 solution; (c) [Rh4(C0)9{HC(PPh2)3}] in 1,2-C2H4C12 solution. The inset show the voltammogram under CO atmosphere. Scan rate (a) 0.2 Vs-'; (b, c) 0.1 Vs-'. For Figure 12c see page 1117.
The relevant redox potentials are listed in Table 7. It is apparent that progressive substitution of cobalt atoms for rhodium atoms induces a shift towards lower potentials. In addition, comparison with the corresponding [ M 4 ( C O ) I # species shows that the strong electron-donating power of the basal tripodal phosphine makes the reductions more difficult by approximately 0.5 V.
3.8 Bimetullic Effects on Transition-metal Curhonyl Clusters
1 1 I7
Volt vs SCE
Figure 12 (continued)
3.8.3.2 Butterfly complexes To introduce the butterfly geometry we discuss the redox behavior of two complexes-the 60-CVE [H30s4(C0)12]- and the 58-CVE [H*Os3Pt(C0)lo { P(C6H11)3}I. Their tetrahedral geometries are illustrated in Fig. 13.[31,321In [ H ~ O S ~ ( C O ) I ~ ] the hydride hydrogens are thought to bridge the Osl -0sl ', Osl-Os2', and Os2-Osl' bonds. In [H20s3Pt(C0)10{P(C6HI1)3}] the hydride hydrogens are located on the Pt-Osl and Os2-Os3 bonds. Although apparently unrelated, some of the redox changes of these complexes are similar. In acetonitrile solution the Os4 complex undergoes a single-step twoelectron oxidation (Ep = +0.78 V), which is coupled to fast coordination of solvent molecules according to the r n e c h a n i ~ m31: ~ ~ Table 7. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the series [ M ~ ( C O ) ~ { H C ( P P (~M~ )=I )Co, ] Rh). Complex
a
CVE changes
Complicated by fast chemical reactions. Under CO atmosphere
Solvent
1 118
3 Dynumics and Physical Properties
As Fig. 14 testifies, the overall 60/62-CVE change induces opening of the os4 tetrahedron to a butterfly arrangement.[331 In a similar manner [H20~3Pt(CO)10 { P ( C ~ H1 )I3 } ] can add two-electron donors L, such as CO or PPh3, to afford the 60-CVE butterfly-shaped complexes [H20~3Pt(C0)10{P(C6H11)3}( L)]. Fig. 15 shows the molecular structure of [HzOs3Pt(CO)ii{ P ( C ~ H ~ I ) ~ } ] . ~ " ' Paralleling its chemical behavior, in dichloromethane solution [H20~3Pt(CO)io. {P(C6H11)3}] undergoes a chemically reversible two-electron reduction to its dianion [H20S3Pt(C0),0{P(C6H11)3}]2(E" = -0.81 V ) . Because of the electrochemical quasireversibility of such 58/60-CVE electron transfer processes, it is reasonable to assume it accompanied by the tetrahedral/butterfly re~rganization.~~ 51
3.8.3.3 [ M ~ ( C O ) ~ O + ~ ( R C - C R(M ) ] ~= - Fe, Ru, Co) A series of homo- and hetero-nuclear 60-CVE alkynyl clusters of formula [ M ~ ( C O ) I RC-CR)IX~+~( ( M = Fe, Ru, Co) has been prepared. In their metallic
3.8 Bimetallic E f e c t s on Transition-metul Curhonjd Clusters
1 1 19
t
..........
Ts
Figure 14. Schematic diagram of [H~OS~(CO),~(NCM~)~]+.
butterfly core, the alkyne lies between the two wings. Fig. 16 shows the molecular structures of [Co4(CO)l0(HC-CH)],r361[ C O ~ R U ( C OPhC-CPh)]-,[371 )~~( and
[Ru4(C0)12(PhC-CPh)].[381 Fig. 17 shows that both [Co4(C0)10(RC-CR)][391 and [Co3M(CO)lo. (PhC5CPh)l- ( M Fe, RU)[~*I undergo two sequential one-electron reductions; in both instances only the first reduction is chemically reversible. In contrast, [ R uCO) ~ 12( PhC-CPh)][401undergoes two irreversible two-electron reductions (and an irreversible four-electron oxidation).
' \
Figure 15. Schematic diagram of [ HzOsiPt( CO)I I t p(C6Hi I ) ? 11.
1120
3 Dynamics and Physical Properties
C
Figure 16. Schematic diagram of: (a) [Co4(C0)lo(HC=CH)] COhlnge-COhlnge = 2.56 A, CowingCOhlnge= 2.45A, CEC = 1.40A; (b) [ColRu(CO)lo(PhC=CPh)]- R u ~ ~ ~ ~ =~2.72A, - C O ~ , ~ ~ ~ COwlng-Mhlnge = 2.50A, C-C = 1.34A; (c) [R~q(C0)12( PhC-CPh)] RUhlnge-RUhrnge = 2.85A, RU,,,g-RUhlnge = 2.73 A, c ~ =c1.46A.
The instability of the redox congeners of the Co3M and Ru4 complexes is due to decarbonylation reactions. Exhaustive one-electron reduction of [Co?Ru(CO)10. (PhC-CPh)]- in the presence of PPh3 affords the stable dianion [ C O ~ R U ( C O ) ~ . ( PhC-CPh)]2-,[281 and exhaustive two-electron reduction of [RQ(C0)12. ( PhC-CPh)] affords the stable dianion [Ru4(CO)11(PhC-CPh)]2-.L411The redox potentials of these electron transfers are reported in Table 8. Although it is apparent that there are large differences between the redox potentials for the isoelectronic jumps, as different solvents have been employed, the starting compounds have different charges, and as the reversibility differs among the examples, it is not possible to make quantitative assessments.
3.8 Bimetallic Eflkcts on Transition-metal Carbonyl Clusters
I 0.4
02
I
I
0.0
-0.2
I -0 4
I -0.6
I -0.8
I -1.0
I
1 121
a
-1.2
Volt vs SCE
Figure 17. Cyclic voltammograms recorded for: (a) [ C O ~ ( C O )HC-CH)] IO( platinum electrode, CHlClz solution, scan rate 0.2 Vs-I; (b) [Co,Ru(CO)lo( PhC=CPh)]- gold electrode, DMF solution. scan rate 0.1 V s-'; (c) [R~q(C0)12( PhC-CPh)] platinum electrode, MeCN solution, scan rate 0.2 Vs-I.
1122
3 Dynamics and Physical Properties
Table 8. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the series [ M ~ ( C O ) ~ O + ~ ( R C - C(M R ) ]=~ Co, - Ru). Complex
[CO~(CO)IO(HC-CH)] [C04(CO)lo( EtC-CEt)] [Cod(CO)lo(PhC-CPh)] [ C ORu( ~ CO)lo( PhC_CPh)][CoiFe(CO)lo(PhC=CPh)][Ru~(CO)I~(P~C-CP~)]
CVE changes
Solvent
60156 EP
6016 1 E"'
61/62 E"'
-
-0.33 -0.36 -0.20 -1.14" - 1.06" -0.81b
-1.03 - 1.05 -0.91 1.68 - 1.66 -0.81b
~
+0.55 +0.50 +1.18
~
CHzClz CHzClr CHzClz DMF D MF MeCN
"Complicated by slow decarbonylation. hComplicated by fast decarbonylation
3.8.3.4 Planar complexes IM4(C0)11(P44-PR)ZI(M = F%Ru) The bis-capped phosphinidene complexes of formula [ M4(CO)11(p4-PR)2] ( M = Fe, Ru) are the unsaturated, 62-CVE, counterpart of the saturated, 64-CVE, series [M4(CO)12(,u4-PR)2].Fig. 18 illustrates the M4P2 octahedral structure of [Fe4(CO)11(P-p-T01)2][~~] and [ R u ~ ( C O1)( IPPh)2].[431 Electrochemically, the complexes [M4(CO)11(PR)2] would be expected to add two electrons to achieve their stable saturated 64-CVE configuration, and this is, indeed, observed for the homonuclear complexes, Fig. 19.[441 The electrochemistry of the heteronuclear complexes [Fe3Ru(CO)11(PR)2] and [Fe2Ru2(CO)11(PR)2] has not been reported, but they probably behave like the homonuclear species because it has been found that their chemical reduction (by cobaltocene) affords the corresponding m o n o a n i o n ~ . [By ~ ~ ]way of confirmation, the isoelectronic and isostructural cluster [Fe3Rh(CO)*(C5Mes)(PPh)2] undergoes two sequential, chemically reversible, one-electron additions, Fig. 20.[451 Table 9 summarizes existing data for the series of [ M4(CO)11 ( PR)2] complexes. Changing iron atoms for ruthenium atoms not only makes the reduction processes more difficult, but also leads to instability of the 64-CVE dianions [RW(CO)iI ( PR)2I2-.
3.8.4 Pentanuclear clusters No comparison between homo- and heteronuclear complexes is available for pentanuclear complexes and only examples of heteronuclear complexes can be discussed.
3.8 Bimetallic Effects on Transition-metal Curhonyl Clusters
1 123
t
h
Figure 18. Schematic diagram of: (a) [ F ~ ~ ( C O ) I ~ ( P - ~Fe- T Fe O ~=) 2.63 ~ ] A; (b) [ R u ~ ( C O ) ~ ~ ~ (PPh)?]Ru-RU = 2.83 A.
3.8.4.1 Bow-tie complexes [Fe4M(CO)1#- (M = Pt, Au) Fig. 21 shows the molecular structures of the 76-CVE anions [Fe,Pt(CO)l,I2- and [Fe4Au(CO) &[46,471 Despite their similar geometry and electron count, the platinum-centered complex can lose one electron to give the relatively unstable corresponding monoanion, Fig. 22a, and then add an immediately framework-destroying electron,[481whereas the gold-centered complex undergoes two consecutive one-electron additions which are chemically reversible, Fig. 22b.[471 The corresponding redox potentials are listed in Table 10, which also quotes the redox potentials of the subsequent class of pentanuclear complexes.
3.8.4.2 Spiked triangulated rhomboidal complexes [M3(CO)io+fl(~-X)(~-Hg(Mo(Co)3(c5H5)})1(M = Mn, n = 2, X = H; M = Fe, n = 0, X = CO) Another comparison between isoelectronic (76-CVE) heteronuclear complexes can be found in [ Mn3(CO)lz(p-H)(p-Hg{Mo(C0)3(C5H5)})]- and [ Fe3(CO)lo(p-CO).
1124
3 Dynamics and Physical Properties
I
I
I
I
I
I
0.6
0.4
0.2
0.0
-0.2
I
-0.4
I
-0.6
a
I
I
-0.8
-1.0
-1.2
Volt vs SCE
I 0.4
0.2
I 0.0
I
I
-0.2
-0.4
I -0.6
I
I
-0.8
-1.0
I -1.2
I
b
-1.4
Volt vs SCE Figure 19. Cyclic voltammograms recorded at a platinum electrode for benzonitrile solutions of: (a) [Fe4(CO)II(PPh)2]and (b) [ R Q ( C O ) I I ( P P ~ scan ) ~ ] , rate 0.2 V s - ' .
(pU-Hg{Mo(CO)3(C~H~)})]-. Fig. 23 shows the triangulated rhomboidal Mn3Hg assembly spiked by a Mo atom in [ M ~ ~ ( C O ) & L - H ) ( ~ - H ~ { M O ( C O ) ~ ( C ~ H ~ ) } ) ] - . [ ~ ~ ~ In dichloromethane solution this class of complexes undergoes a one-electron oxidation to the poorly stable neutral congener.[501[F~~(CO)~~(~-CO)(~-H~{MO(CO) (C5H5)})]- also undergoes a one-electron oxidation, but the electrogenerated neutral species is decidedly more stable than the preceding Both complexes also undergo an irreversible two-electron reduction. The relevant redox potentials are reported in Table 10.
3.8 Bimetallic Effects on Transition-metal Curhonyl Clusters
Figure 20. Cyclic voltammogram recorded at a platinum electrode for a THF solution of [FejRh(CO)g(CsMes)(PPh)2], scan rate 0.01 V S K I .
1 125
~~
2.0
1.o
0.0
-1.0
-2.0
Volt v s SCE
a
Figure 21. Schematic diagram of: (a) [Fe4Pt(CO)16l2-, Fe-Fe = 2.11 A, Fe-Pt = 2.60 A; (b) [ F ~ ~ A U ( C O ) I ~ I - , Fe-Fe = 2.11 A, Fe-Au = 2.59 A.
1126
3 Dynamics and Physical Properties
Table 9. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the series [ M ~ ( C O ) I I ( P R(M ) ~= ] Fe, Ru). Complex
CVE changes
Solvent
62/63 E"'
63/64 E"' -0.17 -1.12 - 1.34 -0.95"
$0.05 -0.06 [F~~R~(CO)X(CSM~~)(PP~)~I -0.46 -0.43 [RQ(CO)II ( P P ~ ) z ]
[ F ~ ~ ( C O )PW21 II( [F~~(CO I ( )PIB ~ ' ) z I
PhCN PhCN THF PhCN
Complicated by slow chemical reactions.
inu7
--
,.
1-
tc / I '
-..
I 0.3
'
-
/---
,''{
4
/- *
__
---A
-,.I
.
I 0.0
_..-.--
--4-
/
'. ./
*/--
-0.3
Volt vs SCE
l
-0.6
a
I
I
0.0
-0.5
'4
,-J
I -1.0
l
b
-1.5
Volt vs SCE
Figure 22. Cyclic voltammetric responses recorded at a platinum electrode in acetonitrile solutions containing [NEt4][C104] (0.1 M ) and (a) [Fe4Pt(C0)16I2-; (b) [Fe4Au(CO)ls]-. Scan rate: (a) 2.0 Vs-I; (b) 0.2 Vs-'.
Figure 23. Schematic diagram of [Mn3(C0)l2(p-H)(pu-Hg{Mo(C0)3(CsHs),)lMnl-Mn2 3.14 A, Mnl-Mn3 = Mn2-Mn3 = 2.95 A, Mn-Hg = 2.10 A, Hg-Mo = 2.19 A.
=
3.8 Binwtullic Efects on Trunsition-metul Curbonyl Clusters
1 127
Table 10. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the complexes [ Fe4M(CO)161' I - and [ M ?( CO)lo+,&-X)(p-Hg { Mo(CO)3(CjH5)))I-. Complex
CVE changes
[ Fe4Pt(C O ) 1 6 ] a [Fe4Au(CO)I 61CO)?(Cs Hs)))][Mnl(CO)I ~ ( P - H ) ( P - H ~ I M O( [Fe?(CO)lo(/L-CO)(P-Hg{ Mo(CO)3(CjH5) I)] a
15/76 E"'
76/11 E"'
+0.01"
-1.51b -0.13 1.46b 1 .40b
-
+0.42' +0.24"
~
~
Solvent 71/78 E"' -
-0.93 - 1 .46b - 1 .40b
MeCN MeCN CH2C12 CH2C12
Complicated by slow chemical reactions. 'Complicated by fast chemical reactions.
3.8.5 Hexanuclear clusters 3.8.5.1 Octahedral complexes Carbonyl complexes [IrsM(CO)l#-
(M = Ir, Fe)
Fig. 24 shows the octahedral geometry of the 86-CVE complexes [Ir6(C0)15]2-[52] and [ IrSFe(CO)153'- .[531 As Fig. 25 illustrates, both the complexes undergo an initial, chemically reversible, one-electron oxidation followed by a second one-electron oxidation; these afford the relatively stable [Ir6(CO)15][541 and the quite unstable [Ir5Fe(C0)15]-,[531 respectively. Despite the rather similar behavior of the two complexes, the redox potentials of their two subsequent electron removals are notably different, Table 11, indicating that the substitution of an iridium atom for an iron atom makes the oxidation easier.
Nitrido-carbonyl complexes [Fe6-,M,N(C0)151a- (M = Rh, Ir; n = 0 - 2) Fig. 26 shows the octahedral geometry of the isoelectronic (86-CVE) cluster anions [Fe6N(C0)15]3-[55], [Fe5RhN(C0)15I2-, and [Fe4Rh2N(C0)15]-.[561 In acetonitrile solution the homonuclear trianion [ Fe6N( CO)151 undergoes three successive one-electron oxidations, with only the first having transient chemical reversibility, Fig. 27a. As a consequence of the three-electron oxidation, [Fe5N(CO)14l3- is formed (peaks-system EF).[551In tetrahydrofuran solution [Fe5MN(CO)15]2-( M = Rh, Ir) undergo two successive oxidations; again only the first has transient chemical reversibility, and is more complex, Fig. 27b.[561Finally, in tetrahydrofuran solution [Fe4Rh*N(C0)15]- undergoes a single two-electron reduction, followed by chemical reaction, Fig. 2 7 ~ . [ ~ ~ ]
'-
1128
3 Dynamics and Physical Properties
Figure 24. Schematic diagram of: (a) [Ir6(C0)15l2-, Ir-Ir = 2.77A; (b) [ I r ~ F e ( C o ) l , ] ~ - , M-M = 2.77 A.
I 1.o
'
I 0.5
a
I
-0.5
0.0
I -0.5
0.0
'
b
-1.0
Volt vs SCE
Volt vs SCE
Figure 25. Cyclic voltammetric responses recorded at a platinum electrode for: (a) TH F solution of [Ir6(C0)15l2-and (b) MeCN solution of [Ir5Fe(C0)15]3-,scan rate 0.2 Vs-'.
Table 11. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the 86 CVE complexes [Ir5M(CO)ls]"-. Complex
[Ir6(C0)l5l2[ IrsFe(CO)ls]3a
CVE changes
Solvent
84/85 E"'
85/86 E"'
+0.36" +O. 1' 7 -0.23'
+0.04 +0.17h -0.57
TH F MeCN MeCN
Complicated by slow chemical reactions. 'Complicated by fast chemical reactions.
3.8 Bimetallic Effects on Transition-metal Curbonyl Clusters
1 129
Figure 26. Schematic diagram of: (a) [Fe6N(CO),513-, mean bond distances Fe-Fe (CO bridged) = 2.54& Fe-Fe (unbridged) = 2.63A, Fe-N = 1.86A; (b) [Fe5RhN(C0)15I2-, mean bond distances Fe -Fe (CO bridged) = 2.55f%,Fe-Fe (unbridged) = 2.66.& Fe-Rh (CO bridged) = 2.83A, Fe-Rh (unbridged) = 2.70A, Fe-N = 1.87& Rh-N = 2.03 A; ( c ) [Fe4Rh*N(C0),5]-, mean bond distances Fe-Fe (unbridged) = 2.64& Rh-Rh = 2.77A, Fe-Rh (CO bridged) = 2.72A2,Fe-Rh (unbridged) = 2.80 A, Fe-N = 1.88 A, Rh-N = 2.02 A.
The formal electrode potentials for these redox changes for the whole series are summarized in Table 12. It is apparent that despite their isoelectronic and isostructural features these complexes have remarkably different redox properties, not only as far as the localization of the redox potentials is concerned, but particularly in their aptitude to lose or gain electrons.
1130
L
I
0.2
0.0
3 Dynamics and Physical Properties
1
l
I -0.8
-0.3
a
-1.2
Volt vs SCE
"\i. ,
u
I
1.5
0.5
1.0
$9
I
I
I
0.0
-0.5
-1.0
I -1.5
1 -20
b
Volt vs SCE
I
I
I
I
'
0.0
-0.5
-1.0
-1.5
-2.0
c
Volt vs SCE Figure 27. Cyclic voltammograms recorded at a platinum electrode for (a) MeCN solution of [Fe6N(C0)15l3-,scan rates top0.2 Vs-l, bottom0.02 Vs-I; (b) THF solution of[FesRhN(C0)15l2scan rates top 0.2 Vs- ', bottom 0.05 Vs-I; (c) THF solution of [Fe4Rh2N(CO)15]-scan rate 0.05 vs-1.
3.8 Binietullic Ejects on Trunsition-metal Carbonyl Clusters
1 131
Table 12. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the 86 CVE complexes [Fe~-,,M,,N(CO)15]"Complex
CVE changes 83/84
4" [F~~N(CO)ISI'[Fe5RhN(C0)l5l2[Fe5IrN(C0)15]'[Fe4RhzN(CO)I 51-
+O.O2
Solvent
84/85 Ep
85/86 E'
-0.19 +O.77
-
f0.80
-0.49' +O. 19' +O. 14'
-
-
-
-
E"'
86/87
87/88 E"'
-
-
- 1.63'
-
-lSb -O.Wb
-
-O.Mb
MeCN THF THF THF
Peak potential value for irreversible processes. bComplicated by slow chemical reactions
3.8.5.2 Raft-like complexes The last comparison of hexanuclear complexes is devoted to the 86-CVE heteronuclear derivatives [Pt3Fe3(C0)1jI2- and [Pt20~4(C0)17]. The raft-like geometry of [Pt3Fe3(C0)15I2- is qualitatively similar to that of the corresponding monoanion is [Pt3Fe3(C0)15]- illustrated in Fig. 28a.[571The X-ray structure of [Pt20~4(C0)17] not available, but that of its cycloocta-l,5-dienyl analog [ P ~ ~ O S ~ ( C O ) I ~ ( C is O D ) ] known, Fig. 28b.[581 Despite their apparent similarity, the two complexes have the opposite redox behavior the iron complex undergoes two reversible one-electron oxidations[591 whereas the osmium complex undergoes two reversible one-electron reductions,r581 Fig. 29. This behavior is reminiscent of that of the isostructural 90-CVE homonuclear series [0s6(C0)2,-,( PR3),l].[601 The relevant redox potentials are listed in Table 13. -
Figure 28. Schematic diagram of: (a) [Pt3Fe3(CO)ls]-. Mean bond lengths: Pt-Pt = 2.66& Pt-Fe = 2.60A; (b) [ P ~ ~ O S ~ ( C O ) ~ ~ (Mean C O Dbond ) ] . lengths: Os-0s = 2.82A, Pt-0s = 2.68A.
1132
3 Dynamics und Physical Properties
I 1.0
I
I
I
0.5
0.0
-0.5
a
-1.0
Volt vs SCE
V 1
0.0
I
-0.5
I
-1.0
-t.5
Volt vs SCE
Figure 29. Cyclic voltammetric responses recorded at a platinum electrode for a CH2C12 solution of: (a) [Pt3Fe3(C0)15I2-, scan rate 0.2 Vs-l; (b) [Pt20~4(C0)17], scan rate 0.1 Vs-'.
Table 13. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes of the 86 CVE complexes [Pt3Fe3(CO)l5I2-, [Pt20~4(C0)17], and [ P ~ ~ O S ~ ( C O ) I ~ ( C in O D ) ] dichloromethane solution. Complex
CVE changes 84/85 E"'
85/86 E"'
86/87 E"'
87/88 E"'
3.8 Bimetallic Effkcts on Tsansition-nietcil Curhonyl Clustess
1 1 33
Figure 30. Schematic diagram of [ R e ~ ( C O ) ~ ~ ( C )Mean ] ' - . bond distances: Re-Re (octahedron) = 2.99 A, Re-Ccarbldr= 2.12 A, Re-Re (capping units) 2.97 A.
3.8.6 Octanuclear clusters
Fig. 30 shows the molecular structure of the homonuclear dianion [Reg(C0)24(C)]2-.["1 It consists of an ReG(C0)lg octahedron encapsulating a carbide atom, bicapped at opposite sides by two Re(C0)3 fragments. A large series of heterocapped dianion clusters of general formula [Re7(C0)z1 (C)M(L)I2- ( M = Pt, Pd, Ir, Rh; L = Mez, allyl, 2-methyl-allyl, 1,5 cyclooctadiene,
Table 14. Formal electrode potentials (in V, relative to the SCE) and CVE variations for the redox changes exhibited by the 110 CVE dianions [Reg(C0)24(C)I2- and [Re7(CO)21(C)Rh(C0)2l2- in dichloromethane solution. Complex
CVE changes 108/109 E"'
a
l09jl10 E"'
Complicated by subsequent reactions. Not reported. '96/97 CVE change. 97/98 CVE change.
1 134
3 Dynamics and Physical Propertie5
(CO)z, (CO)(PPh3) is available.[621To make comparison with the homonuclear complex easier, we discuss here only the isostructural (established by NMR spect r o ~ c o p y [ ~and ~ ] ) isoelectronic [Re7(C0)21(C)Rh(C0)2)l2-. In dichloromethane solution both [Re~(C0)24(C)]~and [Re7(C0)21 (C)Rh(C0)2l2- undergo two successive one-electron oxidations; only the first is chemically r e v e r ~ i b l e . [The ~~~~~] relevant redox potentials are reported in Table 14, and compared with those of the 98-CVE precursor [Re7(C0)21(C)]3-.[641 It is apparent that the insertion of the ,uu,-cappingfragment into the original heptanuclear Re7 complex makes oxidation more difficult, and progressively more difficult on passing from rhodium to the rhenium.
3.8.7 Conclusions It often happens that predictions of the capacity of a cluster to exchange electrons are made simply on the basis of electron-counting rules, sometimes in conjunction with structural features. The aim of this review was to point out how the redox behavior of homo- and heterometallic clusters, even those which are isoelectronic and isostructural, can often be unexpectedly different. Theoreticians are invited to develop new approaches to account for the redox behavior of polynuclear heterometallic compounds. “From a Lecture held to the Workshop on “Bimetallic Efsects in Chemistry”: European Science Foundation, Parma, 26-29 April, 1995
References [ 11 a) J. H. Sinfelt, “Bimetallic Catalysts. Discoveries, Concepts, and Applications”. John Wiley and Sons. New York, 1983; b) M. Ichikawa, Adv. Catal., 1992, 38, 283. [2] a) P. Braunstein and J. Rose, in “Comprehensive Organometallic Chemistry 11”. Vol. 10. E. W. Abel, F. G. A. Stone, G. Wilkinson, eds. Pergamon, Oxford, 1995, pp. 351-385; b) P. Braunstein and J. Rose, in “Catalysis by Di- and Polynuclear Metal Clusters”. R. D. Adams, F. A. Cotton, eds. Wiley, New York, 1997, pp. 346-402. [3] P. Zanello, Stereochemical Aspects of the Redox Propensity of Homometal Carbonyl Clusters, in “Stereochemistry of Organometallic and Inorganic Compounds”. Vol. 5. P.Zanello, ed. Elsevier, Amsterdam, 1994, pp. 163-408. [4] P. Zanello, Stereochemical Aspects Associated with the Redox Behavior of Heterometal Carbonyl, Clusters. Struct. Bonding (Berlin), 1992, 72, 101-214. [5] L. J. Farrugia, J. Chem. Soc., Dalton Trans., 1997, 1783, and references therein. [6] S. M. Owen, Polyhedron, 1988, 7, 253.
171 A. M. Bond. P. A. Dawson. B. M. Peake. B. H. Robinson, and J. Simpson, Inorg. Chem., 1977, 16, 2199. [8] A. J. Downard. B. H. Robinson, J. Simpson, and A. M. Bond, J. Organornet. Chem., 1987, 320. 363. 191 P. A. Dawsoii, B. M. Peake, B. H. Robinson. and J. Simpson, Inorg. Chem., 1980, 19, 465. [lo] F. T. Al-Ani. D. L. Hughes, and C. J. Pickett, J. Organomet. Chem., 1986, 307, C31. [ l l ] C. H. Wei and L. F. Dahl, Inorg. Chem., 1967, 6, 1229. [12] D. L. Stevenson, C. H. Wei, and L. F. Dahl. J. Am. Chem. Soc., 1971, 93, 6027. [13] C. E. Strouse and L. F. Dahl, J. Am. Chem. SOC.,1971. 93, 6032. [ 141 U. Honrath and H. Vahrenkamp, 2 . Naturforsch., 1984, 39b, 545. 1151 H. Beurich, F. Richter, and H. Vahrenkamp, Acta Cryst., 1982. B38, 3012. [I61 J. F. Blount, L. F. Dahl, C. Hoogzand, and W. Habel, J. Am. Chem. SOC.,1966, 88, 292. [I71 S. Aime, L. Milone, D. Osella, A. Tiripicchio, A. M. Manotti Lanfredi, Inorg. Chem., 1982, 21. 501. [IS] F. W. B. Einstein, K. G. Tyers. A. S. Tracey, and D. Sutton, Inorg. Chem., 1986, 25, 1631. [ 191 D. Osella, R . Gobetto, P. Montangero, P. Zanello. and A . Cinquantini, Organometallics, 1986, 5 , 1247. 1201 C. E. Barnes. J. A. Orvis, D. L. Staley. A. L. Rheingold, and D. C. Johnson. J. Am. Chem. Soc.. 1989, 11 1, 4992. [21] L. R. Byers, V. A. Uchtman, and L. F. Dahl, J. Am. Chem. Soc., 1981, 103, 1942. [22] W. L. Olson, A. M. Stacy, and L. F. Dahl, J. Am. Chem. Soc., 1986, 108, 7646. [23] J. J. Maj, A. D. Rae, and L. F. Dahl, J. Am. Chem. Soc., 1982, 104. 3054. [24] R. L. Bedard and L. F. Dahl, J. Am. Chem. Soc., 1986, 108. 5933. [25] C. H. Wei and L. F. Dahl, J. Am. Cheni. SOC.,1966. 88, 1821. [26] M. Hidai, M. Orisaku, M. Ue. Y . Koyasu, T. Kodama, and Y. Uchida, Organometallics, 1983, 2, 292. [27] J. Rimmelin. P. Lemoine, M. Gross, A. A. Bashoun, and J. A. Osborn, Nouv. J. Chim., 1985, 9, 181. [28] R. Jund, J. Rimmelin. and M. Gross, J. Organomet. Chem., 1990, 381, 239. 1291 D. J. Darensbourg, D. J. Zalewski, and T. Delord, Organometallics, 1984, 3, 1210. [30] G. F. Holland, D. E. Ellis, D. R. Tyler, and W. C. Trogler, J. Am. Chem. Soc., 1986, 108, 1884. [31] B. F. Johnson. J. Lewis, P. R. Raithby, and C. Zuccaro. Acta Cryst., 1978, B34, 3765. [32] L. J. Farrugia, J. A. K. Howard, P. Mitrprachachon, F. G. A. Stone, and P. Woodward, J. Chem. Soc., Dalton Trans., 1981, 155. [33] B. F. G. Johnson, J. Lewis, W. J. H. Nelson, J. Puga, P. R. Raithby, M. Schroder, and K. H. Whitmire, J . Chem. SOC.,Chem. Commun., 1982, 610. [34] L. J. Farrugia, M. Green, D. R. Hankey, A. G. Orpen, and F. G. A. Stone, J. Chem. SOC., Dalton Trans.. 1985, 177. [35] L. J. Farrugia, J. A. K. Howard, P. Mitrprachachon, F. G. A. Stone, and P. Woodward, J. Chem. Soc., Dalton Trans., 1981, 162. [36] G. Gervasio, R. Rossetti, and P. L. Stanghellini, Organometallics, 1985, 4, 1612. [37] P. Brdunstein, J. Rose, and 0. Bars, J. Organomet. Chem., 1983, 252, C101. [38] B. F. G. Johnson, J. Lewis, B. E. Reichert, K. T. Schorpp, and G. M. Sheldrick, J. Chem. Soc., Dalton Trans., 1977, 1417. [39] D. Osella, M. Ravera. C. Nervi, C. Housecroft, P. Raithby, P. Zanello, and F. Laschi, Organometallics, 1991, 10, 3253. [40] P. Zanello and D. Osella, unpublished results. 1411 J. Wang, M. Sabat, L. J. Lyons, and D. F. Shriver, Inorg. Chem., 1991, 30, 382. [42] H. Vahrenkamp, E. J. Wucherer, and D. Wolters, Chem. Ber., 1983, 116, 1219. [43] J. S. Field, R. J. Haines, and D. N. Smit, J. Chem. SOC.,Dalton Trans., 1988, 1315.
1 136
3 Dynamics and Physical Properties
[44] J. T. Jaeger, J. S. Field, D. Collins, G . P. Speck, B. M. Peake, J. Hahnle, and H.Vahrenkamp, Organometallics, 1988, 7, 1753. [45] H. H. Host and J. K. Kochi, Organometallics, 1986, 5, 1359. [46] G. Longoni, M. Manassero, and M. Sansoni, J. Am. Chem. Soc., 1980, 102, 3242. [47] V. G. Albano, R. Aureli, M. C. Iapalucci, F. Laschi, G . Longoni, M. Monari, and P. Zanello, J. Chem. Soc., Chem. Commun., 1993, 1501. [48] P. Zanello and L. Garlaschelli, Unpublished results. [49] 0. Rossell, M. Seco, G. Segales, S. Alvarez, M. A. Pellinghelli, and A. Tiripicchio, Organometallics, 1994, 13, 2205. [50] 0. Rossell, M. Seco, G. Segales, R. Mathieu, and D. de Montauzon, J. Organomet. Chem., 1996, 509, 241. [51] R. Reina, 0. Rossell, M. Seco, D. de Mountazon, and R. Zquiak, Organometallics, 1994, 13, 4300. [52] F. Demartin, M. Manassero, M. Sansoni, L. Garlaschelli, S. Martinengo, and F. Canziani, J. Chem. SOC.,Chem. Commun., 1980,903 [53] A. Ceriotti, R. Della Pergola, L. Garlaschelli, F. Laschi, M. Manassero, N. Masciocchi, M. Sansoni, and P. Zanello, Inorg. Chem., 1991, 30, 3349. [54] A. Cinquantini, P. Zanello, R. Della Pergola, L. Garlaschelli, and S. Martinengo, J. Organomet. Chem., 1991, 412, 215. [55] R. Della Pergola, C. Bandini, F. Demartin, E. Diana, L. Garlaschelli, P. L. Stanghellini, and P. Zanello, J. Chem. SOC.,Dalton Trans., 1996, 747. [56] R. Della Pergola, A. Cinquantini, E. Diana, L. Garlaschelli, F. Laschi, P. Luzzini, M. Manassero, A. Repossi, M. Sansoni, P. L. Stanghellini, and P. Zanello, Inorg. Chem., 1997, 36, 3761. [57] G. Longoni, M. Manassero, and M. Sansoni, J. Am. Chem. Soc., 1980, 102, 7973. [58] R. D. Adams, M. S. Alexander, I. Arafa, and W. Wu, Inorg. Chem., 1991, 30, 4717. [59] R. Della Pergola, L. Garlaschelli, C. Mealli, D. M. Proserpio, and P. Zanello, J.Cluster Sci., 1990, 1, 93. [60] R. J. Goudsmit, J. G. Jeffrey, B. F. G . Johnson, J. Lewis, R. C. S. McQueen, A. J. Sanders, and J. -C.Liu, J. Chem. Soc., Chem. Commun., 1986, 24. [61] G. Ciani, G. D’Alfonso, M. Freni, P. Romiti, and A. Sironi, J. Chem. Soc., Chem. Commun., 1982, 705. [62] T. J. Henly, J. R. Shapley, A. L. Rheingold, and J. S. Gelb, Organometallics, 1988, 7, 441. [63] S. W. Simerly and J. R. Shapley, Inorg. Chem., 1990, 29, 3634. [64] C.-M. T. Hayward and J. R. Shapley, Organometallics, 1998, 7, 448.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.9 Electron-sink Features of Homoleptic Transition-metal Carbonyl Clusters Giuliano Longoni, Cristina Femoni, Maria Carmela Iupulucci, and Piero Zanello
3.9.1 Introduction Transition metal clusters (TMC) with several stable oxidation states are of great potential interest. Beyond other considerations, analysis of the structural reorganization brought about by a change in the oxidation state of the cluster has provided the opportunity to test bonding theories with experiments. The effectiveness of this approach, named experimental quantomechanic after Dahl, in gaining a satisfactory bonding picture of the cluster has been elegantly demonstrated several times.['I Furthermore, multivalent TMC provide the opportunity of assembling several new kind of material. For instance, recent yet unpublished results in our laboratory point out that is possible to assemble ionic solids in which both the anion and the cation are TMC entities. So far we have succeeded in building such an ionic lattice with an anion which features different oxidation states and a cation which does not [ ~ ] the [Fe5(p3-S)2(C0)14l2(e.g. [Aug(p3-S)2(PPh3)g]'+[Fe5(p3-S)2(C 0 ) 1 4 ] ~ - ) .Indeed dianion can be chemically and electrochemically reversibly oxidized to the corresponding mono-anion, which turned out to be sufficiently long-lived to enable A cation with similar redox propensity and suitspectroscopic ~haracterization.~~] able tuning of the formal redox potentials of the two could, in principle, enable the assembly of less conventional cluster salts featuring charge transfer and mixed valence phenomena. Eventually the resulting material could show electric and magnetic behavior worthy of investigation. As a further hint, a TMC capable of withstanding several redox changes and containing potential donor E heteroatoms .( y. the sulfur atom of the above example) could be polymerized either by formation of E-E bonds or through the intervention of suitable organic, organometallic or inorganic spacer groups. Some relevant examples of such oligomers or co-polymers have already been r e p ~ r t e d . [ ~Once . ~ ] again, the materials envisioned above could show interesting behavior.
1138
3 Dynumics und Phj~siculProperties
Table 1. Some important examples of homometallic TMC with electron-sponge behavior.[5 91 Range of values of the formal average oxidation state of the metalb
Compound
3/ - 4 7/ - 8 +4/ f 3/ + 2 / 1/0 +3/ 2/ 1/0 -11 - 2/ 3/ 4 1-31 2/ 1/01 - 1 -21 - 3/ 4 +2/ 1/0 +2/ f 1/0/ 1/ - 2 -3/ - 4/ - 5 0/-11-2 +2/ 1/0/ - 1 s1/0/ - 1 +2/ 1/0/ 1 +1/0 -2/
-
-61
-
+
+ + + + + ~
~
~
~
+ +
~
$4 + +3.33 +3.5 + f 3 f3.33 i +2.61 f3.17 + +2.67 f2.75 + +2 +2.75 i +1.75 +2.67 + +2.33 +2.5 + +2 +2.22 + +1.78 +2.12 + +1.87 $2 + +1.5 +2 + + I +1.67 i +l +0.5 ---t -0.25 +0.17 + 0
Figures in bold indicate the species isolated in the solid state. To evaluate the formal average oxidation state of Cp-coordinated metals we have arbitrarily considered the latter as a cyclopentadienyl radical; the alternative consideration as the cyclopentadienyl anion would increase all relevant values by one unit. a
There is widespread belief that TMC behave as electron sponges. This opinion is legitimized by the chemical and electrochemical redox properties of several categories of homo- and heterometallic clusters, almost irrespective of the particular metal(s) involved, their formal oxidation states, and ancillary ligands. Because most of the pertinent literature has been thoroughly reviewed,[6 91 comprehensive reviewing is not the purpose of this contribution. We have, therefore, collected in Table 1 only some of the most relevant examples of the miscellaneous homometallic clusters, which withstand electrochemically reversible redox changes. This should be sufficient to establish the full extent of the phenomenon. Firstly, it is important to notice that the multivalence of the clusters collected in Table 1 encompasses at least three electrochemically reversible or quasi-reversible redox states; occasionally, however, it can also extend to five oxidation states. Moreover, in particular instances a given cluster has been isolated and structurally characterized by X-ray diffraction in different oxidation states. If, however, attention is restricted to homoleptic transition metal carbonyl clusters (HTMCC), the observation of related behavior is rather rare. A comprehensive and critical review of all relevant literature up to 1993 led one of us to refute the widespread opinion that HTMCC could, because of their redox activity, also be
3.9 Electron-sink Feuture.7 qf Hotnolqtic Transition-mrtul Curhotzyl Clusters
1 1 39
considered as electron-sponge~.[~'~~~'~ Since then more examples of HTMCC with redox activity have been reported and a few others have been discovered in our laboratories but have not yet been reported. We will analyze here mainly those latter multivalent clusters, with the aim of gaining a better understanding of the factors which might trigger electron-sink behavior in HTMCC. Before entering into detail, however, it is necessary to remark that in subsequent sections and tables we have decided not to bother too much with the distinction between electrochemically reversible or quasi-reversible p r o c e s ~ e s . [ ~Departure (~)~ from canonical electrochemical reversibility often arises from relatively minor structural changes, such as small distortions of the original symmetry, or small variations in bond lengths, or modification of the carbonyl stereo~hemistry.[~~ These changes often provide a better insight into the electronic structure of HTMCC and do not represent a limitation of potential applications, such as those foreseen at the beginning of this chapter. Therefore, rather than be completely rigorous, we have taken the arbitrary decision also to consider as electrochemically reversible those electrode processes with cyclic voltammetric profiles with peak-to-peak separations which depart somewhat from the theoretical value; we will, moreover, take into account only those oxidation states which seem to have a half-life sufficiently long at least to enable spectroscopic characterization of the chemically or electrochemically generated species.
3.9.2 Metal carbonyl clusters featuring only two chemically and electrochemically reversible oxidation states As shown by some selected relevant examples in Table 2 (in this and subsequent tables main-group elements, whether interstitial or peripheral, are considered as part of the metal framework rather than as ligands), HTMCC at best seem capable of existing in two stable oxidation states. In very few cases only have both components of the redox couple been isolated in the solid state and structurally characredox couple."'] The terized. One such example is the [F~~(CO)I~]'-/[F~~(CO)I~]two tri-iron carbonyl anions differ only slightly in their carbonyl stereochemi ~ t r i e s . [ ~It~is, , ~however, ~] worth mentioning that [ F ~ ~ ( C O ) Ihas ~ ] a- half-life of ca 24 h. Although it could be readily obtained by chemical oxidation of the parent dianion, it has been isolated in the solid state as a byproduct of the reaction of Fe3(C0)12 with F- ions.[241The [M3(C0)11I2- ( M = Ru, 0 s ) congeners, in contrast, undergo only irreversible one-electron oxidation^.[^^,^^] Although stability ]seems awkward and in contrast with trend Fe > Ru 0 s for [ M ~ ( C O ) I I species the trend of M-M bond energies, this might be only an apparent discrepancy, because an increasing M-M bond energy might concomitantly favor condensation processes in second and third row HTMCC.
-
1140
3 Dynamics and Physical Properties
Table 2. Selected examples of HTMCC with one redox change with chemical and electrochemical reversibility (17''in V, relative to the SCE)." Solvent
Compound
I '-
[ h ( C O )I I FeCo2(p3-S)(CO)~ RuCoz(,wSe)(C0)9 FeCo2(,u3-Se)(CO)~ [Co4Sbz(CO)I I [ C O ~ B ~ ~ (11CO)I [F~s(,wN)(CO)I~I[Fes(,wS)dCO)141I [Irs(C0)1sI2[FeIr5(C0)15I3[c09(P*-si)(C0)2112[H ~ O So(ICO)24]2-
I
THF DCEb DCEb DCEb CHIC12 CH2Clz MeCN CH2C12 Me2CO MeCN CH2C12 CH2C12
Reference
En'
+1/0
01-1
-
-11-2
-21-3
-
-0.76
-
f0.44 +O.l
-
-
-
-0.73
-
-
+1.3
-0.58
-
-
-
-
-0.58
-
-
-
-
-
-0.65 -0.90
-
-0.18
-
-
+0.26 +0.50
+0.12
-
-
-
-
-
-
-
-0.57 -0.47
+0.57
$0.35
-
-
-
10 11,12 11 11,13 14,15 15,16 17 3 18 19 20,2 1 22
The redox potential values in bold individuate the two oxidation states which are both chemically and electrochemically reversible and sufficiently long-lived to be isolated in the solid state or spectroscopically characterized in solution. The remaining potential values indicate another oxidation state which might be electrochemically reversible at the higher scan rates, but not sufficiently long-lived to be spectroscopically characterized. Dichloroethane. 'The compound also has a bi-electronic [Co~(,u,-Si)(C0)~1]~--/5-Tedox change.
a
The only other homometallic HTMCC appearing in Table 2 are [Irg(CO)1j]2-[181 and [ H ~ O S ~ ~ ( C O ) * We ~ ] ~delay - . [ ~ ~discussion ] of the latter until the next section, and will examine here only the redox behavior of the former. In contrast with its lighter [Mg(C0)1jI2- (M = Co, Rh) congener^,[^^,^(")^ the first oxidation of [Iq(CO)1 j]*- is chemically and electrochemically reversible and the electrogenerated paramagnetic [Ir6(CO) 151- anion has been characterized spectroscopically.['sl The observed stability trend of the [M6(CO)lj]-congener (Ir >> Co >> Rh) seems more in keeping with the trend of M-M bond energy, and the ease of condensation need be invoked only to account for the inversion between cobalt and rhodium. Not surprisingly, the isoelectronic [FeIrj(CO)1513- trianion undergoes a reversible one-electron oxidation and the electro-generated dianion is sufficiently long-lived to enable EPR characterization." 91 The remaining entries in Table 2 are HTMCC containing either peripheral or interstitial main-group elements. The most notable system featuring two stable oxidation states is probably the pair of compounds [ C O ~ S ~ ~ ( C Oand )~I][Co4Sb2(CO)lI]*-. They have been shown by X-ray diffraction to be isostructural, the main difference being a localized elongation of one Co-Co bond by 0.14 A on
3.9 Electron-sink Features of Homoleptic Transition-metal Curhonyl Clusters
1 141
going from the mono- to the di-anion. EHMO calculations successfully revealed the antibonding nature of the LUMO of the monoanion and the significant orbital contribution from the two Co atom^.^'^.^^] The [Co9Si(C0)21]~dianion is similarly noteworthy owing to its odd number of electrons and redox properties. Not only is it characterized by a chemically and electrochemically reversible one-electron reduction to the stable diamagnetic [Co9Si(C0)~1]~trianion, but also by a subsequent electrochemically reversible twoelectron reduction, which affords a transient [ C O ~ S ~ ( C O ) penta-anion.r20.211 Z~]~The structurally related [RhgP(C0)21]~-[~’’ and [NigC(C0)17]2-[281 dianions are isoelectronic with [CoqSi(C0)21]~-. As far as we are aware the rhodium congener has not yet been investigated electrochemically; [ Ni9C(C0)17I2- undergoes a quasireversible one-electron reduction and an irreversible o x id a ti~ n .[ ~ It (‘)~ seems probable that the 65th cluster valence orbital of these HTMCC is bonding in character and its partial depletion is only favorable for the [CogSi(C0)21]*-dianion, because this might ease the steric demands of the relatively bulky silicon atom in the relatively small square-antiprismatic cage afforded by a first row metal cluster. This interpretation is reminiscent of the stable paramagnetic [Co6C(CO)141- monoanion, in which partial occupation of the 44th antibonding CVO has been suggested to trigger a related synergistic effect.[291
3.9.3 HTMCC featuring three chemically and electrochemically reversible oxidation states Rather few HTMCC are capable of existing in three different chemically and electrochemically reversible oxidation states, each being indefinitely stable or sufficiently long-lived to enable at least spectroscopic characterization in solution or even isolation in the crystalline state. These are collected in Table 3 with the formal potentials of the relevant redox couples. It should be noted that only one homometallic HTMCC is present; two entries are bimetallic HTMCC and most are HTMCC containing either peripheral or interstitial main-group elements. An example of an HTMCC containing alkyl-substituted tin atoms has also been included only for sake of comparison. We will examine these compounds in more detail with the aim of identifying the factors responsible for such expanded electron-reservoir behavior. The first two entries are examples of HTMCC with a planar metal framework. In the [AuFe4(CO)16]”- (11 = 1 ,2 ,3 )system only the parent monoanion has so far been investigated by X-ray diffraction (Fig. la) whereas the corresponding di- and trianions have only been characterized ~ p e c t r o s c o p i c a l l y .The ~ ~ ~ [AuFe4(CO)lh]~ anion can be considered as a square-planar ds Au’+ complex stabilized by two bidentate [Fe2(CO)g]’- ligands, isolobal with diphosphine. Crystal-field theory
'-
Solvent
~
-
-
+0.39
~
-
~
-
-0.93
-0.73 -0.40 +0.74
-0.37
-1.28
~
~
-0.56
~
-1.42 - 1.42
~
~
~
-31-4
~
-
1.39 -0.65 -0.67 -1.18
-
-21-3
-11-2
-0.25 0.00
~
~
f0.19
~
01-1
E"' -5
-0.65
-
-
~
~
-
~
~
~
-41
39 40
31,32 33,34 35 36 37 38
30
Reference
"The redox potential values indicate the three oxidation states which are sufficiently long-lived to be isolated in the solid state or spectroscopically characterized in solution.
MeCN CHzClz CHzClz [o~l"C(Coj2412[Nil I ( P ~ - S ~ ) ~ ( C O ) I ~ ] ~ - MeCN [Nil 1(P6-Bi)?(C0)18I3MeCN [Nil1(Pu,-SnBu)r(C0)18]~THF [Ir14(C0)271CH2C12 MeCN [ Ni I 3 ( ~ -Sb)r 7 (CO)24 I * [Agl3FedC0)32I4MeCN
[A~F~~(CO)I~]Fe3PtdCO)151
Compound
Table 3. HTMCC with two redox changes with chemical and electrochemical reversibility (P' in V, relative to the SCE)."
8
2.
s2
2
5
3.
& 2
7 2
2
s.
3
b
bJ
N
P
-
3.9 Electron-sink Features
of'Homolqtic Transition-nzetal
Figure 1. The structure of [AuFed(CO),(,](a), and a schematic diagram of its L U M O (b; the carbonyl contribution has been omitted for clarity).
Curhonyl Clusters
1
I
i
i
1 143
predicts the presence of an empty higher-lying d+, 2 gold orbital whose progressive population should give rise to the corresponding Au'+ and Au+ complexes, that is the [AuFe4(CO)ls]'- and [AuFe4(C0)16I3- anions, respectively. In partial agreement with this description, EHMO calculations show that the LUMO of [AuFe4(CO)ls]-(Fig. 1b) is ca 0.87 eV above the HOMO and cu 0.56 eV below the second LUMO, and receives a significant contribution from the dyl_!2 gold orbital and is antibonding in character with regard to the Au-Fe interactions. Electrochemically the two reductive processes are only quasi-reversible, having a peak-topeak separation of cu 110 mV. That probably results from the observed change in the carbonyl stereochemistry and the purported departure of gold from square-planar coordination. Indeed, the IR spectrum of the chemically generated [AuFe4(CO)ls]'- shows the presence of bridging carbonyl groups and a pattern identical with that of [CdFe4(C0)16l2-. The latter has been shown by X-ray studies to contain two Fe2(CO)&L-C0)2 moieties.[411
1144
3 Dynamics and Physical Properties
The redox behavior of [AuFe4(CO)161- is unique. Although the isoelectronic dianions can also be and almost isostructural [MFe4(C0)16]*- (M = Pd and Pt)[421 considered as square-planar d8 Pd2+ and Pt2+ complexes, stabilized by bidentate [Fez(CO)g]*- ligands, and their EHMO diagram has features similar to that of [AuFe4(CO)l6]-, they do not have analogous redox proper tie^["^" their chemical oxidation results in condensation to higher nuclearity clusters and their chemical reduction gives rise to fragmentation products. Evidently, the presence of a lowlying LUMO might be a necessary condition for this redox behavior, but it is not sufficient. The best characterized HTMCC system with three stable oxidation states is These three compounds are isostructural and [Fe3Pt3(C0)15In- ( n = 0,1, 2).[43,311 have the overall common geometry depicted in Fig. 2a. The major difference among the three lies in the average Pt-Pt bonding separation, which progressively shortens by ca 0.08A on going from the dianion to the neutral derivative. Although such shortening could result from the reduced free negative charge, it has been taken as more indicative of an antibonding character of the SOMO of [Fe3Pt3(CO)15]-in view of the constancy of all other bonding contacts.[431EPR measurements and electronic spectra of the paramagnetic [Fe3Pt3(C0)15Ipmonoanion show a significant contribution of the Pt d orbitals to the SOM0.[441These conclusions were elegantly validated by subsequent EHMO calculations which revealed that the HOMO of [Fe3Pt3(C0)15l2- (see Fig. 2a) receives a significant contribution from the d , 2 ~ ~Pt 2 orbitals and is, respectively, antibonding and non-bonding in character in respect of the Pt-Pt and Pt-Fe interactions.[311Once more it lies ca 1.25 eV below the LUMO and 0.35 eV above the second HOMO. In the framework of the isolobal analogy,[451a Czv p2-Fe(C0)4group is isolobal with an edge-bridging CO. Therefore, the [Fe3Pt3(CO)15I2-dianion can be regarded as a stabilized counterpart of the very reactive [Pt3(Co)6]*- dianion, which has recently been structurally characterized by liquid X-ray diffraction and multinuclear NMR[461and shown to have the expected [Pt3(CO)3(p-CO)3l2-triangular building block (Fig. 3a) of the Chini [Pt3(CO)3(p-CO)3]n2-( n = 2,3,4,5,10) cluster^.[^^,^^^ The composition of the HOMO of [Pt3(Co)6]*- is, however, completely different from that of [Fe3Pt3(C0)15]2-;[49.501 with regard to the metals, it contains a cyclopropenyl-like bonding combination of pz Pt orbitals (Fig. 3b). As originally sugge~ted,[~'] it seems reasonable that oxidation to [Pt,(CO),]- could be followed by a fast dimerization to [Pt6(C0)12]*-, owing to the absence of steric hindrance to the coupling of such planar moieties along their C3 axis. In contrast, related dimerization of the [Fe,Pt3(C0)1~]-counterpart would be hindered by the presence of axial ligands and could only occur via elimination of a Fe(C0)4 group, as suggested by the SOMO of the resulting [FezPt3(CO)11]-moiety (Fig. 4), and experimentally illustrated by the structure of [Fe4Pt6(C0)22l2-.I4'] The [Fe3Cu3(C0)1zl3- trianionL5'I is isoelectronic with [Fe3Pt3(CO)l~],and has an identical metal framework. EHMO calculations indicate, however, a wide HOMO-LUMO gap of ca 3 eV; the composition of its HOMO is, furthermore, -
3.9 Electron-sink Feutures of Homoleptic Trunsition-metal Curhonyl Clusters
Figure 2. The structure of [FeiPti(CO)~s]*-(d), and d schematic diagram of its HOMO (b, the carbonyl contribution is omitted for cldrity)
1 145
/ (b)
related to that of the LUMO of [Fe3Pt3(CO)15]. In the absence of experimental evidence, it seems unlikely that it could have redox properties similar to those of [ Fei pt,( CO)I 51. A series of carbonyl-substituted osmium clusters of general formula Osg(CO)21- \ L, ( L = P(OMe)3, x = 1-6; L = MeCN, x = 1-3)[52,531 is also structurally related to [FeiPt3(CO)151 and features six extra cluster-valence electrons. As predicted theo r e t i ~ a l l y , [ these ~ ~ I raft-like osmium clusters have a similar capacity to add two more electrons reversibly. The electro-generated anions are stable on the cyclovoltammetry time-scale. An ud lzoc situation is also at the heart of the [Ag13Fe8(CO)3,lf'- ( n = 3 , 4 , 5 ) system. The two derivatives with n = 3 and 4 have been shown to be isostructural
1146
3 Dynamics and Physical Properties
Figure 3. The structure of the [Pt3(CO)6l2-dianion (a), and a schematic diagram of its HOMO (b).
Figure 4. The SOMO of the [Fe*Pt3(CO),11- moiety.
3.9 Electron-sink Features of Homoleptic Transition-metal Carbonyl Clusters
Figure 5. The structure of [Ag13Fe~(CO)~z]'(a), and a scheinatic diagram of its LUMO (b, the contribution of the carbonyl is omitted for clarity).
1 141
(b)
by X-ray diffraction studies. The major difference between the two is a slight elongation of the Ag-Fe bond separation on going from the tri- to the t e t r a - a n i ~ n . ' ~ ~ , ~ ~ ] The structure of the [Ag13Feg(CO)32]3-trianion, which is readily obtained by reaction of [Fe(CO),I2- with AgN03 or AgLNO3 (L = PPh3 or dppe), is shown in Fig. 5a. The redox behavior of this cluster is consistent with the nature of its LUMO (Fig. 5b), which lies CN 0.48 eV above the HOMO and 1.59 eV below the second LUMO. It is essentially non-bonding in character and is delocalized over the whole molecule. It can, therefore, progressively accept two additional electrons without dramatic loss of stability of the resulting tetra- and penta-anion. The results of EHMOr551 and linear combination of Gaussian-type orbital, LCGTO, local density functional, LDF,[561calculations are confirmed by the EPR spectrum of the paramagnetic [Agl3Fez(C0)32l4- tetra-anion. The unpaired electron is strongly coupled
3 Dynamics and Physical Properties
1148
with the interstitial silver atom and only weakly coupled with the peripheral Ag atoms. By comparison with the coupling constant of isolated silver atoms in a noble-gas matrix,I571 the coupling constant of the unpaired electron with the interstitial silver atom of [Agl3Fes(C0)32l4- can be extrapolated to arise from a contribution of ca 25% from the interstitial Ag 5s orbital to the SOMO. Entirely consistent with this, EHMO calculations show that the SOMO indeed receives its greatest contribution (24%) from the 5s orbital of the unique interstitial silver atom. Although the numeric result is certainly too good to be taken too seriously, the agreement between experimental data and the theoretical overall picture nevertheless supports the conclusions drawn in cases not supported by related experimental evidence. The only homometallic HTMCC with three stable oxidation states is the paramagnetic [Ir14(CO)27]- anion.[381This tetradecanuclear compound can be reversibly reduced and oxidized to the corresponding dianion and neutral derivative, respectively; both are sufficiently long-lived to enable their spectroscopic characterization. As shown schematically in Fig. 6, the central Ir12 core of [Ir14(CO)27]- is identical with that of the [H4-nNi12(C0)21]n-anion ( n = 2-4);[”] the Ir14 v2 trigonal bipyramid, on the other hand, might be derived from fusion of two v2 tetrahedra, as is and [ H ~ O S I O ( C O ) ~ ~ ] ~ - [ ~ ~ ] observed for the [MloC(CO)24l2- (M = Ru, Os)i593601 dianions. It might be significant that the latter v2 tetrahedral compounds have chemically and electrochemically reversible redox properties, whereas both [H4-nNi12(C0)21]n-( n = 2-4) and [H4-nNi9Pt3(C0)21]n- ( n = 2-4)[621 do not. It has recently been suggested[631that the p,-Ru(C0)3 caps in the electroactive [H2-nR~1~C(C0)24]n( n = 1,2) might be responsible for the redox activity. The remaining compounds collected in Table 3 share several features and will be discussed together. All these compounds are based on Nil0E2 icosahedral cores, centered on an additional Ni atom and having the heteroatoms E in trans positions, as shown in Fig. 7a by the structure of the [NillBi2(CO)lsl3- paramagnetic trianion.[361The examples reported so far include species with E = Sb,[351Bi,[361
(4
(b)
(4
Figure 6. Relationships between the metal frameworks of [Ir14(C0)271-(a), [Nil2(C0)21I4(b), and [ O S I O C ( C O ) ~(c; ~ ]the ~carbide atom is not shown and occupies the octahedral cavity).
3.9 Electron-sink Features of Hornoleptic Transition-metal Carhonyl Clusters
1 149
Figure 7. The structures of
Se,[641Te [641 Sn-R,[371and Sb + Ni(C0)1.[391Several other closely-related icosahedral nidkel clusters have been isolated and studied by electrochemistry in the last few years. These include examples of an Nil2 icosahedron centered on a heteroatom, e.y. [Nilz(p12-E)(C0)22]'- (E = Ge, Sn)[651or [ N i l ~ ( p ~ ~ - S b ) ( C O ) * 4or ] ~ -non,[~~] centered icosahedral Ni12-vEv (x = 2,3,4) cores as in the [Nilo(pS-E-R)2(CO)~sl2(E = P, As, Sb, Bi; R = alkyl or aryl substituent) All the E-centered and non-centered icosahedral clusters are 8-10 electrons short of the total for the Ni-centered ones. A closer examination of these compounds gives some clue about the factors which might trigger redox activity in HTMCC. Thus, the non-centered [Nil0(ps-E-R)2. (C0)18l2-- (E = P, As, Sb, Bi; R = alkyl or aryl substituent) and E-centered Sn) icosahedral dianions do not have a rever[ N i 1 2 ( p ~ ~ - E ) ( C 0 ) ~(E2 ]=~ Ge, sible chemical or electrochemical redox The recently isolated [ N i l ~ ( p , ~ - S b ) ( C O )dianion, ~ ~ l ~ - which contains an interstitial pI2-Sb atom and a semi-interstitial pu,-Niatom, does, however, display some interesting redox activity with three reversible reduction steps. These electro-generated species were only transient and could not be spectroscopically characterized. In contrast, most Nicentered icosahedral clusters feature electrochemically reversible redox properties and [Nil1E2(C0)18ln- (E = Sb, Bi) and [Ni13Sb2(C0)24In-have been isolated in the solid state in three different oxidation states ( n = 2,3,4) which are perfectly stable under inert conditions. Both the diamagnetic [Ni11Sb2(C0)24I2- dianion and the paramagnetic [Nil3Sb?(C0)24]3- trianion have been crystallographically characterized (Fig. 7b).[391Comparison of the intermolecular contacts in these two isostructural compounds only reveals very slight elongation of most bonding separations in the trianion and is not really diagnostic of the nature of the SOMO - indeed,
1150
3 Dynamics and Physical Properties
such slight swelling might be explained by the increased free negative charge of the anion. From a topological comparison it seems reasonable to ascribe the development of redox properties in this series of icosahedral nickel clusters to the presence of an interstitial nickel atom. This conclusion, and the exceptional electron count of the Ni-centered clusters, has been confirmed by EHMO calculations on model comp o u n d ~ ; [the ~ ~results ] can be summarized as follows. The atomic d orbitals of a late transition metal such as nickel have suitable symmetry and energy to interact weakly with the set of five molecular orbitals of the Nil0E2 cage. As a result, five slightly bonding molecular orbitals are obtained for the Nil I E2 moiety; their antibonding counterparts are, moreover, not sufficiently destabilized to be unavailable for either partial or complete electron occupancy. A related situation occurs in the isolated [Nil5(p,,-Sb)(CO)24l2- dianion described earlier, owing to the presence of a nine-coordinated nickel atom. It should be kept in mind that the well consolidated rule claiming that interstitial atoms do not affect the electron count of clusters is based on the assumption that their atomic orbitals interact strongly with those of the cage so that the resulting in-phase combinations significantly enhance the stability of the metal core, and the out-of-phase combinations are sufficiently destabilized to fall in the antibonding region.r7n1
3.9.4 HTMCC displaying electron-sponge features The HTMCC that might be considered to behave as electron-sponges are collected in Table 4. The nuclearity of their metal core ranges from 8 to 44 metal atoms. The experimental reasons for differentiating between the HTMCC in Table 4 and those listed in Tables 2 and 3 is that their cyclovoltammetric profiles have at least three chemically and electrochemically reversible redox changes. We have arbitrarily taken this as a threshold, because it indicates that more than one molecular orbital can now be depleted of, or populated with, electrons. The entries in Table 4 include examples in which three or four orbitals are involved in the reversible fillingunfilling processes. Therefore, although ad hoc situations could again play a role for a particular example, it becomes increasingly likely that such redox properties begin to arise from the progressive disappearance of a well defined HOMO-LUMO gap, the limiting situation being a quasi-continuum of energy levels between the bonding and antibonding regions of the MO diagram. It is, for example, still possible that an ad hoe situation could be responsible for the redox behavior of [Co&(C0)18]~-and [Col3C2(CO)24ln- ( n = 3,4). The metal framework of the former can be loosely described as a bi-capped trigonal prism centered by the unique carbide atom.[781The structure of the [Co13C2(C0)24lnanions is shown in Fig. 8,r79,8n1 and is formally derived from the fusion of two
~1/-2
E"' -21-3
-31-4
-0.62 -0.60
~
-
~
~
-1.18 -1.06 -1.08 -1.00
-41-5
-0.45 -0.49 -0.60 -0.62 -0.96 -0.93
-
-1.68 -1.20
~
-51-6
-0.17 -0.98 -1.11 -0.91 -1.28 -1.25
-1.98
-61-7
-1.06 -1.33 -1.42 - 1.29 -1.62 -1.58
-
-71-8
~
-1.33 -1.73 -1.11 -1.54
~
-81-9
-
-1.75
~
~
-1.60
-
-91-10
12 73, 9(c) 73, 9(c) 14,15 16 16 16 11 11 77
11
Reference
A
Dichloroethane as solvent. hAcetone as solvent. ' MeCN as solvent. CHzC12 as solvent; the quoted formal redox potentials are recalculated with reference to SCE from the originally published values, which were referred to the AgCl/Ag electrode. Dimethylformamide (DMF) as solvent.
Compound
Table 4. HTMCC with electron-sponge features (only redox changes with chemical and electrochemical reversibility have been taken into account; E"' in V, relative to the SCE).
1152
3 Dynamics and Physical Properties n
n
d
v
Figure 8. The structure of [Col3C2(CO)24]3- the hatched atoms depict one of the two Co8C moieties.
bi-capped trigonal-prismatic C08C moieties. The bi-capped trigonal-prism can be transformed into a square antiprism, through a tetragonal antiprism, via stretching of the unique prism edge shared by the two caps. The square antiprismatic geomeIt seems try requires two additional skeletal electron pairs (e.g. [Ni8C(C0)16]2p1281). possible, therefore, that the property of both [co8c(co)18]2-and [ C O I ~ C ~ ( C O ) ~ ~ ] ~ ~ to accommodate extra electrons reversibly could result from the ability to distort their metal frameworks. As soon as the number of M-M bonding contacts exceeds the number of M-CO bonds (essentially because of the presence of an increasing number of interstitial or quasi-interstitial metal atoms), the cyclovoltammetric profiles of the HTMCC so far investigated become much more complex and their analysis is often not straightforward. In Table 4 we have collected only the high nuclearity HTMCC which have several redox changes with unambiguous features of chemical and electrochemical reversibility. The entries in Table 4 show that electron-sponge properties are a common feature of the behavior of the higher nuclearity HTMCC. For instance, both [Pt19(C0)22I4-and [Pt24(C0),,l2- contain interstitial platinum and have very rich electrochemistry.[73751It should be noted that they undergo a series of redox changes much more extensive than those indicated in Table 4. These are clearly collected in pairs, being alternatively separated by AE"' of ca 100-300 and 500-900 mV, respectively. This is indicative of an energy separation between consecutive molecular orbitals that is greater than the electron-pairing energy in a given orbital. The related [Pt26(C0)32l2- and [Pt38(C0)44]2panions, which contain three and six interstitial platinum atoms, r e s p e c t i ~ e l y , [ ~have ~ , * ~an ]
3.9 Electron-sink Features of' Homoleptic Transition-metal Cuvhonyl Clusters
1 153
Figure 9. The metal core of [HNi38Pth(C0)~8]'-(the six twelvecoordinated platinum atoms are depicted in black).
even richer and more complicated electro~hemistry[~("'.~~~~~~ which warrants further investigation. Finally, the [Ni32C6(C0)36I6-,[76,831 [H6-,~Ni38C6(C0)42Inp( a = 5 , 6),[76,841and [H6-nNi3~Pt6(C0)48]np (n = 5,6) have a set of four or five redox changes which are electrochemically reversible; most of the implied oxidation states are, furthermore, sufficiently long-lived to enable infrared characterization. As shown in Figs. 9 and 10, respectively, the metal core of [H6-nNi38Pt6(C0)48]np ( n = 5 , 6 ) contains six twelve-coordinated platinum atoms whereas that of [Ni32C6. (co)36] 6p contains eight twelve-coordinated nickel atoms (nine-coordinated to other nickel and three-coordinated to carbide atoms). The structure of [H6p,lNi38C6. (CO)42]n- contains six thirteen-coordinated nickel atoms (ten-coordinated to nickel and three-coordinated to carbide) and two nickel atoms twelve-coordinated as above. Notably, the steady-state cyclovoltammetric profiles of the [HNi38Pt6. (CO)48]5- and [HNi38C6(C0)42]5 - mono-hydride penta-anions are significantly different from those of their parent hexa-anions, as found for the [HRu,&(C0)24]and [RuloC(C0)24Izp pair of HTMCC.[631Their consecutive redox couples are, furthermore, separated on average by ca 300 mV, which implies the absence of a well-defined HOMO-LUMO gap and an average energy separation of cu 0.3 eV between their monoelectronic energy levels. This experimental conclusion is in good agreement with previous theoretical results obtained by LCGTO-LDF calculations on [Ni32C6(C0)36In- and
1154
3 Dynamics and Physical Properties
Figure 10. The metals cores of [Ni32C6(C0)36l6-(a) and [HNi38C6(C0)42I5-(b). In both structures one nickel atom has been depicted in black to point out its particular coordination (see text).
[Ni44(CO)48]n- model compounds.[861Several other HTMCC contain bulk metal atoms in their cluster cores.[871Unfortunately, their redox activity have not yet been investigated.
3.9.5 Conclusions The comparative analysis performed in the previous sections draws attention to a set of factors which can serve as guidelines for the identification and synthesis of new HTMCC which could have redox activity and, eventually, lead to electron-
3.9 Electron-sink Feutures of Homoleptic Trunsition-nzetul Curhonyl Clusters
1 155
sink-like behavior. A few have been identified as ad hoe conditions. The electronic basis of redox activity seems to be the presence in the HOMO-LUMO gap of a low-lying highly-delocalized orbital, either non-bonding or slightly antibonding in character, although fulfillment of this condition is only beneficial and often not sufficient. A tightly protecting shell of carbonyl ligands also seems necessary to inhibit clustering upon oxidation, whereas inclusion of main-group elements in the metal cluster core can inhibit de-clustering processes upon reduction. The role of the M-M bond energy is more ambiguous. High M-M bond energy makes the cluster core more robust and, therefore, less prone to de-clustering. It can, however, concomitantly favor condensation reactions and hamper the observation of electrochemically reversible oxidation steps. Electron-sink behavior emerges in the absence of a well-defined HOMO-LUMO gap, which signals that a real increase in metallic character is in progress in the HTMCC. This gradual process is accomplished as soon as the number of M-M interactions overtakes the number of M-CO bonds and the ratio of the number of bulk to surface metal atoms in the metal cores of the HTMCC progressively increases. There is, however, also a need to meet other concomitant requirements such as maximization of M-M bond energy by including a sufficient number of metal atoms of the third transition period and/or the strengthening of the cluster core by partial filling of the interstices of its lattice or surface with suitable maingroup elements. In conclusion, it should be noted that the recently reported [Pd33Nig(CO)41. ( PPh3)h]4-[s81and H,?Pd?gPt13(CO)27(PMe3)(P P h 3 ) 1 2 [ ~carbonyl-substituted ~] bimetallic clusters contain bulk metal atoms (one and four, respectively) but do not have reversible redox behavior.
Acknowledgments We thank the University of Bologna and the HCM program of the EU for a grant. Drawings of the structures and molecular orbitals of HTMCC were produced with SCHAKAL88[901and respectively.
References [ I ] A. C. C. Kharas, L. F. Dahl, Adc. Chem. P/iys., 1988, 70, 1 [2] V. G. Albano. M. C. lapalucci, G. Longoni, M. Monari, S. Zacchini, Inory. Chirn. Acta. in press and ref. therein
1 156
3 Dynamics and Physical Properties
[3] F. Calderoni, F. Demartin, M. C. Iapalucci, F. Laschi, G. Longoni, P. Zanello, Inorg. Chem., 1996, 35, 898 [4] M. R. Jordan, P. S. White, C. K. Schauer, M. A. Mosley, J. Am. Chem. Soc., 1995, 117, 5403 [5] N. Prokopuk, D. F. Shriver, Inorg. Chem., 1997,36, 5609 [6] (a) W. E. Geiger, Prog. Inorg. Chem., 1985, 33, 275; (b) W.E. Geigcr, N. G. Connelly, Adv. Organomet. Chem., 1987,24, 87 [7] S. R. Drake, Polyhedron, 1990, 9, 455 [8] (a) P. Lemoine, Coord. Chem.Rev., 1982,47,55:(b) P. Lemoine, Coord. Chem..Rev., 1988,83,169 [9] (a) P. Zanello, Coord. Chem. Rev., 1988, 83, 199; (b) P. Zanello, Coord. Chem. Rev., 1988, 87, I ; (c) P. Zanello, in Stereochemistry of Organometallic and Inorganic Compounds, P. Zanello Ed., Elsevier, Amsterdam, 1994, 5, 163; (d) P. Zanello, Structure and Bonding, SpringerVerlag, Berlin, 1992, 79, 101 [lo] C. Amatore, J. N. Verpeaux, P. J. Krusic, Organometallics, 1988, 7, 2426 [ l l ] U. Honrath and H. Vahrenkamp, Z. Naturforsch., 1984, B39, 545 [12] B. M. Peake, P. H. Rieger, B. H. Robinson, J. Simpson, Inorg. Chem., 1981, 20, 2540 [13] C. E. Strouse, L. F. Dahl, J. Am. Chem. Soc., 1971, 93, 6032 [14] J. S. Leigh, K. H. Whitmire, K. A. Yee, T. A. Albright, J. Am. Chem. Soc., 1989, I l l , 2726 [I51 T. A. Albright, K. A. Yee, J.-Y. Saillard, S. Kahlal, J.-F. Halet, J. S. Leigh, K. H. Whitmire, Inorg. Chem., 1991, 30, 1179 [I61 S. Martinengo, G. Ciani, J. Chem. SOC. Chem. Commun., 1987, 1589 [I71 R. Hourihane, T. R. Spalding, G. Ferguson, T. Deeney, P. Zanello, J. Chem. Soc. Dalton Trans., 1993, 43 [ 181 A. Cinquantini, P. Zanello, R. Della Pergola, L. Garlaschelli, S. Martinengo, J. Organomet. Chem., 1991, 412, 215 [19] A. Ceriotti, R. Della Pergola, L. Garlaschelli, F. Laschi, M. Manassero, N. Masciocchi, M. Sansoni, P. Zanello, Inorg. Chem., 1991, 30, 3349 [20] K. M. MacKay, B. K. Nicholson, W. T. Robinson, A. W. Sims, J. Chem. SOC.Chem. Commun., 1984, 1276 [21] G. C. Barris, Ph. D. Thesis, University of Waikato (New Zealand), 1990 [22] S. R. Drake, B. F. G. Johnson, J. Lewis, R. C. S. McQueen, J. Chem. Soc. Dalton Trans., 1987, 1051 [23] F. Ragaini, G. L. Geoffroy, A. L. Rheingold, Orgunometallics, 1995, 14, 387 [24] F. Y. K. Lo, G. Longoni, P. Chini, L. Lower, L. F. Dahl, J. Am. Chem. Soc., 1980, 102, 7691 [25] A. A. Bhattacharyya, C. G. Nagel, S. G . Shore, Organometallics, 1983, 2, 1187 [26] A. J. Downard, B. H. Robinson, J. Simpson, A. M. Bond, J. Organomet. Chem., 1987, 320, 363 [27] J. L. Vidal, W. E. Walker, R. L. Pruett, R. C. Schoening, Inorg. Chem., 1979, 18, 129 [28] A. Ceriotti, G. Longoni, M. Manassero, M. Perego, M. Sansoni, Inorg. Chem., 1985, 24, 117 [29] V. G . Albano, P. Chini, G. Ciani, M. Sansoni, D. Strumolo, B. T. Heaton, S. Martinengo, J. Am. Chem. Soc., 1976, 98, 5027 [30] V. G. Albano, R. Aureli, M. C. Iapalucci, F. Laschi, G. Longoni, M. Monari, P. Zanello, J. Chem. Soc. Chem. Commun., 1993, 1501 [31] R. Della Pergola, L. Garlaschelli, C. Mealli, D. M. Proserpio, P. Zanello, J. Cluster Sci., 1990, I , 93 [32] R. D. Adams, I. Arafa, G. Chen, J-C. Lii, J-G. Wang, Organometullics, 1990, 9, 2350 [33] S. R. Drake, B. F. G. Johnson, J. Lewis, R. C. S. McQueen, J. Chem. Soc. Dalton Trans., 1987, 1051 [34] S. R. Drake, M. H. Barley, B. F. G. Johnson, J. Lewis, Organometallics, 1988, 7, 806 [35] C. Femoni, F. Fabrizi De Biani, F. Demartin, M. C. Iapalucci, G. Longoni, M. Monari, P. Zanello, unpublished results [36] V. G. Albano, F. Demartin, M. C. Iapalucci, G. Longoni, M. Monari, P. Zanello, J. Chem. Soc. Dalton Trans., 1992, 497
3.9 Electron-sink Features of Homoleptic Transition-metal Carhonyl Clusters
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[37] J. P. Zebrowski, R. K. Hayashi, L. F. Dahl, J. Am. Chem. Soc., 1993, 115, 1142 [38] R. Della Pergola, L. Garlaschelli, M. Manassero, N. Masciocchi, P. Zanello, Anyew. Chem. Int. Ed., 1993, 32, 1347 [39] V. G. Albano, F. Demartin, M. C. Iapalucci, F. Laschi, G. Longoni, A. Sironi, P. Zanello, J. Chem. Soc. Dalton Trans., 1991, 739 1401 V. G. Albano, F. Calderoni, M. C. Iapalucci, G. Longoni, M. Monari, P. Zanello, J. Cluster Sci., 1995, 6, 107 [41] F. Demartin, F. Fabrizi de Biani, C . Femoni, M. C . Iapalucci, G. Longoni, P. Zanello.
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[42] [43] [44] [45] [46] [47]
1 158
3 Djvzamics and Physical Properties
[73] P. Zanello, A. Ceriotti, L. Garlaschelli, unpublished results [74] G. J. Lewis, J. D. Roth, R. A. Montag, L. K. Safford, X. Gao, S-C. Chang, L. F. Dahl, M. J. Weaver, J. Am. Chem. Soc., 1990, 112, 2831 [75] J. D. Roth, G. J. Lewis, L. K. Safford, X. Jiang, L. F. Dahl, M. J. Weaver, J. Am. Chem. Soc., 1992, 114, 6159 [76] F. Calderoni, F. Demartin, F. Fabrizi de Biani, C. Femoni, M. C. Iapalucci, G. Longoni, P. Zanello, Eur. J. Inorg. Chem., 1999, 663 [77] F. Fabrizi de Biani, C. Femoni, M. C. Iapalucci, G. Longoni, P. Zanello, Inorg. Chem. in press [78] V. G. Albano, P. Chini, G. Ciani, S. Martinengo, M. Sansoni, J. Chem. Soc. Dalton Trans., 1978,463 [79] V. G. Albano, D. Braga, P. Chini, G. Ciani, S. Martinengo, J. Chem. Soc. Dalton Trans., 1982, 645 [SO] V. G. Albano, D. Braga, A. Fumagalli, S. Martinengo, J. Chem. Soc. Dalton Trans., 1985, 237 [81] D. M. Washecheck, E. J. Wucherer, L. F. Dahl, A. Ceriotti,G. Longoni, M. Manassero, M. Sansoni, P. Chini, J. Am. Chem. Soc., 1979, 101, 6110 [82] P. Chini, J. Organornet. Chem., 1980, 200, 37 [83] F. Calderoni, F. Demartin, M. C. Iapalucci, G. Longoni, Angew. Chem. h t . Ed. Engl., 1996, 35, 2225 [84] A. Ceriotti, A. Fait,G. Longoni, G. Piro. F. Demartin, M. Manassero, M. Sansoni, J. Am. Chem. Soc., 1986, 108, 8091 [85] A. Ceriotti, F. Demartin, G. Longoni, M. Manassero, M. Marchionna, G. Piva M. Sansoni, Angew. Chem. Int. Ed. Engl., 1985,24, 696 [86] N. Rosch, L. Ackermann, G. Pacchioni, J. Am. Chem. Soc., 1992, 114, 3549 [87] G. Longoni, M. C. Iapalucci, in Clusters and Col1oid.y: From Theory to Applications (Ed: G. Schmid) VCH, Weinheim, 1994,91. [88] M. Kawano, J. W. Bacon, C. F. Campana, L. F. Dahl, J. Am. Chem. Soc., 1996, 118, 7869 [89] J. M. Bemis, L. F. Dahl, J. Am. Chem. Soc., 1997,119,4545 [90] H. Keller, SCHAKAL88, Graphical Representation Of Molecular Models, University of Freiburg, F R G [91] C. Mealli, D. M. Proserpio, J. Chem. Ed., 1990, 67, 399
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.10 Modeling of Electrode Interactions with Metal Clusters Anna Ignaczak and Jose Alherto Nunes Ferreira Gomes
3.10.1 Introduction A detailed knowledge of the interactions between species near the electrode is of great importance for the understanding of processes occurring in the interfacial region. Electrochemically related phenomena such as specific adsorption, corrosion, electrocatalysis, etc., cannot be analyzed without detailed knowledge of the nature of forces determining these processes. Unfortunately, experimental techniques, although developing rapidly, are still unable to measure directly the interactions between species in the different phases. The values of water-metal and ion-metal interaction energies are very uncertain; for the latter even approximate experimental estimates are not available in the literature. This area has received special attention from theoreticians. The most commonly used methodology for the calculation of the particle(ion)-metal interaction is to approximate the metal surface with a cluster of several atoms with the crystallographic organization typical of the metal studied. Although such an approach has many limitations and introduces certain difficulties, cluster-model calculations are becoming more popular in studies of interfacial interactions. Section 3.10.2 gives a brief review of quantum studies related to adsorption on metal surfaces in the cluster model approximation. In Section 3.10.3 a more detailed analysis of some aspects of this methodology is described, and is related to some recently published work on the problem of specific adsorption phenomenon.
3.10.2 Historical background The metal cluster as a model of the electrode has been used in the theoretical investigations of specific adsorption phenomena for approximately twenty years.
1160
3 Dynamics and Physical Properties
In 1976 Leban et al.[ll used this approach to study the interaction of a platinum electrode with several particles, among them the water molecule and halide ions. The cluster used was taken to be a model of the Pt( 11 1) surface and contained only five platinum atoms. In this work the iterative extended Hiickel molecular orbital (IEHT) method was used. The stability of the adsorption of the water molecule and of the ions was tested by computing the charge transfer to the cluster and the total energy of the system for various positions of the adsorbate on the surface. A similar strategy was used in several other studies in which interactions of the were studied H20 molecule[2-61and of the halide ions, F-, C1-, Br- and 1-,[7-131 at the semi-empirical level. In some cases the metal-particle bond energy was evaluated. The authors of these investigations were usually satisfied by qualitative agreement with experimental results. From most EHT studies, the water molecule was found to be adsorbed with its oxygen end pointing towards the metal and located over the metal atom (top site), as is suggested by experimental data. Experimental results showed, however, that the molecular plane of the water was tilted by approximately 60-70" to the metal surfacer14]whereas in the semi-empirical studies it was assumed to be with its dipole moment perpendicular to the surface plane. the When the CNDO method was applied to studies of water-metal results obtained seemed very different. In studies by Kuznetsov et a[.['] on the adsorption of the H20 monomer by several metals, Cu, Ag, Au, Zn, Cd and Hg, in which the different crystallographic orientations of the surface for each metal were considered, surprisingly the hollow site was reported to be preferred. The stabilization energy in this work was reported to be extremely strong, from -66 kJmol-' for mercury up to -296 kJmol-' for gold. These values seem to be severe overestimates when compared with experimental estimates of the energy of approximately -40 to -60 kJmol-' for the non-dissociative adsorption of the water molecule on Other techniques, for example the atom superposition and electron delocalization (ASED) theory, the many-electron embedding theory, and the density functional theory (DFT), were also used to investigate the adsorption of water on metals.r16p211 The ab initio Hartree-Fock method was also used in studies of this type,[22p271 mostly in the last decade. Although the results of most of these investigations do predict that the water is adsorbed on the single top site, other adsorption sites are also proposed in the literature. Apart from the studies of Kuznetsov et U Z . [ ~ ] mentioned above, other studies also reported non-top positions as favored for the adsorption of water on metals. The top and bridge sites were found to be very close in terms of the interaction energy in the studies of Ba~schlicher,[~~] and the bridge site was proposed for the adsorption of water on the Fe(100) surface."'] In most of these studies the orientation of water was reported to be perpendicular to the surface, either as an assumption in the calculation or as a result of tests made on it. Only in few cases is the tilted conformation reported to be more stable; angles between the dipole moment and the normal to the surface vary from 90°[211and 6075°r5*183191 to only 25" in the studies of Yang et a1.[201 and of Kuznetsov et It
3.10 Modeling of Electrode Interactions with Metal Clusters
1 161
should be noted that the tilted conformation does not always refer to the top site but sometimes, as in the work of Kuznetsov, to the hollow site. An even more inconsistent picture of the properties of the system studied is obtained from calculations performed for halide ions adsorbed on metal surfaces. As mentioned above, the first semi-empirical studies r7-131 seemed to be in good qualitative agreement with experimental data. It is well known that iodide is the one adsorbed most strongly on the metal and fluoride the most weakly; this was confirmed by the results of cluster-model calculations. Quite the opposite picture for the adsorption of ions on metals under vacuum has emerged from more recent studies in which ah initio methods were applied to the cluster calculation^.[^^^^^^ These studies found that fluoride was attracted most strongly to the metal, and reported a generally decreasing trend in ion-metal interaction energies from fluoride to iodide. In the first HF studies of Hg7X- systems (X = F, C1, Br, 1),[28,291adsorption of ions on the (1 11) crystallographic plane of mercury was investigated. The trend of the adsorption energies estimated by the ah initio methods was found to be in a direction opposite to that suggested by electrochemists. A deviation from this tendency, found in the first work for bromide (which seemed to be the weakest adsorbed), was eliminated in the second study. At the same time it was reported that the results were highly dependent on the basis set used. In recent work on Cu7X- K u z n e t s ~ v [confirms ~~] the ordering of the adsorption energies found for mercury on the same ( 1 1 1 ) crystallographic plane but has some difficulty for Br- which deviates slightly from the general trend. In this work it was found that Br- was attracted to the metal more strongly than was chloride, the adsorption energies of both ions being very similar. The HF calculations discussed above for the adsorption of halides on metal surfaces do have some difficulty with bromide which frequently deviates from the expected trend. Structurally, the somewhat unclear interpretation of the results is caused by the strategy used in these calculations. Although there is strong experimental evidence[34-411that the halide ions are adsorbed on the hollow site, this site was not always tested in the cluster-model calculations. In the calculations performed for the Hg7X- systems, the halide ion was located over the central Hg atom of the metal cluster; according to the experimental observations of STM images this is the least stable position for the adsorption of halide ions on metals. Unfortunately, the aut h o r ~ [ ~ *did . ~ not ~ ] perform calculations with the ion placed at the preferred adsorption position, the 3-fold hollow site. In the above mentioned studies on copperhalide interactions,[30]two extreme positions, top and hollow, were probed, but the results also disagree with experimental findings. Only for fluoride was the hollow site preferred; for the other three ions the energetics favored the top position. In a work by P a ~ c h i o n i [ published ~~] very recently, the adsorption energy of chloride, bromide, and iodide on the Ag( 100) and Pt( 100) surfaces was calculated by means of ah initio wave-functions. The three sites on the surface - hollow, bridge, and top - were studied and again the results are not in agreement with experimental data. In the calculations for the Ag( 100) surface the stability of adsorption
1162
3 Dynamics and Physical Properties
was found to be in the order bridge > hollow > top whereas for the Pt( 100) surface the top site was favored. Some ambiguity also appears in the results of Hartree-Fock calculations performed in recent years for Hg(111).[331In this study the authors tested all three positions for adsorption of the halide ions on the mercury surface - hollow, top and bridge. The results obtained suggest the top position as that preferred for the adsorption of halide ions. To the results obtained for this particular site the authors then applied an electrostatic correction because two different clusters were used, one structure modeling the top site and the second modeling the bridge and the hollow sites. This correction reduced the energy values for the top site making it the least stable position on the Hg( 11 1) surface. It should be mentioned that the sizes and shapes of the clusters used in all the investigations discussed above varied from structures containing a few atoms only to quite large aggregates of 50-60 atoms. The size of the cluster used depends on the method used more advanced methods are usually quite expensive computationally and so require limitation of the size of the system studied. This same type of limitation must be taken into account when the quality of the basis set describing this system is considered. Most of the clustermodel calculations were performed at the non-correlated level for a similar reason. Most standard correlation methods, for example Mdler-Plesset corrections to the Hartree-Fock energies, are very expensive in terms of computational time and cannot be used for large systems. The density functional techniques that have been developed and significantly improved in the last decade have become a very tempting alternative to cluster-model calculations. They enable calculations to be performed at an electron-correlated level at a cost similar to that of the standard Hartree-Fock method. Some very recent studies performed in our laboratory by the application of DFT techniques to the calculation of the ~ater-metal[~’]and the i ~ n - m e t a l [ ~ interactions ~,~~] are summarized in Section 3.10.3. ~
3.10.3 The B3LYP method applied to the cluster-model calculations Because the new DFT techniques had not yet been applied to cluster-model calculations, many preliminary tests had to be performed to eliminate most uncertainty factors. First of all, from the variety of DFT functionals and the basis sets proposed in the literature one must select the combination which performs best for the system studied. This usually means that a large set of tests must be performed for a small sample representing the system of interest in the cluster calculations. In the simplest approach, these tests can be performed by assuming that the metal cluster is re-
3.10 Modeling of Electrode Interactions ttith Metal Clusters
1163
duced to only one atom. In the problem of the adsorption of a water molecule and of halide ions on noble metals considered here, the Cu atom was used to represent the metal site. First, a series of calculations was performed to determine the interaction of halide ions with a Cu atom. The simultaneous tests of the basis sets and the functional were performed by use of the Gaussian92 program. Several different D F T variants were tested, for example SVWN, BP86 and B3LYP that are representative of the pure local DFT, pure non-local DFT and the hybrid HF/DFT non-local functional. Other DFT alternatives were also tested, but the trend in results seems to be close to that obtained with the methods mentioned above. The basis sets were chosen to be of a rather limited size, especially for the metal atom, because a more limited basis set might enable the use of larger aggregates of atoms at acceptable cost. Therefore, the pseudo-potentials approximation for the inner electrons together with the minimum basis set for description of the valence electrons, known as the LANL1MB[451basis set, was favored, but a double-[ description of the valence electrons, LANLlDZ, was also tested. For halide ions the valence electrons were always described with double-c quality basis set and for the three larger ions the core pseudo-potentials were applied.[46,471 The nomenclature: MB-DZ and DZ-DZ, used in subsequent discussion, refers then to the LANLlMB or LANLlDZ basis set used for the Cu atom combined with the LANLlDZ basis set used for the halide ion. Because the final goal of the work was to use the metal cluster model, these tests enabled us to select the combination of basis sets and method that will compensate for the loss of quality arising from the limitations which result from the size of the basis set used for the metal atoms. The quality of the results obtained in such tests can be verified in two ways. When experimental estimates of properties of the sample are available, these can be compared with the computed values. When experimental data do not exist, the results of tests are compared with those obtained for the same sample but with the high quality basis sets and methods. For the copper-ion sample, therefore, because experimental data are lacking, additional calculations were performed using the extended basis sets ( LANL2DZr4*]for Cu and LANLlDZ for halide ions) in conjunction with the standard H F as well the MP2 and MP4 methods. The results of the calculations confirmed the need for such careful tests as those described above. The energy values are highly sensitive to the basis set and to the method used. The limited (but still very extended in terms of computational cost) double-c basis set, DZ-DZ, was found to give results close to those obtained with the reference basis set, DZ2-DZ. Much larger discrepancies were found between the results obtained with the DZ-DZ and MB-DZ basis sets. The interaction energy values obtained for the same ion but with various basis sets and methods differ from - 164 to -290 kJ mol-' for fluoride, from - 107 to - 195 kJ mol-' for chloride, from -83 to -157 kJmol-' for bromide, and from -57 to -126 kJmol-' for iodide (the first value refers to the HF/MB-DZ method and the second to the SVWN/DZ-DZ method). The limited description of the copper atom is seen to result always in
1164
3 Dynamics and Physical Properties
underestimation of the atom-ion interaction energy. The results for the Cu-Xsystem always suggest that the SVWN method combined with the MB-DZ basis set give the best approximation to the results obtained at the reference, MP4/DZ2-DZ, level. Nevertheless, to ascertain that the choice made through tests with the ion-atom system performs well for the cluster model, some additional tests were performed. With the Cu5-I- system as a test case, a series of calculations were performed with the MB-DZ and DZ-DZ basis sets and with the DFT variants listed above. For comparison, the H F and MP2 calculations were also performed for this system using the same basis sets. These calculations showed that the behavior of the cluster was significantly different from that of the ion-atom dimer. The most important effect is that, unlike for the atom-ion system, the MB-DZ combination of basis sets now gives stronger interaction energies than those obtained with the DZ-DZ basis set. At the same time the sequence of absolute energy values, which for Cu-X- was IAESVWNI> l A E ~ p 8 6 > ) IAEB~LYPI, is preserved for the Cu5-I- system. The reference MP2/DZ-DZ method gives an iodide-copper interaction energy of approximately -141 kJmol-' (in all cases the adsorption of iodide at the hollow site of the Cug cluster was studied). The method SVWN/MB-DZ suggested by the previous atom-ion tests, severely overestimates the Cu5 ion interaction, giving an energy value of -240 kJ mol-' . The interaction energy of - 157 kJ mol-' obtained using the B3LYP/ MB-DZ combination for the Cu5-I- cluster model was found to be the closest to the reference level. It should be mentioned that this B3LYP result is also very close to the MPYMB-DZ estimate of energy, - 154 kJ mol-', confirming the quality of this method to be close to that of the standard MP2 level. It should be stressed that all values discussed above did not include a correction for the basis set superposition error. As a result of these tests the B3LYP method was selected for use in all clustermodel calculations. It was assumed that the metal cluster was always described with the LANLlMB basis set combined with the LANLlDZ for the ions. For the water-metal interaction, extended tests of the dependency of results on the basis set used were also performed using the Cu-H20 dimer as a test case. The full electron description was used for the reference system. The results, calculated using the B3LYP method combined with different basis sets, were compared with those obtained at the standard H F and MP4 levels. Consequently, the least extended LANLIMB basis set was chosen for copper and the tests were focused on the choice of the basis set for the water molecule. Of all tests performed the B3LYP/ MB-6-31G method was found to give the best estimates for the Cu-HzO system. An energy of -37.5 kJmol-' was obtained with this basis set for the ion-copper interaction; the reference level defined by the MP4/full electron calculations is -42.3 kJmol-'. It should be mentioned that both results are close to the experimentally suggested estimate of the interaction of the water molecule with the copper e l e ~ t r o d e ,ca ~ ~-35 ~ ~kJ~ mol-I ~] . An additional test that is necessary is the careful analysis of the cluster-size effect. It is well known that the results of cluster-model calculations strongly depend on the
3.10 Modeling of Electrode Inteructions with Mrtul Clusters
(a)
,
TOP
BRIDGE
I
(b)
TOP
I
1 165
BRIDGE
I
HOLLOW
TOP
BRIDGE
(d)
BRIDGE
(e)
HOLLOW TOP
BRIDGE
BRIDGE
I
HOL~OW
HOLLOW TOP
Figure 1. The Cu, clusters tested: (a) Cu4 planar, (b) Cu4 pyramidal, (c) Cus pyramidal, (d) cu5 planar, (e) Cu9, ( f ) Cu12.
features of the cluster used and that this effect is difficult to predict. The commonly used method for crude evaluation of this effect is the comparison of the results for several clusters of different size and shape. The existence of an approximate estimate of the strength of the copper-water interaction determined the choice of this system for the study of the cluster-size effect. The set of calculations for the adsorption of the water molecule a t the three sites, top, bridge and hollow, was performed for each Cu, cluster ( n = 1 2 ,4 ,5 ,9 ,1 2 ) shown in Fig. 1. The adsorption energy, the distance of the molecule from the surface, and, for the top site, the tilt angle between the molecular axis of H 2 0 and the normal to the surface were measured for all the clusters. ~
1166
3 Dynamics and Physical Properties BRIDGE
TOP
E
Clusters:
[kJ/mol]
1 - cuz
-20
fj-.
7-
_ . . _ . . _ . . _ . . -.. -40
k
2 - cuq planar (Fig.la)
i
U
3 - Cud pyramidal (Fig.lb) 4 - CUSpyramidal (Fig.lc) 5 - CUSplanar (Flg.ld)
-60
- 4
6 - CUS(Fig.le)
2” f
7 - CUlz (Flg.10
-80
-3 -100
-4
The dashed-dotted line indicates the experimental energy value for adsorption of water on copper
Figure 2. The energies of adsorption of the H20 molecule at the top and the bridge site on the Cu(100) surface modeled by different Cu, clusters. At both sites the oxygen points towards the surface; at the bridge site the molecular plane of the water is perpendicular to the surface; at the top site, for some clusters (n = 1,2,6,7) a tilted conformation (a non-zero angle between the dipole moment of water and the normal to the surface) was found to be the most stable whereas for the other clusters the perpendicular orientation of the water molecule was preferred.
The hollow site was always found to be the least attractive for adsorption of water. The water-Cu, interaction energies for the two more stable positions of water, top and bridge, are shown in Fig. 2. It is apparent that the largest deviations from the level suggested experimentally occur for the very small clusters, for n from 2 to 5. It should be stressed however, that this effect depends on the shape of the cluster used the pyramidal structures give much stronger interaction energies than the planar ones (see Fig. 1). The larger aggregates give results oscillating around the experimental adsorption energy. The preferred position for adsorption of the water molecule on the surface is also determined by the structure used in the calculations. For very small clusters (numbers 1, 2, 4 in Fig. 2) the top site is more stable. For larger structures the 2-fold bridge site seems to be favored. It should be stressed that we also performed many tests with the Cu9 structure to determine whether this result does depend on the basis set or the method of calculation. In those studies the top and the bridge sites were tested in terms of the interaction energy characterizing -
3.10 Modeling of Ejectrode Interactions with Metal Clusters
1167
the adsorption of the water molecule at a certain distance from the surface. Although the energy values were sometimes found to be extremely different, the bridge site was always preferred to the top position. From the tests presented above the M12 cluster (Fig. I f ) was selected for further studies on the adsorption phenomenon. It is characterized by a more planar shape; this should avoid the possibility of an artificial increase of the energy values because of the polarization effect. It also enables one to search the adsorption surface continuously, without reaching the borders. An additional test was performed for this cluster as a model of a (100) surface of the three noble metals by computing the Values of 4.87 eV for energies of the highest occupied molecular orbitals, EHOMO. copper, 4.58 eV for silver, and 5.41 eV for gold were obtained with the B3LYP/ LANL 1MB method; these are in good agreement with experimental and theoretical predictions for the electronic work functions of these metals (references 51-53 and references cited therein). Using this cluster, the adsorption of the water molecule[421 and of the four halide ions[441on copper, silver, and gold was studied by use of B3LYP calculations. The results of these calculations are summarized in Table 1. Several conclusions can be drawn from the systematic calculations performed for the adsorption of the water molecule at different sites of the (100) surface of the three noble metals, and for different orientations of the water monomer at those sites. i) The water molecule interacts with the metal surface more strongly through its oxygen atom; this bonding is accompanied by non-negligible charge transfer from the water molecule to the metal of ca 0.10e to 0.20e. ii) The conformation with oxygen pointing towards the surface is preferred at the top and bridge sites, but the opposite orientation is favored at the 4-fold hollow site, which is the least attractive position for the adsorption of water. iii) On the Cu( 100) and Au( 100) surfaces, the H20 molecule is found to be adsorbed more strongly at the bridge site, whereas on the Ag( 100) surface the top site is preferred. iv) At the bridge site water is adsorbed preferentially with its molecular plane perpendicular to the electrode surface and hydrogens pointing toward the two neighboring hollow sites, whereas at the top site orientation with a tilt angle, CI, of approximately 50-65" between the dipole moment and the surface normal is more stable, in agreement with experimental reports." 41 v) The bridge and the top sites are effectively indistinguishable for adsorption of water on all three metals, because the adsorption energy values differ by less then 1 kJ mol-' when the two positions on the same metal are compared. vi) For the orientation of water with its molecular plane parallel to the surface, the top position is the most stable on all metals. vii) The energies of adsorption of the H20 monomer on the noble metals (Table 1) indicate that the hydrophilicity of the three metals is in the sequence Cu > Au > Ag.
1168
3 Dynamics and Physical Properties
Table 1. The results of DFT calculations performed for the adsorption of the water molecule and of the four halide ions by the M12 cluster (M = Cu, Ag, Au). E is the energy of adsorption at the given site-hollow (H), bridge (B), and top (T); 2 is the optimum distance from the surface defined by the centers of the atoms forming the first metal layer (for the water molecule this is the oxygento-surface distance); and Q is the charge remaining on the molecule or on the ion. For the water molecule, the angle between the dipole moment of H20 and the normal to the surface is given in parentheses, because the optimum energy value depends on this angle.
E (kJ/mol)
H20
IBrC1F-
Z
(4
H
B
T
H
B
T
-17.4 (180") -93 -118 -141 -241
-31.8 (0") -82 -103 -126 -217
-30.8 (55") -80 -94 -117 -193
3.2
1.9
2.3
2.6 2.3 2.0 1.3
2.7 2.5 2.2 1.6
2.9 2.6 2.4 2.0
-15,9 (180") -96 -118 -139 -221
-25.9 (0") -87 -106 -126 -199
-26.6 (50") -83 -98 - I 14 -174
3.5
2.2
2.5
2.7 2.4 2.1 1.4
2.8 2.6 2.4 1.8
3.0 2.8 2.6 2.2
-19.5 (90.) -121 -139 -157 -229
-29.7 (0") -116 -130 -145 -206
-28.7 (65") -109 -120 -131 -178
2.5
2.2
2.6
2.7 2.4 2.2 1.5
2.8 2.6 2.4 1.9
3.0 2.8 2.6 2.2
Q
V
(el
(cm-')
B
H
0.07 -0.35 -0.36 -0.37 -0.55
T
0.15 -0.40 -0.43 -0.42 -0.57
H
0.12 -0.46 -0.45 -0.47 -0.61
-
72 104 178 281
Ag12
H20
IBrC1F-
H20
IBrC1F-
0.04 -0.41 -0.41 -0.43 -0.56
0.09 -0.35 -0.36 -0.39 -0.51
0.12 -0.45 -0.48 -0.49 -0.59
0.14 -0.38 -0.42 -0.43 -0.54
0.10 -0.51 -0.53 -0.53 -0.63
0.10 -0.44 -0.47 -0.47 -0.58
-
61 97 158 252
-
73 104 170 276
The adsorption of the halide ions on the three noble metals was studied similarly using M12X- clusters, where X = F, C1, Br, I. All ion-metal energy values were corrected for the basis set superposition error. In the absence of any experimental predictions of the interaction energy, additional calculations were performed to obtain the vibrational frequencies of these systems; these are available from surface-
3.10 Modeling
ofElectrode Interactions
with Metal Clusters
1 169
enhanced Raman spectroscopy (SERS) measurement~.['~-~ 71 The vibrational frequencies on silver and on gold, respectively, were reported to be 1 15 and 158 cm-I for iodide, 158 and 181 cm-' for bromide, and 238 and 245 cm-' for chloride. Chloride is the most widely investigated halide on the Cu surface and for this ion a vibration frequency of ca 290 cm-' was found experimentally. Of course, these values cannot be compared directly with those computed at the optimum position of adsorption (v values in Table 1), because the conditions in the experimental and the theoretical studies are extremely different. The comparison was therefore made by applying the theory proposed by Nichols['81 whereby this property is expressed as a function of the concentration of the ions on the Ag( 100) surface. The approximation of the zero-coverage made in this latter work, resulted in predicted frequency estimates of 82 cm-' for iodide, 100 cm-' for bromide, 159 cm-' for chloride and 317 cm-' for fluoride, which are very close to our results and give good credibility to the other properties obtained from the cluster-model calculations, listed in Table 1. The general picture of the adsorption of the halide ions on the noble metals is:
i) On all the metals studied the halide ions adsorb preferentialy at the multifold sites on the metal surface; for the (100) crystallographic plane the 4-fold hollow site is preferred to the 2-fold bridge site and the least favored is the 1-fold top site. It should be noted that this ordering, which is in excellent agreement with experimental results, is for the first time, found unambiguously for all the systems studied. ii) The distances predicted for the adsorption of the larger ions at the hollow site are in agreement with experimental estimates, varying between 2.0 and 2.7& depending on the ion and the metal. iii) Significant charge transfer from the ion to the metal surface was found at all the sites studied; charge transfer was highest at the most stable hollow position and lowest at the top site. iv) The strength of adsorption of the ions on the same metal under vacuum was found to follow the order F- > C1- > Br- > I-, which is the reverse of that estimated from electrochemical measurements performed in the aqueous environment. v) The vibrational frequency values, however, do follow this same order; they are largest for fluoride and smallest for iodide, in good agreement with the trend measured experimentally. vi) Adsorption of the three largest ions was found to be strongest on gold; the interactions with copper and silver were very close in terms of energy of adsorption. For chloride, copper is slightly more adsorptive than silver whereas for the two other ions, slightly stronger adsorption was on silver. vii) The behavior of fluoride is very different from that of the other ions. Its interaction with copper is unexpectedly strong; it is much weaker with gold and weakest with silver.
1170
3 Dynamics and Physical Properties
Electrochemical measurements suggest that, of all the noble metals, gold adsorbs halide ions most strongly. It should be possible to confirm this experimental result by calculating the ion-metal interaction under vacuum. In fact, the ion-metal interaction is much stronger than that of water on the same metals and the watermetal energy is not very sensitive to the particular metal considered. This leads to the expectation that the trend among the interaction energies of a given ion with the three noble metals should be similar whether under vacuum or in water. Following this argument, when compared with solution electrochemical data, and assuming that the same trends apply, the vacuum quantum results in Table 1 show some inconsistencies that might be an artifact of the cluster model used. A novel attempt to deal with this difficulty is discussed in Section 3.10.4.
3.10.4 Electrostatic effects Although the cluster M12 used in the quantum calculations is relatively large in terms of computational cost, it is still a very limited representation of an infinite surface even if compared only with the size of the ions studied. The error arising from this is certainly greater for iodide than for fluoride. It is enough to compare the diameter of the ion (approximately 4.3 8, for I- and only 2.7 8, for F-) with the Cu lattice constant of 3.68,. The growing inadequacy of the cluster used (always the same) with the increasing ion radius is also apparent from the charge transfer from the ion to the cluster. There is some experimental evidence that iodide is almost totally discharged when adsorbed on the metal. For the other halide ions charge transfer to the electrode is reported to be much smaller. In the work of Bockris et UZ.[’~] the charge remaining on the ion was predicted to be -0.4e for chloride, and suggested almost complete only -0. le for iodide. Other experimental work[60p621 discharge of the ion when it is adsorbed on the metal surface. DFT cluster-model calculations for the adsorption of C1-, Br-, and I- on copper and gold implied that in the optimum position the charge on the ions is still approximately -0.35e, almost the same for all three ions; even larger values are found for the ions adsorbed on silver (see Table 1). It is well known that small metal clusters are always characterized by an inner charge distribution that depends on the shape and the size of the structure used. It seems that metal atoms closer to the center of mass are positively charged whereas those close to edges are negatively charged. This non-uniform charge distribution is obviously expected to induce an electrostatic interaction, UO,between an ion and the distribution of atomic charges. The importance of this component is usually neglected and not considered in the discussion. An additional effect of a similar nature can be expected to arise as a result of the charge transfer from the ion to the cluster. Detailed analysis shows that an extra
3.10 Modeling of Electrode Inteructions with Metul Clusters
1 171
charge is transferred to the metal atoms which are furthest from the site of adsorption of the ion, whereas on the closest atoms a more positive charge is induced by the negative charge remaining on the ion. The size of the cluster is extremely important to this phenomenon. In a very large structure it would be possible to move the negative charge in the cluster to regions far from the ion, resulting in greater polarization of the metal and reduced electrostatic repulsion between the cluster and the ion. In a relatively small cluster the distribution of charges is very limited and the final effect is a compromise between the tendency to move a negative charge as far as possible from the ion, and the repulsion between these charges inside the cluster. It should be noticed that, again, the charge transfer from the ion to the cluster is largest for iodide and so results for this ion are likely to include the largest error. Until now, the effects described above were assumed to be of a similar magnitude for all ions and were neglected. This assumption is now re-examined by approximate evaluation of the electrostatic interaction between the ion and the metal cluster. To estimate the contribution of this artificial electrostatic interaction to the results of DFT calculations for the optimum position of the ion, a set of simple calculations was performed. The total interaction, Utot,has been crudely computed as the sum of the electrostatic interactions between the charge remaining on the ion and the charges on the metal atoms in the M12 cluster. The charges used in these calculations were taken from the DFT calculations for the optimum position of the ion, i.e. at the hollow site at the energy minimum distance (see Table 1). This total interaction may be considered as the sum of two components; the first, UO,comes from the initial charge distribution in the cluster and the second, U1, is a result of charge transfer from the ion to the cluster and of the polarization of the metal surface by the adsorbed ion. Each of the two components, UOand U1, can be calculated approximately. The UOpart can be assumed to be equal to the interaction of the charge on the ion in its optimum position and the charges in the M12 cluster when isolated. The component Ul is then simply obtained as the difference between U,,, and UO. The results of these calculations are shown in Table 2 for the interaction of the four ions with the Cull cluster. It is apparent from these results that neither the effect of the inner distribution of charge in the cluster itself nor its polarization under adsorption should be ignored. The total electrostatic interaction, Utot, of the ion with the cluster at the most stable position is very attractive for fluoride whereas for the three larger ions it is definitely repulsive in character. Analysis of the two components clearly shows the reason for this effect. The cluster itself, because of its inner charge distribution, is attractive toward halide ions adsorbed at the hollow site, so UOis negative for all ions. It is the largest for fluoride because this ion can come closer to the surface. Analogously, as this distance increases when going from fluoride to iodide the Uo attraction decreases. Thus, because the inner charge distribution of the cluster is an artifact of the model used, the results of D F T calculations for all ions are overestimated by a factor UO.
1172
3 Dynamics and Physical Properties
Table 2. The Coulomb-type interaction of the charge remaining on the ion at its optimum position with the charges in the cluster, when the ion is adsorbed at the hollow site (Utot) and with the charges in the neutral Cu12 cluster (Uo). The difference between the two quantities, U1, is an indication of the interaction resulting from charge transfer from the ion to the cluster and the polarization of the cluster by the negative charge of the ion. All energies are given in kJ mol-' . Property
U,,,
UO Ul
Ion
F-
Cl-
Br-
I-
-71 -56 -15
26 -25 51
34 -21 55
-17
42 59
The contribution of the second component, U1, to the total interaction energy is different, as is apparent from Table 2. Only for fluoride is the U1 interaction attractive, whereas for chloride, bromide and iodide it has large, positive values. So, together with increasing charge transfer, ion-cluster repulsion also increases. This is clearly an effect of the limited size of the cluster; all charge transferred to Cu12 has to stay in relatively close proximity to the ion. As a secondary effect, such repulsion causes back-donation of charge from the cluster to the ion. The results for fluoride are likely also to include some repulsion between the ion and the negative charge transferred to the cluster. Nevertheless, because the charge transferred to the cluster is much smaller, it is dominated by the effect of the polarization of the metal because of the charge remaining on the ion. This might indicate that results for fluoride are much closer to those expected for the infinite surface than for the other ions. For this unique ion the response of the cluster to the presence of the ion is similar to that expected for the infinite neutral metal surface. The results from analogous calculations of the electrostatic interaction between ions with the Ag12 and Au12 clusters are presented in Tables 3 and 4, respectively; as Table 3. The Coulomb-type interaction of the charge remaining on the ion at its optimum position with the charges in the cluster, when the ion is adsorbed at the hollow site (U,",) and with the charges in the neutral Ag12 cluster (Uo). The difference between the two quantities, U I, is an indication of the interaction resulting from charge transfer from the ion to the cluster and the polarization of the cluster by the negative charge of the ion. All energies are given in kJmol-'. Property
U,,,
uo
UI
Ion F-
c1-
Br-
I
-61 -63 2
13 -34 47
21 -28 49
29 -24 53
3.10 Modeling o j Electrode Interactions with Metal Clusters
1 173
Table 4. The Coulomb-type interaction of the charge remaining on the ion a t its optimum position with the charges in the cluster, when the ion is adsorbed at the hollow site (Utot) and with the charges in the neutral Aulz cluster (Uo). The difference between the two quantities, U I ,is an indication of the interaction resulting from charge transfer from the ion to the cluster and the polarization of the cluster by the negative charge of the ion. All energies are given in kJ mol-' . Property
Got
un 9
Ion
F-
c1-
Br-
I
-8 -43 35
34 -23 57
36 -20 56
41 -17 58
for the copper cluster, the components CJtot, c/o and Ul were computed for these clusters also. As might be expected, the range of the Utot interaction energies obtained for adsorption of the ions on silver and gold is smaller than is found for copper. The lattice constants for these two metals are larger than for copper so the Aglz and A U Iclusters ~ are larger. If Tables 2, 3, and 4 are compared no regular pattern can be found in the values presented. This is understandable, because the electrostatic interaction is the combined effect of several factors, including the geometry of the system, the amount of charge transferred to the cluster, the optimum distance from the surface predicted for the adsorption of the ion, etc. The question that arises is whether the interaction energies obtained from the DFT calculations should be corrected by subtracting the Utot values from the final results. The results from such calculations are presented in Table 5. The approximate values listed were obtained by use of the equation AE,,, = AE,,, - Utot, where Etotare the EH values taken from Table 1 and the Utot values were taken from Tables 2-4. It is apparent that the A&,, values are characterized by a clear trend, much better defined than that found for the original DFT results. The adsorption of halide ions on the different metals is strongest on Au, much weaker on Cu and Table 5. The interaction energies, AE,,,, for the four halide ions adsorbed at the hollow site of the M I ? cluster (M = Cu, Ag, Au). All energies are given in kJmol-'. Property
cu A& Au
Ion
F-
c1-
Br-
I-
-170 -160 -222
-167 -152 -191
-152 -139 -175
-135 -125 -162
1174
3 Dynamics and Physical Properties
Table 6. The Coulomb-type interaction of the charge remaining on the ion at its optimum position with the charges in the cluster, when the ion is adsorbed at the bridge site (Utot) and with the charges in the neutral Cu12 cluster (Uo). The difference between the two quantities, U1, is an indication of the interaction resulting from charge transfer from the ion to the cluster and the polarization of the cluster by the negative charge of the ion. All energies are given in kJ mol-I . Property
40,
uo
u, Em
Ion
F-
Cl-
Br-
I-
-88 -56 -32 - 129
24 -28 52 -151
36 -23 58 -138
46 -19 65 -129
weakest on Ag. On the other hand, for each metal, the AE,,, estimates from fluoride to iodide vary much less. To complete this discussion, additional calculations were performed for the Cu12 cluster to test the influence of the electrostatic factor on the preference of the ions for adsorption in the sequence hollow site > bridge site > top site. The electrostatic components Utot, Uo, and U1 of the ion-metal interactions are listed in Table 6 for the bridge site and in Table 7 for the top site. When these two tables are compared with Table 2 it is apparent that the artificial electrostatic effect coming from the cluster used is the largest at the bridge site, followed by the top position; the hollow site is the least affected. An exception to this trend is found for chloride; this ion feels the largest repulsion at the hollow site, and the smallest at the top site. Nevertheless, the interaction energies coming from the electrostatic forces do not differ very much for the ions - the range of U,,, values is largest for fluoride and is only a few kJ mol-' for the three other ions. Again, a gross approximation has been made by subtracting these values from the interaction energies obtained from the DFT calculations and the results of these difference are included in Tables 6 and 7 as ,Fa,,. When compared with the estimated Eappfor adsorption of ions at the hollow site of the copper cluster it is apparent that the trend hollow > bridge > top remains unchanged, in agreement with experiment. One must, of course, realize that all the energy values discussed here are very crude estimates and cannot be treated as true values of the ion-metal interaction. Firstly, the effect of the polarization of the metal by the ion should be evaluated. Also, careful tests are needed to determine whether the repulsion between the charge remaining on the ion and that transferred to the cluster is indeed an artifact of the cluster used; the true contribution of this component to the total ion-metal interaction should also be determined. Thus, much more extensive testing of electrostatic effects in cluster-model calculations are necessary if the ion-metal interac-
3.10 Modeling of Electrode Inteructions with Metal Clusters
1175
Table 7. The Coulomb-type interaction of the charge remaining on the ion at its optimum position with the charges in the cluster, when the ion is adsorbed at the top site (U,,,) and with the charges in the neutral Cu12 cluster (Uo). The difference between the two quantities, U1, is an indication of the interaction resulting from charge transfer from the ion to the cluster and the polarization of the cluster by the negative charge of the ion. All energies are given in kJmol-' .
Property
uioi
UO
9 EaPP
Ion F-
CI-
Br-
1
-76 -51 -19 -116
21 -31 52 -137
35 -26 60 -129
45 -21 66 -125
tion is to be evaluated more accurately. The results presented above can be treated only as a qualitative explanation of some irregularities observed in the results obtained with the present model.
3.10.5 Conclusions The discussion in Section 3.10.4 shows that the effect of cluster-size on the interaction of an ion with a metal cluster might have far wider implications than is commonly understood. These include an additional effect arising from the electrostatic interactions between the ion and the inner charge distribution in the cluster. This interaction, an artifact of the model used, was found in this work to be relatively strong compared with the total ion-metal interaction. This effect seems, moreover, to change significantly when going from fluoride to iodide. The electrostatic interaction between the ion and the cluster was found to be attractive for F- and repulsive for the three other ions. A final qualitative conclusion can be drawn, namely that the DFT-computed energies are somewhat overestimated for fluoride and are underestimated for the three larger ions; the underestimation error increases when going from chloride to iodide. It should be noted that cluster-model calculations are currently a very popular model for the calculation of the interaction of ions and molecules with metals. Usually the electrostatic effects described above are not taken into account when the ion-metal interaction is discussed in the light of results of the cluster-model calculations. It has been shown above that a limited cluster introduces a significant perturbation into the calculation, coming from its inner charge distribution, that
1176
3 Dynamics and Physical Properties
will produce some artificial electrostatic forces. The effect of this perturbation should not be ignored and might be crucial to the interpretation of the final results, because it might be extremely different, even in studies of the adsorption of similar species, as has been shown for the halide ions. To eliminate this source of error, very extensive comparative studies are required. A series of cluster-model calculations with much larger systems is needed, including evaluation of the electrostatic component in each instance. Careful analysis of the results should enable the extraction of a more accurate estimate of the ion-metal interaction energy. These tests should not be limited to one ion only, because, as mentioned above, this effect depends strongly not only on the shape and size of the cluster used, but also on the type of ion. The ideal solution would be to perform calculations with a very large cluster, so that the results could be treated as a description of the total interaction of the ion with the surface. Of course, the problem of an artificial electrostatic interaction between the ion and the cluster can never be totally eliminated. Any structure limited in size will have its own inner charge distribution that affects the results. Nevertheless, calculations with a very large cluster would eliminate the ambiguous interpretation of the interaction of the ion with the distant metal atoms. On the other hand, it should be remembered that the real electrode has its own inner charge distribution that makes the surface more negative than the bulk of the meta1.[51-521 Some evaluation of this effect is also needed. These aspects of the problem clearly show that the interfacial interaction in a cluster model approximation is not a trivial problem and requires still further, very extensive investigation.
Acknowledgment The authors thank Professor S. Romanowski of the University of Lodz (Poland) for discussions that led to this project. The financial support of Praxis XXI through project PRAXIS/PCEX/C/QUI/61/96 is acknowledged. A.I. thanks PRAXIS XXI for a doctoral scholarship.
References [ I ] M. A. Leban, A. T. Hubbard, J. Electroanal. Chem. 74 (1976) 253. [2] S. Holloway and K. H. Bennemann, Surf. Sci. 101 (1980) 327. [3] B. C. Khanra, Chem. Phys. Letters 84 (1981) 107. [4] G. Estiu, S. A. Maluendes, E. A. Castro and A. J. Arvia, J. Phys. Chem. 92 (1988) 2512. [5] An. Kuznetsov, J. Reinhold, W. Lorentz, J. Electroanal. Chem. 164 (1984) 167.
3.10 Modeling oj'lectrode
Interactions with Metal Clusters
1 111
[6] An. M. Kuznetsov, R. R. Nazmutdinov, M. S. Shapnik, Electrochimica Acta 34 (1989) 1821. [7] D. Dohnert, J. Koutecky, J. W. Shultze, J. Electroanal. Chem. 82 (1977) 81. [8] F. Illas, F. Sanz, J. Virgili, J. Electroanal. Chem. 142 (1982) 31. [9] M. Weissmann, N. V. Cohan, J. Electroanal. Chem. 146 (1983) 171. [lo] M. Jauregui, N. V. Cohan, M. Weissmann, J. Electroanal. Chem., 163 (1984) 381. [ 111 G. V. Kulkarni, S. K. Rangarajan, J. Electroanal. Chem., 196 (1985) 375. [I21 An. Kuznetsov, J. Reinhold, W. Lorentz, Electrochim. Acta., 29 (1984) 801. [13] An. Kuznetsov, R. R. Nazmutdinov, R. R. Shapnik, Electrokhimija, 22 (1986) 776. [ 141 S. Anderson, C. Nyberg, C. G. TengstBI, Chem. Phys. Letters 104 (1984) 305. [ 151 P. A. Thiel and T. E. Madey, Surf. Sci. Rep. 7 (1987) 21 1. [I61 A. B. Anderson. Surf. Sci. 105 (1981) 159. [ 171 J. E. Miiller and J. Harris, Phys. Rev. Letters 26 (1984) 2493. [IS] M. W. Ribarsky, W. D. Luedtke and U . Landman, Phys. Rev. B32 (1985) 1430. [I91 S. K. Saha and N. C. Debnath, Chem. Phys. Letters 121 (1985) 490. [20] H. Yang and J. L. Whitten, Surf. Sci. 223 (1989) 131. [21] H. P. Bonzel, G. Pirug, J. E. Miiller, Phys. Rev. Letters 58 (1987) 2138. [22] J. Paul and A. Rosen, Int. J. Quant. Chem 23 (1983) 1231. [23] C. W. Bauschlicher, J. Chem. Phys. 83 (1985) 3129. [24] I. I. Zakharov, V. I. Avdeev, G. M. Zhidomirov, Surf. Sci. 277 (1992) 407. [25] R. R. Nazmutdinov, M. Probst, K. Heinzinger, J. Electroanal. Chem. 369 (1994) 227. [26] P. J. Boussard, P. E. M. Siegbahn, M. Svensson, Chem. Phys. Letters 231 (1994) 337. [27] R. R. Nazmutdinov, M. S. Shapnik, Electrochimica Acta 41 (1996) 2253. [28] M. Blanco. J. Rubio, F. Illas, J. Electroanal. Chem. 261 (1989) 39. [29] M. Blanco, J. M. Ricart. J. Rubio, F. Illas, J. Electroanal. Chem. 267 (1989) 243. [30] An. Kuznetsov, Electrochim. Acta., 40 (1995) 2483. [31] J. Seitz-Beywl, M. Poxleitner. M. M. Probst, K . Heinzinger, Int. J. Quant. Chem, 42 (1992) 1141. [32] G. Pacchioni, Electrochim. Acta, 41 (1996) 2285. [33] G. Toth, E. Spohr, K. Heinzinger, Chem. Phys, 200 (1995) 347. [34] X. Gao, M. J. Weaver, J. Am. Chem. Soc. 114 (1992) 8544. [35] W. Haiss, J. K. Sass, X. Gao, M. J. Weaver, Surf. Sci. Lett. 274 (1992) L593. [36] X. Gao, M. J. Weaver, J. Phys. Chem. 97 (1993) 8685. [37] X. Gao, M. J. Weaver, Ber. Bunsen.-Ges. Phys. Chem. 97 (1993) 507. [38] X. Gao, G. J. Edens, M. J. Weaver, J. Phys. Chem. 98 (1994) 8074. [39] X. Gao, G. J. Edens, F. C. Liu, A. Hamelin, M. J. Weaver, J. Phys. Chem. 98 (1994) 8086. [40] D. W. Suggs and A. J. Bard, J. Phys. Chem. 99 (1995) 8349. [41] T. Yamada, N. Batina. K. Itaya, J. Phys. Chem. 99 (1995) 8817. [42] A. lgnaczak and J. A. N . F. Gomes, J. Electroanal. Chem., 420 (1997) 209. [43] A. lgnaczak and J. A. N. F. Gomes, Chem. Phys. Letters, 257 (1996) 609. [44] A. Ignaczak and J. A. N. F. Gomes, J. Electroanal. Chem., 420 (1997) 71. [45] P. J. Hay and W. R. Wadt, J. Chem. Phys. 82 (1985) 270. [46] W. R. Wadt and P. J. Hay. J. Chem. Phys. 82 (1985) 284. [47] T. H. Dunning and P. J. Hay. Modern Theoreticul C!zerni.Wy, Plenum New York 1976, Chap. I , pp 1-28. [48] P. J. Hay and W. R. Wadt, J. Chem. Phys. 82 (1985) 299. [49] C. Au. J. Breza, M. W. Roberts, Chem. Phys. Letters 66 (1979) 340. [ S O ] B. A. Sexton and A. E. Hughes. Surf. Sci. 140 (1984) 227. [51] S. Romanowski, Phys. Stat. Sol. (b) 145 (1988) 467. 1521 S. Romanowski, Polish. J. Chem. 67 (1993) 729. 1531 S. Romanowski, J. A. N. F. Gomes. J. Electroanal. Chem. 373 (1994) 133. 1541 H. Wetzel, H. Gerischer, B. Pettinger. Chem. Phys. Letters 207 (1993) 455.
1178 551 561 571 581 591 601 611 621
3 Dynamics and Physical Properties
P. Gao, M. J. Weather, J. Phys. Chem. 90 (1986) 4057. B. Pettinger, M. R. Philpott, J. G. Gordon 11, J. Phys. Chem. 85 (1981) 2746. G. Niaura, A. Malinauskas, Chem. Phys. Letters 207 (1993) 455. H. Nichols, R. M. Hexter, J. Chem. Phys. 74 (1981) 2059. J. O’M. Bockris, M. Gamboa-Aldeco, M. Szklarczyk, J. Electroanal. Chem. 339 (1992) 355. X. Gao, M. J. Weaver, J. Am. Chem. SOC.114 (1992) 8544. X. Gao, M. J. Weaver, J. Phys. Chem. 97 (1993) 8685. 0. M. Magnussen, B. M. Ocko, R. R. Adzic, J. X. Wang, Phys. Rev. B51 (1995) 5510.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.11 The Transition from Low- to High-nuclearity Molecular Metal Clusters Followed by X-ray Photoelectron Spectroscopy Roberto Zanoni
3.1 1.l Introduction Photoemission studies in the closely related fields of molecular entities as molecular metal clusters (MMC), ligand-protected metal nanoparticles, and gas-phase 'naked' clusters require different experimental approaches and imply distinct aspects of basic and applied research. MMC have single size and symmetry and have typical covalent metal-ligand bonds. The borderline between MMC and ligand-protected metal nanoparticles has now become subtle. In recent few years many different series of nanoparticles have been produced in which a metal cluster framework is surrounded by one or more shells of ligands, especially surfactants, or is encapsulated in a host matrix, such as a polymer, and often has a narrow size distribution. Gas-phase metal clusters with a selected mass can be produced and characterized more readily and efficiently than hitherto, and their reactivity tested in the gas phase with ligands such as CO. These three main series of compounds have been the object of several photoemission studies, either in the gas phase or as solids, and a general review in a limited space has become much too difficult. In this chapter the focus is necessarily limited; it concentrates mainly on recent advances in solid-state photoemission spectroscopy as applied to series of molecular clusters with high nuclearities. This simply reflects the author's main interest in the field. Extensive review articles are available in the literature,"] and only a short introduction to photoemission studies on clusters is given, to present two case studies of XPS as applied to the related fields of large MMC and nanoparticles.
1180
3 Dynamics and Physical Properties
3.11.2 A brief introduction to applications of photoemission spectroscopy to clusters Photoemission spectroscopy is a flexible and versatile technique with which the electronic, magnetic and chemical structure of matter can be efficiently investigated. In a photoemission experiment absorption of monochromatic photons, with energy hv above a certain threshold, impinging on matter is used to create photoelectrons. A variety of photoemission experiments and corresponding techniques is available, depending on the choice of the detection technique and of the parameters of the ionizing source. In its conceptually simplest set-up, a photoemission experiment is performed at fixed photon energy, by measuring the kinetic energy ( K E )and number of the photoelectrons emitted at a fixed collection angle. Photon energies in the soft-X-ray range ( 102-103 eV) are used in X-ray photoelectron spectroscopy (XPS or ESCA) and both valence and core levels are accessed. Synchrotron radiation light has bridged the gap between energy ranges, enabling new classes of photoemission experiment to be performed,r21also on clusters. The total energy of the ionic system produced, Eion,results from different electronic, vibrational, rotational, and translational energy contributions. Conservation of energy requires that hv Ei, = KE Eionwhere Ei, is the initial energy of the system, for the neutral species, in its ground state. The binding energy ( B E )of the ejected electron is defined as the energy difference between the corresponding excited ionic state, Eion,and the ground state Ei, of the system, i.e. BE = Eion- Ei,. The relevant relationship is, therefore: hv = BE KE. This equation enables derivation of BE values from the experimental photoelectron KE values. Photoionization requires a threshold photon energy which, for solids, is termed the work function, 4, with typical values 1.55 eV, which must be added to the energy balance equation: hv = BE KE 4. Although the ‘one-photon one-electron’ picture is appealing, photoemission is a many-body process which results in manifold electronic states of the ionic system, because of changes in electron correlation as a consequence of the photoemission. Photoelectron spectroscopy is an inherently surface-sensitive technique. The electron inelastic scattering mean-free path (MFP) in solids depends on electron KE, and is very small, typically 0.5-2 nm, at the KE values of interest ( 10-103 eV). For metal particles and large MMC, particle diameters can be close to the MFP for bulk metals. The intensity of a photoemission peak is quantitatively related to the corresponding cross-section for photoionization, 0,defined as the transition probability per unit time for exciting a system with a unit incident flux of photons. Quantitation is provided by the relative atomic ratios between different elements in one sample, from the intensity ratios of core peaks, after normalization to 0 and corrections for instrumental effects.
+
+
+
+
+
3.1I The Transition,fiom Low- to High-nuclearity Moleculur Metal Clusters
1 181
A chemical shift in XPS is the distance in energy between two BE values of the same core orbital of an element, measured in different compounds or atomic environments. The energy required to remove an electron from an atom is inversely related to the valence electron density on it. BE should increase with increasing positive charge. Several factors, however, contribute to a core-level chemical shift and they are usually grouped into initial and final-state effects. Initial-state electronic charge reorganizations following variations in size, number, and type of first neighbors are referred to as ‘initial-state effects’; ‘final-state effects’ refer to all processes connected to core-hole screening of the system. Electron-cloud reorganis. The positive zation acts in the same time-scale as photoemission, typically charge left in the system by a photoelectron will pull all the electronic levels and attract electrons towards the hole, which will be screened. The total energies of the primary final state and the initial state differ by the relaxation energy. For clusters, the departure from metallic screening increases with decreasing nuclearity, because the conduction band progressively separates into molecular orbitals. In small clusters energy reduction in the final state requires contributions from polarization of the surrounding atoms. Metal core lines broaden and shift positively with decreasing cluster size. Although hole lifetime or, alternatively, reduced screening in clusters have been proposed as the source of the broadening, its actual extent is difficult to assign to a specific nuclearity, because of the size distribution of bare metal particles or as a consequence of the complications inherent in ligand bonding in monodispersed MMCs. The existence of an invariably positive chemical shift is not intuitive. Cluster atoms, in fact, are prevalently surface atoms but the BE shift of a core level between surface and bulk metal atoms (the surface-to-core level shift, or SCLSr3]) can be negative, as found inter alia for Au, Pd, Pt. The explanation of this point is ~] still the subject of debate. In the interpretation by Wertheim et U Z . , ~a ~final-state Coulombic charge is primarily responsible for the BE shift. A core hole created by photoemission is screened by the conduction electrons. This will produce a charge +e on the surface of the cluster. If the substrate is not able to neutralize the charge within the typical core-hole life-time, the BE of the entire XPS spectrum of a spherical particle will be increased by the amount e’/2r, where r is the cluster radius. The calculated amount is 1.44 eV for a spherical cluster of 1 nm diameter. Mason[4h1has emphasized the importance of initial-state effects - the electronic levels of small metal particles are different from the bulk and BE shifts are interpreted as mainly a result of different occupancy of the valence d and s levels. On the basis of experiments on small palladium clusters on graphite, Cini et al. ,I4‘]have proposed a cluster-substrate interaction mechanism. In the Wertheim model also cluster-substrate interactions are invoked to explain experimental shifts often smaller than calculated from e’ /2r.I5]
1182
3 Dynamics and Physical Properties
3.11.3 Case studies 3.11.3.1 Experimental resolution of surface and bulk atoms in ligated metal clusters Much experimental and theoretical work has focused on the distinctive behavior of the surface and bulk atoms of pure metals. The fundamental contribution of angle-resolved photoemission studies is widely acknowledged. For clusters, angledependent XPS measurements have been reported for Au55( PR3)&16 only.[61The main reason for this is that XPS experiments have mainly been performed on bare metal particles produced on a substrate. Any distribution of sizes averages out the angular effects of specific cluster dimension. Also, although highly mass-selected clusters can be anchored on to a substrate, for bare clusters the particle-to-particle distance must be relatively large to prevent coalescence. Thus photoelectrons emerging from the surface and the interior of the particles can reach the detector without any shadowing effect from nearest neighbor particles, even at small collection angles from the surface. Core and valence spectra of the full series of Auss(PR3)12C16 clusters have been measured as a function of the photoelectron collection angle.[61No member of the Au55 series has been obtained as a crystal. Extensive characterization by several physical and chemical measurements was conducted,[71 leading to a proposed structure with a core of 55 Au atoms arranged in a cubeoctahedral structure, surrounded by a shell of ligands. The 12 apical Au atoms are bound to the P and the six bound to C1 are located on the square surfaces of the cubeoctahedron, with a different number of nearest neighbors. The actual composition and stability of Au55 clusters, first synthesized by S ~ h m i d , [has ~ ] been debated in the literature. XPS has enabled detailed checking of sample composition and reproducibility to be performed for the first time, as shown for Au4f in Fig. 1. In the following discussion the different Au55( PR3)12C16 species will be named according to the different R3 ligated groups present. 1 = ( C ~ H ~ ) ~ ( C ~ H ~ S O ~ N ~ - W Z ) , 2 = (C6H4Me-p)3, 3 = (CbH4OMe-p)3, and 4 = Ph3. Different line shapes, often with separate components, are obtained for Au 4f, P 2p, and C1 2p spectra from different samples. An essential preliminary is to distinguish between ‘real’ extra components belonging to the Au55 moiety and peaks due to species physically mixed with Au55. A check for decomposition under the action of X-rays enabled the assignment of shoulders in the Au 4f, P 2p, and C1 2p spectra to AuPR3C1 monomemC6]For 3b and 3b’ XPS spectra contain only a spin-orbit split main component. The XPS Au/P and Au/Cl atomic ratios are consistent with the chemical formula. A clear distinction among samples is therefore made. Angle-dependent photoemission measurements have been conducted on all samples. The results for Au 4f7p are shown in Fig. 2.
3. I 1 The Transition.fiom Low- to Hiyh-nuclearity Molecular Metal Clusters
1
I
Figure 1. Au 4f spectra from a full series of Auss( PR3)12C16 samples with different R groups, identified in the text. All samples were measured after different storage times from preparation. 2a and 2b were prepared in different batches. 3a and 3a' are the same sample, measured after different storage times. 3b was measured immediately after preparation and 3b' is the same sample as 3b, re-measured after 1 week. 3c had been stored for 2 years at ambient temperatures. 4a was measured as received. 4b was in the form of a pressed wafer. All samples were prepared and stored in air.
"
'
,
1183
'
Au 4f 512 .?
'
I
I
712
....
92 90 88 86 84 82 Binding Energy/eV
When the electron-escape angle, measured relative to the plane of the graphite substrate, is reduced, a shift towards higher BE and a broadening of Au 4f peak are observed. The line shape is asymmetric. Both effects can be caused by a relative enhancement of the component from Au surface atoms. By taking linear combinations of the 10" and 90" spectra, denoted P( 10") and P(90"), the surface contribution to the Au 4f spectrum ( S )is separated in Fig. 3 from the contribution of the deeper lying Au atoms ( B ) , because S is proportional to [P(10") - yP(90°)] and B to [P(90") - pP( 1 O " ) ] .
.-x Y v)
8-
Figure 2. Normalized Au 4f712 spectra of A U ~ S ( P ( C ~ H ~ O M ~ taken - ~ ) ~ )EI ~ C ~ ~ , at five different photoelectron collec86 tion angles, measured from the graphite holder surface.
1
,
, , , 85 84 83 Binding Energy/eV
82
1 184
3 Dynamics and Physical Properties
Figure 3. Original curves (dots), and two-component fits from linear combinations of the Au 4f7p spectra for Auss( P(C6HdOMe-p)3)&16 taken at three different photoelectron collection angles. B components are at 84.17 eV, S components at 84.45 eV (dashed lines). B and S were fitted to a line shape consisting of a mixture of a Gaussian and a Lorentzian.
From the spectra at five different photoelectron collection angles, the linear combinations for both B and S components yield a consistent set of values of shape, width, and BE.[61The small variation of these values and the symmetry of the spectra imply there is no third component. The B component appears at 84.17 0.015 eV and S at 84.45 i 0.045 eV. The B / S ratios determined are 3.4, 3.5, 3.4, 2.0 and 0.28, respectively at 90", 70", 50", 30°, and 10". Photoelectrons emitted within a certain angle from all 55 Au atoms of a particular Au55 particle reach the detector undisturbed by other particles. A center of mass separation of approximately 2 nm can be shown to eliminate any influence between 90" and 45". This value compares well with the 2.1 nm diameter of the whole Au55 cluster, including the ligands. The overall ratio of bulk-to-surface Au atoms in Au55 is 13/42, = 0.31, much removed from the value of 3.4 at 90". Assignment of S to the 24 unbound surface Au atoms would give 31/24 = 1.2. A B / S ratio at 90" of 49/6 = 8.2 would result if the six C1-bound Au atoms were responsible for the S peak. The C1-bound Au atoms are, moreover, located at the centers of the square faces of the cubeoctahedron and are surrounded by four unbound and four P-bound Au atoms in the same plane. That these Au atoms have eight in-plane neighbors would lead to a B / S ratio of 8.0 at small angles, when photoelectrons from the first layer only reach the detector. The experimental value, however, is 0.28 at 10". The apices of the cubeoctahedron in Au55 are occupied by 12 P-bound Au atoms. The average orientation of a cubeoctahedron on a surface is such that a P-bound Au atom is almost always pointing outwards. The very low B / S ratio at 10" can be explained
+
3. I I The Transition from Low- to High-nuclearity Moleculur Metul Clusters
1 185
-
non-apical
&
-
Figure 4. Apical and non-apical components of the Au valence-band spectra of A u g ( P(C6H40Me-p)3)12C16 compared with Au metal.
<
I
.
12108
1
s
I
6
,
I
4
.
(
2
I
(
Binding Energy/eV
by assigning the S peak to the 12 apical Au atoms, because at that angle only photoelectrons emitted from the apices can reach the detector. The resulting nonapical-to-apical ratio, 431 12 = 3.6 closely reproduces the experimentally obtained B I S ratios of 3.4-3.5 between 90" and 50". The Au 4f712core-level peak at 84.45 eV is then identified with the 12 apical Au atoms and the peak at 84.17 eV with the 43 non-apical atoms. The latter atoms include the 13 core atoms, the 24 unbound surface atoms and the 6 C1-bound surface atoms. The results from the VB, shown in Fig. 4, reinforce this assignment. By using the same p, and y , values, linear combinations of 10" and 90" spectra give the two corresponding valence-band (VB) components. Once resolved, the VB spectra of both components of Au55 resemble those of metallic Au, with a 5d and a 6s band and a Fermi cut-off. The 5d spin-orbit splitting is 2.5 f 0.1 eV, which is only slightly smaller than for metallic Au. The Fermi level of the non-apical atoms is shifted by +0.33 k 0.02 eV compared with that of metallic Au, and f0.60 f 0.04 eV for the apical atoms. Therefore, an energy shift of 0.27 eV separates apical from non-apical atoms. This value is in excellent agreement with the corresponding separation of the Au 4f7/2 core levels (0.28 eV) and calls for a rigid shift of the two components. If 0.60 eV and 0.33 eV are subtracted from the apical and non-apical Au 4f7p core lines, the two peaks coincide at 83.84 eV. This is a relevant finding, for two reasons. It gives experimental evidence for the generally accepted hypothesis that the metal core BE of a small cluster should lie between the values for surface and bulk atoms in the metal; these are
1 186
3 Dynamics and Physical Properties
83.60 and 84.00 eV, respectively, for Au. Moreover, because no BE shift is observed for the surface Au atoms bound to P and C1 (the 0.28 eV shift being assigned to a final-state charge on the cluster), it is concluded that the amount of charge transfer between Au atoms and P and C1 atoms is small. This is at variance with results from XPS studies of the bonding in smaller clusters such as Aul I ( PPh3)7C13,181where the Au atoms bound to C1 and P lose appreciable charge, resulting in positive BE shifts. A conclusive argument against charge loss from the 12 P-bound Au atoms in Au55 is the shape of the VB of these atoms - an Au 6s-band is still clearly visible. The next case study will reinforce the interpretation of this effect as characteristic of the transition from small to large metal clusters. The e2/2r shift arising because of a +e charge on the surface of Au55 should be ca 1.1 eV, yet smaller shifts are observed for apical and non-apical Au atoms. This can be attributed to overall polarization of the ligands and to extra-atomic relaxation involving the surrounding core-holes. Ligand polarization does not explain diferential shifts, producing only a shift of the reference level. The lower coordination number of the apical Au atoms in Au55 will cause extra-atomic relaxation because of the polarization of the nearest neighbors less effective than for the non-apical atoms. As a result, the Au 4f BE of the apical atoms will be larger than for the nonapical atoms. An intriguing point is, however, raised by the above interpretation. Apical and non-apical Au atoms have equal shifts at the Fermi level and at the core Au levels, as if holes were localized at the VB. In case of delocalized holes, in fact, the Fermi-edge shift amounts to the Coulombic term e2/2r,[91larger than the corelevel shift, which is reduced by polarization near the hole. Ley et al. have, however, reported" that, even in metals with highly delocalized valence states, the relaxation energy associated with a localized and a delocalized valence-hole state is not much different. A hole in the VB spectrum can, therefore, be localized, inducing the same polarization of the environment as a core hole, leading to an equal BE shift of valence and core electrons.
3.11.3.2 From molecular clusters to semiconducting particles high nuclearity Cu-Se clusters In this section XPS results for the molecular clusters Cu30Se15(PiPr3)12, CqoSe35(PEt3)22, and Cul46Se73( PPh3)30, (denoted hereafter Cu30Se15, Cu70Se35 and Cu146Se73) are summarized." 'I These complex species have been synthesized as crystalline compounds and their structure determined by Fenske and coworkers.[121 The unique advantage offered by this series in an XPS study is that a wellcharacterized bulk CuzSe stoichiometric phase can be produced in situ from the molecular species, as confirmed by X-ray diffraction analysis. BE values for Cu2Se constitute the 'natural' reference levels for isolation of size effects in species with closely related structures. The geometric structures of Cu146Se73 and Cu70Se35 are
3. I I The Trunsition from Low- to High-nucleurity Moleculur Metul Clusters
,
c-)
d
Figure 5. Cu 2~312spectra from a series of Cu-Se molecular clusters and from the bulk phase P-CuzSe obtained by thermal treatment of Cur&e73( PPh3)30.
,
.. . .. .
1 187
....
..
.. ._ . . ..
.. .
. Hulh , , C'u,Se
.
<j, :.,;
,
,
! ,
'L 936
934
932
930
Binding Energy/eV
similar and they are best described in terms of a packing of Se atoms; Cu$3el5 is built from five parallel layers of Cu and Se atoms. To exemplify this briefly, the Se atoms in Cu146Se73 are organized in three planar layers, with 21, 31 and 21 atoms. 120 Cu atoms are located between the three Se layers, 12 in the middle Se layer and 14 outside the Se layers. The phosphines are terminally bound to Cu atoms and completely cover the cluster. Cu146Se73 has a trigonal prismatic shape, and the copper selenide core has a maximum width of 2.67 nm and a thickness of 0.87 nm. The electron conductivity of the clusters is size dependent, the smaller species are insulators, Cu146Se73 is a semiconductor ( lop2 S c m p l ) ,and Cu2Se a semi-metal. The first striking result from Fig. 5 is that the presence of Cu-P covalent bonds in the clusters is not reflected in complex Cu 2p peaks; this is at variance with Au-P in smaller clusters, e.y. Aull( PR3)&13,['] but consistent with XPS spectra of Au55( PR3)12CI6.It should be emphasized that this effect is strongly related to the high nuclearities and it represents a clear feature of the transition from small to large metal clusters. A second relevant finding is that core line BE values increase (and Auger lines KE values decrease Table 1) in a sequence different from that of decreasing nominal nuclearities. Both effects can be rationalized. In Table 2, experimental and nominal XPS atomic ratios match only for Cu146Se73. Coalescence of Cu30Sel5 and Cu70Se35 into phosphine-ligated particles can be proposed on the basis of their high Cu/P ratios. An estimate of the diameter of Cu,oSe15* and Cu?OSe35*(the asterisk referring to compounds with compositions different from nominal) can be given. ~
1 188
3 Dynamics and Physical Properties
Table 1. Binding energies of core lines and kinetic energies of Auger L3M4.jM45 peaks for the compounds reported. All values are in eV.
c u 21)3/2 Se 3d5p p 213312 Cu Auger Se Auger
932.99 54.26 130.89 916.55 1306.96
932.90 54.23 130.90 916.73 1306.78
933.10 54.28 131.20 916.27 1307.44
932.63 53.99
932.71 ~
~ ~
917.17
918.40
-
-
The width of Cu 2p3p core lines for Cu30Sel~* and Cu70Se35*is closely comparable with the width of both bulk CuzSe and the mono-dispersed Cu146Se73. This implies an unexpected narrow size distribution, which can be estimated as follows. The number of P atoms needed to stabilize a cluster should be proportional to the number of Cu surface atoms. A plot of Cu/P atomic ratios against the cube root of the total number of Cu atoms should yield a straight line; this is, indeed, obtained in Fig. 6. Estimated values of 3 and 5 nm, respectively, are found for Cu3oSel5* and Cu70Se35*. The atomic ratio and the dimension of Cu146Se73* agree with the nominal value. When the BE values in Table 1 are re-ordered in terms of the effective cluster nuclearities, the expected increasing trend of BE values with decreasing size is actually observed. A further relevant result is that the relative energy separation between Cu and Se lines increases with decreasing effective cluster diameter. This implies a complex core-hole relaxation mechanism, by analogy with the relative shift between apical and non-apical Au55 atoms. The VB line shapes in Fig. 7 are closely comparable, suggesting that initial state effects are not relevant. The VB consists of a relatively narrow Cu 3d band at ca
Table 2. Experimental (Exp) and theoretical (Theor) Ncu/Np and Ncu/Nse atomic ratios obtained from the XPS relative intensity of Cu 2p3/2 to P 2p and to Se 3d, corrected for the corresponding atomic cross-sections. The uncertainty in experimental values is f 1OYO. Ncu/Np
Cu30Se15 CwoSe35 cu146se73
Ncu/Nse
EXP
Theor
EXP
Theor
8.1 12.6 4.3
2.5 3.2 4.9
2.0 2.0 1.9
2.0 2.0 2.0
3.1 I The Transition Jiom Loti'- to High-nuclearity Molecular Metal Clusters
1 189
Figure 6. Plot of nominal Cu/P atomic ratios against the cube root of the number of Cu atoms in the various clusters. The experimental XPS Cu/P atomic ratios afford an estimate of the actual dimension of Cuz,Se,* cluster species
Figure 7. Valence-band spectra from a series of Cu-Se molecular clusters and from the bulk phase p-CulSe obtained by thermal treatment of Cu146Se73(PPh3)30.
1190
3 Dynamics and Physical Properties
Valence Band at the Fermi level
....... ...
............Copper ............meta
.........Bulk .......Cu,Se ... L
1
0
“I
Binding Energy/eV
-2
Figure 8. Enlarged view of the top of the valence-band spectra from Cu1&e73( PPh3)30 and from the bulk phase p-CuzSe. Note the finite density of states for the latter at the Fermi energy.
3 eV and two Se 4p bands at ca 1 and 5 eV. The top of the VB is, therefore, mainly Se electronic states. For CuzSe a non-zero density of states at the Fermi level is apparent from Fig. 8, which assigns conductive properties to P-CuzSe. The Cu Auger positions and line shapes, shown in Fig. 9, combined with the results from Cu 2p3p, are indicative of monovalent Cu atoms. Core and VB BE values are shifted positively in the clusters compared with CuzSe. A rigid shift for Cu and Se XPS spectra is therefore implied. For Cu?oSe35+ and Cu30Sels* the shifts are closely comparable for Cu and Se, and confined to restricted ranges. Distinct shifts for Cu and Se are instead found for the smaller cluster, Cu146Se73. A rigid shift is interpreted as a final-state effect because of the finite size of the species. The Coulombic shift reproduces the experimental ABE value for the larger particle only, Cu?oSe35*. Therefore, by close analogy with Au55( PR3)12C16,for the C U ~ clusters ~ S ~also~ a combined mechanism of limited screening and local polarization of the surroundings needs to be invoked for the smaller species. While a surface charge shifts all BE values, the energy required to polarize the surroundings of a charged Cu or Se atom depends on local coordination. ABE values of Cu and Se will, therefore, be different. The KE shifts of the Auger lines of Cu and Se closely follow the trend in ABE values, with expected larger values, because an Auger electron leaves two holes in the system. The Auger lines shift by AKE = -ABE ABE(1,m). Here ABE is the final-state Coulombic shift of the primary core hole and ABE(1,m) is the Coulombic shift which results from the double hole in the
+
3.1I The Trrrnsition from
Lotit- to
1 191
High-nucleurity Moleculur Metal Clusters
I
.
Figure 9. Copper Auger spectra L3M4 5M4 5 from a series of Cu-Se molecular clusters and from the bulk phase P-CuzSe obtained by thermal treatment of Cul4&71( PPh7)io.
I
I
.
.
I
I
I
I
.
/
I
/
I
I
I
I
I
I
I
I
908 912 916 920 924 Kinetic Energy/eV
Cu 3d valence band. Valence holes in the Cu 3d band are known to be localized. If a surface charge only were responsible for the BE shift, the expected result would be AKE = 3ABE.[13]These results, however, indicate that AKE 2ABE (Table 3 ) , again showing the additive effect of local polarization on the photoelectron energies.
-
Table 3. Shifts in binding energy of core lines ( A B E ) and in kinetic energy of Auger L3M4.5M4.5 peaks ( A K E ) for C~1146Se~~ and CulxSex* cluster species relative to the bulk phase p-Cu2Se. All values are in eV.
c u 2P312 VB-CU 3d Se 3dSp VB-Se 4p CuLMM SeLMM Coulomb shift ( e 2 / 2 r )
0.49 0.50 0.29 0.35
-
-
-
-
0.35 0.31 0.28 0.30
-
-
-
0.27 0.24 0.24 0.30
0.48
-
-
-
-0.44
-0.62 -0.48
-0.91 -0.61 0.80
-
-
0.29
-
1192
3 Dynamics and Physical Properties
3.11.4 Perspectives Studies on MMC have been among earliest applications of photoemission spectroscopy to inorganic compounds, with major implications in the study of metal surfaces, molecular adsorption, and catalysis.['] Traditionally important subfields, such as supported catalysts from molecular precursors, still benefit from XPS. A progressively increasing share of studies is, however, contributing to fundamental and applied materials science issues. The electronic properties of small semiconducting particles, the magnetic behavior of metal clusters, and the superconducting nature of metal clusters are being investigated by different g r o u p ~ . [ ' ~ ] While research on clusters has grown to encompass what can be done with these species, and nanophase materials have found commercial applications, photoemission has undergone a new evolution step, represented by the new area of photoelectron spectromicroscopy.[21By adding lateral resolution to the long-standing high-energy resolution of photoemission spectroscopy it is now possible to obtain a chemical image of a surface. The actual resolution is still much lower than the diffraction limit, but projects have started with the intention of approaching it, by making use of the radiation from the existing third (and announced fourth) generation synchrotron rings. Clusters could indeed be seen with a dzerent eye, in the next future, by means of photoemission.
Acknowledgments Collaboration with Dr D. van der Putten is gratefully acknowledged.
References [ I ] S.B. DiCenzo, G.K. Wertheim in Clusters qf Atoms and Molecules ZZ (Ed.: H. Haberland), Springer-Verlag, Berlin, 1995, pp. 361-383; R. Zanoni in Physics and Chemistry of Metul Cluster Compounds (ed.: L.J. de Jongh), Kluwer Academic Publishers, Dordrecht, 1994, pp.159-182; M.G. Mason in Cluster Models jur Surfuce und Bulk Phenomena (Eds.: G. Pacchioni, P. Bagus, F. Parmigiani), Plenum Press, New York, 1992, p. 115-129; L. Guczi in Metal Clusters in Catalysis (Eds.: B.C. Gates, L. Guczi, H. Knozinger), Elsevier Science Publishers, Amsterdam, 1986, p. 209-219; S.B. DiCenzo, G.K. Wertheim, Comments Solid State Phys. 1985,11, 203-219; P.H. Citrin, G.K. Wertheim, Phys. Rev. B 1983.27, 3176-3200. [ 21 G. Margaritondo, Introduction to Synchrotron Radiation, Oxford University Press, New York, 1988, pp. 139-181.
3.11 The Transition from Low- to High-nuclearity Molecular Metal Clusters
1193
[3] W.F. Egelhoff, Jr., Surf: Sci. Reports 1987, 6 , 253-415. [4] a) G.K. Wertheim, S.B. DiCenzo, S.E. Youngquist, Phys. Rev. Lett. 1983, 51, 2310-2313; b) G. Mason, Phys. Rev. B, 1983, 27, 748-762; c) M. Chi, M. De Crescenzi, F. Patella, N. Motta, M. Sastry, F. Rochet, R. Pasquali, A. Balzarotti, C. Verdozzi, Phys. Rev. B 1990, 41, 5685-5695. [5] G.K. Wertheim, S.B. DiCenzo, D.N.E. Buchanan, P.A. Bennett, Solid State Commun. 1985, 53, 377-38 1. [6] D. van der Putten, R. Zanoni, J. Electron Spectrox Relat. Phenom. 1995, 76, 741-745; D. van der Putten, R. Zanoni, Phys. Lett. A 1995, 208. 345-350; D. van der Putten, R. Zanoni, C. Coluzza, G . Schmid, J. Chem. Soc., Dalton Truns. 1996, 1721-1725. [7] G. Schmid in Clusters and Colloids. From Theory to Applications, (Ed.: G. Schmid), VCH, Weinheim, 1994, pp. 178-208; R.C. Thiel, R.E. Benfield, R. Zanoni, H.H.A. Smit, M.W. Dirken, Struct. Bonding 1993, 81, 1-39. [8] G. K. Wertheim, J. Kwo, B. K. Teo, K. A. Keating, SolidStute Commun. 1985, 55, 357-361. [9] G.K.Wertheim, S.B. DiCenzo, D.N.E. Buchanan, Phys. Rev. B 1986, 33, 5384-5390. [lo] L. Ley, F. R. McFeely, S. P. Kowalczyk, J. G. Jenkin, D.A. Shirley, Phys. Rev. B 1975, 11, 600-612. [ 111 D. van der Putten, R. Zanoni, J. Electron Spectrosc. Relat. Phenom. 1995, 76, 207-21 I; D. van der Putten, R. Zanoni, Phys. Lett. A 1995,208, 351-355. [I21 D. Fenske, H. Krautscheid, S. Balter, Anyew. Chem. Int. Ed. Engl. 1990, 29, 796-798; D. Fenske, H. Krautscheid, Angew. Chem. Int. Ed. Engl. 1990,29, 1452-1454; H. Krautscheid, D. Fenske, G . Baum, M. Semmelmann, Angeiu. Clzem. Int. Ed. Enyl. 1993, 32, 1303-1305. [13] S.D. Waddington, in Practical Surjace Analysis, (Second Edition), Vol. I (Eds.: D. Briggs, M. P. Seah), John Wiley & Sons Ltd, Chichester, 1990, pp. 587-594. [14] Science, 1996, 271, 920-941; B. Weitzel, H. Micklitz, Phys. Reo. Lett. 1991, 66, 385-388; A.P. Alivisatos, J. Chem. Phys. 1996, 100, 13226-13239; A.W. Castleman, Jr., K.H. Bowen, J. Chem. Phys. 1996, 100, 12911-12944.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.12 Structure, Morphology, and Interface Structure of Supported Metal and Alloy Particles - HRTEM Studies Suzanne Giorgio, Houda Graoui, Claude Chapon, and Claude Raymond Henry
3.12.1 Introduction Determination of structure and crystalline defects is necessary to understand the physical properties of particles nucleated on crystalline substrates, in a matrix, or directly in the gas phase or in solution. Indeed, the reactivity of small particles and their magnetic and optical properties are related to their size, structure, morphology, and lattice deformations. Different characterization techniques, for example X-ray diffraction,[' EXAFS,['] SEELFS,[31electron diffraction ( LEED,141RHEED,['] SAED,@]and CBEDL7'), and electron microscopy ( H R T E M p WBDFL9I)give information about crystal structure, lattice distances, or morphology. All these techniques give average information about the shapes and the lattice distances of particle collections and of isolated particles. To obtain information about the particles at the atomic level, HRTEM is necessary. Other information about the surface structure of small particles can be obtained by STM."'] Most HRTEM studies have been devoted to determination of the internal structure and defects of single crystalline and multiple twinned particles at the atomic level,"'~l and sometimes atomic diffusion in the surfaces in real Depending on the nature of the sample, different techniques of observations and sample preparations are used in electron microscopy. For particles produced in solution['41 or from inert gas aggregation,[' 51 clusters are generally collected on a microscope grid covered with an amorphous carbon film or with micro-crystals. Particles can, however, diffuse on the substrate and structural instabilities sometimes occur during the measurements.['61 For particles supported on a soluble substrate ( NaCl, KCl, MgO, etc.), a carbon layer is evaporated onto the particles and the carbon layer containing the particles is then stripped from the substrate by interfacial dissolution. The particles are observed directly in top vie^.['^-'^^ The
3.12 Structure, Morphology, and Interjiace Structure of Supported Metal
1 195
3D shapes and, occasionally, lattice strains and stacking faults, can be determined by WBDF for particles larger than 9 nm1.[19*20.213 For insoluble substrates (SiOz, A1203, TiO2, CeO2, etc.) the simplest technique consists in growing the particles on micro-crystals of oxide, small enough to be transparent to the electrons; the particles are then observed directly in top view and in c r o ~ s - s e c t i o n . [ ~ ~ ~ ~ ~ ~ Another way of observing particles supported or embedded on or in thin films is or to prepare the thin films of the substrate on soluble single crystals (NaCI, etc.)[261 on lamellar crystals[271(e.g mica, graphite, or MoS2) and then to remove the films by dissolution or cleavage of the support. The applicability of HRTEM for the determination of the shape and structure and investigation of the interface of supported particles is, however, limited to use with stable systems. For Au particles smaller than 2 nm, it has been shown that the shape can change continuously (in less than 0.1 s) in the electron beam.['6.241 Even supported on alumina spheres,['*] or on amorphous silica,[2y12-4 nm gold particles fluctuate between single crystalline structures and multiple twins, and Au particles deposited on MgO microcrystals produced in air can move on the substrate in the electron beam, fluctuate (quasi melting phenomena), and maintain a stable position after falling down in epitaxy on the surface.[243301 Metallic particles grown epitaxially, in situ, on clean MgO micro-cubes are always stable during HRTEM observation^.[^ This technique of direct profile imaging is powerful for the determination of the 3D shape of very small particles (2-8 nm) and the structural characterization of the interface at the atomic level, as for example the Pd/MgO and Pd/ZnO interface^.[^^,^^] For particles in a matrix, e.y. Au or Fe particles embedded in Mg0,[341even with very thin samples the matrix contrast is still too strong compared with the contrast of the particles. Tilting the sample by a few degrees from the Bragg orientation enables attenuation of the image of the matrix before that of the particles, which are smaller and less sensitive to the orientation. HRTEM is the only technique which can be used to determine the size and the edges of particles of this type of sample. In this paper, we review the determination of the shape and the structure of metal particles by electron microscopy as illustrated by examples of Pd clusters epitaxially oriented on oxide single crystals and thin films of MgO and ZnO. The metal-oxide interfaces are characterized by HRTEM profile-view imaging, numerical analysis of the images, and image simulations by the multi-slice technique. Alloying with other metals can modify crystal structure. Different techniques, e.y. low energy ion scattering[351and HRTEM[361have been used to determine segregation in small particles. We show here the information obtained by HRTEM imaging at the level of the interface. Gas adsorption on particles modifies crystal shape and sometimes the interaction with the substrate and the structure of the interface. The evolution of the particle shape after gas adsorption at high temperature has been studied by TEM137,381 and AFM.[391
1196
3 Dynamics and Physical Properties
We show in this paper that association of HRTEM and WBDF imaging enables quantification of structural and morphological transformations induced by oxidoreduction cycles.
3.12.2 Sample preparation Pd particles (1-15 nm) were prepared by ultra high vacuum (UHV) condensation of the metal on clean MgO or ZnO microcrystals, on ZnO (00.1) thin epitaxial layers synthesized in situ and on clean (001) MgO single crystals, air cleaved and in situ cleaned. The metal was evaporated from a Knudsen cell at a calibrated flux of 1013 atoms cm-2 s-'. The temperatures of the substrates were between 400 "C (for ZnO layers) and 650 "C (for MgO single crystals). Micro-cubes of MgO and prisms of ZnO were obtained by burning a Mg or Zn ribbon in a mixture of pure gases (N2 0 2 ) at 700 torr. The oxide smokes were collected on microscope grids covered with an amorphous carbon film. Thin films of ZnO (00.1) were produced on a lamellar crystal of mica, from an electron-gun evaporator. MgO single crystals were cleaved in dry air and quickly annealed under ultra-high vacuum (UHV) at 900 "C.
+
3.12.3 Shape and structure The preparations were observed by transmission electron microscopy, with a Jeol 2000 FX for selected area diffraction (SAD) and convergent beam (CBED), a Jeol FEG 2010 and a Jeol4000 EX instrument for high-resolution imaging.
3.12.3.1 Pd-MgO (001) Large Pd particles were produced on MgO single crystals, cleaved in air, and cleaned in situ. Fig. 1 shows a collection of Pd particles (10 nm), in top view. They are of three types - square, truncated at the four corners, and rectangles truncated at the corners elongated in the [ 1001 and [ 1 101 directions. Fig. 2 shows a particle in profile view, with re-entrant angles near the interface with the substrate. The HRTEM image in top view (Fig. 3) shows that the structure is fcc and the edges correspond the [loo] and [110] directions. The series of WBDF images taken at increasing tilts enable quantification of the extensions of the (100) and (111) facets. The square particles are octahedra limited by eight (111) faces, asymmetrically
3.12 Structure, Morphology, and Interface Structure of’ Supported Metal
1 197
Figure 1. Collection of 10 nm particles, (001) oriented.
truncated at the top and interfaced with (001) faces, and truncated at the four corners by (100) less extended facets. The shape is drawn in Figs. 4a and 4b (in top view along [OOl] and in profile view along [ 1101, respectively). The ratio between the surfaces (100) and (1 1 1) is estimated to 0.15, the average distance h2/hl is 0.45 and h l / L = 0.47. Because the WBDF technique does not enable determination of the 3D shape of particles smaller than 6 nm, such particles must be observed in top view and in cross-sections on the different faces of MgO r n i c r ~ - c u b e s .Fig. [ ~ ~ ~5a is an overview of a collection of micro-cubes covered with particles on the different faces. The combination of top and profile views in HRTEM in the [ 1001 and [ 1101 directions (5b, c), shows that particles are limited in the [ 1001 and [ 1101 directions only. The shapes are half octahedra limited by four (1 11) faces and truncated at the top by a (001) face. The atomic columns of Pd seen in Fig. 5c near the interface show that the shape does not contain re-entrant angles even at the level of the first Pd layer. The average ratio between the height and the side at the base is 0.4.
Figure 2. Profile view of a large particle.
1198
3 Dynamics and Physical Properties
Figure 3. HRTEM image of a Pd particle (001) oriented, limited by [ 1001 and [ I101 directions.[531
3.12.3.2 Pd-ZnO (100) Pd particles are obtained by UHV condensation on ZnO micro-prisms prepared in situ by burning a Zn ribbon in a mixture of pure gases.[271Fig. 6a is an overview of particles on the different faces of ZnO micro-prisms. As seen in the HRTEM image of a prism with one lateral (10.0) face parallel to the electron beam (Fig. 6b), all the particles are epitaxially oriented on ZnO according to the relationships (21l)Pd // (10.O)ZnO and [OllIPd // [OOlIZnO. Their shape is limited by the (111) and (100) faces.
3.12.3.3 Pd-ZnO (001) Pd particles were condensed under UHV on ZnO (001) thin films epitaxially grown in situ on mica single The epitaxial relationships between Pd and ZnO lattices were determined by electron diffraction. In Fig. 7a, the 200 spots from Pd are close to the 110 reflections from ZnO and 330 from mica. The epitaxial relationships are (1 11) Pd // (001) ZnO
3.12 Structure, Morphology, unnd Interflce Structure of Supported Metul
1 199
(b)
MgO (1 10) Figure 4. Drawings of a particle: (a) in top view [OOI]; (b) in profile view [ 1101.
and [ 1121 Pd // [ 1001 ZnO. For particles larger than 20 nm a tetrahedral shape was observed by use of weak-beam dark-field electron microscopy at increasing angles, as seen in Fig. 7b. The particles are tetrahedra limited by three (1 11) facets at the edges and truncated at the top by a (1 1 1) face and at the corners by (100) faces. The truncations at the edges vary according to the particle sizes, but the truncations at the top are more regular; the average ratio of their height to the side of the triangular base is 0.7.
1200
3 Dynamics and Physical Properties
Figure 5. (a) Overview of particles on the different faces of MgO micro-cubes. (b) Profile view of small Pd particles (2 nm), epitaxially oriented (001) on MgO, observed along MgO[100].L3'1(c) Profile view of small particles, epitaxially oriented (001) on MgO, observed along MgO[ 1
3.12 Structure, Morphology, and Interface Structure o j Supported Metal
1201
Figure 6. (a) Overview of particles on the different faces of ZnO mi~ro-prisrns.[”~ (b) Profile view of a ZnO prism [loo] oriented. The particles are seen in profile view along their [ I 111 direction. They are epitaxially oriented on ZnO according to the relationships (21 1)Pd // (100)ZnO and [OllIPd // [001]Zn0.[331
1202
3 Dynamics and Physical Properties
Figure 7. (a) Diffraction pattern o f a collection of Pd particles (111) oriented on ZnO (001)Mica.[271(b) WBDF images o f a particle tilted through 3.5", 4, and 6".[271
3.12.4 Characterization of the interface HRTEM is the most powerful technique for the study, at the atomic level, of interfaces between continuous deposits of metals and oxide substrates ( Nb-A1203, Ag-CdO, Cu-Al203, Pd-MgO, Pd-A1203)[36p381.For supported clusters, the preparation technique for observations in cross-section does not enable preservation of the isolated particles. Some information was obtained for gold and iron particles embedded in a MgO for example strains near the edges, because of the lattice misfit with MgO. The lattice strains were represented by distortions in the MoirC fringes. For metal particles on oxide surfaces, the most convenient means of direct observation of the interface is the direct profile imaging technique using micro-crystals.
3.12.4.1 Pd-MgO (001) Pd particles larger than 6 nm cannot be produced on micro-crystals. A top view of an 8-nm particle grown on air-cleaved MgO is seen on a carbon replica in Fig. 8. It
3.12 Structure, Morphology, und Interface Structure of Supported Metul
1203
Figure 8. HRTEM image of a particle with a tilt of 2" relative to the perfect (001) orientation. The blurred fringes are marked by arrows.
is tilted by 2" to the Bragg orientation, with the tilt axis parallel to a [ 1001 direction. A set of large fringes with blurred contrast appears after tilting and does not disappear as the focus is varied. The average distance between these fringes was 34 nm. The most plausible interpretation of the origin of these fringes is the existence of dislocations in the Pd lattice at least in the neighborhood of the interface, because of the 8% misfit between Pd and MgO. Indeed, simulations of the Moire fringes corresponding to the superimposition of four expanded Pd layers (by 8%), with 20 layers with the bulk value, do not give a visible contrast. It has, on the other hand, been shown[431that in bulk Pd on bulk MgO lattice of dislocations were formed in the three first Pd layers, with a spacing of 3.7 nm. The HRTEM images of Pd particles smaller than 6 nm, seen in profile view on MgO micro cubes (Fig. 9a), were recorded for numerical analysis. The intensity profiles along the [ 1001 directions parallel to the interface were averaged among particles with the same size to deduce the exact value of the distance between the (200) lattice planes perpendicular to the interface in each Pd layer from the interface to the top of the ~ a r t i c 1 e . I The ~ ~ ' MgO lattice was taken as an internal calibrant. The smallest particles (1 -2 nm), are found with the same lattice parameter as the MgO. The 8% dilation is homogeneous from the interface to the top. The middlesized particles (3-4 nm) are accommodated on the MgO at the interface and the upper layers have a lattice parameter between Pd and MgO. The 5-nm sized particles have the parameter of bulk Pd in the volume and they are progressively accommodated on the substrate at the level of the interface in the three first layers. The average lattice distance measured in each layer as a function of its distance from the interface is shown in Fig. 9b. The increase of the lattice distance measured in the upper layers (from the 10th layer) is only an artifact arising from the edge
1204
3 Dynamics and Physical Properties
t
Figure 9. (a) Pd particle ( 5 nm) in cross-sectional view.'321(b) Averaged distance between the (200) lattice planes of Pd, normal to the interface in the successive layers from the interface with MgO (layer number 1) to the top.'32'
effect. Indeed, on the simulated image of a 5-nm particle with the bulk value, just accommodated by the substrate at the level of the interface, the same increase is observed near the top.[321
3.12.4.2 Pd-ZnO (100) The ZnO prisms [ 1001 oriented with a pair of lateral faces parallel to the electron beam contrast sharply in HRTEM imaging, compared with the Pd particles.[331If the prisms are tilted by 15" around the [001] axis from this orientation and then the particles in (211) epitaxy on ZnO (100) are, as before, seen along their [ 1 1 11 axis, the contrast of ZnO disappears. Fig. 10 shows a large particle (7 nm) viewed along the [ 11 11 axis of observation. All around the particle, in the faintest parts, the contrast is different from that in the center. Indeed, the Pd lattice is dilated between 3 and 5'1/0 in this area. This local deformation can be interpreted as dilatation in the three first deposited layers at the interface, and complete accommodation of the first layer on the substrate.
3.12.4.3 Pd-ZnO (001) From the diffraction pattern corresponding to samples ( Pd-ZnO-mica) with large or small particles, as shown in Fig. 7a, taking as a reference the spots from mica,
3.12 Structure, Morphology, and Interjuce Structure o j Supported Metal
1205
Figure 10. Pd particle (7 nm) seen in the [ 1 I 11 direction with dilation of the lattice at the interface.'"'
the lattice parameter of Pd is measured with a precision better than +2%. Despite the large misfit of 11% between Pd( 11 1) and Zn0(001), no large expansion appears in Pd particles, as was observed on other faces of ZnO. For the interface between bulk Pd and bulk Zn0,[451the Pd lattice was found without dislocations or deformations at the interface.
3.12.5 Alloying effect Bimetallic particles of PdCu and PdCu3 were first prepared by simultaneous condensation of both metals on NaCl (100) single crystals cleaved in situ under UHV c o n d i t i o n ~ . [ The ~ ~ *same ~ ~ ] kind of particles have been produced by decomposition of Pd(acac)2 and Cu(acac)2 in solution, on previously cleaned MgO micro-cubes. It has been s h o ~ n [that ~ ~small . ~ ~ Pd~particles (<6 nm) prepared from solution in this manner had the same characteristics (shape, structure, and structure of the interface) as particles condensed under UHV conditions. After annealing between 350 and 400 "C in a reducing atmosphere of H2 at 400 torr, PdCu particles have the fcc structure, but with undefined shape. Their size varies between 2 and 4 nm. Most are oriented (001) or (110) on MgO (OOl), as is apparent from Fig. 1 1. During annealing between 400 and 450 "C, the particles assume the p ordered structure of the bulk alloy (type CsCl). They are polyhedral and limited at the edges
1206
3 Dynamics und Physical Properties
Figure 11. HRTEM image of PdCu(cfc) particles (2-4 nm) oriented (001) and ( 1 10) on Mg0.'49'
by [loo] and [110] directions as for the -4 nm particle observed in top view in Fig. 12. They are all oriented in the (001) plane on MgO with [11O]PdCu // [ 1001Mg0, which corresponds to perfect accommodation of the lattices without deformation compared with the bulk, despite reduction by H,. By recording the intensity profiles along thc directions parallel to the interface, a slight contraction (by 1%) is measured, compared with the bulk parameter of PdCu. Depending on sample thickness, HRTEM images of p PdCu correspond to chemical images in which it is possible to differentiate between the atomic columns of Pd and the atomic columns of Cu in the [OOl] orientation, and the Pd layers or the Cu layers in the [110] orientation. The simulated images are given in Figs. 13a and 13b, respectively. In the HRTEM images of the interface PdCu-MgO, in the [ 1001 and [ 1101 directions of MgO, respectively, the contrast between Pd and Cu is
Figure 12. Top and profile views of PdCu particles on MgO (001).
3.12 Structure, Morplioloyy, and Interface Structure of Supported Metal
(a)
(b)
1207
Figure 13. (a) Simulated image of PdCu [ 1001 (thickness 2.9 nm) at the Scherzer focus. The bright spots represent the columns of Cu, the Grey spots, those of Pd.[491(b) Simulated image of PdCu in [ 1 101 orientation, thickness 4.2 nm. The origin at the bottom left is a Pd atom. The Grey spots in the dark fringes are Cu
attenuated near the interface with MgO. This attenuation of the contrast between Pd and Cu layers was observed on the simulated images at the interfaces in the [ 1001 and [ 1101 directions.[491According to the number of fringes from the particle center where the contrast is strong, however, the first layer of metal at the interface should correspond to a layer of Pd in contact with the MgO substrate. The PdCu3 particles were obtained with the cx ordered structure, (type AuCu3), after annealing at 500 "C in H2 for 5 days. Particles smaller than 20 nm are found in two main epitaxial orientations (001) and (1 10) on MgO (OOl), with a continuous structure and stacking faults. Larger particles had periodic anti-phase boundaries as observed in the bulk, with a modulation of 4 unit cells, corresponding to an atomic composition between 24.5 and 27% of Pd [491. Particles of PdCu and PdCu3 which have been reduced (under H2) at a temperature below 400 "C have the fcc structure (solid solution). According to the bulk phase diagram they should have the ordered p and CI structures, respectively. They are, however, transformed to the ordered structure by annealing at higher temperatures. The same behavior was also observed for particles prepared by UHV condensation of the two metals. The disordered structure of the alloy at low temperatures is explained by a growth kinetics The existence of the ordered structure at temperatures higher than expected from the phase diagram is, however, probably because of a size effect.
3.12.6 Annealing in gas atmospheres The first stages of oxidation of Pd particles, after annealing in 0 2 , is shown to occur preferentially from the (100) Pd faces.["] If Pd particles are grown epitaxially on MgO micro-cubes, annealing in lo-' torr 0 2 induces complete rounding of the particles and the interface with MgO becomes diffuse,[s11as seen in Fig. 14.
1208
3 Dynamics and Physical Properties
Figure 14. Particle in profile view, after annealing in
0 2
at lo-' torr.
Here, Pd particles prepared by UHV condensation on clean air-cleaved MgO single crystals are annealed in 0 2 at increasing pressures, from lo-* to torr, Extension of the (100) and at high temperatures (between 450 and 600 0C).r529531 faces and of re-entrant angles at the interface is observed and measured as a function of the annealing pressure of 0 2 , and of the particle size. Under these conditions, the increase of the re-entrant angles is the evidence that annealing reduces the wetting of MgO by the metal. torr, A series of WBDF images of a large particle (18 nm) annealed in 0 2 at is shown in Fig. 15; the figure shows important flattening. The particle is limited by (1 1 1) and (100) faces and truncated at the top by a large (001) facet. We have quantitatively measured the variation of the facet extensions for particles in the size range 10-12 nm.[533The particles are still three-dimensional, as is apparent from the series of images in Fig. 16, obtained after annealing in torr 0 2 .
Figure 15. Bright field and WBDF images of a large particle (18 nm) annealed in the particle is completely flattened.
0 2
at
torr;
3.12 Structure, Morphology. und Interfuce Structure of Supported Metal
(a)
(b)
Figure 16. Bright field and WBDF images of a particle (7 nm) annealed in
1209
(c) 0 2
at lo-' t ~ r r . [ ' ~ '
On increasing the pressure of oxygen from lop8 to lop5 torr, the (100) facets extend and the particles become flatter. From torr, PdO starts to appear at the edges of some particles and the complete formation in PdO occurs at lop' torr. The HRTEM images of the particles annealed under UHV or in low pressures ( lop8 torr) of 0 2 show that the junction between the (1 11) and (100) faces is sharp. Comparison of the experimental images with images simulated by the multi-slice technique shows that this junction is atomically sharp.[521On annealing at higher pressures ( lop5 torr) progressive rounding occurs at the corners; this is completely reversible by reduction in H2.
3.12.7 Discussion Pd and bimetallic PdCu and PdCu3 particles have been prepared, both under UHV conditions and by decomposition of organometallic compounds, on MgO and ZnO micro-crystals, single crystals or thin epitaxial layers. The shapes and structures of the particles and the structures of the interfaces, and the dependence of these on particle size and annealing conditions, were determined by HRTEM and WBDF imaging. All the Pd particles are fcc and epitaxially oriented (001) on MgO, (21 1) on ZnO (loo), and ( 1 11) on ZnO (00.1). Their shapes correspond to more or less truncated octahedra and tetrahedra. The octahedral shape of 10-20-nm particles on MgO is limited by eight (1 11) faces, truncated asymmetrically by (001) faces at the top and at the interface and by (100) faces less extended at the corners. The ratio of the height to the diameter of 0.46.
1210
3 Dynamics and Physical Properties
For particles smaller than 6 nm the shape is an half octahedron limited by four (111) faces and truncated by a (001) face at the top, without re-entrant angles at the interface. Particles smaller than 2 nm are dilated by 8% and perfectly accommodated by the substrate. The 5 nm particles are accommodated only at the level of the interface in the first two or three layers of deposited Pd. Pd particles smaller than 6 nm crystallized on the neutral (100) faces of ZnO are also dilated at the level of the interface by 5% and 2% along the two directions [110] and [OOl] of ZnO and accommodated by the substrate. All the particles epitaxially oriented on ZnO (00.1) are tetrahedra truncated at the top by a (1 11) face; the ratio of the height to the diameter is 0.7. Because of a superstructure between the lattices, despite the large misfit (1 lab), the particles retain the lattice parameter of the bulk even at the interface. For bimetallic PdCu particles crystallized on MgO, an ordered structure (type CsCl) is always obtained after annealing. The particles are epitaxially oriented (001) on the substrate, with perfect coincidence between the lattices. The correspondence between experimental and simulated HRTEM images, and numerical analysis, are indicative of a layer of Pd at the interface between the alloy and MgO, with the atomic columns aligned with those of MgO. Particles with the continuous ordered structure of PdCu3, with stacking faults, are obtained after a long annealing. Their are cap-shaped and mainly oriented (1 00) and (1 10) on MgO. For larger sizes (> 10-20 nm), long period superstructures with anti phase boundaries are obtained, with the same spacing as in the bulk alloy for equivalent concentrations. Adsorption of 0 2 at high temperature (before oxide formation) changes the surface free energy of the crystallographic faces and modifies the crystal shape. Increasing the coverage of 0 2 resulted in extension of the (100) faces of 10-nm particles; this was associated with flattening and an extension of the re-entrant angle. Oxygen pressures greater than lop5 torr induce progressive rounding of the particles. Oxide formation occurs at lop' torr, simultaneously with the appearance of a diffuse interface.
References [ l ] P. Guenard, G. Renaud, B. Villette, Physica B 221 (1996) 205; D. Thiaudiere, A. Naudon, J. Physique IV (1996) C4. 553. [2] A. Pinto, A. R. Pennizi, G. Faraci and G. d' Agostino, Phys. Rev. B 51 (1995) 5315. [3] M. Decrescenzi, Surf. Sci. Rep. 21 (1995) 89. [4] H. J. Freund, Angew. Chem. 109 (1997) 444. 151 M. F. Gillet, V. Matolin, J. Cryst. Growth, 134 (1993) 75. [6] M. Pan, J. M. Cowley, Ultramicroscopy 30 (1989) 385. 171 J. M. Cowley, J. C. H. Spence, Ultramicroscopy 6 (1981) 359.
3.12 Structure, Morphologj,, and Interface Structure of Supported Metal
12 11
A. K. Datye and D. J. Smith, Catal. Rev. Eng. 34 (1992) 129. M. J. Yacaman, K. Heinemann, H. Poppa, in Surf. Sci. Recent Progress and Perspective, Ed. R. Vanselow, CRC (1981). A. Piednoir, E. Perrot, S: Granjeaud, A. Humbert, C. Chapon, C. R. Henry, Surf. Sci. 391 (1997) 19. M. Fiueli, R. Spycher, P. Stadelmann, P. Buffat and J. P. Borel, J. Microsc. Spectrosc. Electron. 14 (1989) 351. J. M. Penisson and A. Renou, J. Cryst. Growth 102 (1990) 585. J. 0. Malm and J. 0. Bovin, Surf. Sci. 200 (1988) 67. J. 0. Malm, G. Schmid and B. Morun, Phil. Mag. A, 63 (1991) 487. S. Giorgio. J. Urban and W. Kunath, Phil. Mag A, 60, 5 (1989) 553. S. Ijima. T. Ichihashi. Phil. Rev. Lett. 56 (1986) 616; N. Doraiswamy, L. D. Marks, Phil. Mag. B 71 (,1995)291. C. R. Henry, C. Chapon, C. Duriez and S. Giorgio, Surf. Sci. 253 (1991) 177. J. M. Schwartz, L. D. Schmidt, J. Catalysis 138 (1992) 283. M. L. Sattler. P. N. Ross, Ultramicroscopy 20 (1986) 21. K. Miyazawa, Y. Ishida, Ultramicroscopy 22 (1987) 231. L. D. Marks, Surf. Sci. 150 (1985) 302. L. R. Wallenberg, A. Anderson, M. Sanati, Utramicroscopy. 34 (1990) 33. H. D. Cochrane, J. L. Hutchison. D. White, G. M. Parkinson, C. Dupas and A. J. Scott, Ultramicroscopy 34 (1990) 10. P. M. Ajayan, L. D. Marks, Phys. Rev. Lett. 63 (1989) 279. G. Fuchs, D. Neiman and H. Poppa, Thin Solid Films, 207 (1992) 65. T. Okabe, Y. Kagdwa and S. Takai, Phil. Mag Lett. 63 (1991) 233. S. Giorgio, H. Graoui. C. Chapon, C. R. Henry, Materials Sci.& Engineering A 229 (1997) 169. D. Ugarte, Electron microscopy, Proceedings EUREM 92, 2 (1992) 677. D. J. Smith, A. K. Petford-Long, L. R. Wallenberg, J. 0. Bovin, Science, 233 (1986) 872. S. Giorgio, C. R. Henry, C. Chapon, G . Nihoul, J. M. Penisson, Ultramicroscopy 38 (1991) 1.
S. Giorgio, C. R. Henry, C. Chapon and J. M. Penisson, J. Cryst. Growth 100 (1990) 254. S. Giorgio, C. Chapon, C. R. Henry, G. Nihoul, Phil. Mag. B, 67, 6 (1993) 773. S. Giorgio, C. R. Henry, C. Chapon, Microsc. Microanal. Microstruc. 6 (1995) 237. N. Tanaka, K. Mihama, Appl. Surf. Sci. 33/34 (1988) 472. A. J. Renouprez, K. Lebas, G. Bergeret, J. L. Rousset and P. Delichere, Proc. 1 lth Intern. Congr. Catal. J. W. Hightower, W. N. Delgass. E. Iglesia and A. T. Bell (Eds.), 101 (1996) Elsevier Science B. V. p.1 105. P. L. Gai, B. C. Smith, Ultramicroscopy 34 (1990) 17. N. M. Rodriguez, S. G. Oh, R. A. Dalla- Betta, R. T. K. Baker, J. Catal. 157 (1995) 676. A . Vazquez and F. Pedraza, Appl. Surf. Sci. 99 (1996) 213. R. Erlanddson, M. Eriksson, L. Olsson, U. Hemersson, I. Lundstrom and L. G. Petersson, J . Vac. Sci. Technol. B 9 (1991) 825. M. Florjancic, W. Mader, M. Ruhle and M. Turwitt, J. de Physique, 46 (1985) 129. [41] G . Necker and W. Mader, Phil. Mag. Lett. 58 (1988) 205. 1421 F. Ernt, P. Pirouz and H. Heuer, Phil. Mag. A, 63 (1991) 259. [43] P. Lu and F. Cosandey, Ultramicroscopy 40 (1992) 271. [44] T. Muschick and M. Ruhle, Phil. Mag A. 65 (1992) 363. [45] H. Ichinose, H. Ishii. T. Ichimori et Y. Ishida, Proceeding ICEM 13 Vol. 2A, Ed. B. Jouffrey, C. Colliex Les Editions de Physique, France (1994) 279. [46] F. Gimenez, C. Chapon, C. R. Henry, New J. Chem. 22 (1998) 1289. [47] F. Gimenez, C. Chapon, S. Giorgio, C. R. Henry, p-351, Electron Microscopy 1994, Proceedings ICEM 13- Vol 2A, Applications in Materials Sciences, Ed. B. Jouffrey, C. Colliex Les Editions de Physique, France. ~
~
1212
3 Dynamics and Physical Properties
[48] S. Giorgio, C. Chapon, C. R. Henry, Langmuir, 13 (1997) 2279. [49] S. Giorgio, C. R. Henry, Microsc., Microanal. Microstruct. 8 (1997) 379. [50] H. J. Ou, J. M. Cowley, Phys. Stat. Sol. A. 107 (1988) 719. [51] S. Giorgio, C. R. Henry, C. Chapon, C. Roucau, J. Catalysis 148, (1994) 534. [52] H. Craoui, S. Giorgio, C. Chapon, C. R. Henry, Electron microscopy 1996, Proceedings EUREM 96, Dublin Equilibrium shape of Pd particles (6-20 nm) annealed in 0 2 at low pressures. [53] H. Graoui, S. Giorgio, C. R. Henry, Surf. Sci., 417 (1998) 350.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry Jacqueline Belloni and Mehran Mostafavi
3.13.1 Introduction In recent decades increasing attention has been paid to clusters of a few atoms or molecules only. Because its energy levels are predicted['] to differ both from those of single atoms and from those of crystals, this ultradivided state of matter should have the specific physical and chemical properties of a new phase, denoted mesoscopic. The smallest clusters or oligomers, the properties of which are the most different, are, however, generally short-lived and special equipment is required to observe them. In 1973, it was foundr2]that after radiolytic reduction of copper ions into ultradivided copper in liquid ammonia solution, no metal was detected - as if copper was spontaneously corroded by the solvent as soon as it was formed. The reaction gave rise simultaneously to the evolution of molecular hydrogen, in contrast with metallic copper which is stable under the same conditions. These results were explained by invoking a drastic thermodynamic change owing to the extreme division of this 'quasi-atomic state' of matter generated by irradiation of the solution. This concept implied that for a given metal the thermodynamics depended on particle nuclearity, and enabled us to provide rational interpretation of other observations, for example the oxidation of radiolytic silver by oxygen in Experiments on the photoionization of silver atoms in solution have shown that their ionization potential is much lower than that of the bulk metal.[51It was also shown that the redox potential of isolated silver atoms in water (Eo(Ag'/Ago)) must be lower than that of the silver electrode (Eo(Ag+/Agmet)) by the sublimation energy of the metal - 2.6 V.[61 Later, reactions of Ag" or Ag2+ with different oxidizing molecules were systematically studied by pulse radiolysis; the high rate constants found were indicative of the electron-donating character of atomic silver,
1214
3 Dynamics and Physical Properties
whether free or associated with Ag+.[71In the early eighties, an increasing amount of experimental work with supersonic molecular beams in the gas phase emphasized the nuclearity-dependent properties of clusters of atoms or m01ecules,[~'~~ theoretically predicted by Kubo."] It is also interesting that this very general concept was already known from the radiation chemistry of solutions of metal ions as solvated metal aggregate~[l~-'~] and that it was applicable to all metals. It is no longer a controversial subject and is particularly useful for understanding the exotic phenomena which occur whenever a new phase of oligomeric particles is formed in the bulk of an homogeneous mother phase, phenomena which occur frequently in physics and chemistry. Moreover, owing to their specific electronic properties, these particles play a unique role in catalyzing chemical reaction^."^]
3.13.2 Pulse radiolysis principle Pulse radiolysis of a metal ion solution is a powerful method[l5] of generating isolated metal atoms and then studying their coalescence, and the redox properties of the transient clusters formed, by time-resolved spectroscopy or conductimetry. The short lifetime of metal atoms and oligomers, because of their coalescence or corrosion, usually makes their observation possible by pulse techniques only. As for other methods (chemical, electrochemical), the precursors are metal ions Mt . They are reduced to metal atoms by the rapid scavenging of reducing species generated from the solvent by short-pulse irradiation of the solution. The pulse is intense (the concentration of radicals formed at the end of the pulse is ca lop4 molL-') and lasts for 3 ns, i.e. much shorter than the reaction time of the primary radiolytic radicals. Therefore, immediately after the pulse, the evolution of the transient radicals formed in the solution, then the atoms, and subsequently the oligomers, can be observed directly by time-resolved spectrophotometry or conductimetry. Numerous pulse-radiolysis studies have been devoted to radiation-induced reduction of various metal ions[l6] and aggregation of metal atoms.[lO-'31 In particular, assignment of successive transient optical absorption spectra to the transient solvated oligomers and clusters of increasing nuclearity and of various charge results from the correlation between the kinetics of precursor decay and the formation of a new band. Also, the reactivity of short-lived atoms or oligomers with an added reactant can be measured if the reaction is faster than their decay as a result of coalescence. Thus the method provides important information on the chemistry, for example the redox properties, of small metal clusters, despite their transient character.
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1215
3.13.3 Nucleation and growth mechanisms 3.13.3.1 Principle of radiolytic formation of atoms and clusters Energy deposition throughout the solution ensures the homogeneous distribution of radiolytic radicals formed by ionization and excitation of the solvent, the molecules of which are the most abundant:" 71
The species eaq- and H' atoms are strong reducing agents"'] (E"(HzO/e,,-) = -2.87 VNHE and E"(H+/H') = -2.3 VNHE).Thus, in the following step, they easily reduce monovalent metal ions to the zero-valent state (Fig. 1). In the reactions below, M+ are monovalent metal ions, possibly complexed by a ligand:
+ eiq M+ + H"
M+
4
M"
4
Mo + H i
So 1vent
x
Figure 1. Radiolytic reduction of metal ions. After their formation, the atoms coalesce and also become associated with excess metal ions. The clusters can be stabilized by ligands, polymers or supports. The cluster redox potential increases with the nuclearity n . The smallest oligomers can undergo corrosion by H30+.['']
STABLE AGGREGATE
1216
3 Dynamics and Physical Properties
In contrast, OH' radicals can oxidize the ions or the atoms to a higher oxidation state and can thus counterbalance the previous reductions (Eqs. 2 and 3). For this reason, the solution generally contains a scavenger of OH' radicals. The preferred choice is for solutes whose oxidation by OH' yields radicals which are unable to oxidize the metal ions, but which do have strong reducing properties, for example the radicals of secondary alcohols or of formate anion. The H' radicals are also scavenged by these molecules.
(CH3)2CHOH+ OH'
-+
(CH3)2C'OH
+ H20
+ OH' COO'- + H20 (CH3)2C'OH + H2 (CH3)2CHOH + H' COO'- + H2 HCOO- + H'
HCOO-
-+
-+
-+
The radicals (CH3)2C'OH and COO'- are almost as powerful reducing agents as H atoms (Eo((CH3)2CO/(CH3)2C'OH)= -1.8 VNHE at pH 7, E0(C02/C00'-) = - 1.9 VNHE).['~] They are, therefore, able to reduce various metal ions:
+ (CH3)2C'OH Mo + (CH3)zCO+ HS M+ + COO'Mo + C02 M+
-+
-+
The atoms are formed homogeneously throughout the solution. The binding energy between two transition metal atoms is stronger than the atom-solvent bond energy. Therefore the atoms dimerize or associate with excess ions: (10)
M O + M + ~ M , +
By a multi-step process these species progressively coalesce into clusters according to:
Ma',,
+ M&Y:
4
M;:z
+ z with n + m = p and x + y = z
where n, m and p represent the nuclearities, i.e. the number of reduced atoms they contain, and x, y and z represent the charge of metal ions adsorbed on the clusters (Fig. 1). For free clusters in aqueous solution, in the absence of ligands and polymers, the aggregation process does not cease before high values of the nuclearity n are attained, leading to a colloid and even to a precipitate. It has long been known that the clusters can be stabilized by their interaction with an added surfactant or ligand which, as a result of electrostatic repulsion or steric hindrance, prevents, at an early stage, further coalescence (Fig. 1).
3.13 Radiation-induced Metal Clusters. Nucleution Mechanism and Chemistry
1217
3.13.3.2 Atom and charged dimer formation Soon after absorption of the irradiation pulse by a solution containing the monovalent solvated cation M+, the population of atoms is created by the reaction depicted in Eq. (2). Formation of the atom is correlated with the decay of the solvated electron and this correlation enables determination of the rate constant of the reaction. The silver ion aqueous solution was the first system thoroughly studied by pulse r a d i o l y ~ i s [ ~ , ~and , ’ *has ~ ~recently , ~ ~ ~ been revisited[22p251 (Fig. 2). The optical absorption spectra of transient silver atoms and charged dimers produced by the reaction depicted in Eq. (10) have been observed by pulse radiolysis in various solvents, for example water (Table 1). The rate constants are generally diffusioncontrolled, as are those for the corresponding reactions for formation of Tl0 and T12’ .[261
Solvent effect Although the properties of a bare metal cluster in the gas phase, e.g. the ionization potential, depend solely on the nuclearity, it is clear that any other molecule interacting with the metal atoms influences their behavior. Evidence of the strong influence of the atom-solvent interaction is given by the solvent-dependence of the optical absorption spectra in solution. For example, Table 213’] lists the wavelength maxima of the optical transitions of Ago and Ag2+. The absorption band is blue-shifted with the increasing solvent polarity as measured by the static dielectric constant.[331The maximum is red-shifted by increasing the temperature. These features are comparable with those of the solvated electron[341 and suggest a charge transfer to solvent (CTTS) character of the absorption spectra. Electron spin-echo modulation analysis in which the ion is suddenly converted to the atom by irradiation at 4 K in ice or methanol glasses has shown[351that the structure of the solvation shell of the precursor Ag+ (4D20 molecules) is maintained. After brief warming to 77 K before returning to 4 K, however, the analysis indicates a drastic change with one single deuteron (out of eight) moving much closer to the silver atom. The former structure of Ag+D20 is reversibly obtained by photoionization of the atom by excitation at Aexc = 400 nm. The solvated metal monomer can, therefore, be considered as a tight cation-electron solvent complex. These results suggest that the Ago-solvent interaction resembles an Ag+ core plus one negative charge mostly delocalized on the surrounding solvent molecules according to their thus supporting the CTTS structure put forward by Walker et u Z . [ ~ ’ No direct observation at room temperature has, however, yet been made on the dynamics of the reorganization of the solvent when the solvated cation is replaced by an isolated atom, because this process is much faster than the reduction reaction itself by esolv-.
I
300
lOnin
400
X [nml
-
20
CI
I
E
15
-t 10 U
-
Lr7
0 Y
W
0.5
0 2 40
400
300 A [nml
Figure 2. Absorption spectra of silver oligomers in water. (a) Transient spectra at increasing times after the pulse."'] (b) Calibrated spectra of Ago, Ag2+ and Ag32+.1221
Ligand effect The interaction between an atom or a cluster, even when neutral, and a solvent is n ~ t i c e a b l e [ ~and ~.~ induces ~' specific properties; that between a ligand and a metal is even stronger than the metal-solvent interaction. Thus the complexation has a marked influence on the reaction rate-constants and on the thermodynamics of the
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism clnd Chemistry
1219
Table 1. Formation rate constants and optical absorption maxima of metal atoms, hydrated or complexed. and of the corresponding charged dimers in water. Rate constant of atom formation,
Metal ion
kbI++eaq-
(L moI--l s-')[''] Ag+
3.6 x 10"
TI+
3 x 101"
In+ Ag(CN)r-
Absorption maxima of atoms, I. (nm)
Rate constant of charged dimer formation k M " + M , (LmoI-' s - ' ) [ ' ~ ]
Absorption maxima of charged dimers, 1. (nm)
8 x lo9
290, 308, 315, 325[20.22.2'1 700, 420, 245l26.271 310, 460[271 350, 410, 490[281 310, 340, 400, 475[28,291 3 15, 340
450, 260[26,271 1.4 x 10'
1.5 109 2 x 10'0
-
5 x 109
1.6 x 10'
Ag(EDTA)'-
1.7 x lo9
400, 450[28'
Ag(NH3)zAu(CN)?Cu(CI)3?-
3.2 x 10'O 1.1 x 10"' 2.7 x 10'"
350, 385[29.301 420[3'I 38OLz7] 4.9 -
-
-
107
360[271
species Mo or M2+. Again, silver solutions constitute a model system, because of the possible monoelectronic reduction of the ions to the zero-valent state, their stability as non-complexed cations, and their ready complexation with various ligands. When Agf reduction occurs in the presence of the CN- ligand in water, the reaction rate-constant of eaq- with Ag1(CN)2- is 20 times lower[281than that for the reaction with hydrated Ag+ (3.6 x 10" L mol-' s-1)[16*201 (Table 1). eaq-
+ Ag'(CN)?-
+
Ag"(CN)22-
k12 =
5 x lo9 Lmol-I s-'
(12)
The product of reaction depicted by Eq. (12) was assigned as the transient cyano complex of the silver atom Ago(CN)2'-. Its absorption spectrum is characterized
Table 2. Wavelengths of optical absorption maxima I.,,, Cluster species
HrO
360 308 275
(nm) of transient silver species in various
NH'
EDA
HF
At 23 "C
At -50 "C
435 390
450 380
440 345
300
-
-
-
-
1220
3 Dynamics and Physical Properties
d
0
Figure 3. Absorption spectra of a silver atom Ago (top) and charged dimer Ag2+ (bottom) complexed by CN-, EDTA, and NH3 in solution. The spectra of the uncomplexed hydrated species are shown for comparison.[291
by two bands at 450 and 500 nm (Fig. 3 and Table 1). It seems that the 450 nm band of Ag0(CN)z2-corresponds to the 360 nm CTTS band of Ago red-shifted by complexation, whereas the band at 500 nm results from a metal to ligand charge transfer (MLCT). As already observed in the pulse radiolysis of hydrated Ag+,[7,221 the formation of the atom in the presence of CN- is followed by formation of the charged dimer.[’
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
Ago(CN)22-
+ Ag'(CN)2-
+
(Agz', XCN-)
k13 = 2 x 10"
+ (4
-
1221
x)CN
LmolF' s-I.
(13)
An intense band assigned to (Ag2+, xCN-) occurs at 350 nm with a shoulder at 410 nm (Table 1). This spectrum is red shifted relative to hydrated Agz+.1281 Another maximum at 490 nm is assigned to a MLCT band (Fig. 3). The value of the rate constant kl3 is as large as in the absence of CN- and explains the early formation of the bands already present at 900 ns. In the presence of EDTA , the silver ions Ag'Y3- complexed with the fully deprotonated form, Y4-, of EDTA (four COO- groups) are reduced by eaq-. The optical absorption maxima of the complexed atom Ag'Y4- are at 400 and 450 nm (Fig. 3). eaq-
+ Ag'Y3--
-+
AgoY4- k14= 1.5 x lo9 Lmol-I s - ~ . ~ *
(14)
After association of this complexed atom with an excess ion, the spectrum of the complexed charged dimer (Ag2+, xY4-) has three bands at 310, 340, and 400 nm (Fig. 3).[281 When complexed by NH3, the silver ions Ag'( NH3)2+ are also readily reduced by eaq- to the complexed atom: eaq- + Ag'(NH3)2+
i
Ago(NH3)2 kls
=
3.2 x 10'' Lmol-' sC1.30 (15)
The spectrum of the complexed atom contains three bands at 345, 385, and 435 nrn1301(Fig. 3). Note that Ag" fully solvated in NH3 also has an absorption maximum at 435 nm. The formation of other metal atoms complexed by ligands has been observed; (A, ~ - = 380 nm)12'] these include Auo(CN)2*- (Amax = 410 nm)[311and C U O C ~ ~ (Table 1).
3.13.4 Coalescence processes Time-resolved observations of transient signals of optical absorption, light scattering, or conductivity have been used to investigate the kinetics of the coalescence from atoms to ~ l u s t e r s . ~After ' ~ ~ the ' ~ ~pulse, the total concentration of metal atoms does not change during the coalescence, although the average cluster nuclearity increases and the particle concentration decreases. The time-dependent absorbance is therefore a consequence of the nuclearity-dependence of the optical properties
1222
3 Dynamics and Physical Properties
(spectrum shape and extinction coefficient per atom). The growth kinetics have been systematically treated by numerical simulation studies to derive, by comparison with experimental data, information about the nucleation rate constant and the optical properties of transient c l u ~ t e r s .The ~ ~ coalescence ~ ' ~ ~ ~ rate constant kd for the reaction between two small clusters (Eq. 11) can be assumed to be independent of the nuclearity n. Thus, the value of kd can be derived from kinetic results from the early stages of the growth process. From the general model of growth kineticsc361 resulting from all coalescence reactions between two clusters of any nuclearity n, it has been stated that the kinetics of formation and decay of the different clusters only depend on k d , and xo = [M1],=,. The kinetics are unchanged if the cluster concentrations x, = [M,] are normalized as X , / X O relative to the initial concentration of atoms and the time is expressed as t / z where z = l / ( k d x ~ ) [ ~ ~ ]
3.13.4.1 Hydrated clusters Pulse radiolysis studies of silver solutions with a low radiation dose per have shown that the Ag2+ species, instead of dimerizing into Ag42+ (Eq. 16),c381 reacts with another cation to yield Ag32+ (Eq. 17).c371 Agz+ + Ag2+ -+ Ag42f 2k16 = 4 x los 1 mol-' Ag2+ + Ag'
-+
Ag3+ k17
= 2.0
SKI
(16)
x lo9 1 mol-' s-*.
(17)
The transient Ag3 2+ ion have an intense absorption spectrum with two maxima, at 310 and at 265 nm. Its second-order decay leads to the cluster Ag42+. Under total reduction conditions the neutral dimer Agz is observed at 275 and 310 nm. The optical transitions of low-nuclearity silver oligomers, the rate constants, and the extinction coefficients are derived from adjustmentc3'I between experimental (Fig. 2, bottom) and calculated absorption spectrum evolution. An even number of atoms favors the high stability of the 'magic' hydrated clusters Ag42+ (275 nm), Ags2+ and possibly Ag14~+(Fig. 2, top).c391After a longer time, the plasmon band of larger silver clusters develops at ca 380 nm (~380(Ag,,SO:-) = 1.5 x lo4 Lmol-' s-1).c381 Because the total concentration of atoms is constant during the growth, the observed change with time of the total absorbance, ZE, x n x x,, results from the variation of the extinction coefficient per atom as a function of n and of the above time evolution of the size distribution. It has been shown for silver that the shape of the absorption band and the extinction coefficient value do not change further beyond n > 13.[381 The growth of thallium clusters has been also observed by time-resolved optical spectroscopy. The coalescence steps are comparable with those of silver and the final plasmon band of T1, is located at 300 nm.[261
1223
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
3.13.4.2 Ligand effect on cluster growth A g n CN ~ For silver complexed by CN-, the surface plasmon spectrum of clusters develops with a maximum close to 395 nm, similar to that of hydrated clusters without ligand, but with a higher extinction coefficient per atom ~ 3 9 5 (Agn.cN-)= 2.1 x lo4 Lmol-' s - ' . ' ~ The ~ ' absorption band is much narrower. The coalescence kinetics of small clusters (Fig. 4a) are much slower than for hydrated clusters at the same concentration (Fig. 4b), but the kinetics can be superimposed after normalization in z. A value of the coalescence rate constant kd(Ag,,,, ) = (6 2) x lo6 Lmol-' sP1 in the presence of cyanide was derived according to the growth model; this value is one-thirtieth that for the sulfate.[401
. , . . , . . . , l . . , . l . . . . , , . . .
05
-
04
-2
ddTF-N)-Ty -
03
4dJ
M
6
n 0
Figure 4. The growth kinetics of silver clusters observed by means of their absorbance at
02
-
-
-
01 b
0
"
"
*
.
.
~
.
'
~
~
.
.
'
.
.
r
.
'
.
r
,
.
.
1224
3 Dynamics and Physical Properties
Agn,PA
Pulse radiolysis experiments on a solution containing silver ions complexed by polyacrylate PA enabled us to observe two absorption bands - at 275 nm, already seen for the tetrameric charged clusters Ag42+, and at 350 nm, assigned to the clusterPA i n t e r a ~ t i o n . [ ~The ' . ~ ~further ] dimerization is so slow (one hour) that it can be observed after y-irradiation. The 275 band is shifted progressively to 292 nm while the 350 nm band fades away and is replaced by a 480 nm band. The new spectrum has been assigned to a cluster containing four reduced silver atoms. After a month the visible band is red-shifted to 800 nmr431as a result of the very slow reorganization of the cluster-PA interaction. These 'blue' silver clusters are stable for years, even in the presence of air, and have been imaged by STM. Clear images show that each cluster contains 7 (or 8) nuclei. Because IZ = 4, the clusters correspond to the stoichiometry Ag73+ (or Agg,4+).[441
A&, CN The formation of gold oligomers and, after a long time, of gold particles Au,,cN-, that are also associated with complexed gold ions, is observed at 520 nm.[313451 The extinction coefficient of AU,,CN- increases with nuclearity up to E ~ ~ ~ ( A u , . c=N - ) 4000 Lmol-' s-'. The absorbance increase as a result of coalescence is extremely slow and lasts for 30 s (compared with 20 ms for Ag,,cN-, Fig. 4a), probably because of the stronger complexation of gold with CN-. The estimated kd value is lo4 Lmol-' s-l
Gun. CI The pulse radiolysis study of a solution containing the monovalent copper complexed by C1- has shown an optical absorption band at ca 380 nm as a result of the formation of Cuo,Cl-.[271After association of this complexed atom with excess of the ion, the complexed charged dimer (Cu2+,Cl-) is formed with a new absorption band at 360 nm. This latter species either dimerizes into C Q ~ +C1, or reacts with another cation to yield C U ~ ~Cl-.[271 + , These copper oligomers absorb at 410 nm. The coalescence is very slow, in contrast with the growth after the reduction of Cu2+, and leads to Cu, with a surface plasmon absorption band at 570 nm.[461
3.13.4.3 Effect of dose rate on cluster growth The mechanism depicted by Eqs. (2) and (8-11) combines the steps of reduction (by solvated electrons and radicals), coalescence, and ion adsorption. If the rate at which the dose is absorbed and thus at which the reducing species are generated is low, the coalescence and adsorption resulting from free diffusion in the solution
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1225
might start long before all the ions are reduced. Therefore, the ions attached to clusters formed early in the process are later reduced in situ (also by solvated electrons and radicals), instead of producing free atoms and new nuclei. This contributes to the growth of a small number of clusters with higher nuclearities, even in the presence of a stabilizer, and the final mean nuclearity is higher. In contrast, a quasi-instantaneous reduction of all the ions, achieved before diffusion, by a high dose-rate pulse, or train of pulses, practically prevents the adsorption reactions depicted in Eq. (10). The atoms are initially isolated and after their coalescence the cluster concentration is higher and the mean size is smaller than at low dose rate. A high dose rate favors nucleation rather than growth. This dose-rate effect is systematically apparent from the final size distribution of clusters when imaged by electron or atomic force m i c r ~ s c o p y . [ ~Note ' ~ ~ ~that ] the homogeneous mixing of metal ions with a chemical reducing agent in solution is not as fast as pulse production of radiolytic radicals close to the ions.
3.13.4.4 Cluster formation from multivalent ions Pulse radiolysis has been used to collect many data on the reduction rate constants of multivalent ions M'+, on the transient absorption spectra of unstable valences M(-yp')+,and sometimes on the rate constants of their disproporti~nation.[~~~~~] All transition metal ions are reducible by solvated electrons into M(x-')+. With long pulses or with a repetitive pulse regime, multistep reduction of M"+ to the zerovalent state Mo can be achieved. The coalescence of the atoms then leads to metal clusters. Although stable clusters of numerous metals have been synthesized by r a d i o l y s i ~ , [the ~ ~ ]mechanisms of formation are far less clear than for the clusters arising from monovalent ion reduction, owing to the multi-electronic reduction process and to competition between the different valences. The growth of platinum clusters formed by reduction of Pt"' chloride has been observed,r501as has the growth of Cd,,[51,521 Pdn,[531 and Pb,.r541
3.13.4.5 Coalescence of bimetallic clusters Pulse radiolytic studies of the kinetics of formation of clusters containing two different metals are more readily accessible, for the reasons given above, when both ions may be reduced by a monoelectronic process. This can be achieved with mixed solutions of the monovalent ions Ag' and Au', in the form of KAg(CN)Z and K A U ( C N ) Z . [The ~ ~ ] evolution of the optical absorption spectrum with time was followed specifically at 400 and 520 nm, which correspond to the maxima of the surface plasmon bands of the monometallic silver and gold clusters, respectively. The early steps of the mechanism are rapid reductions of Ag' and Au' into atoms
1226
3 Dynamics and Physical Properties
by esolv-; the rate constants are 5 x lo9 and 9.5 x lo9 Lmol-' s-I, respectively. These atoms then associate with excess ions of either the same or different metal species, according to the probability of the encounters:
The products of the reactions depicted by Eqs. (20) and (21) are identical. These reactions constitute the early binding between the two metals within the same entity. Mixed species including two different metals have already been detected by pulse radiolysis, e.g. ( TlAg)+ in solutions of monovalent-monovalent ionic preand ( CoAg)2+ in divalent-monovalent precursors.[561 c u r s o r ~ , and [ ~ ~ (CdAg)2+ ~ Further coalescence as in Eqs. (22) and (23) of the mixed species formed in Eqs. (20) and (21) yields alloyed clusters of higher nuclearity, (AgjAuj)cN-; this accounts for the increase in absorbance at 400 and 520 nm, at least up to 2 s.[451
The relative absorbances at 400 and 520 nm are very different from those in pure solutions of Ag(CN)2- or Au(CN)z-. These features suggest alloying between both metal atoms in each cluster, possibly as depicted in Eqs. (20)-(23). Beyond 2 s, the absorbance at 520 nm decreases whereas at 400 nm it continues to increase in proportion (Fig. 5). Both absorbances reach a plateau after 20 s. The plateau at 400 nm is nearly the same as for pure silver solutions, as if the reduction equivalents of gold atoms had been all transferred to silver. Therefore, even when formed soon after the pulse, the alloyed metal clusters progressively lose, during the second time period, the zero-valent gold and are enriched in silver until the gold completely disappears, and gold ions are slowly released (Eq. 24).
The substitution of gold atoms by silver atoms in the cluster implies that the redox potential at a given nuclearity in the presence of cyanide is more positive for silver than for gold, as for the respective electrode potentials. Oxidation, after some time,
3.13 Rudiution-induced Metal Clusters. Nucleution Mechanism and Chemistry
1227
0.15
0.10
8 0.05
0.00
-5
0
5
10 t
15
20
(s)
Figure 5. Correlated signals at 400 nm and 520 nm with a single pulse in an equimolar mixed solution of KAg(CN)z and KAu(CN)2 in the presence of 2-propan01.'~'~
of gold atoms formed early in the process by silver ions is confirmed by y-radiolysis experiments.r451If the dose imparted is such that only partial ion reduction is achieved, the spectrum corresponds to that of monometallic silver clusters superimposed on the bands of Au'(CN)l- ions at 232 and 242 nm. Silver is thus clearly reduced first and benefits from electron transfer from transient gold atoms acting as an electron relay. When the dose is sufficient to reduce all the ions in the mixed solution, the spectrum maximum shifts to 450-470 nm because gold ions are now also reduced at the surface of the silver clusters and the final clusters comprise a silver core coated by a gold shell. Because the dynamics observed by means of pulse radiolysis indicated that the displacement process was not instantaneous, it was suggestedL4'] that very short, intense irradiation, with a dose sufficient to achieve the complete reduction of all the ions, could efficiently prevent the segregation, due to electron transfer between the metals. Therefore, the method could enable the formation of alloyed clusters, of major interest for various applications, particularly catalysis. The positive influence of high dose rates, which quench the atoms in an alloyed cluster, has been demonstrated; a bilayered cluster would be obtained from the same system by irradiation at a lower dose rate.[471Moreover, as for monometallic clusters (Section 3.13.4.3), the high dose rate favors nucleation rather than growth, and the final sizes of the alloyed clusters are particularly
1228
3 Dynamics and Physical Properties
3.13.5 Principle of redox potential determination of transient species The transient character of unstable species is intrinsically because of at least one fast reaction which they undergo as soon as they are formed (for example coalescence reaction in the case of atoms and clusters). This reaction therefore induces competition with any redox reaction which could be regarded as determining the redox potential of a transient entity. In particular, the competition does not enable the establishment of a reversible equilibrium of electron transfer with another suitable system. Thus, the redox potential of short-lived species must be evaluated from kinetic methodsr571- the pulse technique enables us to observe whether or not electron transfer involving the transient species and a series of donor/acceptor couples, used as monitors, is effective, and thus to establish by a bracketing method the value of the unknown redox potential. Only elementary monoelectronic transfers are considered. Thus, note that one of the forms of the reference couple, reduced or oxidized, can also be a transient radical. Moreover, because no reversible equilibrium is established, only standard redox potentials may be compared. Among a series of S/S- reference couples, the transition between the occurrence and the absence of reaction indicates the upper limit (cluster oxidation) or the lower limit (cluster reduction) of the standard potential value of the cluster to be measured. The critical nuclearity corresponding to this potential is determined by fitting the kinetics calculated by simulation with the experimental signal.
3.13.6 Reactivity of metal atoms 3.13.6.1 Redox potential of metal atoms The observation of the atom reactivity is mostly restricted to metal atoms formed by reduction of monovalent ions, because they may be generated in one step in a single pulse regime. Silver atoms Ago or charged dimers Ag2+ react readily with even mild oxidizing m o l e c ~ l e s [ (Table ~ . ~ ~ 3). Thus they behave as strong electron donors towards, for example, CH3N02 whose potential is Eo(CH3N02/CH3NOz-) = -1 .O VNHE. This suggests a still more negative value of the potentials Eo(Ag+/Ago) and Eo(2Agf/Ag2+).The hydrated ion Ag+ is reduced by the alcohol radical (Eq. 8), thus confirming that Eo(Ag+/Ago)is at least greater than the radical potential (- 1.8 VNHE under neutral conditions). Actually, the value E"(Ag+/Ago) =
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1229
Table 3. Rate constants for oxidation of Agl and Agz+ by different substances, as determined by Tausch - Treml ei al."] Oxidant
Oxidation rate constant (LmoI-' S K I ) for Agl' for Ag2+ 1.2 x 6.5 x <5 x <3 x 5.0 x 3.5 x 3.0 x
10' lo* 106
106 10' 10' 10'
1 . 1 109 1.1 x 10' 1.5 x 105 2.3 109 2.8 x 10'
3 x 10'
< 105 < 105 < 105 4.6 x 8.0 x 5.0 x <2 1.5 x
10' lo6 lo8 105 107 106 108 10'
- 1.8 VNHE has been derived from the electrode potential E"(Ag+/Ag,,,,,) = +0.78 VNHEand from the sublimation energy of the metal into atoms.[61 Similar observations on the oxidation of the thallium atom or on the reduction of T1+ have been made by pulse radiolysis.[261They are in agreement, as for silver, with the value determined from the electrode potential and the sublimation energy of the bulk metal into atoms, i.e. E"(Tl+/Tlo) = -1.9 VNHE. Silver ions complexed by cyanide, ammonia, or EDTA, Ag'L, are not reduced by the radical (CH3)2C'OH, even under basic conditions, and the redox potential of these complexed forms must be more negative than -2.1 v N H E . [ 2 6 ' 2 7 1 According to SCF calculations for determining the electronic structure of the complexed oxidized and reduced species Ag'(CN)f and Ago(CN)22pand then using the cavity model for the solvation effect on both, the redox potential was evaluated as Eo(Ag1(CN),-/Ago(CN)*2p) = -2.6 V N H E . [ 5 9 1 A similar calculation[301for the amino complex gave E"(Ag'(NH3)2+/Ago(NH3)2) = -2.4 VNHE. These values agree fairly well with the experimental upper limit indicated by the alcohol radical, -2.1 V N H E , and the lower limit arising from the effective reduction of the ions by eaqp(Eq. 2), so that:
-2.87 VNHE< Eo(AgLL/AgoL)< -2.1 VNHE
(25)
Ion complexation by a strong ligand therefore induces a marked shift of the atom redox potential to more negative values compared with the aqueous system (Fig. 6), as is observed for the potentials of bulk metal electrodes. Similarly, the higher
1230
3 Dynamics and Physical Properties
-Ag+
4
1
Ago
Gas phase
Aqueous solution
Figure 6. Ionization potentials of silver atoms in the gas phase, and of different complexed silver atoms in water.
the complexation constant of the ion, the more negative is the redox potential of the atom E"(Ag'L2/AgoL2) with a ligand order CN- < EDTA < NH3 < H20[293601 (Table 4). Because the sublimation energy of metals is usually large, the redox potential of all metal atoms Eo(M+/Mo)is systematically quite negative (Table 4).[637,31,581 For instance, for copper Eo(Cuf/Cuo) = E0(Cuf/Cumet)- AGsub = -2.7 VNHE[~' and for nickel Eo(Nif/Nio)was estimated to be < -2.55 VNHE.[~'] Note that the quite negative redox potential of metal atoms E"(M'/Mo) explains why the very first atom formation process from free or complexed monovalent ions in the bulk is thermodynamically unfavorable, unless the reducing agent is very strong. When the ions are adsorbed on a support, however, their reduction potential is markedly shifted to a higher value and reduction by a moderate electron donor is possible.[211For that reason, in the latter circumstance, the walls and any particle present in the solution play an important role in the nucleation. For example, the Table 4. Redox potentials of hydrated and complexed M'/M" couple in solution. Redox couple
E"(MI/M")
MI/M"
(VNHE)
3.13 Rudiution-induced Metul Clusters. Nucleution Meclzanism and Chemistry
1231
reduction of Ag' by mild electron donors yields exclusively a metal mirror on the walls. The reactivity of the transient metal atoms formed by reduction of multivalent ion precursors is less accessible by time-resolved techniques than for atoms of monovalent metals. After a single short pulse, the reducing radicals are scavenged by M'+, forming M('-Ijf, and they are exhausted. The disproportionation of M('-'j+, possibly followed by other disproportionation reactions, could still lead to lower v a l e n c e ~ : [ ~ ~ . ~ ~ ] M(X-l)f
+
M(X-I)+
--j
MX+ + M(X-')+
(26)
However, the redox potential E"(M'/M") of the last step is usually quite negative (Table 4) and therefore lower than E"( M"/M+), so that the last disproportionation ( Eq. 27) is not favored thermodynamically.[611 MI
+ MI
+ M"
+ M"
(27)
When the monovalent ions M' are fixed on preformed clusters, they instead disproportionate readily, because Eo(M,,+/Mf7)becomes higher than E"( M, 2+/M,7+). The process does not, however. create isolated atoms and thus new nuclei, but favors the growth of preformed clusters rather than nucleation. Therefore, in the absence of preformed clusters, the zerovalent state may be reached only by a multistep reduction process which requires a train of pulses or at so that the unstable lower valence M' least a long pulse (also by y-irradiati~n),[~'] may be itself reduced, as soon as it is formed, by radiolytic radicals.
3.13.6.2 Comparison between ionization potential and optical absorption of atoms The value of a redox potential is directly related to the energy required to abstract an electron from the atom, i.e. the ionization potential, ZPsol,relative to vacuum (or Fermi level) for the atom in solution:[h21 EoNHE(Ag', L/Ag", L)/e
= 4.5
-
IPsC,l
(28)
where e is the electron charge and 4.5 V is the Fermi potential of the normal hydrogen electrode.[621It is worth comparing the CTTS transition energy of a given atom with its ionization potential (Fig. 2 and Table 4). It seems that the more negative the potential. the more important is the red-shift of the CTTS band. Moreover, in both examples shown in Fig. 7 for hydrated and cyano silver atoms, the IP\"l value is lower than the CTTS transition energy. Thus the excited states
1232
3 Dynamics and Physical Properties
-
Figure 7. Energy levels of the silver atom and the silver ion in aqueous solution in the presence and absence of cyanide.[281
(Ago)* and (Ago(CN)22-)*are both autoionizing whereas the ground states are not. Assuming that Ago(EDTA) behaves similarly we can estimate from the CTTS transition energy that the redox potential is close to -2.2 VNHE.
3.13.7 Reactivity of metal clusters 3.13.7.1 Monometallic clusters It was emphasized that cluster redox properties depended on the nuclearity, mostly at low y1 values. The oligomers are spontaneously unstable with respect to coalescence and the determination of the redox properties of these transient oligomers is again accessible only by means of a kinetic approach. The clusters are formed as above by using a pulse to induce atoms which then coalesce; during the coalescence they can react with an added reactant. Depending on the chemical properties of the reactant and on their nuclearity n, the clusters may behave as electron acceptors or donors.
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1233
Reduction processes Radiolytic reducing radicals can be used, during the same pulse, partly to form the metal atoms (Eqs. 2, 8, and 9), and partly to produce, from a precursor S, the electron donor S- (Fig. 8) which will impose the potential threshold for the reaction of electron transfer to the cluster ( Eqs. 29-3 1):
+ eag + SS + (CH3)zC'OH S- + (CH2)zCO + H+ s + coo*-+ s- + coz S
4
(30) (31)
The couple S/S- is selected with a specific and intense optical absorption of S or S-, so that the electron-transfer reaction can be observed directly. In the early stages of atom coalescence, the redox potentials of the atom and of the smallest clusters are generally far below that of the donor S- and the transfer from S- to the oligomer does not occur. The ion reduction is caused exclusively by solvated electrons and alcohol radicals (Eqs. 2, 8, and 9). The nucleation and coalescence dynamics are thus the same as in the absence of S- (Eqs. 10 and 1 1). Beyond a certain critical time, t,., that is large enough to enable the growth of clusters and the increase of their potential above the threshold imposed by the electron donor s-, electron transfer from this monitor to the supercritical clusters is allowed (Eq. 32) and detected by the absorbance decay of S- (Fig. 6). For n > n,.:
If the concentration of S- is high, the reactions depicted by Eqs. (29)-(31) are faster than the coalescence reactions (Eqs. 10 and 11) with a fixed total concentration of atoms and the clusters now grow mostly by successive additions of supplementary reduced atoms (electron plus ion). It has been shown that once formed, a . ~ , ~ ~behaves as a growth nucleus. Alcritical cluster, of silver for e ~ a m p l e , [ ~indeed ] ternate reactions of electron transfer (Eqs. 32 and 34) and adsorption of surrounding metal ions (Eq. 33) make its redox potential more and more favorable to the transfer (Fig. 8), and autocatalytic growth is observed.[211The observation of an effective transfer therefore implies that the potential of the critical cluster is at least slightly more positive than that of the electron-donor system, i.e. Eo(M,+/M,) > E " ( S / S - ) . The value of the nuclearity of this critical cluster enabling transfer from the monitor depends on the redox potential of the selected donor, S-. We studied electron transfer to silver clusters from the decay of different electron donors,
1234
3 Dynamics and Physical Properties
EO[M+/M
1 .
I
Eo [S/S-].
E O [M;/M, ]
-
M:IM,
1
e- pulse
1-
Figure 8. Principle of the determination of the redox potential of a short-lived cluster by the kinetics method. The reference electron donor S- of given potential and the metal atoms are generated by a single pulse. During cluster coalescence the redox potential of the couple E"( M,+/M,) progressively increases, so that effective transfer is observed after a critical time when the cluster potential becomes higher than that of the reference imposing a threshold (n 2 n o . The subcritical clusters M, (n < n , ) may be oxidized by S.['''
e.g. Ni+,12'] the reduced radical anions of sulfonatopropyl viologen SPVp'[211 (E"(SPV[SPV-') = -0.41 VNHE) (Fig. 9a) and methyl viologen MV+'[4s*631 (E"(MV +/MV+') = -0.41 VNHE),and the hydroquinone of naphthazarin QH2 (Figs. 9b and ~ c ) . [With ~ ~ I this last compound the potential of the initial step E" (semiquinone/hydroquinone) which controls the critical nuclearity depends on pH (E"(Q-' + 2H+/QH2) = 0.22 and 0.33 VNHEat pH 4.8 and pH 3.9, respectively). Donor decay and the corresponding increase of supplementary silver atoms from the reaction depicted in Eq. (32) start systematically after a critical time as expected, and the higher the donor potential, the longer is t, (Fig. 9). The critical time and the donor decay rate depend on the initial concentrations both of metal atoms and of the donor. The critical nuclearity, rzc, corresponding to the potential threshold im-
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
08 -
-
-
-
06
Figure9. (a)Transient (b) optical absorption decay signals after t, = 1 ms of the electron donor SPV-' (at 650 nm) and of growth of silver clusters (at 420 nm), after a single pulse in a mixed solution of silver ions and SPV.[*'](b) Kinetic signals from quinone formation (at 5 12 nm) and from silver cluster growth (at 380 nm) in a mixed solution of silver ions and naphthazarin at pH 4.8 (tc = (c) Kinetic signal from silver cluster formation (at 380 nm) in the presence of naphthazarin at pH 3.9 (t,
=
150ms).[64.h61
1235
0.4
. . I
I
I .
.
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
380 nm
8 0.1 -
-0.1 1
-0.2 ~* -0.1
" *
I "
0
"
I '
0.1
'
"
I '
0.2
'
"
I ' '
0.3
"
" '
0.4
"
0.5
t (s)
posed by the donor and the transfer rate constant k32, which is supposed to be independent of n, are derived by fitting the kinetics of the experimental donor decay under various conditions with numerical simulations through adjusted parameters (Fig. The dependence of cluster potential on nuclearity was obtained by changing the reference potential in a series of redox monitors (Table 5). The redox potentials of hydrated silver clusters are seen to increase with n. The data in Fig. 11 indicate that, at least for the redox properties of silver clusters, the transition between the meso-
1236
3 Dynamics and Physical Properties
0.5
~4
(4
L
0.4 0.3 0
m
8 0.2 0.1
-
0 -0.2
3 0
0.2
0.4
0.6
0.8
1
t (s)
Figure 9 (continued)
0.8
v1
0.6
0.4 0.2 0 0
0.05
0.1
0.15
0.2
t (s) Figure 10. Electron donor kinetic signal, concentration-normalized relative to that at the end of the pulse. Experimental signal under same conditions as in Fig. 9b (pH 4.8). (-) Simulated signals at various n, values with k32 = 2.25 x lo8 L m o l - ' s ~ ~The . best adjustment is found for n, = 85 5.[641
-4::r 1237
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
vacuum level
0
3
z
v
!3
0
w
6 3
Figure 11. Size-dependence of the redox potential of silver clusters in water (o)[21,38,63,64,661 and in the gas phase
4.
s0
5
d1 0
15
l
9 200
600
Qo
nuclearity
(A),I67.681
scopic and macroscopic phases, or the limit of a constant potential, occurs for nuclearity n % 500 (diameter z 2 nm). The density of values available so far is not sufficient to prove the existence of odd-even oscillations like those observed for ~ - ~ ' ]are also ionization potentials ZP, of bare silver clusters in the gas p h a ~ e , [ ~ which presented in Fig. 11. However, it is obvious from this figure that the variations of E" or IPsolvand IPgdo have opposite trends as n changes, for the solution (Table 5) and the gas p h a ~ e , [ ~ " "respectively. ~ The difference between the ionization potentials of bare and solvated clusters decreases with increasing n and corresponds fairly Note that for the well to the solvation free energy deduced from the Born single atom, the difference indeed represents the solvation energy of the silver cation (4.4 eV).[691
Table 5. Nuclearity-dependence of E"( M,,+/Mn) Reference system (electron donor)
E" (reference couple) (VNHE)
Metal cluster (electron acceptor)
Nit I-hydroxy-1-methylethyl radical (pH 5 ) SPV-' cu+ Q-' (pH 4.8) Q-' (pH 3.9) MV+'
-1.9 -1.8 -0.41
Agn Agl? Agn Ag,, Agrl Ag,,
?.KTi+.
IV1
v
MV+'
0.16 0.22
0.33 -0.41 nA
I
Agn.cNA-
n, (reduced atoms / cluster) lIZ11 11211
41211 11I381
85 f .5[641 500 & 30[641
5-6[451 .... A1631
1238
3 Dynamics and Physical Properties
Similarly to the redox potential of the bulk electrode or of the atom, the cluster potentials at a given nuclearity are shifted to more negative values under the influence of a ligand (compare Ag, and Ag,+--- in Table 5). The potentials obviously depend also on the nature of the metal. The general trend of redox potential increase with nuclearity is, however, expected for all metals in solution because any isolated atom has strong electron-donor character in solution (Section 3.13.6.1). For Cu,, EO[Cu,+/Cu,] increases from -2.7 to -0.40 and 0.15 VNHEfor y1 = 1, n = 6 & 1, and n + co,respectively.[661
Oxidation processes In the study of oxidation processes the clusters are generated in aqueous solution by means of a short electron pulse in the presence of an electron acceptor S . A required condition is that the metal ions do not react with the acceptor before irradiation. If, after the pulse, an oxidation reaction of the clusters by S is observed, this implies that Eo(M,+/M,) < E " ( S / S - ) . M,
+ s -+
M,+
M,+ + S
(35)
+ M+
(36)
+ M,-1
In fact, the potential of the resulting oxidized cluster M,-l is still lower than that of M,, according to the dependence of Eo(M,+/M,) on n, and the cluster M,-I may undergo also an oxidation by S:
By such a cascade of oxidation reactions, the nuclearity decreases and clusters are progressively corroded. Note that the process still coexists with the coalescence reactions which, in contrast, result in a nuclearity increase. It has, for example, been found that during the very slow coalescence of Ag, inside the cavities of a Nafion membrane,[7o1the smallest clusters could be oxidized by the protons H30+, which are highly concentrated at the surface. In contrast, when, by coalescence, the clusters reach the critical nuclearity for which their potential is higher than Eo(H30+/H2)= 0 VNHE,they escape corrosion and are observed by optical absorption. Numerical simulation of the kinetic signal, including the cascade of coalescence reactions (Eq. 11) and of oxidation reactions (Eqs. 3537), yields the value n = 8 for the upper limit of nuclearity of clusters oxidized by H30+.[711Therefore, E"(Ag,,+/Ag,,) > Eo(H30+/H2)= 0 VNHE(Fig. 11). Note that such a corrosion by H30+ was not observed under conditions of free diffusion of the clusters, as in solution,[321because the coalescence enables the clusters to grow much faster up to the supercritical nuclearities. Oxidation of the smallest clusters is also observed when silver atoms are generated from silver cyanide in the presence of the redox couple MV2+/MV+'.[401
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1239
While supercritical clusters ( n > 6 ? 1) (Table 5) accept electrons from MV+' with a progressive increase of their nuclearity ( Eqs. 32-34), the subcritical clusters undergo a progressive oxidation by MV2' by means of the reactions depicted in Eqs. 35-37). Eq. (35) ( n < a,) is the reverse of Eq. (32) ( n > n,). The reduced ions MV+' so produced also contribute to the growth of the supercritical clusters. Actually, they act as an electron relay favoring the growth of large clusters at the expense of the small ones. The coexistence of reduction by MV+' and oxidation by MV2+ is observed because the coalescence in the presence of ligand CN- is, as shown above, quite slow. For gold clusters complexed by cyanide, the coalescence is still slower and oxidation of Au,,cN- by MV'+ is observed alone.r451 The cluster oxidation reactions seem to be very slow in all three examples described above, and their observation is possible because the competition from coalescence is not too efficient. Oxidation of atoms and small oligomers in the presence of oxygen is often so fast that the corrosion is over before the formation of clusters. Even large clusters such as Ag,,cN-, AU,,CN , or Cu,,cl-, which are stable after their radiolytic formation under anaerobic conditions, are readily oxidized when exposed to air, suggesting that their redox potential is lower than E 0 ( 0 2 / 0 2 - ) = -0.33 VNHE.
3.13.7.2 Reactivity of bimetallic clusters The reactivity of short-lived bimetallic clusters has also been studied by the kinetics method. Under conditions when a transient alloyed cluster of Ag-Au was formed,[451reactivity with the electron donor MV+' was probed and compared with that of monometallic Ag clusters previously observed. Just after the pulse a mixed solution of Ag' and Au' cyanides is partially reduced into atoms Ago and Au', while MV2+ is partially reduced to the redox probe MV+'. It is observed that in the first 20 ms the kinetics, at 400 nm, of cluster growth are the same as in the absence of the probe. Thus the coalescence of atoms to form an alloyed small cluster is, at first, not affected. The mechanism should be the same as in Eqs. (20)-(23). After this period, however, the decay of MV+' at 700 nm starts in correlation with the increase of the cluster absorbance which results from electron transfer (Fig. 12). When the bimetallic cluster formed reaches the critical size nc, where its potential becomes slightly higher than E"( MV2+/MVf'), it acts as a nucleus that initiates a catalyzed growth fed alternately by electron transfer from the donor and the adsorption of excess Ag' or Au' ions. For i + j > n,:
1240 0.8
3 Dynamics and Physical Properties
C"""""'""""""'1
0.6
a 0
0.4
0.2
0.0
-5
0
5
10 t
15
20
(s)
Figure 12. Comparison of kinetics signals at 700 nm in silver cyanide solution and in a mixed equimolar cyanide solution of Ag'/Au' , in the presence of the electron donor MVf' formed simultaneously with Ago and Auo during the pulse. For t < 2 s, the absorbance is exclusively that of MV+'. For t > 2 s, MV+' is completely consumed and the absorbance arises from large alloyed Ag-Au clusters developed by MV+*.[451
or
Actually, the kinetic changes are the opposite of those for pure gold solutions oxidized by MV2+. The general scheme of MV+' decay now shows the same decreasing characteristics as in pure silver solution,[401as if the gold in the alloyed cluster had the same electron acceptor properties as silver. Indeed, we observe that at constant nuclearity, the alloyed cluster has the same redox potential even if gold atoms are replacing part of the silver atoms in the alloyed cluster. Therefore, a clear synergy is observed in the mixed system, which supports the conclusion of strong binding between gold and silver atoms and on the alloyed character of the clusters after the end of the reduction by MV+'. In the interval 0.2 to 2 s we also observe slower decay than for t < 0.2 s which was assigned, as previously for silver,r401to the reverse electron transfer from sub-
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1241
critical clusters to MV*+. For i + j < n,: (Ag,Auj)cN-
+ MV2+
(Ag;Au,)+,,
4
+ MV"
(43)
The MVf' formed can transfer an electron back to supercritical clusters (Eqs. 38, 41, and 42) so that it acts, as for silver, as an electron relay from subcritical to supercritical After 2 s, the absorbance at 700 nm increases drastically (Fig. 12). This absorbance is no longer that of MV+, already consumed, but of an increasing red component of large clusters, which were developed from a small concentration of nuclei.L2'1
3.13.8 Mechanism of electron transfer catalyzed by clusters The size-dependence of redox properties of metal clusters is crucial for their catalytic efficiency, particularly in electron-transfer processes.
3.13.8.1 The thermodynamics of catalysis A metal cluster acting as a catalytic relay behaves alternately as an acceptor and as a donor of electrons (Fig. 13a). The thermodynamics therefore implies that the redox potential of the couple Eo(M,+/M,) is intermediate between that of the donor system D, E"(D+/D) as the lower threshold and that of the acceptor system A, E"(A/A-), as the upper t h r e ~ h o l d . ~ The ~ ' . ~reaction ~] between these systems, negligible in the absence of the catalyst, becomes efficient because of the double electron transfer through the metal cluster relay. If Eo(M,+/M,) is higher than Eo(A/Ap),as for most polished bulk electrodes, the electron is still transferred from D to M,+, but the thermodynamics does not allow a relay from M, to A and no catalytic effect is observed. Conversely, if the clusters are very small and Eo(M,+/M,) is lower than Eo(D+/D), no electron is transferred from D to M,+, and instead the clusters are corroded by A. The high efficiency of ultra-divided metals is, therefore, not only because of their high specific area but essentially because of their appropriate thermodynamic properties. Note that surface roughness and even large clusters also create locally variable potentials which, when selected by the double threshold, promote a fast electron relay mechanism, and are responsible for the catalytic efficiency (Fig. 13b). On the other sites corrosion of the metal by the acceptor (sharp regions) or reduction by the donor (smooth regions) are predominant.
1242
3 Dynamics and Physical Properties
x
A-
A
D
D+
, , , , , ,
Oxidizing site
‘///////A D+
D
Mtl
G+ n D+
D
Figure 13. (a) Mechanism of catalytic electron transfer involving metal clusters as the relay. The thermodynamic conditions to be fulfilled are that the cluster redox potential be higher than the donor D potential and lower than the acceptor A potential. This implies that the size of the cluster is within the size range appropriate for an efficient redox potential.[641(b) Rough catalyst surface with variable local redox potentials. The catalytic efficiency is restricted to regions where the potential is between those of the donor and of the acceptor. On other sites, reduction of A or oxidation of D are predominant.
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1243
3.13.8.2 Autocatalytic growth When reduction of metal ions into atoms is achieved by the strongly reducing radiolytic radicals, especially at high doses, subsequent cluster growth is essentially governed by coalescence (Eq. 1 I), which is then markedly slowed down beyond a certain size owing to the presence of a stabilizer. When reduction of metal ions is achieved partly by use of radiolytic radicals and partly by use of a moderate electron donor, which reduces metal ions fixed on clusters above a critical nuclearity only, cluster growth occurs, irrespective of the presence of the stabilizer, as a result of successive addition of atoms to the same cluster (alternate steps of reduction and ion fixation). Therefore, the final size of clusters developed in this manner might be quite large. In fact, as shown in Section 3.13.7, owing to the increase of cluster redox potential Eo(M,+/M,) with the nuclearity, electron transfer from a donor Sto M,+ is effective only for n > n,. and becomes thermodynamically increasingly favored for M,+l+, Mn+2+and subsequent steps. The mechanism (Eqs. 19, 22, and 23) is thus autocatalytic because the nucleus catalyzes its own development. The cyclic process is repeated up to exhaustion of the electron donor or of M+. The lower the initial concentration of nuclei, the larger is the size of the developed cluster. Pulse radiolytic study of electron transfer from a donor to silver clusters during their formation (Fig. 9) has all the features of a photographic development process. Atoms are produced by radiation of a high-energy in solution instead of action of light on an AgBr emulsion. The electron donor behaves as the developer and transfers electron to clusters exclusively above a critical nuclearity. We therefore that discrimination by a developer between developable and not proposed[' developable crystals (weakly or not exposed crystals) was also a consequence of the nuclearity-dependence of the redox potential of the cluster generated by the light. The development threshold is thus fixed by the developer potential. Beyond nc, the growth by reduction of ions fixed on the supercritical clusters is autocatalyzed up to the total reduction of the exposed AgBr crystal. 1372,731
3.13.8.3 Kinetics of electron-transfer catalysis The catalytic role of metal clusters such as Tl,1,[741 Ag,, Au,,, and Ir,,"'] in electron transfer from free radicals such as COz-' or (CH3)2C'OH to a substrate has been demonstrated. Electrons donated from the radicals are first stored on clusters and then are transferred again, for example pairwise to water producing molecular hydrogen:
1244
3 Dynamics and Physical Properties
Pulse radiolysis also enables the observation of some catalytic electron-transfer reactions other than the autocatalytic growth ( Eqs. 42-44). Electron transfer from MV+' to protons in water requires the presence of a catalyst, for example platinum clusters. The initial electron transfer step (Eq. 42) from MV+' to the cluster was found to be diffusion-~ontrolled!~ MV" Pt,-
+ Pt, + (MV2+,Pt,-) + MV2' + Pt, + H30' + (Pt,, H) + H20
2(Ptn,H)
-+
Pt,
+ H2
For a given amount of metal the transfer rate increases linearly with particle concentration. The rate-determining step is clearly the desorption of molecular hydrogen.[7 The catalysis of the disproportionation of the superoxide anion 0 2 - ' by Pt, clusters in subcolloidal solutions or supported on colloidal Ti02 particles has also been studied by time-resolved techniques.[761The decay of 0 2 - ' follows first-order kinetics in respect of both OZ-' and the Pt, clusters because the catalysis is governed by the proton concentration adsorbed at the cluster surface.
3.13.9 Conclusion The specific approach of the studies described in this review has been to investigate the elementary mechanisms and the thermodynamics of the atom formation, of cluster nucleation and growth, and of their reactions. The dynamics of metal atom formation and coalescence into clusters were studied by pulse radiolysis and by time-resolved spectroscopy. The redox potentials of short-lived metal clusters were also determined by kinetic methods using reference systems. Depending on their nuclearity, the behavior of the clusters changes during their growth from electron-donating to electron-accepting in character, the threshold being controlled by the reference system potential. For a given metal and a given nuclearity, the redox potential and the kinetics are highly dependent on the environment (ligand, surfactant, support). As shown by the time-resolved spectroscopy experiments on the mechanisms of the clusters reaction, the nuclearitydependence of the redox properties of metal clusters in solution plays the most important role, firstly in determining the dynamics of their growth and the distribution of the final sizes, secondly in determining their catalytic efficiency. When the electron-donor character of the reducing agent is strong and the reduction rate is very fast, the initial concentration of small nuclei is highest and the final sizes at complete reduction are generally small. In contrast, a moderate reducing
3.13 Radiation-induced Metal Clusters. Nucleation Mechanism and Chemistry
1245
agent creates fewer new nuclei and mostly develops the existing ones by in situ reduction of adsorbed ions, and so the final sizes are much larger. Once prepared in their final stable state, the metal clusters are often excellent catalysts owing to their specific electronic properties. In the electron transfer catalysis, especially, the clusters can behave as an efficient electron relay between reactants, if the cluster potential, fixed by the nuclearity, is intermediate between those of the donor and of the acceptor.
References [ I ] Kubo R J (1962) Phys SOCJpn 17: 975-981. [2] Delcourt M 0, Belloni J (1973) Radiochem Radioanal Letters 13: 329-338. [3] Baxendale J H, Fielden E M, Keene J P and Ebert M (1965) in Pulse Radiolysis, Keene J P, Swallow A, Baxendale J H, Eds Acad Press London, 207-220. [4] Hai'ssinsky M (1972) in Radiation Chemistry, Dobo J, Hedwig P, Eds, Akad Kiado Budapest 2: 1353-1365 and following discussion. [5] Basco N , Vidyarthi S K and Walker D C (1973) Can J Chem 51: 2497-2500. [6] Henglein A (1977) Ber Bunsenges Phys Chem 81: 556-561. [7] Tausch-Treml R, Henglein A and Lilie J (1978) Ber Bunsenges Phys Chem. 82: 1335-1343. [8] Morse M D (1986) Chem Rev 86: 4049. [9] Halperin W H (1986) Rev Mod Phys 58: 533-606. [lo] Henglein A (1989) Chem Rev 89: 1861-1873. [ 1I ] Belloni J, Amblard J, Marignier J L and Mostafavi M in Clusters of atoms and Molecules (1994) Haberland H, Ed. Springer 11: 290-31 1. [I21 Henglein A (1995) Ber Bunsenges Phys Chem 99: 903-913. [I31 Belloni J (1996) Curr Opinion Coil Interf Sci 1: 184-196. [ 141 Bradley J S (1994) in Clusters and Colloids, Schmid G, Ed. VCH, NY, 459-544. [ 151 Baxendale J H and Busi F (1982) The Study of fast Processes and transient Species by Electron Pulse Radiolysis, NATO AS1 Series 86, D. Reidel Pub. Co. [16] Buxton G V, Greenstock C L, Helman W P and Ross A B (1988) J Phys Chem Ref Data 17: 513-886. [I71 Tabata Y, Ito Y and Tagawa S, (1991) Handbook of Radiation Chemistry, CRC Press, Boca Raton. [18] Hart E J and Anbar M (1970) The hydrated Electron, John Wiley New York. [ 191 Schwarz H A and Dodson R W (1989) J Phys Chem 93: 409-414. [20] Von Pukies J, Roebke W and Henglein A. (1968) Ber Bunsenges Phys Chem 72: 842-847. [21] Mostafavi M, Marignier J L, Amblard J and Belloni J (1989) Radiat. Phys. Chem. 34: 605621. [22] Janata E, Henglein A and Ershov B G (1994) J Phys Chem 98:10888-10890. [23] Janata E, Lilie J and Martin M (1994) Radiat Phys Chem 43: 353-356. [24] Janata E (1994) Radiat Phys Chem 44: 449-454. [25] Kappoor S, Lawless D, Kennepohl P, Meisel D and Serpone N (1994) Langmuir 10: 3018. [26] Butler J and Henglein A (1980) Radiat Phys Chem. 15: 603. [27] Ershov B G and Sukhov N L (1990) Radiat Phys Chem 36: 93-97. [28] Remita S, Mostafavi M and Delcourt M 0 (1996) J Phys Chem 100: 10187-10193. [29] Mostafavi M, Remita S, Delcourt M 0 and Belloni J (1996) J Chim Phys 93: 1828-1842.
1246
3 Dynamics and Physical Properties
[30] Texier I, Remita S, Archirel P and Mostafavi M (1996) J Phys Chem 100: 12472-12476. [31] Mosseri S, Henglein A and Janata E (1989) J Phys Chem 93: 6791-6795. [32] Belloni J, Delcourt M 0, Marignier J L and Amblard J (1987) in Radiation Chemistry, Hedwig P, Nyikos L and Schiller R, Eds., Akademiai Kiado, Budapest, p. 89. [33] Belloni J, Khatouri J, Mostafavi M and Amblard J (1994) in Ultrafast reaction dynamics and solvent effects, Am. Inst. Phys. Rossky P J and Gauduel Y Eds. p. 541-550. [34] Dorfman L M and You F Y (1973) in Electrons in fluids, Jortner J and Kestner N R Eds, Springer, Berlin, 447-459. [35] Kevan L (1981) J Phys Chem 85: 1828. [36] Khatouri J, Mostafavi M, Ridard J, Amblard J and Belloni J (1995) Z Phys D - Atoms; Molecules and Clusters 34: 47-56. [37] Ershov B G, Janata E, Henglein A and Fojtik A (1993) J Phys Chem 97: 4589-4594. [38] Henglein A and Tansch-Treml R (1981) J Coll Interf Sci 80: 84-93. [39] Ershov B G, Janata E and Henglein A (1993) J Phys Chem 97: 339-343. [40] De Cointet C, Mostafavi M, Khatouri J and Belloni J (1997) J Phys Chem 101: 3512-3517. [41] Mostafavi M, Delcourt M 0, Keghouche N and Picq G (1992) Radiat Phys Chem 40: 445450. [42] Mostafavi M, Keghouche N, Delcourt M 0 and Belloni J (1990) Chem Phys Letters 167:193197. [43] Mostafavi M, Keghouche N and Delcourt M.O. (1990) Chem Phys Letters 169: 81-84. [44] Remita S, Orts J, Feliu J M, Mostafavi M and Delcourt M 0 (1994) Chem Phys Letters 21 8: 115-12 1. [45] De Cointet C, Khatouri J, Mostafavi M and Belloni J (1997) J Phys Chem 101: 3517-3522 [46] Khatouri J, Mostafavi M, Amblard J and Belloni J (1992) Chem Phys Letters 191: 351-356. [47] Treguer M, De Cointet C, Remita H, Khatouri J, Mostafavi M, Amblard, Belloni J and de Keyzer R (1998) J Phys Chem 102: 4310-4321. [48] Belloni J, Mostafavi M, Remita H, Marignier J L and Delcourt M 0 (1998) in Synthesis, chemistry and some applications of metal nanoparticles, Bradley J, Chaudret B, Eds, New J Chem 22: 1239-1255. [49] Buxton G V, Mulazzani Q G and Ross A B (1995) J Phys Chem Ref Data 24: no 3. [50] Delcourt M 0, Belloni J, Marignier J L, Mory C and Colliex C (1984) Radiat Phys Chem 23: 485-487. [51] Henglein A and Lilie J (1981) J Phys Chem 85: 1246-1250. [52] Henglein A, Gutierrez M, Janata E and Ershov B G (1992) J Phys Chem 96: 4598-4602. [53] Ershov B G, Sukhov N L and Troistskii (1994) Russ J Phys Chem 68: 734-738. [54] Henglein A, Janata E and Fojtik A (1992) J Phys Chem 96: 4734-4736. [55] Ershov B G, Janata E and Henglein A (1994) J Phys Chem 98: 10891-10894. [56] Ershov B G, Janata E and Henglein A (1994) J Phys Chem 98: 7619-7623. [57] Wardman P (1989) J Phys Chem Ref Data 18: 1637-1712. [58] Marignier J L (1987) These d’Etat, Univ. Paris-Sud. [59] Remita S, Archirel P and Mostafavi M (1995) J Phys Chem 99: 13198-13202. [60] Mostafavi M and Belloni J (1997) Rec Res Dev Phys Chem 1: 457-473. [61] Ershov B G (1994) Russ Chem Bull 43: 16-21. [62] Reiss H (1985) J Phys Chem 89: 3783-3790. [63] Mostafavi M, Delcourt M 0 and Belloni J (1994) J Imaging Sc 38: 54-58. [64] Khatouri J, Mostafavi M and Belloni J (1998) in Photochemistry and Radiation Chemistry, Wishart J and Nocera D, Eds, Adv Chem Ser ACS Washington 254: 293-314. [65] Khatouri J, Ridard J, Mostafavi M, Amblard J and Belloni J, (1995) Z.Phys.D, 34: 57-64. [66] Khatouri J, Mostafavi M, Amblard J and Belloni J (1993) Z.Phys.D, Atoms, Molec., Clusters 26: 82-86. [67] Jackschath C, Rabin I and Schulze W (1992) Z. Phys. D - Atoms, Molecules and Clusters, 22: 517-520.
3.I3 Radiation-induced Metal Clusters. Nucleation Meclianism and Chemistry
1241
[68] Alameddin G, Hunter J, Cameron D and Kappes M M (1992) Chem. Phys. Letters , 192: 122125. [69] Krestow G A (1991) Thermodynamics of solvation, Ellis Horwood Ser Phys Chem, Univ Warwick. 1701 Platzer 0, Amblard J, Marignier J L and Belloni J (1992) J. Phys Chem 96: 2334-2340. [71] Amblard J, Platzer 0. Ridard J and Belloni J (1992) J. Phys Chem 96: 2340-2344. [72] Mostafavi M, Amblard J. Marignier J L and Belloni J (1991) J. Imaging Sci. 35: 68-74. [73] Belloni J, (1997) in Homogeneous photocatalysis , Chanon M, Ed., John Wiley, 2: 169-218. [74] Buxton G V, Rhodes T and Sellers R M (1982) J Chem SOCFarday Trans 1, 78: 3341-3356. [75] Delcourt M 0 , Keghouche N and Belloni J (1983) N J Chim 21: 177-183. [76] Belloni J.. Lecheheb A. (1987) Radiat Phys Chem 29: 89-92.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.14 The Investigation of Heavy Metal Cluster Structures in Powder Samples by the Radial Distribution Function Method Vladimir I. Korsunsky
3.14.1 Introduction To determine the molecular structure of compounds chemists now use single crystal X-ray diffraction routinely. Clusters are no exception, but if one cannot grow a suitable single crystal it is difficult both to understand the nature of the cluster and, often, to detect cluster formation in a reaction with the help of spectral data alone. This is because no spectral method gives direct and unambiguous information about the mutual arrangement of metal atoms, the feature which primarily distinguishes the structure of a cluster from that of any other compound. This review discusses the possibility of obtaining information about the structure of heavy metal clusters by use of the method of the radial distribution function ( R D F ) calculated from the X-ray powder diffraction The samples can be either amorphous or polycrystalline powders. Fig. 1 shows a schematic view of the RDF D ( r ) which is the spectrum of interatomic distances of the substance. It is a superposition of partial peaks Gkjrn(r), each such peak originating from the various interatomic distances. The subscripts 'k' and 'j' indicate the types of atom, and the superscript 'm' indicates a specific distance between these atoms. The peak &jrn(r) is located at a value of r equal to the mean value of the corresponding distance &jm. The peak intensity Ikj" is proportional to the number of these distances N k j " per the chosen stoichiometric unit of specimen to which the experimental RDF is normalized.['p41
The weight coefficient, Wk;, is approximately proportional to the product of the number of electrons in the atoms of each member of the pair:
3.14 The Investigation of Heavy Metal Cluster Structures in Powder Samples
1249
Figure 1. Schematic view of an RDF. The solid line is the total RDF, D(r); vertical lines schematically represent the positions Rk,"' and the integral intensities Zklm of individual peaks produced by different types of inter-atomic distance; short dashes indicate the profiles of the individual peaks and long dashes depict the parabola of an average distribution of inter-atomic distances (see text).
for the RDF obtained as a Fourier transformation of X-ray powder diffraction curve. It is quite clear that a contribution of a subsystem consisting of few heavy metal atoms with high Zk will be prominent in such an R D F compared with the peaks from the more numerous light atoms of the ligands. For instance, the weight of a distance peak between platinum atoms is 100-150 times larger than that of a distance peak between c, N, and 0 atoms. The distances between heavy metal atoms usually manifest themselves in the RDF as very intense peaks and can be readily identified. The position of such a peak at once gives an accurate first approximation for the metal-metal distance. Thus the RDF enables the direct estimation of the key distances which are necessary for understanding cluster structure. It is not of dramatic importance here that the peaks corresponding to different distances origin in the RDF overlap each other. For any substance this last feature precludes the separation of different contributions but, as is emphasized in Fig. 1, a few types of most intense individual contributions determine the form of the observed peaks in the RDF of a heavy atom cluster, the others serving as the background. At the next stage, one should imagine a model structure for the cluster and then test it directly by comparing the RDF calculated for the model and the experimental RDF in detail. To imagine the model, one should use all accessible information; in addition to the metal-metal distances, this includes: i) chemical and spectral data on the types, and sometimes the structural functions of the ligands; ii) the huge amount of crystallographic data available which indicates which metal core structures are likely and which provides typical bond lengths and angles, details of the structures of ligands, and spatial constraints determined by possible non-bonding interactions, etc. ; iii) chemical intuition, previous experience, etc. When a model has been constructed, its exact RDF can be calculated. The form ~ l a e [ ' - -are ~ ] known and have been summarized in their correct f ~ r m . [ The ~ - ~exact ]
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3 Dynamics and Physical Properties
form and intensity of the contribution of each type of inter-atomic distance, &jm(r) in the model are calculated numerically, and then all are summed to give a total model RDF. To calculate all necessary &jm(r), one must specify the mean distances &jm and the numbers of such distances Nkjm.The parameters of both types are determined by the geometry of the model, i.e. the mutual arrangement of the atoms. One can vary &jm somewhat within reasonable limits to achieve a better agreement with experiment. In this way initial approximations of the inter-atomic distances are refined. When making these variations to obtain, finally, the distances which correspond to a spatially realizable model, the limitations mentioned above, which follow from the knowledge about known crystal structures, should be borne in mind. Nkjm are rigid parameters for a given model and are not varied. If one fails to fit the model RDF with the experimental one operating with a given set of Nkjm, this means that the geometry of the trial model is incorrect and must be changed. One more parameter must be specified for each inter-atomic distance. This is Okjm, the root mean square deviation of the distance from its mean value &jm, i.e. the width of distribution of the distance which arises as a result of thermal vibrations and any possible structural disorder. These parameters are analogs of the thermal displacement of atoms in single-crystal X-ray analysis. Like the latter, Okjm are the most uncertain RDF fitting parameters. When these are varied, one should keep in mind both their typical values obtained in specific RDF studies of the various reference compounds, of known structure, and some physically reasonable limitation~.[~,~] The method outlined above for testing a cluster structure is highly informative and convincing because of the high weight of heavy atoms in the RDF. There is no complicated interference from many types of contribution of almost the same weight in both the model and experimental RDFs. It is of importance that only a small number of types of inter-atomic distance are very sensitive to the variations of the cluster structure and contribute strongly in the RDF; these are primarily metalmetal distances. Other noticeable features of the RDF indicate contributions of distances between the metal atoms and lighter atoms and can be used to derive distances between metal and light ligand atoms which have intermediate individual weights Wkj. Their peaks are especially intense when such distances are numerous in the cluster, i.e. corresponding Nkjm are high. This is, on the one hand, an additional complication of an RDF but, on the other hand, it aids the testing of the arrangement of ligands relative to metal atoms. At the same time, the most numerous distances between light atoms of ligands produce a background of low significance. They add no complications in the interpretation of the RDFs of heavy metal clusters. The model RDF can be quantitatively compared with the experimental one only in the region of relatively small r, where the complications from large contributions of the distances between atoms of different molecules do not arise. This is usually the region of r < 3.5-4A in which the RDFs of models will be shown in the figures. The intra-complex inter-atomic distances mainly contribute here; the values and numbers of these are determined accurately by the molecular structure model. The
3.14 The Investigation of Heauy Metal Cluster Structures in Powder Samples
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RDFs of polycrystalline and amorphous samples do not differ in this r region. They usually differ substantially at larger r . The R D F of the former usually contain sharp peaks; this reflects the rigid long-range order in a crystal lattice. The R D F of the latter usually contains smooth oscillations after the first sharp peaks of the intramolecular distances. This reflects the high disorder in an arrangement of molecules. One should keep this in mind when looking the RDFs in the figures. The so-called difference RDFs, G ( r ) ,will be shown in all figures. The total RDF, D(r), oscillates around the trivial growing parabola shown in Fig. 1. The parabola is determined by the mean density of the and reflects a trivial increase of the mean number of inter-atomic distances in a substance when r increases. The subtraction of this parabola from D(r) gives G ( r ) .It oscillates around the r axis, and the new zero line for the absolute intensities of peaks is now the parabola depicted by the long dashes. For this short review, we have chosen the relatively simple results from an R D F study of clusters to illustrate the main potential of the R D F method and to emphasize ‘hampering’ and ‘favoring’ factors. We cannot discuss here spectral and chemical aspects of the investigations in detail. One should understand, however, that this is always an important part of the study, especially specification of the type of ligands. We imply that the authors of the original papers have done their utmost in this respect. Some results will be demonstrated using compounds of known structure which we have specially studied to test the method and to gain experience in interpretation of RDFs. We do not give the complete sets of parameters of the models used in each R D F simulation shown in the figures but only discuss the most important of them. All details are usually given in the original papers.
3.14.2 Simple manifestation of nearest metal-metal distances Fig. 2 shows the simplest example of the direct detection of the Au-Au bond of bis(triphenylphosphinego1d)malonitrilein the R D F of its polycrystalline powder.[’] The structure of the known cluster molecule[71is shown on the right. The first three peaks are actually of similar origin in most of RDFs that will be discussed. All bonds between the atoms of ligands contribute to the common first peak near 1.5 A. This peak is more or less pronounced depending on ligand type. The next peak normally appears near 2 A or at somewhat greater r. Direct bonds between heavy metal atoms and the nearest light atoms of the ligands make contributions here. For = 2.28 A) bonds this species these are the Au-C (RA”c= 2.1 A) and Au-P and the peak maximum location is determined by the distances to the heavier phosphorus atoms. Finally, the third peak, which is usually the strongest, contains
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Figure 2. C(r ) for bis(triphenylphosphinego1d)malonitrile.Here and in analogous figures the solid line is the experimental RDF G ( r ) , the dashed-and-dotted line is the modeled contribution of the distances between the heavy metal atoms (here Au-Au), the dashed line is the sum of all the contributions of near inter-atomic distances taken into account in a model RDF calculation (here Au-Au + Au-P + Au-C), and long dashes show a zero line for absolute intensities of peaks in C ( r ) .When the dashed-and-dotted and the dashed lines coincide in any region of r, only one is shown in the figures. The arrows indicate the positions of the maxima of the G ( r ) peaks. The structure model is shown near the RDF to which corresponds the RDF simulation. All G ( r ) are given in electrons' A-'.
the contribution of the nearest distances between the heavy metal atoms of a polynuclear complex. Here it is the distance Au-Au ( R A ~=A2.93 ~ A). The model calculation indicates that the contribution of the single Au-Au distance in the molecule (dashes-and-dots) accounts almost completely for the intensity of the peak at Y = 2.935 A of the experimental RDF. A scarcely visible contribution is made to this peak by the few distances between Au atoms and the carbon atoms of CN groups = 2.96A). Here, the distance between the heavy metal atoms is manifested in a pure manner as the strongest RDF peak. Often, however, this does not happen. Fig. 3a shows the RDF[5361 of the cluster Re2(CO)lo, the structure of which is knownL8]The Re-Re bond (3.04A) alone in the carbonyl molecule makes the major contribution (dashes-and-dots) to the intensity of the peak at r = 3.085 A.The molecule contains ten R e . . . O distances approximately 3.15 A in length. Their joint contribution is quite apparent although the weight of each R e . . . O distance is much smaller than that of the Re-Re contribution. Only the sum of the Re-Re and R e . . . O contributions (dashes) fits the intensity of the experimental peak. Its maximum is somewhat moved to the right of the Re-Re bond length because of the Re. . . O contribution. Fig. 3b shows the still greater effect of masking of the metal-metal bond by the Re-C1 bonds (2.33 A) in the RDF of ammonium octachlorodirhenate( III).[61The weight of the Re contact with a 'semi-heavy' C1 atom is approximately twice as large as that of R e . . . O in the R D F of the carbonyl. Therefore, the Re-Re bond (2.23A)[91accounts for only half the intensity of the RDF peak at Y = 2.29A. The rest is the contribution of the eight Re-C1 bonds. This is, of course, an extreme
3.14 The Investigation of Heavy Metal Cluster Structures in Powder Samples
Figure 3. G ( r ) for (a) Rez(CO)lo and (b) ( NH4)2[RezCl8].2H20.
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example. One or two Re-Cl bonds per Re-Re bond would not make such a significant contribution. The possibility of this type of interference does, however, predetermine requirements of materials to be studied by the R D F method. One should remove all ‘unnecessary’ heavy and ‘semi-heavy’ atoms that have no direct relationship with the main problem to be solved. For example, when detecting a metal-metal bond, it is preferable to use a derivative containing ligands consisting of the lightest atoms possible, and light counter-ions if they are needed. It is obvious that Br atoms are hardly desirable compared with CI atoms. Usually the metal-metal distance is certain to be identified and measured within an accuracy of 0.02-0.03 A[5,61 even in the presence of interfering contributions, assuming the latter are of reasonable quality and quantity. The metal-metal distance imparts such high intensity to a peak that it cannot be submerged by the contributions of distances of other possible types for a given molecular stoichiometry and with a reasonably sensible model of the molecular geometry. For instance, no more than half of the area under the peak at r = 2.29A in Fig. 3b could be accounted for solely by the contributions of all possible Re-Cl bonds. This directly confirms the need for the metal-metal bond contribution to the R D F because no other explanation of its most intense peak is possible. The problem of ‘hampering’ contributions is not so important when heavy metal atoms have more than one heavy neighbor at a given distance. For example, this is so for simple polynuclear clusters in which the metal atoms adopt compact geometries, e.g. an almost regular triangle, square, or tetrahedron. Then the number of metal-metal distances, N M M calculated , per single metal atom, is 2 or 3 times larger than in a binuclear cluster, and the intensities of the corresponding peaks therefore
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3 Dynamics and Physical Properties
2
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Figure 4. G(Y)for O S ~ ( C O ) ~ ~ .
increase by the same factor. The contribution of distances between metal and ligand atoms is approximately the same in the RDFs of clusters of all types, because of the similar numbers of such distances. So, the interfering contributions look much smaller than the increased contributions from metal-metal distances. This is shown by the RDF[59'01of trinuclear osmium carbonyl["] in Fig. 4. The contribution of 2.89A Os-0s distances (dashes-and-dots) fills in the area of the peak at r = 2.91 A almost completely. The addition contributed by the 0 s . . . O distances is much less noticeable than that in Fig. 3a because of the analogous Re. . . O contribution to the RDF of binuclear rhenium carbonyl. A non-trivial question might be in what proportion should one refer to metalmetal and metal-ligand contributions the intensity of an unresolved peak of the RDF, if its intensity is noticeably larger than that corresponding to the metal-metal contribution alone in a simple binuclear cluster model. One could ascribe the excess either to metal-ligand distances or to additional metal-metal distances in a more complicated cluster core. There is no general answer to this question. In each instance it depends on the composition of the complex, on the type and structure of ligands and their possible functions, and on a more detailed analysis of other RDF peaks based on a more complicated structural model, etc. For example, the contributions from R e . . . Re type interactions could fill in the peak at r = 2.29 A in Fig. 3b if one assumes a chain model or a small triangle in which each Re atom has two Re neighbors. There would remain, however, no place for the contributions of the Re-Cl distances under the peak, i.e. one would have to assume that there are no Re-Cl bonds in the substance. Since the latter assumption is unrealistic in this instance, the model of the binuclear cluster remains the only possibility.
3.14.3 More detailed structure manifestation in RDFs Some high intensity peaks, in addition to those discussed above, can appear in an RDF and can help with the determination of the structure. Fig. 5 shows the RDFs
3.14 The Invrstigution of Heuvy Metal Cluster Structures in Powder Sumplrs
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of polycrystalline powders of ( NH4)2[Re2(HP04)4( H20)2] and K*[Pt2(HP04)4. ( H20)2].r6,'0.'21Both salts contain binuclear anions. The structure of platinum phosphate is known" 31 and that of the rhenium phosphate anion in the ammonium salt was found from the R D F and is the same as that known in the cesium The longer Pt-Pt bond (2.50A) gives a separate peak at r = 2.51 8, in the R D F of K2[Pt*(HP04)4( H20)2]; it is resolved from the peak of the nearest Pt-0 distances at r = 1.998,. The shorter Re-Re bond (2.21 A) gives the unresolved peak at r = 2.18 8, in the R D F of (NH4)2[Re2(HP04)4( H20)2] together with a minor contribution from the Re-0 bonds ( 2.05 A). The symmetric arrangement of four phosphate bridges across the metal-metal bond (Fig. 5) gives rise to many almost equal distances of ca 3A, the distances being from the metal atoms to some of the ligand atoms. These are the Pt( Re) . . . O distances to oxygen atoms coordinated by the neighboring Pt( Re) atom and Pt( Re) . . . P distances. All these numerous distances yield very strong combined peaks near r zz 3.1 A which don't overlap with the peaks from the Re-Re and Pt-Pt distances. A characteristic doublet of intense peaks occurs in the RDF; this undoubtedly confirms that both structures are of the so called 'lantern' type. In this instance, 'semi-heavy' phosphorus atoms in the ligands favor the structural interpretation of the R D F because they increase the weight of the distances Pt( Re) . . . P sensitive to the ligand arrangement. A propos Fig. 3b, one could see the additional strong peak located at r = 3.59 A which is also mainly produced by the large number of metal-ligand type distances, i.e. between the Re atom and C1 atoms bonded with the second Re atom of the cluster. In Fig. 6, we compare the RDFs of the binuclear acetamidate and the tetranuclear trifluoroacetate complexes of Pt, both of which have structurally very similar ligands, to show directly the appearance of the new peak corresponding to Pt . . . Pt distances as a result of the transition from the binuclear to the more complicated cluster nucleus. The RDF of the first complex and the cluster structure obtained
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3 Dynamics and Physical Properties
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Figure 6. G ( r ) for [Pt2(CH3CONH)4(N02)2](a) and [Ph(CF3C00)8](b).
from it['51 are very similar to those of the just discussed 'lantern' type platinum phosphate. Acetamidate bridging fragments are structurally similar to the phosphate groups in the reference compound, and therefore the characteristic doublet is again manifested in the RDF. The first component at r = 2.455A is given by the strong Pt"'-Pt"' bond and the second, at Y = 3.025 A, by the Pt . . .O, Pt . . . N and Pt . . . C (rather than Pt . . . P in phosphate) distances. In the latter complex, the structure of which is known, the Pt atoms are found at the vertices of the square and two carboxyl bridges are located on each square side like the acetamidate bridges in the 'lantern' structure (Ju. T. Struchkov, A. S. Batsanov, private communication). The peak corresponding to the Pt-Pt distances at r = 2.51 A is twice as large as the corresponding peak in the lower curve because each Pt atom has two near Pt neighbors in the square Pt4. The additional peak corresponding to the Pt . . . Pt distances along the diagonals of the square appears in the RDF at r = 3.54A. Such a diagonal peak in the RDF is an indispensable indication of any rigid figure a square, a rectangle, a trapezium, etc.) formed by heavy atoms. The peak at 3.06 again arises from contributions of Pt . . . O and Pt . . . C distances in bridging fragments. Here it is not as intense as on the lower curve in comparison with the larger peak corresponding to the Pt-Pt distances at r = 2.51 A, although the numbers of contributing distances are the same both in the binuclear and tetranuclear clusters (counted per single Pt atom in both examples). Fig. 7 gives a contrasting example. Here the absence of strong R D F peaks other than the first intense peak gives a strong argument for choosing a tetrahedral structure. Fig. 7 shows the RDF of a polycrystalline sample of the oxonitro complex I(4[Pt4IVO4(N02)12] and the model of the structure of its anion as obtained from the RDF.[l6I Each Pt atom has three near-neighbor Pt atoms. The identical distances to these neighbors results in the very strong and narrow peak at r = 3.13 A.No other intense peaks which could be ascribed to longer Pt . . . Pt distances are observed up to r % 8 A. Both facts can be explained simultaneously only
8,
3.14 The Investigation of Heuvy Metal Cluster Structures in Powder Sumples
Figure 7. G(v) for Kd[Ph04(N02)12].4H20.
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by a model in which the Pt atoms form Pt4 tetrahedrons which are positioned far from each other in the crystal lattice.
3.14.4 Polynuclear chains with direct interactions between heavy atoms Having discussed the main components of the RDFs of rather simple clusters, we will devote the last two sections to more complicated examples. The first two are relatively long oligomeric and polymeric chains of heavy atoms which have a rigid and flexible structures, respectively. Fig. 8 shows an impressive example of the R D F of the rigid chain of Pt atoms detected in an amorphous ‘platinum blue’ with inorganic ligands [Pt2(NH~)4(p-HP04)2],.[’71 The structural model is shown next to the RDF, the arrangement of atoms being tested as usual by the reconstruction of the RDF peaks up to r z 3.5 The main structural unit in the model is a binuclear cluster with a direct Pt-Pt bond (2.65A) and two phosphate bridges in cis positions. The Pt-Pt bond makes the main contribution to the peak at r = 2.70 A,and the phosphate bridges give rise to the appearance of a peak at r = 3.23 A in the same way as has been discussed for complexes of the ‘lantern’ type. These binuclear fragments form a rigid and almost linear linear chain, the distance between adjacent Pt atoms being 2.8 A.The latter distance contributes to the combined peak of the closest Pt-Pt contacts at r = 2.70 A.The subsequent distances between the heavy atoms along the chain (uia one, two, etc.) are manifested as a sequence of equidistant sharp peaks at 5.47, 8.13, and 10.87A. Each step is approximately equal to the nearest mean Pt-Pt distance of ca 2.7 A.The short nearest Pt-Pt separations of 2.65 and 2.8 A indicate strong interaction between Pt atoms which makes the delocalization of electrons along the chain possible. The latter property seems to be typical of Pt blue complexes with the metal in a fractional oxidation
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Figure 8. G ( r ) for the ‘platinum blue’ [Pt2(NH3)4(pU-HPO4)2],.Different types of atom are labeled with numbers in the diagram of the model 1, 2, 3, 4 indicate the Pt atoms of the polymetallic chain, 5 is the oxygen atom, 6 the ammonia ligand, and 7 the phosphorus atom. ~
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Here the chains of Pt atoms are noticeably longer than those in tetrameric ‘platinum blues’ of known structure formed with large asymmetric bridging organic ligands (a-pyridone, 1-methyluracil) in which steric hindrance prevents the formation of long chains.[’81The given phosphate ‘blue’ is structurally closer to the Pt octamer consisting of dimers with double acetamidate bridges“’] which do not create such obstacles. In this example the best fit of the RDF peaks in the region of Y < 3.5 A is reached with a chain 6-8 Pt atoms long (3-4 dimers). This was shown by varying the number of dimers in the chain in Fig. 8 and thus changing the mean number of 2.8 A Pt-Pt separations. The chain length thus obtained might be a mean value, with the sample containing both longer and shorter oligomeric chains. The manifestation of a ‘comb’ of peaks arising from large distances between heavy atoms in a chain, as vivid as in the RDF in Fig. 8 and unambiguously interpretable, is possible only in amorphous samples. A disordered arrangement of the chains substantially broadens the distributions of distances between the Pt atoms belonging to different chains. In contrast to polycrystalline samples, they do not manifest themselves as narrow strong R D F peaks in addition to those produced by the sequence of fixed Pt . . . Pt distances along the rigid chain. The latter can be observed directly. Flexible polynuclear chains provide another extreme example of peaks arising from large distances between heavy atoms not being explicitly manifested in the RDF even though these chains certainly exist in an amorphous sample. Because distances between the nearest neighbors in a chain can be rigidly fixed either by bridges or by direct interactions between metal atoms, they give the usual strong narrow peak in the RDF. However, the angle between such metal-metal bonds might have a wide distribution. As a result, the distribution of distances between heavy atoms via one or two sections along the chain will be wide. These distances will produce very broad peaks which are not prominent among other numerous contributions to an RDF tail and so are not observable explicitly. The nature of the amorphous state enables the existence of such disordered chains. Fig. 9a shows the RDFrZo1of the bright-red amorphous modification of the compound 4-( N O ~ ) C ~ H ~ N H A UThe P P ~doublet ~. of strong peaks with maxima
3.14 The Investigation o j Heavy Metal Cluster Structures in Powder Sumnpkes
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2 1 G( r)
Figure 9. G(Y) for (a) amorphous polynuclear bright-red and (b) mononuclear crystalline yellow modifications of ~-(NO~)C~H~NHALIPP~~.
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at r = 3.12 and 3.42A testifies to the existence of direct Au-Au contacts. Their intensities correspond to a model in which each Au atom has one neighbor at the distance of 3.07 A and another at 3.46 A.Thus the Au atoms occur in long chains, a fragment of which is shown schematically in Fig. 9. The chain consists of two different sections. The former looks like dimers with a rather strong Au-Au bond which corresponds to the short separation (3.07;\). The other is formed from the longer and weaker interactions of Au atoms of the dimers (3.46A). The latter might be not so strongly directed as the former and this enables large disordering of the angles between the Au-Au bonds in the chain. Only smooth oscillations, typical of amorphous samples, are observed in the RDF in the region r > 4A, which testifies to this type of disorder. The same compound has a crystalline yellow modification of known structureL211 in which the molecules are packed so that Au atoms are positioned far from each other. The RDF of the polycrystalline powder of this phase is shown in Fig. 9b. It contains no strong peaks near 3 A typical of Au-Au bonds. Comparison of two curves confirms the above interpretation of the strong peaks in the lower curve experimentally. This is a curious example in which the metal-metal interactions occur in a kinetically preferable metastable amorphous phase under conditions of fast precipitation of the substance from a solution, whereas a stable crystalline phase with no metal-metal contacts is formed by slow precipitation.[2032
3.14.5 Heavy atoms in ligands The presence of rather heavy atoms in ligands makes it possible, firstly, to measure some individual metal-ligand distances with confidence - we can refer to our
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Figure 10. G(v) for K4Hz[(SnCitr)4Pt(pSnCitr)2Pt(SnCitr)4]5H20 (Citr3- =(-OOC)CH2C(OH). (COO-)CH2(COO-)). The model reconstruction of the R D F in the main picture corresponds to the structure shown in Fig. 1 I . In this figure the dashed-and-dotted line shows the contribution of the distances between the Sn atoms and the dotted line shows the contribution of the distances between the Pt and Sn atoms. The insert shows a comparison of the R D F in the region 3-5 A with the model simulation based on the trigonal-bipyramidal arrangement of Sn atoms around Pt atom.
work[”] which has applied this property of RDFs. We now discuss briefly another type of example in which it was crucial for the correct reproduction of the range of distances between ligand heavy atoms for selecting a cluster structure model. Fig. 10 shows the RDFIZ3]of a platinum-tin heteronuclear anion complex with citrate ligands (amorphous sample of the potassium salt). The structural model of the anion obtained by interpretation of the RDFLZ3]is shown in Fig. 11. The strongest RDF peak at r = 2.58 A corresponds to the known length of the Pt-Sn bond in [Pt(SnC13)s]3-.[241 Thus the (SnCitr)- ligands are also connected here to Pt centers through their Sn atoms, and the citrate groups are situated at the periphery of the cluster. Sn atoms, however, are not arranged at the vertices of a trigonal bipyramid as occurs in the chloride derivative. Such a model contradicts the R D F in the region of r = 3.3-4A in which the main contribution is made by the distances between the heavy Sn atoms of the (SnCitr)- ligands. The substantial disagreement
OH
Figure 11. The model of structure of the platinum-tin complex anion. The inter-atomic distances (and some related bond angles) specified those determined with certainty on the basis of simulation of the RDF in Fig. 10, because of the sufficiently high contribution of these distances to the RDF.
3.14 The Investigation of Heuvy Metul Cluster Structures in Powder Samples
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between the model (dashes) and experimental (solid) RDFs is shown in the insert in Fig. 10. In the correct model (Fig. l l ) , Sn atoms are situated at the vertices of a somewhat distorted octahedron around the Pt atom; two act as bridges in accordance with the stoichiometry of the complex. The Sn bridges connect two platinum-tin octahedra uia the common edge. The Pt . . . Pt distance of ca 3.9 8, in the binuclear complex makes a substantial contribution to the R D F peak at r = 3.91 8,. Octahedral coordination of Pt atoms increases the number of Sn . . . Sn distances in the region of r < 4 8, (all edges of the octahedron) which is evidently not enough in the bipyramidal model (only short edges of bipyramid). The necessary splitting of the Sn . . . Sn contribution in the R D F (dashed-and-dotted line) is achieved by distortion of the angles between Pt-Sn bonds because of pushing apart of the large citrate ligands at the axial tin atoms (Sn3.4.7,8).The last pronounced peak of the R D F near r = 5.15 8, corresponds to the numerous Sn . . . Sn distances between Sn atoms at the opposite vertices of the octahedra.
3.14.6 Conclusion The R D F method has been ‘well-forgotten’ by chemists. Nowadays they use the much more popular and well-known EXAFS method when trying to solve structural problems such as those discussed. In the EXAFS method, however, the peaks for distances between metal atoms have second-order intensities compared with those of the main peaks for the distances between the metal and the nearest ligand atoms. The former are often not observable, especially if the distances are longer than 3 8,. Special conditions are necessary to make metal-metal peaks detectable, e.y. either very low temperatures or a special type of a complex structure. There is no such limitation of the RDFs obtained by X-ray diffraction. The R D F method requiring an ordinary powder diffractometer is experimentally more accessible than the EXAFS technique. R D F theory does not deal with complications and approximations characteristic of EXAFS because the physics of X-ray diffraction, which is the basis of the R D F method, is much simpler than the physics of the processes generating EXAFS spectra. In the R D F technique, therefore, the measured experimental data are more directly related to structure. Of course, the R D F method has its own limitations and cannot be used to solve some problems for which the EXAFS method gives valuable information, e.y. for dilute systems. The R D F method is, however, preferable for problems such as those outlined above. There is a rather narrow window, or more exactly a slit, in r values from approximately 2 to 3.5-4 A from which one can obtain quantitative structural information from an RDF. This slit does, however, enable one to ‘see’, although not
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3 Dynamics and Physical Properties
always unambiguously clearly, but still to ‘see’, much about the object of interest. At the same time, most spectral methods enable one at best to ‘hear’ something, and not very clearly at that, about the mutual arrangement of metal atoms. As the Russian proverb goes, ‘It is better to see once than to hear a hundred times’.
References [ I ] A. Guinier, Theorie et technique de la radiocristallographie, Dunod, Paris, 1956, Chapters 5 , 11. (Russian edition: X-ray difraction in crystals, Fismatgis, Moscow, 1961). [2] R. W. James, The opticalprinciples of the diffraction of X-rays, London, 1950. Chapters 9-10. (Russian edition: Inostrannaja literatura, Moscow, 1950). [3] B. E. Warren, X-ray Diffraction. Addison Wesley, Reading, 1969, Chapter 10. [4] A. F. Skrishevskii, Structure analysis of liquids and amorphous bodies, High school, Moscow, 1980. [5] V. I. Korsunsky, J. Organomet. Chem., 1986, 311, 357. [6] V. I. Korsunsky, J. Struct. Chem. (Russ), 1987, 28, 80 (NI,Russ), 67 (Engl. tr.). [7] E. I. Smyslova, E. G. Perevalova, V. P. Dyadchenko, K. I. Grandberg, Yu. L. Slovokhotov, Yu. T. Struchkov, J. Organornet. Chem., 1981,215, 269. [8] L. F. Dahl, E. Ishishi, R. E. Rundle, J. Chem. Phys., 1957, 26, 1750. [9] P. A. Kozmin, M. D. Surazhskaja, T. B. Larina, Koordinatsionnaja Khimiju, 1979, 5, 752. [lo] V. I. Korsunsky, Doklady A N SSSR, 1986,291, 1411. [ I l l M. R. Churchill, B. G. De Boer, Znorg. Chem., 1977, 16, 878. [I21 V. I. Korsunsky, G. S. Muraveiskaja, V. E. Abashkin, I. G. Fomina, Russian J. Znorg. Chem., 1992, 37, 2019 (Russ), 1042 (Engl. tr.). [I31 D. P. Bancroft, F. A. Cotton, L. R. Falvello, S. Han, W. Schowotzer, Inorg. Chim. Acta., 1984, 87, 147. [ 141 P. A. Kozmin, M. D. Surazhskaja, T. B. Larina, Doklady AN SSSR, 1985,280, 929. [15] V. I. Korsunsky, G. N. Kuznetsova, Russian J. Inorg. Chem., 1988, 33, 1624 (Russ), 923 (Engl. tr.). [ 161 V. I. Korsunsky, Russian J. Inorg. Chem., 1989, 34, 139 (Russ), 77 (Engl. tr.). [ 171 V. I. Korsunsky, G. S. Muraveiskaja, V. E. Abashkin, Russian J. Inorg. Chem., 1988, 33, 669 (Russ), 374 (Engl. tr.). [ 181 M. Peilert, A. Erxleben, B. Lippert, Z. Anorg. Allg. Chem., 1996, 622, 267. [19] K. Matsumoto, K. Sakai, K. Nishio, Y. Tokisue, R. Ito, T. Nishide, Y. Shichi, J. Am. Chem. Soc., 1992, 114, 8110. [20] L. M. Epstein, E. S. Shubina, L. N . Saitculova, V. I. Korsunsky, E. I. Smislova, K. I. Grandberg, D. N. Kravtsov, Metalloorgan. Khimija, 1989,2, 1009. [21] E. G. Perevalova, K. I. Grandberg, E. I. Smyslova, L. G . Kuzmina, V. I. Korsunsky, D. N. Kravtsov, Metalloorgan. Khimija, 1989, 2, 1002. [22] V. I. Korsunsky, G. S. Muraveiskaja, A. A. Sidorov, Znorg. Chim. Acta., 1991, 187, 23. [23] V. I. Korsunsky, P. G. Antonov, T. P. Lutsko, Polyhedron, 1992, 11, 1403. [24] J. H. Nelson, N. W. Alcock, Inorg. Chern., 1982, 21, 1196.
Metal Clusters in Chemistry Volume 2 : Catalysis and Dynamics and Physical Properties of Metal Clusters Edited by P. Braunstein, L. A. Oro & P. R. Raithby Copyright 0WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
3.15 The Spectrum of an Exciton in Nanocrystal Semiconductor Structures - Theory Sergey I. Pokutnyi and Volodymyr V. Kouulchuk
3.15.1 Introduction The optical properties of different inhomogeneous condensed media of reduced diThe change in the electron mensionality are now being studied spectrum of a semiconductor under the influence of a medium in contact with it, and associated optical effects, have been inve~tigated[~-~] and possible localization of excitons at the interface between a semiconductor and vacuum by electrostatic image forces has been d e m o n ~ t r a t e d . [ ~There , ~ ] is particular interest in the optical properties of nano-structures in the form of real spherical semiconductor microcrystals['31 with radii in the range 1-100 nm, grown in a transparent insulating host. Because the energy gap of a semiconductor is much smaller than in the dielectric host, the motion of carriers in the semiconductor nano-crystal is restricted by its volume. Optical and non-linear optical properties of such multiphase systems are determined from the energy spectrum of space-limited electron-hole pairs (excitons). Quantum-well effects have been found by optical-spectroscopic studies in in such nano-dimensional the energy spectrum of electronsL8-Io1 and excitons" structures. The energy spectrum of an exciton in a small semiconductor nanocrystal (SNc) has been determined"' 241 by assuming that the SNc was in an impenetrable spherical potential well. The formation of a bulk exciton in a small SNc was not considered. Let us note that we define a bulk exciton in the SNc as an exciton whose structure (reduced effective mass, Bohr radius, and binding energy) in the SNc is no different from its structure in an unbounded semiconductor material. In this contribution we compare theoretical and experimental exciton spectra obtained for different SNc radii and thus determine the unknown parameters of the ball-like nano-dimensional structure, i.e., the effective mass of the exciton as function of the SNc radius and the critical radius a, above which a bulk exciton can appear in a semiconductor microcrystal with a 2 a,.[131
1264
3 Dynamics and Physical Properties
3.15.2 The Hamiltonian of an exciton in a nanocrystal A simple model of a nano-dimensional structure in the form of a neutral spherical SNc of radius a and permittivity ~ 1 embedded , in a medium with permittivity ~ 2 has , been discussed e l s e ~ h e r e . [ ' ~An - ~electron ~~ e and hole h with effective masses me and mh were assumed to travel within this SNc (we use re and r h to denote the distances of the electron and the hole, respectively, from the center of the SNc). We assume that the two permittivites are such that ~2 >> E I , and that the electron and hole bands are parabolic in shape. In this model, and subject to these approximations and the effective mass approximation, the exciton Hamiltonian takes the form:" 8 %191
where the first two terms represent the kinetic energy of the electron and hole and Ve,h(Ye, r h ) is the energy of the Coulombic interaction between the hole and the electron,
0 denotes the angle between Th and re, Veet(re,a) and Vhhf(rh, a) are the energies of interaction of the electron and hole with their own images and Vehl( r e ,Th, a)and Vhet(re,rh,a) are the energies of interaction with 'strange' images, and Eg is the . arbitrary and ~ 2 band gap in the infinite semiconductor with permittivity ~ 2 For the terms['] that describe the energy of the polarization interaction of electron and hole with the SNc surface can be written in an analytical form[251that is particularly simple for ~2 >> ~
1
:
[
~
~
3
~
~
l
,
3.15 The Spectrum of an Exciton in Nanocrystal Semiconductor Structures
1265
3.15.3 The spectrum of an exciton in a nanocrystal The spectrum of an exciton in a small semiconductor nano-crystal has been investigated" t~~91 in the case where its size was restricted by the condition: a0
<< a h << a
a,
N
(6)
aex
where ah = E2h/mhe2,a, = E2h/mee2, and aex= c2h/,ue2 are the Bohr radii of the , hole, electron, and exciton in the semiconductor with permittivity ~ 2 respectively; memh is the reduced effective mass of the me mh exciton, and a0 is a characteristic size that is of the order of the inter-atomic separation. When this condition is satisfied, the polarization interaction plays a significant part in the potential energy of the Hamiltonian."] These inequalities also enable us to examine the motion of an electron and a hole in the effective mass approximation. The validity of the conditionc6] further enables us to consider the motion of a heavy hole (mh >> me)in the electron potential averaged over the motion of the electron (adiabatic approximation). The adiabatic approximation can be used in eq. 6 if we assume that the electron kinetic energy is the largest quantity and take the last four terms in eq. 1 together with the non-adiabatic operator as a perturbation. First-order perturbation theory in the electron wave functions in an infinitely deep spherical well has been ~ s e d [to~obtain ~ * ~an ~expression ~ for the in the state (n,, I, = 0; t h ) , where ne and 1, are the exciton spectrum [E:, I, = 0(s)] principal and orbital quantum numbers of the electron, and th is the principal quantum number of the hole in the SNc of radius = i i / a h ) . The exciton spectrum was thus found to be:
e is the charge of the electron, p
=
+
~
(s
where K = 0.67 represents the spread of the SNc radii['71 and = m,/mh. Numerical analysis of X-ray data,"21 which took into account the spread of the SNc radii, shows that the mean radius evaluated over the Lifshits-Slezov distribution[261was = 0.865,['21where is the SNc radius obtained in the small-variance approximation. The coefficients in Ref. 7 were defined in Refs 19 and 22 by the expressions:
s
s
1
z ~ =, ,2 ~ d.xsin2(nn,x)/(l
-
x2)
SO
P,,e,o= 2Ci(2nne) - 21n(2nne) - 2y
+(E~/EI)
-
1
(8)
1266
3 Dynamics and Physical Properties
The last term in the exciton spectrum given elsewhereL7]represents the spectrum of a hole undergoing oscillations of f r e q u e n ~ y . [ ' ~ ~ ~ ~ ~
W(s,n,)
= 2.232( 1
+ 7~ 2 2 n,) 2 1123-312
(9)
in the adiabatic electron potential. The SNc distribution over the Lifshits-Slesov radii[261has been used['] to take into account the SNc size variance. The main contribution to the exciton spectrum[71obtained in the adiabatic approximation is provided by the second term (electron kinetic energy), because of the purely spatial limitation imposed on the quantization region, and the last terms, which are associated with the Coulombic and polarization interactions between electron and hole, are regarded as corrections. The polarization interaction between hole and electron, on the one hand, and the surface of the SNc, on the other, provides, like the size quantization of carriers, a contribution to renormalization of the energy gap of the SNc.17]The polarization interaction, which is of the same order of magnitude as the exciton binding energy in the SNc, is, in this instance, found to be much smaller than the size quantization energy of the charge carriers in the SNc. We also note the exciton spectrum[71is valid only for low-lying states of the exciton (ae,0; t h ) in the SNc, for which (E: - E g ) << VO,where VOis the depth of the potential well for an electron in the SNC (for example, in the cadmium sulfide SNs, VO= 2.3-2.5eV when the condition given in Eq. (6) is satisfied.[61
3.15.4 Bulk exciton in a nanocrystal When the radius a of the SNc is greater than or equal to the critical value a,, a bulk exciton can be formed and become localized as a complete entity, in the SNc. This type of threshold for the appearance of the bulk exciton in the small SNc of radius a 2 a, can be predicted by simple qualitative estimates. The necessary conditions for the formation of a bulk exciton in the SNc is that its binding energy Eb must be greater than the energy of the polarization interaction ( e 2 / & 2 u ) between electron and hole on the one hand, and the SNc surface, on the other:
-
It follows from this inequality that the bulk exciton will appear in the SNc only if its radius a is greater than the critical size a, of the SNc:
3.15 The Spectrum of an Exciton in Nanocrystul Semiconductor Structures
1267
We must now explain, at least qualitatively, the existence of the critical radius a, (equation given in Ref. 11). In an infinite semiconductor medium a large-radius exciton appears as a result of the Coulombic attraction Veh(Y,,rh)[21 between an electron and a hole. The Hamiltonian['] of an exciton moving within a small SNc a),141 contains not only the Coulombic attractionI2] but also the terms V,,,(Y,, Vhh'(Yh, and Vehf(re, rh,u) Vhe/(f,,rhu ) [ 5 1 that describe the interaction of the electron and the hole with their own images and those of other particles, respectively. The terms V,,! (re. produce a repulsion between an and Vhh'(rh, electron and a hole, on the one hand, and the SNc surface, on the other, which leads to effective attraction between quasi-particles. The term Veh' ( r e ,Th, a ) Vh,J(fc rh,L,l 151 is responsible for the attraction between the quasi-particles and the SNc surface, which produces a repulsion between the electron and the hole. The terms which lead to attraction between the q ~ a s i - p a r t i c l e s , 'predominate ~.~~ over the terms responsible for attraction between the electron and the h01e.I~'The result is that the interaction between the electron and the hole in the small SNc consists of the Coulombic attraction V, h(re3 rh)[21 and a further contribution because of the additional effective attraction between the electron and the hole, which is because of repulsion between and the hole Vhh'(rh,a),131 on the one hand, and their rethe electron V,,~(Y,.~)[~~ a,,), spective images, on the other. As the SNc radius is reduced and we have ( a I the magnitude of this additional attraction between the electron and the hole increases as ma-'. This effective polarization attraction ensures that the electron and the hole travelling in the SNc become localized within the volume of the SNc with an effective mass p = p(a) that is greater than the exciton mass p o in the infinite semiconducting material of permittivity Consequently, the bulk exciton aex. The bulk exwith effective mass p,, cannot appear in the SNc with radius a I citon can therefore arise in the SNc only with radius a > a,,. The formation of this bulk exciton has threshold character and is possible only when the size of the SNc exceeds the critical radius As the radius a of the SNc increases, so that a becomes greater than a,,, the effective attraction between the electron and the hole falls as m a - ' . Above a certain radius a = a,, the energy of the effective additional electron-hole attraction becomes small in comparison with exciton binding energy Eb, given elsewhere.["] The measured positions of the absorption lines of small cadmium sulfide SNc that were because of inter-band transitions of electrons to the 2) in the conduction band as functions of the size-quantization levels (n, = 1, I, I SNc radius a were reported elsewhere.['-' Moreover, the dispersion relations for charge carriers at the bottom of the conduction band and the valence band can be assumed parabolic to a good approximation.[s-''l We assume that as the radius of the calcium sulfide SNc reduced to value comparable with the Bohr radius of the bulk exciton, a,, = 25& only the effective masses me and p of the electron and exciton, which depend on the SNc radius a and the position of the exciton level (n,, 1,; t h ) in the SNc, i.e., m, = m,(cl;n,, I,; th),L( = p(6;IZ,, 1,; th) will vary (the hole in the cadmium sulfide is heavy (mh/m, z 2 5 ) , so we may assume that its effective mass m in the small SNc will remain the same). When we interpret the experimental ~
2
.
~
~
~
3
~
~
~
1268
3 Dynamics and Physical Properties
Table 1. Effective mass rn,(a) of an electron and p(C) of an exciton as functions of of the radius, 5, of a nano-crystal. E(A)
25 30 35 40 45 50 55 60 65 69.98
0.332 0.263 0.226 0.190 0.162 0.142 0.125 0.112 0.102 0.093
0.379 0.341 0.283 0.267 0.253 0.240 0.229 0.218 0.209 0.205
0.353 0.319 0.268 0.253 0.241 0.230 0.219 0.209 0.200 0.197
data,[8p101we use the theoretical exciton spectrum E:,o = (s)r71 in the ground state (n, = 1, I, = 0; t h = 0) of the small SNc for 5 2 aex. We assume, as elsewhere,[291that the exciton spectrum[71can also be used for SNc with 5 < 3aex. By with measured positions of the absorpcomparing the exciton spectrum E0l,o(5)[71 tion peaks of the calcium sulfide SNS[~~''] we can determine the electron effective electron mass me = rne(5)and exciton mass p = p(5) of the small SNc as function of the mean SNc radius. Table 1 lists numerical values of me = me(ii)and p = p ( i i ) . The values of Eol,o - Eg given in Table 1 are taken from the experiments reported el~ewhere.[~,~] It is clear from the behavior of these functions that, as the SNc radius increases for a > aex,the exciton and electron effective masses p = p ( 5 ) and me = m,(a) decrease, approaching for 5 = a, = 2.8ae, z 70A the effective exciton and electron effective masses po and neOin infinite cadmium sulfide (see Table 1). The exciton spectrum of small SNc was found in the framework of the variational method,[301where the adiabatic approximation has not been used. Comparison of the calculated exciton spectrum with experimental data obtained for small S N S , [ ~enables - ~ ~ ] determination of the magnitude of the critical radius a, z 3.48a,, of SNs. We point out that the bulk exciton can appear in SNc with radius a 2 a,. The critical radii of small cadmium sulfide SNc a, z 2.8ae, and a, z 3 . 4 8 ~ , , [ ~ 'are ~ in good agreement. The difference, ca 20%, is a simple consequence of the variational approach used in the calculation of exciton ~ p e c t r a , [ ~which ~ ~ ' ] overestimates the exciton energy leading also to higher magnitudes of critical radii a, of SNc. In a previous paper['] we obtained the exciton spectrum for a small SNc on the assumption that the electron could leave the SNc volume for the ambient dielectric host. The problem solved was that of the electron energy spectrum in a small SNc, taking into account the finite height of the potential barrier VOon the spherical separation boundary between the SNc and host dielectric, and the associated penetration by the electron of the ambient host.['] The electron wave function were ob-
3.15 The Spectrum ojan Exciton in Nunocrystal Semiconductor Structures
1269
tainedL3'I for an electron travelling both in the SNc and in the ambient dielectric host. The adiabatic approximation and first-order perturbation theory were used with these electron wave functions to determine the exciton spectrum in a small SNc. A comparison of the exciton spectrum f o ~ n d [ ~ with . ~ ' ] the experimental exciton spectrum in cadmium sulfide SNc has shown[".''] that the bulk exciton could ). appear for radii u greater than the critical radius of the SNc (a, M 1 . 7 ~ ~As expected, the critical radius a, = 2 . 8 ~found ~ by us for the cadmium sulfide SNs under the same conditions as is somewhat greater than the re~ ~ ~a, > a,. We emphasize that the critical ported value of &(a, % 1 . 7 ~ , , ) , [i.e., radii of small cadmium sulfide semiconductor nano-crystals obtained here by different methods, ie., (a,M 2a,x),["] (a,% 2.8a,,), (a, % 3.48a,x),[301and also (a, M 1.7a,,),[311are not very different from each other. Comparison of the theoretical as a function of the small SNc radius a with the exexciton spectrum (EO10(a))[71 perimental absorption spectra of the SNc can be used to determine the unknown parameters of nano-structures such as the exciton effective mass p = p ( a ;n,. le; t h ) in the state (ne,1,; th) as a function of the SNs radius in the simple parabolic band of the small SNc. Such a comparison will also yield the critical SNc radius ac(ne,le;th), above which the exciton can appear in the small SNc of radius a 2 a, > aex.The new method proposed above can be used in this way to determine the unknown parameters of nano-one-dimensional and nano-two-dimensional structure^.[^^^.^^-^^^
Acknowledgment The authors are indebted to Professor V. M. Agranovich and Professor E. L. Ivchenko for helpful discussion of the results reported here. V. K. would like to express his deep thanks to Professor Dr. P. Braunstein for assistance and who made this project possible.
References [ 11 V. M. Agranovich, Theory of E.xciton.c., (in Russian) Moscow, 1968. [2] T. Ando, A. Fowler, and F. Stern, Electronic Properties of Two-dimensioncilSystems, (Russian trans.), Moscow, 1985. [3] V. M. Agranovich, A. G. Mal'shukov, M. A . Mekhtiev, Zh. Eksp. Teor. Fiz., 63, 2274 (1972). [4] M. V. Tkach, V. I. Bojchuk, V. A. Holovatsky, 0. M. Voitsekhivska, Fiz. T o e d Tela, 38, 3161 (1996). [5] V. M. Agranovich, and Yu. E. Lozovik, Pis'mcr Zh. Eksp. Teor. Fi:., 17, 209 (1973) [JETP Letters 17, 148 (1973)l.
1270
3 Dynamics and Physical Properties
S. I. Pokutnyi, V.V. Kovalchuk J. Phys. Studies., 3, (1999) (in press). M. F. Deglen and M. D. Glinchuk, Fiz. Tverd. Tela (Leningrad), 5, 3250 (1963) [Sou. Phys. Solid. State 5, 2377 (1963)l. V. Dnieprovskii, Phys. Lett., A204, 59 (1995). A. Ekimov, V. Markov, A, Efros, J. Phys.: Cond. Matt., 6, 2573 (1994). A. Ekimov, A. Efros, J. Opt. Soc. Am., B10, 100 (1993). D. Chepic, A. Efros, A. Ekimov, J. Lumin., 47, 113 (1990). A. Ekimov, A. Onushchenko, Pis’ma Zh. Eksp. Teor. Fiz., 90, 1795 (1986) [JETP Letters 63, 1054 ( 1986)l. A. Sekiguchi in: The Chemistry of Organic Silicon Compounds, Vol. 2, Ed. Z. Rappoport and Y. Apeloig, John Wiley and Sons, Chichester, 1998. L. Brus, J. Chern. Phys., 79, 5566 (1983). R. Rossetti and L. Brus, J. Chem. Phys., 82, 552 (1985). H. Chestnoy and L. Brus, J. Chem. Phys., 85, 2237 (1986). A. I. Efros and A. D. Efros, Fiz. Tekh. Poluprouodn., 16, 1209 (1982) [Sou. Phys. Semicond., 16, 955 (1982)l. N. A. Efremov, S. I. Pokutnyi, Fiz. Tuerd. Tela (Leningrad),32, 1637 (1990) [Sou. Phys. Solid. State 32, 955 (1990)l. S.1. Pokutnyi, Fiz. Tekh. Poluprouodn., 25, 628 (1991) [Sou. Phys. Semicond., 25, 955 (1991)l. S. I. Pokutnyi, Phys. Status. Solidi, B165, 109 (1991). S. I. Pokutnyi, Phys. Status. Solidi, B172, 573 (1992). S. I. Pokutnyi, Phys. Status. Solidi, B168, 433 (1992). S. I. Pokutnyi, Phys. Status. Solidi, B173, 607 (1992). S. I. Pokutnyi, Fiz. Tuerd. Tela (Leningrad), 34, 2386 (1992) [Sou. Phys. Solid. State 34, 1278 ( 1992)]. N. A. Efremov, S.I. Pokutnyi, Fiz. Tuerd. Tela (Leningrad), 27, 48 (1985) [Sou. Phys. Solid. State 27, 27 (1985)l. I. M. Lifshitz, V. V. Slezov, Zh. Eksp. Teor. Fiz., 35, 479 (1958) [Sou JETP 8, 331 (1958)l. V. Grabovskis, Ya. Dzenis, A.N. Ekimov, Fiz. Tverd. Tela (Leningrad), 31, 272 (1989) [Sou. Phys. Solid State, 32, 272 (1989)l. V. L. Bonch-Bruevich and S. G. Kalasnikov, Semiconductor Physics [in Russian], Moscow, 1990. A. Ekimov, A. A. Onushchenko, and A. L. Efros, Pis’ma Zh. Eksp. Teor. Fiz., 13, 281 (1987) [JETP Letters 13, 281 (1987)l. S. I. Pokutnyi, Fiz. Tverd. Tela, 38, 2661 (1996). S. I. Pokutnyi, Phys. Lett., A203, 388 (1995). V. V. Kovalchuk, V. V. Chislov, V. A. Yanchuk, Phys. Stat. Sol.(b), 187, 47 (1995). V. V. Kovalchuk, L. Yu. Kutsenko, I. A. Polosovskaya, etc. in the book: Computer modelling of electronic und atomic processes in solids. NATO Scientijic Affairs Division (Kluwer Acad. Publ., Dordrecht-Boston-London, 1996).
4 Nanomaterials
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.1 Metal Clusters and Nanomaterials: an Overview Musaru Ichikuwa
4.1.1 Introduction Organometallic cluster compounds with metal frameworks containing more than three metal atoms are akin to metal particles and stepped metal surfaces having neighbouring multi-metallic centers, as represented in Fig. 1. The adjacent metal sites in polynuclear metal clusters, as exemplified by open-butterfly metal clusters, make available multi-metallic coordination environments (e.g., face, edge and kink) that are not possible at a single metal atom or in isolated metal ion complexes. The activity and selectivity of supported metal catalysts generally depend on the state of metal dispersion (ensemble sizes), structure (shape and morphology), metal composi-tions (geometric and spatial distribution), and metal-support (metal oxide/sulfide) interactions. The dispersion and morphology of metals can be controlled by selection of the metal precursors and supporting materials and also by the method of preparation. Conventional catalyst preparation is based on a series of empirical inorganic reactions of metallic salts consisting of impregnation, calcination and reduction, which does not afford a good control of particle size, morphology and metal compositions that would yield high performance catalysts for industrial processes. During the last two decades attempts to prepare tailor-made catalysts using molecular precursors such as metal clusters (see Fig. 1) grafted on metal oxide supports have been made and developed by some pioneering studies, since these methods would give better control of catalyst formation.[’-*] The chemistry, characterization and catalytic application of surface-grafted metal clusters have been extensively reviewed by Gates,“] BassetL2]and I~hikawa.[~] More recently, micro/mesoporous cavities and channels of porous materials such as zeolites and layered clays (Fig. 2) have been used as the “ultimate reaction vessel” in which the template synthesis of metal clusters can be carried out. This is coined as “ship-in-a-bottle” synthesis
1274
4 Nunornaterials
A ph3pB 0s
ph3 p AU
0s
AU
Open ButterJj
Closed Tetrahedron
Catalytically active
Inactive
Figure 1. Pictorial representation of various metal clusters as molecular precursors for the nanomaterials and tailored metal/bimetal catalysts, e.g., Os3(CO)12, Rh(CO),z, Fe2Rh(C0)162-, Pt12(Co)242-, [Ptlg(CO)l8I2-, [ P ~ ~ ~ ( C O ) U[~i38Pt6(C0)48H6-,]"-, ]~-, catalytically active openbutterfly AuOs,(CO)h( PPh3)Cl and inactive AuOsl(CO)h(PPh3)H. It has been later established that both AuOs3 clusters are of the butterfly type, however, with 58 and 60 electrons, respectively. See: P. Braunstein and J. Rose, in Comprehensive Organometullic Chemistry ZZ, E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Pergamon, Oxford, 1995, Vol. 10, p. 351.
of metal complexes by analogy with the tricky preparation of a model ship in a whisky-bottle using building blo~ks.[~,',*~ Metal clusters of uniform composition are prepared and held in each micro/mesopore or channel in preventing facile migration and cluster-sintering under the prevailing preparative and reaction conditions. This introductory chapter, which follows Volumes I and I1 on Molecular Clusters and Catalysis, reviews the recent progress in the synthesis, characterization and catalysis of metal clusters in micro/mesopores. In particular, the "ship-in-a-bottle technique for the rational design of nanomaterials, robust clusters, nanoparticles and nanowires is described and their potential application to the molecular approaches for tailored metal catalysts and electronic/magnetic devices is discussed.[91
4. I Metal Clusters and Nunomuteriuls: an Overview
1275
‘wM picture of ZSM-5
erYsai
Inorganic Micro/Mesnporous Materials
Micro-reactors fnr Synthesis of Organometallic Clusters
SOD
/
NaY
1950
Figure 2. Some examples of micro- and mesoporous materials as the template micro reactors for “ship-in-a-bottle’’ synthesis of metal clusters in micropores and mesoporous channels.
4.1.2 Ship-in-a-bottle synthesis of metal clusters in micropores Various types of zeolites referred to as Faujacite ( N a y , NaX), Mordenite, ZSM-5 and ALPO-5 are aluminosilicate and aluminophosphate crystals consisting of microporous cages of molecular dimensions (5-12 A) which are interconnected by smaller windows and channels, as is represented in Fig. 2. Such micropores can be used as ultimate “nanometer-sized reaction vessels”, providing a “template” for synthesizing nanomaterials such as selected metal clusters and nanoparticles which may fit the interior cages. Such bulky carbonyl clusters as Rhs(C0)16 (van der Waals diameter of 10 A) and [Ptl2(C0)24I2- (8 x 12 A)are unable to enter directly into the pores of NaY (12 A) through its smaller windows (6 A). Nevertheless, Rhs(C0)16, for example, can be synthesized and encapsulated in the cages by the successive carbonylation reactions of precursor Rh ions with CO H2
+
1276
4 Nunomuterials
R hKOhs
NaY zeolite SiOdAko3=5.6
I
“Ship-in-a-Bottle Synthesis”
n=4-6
of “ship-in-a-bottle’’ synthesis of metal clusters, RhG(C0)16 assembled in NaY cages by the successive carbonylation of Rh
which are introduced by the ion-exchange methods and gas admission.
+
or CO H20 as building-blocks accessible to its smaller window, as illustrated in Fig. 3. Metal ions are introduced in the cages by the conventional ion-exchange method, and volatile subcarbonyls and metal complexes by the solid-state dispersion technique.“’] In this context, in situ intrazeolitic preparation of nanomaterials such as metal clusters in micropores is referred to as “ship-in-a-bottle synthesis”. The ion-exchanged Rh3+ in NaY zeolites is reduced with CO H2 or CO H20 at 393-473 K to give initially the mononuclear dicarbonyl [Rh(CO)2; Rh( H20)(C0)2](IR stretching frequencies 21 14, 2048; 2098 and 2022 cm-’), being followed by its migration through the zeolite channels and undergoing a subsequent cluster-oligomerization to form Rhq(C0)12 (v(C0) 2085, 1830 cm-’; 7 A) analogous to that obtained in an aqueous alkaline solution. The reaction eventually forms Rhs(C0)16 (v(C0) 2098, 1760 cm-’, 10 A), which just fits the interior of NaY (12 A).[1131z1 Rhs(C0)16 was selectively (>85%) produced with a minor contribution of
+
+
1277
4.1 Metal Clusters and Nanomaterials: an Overview
Table 1. Results of curve-fitting analysis of Rh K-edge EXAFS data obtained at 300 K for NaYentrapped Rh cluster samples", e.g., Rh6(C0)16/NaY and [Rh6]/NaY before and after the sequential treatments of oxidation, reduction and carbonylation. Clusters in NaY pore
Rh-Rh
Rh -COl
Rh-COf o r b
Rh-0
r(A)
n
r(A)
n
r(A)
n
r(A)
3.1
2.74
1.5
1.88
1.6
2.15
4.6 4.6 3.2
2.70 2.70 2.72
1.4
1.85
1.4
2.15
1.8 6.8 0.7 0.7 0.8
2.06 2.06 2.10 2.09 2.03
6.0
2.05
2.1 2.0
1.87 1.864
2.0 2.0
2.17 2.168
n Rh6(CO)dNaY [Rhs]ox/NaY /Rh6]~~1/NaY(473 K H2) [Rh6]1,d/NaY(673K H2) CO(ads)Rhs/NaY(473K H2) Reference samples Rh foil (fcc) Rh203 (bulk) Rh6 ( c o )I 6 Rh6(CO)16
12 4.0 4.0
2.69 2.76 2.776
"Estimated experimental errors are k0.02 A for atomic distance r and k0.2 for coordination number n in the EXAFS data evaluation. Results based on X-ray diffraction analysis.
residual mononuclear Rh carbonyls inside Nay. Rhs(C0)16 thus encapsulated in a NaY cage by "ship-in-a-bottle'' technique is well characterized by IR bands at 2968 and 1760 cm-' for the linear and face-bridging carbonyls, respectively, which are different from those (2948 and 1800 cm-') of Rhs(C0)Is adsorbed externally on NaY and in solution. The resulting frequency-shifts to higher wavenumber for linear carbonyl bands, but to lower for face-bridging catalysts reflecting capsulation of the cluster inside the NaY cage. The metal core structures of Rh6 clusters inside NaY was characterized by means of Rh-K edge EXAFS. The data shown in Table 1 provides direct evidence for the stoichiometric formation of a hexanuclear Rh carbonyl cluster, and is in good agreement with that for a free molecule in terms of coordination number and atomic distance Rh-Rh (2.74 A,C.N. = 3.1) and for linear/face-bridging carbonyls, Rh-CO ( R = 1.88 A,C.N. = 1.5; R = 2.15 A, C.N. = 1.6). In general, the synthesis of clusters in micropores is analogous to that in solution. The formation of neutral rhodium carbonyl clusters in the micropores of NaY zeolite is very similar to that occurring in neutral and alkaline solutions" 31 and on metal oxide surfaces such as silica and alumina,['41 as shown in Fig. 4. The intrazeolitic walls of NaY(Si02/A1203 = 5.6) and the A1203 surface contain protonic hydroxyl groups, basic oxygen and metal cations such as Na+ and A13+ which promote the reductive carbonylation of Rh cations with CO H2 and CO H20 at 333-523 K. Nevertheless, in comparison with those obtained in solution and even by the surface mediated reaction, the intrazeolitic synthesis of metal clusters
+
+
1278
4 Nanomaterials
Metal Oxide Surface
Solution
Mesopore
Micropore
H&C\
+ FSM-16
1 GO, 25 99
inmethanol pH = 7-6
+
R ( C 0 M m L n=4,5,4
coy^^
I
2056,1873
an’
2059,2046 1853,1846 1877 an ’
g:
JJmL 1
T F
+CO, 25 99
2112,1896, 1841 an’
R ( C 0)JiNaY
CO,
NaOH pH = 5-4
I
co,
NaOH pHc4
R(COu,”2030,1840
+
an’
ptJcoy]:1995,1810 on’ Figure 4. Synthesis of platinum cluster anions, [Pt-,(CO)-,(p-CO)-,],2, in solution, on the basic surface of MgO and in the Micro/mesoporous cages of NaY(12 A) and FSM-16 (28, 48 A).[’ 1 3 4 4 3 4 5 1 The chemistry on MgO and in micro/mesoporous cages is quite similar to that occurring in the basic solution. However, the cavities afford the templating micro reactor for the synthesize of the selected clusters owing to their chemical properties and confinement of the micro cavities. By contrast, a wide distribution of Pt cluster anions having different sizes was formed on the MgO surface by the surface-mediated carbonylation reaction.
proceeds much more readily under the milder conditions to selectively yield those adapted to the interior of the micropores. Similarly, using ion-exchanged Ir4+/NaY, Irg(CO)I,j was synthesized with CO H2 at 1 atm and 323 K in NaY cages[11,15,561 via the intermediate formation of
+
1279
4. I Metal Clusters and Nunomuterials: an Ovevoiew
Ir(C0)2 and Ir4(C0)12 by analogous processes to those described for [Rh6(C0)16]/ Nay. Gates et al. reported[’61that Ir(C0)2(acac) impregnated with NaY was converted with CO at 323 K to Ir4(CO)12 and with CO H2 at 20 atm and 623 K to a mixture of edge/face bridging Irg(C0)16 isomers, respectively. By contrast, cluster anions such as H I ~ ~ ( C O ) Iand I - Ir6(CO)152-were formed by the analogous reductive carbonylation of Ir(CO)z(acac) in basic NaX(SiO~/A1203= 3.2) having the same sized cages as The authors characterized each of these Ir carbonyl clusters in NaY and NaX by means of IR and EXAFS spectroscopy. Moreover, by analogy with [Rh6(cO)16]/NaY and [Ir6(C0)16]/NaY, a hexanuclear Co cluster, Cog(CO)16,in NaY cages was also prepared at 473 K from C02(CO)8 pre-adsorbed in NaY under an atmosphere of CO H2.[”l In this sense, the NaY zeolite pores act as a “common templating micro reactor” to selectively synthesize the hexanuclear homologous clusters due to the confined reaction space. By contrast, sub-carbonyls such as Rh(C0)2+ and Rh2(C0)4C12 adsorbed in ALPO-5(6 A) are selectively converted with CO H20 at 323-392 K to Rh4(C0)12 but not to Rh6(C0)16 owing to the spatial confinement of the ALPO-5 cavities.[’g1 Sachtler et al. reported[”] a new type of palladium carbonyl cluster [Pd13(CO (x is unknown) in the supercage of NaY zeolite (12 A) and [Pd6(CO).] in NaA (5 ), although neither has been fully characterized by IR and EXAFS spectroscopies. The carbonyl clusters were prepared by reductive carbonylation of the ionexchanged Pd/NaY and Pd/NaA which was mildly pre-reduced with hydrogen. The synthesis of some Pd carbonyl clusters has been reported in solution, but they are unstable and easily decompose to metal. Accordingly, the zeolite pores may serve as a confined chemical environment that is good for accommodating such unstable Pd carbonyl clusters. In the presence of aqueous zeolite, Fez(C0)9 adsorbed on the external zeolite NaY or basic NaX was readily converted into HFe3(CO)11- at 297-333 K,[”] in a similar manner to the reaction of FeZ(C0)g or Fe(C0)S in an aqueous alkaline solution. It was proposed that the active Fe(C0)4 radical species generated by decomposing F e ~ ( c 0 migrates )~ inside the zeolite frameworks. This facile migration of the precursor carbonyl results in the rebuilding of the more stable cluster anions such as HFe3(C0)11-, which are accommodated with Naf and A13+ in N a y . Furthermore, Ichikawa et. al. recently extended the “ship-in-a-bottle’’ synthesis for some other metal clusters such as R U ~ ( C O ) ~ ~ / N ~HY~, R ’ ~U’ ~ ( C O ) I ~ / N ~ Y , [ ~ ~ ~ [ HRu6(CO)l8]-/NaY ,Iz4] [Ru6(CO)1812-/NaX,[2s1 [ P ~ ~ ( C O ) I ~ ] ~ - / Nand ~Y[’~~ [ Pt 12 (C0)24]*-/Nay .[*71 R u 3 ( C 0 ) ~is small enough to diffuse into NaY super cages by solid-state monolayer dispersion, the product being referred to as Ru3(CO)12/NaY. Ru)(C0)12encapsulated in NaY is uniformly transformed with H2 at 363 K to yield the hydrido ruthenium cluster H4Ru(C0)12, by analogous chemistry to that which occurs in solution.[281In a similar manner, the intrazeolitic synthesis of [Rhf,(CO),6]/NaY and [Ir6(CO)16]/NaY, [ H R u ~ ( C O ) I ~ ] - / N ~ Y ( V2054 ( C O )s and 1937 vs cm-I) was synthesized by the reductive carbonylation of [Ru(NH3)6I3+/NaYwith CO H2 at 413 K. By comparison [Ru(NH3)6I3+ion-exchanged in the more basic NaX zeolite
+
+
+
8,
+
1280
4 Nanomateriuls
coz
210 is00
I
I
1
2400
2200
2000
Pt2+/NaY
300 K
+co
"PtO(C0)"
I
1900
A
UAl
I
1800
323 K
CO+HzO
Figure 5. IR spectra of carbonyl species successively formed in the reaction of Pt2+/NaY with CO: a) after 3 min at 293 K; (b) after 5 min at 353 K; (c) with CO H20 at 353 K after 10 min; (d) 30 rnin; (e) 2 h; (f) 5 h; at 373 K after (f) 5 h and (g) 10 h.
+
to [Rug(C0)18I2-/NaX(v(CO) 2000 s, 1972 s, 1925 m and 1754 w em-') in the reaction with CO H2 at 1 atm and 298-393 K. A calcined Pt2+/NaY material was heated from 298 to 373 K under CO with a trace of H20, resulting in the successive formation of different carbonyl species which exhibit characteristic carbonyl IR bands as shown in Fig. 5(a)-(g). By analogy with the IR bands of previously reported platinum carbonyl complexes e.g., Pt(CO)C13,cis-Pt(C0)2C12and Pt3(p2-C0)3(PPh3)3 it was suggested that Pt2+/NaY reacts with CO forming PtO(CO)/NaY(v(CO) 2100 em-') and a proposed Pt trigonal intermediate species [Pt3(C0)3(p2-C0)3]/NaY(v(C0) 21 12 s, 1896 m and
+
4. I Metal Clusters and Nanomaterials: an Overview
1281
1824 s cm-I), which are eventually converted by stacking together into the dark] the green Chini complex [Pt12(CO)24]2-(v(CO)2080 vs and 1824 s ~ m - ’ ) . [ ~ ’On other hand, [Pt(NH3)4I2+ion-exchanged with NaY provided stoichiometrically the orange-brown Chini cluster [Pts(CO)lsl2- which shows intense IR bands at 2056 vs Basic NH40H is generated from the ammonia ligands with and 1798 s cm-1.[269573 water in NaY cages and promotes the formation of the smaller Chini complex, rather than that from Pt2+/NaY which is analogous to the reaction in solution.[291 IR bands characteristic of linear carbonyls shift to higher frequencies by 26-40 cm- I , whereas the edge-bridged CO band move to lower frequencies by 40-50 cm- I , compared with those observed for analogous species on the external zeolite surface and in solution. This is also supporting evidence that the Chini Pt cluster anions are encapsulated in micropores of N a y , in a similar manner to the neutral clusters e.g., Rhs(C0)16 and Irg(CO)16 in Nay. This spatial confinement induced by the micropores may hinder cluster migration, and promote cluster isolation and stability. This “ship-in-a-bottle” synthesis may open up new opportunities for the rational design of tailor-made catalysts consisting of discrete metal/alloy clusters, nano-particle or nano-wires. These nanomaterials would have uniform sizes and metal compositions and have sufficient catalyst stability against metal sintering and leaching under the prevailing reaction conditions. Examples of metal clusters in micro- and mesoporous cages are listed by their method of preparation and characterization in Table 2.
Reversible formation of metal clusters in cages After the mild oxidation of [Rhs(CO)16]/NaYwith dry 0 2 by heating from 293 to 473 K, it is suggested by IR and EXAFS data that the hexanuclear Rh carbonyl cluster decomposes and is converted into an oxide cluster via the intermediate formation of “Rh(C0)2”. By successive reduction of the oxidized sample with hydrogen at 473 and 673 K, the naked Rh cluster is formed inside the cavity.“ ’]As shown in Table 1, the EXAFS analysis of the reduced sample demonstrates that the Rh6 unit in the zerovalent state is retained. The meta coordination number is the same as that of the original [Rh6(CO)16]/NaY material and the Rh-Rh distances are close to that in metallic rhodium (Rh-Rh: C.N. = 3.2, R = 2.72 A).Furthermore, it was demonstrated that the exposure of a reduced Rh6 cluster to CO at 300 K (CO/Rh(total) = 2.6) results in a stoichiometric formation of a carbonyl cluster which shows the characteristic IR bands associated with linear (2087(vs), 2042(w) cm-’) and bridging carbonyls (1835(s) cm-I) which differ from those observed for the face bridging COs in Rhs(C0)16]/NaY. Nevertheless, the Fourier transform spectrum of Rh k-edge EXAFS resembled that of [Rh6(CO)16]/NaY. According to the IR and EXAFS results that we have reported previously,“ ‘I a naked Rh6 cluster in NaY reacts with CO to generate a new edge-bridged Rh6(C0)16 isomer. This edge-bridged Rh6(C0)16 isomer has not been synthesized and isolated in solution, but is analogous to the larger cluster edge-bridged Irg(CO)16 (v(C0) 2082, 2040 m and 1816 cm-1).[16v301 It has been found that the edge-bridged Rhs(C0)16 cluster in
Rh’+/NaY(CO H2O) Rh2(C0)4C12/ALPO-S(CO + H20) Rh’+/NaY(CO + H2) [Rh6l/NaY(CO)
+
[Fez(co)9l/NaY(co HzO)
+
Ship-in-a-bottle Synthesis Precursor/pores (reaction)
+
* The metal clusters in pores have been characterized by IR, EXAFS, UV-vis spectroscopy.
+ + [(6-x)Rh3++ xTr4+]/NaY(C0+ H2) (x = 1-4) [HFe3(C0)1I l-/NaY(Rb(C0)12) Ru3+/NaY(Co2(CO)s+ CO/H2)
+
Pt2+/NaX(C0+ H20) Pt2+/NaY(C0 + H20) Pt2+/NaY(C0 H20) [Pt(NH3)4I2+/NaY(CO+ H2O) H,PtCLj//FSM-I6(CO + H20) H2PtCLj/NEt4Cl/FSM-l6(CO H20) H2PtC16//FSM-I6(CO HzO) H2PtCl6/NE~Cl/FSM-l6(CO H20)
1790 m 1896 m, 1841 m 1798 m 1824 m 1882 m 1875 m 1878 m 1879 m
2025 s, 2112 s, 2056 s, 2080 s, 2085 s, 2075 s, 2065 s, 2056 s,
+ +
[Ru3(C0)121/NaY(Hz) [Ru(NH3)4I2+/NaY(CO H2) [Ru(NH3)4I2+/NaX(CO H2)
2085 w, 2070 s, 2031 m, 2008 w 2126 w, 2062 s, 2044 w, 1975 m 2000 w, 1972 v, 1925 m, 1743 m (face)
+
+
Co2(CO)g/NaY(CO H20) Coz(CO)s/NaY(CO H2)
2126 w, 2078 s, 1817 ms (edge) 2080 s, 2064 w, 1719 s (face)
+ +
Ir4+/NaY(C0+ H2) 2095 s, 2048 m, 1744 m (face) 2082 s, 2040 m, 1816 m (edge), 1730 m (face) Ir(CO)z(acac)/NaY(CO H2) 2035 s, 2011 m. 1765 mw Ir(CO)2(acac)/NaX(CO) 2001 s, 1993 s, 1710 m Ir(C0)2(acac)/NaX(CO H2)
2085 s, 1830 m (edge) 2082 s, 1832 m (edge) 2097 s, 2066 w, 1760 s (face) 2098 s, 2050 w, 1835 s (edge)
2044 m, 1987 s, 1950 s, 1645 m
K O [cm-]] (linear, bridging)
[Rhh_xIr,(CO)16]/NaY(x= 1, 2, 3, 4) 2098 s, 2060 m, (1756-1744) m (face) [Fe2Rb(CO)15I2-/NaY 2078 s, 2020 m, 1980 m, 1744 w, 811 m [HRuCo3(C0)121/NaY 2084 s, 2064 m, 1989 m, 1812 m (edge)
Metal/Bimetal Clusters/pores
Table 2. IR spectral data in the carbonyl stretching region for various metal/bimetal clusters in micro/mesopores, which are prepared by the “Ship-in-a-bottle” technique (references in text).
n
+O,, T>473 K
A
Figure 6 . Reversible formation and isomer transformations of Rh6(C0)16 in NaY micropores after the cyclic procedures of oxidation, reduction and carbonylation with CO.
A
-
+CO, RT
t
bridging Rh,(CO),, = 2092,2072,2060,1830
cm Rh-Rh: C. N. = 3.2, R = 2.72 A
Y,
"Rh," Rh-Rh: C. N. = 4.6, R = 2.70
= 2098,2066,1760 cm" Rh-Rh:C. N. = 3.1, R = 2.74 A
Y,
U,O, particle Rh-0: C. N. = 6.8, R = 2.06
gem-dicarbonyls v,, = 2114,2048, 2098,2022 cm"
CO,
w
m
N
a a
&
a
1284
4 Nunomaterials
NaY is readily transformed to a face-bridged one by thermal activation under CO and N Z at 333 K.[3,111 The edge-bridged Ir6(CO)16/NaY material is similarly transformed by heating at 393 K to the thermally more stable face-bridged isomer (v(C0) = 2095 s, 2048 m, 1744 m cm-1).[161 In this sense, the micropores afford preparative and catalytic advantages for encapsulated metal clusters over the conventional surface-grafted metal clusters. Regardless of the treatment and prevailing conditions that may involve several cyclic sequence of oxidation] reduction with H2 and catalytic carbonylation with CO/H2, in the microporous environment retains cluster unity and even at higher temperatures sintering is prevented.
4.1.3 Robust metal clusters in mesopores Recently, mesoporous sieve-materials such as MCM-41 and FSM-16 have been synthesized by using different micelle surfactant templates such as alkyltrimethyl ammonium These materials consist of ordered mesoporous channels of 20-100 A diameter, which are much larger than those in the conventional zeolites such as ZSM-5, AlPO-5, and Nay, as depicted in Fig. 2. They are potential hosts for the inclusion of organometallic complexes and nanoparticles that will align with the ordered channels of the mesoporous materials and are accessible to larger substrates in the catalytic reactions. As described in the previous section, the smaller Chini clusters [Pt3( c 0 ) 6 ] n 2 (n = 3 and 4) can be capsulated in the micropores of NaY (12 A) by the reductive carbonylation of [Pt(NH3)4]2-t/NaY[261 and Pt2+/NaY,[271 respectively. However, owing to the size limitation it has not been possible to incorporate longer Chini clusters in the micropores of NaY and NaX. In an extension of the work some robust Pt carbonyl clusters such as [Pt3(CO)6]n2-(, = 5 and 6; 15-18 A) have been successfully synthesized using the mesopores of FSM-16 (28 and 48 A) as the host reactor. The initial exposure of CO at 300-323 K to H2PtC16 impregnated in FSM16 (28 A) resulted in sharp IR bands appearing around 2000 cm-', which are ascribed to cis-Pt(CO)2Clz (2188 and 2149 cm-') and Pt(CO)C13 (2119 cm-'), respectively. Subsequent addition of H20 vapor at 323 K to the mono-Pt carbonyls in FSM-16 yielded an olive-green complex showing intense carbonyl bands (VCO = 2086 s and 1882 m cm-') and UV-vis reflectance (Amax; 452 and 805 nm). The final IR and UV spectra closely resemble those of [Et4N]2[Pt15(C0)30]in MeOH solution ("0 = 2056 s and 1872 m cm-'; ;Imax = 408 and 697 nm) or in the solid state. Indeed, the Chini complex extracted from FSM-16 by cation metathesis was identified as [Ptl5(C0)30I2- by FTIR and UV-vis spectroscopy. Furthermore, it was demonstrated[331that Ptl5 cluster anions formed in FSM-16 were stabilized by using the quaternary alkyl ammonium cations NR4+ as spacer molecules. The thermal stability was dependent on the size of alkyl group and follows the order: Butyl > Ethyl > Methyl, Methyl viologen (MV) > Hexyl >> no counter cations.
4.1 Metal Clusters and Nanomaterials: an Overview
1285
The larger trigonal prismatic Ptl8 cluster anion is synthesized by the reductive carbonylation of H2PtC16 in the FSM-16 having a larger channel size of 48 p\.[341A dark-green [Pt3(C0)6]62-/FSM-16 material (4.8 nm) (VCO = 2056, 1879 cm-*) was characterized by IR, UV and EXAFS spectroscopy by analogy to the reference complex [NEt4]2[Pt3(co)6]6. As summarized in Table 3, the observed EXAFS parameters for intra-triangularPt-Pt atomic distances ( R I = 2.68 A) and average interfacial distances between adjacent triplatinum planes (R2 = 3.08-3.09 A) for the clusters in [Pt3(C0)6]52-/ NEt4Cl/FSM-16 (28 A) and [Pt3(C0)6]62-/NEt4Cl/ FSM-16 (48 A) are within experimental error (0.02 A) of those of the reference salts [NEt4]2[Pt3(C0)6]5in BN (R1 = 2.68 A;R2 = 3.07 A). In contrast, it is noteworthy that for the Chini clusters the interfacial in micropores, [Pt,(C0)6]42-/NaY and [Pt3(C0)6]32-/NaY['9,201 Pt-Pt distance of a Pt3 triangle is substantially shortened (R(Pt-Pt) = 2.99 A)compared to that in the reference compound [NEt4]2[Pt3(C0)6]3,4in BN ( R = 3.063.07 A) and [Pt3(C0)6]5p62-in the mesopores of FSM-16 (28 and 48 A). This may be caused by the compressing and distorting effect of the bulky Chini clusters due to the intrazeolitic constraint of the microcavity of NaY (12 A). On the contrary, the mesoporous channels of FSM-16 are large enough (28 and 48 A in diameter) to accommodate Ptl5 and Pt18 carbonyl clusters without any appreciable distortion. The 195PtNMR study by H e a t 0 1 - 1on ~ ~[Pt3(C0)6]52~~ in solution showed that the Pt3 triangles were rotating about the three-fold axis of the trigonal prismatic framework, which causes the wide distribution of the Pt-Pt interfacial distances in the Chini clusters. Accordingly, it is suggested that the slight elongation of the Pt-Pt interfacial distance for [Pt3(C0)6]52-and [Pt3(CO)6]62-inmesopores of FSM-16 may result from a fluxional rotation of the [Pt3(CO)6]52-,similar to that which occurs in solution. In order to locate the [Pt3(C0)6]52-cluster anions in the mesoporous channel of FSM-16, the transmission electron micrograph of this sample was measured. TEM observations of the sample [pt3(C0)6]52- in NEt4Cl/FSM-16 revealed that Pt clusters as small specks of 10-15 A size that were uniformly scattered and aligned in the ordered channel of FSM-16.[32334,351 No larger Pt particles or crystal were found on the external surface of FSM-16.
4.1.4 Bimetallic clusters in zeolites A number of bimetallic carbonyl clusters such as Rh6-.IrX(C0)16/NaY (x = 14),[I 5 1 [ Fe2Rh4 (CO)1512-/NaY[361and HRuCo3 (CO)12/NaY[ 3 have also been similarly synthesized by the "ship-in-a-bottle" technique and characterized by IR, EXAFS, UV and Raman spectroscopy, and are listed in Table 2. Recently, a series of RhIr bimetallic clusters in NaY related to [Rh6pxIrx(C0)16] (x = 1,2,3,4)/NaY was prepared by the reductive carbonylation of doubleexchanged [Rh3'+xIr4']NaY with the different compositions in CO H2 at 1 atm and 423-500 K, in a similar manner to that for [Rh6(C0)16]/NaY["9'21and
+
( 12 2.1
A)
1.3 0.1
Ptly-AlzO3
+
-
48
20
-
28
-
40
COz Hz) Ea (kJ/mol)b
Pt-Pt (av.)
Pt-Pt (av.)
Pt-Pt (intra) (inter) Pt-Pt (intra) (intra) Pt-Pt (intra) (inter)
2.72
2.74
2.68 2.99 2.67 2.99 2.68 3.08
6.7
7.8
1.9 1.7 2.2 1.4 2.3 1.7
EXAFS parameters R(A)' C.N.
3.4 x 10-4
7.7 x 10-3
10-3 7.8 10-3 -1.6 x 10-3 7.4 10-3 -1.2 10-3 7.0 x 10-3
- 1.2
02(A2)"
"
T O (200 Torr) + H20 (15 Torr); TOF(C02) (mmol/Pt atom/ min): Pt surface is estimated by CO chemisorption; bActivation energy (kJ/ mol); intra and interfacial Pt-Pt bonds in trigonal Pt clusters in NaY and those avaraged for Pt nanoparticle and nanowires in FSM-16; Debye-Waller factor.
110
23
A) [Ptl5(C0)30I2-/NB~/FSM-l6 (28 A) Pt nanowire/FSM-16 (28 A) Pt nanoparticle/FSM-16 (28 A)
[Ptls(CO)3olz-/NEt4/FSM-16 (28
60
3.8
WGSR (CO + H20 k/min (323 K)"
A)
[ P ~ ~ Z ( C O ) ~ ~ I ~ - / N ~ Y(12
[ P ~ ~ ( C O ) I ~ *-/Nay I
Pt nanomaterials/FSM-16 or NaY
Table 3. EXAFS parameters of Pt nanomaterials and their catalytic performance in the water gas-shift reaction (WGSR) for [Ptls(CO)30]~;, Pt nanowires and Pt nanoparticles in FSM-16 (28 A), compared with those on Pt clusters in micropores, [Pt3(C0)6ln2-(n = 3,4)/NaY(12 A) and the conventional Pt catalyst, 4 wt%Pt/y-A1203 [ l l , 15, 44, 451.
G-
ij.
2
5
5 9
Q
;J
4.1 Metal Clusters and Nunomaterials: an Overview
1287
J IrJ/NaY
Rhs-rIrx(COl~el/NaY (x=O,2,3,4,6)
Figure 7. Pictorial representation of RhIr alloy clusters in NaY cages derived from oxidation and reduction with Hz of a series of hexanuclear bimetal carbonyl clusters [Rhh-,Irx(C0)16] (x = 0-6) which are synthesized by the reductive carbonylation of co-exchanged Rh3+/Ir4+ in NaY with CO + H2.
[Ir6(C0)16]/NaY.['5.5 They showed the linear and face-bridging CO bands characteristic of the hexanuclear metal clusters. EXAFS studies on the Rh K-edge and Ir L-edge of the resulting heterometallic carbonyl samples have been conducted, and they revealed the stoichiometric formation of bimetallic RhIr clusters with the Rh/ Ir metal compositions based on those of the initial double-exchanged N a y . The reduced alloy RhIr clusters in Nay, Rh6p,Ir,/NaY(x = 1-4) were prepared by oxidation and subsequent reduction with hydrogen at 723 K. The novel preparation of hexanuclear RhIr carbonyls forming alloy clusters in NaY is pictorially represented in Fig. 4. It was found that the CO/M values in the Rh-rich alloy clusters were higher, but the H/M ratio decreased, compared with those of the Ir-rich clusters. A
1288
4 Nunomateriuls
study of the lolRh NMR chemical shift suggests that the Rh-rich alloy clusters in NaY exhibit a relatively higher electron-deficiency than the Ir-rich ones. This is reflected in the higher catalytic activities for alkane hydrogenolysis.[' '] A bimetallic RhFe carbonyl cluster was prepared[361by the solid-state reaction of [HFe3(CO)rI]/NaY with Rh4(C0)12 at 343 K as in the analogous solution reaction between [Fe3(C0)11I2- and Rh4(C0)12(or [Rh(CO)*C1]2)dealing with the synthesis of [Rh4Fe2(C0)16I2-. It was demonstrated by an IR study that the active Rh(C0)Z species generates by the decomposition of the precursor Rb(C0)12 migrates inside the NaY zeolite framework. This facile migration of a monometal carbonyl results in the rebuilding of more stable bimetallic cluster anions. The resulting sample gave the IR carbonyl spectra (2093(s), 2044(w), 1985(w), 1741(m), 171l(s), 1698(w)cm-'), similar to those of [ R ~ F ~ ~ ( C O ) I ~ ] [ T formed M B A ]by ~ the reaction in solution.[401Similarly, HCoRu3 (CO)13/NaY has been reportedL37 , 3 81 as the product of the reaction between Co(CO)4- and Ru(C0)2 in NaY cages, and was characterized by IR and EXAFS spectroscopies. A catalytic test of the hydroformylation of ethylene and propene was conducted at 373-453 K on the reduced [Rh6]/NaY and RhFe/NaY material derived from the resulting homo and bimetallic clusters in N a y . The results show that acetaldehyde was selectively obtained as the hydroformylation product on [Rh6]/NaY, whereas it is of interest to note that the bimetallic RhFe/NaY catalyst gave higher activities and selectivities for the alcohols. In particular RhFe/NaX, which was prepared from the solid-state reaction between [HFe3(CO)I1]/NaX and Rh4(C0)12, exhibited higher selectivities for linear alcohols giving up to 83Y0 mole conversion in the propene hydroformylation; possibly due to the constraints imposed by NaX pores. A physically mixed cluster catalyst consisting of [Rh6(C0)16] and [HFe3(C0)11]- encapsulated in NaY was prepared by the solid-state reaction between the presynthesized [Rhs(CO)16]/NaY and Fe2(CO)g, in the presence of water and CO at 343 K.[369391 The resulting sample showed an IR carbonyl spectrum consistent with the presence of both Rhs(C0)16 and [HFe3(C0)11]- in the micropores of Nay. The reduced sample of [Rh6] [Fe3]/NaY was catalytically active for the hydroformylation of ethylene and propene. The specific rates and selectivities were almost the same as those obtained with [Rhs]/NaY. No bimetallic promotion sites of RhFe exist in the reaction. The bimetallic RhFe/NaY(and RhFe/NaX) provided a good yield of oxygenates, mainly consisting of ethanol and methanol at the expense of decreasing the hydrocarbon content in the CO + H2 reaction at 623 K.[401In fact, [Rh6]/NaY and [Rh6] [Fe3]/NaY formed methane and higher hydrocarbons with a minor yield of acetaldehyde. The results are interpreted by assuming that the adjacent RhFe ensembles are the active sites for the promotion of olefin hydroformylation '~'~~ and and CO H2 reaction towards alcohol synthesis. I ~ h i k a w a , [ ~ Sa~htler[~'] S h r i ~ e rproposed ~ ~ ~ ] that the two-site CO activation on the bimetallic clusters e.g., RhFe, RhCo and RuCo grafted on silica and alumina are responsible for the enhancement of the migratory insertion of CO with M-alkyl and M-H bonds for oxygenates in the carbonylation of olefins and CO hydrogenation. The hydrogenolysis of n-butane and ethene and the hydrogenation of benzene
','
+
+
+
4. I Metal Clusters and Nanornaterials: an Overview
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were conducted as test-reactions on the RhIr alloy clusters prepared from Rh6~,IrX(C0)l6 in NaY (see Fig. 7). It is of interest to observe the dramatic decrease of hydrogenolysis activity by four orders of magnitude across the series of RhIr bimetallic clusters as the number of Ir atoms in the active Rh ensemble increases. A slight enhancement of activity for the benezene hydrogenation occurs. The remarkable activity-suppression of hydrogenolysis has been interpreted in terms of the geometric Rh ensemble-size effect (via the separation of Rh clusters by a single Ir atom), and also by the decrease in the electron deficiency of the Rhrich bimetallic clusters in Nay. The alkane hydrogenolysis is a “structure-sensitive” reaction in which relatively large ensembles of metal atoms are required to break the multiple bonds of reactant molecules such as CO, N2 and alkanes. By contrast, benzene hydrogenation displays a quite different trend (basically structure-insensitive) to the metal composition of the RhIr clusters in Nay. These results reflect the crucial advantage of the “ship-in-a-bottle’’ technique which affords bimetallic clusters in micropores as a site isolated systems for heterogeneous catalysis over the conventional catalyst preparation.
4.1.5 Cluster tranformation to nanoparticles in mesopores Robust Pt clusters such as [Ptls(C0)30]*- in FSM-16 (28 A) and [Ptl~(CO)36]~in FSM-16 (48 A) are transformed stepwise to nanoparticles by a gentle heating at ~ , ~ 343 ~ ] K, EXAFS data show that the resulting 300-473 K under v a c u ~ m . [ ~Below Pt clusters retain the prismatic triangular Pt framework (CN’ = 2.0; R1 = 2.68 A; CN2 = 1.5; R1 = 3.10 A) regardless of the removal of their bridging carbonyls. At temperatures in excess of 363 K, a substantial transformation to the spherical aggregates of 15 A diameter occurs. These were characterized by EXAFS (CN(Pt-Pt) = 6.7; R = 2.74 A) and TEM images of Fig. 8.[321The Pt nanoparticles are uniformly located and aligned in the ordered mesoporous channels of FSM-16. Similarly for the larger clusters e.g., [ P ~ I ~ ( C O ) ~ ~ ] ~ - / (48 FSM ~ thermal A),- Ithe activation yields nano Pt particles of 20 A (C.N. = 7.9: R = 2.74 A) similarly aligned in the mesoporous channels of FSM-16 (48 A).[343351 Recently, Thomas and Johnson in Cambridge reported the elegant example of using giant bimetallic clusand [ A ~ ~ R u ~ o C ~ ( C Oas) ~the ~ Cpre~]*~ ters such as cursors for the formation of discrete alloy nanoparticles which were loaded in the mesopores of MCM-41 (28 A).[441TEM and EXAFS studies suggest that after pyrolysis of the precursor clusters in MCM-41 there was no evidence for either segregation or aggregation. The RuCu clusters are anchored by Cu ions bound to four oxygen bridged silanols of the mesoporous lining.[451This anchoring model of RuCu clusters on MCM-41 is analogous to the RhFe and RuCo bimetallic clusters grafted or encapsulated in NaY,[36,41,43,551 which are anchored by F e e 0 and Co-0 bonds from the Si or A1 oxide surface. The RuCu bimetallic nanoparticle
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Figure 8. Transmission electron micrograph of Pt nanoparticles prepared by the controlled removal of CO at 573 K for 5 h from the Chini cluster anion [Pt3(CO)h]52- synthesized in FSM-16. The nanostructured particles (1 5-20 nm diameter) were uniformly aligned in the ordered channels of FSM-16 (28 A).
catalysts exhibited high TOF activities in the hydrogenation of bulky substrates such as 1-hexene, diphenylacetylene and D-limonene at 65 bar H:! and 373 K.
4.1.6 Nanowires in mesoporous channels and their unique properties Ichikawa et al. recently reported[343351 the templating of Pt nanowires in mesoporous channels of FSM-16 by the photoreduction of H*PtC16/FSM-16 in the presence of 2-propanol and water, as illustrated in Fig. 9. Figure 10 shows typical TEM images of Pt nanowires in FSM-16 (28 A).The nanowires have diameters of ca. 3 nm consistent with the pore size of FSM-16, and their lengths range from 50 to 200 nm. In the TEM observations, the nanowires were not seen on the external surface of FSM-16 but were observed in the internal channels. The TEM also
4.1 Metal Clusters and Nanornatcrials: an Overview,
1291
1 HzO 2-PrOH+H20 y-Ray or hv
T=300K
evacuated at 473K
/ P-J-
1
-CO
Pt Nano-Wires
Pt Nano-Particles
Figure 9. Templating fabrication of Pt nanoparticles prepared by the controlled removal of CO from the Chini cluster anions [Pt3(CO)6],2-(n= 5,6) synthesized in FSM-16, and the Pt nanowires by the exposure of H*PtC16/FSM-I6 with 2-propanol and water to y-rays or UV-light.
showed crystal faces for the nanowires, and the high resolution electron diffraction gave a clear fringe pattern of Pt( 1lo), implying that the Pt wires consist of a single crystal phase. In this sense, mesoporous channels of FSM-16 play a role for the templating fabrication of Pt nanowires by a one-dimensional elongation of the Pt crystal. On the other hand, Pt nanoparticles formed from the robust carbonyl clusters were ca. 2-3 nm in size in FSM-16, and these were homogeneously distributed in the mesoporous channels (Fig. 8). From the EXAFS data, Pt nanowires and nanoparticles in FSM-16 gave different parameters from those of Pt foil; the Pt-Pt distance and coordination number were 0.274 nm and 7.8 for Pt nanowires in FSM16 and 0.272 nm and 6.7 for Pt nanoparticles in FSM-16,[351as summarized in Table 4. However, XANES and XPS indicate that the nanowires in FSM-16 are slightly electron-deficient compared to those in the Pt foil and the nanoparticles in
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Figure 10. Transmission electron micrograph of (a) Pt-nanowire/FSM-16 prepared by reduction of H,PtCl,/ FSM-16 with 2-propanol and water exposed to y-ray irradiation at 300 K for 5 h. The nanostructured wires (3 x 50-100 nm long) were aligned in the ordered channels of FSM-16 (28 A). (b) a close view of a single strand of Pt wire in FSM-16 (28 A) which shows a clear diffraction image characteristic of the (1 10) bcc lattice fringe; d = 2.256 A.
FSM-16, which may be due to the dominant interaction of nanowires with the inner acidic surface of the FSM-16 pores. The adsorption of hydrogen for Pt nanowires was one or two-orders of magnitude smaller than that for nanoparticles. In the IR carbonyl adsorption spectrum, Pt nanowires gave a weak band at 2080 cm-' corresponding to journal carbonyls, which was shifted to higher frequency than in the Pt nanoparticles (2060 cm-'). This shift may reflect the electron-deficiency of the Pt atoms or the dipole coupling of CO on Pt nanowires in me sop ore^.^^^] By varying the exposure time to UV-light, it is found by TEM and XAFS that the Pt nanoparticles are initially formed by the photo-reduction of the impregnated Pt cations in the mesopores. Subsequently, [PtC16I2- ions migrate in the vicinity of the nanoparticles and are reduced on the surface and extend into the nanowires in the confined channels of FSM-16, as represented in Fig. lO(a). Ryoo reported the thermal preparation of the Pt nanowires in MCM-41 by the stepwise Hz-reduction of Pt ions that were added to the pre-reduced Pt/MCM-41 above 823 K.[461 As catalytic test reactions for Pt nanowires/FSM-16, Ichikawa et al. studied
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4.1 Metal Clusters and Nunomateriuls: an Overview
Table 4. Structural EXAFS parameters for the samples of Pt nanowires in FSM-16 having 28 and 48 A channel size prepared by photoreduction of H2PtC16/FSM-16 under 2-propanol and water at 300 K, compared with those for Pt nanoparticles in FSM-16.
(a) Pt nanowire/FSM-16 (2.8 nm) 5% wt. Pt loading mass (b) Pt nanowire/FSM-16 (4.7 nm) 5% Pt loading mass (c) Pt nanoparticle/FSM-16 (2.8 nm) 5% Pt loading mass (d) Pt nanoparticle/FSM-16 (4.7 nm) 5% Pt loading mass
Bond
R(A)
C.N.
E(eV)
a2(A2)
R factor
Pt-Pt
2.74
7.8
-1.6
7.7 x
9.8
Pt-Pt
2.75
8.9
3.8
4.7 x
3.0
Pt-Pt
2.72
6.7
1.23
Pt-Pt
2.74
7.9
-1.8
4 x 10-4 7.4 x
0.1 0.2
R(A): interatomic distance of Pt-Pt , C.N.: coordination number of Pt atoms.
water-gas-shift ( WGS) reaction and butane hydrogenolysis. In the WGS reaction at 323 K, Pt nanowires/ FSM-16 exhibited a formation rate of COz 10 times higher than that of Pt nanoparticles in FSM-16 based on the surface Pt atoms, as shown in Table 4. The higher rate may be related to the anisotropic morphology of the Pt nanowires in contrast to the nanoparticles, possibly due to the stronger interaction with the pore walls of FSM-16.[351 The isolation of the Pt nanowires from FSM-16 was also attempted by dissolving FSM-16 in aqueous HF or NaOH solution, but the nanowires decomposed to form Pt aggregates. However, the Pt nanowires were successfully extracted from the mesopores using [NBu4]Cl in a benzene/ethanol mixture. Probably, the extracted wires are stabilized by [NBQICI, which covers the surface of wires to form organoMagnetic susceptibility measurements on 5% Pt loading samples of nano Pt wires and particles having the same size of ca. 2 nm, encapsulated in the ordered channels of FSM-16 host, were conducted by varying the temperatures from 5 to 300 K at 30 KOe and a magnetic field of 0-50 KOe at 5 K using a Quantum Design MPMS SQUID magnetometer. The inverse susceptibility of the Pt particle/ FSM-16 and Pt-wire/FSM-16 is shown in Fig. 11. A Curie-Weiss law behavior was measured for the Pt clusters/FSM-16 with approximately 1.54 ,ub per gram of Pt and a Curie temperature B = 0.05 K. In contrast, for the Pt wire/FSM-16 below 70 K, a Curie-Weiss law dependency was observed with 0.13 1 ,ub per Pt gram and a Curie temperature B = 0.48 K, while there is quite obvious deviation from CurieWeiss law above 100 K.[341This unique magnetism on the Pt-nanowire/FSM-16 could be explained by the quantum size effects or spin-coupling due to the morphology characteristics of a nanowire. By contrast to the Pt nanoparticle/FSM- 16 samples, such a new and unique magnetic behavior of the Pt wire/FSM-16 samples regardless of their diameters may be based on the low dimensional quantum con-
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4 Nunornaterials
3.5
j
3 -
A
Pt nano-wire / FSM- 16(4.8)
A
R nano-wire I FSM-16(2.8)
A
Pt nano-particleI FSM-16(4.8)
. A
Pt nano-particle/ FSM-l6(2.8) 0
'
A
A A
2.5
A
h
A
A
A
\
A
'A
A
A
3
A A
2 -
E. M 1.5-
A
A
A A
A A'
A 4
W
A
x
\
A A
A
Q)
3
A A A
A
A
1-
A '
m
a
m
'
-
m A
A A A%
0
a a a . m . 8 n a
AA
A
A
0.5
'
o o o o 0
'AA
m
0 0 0 0 0 0
o o o n n
: A o o ~ o o o
:geeoo I
I
I
I
I
I
Figure 11. Temperature dependency of 1/x (magnetization) for samples of Pt nanoparticles in FSM-16 (2.8 nm)(-n-) and in FSM-16 (4.8 nm)(-m-), and samples of Pt nanowires in FSM-16 (2.8 nm)(-A-) and those in FSM-16 (4.8 nm)(-A-) which were prepared by the photo-reduction of H2PtC16/FSM-16 (2.8 nm and 4.8 nm), respectively, with propanol CO under y-ray irradiation at 300 K for 5 h. All the samples were measured at 5-300 K at 30 KOe.
+
finement effect or the hetero-coupling between silica surface of mesoporous channels and Pt wires.
4.1.7 Metal catalysts derived from zeolite-entrapped metal clusters The metal catalysts derived from the zeolite-entrapped metal cluster complexes have been studied because of the interest in a uniform distribution and a high degree of metal dispersion through the zeolite frameworks. Nevertheless, little information is available on the structural and chemical behavior of the entrapped metal cluster complexes, particularly on the retention of the cluster character under the reaction conditions, e.g., CO + Hz, alkane hydrogenolysis and methane homologation re-
4.1 Metal Clusters and Nanornaterials: an Overview
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actions. The stability of the entrapped clusters in micro/mesoporous cages is much higher than that of the clusters bound to the amorphous oxide supports, such as alumina and silica.
4.1.7.1 Water-gas shift reaction (WGSR)
Some metal carbonyl clusters such as HFe3(CO)ll-, Ru3(C0)12and Irq(CO)12 have been studied in an alkaline solution as possible catalysts for the water gas shift reaction. A plausible mechanism has been proposed that involves nuleophilic attack by H20 or OH- on an electrophilic metal center of the cluster to form an unstable carbohydroxy metal complex, which is then decarboxylated to give a metal hydride from which H2 is eliminated. Some metal carbonyl clusters in zeolites have been reported as active phases for the water gas shift reaction under mild reaction conditions. Iwamoto et al. suggested[''] that [HFe3(CO)ll]- in NaY yielded a catalytic activity for the reaction, at 337--453 K and 1 atm, which was comparable to that in solution at 453 K and 40 atm. Kinetic and spectroscopic results indicated that the reaction between [HFe3(C0)11lp and H2O is rate determining, as it is in solution. To investigate the catalytic performance of the Pt nanomaterials encapsulated in micro- and mesopores in the context of their structural confinement and clustersupport interactions, Ichikawa et al. have recently studiedc31 , 3 2 , 3 5 1 the water gas shift reaction (WGSR) on the powder samples e.g., [P~~~(CO)~O]~-/[E~N+]/FS and Pt nano particle/FSM-l6(28 A), Pt nanowire/FSM-16, [Ptl2(C0)24I2-/NaY and [Ptg(C0)l8l2-/NaY. As shown in Table 3, the [Pt15(C0)30]~~ in FSM-16 exhibited high activities in the WGS reaction, to form an equimolar mixture of C02 and H2 at 298 K-323 K compared with the Ptl2 and Ptg cluster anions in NaY or in FSM-16. The IR and EXAFS data showed that the reaction proceeds whilst the clusters retain their integrity under the steady-state conditions. The higher activities of the Ptls cluster anions in FSM-16 are probably due to the flexibility of the cluster framework similar to that found in solution, rather than to the increased diffusion of the reactant gases in the mesoporous channels of FSM-16 (28 A), compared with the Ptg and Pt12 carbonyl clusters which are strongly restricted in NaY micropores. This is reflected in the slight elongation of the intermarginal Pt-Pt distance (R(intermarginal) = 3.08 A) of [Pt15(C0)30I2-in mesopores of FSM-16, which is in contrast to a substantial compression of the cluster frameworks ( R = 2.99 A) in the Ptg and Ptl2 cluster anions encapsulated in micropores of N a y . Moreover, it is interesting to find that the WGS reaction is greatly enhanced (30 times larger) at 300 K by the photo-illumination with a high-pressure Hg lamp on the [Ptl2(C0)24I2-/NaY material.I3'1 It is noteworthy that the turnover rates (TOF; mmol(C02)/Pt surface atom/min) for the WGSR at 323 K on the sample of Pt nanowires (2.8 nm x 100-200 nm long) in FSM-16 (28 A) are 60-90 times higher than those on the Pt nanoparticles (ca. 20
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A size) in FSM-16 (28 A).[351As indicated in Table 3, the turnover rates (TOF) of the WGS reaction at 323 K on [Pt1s(C0)30l2- in FSM-16 are comparable with those on the Pt nanowires in FSM-16, which are higher (20-30 times) than those on Pt nanoparticles in FSM-16. From this evidence, it is suggested that the surface Pt atoms on the nanowires are extremely reactive, compared to the nanoparticles formed in the confined channels of FSM-16 (28 A).The unusually high turnoverrates of the Pt nanowires in FSM-16 for WGSR may be due to a partial electron deficiency as suggested from the XPS and XAFS data. This is reflected in a relatively weak CO chemisorption on the exposed surface of the Pt nanowires in FSM16 which promotes the WGSR.r35,471
4.1.7.2 NO + CO reaction The Pt9 and Pt12 carbonyl cluster dianions in NaY microcavities exhibit high catalytic activities (10-15 times higher) for NO reduction with CO towards N2 and N20 at the lower temperatures of 300-473 K, compared with those of the conventional Pt/A1203 catalyst.[269271 It is interesting to find that on exposure of [Ptl2(C0)24l2-/ NaY to NO (150 Torr), in situ FTIR studies demonstrated that NO breaks the intra- and inter-trigonal Pt-Pt bonds in the Pt12 carbonyl cluster even at 298 K to give a trigonal intermediate “Pts(C0)6” (2112,1896 and 1841 cm-’) and PtO(C0) (2110 cm-’) in forming N2 (N20) and COz. Furthermore, it was found that [Ptlz(C0)24l2- was reversibly regenerated from the Pt carbonyl fragments such as ‘‘Ptj(C0)h” and PtO(C0) inside NaY by the reaction with CO and water at 300-353 K to produce COz, as shown in Fig. 5. In situ FTIR/mass spectral studies suggest that NO reduction with CO proceed catalytically to give N2/N20 and CO2 through the redox cycles consisting of cluster-breaking-regeneration (Ptl2 + Pt9 Pt3) under an atmosphere of NO CO. In fact, the framework of Pt12 clusters in NaY is apparently retained under the steady state conditions of the
+
+
4.1.7.3 CO hydrogenation The carbonyls in [ R u(CO) ~ 12]/NaY are eliminated by heating under vacuum at 593 K, leaving the Ru particles with a size of 15-20 A. No further aggregation occurs under the CO + H2 reaction conditions and the resulting catalysts yields a product spectrum centered on C4 hydrocarbons with a sharp cut-off at C9.r481BallivetTkatchenko et al. also reported[4y1the similar observation that Fes (C0)12 incorporated in NaY decomposed to give highly dispersed Fe particles which chemisorbed CO to form the carbonyl species characterized by IR spectra. The resulting catalyst provided higher selectivities toward lower olefins with an upper limit of C9-Cl0,
4.1 Metal Clusters and Nanornaterials: an Overview
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since the CS hydrocarbons fit the supercage diameter due to the molecular shape selectivity of the zeolite framework. They claimed no further appreciable aggregation of the Fe particles under the syngas reaction conditions. Another example is metal-vapor condensation the “metal atom solvation” method, used by Nazar and O ~ i n , ~ for ’ ~ ]entrapping small Fe and Co particles within the supercages of faujasite zeolite. Using 57FeMossbauer and ferromagnetic resonance methods they found that Fe/NaY species contain Fe particles of 5-12 8, diameter with superparamagnetism between the adjoining zeolite supercage and its 12-membered ring entrance. The resulting Feo/NaY and Co”/NaY incidents in the reaction with 1-5 atm of CO H2 showed a non Schulz-Flory distribution of olefin and paraffin products in the C I - C range. ~ In particular, the more stable Co”/NaY samples exhibited notable selectivities toward the formation of Cq hydrocarbons (around 70% but-1-ene), although the CO conversion was less than 0.25%. The conversion and the product distribution remained unchanged after 60 h on stream. Additionally, it is of interest to find that the bimetallic RhFe/NaY (and RhFe/ NaX) species provided a good yield of oxygenates, mainly consisting of ethanol and methanol at the expense of the hydrocarbon contents.[’’l By contrast, [Rhs]/NaY and [Rh6] + [Fe3]/NaY gave the preferential formation of methane and higher hydrocarbons with selectivity toward acetaldehyde, which is the only oxygenated product in the CO + H2 reaction. The results also indicate that the adjacent RhFe bimetal ensembles are the active sites for the promotion of hydroformylation of olefins and oxygenates from CO + HZ due to the enhancement of migratory insertion of CO with M-alkyl and M-H bonds. We have made similar proposals for the catalysts derived from the oxide-support RhFe, RhCo and RuCo bimetal carbonyl clusters.r581A CO + HZ reaction was conducted at 5 bar and 498-548 K on Ru, RuCo and Co clusters in NaY.[22,251 Interestingly, it is found that oxygenates such as methanol and ethanol were produced with relatively higher selectivities on the catalysts prepared from HRuCo3 (CO),2/NaY, while Rus/NaY derived from [ HRus(CO)lg]-/NaY preferentially produced methane and higher hydrocarbons reported that with a poor yield of oxygenates. Furthermore, Pinnavia et al.1521 HM3(C0)12cations (M = Ru, 0 s ) were formed inside the galleries of pillared clay(a1umina montmorillorite) which afford Ru (or 0 s ) particles (<50 A) embedded within the clay sheets after HZ reduction. The Ru clusters/clay catalysts exhibited a marked selectivity for branched hydrocarbons in CO hydrogenation.
+
4.1.7.4 Methane homologation reaction Methane is chemisorbed below 723 K by dehydrocondensation on the metal clusters [M,] in pores of Ru3/NaY to yield metal carbide species [MxCn](n> 1). By
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replacing methane with hydrogen, the resulting surface carbon in Ru3/NaY is almost quantitatively converted above 300 K to C1 and C2+ hydrocarbons according to equations (1) and (2):[53,541 CH4
+ [M,]/NaY ==+ [M,C,]/NaY + 2nH2
[M,C,]/NaY
+ H2 ==+ [M,]/NaY + CH4 + C2+ hydrocarbons
(1) (2)
The reactive C, carbon is effectively converted with hydrogen at 300-373 K to provide methane and ethane (12-23% selectivity) as the C2+ products. Additionally, the less reactive Cp species gave methane and C2+ hydrocarbons including ethane and c 3 - C ~fraction (2-59% selectivity) at 373-523 K.IS4] The yields of higher hydrocarbons ( c 2 - C ~fraction) increased on the larger Ru clusters in NaY as follows,[53’ Ru3/NaY < Ru4/NaY < Rus/NaY << Ru (50 A) on Nay.
4.1.8 Future prospects To promote our understanding of microscopic processes in catalysis by metal clusters, it is important and useful to study their in-situ chemistry and dynamic behavior on an atomic/molecular level. In this introductory chapter, we have reviewed some strategies for the synthesis of metal clusters, nanoparticles and nanowires in micro- and mesoporous cavities, together with their structural characterization and catalytic perfomance. The extension of the present approach to multimetallic clusters encapsulated in micro/mesopores may lead to the tailored design of super active, selective and stable metal catalysts applicable to industrial processes. Few studies have been carried out with clusters having ligands other than CO. Future developments in this area are therefore most likely to involve new synthetic approaches to novel multimetallic clusters and provide an insight into the local surface chemistry of metal clusters. In fact, the organometallic cluster precursors offer exciting opportunities for the creation of new catalysts for industry. Structures of a variety of multinuclear metal clusters, alloy particles and nanowires grafted on oxidesurface or encapsulated in cavities would help in the understanding of the structural/ morphological dependencies upon their catalytic performances. These are relevant to the rational design of heterogeneous cluster catalysts that bridge the gap between homogeneous metal complexes and conventional metal surfaces.
4. I Metal Clusters and Nanomaterials: an Overview
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References [ I ] a) B.C. Gates and J. Lieto, CHEMTECH, 195 (1980); b) “Metal Clusters in Catalysis” B.C. Gates, L. Guczi and H. Ktzinger (eds), Elsevier, Amsterdam (1986); p 28; c) B.C. Gates, in “Catalyst Design” (L.S. Hegedus, ed.), p. 71. Wiley, Chichester, (1987). [2] J.-M. Basset, et al. (eds), “Surface Organometallic Chemistry, NATO Ser., vol 231, p. 75-90 (1988); J.M. Basset and A.K. Smith, Fundamental Research in Homogeneous Catalysis (M. Tsuji ed.) Plenum Pub., 1, 69-98 (1977). [3] M. Ichikawa, Adv. Catal., 38, 283 (1992). [4] H. Vahrenkamp, Adv. Organometal. Chem., 22, 674 (1983). [5] B.C. Gates in “Metal Clusters” M. Moskovits (ed.), Wiley, N.Y. (1986), p. 283. [6] a) M. Ichikawa, “Tailored Metal Catalysts” (Y. Iwasawa, ed.), Reidel Pub., Dordrecht, The Netherlands, (1984), b) M. Ichikawa, CHEMTECH, 12, 674 (1982). [7] a) M. Ichikawa, Chemisorption and Reactivity on Supported Clusters and Thin Films, (R.M. Lambert and G.G. Pacchioni, eds.), Kluwer, Dordrecht (1997), pp 153-192; b) M. Ichikawa, Polyhedron, 7, 2351 (1988). [8] S. Kawi and B.C. Gates, “Clusters and Colloids -From Theory To Application- (ed. G. Schmid), VCH Publisher, N.Y., p 300-372 (1994). [9] a) T.M. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature, 378, 159 (1995); b) G.A. Ozin and S. Ozkar, Chem. Mater., 4: 511 (1992); c) G. Schmidt, Struct. Bonding (Berlin) 62, 51 (1985) and Mater. Chem. Phys., 29, 133 (1991). [9] S. Martinengo, P. Chini, G. Giordano, V.G. Albano and G. Cianti, J. Organomet. Chem., 88, 375 (1975). [lo] Y.C. Xie and Y.Q. Tang, Adv. Catal., 37, 1 (1990); S. Qiu, R. Ohnishi and M. Ichikawa, J. Chem. SOC.,Chem. Commun., 1425 (1992); T.M. Salama, T. Shido, R. Ohnishi and M. Ichikawa, J. Phys. Chem., 100, 3688 (1996). [ I I ] M. Ichikawa, L.F. Rao, N. Kosugi and A. Fukuoka, Faraday Dis., 87,232 (1989). [I21 L.-F. Rao, A. Fukuoka, N. Kosugi, H. Kuroda and M. Ichikawa, J. Phys. Chem. 94, 5317 (1990). [I31 a) S. Martinengo, P. Chini, G. Giordano, V.G. Albano and G. Cianti, J. Organomet. Chem., 88, 375 (1975); b) S. Martinengo, P. Chini and G. Giargano, Gazz. Chim. Ital. 102, 330 (1972). [I41 a) A.K. Smith, A. Theolier, J.M. Basset, R. Ugo, D. Commereuc and Y. Chauvin, J. Am. Chem. Soc. 100, 2590 (1978); b) F. Hugues, B. Besson, P. Bussiere, J.A. Delmon, M. Leconte and J.M. Basset, ACS Symp. Ser. 192, 255 (1982). [I51 M. Ichikawa, L.-F. Rao, T. Kimura and A. Fukuoka, J. Mol. Catal. 62, 15 (1990); G. Bergeret, P. Gallezot and F. Lefevre, Stud. Surf. Sci. Catal. 28, 401 (1986). [I61 a) S. Kawi and B.C. Gates, J. Chem. Soc., Chem. Commun., (1991), 994; b) S. Kawi, J.R. Chang and B.C. Gates, J. Am. Chem. SOC.,115,4830 (1993). [17] S. Kawi and B.C. Gates, Inorg. Chem., 31, 2939 (1992). [l8] G.C. Chen, T. Shido and M. Ichikawa, J. Phys. Chem., 100, 16947 (1996). [19] A. Fukuoka, L.-F. Rao, N. Kosugi, H. Kuroda and M. Ichikawa, Appl. Catal. 50, 295 (1988). [20] L.L. Sheu, H. Knozinger and W.M.H. Sachtler, Catal. Lett., 2, 35 (1989); J. Mol. Catal., 57, 61 (1989); J. Am. Chem. Soc., 111, 8125 (1989). [21] a) M. Iwamoto, H. Kusano and S. Kagawa, Chem. Lett. 1483 (1983); b) M. Iwamoto, S. Nakamura, H. Kusano and S. Kagawa, J. Phys. Chem., 90, 5244 (1986). [22] M. Ichikawa, A.M. Liu, G.S. Shen and T. Shido, Topics. Catal., 2, 141 (1995). [23] G.C. Shen, A.M. Liu and M. Ichikawa, J. Chem. SOC.,Faraday Trans., 94, 1353 (1998).
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A,-M. Liu, T. Shido and M. Ichikawa, J. Chem. SOC.,Chem. Commun., 507 (1995); G. Shen, T. Shido and M. Ichikawa, J. Phys. Chem., 100, 16947 (1996). J.G.C. Shen, A. Liu and M. Ichikawa, J. Phys. Chem., B, 102, 7782 (1998). G.J. Li, T. Fujimoto, A. Fukuoka and M. Ichikawa, J. Chem. SOC.,Chem. Commun., 1337 (1991). G. Li, T. Fujimoto, A. Fukuoka and M. Ichikawa, Catal. Lett., 12, 171 (1992). S.A.R. Knox, J.W. Koepka, M.A. Andrews and H.D. Kaesz, J. Am. Chem. SOC.,97, 3942 (1997). J.S. Beck, J.C. Vartuli, W.J. Roth, C.T. Leonowicz and S.K. Kreag, J. Am. Chem. SOC.,114, 10843 (1992). T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. SOC.,Japan., 62, 763 (1990); Y. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. SOC.,Chem. Commun., 680 (1993). R. Wang, T. Fujimoto, T. Shido and M. Ichikawa, J. Chem. SOC.,Chem. Commun., 962 (1992). T. Yamamoto, T. Shido, S. Inagaki, Y. Fukushima and M. Ichikawa, J. Am. Chem. SOC.,118, 5810 (1996). T. Yamamoto, T. Shido, S. Inagaki, Y. Fukushima and M. Ichikawa, J. Phys. Chem. B, 102, 3866 (1998). M. Sasaki, M. Osada, N. Higashimoto, S. Inagaki, Y. Fukushima, A. Fukuoka and M. Ichikawa, Micro. Mesoporous Mater., 21, 597 (1998). M. Sasaki, M. Osada, N. Higashimoto, T. Yamamoto, A. Fukuoka and M. Ichikawa, J. Mol. Catal., A, 141, 223 (1999). A. Fukuoka, L.-F. Rao, N. Kosugi, H., Kuroda and M. Ichikawa, Appl. Catal. 50, 295 (1988). J.G.C. Shen and M. Ichikawa, J. Phys. Chem., 102, 5602 (1998). A.M. Liu, G.S. Shen and T. Shido, Topics. Catal., 2, 141 (1995). A. Fukuoka, T. Kimura and M. Ichikawa, J. Chem. SOC.,Chem. Commun (1988) 428. M. Ichikawa, A. Fukuoka and T. Kimura, Proc. Int. Congr. Catal., 9th Vol. I, 569 (1988). M. Ichikawa, A.J. Lang, D.F. Shriver and W.M.H. Sachtler, J. Am. Chem. SOC.107, 7216 (1985). W.M.H. Sachtler and M. Ichikawa, J. Phys. Chem. 90,475 (1986). A. Fukuoka, M. Ichikawa, J.A. Hriljac and D.F. Shriver, Inorg. Chem. 26, 3643 (1987). D.S. Shephard, T. Maschmeyer, B.F.G. Johnson, J.M. Thomas, G. Sankar, D. Ozkaya, W. Zhou, R.D. Oldroyd and R.G. Bell, Angew. Chem. Int. Ed. Engl., 36,2242 (1997). D.S. Shephard, T. Maschmeyer, G. Sankar, J.M. Thomas, D. Ozkaya, B.F.G. Johnson, R. Raja, R.D. Oldroyd and R.G. Bell, Chem. Eur. J., 4, 1214 (1998). C.H. KO and R. Ryoo, Chem. Commun., 2467 (1996). M. Sasaki, N. Higashimoto, M. Sasaki, A. Fukuoka and M. Ichikawa, Inorg. Chim. Acta., in press (1999). P. Gallezot, G. Coudurier, M. Primet and B. Imelik, ACS symp. Ser., 40, 144 (1977). D. Ballivet-Tkatchenko and G. Coudurier, Inorg. Chem. 18, 558 (1979); D. BallivetTkatchenko and I. Tkatchenko, J. Mol. Catal. 13, 1 (1981). L.F. Nazar, G.A. Ozin, F. Hugues, J. Gadbet and D. Rancoutt, J. Mol. Catal. 21, 313 (1983). L.F. Rao, A. Fukuoka and M. Ichikawa, J. Chem. SOC.,Chem. Commun. 458 (1988); M. Ichikawd, L.-F. Rao and A. Fukuoka, Catal. Sci. Tech., 1, 111 (1991). a) E.P. Giannalis, E.G. Rightov and T.J. Pinnavaia, J. Am. Chem. SOC.110, 3880 (1988); b) M. Rameswaram, E.G. Rightov, E.D. Dimotakes and T.J. Pinnavaia, Proc Int. Congr. Catal., 9th Vol. 2 p. 783 (1988). M. Ichikawa, T. Tanaka, W. Pan, T. Ohtani, R. Ohnishi and T. Shido, Stud. Surf. Sci. Catal., 101, 1075 (1996).
4. I Metal Clusters and Nunomuterials: an Overview
130 1
[54] a) M. Ichikawa, W. Pan, Y. Imada, M. Yamaguchi, K. Isobe and T. Shido, J. Mol. Catal., 107, 23 (1996); b) T. Koerts, M.J.G. Deelen and R. van Santen, J. Am. Chem. Soc., 138, 101 (1991). [55] M. Ichikawa, “Dynamic Aspects in Heterogeneous Catalysis” K. Tamaru, ed.) Plenum Pub., pp 149-216 (1994). (561 G . Bergeret, P. Gallezot and F. Lefevre, Stud. Surf. Sci. Catal. 28, 401 (1986). [57] De Mallmann and D. Barthomeuf, Catal. Lett., 5, 293 (1990). [58] A. Fukuoka, T. Kimura, N. Kosugi, H. Kuroda, Y. Minai, Y. Sasaki, T. Tominaga and M. Ichikawa, J. Catal., 126, 434 (1990).
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.2 Silver-Tellurium Clusters from Silylated Tellurolate Reagents Dieter Fenske and John F. Corrigan
4.2.1 Introduction Silylated chalcogen and pnictogen reagents are excellent starting materials for the synthesis of binary phase colloidal and nanocrystalline materials. They are relatively easy to prepare and handle, and react readily with a variety of metal salts to form metal-chalcogen (pnictogen) bonds via the generation and elimination of SiMe3X (X = halogen, OAc, etc.). Independent work by numerous researchers[lp6]has shown that metal-alkyls and salts react with silylated reagents of the heavier group-15 and 16 elements [As(SiMe3)3,RP(SiMe3)2, Se(SiMe3)2, etc.] to yield ligand-stabilized clusters and colloids. These reactions are driven by the formation and elimination of volatile ‘Me3SiX’ ( X = C1, Me, etc). Recent investigations of ‘hot injection’ techniques by Bawendi and co-workers with a phosphine selenide/oxide stabilizing sphere have led to the formation of near mono-disperse colloidal CdSe nanoparticles.[’] The use of silylated chalcogenolate and related reagents has recently led to the isolation of numerous group- 12-1 6 nanocluster complexes as single crystals, from which complete structural information was obtained via X-ray analyses. The complexes [Cd&14(SR)36], [Hg32Se14(SePh)36]and [Cd32Se14(SePh)~6( PPh3)4] have been characterized.17p91 The reactions of bis(trimethylsily1)sulfur and bis(trimethylsily1)selenium with copper( I) salts have been extensively explored in our laboratories and have been shown to produce a variety of structural types of size and shape depending on the reaction conditions employed.[’01The ratio Cu/E/PR3 ( E = chalcogen), the steric requirements of the ancillary phosphine ligands employed, and the conditions and solvents used both for the syntheses and for crystallization all play significant roles in governing the structures of the products formed. The particles produced using this synthetic strategy range in size from ‘molecular type’ [Cu12E6(PR3)6][loa1to the nanoparticle [Cu146Se73(PPh3)30].[’Obl Although this strategy works well for copper-sulfide, copper-selenide, and copper-telluride particles, it has not been shown that related silver- and gold-chalco-
4.2 Silver- Tellurium Clusters from Silyluted Tellurolute Reagents RMgX+Te RLi + Te
-
RTeMgX
ClSiMeJ
RTeLi
A
2 CISiMe3
Na2Te *
Te + 2 LiB(H)Et3*
I
SiMe3
ClSiMe3
- -
Te + 2 Na
Te
-
Te I
SiMe3
LizTe
Te I
SiMe3
1303
RTeSiMe3
RTeSiMe3
Te(SiMe3)2
2 CISiMe3
Te(SiMe3)p
Te I
SiMe3
Scheme 1
genide clusters can be similarly produced. The phosphine (kinetically) stabilizing sphere is apparently insufficient in preventing the formation of the binary solid state materials in lieu of molecular clusters. The binary solid AgzTe is a semi-conducting solid of the electron/hole impurity type with a band gap of only 0.05 eV (0 K)["I and we were interested in developing a research program to synthesize well defined, nanosized clusters of this material. The strategy at the onset of this work was to use monosilylated chalcogen reagents because the hydrocarbon moiety is relatively inert, and unreactive towards metal salts and could thus inhibit the formation of the bulk, amorphous solids. The chemistry of tellurolate (RTe-) ligands remains surprisingly unexplored for the synthesis of polymetallic metal-main group (cluster) compounds despite their widespread use as precursors to semiconducting solids.[l2] We reasoned that the combination of PR3 ligands and alkyl/aryl groups on the tellurolate ligands would aid the solubilization and stabilization of polynuclear complexes. The synthesis of silylated tellurolate reagents is well developed and involves insertion of Te(0) into a metal-carbon bond, then reaction with ClSiR3 (Scheme l).[I3]The carbon chain on the chalcogen center is thus readily modified and, as will be discussed herein, this plays an important role in governing the structures formed.
4.2.2 Silver-tellurium clusters Homogenous ether or hydrocarbon solutions of silver chloride are prepared by using excess trialkylphosphine ligands to solubilize the metal salt. At room tem-
1304
4 Nunomaterials
x =3
t
C5H12,4 0 ° C
x=A=2 C5H12. 4 0 ° C
perature, the addition of Te( Bu")SiMe3 to these solutions proceeds with the immediate precipitation of brick-red, amorphous materials or the formation of bulk silver telluride. At lower temperatures, however, crystalline products suitable for single crystal X-ray analysis can be obtained (Scheme 2).[14' Typically, the addition of Te( Bu")SiMe3 to pentane solutions of AgCI/PEt3 is performed at -40 "C or lower. With a AgCI/PEt3 ratio of 1 : 3, a gold-colored solution is produced from which bright yellow crystals of [Ag6(p3-TeBun)4(p2-TeBun)2( PEt3)4] 1 grow within several days at slightly higher temperatures (-30 "C) in fair yield (30%). The molecular structure of 1 is shown in Fig. 1. The six tellurolate ligands are arranged about the cluster core to define an octahedron although the long Te...Te contacts observed (4.314(2)-5.057(2) A) suggest little bonding between the chalcogen centers. The Ag-Te distances lie within a relatively wide range (2.760(2)-3.046(2) A) with four of the RTe- ligands bonded in p3-fashion [Te(1)- Te(4)] and atoms Te(5) and Te(6) bridging only two silver centers. This flexible coordination geometry about the tellurolate ligands is well documented,[12"]and is what makes them so useful in stabilizing polynuclear frameworks. The former set has one lengthened Ag-Te contact (-3.0 A) with an average Te-Ag distance (2.85 A) markedly longer than in their p2 counterparts in 1 (2.73 A) and in the related, homoleptic complex [Ag4(p2-TeC4H3S)6]*-(av. 2.75 A).[''] Atoms Ag(3)-Ag(6) lie in deltahedral faces of the Te6 frame (maximum deviation 0.10 A)with planar coordination geometries (sum of the angles = 359.6-360.0"), whereas Ag(1) and Ag(2), each bonded to two phosphine ligands, span Te...Te edges and have local tetrahedral coordination environments. The distances between the silver(I) metal centers (2.888(2)-3.140(2) A) are not indicative of strong bonding interactions within the bi-spiked tetrahedral Ag6 framework. Assuming two eighteen-electron (tetrahedral) and four sixteenelectron (trigonal) silver centers, a total cluster valence electron count of 100 is required. This is achieved if the p3-Te(Bun)- ligands are considered as six-electron
4.2 Silver- Tellurium Clusters Jiom Silylated Tellurolate Reugents
Te
1305
I
Figure 1. The molecular structure of [A&(~ ~ - T e B ~ " ) 4 ( ~ ~ - T e( PEt3141 B u " ) 2 1. For clarity, the alkyl chains on Te and P are represented as lines only.
donors, and their p 2 counterparts each donate four electrons (6 x Ag(1) = 60; 4 x PEt3 = 8). If the ratio of phosphine to silver is only 1 : 1, there is no evidence for the for2a is formed mation of 1 (Scheme 2) and the cluster [Ag32(p3-TeBun)1~Te7(PEt3)6] in low yield. The layered cluster 2a is centered (Te13) on a crystallographic inversion center ( P 1), which relates the two halves of the molecule (Fig. 2). There is no obvious structural relationship between 1 and 2a. The top half of the cluster core consists exclusively of three pu,-Te(Bun)- groups whereas the second tellurium rich layer comprises three p3-tellurolate [Te(5),Te(7), Te(9)l and three p5-telluride ( Te2-) ligands. The latter form a nearly planar hexagonal Te3Ag3 ring (maximum deviation 0.10 A) which lies slightly above (0.61 A) tetrahedral Ag(l0). The central tellurium layer contains six p3-Te(Bun)- ligands and a central Te2- atom which is within bonding distance of eight silver atoms. The longest contacts associated with the central tellurium are those to Ag(l0) (2.938(2) A compared with 2.789(2)2.8 14(2) A). The distorted tetrahedral coordination geometry of these two silver atoms is unique in 2, the remaining Ag( I ) centers being planar. Thus 24 Ag atoms all form three bonds to neighboring Te atoms and Ag(l), Ag(3) and Ag(5) achieve trigonal planar coordination by bonding to two tellurolate and one phosphine ligands. Each butyltellurolate ligand in 2 symmetrically bridges three metal centers Te( Bun)-Ag bonding distances vary little from 2.753(2)-2.783(2) for Te( 12) to 2.781(2)-2.848(2) for Te(9).
~
1306
4 Nunomaterials
Figure 2. The molecular structure of [Ag32(,u3-TeBu")lgTe7(PEt3)6] 2a. For clarity, the carbon atoms have been omitted. Atoms labeled Tel-Te3, Te5, Te7, and Te9-Te12 are the Te(Bu")ligands. Atoms Te4, Te6, Te8 and Te13 are the telluride ligands. The molecule sits on a crystallographic inversion center.
The combination of eighteen tellurolate (18-) and seven telluride (14-) ligands again suggests a +1 oxidation state for the thirty-two silver atoms. The effectiveness of the eighteen butyl chains and six trialkylphosphine ligands in effectively enveloping and stabilizing the inner AgTe core is illustrated in Fig. 3. Although the yield of 2a is low ( - 5%), the synthesis is completely reproducible and there seems to be some inherent stability associated with the Ag-Te framework as analogous [Ag32(TeBu")lsTe7(PR3)6] clusters can be isolated using other tertiary phosphine ligands. The complexes [Ag32(TeBun)lsTe7(PEt2Ph)6] 2b, [Ag32(TeBun)18.
4.2 Silver- Tellurium Clusters from Silyluted Tellurolute Reagents
1307
Figure 3. A space filling projection of the cluster [Ag32(p3-TeBun)l8Te7( PEtj)6] 2a illustrating the shielding ability of the alkyl chains on the cluster surface. The projection of the molecule is identical to that in Fig. 2.
Te7(PPrn3)6] 2c, and [Ag32(TeBun)lgTe7(PPri3)6] 2d are prepared similarly. That identical cluster frameworks are observed for a range of phosphine ligands with cone angles['61varying from 132 to 160" illustrates the primary role the alkyl chains on the Te centers play in stabilizing the cluster core. The non-bonded (4.03-5.19 A) Te25 polyhedron in 2 (Fig. 4) can be described as consisting of three parallel Teg -Teg-Teg layers (Te1,2,4,5,9,10,12,7A; Te3,6,7,11, I 3,3A,6A,SA, 1 1A; Te7,1A,2A,4A,5A,9A, IOA, 12A). With the exception of the tetrahedral silver centers AglO and AglOA, all silver sites within the core of the molecule lie in various Te3 deltahedral faces. Unlike 2a-c for which yields are low, cluster 2d (R=Pri) is produced in good yield (40%) by stirring the reaction mixture (of AgCl, PPri3, and Te(Bu")SiMe3) at room temperature for a few hours before inducing crystallization. This increases the amount of telluride ligands produced by tellurium-carbon bond cleavage in butyltellurolate moieties. The relative weakness of the tellurium-carbon bond" 7 1 might account for the formation of the telluride ligands in 2a-c, although Te(SiMe3)2, invariably produced in small quantities during the synthesis of Te(Bu")SiMe3,[I8]also acts as the source of Te2-. We, and others, have, however, previously noted such facile Te-C bond cleavage during the synthesis of M-TeR Curiously, however, attempts to improve the yields of clusters 2 by using a defined mixture of Te( Bu")SiMe3 and Te(SiMe3)~does not yield crystalline material.
1308
4 Nanomaterials Te
Te
Figure 4. The (non-bonded) telluirium polyhedron in 2a.
This strategy does, however, produce silver-telluride nanoclusters in good to excellent yields if the substituent on the tellurolate ligand is replaced with an aryl group. AgCl, dissolved with tertiary phosphine ligands in ether solvents, reacts with a combination of the silylated reagents Te(SiMe3)z and Te(R)SiMe3 (R=Ph, mes; 3a and mes = C6H2Me3) to yield the cluster complexes [Ag3o(TePh)12Te9(PEt3)12] [Ag46(Temes)l2Tel7(PEt3)16] 4a. The preparation of 3a and 4a are summarized in Scheme 3.['01 When a 1 : 2 solution of AgCl/PEt3 is treated with 0.5 equiv of Te(Ph)SiMe3 and 0.25 equiv of Te(SiMe3)z at -40 "C in tetrahydrofuran, crystals of 3a suitable for X-ray analysis form within a few days at -30 "C in fair yield (20%). The cluster core comprises 12 tellurolate (Te(Ph)-, Tel-Tel2) and nine telluride (Te2-, Te13Te21) ligands in addition to the 30 silver atoms (Fig. 5). The order of addition of the silylated reagents does not affect the yield of product produced. Unlike 2, in which the RTe- binds to three metal centers only, the tellurolate ligands in 3a adopt both p3 (TelLTe9, Tell) and p4 (TelO, Te12)
4.2 Silver- Tellurium Clustersfrom Silyluted Tellurolute Reagents
R‘ = Ph
1309
~ 4 O D C
AgCl + 2 PR, + 0.25 Te(SiMe& + 0.5 RTeSiMe,
R=mes
Scheme 3
I
40%
Ag,,{Te(rnes)},~Tel,(PR3),~4
bonding modes. The p,-ligands have an unsymmetrical pattern of one ‘shorter’ (2.717(3)-2.826(3) A), one longer (2.842(3)-2.981(3) A) and one intermediate Ag-Te bonding distances. Atoms Te9 and Tell have the least deviation with Ag-Te = 2.817(3)-2.853(3) and 2.790(3)-2.842(3) A, respectively. The average difference in length between the longest and shortest Ag-Te contact is 0.14 A;this contrasts with cluster 2a for which the average difference in Ag-TeR bonding distances is only 0.03 A.The p,-Te(Ph)-Ag bond lengths in 3a are noticeably longer (2.933(8) A average) than their p , counterparts. The nine Te2- ligands (Te13-Te21) all form six bonding contacts to six Ag centers (Ag-Te = 2.703(3)-3.185(3) A)and atoms Te14 and Te16 also have a longer interaction with a seventh metal site (3.260(3) and 3.229(3) A,respectively). The combination of twelve Te( Ph)- and nine Te2- ligands suggests a +I oxidation state for the silver atoms, and the Ag.-.Ag contacts (2.849(3)-3.351(3) A) preclude any strong bonding interactions between them. Of the twelve phosphine-bonded silver sites, all have tetrahedral coordination with the exception of Agl and Ag2 which are trigonal planar. The phosphine-free silver atoms Ag18 and Ag19 form two bonds each (LTeAg-Te = 153.1(1) and 153.22(9)”, respectively); atoms Ag3, Ag6, A g l l , Ag12, Ag17, Ag20, Ag25, and Ag26 are three-coordinate and Ag9, AglO, Ag13, Ag14, and Ag21-Ag24 are tetrahedral. This varied coordination geometry about silver in the cluster contrasts with the trigonal planar geometry of the majority of the silver centers in 2 and with the solid-state structure of the mineral hessite (AgzTe) in which all Ag atoms are in tetrahedral sites.[”] Although the structures of the more covalent 12-16 clusters can be viewed as fragments of the corresponding bulk solid~,[’-~] similar observations for varied coordination geometries have been observed for copper-chalcogenide clusters - for copper sulfide clusters ( E = S) containing up to 28 Cu centers, near linear S-Cu-S and trigonal planar S2-Cu-P arrangements are always The copper centers in larger selenium analogs, on the other hand, have tetrahedral and trigonal planar coordination. Similarly for Cu-Te polynuclear complexes tetrahedral and trigonal planar coordination are observed
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4 Nanomuterials
Figure 5. The molecular structure of [Ag3o(TePh)l~Teg( PEt3)12] 3a. Tellurium atoms are represented as dark spheres and the silver atoms, with horizontal hatching, are labeled with numbers only for clarity. Tellurium atoms labeled TelLTel2 are the tellurolate ligands.
about C U , [ ~with ~ ] the exception of the cluster [C~58Te32(PPh3)16][~~] in which all Cu atoms are tetrahedral. It is interesting to note that the structurally related copper-tellurium cluster [C~29(TePh)12Teg(PEt3)s][PEt3Ph] 5 can be prepared not from a combination of copper salts and Te(SiMe3)z and Te(Ph)SiMe3, but rather from the virtually quuntitative co-condensation reaction of the two smaller clusters [CulzTes(TePh)n(PEt3)6] 6 and [Cus(TePh)6(PEt3)3][PEt3Ph] 7 via elimination of TePh2 (Scheme 4).r231This is the only example yet isolated with this distinct structural relationship between a copper-telluride-tellurolate cluster and its silver analog. The molecular structure
4.2 Silver-Tellurium Clusters from Silyluted Tellurolute Reagents
2 Cu,,(TePh),Te,(PEt,),
131 1
+ [Cu,(TePh),(PEt,),][PEt,Ph]
1
THF,RT -1.5 TePh,
Scheme 4
[Cu,,(TePh),,Te,(PEt,)~I[PEt,Phl5
of 5 is illustrated in Fig. 6 and, as for 3a, consists of twelve Te(Ph)- and nine Te2ligands. A total of 29 Cu sites and the anionic nature of the cluster core again suggests a +1 oxidation state for the coinage metal centers. Although the overall geometries of clusters 3a and 5 are similar, there are marked differences in the manner in which the Cu and Ag centers are distributed and bonded within the cluster frame. Ten p3-TePh ligands adopt an unsymmetrical pattern of one ‘shorter’, one longer and one intermediate Cu-Te bonding distances. These distances (2.552(3)-2.708(3) A) are, expectedly, shorter than their Ag-Te counterparts, and reflect the difference between the ionic radii of the two metals.[241This ‘contraction’ of the metaltellurium bonding distances is reflected in the overall size of the cluster frame. Thus, for comparison, the Te4...Te7 distance is 12.29 A in 5 and 12.95 A in 3a; similarly the telluride ligands Te15...Te16 are 7.72 8, apart in the copper complex whereas they are separated by 8.81 A in the silver cluster 3a. The most noticeable difference between the two frameworks is, however, at the ‘top’ of the molecule - in cluster 3 there are four phosphine-bonded silver atoms (Agl, 2, 4, 5) that are not represented in 5. Atoms Agl and Ag2 adopt a trigonal planar coordination geometry. Each is bonded to one phosphine, one tellurolate and one telluride ligand, whereas Ag4 and Ag5 achieve tetrahedral coordination with one PEt3, two Te( Ph)-, and one Te2- bonds. As is the general rule rather than the exception in copper- and silver-chalcogenide cluster chemistry, the phosphinebonded metal centers lie well ‘outside’ the tellurium polyhedron. In cluster 5, however, there are four fewer PEt3 ligands than in 3a - the copper atoms ‘missing’ the phosphine (Cu3, Cu4, Cu6, Cu15) are drawn into the cluster framework and is each tetrahedrally coordinated. Atoms Cu17 and Cu18 are trigonal planar (sum of the angles = 356” and 357”, respectively) and contrast with the near linear arrangement
1312
4 Nunomuterials Te
Figure 6. The anionic cluster core in [Cu29(TePh),2Te9(PEt3)8][PEt3Ph] 5. For clarity, carbon atoms have been omitted. Tellurium atoms labeled Tel-Tel2 are the tellurolate ligands.
of the two related silver sites in 3. This is accompanied by an increase in the coordination numbers of Te17 and Te19, which bridge seven copper atoms. The anionic nature of 5 results in excellent solubility in ether solvents compared with that of the neutral, silver complex 3. Although we have isolated several silvertellurium complexes 3 with different phosphine ligands (PEt3, PEt2Ph, PPrn3),[251 the same cannot be said for 5 for which, to date, only the PEt3 derivative has been isolated and characterized via the condensation reaction outlined in Scheme 4. One distinct advantage this route does offer is, however, the possibility of synthesizing ternary and quaternary phase nanoclusters by promoting condensation of different cluster molecules. Work by Bochmann and co-workers has illustrated the utility of 2,4,6Me3C6H2E- ( E = Se, Te) ligands for accessing homoleptic metal-chalcogen complexes.[261We reasoned that ligands of this type would also be suitable for stabilizing polymetallic frameworks. Thus, in a reaction analogous to that used to produce 3, when AgCl : PR3 (PR3 = PEt3, PPr"3, PEtzPh) solutions in ether solvents are reacted with a combination of Te(mes)SiMe3 and Te(SiMe3)z (Scheme 3) the
4.2 Silver- Tellurium Clusters from Silyluted Tellurolute Reagents
131 3
Figure 7. A projection of the cluster of [Ag4h{Te(mes)}IzTel,(PEt3)ls] 4a.
clusters [Ag46jTe(mes)}~zTel;l( PR3)16] 4 are formed as dark red/black crystals in excellent yield (80-85%). As in 3a, cluster 4a (PR3 = PEt3) contains a distribution of telluride ligands in the central portion of the cluster and tellurolate ligands around the periphery of the molecule (Figs 7 and 8). About the central telluride are 12 additional Te2- (Te15-Te18, Te21-Te26, Te28, Te29) arranged to form a distorted (non-bonded, Te...Te = 4.523(2)-5.288(2) .$) centered icosahedron (Fig. 8, insert). The central atom, Te27, is bonded to eight silver metal atoms (2.814(2)-3.198(2) A) which reside within the Tel3-centered icosahedron with six additional Ag atoms (Ag4, Ag5, Ag25, Ag26, Ag44, Ag45) lying within the deltahedral faces. Ag atoms span twenty of the thirty Te2 edges of the icosahedron. This central Ag34Te13 unit is surrounded by 16 tellurium (12 x Te(mes)-, 4 x Te2-) and 12 Ag centers. The projection illustrated in Fig. 8 emphasizes the common structural features of [Ag3o(TePh)12Teg(PEt3)l2] 3a and 4a. The relative arrangement in 3 of the ligands Te3-Te5, Te9, TelO, Te12, and Te13 and the silver atoms that are bonded to them can also be seen in 4. Two of these fragments come together to share a common ‘edge’ in 3 whereas in 4 they are separated by a larger inner core. The more condensed nature of the framework in 4 is reflected in the dark red/black color of the
1314
4 Nunomuterials
Figure 8. The molecular structure of the cluster [Ag46{Te(mes)}12Te17(PEt3)16] 4a emphasizing the structural relationship with 3a (Fig. 5). The insert shows the internal, Telj-centered icosahedron.
material compared with the more lightly colored (red) 3. As observed in 3, the coordination number about the silver centers in 4 varies from two to four. The coordination geometry of the silver atoms in the 'common' portions of the two clusters is identical. To date the largest silver-tellurium cluster isolated and structurally characterized, albeit in moderate yields, is the complex [Ag48(1~~-TeBu~)24Tel2( PEt3)14] 6. Cluster 6 is obtained as the only crystalline product from the reaction of AgCl/PEt3/Te( Bu")SiMe3 (1 :2 : 1); it is isolated as dark red crystals after several days (Scheme 2, Fig. 9).[l4] The cluster is best described as comprising Ag24Te16 units linked by four p3Te(Bu")- ligands (Te5 and Te17 and their symmetry equivalents), centered about a crystallographic inversion center (in Pi). These Te-Ag distances range from 2.701(2)-3.109(3) A. The other tellurolate ligands also bridge three metal atoms (Ag-Te = 2.701(2)-3.404(2) A). Twelve telluride ligands all bond to six metal atoms (2.775(2)-3.403(2) A)yielding a layered type structure as observed in 2 (Fig. 9, insert). Unlike in 2, however, the fourteen Ag-PEt3 centers in 6 are all tetrahe-
4.2 Silver- Tellurium Clustersfrom Silyluted Tellurolute Reagents
13 15
Figure 9. The molecular structure of [Ag48(TeBun)24Te12( PEt3)14] 3 (carbon atoms omitted for clarity). Tellurium atoms are represented as dark spheres and the silver atoms, with horizontal hatching, are labeled with numbers only for clarity. A crystallographic inversion center relates the two halves of the molecule.
dral. The remaining silver atoms within the cluster core have co-ordination number three (trigonal planar) or four (tetrahedral) and longer Ag...Te contacts (cJ Ag(l3)...Te(lO) = 3.378(2); Ag(lS)...Te(lO) = 3.388(2) A)also suggest intermediate bonding descriptions. The 24 Te(Bu")- and 12 Te2- enable a +1 oxidation state to be assigned to the Ag sites, and the Ag( I)...Ag(I ) contacts vary between 2.802(2) and 3.222(2) A. Unlike the chemistry of alkyltellurolate-silver complexes, which must usually be formed at low temperatures to avoid the precipitation of amorphous solids and of which we have isolated relatively few structural types (vide ~ u p r a )the , chemistry of aryltellurolate silver complexes is rich and varied in respect of the polynuclear frameworks formed. The reagents can also be classified as 'gentler' in that reactions between the phosphine-solubilized metal salt and the silylated chalcogens can often be performed at room temperature to yield medium sized polynuclear complexes.
1316
4 Nunomuterials
Te 1
P7
Te
Figure 10. Two projections of the cluster [Agl4{ p,-Te(mes)}12(p6-Te)( PEtzPh)4] 7 (carbon atoms omitted). The lone telluride is labeled Te13.
Thus addition of one equivalent of Te(mes)SiMe3 to AgC/PEtzPh (1 : 2) in diethyl ether results in a yellow solution from which orange crystals of [Ag14. {y3-Te(mes)}12(y6-Te)(PEt2Ph)4] 7 (Fig. 10) are obtained.[251The molecule has six silver atoms (Ag3, Ag6-Ag9, Ag12) clustered about the central telluride (Te13) in an octahedral array (Ag-Te = 2.831(2)-2.906(2) A). Each of these silver atoms is also bonded to two tellurolate ligands (2.720(7) A average) to yield a near trigonal planar coordination environment. The twelve tellurolate ligands all have a distinct
4.2 Silver-Tellurium Clusters from Silyluted Tellurolute Reagents
5
1317
7
10
T
Te
Te
Figure 11. The cluster core in [ Ag ~ ( ,q T e P h12 ) h6-Te) ( PEt2Ph)~] 8. The insert is of the Tel3 (non-bonded) polyhedral arrangement of the tellurium atoms.
pattern of one short (2.700(2)-2.729(2) A),one intermediate (2.781(1)-2.826(1) A) and one longer (2.808(2)-2.919(2) A) Ag-Te contact. The remaining four core Ag centers (Ag4, Ag5, Ag13, Ag14) are also trigonal planar, bonded to three different Te(mes)- ligands (Ag-Te = 2.803(7) A average). In contrast, the four unique silver atoms bonded to PEt2Ph ligands (Agl, Ag2, AglO, Agll) are tetrahedral. The core of the molecule has overall pseudo-tetrahedral geometry (Fig. 10, insert). The Tel3 non-bonded polyhedron in 7 is that of a centered icosahedron and parPEtzPh)s] 8 (Fig. allels that observed in the related cluster [Ag14(,u3-TePh)Iz(p6-Te)( 11, insert). Cluster 8 is obtained in moderate yields (15%) in a reaction similar to
1318
4 Nunomaterials
AgCl + 2 PEt,Ph + Te(Ph)SiMe, THF, RT
I
Agl,(TePh)12Te(PEt2Ph)8 8
+
that used to obtain 7, with the reagent Te(Ph)SiMe3 (Scheme 5).Iz7] Compared with 7, cluster 8 contains four additional phosphine ligands, the eight PEtzPh ligands are arranged at the corners of a cube. This results in a total of eight tetrahedral silver centers, each capping a deltahedral face of the Telz icosahedron. These Ag-Te contacts range from 2.838(1) to 3.026(1) A.There are six metal atoms bonded to the central telluride (Ag-Te9 = 2.791(1)-2.825(1) A). The Ag-Te bond distances for the three coordinate silver centers in 8 (Ag5-Ag9, Ag14) range from 2.660(1) to 2.825(1) 8, with near planar coordination around silver (sum of the angles = 359.4360.0'). Space filling projections of the two clusters (Fig. 12) illustrate the effectiveness of
Figure 12. Space filling projections of cluster [Ag14{ ,u,-Te(mes)},z(,u6-Te)( PEtzPh)4] 7 (left) and [Ag14(,uu,-TePh) 12 (,u6-Te)(PEt2Ph)sl8 (right).
4.2 Silver-Tellurium Clustersfrom Silylated Tellurolute Reuyents
1319
Figure 13. The molecular structure of (P3 -TePh)9( PPhEt2)6] 9a. Te atoms are drawn as dark spheres and Ag atoms are drawn with horizontal hashing.
the mesityl groups in shielding the cluster core compared with the CsH5 rings the methyl substituents on the phenyl rings limit the amount of exposed surface on the silver-tellurium core. Thus in 7 there are fewer surface Ag sites available for PEt2Ph ligands. The major product (60%) from the reaction of AgCI/PEt*Ph with Te( Ph)SiMe3 9a (Scheme 5 , Fig. 13). The cluster is the planar cluster [Agg(p3-TePh)g(PEtzPh)6] sits on a 3-fold axis of rotation that bisects atoms Ag2, Ag3, and Ag5 a noncrystallographic mirror plane relates the two halves of the molecule. Two of these ligands (Tel and Te3) symmetrically bridge three silver atoms (2.867(1)-2.879(2) and 2.868(1)-2.882(2) A, respectively); the Te-Ag bonding distances about Te2 vary from 2.730(1) to 2.932(2) A. The coordination environment about Te2 consists of a near linear (168.80(5)") Agl-Te2-Ag4 arrangement. The six silver atoms bonded to the PEtzPh ligands are tetrahedral and atoms Ag2, Ag3, and Ag5 are ~
1320
4 Nanomuteriuls
Figure 14. Two projections of the crystal packing repeat in [Ag9(p3-TePh)9(PPhEt&] 9a illustrating the alignment of the Ag(TePh)? middle layers of the clusters. The top projection is viewed down (001).
4.2 Silver- Tellurium Clusters from Silylated Tellurolate Reugents
132 1
AgCl + 2 PPt3 + Te(Ph)SiMe, Et20, RT
J
[Ag,(TePh),][HPPi,]
10
+
Scheme 6
trigonal planar. There are no significant bonding interactions between the nine silver(1) metal sites (2.809(1)-3.126(4) A). For its part, the central layer comprises a planar arrangement of one Ag and three Te centers with the three phenyl rings of the tellurolate ligands also lying in this plane. The crystallographic packing arrangement of 9a in the lattice is such that the central Ag( TePh)3 layers are aligned, resulting in a mixed aromatic-Ag-Te infinite repeat (Fig. 14). The related cluster [Ags(TePh)g(PPri3)6]9b can be prepared by a method analogous to that used for 9a, with the bulkier phosphine PPr‘3 in ether solvent. The structural features of 9b are virtually identical with those observed for 9a. Also formed however is the homoleptic, anionic cluster [Ag7(TePh)9][HPPri3]2 10 (Scheme 6, Fig. 15).[251There is a clear structural relationship between 9b and 10 (Fig. 15) - both structures contain three Ag-Te ‘layers’. In 10 however, the top and bottom layers contain only three p2-TePh ligands and three silver atoms, in contrast to the Ag4Te3 arrangement in 9. The two ‘missing’ Ag metal atoms thus account for the 2- charge of the cluster core. The p2-Te(Ph)-Ag bonding distances (2.733(3) A) in 10 are much shorter than their p3 counterparts in 9b (2.878(1) A), resembling those observed in the related cluster [Ag4(p2-TeC4H3S)6I2- (average 2.75 A).[151 The absence of phosphine ligands in 10 results in all of the silver(I) centers adopting trigonal planar geometry. Also noteworthy is that atoms Agl -Ag4 and Ag7-Ag9 are highly ‘exposed’, as the relative orientation of the phenyl rings bonded to tellurium do little to mask these metal sites, in contrast to all other observed structures. The nine tellurolate ligands are so arranged as to form two non-bonded (4.326-5.21 1 A) face-sharing octahedra (Fig. 16) with the silver atoms lying within seven of the fifteen deltahedral faces.
I322
4 Nunomaterials
Figure 15. The molecular structure of [Ag9(p3-TePh)g(PPr'3)6] 9b (top) and the anionic core in [Ag, (TePh)g][ HPPri3]2 10 (bottom).
4.2 Silver- Tellurium Clusters ,from Silyluted Tellurolute Reagents
I323
Figure 16. The non-bonded Te9 framework in [Ag7(TePh)9I2- 10.
References (a) X. Peng, T. E. Wilson, A. P. Alivizatos and P. G. Schultz, Angew. Chem. Int. Ed. Engl. 1997, 36, 145. (b) J. E. B. Katari, V. L. Colvin and A. P. Alivizatos, J. Phys. Chem. 1994, 98, 4109. ( c ) M. A. Olshavsky, A. N. Goldstein and A. P. Alivizatos J. Am. Chem. S o c , 1990,112, 9438. C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc. 1993, 115, 8706. D. Fenske in Clusters and Colloids, From Theory to Applications, G. Schmid (Ed.), VCH, Weinheim, 1994, Chapter 3.4. J. G. Brennan, T. Siegrist, P. J. Carroll, S. M. Stuczynski, L. E. Brus and M. L. Steigerwald, J. Am. Chem. Soc. 1989, I l l , 4141. R. J. Wehmschulte and P. P. Power, J. Am. Chem. Soc. 1997, 119, 9566. K. Merzweiler, Z. Anorg Allg. Chem., 1997, 623, 478. (a) S. Behrens, M. Bettenhausen, A. C. Deveson, A. Eichhofer, D. Fenske, A. Lohde, U. Woggon, Angew. Chem. Int. Ed. Engl. 1996,35, 2215. (b) S. Behrens, M. Bettenhausen and D. Fenske, ibid. 1997, 36, 2797. N. Herron, J. C. Calabrese, W. E. Farneth, Y . Wang, Science, 1993, 259, 1426. T. Vossmeyer, G. Reek, B. Schulz, L. Katzikas and H. Weller, J. Am. Chem. Soc., 1995, 117, 12881. (a) D. Fenske, A. C. Deveson and S. Dehnen, J. Cluster Sci., 1996, 7, 351. (b) H. Krautscheid, D. Fenske, G. Baum and M. Semmelmann, Angew. Chem. Int. Ed. Engl. 1993, 32, 1303. (c) S. Dehnen and D. Fenske, Chem. Eur. J. 1996,2, 1407. C. Wood, V. Harrdp and W. M. Kane, Phys. Rev. 1961, 121, 978. (a) J. Arnold, Prog. Znorg. Chem. 1995, 43, 353. (b) M. Bocchmann, Chem. Vup. Deposition 1996,2, 85. (a) J. E. Drake and R. T. Hemmings, Inorg. Chem. 1980, 19, 1879. (b) L. Engman and M. P. Cava, Synth. Commun. 1982, 12, 163. ( c ) K. Praefcke and C. Weichsel, Synthesis 1980, 216. (d) M. Detty and M. D. Seidler J. Org. Chem., 1982, 47, 1354. (e) H. Buerger and U. Goetze,
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Inorg. Nucl. Chem. Lett. 1967, 3, 549. (f) F. Feher, in Handbuch der Praparativen Anorganischen Chemie, G. Brauer (Ed.), Enke-Verlag, Stuttgart, 372. [14] J. F. Corrigan and D. Fenske, J. Chem. Soc., Chem. Commun. 1996, 943. [15] J. Zhao, D. Adcock, W. T. Pennington and J. W. Kolis, Inorg. Chem. 1990, 29, 4358. [16] C. A. Tolman, Chem. Rev. 1977, 77, 312. [ 171 L. Blatt in The Chemistry of’ Organic Selenium and Tellurium Compounds, Vol. I , S . Patai and Z. Rappoport (Eds.), John Wiley and Sons, Chichester, 1986, p.157. [18] J. F. Corrigan, S. Balter and D. Fenske, J. Chem. Soc., Dalton Trans., 1996, 729. [19] (a) P. J. Bonasia, G. P. Mitchell, F. J. Hollander and J. Arnold, Inorg. Chem. 1994, 33, 1797. (b) M. Semmelmann, D. Fenske and J. F. Corrigan, J. Chem. Soc., Dalton Trans. 1998, 2541. [20] J. F. Corrigan and D. Fenske, J. Chem. Soc., Chem. Commun. 1997, 1837. [21] A. J. Frueh, Jr., Am. Minerol., 1961, 46, 655. (b) A. J. Freuh, Jr., Zeit. Krist., 1959, 112, 44. [22] D. Fenske and J.-C. Steck, Angew. Chem. Int. Ed. Engl. 1993,32, 238. [23] J. F. Corrigan, to be published. [24] R. D. Shannon, Acta Crystallogr. 1976, A32, 751. [25] J. F. Corrigan and D. Fenske, unpublished results. [26] (a) M. Bochmann, K. J. Webb, M. B. Hursthouse and M. Mazid, J. Chem. Soc., Dalton Trans. 1991, 2317. (b) M. Bochmann, A. P. Coleman, K. J. Webb, M. B. Hursthouse and M. Mazid, Angew. Chem. Int. Ed. Engl. 1991, 30, 973. [27] J. F. Corrigan, D. Fenske and W. P. Power, Angew. Chem., Int. Ed. Engl. 1997, 36, 1176.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.3 Nanosized Clusters on and in Supports Perspectives for Future Catalysis G n t e r Schmid
4.3.1 Introduction The historical role of organometallic chemistry in catalysis is unquestioned and the development of catalysis would never have become so impressive without the parallel evolution of organometallic chemistry since approximately 1960. Beyond all doubt, mononuclear complexes played, and still play, the most important part in this development. Oligo- and polynuclear organometallics, hence clusters, are, however, becoming increasingly interesting for several reasons. Firstly, clusters containing a few up to a dozen or more metal centers have been considered as models mimicking metal surfaces with catalytically active centers. In contrast to a real metal they can be used in solution owing to the nature of their ligand shell. Many complications, however, accompanied the use of clusters in homogenous catalysis, in particular the tendency to decompose, forming colloidal or even larger metal particles. Very few examples are known of original cluster molecules being identified as the catalytically active species and being found unchanged after several catalytic cycles.['-31 Many efforts have, therefore, been made to immobilize clusters and so to prevent this rapid degradation. Supported clusters are more long-lived and have other advantages, for example easy separation from the products. Comparison of oligonuclear clusters, whether dissolved or immobilized, with a metal particle surface is, however, not acceptable because of our knowledge about the electronic situation in a metal compared with that in a Mq, Mg, or even a M12 cluster. To elucidate that statement it might be useful to discuss briefly the electronic situation in a cluster belonging to the size regime between molecule and bulk. With reference to catalysis it is important to distinguish between a metal atom in a complex or in a small cluster on the one hand, and an atom which is part of a bulk metal with all its specific properties, on the other hand. This is why not only the size of a cluster as a geometric factor must be taken into consideration but also the dramatic change in the electronic situation going from
1326
4 Nanornuteriuls
a complex or a small molecular cluster to a nanosized particle consisting of a few dozen or even thousands of atoms. This effect has in the past completely been neglected. Finally it should be mentioned that negative experiences with small organometallic clusters in homogenous catalysis implies that larger clusters should only be used in a supported form.
4.3.2 Between molecule and bulk - the position of nanosized clusters The electronic situation in a typical molecule is determined by strongly localized bonding electrons. In principle there is no difference between the C-C bond in ethane and the Mn-Mn bond in (C0)5Mn-Mn(C0)5 (isolobal principle).r41Welldefined molecular orbitals characterize the mode of bonding. The metallic bond is, however, described by the band model which assumes the presence of delocalized electrons, freely mobile between the positively charged atomic nuclei. Although surface atoms of a piece of metal participate only partially in the metallic state, as part of the metallic system their electronic behavior must be different from that of atoms in an oligonuclear cluster with localized bonds. These considerations consequently lead to the question of formation of a metal. How many atoms are necessary or how large must a cluster become before it begins to have metallic behavior? Although numerous investigations have been performed to answer this fundamental question, only in the last decade has the existence of well defined clusters of different sizes enabled such s t u d i e ~ . [ ~Details ,~] will be discussed in sections 4.4 and 4.7. Here a few hints are given to clarify the relationship between catalysis and cluster size. Differentiation between a bulk metal and the beginning of the formation of discrete energy levels is related to the emergence of so-called quantum size effects. Quantum size effects indicate the domain of quantum mechanical rules instead of the laws of classical physics. How can quantum size effects be demonstrated? In principle by any method that gives information about the electronic situation in a particle. If deviations from bulk behavior are observed, quantum size effects can be assumed. It has recently become apparent that ligand-stabilized particles of sizes > 10 nm exhibit bulk-like properties whereas particles smaller than 1 nm behave more or less like molecules. It must, however, be kept in mind that all the particles investigated have a shell of protecting ligand molecules on at least some of the surface atoms. These ligands influence the electronic behavior in a way which is not completely understood but makes ligand-protected clusters different from naked clusters. As unprotected clusters can not be investigated in the solid state, discussion of numbers of atoms or cluster size must always be considered under these conditions.
4.3 Nunosized Clusters on and in Supports
-
Perspectives for Future Catalysis
1327
This contribution will elucidate the decisive difference between a molecular cluster and a metal-like particle. Among numerous physical investigations, indicating quantum size effects, the size-dependent electronic relaxation observed in some gold and platinum clusters is highly suitable for showing the bulk + molecule transition.[71Optical investigations also indicate the size-determined electronic situation in two different gold clusters.[*] A short laser pulse (-200 fs) generates hot electrons in the metal clusters. A second laser pulse monitors the hot electron population. This technique enables the direct observation of electron relaxation as a result of electron-phonon coupling. Electron-phonon coupling is a decisive property enabling differentiation between bulk and molecular properties. Although this possibility has been exploited by different authors,['] the particles used have not been single-sized, which is necessary if valid results are to be obtained. The use of clusters of a distinct size such as A U ~ ~ [ ' ~ I and A u ~ ~ [ and " ] almost monodisperse 15-nm gold colloids[121and 3 nmrI3]and 35 nm[l4l Pt particles recently enabled more precise measurements to be made and better results to be obtained than in the past. It could be shown that the hot electron relaxation depends on the particle size in two different ways: i) weakening of electron-phonon coupling with decreasing cluster size, slowing down electronic relaxation; and ii) the facilitation of electronic relaxation with decreasing cluster size because of the enhanced surface collision rate. These effects compete with each other. The domination of the latter effect for smaller particles can be seen from the transition from the 15-nm particles to the Au55 cluster (cluster core = 1.4 nm) in Figs l a and lb. The behavior of Aul3(dppmH)6(N03)4 (dppmH = bis(dipheny1phosphino)methane) deviates completely from this tendency indicating single electron excitation in a typical molecular particle. (Fig. lc) The conclusion of this fundamental observation is that for gold the metal + non-metal (molecule) transition takes places in the size range between 55 and 13 atoms. Comparison between the behavior of 35-nm and 3-nm platinum particles confirms the observations with gold clusters. Unfortunately, molecule-like Pt clusters have not yet been investigated. Assuming that the metal + non-metal transition occurs over a size range similar to that for gold it is obvious that catalysis by a particle 2 M55 must be different from that by a cluster consisting of only a dozen or even fewer atoms. Study of the optical extinction spectra of the same two gold clusters, Au13 and Au55, in solution leads to the same result with respect to the metal + non-metal transition. The Au55 cluster core forms a metallic system characterized by collective excitation of the s electrons. There is no indication of any molecular fine structure even at 2 K. The observed smearing of the band edge is responsible for damping
1328
4 Nanomaterials
6 ’ 0 d
w
W
Q)
z
Q
e 0
2
4 E
.CI
& E
Q
E
u
I
0
I
2 4 Time (ps)
I
6
Figure 1. Relaxation behavior of photogenerated hot electrons (a) in 15-nm gold colloids, (b) in 1.4-nm Auss clusters, and (c) in molecular Au13 clusters. Solid lines are fitted.”’
away excitation. In contrast, the spectrum of the Au13 cluster is characterized by a multiband spectrum typical of that of a molecule. As already mentioned, these two electronic investigations of hot electron relaxation and the optical extinction of various clusters, are only part of many attempts to visualize quantum size behavior and, as a consequence, the transition from metal to molecule. Other chapters will provide more information about the results obtained. Catalysis using metal clusters, whether homogenous or heterogeneous, must be influenced by this fundamental difference between molecular clusters and clusters with a metallic ‘inner life’. It is surprising that these aspects have not been discussed previously in relation to catalytic effects. The purpose of this contribution is, there-
4.3 Nunosized Clusters on and in Supports - Perspectives for Future Cutulysis
1329
fore, to provide information about these findings and to initiate further studies that might lead to a better understanding of effects already observed. It will become obvious that small molecular clusters cannot be used as models of catalytic processes on metal particles in heterogeneous catalysis. Section 4.3.3 deals with another effect that can play a decisive role in catalysis by immobilized clusters - the influence of the ligands present on the cluster surface.
4.3.3 The effect of the ligand shell in immobilized clusters on activity and selectivity The function of ligands in simple catalytically active transition metal complexes is well known. They decisively determine the activity and - even more importantly the selectivity, culminating in the enantioselective synthesis of many valuable products. The nature of ligand molecules on the surface of nanosized clusters should, in principle, be equally important. The use of a series of variously ligated Pd clusters in the size range 3.0-3.6 nm on Ti02 or active carbon as catalysts for the selective hydrogenation of hex-2-yne to cis-hex-2-ene impressively shows the function of the ligands." 5,161 Figs 2a-d show results from the semihydrogenation under very mild conditions. The catalysts contain 1% w / w palladium; hex-2-yne is dissolved in n-octane and hydrogen is used under normal pressure and at room temperature. When phenanthroline is the ligand, a maximum of cu. 93% cis-hex-2-ene is formed after 75 min; this corresponds to a turnover frequency (TOF) of 35 min-' (Fig. 2a). If the phenanthroline is substituted in the 3-position by n-decyl, the activity is remarkably reduced, probably because of the reduced availability of free surface atoms. The cis-hex-2-ene maximum is reached only after ca. 200 min (Fig. 2b). As in the former example the product maximum is followed mainly by the formation of trans-hex-2-ene and n-hexane. A completely different ligand shell is used to give the similar result shown in Fig. 2c (-)-cinchonidhe leads to the cis-hex-2-ene maximum after 150 min. 3-Butyl- and 3-heptylphenanthroline on the cluster surface, however, reduce the activity dramatically (Fig. 2d). The maximum is reached only after cu. 500 min, but with the big advantage of almost exclusive selectivity for cishex-2-ene formation. The generation of consecutive products can be observed only after cu. 2000 min, and then at very low concentrations only. These clear influences of the ligands on speed and selectivity can be understood as arising from simple geometric effects which depend on the coverage of the cluster surface by the ligands, although there is still no quantitative explanation as to why the change from n-decyl to n-heptyl or n-butyl substituents induces such a drastic change in activity. The reasons lie in the chain length responsible for opening or closing reaction paths for reactants and products. ~
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4 Nunomaterials
100
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8 75 50
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0 0
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a
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s 75
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500
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1000 b
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Figure 2. Hydrogenation of hex-2-yne by variously ligated 3-4 nm Pd clusters. (1% W / M ? cluster on active carbon, room temperature, 1 atm H2). Ligands: (a) phenanthroline; (b) 3-n-decylphenanthroline; (c) (-)-cinchonidine; (d) 3-n-butylphenanthroline as ligands.[15,'61
4.3 Nanosized Clusters on and in Supports
-
~
Perspectives for Future Catalysis
100
3
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C
100 e
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n -hexane
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Figure 2 (continued)
2000
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3000 min
133 1
1332
4 Nanomaterials
1.5-nm Pd clusters, either unstabilized or stabilized by phenanthroline, both supported by active carbon, have been used to study the influence of ligands on activity Figs 3a and 3b show results from the semicompared with that of bare hydrogenation of hex-2-yne to cis-hex-2-ene under conditions identical with those in the experiments described above. It is apparent from Fig. 3a that the activity of the ligand-protected clusters on active carbon is rather low. After ca 2 h only -80% cis-hex-2-ene is formed. Removal of the ligand shell by heating the catalyst under high vacuum to 180-200 "C gives the result shown in Fig. 3b. cis-Hex-2-ene is obtained quantitatively after 2 h. Transmission electron microscopy (TEM) studies showed that the size of the clusters on the carbon support was not changed by heating.
4.3.4 Bimetallic shell-structured particles Alloy-like bimetallic particles play an important role in heterogeneous catalysis. Numerous papers have discussed the gold-palladium system.[' 8-261 The mutual influence of different neighboring atoms leads to catalytic behavior which is often considerably different (and sometimes better) than that of monometallic species. Electronic effects in shell-like structured bimetallic systems have, however, rarely been investigated. Palladium-covered gold nuclei (Au/Pd) and gold-covered palladium particles (Pd/Au) can be prepared by the so-called seed-germ process,[271i.e. gold (or palladium) particles without protecting ligands are used to grow a second metal on their surfaces by means of second reduction step. The thickness of the second metal layer can be varied over a relatively wide range. Finally, the outer metal can be protected by a shell of appropriate ligand molecules.[281The question arising from such a system is whether or not the inner metal has an observable electronic influence on the outer metal and hence on the catalytic behavior. Again, the selective hydrogenation of hex-2-yne to cis-hex-2-ene is used to study the catalytic behavior of supported bimetallic Au/Pd particles in a ligated The influence of the inner metal on the outer shell should be more pronounced the thinner the shell is. If the thickness of the outer metal layer is substantial, this must suppress the effect of the inner metal. Extensive studies of such particles by high-resolution electron microscopy (HRTEM) have confirmed the shell structure and the almost perfect coverage of the inner metal by the outer shell. Fig. 4 shows a HRTEM image of Pd-covered gold colloids as used for catalysis.[291 Some results from catalysis studies are shown in Figs 5 and 6. Fig. 5 shows the course of the hydrogenation of hex-2-yne to cis-hex-2-ene with
4.3 Nunosized Clusters on and in Supports
-
Perspectives for Future Catalysis
1~
-
1333
hex-2-yne cis-hex-2-ene
1
trans-hex-2-ene trans-hex-3-ene x n-hexane -A-
x
45
0
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90
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155
175
195
215
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Urnin
a
100
75
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0 0
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b Figure 3. Hydrogenation of hex-2-yne by 1.5-nm Pd clusters, (a) stabilized by phenanthroline, (b) after removing the ligands.
1334
4 Nanornuteriuls
Figure 4. High-resolution electron microscopy (HRTEM) of bimetallic, shell structured goldpalladium colloids. The 18-nm gold cores (dark areas) are covered by a Pd shell cu 4-5 nm thick.'*']
-
tcis-hex-Ben@
100 80
s -
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0 0
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t[hI Figure 5. The course of the selective hydrogenation of hex-2-yne to cis-hex-2-ene with a bimetallic Au/Pd colloid (0.50/, w/w Au/Pd 18/30 P,N on TiOz, ethanol, room temperature, 1 atm H2) as
4.3 Nunosized Clusters on and in Supports - Perspectives f o r Future Catalysis
1335
d
Catalysts Figure 6. The influence of the thickness of the Pd layer in bimetallic AujPd particles on the activity of the hydrogenation of hex-2-yne to cis-he~-2-ene.[~~]
0.5% w/w of a Au/Pd catalyst on Ti02, consisting of a 18 nm gold core, enlarged to 30 nm in diameter by palladium. The Pd surface is finally protected by a shell of mixed ligands, P(m-C6H4S03Na)3 (TPPTS) and sodium sulfanilate P - H ~ N C ~ H ~ S(abbreviation O~N~ Au/Pd 18/30 P,N). From Fig. 6 it can be concluded that the gold core does, indeed, exert a remarkable influence on the palladium and, furthermore, as might be expected, that this influence decreases with the thickness of the Pd layer. For comparison, 20-nm Pd particles have also been tested. Pd particles have considerably less activity than the bimetallic particles, even if the Pd shell is 9 nm thick (as for Au/Pd 18/36). Reducing of the Pd shell thickness by 6 nm in Au/Pd 18/30 to 3.5 nm in Au/Pd 18/25 results in a continuous increase in activity, clearly indicating the electronic influence of the gold core on the outer palladium layers. It should be remarked that the ligand molecules on the two species are not identical. For Pd 20 the ligand is sulfanilate ( N ) only; for Au/Pd 18/36 and Au/Pd 18/30 it is mixture of TPPTS (P) and N; and for Au/Pd 18/25 it is only P. The increase of activity from Au/Pd 18/36 P,N to Au/Pd 18/30 P,N, which have the same ligand shell, does, however, clearly indicate the tendency.
1336
4 Nunomaterials
4.3.5 Clusters in nanotubes The fixing of metal particles on different supports, e.g. alumina, silica, titania, active carbon, etc. is a very useful, traditional, way of producing highly effective catalysts in science and in industry. The purpose of the support is to offer a large surface area and so to bind many metal particles on a small amount of material. This section deals with a novel type of a support on which the clusters are organized into nanotubes, all oriented in the same direction - thus enabling gas-phase catalysis of a very special type. The material offering these facilities consists of nanoporous transparent alumina membranes the production of which has been described elsewhere.r30p351 The attractiveness of the material is its ready availability and its architecture, which has 109-101' uniform pores running in a parallel direction through the membrane, as is illustrated schematically in Fig. 7.
pores
alumina barrier layer Figure 7. Schematic diagram of a nanoporous alumina membrane.
Fig. 8 shows a view on the surface of such a porous membrane, obtained by atomic force microscopy (AFM).[361A membrane, sectioned along the pores is shown in Fig. 9, obtained by transmission electron microscopy (TEM).[361 In addition to the attractive arrangement of the pores, there is also the advantage of variable pore diameters and lengths, depending on the experimental conditions aluminum plates, used as anodes, are oxidized in di- or triprotic acids; the applied voltage directly determines the pore width and the anodization time is responsible for the thickness of the oxide layer and hence the length of the pores. The pore walls are not formed from pure A1203 alone - it has been shown they are covered with OH groups. This enables their chemical modification such that metal particles can be trapped. Scheme 1 depicts one of several means of attaching metal clusters to the pore walls.r371 ~
4.3 Nunosized Clusters on and in Supports
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Perspectives for Future Catalysis
1337
Figure 8. Atomic force microscopic (AFM) image of the surface of a nanoporous alumina membrane.
The highly active OH groups react, for instance, with alkoxysilane molecules to give strong AI-0-Si bonds. The functional substituents Y can be selected according to the affinity of the metal of choice. The transparency of the membranes enables direct spectroscopic investigation of modified membranes to study the inner parts.
Figure 9. TEM image of a nanoporous alumina membrane, sectioned along the pores.
1338
4 Nunornaterials
Aluminum oxide membrane
OH
OH OH
OH
Cluster
(RO),Si(CHJ,-Y
0 '
Scheme 1
The IR spectrum of a membrane reacted with 3-aminopropylmethyldiethoxysilane has signals at 2960 and 2930 cm-' indicating the C-H vibrations of the alkyl chains. By following peak intensity, saturation after ca. 14 h can be recorded. The reactivity of the silanes depends on the substituent Y and the number of alkoxy groups. Trialkoxysilanes are more reactive than dialkoxysilanes, and the reactivity with Y = NH2 is greater than that with Y = SH. Gold and palladium particles have to be trapped inside the tubes. Two different routes for transporting the clusters into the pores were found to work equally successfully: i) a cluster solution is sucked through the membrane which acts as a kind of a molecular sieve; or ii) the modified membrane is evacuated in a flask and then covered by a solution of the cluster. UV-Vis spectra of a gold cluster containing a membrane modified by 3aminopropylmethyldiethoxysilane have maxima depending on the particle size, as is known from aqueous solutions, indicating individually fixed particles without any aggregation. Surprisingly, SH-functionalized silanes are less suitable for binding gold particles, despite the well known aurophilicity of thiols. As the 13 nm clusters have been synthesized by means of citrate as a reducer it can be assumed that citric acid molecules on the surface of the particles enable rapid interaction with the basic amino groups on the pore walls. The dispersion of the colloids on the derivatized pore walls can be best investigated by TEM. Depending on the section different TEM images are obtained. Fig. 10a shows two opened pore types of different appearance containing gold clusters. Sectioning has removed both the back and the front from the top pore whereas the front only has been removed from the lower pore and the particles lying on its rear are clearly apparent. Both situations are illustrated schematically in Fig. lob.
4.3 Nanosized Clusters on and in Supports
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Perspectives f o r Future Catalysis
1339
Figure 10. (a) TEM image of two pores in an alumina membrane the walls of which are covered with 13-nm gold colloids. Sectioning has removed both front and back of the upper pore; the other (lower) pore has been opened by removing the front only. (b) Schematic illustration of both situations.
Although catalytic reactions have not yet been performed with such cluster-tube arrangements it can be foreseen that novel types of catalysis will become available in the near future.
4.3.6 Conclusions and perspectives Quantum size behavior, substituting classical physics, is observed when metal particles reach a certain size range; at present this is manifested in ligand-stabilized metal clusters ca. 1.5 nm in diameter. Significantly smaller particles behave like molecules, larger particles have typical bulk behavior. This fundamental sizedependent electronic change when passing from a molecular complex to a ca. 1.5 nm cluster, or, vice versa, from bulk to the 1.5 nm range, must have consequences on the catalytic behavior of these three types of particle. Furthermore, the catalytic properties of larger or smaller particles can be influenced by the nature of the ligand
1340
4 Nanomaterials
molecules. We are, however, still far from understanding the details of this matter. The future of nanosized clusters in catalysis will depend on our extending fundamental research to learn more about the relationship between activity, selectivity, and particle size and shape. It is most important to point out that we are openminded about the interrelationship between particle size and catalytic behavior, which in the past has only been considered from a geometrical rather than electronical viewpoint. A catalytically active atom in a particle ‘feels’ that it is part of the bulk metal with a band structure, and an active atom within a molecule ‘feels’ that it has with localized bonds and, of course, the atom will ‘feel’ as if it is ‘something’ in between.
References [ I ] Lin Y, Finke R G, Inorg. Chem. 33 (1994) 4891-4910. [2] Lin Y, Finke R G, J. Am. Chem. SOC.116 (1994) 8335-8353. [3] Pohl M, Lyon D K, Mizuno N, Nomiya K, Finke R G, Inorg. Chem. 34 (1995) 1413-1429. [4] Hoffmann R, Angew. Chem. Int. Ed. Engl. 21 (1982) 711-724. [5] Schmid G (ed): Clusters and Colloids. From Theory to Applications, VCH, Weinheim 1994. [6] de Jongh J (ed): Physics and Chemistry of Metal Cluster Compounds, Kluwer Academic Publishers, Dordrecht 1994. [7] Smith B A, Zhang J Z, Giebel U, Schmid G, Chem. Phys. Lett. 270 (1997) 139-144. [8] Hermann M, Krcibig U, Schmid G, Z. Phys. D. 26 (1993) 1-3. [9] Ahmadi T S, Logunov S L, El-Sayed M A, J. Phys. Chem. 100 (1996) 8053-8056. [lo] van der Velden J W A, Vollenbroeck F A, Bour J J, Beurskens P T, Smits J M M, Bosman W P, J Roy. Netherlands Chem. SOC.100 (1981) 148-153. [ l l ] Schmid G, Klein N, Korste L, Kreibig U, Schonauer D, Polyhedron 7 (1988) 605-608. [ 121 Schmid G, Lehnert A, Angew. Chem. Int. Ed. Engl. 6 (1989) 780-781. [13] Schmid G, Morun B, Malm J 0, Angew. Chem. Int. Ed. Engl. 28 (1989) 778-780. [ 141 Wilenzick R M, Russell D C, Morris R H, Marshall S W, J. Chem. Phys. 47 (1967) 533 -536. [I51 Schmid G, Emde S, Maihack V, Meyer-Zaika W, Peschel St, J. Mol. Catal. A 107 (1996) 95104. [I61 Schmid G, Maihack V, Lantermann F, Peschel St, J. Chem. SOC.Dalton Trans. (1996) 589595. [ 171 Schmid G, Baumle M, unpublished results. [ 181 Joke B J, Rooney J J, Wells P B, Wilson G R, Discuss. Faraday SOC.41 (1966) 223-236. [19] Jusczyk W, Karpinski Z, Lomot D, Pielaszek J, Sobczak J W, J. Catal. 151 (1995) 67-76. [20] Cinneide A 0, Clarke J K A, J. Catal. 26 (1972) 233-241. [21] Inami S H. Wise W, J. Catal. 26 (1972) 92-96. [22] Visser C, Zuidwijk J G P, Ponec V, J. Catal. 35 (1974) 407-416. [23] Toshima N, J. Macromol. Sci. A27 (1990) 1225-1238. [24] Cinneide A 0, Gault F G, J. Catal. 37 (1975) 311-323. [25] Toshima N, Harada H, Yamazaki Y, Asakura K, J. Phys. Chem. 96 (1992) 9927-9933. [26] Liu H, Mao G, Meng S, J. Mol. Catal. 74 (1992) 275-284. [27] Turkewitch J, Kim G, Science 169 (1970) 873-979. [28] Schmid G, Lehnert A, Malm J 0, Bovin J 0, Angew. Chem. Int. Ed. Engl. 30 (1991) 874-876. [29] Schmid G, West H, Malm J 0, Bovin J 0, Grenthe C, Chem. Eur. J. 2 (1996) 1099-1 103.
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Diggle J W, Downie T C, Goulding C W, Chem. Rev. 69 (1969) 365-405. Furneaux R C, Rigby W R, Davidson A P, Nature 337 (1989) 147-149. Masudd H, Fukuda K, Science 268 (1995) 1466-1468. Hoyer P, Baba N, Masuda H, Appl. Phys. Lett. 66 (1995) 2700-2702. Huber C A, Huber T E, Sadoqi M, Lubin J A, Manalis S, Prater C B, Science 263 (1994) 800802. [35] Hulteen J C, Martin C R, J. Mater. Chern. 7 (1997) 1078-1087. [36] Schmid G, Sawitowski Th, unpublished results. [37] Hanaoka T, Kormann H P, Kro11 M, Sawitowski Th, Schmid G, Eur. J. Inorg. Chem. (1998), 807-8 12.
[30] [31] [32] [33] [34]
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.4 On the Possibility of Single Electronics Based on Ligand-Stabilized Metal Clusters Ulrich Simon
4.4.1 Introduction Advances in cluster chemistry have enabled the synthesis of molar quantities of well defined nanoscaled metal or semiconductor particles with high reproducibility."] The sizes of these particles fall within the range 1-10 nm and because these dimensions are much smaller than characteristic length scales, like the de Broglie wavelength, the mean free path, and the phase relaxation length they lead to socalled size quantization effects or quantum size effects, Although these length scales vary widely from one material to another, dimensionality determines the material properties in this size range and thus these clusters are often denoted as 'quantum dots'.[2p61 The minute size also results in unique properties with regard to electronic interparticle interaction. If such small (i.e. a few nanometers) objects, e.g. metal or semiconductor nanoparticles are arranged at small spatial distances of, approximately, 1 nm in one, two or three dimensions, electrical capacitances of less than lo-'* F are generated.[7p91This enables controlled transport of single charges between neighboring particles. This process, called 'single electron tunneling', has been recognized to be a concept fundamental to ultimate miniaturization in electronics." op121 In contrast to lithographically fabricated tunnel junctions in the sub100 nm range, which have to be cooled down to the milli-Kelvin range to suppress thermal fluctuations, ligand-stabilized metal or semiconductor clusters can be used 31 at temperatures up to room temperat~re.[~,' Thus cluster chemistry enables the control of a specific size-property relationship for many solid-state nanomaterials and helps overcome the size limitations of lithography, because it enables the tailoring of building units for nanoscale devices. It will be shown in the following discussion that ligand-stabilized metal clusters are promising building blocks for a new nanoscaled architecture which can be used to construct microelectronic devices. It will be pointed out that use of the principles
4.4 Possibility of Single Electronics
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of self-assembly by controlling intermolecular interactions will be a key feature in this development.
4.4.2 What is single-electron tunneling? Charge transport, or the passage of electric current through bulk metal connected to outer electrodes, is attributed to the motion of a huge number of free electrons. Despite the discrete nature of the charge carriers, the current flow is averaged over all the charge carriers and is continuous; resistance thus obeys Ohm’s law. If, in contrast, one deals with an isolated metallic nanoparticle lying between two electrodes the number of electrons available is always integer and can be changed by adjustment of the voltage. Because only electrons as a discrete entity will enter or leave the nanoparticle, their number is also integer. One can, therefore, think in terms of the controlled transfer of single electrons in an array built up of metallic nanoparticles. These principles are illustrated in Fig. 1. Physically the transfer of single electrons can be achieved by means of sequential quantum tunneling. Here the probability of a tunneling event is determined by an external voltage or current source applied to the circuit and by the distribution of excess electrons over the constituent sites. Therefore a microelectronic circuit could consist of a certain number of reservoirs of free electrons. These reservoirs should
electrode - conductor Figure 1. Metallic conductors placed between two metallic electrodes across which an external voltage is applied. (a) The conductor is connected directly to the electrodes. Assuming reflectionless contacts the resistance of this array obeys Ohm’s law. (b) The central conductor is macroscopic and Ec is determined by junction size. (c)The central electrode is a metallic nanoparticle and EC is determined by the size of the particle.
a>
b)
c)
H
-
- electrode
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4 Nanomaterials
be small, but well-conducting, islands separated by poorly permeable tunneling barriers. Within the reservoir handling of individual charges will be possible if the characteristic electric capacitance C of the islands is very small. Then, the charging of a nanoparticle by only one excess electron will change the electric potential. If the particle is small enough, the enhanced voltage V could prevent further charging, essentially reducing the probability of corresponding tunneling. If the potential of neighboring islands or electrodes is then changed, the probability of tunneling could be restored and the tunneling of strictly one electron will occur. This process is, in general, denoted 'single electron tunneling' (SET).["-' 21 This single electronics (SE) deals with small numbers of excess electrons on islands changing their distribution over the islands in time in a desired manner. 31 To realize this in practice two prerequisites must be i) The properties of insulating barriers separating the conducting islands from each other and from the electrodes determine the transport of electrons. If the energy barrier is high and sufficiently wide, it provides an essential decay of the electron wave function outside the island and only weak overlapping of the wave functions of the inter-island space will occur. If, also, the number of electronic states contributing to tunneling is sufficiently small then the total exchange of electrons between the islands will become negligibly small. This situation is often referred to the case of small quantum fluctuations of charge. Despite relatively complex rigorous quantum mechanical consideration, quantitatively this condition can be clearly formulated using the tunneling resistance RT as a characteristic of the tunneling junction. It must exceed the resistance quantum R, = h/e2 = 25.8 kf2, i. e. RT >> RQ
The electrons in the island can then be regarded as being localized and they behave classically, although they undergo the same thermodynamic fluctuations as every statistical variable. ii) To suppress thermal fluctuations the energy EC required to add an extra electron to the island, in terms of the characteristic thermal energy, kBT, should be given by: EC = e2/2C >> kBT
(2)
where C is the capacitance of the junction. If the metallic island is sufficiently small its capacitance can be approximated by:
where d is the diameter of the island and
E
is the dielectric constant of the in-
4.4 Possibility of Single Electronics
1345
sulating surrounding medium. The limits of the validity of this approximation with respect to very small particle diameters are discussed by van Staveren et a1."4"51 (see also Chapter 4.7 of this book). For a large central island, C is the junction capacitance according to
where A / d is the geometric parameter and E is the dielectric constant of the dielectric in a parallel-plate capacitor. In practice these conditions can be fulfilled by present-day lithographic fabrication techniques, characterized by sub- 100 nm patterning. Because the geometrical capacitances of junctions of this size are in the range 10-'5-10-'6 F, suppression of thermal fluctuations appears in the temperature range of some mK.[16] Then the SET junction is characterized by just two parameters,["] its capacitance C and its resistance RT.If the array is connected to a voltage source, the charge Q on the junction will increase with increasing voltage until a tunneling event occurs. As long as lQl < e / 2 the junction is in the Coulomb blockaded state (Fig. 2). When lQl > e / 2 tunneling is allowed and will happen after a random amount of time because of the stochastic nature of the process. This is reflected in the current-voltage characteristic of the junction, because Coulomb blockade appears at voltages below 0.5 e/C, and a corresponding shift of the tunnel current characteristic. If the array is biased by a constant current I, at lQl > e / 2 tunneling will occur making lQl jump to -e/2 and a new charging cycle starts. This leads to an oscillation of the voltage across the junction in a sawtooth-like manner with the fundamental frequency:
In this state tunneling of single electrons is correlated. This enables manipulation of charges in a circuit at the single electron level and therefore to the creation of, e.g., sensitive amplifiers and electrometers, switches, current standards, transistors, ultrafast oscillators, or generally of digital electronic circuits,[121in which the presence or absence of a single electron at a certain time and place provides the digital information. The basic element of simple or complex circuits is single-electron transistor. It consists in its simplest form of only one Coulomb island with two leads (electrodes) attached to it. If a gate electrode is capacitively coupled to the central island the current through it can be controlled by an outer gate voltage. The dependence of the transistor current on the gate voltage provides the opportunity of fabricating a sensitive device which measures directly either an electric charge on the island or the charge induced on the island by the charges collected on
1346
4 Nanomaterials
I = (U-el2C)IR
05
10
15
Ul(e/C) A
I
Time
Figure 2. (a) The dependence of the time-averaged current I on U (the I ( U ) characteristic) for a single tunnel junction. Coulomb blockade appears at voltages below e/C. (b) Time development of the junction charge imposed by a constant current source.
the gate, i.e. a highly sensitive electrometer.['61 The sensitivity of such an electrometer, which has already been achieved in practice by lithographic fabrication techniques, is approximately 10-4-10-5parts of the electronic charge and thus exceeds the charge-sensitivity of conventional devices by several orders of magnitude. The opportunity of frequency-controlled single-electron tunneling by means of an alternating gate voltage enables realization of a dc current in a lead, the value of which is determined by the frequency applied. This is of great importance for modern quantum metrology which could then determine the unit of dc current via the unit of the frequency using one universal constant, i.e. the electronic charge.['61 Then, taking into account the very high accuracy of the atomic standard of frequency, the accuracy of this current standard would be basically limited by the accuracy of the electron-transfer cycle. At present it is approximately parts of 1% for
4.4 Possibility o j Single Electronics
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currents of several PA, because of several factors which make the operation of the devices far from being as ideal as described above. Among these factors are: i) the influence of thermal fluctuations; ii) uncontrollable higher-order tunneling of an electron through all the junctions of the circuit at the same time; and iii) the rare intermission of tunneling events because of their statistical character. The solution to this problem, which necessarily involves reducing the size of structure and thereby increasing the Coulomb charging energy, seems conceivable if chemical nanostructures are taken into consideration. Accordingly, at present, ligand-stabilized metal nanoparticles with their well defined structure seem to be the most promising candidate^.^'.'^]
4.4.2.1 Single-electron tunneling in nanoparticles To satisfy the conditions described above, arrays with characteristic capacitances of 10-15-10-16F, typical values for junctions fabricated lithographically with presentday technology, one must deal with very low temperatures, which is laborious. The utilization of SET events for applications up to room temperature leads to the need to reduce the junction capacitance by at least two orders of magnitude. As predicted few years ago this can be realized by use of chemically tailored ligand-stabilized metal clusters a few nanometers in size.”] On the one hand they contain a selected ‘magic’ number of metal atoms, 13, 55, 147, 309, 561,... and perfect polyhedral shapes of high symmetry. On the other hand these clusters are stabilized by organic molecules, the ligands, which surround the metal cores and play the role of a separating layer in contacts with neighboring clusters and with conducting objects (Fig. 3). The size of cluster cores is determined by the number and the size of atoms they contain. For example, the lateral extension of the core for the cluster Au55(PPh3)12C16 is approximately 1.4 nm; for the cluster Pd561 (phen)300200 it is approximately 2.5 nm, i.e. merely one-order of magnitude larger than the size of constituent atoms. Therefore the volume of space occupied by the electrons is restricted and the number of electrons is already countable; this makes the physics of such systems more complicated and requires slight modification of the two principal conditions for ‘classical’ SET.[’31 i) The macroscopic electrostatic approach, which treats a conductor as a continuous body with infinitely thin screening depth, fails, because the latter almost reaches the size of the object. Moreover, the number of conductance electrons becomes small. As a result electron-electron interaction is not completely screened out, and so the concept of electric capacitances no longer strictly works.
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4 Nunomuterials
Figure 3. Schematic diagram of the ligandstabilized cluster Auss(PPh3)&12. The metal core, a regular polyhedron, has a diameter of 1.4 nm and it is shrouded by umbrella-shaped PPh3 ligands to give a total diameter of 2.1 nm (reproduced, with permission, from Ref. 7).
In this circumstance calculation of the charging energy should be achieved by direct counting of the energy of interacting charges, although the Coulomb energy can be still be roughly described with the elementary formula mentioned above. Then the capacitance already denotes an amount which generally depends on the total number of interacting electrons occupying the cluster. ii) The small size of a cluster changes its energy spectrum, essentially because of the quantum size effect described above.['81In contrast to a conductor with a quasicontinuous density of states a small metal cluster can have a discrete energy spectrum. The density and distribution of the energy levels depends on the size and shape of cluster and tunnel junctions, where one of its electrodes is a small cluster; it can, therefore, hardly be described by such a simple parameter as a constant tunneling resistance RT. The two peculiarities of metal clusters discussed above do not, however, eliminate charging effects. Theory and experiment have shown that basic results of singleelectron tunneling still qualitatively remain and acquire new features.['-'91 For example, the I ( U ) characteristic of a double barrier junction with a central quantum dot reflects the fine structure of the energy spectrum besides the Coulomb staircase.['] The situation will, furthermore, become more complicated when the characteristic time RTC becomes as short as the tunneling time itself, the uncertainty time and the characteristic time of the energy relaxation inside the d ~ t , [ ~ so, ~ ~ , ~ that the distribution of electrons is no longer the pure Fermi distribution. The single electron charging effect, which turns out to be a classical size effect, and the quan-
4.4 Possibility of Single Electronics
1349
tum size effects do not, however, contradict each other, and can co-exist and provide the opportunity for Coulomb blockade and ballistic transport within the same system.
4.4.3 Evidence of SET in ligand-stabilized metal clusters 4.4.3.1 Single cluster properties The first results obtained on single ligand-stabilized metal clusters have been reported by van Kempen et al. [8-201They performed scanning tunneling spectroscopy at 4.2 K on a Ptyjg(phen)36020 cluster deposited from a droplet of an aqueous solution of the clusters on a flat gold substrate (Fig. 4). The I ( U ) characteristics exhibit charging effects, which indicates that the ligands are electrical insulators, as desired. They act as tunnel barriers between the cluster and the substrate. The experimentally observed charging energy varies from 50 to 500 meV; 140 meV would be expected from the classical formula for Ec of a spherical particle with a dielectric constant E, = 10 for the surrounding ligands (see
STM tip
3 s
I
Ligand shell Pt309-cluster
2
o -100
-200 -08
-04
0 04 wltage (v)
0.8
Figure 4. (a) A single-ligand-stabilized Pt3og-cluster between STM tip and a Au(l11) facet. In this double junction geometry the cluster is surrounded by the ligand shell which acts as tunnel barrier. (b) A measured I ( U ) characteristic of this geometry at 4.2 K (curve 1). Also indicated are two calculated curves, one without level splitting of the cluster (2) and one with a level splitting of 50 meV (3), (reprinted from H. van Kempen et ul. Physica B 204 (1995) 51, ‘Small metallic particles studies by scanning tunneling microscope’ with kind permission of Elsevier Science NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands). ~
1350
4 Nanomuteriuls
Eq. 3 ) . The authors discussed different reasons for the variation in Ec. Since the clusters have different facets (squares and triangles) EC will depend on the exact way in which the cluster is oriented on the substrate. Furthermore the ligands, which for simplification have been assumed to be a spherical covering of the cluster, might have different orientations varying from cluster to cluster relative to the underlying substrate, thus resulting in a different probability of tunneling between the cluster and the substrate. Finally the amount of residual solvent molecules on the cluster surface or on the ligand shell might differ for different clusters. Additional fine structure on the charging characteristic has been observed for the same clusters; this might be expected from a discrete electron level spectrum for the cluster, as result of quantum size effects. The effect of discrete levels on the charging characteristics has been treated theoretically by Averin and Korotk~v,[’~] who extended the existing ‘orthodox’ theory of correlated SET in a double normal-metal tunnel junction to a nanoscaled central electrode. According to this the I ( U ) characteristics should exhibit small-scale singularities reflecting the structure of the energy spectrum of the central electrode, i. e. the cluster. Furthermore the energy relaxation rate becomes evident because of the small recharging time RTC,resulting from the small junction capacitance. Whereas the theoretical prediction of the level splitting is approximately 8 meV spectral fit gave values between 20 and 50 meV. Although different reasons are discussed in the paper cited, the measurements reflect the presence of discrete levels in the spectrum of the 2.2 nm Pt3o9-clusters. On smaller (1.4 nm) ligand-stabilized metal clusters, i. e. Au55(PPh3)&16, scanning tunneling spectroscopy (STS) has been performed up to room temperature. Three-dimensional compacts taken for the clusters at room temperature show I ( U)-curves[221 that reflect the occurence of single-electron-transfer processes. In this investigation, however, neither the vertical nor the lateral arrangement of the clusters is well defined, as would be the case for a one-, two- or three-dimensional superlattice. As a consequence, the charging energy is distributed over the different cluster sites. To avoid this problem Chi et al. reported STS on Au55 monolayers prepared on various substrates[231(Fig. 5 ) by means of a two-step self-assembly (SA) process and by a combined Langmuir-Blodgett/SA-process. The spectroscopy gives clear evidence of Coulomb blockade at room temperature originating from the double barrier at the ligand-stabilized cluster as the central electrode. The capacitance of the cluster/substrate-junction was calculated to be 3.9 x F. This value is in agreement with the value of the cluster capacitance previously determined by temperature-dependent impedance rneas~rements.[~] These results are, furthermore, in good agreement with capacitance data obtained from self-assembled gold nanoparticles on a dithiol-modified Au surface, reported by Andres and c ~ w o r k e r s . [51~Here ~ ’ ~ tunneling spectroscopy has been performed on 1.8 nm Au particles which were grown in the gas phase and a cluster-substrate caF was obtained. Thus, the small capacitance enables the pacitance of 1.7 x observation of Coulomb blockade phenomena at room temperature.
4.4 Possibility of Single Electronics
1351
STM tip
T
Monolayer of A~~~-clusters Substrate
- _ _ - _ _ ~ _ _ ~ -
Figure 5. (a) Monolayer of a ligand stabilized Au55 cluster on a conducting substrate. (b) Z ( U ) characteristic of a single-ligand-stabilized Au55 cluster at 90 K. The junction capacitance was calculated to be 3 x F by fitting.
4.4.3.2 One-dimensional arrangements As far as the author is aware experimental data relating to the electrical properties of one-dimensional (1D) arrays of ligand-stabilized nanoparticles are not available, mainly because of the difficulty is the application of suitable contacts for transport measurements, and so their electrical properties have only been studied theoreti~ally.[~~J~] Most recently Schmid et al. reported the intercalation of gold clusters and colloids into the quasi-parallel channels of anodically generated porous alumina.[281 For example, pores of 7 nm diameter were filled electrophoretically by ligand-stabilized Ausj clusters. Fig. 6 shows the alignment of 16 or 17 clusters within a pore 70 nm long; this might provide a means of creating 1D quantum wires. So far the diameter and parallel nature of the channels and the packing of the clusters have shown no periodicity in the strict sense, i.e., disorder cannot be avoided. An exact analytical solution of the effect of disorder has been found in terms of Green's function (GF) for the potential distribution in a finite 1D array consisting of N-ligand-stabilized metal clusters (Fig. 7).[26.271 The GF approach enables the formulation of the so-called partial 'solitary' problem of small mesoscopic tunnel junctions similarly to the problem of the behavior of an electron in 1D tight-binding and in a set of random delta-function models. Discussion of 1D metal cluster arrangements has assumed either that: i) the capacitance C is the same for all junctions whereas the self capacitance Co can fluctuate from site to site because of a finite size distribution; or ii) Co is constant and C can fluctuate from site to site owing to packing defects.
I352
4 Nunomaterials
Figure 6. (a) Transmission electron micrograph of the ID arrangement of ligand-stabilized Auss clusters in porous alumina. Because the diameters of the pore channel and cluster were 7 nm (approximately) and 4.2 nm, respectively, a helical structure was formed. (b) Schematic representation of the probable arrangement. (Reprinted, with kind permission, from Ref. 28.)
Electrode
1
2
3
.
N rr
Figure 7. (a) 1D array of N ligand-stabilized metal clusters and (b) the corresponding circuit equivalent.
4.4 Possibility of Single Electronics
1353
It has further been assumed that the metal nanoparticles have a continuous density of states, i.e. quantum size effects; the influence of this on the capacitance has been excluded from consideration. These simplifications illustrate the practical implications of disorder with respect to future applications in microelectronics, but generally, they are not necessary assumptions for the method discussed. The main results are that a reduction in particle size at one position of the array increases the potential at this point which may lead, at least, to localization, i.e. the single excess electron in the array might be trapped. At a packing defect, which affects the inter-particle capacitance at one point and acts like an inhomogenity, the soliton will interact with its mirror-image soliton (or anti-soliton) and will therefore be attracted. Concerning the practical use of this method, it was emphasized that the total reflection amplitude obtained from these calculations is directly related to the Landauer resistance,[291and reflects the electrical characteristics of such multijunction arrays.
4.4.3.3 Two-dimensional arrangements Janes et al. reported charge transport through a network of 4-nm gold nanoparticles interconnected by 1,4-di(4-isocynaophenyIethynyl)-2-ethylbenzeneligands. The ligand is a conjugated, rigid molecule, giving an approximate interparticle spacing of 2.2 nm.L301 The assembly in two-dimensions results from the deposition of the nanoparticles from solution, followed by reactive addition of the linking molecules. Electrical measurements were performed on layers deposited on an SiOz-supported GaAs wafer with gold contacts separated by approximately 500 nm and 450 nm. The I ( 17) characteristic of the arrays is linear over a wide voltage range. AsF, the total dot capacitance would be suming a dot-to-dot capacitance of 2 x 1.2 x F, if each cluster is assumed to have six nearest neighbors. The corresponding charging energy will thus be approximately 11 meV, which is only ca. half of the characteristic thermal energy at room temperature. Therefore a Coulomb gap at room temperature could not be observed, although the capacitance is two orders of magnitude smaller than in lithographically made junctions. In contrast, the evidence of charging energy in comparable samples with twodimensional cluster linkage has been deduced from low-temperature DC measure. 8).~ ~ ~ ments by Andres et ~ 1(Fig. According to the Arrhenius relationship:
where G , is the conductance at T + m, EA is the activation energy, and kB is Boltzmann’s constant, Coulomb charging behavior with a charging energy (which corresponds to the activation energy) of EA = 97 meV was observed. The interparticle resistance was calculated from G, and revealed to be 0.9 M a , from which
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4 Nunomuteriuls
I
0
1
2 Voltage (V)
3
4
Figure 8. (a) Brightfield transmission electron micrograph of a monolayer film of 3.7-nm gold clusters supported on a thin flake of MoS2. (b) Measured I ( U ) characteristic of a linked cluster network at different temperatures. (Reproduced, with kind permission, from Ref. 24).
a single molecule resistance of 29 MQ was predicted. This value is in good agreement with the prediction of 43 M a obtained from Huckel MO calculations.
4.4.3.4 Three-dimensional cluster networks The chemical tailoring of the charging energy in three dimensional networks of metal clusters has recently been d e ~ c r i b e d1,321 . ~ ~The basic idea was to increase the
4.4 Possibility
of'Sinyle Electronics
1355
Figure 9. Schematic representation of the insertion of bifunctional spacer molecules into a dense packing of ligand-stabilized Pd561 clusters. The stretching of the packing results in a 3-D network.
interparticle spacing by interconnecting the clusters with spacer molecules of defined length. This should lead to an increase in the charging energy, i.e. a reduction of the electrical capacitance between the clusters (Fig. 9). Most recently the insertion of bifunctional amines into arrangements of Pd561 (phen)360200 clusters was reported.r321The spacing started with deoxygenation of the cluster by hydrogen in a water-pyridine solution at room temperature with formation of H202. The oxygen-free cluster Pd561 (phen)3o provides active surface sites which can be coordinated by the NHz-groups of 4,4'-diamino- 1,2-diphenylethane, used as the spacer molecule. The spacing procedure leads to an insoluble precipitate with a three-dimensional cluster linkage; the interparticle spacing of this is greater than the closest sphere packing of the unmodified cluster. As described above, the charging energy Ec, which is dependent on the cluster size and on the inter-particle capacitance C, can be determined directly from the temperature-dependence of the dc conductivity. Previous investigation of the electrical properties of these clusters has shown that even at high temperatures thermally activated electron hops instead of hops of variable range dominate charge transport through the samples.['41 Whereas compact packing of the cluster material results in an activation energy of 20 meV, insertion of the spacer molecules increases this to 50 meV. The corresponding capacitance decreases from an initial value of 4.0 x F to 1.6 x lo-'* F. The specific conductivity follows the same trend, the volume fraction of the metal in the total sample volume decreases. Comparable results where obtained by Schiffrin et al.[311 who investigated 2.2- and 8.8-nm colloidal gold nanoparticles with interconnecting alkyldithiols of different length.
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4 Nanornuteriuls
4.4.4 Clusters as building blocks for SET devices 4.4.4.1 Doubts and chances As described above the progress of present-day lithographic techniques guided by the miniaturization of conventional electronic circuits fails to fulfil the requirements of single electronics at elevated temperature, a requirement which extends to a size range of a few nanometers or l e s ~ . [ ~ , ’ ~ ] Consequently the use of ligand-stabilized metal clusters should solve this problem and embody suitable building blocks for a new nanoscale architecture. Furthermore, new techniques to produce a defined organization of clusters must be developed to build up single-electron circuits of different complexity. So far, it has become evident that utilizing the principles of self-assembly by controlling intermolecular interactions, one of the main interests of supennolecular chemistry, will be a key feature in this development.[’31This relatively simple theory of tunneling, which has been successfully applied to larger arrays, might now be applied with much caution; it should be certainly revised for clusters, where quantum size effects may appear. In particular, standard diffusive single-electron transport, which arises from the particle nature of electrons, could be converted into ballistic transport or even into resonant tunneling reflecting the wave nature of electrons. These could drastically modify the Coulomb blockade, but still leaving the important role of Coulomb repulsion. Despite these problems, it should be emphasized that the transition to the scale of clusters promises to provide an incredibly high density of functional units on a chip, which is extremely important for computer circuits.[16]Thus, assuming a ‘3-nanometer design rule’ (assumed to be the approximate size of a cluster including its ligand-shell) and the need for 10 by 10 clusters for every reliable gate, for two-dimensional architecture the number of gates on a 1 cm2 chip might be ca. 10”. The extension of such a hypothetical structure into the third dimension would increase the number to ca. 10l6. This is a rather cautious estimate based on a traditional paradigm for information processing. On the other hand, a locally-interconnected architecture, such as cellular automata with neural networks, seems to be more appropriate for cluster networks. This will give rise to new problems in design and principles of operation, but solving these could provide a bridge to structures on the true atomic scale.
4.4.4.2 First devices Successful research utilizing Coulombic charging in a SET-transistor based on chemically prepared nanoparticles has been reported. This simplest circuit reveals
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4.4 Possibility ojSing1e Electronics
the peculiarities of SET because it includes only one nanoparticle. Sato et al.r291 reported the electrical characteristics of the first SET-transistor utilizing charging effects on single ligand-stabilized gold nanoparticles. They developed a device which was fabricated by means of metal electrodes formed by electron beam lithography to which a self assembled chain of colloidal gold particles was connected. Interparticle connection and connection to the electrodes was achieved by linkage with bifunctional organic molecules. The latter provide the tunnel barriers and between the particles the self-assembling nature of the gold nanoparticles helped to overcome the size limitations of lithography. The fabrication is described as follows. A sub-monolayer of gold nanoparticles with an average diameter of 10 nm was deposited on a thermally grown SiOz surface on a Si substrate by using alkylsiloxane molecules as the adhesion agent. The gold particles retained their ionic charges on the surface because they were obtained by the citrate method.[331Thus nanoparticle deposition stops automatically before it reaches close-packing density, leaving an interparticle spacing of 10-50 nm. 1,6Hexanedithiol was then added to interconnect the particles with a more regular spacing. A second immersion into a gold particle solution increased the coverage; again the distance between the particles was maintained by the dithiol molecules. The second layer filled the gaps between the particles of the first, forming chains of 2 4 particles. When this procedure was performed on an SiOz substrate, equipped with source, drain, and gate metal electrodes defined by electron-beam lithography, the particles form a chain of at least three particles bridging the gap between the outer driving electrodes. Because not all the steps of this procedure could be precisely controlled the number of nanoparticles in the bridge chain differed from device to device. Despite this, electron conduction dominated by single electron charging indicated by a Coulomb gap could be observed up to 77 K. The capacitance of all junctions in the chain was 1.8 - 2 x 10-” F and the calculated Coulomb gap was in reasonably good agreement with the value of 150 mV obtained from the measured Z( V )characteristics, which was systematically squeezed, when a gate voltage of -0.4 to 0.4 volt was applied. This proved that the device functioned as a single-electron transistor. The transmission function of electrons across the dithiol ligands was calculated by use of Green’s function-based method, as proposed by Samanta et a1.;[341 the resistance R per molecule could then be obtained by use of the Landauer formula, i. e. :
where T ( E g )is the transmission function, i.e. T z e~p[-(2rnE,)”~/h].Assuming a barrier height Eg z 2.8 in the dithiol molecules, the resulting resistance was estimated to be R z 30 GR. More recently Klein et al. used a 5.5-nm CdSe nanoparticle to fabricate a singleelectron transistor.[351The nanoparticle is bound to two closely spaced ( 5 nm)
-
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Figure 10. (a) Scanning electron micrograph of a chain consisting of three gold nanoparticles incorporated in a system of source, drain, and gate metal electrodes. (b) Schematic diagram of the electrode pattern defined by electron beam lithography. (c) Drain current (ID)-source-drain voltage ( V ~ Dcharacteristics ) of the SET transistor measured at 4.2 K for different gate voltages ( VG). (Reproduced, with kind permission, from Ref. 29.)
gold electrodes by means of bifunctional molecules. The function of this SET transistor element was proved by the dependence of the current through a single nanoparticle from a gate voltage, which was applied to an underlying gate electrode. Instead of chemically tailored metal or semiconductor nanoparticles, however,
4.4 Possibility qf Single Electronics
1359
STM tip
Figure 11. SET transistor, based on 1,7-(CH3)2-1,2C2BloH9TI(OCOCF3)2carborane molecules in a LB film on a gold electrode array deposited on graphite ( HOPG).[361
film -Au --. .- -A1203 --- --HOPG
even cluster-like molecules of the same size seem to be suitable building blocks in the fabrication of SET devices. Soldatov et al. reported the fabrication of a SET transistor, which worked at room temperature,[361from a carborane cluster. They deposited Langmuir-Blodget (LB) monolayers of the spherical carborane 1,7(CH3)2-1, ~ - C ~ B ~ O H ~ T I ( O Con O CaFpreformed ~)~ gate electrode system, which was formed by the electron lithography technique. It consists of thin and narrow bilayer strips in which 50 nm gold on 50 nm A1203 is deposited on a graphite sub0 The electron transport strate (HOPG) with a strip width and distance of ~ 4 0 nm. through the layers was probed by a STM tip at room temperature (Fig. 11). When the tip was positioned above the single carborane molecules, a transistor, consisting of a double junction (tip/molecule/HOPG) and closely situated gate electrodes (Au strips) was obtained. In this arrangement the junction capacitance F. was estimated to be 1 x In the discussion of their results the authors pointed out that the explanation of the experimental data they obtained has to involve the discrete nature of the electronic structure of the carborane molecules, which act as the central electrode.
4.4.5 Summary and outlook To summarize this chapter, it has become evident that chemically tailored metal and semiconductor clusters in the sub-10 nm range are suitable building blocks for SET devices. Their minute size and the surrounding insulation barrier, which is defined by the size and the chemical nature of the ligand shell, make ultra-small electrical capacitances in the range of F accessible. Such small capacitances overcome the size limits of current technologies hopefully enabling the use of single cluster electronics at room temperature.
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4 Nunomaterials
Array
Dimension
OD
Function
SET Transistor Elsdnde
7
Eklmde
Gale
ID
SET Turnstile
I Gate
2D
I
3D
Cluster Solids, Superlattices and Networks
Figure 12. Potpourri of SET arrangements and device ideas based on ligand stabilized metal clusters.
This opens a broad field of possible applications where the use of clusters seems to be profitable. It is not the purpose of this chapter to detail specific microelectronic devices, because this would far exceed the knowledge of the author. Here the reader should be guided by other comprehensive reviews.[1op121 According to the ideas outlined there, the following examples could be tentative suggestions as to how to construct simple devices with metal clusters, suitable for experimental examination of SET effects and for special purpose applications (Fig. 12). Single-cluster arrays should be denoted zero-dimensional (OD), because they are built up by means of single ‘quantum dots’. As already explained in Section 4.4.4.2 such arrays have already been obtained. This element works as a single electron transistor, which is a key element for various analog and digital circuits. The electrode system connecting the cluster to the outer world must be fabricated by lithographic techniques; self-assembly of the clusters into the residual ‘space-gap’ makes the step down to the molecular scale. Because the properties of the central electrode are adjustable chemically this arrangement could be suitable for the utili-
4.4 Possibility of Single Electronics
1361
zation of correlated SET in supersensitive electrometry, in which the current through the system should be sensitive to sub-single-electron changes of the charge injected on to its gate electrode.['61 One dimensional arrays of mesoscopic tunnel junctions, built up from clusters, have been treated t h e o r e t i ~ a l l y [(see ~ ~ ,also ~ ~ ~Section 4.4.3.2). There are different means of arranging clusters in one dimension, e.g. inclusion into 1D channel structures,[261connection to rod-like molecules, such as DNA strands,[371 or deposition on to preformed substrate^.[^,^^^ The preparation techniques will cover a broad field ranging from self-assembly or LB techniques, through nanoreactor synthesis, up to electrodeposition. Delsing has pointed that according to theory tunneling events in arrays without an external ac signal should be self correlated. To build up an array which make this inherent property observable would be of great importance. If a gate electrode is coupled capacitively to one or several clusters in the middle, the array will be a turnstile for single electrons, where the time correlation is determined by the outer ac signal applied to the gate.[401The large number of identical junctions should reduce the amount of quantum leakage and the device should be less sensitive to background charge.r391 Two-dimensional arrangements might be monolayers of clusters on a suitable substrate or two or more coupled 1D arrays. While layers are accessible uiu selfassembly, LB, or electrodeposition, coupled arrays could be obtained by filling clusters into the parallel channels of a crystalline nanoporous solid. 2D networks of clusters might be precursors for simple neural networks, utilizing the Coulombic interaction between ballistic electrons in a 2D electron gas. This concept has been discussed by N a r ~ s e [ ~and ' ] in general introduces new possibilities for the interconnection approach in various fields, e.y. parallel processing and quantum functional devices. A coupled array consisting of two or more parallel ID arrays, which are isolated from each other, could work as a quantum current mirror or even as a quantum current transformer.[391Because each cluster of the array is capacitively coupled to a corresponding cluster in the parallel chain, a current flow in one array could induce an equally large current of the opposite sign in the other array. Three-dimensional arrays such as networks or crystalline solids should be suitable for studying the properties of superlattices or of Coulombic interaction, as in 2D arrays, but on an even more complex scale involving the third dimension. Despite these promising perspectives one must be aware that the utilization of the minute size requires new and comprehensive fabrication and addressing techniques. Extension of the present-day technologies to smaller structures or the development of techniques which treat chemical nanostructures with conventional technologies will not reach the true molecular size scale and will therefore waste much of the advantage of these structures. But even in arrays which are preformed by lithographic techniques, the self-assembling nature of chemical nanostructure helps overcome current size limits.
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Although there is nothing unbelievable about the concept, the technical relevance of chemical nanostructures in single electronics still needs to be proved. The creation of more complex circuits with low error rates requires automation of design and fabrication. Simulation within the semiclassical approach and quantitative calculations are still a problem, even for relatively simple systems['61 and supermolecular chemistry must be developed to organize clusters in the desired manner. Solving both problems will need much interdisciplinary effort and is awaiting those scientists who are willing to cross the frontiers between the classical subjects of science.
Acknowledgments The author is grateful to V. Gasparian, G. Schon, and H. Wiggers for helpful discussions and careful reading of the manuscript, which was completed during a current national research project 'Metal and metal chalcogenide clusters as building blocks for quantum devices', supported by the Bundesminister fur Bildung, Wissenschaft und Forschung (BMBF) under contract 03N 1012A7, which is appreciated.
References [ 11 G. Schmid (ed.), Clusters and Colloids, VCH Weinheim, Germany (1994) [2] U. Simon, G. Schon and G. Schmid, Angew. Chem. Int. Ed. Engl. 2, 32, 250-254 (1993) [3] H. Weller, Angew. Chem. 105, 43-55 (1993); ibid.108, 1159-1161 (1996) [4] G. Schmid, Chem. Rev. 92, 1709-1727 (1992) [5] N. Herron, Y. Wang, M. M . Eddy, G. D. Stucky, D. Cox, K. Moller and T. Bein, J. Am. Chem. SOC.111, 530 (1989) [6] L. E. Brus, J. Chem. Phys., 79, 5566-5571 (1983) [7] G. Schon and U. Simon, Colloid Polym. Sci. 273, 101-117 (1995); idid. 273, 202-218 (1995) [8] H. van Kempen, J. G. A. Dubois, J. W. Gerritsen and G. Schmid, Physica B 204, 51-56 (1995) [9] R. P. Andres, J. D. Bielefeld, J. I. Henderson, D. B. Janes, V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney and R. G. Osifchin, Science 273, 1690-1693 (1996) [lo] K. K. Likharev, IBM J. Res. Develop. 32, 144-157 (1988) [ 111 L. S. Kuzmin and K. K. Likharev, Jpn. J. Appl. Phys. 26, Suppl. 3, 1387 (1987) [12] M. H. Devoret and H. Grabert (eds.), Single Charge Tunneling Coulomb Blockade Phenomena in Nanostructures, NATO AS1 Series Vol. 294, Plenum Press, New York (1992) [13] U. Simon and G. Schon, in H. Nalwa (ed.), Handbook of Nanostructures and Nanotechnology, Academic Press, San Diego, in press (1999) [14] M. P. J. van Staveren, H. B. Brom and L. J. de Jongh, Physics Reports 208, 1-96 (1991) [ 151 M. P. J. van Staveren, H. B. Brom, L. J. de Jongh and Y. Ishii, Phys. Rev. B 35, 7749 (1987) -
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[I61 D. V. Averin and K. K. Likharev in H. Grabert and M. H. Devoret (eds.), Single Charge Tunneling - Coulomb Blockade Phenomena in Nanostructures, Nato AS1 Series Vol. 294, Plenum Press. New York. (1992), pp. 311-332 [17] M. H. Devoret and H. Grabert in H. Grabert and M. H. Devoret (eds.), Single Charge Tunneling - Coulomb Blockade Phenomena in Nanostructures, Nato AS1 Series Vol. 294, Plenum Press, New York. (1992), pp. 1L20 [IS] W. P. Halperin, Rev. Mod. Phys. 58, 533-606 (1986) [I91 D. V. Averin and A. N. Korotkov, J. Low Temp. Phys., 3/4, 173-185 (1990) [20] J. G. A. Dubois, J. W. Gerritsen, S. E. Shafranjuk, E. J. G. Boon, G. Schmid and H. van Kempen, Europhys. Lett. 33, 279-284 (1996) [21] V. Gasparian, M. Ortuno, G. Schon and U. Simon, in H. Nalwa (ed.), Handbook of Nanostructures and Nanotechnology, Academic Press. San Diego, in press [22] R. Houbertz, T. Feigenspan, F. Mielke, U. Memmert, U. Hartmann, U. Simon, G. Schon and G . Schmid, Europhys. Lett., 28, 641-646 (1994) [23] L. F. Chi, M. Hartig, T. Drechsler, Th. Schaak, C. Seidel, H. Fuchs and G. Schmid, Appl. Phys. A 66, 187-190 (1998) [24] R. P. Andres, Th. Bein, M. Dorogi, S. Feng, J. I. Henderson, C. P. Kubiak, W. Mahoney, R. G. Osifchin and R. Reifenbereer. Science 272. 1323-1325 (19961 [25] M. Dorogi, J. Gomez, R. Osifchin, R. P. Andres and R. Reifenberger, Phys. Rev. B 52. 90719077 (19951 [26] V. Gasparian and U. Simon, Physica B. 240, 289-297 (1997) [27] U. Simon and V. Gasparian, Phys. Stat. Sol. (B), 205, 223 (1998) [28] G. Hornyak, M. Kro11, R. Pugin, T. Sawitowski, G. Schmid, J.-0. Bovin, G. Karsson, H. Hofmeister and S. Hopfe, Chem. Eur. J. 3. 1951-1956, (1997) [29] T. Sato, H. Ahmed, D. Brown and B. F. H. Johnson, J. Appl. Phys. 82, 696 (1997) [30] D. B. Janes, V. R. Kolagunta, R. G. Osifchin, J. D. Bielefeld, R. P. Andres, J. I. Henderson and C. P. Kubiak, Superlattices and Microstructures 18, 275-281 (1995) [31] M. Brust, D. Bethell, D. J. Schiffrin and Ch. J. Kiely, Adv. Mater. 7, 795-797 (1995) [32] U. Simon, R. Flesch, H. Wiggers, G. Schon and G. Schmid, J. Mater. Chem., 8, 517-518 (1998) [33] M. Mabuchi, T. Takenaka, Y. Fujiyoshi and N. Ugeda, Surf. Sci. 119, 150 (1982) [34] M. P. Samanta, W. Tian and S. Datta, Phys. Rev. B 53, R7626 (1996) [35] D. L. Klein, R. Roth, A. K. L. Lim, A. P. Alivizatos and P. L. McEuen, Nature, 389, 699-701 (1997) [36] Soldatov, E. S., Khanin, V. V., Trifonov, A. S., Gubin, S. P., Kolesov, V. V., Presnov, D. E., Iakovenko, S. A. and Khomutov, G. B., Moscow State University, Department of Physics, 111 (1996) [37] A. P. Alivizatos, K. P., Johnsson, X. Peng, T. E. Wilson, C. J. Loweth, M. P. Bruchez, Jr. and P. G. Schultz, Nature, 382, 609-611 (1996) [38] G . M. Francis, I. M. Goldby, L. Kuipers, B. von lssendorff and R. E. Palmer, J. Chem. SOC. Dalton Trans., 665-671 (1996) [39] P. Delsing in H. Grabert and M. H. Devoret (eds.), Single Charge Tunneling Coulomb Blockade Phenomena in Nanostructures, NATO AS1 Series Vol. 294, Plenum Press, New York. (l992), pp. 249-274 [40] G. Y. Hu and R. F. O’Connell, Phys. Rev. B 49, 16773-16776 (1994); Phys. Rev. Lett. 74, 1839-1842 (1995) [41] Y. Naruse, IEICE Trans. Electron. E76-C, 9, 1362-1366 (1993) -
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.5 Strategies for Assembling Pd and Pt Atoms* Michael N. Vargafik, Natalia Yu. Kozitsyna, Natalia V. Cherkashina, Rimma I. Rudy, Dmitry I. Kochubey, and Ilya I. Moiseev
4.5.1 Introduction One of the challenging aims of cluster chemistry is to elucidate the factors controlling the formation of cluster molecules and small metal crystallites. Despite remarkable achievements in the synthesis and structural characterization of metal clusters, the pathways to the assembly of large numbers of metal atoms in the course of the synthesis of high nuclearity metal clusters remains rather mysterious. Some insight into this problem has been gained by recent studies of so-called giant clusters of palladium and platinum. The term 'giant cluster' appeared in the literature in the middle of the 1 9 8 0 ~ , [ ' ~ ~ ] when a race for increasingly larger metal clusters overcame the limit which had hitherto prevented the characterization of these materials by conventional singlecrystal X-ray analysis. Besides the experimental and computational problems of Xray structural studies, synthetic chemists faced progressively increasing difficulties in the preparation of cluster single crystals as their metal skeleton contained more than 40-50 metal atoms and the size of the metal unit exceeded 20-25 A. By this time, substantial experience in the synthesis of low-valence noble metal complexes in the form of non-crystalline, colloid-like samples has been accumulated. For instance, a series of amorphous, high-molecular weight palladium complexes, which had remarkable catalytic capability, had been obtained starting from low-nuclearity Pd( I) c l ~ s t e r s [and ~ ~ ~Pd( ' 11) complexes.[5-'21 An early attempt to establish the nature of such species was made by G. Schmid et al. in the study of the non-crystalline phosphine cluster A~55(PPh3)12C16.[~~] The structure of this cluster was proposed on the basis of data from high-resolution
* Dedicated to the memory of our friend Professor Kirill I. Zamaraev in recognition of our longterm collaboration.
4.5 Strategiesjor Assembling Pd and Pt Atoms
1365
electron microscopy, electron diffraction, and Mossbauer spectroscopy combined with elemental analysis and molecular weight measurement~.['~.'~~ The basic idea of this structure was derived from Chini's hypothesis" 'I that arrangement of the metal atoms in the skeleton of a large cluster might be similar to the atoms packing in a bulk metal (Scheme 1 ) and the cluster metal skeleton can be shaped as a polyhedron typical of metal crystallites (Scheme 2). Somewhat later the structure of a Pd561 phenanthroline cluster was determined by a similar approach."." Since this work the methods of investigation of the structure of large metal clusters have changed from primarily single-crystal X-ray diffraction studies to indirect methods such as transmission electron microscopy (TEM, HREM), electron diffraction (ED), scanning tunneling microscopy (STM), Mossbauer spectroscopy, small-angle X-ray scattering (SAXS), and extended X-ray absorption fine structure spectroscopy (EXAFS). By use of such strategies several non-crystalline giant clusters have been structurally characterized during the past decade." 7p201 Thus, a series of Chini type clusters have been identified: i) 13-atom (one-layer) clusters, e.g., [A~,3(diphos)6](N03)4[~~] with icosahedral and [Rh13(C0)24H3Izp[221 with anti-cuboctahedral metal cores; Rh55(PPh3)12C16, ii) 55-atom (two-layer) clusters, e.g., A~55(PPh3)12C16,[~~,~~~ Pt55[A~( B~')3]12C120,['9,241 Pt55 (phen)12(OAc)30(AcOH)33 ,c2 '1 and Ru55[P( Bu')3]12C120[~~] with cuboctahedral metal cores; iii) 309-atom (four-layer) cluster Pt3o9(phen*)3603ok10 (phen* = sodium 4,7-bisphenylsulfate-],lo-phenanthroline)with a cuboctahedral metal core;[271 iv) 561-atom (five-layer) clusters such as Pd561 (phen)60(OAc)lgo with icosahedral,['%2.' 71 and [ Pd561 (phen)60060](p F 6 ) 6 0 , [ ' ~ ' ' andPd561 ~'~~' (phen)3g0,200['~.~~~ with cuboctahedral metal cores; and v) 1415-atom (seven-layer) Pd1415(phen)600-1100and Pd2057(phen)840-1600 (eightlayer) Pd-phen giant clusters with cuboctahedral metal c o r e ~ . [ ' ~ ' ~ ~ ] Only for the first members of this series, the 13-atom clusters, has the molecular structure been determined by single-crystal X-ray analysis. The other clusters have been characterized by the indirect methods mentioned above. In this review the results of such combined studies are presented, using as examples palladium and platinum giant clusters with phen, dipy, and phospinefphospide ligands.
First layer (i = 1): N, = 10; I* + 2 = 12 Total number: Nx = I + 12 = 13
+
Second layer (i = 2): Ni = 10:2* 2 = 42 Total number: Nz = 13 + 42 = 55
Third layer (i = 3) : N , = 10;3* + 2 = 92 Total number: Nz = 55 + 92 = 147 d) Fourth layer ( i = 4 ) . . . . . .NX = 147 + 162 = 309
e) Fifth layer (i = 5 ) . ..... Nx = 309 + 252 = 561
.................................... Continuing this procedure, one obtain a series of “Chini’s Magic Numbers”:“
5*161
Nx = 13; 55; 147; 309; 561; 923; 1415; 2057; 2869 ...... m= 1 2 3 4 5 6 7 8 9
......
A number of spheres, N,, in each ith close-packed layer: Ni = 10i2 + 2 Total number, Ns, of spheres close-packed in m layers around a central sphere:
N s = $(lorn3 + 15m2 + l l m + 3) B.K. Teo, N.J.A. Sloan (Ref. [16]) Scheme 1. A schematic representation of the formation of a cluster metal core (cross-section is shown): a) from a single metal atom (shadowed circle) and the first 12 metal atoms surrounding the first atom b) - the same for the stage of 13-atomic cluster surrounded by 42 atoms c) the same for the stage of 55-atomic cluster surrounded by 92 atoms d), e) the same for the next stages. ~
~
~
4.5 Strategies for Assembling Pd and Pt Atoms
Cuboctahedron
1361
Icosahedron
Anti-
cuboctahedron
Packings:
j c . c.
hexagonal
non-crystallographic
Scheme 2. A schematic representation of solids of different shapes of 12-vertices polyhedrons.
4.5.2 Giant clusters 4.5.2.1 Polymeric precursors of giant clusters Among the known preparative methods for the synthesis of large clusters of the low-valence platinum group metals, the most frequently employed involve the condensation of low-nuclearity clusters and the reduction of complexes of the metals in Organic phosphines, particularly higher oxidation states, e.g., Pd( II), Pt( PPh3, are in common use both for the synthesis of noble metal clusters and as the auxiliary ligands in catalytic systems for h y d r o g e n a t i ~ n . [ ~A ~ -variety - ~ ~ I of low- and high-nuclearity Pd complexes can be formed by the action of H2 on Pd2+ complexes, depending on the nature of acid ligand, solvent, and the ratio of organic phosphine to Pd.[8335p371 For instance, the absorption of H2 with [ (Ph3P)Pd(OAc)2]2 in CH2C12 solution results in the formation of the oligomeric low-valence Pd complex 1 (Eq. 1):13’] II).[303311
Ph3P OAc OAc ’Pd’ ‘Pd’ Acd b A < \PPh3
--
H2
-2AcoH
OAc / \ Ph3P-Pd-Pd-PPh3 \ / OAc
Hz/[O] or Pd 2+ -AcOH, -C&
-
[PddPPhh],
(1)
1
where n is most probably equal to 4. Complex 1 is apparently the species responsible for the efficient catalysis of the hydrogenation of different organic substances.r361 Evolution of benzene as a result of the dephenylation of phosphine ligands under the action of reductants (e.g., sodium amalgam, H2, HCOOH) seems to be a general occurence. For instance, the high-nuclearity substance formulated
1368
4 Nanomaterials
as [(Pd3P)sPPh3], was obtained with benzene by reduction of Pd(acac)2 with HZ in solutions containing PPh3 .[381 A series of cationic trinuclear clusters [ P ~ ~ ( P P ~ z ) ~ ( P R ~ ) ~(R X ]=+Ph, B FEt, ~ -X = C1, Br, SCF3) have been synthesized and their structures determined by X-ray diffraction; one example is the cluster [ Pd3(PPh2)2(PEt3)3Cl]+BF4(2).1391
PEt3
I
l+
2
Hydrogenolysis of the P-C bond to form an alkane and a PR2 ligand can also occur when Pd( 11) trialkylphosphine (e.g., PBu3) complexes are treated with H2.[351 Analogously, dephenylation of PPh3 was observed during the reduction of complexes [ (Ph3P)PtC12]2 and [ (Ph3P)Pt(OAc)2]2with Na/Hg:131.401
[(Ph3P)Pt(O Ac)~] 2
NdHg, THF
-GH6
lJ’h2PPtIm
(3)
Molecular weight measurements showed IZ = 6 and m = 8. On the basis of ESCA data (Pt4f7p72.1 eV), the oxidation state of Pt atoms in these complexes is (+l). Reduction of [ (Ph3P)Pt(OAc)2]2with formic acid accompanied by dephenylation affords complexes with Ph2P and PhP l i g a n d ~ : ~ ~ ~ , ~ ~ ] [(ph3p)pt(OAc)d2
HCOOH -AcOH
[(Ph3P)Pt(OCOH)212
-
-a -mc.h/
[(Ph3P)Pt&]n
[PhPPtIm
\C&
(4)
[Ph2PPt18.,0
According to SAXS data, the molecules of platinum phosphide complex [(PPh2)Pt]*-loare almost spherical in shape with a mean diameter of 15.2 A. Unlike the platinum analog, treatment of the palladiumcomplex [ (Ph3P)Pd(OAc)2]2
4.5 Strutegies for Assembling Pd and Pt Atoms
I369
Figure 1. X-ray structure of the complex [ Pd( P ~ ) ( , L L - O ~ P P ~ ~ )(reproduced ( P P ~ ~ ) ]with ~ ' ~permis~~ sion).
with formic acid leads to the formation of the polyhydride complex [ Pd2(PPh2)H,], as a main product. The presence of hydride ligands was revealed by ' H NMR spectroscopy a broad signal with a maximum at 6 = -9.5 ppm was detected in the region typical of hydride complexes. The minor product is a binuclear phenyl PPh3)]2. According to a single-crystal derivative of palladium [ Pd( Ph)(pU-O2PPh2)( X-ray diffraction the Pd atoms in the last complex are bridged by two phosphinate groups at a distance of 5.35 A (Fig. 1). SAXS data showed that the molecules of the palladium complex [ Pd2(PPh2)HX], are nearly spherical in shape with a diameter of 15.6 A.EXAFS data showed that both the platinum, [( PPh2)PtIs -10, and palladium, [ Pdz(PPh2)HX],,complexes contained only phosphorus and metal atoms in the first coordination sphere. The interatomic distance between Pt( Pd) and the phosphorus atoms is 2.26 A,which is typical of the Pt( Pd)-P distances in platinum and palladium complexes. Comparison with the spectral data of the reference compound, Pd( PPh3)4, showed that each platinum atom is surrounded by four phosphorus atoms (the coordination number, n( P/Pt), is four). The corresponding value for palladium atoms, n( P/Pd), is three.[401 It follows that reduction reactions of the Pd and Pt phosphine complexes are often complicated by destructive splitting of the phosphine ligands, resulting in non~
1310
4 Nunomaterials
crystalline mixed-ligand ( phosphine, phosphene, and phosphide) oligonuclear complexes, the structures of which remain unclear. In contrast, heteroaromatic auxiliary ligands, e. g., 1,lO-phenanthroline and its derivatives seem to be less sensitive toward reductants, and a clearer picture can be expected for such ligands. The resulting materials seem, moreover, to be more resistant toward dioxygen; thus clusters and colloids containing such ligands can be used not only for catalysis of reduction reactions but also for oxidation reactions. In one of our first studies,[31palladium (11) acetate was found to be readily reduced by CO in glacial acetic acid to form the tetranuclear Pd( I) cluster (3) (Eq. 5):
According to X-ray diffraction data,I3I cluster 3 has a rectangular Pd4 metal skeleton (the Pd-Pd distances are 2.66 and 2.91 A). The CO ligands in cluster 3 can be readily replaced by mild bases L such as phosphines or heteroaromatic N-bases (e.g., dipy, phen) to form, depending on the L/Pd ratio, either crystalline lownuclearity cationic clusters with a nearly tetrahedral metal skeleton of type 4,[421 (Eq. 6) which are fairly stable and inactive in catalysis, or non-crystalline, colloidlike Pd-containing products of the condensation of primary low-nuclearity clusters that have high catalytic performance in various redox reaction^.[^'^^^^^ In all these compounds phen and dipy are chelating ligands.
4
N:N-Base
Substances similar to those just described in terms of their chemical composition and catalytic capabilities can also be obtained by a more convenient procedure, without isolation of clusters 3 and 4, by reduction of Pd(OAc)* with CO in the presence of small quantities (<; mol per Pd atom) of the L = phen, dipy additives. The more facile synthetic way to low-valence polynuclear Pd complexes is reduction of Pd(OAc);?by H;?in the presence of L l i g a n d ~ . [ ~ ~The ~ ~colloids ” ~ . ~ obtained ~l in this way have been shown to be catalytically active in a number of redox reactions involving those of alkenes, alkylarenes, CO, alcohols, nitroarenes, etc.[’ 7320,431
4.5 Strategies for Assembling Pd and Pt Atoms
1371
The reduction of Pd (11) acetate by H2 (1 atm, 20 "C) in AcOH solution (containing mol phen per Pd atom) afforded an X-ray-amorphous substance of empirical composition Pd4phen(OAc)2 (5) (Eq. 7):
1
4nPd3(OAc),
+ 3nL + 15nH2
+
~[P~~L(OAC)~H ~ ]100 ~ nz
(7)
(5)
The occurence of hydride ligands in the composition of 5 is revealed by the uptake of excess H2 during this reaction (compared with the quantity of H2 necessary to reduce Pd(+2) to Pd(+i)) and a broad hydride line at 6 = -35 to -50 ppm, in the 'H NMR spectrum of complex 5 and its dipy analog both in MeCN solutions and in the solid state (recorded with the MAS t e ~ h n i q u e ) . [ " . ' ~ .Th ~ ~eJ CH3COO protons appear as a narrow singlet at 6 = 2.1 ppm. The broadening of the hydride signal in the ' H NMR spectrum could be an indication of the polymeric character of molecule 5.
5
According to SAXS data, molecule 5 is nearly spherical in shape with the mean diameter of 20 f 5 A. These data are in accord with results from a TEM study which showed molecules of 5 to be nearly spherical metal particles with a mean Electron diffraction of the same samples of 5 showed no diameter 20 f 4 p1.['7*441 diffraction rings, suggesting the absence of close metal ordering within the observed 20 A particles. Additional information about the structure of complex 5 was obtained from extended X-ray absorption fine structure (EXAFS) ~ p e c t r a . ~ 'The ~ . ~curve ~ ] of radial distribution of atoms, RDA, which was calculated on the basis of the EXAFS spectrum of complex 5 contained only two maxima - 2.1 f 0.1 A for the Pd-light atom distance and 2.6 i 0.1 A for the Pd-Pd distance. Maxima corresponding to the distances between the more remote Pd atoms were not detected. On the basis of combined data from elemental analysis, NMR, TEM, and as being formed from positively charged EXAFS, complex 5 can be filament-like chains . . .{[(Pd4H3L)HI2' (OAC-)~}. . . which are either curled into a ball or shaped as a fractal aggregate; the positive charge of the cluster cations is counterbalanced by the OAc- anions in the outer coordination sphere:
-
1312
4 Nanomuteriuls
4.5.2.2 Giant Pd clusters Complex 5 is unstable in air. It readily loses all hydride atoms under the action of 0 2 to form H20 and a polynuclear compound of empirical formula Pdgphen(OAc)3 (6); this compound is highly soluble in water and acetic acid, less so in other polar solvents (e.g., MeCN, DMF, DMSO) and their mixtures with water.['923171 The molecular mass of 6 was estimated to be (1.0 0.5) x lo5 from the data on the rates of sedimentation of its aqueous solution on ultracentrifugation, by use of the Stokes-Einstein la^.['.^*^ 71 This method is analogous to the technique used for determination of the molecular mass of the cluster Au55(PPh3)12C16 in the pioneering work by G. S ~ h m i d . [ ~ ~ % ' ~ ] The more accurate data on the size of molecules 6 were obtained by SAXS, In TEM micrographs the metal skeleTEM, and electron diffraction ( ED).[1,2,171 tons of 6 were observed as nearly spherical particles of diameter 26 3.5 A.According to SAXS measurements, mean diameter of 6 is 20 k 5 A,whereas estimation based on the half-width of the ED rings gave the similar value of 25 A.[1,2,171 To elucidate the molecular formula of cluster 6 it was necessary to estimate the number of Pd atoms within the cluster metal core. For this purpose, the packing mode of metal atoms was investigated by two different techniques, HREM and EXAFS. The radial distribution curve calculated from the EXAFS spectrum revealed a set of four Pd-Pd distances within the range 2.6 to 4 A,which is consistent with icosahedral packing of Pd atoms (Table 1). Note that the shortest Pd-Pd distance is very close to that in bulk Pd metal. The HREM study showed some additional features of the packing of the Pd atom^.[^^-^'] Although some cluster metal particles have a five-fold axis typical of an icosahedron (Fig. 2a), the more abundant metal particles seemed from the micrographs to be arranged according to face-centered cubic (f.c.c.) packing (Fig. 2b), and this was confirmed by the electron diffractograms for the same cluster samples. One possible reason for this structure might be the non-uniformity of the material under study. The HREM study of several samples of 6 obtained after the same
+
+
-
Table 1. The Pd-Pd distances in the metal skeleton of giant cluster 6 found from EXAFS data compared with the distances expected for different packings of Pd atoms.['] Pd-Pd distances (A)
Packings Found from EXAFS Expected for packings* Face-centered cubic (f.c.c.) Hexagonal close (h.c.p.) Icosahedral *With 2.60
2.60k0.04 2.60 2.60 2.60
A the shortest Pd-Pd
distance.
3.1i0.1 -
-
3.10
3.66k0.1 3.66 3.66 3.66
4.08iO.l
-
-
-
-
4.50
4.10
-
4.5 Strategies f o r Assembling Pd and Pt Atoms
1373
Figure 2. HREM micrographs, obtained by Dr V.V. Volkov in the laboratory of Professor G. Van Tendeloo, the University of Antwerp, of the metal cores of Pd cluster 6 with the different packings f.c.c. (a) and icosahedral/multiply twinned (b).
synthesis by precipitation from an AcOH solution with successive amounts of benzene showed that the cluster particles of different samples do not differ noticeably in average size and size d i s t r i b ~ t i o n . [ ~ ’The , ~ ~ ’clusters from the first, most soluble, fraction have nearly structureless metal cores (almost no fringes in the HREM micrographs and no ED rings point to the absence of close order in the arrangement of Pd atoms within the metal cores); the next, less soluble, fractions give rise to sharp fringes and ED rings which attest to close ordering of the metal cores, but with a different arrangement of the metal atoms. Both the f.c.c.-packed and icosahedral (or multiply-twinned) metal cores are present in all the samples, and the less soluble samples contained more f.c.c.-packed metal Another reason for the observed discrepancy between EXAFS and HREM-ED data might be the different experimental conditions of the two techniques. The EXAFS experiments were performed at normal temperatures and in air, and no noticeable damage to a sample exposed to a filtered, low-intensity beam of syn-
1374
4 Nunomaterials
Figure 2 (continued)
chrotron X-radiation has been found during experimental runs in air or under an Ar atmosphere." 71 On the other hand, under the conditions of electron microscopic experiments, uncontrolled heating by the electron beam under high vacuum could have damaging effects on the cluster sample (e.g., loss of the ligands and relaxation of the cluster metal core to a more stable structure). Because icosahedral packing seems to be somewhat less favorable energetically than cuboctahedral, such particles could be transformed to the more stable f.c.c. packing, whereas the opposite transformation is hardly possible under HREM, ED experimental conditions. This seems to be a realistic explanation of the disagreement found, and the occurence of at least some part of the icosahedral (or multiplytwinned) metal cores is beyond any doubt. Whatever the reason, the packing densities of both arrangements are very similar and, therefore, one can calculate the total number of metal atoms within the 25 A cluster metal core on the basis of either of the two packings (icosahedral or cuboctahedral); the value observed is 2.60 8,for the shortest Pd-Pd distance, indicative of the
4.5 Strategies for Assembling Pd and Pt Atoms
1375
Figure 3. Idealized structure of Pd cluster 612](reproduced with permission).
presence of 570 k 30 Pd atoms. On the basis of this value and elemental analysis data molecule 6 was formulated approximately as P d 5 7 0 ~ 3 0 p h e n 6 3 ~ 3 ( 0 A c1) 10 ~. 0~~ ' ~ At this point it is appropriate to turn to the model of the Au55 cluster,['31 recalling the Chini 'magic n u m b e r ~ ' . [ ' ~ -The ' ~ ] number of Pd atoms found in cluster 6 matches this well, within experimental error. The number of metal atoms in the fivelayer 12-vertex solid, icosahedron or cuboctahedron is 1 12 42 92 162 252 = 561 atoms (see Schemes 1 and 2). Therefore, molecule 6 can be represented by the model (idealized) formula Pd561phen6o(OAc)lso (Fig. 3). In fact the idealized formula corresponds to an average size, shape, and composition of the cluster rather than to a definite fixed structure. There is some distribution in these parameters, and the real cluster contains nearly a continuous range of species, the most abundant of which almost coincide with the idealized structure in their chemical composition, shape and size. This essentially distinguishes the giant clusters from individual, smaller clusters the structures of which have been established by single-crystal X-ray analysis. Because of the dense packing of the metal core, the inner-layer metal atoms are inaccessible to coordination by any ligands except, possibly, hydride, if any are
+ + + +
+
~ ' ~ ~ ~
1376
4 Nanomaterials
available. Only 252 Pd atoms, occurring in the outer layer of the metal core, are capable of bonding phen and OAc- ligands. Taking into account the van der Waals' shapes of phen molecules, -60 phen ligands could be coordinated in a bidentate manner at the vertices and edges of an icosahedron or cubeoctahedron without substantial steric interference between them. With this arrangement, almost the whole surface of the metal core must be sterically screened by the bulky phen ligands, leaving only a small number of sites little space for coordinate some of 180 OAc- ligands. A similar steric situation has been found for the tetranuclear cluster 4 (see above). Single-crystal X-ray structural analysis has shown that the OAc- anions are located Some evidence of in the outer sphere of the [ Pd4(C0)2phen4I4+ cluster the outer-sphere location of OAc- anions in the giant cluster 6 was obtained by NMR.r17,441 In the ' H NMR spectrum the protons of the phen ligand appear as a broad unresolved multiplet, apparently because of the effect of the paramagnetic metal core (see below) and/or some irregularity in their arrangement at the surface of the Pd metal core. Unlike this, the CH3COO- protons appear in the spectrum as a fairly narrow signal at 6 = 2.0 ppm, testifying to their outer-sphere location in molecule 6. This suggestion is also supported by the IR spectrum of cluster 6 in AcOH solution.[501 The outer-sphere coordination of OAc- ligands in cluster 6 is in line with the ease of their replacement by another anions. This cluster can be precipitated from aqueous solution by various salts, e.g., alkaline halides, perchlorate, and fluorophosphate. For example, complete substitution of OAc- anions, accompanied by some hydrolysis, was observed on treatment of a solution of cluster 6 with K[PF6] to form a cluster with the idealized formula Pd561phen6o06o(PF6)60(7):
+ 60PF6- + 6OH2O = [Pd56lphen6o060](PFs),, + 120AcOH + 60Ac0-
[Pd56lphen60](OAC)180
(8)
(7) The structure of cluster 7 has been studied by means of the same set of indirect These data show the main structural methods as those used for 6.r17,18,20,28,45-491 features of cluster 7 to be similar to those of cluster 6. The phen ligands are also coordinated with a metal core of 28 i-5 A diameter, about sixty 02-ligands are coordinated, probably to the core surface, and 60 of the more bulky PF6- anions substitute 180 OAc- in the outer sphere of the giant cluster. There is no disagreement between HREM and EXAFS data, as was found for cluster 6 - both techniques pointed to the f.c.c. packing of the metal core, suggesting that structural relaxation of cluster metal skeleton to the most stable f.c.c. packing can occur during ligand substitution." 7,181 The giant clusters 6 and 7 have been also studied by scanning tunneling microscopy (STM).r5'1Unlike transmission electron microscopy, STM data give no information on the structure of the metal core of the cluster, but it is sensitive to the
4.5 Strategies for Assembling Pd and Pt Atoms
1377
Although the STM image does not overall shape and size of the cluster molecule.[521 exactly follow the van der Waals’ contours of the cluster molecule under study, that has been demonstrated by the example of the Schmid Pd561phen380-200 giant c l ~ s t e r , [that ~ ~ *the~ ~sizes ~ of clusters 6 and 7 which were observed in our STM study[”] considerably exceed the sizes of their metal cores found by the TEM, HREM measurements. The observed particles of the Pd giant clusters are distributed according to their sizes, and all particles are observed by STM as nearly round in a shape (Fig. 4).
Figure 4. STM images of Pd clusters 6 (a) and 7 (b) made by Dr. J.C. Poulin in the laboratory of Professor H.B. Kagan, UniversitC de Paris Sud.
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The observed average diameters of cluster molecules 6 and 7, d, = 33-36 A,are somewhat less than those expected for the van der Waals contour (45-50 A),but they are noticeably larger than the size of the cluster cores (26-28 A).These STM data are essentially in line with those for the giant cluster Pd561phen3@-200 studied by the same t e ~ h n i q u e s . [ ~ ~ , ~ ~ ] Unlike the smaller diamagnetic Pd clusters containing 4 to 38 metal a t o m ~ , [ ~ ~ * ~ clusters 6 and 7 exhibit low, temperature-independent paramagnetism - ~ ~ ( 3 0 0 K) = (+1.3 0.2) x ~ ~ ( 7K)7= (+1.8 f 0.3) x lop6 CGSU.[28,471These data show that Van Vleck paramagnetism, a property of a bulk metal, can arise when the cluster skeleton comprises no more than several hundred metal atoms. In line with this, the magnetic properties and heat capacity of the giant cluster 6 were found to have unusual temperature dependence near absolute zero (0.01-1 K) - the magnetic susceptibility passes through a maximum at 1 K, and the dependence of the heat capacity on temperature, especially in high magnetic fields, is abnormal in character.[561Both characteristics differ sharply from those observed for the bulk Pd metal. These quantum size effects for small metal particles have previously been predicted theoretically, but their experimental observation became possible only when well-characterized uniform cluster particles with a narrow unimodal size distribution, e.g. the giant palladium clusters were synthesized. Thus, in the genetic sequence ‘mononuclear complex-oligonuclear cluster-giant clustercolloidal metal-bulk metal’, the giant Pd561 clusters are species which retain the properties of molecular clusters but are beginning to have properties inherent to the metallic state of a substance. Thus, studies of phenanthroline-containing giant palladium clusters revealed some intermediates in the assembly of the metal during formation of the metal microphase from low-nuclearity metal complexes. Because of the presence of L ligands (phen, dipy) we were able to isolate the polymeric intermediate complex 5 which contains only small cluster moieties (e.g. Pd4 or even Pd3) rather than an extended Pd-Pd metal system stabilized by the L and H ligands. Removal of the hydride ligands results in the coupling of the small clusters to form the metal core of the giant cluster 6 (Scheme 3). It seems that the material obtained experimentally consists of the giant Pd cluster species the metal core of which is far from the perfect f.c.c. packing. The less stable icosahedral and/or multiply-twinned polyhedra form initially. The more stable f.c.c.-packed cluster metal cores are, however, formed either on heating (e.g. under conditions of the electron microscopic experiments) or in the course of chemical transformations such as substitution of OAc- ligands under ambient conditions, affording cluster 7. As a consequence of the reduction/oxidation stages during Pd cluster formation some important intermediates could be missed because of general lability of palladium compounds. In this context, it was of interest to follow analogous chemical transformations, using platinum complexes as an example because platinum is
-
4.5 Strutegies f o r Assembling Pd and Pt Atoms
1379
B 5
PF6-1 H20
- AcOH
Scheme 3
known to be the more kinetically 'inert' element, and there is a chance of following those stages of the reactions which occur too quickly with palladium compounds.
4.5.2.3 Platinum clusters and colloids
Unlike palladium, data on large platinum clusters is still scarce and contradictory. The clusters with the empirical formulas Pt~sphen12(0Ac)30(AcOH)33[~~' and Pt30gphen*36030 (phen* = sodium 4,7-bis-phenylsulfate-l,1O-phenanthr~line)'~~] have been stated as the reaction products in two independent syntheses performed under almost identical conditions which almost reproduced the original preparation procedure for the synthesis of the giant palladium cluster Pd561phenm(OAc)lso.[1'2"7 1 With the intention of gaining insight into the nature of platinum analogs of Pd clusters, the reactions of platinum acetates with H2 in acetic acid solutions containing phen were studied in an attempt to follow the stages of the synthesis of phenanthroline-containing platinum giant c l ~ s t e r s .Platinum [ ~ ~ ~ ~acetates ~~ with different oxidation states, P t d ( O A ~ ) g , [ P~ ~~l ( O A C ) ~ and ~ , [ [~P ~~ ~Z I( O A C ) ~ ] , [ ~ ~ ~ , ~ ~ were used as the starting compounds. It was found that the standard preparation p r o ~ e d u r e [ ~ , ~resulted , ' ~ ] in the same reaction product when using any of the Pt
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4 Nanomaterials
acetates. A series of experiments were also performed with different initial Pt/phen molar ratios (3 : 1; 4 : 1; 6 : 1, and 8 : 1) under the same reaction conditions. In a typical experiment, a solution of Pt4(OAc)s and phen monohydrate (2: 1 initial Pt/phen mole ratio) in glacial AcOH was stirred successively under H2 and 0 2 (1 atm) at 20 "C:
Hz/phen
Ptd(OAc)s 8
[expected Pt polyhydrido compound]
0 [expected 'giant Pt cluster']
(9)
Contrary to expectation, the reaction product 8, a black amorphous powder soluble in water and acetic acid, was found to have composition and properties quite different from those expected for the platinum analog of the Pd561 giant cluster.[5'I According to elemental analysis, substance 8 has the reproducible elemental composition Ptsphen3(OAc)4(OH)4(H20)6, which is quite different from the expected minimum formula Pt9(phen)o.96(0Ac)2.89for the Pt analog of palladium giant cluster 6. On the basis of the elemental composition, the formal oxidation state of platinum in substance 8 is close to ( f l ) . This value was confirmed by the EXAFS data.[581 Because of difficulties in determining the molecular mass of complex 8, the size of its molecule was estimated on the basis of SAXS data, from which particles 8 were found to be nearly spherical in shape with a mean diameter of 30 5 A. Transmission electron microscopy (TEM, HREM), on the other hand, revealed the metal cores of 8 to be particles also close to spherical in shape but with the mean diameter 14-18 A; fringes typical of regular metal packing within the observed particles were also faintly visible on the micrographs (Fig. 5a). In accordance with this, the electron diffraction pattern of sample 8 contains nothing but barely perceptible, poorly reproducible diffraction rings without reasonable regularity in their arrangement.[5'I These facts suggest that the molecules 8 are somewhat larger and looser in their structure than is apparent from TEM and HREM data, but undergo deterioration during electron microscopic experiments because of uncontrolled heating in high vacuum. Such precedents are not an infrequent occurrence in TEM studies of noble metal cluster^.['^"^^ It is worthy of note that in the above-mentioned s t ~ d i e s [ ~ ' , ~ ~ ] of giant Pt clusters the main body of structural evidence comes from electron microscopic data. Study of the thermal stability of complex 8 by thermal analysis (DTA-TG)[57,581 showed a smooth endothermic effect accompanied by small mass loss ( - 5%) on heating a sample of 8 in Ar atmosphere at surprisingly low temperatures in the range of 25-125 "C (Fig. 6). In modeling experiments complex 8 was heated stepwise to specified temperatures
4.5 Strategies f o r Assembling Pd and Pt Atoms
1381
Figure 5. HREM micrographs of Pt samples 8 (a) and 10 (b) made by Dr G.A. Kryukova, Institute of Catalysis, Novosibirsk.
between 20 and 120 "C in an Ar atmosphere under the conditions close to those of the DTA-TG study. The elemental analytical data of the thermally treated sample 8 pointed to complete elimination of OAc groups and a change of composition to one with the empirical formula Pt4phen,.5(OH)4 (8a). Further heating of 8 above 125 "C in Ar resulted in an intense exothermic effect with a maximum at 240 "C accompanied by a substantial mass loss. The samples obtained after heating to temperatures above 125-130 "C are insoluble in any solvents, and their compositions are shifted to higher platinum content. Elemental microanalysis data showed that after heating to 240 "C in an inert atmosphere, not only were the acetate groups removed but also the phen ligands underwent appreciable transformations the C/N mass ratio in heated sample 8b is 3.8 (cf 5.1 for phen), which suggests destruction of the coordinated phen ligands to form, e.g., either a quinolylnitrene (A) or a product of the oxidative condensation of phen (B) or even the more complicated dendrimeric structures (Scheme 4).
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Figure 5 (continued)
Although degradation of the coordinated heteroaromatic ligands seems to continue during further heating of 8, even after heating to 300-400 " C for 3-5 h under Ar 8 was not pure platinum but contained no less than 5-7% carbon and nitrogen (in -2 : 1 mass ratio) and only 92-95% Pt.[581These facts point to the thermal instability of complex 8 even at 100 "C in an inert atmosphere, i.e., under milder conditions than those expected during the electron microscopic study. Therefore, when studying such platinum complexes with TEM and HREM, a variety of species, e.g. 8a and 8b, of different composition could be formed, depending on the operating conditions. In addition, even small variations in the synthetic procedure and isolation of the reaction product could change its elemental composition and structure. This explains the disagreement between our r e s ~ l t s and ~ ~ those ~ , ~ of~ ~ others.[' 5,271 To reveal the nature of temperature-induced transformations of complex 8, its decomposition was monitored by temperature-programmed EXAFS in s i t ~ . ~At ~ *room ] temperature the X-ray absorption spectrum of complex 8 gave the following information.
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4.5 Strategies jor Assembling Pd and Pt Atoms
1383
t/”c
I: 200
100
,”1 Figure 6. Thermal analysis (DTA-TG curves) of Pt cluster St5’] (reproduced with permission).
1
4 Time/&
1
i) The intensity of the L3-edge (the p3/2+ d5l2electron transitions from the inner to the highest vacant atomic levels near the Pt ionization potential), which is proportional to the effective charge of a metal atom, enabled estimation of the average oxidation state of Pt in complex 8 as equal to +1, in line with the value
--
& \O W Scheme 4
6
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4 Nanomaterials
derived from elemental analysis data (note that +0.3 was found for giant Pd cluster 6['?]). ii) The curve of the radial distribution of atoms (RDA) for complex 8 calculated from the EXAFS spectrum at 20 "C with the use of the EXCURVE-92 simulation programr611was indicative of only one short Pt-Pt distance (2.68 k 0.03 A), which is typical of Pt( I ) c o m p l e x e ~ . No [ ~ ~other ~ ~ ~short ~ Pt-Pt distances were apparent from this data. The average Pt-light atom ( N or 0) distance was found to be the normal 1.98 k 0.01 A,but the intensity of this peak considerably exceeded that for the Pt-Pt distance. According to the calculations, the average number of platinum atoms in the close vicinity of each Pt atom (-2.7 A)is 1, and that of the light atoms in the nearest coordination sphere of a Pt atom ( 2 A) is -3.5. It is noteworthy that the expected coordination numbers for the idealized model of the hypothetical Pt561 giant cluster are -9 for the Pt-Pt and 0.5-1 .O for the Pt-light atom coordination spheres. Heating of sample 8 in the temperature range 25-240 "C resulted in a decrease in the oxidation state of the Pt atoms and in the average number of light atoms (0, N ) coordinated to platinum, whereas the average number of Pt atoms arranged at a short Pt-Pt distance (2.7 A) increased to the value which is reasonably close to that expected for a close-packed metal core with an extended system of metal-metal bonds. These data suggest that giant clusters with a metal nucleus of Pt atoms in the oxidation state of +0.3 and a Pt-Pt coordination number of 6 can be formed by transformations above 180-200 "C (Fig. 7a).
--
-
Microanalysis data showed that the chemical composition of species formed at these temperatures is different from that expected for the platinum analog of the giant Pd cluster 6. The substance with the chemical composition Ptll (C9H6N2)503 (9), which was obtained by heating sample 8 to 240 "C, seems to resemble the Pt309phen*36030 cluster described elsewhererz7] 28 simplest units of 9 (28 x P ~ I I ( C ~ H ~ N correspond ~ ) ~ O ~to) the formula Pt308L140084 (Scheme 4, L = A) or Pt3ogL70 Og4( L = B). The metal core of such a cluster should have a diameter of 20-25 A,which is close to the value found in our TEM Another route to giant Pt clusters was found by changing the initial Pt/phen ratio in the synthetic procedure. The chemical composition of the reaction products changed systematically when the Pt/phen ratio was increased from 2 : 1 to 6 : 1, close to that expected for the platinum analog of giant cluster 6. A further increase in the Pt/phen ratio ( 2 8 : 1) resulted in substances containing 2 92% Pt. These samples are quite insoluble in any solvents and resemble the coarse-dispersed platinum metal in their properties. According to EXAFS data, coordination numbers Pt-Pt and Pt-light atom also change systematically with variation of the Pt/phen ratio (Fig. 7b). HREM photographs showed a noticeable increase in the average diameter of the observed cluster particles, from 15 to -25 A,on increasing the Pt/phen ratio from 2 : 1 to 6 : 1. -
-
4.5 Strategies for Assembling Pd and Pt Atoms
1385
1
6-
5-
.;I-
d
c: 4 -
p:
dm
g
-
V
3-
g F:
-g
2-
3
1-,
Furthermore, sharp fringes, which point to a regular arrangement of the metal atoms within the observed particles, appear in the micrographs of these samples (Fig. 5b). On the basis of chemical analysis, and on HREM, SAXS, and EXAFS data, substance 10 obtained at the Pt/phen ratio of 6 : 1 is the closest to that expected for the giant Pt cluster.[581The accuracy of the SAXS and HREM measurements is inadequate to enable confident distinction between the Pt561 and Pt309 cluster cores. Therefore, the formulas Pt306phen34(0Ac)1 0 2 0 1 3 6 and Pt55~phen62(OAc)1~60248 (which can be obtained by multiplication of Ptgphen(OAc)304, the empirical formula of substance 10, by a factors of 34 or 62, respectively) could be applied to cluster 10 as almost equally probable for the giant clusters with the idealized Pt309 and Pt561 cores, i.e., Pt309phen34(0Ac)100013oor Pt561phen~0(OAc)1~002~0, respectively.
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4 Nunomaterials
2
o l l , 2
1
I 3
I
I 4
I
1
1
5
I
6
I
I
I
I
I
I
8
Pt:phen ratio b
Figure 7 (continued)
Taking all these data into consideration, the transformation of the precursor complex 8 to the giant cluster 10 can be presented by Scheme 5. This scheme tentatively reflects the formation and gradual growth of the system of Pt-Pt short bonds to form the metal core of the giant Pt cluster. It is still unclear, however, whether 140 or 70 bulky ligands of type phen, A or B can be coordinated at the surface of the Pt metal particle 25 A in diameter, along with 84 oxygen atoms. The possibility cannot be eliminated that at least some of these 0 atoms are located within the cluster core as non-stoichiometric PtO, inclusions. Examination of the idealized models for giant Pd cluster 6 showed that because of steric hindrance only 60-62 phen molecules can be coordinated in a bidentate manner at the outer layer of the 561-atom metal luster.[^^"'^ The OAc- ligands constituting the cluster can be arranged mainly as outer-sphere anions. Another
1387
4.5 Strategies for Assembling Pd and Pt Atoms
4n+
4n OAc-
I
10
Scheme 5
example is the platinum giant cluster, whose composition contains not only OAcanions but also 0 atoms. Because the atomic radii of Pt and Pd metals are virtually equal, the idealized models of their giant clusters could be of similar dimensions. Hence, it is reasonable to suggest that only a small portion of these 0 atoms can be coordinated to the surface of the Pt cluster core, and that the remaining 0 atoms should be located within the cluster core as a PtO, rather than a neat metal core of cluster 10. Note that the thermally unstable PtO, phases are well known in platinum chemistry and are more typical of the platinum chemistry than those of palladium.
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4.5.3 Conclusions The nature of highly dispersed metal substances and materials has been investigated since pioneering studies by M. Faraday on gold colloids.r641During the past four decades the chemistry of transition metal clusters has developed as a branch of the science of dispersed materials based on clear-cut concepts of coordination chemistry. However, a natural limit of the use of a rigorous approach of coordination cluster chemistry occurs when the size of cluster molecules reaches 30-40 A.As far as we are aware the largest cluster so far structurally characterized by single-crystal X-ray diffraction is Cu14,$e73(PPh3)30, the overall dimensions of which are ca 2040 The less rigorous approach based on diverse techniques widely applied to the physics of solids seems to be more promising for characterizing the larger cluster species. Although more laborious, these methods provide valuable, although indirect, information about the structure of such species. An important point in these studies is the use of chemical composition data. In fact, such cluster-like substances as metal blacks and colloidal metals have been under investigation for a long time with no substantial progress despite the use of various physical techniques. The chemical nature of giant clusters was determined in the 1980s because elemental analysis data were taken into consideration, along with data from another techniques. Giant clusters can serve as useful models for understanding the structure and chemical behavior of dispersed metals. Magnetic and thermodynamic measurements in the vicinity of absolute zero showed the Pd561 species to be the smallest particles which still have the properties of molecular clusters that distinguish them from bulk metal. The chemistry of giant metal clusters is still at the start of its development. The studies outlined in this survey show that assembling of palladium and platinum atoms to form giant clusters proceeds via polynuclear complexes, which are precursors difficult to isolate for palladium but are more stable for platinum. Unlike giant palladium clusters with close-packed metal cores, the Pt clusters apparently contain a PtO, cluster core. These facts give more evidence for distinguishing between the cluster chemistry of palladium and platinum and provide a better understanding of the chemical processes occurring during the formation of noble metal colloids. Because palladium and platinum colloids are typical active species or precursors of homogeneous and supported metal catalysts for various chemical reactions, e. g. enantioselective hydrogenation, hydrosilylation, and C-C bond formation,[661their study is expected to provide useful information on the nature of catalysis.
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Acknowledgments Financial support from the Foundation of the Government of Russian Federation for leading scientific schools (grant 96-15-97577), the Russian Foundation for Basic Research (projects no. 95-03-10746 and 99-03-32292), and the International Science Foundation (grants MGP 000 and MGP 300) is gratefully acknowledged. The authors are also thankful to Drs 1.P. Stolarov, V.P. Zagorodnikov, T.A. Stromnova, L.G. Kuz’mina, L.K. Shubochkin, A.L. Chuvilin, V.V. Volkov, G.A. Kryukova, B.N. Novgorodov and V.N. Kolomiychuk for their generous help in these studies.
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[48] V. Oleshko, V. Volkov., R. Gijbels, W. Jacob, M. Vargaftik, I. Moiseev, G. van Tendeloo, Zeitschr. f u r Pl?ysik D 1995. 34, 283. 1491 V. V. Volkov. G. van Tendeloo, G. A. Tsirkov, N. V. Cherkashina, M. N. Vargaftik, I. I. Moiseev, V. M. Novotortsev, A. V. Kvit, A. L. Chuvilin, J. Cryst. Growth 1996, 163, 377. [SO] E. S. Stoyanov, Yu. A. Chesalov, D. I. Kochubey, I. P. Stolarov, M. N. Vargaftik, Inorg. Chem. (to be published). [Sl] J. C. Poulin, H. B. Kagan, M. N. Vargaftik, I. P. Stolarov, I. I. Moiseev, J Mol. Catal. A, 1995, 95, 109. [52] J. Frommer, Angeiv. Clwm., lnt. Ed. Engl. 1992, 31, 1298. [53] H. A. Wierenga, L. Soetout, J. W. Gerritsen, L. E. C. van de Leemput, H. van Kempen, G. Schmid, Ado. Muter. 1990,2, 482. [54] L. E. C. van de Leemut, J. W. Gerritsen. P. P. H. Rongen, R. T. M. Smokers, H. A. Wierenga, H. van Kempen. G. Schmid, J. Vac. Sci. Technoll991, 9, 814. [55] E. G. Mednikov, N. K. Eremenko. Yu. L. Slovokhotov, Yu. T. Struchkov, J. Chem. Soc. Clieni. Commun. 1987, 218. [ 561 Ya. Volokitin. J. Sinzig, L. J. de Jongh, G. Schmid, M. N . Vargaftik, and I. I. Moiseev, Nature 1996. 384, 621. [57] R. I. Rudy, N. V. Cherkashina, L. K. Shubochkin, M. N. Vargaftik, D. I. Kochubey, B. N. Novgorodob, V. N. Kolomiychuk, 1. I. Moiseev, Doklady R A N 1996, 349, 490 [Engl. Trans]. Doklady Chemistry 1996, 349, 4901. (581 I. I. Moiseev. R. I. Rudy, N. V. Cherkashina, L. K. Shubochkin, D. I. Kochubey, B. N. Novgorodov, V. N. Kolomiychuk. M. N. Vargaftik, lnorg. Clzim. Acta 1998, 280, 339. [59] C. T. Carrondo, A. C. Skapski, Acra Crystullogr. B 1978, 34, 1857. 1601 (a) R. 1. Rudy, N. V. Cherkashina, G . Ya. Mazo, Ya. V. Salyn, I. I. Moiseev, lzv. Akad. Nriuk SSSR, Ser. Kliim. 1980, 754 [Engl. Transl. : Bull. Acud. Sci. USSR,Div.Chem. Sci. 1980. 5101; (b) R. I. Rudy, N. V. Cherkashina, Ya. V. Salyn, I. I. Moiseev, Izv. Akad. Nuuk SSSR, Ser. Khim. 1983, 1866 [Engl. Trans]. : Bull. Acad. Sci. USSR,Div. Chem. Sci. 1983, 18661; (c) D. 1. Kochubey, M. A. Kozlov, N. V. Cherkasina, R. I. Rudy, K. I. Zamaraev, I. I. Moiseev, Koordin. Kliini. 1985, 11, 846 [Engl. Trans]. : Russ. Coordin. Chrm. 1985, 11, 4791. [61] N. Binsted, J. V. Campbell, S. J. Gurman, P. C. Stephenson, Laborutory EXCURV92 proyran7, SERC Daresbury, 1991. [62] L. Manojlovic-Muir, K. W. Muir, T. Solomun, Acta Clyst. 1979, 35, 1237. [63] S. I. Al Resayes, P. B. Hitchcock, J. F. Nixon, J. Orqanomet. Chem.; 1984, 267; C13. [64] M. Faraday. Philos. Trans. Roy. Soc. 1857, 147, 145. [65] H. Krautscheid. D. Fenske, G. Baum, M. Semmelmann, Angew. Chem. Int. Ed. 1993, 32, 1303. [66] (a) L. N. Lewis, Chelern. Rev. 1993, 93, 2693; (b) H. Bonnemann, G. A. Braun, Chem. Eur. J . 1997, 3, 1200; (d) S. Klingelhofer, W. Heitz, A. Greiner, S. Oestreich, S. Forster, M. Antonietti, J. Am. Cliem. Soc. 1997, 119, 10116.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.6 Electronic Structure of Naked, Ligated and Supported Transition Metal Clusters from ‘First Principles’ Density Functional Calculations Gianfranco Pacchioni, Sven Kriiger and Notker Rosch
4.6.1 Introduction Important experimental and theoretical activities in the coming years will include the study of the relationships between the structure and properties of nanosized materials. Clusters belong to this class and exist in the gas phase, in solution, and in the solid state.[’-31 They can exist as free entities or in contact with an ‘environment’, e.g. on a support or encapsulated in a sphere of surrounding molecules, the ligands, or in granular materials. Clusters can contain from a few atoms to thous a n d ~ . [They ~ ] can be produced in very small quantities by physical methods in a very complex experimental apparatus or can be obtained by relatively simple chemical synthesis in macroscopic quantities. All these aspects make clusters very special and very appealing theoretically. The electronic properties of metal clusters are in fact very diverse, depending on their size and shape, on their preparation and aggregation, and on the constituent atoms. For all these reasons the theoretical description and the fundamental understanding of the structure-property relationships of clusters are of crucial importance. The fundamental theory of clusters is, of course, quantum mechanics. Different ways of solving the electronic Schrodinger equation have been proposed and implemented and intense application to concrete problems has revealed which are the best approaches for a given problem. It has become clear that density functional (DF) theory provides one of the most useful means of studying the ground-state properties of complex s y s t e m ~ . [ ~D- ~F]theory offers a ‘first principles’, parameterfree approach which has been shown to describe molecular and solid-state properties with the desired high, sometimes even close to chemical accuracy. D F methods provide an excellent compromise between computational efficiency and numerical accuracy, thus enabling satisfactory treatment of rather complex systems. Although simplified computational methods can provide insights into the nature of the bonds
4.6 Naked, Ligated and Supported Transition Metal Clusters
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in clusters and cluster compounds,[71their application to unknown systems and their predictive power are restricted by the choice of a parameterized Hamiltonian. The rapid development of quantum chemistry and computer power has led to the birth of a new research area, computational chemistry. This new discipline is just one subfield of the more general area of scientific computing, a relatively new field of research which complements the experimental approach to scientific problems. It should not be confused with the more classical areas of theoretical chemistry and theoretical physics. Experiments are supposed to produce new facts and discoveries; theory is supposed to provide a firm general framework for the explanation of observed phenomena through a series of mathematical laws. Scientific computing complements experiment and theory alike: it is based on well established theories and formalisms but, like experiments, provides new data. Thanks to the combined use of advanced software and powerful computers, it is now possible to some extent to ‘simulate’ experiments on a computer, thus often providing a firm basis for the interpretation of experimental results. In this spirit, we will review some of the most important results obtained in recent years in our groups on the structure-property relationships for metal clusters. Modern quantum-chemical techniques, in particular density-functional methods, have been employed to describe the electronic structure and related properties of metal clusters in different environments - free clusters in the gas phase, mono- and bimetallic clusters stabilized by organic ligands, clusters supported on ‘inert’ substrates, e. g. oxide surfaces. All calculations have been performed from first principles, i.e. without making a priori assumptions about the system or resorting to empirical, experiment-derived parameters. Several spectroscopic features of the systems investigated have been determined from calculations and compared with experimental data. From the combined use of theory and experiment much information on the nature of ‘clusters’ has been deduced. In the following article we try to formulate some of the basic ideas and principles which determine the general behavior of free, ligand-stabilized, and supported transition metal clusters.
4.6.2 Density functional theory Density functional theory starts from the assertion that the ground state energy of an electronic system can be expressed as a unique functional of the electron density p and that this functional fulfils a variational p r i n ~ i p l e . [ A ~ , convenient ~] technique for solving this minimization problem is provided by the Kohn-Sham formalism which results in an effective one-electron Schrodinger equation with a density dependent effective local potential.[’] Because of its simplicity this formalism is very appealing; yet it incorporates exchange and correlation effects on an equal footing. Limitations of DF methods are that the fundamental form of the energy functional is only approximately known and that, in contrast with wave-function-based meth-
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4 Nunomuterials
ods like the Hartree-Fock and post-Hartree-Fock approaches, there is no hierarchy of approximations which would enable systematic improvements of the results. For the calculations we used the Munich version of the linear combination of Gaussian-type orbital density functional (LCGTO-DF) c ~ d e . [ ~ *The ' ~ l computationally economic local spin-density approximation (LSDA) to the exchangecorrelation functional has been successfully used in chemical applications since the seventies. This functional (employed here in the parameterization suggested by Vosko, Wilk, and Nusair, VWN" 'I) has been shown to describe accurately important properties of clusters and other molecular species. However, calculated LSDA binding energies are often significantly overestimated. In the past decade it has become apparent that density gradient corrections to the LSDA exchange-correlation functional considerably improve the calculated molecular energetics." 21 Several variants of generalized gradient approximations (GGA) have been implemented in the LCGTO-DF program. GGA expressions suggested by B e ~ k e [ ' ~or' by Perdew and War~g['~I were used to calculate density gradient corrections to the exchange energy functional. The corresponding correlation contribution was computed after work by Lee et aZ.["] and Perdew.[''] Thus three GGA variants were used in the studies to be reviewed - Be~ke-Lee-Yang-Parr,['~.' 'I Becke-Perdew," 3 , 1 6 1 and PerdewWa~~g.['~l A self-consistent scalar-relativistic (SR) version of the LCGTO-DF method has also been developed recently.1'0,'7p191The SR variant employs a unitary secondorder Douglas-Kroll-Hess (DKH)[20,211transformation for decoupling large and small components of the full four-component spinor solutions to the DiracKohn-Sham equation. The approximate DKH transformation, very appropriate and efficient for molecular calculations, has been implemented; this variant utilizes nuclear potential-based projectors and leaves the electron-electron interaction untransformed. Relatively large Gaussian basis sets were used to generate the Kohn-Sham orbitals in the LCGTO-DF all-electron calculations to be discussed below. Application of the spin-polarized version of the LCGTO method is particularly important for the study of clusters of transition metal atoms with magnetic character, e.g. Fe, Co, and Ni, and in general for the study of paramagnetic or other open-shell systems. In metallic clusters one often finds situations where very many one-electron energy levels are separated by only small energies. For metal clusters containing a few tens of atoms (and even for smaller systems) the description in terms of a discrete molecular orbital spectrum is no longer feasible and the language of band theory becomes appropriate. One convenient computational strategy is to use fractional occupation numbers, FON, for the levels near the Fermi energy (the cluster HOMO). In practice, one formally broadens each one-electron level by a Gaussian and fills the resulting density of states, DOS, up to the cluster Fermi energy which is determined self-c~nsistently.[~~ This technique may be viewed as equivalent to computing an average energy for a few close-lying one-electron configurations of a metal cluster. Further technical details are given elsewhere.['0~221
4.6 Nuked, Ligated and Supported Transition Metal Clusters
1395
4.6.3 Naked clusters The study of naked transition metal clusters forms the basis and reference for understanding the properties of the corresponding ligated and supported species. Most of the experimental evidence so far available deals with ligand-stabilized or supported clusters which are the species commonly encountered in colloidal solutions, catalytic materials, and cluster based nanostructured material^."^^] Thus, systematic theoretical work on free unperturbed transition metal clusters is especially needed to provide fundamental information, since these clusters are not so easily accessible experimentally. We have performed a number of density-functional studies on series of transition metal clusters for the elements Ni,[231Pd,[24-251 and Au.[’~]The aim of the studies was to characterize the evolution of cluster properties from the molecular to the bulk-like regime and to search for scaling relationships elucidating how various properties change with cluster size. To this end, highly symmetric clusters belonging to the point groups I/, and 01, have been considered. This idealization enables accurate determination of the electronic structure by means of all-electron calculations, even for heavy transition metal-clusters with more than 100 atoms. Examples of different cluster geometries are shown in Fig. 1. The largest cluster, M147, consists of a central atom surrounded by three icosahedral atomic shells comprising 12, 42, and 92 atoms, respectively. As shown for Au,, and Pd, clusters ( n = 6-147) by breathing mode optimizations, the average bond length of transition metal clusters lies between that of the dimer MZ as lower limit and that of the nearest neighbor distance in the bulk as upper limit.[24,261 The average bond length is found to increase with growing cluster size. A remarkably linear trend is found if the bond length is reported as a function Because of the average atomic coordination of the various clusters (Fig. 2).[24,261 the inner atoms of larger clusters have a bulk-like coordination of 12, this trend can be traced back to the surface-to-volume ratio, which drops with increasing cluster size. Thus, we have a first indication that noteworthy effects of the cluster surface are essentially restricted to the outer atomic layer. The extrapolation of the average bond distance to the bulk (coordination 12) was found to be in good agreement with experiment when the LSDA is applied. The values obtained in relativistic calculations, 2.76A for Pd[251and 2.89A for A u , [ ~are ~ ] only 0.01 A longer than the corresponding experimental data.12’I Icosahedral and octahedral clusters of comparable size (Fig. 2) are only slightly different; the icosahedral clusters tend to have longer bonds, a trend which is more obvious for Pd. Surprisingly, the small deviations of individual clusters from the general trend are very similar for Pd and A u , [ ~although ~] these metals have different electronic structures (see below). Effects of extending the geometry optimization to all degrees of freedom under a given symmetry constraint have been exemplified for the octahedral cluster Au19
1396
4 Nunomaterials
A
M13 (oh)
M147 (Ih)
Figure 1. Structures of metal clusters with octahedral (oh)or icosahedral (Zh)symmetry. Atoms at the corners, edges, and faces are distinguished by shading of increasing density.
(Fig. 1):[261 corner atoms move inward, edge atoms outward, leading to overall rounding of the structure. The average bond distance shrinks by 0.0.5A. For the larger clusters Au38 and Au44, the corresponding relaxation is smaller[281as might be expected for larger species with more spherical shapes. The overall stability of naked clusters has been characterized by the average binding energy per atom. As an example, Fig. 3 shows the calculated binding energy per atom for a series of Ni clusters[231in comparison with experimental results
4.6 Nuked, Ligated and Supported Trclnsition Metal Clusters
2.90
1391
1
2.80
2.70 Figure 2. Comparison of the average bond distances (LSDA results) of Pd and Au clusters for breathing mode-optimized structures as a function of the average coordination. Open and filled symbols denote oh and I h symmetric species, respectively.
2.60 _ _ ~
E,/n (eV)
Figure 3. Plot of average binding energy per atom against n-'I3 for Ni clusters (circles) compared with experimental results (triangles). Open and filled circles denote LSDA results for oh and symmetric species, respectively.
4 6 8 10 12 av. coordination
1398
4 Nanomaterials
obtained by collision induced dissociation.[291The observed increase of the average binding energy with growing cluster size is remarkably linearly dependent as a function of the surface-to-volume ratio S / V K nP1l3 K R-', where R is the effective radius of a cluster M,. The same behavior has been observed for Pd and Au clust e r ~ . " ~In] agreement with the scaling of the average bond distance, this behavior shows the influence of the cluster surface to be essentially restricted to the first atomic layer. Although in this study on Ni, clusters[231the bulk nearest neighbor distance has been employed to set up the cluster geometries, good agreement with the experimental results is found for the evolution of the binding energy with cluster size - linear fitting of experimental and computational results leads to very similar slopes (Fig. 3). It is remarkable that the nearly linear increase in the average binding energy per atom with growing cluster size is experimentally (and to some extent also computationally) found even for very small clusters containing fewer than 10 atoms (Fig. 3 ) . Because Pd and Au both have a face-centered cubic (fcc) crystal lattice in the bulk, one should expect Oh-symmetric clusters to be more stable than &symmetric species, at least for larger clusters. For clusters with up to 147 atoms, no clear preference was found and differences in average binding energies per atom amount to only some hundredths of an eV.r25,263 Deviations from idealized cluster geometries because of symmetry-conserving relaxation or even symmetry reduction are expected to lead to small stabilization only. The binding energy per atom for Au19 increases by 0.04 eV when the perfect octahedral structure with equal nearestneighbor distances is relaxed under o h symmetry constraint.[261An even smaller increase of the binding energy per atom compared with the breathing modeoptimized structure has been obtained for Au38 and A U M , [ ~leading ~I to overall stabilization of the clusters by less than 1 eV. Model potential calculations show that no noteworthy stability enhancement results even if disordered structures are taken into account.[301The analysis of the electronic structure of high-nuclearity transition metal clusters is complicated by two aspects - the sd valence manifold comprises many one-electron levels and a large number of electronic states are energetically close to the ground state. Some insight is conveniently gained by borrowing the concept of the density of states (DOS) from solid state physics. Thereby, each one-electron level is formally broadened to a narrow Gaussian shaped normalized peak; the sum of all these curves yields the DOS. As shown in Fig. 4, the DOS of smaller clusters (Aul3) consists of groups of levels which broaden to bandlike quasi-continuous manifolds with increasing cluster size (e.g. A~147,Fig. 4). Even for a cluster as large as Au147, the DOS is dominated by contributions from the surface atoms,1261thus only qualitative agreement with the bulk DOS is to be expected. For gold the effect of the spin-orbit interaction is missing in the scalarrelativistic results, providing a further discrepancy to a DOS obtained from a bulk band structure calculation.[311In general, a spatially local feature like the d-band width rapidly approaches the corresponding bulk value. For Auss a d-band width of about 6 eV has been calculated for both Ih and 01, symmetric geometries,[191
4.6 Naked, Ligated and Supported Transition Metal Clusters
1399
Figure 4. Density of states (DOS) for the clusters Au13 and Auld,. For Au147 increasing shading indicates contributions from the surface, subsurface, and more interior atoms, respectively. The Fermi edge is indicated by a horizontal line.
which agrees well with the experimental result of 5.7 t- 0.3 eVL3']for the bulk. On the other hand, the overall shape of the DOS is dominated by surface atoms which amount to at least 60Y0 of the cluster nuclearity (Fig. 4).[261 Their contribution is responsible for the triangular shape of the d-band while atoms from inner shells give an approximately constant contribution to the d-manifold (Fig. 4). The same qualitative characteristics have been observed for the DOS of Pd clusters.[251 A characteristic difference between the various transition metals, apart from the number of valence electrons, is their distribution between the d and s shells. In the atomic ground state configuration, Pd and Au both have a closed d shell (Pd dlO, Au d'Os'). In the clusters, an 'opening' of the d shell is observed because of s-d hybridization. In qualitative agreement with the bulk DOS of Pd, where the Fermi level lies at the upper edge of the d band,[331an average atomic configuration 5s0.885p0.164ds.96 is calculated for the cluster Pd147.[~~] Although the Fermi edge of and for clusters (Fig. 4), the d Au is situated well above the d band in the manifold has noticeable depletion. For Au147 an effective atomic configuration 6s0.936p0.475d9.575f0.03 has been determined.r261This reduced occupation of the Au 5d shell because of relativistically enhanced s-d hybridization has been shown to be important in gold-ligand interactions (cf Section 4.6.4.5).
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4 Nanomateriuls
Whereas Pd and Au clusters have low-spin ground states by analogy with the non-magnetic nature of the corresponding bulk metals, a particular situation arises for Ni because of the open d shell (atomic ground state configuration d9s1,almost degenerate with dss2). Free, gas-phase Ni clusters have a high-spin ground state, irrespective of cluster size.[231Their magnetization is even larger than in bulk Ni because the average coordination of atoms in a cluster is lower than in the bulk metal, an effect which is known to lead to an increased magnetic moment.[341 Although different experiments give different values for the absolute magnetic moment for Ni c l ~ s t e r s , [ ~there ~ , ~ is ~ ]a clear tendency for the average magnetic moment to converge to the bulk value, 0.6 pB, as the cluster size increases,[353361 The high number of unpaired electrons in the ground state of Ni clusters is a feature which is correctly reproduced by all-electron local spin density calculations. For an octahedral cluster of 44 Ni atoms the average magnetic moment per atom is of 0.7 pB,with little difference between surface and internal atoms of the cluster.[231The magnetization originates from the partial filling of the narrow 3d band. The Ni atoms assume an average configuration close to 3d94s', similar to that of atoms in bulk Ni. The 4s orbitals are very diffuse and their overlap gives rise to a wide band. The electrons occupying this band are all spin-coupled and do not contribute to the total magnetic moment. The 3d orbitals, on the other hand, are very localized and their mutual overlap is small. The presence of holes in the 3d band results on average in about 0.7 unpaired spins localized on the 3d orbitals for each Ni atom.[231 In addition to the valence electronic structure, which is essential for the chemical properties of clusters, the core levels are also of interest because they are often used as indicators of different atomic species and their chemical surroundings by X-ray photoemission spectroscopy. As an approximation to core-level binding energies, the (negative values of) Kohn-Sham orbital energies of Au and Pd clusters have been examined. Although final state effects are neglected by this approach, surprisingly good agreement has been found between the absolute core-level binding energies of central atoms of larger clusters and the experimental bulk values for Pd and A u . [ Similar ~ ~ ~to ~transition ~ ~ metal surfaces,[371for Pd 3d and Au 4f levels of 1 eV is ~ a l c u l a t e d .According ~~~~~~] atoms at cluster surfaces a destabilization of to the decreasing surface-to-volume ratio with growing cluster size, an increase of the average core level binding energy is obtained for larger clusters. The opposite trend has been found experimentally for supported Pd and Au cluster^,[^*^^^^ demonstrating the high sensitivity of this local probe to the surrounding cluster. s-d Hybridization has been identified as an important mechanism for the shifts of core levels in Pd clusters.r251 On the cluster average the d population per atom decreases, leading to a stabilization of the core levels owing to increasing delocalization of the valence electrons,[401which are partially transferred from d- to sp-type orbitals. Interestingly, not only differently coordinated cluster atoms (corner, edge, or face positions, Fig. 1) are discernible by core level shifts, but even atoms of differently oriented faces, for example the (1 1 1) and (100) faces for &-symmetry clusters.[261 A central question about the electronic structure of a metal cluster is how large it
-
1401
4.6 Naked, Ligated and Supported Transition Metal Clusters
Figure 5. Plot of first ionization potentials (IP) and electron affinities (EA) of Au, clusters against the inverse cluster radius R. Open and filled symbols denote o h and Ih, respectively. The bulk value is indicated by a dashed line.
- 2
0
1
2
3
4
1/R [nm-’1
has to be to have metallic properties. Although there is no uniform size threshold valid for all types of property, one way of addressing this question is by a comparison of experimental or accurate theoretical results to simple models of metallic behavior. For a metallic sphere of radius R and work function W , the classical spherical droplet m ~ d e l [ ~ ’yields , ~ ~ Ithe following expressions for the ionization potential (IP) and electron affinity (EA):
+ cte2/R
(1)
EA(R) = W , -pe2/R
(4
IP(R) = W ,
+
with the constraint CI p = 1. Calculated first IPS and EAs of Au clusters are shown in Fig. 5. For both IP and EA a remarkably linear dependence on the inverse of the cluster radius R is obtained, suggesting that metal-like behavior is already apparent even for quite small Au clusters.IZ6]This conclusion is further confirmed by extrapolation of the results to the bulk; this reproduces the metal work function to within less than 0.1 eV. It is, furthermore, important to note that the computed ‘gap’ values IP(R) - EA(R) yield a value of the a p constraint, 1.008, which is extremely close to the theoretical value. Both these observations strongly support the description of the clusters considered as spherical metal droplets. As demonstrated for Auss, the droplet model is also applicable with high accuracy to 2nd, 3rd, etc. IPS and EAs,[~’]confirming that a cluster of this size can be regarded as metallic, at least in respect of these properties. Further confirmation of the metallic
+
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4 Nunomaterials
behavior of Au55 is provided by comparison with the spherical jellium model for delocalized metal electrons? 91 the ordering of the 6s-dominated levels of the cluster follows very closely that of the jellium model, although the Au d orbitals do contribute to the valence levels and cannot be excluded when one analyzes the valenceelectronic structure of gold clusters.
4.6.4 Clusters in ligand shells 4.6.4.1 Low-nuclearity gold compounds - complexes or clusters? Before discussing various high-nuclearity clusters and cluster compounds we want to address the general question of what can be defined as a metal cluster. To this end we consider two examples of low-nuclearity Au compounds. According to a widely used definition, a metal cluster is characterized by direct chemical bonds between metal atoms.1441Here, we want to correlate the metal-metal distance in the cluster with the strength of the metal-metal bond. The clusters considered are the trigold oxonium complex[451and the recently synthesized gold-indium cluster^[^^,^^] all of which have a central A u ~unit. The ligand-free cluster Au) has an isosceles triangular shape as a result of Jahn-Teller distortion, with Au-Au bond distances of 2.52 and 2.64 A. For A u ~ +an equilateral triangular structure is calculated with an edge of 2.56 All these bond lengths are longer than in the dimer, 2.47 A, and shorter than the average bond distance in clusters (see Section 4.6.3) and in bulk gold, 2.88 A. Trigold oxonium cations consist of three Au-phosphine units bound to an oxygen center in a pyramidal geometry (Fig. 6). This compound had been synthesized from different phosphine ligands of varying steric bulk. For large phosphine ligands, L
L
Figure 6. Structure of a dimer of trigold oxonium, [(LAu)JO]Z~+.
4.6 Nuked, Ligated und Supported Transition Metal Clusters
1403
trigold oxonium is found as a monomer in the ~ r y s t a l , [ ~for ~ ,smaller ~ ~ l ligands, two different dimer geometries have been o b s e r ~ e d . ‘I [ ~The ~ ~Au-Au ~ ~ * ~distances and the oxidation state of the Au atoms are of crucial importance to the question under discussion. Optimization of trigold oxonium without ligands, [Au30]+, yields an Au-Au bond length of 2.78 A,whereas in the ligated cation [(AuPH3)30]+the distance is 2.88 Experimentally even larger distances, up to 3.3 A, have been det e ~ m i n e d , ~reflecting ~ the steric repulsion of voluminous ligands. All these ‘bond lengths’ exceed the values for Au3 to a considerable amount, clearly suggesting no or very little direct chemical bonding between the Au atoms. This is not surprising because an electron count and a population analysis give an oxidation state +I for Au, formally corresponding to a closed-shell species (d”). On the other hand, it is well known that there is a weak attraction between Au(1) species (‘d*O-d*Ointeract i ~ n ” ~which ~ ] ) is the driving force behind the dimerization of trigold oxonium cations. The resulting inter-unit Au-Au distances are of the same size as the intra-unit distances.[451Thus, no gold-gold contact encountered in trigold oxonium crystals resembles a direct bond in clusters of comparable size; therefore, we will not consider it as a metal cluster in this context. With regard to Au-Au bonding, the gold-indium clusters are a particularly interesting s p e ~ i e s . [ ~ ~In. ~both ’ l the clusters known an isosceles A u ~unit is present (Fig. 7) which is capped at both sides by In atoms. A third In atom bridges the short edge. The long edges are bridged by bidentate phosphine ligands (dppe). The In atoms are ligated by halogen atoms (C1 or Br) and tetrahydrofuran groups. For the C1 ligated compound,[461two very different Au-Au distances have been measured as 2.56 and 2.94A. A comparable difference is found for the Br ligated species.[471
Figure 7. Structure of the Au3In3 core of gold-indium clusters. Surrounding ligands are indicated schematically. The gold atoms are distinguished by their oxidation state.
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4 Nanomaterials
Oxidation state zero has been assigned to the two Au atoms forming the short edge, whereas the third Au center is a +I species. The two In centers were assigned an oxidation state intermediate between I and I1 because their Mulliken charges are very similar.[471The short Au-Au distance between the Au(0) centers is a good fit to the corresponding bond length in Au3+ and must be regarded as a direct metalmetal bond. The longer Au(0)-Au(1) distance, on the other hand, is not consistent with the interpretation of the central gold triangle as a A u ~ +unit; rather, this Au-Au contact is similar to that in trigold oxonium cations, exceeding even the bulk nearest-neighbor distance, 2.88 A. The origin of this bond elongation, relative to the structure of ‘naked’ Au3+, has been traced back by geometry optimizations of various cluster fragments;1471it is found that ligation of Au(1) by two phosphines induces a charge imbalance whereas both Au(0) centers are only singly connected to phosphine ligands. The difference between the two Au-Au bonds is further enhanced by the face-capping In groups. From these results it can be concluded that these Au3In3 compounds are at the borderline between complexes and clusters, incorporating both a direct chemical Au-Au bond and weaker intermetallic contacts. In conclusion, analysis of bond distances is a useful tool for describing the bond character in a polynuclear complex. Other examples of this approach are given below.
4.6.4.2 Metal-ligand interaction Ligand-stabilized clusters are a very important class of inorganic material. They can be regarded as formed by a metal core surrounded by a shell of more or less ‘inert’ ligands which further contribute to the stability of the system. The ligands also prevent the coalescence of the small metal cores into bigger particles. The role of the ligands in these compounds, often called molecular clusters, has been a topic of discussion from the very beginning of the cluster era. The classical view of a molecular cluster is that of a small piece of metal surrounded by ligand molecules ‘adsorbed’ on it.r441This view is indirectly supported by the fluxional nature of the ligands which at finite temperatures move around the metal core in the same way as molecules adsorbed on metal surfaces can diffuse from site to site, depending on the energy barriers to diffusion.[531As will be shown below, the idealized picture of a cluster core with metallic character surrounded by weakly perturbing ligands is at least incomplete, if not inadequate. To better understand the nature of the metal-ligand interaction it is useful to analyze the bonding of a molecule with a single metal atom. In this review we focus only on the late transition metal (TM) atoms on the right of the periodic table. Clusters of these atoms interact preferentially in low oxidation states with neutral molecular ligands like CO, NO, phosphines, etc. All these ligands are bound to metal atoms through the classical donation-back donation m e ~ h a n i s r n . [51~Ac~,~ cording to this scheme the ligand donates charge to the metal through a lone pair
4.6 Naked, Ligated and Supported Transition Metal Clusters
1405
orbital, and the metal d orbitals donate charge back to the low-lying empty orbitals of the ligand. One important interaction, not included in the original Dewar, Chatt, . ~that and Duncanson d e s c r i p t i ~ n , [ ~is , ~ ~of ] the non-bonding occupied orbitals on the interacting fragments. The Pauli exclusion principle ensures that two electrons of the same spin cannot occupy the same spatial region. Thus, when two occupied orbitals on different fragments point at each other in close contact, strong repulsion occurs (also called Pauli or exchange repulsion). An important manifestation of this effect can be illustrated by the mechanism of the interaction of a lone pair ligand L (CO, NO, PR3, etc.) with a transition metal atom with partially filled nd and (n + 1)s shells. This is indeed the most common situation for TM atoms, with the only exception of Pd which has a 4d1°5s0 atomic ground state.r561The nd, (n + 1)s and (n + l)p orbitals of a TM atom have very different spatial extensions, with the nd shell being much more contracted than the (n 1)s and (n l)p orbitals (Fig. 8). This means that when the distance between a TM in its ground state and a lone pair ligand decreases, the first orbital interaction involves the diffuse (n + 1)s and the lone pair orbitals. Both these orbitals are occupied. If the ( n + 1)s orbital is doubly occupied the interaction with the ligand will be very repulsive because four electrons are forced to occupy the same region; if the (n 1)s orbital is singly occupied the interaction will still be repulsive although to a lesser extent (Fig. 9).
+
+
+
Y-
O
Figure 8. Schematic representation of the radial extension of the valence orbitals of a transition metal atom.
0
1.5 3.0 4.5
Radius 1 A
1406
4 Nanomaterials
w
I
Bond length M-L
Figure 9. Potential energy curves of a transition metal in different configurations interacting with a CO molecule.
In general, formation of a ligand-metal bond entails a rearrangement of charge. The energy required to promote the lone pair electrons to a previously unoccupied orbital on the ligand itself is usually much larger than the energy necessary to redistribute the (n + 1)s electrons into the partially filled nd shell of the TM atom (Pd and the coinage metal atoms with their completely filled d shell provide exceptions to this statement). Actually an internal excitation of the type ndqPr(n+ 1)s' + ndq-'+l (n + I)&' occurs. This internal rearrangement is of fundamental importance for bond formation. In fact, the interaction between a TM atom in the state ndq(n + l)so and a lone pair ligand is attractive because of the efficient overlap between ligand and metal d orbitals at a relatively short distance. On the other hand, if the (n + 1)s level of the metal remains occupied, then the repulsion with the lone pair prevents the ligand from coming close enough to the metal to interact directly with the contracted nd level^.^^^,^*^ To summarize, one can schematically divide the interaction between a lone pair ligand, L, and a TM atom into three steps:[591 i) the initial repulsion between L and the metal (n + 1)s electrons; ii) electron redistribution within the TM orbitals which increases the nd character and reduces the (n + 1)s character; and iii) the formation of a classical donation-back donation bond which involves the direct overlap between the lone pair orbital on L and the emptied (n + 1)s TM orbital as well as the TM nd levels and the empty orbitals of L.
This bonding mechanism is quite general and has important consequences for understanding the properties of mononuclear or polynuclear organometallic complexes. In fact, it is quite obvious that bonding with ligands causes a substantial change in the nature of the metal atom. This is clearly shown by the simple case of
4.6 Naked, Ligated and Supported Transition Metal Clusters
1407
the interaction between a Ni atom, which has a 3F(3ds4s2)ground state almost degenerate with the 3D(3d94s') state in g a s - p h a ~ e , ' ~and ~ ] four CO molecules to give Ni(C0)4. The resulting Ni(C0)4 complex is, in fact, diamagnetic and has a closed shell *Al, ground state. This means that, without change in multiplicity, the asymptotic dissociation limit for the reaction Ni(C0)4 ('A',) + Ni ('S) + 4 CO (IC,') must lead to singlet fragments and in particular to a closed shell, excited state Ni 3d1° atom. Conversely, this implies a crossing of triplet and singlet electronic states during bond formation and thus a change in the TM atom electronic configuration. It is interesting to mention that this configuration change occurs also when a single CO molecule interacts with a Ni atom to form the molecule Ni-C0.[571 This digression about metal-ligand interaction is essential if one wants to understand the role of the coordination sphere in stabilizing the metal core of a ligated metal cluster. It is also essential for understanding the changes in the electronic structure of a metal particle induced by ligand molecules.
4.6.4.3 Magnetic quenching in carbonylated Ni clusters In a series of papers we have analyzed the influence of the ligands on the electronic properties of a small metallic particle in detai1.[60-641The study of the effect of metal-ligand bonding on cluster electronic structure is not restricted to purely academic interests. In fact, large metal clusters of a few tens or hundreds of atoms stabilized by ligand shells in principle represent an important class of monodispersed colloids with potential useful applications in material science and catalysis. The key question, however, is the extent to which the presence of the ligand shell, which is essential to prevent the coalescence of the metal cores into large metallic particles, does perturb the electronic structure of the metal core. Several physical measurements under controlled conditions have been performed on clusters and colloids in ligand shells to answer this question.[6s1The general message emerging from these measurements is that the ligands do perturb the outermost shell of metal atoms in a spherical particle. The metal atoms at the cluster surface, which directly interact with the ligands, have distinct behavior, different from the rest of the metal atoms inside the cluster. In a very simplified picture, one can say that whereas metal atoms in the core of the cluster maintain most of their 'metallic' properties (at least to the extent that they exhibit such behavior, Section 4.6.3), metal atoms on the cluster surface lose part of these metallic properties as a consequence of their interaction with the ligands. Below we briefly summarize the fundamental reasons for this behavior taking the case of carbonylated Ni clusters and of their magnetic properties as an example to illustrate these concepts. We have seen that ligand-free, gas-phase Ni clusters have magnetic ground states (Section 4.6.3). This is completely different from carbonylated Ni clusters of the same size. Accurate magnetic measurements on these systems have unambiguously
1408
4 Nunomaterials
shown that the cluster ground state is diamagnetic.[651Thus, a strong change in the bare cluster electronic structure occurs when one adds the ligands to the metal core. This effect has been reproduced by LSDA calculations on a series of free and carbonylated Ni c l ~ s t e r s . ~These ~ ~ - ~systematic ~] studies have enabled us to formulate a general model for the quenching of the magnetic moment.[641This model is essentially derived from the basic concepts of the metal-ligand interaction described in the previous paragraph; an extension to clusters is quite straightforward. Instead of having the metal atom nd and ( n + 1)s orbitals, in a cluster we have a set of molecular orbitals with dominant d or s character (for Ni 3d and 4s, respectively). Resorting to the terminology of ‘bands’, we can state that Ni clusters in the gas phase have holes, i.e. unpaired electrons, in the 3d band which are the origin of their large magnetic moments.[661The 4s ‘conduction’ band electrons are spatially more diffuse than the 3d band; thus, when lone pair ligands like CO are added to the Ni, cluster, the 4s-derived levels are destabilized by the repulsive interaction and moved above the Fermi level (Fig. 10). The 4s-band electrons are promoted into the 3d shell where they fill the holes with consequent quenching of the magnetic moment. The mechanism is the same as for a single metal atom, only the rearrangement of the electrons within the metal core involves several (cluster) energy levels and is easier energetically. Formally, a configuration change from 3d94s’ to 3d1° is the cause of change in magnetization (from magnetic to diamagnetic), in a carbonylated Ni cluster just as for a single atom in Ni(C0)4. The CO ligands bind to the Ni atoms via the usual 0-donation z-back donation mechanism. There are a few important consequences of this interaction, besides the magnetic quenching. While free Ni clusters have virtually no gap at the Fermi level, a typical sign of developing metallic character, carbonylated Ni clusters do have a gap of cu 1-2 eV, typical of semiconducting materials (Fig. 11). Calculations on large Ni clusters containing up to 147 metal atoms and a simple model of a ligand shell have shown that the interaction with the ligands induces electron rearrangement only within the surface Ni atoms of the cluster, while the internal atoms are virtually unperturbed by the presence of the ligand~.[’~] This is a very important conclusion. It shows that two kinds of metal atoms can be distinguished in a high-nuclearity ligated cluster, the surface metal atoms and the internal metal atoms, in agreement with experimental findings discussed above. In this respect, it is important to stress that even in an icosahedral particle of 147 atoms most of the atoms, 92, are on the surface and only 55 occupy internal positions, Fig. 1. Clearly, the extent to which a ligated cluster has ‘metallic’ properties depends in general on the ratio of ‘bulk’ to surface atoms. Another noteworthy consequence of the metal-ligand interaction in polynuclear cluster complexes is the reduction of the metal-metal bond strength in a ligated cluster compared to the bare metal core without ligands. To illustrate this effect we consider again Ni clusters and in particular a series of Nig clusters stabilized by terminal CO and p4-X ligands where X = PR, S, GeR, Te, As, etc. Several clusters
4.6 Nuked, Ligated and Supported Transition Metal Clusters
1409
Energy [ e v ]
-2
-4
-6
-8
-10
-12 Figure 10. Schematic orbital interaction diagram of Ni, interacting with a (CO), polyhedron to form Ni,(C0),.[621
I bare Ni,
4a
Ni,(CO),
n (CO),
of general formula Ni8(p4-X)6(L)8 where L = CO or PR3 have been synthe~ized.[~’-~’] All have the same structure with eight Ni atoms at the vertices of a regular cube, the terminal ligands, CO or phosphine, bound to the Ni atoms, and six phosphido groups capping the square faces of the cube (Fig. 12). The Ni-Ni distances, determined by X-ray diffraction, are ca. 2.61-2.67& i.e. considerably longer than the Ni-Ni distance in the bulk metal, 2.49A. The geometric structure of Nig (p4-PH)6(CO)8has been optimized at the LSDA level;[701the resulting optimum Ni-Ni distance is 2.65 A, in close agreement with experiment. When the same distance is recomputed after stepwise removal of the ligands, a progressive reduction, hence an increase of the metal-metal bond strength is found
1410
4 Nanomaterials 6
4
-2
-4
Figure 11. Calculated DOS of the cluster [Nis(C0)12l2- (in arbitrary units).
Figure 12. The structure of the model cluster compound “ix (P‘l -PHI6(COhI.
4.6 Nuked, Ligated and Supported Transition Metal Clusters
1411
Table 1. Nearest-neighbor Ni -Ni distances in cubic Nig clusters from theory and experiment. Cluster
r(Ni-Ni)
(A)
(Table 1). Under cubic symmetry constraints, the bare Nig cluster nearest-neighbor distances are 2.19 A,i.e. 0.3 A shorter than in the bulk. The addition of the terminal CO ligands or of the four-coordinated PH ligands results in an increased Ni-Ni distance; when the ligand shell is complete the elongation of the Ni-Ni distance is substantial, indicating progressive weakening of the metal-metal bond. It is worth noting that in the experimentally isolated cluster compound Nig(,u4-PPh)6(PPh3)4,in which the ligand coordination sphere is not complete, average Ni-Ni distances are shorter by more than 0.1 A than in the other clusters which are coordinatively saturated.[691Thus, both theory and experiment indicate the existence of a direct correlation between the presence of the ligands and the strength of the metal-metal interaction, hence the length of the metal-metal bond. The presence of the ligands has clearly the effect of reducing the amount of metalmetal bonding in the metal cage. Indeed, this interaction scenario brings to mind the principle of bond-order conservation, annunciated to describe the effects of adsorption on bonding, both within the metal substrate and the adsorbate.[711
4.6.4.4 Bimetallic clusters Fe- Ag bimetallic clusters Bimetallic cluster compounds containing coinage metals and peripheral Fe(C0)4 groups have been synthesized and show a considerable structural ~ a r i e t y . [ ~ ~ - ~ ~ I Particularly interesting are the clusters of composition [M4{Fe(C0)4}4I4- (M = Ag, Au). Clusters of other stoichiometry have been isolated with M = C U . [ ~The ~ ] metal core of clusters containing up to five coinage metals is planar. D F calculations performed at the scalar-relativistic have shown that the optimized structure of the clusters [M4{Fe(C0)4}4l4- (M = Ag, Au) is close to that obtained from X-ray crystallographic data (Fig. 13). The main difference lies in the Ag-Ag distance which in the calculations is ca. 0.2 A shorter than in the experiment.[731It should be noted, however, that changes
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4 Nunomaterials
V
Figure 13. The structure of the mixed metal cluster [M4{Fe(C04))414- (M = Cu, Ag, Au) Noble metal atoms are shaded, iron atoms are black, and CO ligands white.
in the Ag-Ag distance (while keeping the other structural characteristics of the cluster fixed) result only in very small energy changes. This indicates that the Ag-Ag contacts are very soft and that solid-state effects (crystal field, packing, etc.) can easily be responsible for non-negligible modifications in the Ag-Ag distances within the cluster. The role of the ligands has been investigated by calculating the electronic structure of the bare M4Fe4 clusters.[781One major effect can be observed - the metalmetal distances in the bare clusters are much shorter than those in the carbonylated analogs. This seems to indicate weakening of the metal-metal bond because of the presence of the ligands, in line with the observations on Ni clusters (Section 4.6.4.3). This effect is a consequence of the destabilization of the (n + 1)s-derived metal orbitals owing to interaction with the CO ligands. This weakening of the metal-metal bonds in the ligated clusters is also shown by the fragmentation energies and by the fact that the cluster magnetization is quenched. The analysis of the electron configuration of the cluster compounds and of the charge on the M atoms helps to clarify further the nature of the bond in these syst e m ~ . [In ~~ naked ] M4Fe4 clusters there is no evidence of appreciable charge transfer from the coinage metal to Fe or vice versa. This is typical of intermetallic bonds in alloys. The situation is considerably different when the carbonylated clusters are considered where, as a major effect, noticeable charge transfer occurs from M to Fe, decreasing along the series Ag > Cu > Au. Formally, the M atoms change their oxidation state from 0 to +I. This has important consequences for the rationalization of the geometry of the clusters. In fact, Cu(I), Ag(I), and Au(1) compounds show a general tendency to form linear complexes of the type X-M-X (X = NH3, C1-, etc.). This is the same arrangement assumed by the Cu, Ag, and Au atoms in the [M4{Fe(C0)4)4l4- clusters (Fig. 13). The charge transfer from M to Fe is well illustrated by the orbital interaction scheme between Ag4 and four Fe(C0)4 units (Fig. 14). The 4d orbitals of A& are highly stabilized by the interaction with the Fe(C0)4 groups and provide the first clear evidence of the electron flow, because the rather localized d orbitals are very
4.6 Naked, Ligated and Supported Transition Metal Clusters
1413
Figure 14. Orbital interaction energy diagram of Ag4 (a) and Fe(CO)4 (c) to give [AD (Fe(CO4)M4- (b).
sensitive to changes in the atomic charge. A reduction of the number of electrons occupying the d levels results in less screening of the nuclear charge and in lowering of the orbital energies. The frontier orbitals of the Fe(C0)4 units basically maintain their character and are stabilized by interaction with Ag4 while the s-derived levels of the Ag4 cluster are significantly destabilized. Consequently, four electrons are transferred from the 5s-type levels of A& to the Fe(C0)4 units and the Ag atoms become formally oxidized to Ag(1). This charge transfer renders the Fe(C0)4 units closed shell and demonstrates their Lewis acid character. That the charge transfer from Ag4 to the Fe(C0)4 'ligands' is real is demonstrated by the shifts of the core levels of the Ag atoms to higher binding energies.r781 This shift is a direct consequence of the reduced electron density in the valence shell of the atoms. When the isovalent [M4{ Fe(C0)4}4I4- clusters are considered with M = Cu, Ag, and Au, it is found that the shift of the metal 1s core levels follows the trend Cu > Ag > Au, suggesting more pronounced charge transfer for the lighter elements. The bonding mechanism described for the bimetallic [M4{ Fe(C0)4)4l4- clusters is not restricted to the tetramers. The largest carbonylated Ag-Fe cluster synthesized so far is [Ag13{Fe(C0)4}gl4- (Fig. 15).[771The cluster core consists of 12 Ag
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4 Nanomaterials
Figure 15. Structure of the mixed metal cluster [Agl3{Fe(C04)}8l4-. Silver atoms are shaded, iron atoms black, and CO ligands white.
atoms in a cuboctahedral arrangement with a central Ag atom. The average radial and tangential Ag-Ag distances are both ca. 2.9A. In this cluster, the Fe(C0)4 ligands cap the triangular faces of the Ag13 cuboctahedron. Transfer of 13 Ag 5s electrons to the eight Fe(C0)4 units leaves one unpaired electron which interacts only weakly with the Ag atoms of the cluster. Spin density plots computed at the D F level show that this electron is largely localized on the Fe centers and that the spin density on Ag is in broad agreement with results from ESR spectroscopy.f771 These results indicate that Ag-Fe bimetallic clusters can hardly be considered as being formed by a Ag-Fe metal core surrounded by neutral CO ligands. Analysis of the electronic structure has clearly shown that partitioning of the cluster into an M, core ligated by Fe(C0)4 units is more suitable. These complex ligands act as electron acceptors from the Ag atoms which formally change their oxidation state from 0 to +I. This is partly because of the repulsive interaction with the diffuse 5s levels of the Ag centers which leads to a destabilization of these orbitals, and in part to the electron-acceptor character of the CO ligands. The possibility of delocalizing the transferred charge over four CO ligands of Fe(C0)4 provides an additional mechanism for stabilization. Thus, the metal-metal bonds within the Ag-Fe units are weakened and the Ag-Ag modes are very soft. This results in the easy deformation of the internal Ag cluster with rather large variations in the Ag-Ag distances.
4.6 Naked, Ligated and Supported Transition Metal Clusters
1415
Figure 16. The structure of the carbonylated bimetallic cluster [Ni38Pts(C0)d8l6-.
Ni-Pt bimetallic clusters As a second example of bimetallic cluster properties we consider the high-nuclearity cluster [ ~ i ~ ~ ~ t 6 ( ~ 0(Fig. ) 4 8 16), ] ~ -one of the largest organometallic clusters isolated and characterized by X-ray diffraction technique^.^'^] It consists of an octahedral core of six Pt atoms embedded in a larger octahedron of 38 Ni atoms. 48 CO molecules are bound to the surface Ni atoms and form the ligand shell. Very accurate physical studies were performed on this cluster to elucidate its electronic structure.[80.811In particular, low-temperature measurements on a single crystal of [Ni38Pt6(C0)48I6-clearly demonstrated a very Small residual magnetization of 0.0 1 pB/atom at T = 1.7 K in a field of 3 T.[*ll This magnetization arises because one out of five [~i&6(CO)48]~-clusters carries a net spin of 1 pB.The whole crystal behaves as a dilute Heisenberg antiferrornagnet.["] What is rather difficult to explain is why only a fraction of the clusters have a weak magnetic ground state when most are diamagnetic. Several mechanisms could account for this phenomenon (traces of magnetic impurities, deviations from exact stoichiometry, contamination at the sample surface, etc.). However, a quantum-mechanical analysis of the problem provides a simple yet convincing explanation.@'] Spin-polarized LDA all-electron scalar-relativistic calculations were performed
-
1416
4 Nanomateriuls
Figure 17. DOS of [Ni38Pts(CO)48l6-. The contributions of bulk nickel atoms (. . .), of Pt atoms (-), and of surface Ni atoms (---) are shown separately. In the sketch of the cluster structure Ni atoms are indicated white, Pt atoms black.
on [Ni38Pt6(C0)48]6-;[811 the cluster ground state was found to be diamagnetic, in agreement with the experimental observation that this is the character of most clusters in the crystal. The bimetallic nature of the [Ni38Pt6(C0)48I6-cluster, however, suggests possible random interchanges in the position of the Ni and Pt atoms. If one formally replaces all six internal Pt atoms by Ni atoms, one obtains the hypothetical cluster [Ni4(C0)48I6- which has a total magnetization corresponding to 3.6 unpaired electrons according to DF calculations.[641Analysis of the spin distribution indicates complete localization of the unpaired electrons on those ‘internal’ Ni atoms which have a complete bulk Ni nearest-neighbor shell. Only these internal Ni atoms contribute to the density of states at the Fermi level (Fig. 17); hence, the ‘metallic’ properties of the cluster are entirely dependent on these atoms. Thus replacement of the internal Pt atoms by Ni atoms results in a cluster with a magnetic ground state. This suggests the following argument - a random positional interchange of a Pt atom in the interior of [Ni38Pt6(C0)48j6-and a surface Ni atom could result in a residual magnetic moment of the cluster. In fact, such an interchange can occur in the ‘alloy’ cluster Ni38Pt6 at low energy cost. To maintain the octahedral symmetry of the cluster, calculations have been performed for a model
4.6 Nuked, Ligated and Supported Transition Metal Clusters
1417
isomer of [Ni38Pt6(C0)48I6- where all the Pt atoms reside at the vertices of the external octahedron.[8'I The ground state of this isomer is calculated to be magnetic, with 4.1 unpaired electrons per cluster, in agreement with the fact that the 'internal' Ni atoms are embedded in an almost complete Ni environment. An assumed interchange of only one 'internal' Pt atom with a surface Ni atom would lead to an isomer of [Ni3sPt6(C0)48l6- with net magnetization close to 1 pB. The magnetic measurements suggest that this occurs every five cluster units.["] These results show that different isomers of bimetallic clusters can behave quite differently as a function of the structure. In fact, Pt atoms, thanks to the larger radial extent of the 5d orbitals, have a smaller tendency to form ferromagnetic domains, at variance with Ni atoms. Positional interchange of Ni and Pt atoms can be the origin of unusual variations in magnetization for different isomers of the same organometallic cluster, a possibility that has yet to be explored for deriving molecular systems with unusual magnetic properties. In this respect bimetallic clusters can also be considered as useful models of metal alloys.
4.6.4.5 Interstitial atoms in clusters Clusters with interstitial atoms are an interesting class of polynuclear complexes. Several examples of compounds with interstitial atoms (H, C, Ge, N, P, S, etc.) have been reported.[821These compounds exhibit special behavior because the presence of main-group atoms in the cluster cage influences the chemical reactivity and stability of the system and its geometric structure. In the following section we consider some of these peculiarities, related both to the stability and to the electronic properties of clusters with interstitial elements.
Main-group element centered Au clusters Among main-group element centered metal clusters, gold clusters have recently received considerable a t t e n t i ~ n31. ~Tetrahedral, ~ trigonal-bipyramidal and octahedral clusters of constitution [(LAu),XIn+ with interstitial X = As, C, N, and P atoms have been s y n t h e s i ~ e d , [ ~ ~(Fig. - * ~ ]18). The phosphine ligands L stabilize these Au clusters, which can be regarded as formed by Au(1)L building blocks, arranged around a main-group element X. The total charge of the clusters varies from 0 to f3, depending on the central atom X and the number of AuL units. The electronic structure of the isoelectronic series of six-coordinated clusters [(LAu)6XIn+ (X = B, C, N and n = 1, 2, 3, respectively) and of their trigonalbipyramidal and tetrahedral analogs has been studied by means of relativistic LDA calculations.[901The triphenylphosphine ligands used experimentally have been modeled by PH3 to determine the cluster geometry; for more accurate evaluation of binding energies, PMe3 ligands have been employed. This combined strategy has been demonstrated as being reliable for smaller Au complexes.[9 Of the octahedral
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4 Nanomaterials
\
L
L
L
X = B C N g = +I+2 +3 Figure 18. Structures of main-group element (X) centered gold phosphine (L) clusters.
clusters only the carbon-centered member has been synthesized;[891 the synthesis of the corresponding nitrogen compound is still under discussion.[8 To understand the interaction of the atoms or groups of atoms forming the cluster [(LAu)gCI2+,we analyzed the bonding in the metal core [Au&]2+.r90.921 One can start by assuming the Au 5d shell is completely filled and that the effect of the phosphine ligands can be neglected in this simplified analysis. In fact, it has been shown that this is not correct - after proper inclusion of relativistic effects there is an opening and rehybridization of the 5d shell which leads to an effective configuration for Au that is close to 5d9.56(sp).[93,941 For a qualitative one-electron analysis we can, however, think of the system as formed by interaction of Au 6s orbitals with those of the central atom. More accurately, each Au atom contributes one valence orbital formed by hybridizing the Au 6s and 6p orbitals. In octahedral symmetry these orbitals form a set of alg, tl,, and eg levels. The alg and tl, orbitals have the proper symmetry to interact with the s and p orbitals of the central atom, in this example a C atom. The C atom contributes four valence electrons; each of the six Au atoms provides one valence electron; because the total charge is +2 only the four lowest levels alg and tl, are occupied (Fig. 19) resulting in a stable closedshell configuration. If one changes to a central B atom, with only three valence electrons, the net charge should be f l to achieve an isoelectronic situation; for a central N atom, with five valence electrons, the cluster charge should be +3. In the naked Aug cluster core the tangential metal-metal bonding dominates. With addition of the central atom X and the formation of the charged species, strong radial bonds are formed between the Au atoms and the interstitial atom X.r901When going from a neutral Aug octahedral cluster to [AugCI2+there is, however, an expansion of the octahedron; this effect is found also with B and N and is more or less of the same magnitude, ca. 5%. Among the three interstitial atoms considered, B is most strongly bound to the bare Aug unit, followed by C and N, the latter being considerably less strongly bound (Table 2). There are various rea-
4.6 Nuked, Ligated and Supported Transition Metul Clusters
I
1419
\-I
Energy
Figure 19. Orbital interaction diagram for a central carbon atom with an Aug cluster to give [Au6C12+.
sons for this behavior. Firstly, the size of the atoms decreases from B to C to N; consequently the N orbitals have less overlap with the Au orbitals. Also the energetic position of the valence orbitals plays a role; N has the lowest-lying orbitals in the series and thus gives rise to the weakest interaction with the metal atoms. A very important destabilizing factor (that goes beyond the simple orbital picture) is, however, the increase in cluster charge, thus in Coulombic repulsion, when going from B to N. The phosphine ligands L are strongly bound to the [Au,jX]"+ clusters (Table 2). Notice that PMe3 ligands have larger binding energies than simple phosphine
Table 2. Binding energies (eV) of main-group element centered Au clusters.
Cluster charge, n central atom, X
+1
+ 6 PH3 [Au~]"+ + 6 PMe3 [AQX]"++ 6 PH3 [Au6X]"++ 6 PMe3 [A%]'+ f x [ ( H ~ P A u ) ~ ] " +X [(Me3PAu)6]"+ X
-11.2 -13.4 -12.4 -14.5 -8.2 -9.4 -9.3
+ +
B
+2 C
+3 N
-15.6
-21.1 -26.8 -25.7 -31.5 -2.2
-19.2 - 18.5
-22.3 -6.9 -9.8 -10.0
-6.8
-7.0
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4 Nanomaterials
ligands PH3. An important question, however, is how the presence of the central atoms affects the binding energy of the ligands. First we note that the phosphine ligands bind ca 20% more strongly to [AUgX]"' than to the 'empty' cluster [AUs]"'. Secondly, by increasing the cluster charge from +1 to +3 there is a corresponding increase in the energy of interaction with the ligands. This is not surprising because phosphines act as a 0 donors and the donation mechanism is reinforced when the cluster is electr~n-deficient.[~~~ An important difference among the three clusters containing B, C, and N emerges when one considers the bond strength of the central atom to the rest of the cluster with and without the phosphine ligands. For the naked cluster, we have seen that the B-centered structure is by far the most stable, followed by C and by N (Table 2). The order changes, however, with the C-centered cluster being most stable, when the ligands are present (Table 2). Thus, two trends can be clearly identified by comparing the B-, C-, and Ncentered Au clusters.1901 i) The binding energy of the central atom to the metal framework without ligands decreases by increasing the atomic number of the central atom, because of reduced overlap between the main-group element orbitals and the Au orbitals and decreasing electrostatic stability because of the increasing cluster charge. ii) The binding of the ligands to the centered Au cluster becomes stronger as the net cluster charge increases (from B, to C to N); this is readily explained by the increased donation from the phosphines to the empty cluster orbitals. Thus, these two trends work in opposite directions - their combined effect results in maximum stability for the C-centered Aug cluster, the only octahedral structure which has been synthesized so far. The stability differences among the three structures are, however, not so large that the formation of the B- and N-centered compounds is excluded. The relative stability of the analogous tetrahedral and trigonal bipyramidal element-centered compounds [(LAu)5X]("-')+ and [(LAU)~X]("-~)+ (X = B, C, N and n = 1,2, 3, respectively) is governed by the same interplay of Au-X bonding, ligand stabilization, and total charge as for the octahedral clusters discussed above.r901For the trigonal bipyramidal compounds, the C-centered species has also been found to be the most stable. For the tetrahedral clusters, the stability maximum shifts to the N-centered species. Both these findings are in agreement with experiments. Concerning possible dissociation of the octahedral or trigonal bipyramidal clusters into smaller species by abstraction of an AuL+ unit (Fig. 18), one notes that all clusters have been found to be stable with the exception of [ ( L A U ) ~ N ] ~ This + . [ ~result ~ ] is in accord with the observation that in this process the stabilization of the clusters by the ligand shell decreases as the overall charge is reduced. The exception of [ (LAu)5NlZ+ is related to the particular stability of the N-centered tetrahedral cluster mentioned above. It is interesting to note that cluster stability is reduced if Au is substituted by Cu, forming the hypothetical isoelectronic cluster compound [ ( R ~ P C U ) ~ C ] Most ~+.[~~]
4.6 Nuked, Ligated und Supported Transition Metal Clusters
1421
of the difference in stability between the Au and the Cu complexes is because of weaker bonding of the phosphine ligands to the cluster, 2.8 eV for Cu and 3.1 eV for Au, while the [MgCI2+ cluster cage has almost the same stability toward fragmentation 20.4 eV for Cu, 20.0 eV for Au. This result further demonstrates the importance of the metal-ligand interaction for the global stabilization of organometallic clusters. ~
Ni32C,j(C0)36 nickel carbido cluster In this paragraph we consider another aspect of the presence of interstitial atoms within the cage of metal clusters. Examples of high-nuclearity metal clusters with incorporated main group atoms have been reported. One of these clusters is [ N i & 6 ( C 0 ) ~ ]n - - ; [ 9 7 1 if one idealizes the structure by eliminating the six Ni-CO groups which cap some of the triangular faces of the regular Ni32 polyhedron, one obtains the more symmetric [Ni32C6(C0)36In- cluster. It consists of eight internal Ni atoms in a cubic arrangement surrounded by 24 Ni atoms which form a truncated octahedron. The cluster can be viewed as formed by six edge-sharing Nig square antiprism units; six interstitial C atoms occupy the center of each Nig square antiprism (Fig. 20) This ‘hypothetical’ cluster was originally studied by LSDA calculation^;[^ it has recently been synthesized, and characterized by X-ray diffraction.[991 13981
Figure 20. The structure of the cluster [Ni&j(C0)36In
1422
4 Nunornaterials
Table 3. Total number of unpaired electrons, It, in naked and carbonylated Ni32 clusters with and without interstitial atoms.
The interstitial C atoms give the structure additional stability by forming strong radial bonds with the Ni 4s and 3d orbitals; the fragmentation energy per atom increases from 4.1 to 4.9 eV when they are added to the 'naked' Ni32 cluster to formally yield the cluster Ni32C6. The C atoms, however, also have the effect of inducing a substantial change in the overall electronic structure of the cluster as shown for instance by the computed magnetic moments listed in Table 3. Addition of the interstitial C atoms to Ni32 results in a ligand-free nickel-carbide cluster which still has a magnetic ground state (ca. 10 unpaired electrons). The partial reduction of the initial magnetization of the bare Ni32 cluster is the consequence of the direct orbital interaction between C s and p orbitals and Ni 3d levels. Strong Ni-C covalent bonds are formed and the number of unpaired electrons is reduced. Addition of the CO ligands to Ni32 also reduces the magnetization, an effect already analyzed in Section 4.6.4.3, so that the cluster N i ~ ( C 0 ) 3 6has a residual magnetization of ca. four unpaired electrons which are located on the internal Ni atoms. Thus, the Nis cluster core keeps some 'magnetic-metallic' character, at variance with the surface Ni atoms which have their magnetic moments completely quenched by the interaction with the ligands. When, however, the six interstitial C atoms are added to complete the 'model' cluster, a diamagnetic ground state results, as indeed is observed for the parent cluster [Ni3*Cs(C0)42]"-.[971 This result emphasizes the role of interstitial atoms in determining the electronic structure and the properties of these materials. The Ni& core of the cluster can be regarded as a microscopic fragment of a nickel carbide. The interstitial main group atoms form strong bonds with the surroundings; these bonds may be viewed as initial step in the formation of a different phase (carbides, nitrides, etc.) within the cluster complex.
4.6.5 Supported metal clusters A discussion of the electronic structure and properties of clusters, naked or ligated, would not be complete without addressing the problem of supported metal clus-
4.6 Naked, Ligated and Supported Transition Metal Clusters
1423
ters*"00.10 11 Ve ry often, in fact, clusters are deposited from the gas-phase or from solution on to an 'inert' support. The clusters are then anchored to the substrate (usually an insulating oxide such as SiOz, A1203, or MgO) by chemical or physical forces. If the precursor is a ligated cluster, the ligands can be removed by thermal treatment and ligand-free supported metal particles can be formed on the surface. New techniques have, recently, been developed for depositing clusters from the gasIn particular, clusters produced by laser phase under controlled evaporation can be mass-selected and then deposited on to a substrate; in this way well defined, monodispersed supported particles can be formed. Cluster deposition by other techniques has also been proposed with the aim of investigating the growth of thin metal films on insulators or Supported clusters, thin metal films, etc. are of great technological importance in catalysis (see Chapter 2.2 in this book), microelectronics, coating of materials, electrochemistry, etc. Ideally, the substrate should be sufficiently inert to guarantee that the metalsubstrate interaction will not affect the properties of the metal particle. Often, however, a relatively strong interaction can occur between substrate and metal species, in particular for small clusters. In a sense, the oxide surface can be considered to be a very special case of a 'ligand' environment stabilizing the metal cluster. The metal electronic states are, however, always perturbed by interaction with the substrate, just as for proper ligands. Very little is known theoretically about the metal-oxide interface although a rather limited number of 'first principles' theoretical studies have dealt with the general problem of metal-ceramic i n t e r a ~ t i o n . [ ' ~ ~ -The " ~ ]aim of this section is to discuss some fundamental aspects of the bonding of metal clusters deposited on the surface of simple binary oxides, in particular MgO and A1203. Before addressing the specific problem of cluster-substrate interaction, we briefly summarize the main features of the mechanism of bonding of individual metal atoms with the surface of a simple oxide such as MgO. Two types of interaction are usually assumed, chemical bonds (mainly with the surface oxygen atoms), or van der Waals interactions and/or weak polarization bonds with no metalization of the The formation of a metallic film is initiated by metal atoms impinging on the substrate. These atoms can be reflected from the surface, or they might stick to and diffuse on it, and they might eventually re-evaporate. Condensation occurs when the flux of adsorbed atoms is larger than the flux of re-evaporated atoms and it is clear that the strength of the bond with the surface plays an essential role in this process. In a first systematic computational study we have considered three triads of transition metal atoms, namely Cr, Mo, and W; Ni, Pd, and Pt; and Cu, Ag, and Au adsorbed on an ideal MgO(001) surface.['I3] Only the W and the Ni triads are found to form strong interfacial bonds. The adhesion energy of these atoms on top of the surface oxygen centers is ca 1 eV/atom or more. This tendency to form strong bonds is connected to the hybridization of the s and d orbitals of the metal and the mixing of these hybrid orbitals with the p orbitals of surf'dce oxygens. The bonding of Cr, Mo, Cu, Ag, and Au to a crystalline MgO substrate can be classified as weak,
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4 Nanornaterials
arising mainly from polarization and dispersion effects with only minor orbital mixing with the surface oxygen orbitals. This explains the very long bond distances occasionally found, and the flat potential energy curves which result in very small force constants. Thus, the TM atoms considered can be classified into two groups, atoms which tend to form relatively strong, chemical bonds with the surface oxygen anions of MgO (Ni, Pd, Pt, and W), and atoms which interact very weakly with the surface, with adsorption energies of the order of 0.3 eV or less (Cr, Mo, Cu, Ag, and Au). For the first group the interaction is covalent in nature and does not imply significant charge transfer from the metal to the surface. This is an important conclusion, connected with the highly ionic nature of the MgO surface in which the surface oxygen atoms have their valence almost saturated. To a first-order approximation MgO can be described in terms of a classical ionic model, Mg2+-02-. Oxygen centers at the regular surface sites of MgO(001) are almost completely reduced, unable to oxidize adsorbed metal atoms. Bearing in mind this important conclusion we now consider the interaction of small Ni, Cu, and Pd clusters with the MgO(001) surface. Things are different when the metal is deposited on a surface with some oxidizing power, e.g. that of A1203. Calculations have shown that an isolated Ni atom adsorbed on a model of oxygen-terminated alumina is substantially oxidized by the substrate,[loglat variance with MgO.
4.6.5.1 Ni4, Cuq, and Pd4 clusters on MgO(001) We consider here the deposition of Ni4, Cu4, and P& clusters at various sites of the MgO(001) surface. The cluster models used to simulate the MgO(001) surface are Mg909 and 09Mg9 (Fig. 21). These clusters represent the adsorption sites on an oxygen anion or a magnesium cation, respectively. The cluster models are embedded in an array of 17 x 17 x 6 point charges. Further details can be found elsewhere." 1 0 , 1 1 4 1 To model the metal/oxide interface we have considered square planar ( D 4 h ) clusters which can be viewed as precursors of an epitaxially growing metal
Mg 2+
0 0209Mg 9
Mg 9 0 9
Figure 21. Cluster models of the 02-and Mg2+ adsorption sites on the MgO(001) surface.
1425
4.6 Naked, Ligated and Supported Transition Metal Clusters
Figure 22. Various idealized adsorption geometries of a square planar metal cluster on the MgO(001) surface.
A
C
E
B
D
F
film. Unsupported, gas-phase, M4 clusters have different geometries, either tetrahedral or rhombic, with metal-metal distances shorter than in the bulk metal.["01 Various idealized adsorption geometries are possible for the deposition of planar Mq clusters on the MgO(001) surface (Fig. 22). There are two important open questions, related to the mode of adsorption and the metal-metal bond strength in the supported clusters. In principle, a metal cluster deposited on the surface of an inert oxide can interact with the surface cations or anions, i.e. it can bind to the acid or basic sites of the surface. The second question is the extent to which the interaction with the substrate affects the electronic structure of the metal cluster. As for the ligand-stabilized organometallic clusters, changes in the metal-metal bond distance provide an important measure of changes in the metal-metal bonding (Sections 4.6.4.1 and 4.6.4.3). Another important aspect of cluster deposition is the analysis of the metal nearest-neighbor distances of the deposited cluster compared to the lattice constant of the substrate. To a good first approximation, the substrate structure may be considered unchanged by the metal deposition.["51 Metal films or clusters can then grow in register with the substrate, i.e. with a lattice constant identical with that of the substrate, or there can be a lattice mismatch at the interface. In the first instance one is dealing with epitaxial growth (or pseudomorphic growth if the monolayer is not complete). Study of the deposition of Ni, Cu, and Pd tetramers on MgO(001) is intended to answer these questions. The structure of the adsorbed clusters has been partially optimized.["0,' 14] In two of the adsorption geometries, denoted A and B (Fig. 22), the interaction is primarily between the cluster metal atoms and the surface oxygen atoms. In geometry B each metal atom is placed on-top of the surface oxygen centers; this corre-
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4 Nanomaterials
sponds to pseudomorphic growth of a metal overlayer. In geometry C the metal atoms are closer to the surface cations; on-top adsorption on Mg2+ (geometry D) has not been considered, because the metal atoms interact only weakly with these centers.[1'0,1'31Sites E and F are possible alternative orientations of the M4 cluster with the metal atoms near the fourfold hollow sites of the (001) surface. In general, adhesion of the metal clusters to the surface is moderately strong for a direct interaction with the oxygen anions, whereas it is very weak when the interaction primarily occurs with the substrate cations or in the fourfold hollow sites.[''0*"41 The competition between the metal-metal and the metal-oxygen bonds is an important issue. If the adsorptive metal-oxygen bond strength is larger, the metal cluster can grow pseudomorphically until completion of the first monolayer (Frank-van der Menve mechanism). If the intracluster metal-metal bond prevails, growth of three-dimensional metal particles is expected (Vollmer-Weber mechanism). When the square-planar Ni4 and Cu4 clusters are adsorbed on site A, the bond strength is ca 0.6 (Ni) or 0.4 (Cu) eV/atom at the gradient-corrected D F level. The surface-cluster distance is similar for the two metals, ca. 1.95-2.0 A.Upon adsorption the metal-metal bond length expands from 2.2 to 2.5 A for Ni and from 2.4 to 2.65A for Cu. In MgO the 0-0 distance is 2.97A. This expansion of the metal-metal distance reflects a weakening of the corresponding bonds because of the interaction with the surface oxygen atoms. Notice that the same effect occurs when a metal cluster is surrounded by a ligand shell (Section 4.6.4.3). The metalmetal distance in the supported clusters is similar to those in the bulk metal (2.49 A for Ni and 2.56A for Cu). If the metal-oxygen bonds were strong, the M4 cluster grown pseudomorphically on-top of the surface oxygen atoms, see site B in Fig. 22, should have a lower total energy; this however, does not occur. When the four metal atoms are placed directly above the four oxygen atoms of the MgO surface, geometry B, the dissociation into free square-planar Ni4 and Cuq clusters is calculated to be easier than for site A. In site B, with the metal-metal distance at 2.97 A, there is still some metal-metal bonding. Thus, the calculations show that for Ni and Cu the metal-oxygen bonding is not strong enough to prevail over the metal-metal interaction and that the adsorbed clusters maintain some intermetallic bonding.[' For adsorption on site C, with the metal atoms close to the Mg ions, or for sites E and F, where the metal atoms sit on fourfold hollow sites, the bonding is weak as expected." l o ] Things are somewhat different for Pd. The calculations on Ni and Cu clusters have clearly shown a preference for the interaction at the oxide anions of the surface while experimental data on small Pd particles supported on MgO have been interpreted in terms of adsorption of the Pd atoms on top of the Mg2+ ions.["61 Furthermore, Pd has a larger atomic radius than Ni and Cu and it is interesting to analyze the expansion of a Pd cluster adsorbed on MgO. Relativistic GGA calculations have been performed for Pd4/Mg0.[1'41 They show as an important first result that for Pd also the preferred adsorption sites are the oxide anions of the surface.["41 Much weaker bonding is found when the Pd4 cluster interacts with the
4.6 Naked, Ligated and Supported Transition Metal Clusters
1427
surface cations. Very recent experiments for Ag on MgO have proven unambiguas ously that the atoms of a metal cluster interact directly with the 0 suggested by theory. The supported Pd4 cluster has Pd-Pd distances which are elongated compared with those of the compound in the gas-phase. The optimum geometry of Pd4 adsorbed on the oxide anions of MgO has metal-metal distances of 2.93 A, somewhat shorter than the MgO lattice parameter.["41 The potential energy for the Pd-Pd stretch is, however, very flat; with very little energy it is possible to expand the cluster to a pseudomorphic structure on the MgO surface. Because the clusters experimentally deposited on MgO are usually much larger than the species considered here, it is conceivable that a thin Pd metal film will grow epitaxially on MgO, at variance with Ni and Cu. The perturbation induced on the metal cluster by the interaction with the substrate has been analyzed by adsorbing a CO probe molecule at the center of the M4 cluster square deposited on MgO; various adsorption properties were compared with those of the free, unsupported clusters. With Ni, Cu, and Pd it was always found that the interaction of CO with the metal tetramer is only weakly perturbed by the substrate." O1 The changes in CO adsorption properties can be interpreted by saying that the bonding at the metal-MgO interface does imply a moderate charge transfer (or polarization) of the metal electrons towards the oxide. The metal cluster, however, is not oxidized by the oxygen atoms of the surface. Still, the substrate induces non-negligible changes in the cluster electronic structure like, for instance, a partial quenching of the magnetic moment of a Ni4 cluster,["O1 in full analogy with what is observed for carbonylated species (Section 4.6.4.3).Therefore, although the (001) surface of MgO is a rather inert support with poor oxidizing power, it can be seen as a perturbing 'ligand'. This conclusion is not true for all supporting oxides. In the next paragraph we consider alumina, the behavior of which is different.
4.6.5.2 Ni clusters deposited on A1203 Modeling an alumina surface is complicated by the absence of reliable structural information, yet conclusions about the nature of the metal-Al203 interaction vary dramatically depending on the assumptions made on the type of termination and the oxidation state of the A1203 surface.['091 Not surprisingly, an aluminiumterminated A1203 surface behaves differently from an oxygen-terminated surface and the chemical nature of the surface oxygen atoms, fully reduced, 02-,partially reduced, 0-, or in zero oxidation state, 0, will result in a completely different interaction with the adsorbed metal. The discussion starts with a simple attempt to describe the interaction of a TM cluster with an alumina surface. The calculations for Ni clusters supported on A1203 have been performed at the LDA differently from the studies of metal deposition on MgO previously
1428
4 Nanomateriuls
a)
Figure 23. The structures of one- (a) and two-layer (b) Ni6 clusters deposited on a model of the alumina surface.
described. To represent the A1203 surface a simplified model was used in which 12 oxygen and 6 aluminium centers form a two-layer cluster with overall C3" symmetry, as shown in Fig. 23. This can be considered as a model of an oxygen-covered Al( 111) surface rather than as one of a basal (0001) plane of A1203. For these reasons, the results are of qualitative nature only. One- and two-layer Nis clusters have been deposited on this model of the substrate (Fig. 23). Except for the cluster 'embedding', however, no u priori assumption was made about the oxidation state of the surface oxygen centers. A Ni atom adsorbed on alumina gives rise to significant charge transfer to the substrate, indicative of the substantial oxidizing power of the oxide surface compared with the MgO(001) surface previously described.['091Of course, the character of the bonding might change considerably when a small Ni cluster interacts with the surface; in fact, in this case strong metal-oxygen bonds are formed at the expense of the bonds within the metal cluster. We consider first the interaction of a one-layer cluster Ni6(6,O) of triangular shape (Fig. 23; the numbers in parentheses indicate the number of Ni atoms in each layer). The optimum Ni-Ni distance for the free cluster, 2.22 A,is much shorter than the Ni bulk distance, 2.49 A,but for adsorption on alumina we assumed pseudomorphic growth of the metal 'film' with the Ni atoms occupying the threefold hollow positions (Fig. 23); this corresponds to an intralayer Ni-Ni distance of 2.52A, very close to the bulk value of Ni metal. The optimum Ni-0 distances between Ni6 and the oxide surface are ca. 2 A. The Ni6(6,O) cluster is strongly bound to the substrate, but the Ni atoms are also involved in bonding to each other. Analysis of the Mulliken charges and of the shifts of the Ni 2p core
4.6 Naked, Ligated and Supported Transition Metal Clusters
1429
levels suggests that charge transfer occurs from the cluster to the oxygen atoms, in a similar way as for a single Ni atom.['091 We have also considered an adsorbed two-layer Ni cluster, Ni6(3,3) of D3d symmetry (Fig. 23). This cluster also is strongly bound to the substrate. The estimated charge transfer per atom from the first Ni layer to the oxygen centers is higher than for Ni6(6,0). On the other hand, we found a moderate perturbation of the Ni atoms in the second layer of the cluster; essentially, the electronic environment of the Ni atoms in the second layer is not too different in free and supported Ni6(3,3) moieties. To investigate further the change in electron distribution within the metal cluster we used CO as a probe molecule to monitor the modifications induced in the Ni6 clusters by the interaction with the support. We found that when CO is adsorbed on the one-layer Ni6 cluster deposited on alumina the CO vibrational frequency is substantially blue shifted compared with free Ni6. This indicates that charge transfer from the Ni overlayer to the oxide occurs, with a consequent reduction of backdonation into the Ni-CO bond. When, on the other hand, CO is adsorbed on the two-layer Ni6 cluster deposited on alumina we found little change in the CO adsorption properties compared with the situation without substrate.[loglThe metal layer in direct contact with the substrate is partially oxidized whereas the second Ni layer is almost unperturbed by the substrate. The change in electronic structure at the interface is rapidly screened for the upper metal layers; thus, Ni atoms of the second layer where CO is adsorbed behave similarly in supported and unsupported clusters.
4.6.6 Concluding remarks The chemical and physical behavior of naked, ligand-stabilized, or supported clusters is highly varied. In this review, based on density functional calculations, we have presented some general concepts which enable a better understanding of the electronic structure of these systems and the resulting properties. We have shown that, in many ways, the properties of ligand-free transition metal clusters evolve rapidly toward bulk metallic behavior. On the other hand, because of the high surface/volume ratio several properties are highly dependent on cluster size. Often, this size dependence can be quantified with the help of simple scaling laws which, surprisingly, are valid down to quite small clusters. It is possible to distinguish the characteristics of the first layer of metal atoms from that of the interior atoms of the cluster. The atoms at the interface with the vacuum have somewhat peculiar behavior, resembling atoms on a metal surface, with weaker metal-metal bonds, enhanced magnetization, and different core-level shifts. Ligated clusters undergo much stronger electronic rearrangement as a conse-
1430
4 Nunornaterials
quence of the addition of the ligand shell. The outermost layer of metal atoms in the cluster is strongly perturbed by chemical interaction with the ligands, thus featuring a distinct character. In a simplified view, one might say that the ‘surface’ atoms lose part or most of their metallic character whereas the internal atoms still have metallic properties. When the clusters are very small and all the metal atoms are at the cluster surface, the nature of the metal-metal interaction can become unclear. Under these circumstances the system is better regarded as a polynuclear organometallic complex with metal-metal bonds which are much weaker and substantially different from those of bare metal clusters. Finally, it is interesting to note also that the properties of supported metal clusters also are modified by interaction with the substrate. In a general view this interaction is not too different from that occurring in ligand-stabilized clusters. In fact, metal atoms at an interface, with an oxide surface or a ligand shell, undergo a substantial change of their electronic structure. This perturbation caused by the bonding at the interface decays very rapidly so that the metal atoms which are not in direct contact with the ligands or the support maintain typical metallic behavior. In the same way as ligands can be classified as neutral (CO, PR3, etc.) or reduced (C1-, etc.) so also can the oxide surfaces be regarded as more or less oxidizing (cf. MgO compared with A1203). Direct contact with the metal leads to modified properties of the first metal layer in either instance. Thus, very similar concepts can be useful for understanding seemingly very different situations.
Acknowledgment We would like to thank a long series of gifted graduate students, post-doctoral coworkers, and visitors from the Munich group for numerous contributions to the present work; their work is identified by references in this review. We gratefully acknowledge support from the ‘Vigoni Program’ of CRUI and DAAD for mutual visits at our institutions. This work has been supported by the Deutsche Forschungsgemeinschaft, the European Community (through the Science, HCM, and INTAS programs), and the Fonds der Chemischen Industrie.
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[86] H. Schmidbaur, G. Weidenhiller, and 0. Steigelmann, Angew. Chem. Int. Ed. Engl. 1989,28, 463. [87] F. Scherbaum, A. Grohman, B. Huber, C. Kriiger, and H. Schmidbaur, Angew. Chem. Int. Ed. Engl. 1988, 27, 1544. [88] H. Schmidbaur, Pure Appl. Chem. 1993, 65, 691. [89] A. Brodbeck, J. Strahle, Acta Crystallogr. 1990, 46, C-232. [90] O.D. Haberlen, H. Schmidbaur, and N. Rosch, J. Am. Chem. Soc. 1994,116, 8241. [91] O.D. Haberlen and N. Rosch, J. Phys. Chem. 1993, 97, 4970. [92] D.M.P. Mingos, J. Chem. Soc. Dalton Trans. 1976, 1163 [93] N. Rosch, A. Gorling, D.E. Ellis, and H. Schmidbaur, Angew. Chem. Int. Ed. Engl. 1989,28, 1357. [94] A. Gorling, N. Rosch, and D.E. Ellis, H. Schmidbaur, Inorg. Chem. 1991, 30, 3986. [95] G. Pacchioni and P.S. Bagus, Inorg. Chem. 1992, 31, 4391. [96] G.A. Bowmaker, M. Pabst, N. Rosch, and H. Schmidbaur, Inorg. Chem. 1993,32, 880. [97] A. Ceriotti, A. Fait, G. Longoni, and G. Piro, J . Am. Chem. Soc. 1986, 108, 8091. [98] L. Ackermann, N. Rosch, B.I. Dunlap, and G. Pacchioni, Int. J. Quant. Chem. Quant. Chem. Symp. 1992,26,605. [99] F. Calderoni, F. Demartin, M.C. Iapalucci, and G. Longoni, Angew. Chem. Int. Ed. Engl. 1996,35, 2225. [IOO] Chemisorption and Reactivity on Supported Clusters and Thin Films, R.M. Lambert and G. Pacchioni (Eds.), NATO AS1 Series E, Vol. 331, Kluwer, Dordrecht, 1997. [I011 K.M. Neyman, G. Pacchioni, and N. Rosch, in: Recent Developments and Applications of Modern Density Functional Theory, J.M. Seminario (Ed.), Elsevier, Amsterdam 1996, p. 569. [I021 J. He, P.J. M Her, Surf. Sci. 1987, 180, 411. (1031 J.B. Zhou, H.C. Lu, T. Gustafsson, and E. Garfunkel, Surf. Sci. 1993, 293, L887. [I041 M.C. Wu, W.S. Oh, and D.W. Goodman, Surf. Sci. 1995, 330, 61. [lo51 F. Didier and J. Jupille, Surf. Sci. 1994, 307-309, 587. [I061 M. Meunier, C. Henry, Surf. Sci. 1994, 307-309, 514. [ 1071 A.B. Kunz, Philosoph. Mag. 1985,51, 209. [I081 C. Li, A.J. Freeman, Phys. Rev. B 1990, 43, 780. [I091 G. Pacchioni and N. Rosch, Surf. Sci. 1994, 306, 169. [ 1101 G. Pacchioni and N. Rosch, J. Chem. Phys. 1996, 104, 7329. [ 11 11 Y. Li, D.C. Langreth, and M.R. Pederson, Phys. Rev. B 1995,52, 6065. [ 1121 M. Gautier and J.P. Durand, J. Phys. I11 France 1994, 4, 1779. [ I 131 I. Yudanov, G. Pacchioni, K. Neyman, and N. Rosch, J. Phys. Chem. B 1997,101, 2786. [ 1141 I. Yudanov, S. Vent, K. Neyman, G. Pacchioni, and N. Rosch, Chem. Phys. Lett. 1997,275, 245. [ 1151 L. Spiess, Surf. Rev. Lett. 1996, 3, 1365. [ 1161 C. Goyhenex and C. Henry, J. Electron Spectrosc. Relat. Phenom. 1992, 61, 65. [I171 A.M. Flank, R. Delanay, P. Lagdrde, M. Pompa, and J. Jupille, Phys. Rev. B 1996, 53, R1737. [I181 P. Guenard, G. Renaud, and B. Villette, Physica B 1996,221, 205.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.7 Physical Properties of Metal Cluster Compounds. Model Systems for Nanosized Metal Particles L. Jos de Jongh
4.7.1 Introduction: Why are metal nanoparticles of interest? Metal nanoparticles or clusters, that is very small pieces of bulk metal of size 1100 nm, are of great interest for fundamental science as well as for potential future applications. This size-range implies a number of metal atoms per particle varying from ten to ten million or more; concomitantly the surface-to-volumeratio becomes very large for the smallest sizes. For instance, for cubic-close-packing,the percentage of surface atoms is about 80% for a 50-atom particle, and is still 20% for a particle containing as many as 2000 atoms. This simple fact has enormous consequences for the chemical and physical properties, which are in between (and might differ quite strongly!) from those of atoms on the one hand and of bulk materials on the other. Most, if not all chemical reactions occur at the surfaces (interfaces) of materials. This explains, at least in part, why metal particles can be used advantageously as industrial catalysts, and, more generally, their special role in chemical synthesis and reactivity. In addition, important problems such as the chemisorption and physisorption of molecules on metal surfaces can be fruitfully investigated, both experimentally and theoretically, by studying metal nanoparticles. The size-evolution of the physical properties from atom to bulk might also be related in part to the variation of the surface-to-volume ratio. In addition to these ‘classical’effects, however, the quantum mechanical properties of the electrons play an equally important role. These so-called quantum-size effects[’]can be understood most simply by realizing that a conduction electron in a metal has both particleand wave-like properties, according to the famous particle-wave duality of quantum mechanics. Treated as a wave-phenomenon, the electron in a metal has a wavelength of one to a few nanometers. The wave-character of the electron will
4.7 Physical Properties of Metal Cluster Compounds
1435
become predominant as soon as it is confined to a region in space of similar dimension. This is precisely what occurs in a metal nanoparticle, and is the underlying reason why the familiar bulk metallic properties are lost when the size of the particle is reduced to the nanometer range. In terms of the energy-level diagram of conduction electrons in a metal, this metal-nonmetal transition corresponds to replacement of the (pseudo-) continuous energy band structure appropriate for the bulk solid, by the discrete energy-level structure characteristic of electrons confined to such a small volume. The terms ‘quantum-wells’ or ‘quantum-dots’ are used for these restricted geometries. The average level separation in the electronic energy diagram is inversely proportional to the volume of the particle and is approximately 10 meV for a particle of about 1000 atoms, when each atom contributes one conduction electron to the cluster. In terms of the quantum-well picture, a small particle of, e.g., an alkali metal, can be regarded in many respects as a giant atom (or molecule). The electrons are confined by the outer surface of the particle, which presents an approximately spherical potential, similar therefore to the spherically symmetric Coulombic potential in the atom arising from the electron-nucleus electrostatic interaction. Thus, the buildingup principle of electrons in such a cluster is quite similar to that underlying the periodic system of the elements, with the characteristic shell-structure for the electrons. Indeed, large differences in reactivity have been observed for clusters with filled or unfilled electron shells! An attractive feature of clusters in this respect is, evidently, that the number of electrons (atoms) per cluster can surpass by orders of magnitude the number of elements in the periodic system. The discrete electronic energy level spectrum of clusters also forms the basis for several of the expected possibilities for future applications. For instance, lightinduced transitions between occupied and unoccupied energy levels are responsible for the optical properties of materials, the energy level distances determining in which region of the electromagnetic spectrum these properties will be located. The possibility of varying the level separations in clusters by changing their volume, opens the way to tunable optical properties. Several research groups hope to use clusters eventually to obtain lasers operating in so far unavailable spectral ranges. It should be added here that these aspects are by no means restricted to metal nanoparticles, but also apply to semiconducting clusters.r21In the latter the semiconducting energy gap (HOMO-LUMO gap) has been convincingly shown to be a sensitive function of size in several cluster materials, e.g. CdS and CdSe. So far, the discussion has been concerned with properties of single particles, that is particles isolated from one another and from their (direct) environment. Does this correspond to the situation met in actual practice? To check this it is of interest first to summarize briefly the various methods by which small metal particles can be produced. In fact, it then transpires that the only method of producing and studying fully isolated individual clusters is by means of atomic or molecular beams. In this method the metal (or other material) is evaporated under high-vacuum conditions or in an inert gas atmosphere. The beam of atoms is subsequently expanded
1436
4 Nanomaterials
through a nozzle, whereupon condensation into clusters occurs. These clusters can be studied in flight (or after deposition on to a suitable substrate, e.g. graphite). The freely suspended clusters in the beam are indeed fully isolated, but with the disadvantage that they are amenable to few experimental probes only, being limited mainly to spectroscopic measurements. We hasten to add that such experiments have proven of immense value, and have greatly advanced our fundamental knowledge of clusters, however, as regards applications the possibilities are obviously quite limited. For more practical purposes, therefore, one should take recourse to metal particles as produced by other means, in particular on supports or in matrices. The advantage is the availability of macroscopic amounts of sample; the disadvantage is that interaction with the supporting medium must be assessed. A great variety of synthetic methods exists, of which we can mention only a few. Metal clusters can be produced by aerosol techniques, by vapor deposition, by condensation in rare-gas matrices, by chemical reactions in various supports, e.g. zeolites, SiOz, A1203, or polymer matrices. Many different metal-nonmetal composites, such as the ceramic metals (cermets) have been obtained with metal particles with sizes varying from nanometers upward. In alternative approaches, metal particles are stabilized by chemical coordination with ligand molecules, as in metal colloids and metal cluster compounds. Fortunately, our understanding of cluster-support interactions has advanced greatly. In recent decades very extensive efforts have been devoted in surface science to experimental and theoretical studies of the interactions between adsorbate molecules and metal surfaces (ligand-metal interactions). As a general rule of thumb, it seems that these interactions, although quite important, are limited for the most part to the outermost layer of metal atoms, i.e. those directly bonded to the ligands. This can be understood on the basis of the extreme screening power of the conduction electrons in a metal, as a consequence of which all ‘memory’ of the presence of the surface is lost one to two layers deep inside the metal. Also for metal clusters it has been found that for particles of about 100 atoms upward this screening is already quite well developed, to the extent that, apart from the surface layer, the remainder of the metal particle will be little affected by the contact with the surrounding medium in or upon which it occurs. The conclusion, therefore, seems to be that such metal cluster assemblies, or cluster solids, might indeed be regarded as collections of metal particles, be it with a smaller effective diameter as a consequence of the interaction with the surrounding. This is important for application purposes, in which assemblies (arrays) of metal particles will probably always be involved. Until now the stacking of the metal particles in experimental cluster assemblies is usually random, as in metal-nonmetal composites, metal colloids, and matrixstabilized clusters. Increasing efforts are, however, being made to obtain regular, periodic arrays of, preferably identical, metal particles. Such possibilities are offered by the zeolites, in which the metal particles sit in regularly spaced holes in the structure, and in particular by the metal cluster compounds, many of which can be
4.7 Physical Properties of Metal Cluster Compounds
1437
obtained in crystalline form. Because these are stoichiometric chemical compounds, the metal particles in a given metal cluster compound are identical (with sizes up to a few hundred atoms), and are chemically stabilized by ligand s h e l l ~ . [Thus, ~ ~ ~ ]they can be regarded as ideal model systems for (macroscopically large) assemblies of identical metal nanoparticles. In this same respect the recent great advances in nanolithography should not be forgotten it has already proven possible to obtain 2-dimensional arrays of regularly spaced, very small metal particles (size about 10 nm) by evaporation techniques. Alternatively, surfaces can be prepared by etching techniques or use of a chemical surfactant, to enable the anchoring of metal cluster molecules or colloidal particles deposited with equal spacing. Such a 2D array of metal particles should enable individual addressing of particles, e.g. with an STMtip, and this might play an important role in nanoelectronics. More generally, metal-nonmetal composites can be expected to have novel conducting and dielectric properties, particularly if the particles are identical in size and regularly spaced. We end this introduction with a few remarks on magnetic particles, which do not necessarily relate to bulk magnetic metals (e.g. Fe, Co, Ni), but might also be pieces of bulk magnetic insulators (e.g. ferrites or ferromagnets). Although small particles of antiferromagnetic materials do exist, most effort has been concentrated on ferromagnetic or ferrimagnetic counterparts. The particles are usually single-domain, so that a net magnetic moment per particle results. These materials are called superparamagnetic, and are, for example, of importance in the magnetic recording industry. Challenging fundamental problems that are still under investigation include surface magnetism, the effects of ligand-metal interactions on magnetic properties (surface pinning, exchange anisotropy), the magnetism of macromolecules, and the macroscopic magnetic quantum tunneling of superparamagnetic ~
4.7.2 Giant magic-number metal clusters Metal cluster compounds with (relatively) small metal cores have long been known in chemistry - even a decade ago several hundred such compounds had been synthesized from many different transition metal elements (Fe, Co, Ni, Mo, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, . . .), with core sizes of up to 10-20 atoms. In the last 10 years, however, a few synthetic chemical groups, notably in Germany, Italy, and England, have initiated successful quests to create ever larger metal cluster molecules, inspired not only by the synthetic challenges involved, but also by the growing interest of solid-state physicists in their product^.^^^^] For example, within the subgroup of metal cdrbonyl clusters (metal cores coordinated by CO ligands), very large metal cores containing up to 38 Ni, Pt, or Pd atoms have been synthesized, as have bimetallic cores consisting of six Pt atoms surrounded by 38 Ni atoms, as in the example shown in Fig. 1.
1438
4 Nunomaterials
Figure 1. The structure of the ionic metal-cluster [Ni~8Pt6(C0)48H]5-.'81
By use of phosphine (or related) groups as ligands, neutral cluster macromolecules with large pieces of e.g. Ni2Se3 or Cu2Se as cores have been obtained;"'] examples include [Ni34Se22(PPh3)10],[Cu70Se35(PEt3)22], and [Cu146Se73(PPh3)3o]. We note that bulk Cu2Se has semimetallic properties. These materials form molecular crystals, and, in fact, Cu146Se73 is the largest cluster compound characterized by X-ray analysis. Much larger clusters have, however, been found in a series of giant metal-cluster molecules, the metal cores of which are members of the series of so-called magicnumber (full-shell) clusters depicted in Fig. 2.[71They are obtained by surrounding
4.7 Physical Properties of Metal Cluster Compounds
1439
0 M13
M55
MI47 M309
Figure 2. Magic-number clusters M, obtained by surrounding a given atom by successive shells of atoms (the illustration is for cuboctahedral packing).
an atom progressively with additional shells of atoms of its kind. The resulting I-shell, 2-shell, etc. clusters have the magic numbers of atoms 13, 55, 147, 561, etc. These numbers are the same for icosahedral or cubeoctahedral (fcc) packing of atoms; the experimental examples described below have cuboctahedral structures. For instance, the 2-shell M55 core is found in a series of cluster molecules MssL12Cl,, in which the metal atom M can be Au, Pt, Ru, Rh, or Co. Depending on the metal element, the ligand L is, e.g., PPh3. PMe3 P(Bu')3 or P(p-tolyl)3, and x is 6 or 20. Although isolation and characterization of the 3-shell cluster is still lacking, 4shell and 5-shell clusters have been found to be the metal cores in, respectively, Pt309Phen*6030 and Pd561Phen360200,and the record to date seems to be held by the 7- and 8-shell Pd clusters found in Pd1415Phen5401000 and Pd2057Phen7801600. It should be noted that no single-crystal sample of these giant cluster compounds has yet been obtained. The neutral macromolecules form dense but randomly packed solids with only short-range order, as in a glass. In the absence of full X-ray analyses, the precise chemical stoichiometry is uncertain. Notwithstanding this, a wealth of direct or indirect physical data is available, e.g. from high-resolution electron microscopy, EXAFS, Mossbauer spectroscopy, NMR, calorimetry, etc, which seems to agree consistently with the formulas given above. It should also be realized that, from the 3-shell cluster onward, the metal cores in these macromolecules are already large enough to be studied individually by X-ray scattering! The observed reflections confirm the cubic-close-packing, with metal-metal distances indistinguishable from the corresponding bulk values. In addition to these giant cluster compounds, it has proven possible to synthesize new types of Pd and Pt colloids using the same ligands. These colloids are available for experiments in the powder (solid) form, with a very high metal content and small size distributions (< 10%); they thus nicely complement the metal cluster molecules. Taken together, colloids and molecular clusters offer a scale of metal particle sizes ranging indeed from 10 to 100000 atoms.
1440
4 Nanomaterials
4.7.3 Evolution to magnetic metallic properties. Some examples The first item to consider in this regard is the effect of the ligand shell. It is obvious that the same ligands, which are so beneficial in stabilizing the metal cores, should at the same time have a strong influence on the electronic structure, notably that of the surface metal atoms of the cluster, to which atoms they are chemically bonded. Such effects should come in addition to the bare surface effects which occur in naked (unligated) metal clusters, or at the surfaces of bulk metals. Indeed, it is already well established that, because of a reduced number of neighboring atoms, the electronic structure of an atom at a metal surface will be different from that of an atom in the bulk. From recent physical experiments on large ligated metal clusters,[41it has become apparent that the presence of the ligand shell strongly affects the surface atoms, but that at the same time its influence is to a very good approximation restricted to these. Therefore, the inner-core atoms, being unaffected, can be viewed as forming a minute, 'embryonic' piece of bulk metal with strong quantum-size effects, evolving to bulk behavior with increasing size. The rate of this evolution obviously depends also on the number of valence electrons per atom, which is rather high (ca. ten) for the transition metals of the second half of the 3d, 4d, and 5d series considered here (Ni, Cu; Ru, Rh, Pd; Os, Ir, Pt, Au). With Fermi-energies of approximately 5 eV, this implies average level spacings 6 % ~ E F / Nof ca. lo2 K for an inner-core of approximately lo2 atoms. Several results have already been obtained which seem to agree with this simple analysis. For instance, 1 9 7 AMossbauer ~ spectroscopic studies on the Schmid-cluster Au55(PPh3)12C16 could be interpreted["] in terms of four different contributions from the various identifiable Au sites, namely the 13 Au atoms forming the inner core of the Au55 cuboctahedron, the 24 uncoordinated surface Au atoms, the 12 surface atoms coordinated by the PPh3 ligands, and the six surface atoms ligated by C1. It has, indeed, been found that the Mossbauer parameters of the 13 inner-core sites are close to those of bulk metals, the quadmpole splitting parameter is zero because of the high symmetry, and the isomer-shift parameter being near to the bulk value (the isomer shift is a measure of the 6s electron density seen at the nucleus). By contrast the Mossbauer parameters for the surface sites were found to be completely different, lying in fact in the ranges of literature values known for non-conducting Au compounds. An even more dramatic confirmation of the bulk properties of the inner-core metal atoms was obtained from recent Mossbauer studies of the giant Pt309 cluster, which yielded a value of the isomer shift of the inner-core atoms equal to the bulk value, within experimental error." 'I Again, the surface atoms were found to be widely different. Although the extreme screening of the surface might initially seem surprising, it
4.7 Physical Properties of Metal Cluster Compounds
1441
is, in fact, a hallmark of delocalized electrons, as in a metal. It is confirmed by both jellium model calculations and quantum-theoretical studies. Within the jellium model, the variation of the charge density close to the surface of a metal has been studied.[l3]It shows the well known spill-out of the charge over a small distance into the vacuum. Near the surface inside the metal, the charge-density oscillates around the bulk value. The range of both the spill-out and these oscillations is, however, of the order of the Fermi wavelength of the electron only, i. e. of the order of the size of the metal atom. Consequently, at a distance of one atom inside the metal, the charge density is already almost at the bulk value. Molecular-orbital calculations on ligated clusters have confirmed the drastic effects of the ligation of a metal cluster. To show that the influence is restricted to the surface, calculations on large cluster molecules were needed, and it has taken some time for these to become available. Re~ently,"~] the electronic structure of two high-nuclearity carbonylated Ni clusters, [Ni32C6(C0)32In-and [Ni4(C0)48In-,with n = 0-6, has been investigated by means of the linear combination of Gaussiantype orbitals (LCGTO) and local density functional (LDF) method. All-electron, spin-polarized calculations were performed to determine the magnetic nature of the ground states of both the bare and the carbonylated Ni clusters, and thereby study the effect of the ligand coordination. In agreement with earlier work on smaller Ni it was found that the bare Ni clusters are in high-spin ground states. The average magnetic moment per Ni atom is found to be approximately 0.7-0.8 p, for both surface and core Ni atoms. The fact that this value is somewhat higher than that of the bulk (0.60 ,us) is attributed to the larger Ni-Ni distances used in the calculations (as appropriate for the experimental examples[*]of such clusters). This agrees with the notion that the electronic configuration for Ni in small, bare Ni clusters is close to 3d94s', the metal-metal bond resulting, primarily, from the 4s conduction-band electrons. This would leave one hole in the fairly localized 3d shell of each Ni atom; as in the above example of the Au55 cluster, there is, however, additional substantial dd overlap between nearest-neighbor metal atoms, which reduces the average number of unpaired electrons per atom. It is quite interesting to note that the bonding of the Ni atoms in the bare Ni clusters already resembles that of bulk Ni to such an extent that it leads to ferromagnetic spin-ordering inside the cluster, with an average moment per atom comparable with that in the bulk. The addition of the CO shell to the Ni metal cores is found to have dramatic effects on the magnetism. The magnetic moments of the surface Ni atoms are completely quenched, those of the core atoms still remain at a sizable value of 0.5 pB per atom. These results are in keeping with previous calculations on smaller clusters,['5] from which it could be concluded that carbonylation of Ni clusters so small as to have only surface atoms leads to a complete suppression of the magnetic moment of the cluster. The quenching of the Ni moments because of interaction with CO molecules is an effect which also occurs when CO is chemisorbed on to Ni metal surfaces. The electronic mechanism behind this effect is ascribed to a transfer
1442
co
4 Nunornaterials
gas
A
Nickel
Figure 3. Schematic energy-level diagram for the interaction between Ni atoms and carbon monoxide (adapted from Salahub and R a a t ~ [ ' ~ ] ) .
of electrons from the 4s orbitals into the 3d shell of the Ni atoms. The process is illustrated schematically in Fig. 3. Such a transfer is induced by the combination of: i) the repulsive interaction of the 4s-derived metal MO with the 50 MO of the CO (destabilizing the 4s orbitals well above the cluster HOMO level, so that the electrons are excited into the 3d band), and ii) the z back-donation from d, metal orbitals to the 2z* MO of CO, which leaves the Ni atoms slightly positive and causes a further electron transfer from 4s orbitals into the 3d shells. The net result of this ligand-induced transfer mechanism can be interpreted as a change in the electronic structure of the surface atoms into a formal 3d'O-like atomic configuration (diamagnetic). The volume Ni atoms, on the other hand, retain most of their original 3d94s' character, so that they still show magnetic behavior similar to that of the bulk metal. These calculations provide a satisfactory explanation of the magnetic properties of a series of high-nuclearity Ni clusters studied in our l a b o r a t ~ r y . [ ' ' ~ 'In ~ ~fact these real Ni carbonyl clusters served as model systems for the above mentioned theoretically considered Ni clusters. From the earlier experimental results for the temperature-dependence of the magnetic susceptibility and of the high-field magnetization of powdered samples, it followed that the total magnetic moment per cluster was only of the order of 4-9 pB, even though the clusters contained 34 to 38 Ni atoms."'] The strong reduction of the cluster magnetic moment was at the time tentatively ascribed to the effect of the ligands, an interpretation that now seems
4.7 Physical Properties o j Metal Cluster Compounds
1443
to be fully corroborated by the LDF ~alculations.[”~From careful of a single-crystal sample (- 18 mg) of a [Ni38Pt6(C0)48H]5-cluster it was recently concluded that the intrinsic magnetic moment of this species is most probably zero. Because the Ni atoms are all on the surface, while the (presumably non-magnetic) Pt atoms form the inner-core of the metal cluster, this is in accord with complete quenching of the moment of the surface Ni atoms, as predicted by theory. We note that the destabilization process described above, with the accompanying change in the magnetic moment, is the (macro) molecular analog of the high spinlow spin transition familiar in transition metal complexes of, e. g., Mn and Fe ions. The chemical bonding of ligands to a (single) metal atom in such complexes is taken into account in the form of an electric field acting on the energy levels. This field is called crystal-field or ligand-field, and its symmetry reflects the coordination of the metal atom. In the ligand-field theory, a low-spin complex results when the crystal field is sufficiently strong to produce a splitting of the d orbitals large enough to overcome Hund’s rule. The latter favors the distribution of electrons with parallel spin in different orbitals (intra-atomic ferromagnetism!), thereby minimizing the Coulombic energy. In metal cluster molecules the ligand shell coordinates a cluster of metal atoms instead of a single atom. Nevertheless, in this case also one can envisage taking account of the chemical metal-ligand bonds in terms of an electric field acting on the energy levels of the bare metal clusters. As metal-cluster compounds of sufficiently large size have become available, it has proved possible to observe Pauli spin-paramagnetism. Systematic size-dependence of the Pauli susceptibility was observed in a series of large Pd clusters and colloids.[201The study involved the 5-shell Pd561Phen360200 ‘Schmid’ cluster, the metal core of which consists of a central atom surrounded by five layers of close-packed nearest neighbors. Further, a sample containing a 50: 50 mixture of 7- and 8-shell clusters was used, in addition to a 150 A colloid stabilized using the same type of ligands. For the smallest system in this series, Pd561, the average energy-level separation 6 / / k Bcan be estimated to be approximately 20 K. Above this temperature Pauli paramagnetism should develop. Observation of quantum-size effects below this temperature is probably inhibited by the strong spin-orbit coupling. The susceptibility as a function of temperature measured for these three systems, by use of a SQUID magnetometer, is reproduced in Fig. 4, and compared with the behavior of bulk Pd. As was mentioned in Section 4.7.2.2, the Pauli susceptibility of bulk Pd is exchange-enhanced by a Stoner factor S = 9.4. Its temperature-dependence is a result of marked energy-dependence in the density of states. The susceptibility of the clusters is indeed indicative of enhancement of the Pauli susceptibility, but one which is reduced. Temperature-dependence is also observed, again reduced relative to that of the bulk. The size-dependence was described with a model which assumes a reduction of the density of states at the cluster surface as a result of the ligand bonding, by analogy with similar effects on nickel surfaces, as described above. Accordingly, the susceptibility without enhancement effects was taken to depend on
1444
4 Nunomaterials
I0 9
-a
8
a
7
h
M
\
1
6
$
5
l4 o
4
(D
colloid 15 n m
v
x
3 2
1
Pd 5 shells
t
j
0 0
50
150
100
200
250
300
T (K) Figure 4. The temperature-dependent susceptibility (in 0.1 T) of Pd clusters of different size compared with that of bulk Pd. The values are normalized to the (estimated) weight of the Pd cores, and corrected for various diamagnetic contributions. The bulk measurements are adapted from Manuel and St Quinton.[211
the radial coordinate as:
xpauli(Y) = xPauli(bulk)( 1
-
Ae(r-R)/A)
Here, A is the reduction factor at the surface, which is restored over a characteristic length, A, away from the surface of the cluster, and R is the cluster radius. Then, the where xPauliis the Stoner enhancement factor is calculated from S = 1/( 1 - ZxPauli), average value of xpauli(r)over the cluster, and Z is the interaction constant determined from the bulk susceptibility. A fit of this model to the low-temperature data (Fig. 5) gives A = 0.32 and il = 0.68 nm, which seem to be reasonable numbers. Note that the resulting size-induced reduction of x extends to very large diameters. In view of the limited number of points, it is reassuring that the experimental temperature dependence dx/dT scales as S 2 , is in agreement with the model. Measurements on a larger range of cluster sizes are still required to test the validity of the model.
4.7 Physical Properties of Metal Cluster Compounds
1445
a
0.9 0.8
0.7 Y 0.6 a
x" 0.5 \ X
0.4
0.3
0.1
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0 . 0 ~ " " '
0
"
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'
'
I
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"
'
15
"
"
'
20
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R (nm> Figure 5. The T = 0 extrapolated values of the susceptibility in Fig. 4. The curve is a fit of the model describing the size-dependence.
4.7.4 Odd-even electron numbers and energy level statistics in cluster assemblies As mentioned in the introduction, nanosize metal clusters are quantum wells for electrons, because their size is comparable with that of the wavelength of an electron at the Fermi energy (EF).In an extensive study of a series of Pd metal cluster compounds and metal colloids, ranging in size from ca. 500 to ca. 125000 atoms/ cluster, we have recently succeeded in obtaining compelling evidence (for the first time) of the manifestation of quantum-size effects (QSE) in the electronic specific heat and susceptibility of cluster assemblies, and for the closely related odd-even electron-number effects arising from the Coulombic charging energies of the clusters[221(Fig. 6). The QSE arise from the wave nature of the electrons, and the ensuing discrete energy-level structure of the individual clusters. An important point here is that,
1446
4 Nanomaterials
1 o-2
I
. ' ' ' ."'" ' ' ' ' comparison with the QSE theory ' .'.''I
1o
-~
1o
-~
0.1
'
1
Temperature, K Figure 6. Specific-heat data of Pd clusters of different size compared with those of bulk Pd. The number of atoms/cluster is 561, 1415, 2057 and 1.25 x lo5 for Pd5, Pd7, Pd8 and Pd coll., respectively. The insert shows the electronic contribution (predominant below 1 K) fitted to the QSE theory for the orthogonal distribution. The average level distance 6 is 12 K, 4.5 K, 3.0 K, and 0.06 K for Pd5, Pd7, Pd8 and Pd coll. respectively. The transition from high-temperature, bulk-like behavior (linear Tdependence) to the QSE regime (quadratic Tdependence) can be clearly seen in the theoretical curve (solid line) and the experimental data.
even for an assembly of nominally identical metal particles, as in these monodispersed metal cluster compounds, the energy level structure will still differ from one cluster to the other. The reason is that for clusters containing 1000-10000 d-metal atoms, the average energy separation, 6, between neighboring levels will be quite small, i.e. of order 1 to 0.1 meV. This is readily apparent from a back-of-theenvelope argument; when this distance is equated to the density of states at EF, N the , bulk metal. When EF = 5-10 eV, and N is the number of D ( E F )= ~ E F / ~for valence electrons in the cluster, one indeed obtains ca. 1 meV taking into account that each d-metal atom contributes ca. 10 valence electrons to the cluster (s and d electrons together). Because this energy scale is so small compared with chemical binding energies (ev), it follows immediately that even the slightest surface irregu-
4.7 Physical Properties of Metal Cluster Compounds
1447
larity of a cluster will largely affect its energy-level structure. Such irregularity will unavoidably be present at the atomic length scale of 0.1 nm, for the simple reason that metal clusters are composed of metal atoms (even without considering the presence of the ligands). Because 0.1 nm is a sizable fraction of the electron wavelength at EF (1 nm), it immediately follows that to calculate the electronic thermodynamic properties of an assembly of monodispersed metal clusters one must take recourse to statistical theories for the distribution of energy level structures in such an assembly. Such theories have been developed for the fully analogous problem of the description of the energy level structure of atomic nuclei by Wigner and Dyson, and were first applied to metal clusters by Gor’kov and E l i a ~ h b e r g .On ~ ~ ~the ] basis of random-matrix theory it can be shown that only three possible statistical level distributions need to be considered, the orthogonal, the unitary, and the symplectic distribution.r241Which of these is applicable depends on the basic symmetry properties of the Hamiltonian describing the electron system. For a zero or small applied magnetic field and small spin-orbit coupling, as appropriate for our experiments on Pd clusters, the orthogonal distribution should apply. For strong magnetic fields, on the other hand, the unitary distribution should be used. In the experiments described by Volokitin et a1.[221 (see also Fig. 6) we could convincingly show the transition ) QSE from bulk-like metallic behavior at high temperatures ( T > 6 / k ~ towards behavior at low temperatures ( T < 6 / k ~ )In. the QSE region the electronic susceptibility and specific heat were both well described by the predictions from the orthogonal distribution. It should be realized that these statistical distributions lead to the conclusion that the levels will repel one another, i.e. there is zero probability of finding two levels infinitely close together. This contrasts with the classical Poisson distribution, for which the probability is a maximum for zero-level separation. Thus, our findings can also be regarded as (the first) experimental evidence for level repulsion in clusters, in other words the applicability of random-matrix theory to describe the level statistics of cluster assemblies. A brief summary of the theoretical concepts is given below (Section 4.7.5). It was also found that in the Pd cluster samples half of the metal particles carries an odd number of electrons and half an even number. Such an even-odd distinction was predicted by K ~ b o [ ~as ’ ] a characteristic cluster property, but had never previously been observed. This might be understood in terms of classical electrostatics - it takes energy Q2/4neoRto put a charge Q on a metal sphere of radius R. For particles in the nm range, this Coulombic charging energy becomes quite large, ca. 0.1 eV or larger. It follows that for such particles the energy involved to change the number of electrons might exceed the thermal energy, so that at low temperatures charge fluctuations become completely suppressed, and one must distinguish between particles with even and odd numbers of electrons. The difference shows up quite markedly in the magnetic susceptibility, x, because for the even particles all electrons are spin-paired, giving a very small x, whereas the odd particles carry an unpaired electron spin, leading to a diverging (Curie-type) x for T + 0. These effects could be clearly seen in our measurements, as is apparent from Fig. 7.
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I
/
0.0
0.1
0.2
0.3
0.4
Figure 7. Electronic magnetic susceptibility ,ye, of the Pd clusters in the quantum-size range. In the temperature range 2-300 K the measurements were made with a commercial SQUID magnetometer; in the temperature range 50 mK to 4 K, a SQUID ax. susceptometer made by the authors was used. The estimated diamagnetism of the ligand shells (a few percent) has been subtracted from the susceptibility data. The inset shows the high temperature behavior, compared with that of bulk Pd. The low temperature data have been normalized by dividing the susceptibilities by their high temperature values, which will correspond to the electronic contributions unaffected by the presence of quantum gaps near EF. Temperature is scaled by the average energy gap value 6 appearing in the theory. The dotted curve shows predictions of the QSE model in zero field, the solid curves are fits to the QSE model in field, yielding 6/kB values of 12.5 K, 5 K, and 2.0 K for Pd', Pd7 and Pd', respectively. To explain the observed susceptibility maxima, interactions between spins on neighboring clusters must be invoked, yielding internal fields of the order of 0.2 T, namely ,uBB/6= 0.025, 0.06, 0.15 for Pd5, Pd7, and Pd'.
4.7 Physical Properties of Metal Cluster Compounds
1449
4.7.5 Energy-level statistics in assemblies of small metal particles: summary of theoretical background Traditional theoretical approaches to quantum size effects (QSE) in metal particles are based on random matrix theory (RMT), which was first established by Wigner and Dyson to describe the spectrum of heavy nuclei.[24~261 It is assumed that the (random) Hamiltonian of the system is a random N x N Hermitian matrix, with a Gaussian probability distribution of the form: P ( H ) = cexp(-PTrH2) The symmetry index j? counts how many real numbers define the matrix elements, which can be real, complex, or quaternion numbers, corresponding to, respectively, p = 1, 2, and 4. (We recall that a quaternion is a 2 x 2 matrix which is a linear combination of the unit matrix and the three Pauli spin matrices). Because the transformations H 4 UHU-’, with U an orthogonal ( p = l), unitary ( p = 2), or symplectic (p = 4; quaternion elements) matrix leaves the distribution P ( H ) invariant, the corresponding ensemble of Hamiltonians is called orthogonal, unitary, or symplectic. Thus, only these three distribution types are possible, depending on the transformational and corresponding symmetry properties of the Hamiltonian. If time-reversal symmetry is broken, which physically would correspond to the presence of a strong enough magnetic field or of magnetic impurities, then the Hamiltonian matrix must be Hermitian, and the unitary ensemble should be used. When the applied magnetic field is small enough time-reversal symmetry should hold. Then the orthogonal ensemble applies if the electron spin is conserved and the symplectic ensemble if spin-rotation symmetry is broken. In the presence of spinorbit coupling, the spin enters explicitly into the Hamiltonian. For small enough spin-orbit coupling the orthogonal ensemble might still be used, and for large spinorbit coupling if the spin is integer. For half-integer spin and large spin-orbit coupling the symplectic ensemble should apply instead. From the distribution P ( H ) the distribution of the eigenvalues of the Hamiltonian can be derived for the three ensembles. To a high degree of approximation these distributions should be given by the following formulas (x = A/8): Orthogonal
unitary
symplectic
The probability distribution functions p(x),where x = A/6, give the probability of finding a certain energy difference A between successive energy levels in a spectrum
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1.4
.o -
t
1.2
1
0.8
I
I‘
; ; I
II
I
I
......... Poisson
’
I, )I *’
orthogonal _-----
synplectic -- unitary
h
s.
-
-
Q 0.6
-
0.4
-
0.2
-
0.0
-i----.....................
Figure 8. Probability-distribution functions which determine the chance of finding the energy difference, A, between successive levels in the spectrum with average level spacing, 6, in ensembles with different symmetries.
with average level spacing 6 . These have been plotted in Fig. 8 for the three ensembles. For comparison the distribution function p(x) = exp(-x) of the Poisson distribution, as proposed by K ~ b o , [is~also ~ ] plotted. This distribution would apply if the level spacings were completely random (instead of reflecting the symmetry properties of the Hamiltonian). The above formulas and Fig. 8 reveal the celebrated property of Wigner-Dyson level statistics that the levels repel each other, i.e. the probability of finding two levels infinitely close together vanishes as xp for x + 0. In Poisson statistics the reverse is obviously true (Fig. 8). Mathematically, this repulsion follows from the exact mapping which exists between these level distributions and the statistical mechanical Gibbs distribution function describing a 1-dimensional Coulombic gas of classical particles, interacting by a logarithmic pair potential. A general necessary requirement for the applicability of Wigner-Dyson statistics to physical systems is that there should be no constants of motion other than the energy itself, to eliminate crossing of levels. In classical mechanics such systems are known as non-integrable or chaotic systems (no stable periodic orbits in phase space). Besides impurity scattering, scattering at boundaries can make the system
4.7 Physical Properties of Metal Cluster Compounds
1451
chaotic, if the spatial symmetry is sufficiently low. This was the assumption underlying the proposal of Gor’kov and Eliashberg that the Wigner-Dyson statistics should describe the level distributions of an assembly of small metal particles. Because metal particles are composed of atoms, they will unavoidably have surface roughness on an atomic length scale. On the other hand, the wavelength of an electron at the Fermi level, EF, is also of the order of the size of an atom. For sufficiently large particles (> 100 atoms) one thus expects that electron-boundary (and electron-electron interactions) will lead to sufficient randomness. Then, even in an assembly of particles of the same volume, the energy levels would be distributed within each particle and between particles. In principle two kinds of averaging are possible - an average over the energy levels of a single particle (spectral or vertical average) or an average over the particles in the assembly (horizontal average). The assumption that both types of averaging will be equivalent amounts to the assumption of ergodicity. In this respect it should be noted, however, that in experimental particle systems the single particle excitation spectrum is likely to be broadened for the levels farther away from EF, so that spectral averaging for a single particle will not be meaningful. So, in calculating QSE for the thermodynamic properties of an applied the Wigner-Dyson distribution only assembly of particles, Denton et aZ.[271 to the first few excitations above EF, averaging over an assembly of particles of the same volume, the same average level separation, 6. Finally, theoretical justification for the applicability of RMT to small metal particles were given on basis of microscopic theories by Efetov, using the supersymmetric field theory, and by Altshuler and Shklovskii, on the basis of diagrammatic perturbation theory.[261 Thus, although it seems that the theoretical basis for applying Wigner-Dyson statistics to small metal particles is well founded, the amount of direct experimental evidence has been almost completely lacking until very recently. A few years ago, Sivan et a1.[281measured the level spacing in a semiconducting quantum dot and found agreement with Wigner-Dyson statistics for the first few low-lying levels (vertical averaging), the levels farther away being broadened into a continuum, presumably because of electron-electron interactions. The only other evidence seems to be the specific heat and susceptibility studies on the series of monodisperse Pd clusters and the Pd colloid, as described above. The lack of success of earlier attempts can clearly be attributed to sample problems, in particular the presence of size-distribution in assemblies of particles, which will easily wash out the QSE.
Acknowledgments The research described briefly on the preceding pages has been the result of an ongoing collaboration between physicists and chemists sponsored by the European Community stimulation programs. I mention Gunter Schmid (Essen), Giuliano
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Longoni (Bologna), Alessandro Ceriotti (Milano), Bob Benfield (Canterbury), Roberto Zanoni (Roma), Notker Rosch (Munich), and Herman van Kempen (Nijmegen). At Leiden the research has involved Hans Brom, Roger Thiel, Jan van Ruitenbeek, Yakov Volokitin, Joachim Sinzig, Bart van de Straat, Fokko Mulder, Huub Smit, David van Leeuwen, Daniel van der Putten, and Jaap Baak. I wish to express my thanks to all these colleagues and students.
References W. P. Halperin, Rev. Mod. Phys. 58 (1986) 533-606 See e.g. H. Weller, Angew. Chem. Int. Ed. Engl. 32 (1993) 41-53; C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. SOC.115 (1993) 8706-8715 Clusters and Colloids. From Theory to Applications, ed. Giinter Schmid, VCH, Weinheim, 1994 Physics and Chemistry of Metal Cluster Compounds. Model Systemsfor Small Metal Particles, ed. L. Jos de Jongh, Kluwer Academic (Dordrecht), 1994 L. Thomas, F. Lionti, R. Ballore, D. Gatteschi, R. Sessoli and B. Barbara, Nature 383 (1996) 145-147 J. M. Hernandez, X. X. Zhang, F. Luis, J. Bartolome, J. Tejada and R. Ziolo, Europhys. Lett. 35 (1996) 301-306 G. Schmid, Chem. Rev. 92 (1992) 1709; Struct. Bond. 62 (1985) 5285 G. Longoni, A. Ceriotti, M. Marchionna and G. Piro, (1988) in Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis, eds. J. M. Basset et al., Kluwer, Dordrecht G. Schmid, Polyhedron 7 (1988) 2321; Endeavour, New Series 14 (1990) 172; Aspects of Homogeneous Catalysis 7 (1990) 1, ed. R. Ugo, Kluwer, Dordrecht H. Krautscheid, D. Fenske, G. Baum and M. Semmelmann, Angew. Chem.Int. Ed. Engl. 32 (1993) 1303-1305 H. H. A. Smit, P. R. Nugteren, R. C. Thiel and L. J. de Jongh, Physica B153 (1988) 3352 F. M. Mulder, T. A. Stegink, Thiel, R. C., L. J. de Jongh and G. Schmid, Nature 367 (1994) 716 N. D. Lang and W. Kohn, Phys. Rev. B1 (1970) 4555 N. Rosch, L. Ackermann, G. Pacchioni and B. I. Dunlap, J. Chem. Phys. 95 (1991) 7004 D. R. Salahub and F. Raatz, Intern. J. Quantum Chem. 18 (1984) 173; F. Raatz and D. R. Salahub, Surface Science 176 (1986) 219 G. F. Holland, D. E. Ellis and W. C. Trogler, J. Chem. Phys. 83 (1985) 3507 G. Pacchioni, N. Rosch, Inorg. Chem. 29 (1990) 2901; N. Rosch, L. Ackermann, G. Pacchioni and B. I. Dunlap, J. Chem. Phys. 95 (1991) 7004; L. Ackermann, N. Rosch, B. I. Dunlap and G. Pacchioni, Int. J. Quant. Chem. 26 (1992) 605 B. J. Pronk, H. B. Brom and L. J. de Jongh, Solid State Commun. 59 (1986) 349; L. J. de Jongh, Physica B155 (1989) 289 D. A. van Leeuwen, J. M. van Ruitenbeek, L. J. de Jongh, A. Ceriotti, G. Pacchioni, G. Longoni, 0. D. Haberlen and N. Rosch, Phys. Rev. Letters 73 (1994) 1432-1435 D. A. van Leeuwen, J. M. van Ruitenbeek, G. Schmid and L. J. de Jongh, Phys. Lett. A 170 (1992) 325 A. J. Manuel and J. M. P. St Quinton, Proc. Roy. SOC.A273 (1963) 412 Y. Volokitin, J. Sinzig, L. J. de Jongh, G. Schmid, M. N. Vargaftik and I. I. Moiseev, Nature 384 (1996) 621-623
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L. P. Gor’kov and G. M. Eliashberg, Sov. Phys. JETP 21 (1965) 940 For review see J. A. A. J. Perenboom, P. Wyder and F. Meier, Phys. Rep. 78 (1981) 173-292 R. Kubo, J. Phys. Soc. Jap. 17 (1962) 975 For a review, see e.g. C.W.J. Beenakker, Random-matrix theory of quantum transport, to be published in Rev. Mod. Phys. [271 R. Denton, B. Muhlschlegel and D. J. Scalapino, Phys. Rev. B7 (1973) 3589 [28] U. Sivan, Y. Imry and A. G. Aronov, Europhys. Lett. 28 (1994) 115
[23] [24] [25] [26]
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
4.8 On the Size-Induced Metal-Insulator Transition in Clusters and Small Particles Peter P. Edwards, Roy L. Johnston and C.N. R. Rao
4.8.1 Introduction - divided metals There are countless examples in nature where highly conducting metals or metallic materials can be continuously transformed into stubbornly resistive insulators or non-metals by relatively small changes in variables such as elemental or chemical composition, pressure, temperature etc.[l] Representative examples ‘expanded’ elemental metals (e.g. Cs, Hg) above their critical point, doped group-IV semiconductors, solutions of alkali metals in liquid ammonia and other non-aqueous solvents, transition metal oxides, and low-dimensional organic and inorganic conductors. It has also been proposed that the venerable periodic system of the elements represents perhaps the most fundamental and wide-ranging example of the metal-insulator transition in condensed p h a s e ~ . [ ~To % this ~ ] list we now add here the possibility of a size-induced metalinsulator transition (SIMIT) within a single, isolated cluster or small particle of a bulk For surely one imagines that the inevitable consequence of the successive fragmentation, or division, of a single, macroscopic grain of bulk metal must be the ultimate cessation of conducting behavior within the resulting microscopic particle or cluster; clearly the metallic (macroscopic) and the insulating (microscopic) extremes are composed of the same element. An artist’s impression of the issue at hand is given in Fig. 1; this attempts to set out the broad, global features derived from the successive fragmentation of a while at the same time it will form the basis of our approximate classification scheme linking the two extremes. Perhaps the earliest discussion of this problem was that given in 1857 by Faraday,[’31whose researches on colloidal gold, silver, and other metals led him to conclude that “. . . the gold is reduced in exceedingly fine particles which, becoming diffused, produce a beautiful fluid . . . the various preparations of gold, whether ruby, green, violet or blue . . . consist of that substance in a metallic divided state”.[’]
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From the current perspective, part of the continued interest and fascination in metal clusters and metal particles over the past 20 years or so is derived from their enviable position as natural mesoscopic intermediaries between the atomic (nonmetallic) and bulk (metallic) regimes (Fig. 1).[8,9,11,14-171 With these systems in mind, the old and venerable question[’61“What is a Metal?” can now be neatly recast into the equally searching enquiry “When is a Metal?’ By this we mean “How many atoms are needed in a single particle or cluster of atoms to efectively mimic the bulk properties of a metal?” Clearly, the answers to these related inquiries rest, in part, upon which particular characteristic property of the metallic state one wishes to consider.[11~16~’71 For example, from common experience, it is manifestly obvious that the macroscopic metal is unquestionably a magnificent conductor of electricity. But what of the situation in an isolated finite fragment of metal comprising, say, 102-104 atoms? And what does electrical conductivity mean in the context of such an isolated particle of finite size? Furthermore, how can one begin to probe the property of conductivity (and other fingerprint physical properties such as magnetism) in this mesoscopic-size regime? (Mesoscopic is here defined as subpm dimensions - Fig. 1) In essence, how can we begin to describe and interrogate the electronic and physical properties of a single particle - possibly metallic, possibly insulating - which is neither quite microscopic nor quite macroscopic in nature? Equally, we would wish to review the situation of arrays of metal particles to probe electron tunneling and transfer between the constituent centers.[’81 With this in mind, our fundamental tenet is that an electronic transition from metal-to-insulator must inevitably occur as a result of the successive fragmentation or division of a single grain of bulk, metallic matter. Our proposal, then, is that a SIMIT is a safe prediction for any cluster or particle of an element that is a metal in bulk form (Fig. 1). Clearly, a stringent lower limit for this critical size or nuclearity must then be a ‘particle’ consisting of a single atom. One would, however, clearly wish to probe this electronic transition in more searching detail. For example, is the SIMIT within our finite system continuous or discontinuous? At what critical particle size (nuclearity) can one expect to see a SIMIT in the characteristic physical properties of our ‘divided metal ’? These vexing issues and questions constitute the major thrust of this Chapter. In Section 4.8.2 we describe what we believe constitutes metallic or non-metallic behavior within an isolated cluster or particle. As in the case of the corresponding problem in the macroscopic regime, it turns out that the precise definition of metallicity in finite systems is far from straightforward. One approach which at least identifies some of the key problems of the SIMIT revolves around the consequences of the division or fragmentation of an isolated grain of metal within the framework of what one might term (macroscopic) solid-state theory. Such an approach would obviously be inappropriate for clusters and particles containing a small number of metal atoms, say 2-10, which undoubtedly have a substantial parentage in the atomic or molecular limit. It has, however, been known for some time that this approach - which has its roots in the solid-state theory of macroscopic metals -
4.8 On the Size-Induced Metal-Insulator Trunsition in Clusters
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gives useful and intuitive information on the problem of divided metals within the size regime of interest. In Section 4.8.3 we will attempt to compare and contrast the limiting situations of the transition from metal to insulator in both microscopic and macroscopic systems. Finally, in Section 4.8.4 we present a brief overview of recent experimental studies of single particles, and arrays of particles.
4.8.2 The electronic structure of divided metals Metals are unique in that they have one or more partially filled electronic energy bands."'] At a temperature of absolute zero ( T = 0 K) the highest occupied state represents the so-called Fermi level, having a characteristic Fermi energy, EF (Fig. 2). Above EF there are an infinite number of infinitesimally separated empty energy levels which can easily be populated, even at low temperature. Because the constituent orbitals which make up each electronic band are spatially delocalized over the entire macroscopic crystal, any partial occupation of states above and below EF at T > 0 K creates an electron-hole pair, with essentially no constraints on their motion. The application of an external electric field, therefore, gives rise to a spontaneous flow of electric current across the entire crystal; this constitutes the conducting, metallic state. These (conduction electron) carriers are also responsible for the characteristic finite (but small) magnetic susceptibility of bulk metals - the so-called Pauli paramagnetism - because in the presence of an external magnetic field they acquire a net imbalance of up and down spins. Of course, because EF >> kT for most accessible temperatures and the number of spins contributing to the magnetism is very small, the Pauli paramagnetism for macroscopic bulk metals is usually weak and temperature-independent. In contrast, when the sample of metal is reduced to microscopic or mesoscopic size (Fig. l), the assumption of an electronic energy continuum eventually breaks 'I The average spacing between the electronic energy levels is approximately ( E F I N ) ,where N is the total number of atoms in the cluster or particle. When this energy level separation, 6, becomes comparable with the (ambient) thermal energy, kT, the electronic energy levels now become discrete, rather than continuous (Fig. 2). Interestingly, the phenomenon of discrete electronic energy-level separation in small metallic particles was first highlighted['s1 by Frohlich in 1937 as a test of the new quantum theory of Widespread interest in the study of small metallic particles stems, however, from the work of Kubo, who proposed in the early 1960s that the discreteness of energy levels should lead to anomalies in the basic thermodynamic and electronic properties of small metallic particles at low ternperature~.[''~For particles with diameters less than 100 A, Kubo predicted significant deviations from the behavior of the bulk material at low temperatures." 8 9 2'I For example, he predicted that because of the emerging discreteness of the elec-
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i MACROSCOPIC
DIVIDEDMETALS
7
I
1 MESOSCOPIC 7
1 MICROSCOPIC
Size-Induced Metal-Insulator Transition ............. .............
............. .............
............. ............. ............. ............. ............. .............
.............
............... ........................ .............
6
BULK METAL
=o
............. .............
............. ............. .............
6>kT
k k T
METALLlC CLUSTERS & PARTICLES
6 >>kT
INSULATING CLUSTERS & PARTICLES
- increasing
decreasing
Nuclearity
Figure 2. The effect of decreasing particle size (particle nuclearity) on the electronic structure of a metal. The figure attempts to link the bulk, macroscopic regime (with the associated Fermi energy) with the situation of an emerging energy (Kubo) gap arising from the finite size of the particle. The various metallic and insulating regimes of the mesoscopic regime are identified. Modified from Johnston.["]
tronic energy levels of particles of finite size, it should now be possible to distinguish between particles with odd and even numbers of conduction electrons. The oddnumber small particles of a monovalent atomic system (e.g. Na) would thus exhibit Curie paramagnetism in the magnetic susceptibility at low temperatures, rather than the Pauli paramagnetism characteristic of the bulk metal. Similarly, the resulting spin-pairing of electrons in even-number particles at low temperatures tends to reduce their intrinsic magnetic susceptibility below the characteristic Pauli value.
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
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These expected changes in magnetic behavior are just one example of so-called 'quantum-size effects' arising from situations in which the 'Kubo gap', 6, is comparable with, or greater than, the thermal energy, kT, at any particular temperature, uiz.6 2 kT.[8,11,15.16~18.221 One ~ p i n i o n , [ ~accredited ~ * ~ ~ ] to Kubo (although it is not clear whether the interpretations noted in these references represent the concept of quantum-size effects as originally envisaged by Kubo) argues that genuine 'metallic' properties can only be sustained in finite clusters or particles at finite temperatures when 6 < kT, a situation enabling the facile creation of electronic charge carriers by thermal activation.[251From this viewpoint, the following simple criteria:
6 < k T : metal
(1)
6 > k T : insulator
(2)
would therefore define the experimental conditions for a SIMIT in a finite, microscopic solid at temperatures above T = 0 K. A schematic representation["] of the emerging discretization of electronic energy levels with decreasing metallic particle size is shown in Fig. 2. The cluster size appropriate for a SIMIT can thus be readily calculated. Assuming the cluster or particle is approximately spherical, it can be shown that the diameter, D, of a particle containing N atoms is given by:
where r,, is the Wigner-Seitz radius of the element under consideration, i.e. the radius of a sphere of volume equal to the volume per atom in the solid. For a prototypical s-band metal such as sodium with EF = 3.2 eV at room temperature (kT z 0.025 eV), the condition 6 < k T (and presumed metallic conduction on the basis of Eq. l), would then be realized in a metal cluster or particle containing more than 200 atoms, or a diameter of ca 20 A. Of course, this also leads to the inescapable conclusion that a sodium cluster with a diameter below ca 20A would be insulating at room temperature! Fig. 3 shows the size-dependence of the (average) energy level spacing of sodium metal for varying particle diameters." 71 Here, the relevant Kubo gaps ( E F / N )are given in K. Table 1 lists a range of diameters[l7] for several characteristic size regimes, taking as an example clusters and particles of sodium; it shows values for N , the number of atoms, and 6 the Kubo gap (in K) for sodium particles of D = lo4, lo3,lo2, and lOA. Of course, the finite size of the system also leads to a significant number of atoms being located on the surface of the cluster or particle.['81We can estimate the percentage of atoms, P s ( N ) ,which lie on the surface of a cluster of nuclearity N . For
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1o8 - Microscopic
-
Mesoscopic
-
Macroscopic -
1o6 1o4 n
Li
& lo2 i$ 0
2
1 1o-: 10" 1o-(
1
10
lo2
103
Particle diameter (D /
lo4
A)
Figure 3. The fragmentation of bulk sodium; a plot of the average electronic energy-level spacing (the Kubo gap), 6, as a fraction of the particle diameter. The figure also shows the calculated percentage of surface sodium atoms, Ps,as a fraction of the particle diameter. Modified from Harrison and Edwards.["] The approximate location of the microscopic, mesoscopic, and macroscopic regimes are also indicated.
pseudo-spherical clusters, this quantity is given by: Ps(N)% = 4Np'I3 x 100%
(4)
We also show in Fig. 3 the effective percentage of surface atoms for sodium particles as a function of particle diameter.[l7] Ranges of P s ( N ) values for the various cluster size regimes are also given in Table 1, from which it is apparent that clusters of as many as 10000 atoms still have nearly 20% of their atoms on the surface. In fact, & ( N ) only drops below 1% for a system with N > 6.4 x lo7 atoms (corresponding to a diameter of approximately 0.16 pm for sodium clusters).
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
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Table 1. The Kubo gap (8) for sodium particles and clusters as a function of diameter (D)and nuclearity ( N ) .The corresponding values ( P s )for the percentage of surface sodium atoms are also given. Diameter
(D/ 4 104 103 102
10
Number of atoms (N)
Kubo gap (8/K)
Percentage of surface atoms
1.6 x 10" 1.6 x 107 1.6 x 104 16
5 x 10-6 5 x 10-3
0.16 1.6 16 100
5 5
103
(PSI
The onset of quantum-size effects in a finite system might indeed signal the occurrence of a genuine metal-insulator transition, but this remains to be rigorously justified. Part of the issue (as elaborated below) is the obvious difficulty inherent in the problem of applying concepts and models derived for (continuum) macroscopic systems, and then applying these directly to the electronic properties of mesoscopic systems (Fig. 1). For example, as elegantly elaborated by Koutecky and Fant ~ c c i , [one ~ ~ might ] assume that a one-electron energy gap, 6, between occupied and unoccupied levels in an isolated cluster or particle (Fig. 2) should be taken as a key indicator in dictating the magnitude of the electrical conductivity within a particle. It is, however, manifestly dangerous to draw firm conclusions about the magnitude of this conductivity by making an analogy between the highest occupied and lowest unoccupied energy levels for a cluster, and the existence or non-existence of an energy gap between the valence and conduction bands of a bulk, macroscopic solid. This approach (as sketched in Fig. 2) derived from continuum solid-state physics of macroscopic metals is, however, particularly useful in our attempts to quantify, or at least identify, the various electronic regimes of interest in the problem of divided metals,[8.9.11-14-17] We are thus able to link the characteristic property of physical size ( D , N ) to the question of the Kubo electronic energy gap, and the emergence of quantum-size effects (at a characteristic temperature, Fig. 3 ) and the inevitable size-inducedmetal-insulator transition (as identified in Eqs. 1 and 2). Thus, the macroscopic regime extends down to particle sizes of lo4 A ( N zz 1O'O) and a negligibly small Kubo gap (8 << 10@ K). The mesoscopic regime begins in the sub-pm range (D < lo4A), and terminates in the microscopic regime ( D < 10 A). Interestingly, the calculated Kubo gap for our prototypical metal, sodium, also provides an effective 'coarse-grained' physical parameter for delineating the various electronic regimes of interest. Clearly, for bulk metals (D>> lo4A), we have 6 << K, and a continuum of electronic energy levels. Within the mesoscopic regime, we see the emergence of a rapidly increasing Kubo gap (6 zz 1OP6-1 K for D zz 104-102A, to 6 zz 1-103 K
1462
4 Nanomuteriuls
for D E lo2 - lOA). Within this size regime one would clearly expect anomalies in the characteristic physical properties of particles and clusters arising from the onset of quantum-size effects. Equally, this is the same regime for which we might expect a SIMIT at relatively accessible temperatures, uiz. 1 K and above (see Eqs. 1 and 2). This is precisely the size regime of colloidal metals and metal nanoclusters. As indicated above, as we move closer and closer to the microscopic size regime, we become increasingly concerned at the validity of such an approach derived from the continuum physics of macroscopic metals. Conversely, any theoretical treatment originating from the quantum chemistry standpoint (the ‘Atoms and Molecules’, regime in Fig. 1) would inevitably become unreasonable as we move from the microscopic to the mesoscopic regimes. This, of course, is the fascination - and the challenge - of the science of divided metals! Gor’kov and Eliashberg[261investigated the problem of a size-induced metalinsulator transition in terms of the location of the gas of conduction electrons in a metal through the (finite) size-induced confinement of the electron wave packet. The de Broglie wavelength A of electrons is given by: 1, = h/mvF = h/(2rnE) ‘I2
(5)
where E = 1/2rnu; and h = 2774 Planck’s constant, and 1, is typically 5-1OA in metals. Left to themselves conduction electrons in a bulk metal behave as a delocalized Bloch (travelling) wave extending throughout the crystal. Gor’kov and Eliashberg noted[261that when the geometric dimensions of the metal particle or cluster become smaller than the characteristic phase coherence length of the conduction electrons, one expects a reduction in the (single-particle)d.c. electrical conductivity. Eventually, as the particle dimensions are continuously reduced, they predicted an inevitable size-induced metal-insulator transition because of the confinement (and collapse) of the conduction electron wavepacket. This, then, is the size-dependent confinement of conduction electrons, equivalent to electron localization within the particle, resulting in a significant reduction in the d.c. electrical conductivity. Over thirty years ago, Ioffe and Rege1[271predicted the occurrence of a metalinsulator transition[’] in a macroscopic system when the criterion
kFI,
E
1
(6)
is satisfied. Here kF = ( 3 7 ~ * n ) ’is/ ~the Fermi wavenumber expressed in terms of the carrier density, n, and I, = V F Z ~= hkFZe/rn* is the (elastic) mean free path of the conduction electrons. This simple criterion has been used extensively - and successfully - in a wide range of macroscopic systems to locate the experimental conditions for a metal-insulator transition.[281Wood and A s h c r ~ f t [have ~ ~ ] noted that if we crudely interpret the electron mean free path (Ie) as the characteristic dimension (diameter = D)of a small particle or cluster, then the Ioffe-Regel criterion, can be modified to kFIe = kFD E 1 to describe the cessation of metallic (conducting)
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
1463
behavior within an individual particle. This approach, therefore, links the issue of the metal-insulator transition in both macroscopic and mesoscopic systems, a theme which forms the basis of the next section.
4.8.3 The metal-insulator transition in mesoscopic and macroscopic systems Transitions from metallic to insulating behavior occur in a wide range of condensedphase systems as a characteristic physical variable or parameter is continuously ~ a r i e d . [ ’ - ~For . ~ ~example, ’ it was recognized some time ago that carefully increasing the concentration of ‘donor’ or ‘impurity’ atom states (e.g. phosphorus) in a single crystal of a group-IV semiconductor (e.g. silicon) could bring about a continuous transition from insulator to metal.[301For an isolated donor or impurity state in Si : P (Si doped with P) the additional valence electron of the P atom moves in the Coulombic field of the parent donor ion (P’) in much the same way that an electron moves about the proton in an isolated hydrogen atom. However, because the P+ cation is itself embedded in a host (Si) dielectric medium, the Coulombic force of attraction between the electron and cation is reduced from (-e2/r) to (-e2/Er) via the background dielectric constant ( E ) of the Si lattice ( - 13 for pure Si). As a result the electron moves in a highly expanded ‘Bohr orbit’ the Bohr radius of which, a H * ,is considerably larger than that of a 1s electron in a hydrogen atom, 2112. :
ah = Eh/m*e2
(7)
where m* is the effective mass of an electron in the Si conduction band. At low donor densities, the system is insulating. Above a critical donor or impurity density (ca 10” atoms cmP3),we have very substantial overlap of the hydrogenic wavefunctions of the entire assembly of donor states, in the crystal. The system is thus transformed into a metallic conductor, as the valence electrons of the constituent donor states are now completely ionized from each center, and these move collectively as a gas of free, or itinerant, conduction electrons throughout the entire crystal. In a series of seminal contributions beginning in 1949, Sir Nevi11 Mott first posed the key question of how such an insulating, non-metallic system could naturally evolve into a metallic His conclusion, that such a system could undergo an insulator-to-metal transition, was perhaps not surprising; what was remarkable, however, was Mott’s proposal that at the very transition from insulator to metal, all the outer (valence) electrons would become free at once, not just a few
1464
4 Nanomaterials
of them. Mott presented persuasive arguments to illustrate that the change from a non-conducting (non-metallic) insulator to a conducting (metallic) state must be very sharp; in essence, “either none of the electrons are free to move, or all are”. This led to the tantalizing and controversial prediction[’,313321 of a discontinuous (firstorder) transition from a metal to insulator at the absolute zero of temperature. This kind of discontinuous quantum phase transition has long been known as ‘The Mott Transition’. The ramifications for a ‘Gedanken experiment’ at T = 0 K are sketched in Fig. 4a, revealing the d.c. electrical conductivity for a macroscopic system such as Si : P in which d, the average distance between one-electron centers, can be continuously tuned by changes in the composition of the system.[5381 For values of d below a critical distance, d,, (i.e. d < d,) the system is metallic and the electronic wavefunction is completely delocalized over the entire sample. For very large d ( d > d,), we have an insulator with a valence electron wavefunction that is completely localized at the individual atomic sites. At a critical distance, d,, we then have, according to Mott, a first-order (discontinuous) metal-insulator transition. Thus, at T = 0 K one either has a non-metal or an insulator, for which the limiting (low temperature) d.c. electrical conductivity is zero, or a metal, with a finite conductivity at this base temperature. Whether the metal-insulator transition in Si : P (Fig. 4a) is continuous or discontinuous is still a source of controversy. It is, nevertheless, clear that in the macroscopic assembly of one-electron centers originally envisaged by Mott (with extremely high densities approaching 101s-1020 electrons cmP3),one is dealing with solid-state physics appropriate to the thermodynamic (bulk) limit. Thus, one generally measures the properties of macroscopicsized crystals of a doped semiconductor which are of dimensions considerably larger than any characteristic length scale of the system, such as the de Broglie wavelength of the electrons. There is therefore a clear distinction to be made here between the nature of the metal-insulator transition in a macroscopic system such as Si : P and our (assumed) inevitable metal-insulator transition occurring solely within an isolated microscopic cluster or particle, typically containing between lo2 and lo6 atoms (Fig. 1). In his celebrated paperr3’] of 1961, ‘The Transition to the Metallic State’, Mott also alluded to the fundamental differences between these two situations. Concerning the proposed first-order (discontinuous) nature of the metal-insulator transition in macroscopic systems, he noted “ . . . the sharp transition described here is only expected in an infinite lattice. It goes without saying that for aJinite number of atoms there will be a gradual decrease in the weight of the ionized states in the wavefunction as the interatomic distance is increased, or, in other words, a gradual transition.. . .” In Fig. 4 (a) and (b) we compare, and ~ o n t r a s t , [ ~the ’ ~metal-insulator ~] transition at T = 0 K in our divided metal with that of a macroscopic sample of a doped semiconductor, Si : P. For the latter the average dopant separation, d, (and thus the electron carrier density) provides the key experimental control parameter separating the metallic (d < d,) and non-metallic (d > d,) regimes. The metal-insulator tran-
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
1465
1466
4 Nanomaterials
sition in Si : P occurs at a donor concentration (n,) of ca3 x lo'* electrons cmP3, corresponding to d, z lOOA. The corresponding SIMIT within an individual microscopic particle or grain of metal (Fig. l ) will reflect the qualitative change in the characteristic electronic structure of each particle arising from a gradual reduction in the average diameter, D. Here, the critical experimental control parameter would be D,, such that clusters and particles for which D > D, will be metallic, whereas those for which D < D, will exhibit non-metallic behavior at the particular temperature of interest (Fig. 4(b)). Recall, that so far we are considering metal particles and clusters only, in isolation, with no possibility of interparticle electron (quantum) tunneling. In Fig. 4, we identify a discontinuous transition (2la Mott) for Si : P (a) while for the divided metal (b), we illustrate both continuous (solid-line) and discontinuous (dotted-line) scenarios for the size-induced metal-insulator transition. In discussing the issue of interparticle interactions, it is generally assumed that an individual metal particle will remain electrically n e ~ t r a l . " ~This ~ ~ ~innovation, ] originally proposed by Kubo,[2 has its atomic counterpart in the Mott-Hubbard correlation energy, U, for macroscopic systems"] (Fig. 5 ) . For our present purposes
Figure 5. The metal-insulator transition in the doped semiconductor (Si : P) and a divided metal. Here Us, is the Mott-Hubbard correlation energy in the doped semiconductor, and U,l,, is the corresponding 'charging energy' of the divided metal.
4.8 On the Size-Induced Metal-lnsulator Transition in Clusters
1467
we designate the Mott-Hubbard U parameter for doped semiconductors as Us, z e2/EaH*. The corresponding energy required to charge a metallic particle of diameter D is Uclus % e2/2moD, where EO is the vacuum permittivity. This charging energy can only originate from the surrounding medium which is at a temperature T. Therefore when kT ( eV at 1 K, -0.03 eV at 300 K) is less than e2/2moD (-0.2 eV for D = lOOA), the probability of electron transfer between particles is negligible, and charge fluctuations on individual particles are highly improbable. The divided metal or cluster charging energy, Uclus,can be viewed as the difference between the ionization energy and the electron affinity ( I P , I ~-~EAclUs)of the isolated particle or cluster.[81 Therefore, if metal particles or clusters are physically well separated or insulated electrically from each other, thermal fluctuations are themselves never sufficient to enable an individual metal particle to lose or gain an electron, even though the total number of electrons within each particle might amount to hundreds or thousands. In many respects, this situation is reminiscent of the problem of the Mott-Hubbard correlation energy,['] which acts to suppress charge fluctuations in isolated centers. In an aggregate or assembly of such metallic particles, however, the Coulombic interaction between oppositely charged particles might serve to reduce the charging energy Uclus when significant inter-particle contacts exist. Electronic conduction with appreciable activation energies, often observed in early work on aggregated or connected particles, supports this basic idea." 81
4.8.4 Experimental studies Within the context of the metal-insulator transition in metal particles and clusters, two closely interwoven problems can be identified. The first is the inevitable transition to insulating (non-metallic) behavior within an individual grain or particle of metal as its geometric dimensions are continuously reduced. Throughout this process, the electron wavefunction is assumed to be completely confined within the single particle; this is the size-induced metal-insulator transition (Fig. I). The second point of interest relates to the electronic structure of ordered (macroscopic) arrays of small particles which are themselves, individually metallic. By the controlled synthesis of such arrays in one, two, or three dimensions, one can hope to engineer significant electron tunneling and transport between neighboring metallic particles or metallic filamentary structures; the resulting electron wavefunction can ultimately be completely delocalized over macroscopic distances. We now review just some of the key literature in relation to the SIMIT in individual particles and clusters, and the complementary subject of the metal-insulator transition in connected arrays of metallic particles.
1468
4 Nunomaterials 4 00
1000
Hgx
X
70
17
8
4
2
1
IP. (eV]
Figure 6. The variation in the measured ionization potential of mercury clusters as a function of cluster size. The work function for bulk Hg (4.49 eV) is indicated. The dashed line is a plot of the ionization potential calculated for the classical (liquid drop) electrostatic model for a metallic sphere of diameter d. Region 111 contains clusters which are classified as insulating. Region I1 denotes the size-induced metal-insulator transition, in which overlap of the 6s and 6p states sets in at around Hg13. The larger clusters, located in Region I, have valence electronic structures that closely resemble the band structures of liquid and crystalline mercury. Adapted from Rademann.r341
4.8.4.1 Single particles A series of important experiments probing the electronic structure of individual (gaseous) metal clusters and particles was performed by Rademann and co-worke r ~ [in ~the~early , ~1980s. ~ ~ These authors interpreted the size-dependent variation of the ionization energies (ZP)of mercury clusters (Fig. 6) in terms of a continuous transition from van der Waals to metallic bonding in the region of N = 13-70 atoms. For N < 13, the approximate straight-line dependence of ZP on l/d extrapolates to ZP(co)% 6.5 eV. This extrapolated value is significantly higher than
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
1469
the bulk work function of elemental Hg (4.49 eV) and reflects a fundamentally different type of bonding in these small clusters. For N > 13, the measured ZP decreases more rapidly and converges on the straight line predicted by the so-called liquid drop at N z 140. Other observables, such as the size-dependence of the 5d-6p autoionization spectrum, and the appearance of the surface plasmon mode, have been proposed as providing supporting evidence for a SIMIT in this size regime.[37,381 However, following Jortner"'] and Rosenblit and J ~ r t n e r [ ~we ~ ]note , that although the size-dependence of ZP provides direct information on specific cluster and size effects on the electronic level structure, it does not elucidate completely the central issue of the precise nature of the SIMIT within an isolated cluster. General comments about the possible description of the transition itself rely on issues relating to electronic band-width considerations. The closed sub-shell electronic configuration, 6s2, of the free Hg atom causes small mercury clusters to be non-metallic and held together by relatively weak van der Waals dispersion forces (as found, for example, in noble gas clusters). As the cluster grows, the atomic 6s and 6p levels broaden into electronic energy bands (Fig. 7). A metal-insulator transition within the cluster is presumed to occur at a critical nuclearity (N,) because of 6s-6p band overlap (as shown in Fig. 7), al-
Figure 7. The transition from atomic to metallic (bulk) mercury through the intermediary of mercury clusters. The occupied bands are indicted by shading. Here we approximate the energy separation between occupied and unoccupied bands by asp,equivalent to the Kubo gap.
1470
4 Nanomaterials
though there is probably a transition to a semiconducting (covalently bonded) state (owing to s-p band hybridization, before band overlap) before the metallic state is reached.[401For an isolated cluster, the individual s- and p-band widths ( W,and W, respectively) are related to the mean atomic coordination number (Z,,) by:
where the square root dependence is derived from a tight binding analysis. The metal-insulator transition within the cluster can therefore be viewed as occurring at a critical mean coordination number (2,”)- and implicitly a critical nuclearity (N,) - rather than a critical density (p,) which is clearly the situation in the macroscopic condensed phase system.[411Here we would identify the analogue of the Kubo gap (Fig. 2) as asp,derived from the 6s-6p gap (Fig. 7). Building upon an analogy with expanded liquid mercury, for which a density-induced metal-insulator transition occurs at pc = 5.75 g cm-3,[411corresponding to Z,, % 6-7, Tomanek et al. predictedr4’] a transition to the metallic state for HgN clusters in the range 20 I N, I 50. Tight binding calculations by Pastor and B e n n e r n a n r ~ [ ~predicted ~,~~] a fundamental change in chemical bonding from van der Waals to covalent at approximately N = 13, with a transition to metallic bonding at approximately N = 80, in good agreement with the experimental data for the measured ZP as a function of N . Pastor and B e n n e ~ n a n n [ ~have ~ , ~also ~ ] considered the detailed process of electronic delocalization within an isolated cluster as the nuclearity increases. When the electrons delocalize within a cluster, they jump from one atom to one of its neighbors (again within a single cluster or particle), and pairs of ‘charged atoms’ (Hg+ c) Hg-) would form instantaneously. There is thus a close link with the Mott-Hubbard energy (Fig. 5). This hopping process within each cluster is enhanced by the 6s-electronic bandwidth arising from the interaction of Hg atoms an important within the particle (Fig. 7 ) . Pastor and Bennemann propose[409431 criterion for electron delocalization within the cluster which corresponds rather closely to Mott’s criterion for the metal-insulator transition in the macroscopic problem“] and this reflects the usual competition between kinetic and Coulombic energy. As in Mott’s theory, delocalization occurs when a critical electronic bandwidth and efficient screening are achieved. Clearly, the fundamental difference with Mott’s famous ‘Gedanken experiment’ (Fig. 4) is that for isolated particles and clusters the critical electronic bandwidth is now related to a critical coordination numberr441,instead of being related to a critical interatomic distance (critical density) for the macroscopic regime. As noted earlier, these a ~ t h o r s ~obtain ~~,~~] N, z 13 for the critical nuclearity for Hg clusters; thus for N < 13, the properties of HgN clusters result mainly from localized electrons occupying atomic-like orbitals, whereas for N > 13, they result from delocalized electrons occupying molecular orbitals with spatial extension across the entire cluster. In the early and mid 1980s Edwards and coworker^[^',^^^ proposed that transi-
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
1471
tion metal carbonyl compounds might be ideal model systems for monitoring the genesis of a particulate metal from individual molecular units. The transition-metal cluster carbonyl compounds of, for example, Os, Rh, and Pt, of generic formula M,(CO),Q- (with Q 2 0 and n > m ) , are well characterized molecular species which can be obtained in exceptionally high purity. The cluster carbonyls of these three elements have been shown by single-crystal studies to assume many different geometries (Fig. 8), which are fragments of bulk metallic lattices. Johnson et al. reported[451a systematic investigation of magnetism in the cluster carbonyls of osmium. The experimental quantity of most direct interest was the 'excess molecular susceptibility' kerns) of the 0 s cluster compounds, which is the difference between the experimentally determined magnetic susceptibility of the molecular complex (Fig. 8) and the sum of the diamagnetic corrections of those species from which the high-nuclearity cluster is composed. The experimental xemsvalues for six cluster compounds of 0 s are plotted as a fraction of cluster n ~ c l e a r i t y in ~ ~Fig. ~ ] 9. These excess molecular susceptibilities are most naturally interpreted as arising from a Van Vleck (temperature-independent) paramagnetic contribution to the magnetic susceptibility. Of particular interest is the marked increase in xemswith increasing cluster size and how this relates to our view of the electronic structure of these materials. The temperature-independent van Vleck paramagnetism increases continuously with cluster size (Fig. 9) as the molecular electronic states begin to approach the electronic band or continuum regime, which ultimately results in the characteristic Pauli paramagnetism in the bulk, metallic state (Figs 1 and 2). Johnson et al. also addressed[451the issues raised in the introduction to this Chapter (Section 4.8.1)) namely, that the answer to the question 'How many atoms maketh metal ?'[l6] depends critically upon which particular property of the metal one wishes to describe." ' , l 2 ] Transport properties (for example, d.c. electrical conductivity) require, as a prerequisite for the detection of metallic character, the complete ionization of an electron from its parent site, which typically involves energy changes in the range 1-10 eV. In contrast, as far as magnetic properties are concerned, modifications of the intra- and inter-unit exchange energy in the developing metal cluster need only be of the order of magnitude of the magnetic Zeeman or hypefine energies (both typically in the range lop3to lop4 eV) for changes to be detectable. Important work relating to the continuing growth of interest in metal cluster compounds as models of divided metals has been elegantly reviewed by Schmid in 1992E4'1 and 1998.L4'] In relation to the gold cluster Au55(PPh3)12C16, with a cluster 'nucleus' ca 14A in diameter, Schmid notes that this system has reached the borderline of the metallic state. Measuring the d.c. conductivity of isolated metal particles, of course, is arguably the most direct method of investigating the size-induced metal-insulator transition within a single particle.['] Marquardt, Nimtz and coworkers believe they have succeeded in measuring the quasi-d.c. conductivity of individual indium nanoparticles
1472
4 Nanomaterials
OS$O),, triangular
RhG(CO)l, octahedral
OS,(CO),, c a p p e d trigonal bipyramid (polytetrahedral structure)
[os 0 c (CO),,] tetra-copped octahedron (fragment o f f.c.c. structure)
[R h , 4 (CO) 2514-
fragment
of
b.c.c.
structure
[H~R~I,(CO),~]~anticubeoctahedron ( f r a g m e n t o f h.c.p. structure)
[R h , 5 C 2 ( C O ) 2 8 1 pentagonal structure
[P t ,cJco)2z]4pentogonal structure
Figure 8. A representationof typical metal cluster geometries in transition metal carbonyl clusters. For clarity, the carbonyl ligands have been omitted. These compounds can profitably be viewed as ‘metallic’cluster fragments which are effectively isolated from adjacent cluster units by inert sheaths of carbonyl ligands. Taken from Johnson et aLr451
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
It
*0-
I
I
I
I
I
I
1
I
I
I
I
I
1473
I
3001
C I uster
N u c l ear i t y
Figure 9. Magnetism in osmium cluster carbonyls. The variation of the high-temperature (298 K) excess molecular susceptibility, xems,as a function of cluster nuclearity. Taken from Johnson et aL[451
by performing contactless microwave absorption experiment^.^^^,^^^ These authors studied metal particles with diameters ranging from 100 8, to ca microns and found that the electrical conductivity within each particle decreases rapidly with decreasing particle diameter, below a few microns according to the approximate relationship G cc D 3 ,giving rise to a presumed size-induced metal-insulator transition (Fig. 10). Scanning tunneling spectroscopy (STS) is an extremely powerful technique for the direct measurement of the I- V characteristics of individual clusters and particles deposited on a surface (usually graphite), and hence for probing the vexing question of the metallic or insulating status of individual particles. Rao and coworkers have demonstrated that in bare gold clusters with diameters larger than 40 8, finite currents are induced by very small applied voltages, implying genuine ‘metallic’ character within the particle.[511In contrast, clusters with diameters of ca 14A have rectifying behavior, which is consistent with insulating status within the particle and on particles of Pd, Ag, Cd, a so-called Coulomb gap. Recent STS and Au by this same group have shown there is a general marked decrease in
1474
4 Nunornaterials
I r
1
bulk
.= ’/
classical
E
r
u
c
I
/5/
I
I
I /
‘E
-c
0
c.- 10’2 >
c
u
I
-0
C
Indium
0
u
(T-300 K)
lo-* 1
10’ lo2 diameter 61
lo3
Figure 10. The size-induced metalinsulator transition in mesoscopic crystals of indium. The size-dependent (quasi)d.c. conductivity versus particle diameter. For comparison the bulk conductivity and the ‘classical’ (surface) size-effect are also displayed. Modified from Nimtz et
the measured conductance for diameters below lOA, at which sizes the measured density-of-states has a band gap. These authors have examined the cluster sizedependence of the slope of the I- V curves in some detail. In Fig. 11 we show the derived slope, the conductance (after suitable normalization relative to the support) plotted against the volume of the particle or cluster. We note that as the cluster size increases the normalized conductance increases linearly up to a volume of some 4 nm3 (Dz 2 0 4 and reaches a constant value for larger particles and clusters. The derived electron-energy gap for clusters of Pd, Ag, Cd and Au of different sizes is also shown in the top portion of Fig. 11. Throughout, the measured value of the conduction gap lies between 10 and 70 mV. This electronic conduction gap thus decreases with increasing cluster volume and there is clearly no conduction gap for clusters larger than ca lOA. This observation suggests that small particles or clusters with diameters below lOA are indeed insulating, in that the extrapolated zero-temperature d.c. conductivity is zero. The data assembled in Fig. 11 provides a striking experimental manifestation of the size-induced metal-non-metal transition in isolated particles. This experimental study is also consistent with recent work by Rosenblit and J ~ r t n e r [who ~ ~ ]predict that electron localization occurs within single metal clusters with diameters of ca 6 8, or less. These authors make the important point that for clusters of metal atoms, the evolution of the electronic band structure with increasing cluster size might indicate a transition to a metallic state which might be manifest in a size-dependent dielec-
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
I415
>
E"
+
C
.-0
u
Y
A
Y U
+
C 0
0
OL
!
I
0.4
0.6
I
0.2
0
- 1.0 0.8 l
a
1
Volume (nm 3 2 .L 7
2.2
mrnerci fmeiai I
;
2.0
n!?
'
+:
1.8 1.6 1.4
1.2
0 Au I
OPd
'
+ Cd
&?A I+
I
A Ag
1 .o
0
50
100
150
200
250
Cluster volume ( n m 3 ) Figure 11. (a) The conduction gap observed in small clusters of gold, palladium, cadmium, and silver as a function of cluster volume. (b) Normalized slope of the I-Vcurves (the conductance) as a fraction of cluster volume for the four metals studied. Above a critical volume of ca 4nm3 the slope becomes size-independent. This is possibly a direct indication of a size-induced-metal-insulator transition, as indicated. Taken from Vinod et a1.'511
1476
4 Nunomaterials
tric constant. Studies of the energetics of surface excess electron states of clusters provides direct information on the real part of the dielectric function, and these should be an effective probe for identification of the metal-insulator transition. The concept of the divergence of the dielectric constant at the metal-insulator transition thus links the macroscopic and the mesoscopic situations (Fig. 4). Studies of the dielectric properties of metal clusters and particles of increasing size and nuclearity might provide important insights into the SIMIT in mesoscopic systems.
4.8.4.2 Arrays of particles Recent developments in the synthesis of small metallic particles have involved the production of relatively monodisperse, organically-functionalized metal particles. In an important contribution, Shiffrin and c o - w o r k e r ~first [ ~ ~produced ~ 2D arrays of gold particles capped with thiols. There is considerable interest now in the crystallization of these particles to form ordered arrays of ‘artificial s o l i d ~ ’ [ ~ ~In- ~Fig. ~]. 12a we show the TEM image of a thiol-derivatized gold nanoparticle array.[541The particles are nearly spherical with a mean diameter of ca 40 8,;the size distribution is shown in Fig. 12b as a histogram. The gold nanoparticles assemble into a hexagonal superstructure, and the (center-to-center) nearest neighbor distance between the particles is nearly constant at ca 5.5 8, throughout the superstructure domain. In principle, chemical control over the composite particle size and the inter-particle separation provides a direct route for investigating the coupling between individual particles. Indeed two dimensional, ordered arrays of such assemblies have been prepared at the air-water interface on a Langmuir trough, and the Langmuir technique can be used for continuous tuning of the interparticle separation d i s t a n ~ e . [ ~ ~ , ~ ~ ] Heath and coworkers have recently reportedL55,561 the observation of a reversible, room-temperature metal-non-metal transition in organically-functionalized silver particle monolayers. A useful parameter for characterizing these monolayers is the quantity ( d / D ) ,where d is the interparticle separation, as measured between particle centers, and D is the particle diameter (c.f. Fig. 4). The ultraviolet-visible reflectance spectrum from a Langmuir layer of 40 8, diameter silver particles, collected in-situ as the film is compressed, is shown in Fig. 13. Upon initial compression, the film becomes more reflective (see (l), (2), and (3) in Fig. 13). The final reflectance spectrum is similar to that reported for thin, metallic silver films ((4) and ( 5 ) in Fig. 13) and indicates conclusively that the Langmuir film has finally become metallic. A reasonable working picture highlights the energy gain from electron delocalization; when this energy gain becomes sufficient to overcome the site (single-particle) charging energy (Fig. 5 ) , an insulator-metal transition occurs throughout the entire array of particles. The experimental data provide strong evidence of the onset of strong quantum mechanical interactions between adjacent silver particles when the interparticle separation distances are reduced below 128,; the reversible insulatormetal transition occurs when the interparticle separation is reduced below 5 A.
1471
4.8 On the Size-Induced Metal--Insulator Transition in Clusters
v)
0
30
-
25
-
20
-
._ K
g15 8 s 10
Figure 12. (a) TEM image of a thin film of thiol-derivatized gold particles. (b) Histogram showing the size distribution of the gold particles. Taken from Vijaya Sarathy et u ~ . [ ’ ~ I
50-
I 1
2
3
4 5 6 Diameter I nm
7
8
These studies have led to the interesting proposal that this reversible metal-insulator transition represents a first-order Mott transition at room temperat~re.[’~~’~] Earlier studies by Kreibig and c o - w ~ r k e r s ~also ’ ~ ~impact on the nature of the insulator-metal transition throughout extended arrays of mesoscopic particles. These authors investigated the effect of cluster-particle aggregation by monitoring the extinction spectrum from gold clusters all the way up to a thin film of the same element. The effect of aggregation on the optical response is drastic; not only are the
1478
4 Nunomuterials
(1) Compression
Figure 13. Ultravioletvisible reflectance spectra of a Langmuir monolayer of 40 8, diameter silver particles, collected in-situ as the film was compressed. Upon initial compression, the film becomes more reflective [(I), (4,and (31;below an interparticle separation of 5 A, the reflectance spectra [(4) and ( 5 ) ] begin to resemble those of a thin, metallic film. Taken from Collier et al. r5’1
spectral intensities changed but the qualitative nature of the spectrum is altered. The extinction spectra of aggregates of gold clusters with various mesoscopic structures are shown in Fig. 14. As aggregation increases the spectrum changes continuously from the relatively narrow plasma absorption of a mesoscopic particle to the broad extinction features which resemble the optical absorption of a continuous thin film of gold. The underlying physical model, reflecting the transformation from an individual mesoscopic cluster or particle, through the interacting particle regime to the thin film, has been investigated.
4.8.5 Concluding remarks If an individual grain of a metallic element or compound is sufficiently reduced in physical size, we expect a size-induced-metal-insulator transition (SIMIT). We have attempted here to highlight certain key features of the problem in terms of the
4.8 On the Size-Induced Metal-Insulator Transition in Clusters
1479
EXTINCTIONOF GOLD CLUSTER3 AND FILM3
:-10'i Figure 14. The measured extinction spectra of gold-cluster matter with different mesoscopic structures. The cluster diameter is approximately 100 A. The extinction spectra change continuously with increasing cluster aggregation. The uppermost curve is that of a thin film of evaporated gold. Different spectra are separated by moving them along the ordinate. Taken from Kreibig and Quentin.'' 'I
.
I
I n
c
r
e
y
.
F
aggregation
1 I1 ' -lo2
single clusters-/ 1
2
successive fragmentation of such a grain of metal . . . a divided metal (Fig. 1). This process can lead to important changes in electronic properties because of the appearance of a respectable energy gap in an (assumed) continuum of energy levels. Further fragmentation of a metal particle can lead to an even more drastic modification; namely, the cessation of metallic conductivity within a single grain of matter. At this stage, presumably, one has not only a divided metal, but also an erstwhile metal! We have attempted to link both the issue of the SIMIT in single particles and the related transition in arrays of particles, with ideas and concepts from the mature field of doped semiconductors We hope that this contribution illustrates the fundamental importance of the size-induced electronic (quantum) phase transition from metal-to-insulator in mesoscopic cluster systems.
References [ I ] N.F. Mott, Metal-Insulator Transitions, Taylor and Francis, London (1990). [2] P.P. Edwards and C.N.R. Rao, eds. The Metallic and Non-metallic States of Matter, Taylor and Francis Ltd., London (1985).
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P.P. Edwards and C.N.R. Rao, eds. Metal-Insulator Transitions Revisited, Taylor and Francis Ltd., London (1995). P.P. Edwards, R.L. Johnston, C.N.R. Rao and D.P. Tunstall, eds. The Metal-nonmetal Transition in Macroscopic and Microscopic Systems, Phil. Trans. R. Soc. Lond., A356 (1998). (a) P.P. Edwards, R.L. Johnston, C.N.R. Rao, D.P. Tunstall and F. Hensel, Phil. Trans. R. Soc. Lond., A356, 5 (1998). (b) P.P. Edwards, R.L. Johnston, F. Hensel, C.N.R. Rao and D.P. Tunstall, Sol. State Phys. (in press). E. Wigner and H.B. Huntington, J. Chem. Phys., 3, 764 (1936). P.P. Edwards and M.J. Sienko, Acc. Chem. Res., 15, 87 (1982). P.P. Edwards, Proc. Indian Nut. Sci. Acad., 52, 265 (1986). P.P. Edwards, in Chemical Processes in Inorganic Materials: Metal and Semiconductor Clusters and Colloids, eds. P.D. Persans, J.S. Bradley, R.R. Chianelli and G. Schmid, Mat. Res. Soc. Symp. Proc., 272, 31 1 (1992). P. Nimtz, P. Marquardt and H. Gleiter, J. Cryst. Growth, 86, 66 (1988). R.L. Johnston, Phil. Trans. R. Soc. Lond., A356, 211 (1998). J. Jortner, Z. Phys. D., 24, 247 (1992). M. Faraday, Phil. Trans., 147, 145 (1857). E. Schumacher, F. Blatter, M. Frey, U Heiz, U. Rothlisberger, M. Schar, A. Vayloyan and C. Yeretzian, Chimia 42, 357 (1988). W.P. Halperin, Rev. Mod. Phys., 58, 535 (1986). P.P. Edwards and M.J. Sienko, Int. Rev. Phys. Chem., 3, 83 (1983). M.R. Harrison and P.P. Edwards, in The Metallic and Non-metallic States of Matter, ed. P.P. Edwards and C.N.R. Rao, Taylor and Francis Ltd., London, (1985) p. 389. R. Kubo, A. Kawabata and S. Kobayashi, Ann. Rev. Muter. Sci., 14, 49 (1984). N.W. Ashcroft and N.D. Mermin, Solid State Physics, Holt, Rhinehart and Winston Inc., New York (1976). H. Frohlich, Physica, 4, 406 (1937). R. Kubo, J. Phys. Soc. Jap., 17, 975 (1962). J.A.A. Perenboom, P. Wyder and F. Meier, Phys. Rep., 78, 173 (1981). S.B. Di Cenzo and G.K. Wertheim, in Clusters of Atoms and Molecules II: solvation and chemistry of free clusters, and embedded, supported and compressed clusters, ed. H. Haberland, Springer-Verlag, Berlin (1994) p. 362. M. Broyer, J. Non-Cryst. Solids, 156-158, 787 (1993). J. Koutecki and P. Fantucci, Chem. Rev., 86, 539 (1986). L.P. Gor’kov and G.M. Eliashberg, Sou. Phys. JETP, 21, 940 (1965). A.F. Ioffe, and A.R. Regel, Proc. Semicond., 4, 239 (1960). P.P. Edwards, T.V. Ramakrishnan and C.N.R. Rao, J. Phys. Chem., 99, 5228 (1995). D.M. Wood and N.W. Ashcroft, Phys. Rev. B., 25, 6255 (1982). M.N. Alexander and D.F. Holcomb, Rev. Mod. Phys., 40, 815 (1968). N.F. Mott, Proc. Phys. Soc., A62, 416 (1949). N.F. Mott, Phil. Mag. 6, 287 (1961). A. Kawabata, J. de Phys., 38, C2-83 (1977). K. Rademann, Ber. Bunsenges. Phys. Chem., 93, 653 (1989). K. Rademann, 0. Dimopoulou-Rademann, M. Schlauf, U. Even and F. Hensel, Phys. Rev. Lett., 69, 3208 (1992). See for example: C. Brechignac, in Clusters of Atoms and Molecules I: theory, experiment and clusters ofatorns, ed. H. Haberland, Springer-Verlag, Berlin (1994) p. 255. C. Brechignac, M. Broyer, P. Cahuzac, G. Delacretaz, L. Labastie, J.P. Wolf and L. Woste, Phys. Rev. Lett., 60, 275 (1988). H. Haberland, B. von Issendorf, J. Yufeng, T. Kolar and G. Thanner, Z. Phys. D, 26, 8 (1993). M. Rosenblit and J. Jortner, J. Phys. Chem., 98, 9365 (1994).
4.8 On the Size-Induced Metul-Insulutor Transition in Clusters
148 1
[40] G.M. Pastor and K.H. Bennemann, in Clu.rters of Atoms and Molecules I: thcwy, e.xperitnent und clusters ufutoms, ed. H. Haberland, Springer-Verlag, Berlin (1994) p. 86. [41] W. Freyland and F. Hensel, in The Metullic and Non-metallic States qf Mutter, ed. P.P. Edwards and C.N.R. Rao, Taylor and Francis Ltd., London, (1985) p. 93. [42] D. Tomanek, S. Mukherjee and K.H. Bennemann, P h p . Rev. B, 28, 665 (1983). [43] G.M. Pastor, P. Stampfli and K. H. Bennemann, Europhys. Lett., 7, 419 (1988). [44] R.E. Benfield, J. Chem. Soc., Furuduy Trans., 88, 1107 ( I 992). [45] D.C. Johnson, R.E. Benfield, P.P. Edwards, W.J.H. Nelson and M.D. Vargas, Nuture, 314, 231 (1985). [46] S.R. Drake, P.P. Edwards, B.F.G. Johnson, J. Lewis, E.A. Marseglia, S.D. Obertelli and N.C. Pyper, Chem. Phys. Lett., 139, 336 (1987). [47] G . Schmid, Chem. Rev., 92, 1709 (1 992). [48] G. Schmid, J. Chem. Soc., Dulton Trans., 1077 (1998). [49] P. Marquardt and G. Nimtz, Festkiirperprohleme, 29, 317 (1989). (501 P. Marquardt and G. Nimtz, Phys. Rev. B, 43, 14245 (1991). [51] C.P. Vinod, G.V. Kulkarni and C.N.R. Rao, Chem. Phys. Lett., 289, 329 (1998). [52] M. Brust, M. Walker, D. Bethell, J.D. Shiffrin, J. Chem. SOC.Chem. Commun., 801 (1994). [S3] R.L. Whetten, J.T. Khoury, M. Alvarez, S. Murthy, I. Vezmar, Z. I. Wang, P.W. Stevens, C.L. Cleveland, W.D. Luedtke and U. Landman, Adv. Muter., 8,428 (1996). [54] K. Vijaya Sarathy, G.U. Kulkarni and C.N.R. Rao, Chem. Commun., 537 (1997). [ S S ] C.P. Collier, R.J. Saykally, J.J. Shiang, S.E. Henrichs and J.R. Heath, Science, 277, 1978 (1997). [56] G. Markovich, C.P. Collier and J.R. Heath. Phys. Rev. Lett., 80, 3807 (1998). [S7] U. Kreibig and M. Quinten, in Clusters 0f’Atoni.s und Molecules II: solvation and chemistry of’ free clusters, und embedded, supported and compressed clusters, ed. H. Haberland, Springer, Berlin (1994) p. 321.
5 Solid-state Cluster Chemistry
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.1 Solid-state Cluster Chemistry - An Overview Taro Saito
Many of the lower oxidation-state halides, oxides, and chalcogenides of the group-4 to group-7 transition metals exist as solid-state cluster compounds. Other anions, e.g. phosphide, can also aggregate the metal-cluster units in the solid state, but so far there have been few such examples. Solid-state clusters have any of three dimensionalities (D) - 1D (chain), 2D (layer), or 3D (network) - depending on the number of anions. There are also compounds in which the metal-cluster skeletons are linked by the sharing of one (vertices), two (edges), or three (faces) metals to form condensed clusters."] The anions bind to the cluster frameworks in terminal, edge-bridging, or face-capping positions. The discrete (OD) clusters assembled by ionic bonds or by weaker bonding forces are ionic or molecular crystals and their solid-state properties are to some extent also characteristic of solid-state cluster compounds. Main group element clusters without ligands belong to another class of cluster compounds and can also be classified into discrete cluster ions and solid-state clusters. Some of their bonding properties are similar to those of the transition-metal clusters, and in addition to being important in their own right, information obtained from compounds such as Zintl phases is very useful for an overview of metal-metal bonding. Metal-cluster skeletons are defined by the number of metal cluster electrons (MCE); this in turn is determined by the positions of the metallic elements in the periodic table, and by their oxidation states. A general principle is the availability of vacant metal coordination sites for the formation of metal-metal bonds and of the valence electrons to be used in the formation of these bonds. Metals in relatively low oxidation states are, therefore, favored and the value of MCE corresponds to the number of M-M bonds if electron-precise 2c-2e bonding is assumed.121Several empirical rules have been established which describe the relationships between MCE and cluster ~ h a p e . [ ~There . ~ ] is some variation in the modes by which the clusters are linked, depending on the oxidation states of the component metals and on the number of vacant coordination sites on metals.
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5 Solid-state Cluster Chemistry
Seven contributions covering aspects of transition metal cluster chemistry and one concerning group- 14 element clusters constitute this chapter on solid-state cluster chemistry. The general topics discussed are the structures of the cluster compounds and the electronic bonding schemes used to describe them. The importance of the bridges between the molecular units which form the solid-state cluster because of the general treatments of the elecstructures should be tronic structures by use of modified molecular orbital calculations that are based on discrete cluster units. The synthetic relationship between the solid state and molecular clusters is twofold. One is the so-called dimensional-reduction method that is used to prepare molecular clusters from solid state clusters by excision or extrusion.['] The other is cluster condensation to prepare solid state clusters (or at least cluster polymers) from molecular clusters by abstraction of anionic or neutral ligands.[s,'O1Amalgamation of solid-state high-temperature syntheses and solution methods provides good links between solid-state chemistry (and chemists) and molecular chemistry by producing common cluster frameworks." Molecular orbital schemes of discrete cluster compounds can sometimes be good starting points for deriving the band structures of solid-state clusters with common cluster units.[' 21 The principles of packing molecular clusters in crystals are described in Section 5.2.[l3,l4]The packing of molecular transition-metal clusters in the crystalline state is regulated by both steric and electronic factors. In carbonyl cluster compounds the holes created by the carbonyl groups in one cluster molecule are filled by the carbony1 groups of the adjacent cluster molecule in the key-keyhole interaction. They are held together by van der Waals-type interactions. The carbonyl group is also sufficiently basic for the oxygen atom to act as a hydrogen-bond acceptor, thus forming networks of hydrogen-bonded cluster molecules. These self-assembling molecular-recognition processes are the main factors in the formation of regular arrays of metal-cluster compounds in the solid state and provide the basis of crystal engineering of metal carbonyl clusters. The intermolecular interactions are not as strong as the covalent bonds (with some ionic contribution) in solid-state metal oxides or chalcogenides and are much more flexible. Because of their plastic nature, molecular packings can be subtly adjusted to obtain desirable collective solid-state properties. This is very different from the crystals of solid-state compounds, the atomic packing of which is determined almost solely by atomic radii and favored bond angles, and stable polytypes are formed under thermodynamic control. This section emphasizes the need to 'make crystals with a purpose' to find molecular crystalline materials with desirable functions on the basis of intercluster interactions. The structural chemistry described in Section 5.3 is concerned with reduced niobium oxides in which various Nb6012 clusters are d i s ~ e r n i b l e . [ ' Phases ~ * ~ ~ ~with ordered structures have been prepared by careful choice of preparative conditions. Reduced niobium oxides have Nb6012 cluster cores which are discrete (OD) or linked together through apical oxygen atoms to form 1D (chain), 2D (layer), or 3D
5.1 Solid-State Cluster Chemistry An Overview ~
1487
structures. The limiting structure is represented by NbO ( N b = +2.0) with the so-called defect NaCl structure which can also be regarded as a three-dimensional network of apex-shared Nb6O12 clusters. These cluster structures contrast with the reduced molybdenum oxides with edge-sharing octahedral clusters" 'I and molybdenum chalcogenides containing face-sharing octahedral clusters" 81 both of which are only one-dimensional chains. The highest oxidation limit is a perovskite-type BaNbO;(Nb = f4.0) which no longer has a cluster structure. The oxoniobates are considerably disordered because of the high melting points of niobium oxides and BaO during preparation in the temperature range 1300 -1500 K. The disordered structures have been investigated by high resolution electron microscopy (HREM). The images show intergrowth of NbO and perovskite structures. The interpretation of the HREM images is facilitated by further information from accurate X-ray parameters of the ordered structures. X-ray crystallography and HREM are, therefore, complementary. For the corner-sharing Nb6O12 cluster units, the number of electrons for the different kinds of N b atoms has been assigned. Generally a magic number of ca. 14 metal-cluster valence electrons are available for the M-M bonds in the NbgOl2 clusters. EH band calculations have been performed to assign the nature of the valence electrons. Section 5.4 reveals that investigation of the octahedral cluster complexes of niobium and tantalum, and the MgXl2 units which have attracted the attention of many inorganic chemists since the studies of their structures in the 196Os,[l9231 are still in an early stage of development. In these compounds, the Mg core is bridged by twelve edge-bridging halogen ligands and their apical positions are occupied either by anionic or neutral ligands. They are usually 16-electron clusters. The section describes the preparation and structures of cluster compounds containing two mixed-charge cluster units of Nb, Ta, and Mo. Representative examples are the homonuclear [Ta6C112(PrCN)g][(TagC112)CIgl and the heteronuclear [ M ~ X I ~ ( E ~ O H ) ~ ] [ ( M(O M~=CNb, ~ ~ Ta) ) C ~systems. ~] The cluster connectivities of the MgLl4 and MsL18 units in early transition-metal cluster compounds are discussed in Section 5.5.[24 261 Molybdenum, tungsten, and rhenium form M6L14 type clusters and the bridging modes depend on the number of anionic ligands, which is determined by the oxidation states and by the ratio of the halide (1-) and chalcogenide (2-) ligands. Niobium and tantalum form MgL18 type clusters; these usually have only edge-bridging or terminal halides as anionic ligands. The edge-bridging and terminal halide ligands can also bind one-, two-, or threedimensionally to vacant coordination sites of adjacent metal clusters. By utilizing extensive structural data, stability and electronic and steric effects on the intra-cluster distances are discussed in terms of the valence electron concentration per cluster. The alkali metal rhenium- and technetium chalcogenide clusters described in Section 5.6 are also o ~ t a h e d r a l . [ ~These ~ ~ ~ *clusters ] have M6E8 units with eight face-capping anions on the octahedral faces in contrast to the niobium halide or oxide clusters. The A4MgS14 (A = alkali metal, M = Re, Tc), CsgRe6S12, and
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5 Solid-state Cluster Chemistry
A4M6Ex (E = S, Se; x = 11, 12, 13, 13.5, 15) clusters have discrete 2D and 3D structures. Preparation conditions, characterization, and the unique structures are discussed. Because rhenium is a group-7 element, the number of valence electrons in the same oxidation states is one larger than for molybdenum which is located diagonally to the upper left in the periodic table. If the isoelectronic analogy holds, we can expect compounds of rhenium in a one electron higher oxidation state than those of molybdenum. In contrast with similar octahedral chalcogenide cluster compounds of molybdenum,[29]only 24-electron clusters have been extensively s t ~ d i e d . [The ~ ~chemistry , ~ ~ ~ ~of ~the ~ octahedral cluster compounds of rhenium with fewer MCE should be very interesting in comparison that of the molybdenum compounds. A review of discrete and condensed homoatomic clusters of germanium, tin, and lead is presented in Section 5.7. These main group cluster compounds are usually called Zintl ions or Zintl phases in recognition of extensive studies of Zintl and coworkers in the 1930s." 1,34p361 The science of nanoparticles with conducting and other physical properties is currently of interest and it is believed that Zintl ions or linked homoatomic clusters might provide us with good materials for basic studies and for application. The first part of the article is concerned with the structures and properties of 'isolated' molecular clusters prepared by solution methods. In the second part, linked clusters in intermetallic phases are summarized. Structureproperty relationships are proposed on the basis of the formulation of lone pairs in intermetallic compounds. Some of the compounds have superconducting properties and the influence of lone-pair interactions is discussed. Section 5.8 describes analysis of the bonding in cubic cluster compounds [Ms(,u4-E)6L,](M = Ni, Co, Fe, E = P, S, Se, n I 8) by means of EHMO and selfconsistent field multiple-scattering Xa calculation^.[^ 7 3 3 8 1 The clusters can be divided into one group with n = 8 and another with n < 8. This MsE6 architecture has a reciprocal structural relationship to that of the M6Es cluster compounds described above they have topologically the same structures except for reversed magnitudes of the interelement distances, and the number of metallic valence electrons (MVE) lies in a much wider range of 120-199 for the same relatively regular cubic units. The optimum number of MVE is 120 for the electronegative metals and/or terminal n-acceptor ligands and the count of 76 is the lowest hypothetical limit which preserves the cubic structures. Another series of cubic clusters of general composition M9(p4-E)& (M = Ni or Pd; E = Ge, P, As, Te) incorporates a metal atom in the center of the cube. The bonding in these cluster compounds is also analyzed by means of EH and SCF-MSXa calculations.[391The number of MVE ranges from 130 to 121. Examination of the electronic structures has shown that the cluster compounds are at the interface between molecular and solid state materials. In the cubic clusters, closed-shell electron configurations of stable molecular systems with a significant HOMO-LUMO gap coexist with open-shell electron configurations of solid-state systems with no significant gap between the skeletal frontier orbitals. -
5. I Solid-State Cluster Chemistry - An Overview
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An extensive theoretical review of cluster metallocarbohedrene (met-cars) chemistry is presented in Section 5.9. The first examples of met-cars M&12 ( M = Ti, V ) were discovered by Castleman et al. in 1992[401in the course of a mass spectrometric study of dehydrogenation of hydrocarbons by titanium atoms, ions, and clusters. Initially a dodecahedra1 structure with a Th point group symmetry was proposed to account for the unusual stability of this molecular cluster.[401Similar cluster compounds with 4,5, 6 group metals and Fe have since been discovered, and the structure of small fcc crystallites has been assigned to these stable clusters with higher nuclearity. The cluster shapes and stabilities have been the subject of theoretical treatment by various computational methods in the absence of pure samples for X-ray diffraction or spectroscopic analysis.[41-431 The calculations reported here have been conducted mainly by DFT (density functional theory) and provide the most plausible geometries based on the energy minima of the possible structures. The properties and reactivity of met-cars assuming the new structure are discussed. The articles in this chapter on solid-state clusters describe many important and interesting features of the new inorganic chemistries which depend on determination of crystal structure. The remarkable progress of X-ray crystallography in recent years has changed not only the speed but also the character of the investigations. Study of chemical systems, the structures of which were too complex to tackle in the past, has become feasible. If solid-state cluster compounds are assumed to be composed of discrete cluster units, it might be easier to treat them experimentally and theoretically from the molecular point of view. This must be one reason why solidstate and molecular chemists have common interests these days and have revived cluster chemistry which deals with every kind of inter-element linkage.[3,44-501
References [ I ] A. Simon, in Clusters and Colloids (Ed.: G. Schmid), VCH, Weinheim, 1994, pp. 373-458. [2] D. M. P. Mingos, Nuture 1972, 236, 99-102. [3] D. M. P. Mingos, D. J. Wales, Introduction to Cluster Chemistry, Prentice-Hall, London, 1990, pp. 1-318. [4] N. Rosch, G . Pacchioni, in Clusters and Colloids (Ed.: G. Schmid), VCH, Weinheim, 1994, pp. 5-88. [5] S. A. Sunshine, D. A. Keszler, J. A. Ibers, Acc. Chern. Res. 1987, 20, 395-400. [6] S. C. Lee, R. H. Holm, Anyew. Chem. Int. Ed. Engl. 1990, 29, 840-856. [7] J. Rouxel, Acc. Chenz. Res. 1992, 25, 328-336. [8] T. Saito, Ado. Inorg. Cheni. 1996, 44, 45-91. 191 J. R. Long, A. S. Williamson, R. H. Holm, Angew. Chem. Int. Ed. Engl. 1995, 34; 226-229. 101 T. Saito, H. Imoto, Bull. Chem. Soc. Jpn. 1996, 69, 2403--2417. 1 I] H. G. von Schnering, Anyew. Chem. Int. Ed. Engl. 1981, 20, 33-51. 121 T. Hughbanks, R. Hoffmann, J. Am. Chem. Soc. 1983, 105, 1150-1162. 131 D. Bragd, F. Grepioni, Arc. Chem. Res. 1994, 27, 51-56.
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[ 141 D. Braga, F. Grepioni, G. R. Desiraju, Chem. Rev. 1998, 98, 1375-1405. [15] J. Kohler, G. Svesson, A. Simon, Angew. Chem. Znt. Ed. Engl. 1992, 31, 1437-1456. [I61 V. G. Zubkov, V. A. Pereliaev, A. P. Tyutyunnik, J. Kohler, A. Simon, G. Svensson, J. Alloys Compd. 1997,256, 129-139. [ 171 C. C. Torardi, R. E. McCarley, J. Solid State Chem. 1981, 37, 393-397. [ 181 R. Chevrel, P. Gougeon, M. Potel, M. Sergent, J. Solid State Chem. 1985, 57, 25-33. [ 191 D. L. Kepert, The Early Transition Metals, Academic Press, London, 1972, pp. 233-249. [20] N. Brnicevic, D. Noetig-Hus, B. Kojic-Prodic, Z. Ruzik-Toros, Z. Canilovic, R. E. McCarley, Inorg. Chem. 1992,3I, 3924-3928. [21] N. Brnicevic, B. Kojic-Prodic, M. Luic, A. Kashta, P. Planinic, R. E. McCarley, Croatia Chem. Acta 1995,68, 861-875. [22] U. Beck, A. Simon, N. Brnicevic, S. Sirac, Croatia Chem. Acta 1995, 68, 837-848. [23] U. Beck, A. Simon, S. Sirac, N. Brnicevic, 2. Anorg. Allg. Chem. 1997, 623, 59-64. [24] A. Perrin, C. Perrin, M. Sergent, J. Less-Common Met. 1988, 137, 241-265. [25] C. Perrin, S. Cordier, S. Ihmaine, M. Sergent, J. Alloys Compd. 1995, 229, 123-133. [26] S. Cordier, C. Perrin, M. Sergent, Muter. Res. Bull. 1997, 32, 25-33. [27] W. Bronger, M. Kanert, M. Loevenich, D. Schmitz, Z. Anorg. Allg. Chem. 1993, 619, 20152020. [28] W. Bronger, C. Koppe, D. Schmitz, Z . Anor. Allg. Chem. 1997,623,239-242. [29] T. Saito, in Early Transition Metal Clusters with n-Donor Ligands (Ed.: M. H. Chisholm), VCH, New York, 1995, pp. 63-164. [30] V. E. Fedorov, A. V. Mishchenko, V. P. Fedin, Russ. Chem. Rev. 1985,54,408-423. [31] A. Perrin, M. Sergent, New. J. Chem. 1988, 12, 337-356. [32] J. R. Long: L. S. McCarty, R. H. Holm, J. Am. Chem. Soc. 1996, 118,4603-4616. [33] T. Saito, J. Chem. Soc. Dalton Trans. 1999, 97-105. [34] H. Schafer, B. Eisenmann, W. Miiller, Angew. Chem. Znt. Ed. Engl. 1973, 12, 694-712. [35] J. D. Corbett, Prog. Znorg. Chem. 1976, 21, 129-158. [36] J. D. Corbett, Chem. Rev. 1985, 85, 383-397. [37] E. Furet, A. L. Beuze, J-F. Halet, J-Y. Saillard, J. Am. Chem. SOC. 1994, 116, 274-280. [38] J.-F. Halet, J.-Y. Saillard, Struct. Bond. 1997, 87, 87-109. [39] E. Furet, A. L. Beuze, J.-F. Halet, J.-Y. Saillard, J. Am. Chem. SOC.1995, 117, 4936-4944. [40] B. C. Guo, K . P. Kerns, A. W. Castleman, Jr., Science 1992, 255, 1411-1413. [41] I. Dance, J. Chem. SOC.Chem. Commun. 1992, 1779-1780. [42] M-M. Rohmer, M. Benard, C. Henriet, C. Bo, J-M. Poblet, J. Chenz. Soc. Chem. Commun. 1993, 1182-1185. [43] J. Munoz, C. Pujol, C. Bo, J-M. Poblet, M-M. Rohmer, M. BCnard, J. Phys. Chem. A 1997, 101, 8345-8350. [44] E. L. Muetterties, Science 1977, 196, 839-848. [45] E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, W. R. Pretzer, Chem. Rev. 1979, 79, 91-137. [46] Transition Metal Clusters (Ed.: B. F. G. Johnson), Wiley, Chichester, 1980, pp. 1-681. [47] The Chemistry of Metnl Cluster Complexes (Ed.: D. F. Shriver, H. D. Kaesz, R. D. Adams), VCH, New York, 1990, pp. 1-439. [48] G. Gonzalez-Moraga, Cluster Chemistry, Springer, Berlin, 1993, pp. 1-302. [49] Clusters and Colloids (Ed.: G. Schmid), VCH, Weinheim, 1994, pp. 1-555. [50] Early Transition Metal Clusters with n-Donor Ligands (Ed.: M. H. Chisholm), VCH, New York, 1995, pp. 1-289.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.2 Transition Metal Clusters - The Relationship between Molecular and Crystal Structure Maria JosP Calhorda, Dario Braga, and Fahrizia Grepioni
5.2.1 Introduction Materials chemistry is a frontier field of research. The possibility of designing, modeling, synthesizing, and exploiting solids with a predefined aggregation of molecules or ions is attracting the attention of an increasing number of scientists."] The most appealing fields of application are those of optoelectronics, in particular second harmonic generation (SHG) for optical devices,['] conductivity and supercond ~ c t i v i t y ,charge ~ ~ ] transfer and magnetism,[3341 and biomimetic materials.[51 Transition metal clusters have long been regarded as potentially useful because they combine a feature typical of bulk metal or metal surface, uiz. the interaction between several metal atoms, with that between ligands (most often organic-type fragments) and the metals.[6-81 The interaction between neutral transition metal cluster molecules in the solid state is governed by the interaction between the peripheral atoms belonging to the ligands,['] whereas the molecular bulk contributes solely to cohesion via long-range van der Waals interactions that depend on the number of electrons (which is large with metal atoms) but 'fade away' very rapidly as the distance between the atoms increases. Notable exceptions are clusters formed by metals such as gold,['01 in which intermolecular metal-metal interactions are possible. Furthermore, van der Waals forces are non-directional whereas those between nearest neighbors can have vectorial properties, e. g. hydrogen-bonds. Another relevant aspect is the intrinsic structural non-rigidity of many transition metal cluster compounds. Carbonyl fluxionality in solution is a well established phenomenon. The barrier to interconversion between terminal, double, or triple bridging bonding geometries of the CO-ligands is usually low and the ligands can move from one metal atom to a neighboring atom via the 'merry-go-round' or similar processes.[' '1 The occurence of CO-ligand fluxionality processes in the solid state is still debated (see below)."']
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The solid-state dynamic behavior of transition metal carbonyl clusters clearly depends on the way the molecules are interlocked.['21 If the structure is non-rigid, i.e. if there is more than one minimum on the molecular potential energy hypersurface corresponding to a different distribution of the ligands, the relationship between molecular and crystal structure depends critically on the compromise between intramolecular bonding and intramolecular non-bonding interactions (internal interactions) and intermolecular interactions in the crystal (external or extra-molecular interactions). These energetic terms need not be convergent, i. e. need not necessarily lead to a unique solution. Indeed, organometallic molecules can often be isolated in isomeric forms and it is often possible to account for the energy difference between the most and least (thermodynamically) stable isomers with external interactions in the solid state.[13]Separating the relative contributions of internal and external interactions is a difficult problem. In the presence of isomeric forms, however, comparison on a relative scale of the isomer internal energies with the energies of cohesion of the corresponding crystals can enable evaluation of the contribution of external interactions to the stabilization of less stable isomeric forms" 31 (see also below). In this work, we describe our joint research efforts aimed at understanding the relationship between the structures of the isolated (gas-phase) transition metal complexes and those of the corresponding crystals.
5.2.2 Experimental and theoretical methods The first problem to address is that of the experimental and theoretical 'tools'. Single-crystal X-ray crystallography has, of course, been the method of choice for the determination of the solid-state structures of transition metal clusters. Their structural variability, complexity, and even, at times, unpredictability preceded, in terms of experimental challenge, that of large biomolecules and proteins.[" New scientific challenges such as, on the one hand, the advent of area detector techniques and powerful X-ray radiation sources and, on the other hand, the use of variable temperature techniques, combined with spectroscopic solid-state methods enables in depth investigation of the temperature-dependence of solid-state molecular and crystal structures and the study of their phase transitional behavior. In terms of crystal structure modeling, much of the knowledge accumulated in the study of organic molecular solids['41can be transferred to the investigation of solid transition metal clusters,[' 51 and the atom-atom potential energy method, in use for more than 50 years in organic solid-state chemistry, still represents a simple and highly transferable method of estimating non-covalent van der Waals-type interactions in crystals.[' 61 We have shown['71 that empirical packing energy calculations, within the atom-atom approach, and theoretical packing generation pro-
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cedures, developed in the field of organic solid-state chemistry, can be applied (although with some additional far from trivial approximations) to solid transition metal clusters. The most commonly used expression for empirical estimation of the packing potential energy (PPE) of a molecular crystal is called 6-exp-lpotential, with PPE = C,C,(AepBr'J - Cr/ q,q,ro-'). In this expression, index i runs over all atoms of the reference molecule in the lattice and indexj over the atoms of the surrounding molecules distributed according to crystallographic symmetry; ro is an atom-atom intermolecular distance and ql and q, are the formal atomic charges if a Coulombic term is included in the calculations. The basic assumption is that only central forces operate between pairs of atoms and that the total interaction energy is the sum of the interactions between all atomic pairs. Excessively close approach of atoms, giving rise to repulsion, is taken into account by the exponential function. Several independent tabulations of the coefficients A , B, and C, for each type of atom-atom contact for organic substances, are available in the literature. They are obtained either by fitting observed crystal properties (heat of sublimation and known crystal structures) or by ab initio calculations of the intermolecular potential energy.['61 The generation of hypothetical crystal structures[18] is one of the major challenges of modern structural science. The problem of crystal structure prediction"91 has been tackled in different ways. As for atom-atom potential-energy calculations, the algorithms and software for theoretical crystal structure generation were initially developed in the field of organic solid-state chemistry.[201In our work on theoretical crystal structure generation we have adopted Gavezzotti's so-called static approach which is based on an elaboration of Kitaigorodsky's aufbau process.[2 The study of mononuclear complexes[221has contributed to the revelation that a large number of polymorphic modifications are available, even for the simplest molecules with energy differences in a very narrow range. As for organic crystals, transition metal cluster crystals are re-constructed starting from pairs of molecules generated via the most common symmetry operators. Energy analysis enables recognition of the most relevant structural sub-units in the experimental crystal structures. The generation of theoretical structures in all 230 space groups is a formidable task which can, however, be drastically reduced if only the most populated space groups are considered.[231 Theoretical organometallic crystal structures have been calculated for the simple binary carbonyl complexes Ni(C0)4, Fe(C0)5,[22"1 and Cr(C0)6[22b1 as representatives of the classes of tetrahedral, trigonal bipyramidal, and octahedral complexes. It has always been possible to retrace, among other solutions of similar cohesive energy, the experimentally determined crystal structures. For the dinuclear complexes CO~(CO)S and F e ~ ( C 0 ) 9 , [the ~ ~ similarity ] between the molecular arrangements in the crystals of the two species arises from the stereoactivity of the lone pairs on the Co species which occupy the space taken by the two additional carbony1 ligands in the structure of the Fe complex. Analysis of the isomers of isolated cluster molecules requires a different kind of
+
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theoretical approach. As mentioned above, one of the sources of isomerism in carbonyl clusters arises from carbonyl fluxionality so that the same structural unit, such as M3(C0)12 or Mq(C0),2, can be found in several internal arrangements, depending on the metal. As a general rule it can be stated that the number and multiplicity of bridging carbonyl groups tend to decrease as one moves from 3d, to 4d, and to 5d metal clusters. For instance, whereas Fe3(C0)12 has a molecular structure with two bridging carbonyls spanning one Fe -Fe edge,” 51 the analogous Ru and 0 s clusters have terminal groups only.[261 In the days when the growing importance of clusters gave rise to many theoretical studies, ab initio quantum chemistry methods could not be widely applied for such ‘large’ molecules and the extended Huckel methodL2’]was extensively used to trace the role of metal-ligand and metal- metal interactions in determining the molecular shape. What this method lacks in quantitative reliability is compensated by the simple explanations it can provide based on the fragment decomposition methodology[281and simple symmetry arguments. More recently, advances in computer technology and in theoretical methodology have enabled the progressive use of ab initio methods, among which density functional (dft) calculations [291 have proved specially reliable and suitable for the study of organometallic molecules, including
Our complementary approach combines the analysis of the crystal structure with analysis of the molecular structure of the component systems, using experimental data from X-ray crystal analysis, crystal packing studies, extended Huckel and density functional calculations, as described above.
5.2.3 Molecular and crystal structures of transition metal carbonyIs The molecular structure, fluxional behavior, and solid-state disorder of binary carbonyl clusters and their ligand substitution derivatives still offer material for (animated) In this section we briefly describe our theoretical analysis of the molecular structures and isomers of several systems, sharing the possibility of exhibiting terminal or bridging carbonyl groups, and use that knowledge to interpret their crystal structures and solid-state behavior. The M3(C0)12 family was our starting point. Two limiting geometries are observed, one with only terminal carbonyls and D 3 h symmetry (M = Ru, Os), la, and another where two carbonyls bridge one M-M edge (M = Fe), lb, with approximately C2” symmetry. In reality, these bridges are not symmetric. Although these molecules have been widely the calculation of accurate relative energies and the mechanism for carbonyl scrambling remain open
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b
a
1
questions. Our extended Huckel calculations always predict greater stability for structures containing the greatest number of bridges, as more bonds are formed, but they are much less favored when the real structure is known to have no bridges (see below). A recent energy optimization of Fe3(C0)12, using dft calculations,[331including exchange and correlation corrections, under D 3 h and Czv symmetry constraints, showed the latter configuration to be more stable by 0.315 eV. The same result can also be obtained at the MP2 level but involves greater computational effort.[331What the extended Huckel calculations could give us, however, was a hint of the origin of preference for bridging carbonyls in the clusters of lighter metals.[341 Let us consider a simple model for binding a bridging carbonyl to two metal atoms. The types of orbital involved in ligand to metal donation are shown schematically in 2a and 2c, whereas 2b and 2d represent back-donation from the metals to the n-acceptor ligands.
op
93 C
2
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D3tl
C,"
-9
2.-I0 3-=
F
E
UJ
-11
-1 2
-13
-14
-15
Figure 1. Interaction diagram between Fe3(CO)lo and two bridging carbonyls, either terminal (left) or bridging (right).
The linear combinations of metal orbitals shown in 2b and 2d have strong metalmetal antibonding character. Such interactions can be found in the molecular orbital diagram depicting the interaction of an Fe3(CO)lo fragment with two bridging carbonyls (Fig. 1) which was derived from extended Hiickel calculations. There are, as expected, two donation components of a1 and bl symmetry and two back donation components (a2 and b2). Our interest lies in the a2, which involves, besides the antisymmetric combination of the z* of the two carbonyls, the combination of the two out of phase metal orbitals, similar to 2d (1la2 on the right of the diagram). This fragment orbital is Fe-Fe antibonding and becomes partially occupied after interaction with r ~ * .The resulting bonding molecular orbital is the HOMO of the complex. Removing two electrons leads to an increase in the Fe-Fe overlap population, as expected. This effect is much more pronounced for the
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Ru3(C0)12 cluster, thus making the bridging structure less favorable than that with only terminal carbonyl groups. The crystal structure contains the most stable isomer of each cluster. The solid-state structure of Fe3 (CO)12 has recently been reinvestigated by variable-temperature diffraction;[351this showed that the molecular structure underwent progressive symmetrization of the two unique bridging COs as the temperature is decreased from room temperature to 120 K. Also, the crystal undergoes a phase transition when the temperature is further reduced.[361Spectroscopic, theoretical, and crystallographic evidence of molecular and crystal dynamics is available for ’I and for the related complexes the almost isostructural analogs Fe2Os(C0)12,[~ FezRu(C0)12 and FeRu2(C0)12.[~~] When crystal-structure generation procedures are attempted on high-nuclearity complexes, the limitations inherent in the description of the metal atoms become increasingly severe. With crystalline Fe3(C0)12 and R U ~ ( C O ) I ~the , [ experimental ~~] crystal structures could be compared with other theoretical structures, although the PPE of the theoretical crystals were invariably higher than the range expected. A comparison of the observed crystal packing of Fe3(C0)12 (constructed with only one of the two disordered molecules related by centers of inversion) and the ‘best’ solution of the theoretical approach is shown in Fig. 2. One might object that the intrinsic structural flexibility of the molecule should discourage attempts at crystal-structure generation with a rigid molecular building
a
b
Figure 2. Comparison of the observed crystal packing of Fe3(CO),*, constructed with only one of the two disordered molecules related by centers of inversion (a), and the ‘best’ solution of the theoretical approach (b).
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block. Indeed, the minimization should take into account the simultaneous optimization of molecular and crystal structure because each affects the other implicitly for organometallic crystals. An attempt in the direction of a simultaneous molecule-crystal energy optimization has been made by S i r ~ n i [in~ the ~ ] modeling of the dynamic behavior of Fe3(C0)12 in the solid state. The structure of the molecule has been optimized by molecular mechanics under the action of the intermolecular atom-atom force field. A second example of the relationship between molecular and crystal structures concerns the family of R u ~ ( C O ) ~ ( L derivatives, ) where L = (co)3,C6H6, and S3C3H6.[411In the dodecacarbonyl cluster, all the CO groups are terminal, as explained above, and the same happens for the benzene derivative. When three carbonyls or benzene are substituted by 1,3,5-trithiacyclohexane,the structure of the molecule changes and three CO bridges span the three Ru-Ru bonds. In this family of compounds, when benzene and 1,3,5-trithiacyclohexaneare introduced, another important effect has to be taken into account when studying the crystal structure, namely the formation of hydrogen-bonds, the key intermolecular interaction in crystals formed from discrete molecules or ions.[421Hydrogen-bonds combine strength with directionality, which makes this the interaction of choice in the design of crystals with predefined a r c h i t e c t u r e ~ . [They ~ ~ ~ have ~ ~ ] recently begun to be investigated in a systematic manner.[451Strong hydrogen-bonding donor or acceptor groups (COOH, OH, CONH2 etc.) are not common in transition metal clusters, whereas weak acceptors, such as CO ligands, or weak donors such as C-H groups belonging to arenes, cyclopentadienyl, or other organic ligands, are almost ubiquitous. Hence, the intermolecular interactions established for carbonyls in clusters are of interest if design tools for molecular and crystal construction are being sought.[461The oxygen atom of the carbonyl group becomes more negatively charged and therefore better suited to act as a donor, as the amount of back donation from the metals increases. Triply bridging carbonyls are the best donors, followed by the doubly bridging carbonyls and finally the terminal carbonyls. This decreasing basicity of CO is reflected in the average length of the H. . .O separation, which follows the order p3 < p2 < terminal for the CO coordination mode.[471 There is a possibility that the structure containing more bridging carbonyls leads to a more cohesive crystal. In R~3(C0)6(p-C0)3(p~-S3C3H6)[~~~’~~ the bridging CO ligands form shorter C-H. . .O interactions than terminal COs, in keeping with the order of basicity. The former ligands form the shortest interactions (2.330and 2.538A). A strong bifurcated interaction is also present (2.369and 2.262A).C-H. . .O bonds are not confined to these bridging ligands, however. Two interactions involve two terminal COs trans to the S atoms, but they are appreciably longer than those involving the bridging ligands. A further point of interest arises from the participation of the lone-pairs of an S atom in an interaction with an H atom (S...H 2.924A).The benzene cluster Ru3(C0)9(p3: q2 : q2 : q2-C6H6)[48a1participates in an intricate network of intermolecular bonds between the facially bound benzene ligands and
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Figure 3. The network of intermolecular bonds between the facially bound benzene ligand and the CO ligands in crystalline Ru3(C0)9(q3 : q2 : q2 : q2-CsHs).
the CO ligands (Fig. 3 ) . Four of the six benzene hydrogens form C-H. . .O bonds that are shorter than 2.6A ( i e . shorter than the sum of the van der Waals radii for H and 0 atoms: 1.20 and 1.50% respectively). Two of these are short (2.416 and 2.372A) and approach the lower limits for C-H. . .O interactions in organic and organometallic crystals. We disWhy are there bridging carbonyls in RU~(CO)~(L~-CO)~(L~~-S~C~H~)? cussed above the disadvantages of bridging carbonyls when dealing with 4d metal clusters. The sulfur ligand differs from both benzene and three COs, however, in being a n-donor instead of a n-acceptor. Bridging carbonyls are better n-acceptors than terminal carbonyls and therefore they compete more effectively with benzene for back-donation from the metal. When a n-donor ligand is present, no competition occurs. On the contrary, it leads to a better fulfillment of the electron-accepting requirements of the CO ligands. Part of the interaction diagram between one ben-
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le CO-RrCO \ / \
RY-p” co
/
RU /I
Ru-Ru
Figure 4. Part of the interaction diagram between one benzene ligand and Ru(CO)6(pc-CO)3(lefthand side) or R q ( C 0 ) g (right-hand side).
zene and either Ru3(C0)9 (right-hand side) or Ru(C0)6(p-C0)3 (left-hand side) is shown in Fig. 4. Benzene has six 71 molecular orbitals. The three with lower energies (alg, elg) are occupied and donate electrons to empty orbitals of the cluster, whereas back donation to the higher energy (eZu, aZu) also takes place to some extent. Only the most relevant interaction, that one involving the elg and ezU sets, is represented. They have the same symmetry when binding and can mix. It can be seen on the righthand side, where there are no bridging COs, that there is both donation from the le into the cluster and back donation from the metal into the empty 2e. On the lefthand side, however, the metal le set is only slightly destabilized, no strong interaction with 2e taking place, as the overlap population is very small. No significant back-donation occurs. The reason lies in the much smaller contribution of ruthenium to the Ru(C0)6(pu-CO)3fragment le set, than to Ru3(C0)9. It is strongly localized in the CO groups, not in the metal atoms. The overlap with benzene orbitals
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n n
U
b
a
3
becomes negligible. For Ru3(C0)9, Ru is the main contributor to the le fragment orbitals. The conclusion is that, once again, the isomer found in the crystal is the most stable. In this particular case, the presence of bridges also leads to stronger interactions in the crystal, but this is a consequence of the electronic requirements of the isolated cluster molecule. We find the same competition in the M4(CO)12 system (M = Co, Rh, Ir), 3.[491 The totally non-bridged structure (Td) is found for iridium only, whereas for both Rh and Co three bridging carbonyls span the three edges of one M3 triangle (C3v), leaving one metal atom coordinated to three terminal COs. When one carbonyl in is replaced by a n-acceptor ligand, such as CNR, the T d structure is unaltered, but when a strong donor is coordinated, the cluster adopts the C3v geometry. The reasons are the same as pointed out above. Intermolecular interactions become more interesting when the substituent of the carbony1 is a negative anion and a counterion must be present in the crystal. Among these, we shall concentrate on the SCN derivative, which is observed in both the Td and C3" geometries. The S-bound thiocyanate is a weak n-donor and therefore not strong enough a donor to make CO bridge formation very favorable. Both isomers exist in the solid (crystallized with different counterions). It has been found that this ligand is bent relative to its most stable conformation, the distortion being more pronounced in the derivative carrying bridging COs where the N atom is involved in C-H...N interactions in the range 2.415-2.616A. This seems to be the only situation so far observed in which the isolated cluster distorts to promote stronger intermolecular interactions. The bending of the SCN ligand takes place over a soft potential-energy surface for both clusters and is therefore not unexpected. The family of clusters of the type (CpR)3M3(C0)3 (M = Co, Rh, Ir; CpR = C ~ H SC5MeS, , C5H4Me) has also been used for a comparative study of molecular
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5 Solid-state Cluster Chemistry
n
-
0
Figure 5. The four structural types of (CpR)3M3(C0)3(M = Co, Rh, Ir; CpR = CsH5, CSMe5, C5H4Me) clusters.
and crystal structures.[501The major structural difference at the molecular level is in the number and type of CO bridges in the structure. Molecules belonging to this family provide examples of all three types of link, namely C-H...O(p3-CO), C-H. . .O(pz-CO), and C-H. . .O(terminal CO). The eight molecules which constitute the sample fall into four different structural types differing in the number of CO bridges, as shown in Fig. 5. The iridium cluster contains only terminal carbonyls, whereas the cobalt derivative has a triply bridging and two doubly bridging carbonyls, and the other structures contain an intermediate number of bridges. We explained the decreasing tendency of these compounds to form CO bridges in the order Co > Rh > Ir, by use of a binuclear model. It parallels the results of the calculations for the trinuclear clusters, but is easier to understand. Again, we found there is one metal-metal antibonding cluster orbital which is involved in metal-carbon-bond formation and becomes occupied (Fig. 6). Its antibonding character is more pronounced for the heavier elements, making bridge formation an unfavorable process. In a similar manner we have addressed the relationship between the stability of the individual arene cluster molecules and that of the same molecules in the solid state for the isomeric pairs RusC(CO)12(q6-C6H6)and RusC(C0)12(p3-y2: q2 : q2C6H6), and RU6C(CO)1i(r6-C6H6)2 and RU6C(CO)11(q6-C6Hg)(p~-r2 : q2 : q2C6H6) with face-bridging and terminal arene clusters.[”] The bonding of a benzene to a face seems to be slightly more favorable than the bonding to one metal atom, but the differences are very small, as expected from the coexistence of several iso-
5.2 Transition Metul Clusters
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Figure 6. Three-dimensional representation of the HOMO in the model system (Cp)2Ir2(CO)z.
mers. The benzene ligands in the terminal coordination site seem to be preferentially involved in C-H. . .O-C interactions compared with the facial ligands. Whether there are electronic reasons for this, which depend on the different types of cluster-ligdnd bonding interaction, or steric reasons, because terminal benzene is more ‘exposed’ than facial and, therefore, is more accessible to the surrounding COs, is, however, difficult to determine. In addition, molecules of the bis-terminal isomer of Ru6C(CO)11(q6-C6H6)2form linear chains with the benzene rings from adjacent molecules in close contact and extended Huckel tight-binding calculations indicated a weakly bonding interaction.1521Similar interactions occur in the terminal-facial isomer, which forms zigzag chains (Fig. 7).
5.2.4 Conclusions Analysis of the interplay between molecular and crystal structures is analysis of the competition between covalent bonds between ligands and metals and non-covalent bonds between ligands in the crystal. We have investigated some of these. Our work has been confined to systems containing simple ligands; it could not be different because the experimental and theoretical difficulties to overcome in the study of large and heavy molecules and of their interactions are still enormous and result in lack of accuracy in the result. Test cases have been discussed in some detail above. The general picture that emerges from our study is relatively simple there is no unique solution to the problem of global energy minimization as cluster and crystal can adapt to each other to some extent. Cluster flexibility, for example, is often transferred to the solid state as crystal-structure flexibility, which might well manifest itself in solid-state dynamics or in phase-transition behavior. In the preceding examples, we analyzed the electronic structural preferences for a few isomeric cluster molecules. In carbonyl cluster families the most important factor is the fluxionality of the carbonyl groups, which can occupy terminal, doubly -
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bridging, or triply bridging sites. Even these situations are not clear cut, because, for instance, in Fe3 (CO)12 the two CO bridges are definitely asymmetric, becoming more symmetric as the temperature is reduced. The carbonyls are very mobile and interchange very easily. The electronic tendency is, however, that bridge formation becomes increasingly less favored on moving from first row to second row and then to third row transition metal derivatives. The substitution of carbonyl ligands by other types of ligand, e.g. strong cr-donors or 0-and n-donors, contributes to the possible existence of bridging COs in heavier metal derivatives. Indeed, as doublybridged carbonyls are better n-acceptors than terminal carbonyls, the presence of better donors enables better back-donation from the metal and the formation of stronger M-C bonds, overcoming the weakening of the M-M bonds. We saw this happen in the R u ~ ( C O )L-L) ~ ( and Ir4(C0)11Lsystems. Intermolecular interactions in the crystal are weaker than these covalent bonds and, from our experience, we can state that the molecule in the crystal will adopt the geometry favored by the isolated molecule. Modulation of the shapes of cluster molecules can be achieved by
5.2 Transition Metal Clusters
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changing the metal or by replacing one or more ligands and this will, in turn, enable interplay of different van der Waals interactions and specially hydrogen-bond formation. What, then, makes cluster chemistry distinct from organic and coordination chemistry? Of course the presence of a large number of metal atoms, which might tune, with their specific electronic characteristics, the non-covalent metal-ligand bonding (e.g. bridging compared with terminal CO bonding) and, consequently, the acid-base behavior of the peripheral atoms. The size and geometry of the cluster core has a topological effect on the organization of the ligands in space as the interaction with one or more metal centers introduces ‘constraints’ on the relative orientation of ligands belonging to the same molecule, and hence on intermolecular interlocking. It has been amply discussed that transition metal clusters can be viewed as ‘nut in a nutshell’ systems in which the metal atom polyhedron is optimized within an outer ligand polyhedron.[531 What are the future goals? The chemistry of transition metal clusters has gone through the booming stages of synthesis and characterization in the 70s and 8 0 ~ , [ ’ ~ ] followed by that of homogeneous-heterogenous catalysis and surface-cluster analo g ~ . [These ~ ~ ] areas are still being intensively explored, as is cluster reactivity. Transition metal clusters, however, need still to be tested in the engineering of crystalline materials.[s61 Crystal engineering has been defined as the capacity to make crystals with a purpose. In transition metal cluster chemistry this purpose is that of utilizing the distinct characteristics mentioned above to construct crystals that can function as the result of the inter-cluster interactions. To do this the experimentalist needs to conceive ways of directing the crystal-building process towards given architectures, i.e. needs to learn how to make non-covalent crystal synthesis. Clearly, the growth and success of a solid-state chemistry of transition metal clusters depends crucially on a close interaction between synthesis, theory, solid state characterization, and evaluation of properties.
Acknowledgments The authors acknowledge CNR (Italy) and JNICT (Portugal) for the financial support that made possible the development of fruitful collaboration. We wish to thank E. Tedesco (Bologna), L. F. Veiros (Lisbon), and P. E. M. Lopes (Lisbon) for their contribution to the work described herein. Collaboration with H. Wadepohl and s. Gebert at Heidelberg University has also been extremely useful. B. F. G. Johnson is thanked for his continuous encouragement and his relentless enthusiasm through many years of joint efforts in the field of cluster chemistry.
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References [I] Inorganic Materials Ed. D. W. Bruce, D. O’Hare, John Wiley & Sons Chichester, U.K., 1992. [2] (a) D. J. Williams, Angew. Chem. Znt. Ed. Engl. 1984, 23, 690. (b) N. J. Long, Angew. Chem. Int. Ed. Engl. 1995, 34, 21. (c) T. J. Marks, M. A. Ratner, Angew. Chem. Int. Ed. Engl. 1995, 34, 155. (d) D. R. Kanis, M. A. Ratner, T. 1. Marks, Chem. Rev. 1994, 94, 195. (e) S. R. Marder, Inorg. Muter. 1992, 115. [3] (a) J. S. Miller, A. J. Epstein, W. M. Reiff, Acc. Chem. Res. 1988,21, 11. (b) J. S. Miller, A. J. Epstein, Angew. Chem. Int. Ed. Engl. 1994, 33, 385. (c) J. S. Miller, A. J. Epstein, Chem. Eng. News 1995, 73, 30. [4] (a) 0. Kahn, Molecular Magnetism, VCH: New York, 1993. (b) D. Gatteschi, Adu. Muter. 1994, 6, 635. [5] (a) S. Mann, J. Chem. SOC.Dalton Trans. 1993, 1. (b) C. L. Bowes, G. A. Ozin, Adv. Mater. 1996, 8, 13. (c) G. Ozin, A. Acc. Chem. Res. 1997, 30, 17. [6] (a) E. L. Muetterties, Chem. Eng. News 1982, 60, 28. (b) E. L. Muetterties, Science 1977, 196, 839. (c) E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, W. R. Pretzer, Chem. Rev. 1979, 79, 91. (d) E. Sappa, A. Tiripicchio, P. Braunstein, Coord. Chem. Rev. 1985, 65, 219. [7] (a) G. A. Somorjai, J. Phys. Chem. 1990, 94, 1013. (a) G. A. Somorjai, Chemistry in Two Dimensions: Suvfaces,Cornell University Press, Ithaca, 1981. (b) B. E. Koel, G . A. Somorjai, Surface Structural Chemistry, Catalysis Science and Technology, J. R. Anderson and M. Boudart Eds., Springer Verlag, Berlin, 1984. [8] (a) K. C. C. Kharas, L. F. Dahl, Adu. Chem. Phys. 1988, 70, 1. (b) D. C. Johnson, R. E. Benfield, P. P. Edwards, W. J. H. Nelson, M. D. Vargas, Nature 1985, 314, 231. (c) B. K. Teo, F. J. DiSalvo, J. V. Waszczak, G. Longoni, A. Ceriotti, Inorg. Chem. 1986, 25, 2265. (d) L. J. de Jongh, Physica B 1989, 155, 289. (d) P. P. Edwards, Acc. Chem. Res. 1996,29, 23. [9] D. Braga, F. Grepioni, Acc. Chem. Res. 1994, 27, 51. [lo] P. Pyykko, J. Li, N. Runeberg, Chem. Phys. Lett. 1994,218, 133. [ 111 F. A. Cotton, B. E. Hanson, Rearrangements in Ground and Excited States, P. De Mayo, Ed; Academic Press: New York, 1980, p. 379 [12] D. Braga, Chem. Rev. 1992, 92, 633. [13] (a) D. Braga, F. Grepioni, J. Chem. SOC.Dalton Trans. 1993, 1223. (b) D. Braga, F. Grepioni, P. J. Dyson, B. F. G. Johnson, P. Frediani, M. Bianchi, F. Piacenti, J. Chem. SOC.Dalton Trans. 1992, 2565. [I41 A. I. Kitaigorodsky, “Molecular Crystal and Molecules”, Academic Press, New York, 1973 [15] see, for example, (a) D. Braga, F. Grepioni, Organometallics 1991, 10, 1254. (b) D. Braga, F. Grepioni, S. Righi, B. F. G. Johnson, P. Frediani, M. Bianchi, F. Piacenti, J. Lewis, Organometallics 1991, 10, 706. [ 161 (a) A. J. Pertsin, A. I. Kitaigorodsky, “The Atom-Atom Potential Method”, Springer-Verlag, Berlin 1987; (b) A. Gavezzotti, M. Simonetta, Chem. Rev. 1982, 82, 1. (c) A. Gavezzotti, G. Filippini, J. Am. Chem. SOC.1995,117, 12299. (d)A. Gavezzotti, J. Am. Chem. Soc. 1983,195, 5220. [I71 (a) D. Braga, F. Grepioni, P. Sabatino, J. Chem. SOC.Dalton Trans. 1990, 3137. (b) D. Braga, F. Grepioni, Organometallics 1992, 11, 71 I. [18] J. Maddox, Nature 1988, 335, 201. [19] A. Gavezzotti, Acc. Chem. Res. 1994, 27, 309. [20] (a) H. R. Karfunkel, R. J. Gdanitz, J. Comput. Chem. 1992,13, 1171. (b) R. J. Gdanitz, Chem. Phys. Lett. 1992, 190, 391. (c) S. J. Maginn, Acta Cryst. 1996, A52, C79. (d) Theoretical Aspects and Computational Modeling of the Molecular Solid State A. Gavezzotti, Ed., Whiley: Chichester, 1997. (e) A. Gavezzotti, Current Opinion in Solid State and Materials Science, A. K. Cheetham, H. Inokuchi, J. M. Thomas, Eds, 1996, I , 501.
5.2 Trunsition Metal Clusters
1501
[21] A. Gavezzotti, PROMET “A Program f o r the Generation of Possible Crystal Structuresfrom the Molecular Structure of Organic Compounds”; A. Gavezzotti, J. Am. Chem. Soc. 1991, 113, 4622. [22] (a) D. Braga, F. Grepioni, A. G. Orpen, Organometallics 1994, 13, 3544. (b) D. Braga, F. Grepioni, E. Tedesco, A. G. Orpen, J. Chem. Soc. Dalton Trans. 1995, 1215. [23] D. Braga, F. Grepioni, Comm. Inorg. Chem. 1997, 19, 185. [24] D. Braga, F. Grepioni, P. Sabatino, A. Gavezzotti, J. Chem. Soc. Dalton Trans. 1992, 1185. [25] (a) D. Braga, F. Grepioni, J. Farrugia, B. F. G. Johnson, J. Chem. Soc. Dalton Trans 1994, 2911. (b) C. H. Wei, L. F. Dahl, J. Am. Chem. Soc. 1969, 91, 1351; (c) F. A. Cotton, J. M. Troup, J. Am. Chem. Soc. 1974, 96,4155. [26] (a) R. Mason, A. I. Rae, J. Chem. Soc. A 1968, 778. (b) M. R. Churchill, F. J. Hollander, J. P. Hutchinson, Inorg. Chem. 1977, 16, 2655. (c) E. R. Corey, L. F. Dahl, Inorg. Chem. 1962, I , 521. (d) M. R. Churchill, B. G. DeBoer, Inorg. Chem. 1977, 16, 877. [27] (a) R. Hoffmann, J. Chem. Phys. 1963,39, 1397. (b) R. Hoffmann, W. N. Lipscomb, J. Chem. Phys. 1962, 36, 2179. (c) J. H. Ammeter, H.-J. Biirgi, J. C. Thibeault, R. Hoffmann, J. Am. Chem. Soc. 1978, 100, 3686. (d) C. Mealli, D. M. Proserpio, J. Chem. Ed. 1990, 67, 39. [28] (a) R. Hoffmann, H. Fujimoto, J. R. Swenson, C. C. Wan, J. Am. Chem. Soc. 1973, 95, 7644. (b) R. Hoffmann, H. Fujimoto, J. Phys. Clzem. 1974, 78, 1167. [29] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, New York, 1989. [30] T. Ziegler, Chem. Rev. 1991, 91, 651 [31] (a) B. E. Mann, J. Chem. Soc. Dalton Trans. 1997, 1457. (b) B. F. G. Johnson, J. Chem. Soc. Dalton Trans. 1997, 1473. (a) B. E. R. Schilling, R. Hoffmann, J. Am. Chem. Soc. 1979, 101, 3456. (b) J. Li, K. Jug, Inorg. Chim. Acta. 1992, 196, 89. (c) E. J. Baerends, A. Rosa, New J. Chem. 1991, 15, 815. (d) D. G. Evans, J. Chem. Soc., Chem. Comm. 1983, 675. (e) B. Delley, M. C. Manning, D. E. Ellis, J. Berkowitz, W. C. Trogler, Inorg. Chem. 1982,21, 2247; (f) J. W. Lauher, J. Am. Chem. Soc. 1986, 108, 1521. (6) C. Mealli, The Synergy between Dynamics and Reactiuity at Clusters and Surfaces, Ed. L. J. Farrugia, Glasgow, 1994. (h) A. Sironi, Inorg. Chem. 1992,31,299.(i) P. T. Chesky, M. B. Hall, Inorg. Chem. 1983;22,2998. (j) R. D. Barreto, T. P. Fehlner, L.-Y. Hsu, S. G. Shore, Inorg. Chem. 1986, 25, 3572. (k) M. A. Gallop, M. P. Gomez-Sal, C. E. Housecroft, B. F. G. Johnson, J. Lewis, S. M. Owen, P. R. Raithby, A. H. Wright, J. Am. Chem. Soc. 1992, 114, 2502. (1) J.-F. Halet, J.-Y. Saillard, R. Lissillour, M. J. McGlinchey, G. Jaouen, Inorg. Chem. 1985, 24, 218. (m) G. L. Griewe, M. B. Hall, Inorg. Chem. 1988, 27, 2250. (n) F. A. Cotton, X. Feng, Inorg. Chem. 1991, 30, 3666. (0) J.-F. Riehl, N. Koga, K. Morokuma, Organometallics 1993, I2,4788. (p) R. L. DeKock, K. S. Wong, T. P Fehlner, Inorg. Chem. 1982, 21, 3203. (9) C. Mealli, J. Am. Chem. Soc. 1985, 107, 2245. (r) A. R. Pinhas, T. A. Albright, P. Hofmann, R. Hoffmann, Helv. Chim. Acta 1980, 63, 29. [33] E. Hunstock, C. Mealli, M. J. Calhorda, J. Reinhold (submitted). [34] D. Braga, M. J. Calhorda, F. Grepioni, P. Lopes, E. Tedesco, J. Chem. Soc. Dalton Trans. 1995, 1215. [35] D. Braga, F. Grepioni, B. F. G. Johnson, J. Farrugia, J. Chem. Soc. Dalton Trans. 1994, 2911. [36] J. Farrugia, personal commnunication [37] D. Braga, J. Farrugia, F. Grepioni, A. Senior, J. Chem. Sac. Chem. Commun. 1995, 1219. [38] D. Braga, J. Farrugia, A. L. Gillon, F. Grepioni, E. Tedesco, Organometallics 1996, 15, 4684. [39j D. Braga, F. Grepioni, E. Tedesco, P. J. Dyson, C. M. Martin, B. F. G. Johnson, Trans. Met. Chem. 1995,20, 615. [40] A. Sironi, Inory. Chem. 1992, 31, 2467. [41] D. Braga, F. Grepioni, M. J. Calhorda, L. Veiros, Organometallics 1995, 14, 5350. [42] (a) G. A. Jeffrey, W. Saenger, Hydrogen Bonding in Biological Structures Springer-Verlag: Berlin, 1991. (b) C. B. Aakeroy, K. R. Seddon, Chem. Soc. Rev. 1993, 397. (c) J. Bernstein, M. C. Etter, L. Leiserowitz. in Structure Correlation H.-B. Biirgi, J. D. Dunitz, Eds, VCH: Weinheim, 1994, p. 431.
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5 Solid-state Cluster Chemistry
[43] (a) A. D. Burrows, C.-W. Chan, M. M. Chowdry, J. E. McGrady, D. M. P. Mingos, Chem. SOC.Rev. 1995, 329. (b) S. Subramanian, M. J. Zaworotko, Coord. Chem. Rev. 1994,137, 357. [44] (a) C. V. K. Sharma, G. R. Desiraju, in Perspectives in Supramolecular Chemistry. The Crystal as a Supramolecular Entity; G. R. Desiraju, Ed., Wiley: Chichester, 1996. (b) G. R. Desiraju, Crystal Engineering: The Design of Organic Solids; Elsevier: Amsterdam, 1989. (c) G. R. Desiraju, Angew. Chem. Int. Ed. Engl. 1995, 34, 23 l l . [45] (a) D. Braga, F. Grepioni, P. Sabatino, G. R. Desiraju, Organometallics 1994, 13, 3532. (b) K. Biradha, G. R. Desiraju, D. Braga, F. Grepioni, Organometallics 1996, 15, 1284. (c) D. Braga, F. Grepioni, E. Tedesco, K. Biradha, G. R. Desiraju, Organometullics 1997, 16, 1846. (d) D. Braga, F. Grepioni, E. Tedesco, K. Biradha, G. R. Desiraju, Organometallics 1996, 15, 2692. (e) D. Braga, F. Grepioni, K. Biradha, G. R. Desiraju, J. Chem. Soc. Dalton Trans. 1996, 3925. [46] D. Braga, F. Grepioni, Acc. Chem. Res. 1997, 30, 81. [47] D. Braga, F. Grepioni, K. Biradha, V. R. Pedireddi, G. R. Desiraju, J. Am. Chem. Soc. 1995, 117, 3156. [48] (a) D. Braga, F. Grepioni, B. F. G. Johnson, J. Lewis, C. E. Housecroft, M. Martinelli, Organometallics 1991, 10, 1260. (b) L. Hoferkamp, G. Rheinwald, H. Stoeckli-Evans, G. SussFink, Helv. Chim. Acta 1992, 75, 2227. (c) S. Rossi, K. Kallinen, J. Pursiainen, T. T. Pakkanen, T. A. Pakkanen, J. Organomet. Chem. 1992,440, 367. [49] D. Braga, F. Grepioni, J. J. Byrne, M. J. Calhorda, J. Chem. SOC.Dalton Trans. 1995, 3287. [50] D. Braga, F. Grepioni, H. Wadepohl, S. Gebert, M. J. Calhorda, L. F. Veiros, Organometallics 1995, 14, 24. [48] (a) M. P. Gomez-Sal, B. F. G. Johnson, J. Lewis, P. R. Raithby, A. H. Wright, J. Chem. SOC. Chem. Commun 1985, 1682. (b) P. J. Dyson, B. F. G. Johnson, J. Lewis, M. Martinelli, D. Braga, F. Grepioni, J. Amer. Chem. SOC.1993. 115, 9062. (c) R. D. Adams, W. Wu Polyhedron 1992,2, 2123. [52] D. Braga, P. J. Dyson, F. Grepioni, B. F. G. Johnson, M. J. Calhorda, Inorg. Chem. 1994,33, 3218. [53] B. F. G. Johnson, R. E. Benfield, Trunsition Metal Clusters ed. B. F. G. Johnson, Wiley: New York, 1980, 471. [54] (a) Introduction to Cluster Chemistry, D. M. P. Mingos, D. J. Wales, Eds. Prentice Hall R. N. Grimes Ed. 1990. Englewood Cliffs, N. J. (b) Metal-Metal Bonds and Clusters in Chemistry and Catalysis Ed. J. P. Fackler, Plenum Press, New York 1989. (c) The Chemistry of Metal Cluster Complexes Eds. D. F. Shriver, H. D. Kaesz, R. D. Adams, VCH, New York 1990. [55] (a) E. Sappa, A. Tiripicchio, P. Braunstein, Chem. Rev. 1983,83, 203. (b) P. R. Raithby, M. J. Rosales, Ado. Inorg. Chem. Radiochem. 1985,29, 169. (c) H. Wadepohl, Angew. Chem. Int. Ed Engl. 1992, 31, 247. (d) D. Imhof, L. M. Venanzi, Chem. SOC.Rev. 1994, 185. (e) D. Braga, F. Grepioni, P. J. Dyson, B. F. G. Johnson, Chem. Rev. 1994, 94, 1585. (f) Catalysis by Diand Polynuclear Metal Cluster Complexes,R. D. Adams and F. A. Cotton, Eds., Wiley-VCH: Weinheim, 1998. [56] D. Braga, F. Grepioni, Coord. Chem. Rev. 1999, 183, 19. D. Braga, F. Grepioni, G. R. Desiraju, Chem. Rev. 1998, 98, 1375.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.3 Discrete and Condensed Clusters in Low Valent Niobium Oxides Cunnar Svensson, Jurgen KohIer and Arndt Simon
5.3.1 Introduction Isolated or condensed metal clusters are found in many reduced transition metal halides and chalcogenides.“] In recent decades it has become clear that metal cluster formation is also a common feature of transition metal oxides, especially those of m o l y b d e n ~ m . [ ~The - ~ ] structures of these reduced oxomolybdates are mainly built from M06O12 clusters, which are condensed via common edges of the M06 octahedra to oligomeric, 1D or 2D infinite cluster units. NbO,[’] which has a ‘defect rocksalt structure’ with 25% ordered vacancies on the anion and cation sites, can be understood in terms of a ‘condensate’ of Nb6O12 clusters linked via all apex atoms of the Nb6 octahedra.r61 The first discrete Nb6012 cluster was found in Ba3Si2Nb20.8044[~] and the search for a rich chemistry of intermediates between this compound containing isolated clusters and the final member, NbO, with all apexjoined clusters was rewarded with the discovery of many compounds containing discrete and condensed clusters.[’] In the last five years many new results have been obtained in this field and at this stage a review seems desirable, which summarizes the structures and new insights into structure, bonding, and property relationships for this class of ionic compound containing metal-metal bonded units.
5.3.2 Oxoniobates containing discrete Nb6012 clusters The discovery of Ba3Si2Nb20,80~,~’I the first compound containing discrete Nb6012 clusters, dates back to the beginning of the seventies, and it took decades until a whole series of reduced oxoniobates were synthesized and characterized (Table 1). The preparations of these oxoniobates have been performed as solid-state reactions
1510
5 Solid-state Cluster Chemistry
Table 1. Reduced oxoniobates containing discrete Nb6012 clusters.
P3m1 P3ml P 63/m R3 R5 R3 R3 P3 P 63 P3 P3 P3 P 63 R3 Pbam Pbam Pbam Pbam Cmca Pbca Pbam Pbam p 2llC p 21Ic
604.2 608.0 1603.4 784.4 784.1 783.7 783.0 782.2 779.2 782.1 781.5 777.4 777.7 771.7 935.2 932.7 925.7 931.1 2371.0 942.2 919.4 927.7 1076.2 784.9
~
~
~
~
~
~
~
1026.8 1028.0 1030.1 1033.1 1036.0 1036.5 1033.7 1031.1 919.2 931.6
746.6 762.7 1807.9 7065.0 4221.8 4236.0 7051.O 2632.0 1443.0 2651.3 2639.2 1676.5 1451.0 3551.8 592.9 593.7 595.4 595.4 942.2 3714.6 596.2 596.4 1031.4 B = 104.25" 1028.1 p = 110.48"
14 14 14 14 14 14 14 14 14 14 14 14 14 14 13 13+x 14 14 14 14 15 14
9, 27 9 28 10 10 16 16 16 16 17 18 7 19 15 20 20 11 13, 21 13 15 20 21 14, 22 15, 12
from appropriate mixtures of binary or ternary starting compounds with Nb and NbO as reducing agents at temperatures above 1300 K. The reducing conditions require special container materials, such as Au, Pt, or Nb ampoules and protection against oxidizing gases, e.g. by use of vacuum or an Ar-filled quartz glass ampoule. The crystal structures of many reduced oxoniobates proved to be extremely complicated and the availability of single crystals was essential for solving their structures and for the determination of their compositions. So, the use of mineralizers or fluxes was often necessary to obtain single c r y s t a l ~ . [ ~ - ~ ~ ] The characteristic building unit occurring in all low-valent oxoniobates listed in Table 1 is a Nb,j012i06a cluster (Fig. la). The Nb-Nb bond distances within the
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
151 1
Oa
cz
-
0
Figure 1. (a) Projection of an Nb60'120"6 cluster (according to notation given elsewhere[61);(b) projection of an Nb6CI'12CIa6 cluster and (c) schematic representation of the condensation of two Nb60'12 clusters via a common corner of the Nb6 octahedra.
discrete Nb6O12 clusters fall in the narrow range of 279-289 pm, and the distortions are minor. There are twelve so-called inner oxygen atoms (0')above the edges of the Nb6 octahedron, which form a cuboctahedron. The six 0 atoms above the apices of the octahedron constitute the outer ligand sphere and are named (Oa).[61 The distances to the Oa atoms are usually between 215 and 235 pm and longer than the Nb-0' distances, ranging between 202 and 213 pm, which is ca. ,/2 x dNb-Nb (285 pm). Therefore, each Nb atom lies closer than 5 pm to the plane formed by four surrounding 0' atoms (inside the cuboctahedron). This situation is different for Nb6X12 clusters with X = c1 or Br, in which the distances dNb-Nb are much smaller than J2 x dNb-X resulting in a contraction of the M6 octahedron relative to the X12 cuboctahedron (Fig. lb). Therefore, for topological reasons the Nb6012 clusters are much more likely to condense via apex atoms than are the corresponding halide clusters (Fig. lc). The structures of oxoniobates containing discrete Nb6012 clusters can be described and compared in terms of a close packing of 0 atoms, Nb6 octahedra and, if present, A atoms (A = Na, K, Rb, Sr, Ba, La, Eu). Smaller M atoms fill tetrahedral (M = Al, Si, Mg, Mn), octahedral ( M = Nb, V, Mg, Mn) and trigonal bipyramidal voids ( M = V, Nb) within the 0 sublattices, which stabilize the structures and formally act as electron donors for the Nb6O12 clusters. This structural principle of a common close packing is outlined in more detail in an earlier review.['] A remarkable difference between the 0x0 cluster and the corresponding halide cluster, i. e. the molecular orbital schemes, becomes obvious when their electronic
1512
5 Solid-state Cluster Chemistry
- 13
- 14
alg
Figure 2. Molecular orbital scheme for the metal-centered orbitals of a Nb6012 cluster.
structures are compared. Analysis of an Nb6012 cluster on the basis of extended Huckel (E.H.) calculations1231leads to the M-M bonding levels alg, tlu, t2g and a2u (Fig. 2). Because of a stronger antibonding metal-ligand interaction the occupancy of all M-M bonding states by 16 electrons has not been observed experimentally for the Nb6012 cluster - in fact, the 14-electron species seems much preferred in agreement with the calculation. In that respect the energy level scheme of the 0x0cluster differs from that of the M6C112 cluster^,^^^^^^^ where the azUis overall nonbonding and, hence, is easily occupied, e.g. in a [Nb&112l2+ unit.[261 The assignment of the number of valence electrons for Nb-Nb bonding states in Nb6O12 clusters of reduced oxoniobates is often obvious, but occasionally the structures are characterized by an intricate distribution of the excess valence electrons between the different structural units. A discussion of such adjustments of electronic balances will be presented below for selected examples of increasing complexity and discussed with regard to possible conclusions. The crystal structures of M3Nb6011 (M = Mn, Mg)[9,271are characterized by close packing of layers containing Nb6O12 clusters which are surrounded by pure 0atom layers (Fig. 3a). Mg atoms occupy one quarter of the tetrahedral and one quarter of the octahedral holes between the 0-atom layers. The particularly simple electronic structure of Mg3Nb6011 is highly suitable for illustrating the charge distribution. With the reasonable assumption that oxygen has the oxidation state -2 leads to 14 electrons in M-M and Mg 2 the ionic limit (Mg2+)3(Nb2.66+)6(02-)~1 bonding states for the Nb6012 cluster. This is confirmed by the band structure of Mg3Nb6011 (Fig. 3b). Rather narrow bands appear in the density of states (DOS) for the Nb atoms of the cluster below the Fermi level at approximately -14, -13, -12.5 and -12.2 eV which are essentially metal-centered and correspond to the
+
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
1513
000 0000
mv1
4
- 10.0 -12 0
- 14.0 - 16.0 000 0 000
Figure 3. (a) Projection of the crystal structure of Mg3NbhOll. Small black circles represent Nb and Mg atoms, the Nbs octahedra are graphically emphasized. (b) total density of states (DOS) for MgiNbbOII together with the atomic orbital projection (AOP) for the Nb cluster atoms.
seven low-lying states of the discrete cluster. The band near - 11.5 eV corresponds to the a2,, orbital and lies significantly above the Fermi level. The relatively large band gap is in agreement with the greenish-black color of Mg3Nb6011 and Mn3Nb6011, and with their semiconducting properties. Mn3Nb6011 has a magnetic moment of peff,= 5.83pB, near that expected for Mn2+. Mg3Nb6011 is nonmagnetic, i. e. the diamagnetism is nearly compensated by temperature-independent paramagnetism, as often found with cluster compounds containing only paired electrons. The electronic situation for NaV2Nb7014[281(simplified formula) is somewhat more complicated. The structure contains the same kind of cluster sheets as found for Mg3Nb6011 (Fig. 4). As a consequence of the hexagonal stacking of additional 0, NaO,, 0 atom layers, trigonal bipyramidal and face sharing octahedral voids are present. The bipyramidal voids are occupied by NbSf atoms. The pairs of facesharing 0 6 octahedra are filled by V3+ atoms resulting in V209 clusters. The V-V distance is so short (256 pm) that M-M bonding must be assumed, as for the wellknown halides Cs3M2C19 (M = Mo, W).[29,301According to the ionic limit NafV3+2NbSfNb2.6+602p 14 there are 14 valence electrons in Nb-Nb bonding states of the Nb6012 cluster, and the remaining excess valence electrons are located in the d-states of the V atoms in the V209 clusters. The latter can be formally considered as an electronic buffer system for the Nb6012 cluster, and vice versa. Other such examples will be presented below. In NaSiNbloOlg another mode of cluster formation is observed. The occupation of two neighboring octahedral voids by Nb4+ atoms sharing a common edge leads
1514
5 Solid-State Cluster Chemistry
0000000
0
- - - - -> 0 I
0
n
Figure 4. Projection of the crystal structures of NaV~Nb7014(left) and a V209 cluster (right).
to a Nb2010 unit (Fig. 5). This bonding situation is typical for Nb4+ with a d' ~ b274 pm) are configuration and, as expected, these pairs of Nb atoms ( d ~ b - = single-bonded as in the low-temperature form of NbO2. The important role of electronic buffer systems becomes even more clear, when discussing the occurence of Nb3013 clusters, which are found in BaSiNbl0019['~] and also in many other reduced oxoniobates (Fig. 6). Each of these Nb3013 clusters is formed by three edge-sharing Nb06 octahedra. The Nb atoms can adopt different oxidation states and, depending on the number of excess valence electrons available for Nb-Nb bonding states, the Nb-Nb distances can vary between 282 and 340 pm. Such variability of the number of valence electrons in M-M bonding states is also well known from the Mo3013 clusters in molybdenum oxides, such as M2M03O13 (M = Li, Zn, Sc, Ni).[319321 In BaSiNbl0019[~~] the common edges of the coordination octahedra are rather long (do-0 = 314 pm) and the Nb3013 clusters have short Nb-Nb distances of 282 pm. The occupancy of Nb-Nb bonding states in these units becomes obvious from inspection of the DOS for BaSiNbloOl9 (Fig. 7a-c). Furthermore, the familiar
5.3 Discrete and Condensed Clusters in LOWVulent Niohiuni Oxides
Figure 5. Projection of the crystal structure of NaSiNbloOlg and an Nb2010 cluster.
15 15
15 16
5 Solid-state Cluster Chemistry
Figure 6. Projection of the crystal structure of BaSiNbloOlg and a Nb3Ol3 cluster.
-4.0
E
[eG6'0
T
-10.0 -12.0
-14.0 -16.0
-18.0
Figure 7. Total density of states (DOS) for BaSiNbloOlg and atomic orbital projections (AOP) for (a) the Nb atoms of the NbsOlz clusters; (b) the Nb atoms of the Nb3Ol3 clusters; (c) the Nb atoms of the Nb06 octahedra; and (d) crystal orbital overlap population (COOP) for the Nb-centered orbitals of the Nb3013 clusters.
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
Figure 8. Projection of the crystal structure of Rb4 (B,Nb)ZNb350 7 0 (left) together with two different kinds of occurring Nb3Ol.i clusters (right).
1517
0 odo b b 00 000
energy level scheme (a*”, e’, al’) for such molecular is reflected in the COOP curve for the Nb3 units in BaSiNbloOlg (Fig. 7d). Here the Nb-Nb bonding states are filled to approximately 60% corresponding to four valence electrons in M-M bonding states of the Nb3 unit instead of the six to eight usually found in oxomolybdates, e.g. M 2 M ~ 3 0 8 . [ 3 5 1 More can be learned about the Nb3013 unit from Rb4(Si,Nb)2Nb35070.“’] As is apparent from Fig. 8 there are two different kinds of such units with ‘short’ (306 pm) and ‘long’ (335 pm) Nb-Nb distances. Band-structure calculations for Rbd(Si,Nb)2Nb35070indicate that the Nb-Nb bonding bands of the Nb6012 clusters and the Nb3013 clusters with short distances are occupied. The d states of the remaining octahedrally surrounded Nb atoms lie high above the Fermi level indicating an oxidation state of +5 for these atoms.
1518
5 Solid-State Cluster Chemistry
0
0
6
0
8
8
0
608
0
6
0
Figure 9. Projection of the crystal structure of SrNbgOla.
According to Sr2+(Nb6) 16f(Nbs+)202-14there are two octahedrally coordinated Nb5+ ions in the structure of SrNb~014[1'1 whereas LaNb7012[221contains only one Nbs+ ion in addition to the Nb6 octahedra. The filling of adjacent voids by Nb atoms in SrNbs014 is such that the NbO6 octahedra share only common corners (Fig. 9), which is also true for the La compound. Together with the surrounding A atoms the part of the structure between the clusters represents a fraction of the perovskite structure. The optimum stability is reached with 14 valence electrons in Nb-Nb bonding bands which are entirely filled in agreement with the DOS and the semiconducting properties of the compounds. There is thus no significant electron transfer to the octahedrally surrounded Nb atoms, as is obvious from inspection of the AOP (atomic orbital projection) for these atoms. In NaNbloOl&'21there are more than 14 electrons available for M-M bonding in the Nb6O12 cluster. Four octahedral sites are occupied by Nb atoms with edgesharing of the NbO6 octahedra (Fig. 10). Other bands which originate from interactions between clusters, clusters and adjacent Nb atoms or between non-cluster Nb atoms may then be filled. These cases are of special interest, because the excess valence electrons cannot be localized in direct M-M bonding states. So, in NaNbloOlg a band which corresponds to the azUlevel is partly occupied as is the d band originating from the octahedrally surrounded Nb atoms, in agreement with the metallic property of the compound. The corresponding COOP curves are indicative of strong bonding overlap between the d orbitals of the octahedrally surrounded Nb atoms in a chain, but not with the Nb atoms of the Nb6012 cluster. The important role of electron balance between different structural regions, as depicted for the compounds containing discrete Nb6O12 clusters, is of even greater importance for oxoniobates containing condensed Nb6012 clusters. This will be outlined in detail in the next section.
5.3 Discrete and Condensed Clusters in Low Valent Niobium Oxides
Figure 10. (a) Projection of the crystal structure of NaNbloOls. (b) Total density of states (DOS) for NaNbloOlg and the atomic orbital projection (AOP) for the Nb cluster atoms. (c) Total density of states (DOS) for NaNb,oOlg and the atomic orbital projection (AOP) for the octahedrally coordinated N b atoms.
-8.0
mv1
T
,
1519
FEF ,v .. ......................... . . .......................
-12.0
-16.0
b)
!
C)
5.3.3 Reduced oxoniobates with condensed Nb6012 clusters The structures of reduced oxoniobates frequently contain Nb-Nb bonded units which can be discussed in terms of NbGOI:! clusters condensed in one, two or three dimensions. Some representative compounds with such condensed Nb6O12 cluster units are shown in Fig. 11. Although more than ten years have passed since the first compound with condensed Nb6O12 clusters was reported only one representative of oligomeric clusters is known, K4A12Nb11021 .[361 The [Nb11020']010acluster found in this compound
1520
5 Solid-state Cluster Chemistry
cluster unit
2D
... 2D ..:*
3D
...
Nb406
.'...
.* Nb,09 ...
'' '
Nb,03
[a*a*l]
[a*m*21
[a* a* a ]
Figure 11. Examples of OD, lD, 2D, and 3D condensation of Nbs octahedra together with the cluster unit and notation according to Kohler et U Z . ' ~ ]
consist of two Nb6012 units condensed via vertex linking of the central Nb6 octahedra. By extending the condensation in one dimension chains form as in BaNbsOg.[371 These chains can condense forming triple and quadruple chains as in Ba4Nb14023[381 and Ba3Nb160~3,[~~] respectively, or single layers as in BaNb406.[40,411 The condensation continues via the double layers found in BaNb709[421to the threedimensional net in NbOr6](In principle only corner-linked Nbb octahedra have been found, although isolated 'defects' of edge-sharing Nb6 octahedra have been found in high resolution electron microscopy (HREM) images.) The extent of condensation is a function of the number of electrons available for Nb-Nb bonding. A detailed study of the cluster units in these compounds reveals that only three different functionalities of Nb atoms need be considered, which leads to a simple procedure for calculation of the optimum number of electrons for any specific condensed cluster arrangement. In discrete Nb6012 clusters there are usually 14 electrons in M-M bonding states corresponding to 14/6 = 2.33 electrons per Nb atom. The other extreme, NbO, with 3D-condensed clusters then has 3.0 electrons per Nb
5.3 Discrete and Condensed Clusters in Low Valent Niobium Oxides
n
tionalities, and the distribution of electrons available for bonding.
1521
2.33 e-
v
atom. The third type of Nb atom is found in the dimeric cluster of Nbl1020 with 24 electrons in M-M bonding states, shown in Fig. 12. The two peripheral Nb atoms which are bonded as in the Nb6012 cluster are assigned to contribute 2.33 electrons to three-center M-M bonding in four faces. The central Nb atom in the Nb11020 cluster has the same environment as the Nb atom in NbO and therefore donates three electrons to bonds in a total of eight faces. The eight equivalent basal Nb atoms remaining need to contribute 16.333 electrons (-2.0 per Nb atom) to M-M bonding in order to reach the number observed, 24. (Band-structure calculations have shown this to be the optimum number.[']) An application of this counting schemetoatrimericcluster, Nb601280a14, leadsto2 x 2.33 + 2 x 3.0 + 3 x 4 x 2.0 = 34.66 electrons as an optimum number of electrons in M-M bonding states. In this way the optimum number of electrons for different levels of condensation can be predicted as shown in Table 2. The extent of condensation is reflected in the electrical resistivity of these compounds, which is shown for some selected barium oxoniobates in Fig. 13. B a N b s 0 1 4 [ ~contains ~] discrete Nb6O12 clusters and is semiconducting whereas the extended condensate, the single chain as in BaNbsOs, leads to metallic properties. The resistivity is lowest for BaNb406 with alternating single slabs of NbO and perovskite (NbO is a metallic conductor and superconducting below 1.25 K[441).
5.3.3.1 Intergrowths between perovskite and NbO All compounds containing condensed Nb6012 clusters are of the intergrowth type, the second building block being of the perovskite type.['] The common intergrowth plane of the subunits of NbO and perovskite is { IOO}. The structures of Nb0[5,451
1522
5 Solid-state Cluster Chemistry
Table 2. Magic electron numbers for condensed Nb6O12 clusters. ~~
Compound
Cluster unit
NVE/cluster Predicted
0bserved
[I x 1 x I]
14
13-15
[I x 1 x2]
23.67
23-24
11 31 40 40
11 40-41 40-4 1 40
[lx co x co] [I x co x co]
10 10
10 10
[I [l [I [I
x co x m]
10 10 10 10
10-1 1 10-1 1 10-11
[ 2 x co x m ]
19
19
[2 x
19
19-20
~
[l x [l x [2 x [2 x
1 x 001 3 x m] 2 x m] 2 x co]
x 00 x
001
x co x m] x co x a]
00 x
co]
[co x co x co]
9
9
9
and BaNb03[46-481fit well together as shown in Fig. 14 (Ba in BaNbO3 can be replaced with K, Sr or Eu). By use of these building blocks an infinite number of structures can be imagined. They are easily systematized in homologous series;[83491 the four shown in Fig. 15 all have infinitely condensed clusters. The size of the NbO type units are given as [ p x q x Y], where p , q and r refer to the number of condensed Nb6 octahedra in the three directions of space. The discrete Nb6O12 cluster and NbO are then represented by [l x 1 x 11 and [GO x co x GO], respectively. The corresponding description for the perovskite units is {s x t x u } , where s, t and u correspond to the number of perovskite-type units. In these homologous series each NbO-type block is linked to four neighboring blocks via 0'-" bridges, resulting in rather short Nb-Nb intercluster distances (ca. 300 pm). The homologous series are described as [ p x q x r] with 1 i 9 2 2 and p = constant according to the notation given above. Increasing values for q correspond to an increase of the NbO-type partial structure and simultaneously the perovskite-type block sizes. The end members of all series have block sizes of the [ p x co x GO] kind and thus consist of alternating slabs of NbO (infinitely two-dimensionally condensed Nb6012 clusters) and perovskite. They form an additional homologous series: AsNbs+3p03F+3p with s = the number of
5.3 Discrete and Condensed Clusters in Low Valent Niobium Oxides
1523
1500
1000
3
0
5 500
0 0
50
100
150
200
250
300
T(K)
--r--BaNb8014 [1*1*1] --e--BaNb508 [1*1*w] --e--Ba4Nb17026[2*2*w] --v--Ba2Nb509[~*M*M] --a--BaNb40s [~*co*co] Figure 13. Dependence of electrical resistance on temperature for various oxoniobates containing Nb6012 clusters. The conductivity of the samples increases with the extent of condensation.
Figure 14. Topological similarity of the structures of NbO (left a = 421 pm) and BaNb03 (right a = 408.5 pm for Ba0.9sNb03).
1524
5 Solid-State Cluster Chemistry
d 0
0 0 0
m
0
0
0
0
d
0 0
Q 0 0
o
d m
0x0o d
Q
5.3 Discrete and Condensed Clusters in Low Valent Niobium Oxides
0 v)
.-a
ZI f 8 cn 2s Z ’
Qci.
0 0 0 0
0
B
0 0 0 0
0 0
0 0
e
e
0 0
0 0
1525
1526
5 Solid-state Cluster Chemistry
perovskite-type slabs {s x [ p x 00 x co].
00
x a} and p = the number of NbO-type slabs
5.3.3.2 The formation of intergrowth compounds During the last 10 years several compounds predicted in the homologous series have been synthesized and structurally characterized. They are listed in Table 3 and Fig. 15. The extent of condensation of the Nb6012clusters ranges from single chains in BaNb508[371to double layers of infinitely condensed Nb6O12 clusters in SrNb406[501and BaNb709.[421All the compounds have at least one short axis (410-419 pm). This corresponds to the heights of the Nb6 octahedra (diagonal of the octahedron) and the NbO6 octahedra in that direction. The atomic positions in the structures are thus well resolved in one projection and the compounds are therefore highly suitable for HREM studies. The structures of most of the compounds listed in Table 3 were first confirmed by this method. The structure models were then refined by use of X-ray diffraction data from single crystals or powders or data from neutron diffraction on powders. Most compounds have been found in the Ba0-Nb0-Nb02 system and the corresponding compositional diagram is shown in Fig. 16. Not all the K, Sr, and Eu analogs have been prepared (Table 3). The difficulties in preparation are probably because of the smaller size difference between NbO and BaNb03 compared with ANb03 A = K, Sr, and Eu, see Table 4. The syntheses of these compounds often have been far from straightforward in the Ba-Nb-0 system. At moderate temperatures (1050-1250 K) disordered crystallites with a Ba/Nb ratio of approximately 1 : 4 exist as a single 'phase' in the shaded region on the right hand side of the BaNb03-NbO line,[531in Fig. 17. One-dimensionally extended blocks of both NbO and BaNb03 types can be recognized in the HREM images of the crystals (Fig. 18). Similar crystallites are also found in the Sr-and Eu-Nb-0 systems. The dark crosses in the HREM image correspond to columns of Nb6 octahedra in the NbO slab and the smaller spots in between denote the strings of Ba and Nb 0 atoms in the perovskite-type lattice. The number of defects seems to be very limited along these columns, which can be considered to be uninterrupted along the viewing d i r e ~ t i o n . [ ~These * , ~ ~ ]disordered crystallites formed at an early stage of the reaction are, of course, thermodynamically unstable but nevertheless kinetically stable. They are thus not true phases in a thermodynamic sense and therefore the term phasoid has been used introduced by Magneli" 5 , 5 6 1 for their description. They form because the difference between the local structures of these disordered crystallites and that in possible line phases is very small. To obtain the ordered compounds higher temperatures and prolonged annealing have studied in detail the formation of these are needed. V.G. Zubkov et u1.[39,571 compounds via reduction of mixtures of ACO3 (A = Sr, Ba, and Eu) and Nb2O5.
+
1201.4 2093.0 2050
416.6 413.8 414.1 417.2 415.6 416.2 413.8 419.5
P4/mmm P4/mmm Cmmm Cmmm P4/mmm P4/mmm P4/mmm P4/mmm P4/mmm P4/mmm P4/mmm P4/mmm P4/mmm P4/mmm P4/mmm
(2X2XXJ (2x2xa) {2x2xm} (1 x 3 x m } ( 2 x 3 x} ~
(I x m x m} ( 1 x cc x x} (1 x x x m}
(2 x (2 x (2x (2x ( 2 x cc x m } ( 2 x cc x m } ( 2 x x x cc} ( 1 x x x cc}
[ I x cc x m ] [ I x x x cc]
m x m]
m x x] x x x]
cc x a ] cc x m]
x x]
[2 x [2x [I x [I x 11 x
[2 x
[ I x ic x CG] [ l x cc x m] [2 x m x x ]
Iccxxxm]
KNb406 BaNb406 BaNbd-,Ti, 0 6 ( Y = 0.53) SrNb406 bNb509 Sr2Nbs09 Ba2N bs 0 9 BazNb5 _xTi,09 ( x < 1.O) Ba2- ,Y ,N bs09 (.W < 0.2) Eu2Nbs09 BaNb709
NbO i
= insulator
(OXOXO}
* Conductivity: 1y1 = metallic, s = semiconductor,
00
1202.3
P4/mmm
(1 x 1 x m ) (2X2XccJ (2X2XXJ
m} cc} cc} cc}
1201.4
I4/mmm I4/mmm P4/m Cmmm P4/mmm
m x cx: x m x xx
879.9 878.6 660.8 2080.7 1213.1
413.9 418.3
u
Space group
Perovskite
NbO
Compound
1247.9
-
1245.2
h
Table 3. Unit cell parameters (pm) and properties of compounds containing condensed Nb6012 clusters.
1220.8 1203.7 1242.6
1221.0
1622.3 1210.1 1204.4 1222.4
825.4 821.3
413.5 416.9 414.0
414.1
414.0
1252.7 1263.7 410.7 41 5.1 415.6
c ~
67 21, 52 42 m m
5 , 45
68
m
m m m
69 50 51 64 40, 41
51, 65 40,41
51 39 58
m
m m
57
57
36 36 37 38 43
Ref.
m
m
m m m
S
S
C*
6P
8’
1528
5 Solid-state Cluster Chemistry
“Phasoid
NbO ”
A
Figure 16. Compositional diagram for the BaO-NbO-NbO2 system. A - BaNbO3 {co x co x 00); B BaNbsOs. [ l x 1 x a], (1 x 1 x co}; C Ba4Nb14023, [ l x 3 x co], (2 x 2 x 03); D - B~Nb17026,[2 x 2 x a], ( 2 x 2 x co}; E - Ba3Nbl6023, [2 x 2 x co], { 1 x 3 x cc}; F - Ba2Nbs09, [l x co x co], (2 x co x co};G - BaNbdOh, [ l x co x co], (1 x co x co}; H BaNb709, [2 x co x co], (1 x co x co}; I Ba2Nb15032,[ l x 1 x 11 -
-
-
NbO
BaO
-
The reducing agents were pyrolytic carbon or niobium metal. Sometimes up to 20 regrindings and reheating at 1200-1500 “C for 20 h at each cycle were needed to obtain single phase or close to single-phase samples with crystallites with ordered crystal structuresr571During the initial stages of the synthesis a-phasoid, NbO and often also ANb03 ( perovskite) are formed as discrete phases. As mentioned above, the a-phasoid crystallites are very disordered, however, small ordered domains can be found, as shown in Fig. 18. They can be regarded as nutrients for possible phases. As the reaction proceeds NbO reacts into the phasoid crystallites and the ordered areas grow. An HREM image of such a crystallite obtained during the synthesis of SrqNb17026 is shown in Fig. 18. In the HREM images small domains of Sr4Nb17026 (marked A in Fig. 18), separated by disordered block size distributions, can be seen, but also two regions with two other phases, Srl0Nb40064 and Sr6Nb25038, marked B and C, respectively. Idealized structure models of these are shown in Figs 18b-d. The structure of SrloNb40064 consists of mixed block sizes of NbO- and perovskite-type of the order 2 x 2 x 03 and 2 x 3 x co; Sr6Nb25038 has single size 2 x 3 x co blocks. The idealized structure of both is monoclinic. In
Table 4. Unit cell parameters (pm) for NbO, KNb03, Sro.g~Nb03,Bao.y5Nb03and EuNbO,. Compound
acub
Ref.
NbO KNbO3 * Sro.9sNb03 Bao.ssNb03 EuNbO3
421.0 401.5 402.5 408.5 402.6
45 61 59 48 60
*KNb03; u = 569.7, b = 397.1, c = 572.1[6’1.
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
1529
Figure 17. HREM image of the a-phasoid in the Ba-Nb-0 system. NbO blocks are seen as large spots or crosses, and the Ba and N b atoms in the perovskite blocks as smaller spots in between.
another crystallite an orthorhombic modification of Sr6Nb25038 was found. A HREM image of a small region of this phase is seen in Fig. 19, together with an interpretation. A vanadium-containing analog Sr6(Nb,V)25038, has also been reported.[581 Neither Sr10Nb40064nor SrgNb25038 have so far been synthesized in pure form. Very often, however, reminiscences of the a-phasoid can be found in the crystallites. After several heating cycles the final product is ordered Sr4Nb17026 as shown in Fig. 20. Although quite a few compounds with condensed clusters have been synthesized, many remain to be synthesized. The compounds found as small domains in the HREM images are promising candidates for new bulk phases.
5.3.3.3 Structural considerations When we introduced a way of calculating the optimum number of electrons in the structures of condensed Nb6O12 clusters three types of niobium atom with different structural functionalities were considered. In intergrowth compounds with infinitely condensed Nb6012 clusters only two of these are found, those which have been assigned 2 and 3 electrons above. In several of the compounds yet another type of niobium atom in Nb06 octahedra is found, for example in BaZNb509 (Fig. 15). We thus have three different groups of niobium atom referring to their nearest coordination sphere:
1530
5 Solid-state Cluster Chemistry
Figure 18. (a) HREM image of an a-phasoid crystallite viewed along the short subcell axis. (The image was taken with a Jeol2OOCX with 2.4A point resolution.) Three small ordered domains are marked A, B, and C. A is a domain of Sr4Nb17026, B is a domain of SrI"Nb40064 with mixed, 2 x 3 x co and 2 x 2 x m blocks of both perovskite- and NbO-type. C is a domain of Sr6Nb25038 with 2 x 3 x 00 blocks of perovskite and NbO. (b) A structure model of SrdNb17026 (A). (c) A structure model of SriONb40064 (B) with mixed block sizes. (d) A structure model of monoclinic Sr6Nb25038 (c).
Figure 18 (continued)
1532
5 Solid-state Cluster Chemistry
Figure 19. (a) HREM image of an orthorhombic analog of Sr6Nb25038. (b) An idealizcd structure model of SrgNb25038.
i) Niobium atoms (assigned 2 electrons above) linking Nb6 octahedra, having a near-cubic coordination of niobium atoms and a planar coordination of four oxygen atoms are denoted Nbi4} where the superscript represents the number of bonded oxygen atoms.
5.3 Discrete und Condensed Clusters in Low Vulent Niobium Oxides
Figure 20. An HREM image of
Sr4Nb17026 viewed
I533
along the c axis.
ii) The niobium atoms (assigned 3 electrons above) bonded to four niobium and 4 1 oxygen atoms (square pyramidal coordination) are denoted Nbi5}. iii) The niobium atoms in the perovskite slabs bonded to six oxygen atoms forming NbOh octahedra are denoted Nb{6}.
+
The barium atoms in these structures have a cubeoctahedral coordination of oxygen atoms. The oxygen atoms can be grouped into five kinds: i) 0i2} are those in the perovskite block linking corner-sharing Nb06 octahedra. The notation 12) stands for the number of bonded niobium atoms; links Nb06 octahedra with niobium atoms of Nb6012 clusters in the NbO ii) 0{2}“ block (‘a’ stands for ‘auDen’ (outer)); iii) O{3}’-aare shared between two NbO blocks (‘i’ stands for innen (inner)); iv) O{3}’-iare those bonded to three Nb atoms within the same NbO block; and finally v) O{4}i-iare those with a planar coordination of four Nb atoms, as in NbO.
A comparison of the short Nb-Nb interatomic distances in compounds with condensed clusters is given in Table 5. There seems to be a correlation between the number of valence electrons available per two-electron three-center bond and the average Nb-Nb bond length in the Nb6012 clusters (Fig. 21). The average Nb-Nb bond length is 287 pm in BaNb5Os with 1.37 electrons per three-center bond whereas dNb.Nb = 297 pm in NbO with 1.13 electrons. It must, however, be remembered that in all compounds there is a balance between A-0, Nb-0, 0-0, and Nb-Nb distances. The two types of niobium atom in the condensed clusters give rise to three kinds of Nb-Nb bonding; Nb{4}-Nb{4}, Nb{4}-Nb{5} and Nb{5}-Nb{5} and as a result the Nb6 octahedra are always distorted. The
1534
5 Solid-State Cluster Chemistry
1.8
-
1.6
-
U
c 0
2m --. a, L
1.4
a,
Q v)
C
g
+ V
1.2
a, W
1.o
-
discrete Nb6012clusters
+
U
*-.
dimeric Nbj 1 0 2 0 cluster
-
Ba2Nb509
NbO I
I
I
I
I
I
I
I
I
Average Nb-Nb distance Figure 21. Diagram showing the numbers of valence electrons available per 3c/2e bond and Nb-Nb interatomic distances (in A).
Nb{41-Nb{4} interatomic distances are typically longer than the Nb{4}-Nb{5} and Nbi5)-Nb{’} distances (Table 5). This is not surprising considering that the Nb14} atoms in the longer Nb{4}-Nb{4}bonds have a local environment similar to that in NbO and a smaller number of electrons is probably involved in the bonding. The inter-NbO-block distances, Nb{5}-Nb{5}in the idealized structures are so short that direct Nb-Nb bonding must be considered. In most of the compounds, however, dNb-Nb > 302 pm which is slightly too long for significant bonding. A simple indicator of an Nb-Nb bond is provided by the distances between the oxygen atoms ‘capping the bond’. These oxygen atoms will be shifted apart when Nb-Nb bonding occurs, resulting in a longer 0-0 distance.[81This trend is obvious for the low-temperature modification of NbOz. In the rutile type structure of LT-Nb02, chains of edge-sharing Nb06 octahedra with alternating long (336 pm) and short (269 pm) Nb-Nb distances are found.r621The 0-0 distances between oxygen atoms capping the latter bonded pair are longer (do-0 = 305 pm) than between those capping the non-bonded Nb2 pair, d ~ - 0= 275 pm. The only example for which the Nb-Nb distance between NbO blocks strongly indicates bonding is Sr4Nbl7O26, for which dNb-Nb = 295 pm. In this compound the distance between the oxygen atoms capping the bond is do-0 = 310 pm, which should be compared
Compound
295-298 293 295 294 297 298 298
285-298 287-293 289-295 290-294 290-299 298
289 29 1 292 295 298
-
290
287 289 290
285
297 29 1
293 296
293
-
-
293-297 291-296
296 293
-
286-288 284-291 280-296 2 82 293 284-287 286-292
304 302 302 295 302 307
286-288 284-300 281 -300 282-302 284-302 284-298
287 29 1 292 292 293 293
280-289
Nb[4!-Nb[sl
300 294-298 296-302 290-305 297-298
308
280-289
Nb[41-Nb[4]
285
Inter NbO block
Intra NbO block
Nb-Nb,,,
286 284 280 287 283 284
Nb151-Nb!51
5
42
64 41 68
65 41 69 50
37 38 43 57 57 39
36
Ref.
Table 5. Nb-Nb interatomic distances (in pm) in compounds with condensed clusters. Nbt4} denotes niobium coordinated with 4 oxygen atoms, Nb{s} denotes niobium coordinated with 5 oxygen atoms, and Nbi61 denotes niobium coordinated with 6 oxygen atoms.
(A
w
(A
1536
5 Solid-State Cluster Chemistry
with that found for the Nb2 pair in LT-Nb02 (do-0 = 305 pm).r621It can be assumed that the cluster compounds which are without bonding between NbO blocks are characterized by significantly shorter 0-0 distances, as in Ba4Nb14023 The weak inter(do-0 = 300(2) ~ r n ) [ ~ and ' ] BaNb5Og (do-0 = 295(1) actions between NbO blocks for Ba4Nb14023[~~] and BaNbsOg have also been confirmed by band-structure calculations.['] The NbO6 octahedra in the perovskite blocks are expanded parallel to the columns of infinitely condensed Nb6O12 clusters. In the compounds with 2D infinitely condensed clusters (slabs of NbO blocks) the NbO6 octahedra are expanded within the planes. This ubiquitous expansion (e.g. in Sr2Nb~09with d ~ b - 0= 192 and 207 pm)[641is because of an adjustment of the large NbO units compared with the smaller perovskite units. The average Nb-0 distances in the NbO6 octahedra in all compounds lie well within expectation for niobium having a valence between +4 and +5. The Nb atoms in the Nb06 octahedra are very important in regulating the valence electron concentration in the blocks/slabs of condensed Nb6012clusters.
5.3.3.4 Two pairs of compounds In this short review it is not the intention to discuss all compounds with condensed Nb6OI2clusters in detail, however, there are two pairs of compounds which we find particularly worthy of mention. The first pair is Ba3Nb16023[~~] and Ba4Nb14023[~~] the structures of which are inverse of each other. Two HREM images of these structures and the idealized structure models are shown in Fig. 22. Ba3Nb16023 and Ba4Nb14023 both crystallize in space group Cmmm with very similar unit cell parameters a = 2093 pm, b = 1248 pm, c = 416 pm and a = 2081 pm, b = 1245 pm, c = 415 pm, respectively. In Ba3Nb16023 there are [2 x 2 x m ] NbO columns and (1 x 3 x co} perovskite columns; these are reversed, [l x 3 x co]and (2 x 2 x m } , in Ba4Nb14023. The other interesting pair of compounds is SrNb406r501and ANb406 A = K,I6'] Ba.[40,411 The barium and potassium analogs have alternating single slabs of NbO ([l x m x co])and perovskite-type ({l x co x co}) while SrNb406 has double ones of both [2 x m x co] and {2 x GO x 00). It is obvious that the origin of the structural difference between BaNb406 and SrNb406 is the smaller ionic radius of Sr2+ ( r = 118 pm, C.N. = 6) compared to that of Ba2+ (r = 135pm, C.N. = 6) and Kf ( r = 138 pm, C.N. = 6).[661The size difference between the building blocks of perovskite and NbO is larger in Sr2Nb509 and SrNb406 than in the barium analog. The valence of the Nb{6} is +4 in SrNb03 with no vacancies. The existence of double layers in SrNb406 can therefore reduce the mismatch by means of charge transfer 6, (0 < 6 < I), between the Nb{6} atoms in the perovskite slab, and the niobium atoms in the NbO slab. This charge transfer increases the number of electrons available for bonding in the Nb6O12 clusters, which results in a shortening of the Nb-Nb bonds (see above). The Nb{6}sites should, therefore, have a higher ox-
5.3 Discrete and Condensed Clusters in Low Valent Niobium Oxides
1531
Figure 22. Projection of the structural models of (a) Ba3NbI6023 and (b) Ba4Nbl4023, with the corresponding HREM images (c) and (d).
1538
5 Solid-state Cluster Chemistry
Figure 22 (continued)
idation state in the strontium compounds than in the barium corn pound^.[^^^^^ An electron transfer from the Nb{6)atoms in the perovskite slab into Nb-Nb bonding states in the NbO slabs results in shorter Nb-Nb bonds, and it has also been observed for other pairs of compounds with and without NbO6 octahedra: Ba2Nb509 (dNb-Nb(ave) = 291 pm)r4'1-BaNb406 (dNbPNqave) = 293 pm)[411and Ba4Nb17026 (dNb-Nqave) = 292 pm)[431-Ba3Nb16023(dNb-Nqave) = 293 pm).[391
5.3.3.5 Attempts to change the properties by doping The structures of these intergrowth compounds are comparable to those found among the high T, superconductors. This suggests that the properties of the compounds discussed here can be tuned by suitable substitution, in particular, because superconductivity has been reported to occur in oxoniobates. There are essentially two possibilities, anion doping and cation doping. The replacement of oxygen by fluorine has so far only been reported for &A12Nb11021-~F~(x < 0.2)[361and KNbs014-,Fx (x < 1.O).[zO1 Bond-order summations indicate that the fluorine atoms enter the 0{2}apa positions. Both compounds (x = 0.0 and 0.2) are, however, semiconducting. There are no reports of anion substitution in compounds with infinitely condensed Nb6O12 clusters. For Ba2Nb509 two examples of heterovalent cation substitution known - Yr671and Ti,[681see Table 2. It is possible to replace 0.2 Ba by Y in Ba2-,YxNb509. The substitution of Ba ( r = 135 pm, C.N. = 6) by the smaller Y ( r = 90 pm, C.N. = 6) results in decreasing a and c axes, as expected.
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
1539
Raman studies have shown that Y enters the Ba position. The compound is metallic but non-superconducting for all compositions. Studies of the BazNb~-,Ti,09 system have shown the existence of a solid solution for 0 < x < 1.0.[681The substitution of Nb by the smaller Ti atoms results in a significant decrease of the a axis and a minor decrease of the c axis. The compounds for 0 < x < 0.75 are all temperature-independent paramagnetic and metallic. Ti doping gives the resistivity a more pronounced metallic behavior, but the change is not continuous with Ti concentration. Structural studies of Ba2Nb5_,TilO9 show that Ti enters the Nbf51 and Nbf6}positions with no preference. (A similar preference for the Nb{5} positions has been found in a single-crystal structural study of BaNb4-,Ti,Os, x = 0.53.[691)The preference for the Nbf6} position is easily understandable considering that both BaNb03 and BaTi03 have the perovskite type structure, and that there seems to be a solid solution between The preference of the Nb{’} position over the Nb{4}position in the NbO slab might also be explained by considering the structures of TiOc7l1and Nb0.[51NbO and ordered T i 0 both have rock-salt structures with 25% and 12.5% anion and cation vacancies, respectively. In NbO all atoms have square planar coordination by oxygen atoms, whereas in T i 0 20%)of the atoms have a square planar coordination and 80% are coordinated by five 0 atoms. Band-structure calculations have shown that the NbO structure is preferred for a metal with a formal d3 configuration, and T i 0 for a d2 configuration.[721A d’ Ti atom in Ba2Nb5_,Ti1O9 and BaNb4-,Tix06 then prefers the Nbi5} position with a fivefold coordination of oxygen atoms which occurs frequently in TiO, over the Nb{4} position with a planar fourfold coordination of oxygen atoms as in NbO. An alternative approach that might be used to explain the preference of the titanium atoms is the hard base character of oxygen.[731The harder acid titanium compared with niobium should prefer sites with higher 0coordination such as the Nb{5}and Nbf6}positions. The bond lengths decrease only slightly upon replacing niobium by titanium except for d( Nb{4}-Ti{5})which is 298 pm compared with d( Nb{4}-Nb{51)= 290 pm in BaNb4 .Ti,06.[691 The reason for this increase is probably a smaller number of bonding electrons in the Nb-Ti bond. The hope of finding superconducting transitions at low temperatures for the previously discussed compounds in the M-Nb-Ti-0 systems has not been fulfilled.
5.3.3.6 What is the valence of the niobium atoms?
Most reduced oxoniobates are mixed valence compounds containing niobium in various oxidation states. A direct way of determining the valences of the atoms is by photoelectron spectroscopy studies (Fig. 23). The binding energies of the electrons depend on the valences of the atoms. Studies of reduced oxoniobates containing Nb6012 clusters have been r e p ~ r t e d . ~ ~ ~ , ~ ~ , ~ ’ Photoemission studies of EuNb03, Euo.7NbO3, and Eu2Nb509 show the valence of europium in Eu2Nb509 and EuNb03 to be 2+. In E ~ o . 7 N b 0 3 [Eu ~ ~is] partly oxidized, which is in agreement with the slightly lower magnetic moment observed for
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5 Solid-State Cluster Chemistry
3
'....
i-
i
-
?
...-
..
.. 1
.
L
-
W
202 204 206 208 210 212 214
Figure 23. Niobium 3d photoemission spectra obtained with aphotonenergyof360eV for EuNb03, Euo.7NbO3, and EU2NbS09.
Euo.7Nb03, p,, = 7.0 B.M. (EuNb03 has peK= 7.4 B.M.). The valences of the niobium atoms in the these compounds were determined by measuring the binding energies of the 3d levels, (3d5/2,3d3j2 doublet), using a photon energy of 360 eV. The corresponding spectra for the three oxides EuNb03, Euo.7NbO3 and EuzNb509 are shown in Fig. 23. The 3d doublet is shifted to lower binding energy when the oxidation states decrease. A collection of selected experimental data from compounds with various valences of the niobium atoms is given in Table 6. It should be mentioned that the experimental values are very sensitive to surface effects, especially contamination. In EuNb03 and E ~ . 7 N b 0 3only one 3d5/2, 3d3p doublet for the Nb 3d core level was found, showing that the valence of the niobium is the same in each compound. The 3d5/2 peak at 209.7 eV in EuNb03 can be assigned Nb4+ nominal valence and the doublet in Euo.7Nb03 found 0.5 eV higher can be associated with a Nb4+& chemical state. In the spectra of Eu2Nb509 two 3d doublets were observed corre-
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
1541
Table 6. Niobium 3d3/2 binding energy of different oxoniobates. Compound
3d3/2(eV)
Suggested valences of the Nb-atoms
Ref.
206.5 208.4 210.3 211.7 211.7? 210.2 209.7 209.1 206.9, 210.1 205.0, 206.0, 209.2
0 2+ 4+ 5+ 5+ 4+s 4+ 4+ 2+, 4 + 6 2+, 3+, 4+
16, 71 76, 71 16, 17 16,71 78 14 14 68 14 68
sponding to two distinct types of Nb atom in this compound. One of the doublets has a bonding energy of 210.1 eV which is nearly the same as for Euo7Nb03. This originates from Nbi6} in the perovskite slab assigned a valence 4 6. The stronger peak at 206.9 eV is assigned to the Nb atoms in the Nb6O12 clusters. Comparison of the values given in Table 6 suggests that the niobium atoms in the NbO slabs have a valence close to 2+. (Although the actual value for d3I2is close to that for Nb(s), a valence below 2+ is not reasonable for Eu2Nb509.). The Nb{4} and Nb{j} in the NbO slab should have slightly different valences, the former being closer to two and the latter to three. This was found in an ESCA study of B a ~ N b j 0 9 . [Although ~~] the photoemission spectrum for niobium in that study was very similar to that of Eu~Nb509discussed above, deconvolution of the spectrum into three double peaks suggests Nb{4}to be 2+, Nb{5}to be 3+ and Nb{6}to be 4+. The exact valences of the niobium atoms in Eu2Nbj09 and Ba2Nb509 cannot be obtained because of systematic errors. General results from these studies are, however, in good agreement with those found by other methods and obtained by LMTO calculations indicating the valence of the Nb{6}to be between +4 and +5.r8,741 Photoemission studies on BaNbsOs, BaNb406, Ba2Nb509, Li,NbOz (x = 0.97, 0.91) have also been reported by Cherkashenko et ul. [751 in a paper discussing the electronic structure of the low-temperature superconductor Li,Nb02 (x = 0.97, 0.91). No valences of the Nb atoms were reported, however.
+
5.3.3 Oxotantalates containing discrete Ta6012 clusters The similar chemistry of Hf and Zr and also of Nb and Ta is well-known, and it seemed promising to look for oxotantalates, which are analogs of the oxoniobates
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5 Solid-state Cluster Chemistry
containing Nb6012 clusters discussed above. The stability of high oxidation states of the transition elements increases with increasing row number from 3d over 4d to 5d, however. This is especially reflected by the chemistry of oxides containing lowvalent transition metals. As an example, many reduced oxomolybdates containing the Mo6012 clusters are known, whereas Snl0W16044[~~] is the only oxide with W6O12 clusters. Also, in Zn2Mo308 only two of the three Mo atoms can be replaced by W to form Z ~ ~ M O W ~and O a~ hypothetical [ ~ ~ ] 'Zn2W308' disproportionates into W and 2ZnW04 during high-temperature syntheses. This shows that the highly oxidized tungstates are more stable than the comparable oxomolybdates. A similar situation is found for the neighboring elements Nb and Ta. Binary reduced Ta oxides are unknown, in contrast to the situation in the system Nb-0, where one finds NbO2 and NbO in addition to Nb2O5 and numerous slightly reduced phases of the last compound.["] Many examples are known of oxoniobates containing Nb atoms in an oxidation state between +4 and +5[82-901which are all built from NbO6 octahedra. Only recently, a small number of reduced complex phases with Ta atoms in oxidation states less than +5 have been found. The dark bluish compound CaTa206Fr9'] which crystallizes in the pyrochlore structure, has been synthesized by reduction of Ta2O5 by Ta in the presence of CaF2 at 1650 "C. Although an oxidation state +4 has been assigned to the Ta atoms, there are good reasons to doubt this, because the Ta atoms are octahedrally coordinated by 0 atoms with Ta-0 distances of 197.1 pm (6x), which are not significantly longer than are normally found for Ta5+.It would, therefore, be better to describe the compound by the formula CaTa2O7_,FX,especially as the F content has not been determined by chemical analysis or by any other method. The new compound Sr3-,Ta5015 (x = 0.16) crystallizes in the tetragonal tungsten bronze-type structure (TTB)[84,851 and contains Ta atoms in oxidation state $4.88. The average Ta-0 distances within the TaO6 octahedra are 194 pm. Direct Ta-Ta bonding cannot be expected in CaTa2O7-,FX, and S1--~Ta~015, because there is only corner sharing of the TaO6 octahedra. Two new phases containing Ta6012 clusters - Ba2Ta15032[~'] and AA12Ta350d921(A = Na, K, Rb) - have been obtained by high-temperature syntheses (1650 "C, Ta ampoule), see Table 7. The oxides are isotypic with the corresponding niobate cluster compounds, Ba2Nb15032"~Iand Rb4Si2Nb35070,['~]respectively. In these tantalates 0 atoms, Ta6 octahedra and large A atoms (A = Na,
Table 7. Reduced oxoniobates containing discrete Ta6012 clusters. Compound
Space group
U
[pml
b [pml
c
[pml
n
Ref.
5.3 Discrete and Condensed Clusters in Low, Vulent Niobium Oxides
1543
K, Rb, Ba) form a conventional close packing by analogy to the niobates discussed above. The intra-cluster Ta-Ta distances fall within the range 276-280 pm and are shorter than the corresponding Nb-Nb distances indicating a tendency towards stronger metal- metal bonding of Ta compared to Nb. The Ta-0 distances from the Ta cluster atoms to the 0" and 0' atoms are similar to the Nb-0 distances in the oxoniobates. Additional Ta atoms, which are not part of the clusters, occupy octahedral voids within the 0 sublattices. The average Ta-0 distances within these TaO6 octahedra are 194 pm and the assumption of an oxidation state t 5 for these atoms seems reasonable. The tendency towards higher oxidation states for Ta than for Nb becomes obvious when comparing the electronic situation of the isotypic compounds NaAl~Ta35070 and Rb4Si2Nb35070. In NaA12Ta3507" three electrons for M-M bonding states are provided by the A1 atoms instead of the four for Si in Rb4Si2Nb35070. Furthermore, because of the cation deficiency in cubeoctahedral voids (only 1 Na instead of 4 Rb) even fewer electrons are present in the tantalate. Altogether there are five fewer electrons in the Ta-0 framework in NaAIzTa35070 than in the Nb-0 framework in Rb4Si2Nb35070.As a result of this the Ta-Ta distances within the Ta3013 cluster in NaAl2Ta35070 are rather long (320 and 335 pm, respectively) and the oxidation state f 5 must be assigned to these Ta atoms. Therefore, excess valence electrons in NaA12Ta35070 are only localized in the Ta-Ta bonding states of the Ta6Ol2 clusters. That there are such differences between the structural chemistry of reduced Nb and Ta oxides is slightly surprising and two basic questions arise. Firstly, is there also a chemistry of condensed Ta6O12 clusters as found for reduced oxoniobates and, secondly, how do the physical properties of such tantalum compounds compare with those of the niobates? Future studies are required to answer this question.
5.3.4 Reduced oxoniobates containing NbzOS clusters An Nb2Og cluster was discovered in the structure of NaNb306-,F, (0 < x < 0.5),[931(Fig. 24). Later it was also found in the structures of the isotypic Ca compounds, Cao.75Nb306[941 and Cao.95Nb306.r951 A comparable unit [Re208]"is known from the crystal structures of reduced rare earth rhenium oxides La4Re2010[~~] and La4Re6019,[971which is an analog of the well-known species [Re2Clg]"-.r981 It is remarkable that the synthesis of the pure oxide NaNb306 is not straightforward and cannot be performed by the so-called 'shake and bake' reactions, as has been found for syntheses of oxoniobates containing the Nb6Ol~cluster. Below 1050 "C a mixture of NaNbO3, NbO and NbO2 does not react. Above 1050 "C the
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5 Solid-state Cluster Chemistry
Figure 24. Projection of an Nb208 cluster in NaNbsOs and related compounds together with the surrounding Nb atoms and a coordination polyhedron for one of these atoms. The broad line corresponds to Nb-Nb multiple bond, dotted lines indicate weak Nb-Nb bonding.
formation of Nag is observed; this results in a product consisting of known binary Nb oxides. Only on addition of mineralizers or fluxes, such as NaF or Na2B407, does the above mentioned mixture react at 850 "C with the formation of the reduced complex salt NaNb306. Partial replacement of oxygen by fluorine or the inclusion of boron can be excluded according to chemical analyses. Systematic investigation of the formation of Cao.75Nb306 has shown that this compound can be synthesized by heating an appropriate mixture of CaNbzO6 and Nb in a small temperature range in different container materials, e. g. corundum crucibles, Nb ampoules, or quartz glass tubes.[991Below 1300 "C no reaction is observed and above 1500 "C the product obtained is a mixture of Ca0.9Nb03,[1001NbOz and NbO. Obviously, Ca0.75Nb306 is thermodynamically unstable at temperatures higher than approximately 1800 K and can only be grown as single crystals below this temperature in the presence of appropriate fluxes. The principle of the crystal structure of NaNb306 is illustrated by Fig. 25. Chains of trans-edge-sharing NbO6 octahedra, similar to the Ti06 octahedra in rutile, are connected via free corners to undulated sheets. Between these sheets rows of Nb( 1) and Na atoms are embedded. Along the c direction the Nb atoms are painvise connected with short distances of 262 pm, whereas the Na atom chains are equidistant. Together with the surrounding 0 atoms the Nb(1) atoms form a Nb208 cluster, which is surrounded by four additional octahedrally coordinated Nb atoms (Fig. 24). In NaNb3Oh the Nb-Nb distances within the dumbbells of Nb(1) atoms are 262 pm, and they are 308 pm between atoms Nb(1) and Nb(2). The corresponding distances in the isotypic Ca compounds are slightly shorter. Analyses according to the bond-lengths bond-strengths concept result in an oxidation state +4 for Nb(2) and +3 for Nb(1) in NaNb306. By analogy with molecular species, such as M02(OR)6 ( R = CzH5) a Nb-Nb double bond must be assumed for the Nb-Nb dumbbell. An extensive discussion by Calhorda[lO'] of the bonding in NaNb306 on the basis of extended Huckel calculations verifies these conclusions that the Nb-Nb bond within the dumbbell can be considered as a double bond and some additional
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
1545
Nb, dumbbells f
Figure 25. Projection of the crystal structure of NaNb3Os along [ IOO]. Undulated sheets of NbOh octahedra are drawn together, with N a atoms and Nbl dumbbells in between.
Na
Nb-Nb bonding to the four 'side-on' coordinated Nb atoms is present, leading to an extended metal-metal bonded 3D network.
5.3.6 Superconductivity in low-valent oxoniobates The discovery of the Cu-based high temperature superconductors initiated the search for new superconducting materials, especially oxide systems. The idea that superconductivity is related to pairwise coupling of some of the conduction electrons, i. e. to a tendency for dynamic electron-pair formation,['021has motivated the investigation of reduced metallic oxoniobates. Transitions from a delocalized to a localized electronic system are well-known from binary and ternary niobium oxides, e.y. for the transition of HT-Nb02r'031to LT-Nb02.[62.'041 In the HT-form the chains of edge-sharing Nb06 octahedra are undistorted, the electrons are delocalized and the compound is metallic. In the LT-form alternate joined edges are lengthened and the Nb atoms form pairs with short Nb-Nb distances (274 pm) corresponding to Nb-Nb single bonds, as mentioned earlier. A similar situation is found for LiVO2; this undergoes a phase transition forming V triangles with short V-V distances. Such a transition is not observed for the corresponding reduced
1546
5 Solid-state Cluster Chemistry
A Nb
A Li
B 0
B
Figure 26. Projection of the crystal structure of LiNbOz along [ 1001. Nb06 octahedra are drawn together with Li atoms (small black circles).
nonmetallic oxides ANb02 (A = Li, Na)[10s91n51 which adopt a filled MoS2 structure in which the A atoms occupy octahedrally coordinated sites between trigonal prismatic NbO2 layers (Fig. 26). Nevertheless, the short Nb-Nb distances (LiNbOz 290 pm, NaNbO2 295 pm) give evidence of M-M bonding, which has been confirmed by extended Hiickel calculations.['n71 Deintercalation of LiNb02 to Li,NbO;? is possible to a value of x z 0.45, and such a phase is superconducting at temperatures below 11 K.['"] This property seems closely related to a tendency towards structural instabilities, i.e. charge density waves which, if they became static, would result in cluster formation. Such a locking-in of charge density waves causing the formation of Nb7 clusters with ordered vacancies of Li and Na, respectively, and complicated superstructures is shown by electron diffraction studies on both Li,NbOz and NaxNb02.['091 So far, superconductivity in reduced oxoniobates has been confirmed only for the two examples mentioned above and superconductivity below 12 K has been claimed for one other reduced oxoniobate.r"O1 Transition to superconductivity below 8 K and 6 K has been found in phases with the composition Baz-,La,Nb509 (0.2 < x < 0.5),['"] although the transition at 8 K probably originates from NbO, (0.0 < x < 0.125).[1121 The following example indicates that the formation of NbN, NbC or related mixed phases is a general problem, if syntheses of oxoniobate samples are not performed with careful exclusion of N and C. Heating of a mixture of appropriate amounts of Nb and Ba02 to cu. 500 "C in air (or a mixture of oxygen and nitrogen) for a short time leads to powder samples of nominal composition Reinvestigation has, Ba,NbOz-s which is superconductive below 23 K however, shown that the occurence of superconductivity in such samples is observed at temperatures below 20 K and originates from a minority phase. According to results from chemical and X-ray analysis the superconducting phase belongs to the system NbO,NI-, and is not a ternary o ~ o n i o b a t e . [ " ~ ~ The structure of the low-temperature modification of BaNbzOh should be ideal for the study of changes of electrical properties as a function of the extent of re-
5.3 Discrete and Condensed Clusters in Low Vulent Niobium Oxides
1547
Figure 27. Projection of the crystal structure of LTBaNbzOb along [ 1001. Nb06 octahedra are drawn together with Ba atoms (large circles).
duction, because it contains 'preformed pairs' of NbO6 octahedra (Fig. 27). Attempts to reduce BaNbzOh with hydrogen should lead to a compound BaNb206-, which contains an appropriate amount of Nb4+ which then should pair uiu metalmetal bonding," 1 5 ] as is found in LT-Nb02. Experiments were, however, unsuccessful. Because of the volatility of Ba(OH)2 loss of Ba is observed at temperatures higher than 700 "C. Below that temperature no reaction occurs, and Ba3-xNb5015 (x > 0 5)"" with the TTB-type structure["61 is formed; this has a Ba/Nb/O ratio similar to that of BaNbzO6. Partial replacement of Ba by La affording Bal-,LaYNb206 to generate Nb4+ failed, because the smaller La also induced the formation of the TTB-type structure and black powder samples of Ba2-,LaANb5015 were obtained. According to recent investigations the tetragonal tungsten bronzetype compound BasNblo030 is semiconducting. Partial replacement of Ba in Ba3Nbs015 by Ln (Ln = La, Ce, Nd) leads to a series of metallic niobates Ba2-,LnYNbsO15,['1 7 , 1 which are metallic for x = 1 , 2 , 3 but do not become superconducting down to 2 K. This is also true for the reduced niobates Srl-,Ln,NbzO6, Ln = La, Nd,[l19] which are isotypic with the high-temperature modification of BaNbzO6, the perovskite variants K1-,BaYNb03 (0.2 5 x 5 0 . 5 ) and KO5Sro sNb03,[1201 and CaLnNb207['211Ln = Y, Nd with a pyrochlore structure.
Acknowledgments This work was supported financially by the Swedish Natural Science Research Council.
1548
5 Solid-state Cluster Chemistry
References [ l ] A. Simon, Angew. Chem. 1981, 93, 23; Angew. Chem. Int. Ed. Engl. 1981, 20, 1. [2] R. E. McCarley, K.-H. Lii, P. A. Edwards, L. F. Brough, J. Solid State Chem. 1985,57, 17. (31 J. D. Corbett. R. E. McCarley, in Crystal Chemistry and Properties of Materials with QuasiOne-Dimensional Structures (Ed: J. Rouxel), Reidel, Dordrecht, The Netherlands, 1986, 179. [4] H. Mattausch, A. Simon, E.-M. Peters, Inorg. Chem. 1986, 25, 3428. [5] G. Brauer, Z. Anorg. Allg. Chem. 1941,248, 1. [6] H. Schafer, H. G. Schnering, Angew. Chem. 1964, 76, 833. [7] D. M. Evans, L. Katz, J. Solid State Chem. 1973, 6, 459. [8] J. Kohler,G. Svensson. A. Simon, Angew. Chem., 1992, 104,1463; Angew. Chem. Int. Ed. Engl., 1992, 31, 1437. [9] R. Burnus, J. Kohler, A. Simon, Z. Naturforsch. B 1987, 426, 536. [lo] J. Kohler, A. Simon, Z. Anorg. Allg. Chem., 1987, 553, 106. [l I] J. Kohler, A. Simon, S. J. Hibble, A. K. Cheetham, J. Less-Common Met. 1988, 142, 123. [12] J. Kohler, A. Simon, Z. Anorg. Ally. Chem. 1989, 572, 7. [13] S. J. Hibble, A. K. Cheetham, J. Kohler, A. Simon, J. Less-CommonMet. 1989, 154, 271. [I41 G. Svensson, J. Kohler, A. Simon, J. Alloys Comp. 1991, 176, 123. [I51 B. Hesseu, S. A. Sunshine, T. Siegrist, G. Waszcak, A. T. Fiory, Chem. Materials 1991, 3, 528. [16] R. Tischtau, Dissertation, Stuttgart 1991. [17] J. Kohler, R. Tischtau, A. Simon, J. Alloys Compounds, 1992, 182, 343. [18] M. J. Geselbracht, A. M. Stacy, J. Solid State Chem. 1994, 110, 1. [I91 K. B. Kersting, W. Jeitschko, J. Solid State Chem. 1991, 93, 350. [20] J. Kohler, R. Tischtau, A. Simon, J. Chem. Soc., Dalton Trans. 1991, 829. [21] V. G. Zubkov, A. P. Tyutyunnik, V. A. Pereliaev, G. P. Shveikin, J. Kohler, R. K. Kremer, A. Simon, G. Svensson, J. Alloys Comp., 1995, 226, 24. [22] J. Xu, T. Emge, M. Greenblatt , J. Solid State Chem. 1996, 123, 21. [23] R. Hoffmann, J. Chem. Phys. 1963,39, 1397. [24] F. A. Cotton, T. E. Haas, Inorg. Chem. 1964, 3, 10. [25] F. A. Kettle, Theoret. Chim. Acta 1965, 3, 211. [26] B. Spreckelmeyer, H. G. von Schnering, Z. Anorg. Allg. Chem. 1971, 386, 27. [27] B. 0. Marinder, Chem. Scriptn, 1977, 11, 97. [28] J. Kohler, G. Miller, A. Simon, Z. Anorg. Allg. Chem. 1989, 568, 8. [29] W. H. J. Watson, J. Waser, Acta Crystallogr. 1958, / I , 689. [30] G. J. Wessel, W. J. D. Ijdo, Acta Crystallogr. 1957, 10, 466. [31] W. H. McCaroll, L. Katz, R. Ward, J. Am. Chem. SOC. 1957, 79, 5410. [32] G. B. Ansell, L. Katz, Acta Crystallogr. 1966, 21, 482. [33] F. A. Cotton, fnorg. Chem. 1964, 3, 1217. [34] F. A. Cotton, X. Feng, fnorg. Chem. 1991, 30, 3666. [35] C. C. Torardi, R. E. McCarley, fnorg. Chem. 1985, 24, 476. [36] J. Kohler, A. Simon, R. Tischtau, G. Miller, Angew. Chem. 1989, 101, 1695; Angew. Chem. fnt. Ed. Engl. 1989,28, 1662. [37] V. G. Zubkov, V. A. Pereleyaev, I. F. Berger, I. A. Kontsevaya, 0. B. Makarova, S. A. Turshevskii, V. A. Gubanov, V. I. Voronin, A. V. Mirmelstein, A. E. Karkin, Sverkhprovodimost. Fiz. Khim. Tekh. 1990, 3, 2121. [38] G. Svensson, J. Grins, Acta Crystallogr. 1993, B49, 626. [39] V. G. Zubkov, V. A. Pereleyaev, A. P. Tyutyunnik, I. A. Kontsevaya, V. I. Voronin and G. Svensson. J. Alloys Comp. 1994,203, 209. [40] V. G. Zubkov, V. A. Pereleyaev, I. F. Berger, V. I. Voronin, I. A. Kontsevaya, G. P. Shveikin, Dokl. Akad. Nauk. SSSR. 1990, 312, 615.
5.3 Discrete und Condensed Clusters in Low Vulent Niobium Oxides
1549
[41] G. Svensson, J. Kohler, A. Simon, J. Alloys Comp. 1991, 176, 123. [42] G. Svensson, J. Kohler, A. Simon, Angew. Chem. 1992, 104, 192; Angew. Chem. Int. Ed. Engl. 1992, 31, 212. [43] V. G. Zubkov, V. A. Pereleyaev, I. A. Kontsevaya, A. P. Tyutyunnik, 0. V. Makarova, G. P. Shveikin, Sou. Phys. Dokl. 1992, 37, 386. [44] W. Meissner, H. Franz, H. Westerhoff, Ann. Phys. 1933, 17, 593. [45] L. Bowman, T. C. Wallace, J. L. Yarnell, R. G. Wenzel, Acta Crystallogr. 1966, 21, 843. [46] R. R. Kreizer and R. Ward, J. Solid State Chem., 1970, I , 368. [47] S. Hessen, A. Sunshine, T. Siegrist and R. Jimenez, Muter. Res. Bull., 1991, 26, 85. [48] G. Svensson, P.-E. Werner, Muter. Res. Bull., 1990, 25, 9. [49] G. Svensson, Microsc. Microanal. Microstruct. 1990, I , 343. [50] G. Svensson and L. Eriksson, J. Solid State Chem. 1995, 114, 301. [51] C. F. Michelson, P. E. Rauch, F. J. Di Salvo, Mat. Res. Bull. 1990, 25, 971. [52] V. G. Zubkov, V. A. Perelyayev, D. G. Kellermann, V. E. Stapsev, V. P. Dyakina, A. Kontzevaya, 0. V. Makarova, G. P. Sveikin, Dokl. Akad. Nauk. SSSR, 1990,313, 367. [53] Left of the line BazNbsOs, NbO and Nb coexist. [54] L. Kihlborg, Prog. Solid. State Chem. 1990, 20, 101. [55] A. MagnCli, Chem. Scripta, 1986, 26, 535. [56] A. Magneli, Microsr. Microanal. Microstruct. 1990, I , 1. [57] V. G. Zubkov, V. A. Pereleyaev, A. P. Tyutyunnik, J. Kohler, A. Simon and G. Svensson, J. Alloys Comp. 1997, 256, 129. [58] H. Friedrich, Diplomarbeit, Stuttgart University 1997. [59] D. Ridgley, A. Ward, J. Am. Chem. Soc. 1955, 81, 6132. [60] K. Ishikawa, G. Adachi, I. Shiokawa, Mat. Res. Bull. 1983, 18, 653. [61] L. Katz, H. D. Megaw, Acta Crystullogr., 1967,22, 639. [62] H.-J. Schweitzer, R. Gruehn, Z. Naturforsch. 1982, B37, 1361. [63] J. Kohler, G. Svensson, unpublished results. [64] G. Svensson, J. Kohler, A. Simon, Acta Chem. Scand. 1992, 46, 244. [65] G . Svensson, G. Svensson, J. Solid State Chem. 1991, 90, 249. [66] R. D. Shannon, Acta Crystallogr. 1976, A32, 751. [67] H.-R. Lee, S.-J. Kim, I.-S. Yang, J.-H. Choy, J. Solid State Chem., 1994, 108, 253. [68] G. Svensson, L. Eriksson, C. Olofsson and W. Holm. J. All. Comp. 1997, 248, 33. [69] G. Svensson, L. Eriksson, Acta Crystallogr., 1999, C55, 17. [70] J. F. Marucco, M. Ocio, A. Forget, D. Coulson, J. Alloys Comp. 1997, 26, 454. [71] D. Watanabe, J. R. Castles, A. Jostsons and A. S. Malin, Acta Crystallogr., 1967, 23, 307. [72] J. K. Burdett and T. Hughbanks, J. Am. Chem. SOC.1984,106, 3101. [73] R. G. Pearson, Inorg. Chem., 1988, 27, 734. [74] C. Felser, J. Kohler. A. Simon, 0. Jepsen, G. Svensson, S. Cramm and W. Eberhardt, Phys. Rev. 1998, 57, 1510. [75] V. M. Cherkashenko, M. A. Korotin, V. I. Anizimov, V. V. Shumilov, V. R. Galakhov, D. G. Kellerman, V. G. Zubkov and E. Z. Kurmaev, Z. Phys. 1994, B 93,417. [76] M. Grudner and J. Halbritter. J. Appl. Phys., 1980, 51, 397. [77] M. K. Bahl, J. Phys. Chem. Solids 1975, 36, 485. [78] K. Szot, J. Keppels, W. Spicer, K. Besocke, M. Teske and W. Eberhardt, Surface Sci. 1993, 280, [79] S. J. Hibble, S. A. McGrellis, J. Chem. Soc., Dalton Trans. 1995, 1947. [80] S. J. Hibble, I. A. Fawcett, J. Chem. Soc., Dalton Trans. 1995, 2555. [81] A. Hibst, R. Gruehn, Angew. Chrm. 1981, 93, 23; Angew,. Chem. Znt. Ed. Engl. 1981, 20, 1. [82] R. Kreizer, A. Ward, J. Solid State Chem. 1970, I , 368. [83] D. Ridgeley, A. Ward, J. Am. Chem. Svc. 1955, 77; 6132. [84] F. Galasso, L. Katz, A. Ward. J. Am. Chem. Soc. 1959, 81, 5898. [85] A. Feltz, H. Langbein, Z. Anorg. Ally. Chem. 1976, 425, 47.
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5 Solid-state Cluster Chemistry
[86] D. Groult, J. M. Chailleux, J. Choisnet, B. Raveau, J. Solid State Chem. 1976, 19, 235; [87] J. M. Chailleux, D. Groult, C. Mercy, F. Studer, B. Raveau, J. Solid State Chem. 1981, 37, 122 [SS] H. Schafer, R. Gruehn, F. Schutte, Angew. Chem. 1966, 78, 28. Angew. Chem. Znt. Ed. Engl. 1966, 5,40 [89] K. Naito, N. Kamageshira, N. Sasaki, J. Solid State Chem. 1980, 35, 305. [90] H. Hibst, R. Gruehn, Z. Anorg. Ally. Chem. 1978, 442, 49. [91] T. Siegrist, R. J. Cava, J. J. Krajewski, Mat. Res. Bull. 1997, 32, 881. [92] B. Harbrecht, V. Wagner, A. Ritter, Z. Anorg. Allg. Chem. 1998, 624, 457. [93] J. Kohler, A. Simon, Angew. Chem. 1986, 98, 1011; Angew. Chem. Znt. Ed, Engl. 1986, 25, 996. [94] S. J. Hibble, A. K. Cheetham, D. E. Cox, Znorg. Chem. 1987, 26, 2389. [95] P. Alemany, V. G. Zubkov, S. Alvarez, V. P. Zhubkov, V. A. Perelyayev, I. Kontsevaya, A. Tyutyunnik, J. Solid State Chem. 1970, 1, 368. [96] K. Waltersson, Acta Cryst. 1976, B 32, 1485. [97] J. M. Longo, A. W. Sleight, Znory. Chem. 1968, 7, 108. [98] F. A. Cotton, Chem. SOC.Rev. 1975, 4, 27. [99] H. Friedrich, J. Kohler, A. Simon, unpublished results. [loo] J. Lamure, J. L. Colas, Compt. Rend. 1970,270 C, 700. [loll M. J. Calhorda, R. Hoffmann, J. Am. Chem. SOC.1988, 110, 8376. [lo21 A. Simon, Angew. Chem. 1997,109, 1872; Angew. Chem. Int. Ed. Engl. 1997,36, 1788. [lo31 B. 0. Marinder, Ark. Kem. 1962, 19, 435. [I041 A. K. Cheetham, C. N. R. Rao, Acta Crystallogr. Sect. B 1976, 32, 1579. [I051 G. Meyer, R. Hoppe, J. Less-CommonMet. 1976, 46, 55. [lo61 G. Meyer, R. Hoppe, Z. Anorg. Allg. Chem. 1976,424, 128. [I071 J. K. Burdett, T. Hughbanks, Znorg. Chem. 1985,24, 1741. [I081 M. J. Geselbracht, A. M. Stacy, Znorg. Synth. 1995, 30, 222. [I091 A. P. Tyutyunnik, V. G. Zubkov, D. G. Kellermann, V. A. Perelyayev, A. E. Kar’kin, G. Svensson, J. Solid State Chem. 1996, 331, 53. [I101 J. Akimitsu, J. Amano, H. Sawa, 0. Nagase, K. Gyoda, M. Kogai, Jap. J. Appl. Phys. 1991, 30, 1 [ I 111 J. Long, J. Kohler, G. Svensson, R. K. Kremer, A. Simon, unpublished. [I121 C. D. Wiseman, J. Appl. Phys. 1966, 37, 3599. [I 131 V. A. Gasparov, G. K. Strukova, S. S. Khasanov, Pis’ma Zh. Teor. Fiz. 60, 1994, 6, 425. [I141 P. E. Bacon, J. G. Hou, A. W. Sleight, J. Solid State Chem. 1995, 119, 207. [ 1151 M. J. Calhorda, J. Kohler, A. Simon, unpublished results. [116] A. Magneli, Arkiv Kemi, 1949, I , 215. [I 171 Y. K. Hwang, Y.-U. Kwon, Mat. Res. Bull. 1997, 32, 1495. [ 1181 0. G. D’yachenko, S. Ya. Istomin, M. M. Fedotov, E. V. Anipov, G. Svensson, M. Nygren, W. Holm, Mat. Res. Bull., 1997, 32, 409. [ 1191 S. Ya. Istomin, 0. G. D’yachenko, E. V. Antipov, G. Svensson, M. Nygren, Mat. Res. Bull., 1994, 29, 743. [I201 E. M. Kopnin, S. Ya. Istomin, 0. G. D’yachenko, E. V. Antipov, P. Bordet, J. J. Capponi, C. Chaillout, M. Marezio, S. de Brion, B. Souletie, Mat. Res. Bull., 1995, 30, 1379. [I211 S. Ya. Istomin, 0. G. D’yachenko, E. V. Antipov, G. Svensson, Mat. Res. Bull., 1997, 32, 421.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.4 Cluster Compounds Containing MixedCharge Cluster Units: [M6X12I2+ and [M6X12I4+or [MgX12l2+and [M06C1g]4+ (M = Nb, Ta; X = C1, Br) Neuenka Brnic'evii.
5.4.1 Introduction The solid state chemistry of the M6 (M = Zr, Nb, Ta, Mo, W ) halide clusters has been developed further than their solution chemistry. This is certainly because of the method of synthesis of these clusters, because they are always prepared by redox reactions at high temperatures. The products of these reactions are usually watersoluble discrete clusters, although occasionally polymeric species with Xa and/or X' bridging halide ligands"] (Fig. l ) , which are less soluble in common solvents, are formed. The hexanuclear halide clusters of niobium and tantalum are usually prepared as M ; [ ( M ~ X I ~ ) X (M' ~ ]= alkali These compounds are readily soluble in water from which, in the presence of mineral acids, octahydrates of composition [( M6X12)X2(H20)4].4H20 are precipitated. In the solution chemistry of Nb6 and Ta6 halide clusters, octahydrates are usually used as the starting materials. Whereas the bromo clusters [(M6Br12)Br2(Hz0)4].4H20 are soluble in water, methanol or ethanol, the chloro derivatives are soluble in methanol or ethanol only. The [ ( M ~ X I ~ H20)4].4H20 )X~( compounds are very stable and attempts to obtain the anhydrous species from octahydrates (e.g. thermogravimetric analysis) have resulted in the decomposition of the cluster units and in the formation of the corresponding pentoxides, M205. This has limited the development of the solution chemistry of these clusters in non-aqueous solvents. As a consequence, the cluster m e t h o ~ i d e s ~and ~ ~the ' ~ compounds with nitrogen-donor ligands[*]of Nb6 and Tab halide clusters have been prepared much more recently than the M06 analogs.['-' The [ M ~ X I ~(]M~ = ' Nb, Ta; X = C1, Br) units with n = 2, 3, 4 are known to have 16, 15, or 14 electrons, respectively, available for metal-metal bonding. The species with n = 3 are paramagnetic and can have interesting magnetic behavior.['21 Cyclic oxidation-reduction processes occur in solution in which the hexanuclear cluster units are retained. This is the background to our research efforts in the development of Nb6 and Ta6 halide cluster chemistry in solution.
1552
5 Solid-state Cluster Chemistry
Xa I
*
Figure 1. Schematic diagram of [(M6X'12)Xahln clusters. The notation is as given by Schafer and von Schnering."]
5.4.2 Hexanuclear [M6Xl2ln+(M = Nb, Ta; X = C1, Br; n=2, 3, 4) cluster units and reactions in non-aqueous solvents 5.4.2.1 Behavior of
[M6X12l2+ in
methanol
Upon dissolution of [(M6X12)X2(H20)4].4H20 in aliphatic alcohols, two main reactions occur: i) dissociation of the Xa ions and the exchange of their coordination sites by aliphatic alcohol molecules, and ii) substitution of the four-coordinated water molecules by aliphatic alcohols.
These two processes occur simultaneously and are time-dependent. Specific conductance (Asp) data and halogen NMR studies are indicative of loss of two Bra ions in [(M6Brl2)Br2(H20)4].4H20faster than two C1" ions in the chloro anal o g ~ . " ~In] a solution mol dm-3) of [(M&112)C12(H20)4].4H20in methanol at 25 k 0.01 "C, As,, changes continuously with an equilibrium not being established even after a few weeks. The values of Asp obtained for [ ( T ~ ~ B I - ~ ~ ) B I - ~ ( H ~ O ) ~ I . ~ and [(Nb6Br12)Br2(H20)4].4H20 in the same solvent are considerably higher than those for the chloro clusters and change little with time. This behavior is certainly connected with the complete dissociation of two Bra ions in bromo clusters
5.4 Cluster Compounds Containing Mixed-Charge Cluster Units
1553
compared with the incomplete and slow dissociation of two C1" ions in [(M6C112)C12(H20)4].4H20. Further, these data are in agreement with the amount of free Br- and C1- found in methanolic solution and determined by 35Cl and *'Br NMR spectroscopy. The observed value of 2.02 mol free Br- mol-' [ (Ta6Br12)Br2(H20)4].4HzO in methanolic solution indicates complete dissociation of the two Bra ligands. The quantities of dissociated C1- mol-' cluster, 0.92 and 0.80 for [(Nb6C112)C12( H20)4].4H20 and [(Ta6C112)C12(H20)4].4H20, respectively, from the solutions set aside for three days, indicate that less than one C1- is dissociated in methanolic solutions and the remainder is coordinated to the cluster unit. The most illustrative picture of what is happening in methanolic solutions of [(TasBrl2)Br2(H20)4].4H20 and [(Ta6C112)C12(H20)4].4H20 is obtained from ' H NMR spectroscopy.[' 31 The expanded spectra for several different solutions in the 3.7-3.8 ppm region are presented in Fig. 2. A careful analysis of the spectra enables identification of each species in the substitution series, including all possible isomers. The pattern and intensities of the ten resonances shown in Fig. 2a, result from nine different isomeric species containing from one to six coordinated methanol molecules, as represented by the formula [ M ~ B ~ I ~ ( H ~ O ) ~ - , ( C H ~(1OI Hx) 16). , ] ~ + If a small amount of water is added to the solution, the equilibrium is shifted such that the concentrations of the species with x 2 4 become very low whereas those of the species with more water increase, as shown in Fig. 2b. The equilibrium for methanolic solutions of [(Ta6C112)C12(H20)4].4H20 is reached much more slowly. As shown in Fig. 2c, the process is incomplete after 1.5 h; the equilibrium spectrum of the solution (Fig. 2d) is obtained after three days. This spectrum is even more complicated than that of [TasBr12(H20)6-,(CH30H),]2+ because of the participation of C1- with H2O and CH30H in the substitution process. Evidently, the strong coordination of C1- ligands in terminal positions is responsible for very low water solubility of the niobium and tantalum chlorooctahydrates. Although the behavior of niobium and tantalum clusters in ethanolic solutions is expected to be similar to that in methanol, experiments indicate significantly faster reaction with methanol.
5.4.2.2 The [(M6X12)X2Ia6ROH ( R
= Me, Et) clusters as precursors
In solutions of [ (M6C112)C12(H20)4].4H20 in aliphatic alcohols the concentration of the different water-alcohol species is a function of the water/alcohol molar ratio in the solution. To increase the concentration of species rich in coordinated alcohol molecules it is necessary to reduce the amount of water in the solution. This can be achieved by high-vacuum evaporation of the solutions and by drying the remaining solids under vacuum. After this procedure the amount of water in the system is only partly reduced. The alcohol is vacuum distilled once more. Repeating the distillation-evaporation and drying process under vacuum minimizes the amount of water
1554
5 Solid-Stnte Cluster Chemistry
l
3.82
3.00
3.78
'
'
3.76
'
3.74
~
3.72
'
'
3.70
~
~
~
I
PP"
Figure 2. ' H NMR spectra of CH3OH coordinated on [TdfjX12I2+units: (a) [(TasBrl2)Br2(H20)4]. 4H20 in methanol (3 h after preparation); (b) solution under (a) after addition of a few drops of water; (c) [(Ta6C112)C12(Hz0)4].4H20in methanol (1.5 h after preparation); (d) solution under (c) after 3 days. Reproduced with permission from ref. [ 131
in the system and species of composition [ (M6C112)C12].6ROHare gradually formed. Details are given elsewhere.['41Compounds with R = Me are insoluble in aliphatic alcohols, whereas those with R = Et are more soluble and therefore more suitable for reactions in non-aqueous solvents. The behavior of [(M,jBr12)Br2(H20)4].4H20 in methanol differs from that of chloro clusters in the same solvent. The clusters [(M6Br12)Br2].6MeOH crystallized from concentrated ( 25 x lop2 mol dmP3) methanolic solutions within a few hours only. For less concentrated methanolic solutions, slight reduction of the volume under vacuum is needed for the conversion
'
f
1555
5.4 Cluster Compounds Containing Mixed-Charge Cluster Units
of octahydrates into methanol derivatives. The process of conversion of octahydrates into ethanol derivatives is slower and three distillation-evaporation cycles must usually be applied. So far [(M6X12)X2].6MeOHclusters have been used for preparation of cluster m e t h o ~ i d e s ; [ ~M6X12)X2]. .~l[( 6EtOH clusters have been used to prepare new substances with N , N-dimethylformamide ([M6X12(C3H7N0)6]X2['51) and alkylcyanides ( [ M ~ X L ~ ( R C N()R~= ] XMe, ~ Et, Pr),[83161 and [Ta6C112(PrCN)6]. [(Ta,$112)C16] . 2PrCN[171) and for the preparation of the [M6X12(EtOH)6]. [(Mo6Cls)C14X2].nEtOH.mEt20 series of clusters." 8 3 1 9 i
5.4.3 Clusters with mixed-charge cluster units The literature dealing with Nb6 and Ta6 halide cluster chemistry contains no information about the existence of mixed-charge cluster compounds either homocluster is nuclear or heteronuclear. The [Ta6C112(PrCN)6][(Ta6C112)Clh].2PrCN the first example of a homonuclear mixed-charge cluster compound and the [ M ~ X ~ ~ ( E ~ O H ) ~ ] [ ( M O ~ C ~ S )(M C~ =~ Nb, X ~Ta; ]. X ~=ECl, ~O Br) H clus.~E~~O ters are the first examples of the heteronuclear variety. All these compounds crystallize in the triclinic space group P i (no. 2). The preparation of these substances was X ~ ] . clusters with substitutionally labile ethanol molecules based on [ ( M ~ X I ~ ) 6EtOH in the Xa positions. The existence of homonuclear mixed-charge [M6Br12In+ (M = Nb, Ta; n = 2, 4) bromo derivatives is also expected.[201
5.4.3.1 [Ta&112( PrCN)6][(Ta&l12)Cl6]- 2PrCN, a compound with homonuclear mixed-charge cluster units - [Ta6C112]~ in the cation and [Ta6C112]4+ in the anion +
The alkylcyanides RCN ( R = Me, Et, Pr) are suitable solvents for dissolution of [(M6X12)X2].6EtOH precursors. In these solutions exchange of coordinated ethanol molecules for the alkylcyanide molecules takes place and the solids [ M6X I 2( RCN )6]& are precipitated.[8,'61 Thus, the [ Ta6Cl12(PrCN)6]. [( Ta6C112)C16].2PrCN cluster is prepared from the PrCN s~lution.["~ Single crystals of this complex are grown in the presence of a limited amount of air so that selective oxidation of some clustcr units from [Ta6C112I2+to [Ta6C112I4+might occur, a sufficient amount of C1- being set free (from partial decomposition of some cluster units) to form the [(Ta6C112)C16]2p The compound crystallizes in the triclinic space group P1 (no. 2) with Z = 1 and unit cell parameters: a = 12.160(2)A, b = 12.587(2)A, c = 13.172(2)A, CI = 93.814(1)", B , = 93.344(1)", y = 93.861(1)'. The crystal structure of the compound is indicative of two types of
1556
5 Solid-state Cluster Chemistry
~a6C112cluster the [ T ~ ~ c ~ I ~ ( P ~ Ccation N ) ~ and I ~ +the [ ( ~ a 6 ~ 1 1 2 ) ~ 1 6anion, ]~each having imposed 1 symmetry. Selected bond lengths for the cluster cation and the cluster anion are given in Table 1. The considerable difference between the interatomic distances in [Ta,jC112(PrCN)6]~+ and [(Ta6C112)C16]2-is obvious. The average Ta-Ta distance in [ ~ a 6 ~ 1 1 2 ( ~ r ~(2.870A) ~ ) 6 ] ~is+shorter than that in [(Ta,jC112 C16l2- (2.982A) but it is identical to the average Ta-Ta bond length (2.872 ) found in the (Table [Ta6C112(EtOH)6I2+ cation Of [Ta6C112(EtOH)6][(MO6Cls)C16].6EtOH l).[183191 It is also close to the value of 2.901 A found for the Ta-Ta bond length in trans-[ (Ta&112)C12( PEt3)4].CHC13.["] Although the Ta-Ta interatomic distance is not comparable with the corresponding values in (2.982 A) in [(Ta6C112)C16]~it is very close any of the compounds listed in Table 1 containing [Ta6C112]3+,[21-231 to the value of 2.962A found for the average metal-metal interatomic distance in the H2[(~a6C112)C16]. 6H2O containing [~a&112]~+. In a series of [Ta6C112I4+compounds with oxygen-donor ligands in the Xa positions the Ta-Ta bond lengths lie in the range 2.940 A to 2.985 The mean Ta-Cl' interatomic distance in [Ta&112(PrCN)6]2+(2.458 A) is, furthermore, nearly the same as those (2.462A) and [Ta&112( EtOH)6]. found for trans-[(Ta~Cl12)C12(PEt3)4].CHC13 [(Mo&lx)C16].6EtOH 2.464A) (Table I). The average Ta-Cl' distance in [(Ta6C112)C16]~(2.430 ) is, on the other hand, close to the value of 2.414A found for the corresponding distance in H2[(Ta6C112)C16].6H20(Table 1). As expected, the Ta-Cla bond distances for [(Ta6C112)C16I2- (2.487 A) and H2[(Ta6C112)C16].6H20 (2.507A) are significantly longer than the Ta-Cl' distances in the same cluster units. All these intracluster bond lengths together with local charge neutrality requirements are consistent with the presence of the [Ta6C112I2+ unit in the [Ta6C112(PrCN)6I2+cation and with the presence of the [Ta6C112I4+ unit in the [(Ta6C112)C16]2-anion of the compound [Ta6C112(PrCN)6][(Ta6C112)C16].2PrCN. In the chemistry of hexanuclear niobium and tantalum halide clusters, this compound is the first example of two coexisting homonuclear mixed-charge [M6X12In+(n = 2, 4) units in the same molecule. The packing of the octahedral [Ta6C112(PrCN)6I2+ cluster cations and the octahedral [(Ta&112)C16l2- cluster anions in the crystal structure of [Ta6C112(PrCN)6][(Ta6C112)C16].2PrCN is shown in Fig. 3. -
8,
s
5.4.3.2
[M~X~~(E~OH)~][(MO~CI~)CI~X~]*~ZE~OH compounds with heteronuclear mixed-charge cluster units [ M ~ X I ~and ] ~ +[Mo6Cl8l4+(M = Nb, Ta; X = C1, Br)
The hexanuclear molybdenum halide clusters (Mo6Xs)X4, i. e. (M0gX~g)C1"2Cl~-~~,2, are known to react with some transition metal halide compounds forming two different types of cluster: polymeric clusters with bridging outer halide ligands (X"-") and discrete clusters with terminal halide ligands (X").
Cluster
Table 1. Correlation of selected bond distances Unit
PrCN CICI-
C1-, PEti CI-, PEti H2O MeOH MeOH
C1-, PEti
EtOH
L“
2.458 2.430 2.414
2.464 2.462 2.441 2.445 2.449 2.439 2.439
2.872 2.901 2.940 2.942 2.9059 2.9048 2.898 2.870 2.982 2.962
Ta-CI’
Ta-Ta
(A, mean values) in some [Ta6Cl12]”+clusters ( H = 2, 3, 4).
2.487 2.507
2.53 1 2.482 2.494
Ta-CI”
24
18, 19 21 21 21 22 23 23 16. 17
Ref,
1558
5 Solid-State Cluster Chemistry
Figure 3. The crystal structure of [Ta6C1I 2 ( PrCN)6][ (Ta6C11,)c16].2PrCN showing the alternating arrangement of the cluster cations and cluster anions rows. Note: Octahedra are constructed from atoms in the Xa positions; CI', C and H atoms are omitted for reasons of clarity; darker octahedra correspond to the [Ta6C112(PrCN)6I2+cations.
In the M[(Mo6C118)C1"6](M = Hg, Pb) cubic compounds six C1" atoms bridge with neighboring [M0&18]~+ cations.[263271 The same is found for C U ~ ( M O ~ X ~ ) X ~ , X = C1, Br.[281The triclinic phases M(Mo&l8)Cl~,M = Na, Ag are examples of clusters in which the Xapaand Xa ligands are both present. The structures are based on (M06X'8)Cl"4C1"~"2,2 fragments arranged in one-dimensional c h a i n ~ . [ ~ ~ , ~ ~ ] Discrete clusters with all-terminal Xa ligands are found in compounds ( Bu~N)~[(Mo~C~'~)X"~][~~' and A2[(Mo6Br'8)Xa6],A = Bu4N ' , Ph4Pf, Ph4As+; X = F, C1, Br, I.[321 Compounds of general formula [M6X12(EtOH)6][(M o ~ C ~ ~ ) C L X ~ ] . ~ E ~ O H . ~ E (M = Nb, Ta; X = C1, Br) are formed by the reactions of (M6X12)X2.6EtOH and (Mo6Cl8)C14 in ethanol as solvent under an inert atmosphere and consist of [M6X12(EtOH)6I2+ cluster cations and [(M06C18)C14X2]~-cluster Chloro derivatives of composition [NbsC112(Et0H)6][(Mo6Cl8)C16]. 3EtOH. 3Et20 and [Ta6C112(E ~ O H ) ~ ] [ ( M O ~ C ~ ~ crystallize )C~~]. in~the E ~triclinic O H space group Pi (no. 2) with Z = 1 and unit cell parameters a = 10.641(2)A,b = 13.947(2)A, c = 15.460(3)A, a = 65.71(2)", /3 = 73.61(2)", y = 85.11(2)' and CI = 11.218(2)A, b = 12.723(3)A, c = 14.134(3)A, = 108.06(2)", p = 101.13(2)", y = 91.18(2)", respectively. Cations and anions are arranged to form distorted CsC1-type structures. The packing of the cluster cations and cluster anions in the crystal structure of [Ta~C1~2(EtOH)~][(Mo~Cl~)C16]~6EtOH is given in Fig. 4. The rows of [TasC112(EtOH)6I2+ cluster cations and [(Mo6C18)C16I2- cluster anions are connected into a three-dimensional network by hydrogen bonds formed by atoms in the Xa positions (from cations and anions) and six molecules of ethanol of crystallization. The unit cell volumes and selected interatomic distances for these two chloro compounds, and for two additional pairs of isostructural bromo clusters, namely [M6Brl2(H20)6][HgBr4].12H20[331and CsEr[(M6Br12)Br6](M = Nb, Ta)[34,351 are given in Table 2.
5.4 Cluster Compounds Containing Mixed-Charge Cluster Units
1559
Figure 4. Crystal structure of [Ta&112( EtOH)~][(Mo,jCl~)Clb].6EtOH showing the three-dimensional arrangement of cluster cations and cluster anions uiu oxygen bridges (oxygen atoms from crystalline ethanol molecules). Note: Octahedra are constructed of the oxygen (or chlorine) atoms in the X d positions; CI', C , and H atoms are omitted for reasons of clarity; darker octahedra correspond to the [ ( M O ~ C ~ ~ ) anions. CI~]~-
Each pair of clusters listed in Table 2 crystallizes in different space groups with different numbers of cluster molecules in the unit cell. The common feature of all the niobium cluster compounds listed in Table 2 is that the metal-metal bond distances are longer than those in the Ta6 analogs. The observed differences between Nb-Nb and Ta-Ta bond lengths of 0.032, 0.049, and 0.056 A, respectively, for the three pairs of clusters listed in Table 2 include chloro and bromo derivatives. Further, the unit cell volumes for all Nb6 clusters are larger than those of the Ta6 analogs. It follows, for the isoelectronic Nb6 and Ta6 halide clusters with Mg units having the same coordination sphere and the same cation (or anion) environment, that somewhat shorter metal-metal interatomic distances (lanthanoid contraction) and slightly reduced unit cell volumes for the tantalum analogs might be expected. The interatomic distances in the [( M0&18)C16]~- cluster anions of [Nb6C112. (EtOH)6][(MOgC18)C16].3EtOH. 3Et20 and [Ta6C112(EtOH)6][(MO6Cls)Cl6].6EtOH are comparable. The mean values of the Mo-Mo (2.603 and 2.609 A) and Mo-C1' (2.468 and 2.473 A) interatomic distances for the Nb6/M06 and Ta6/Mo6 clusters,
= Nb,
Ta; X = C1, Br
Pi Fdjm Fd3m Pjlc Pjlc
[Nb6Br12(H20)6][(HgBu)]'12H20 [Ta&12(H20)6] [(HgBr4)1. 12Hz0 CsEr[(NbsBrl2)Br6] CsEr[(T@r12)Br6]
*M
1 8 8 2 2
Pl
[Nb6C112(EtOH)6][(MOgC18)C16]'3EtOH. 3Et20 [Ta6C112(EtOH)6][(Mo6C18)C16]'6EtOH 1
Space group
Cluster
Z
14.9
49.4
130.3
2005.1(8) 1874.8(7) 9270.5(4) 9221.1(1) 1472.7 1457.8(3)
AV
2.949(1) 2.9000(8) 2.954 2.898
2.904 2.872
M-M*
2.599(1) 2.606(1) 2.587 2.587
2.454 2.464
M-X',*
2.885 2.892
M-Bra
33 33 34 35
18, 19 18, 19
Ref.
(A,mean values except for cubic clusters) for three pairs of
Unit cell volume ( V )
Table 2. The space-group, unit-cell volumes (A3) and selected bond distances clusters with coordination environment identical within each cluster pair.
2
s
3
g
0
5
3
b
0
t ; o\
5.4 Cluster Compoun& Containing Mixed-Charge Cluster Units
1561
respectively, are essentially identical to those for the previously characterized [ ( MogC18)C16I2- examples.t311 Significant differences have been observed for Mo-Cla bond lengths (2.407 to 2.438 A and 2.410 to 2.461 A for Nb6/Mo6 and Tas/Mo6 derivatives, respectively), indicating the sensitivity of the Mo-Cla bond to the hydrogen-bond system involved. The longest distance of 2.461 A found for Mo-Cla in the Tas/Mos cluster is related to the terminal chlorine atom which is involved in two hydrogen bonds. The shortest value of 2.407(1) A is related to the Mo-Cla bond in the Nb6/Mo6 cluster that is not involved in hydrogen bonding.
5.4.4 Conclusion In the chemistry of hexanuclear metal clusters the coexistence of mixed-charge cluster units within homo- and heteronuclear clusters is possible. The synthetic routes have been identified and include: i) the carefully controlled selective oxidation of some cluster molecules; ii) the synthesis of less stable species that might disproportionate into species with different charges; and iii) the reaction of two different hexanuclear cluster species in a suitable solvent.
So far, only the hexanuclear clusters have been studied. What about different nuclearity clusters of other transition metal elements? In other words, is it possible that a new chemistry within transition metal clusters has just started?
References H. Schafer, H. G. von Schnering, Angew. Chem. 1964,20, 833-849. P. B. Fleming, L. A. Mueller, R. E. McCarley. Znorg. Chrm. 1967, 6 , 1-4. A. Simon, H. G. von Schnering, H. Schafer, Z. Anorg. Allg. Chem. 1968, 361, 235-248. F. W. Koknat, J. A. Parsons, A. Vongvusharintra, Znorg. Chem. 1974, 13, 1699-1702. N. Bmitevic, F. MuStoviL, R. E. McCarley, Inorg. Chem. 1988, 27, 4532-4535. N. BrniCeviC, R. E. McCarley, S. Hilsenbeck, B. KojiC-Prodic, Actu Cryst. 1991, C47, 315318. [7] U. Beck, H. Borrmann, A. Simon, Actu Cryst. 1994, C50,695--697. [8] S. Sirac, R. Trojko, Lj. Marie, R. E. McCarley, 0. Tolstikhin, N. Brnitevic, Crout. Chem. Actu, 1995, 68, 905-907. [9] a) P. Nannelli, B. P. Block, Znorg. Chem. 1968, 7, 2423-2426; b) ibid, 1969, 8, 1767--1771. [lo] M. H. Chisholm, J. A. Heppert, J. C. Huffman, Polyhedron, 1984, 3,475-478. [ l l ] K. Lindner, H. Helwig, Z. Anorg. Allg. Chem. 1925, 142, 180-188. 11 21 31 41 51 61
1562
5 Solid-State Cluster Chemistry
[ 121 M. Miljak, N. BmiEeviC, I. Aviani, in preparation. [ 131 N. BmiEeviC, P. PlaniniC, I. BaSic, R. E. McCarley, V. Rutar, B. Xie, Inorg. Chem. 1993, 32, 3786-3788. [14] A. Kashta, N. BrniEeviC, R. E. McCarley, Polyhedron, 1991, 10, 2031-2036. [15] S. Sirac, P. PlaniniC, Lj. MariC, N. BrniEeviC, R. E. McCarley, Znorg. Chim. Acta, 1998, 271, 239-242. [ 161 S. Sirac, Dissertation, University of Zagreb, Zagreb, 1997. [17] N. BrniEeviC, S. Sirac, I. Basic, Z. Zhang, R. E. McCarley, Inorg. Chem., in press. [18] I. Baiic, Di.ssertation, University of Zagreb, Zagreb 1997. [19] I. BaSic, N. BrniEeviC, U. Beck, A. Simon, R. E. McCarley, Z. Anorg. Allg. Chem. 1998, 624, 725-732. [20] N. BmiEeviC, M. VojnoviC, I. BaSic, M. VinkoviC, work in progress. [21] H. Imoto, S. Hayakawa, N. Morita, T. Saito, Inorg. Chem. 1990, 29, 2007-2014. [22] N. BmiEeviC, Z. RuziC-ToroS, B. KojiC-ProdiC, J. Chem. Soc., Dalton Trans. 1995, 455-458. [23] N. BmiEeviC, D. Nothig-Hus. B. KojiC-ProdiC, Z. RuiiC-Toroi, Z. DaniloviC, R. E. McCarley, Inorg. Chem. 1992,31, 3924-3928. [24] C. B. Thaxton, R. A. Jacobson, Inorg. Chem. 1971, 10, 1460-1463. [25] U. Beck, A. Simon, S. Sirac, N. BrniEeviC, Z. Anorg. Allg. Chem. 1997, 623, 59-64. [26] H. G. von Schnering, Z. Anorg. Allg. Chem. 1971, 385, 75-84. [27] S. Boschen, H.-L. Keller, 2. Kristallogr. 1992, 200, 305-315. [28] A. Peppenhorst, H.-L. Keller, Z.Anorg. Allg. Chem. 1996, 622, 663-669. [29] S. Boschen, H.-L. Keller, Z.Kristallogr. 1991, 196, 159-168. [30] M. Potel, C. Perrin, A. Perrin, M. Sergent, Mat. Res. Bull. 1986, 21, 1239-1245. [31] W. Preetz, K. Harder, H. G. von Schnering, G. Kliche and K. Peters, J. Alloys Comp. 1992, 83,413-429. [32] W. Preetz, D. Bublitz, H. G. von Schnering, J. SaOmannshausen, 2. Anorg. Allg. Chem. 1994, 620,234-246. [33] M. VojnoviC, S. AntoliC, B. KojiC-ProdiC, N. BmiEevi6, M. Miljak, I. Aviani, Z. Anorg. Allg. Chem. 1997, 623, 1247-1254. [34] S. Cordier, C. Perrin, M. Sergent, Z. Anorg. Allg. Chem. 1993, 619, 621-627. [35] S. Cordier, C. Perrin, M. Sergent, J. Solid State Chem. 1995, 118, 274-279.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.5
and M6L18 Units in Early Transition Element Cluster Compounds
M6L14
Christiane Perrin
5.5.1 Introduction The early transition elements favor the formation of clusters, defined as aggregates of specific geometry, built from metallic atoms linked together by metal-metal bonds. These metallic cores are surrounded by a ligand shell giving units which form the building blocks of the crystal structures. After the early work by C. Brosset['] and L. Pauling[21on octahedral clusters, many early transition-metal compounds based on such clusters have been isolated in solid-state chemistry.[31The well known compounds discovered during the last forty years have been more recently augmented by a number of original materials characterized by a large variety of cluster packing motifs. The detailed structural data obtained from single crystal X-ray diffraction has subsequently enabled precise explanations of the fine structural or electronic effects influenced by the various anionic or cationic substitutions possible in such materials to be made. We have chosen to focus on octahedral clusters of niobium or tantalum and molybdenum or tungsten in the solid state. They form two structural types, M6L18 or M6L14 respectively; in the first, the M6 cluster is bonded to six apical ligands (La) and is additionally edge-capped by 12 inner ligands (L') (Fig. la); in the second the cluster is similarly bonded to six apical ligands and is now face-capped by eight inner ligands (Fig. 1b).[41Indeed, although many compounds based on these two types of unit have been known for a long time, there was a lack of systematic comparisons between their types of packing, intra-unit bonding, and the relative stability of the various structures. We therefore intend to discuss and compare some specific effects appearing in these two M&18 or M6L14 units, by use of structural data from our recently synthesized compounds augmented by data from the literature. For this purpose, several important series of clusters suitable for anionic and cationic substitutions will be described. Compounds in which the inner or apical
1564
5 Solid-state Cluster Chemistry
9
"A a)
0
I
@ b)
L(
0 @
unit.
ligands are shared between adjacent units will not be considered here, to avoid any possible biasing of such bonding modes on intra-unit bonding; they will be used only as particular examples in some specific discussions.
5.5.2 Preparation and characterization The materials reported in this review have been prepared by solid-state reactions, usually starting from stoichiometric amounts of binary compounds and elements according to the formula of the target compound. The mixture is heated in sealed
5.5 M ~ L and I ~A46L18 Units in Early Transition Element Cluster Compounds
1565
silica tubes (or in metallic containers, for example Nb tubes, when, in specific instances, silica reacts with the mixture during the synthesis) at temperatures ranging most frequently from 600 to 1000 "C depending on the various systems. Usually the Mg cluster is not formed at lower temperatures. For some systems the temperature range suitable for the synthesis is very narrow and has to be determined accurately to obtain a pure compound. The time of reaction is also important, because it has occasionally been shown that a phase transition into another allotrope can occur, depending on the reaction time - for instance, at 600 "C, the chloride RbLaNb6Cllx changes progressively from the metastable R3 polymorph to the thermodynamically more stable P31c polymorph when the reaction time is increased.[51 Similarly, K~W6C114crystallizes in space group C2/c but when heated for 36 h at 400 "C changes progressively to the Pn3 structure after 3 days of reaction at the same temperature.I61 The M6 cluster corresponds to a low oxidation state of the metal and, particularly for Nb6 and Ta6 compounds, the reaction is more efficient under reducing conditions. They are obtained by adding excess metallic element, in the form of powder or as metallic foil, to the starting mixture. This technique is not usual for the preparation of M06 and w6 cluster compounds. So, the synthesis of these materials needs carefully controlled reaction conditions. For this reason, measurements performed to study their physical properties ideally require single crystals, to avoid artifacts arising from the presence of secondary phases in the sample. Small single crystals, suitable for structural determinations, are usually obtained directly during the synthesis. Larger single crystals can be formed by increasing the reaction time or by the use of other, conventional, crystal growth techniques, particularly chemical transport reactions.
For Nb or Ta clusters M6L18 units are formed when the ligands are C1 or Br for niobium, and C1, Br or I for the tantalum. In contrast, the niobium iodides have the M6L14 unit, owing to the important steric effect of the iodine, as discussed later. Such M6L18units can also be prepared from valence-electron-poor transition metals such as zirconium, but in this case an interstitial element located at the center of the octahedral cluster is necessary to stabilize the cluster.[']
5.5.3.1 Electronic properties of the M6L18 unit For the M6L18 unit, the highest occupied molecular orbitals (alg, tlu, tlg, aZu) have M-M bonding character.[8p111These levels are fully filled for 16 valence electrons
1566
5 Solid-state Cluster Chemistry
(valence electron count (VEC) per M6 cluster = 16), which corresponds schematically to eight 2-electron-3-center bonds. The HOMO level of a2u symmetry has an M-L' antibonding contribution, but the M-La bond does not participate at this level. Recent density functional theory (DFT) calculations have shown that the position of the a2" level is significantly closer to the other levels with metallic bonding character than previously found by extended Hiickel (EH) calculations.["] These new DFT results, which clearly suggest the high stability of the 16 VEC phases, are in better agreement with experience, i. e. the occurence of a large number of 16 VEC materials, than the EH calculations, from which maximum stability is expected for the 14 VEC compounds. These theoretical calculations have all been extensively developed for L = halogen. They have been extended to the three new series of oxyhalides that have been recently discovered, and the results are comparable with those found for the pure halides.[l2]For these compounds, however, the M12M111M6X1503and MIIIMgX1303 ( X = halogen) series described in the next section, the a2u level is destabilized and lies closer to the metal-metal antibonding levels, in good agreement with the preferential VEC = 14 experimentally found for these oxyhalides. No direct interaction between clusters can occur for compounds based on such discrete M6L18 units, owing to their large separation. The molecular character of such materials is strong and consequently no metallic behavior has been encountered. The valence electrons remain localized on the cluster and are responsible for the magnetic properties. Indeed, for VEC = 16 or 14 the non-degenerated a2u HOMO orbital is either filled with two electrons or is empty, respectively, and the cluster is not magnetic; for VEC = 15 this orbital accommodates only one unpaired electron, leading to paramagnetic behavior." 31
5.5.3.2 Dependence of M6Lls packing on the size, charge, and stoichiometry of the countercations The general formula of compounds based on such discrete M6L18 units can be written MLM;M6Llg. According to the scheme described above, the most stable and the most frequently encountered phases in solid-state chemistry are obtained for VEC = 16 corresponding to a 4--charged (M6x18)~-unit (X = halogen). The intermediate value VEC = 15 (n = 3) is less common, and VEC = 14 ( n = 2) had been obtained only from solution chemistry with organic countercations (for example [(P~H)2Nb6C118]['~]) until the recent discovery of the new M6L18 based oxyhalides which are stabilized with this 14 VEC. Thus, a large variety in both the nature and the stoichiometry of countercations can be used to stabilize such compounds. The different families of compounds based on these discrete units so far obtained in solid-state chemistry are reported, with their crystal structures, in Table 1. Relevant examples are included of all the possible combinations of mono-, di-, and trivalent countercations, giving VEC of 16 and 15, that have been obtained. In contrast, compounds with VEC of 14 appear only in one oxyhalide series as stated above.
* MI, M",
pseudo-Hexagonal
R3 R3 P31c
16
16
= Nb
or Ta; X
= C1 or
Br.
f.c.c.
P31c
15
f.c.c. f.c.c. pseudo-Hexagonal
Hexagonal pseudo-Hexagonal f.c.c.
PI
16
R3 R3 P31c
Distorted primitive stacking
Pi
16
15 16 14
Tetragonally distorted f.c.c.
C2/m
16
Type of M6Ll8 unit stacking
spacc group
VEC
MI1': monovalent, divalent or trivalent countercation; M
M'M~~IM 6 x 1 8
Family of compounds*
Table 1. Niobium and tantalum halide and oxyhalide series, based on discrete M6L18 units. Determined structure with [ref.]
3
g.
2 $.
Y
1568
5 Solid-state Cluster Chemistry
When considering, to a first approximation, the units as large anionic spheres, their packing is systematized in Table 1 in terms of the classical packings usually encountered in solid-state chemistry. These packings provide M6L18 unit sites where the various countercations are accommodated. In the following sections we will focus on the prominent families of halides obtained with divalent or trivalent rare earths - 1 M'2M1'M6X18 (M' = monovalent cation, M" = divalent rare earth ion, M = Nb or Ta, X = C1 or Br, space group R3); 2 M1M1"M6X1g (M"' = trivalent rare earth ion, R3 or P31c); 3 M111M6X18, R3 - and the two families of oxyhalides 4 M12M111M6X170, R3 and 5 M12MI1'MgX1503, P31c which are isotypic (R3 family) or closely related (P31c family) to the halides. In the former 1 and 2 P31c compounds, the divalent or trivalent rare earth ion has been replaced very recently by another divalent cation and, in some examples, the trivalent rare earth ion by trivalent uranium. These R3 and P3 lc series are of special interest, because many anionic or cationic substitutions can be performed to influence the intra-unit distances in strictly isotypical compounds, thereby providing accurate structural data for relevant comparisons. -
-
M'M"'M6X18 and the related M1MT1M6X18series (structural type CsLuNb&llS, P31c) With a large monovalent countercation such as cesium, the stacking of M6X18 units is pseudo-hexagonal . . .AA'A . . . The M' and M'" countercations are located on the prismatic sites of the units (see, for example, Fig. 2, the structure of CsPbTa6Cll8, in which MI1' is replaced by Pb as mentioned below). However, rotation of the units located in the A layer relative to the units of the A' layer (ca 20") gives different coordinations for M' and M"'. Indeed, M' is bonded to twelve inner and apical halogens forming a complex site that is largely open above and below this cation, whereas MI1' is bonded to six apical halogens forming a distorted octahedron. This structure can be compared with the hexagonal Ba2Nb6Cll8 structure type[211in which the stacking of the units is also hexagonal but without any rotation in adjacent layers, thus forming identical cationic sites in which two identical cations (2 x Ba" instead of 1 x M' 1 x MI") are located. In this P31c structural type, the two cationic sites are always fully occupied by M' and MI'', respectively, giving systematically a VEC = 16. These compounds exist in the Nb-CI, Nb-Br, Ta-Cl, Ta-Br systems with all of the trivalent rare-earth ions and trivalent uranium but only cesium (occasionally rubidium) as the monovalent countercation. Very recent work on the Ta-Cl system has, however, realized the possibility of replacing the trivalent rare earth ion by divalent cations, giving the CsM"Ta6C118 series with MI' = Ba, Pb, Sr, Eu, Ca.[221This new series, for which VEC = 15, constitutes the first example of quaternary halides with an odd VEC; as a consequence they contain magnetic clusters. Indeed, magnetic measurements confirm this VEC value derived from structural data. As an example, the calculated
+
5.5
M6L14
and M6Llb: Units in Early Transition Element Cluster Compounds
1569
Figure 2. Unit-cell of CsPbTasC1,s. Only the chlorine atoms involved in the coordination sphere of Cs and Pb are represented.
effective moment for CsEuTa6Clls is in accord with the presence of a divalent europium, and this compound behaves as a cluster with one-unpaired-electron.
The M'z M" M6X18, M'M" KLuNb6Cll8, R3)
M&18,
M"' M6X18 series (Structure type
With smaller monovalent countercations, e.g. Na+, K+, Rb+, In+ or T1+, the packing of the M6X18 units is f.c.c.; the monovalent cations occupy the tetrahedral sites of the units and the divalent or trivalent cations lie in the octahedral sites (see, for instance, the structure of Cs2PbTa6Clls in Fig. 3). These countercations have two different coordination spheres MI is bonded to twelve inner and apical halogens forming a complex site smaller and more isotropic than the corresponding site in the P31c structure, whereas MI1 or M"' are bonded as above to six apical halogens forming a slightly distorted octahedron. Until now, in this type of structure the M' site is either fully occupied, giving the MiM"M6Xlg series, or half filled in the M'M"'M6Xl8 series (with the same formulation as the compounds described in the last section), for both VEC = 16. This site can also be empty without destroying the unit stacking, giving the M"'M6x18 ~
1510
5 Solid-state Cluster Chemistry
a)
Figure 3. (a) Unit-cell of Cs2PbTa6C118. The chlorine atoms are omitted for clarity; (b) Pb environment; (c) Cs environment.
ternary compounds with a VEC = 15. The latter compounds are of special interest because the magnetism of the cluster and the magnetism of the rare earth ion can coexist in the same material. Such ternary compounds have been stabilized in Nb-Cl, Ta-Cl or Ta-Br systems, but not in the Nb-Br system. In the latter system
5.5 M6LI4and M6LltlUnits in Early Transition Element Cluster Compounds
1511
the larger size of the units, which can result in large empty sites, and the long Nb-Nb intracluster distances related to both the 15 VEC and the matrix effect of the bromine (see below) are unsuitable for the stabilization of such hypothetical 'MrrrNbsBr18'bromides. For the three other systems, the compounds are only stabilized when there is a favorable compromise between the MI1' size, the M' empty site size, and the size of the unit. No solid solution of the type Mr,Mrr1M6X18with 0 I x I 1 has been observed, although the M' site can be partially occupied in this type of structure. In fact, only the integer values x = 0 or 1 corresponding to VEC = 15 or 16 have been obtained experimentally. Thus it can be assumed that in these compounds all the M6XIg units have the same VEC, 16 or 15, locally. The coexistence of two different VEC values in the same M6Llg compound has not yet been observed.
The M12M"'M6X170 oxyhalides (structure type KLuNb6Cll8, R3) These oxyhalides are strictly isotypical with the M12M'IM6X1g halides (R3) and are obtained when one halogen in the halides is replaced by one oxygen. The oxygen ligand is statistically distributed on the twelve inner positions. The increased anionic charge around the cluster resulting from the oxygen substitution is counterbalanced by an increase of the cationic charge, which maintains a VEC of 16, as in the parent halides. Indeed, two M' countercations per unit are present in these oxyhalides and the monovalent site is fully occupied, as in the parent M ~ I M ' I M ~ X IThe ~ . coordinations of the M' and M"' countercations are, of course, similar to those in the parent halides, but now the oxygen participates to the environment of MI. The ( M ~ L I ~units ) ~ -have an important charge, n = 5, never previously encountered in Nbs or Tas chemistry.
The M1&I1I1M6X1503oxyhalides (structure type Cs&aTa6BrlsO3, P31c) These oxyhalides, in which the units again form a pseudo-hexagonal stacking AA'A, as in the M'M"'M6Xls halides (P31c structure, above) are not isotypic, but closely related to this structural type. As an illustration, the structures of Cs2LaTa6Brl503 and CsErTasBrlg are compared in Fig. 4. The three oxygens are now ordered in the inner positions, alternating around the cluster, and are related by a threefold axis, giving the new MsX'9O13Xa6 unit. When comparing these oxyhalides with the parent halides, a shift of the M6Llg units by 1/4 of the c unitcell axis in the direction of the threefold axis gives new prismatic unit sites in which two M' countercations per formula unit can be arranged, in place of one M r in the parent halides. Then, the increased anionic charge around the cluster which results from the presence of the three oxygens is not fully counterbalanced by the increased cationic charge, and the VEC is only 14 instead of 16 in the halides. This is the first time that (M&lg)'--based compounds with such a 14 VEC have been stabilized under solid-state reaction conditions. The anionic charge n is 5 , as in the above oxyhalides. The M"' site is now formed from only three units instead of six in the
1572
5 Solid-state Cluster Chemistry
I
Figure 4. The structures of (a) Cs2LaTasBrl503 and (b) CsErTasBrlB for comparison. Top, the unit cells of the two structures (the ligands are omitted for clarity); bottom, the cation environments in the two structures.
5.5
A46L14
and MsL18 Units in Early Transition Element Cluster Compounds
1573
halides; this cation is nine-coordinated by six halogens and three oxygens and is located at the center of the triangle formed by the oxygens. The MI cation is 12halogen-coordinated as previously. Another new series of oxyhalides, M'"MgX1303, is based on an isomeric MgXigOi3X6aunit.[321The three oxygens, arranged in inner position, are now adjacent around the cluster, one located on the twofold axis and the two others related to one another by this axis. In these compounds, the units are linked together by four apical-apical halogen bridges to form helices of units; this series is not covered by this review. These oxyhalides also have a VEC of 14.
The M6L14 unit is formed by Mo6 or Wg clusters when the ligands are C1, Br or 1. Re6 clusters form similar units, but because rhenium has seven valence electrons instead of six for molybdenum and tungsten, no pure Re6 halides corresponding to the stable VEC = 24 can be obtained. For this reason only Re6 chalcogenides and chalcohalides have been isolated. Few examples of discrete ResL14-based chalcogenides are known; in the chalcohalides various halogen and chalcogen statistical distributions over the unit occur frequently and preclude a precise discussion of interatomic bonding. This review does not, therefore, consider the Re6 cluster compounds in detail, merely some relevant examples containing isolated Re6L14 units in which the various ligands are ordered around the cluster, as, e.y., in or M5(Br0.5)2Re,&Br6.[~~] M6L14 units also appear in very specific examples of niobium iodides, instead of M&18 which is usual for this transition element. In these Nb6 iodides, for example Nb6111[351 or Nb619S,[361the units are not, however, discrete and they will not be discussed in detail.
5.5.4.1 Electronic properties of the MbL14 unit In the M&14 unit, the highest levels occupied by the electrons (alg, tlu, tlg, t2u, e,) have M-M bonding character.[371These levels are fully occupied by 24 valence electrons per Mg cluster (VEC = 24), which corresponds schematically to twelve 2electrons-2-center bonds. The metal-metal bond is stronger than in the M6L18 compounds and the corresponding M-M distances are considerably shorter (cu. 2.6 A compared with 2.9 A for M6L18). Compounds based on such discrete units, obtained by solid-state chemistry syntheses, usually have VEC = 24 and the HOMO level of e, symmetry is fully filled. When the VEC = 23, as in a recent example,[381 the e, HOMO level accommodates three electrons.
1574
5 Solid-state Cluster Chemistry
In these discrete M6L14 materials, no direct interactions between the clusters can occur and they are insulating. The valence electrons remain localized on the cluster and, when the VEC = 23 the cluster is paramagnetic owing to one unpaired electron, as for the MsLls-based compounds when the VEC is 15.
5.5.4.2
M6L14
stacking in the various structures
The MiM6X14 (MM: = Mo or W, X = C1, Br, I ) series based on discrete units appears with one divalent or two monovalent countercations corresponding to VEC = 24, although Agw6B1-14and NaWsB1-14, with the unusual VEC of 23, have been reported very recently.[391This unexpected value of VEC is confirmed by the interatomic distances and by magnetic measurements which indicate paramagnetic behavior with a moment corresponding to one unpaired electron (NaWsB1-14 peff= 1.65 pB, 8 = 0 K). These compounds are the first examples of 23 VEC cluster compounds with isolated M6L14 units stabilized in the solid-state. The different M:M6X14 structures are listed in Table 2. When possible they are described in terms of classical unit packings. The following sections cover only the Pn3 structure type for which a number of structural determinations have been performed on anionic- or cationic-substituted compounds; these have revealed a wide variety of interatomic distances that are useful for comparative purposes.
The MnM&14 series (structure type PbMo6C114, Pn3) In these compounds the M6X14 units form a f.c.c. packing (see, for instance, the structure of CdW6Br14 in Fig. 5 ) , as in the M;M1''M6Xls series (R3), but in contrast with the latter series, the packing is now very regular. The divalent cation, located on the origin of the cubic unit-cell, occupies the octahedral sites formed by the units and is bonded to six apical halogens which form a very slightly distorted octahedron. The tetrahedral sites are vacant, as in the M'''M6X18 series (R3). However, the latter sites can be partly occupied in some isotypical Re6 thiobromides, e.g. in CdRegSgBr~,[~~' although the corresponding selenochloride has the usual ordering.[481Other Pn3 isotypical Re6 compounds have also been reported, for example KRe6Se5C19.r491 The corresponding thiobromide is, however, monoclinic C ~ / C , [ ' ~ ] but the solid solution K1+,RegS5+,Brg-, with 0.1 < x < 1 is again found to be Pn3 with the tetrahedral sites progressively filled.[' 'I
The M2IrnX14 series (structure type Cu~MogC114,Pn3) This series is again of the structural type M1'M6X14, with the same f.c.c. unit packing, but now MI1 is replaced by an MI2 pair with short M-M distances, which is centered on the origin of the unit-cell (see the copper environment in Cu2W6B1-14 in Fig. 6). The octahedral M" site formed by the six apical halogens in the
* MI, M";
23 24
Pn3
24
Pn5 Pn3
Pbca c2/c P21Ic P31c
Space group
VEC
f.c.c. f.c.c. f.c.c. f.c.c.
Hexagonal close-packed
~
-
-
f.c.c. f.c.c.
Type of unit stacking
monovalent or divalent countercation; M = Mo or W; X = C1, Br or I.
Family of compounds*
Table 2. Molybdenum and tungsten halide series, based on discrete M&14 units. Determined structure with [ref.]
n
S'
$ c
CC
c
t?
h
%
s.
a
1516
5 Solid-state Cluster Chemistry
Figure 5. The structure of CdW6Br14. (a) Representation of the unit-cell (the Br’ atoms are omitted for clarity); (b) The Cd environment.
5.5 MgL14 and M ~ L IUnits X in Early Transition Element Cluster Compounds
1577
Figure 6. The environment of the Cuz pair in the structure of Cu2W6Br14.
MI1M6X14 halides is now split into two triangular M' sites. So far, in this structural type the tetrahedral sites within the structure have not been found to be occupied. With very large counter cations, e. g. cesium, this structure becomes hexagonal with two different sites for the alkaline ions, one being located at the origin of the unit-cell with a very large thermal factor, suggesting a delocalization along the c axis.[431The same result has very recently been obtained for the corresponding Re6 compound, CS2Re6S8Br6.[471
5.5.5 Evolution of the intra-unit distances with electronic or steric factors - comparison of M6L14 and MhL18 units Tables 3-5 list the relevant intra-unit distances determined from the various structure types described above - the R3 and P31c compounds for the M6L18 unit and the Pn3 compounds for the M&14 unit - on which anionic or cationic substitutions have been performed. Comparison of these enables valuable discussion on the dependence of intra-unit distances on electronic and steric influences.
VEC
Rj Pjlc Pjlc P?lc P3lc
Rj
2.915(1), 2.920(1) 2.914(1), 2.918(1) 2.951(1), 2.961(1) 2.910(1), 2.917(1) 2.873(1), 2.876(1) 2.950( l), 2.958(1) 2.893(2), 2.903(2)
Rj
(A)
M-M
Space group 2.917 2.916 2.956 2.913 2.874 2.954 2.898
Average
(A)
K2MnNb6C118r241 Cs2PbTa6C118[221 CsPbTa6CI18 'z21 Cs2EuNb6Br18[231
16 16 15 16 R3 R3 P31c R3 2.932(1), 2.933(1) 2.887( I), 2.889( 1) 2.923( l), 2.927( 1) 2.967( l), 2.974( 1) 2.932 2.888 2.925 2.970
Interatomic distances for compounds of the M12MT1M6X18and M1Mr1M6X~g families
16 16 15 16 16 C ~ E r N b 6 B r l 8 ~ ~ ~ ~ 16 CsErTa6Br18r281 16
KGdNb6C1I8" 51 KLUNbhC118'261 LUNb6Cl18[261 CsLUNb6Clls[Z71
Compound with [ref.]
Table 3. Interatomic distances for compounds of the M1M'11M6X18 family.
2.447( 1) to 2.462( 1) 2.456(2) to 2.471(2) 2.435(2) to 2.445(2) 2.586(1) to 2.603(1)
2.449(1) to 2.459(1) 2.449(1) to 2.456(1) 2.424(2) to 2.436(2) 2.438(1) to 2.452( 1) 2.452(2) to 2.471(2) 2.576(1) to 2.595( 1) 2.578(3) to 2.594(3)
M-L' (A)
2.454 2.462 2.439 2.595
2.455 2.452 2.431 2.445 2.463 2.587 2.587
Average (A)
(A)
2.615(1) 2.595(2) 2.574(2) 2.804( 1)
2.648(1) 2.654( 1) 2.623(2) 2.667(1) 2.69 1(2) 2.885(2) 2.892(3)
M-La
F
G
5
R 3
0
3
0 F 2
2
5
g
3
bl
00
4
v1
5.5
A46L14 and
MgL18 Units in Early Transition Element Cluster Compounds
1579
Table 4. Interatomic distances for compounds of the MJ2M111MgX170and M12MIJ1MgX1503 families. Compound with [ref.]
VEC
space group
C S ~ L U N ~ ~ C ~ I ~16O ' ~ ~R3 ' C~>UNbgC11503'~"] 14 P31C C ~ 2 U T a 6 C 1 1 5 0 3 ~ ~ ~14 ~ P31c Cs~LaTagBrl503[~'] 14 Pjlc
M-M (A)
Average (A)
M--L" (A)
2.913(1). 2.918(1) 2.777( 1) to 3.024( 1) 2.760(1) to 2.984(1) 2.753( 1) to 3.033(1)
2.916 2.948 2.912 2.945
2.692( 1) 2.58 l(2) 2.578(2) 2.738( 1)
5.5.5.1 Evolution of the metal-metal intracluster bond depending on the cluster oxidation state Metal-metal intracluster distances are highly dependent on the Mg cluster oxidation state. Indeed, when electrons are removed from the a2" or eg HOMO levels of the MgLlg or MgL14 unit, respectively, which both have M-M bonding character, the M-M bond is weakened and the corresponding distance increases. This effect is illustrated by comparison of KLuNb6C11g(VEC = 16) with LuNbgCllg (VEC = 15) or of CdWgBrl4 (VEC = 24) with AgWgBrl4 (VEC = 23). For the former pair an average Nb-Nb bond-length increase of 0.04 A is observed when the VEC decreases from 16 to 15, in comparison with a W-W increase of only 0.016 A for the latter pair when it decreases from 24 to 23. This difference arises because in the latter pair of compounds more electrons are involved in the M-M bonding states and removing only one reduces the metal-metal bond strength to a lesser extent than for the first pair of compounds. Another interesting feature is the dependence of cluster distortion on the VEC. For the MgLl4 unit it has been suggested that the closer VEC is to the magic number of 24, more regular is the cluster because VEC = 24 corresponds schematically to twelve two-electron metal-metal bonds in the M6 cluster. Indeed, examples are known of this cluster distortion in compounds with VEC lower than 24, but in which the units are not discrete, for instance Nbg iodides and thioiodides with VEC = 19 or 20[521or Chevrel phases in which VEC can vary from 20 to 24.[53,541 In these compounds, however, the connection between the units introduces additional features which can also influence the dist0rti0n.I~ For compounds based on discrete M6L14 units, slight evolution of the distortion (AM-M, dispersion of the distances in the cluster) with the VEC can be also observed when comparing [(nC4H9)4N]2WgBr14 (VEC = 24, AW-W = 0.018 A)[561with [ ( ( C ~ H ~ ) ~ P ) ~ N ] W ~ B T I ~ (VEC = 23, AW-W = 0.026 A).[571A similar tendency is observed for the M6LIg unit when KLuNbsClIg (VEC = 16, ANb-Nb = 0.004 A) is compared with LuNb6ClI8 (VEC = 15, ANb-Nb = 0.010 A). This feature is not, however, systematic (compare, for instance, CdWgBr14 with AgW6B1-14 or CsErTagCllg with
VEC
(A)
2.451 (3) 2.612( 1) 2.613(1) 2.574 Ave. 2.587 Ave. 2.538
M-La (A)
W
9
t.,
3 @
b
24 24 24 24
Pn? Pnj Pn3 Pn3
2.598(2), 2.603(2) 2.614(2), 2.628(2) 2.629(1), 2.636(1) 2.672(3), 2.673(3)
2.600 2.621 2.632 2.672
2.442(3)-2.466(3) 2.574(3)-2.603(3) 2.601(2)-2.629(2) 2.752(2)-2.786(2)
2.455 2.587 2.614 2.767
2.442(3) 2.616(3) 2.6 14(2) 2.858(2)
.2
s.
n
2
2.465 2.592 2.617 2.616 2.628 2.613
Average (A)
PbM06C114[42'441 PbM06Br14[~~] CdWsBr14[~~] PbM06114[441
2.447(3) to 2.487(4) 2.576(1) to 2.610(2) 2.604( 1) to 2.636( 1) 2.592 to 2.639 2.610(4) to 2.638(4) 2.596(2) to 2.630(2)
M-L1 (A)
iF
2.605 2.628 2.631 2.648 2.635 2.649
Average (A)
Interatomic distances for compounds of the M"M6X14 family
2.602(2), 2.608(2) 2.626(1), 2.631(1) 2.629(1), 2.633(1) 2.647, 2.649 2.625(2) to 2.643(2) 2.636( 1) to 2.662( 1)
M-M
0
59
Pn3 Pn3 Pn? Pn3 P&/n P2l/n
Space group
wl
-
* These two compounds, obtained by solution chemistry, are given for comparison.
CU2M06C114[401
24 24 C U ~ W ~ B ~ I ~ ~ ~ ' ] 24 AgW6Br14[38' 23 [(n-C4H9)4N]2W6Br14[561* 24 [((C6H5)3P)2N]W6Br14[571* 23
Compound with [ref.]
Table 5. Interatomic distances for compounds of the M12M6X14 and M'MgX14 families.
5.5
A46L14and A46Ll~Units
in Eurly Trunsition Element Cluster Compounds
1581
CsPbTa6C118) and can be discussed precisely only for compounds with the same core environment. Indeed, as shown below, several other electronic features, for example, the charge of the cation bonded to the apical ligands, indirectly influence M -M intracluster distances.
5.5.5.2 Relativistic effects on M-M intracluster distances It is well known that niobium and tantalum atoms are of the same size.["] When, however, M-M intracluster distances are compared in CsErNbsBrls and CsErTa6B1-18, or in CsLuNb6C118 and CsErTa6C118, for instance, it is clear that Nb-Nb distances are significantly larger than Ta-Ta distances (by ca. 0.05 A).The other intra-unit distances are slightly affected and the M6L18 unit volume is smaller for Ta6X18 than for Nb6Xlg. This is why the unit-cell constants are systematically smaller for Ta6 compounds than for the isotypic Nb6 compounds (Fig. 7). Theoretical calculations have shown that this Ta6 cluster contraction can be explained by relativistic effects which appear on going from second row to third row transition metal elements.['21Indeed, the Nb-Nb and Ta-Ta intracluster distances calculated from geometrical optimization of the structures are in good agreement with the experimental structural data only when the relativistic effects are taken into account in the calculations. These effects are more important for tantalum than for niobium. The M-ligand distances are only slightly affected by the relativistic effects. A similar relativistic feature should be expected for the Mo6 and w6 compounds. When, however, the CU2M06Br14 is compared with isotypic Cu2W6B1-14, or PbMo6B1-14 with CdW6Br14, we do not observe any important difference between Mo-Mo and W-W intracluster distances. In contrast, the M-Br' distances are significantly longer for the tungsten compound, and the M-Bra distances are the same. The W6Brl4 unit is slightly larger than the Mo6B1-14unit. This feature can be extended to other compounds because we observe a systematically greater unit-cell volume for w6 compounds than for the isotypic M06 compounds (Fig. 8). This difference, when compared with the situation for Nb6 and Ta6 cluster compounds, can be related to the difference between the M6 cluster local environments in the two types of unit (face-capped cluster with VEC = 24 instead of edge-capped cluster with VEC = 16), which influences the nature of atomic orbitals.
5.5.5.3 Matrix effects of ligand size For the M ~ L Iunit, s the matrix effect can be discussed by comparing CsLuNb6Clls with CsErNb6Brl~(the sizes of Lu and Er are quite similar) and CsErTa&118 with CsErTasBrls. The increase in the M-M distance when going from the chlorides to the bromides results solely from the steric effect of the halogen. Indeed, when the size of the inner ligands increases, their van der Waals contact distances increase,
0
1450
Lll
d> d;
d-
1300-
Figure 7. Dependence of the unit-cell volume on the ionic radius of the trivalent rare earth ion for the series CsREMsX18 (RE = rare earth ion from La to Lu, M = Nb or Ta, X = C1 or Br).
and correlates with the size of the cluster which is bonded to these inner ligands. This variation is ca. 0.04 A and 0.025 A for Nb6 and Ta6 compounds, respectively, when going from chlorine to bromine. This matrix effect on clusters in an identical environment can constitute a measurement of the strength of the metal-metal b ~ n d . [ ~ , From ~ ’ ] these results it seems that the interaction is stronger for Ta-Ta than for Nb-Nb, in good agreement with the results of theoretical calculations.[l2] For the M6L14 unit the matrix effect increases the Mo-Mo intracluster distance by only 0.02 A when going from chlorine to bromine, because of the smaller 14ligand coordination number, and the larger concentration of valence electrons per
5.5
M6L14
und M6L18 Units in Early Trunsition Element Cluster Compounds
1583
2230 MMo6C114 MW6C114
Figure 8. Dependence of the unit-cell volume on the ionic radius of the divalent cation for the M"MgC114 series (MI' = Zn, Mn, Cd, Pb, Ba).
2130
-
2030
-
1930 u,6
03
1
,o
1
I
1,2
1,4
M 2 +ionic radius (A)
cluster, which gives stronger M-M bonds compared with those in the M6L18 unit. This lengthening reaches 0.07 A when going from chlorine to iodine. The size effect of iodine is obviously important and might explain why 'Nb6118' units have never been obtained. Indeed, in the Nb6L18 units, the Nb-Nb distances are large because of the smaller concentration of valence electrons in the metal-metal bonding states (VEC maximum = 16); this cannot be substantially increased without destabilization of the cluster. This might also explain how, in contrast, TasI18 units have been obtained (for instance in the Ta6114 compound[601),the size of the Ta6 cluster being smaller than that for the Nbs one (see relativistic effects, Section 5.5.5.2).
5.5.5.4 Effects of charge on intra-unit distances The effects of charge on intra-unit distances are clearly apparent from comparison of the distances in Cs2LuNb&1170 and KLuNbhC118 for which the valence electron concentrations are similar (VEC = 16) but the units are differently charged [ ( N ~ ~ C I ' I I O ' ) Cand ~ ~ ~[( Nb6C1'12)C1a6]4]~respectively. When comparing the Nb-L' distances in the two compounds, we should expect smaller distances in the oxychloride than in the chloride because of the matrix effect of the small oxygen. In fact, the average Nb-L' distances are significantly larger in Cs2LuNb6C1170 (2.465 A) than in KLuNb6C118 (2.452 A). It is because of more important Coulombic attraction between ligands and counter cations in the oxychloride, which tends to -
1584
5 Solid-state Cluster Chemistry
increase Nb-L' distances and, indirectly, Nb-Nb distances. Consequently, the matrix effect of the oxygen is counterbalanced and cannot be seen in the oxychloride compared with the chloride. This is why Nb-Nb distances are not shorter in the oxyhalide. This charge effect is clearly apparent from the Nb-Cl" distance, which is not affected by oxygen in the oxychloride and increases by 0.038 A when the unit charge increases from 4- to 5-. The charge effect on M-La can also be observed by comparing compounds with the same unit charge but with a differently charged counter cation in a similar apical ligand environment (compare for instance the M-La distances in Cs2EuNb6Clls and CsErNb6C118 or in Cs2PbTa&llx and CsErTasC118). In the M6L14 based halides the charge of the unit cannot be modified without changing the VEC and we have no example of charge effects on the M6L14 unit in the molybdenum and tungsten halides. This feature is, however, encountered in Re6 chemistry for which differently charged units have been obtained with the same VEC of 24 for the Re6 cluster. For instance, in M10Re6S14,[611with 10- charged units, a tendency similar to that described above for M6L18 is observed, but to a lesser extent than for other Re6 compounds with lower charged units. In the rhenium chalcohalide series, any unit charge change is correlated to halogen-chalcogen substitution on L' sites, so any discussion on Re-L' distance is fallacious. A clear effect on Re-La distances is, however, apparent - for instance, the Re-Cl" distance increases from 2.373 to 2.406 A when going from KResSesC19 to isotypic PbResSe6Cls; it is only 2.330 A (on average) in the parent compound Re6Se~Cllo with a neutral unit. A similar trend has been observed in brominated compounds with an anionic charge up to IZ = 4.[341
5.5.6 The oxyhalides in M6 cluster chemistry When considering the M6 oxyhalides, the most striking feature is the strong distortion of the cluster as a result of the different steric effects of halogen and oxygen. For example, a difference as large as 0.28 A is observed between M-M bonds bridged by oxygen and by bromine in Cs2LaTasBrl503. Such different effects of the matrix on M6 clusters are not encountered in the chalcohalides in which the difference between halogen and chalcogen size is not so important. As discussed above, cluster distortions have also been observed for compounds with unfilled metalmetal bonding states, but usually to a lesser extent. Another interesting observation is the VEC obtained for these oxyhalides. Indeed, a classical VEC = 16 is obtained for the first series, M12MrrrM6X170, whereas VEC = 14 for the two other series, M12M'1rM6X~503and M1"M6X1303, the latter value of which is unusual in M6 solid-state chemistry. Theoretical calcu-
5.5
M6L14 and
M ~ L IUn,i~ts in Early Transition Element Cluster Compounds
1585
lations for the first series have shown that the metal-metal bonding states are similar to those in the corresponding halides, without any significant contribution from oxygen at the a2u HOMO level.[121It is, then, expected that for these compounds VEC = 16 corresponds to the most stable state, in good agreement with experimental results. In contrast, for the last two series, the azUHOMO level is destabilized and closer to the antibonding states, which explains why a VEC of 14 is systematically observed.['21Indeed in several instances when a VEC of 15 would reasonably be expected, a VEC of 14 is experimentally obtained for instance in Cs2UTagC11503 the uranium is found to be trivalent giving a VEC of 14, even though it could be tetravalent which would have resulted in a VEC of 15. Then, if the presence of one oxygen at the inner position is insufficient to change the M-M bonding states compared with the halides, three oxygens around the Mg cluster change these states significantly and the VEC of the compounds get closer to those of the corresponding cluster oxides. Indeed, in the latter example an important M-0 antibonding contribution at the HOMO level does not enable VEC = 16 or 15 and the VEC of 14 is favored. Further comparison of oxyhalides and oxides is difficult because in such Nb6-based oxides, octahedral clusters, triangles, isolated niobium, with different niobium oxidation states, often coexist in a same compound.[6*I -
5.5.7 Physical properties of discrete M&8- and M&4based compounds Because no interaction is possible between the clusters for these discrete M6L18- and MgL14-based compounds, no metallic properties are expected, irrespective of the electron levels filling, and these materials are insulators. The unit acts as a permanent dipole and dielectric relaxations can be observed when the local distribution of the anionic charges around the cluster core is not symmetrical. This behavior has been already extensively studied in Mog and Re6 chalcohalides with discrete or connected M6Lt4 units,1631but not, so far, for the oxyhalides described above. It probably occurs for M:M111MgX170 and M1IrMgX1303, but not for Cs2M111MgX1503 for which the local distribution of the three oxygens is completely symmetrical. As no intercluster interactions occur in these compounds, the valence electrons remain localized on the M6 clusters and paramagnetism arises when valence electrons are unpaired (for instance, VEC = 15 or VEC = 23 for MgLlg or MgL14 respectively). The M ~ ~ M ' l r M g X series l ~ is of particular interest because several situations can be encountered owing to the coexistence of both Mg and rare earth ion networks in the same compound:
1586
5 Solid-state Cluster Chemistry
t
0
***
T I
10
20
30
40
u
I
50
T (K) Figure 9. Dependence of magnetic susceptibility on temperature for LuNb6C118 single crystals and KLuNb6Cllg powdered sample. The insert shows the reciprocal susceptibility of LuNb6C118.
i) both the cluster and the rare earth ion are non-magnetic when x = 1, VEC = 16, and M"' = Lu or La; ii) the cluster is magnetic and the rare earth ion is non-magnetic when x = 0, VEC = 15, and MI1' = Lu or La; iii) the cluster is non-magnetic and the rare earth ion is magnetic when M"' = all the rare earth ions except Lu and La; iv) both the cluster and the rare earth ion are magnetic. Situations (i) and (ii) are depicted in Fig. 9 which illustrates the magnetic susceptibility of KLuNb&llp and LuNb6Cll8. From these data the magnetism of the cluster can be quantified - the LuNb6Cllg cluster is paramagnetic (pen.= 1.5 p B ) because of one unpaired electron, whereas nearly temperature-independent behavior is observed for KLuNb6Cllg. For the latter compound, the constant value of 7.8 x lop4 emu mol-' above 200 K is in good agreement with the van Vleck independent temperature term estimated for other M6L18 compounds.[L31For the former compound, a maximum of susceptibility at 2.5 K has been attributed to interactions between the paramagnetic Indeed, in this compound the clusters are relatively close to each other because of the small size of the lutetium; this behavior has not been observed with larger rare earth ions.
5.5 MgL14 and M ~ L Units ~ R in Early Transition Element Cluster Compounds
I
1587
.*
a)
Figure 10. Reciprocal dependence of magnetic susceptibility on temperature for single crystals of (a) KTmNb6ClI8 and (b) TrnNbsClIR; (c) Magnetic susceptibility of the cluster Ax (%(TmNb6C118) ~(KTmNb6C11~)) below 100 K. The insert shows the reciprocal susceptibility of the cluster. -
Cases (iii) and (iv) are exemplified on Fig. 10 for KTmNb6ClI8and TmNb6C1l8. For these compounds the magnetic susceptibility follows Curie-Weiss behavior with an effective moment close to the Tm'+ value for the quaternary compound; it is larger for the ternary compound owing to the presence of the magnetic cluster in this compound. From these data it has been possible to extract the magnetic susceptibility of the cluster in the ternary compound; as expected this corresponds to one unpaired electron. Other effects, for instance crystal field effects, have been observed in this M:M"'M6Xl8 s e r i e ~ . ' ~Few ~ . ~magnetic ~] data have been reported for
1588
5 Solid-State Cluster Chemistry
the M6L14 based compounds which, of course, have no special magnetic properties. The most prominent result is the paramagnetism observed for Naw6B1-14,corresponding to one unpaired electron and confirming the VEC of 23.
5.5.8 Concluding remarks Structural data, summarized from many isotypical discrete M ~ L ~and R - M6L14based compounds, shows that electronic or steric effects related to various anionic or cationic substitution patterns act similarly on the two types of unit. Of course, if the first effect is greatly dependent on the VEC the second depends mainly on the ligand field and thus, they are more or less important in the two units because of their respective VEC and ligand environment. These units behave as well-defined entities in which the M6 cluster seems quite flexible, and they can be used in a variety of crystal stacking arrangements, without cleavage of the metallic core, to give many original types of structure. Thus, such materials can act as precursors in solution chemistry in which the units behave as anionic building blocks[66-681 or in soft chemistry enabling a number of exciting new synthetic techniques extensively developed recently, to form original organo-mineral materials.
Acknowledgments Dr. S. Cordier, Dr. S. Ihmaine and C. Loisel are greatly acknowledged for their contribution to the crystallochemistry of Nb6, Tas and w6 compounds. Professor J.-Y. Saillard, Dr. J.-F. Halet and Dr F. Ogliaro are thanked for theoretical studies on many of these cluster compounds. All these and Dr. M. Sergent and Dr. A. Perrin are acknowledged for helpful discussions.
References [ l ] C. Brosset Ark. Kem. Mineralog. Ser. A, 20 (1945) 16 [2] P.A. Vaughan, J.H. Sturdivant, L. Pauling J. Amer. Chem. SOC.72 (1950) 5477 [3] A. Simon in Clusters and Colloids. From Theory to Applications, G. Schmid Ed., VCH, Weinheim (1994) [4] H. Schafer, H.G. von Schnering Angew. Chem., 76 (1964) 833 ~
~
~
5.5 M6L14 and M6Lln Units in Early Transition Element Cluster Compounds
1589
S. Cordier Thesis of University, Rennes (1996) S. Ihmaine, C. Perrin, M. Sergent, El Ghadraoui Ann. Chim. 23 (1998) 187 J.D. Corbett J. of Alloys and Compounds, 229 (1995) I0 F.A. Cotton, R.E. Haas Inorg. Chem., 3 (1964) 10 D.M.P. Mingos J. Chem. SOC.Dalton, (1974) 133 D.J. Robbins, A.J. Thomson J. Chem. SOC.Dalton Trans., (1972) 2350 J.D. Smith, J.D. Corbett J. Am. Chem. SOC.,107 (1985) 5704 F. Ogliaro, S. Cordier, J.-F. Halet. C. Perrin, J.-Y. Saillard, M. Sergent - Inorg. Chem., 37 (1998) 6199 J.G. Converse, R.E. Mc Carley - Inorg. Chem., 9 (1970) 1361 B. Spreckelmeyer, H.G. von Schnering Z. Anorg. Allgem. Chem., 386 (1971) 27 A. Simon, H.G. von Schnering, H. Schafer - Z. Anorg. Allgem. Chem., 361 (1968) 235 F. Ueno, A. Simon - Acta Cryst., C41 (1985) 308 0. Reckeweg, H.-J. Meyer - Z. Kristallogr., 211 (1996) 396 B. Bajan, H.-J. Meyer Z. Naturforsch., 50b (1995) 1373 A. Lachgar, H.-J. Meyer J. Solid State Chem., 110 (1994) 15 J. Silar, A. Lachgar, H.-J. Meyer Z. Kristallogr., 211 (1996) 395 A. Broll Thesis (communicated by A. Simon) S. Cordier, C. Loisel, C. Perrin. M. Sergent J. Solid State Chem., in press S. Cordier, C. Perrin, M. Sergent - Z. Anorg. Allgem. Chem., 619 (1993) 621 J. Silar, A. Lachgar, H. Womelsdorf, H.-J. Meyer - J. Solid State Chem., 122 (1996) 428 S. Ihmaine. C . Perrin, M. Sergent Acta Cryst., C43 (1987) 813 S. Ihmai'ne, C. Perrin, 0. Pefia, M. Sergent J. Less Common Met., 137 (1988) 323 S. Ihmai'ne, C. Perrin, M. Sergent Acta Cryst., C45 (1989) 705 S. Cordier, C. Perrin, M. Sergent J. Solid State Chem., 118 (1995) 274 S. Cordier, C. Perrin, M. Sergent Mat. Res. Bull., 31 (1996) 683 S. Cordier, C. Perrin, M. Sergent - Mat. Res. Bull., 32 (1997) 25 S. Cordier, C. Perrin. M. Sergent - J. Solid State Chem., 120 (1995) 43 S. Cordier, C. Perrin, M. Sergent Eur. J. Solid State Inorg. Chem., 31 (1994) 1049 W. Bronger, M. Kanert, M. Loevenich, D. Schmitz Z. Anorg. Allgem. Chcm., 619 (1993) 2015 A. Slougui, S. Ferron, A. Perrin. M. Sergent - Eur. J. Solid State Inorg. Chem., 33 (1996) 1001 A. Simon, H.G. von Schnering, H. Schiifer - Z. Anorg. Allgem. Chem., 355 (1967) 295 H.-J. Meyer, J.D. Corbett - Inorg. Chem., 30 (1991) 963 T. Hughbanks, R. Hoffmann - J. Am. Chem. SOC.,105 (1983) 1150 Y.Q. Zheng, Yu. Grin, K. Peters, H.G. von Schnering Book of Abstracts, 26th General Meeting of GDCh, Vienna (l997), p. 187, AC-NM 33 Y.Q. Zheng, Yu. Grin, K. Peters, H.G. von Schnering Proceeding of VIth European Conference on Solid-state Chemistry, Zurich (1997) A. Peppenhorst, H.-L. Keller Z . Anorg. Allgem. Chem., 622 (1996) 663 I S. Ihmaine, C. Perrin, M. Sergent - Eur. J. Solid State Inorg. Chem., 34 (1997) 179 M. Potel, C. Perrin, A. Perrin, M. Sergent - Mat. Res. Bull., 21 (1986) 1239 P.C. Healy, D.L. Kepert, D. Taylor, A.H. White J. Chem. SOC.Dalton Trans. (1973) 646 S. Boschen, H.-L. Keller Z. Kristdllogr., 200 (1992) 305 H.G. von Schnering ~- Z. Anorg. Allgem. Chem., 385 (1971) 75 S. Ihmai'ne, C. Perrin, M. Sergent Croatica Chem. Acta, special issue on Metal Cluster Theory and Experiment, 68 (4) (1995) 877 A. Slougui, A. Perrin to be published L. Leduc, Thesis Rennes 1983 A. Perrin, L. Leduc, M. Potel, M. Sergent Mat. Res. Bull., 25 (1990) 1227 1 A. Slougui. A. Perrin. M. Sergent Acta Cryst. C, 43 (1992) 1917 J. Cluster Sci., 8 (1997) 349 b l i A. Slougui. - S. Ferron. A. Perrin, M. Serpent -
~
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-
~
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-
-
I590
5 Solid-state Cluster Chemistry
[52] H.-J. Meyer, J.D. Corbett - Inorg. Chem., 31 (1992) 4276 [53] R. Chevrel, M. Sergent Topics of Current Physics, vol. 32 Superconductivity in Ternary Compounds I, Ed. 0. Fischer, M.B. Maple, Springer Verlag, Berlin, Heidelberg (1982) 25. [54] K. Yvon Current Topics, 3 (1979) 55 [55] J.D. Corbett J. Solid State Chem., 39 (1981) 56 [56] T.C. Zietlow, W.P. Schaefer, B. Sadeghi, N. Hua, H.B. Gray Inorg. Chem., 25 (1986) 2195 [57] T.C. Zietlow, W.P. Schaefer, B. Sadeghi, D.G. Nocera, H.B. Gray Inorg. Chem., 25 (1986) 2198 [58] J. Emsley -~The Elements, Clarendon Press, Oxford (1989) [59] J.D. Corbett - J. Solid State Chem., 37 (1981) 335 [60] D. Bauer, H.G. von Schnering, H. Schafer - J. Less Common Met., 8 (1968) 388 [61] W. Bronger, M. Kanert, M. Loevenich, D. Schmitz Z. Anorg. Allgem. Chem., 619 (1993) 2015 [62] J. Kohler, G. Svensson, A. Simon Angew. Chem. Int. Ed. Engl., 31 (1992) 1437 [63] J.C. Pilet, F. Le Traon, A. Le Traon, C. Perrin, A. Perrin Surface Science, 156 (1985) 359 [64] 0. Peiia, S. Ihmaine, C. Perrin, M. Sergent Solid State Comm., 74 (1990) 285 [65] S. Ihmaine, C. Perrin, 0. Peiia, M. Sergent Physica B, 163 (1990) 615 [66] S. Uriel, K. Boubekeur, P. Batail, J. Orduna - Angew. Chem. Int. Ed. Engl. 35 (1996) 1544 [67] U. Beck, A. Simon, S. Sirac, N. BrniEeviC - Z. Anorg. Allgem. Chem., 623 (1997) 59 [68] S.J. Hilsenbeck, V.G. Young, R.E. McCarley Inorg. Chem., 33 (1994) 1822 [69] V.P. Fedin, H. Imoto, T. Saito, V.E. Fedorov, Y.V. Mironov, S.S. Yarovoi Polyhedron 15 (1996) 1229 ~
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Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.6 Ternary Rhenium and Technetium Chalcogenides Containing Re6 or T c ~ Clusters WelJ'Bronger
Dedicated to Dr. Marcel Sergent on the Occasion of his Retirement*
5.6.1 Introduction In the final chapter of a review on cluster compounds of the heavy transition metals by H. Schafer and H.G. von Schnering in 1964,"l issues which still remained unsolved were raised. One unanswered question at that time concerned the possibility of the existence of an Re6 cluster. Fourteen years later we synthesized and structurally characterized the first Re6 cluster compounds in our research group in the form of the ternary alkali-metal rhenium sulfides.[21After this we investigated this new group of compounds in detail. The results obtained since that time now enable a synopsis to be made within which, in addition to the ternary rhenium compounds, the analogous compounds of technetium can also be included. In parallel with our investigations, ternary rhenium chalcogen halides also containing Re6 clusters as the characteristic structural unit have been investigated, mainly in the Laboratoire de Chimie Minerale B, Unite Associee au C.N.R.S. in Rennes. A summary of this work has already been reported;[31and parallels between the properties of both groups of compounds will be discussed.
5.6.2 The structural systems of the ternary chalcogenides of rhenium and technetium The crystal structures of every known ternary alkali-metal rhenium sulfide and selenide all contain [ResXg] structural units in which X = S or Se. These structural
* This dedication is in agreement with the publishing editor Dr. Gndum Walter.
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5 Solid-state Cluster Chemistry
Figure 1. The structural unit [Re6X8] and six linking X atoms.
groups are extended by six further chalcogenide ligands which form the bonds to the rhenium atoms of each Re6 octahedron and, in addition, facilitate linkage between the structural units (Fig. 1). Without these linkages, {[Re6Xs]X6}lo& ions result if one assumes that the rhenium atoms are in the +3 oxidation state, thus stabilizing the Re6 cluster with 24 valence electrons. For a one-dimensional linkage model, one obtains under corresponding conditions { [Re6Xs]X4X2/2} chains; a 6- layers; and a three-dimensional two-dimensional linkage yields { [Re6Xg]X2X4/2} linkage results in a {[Re6Xs]X6/2}4-framework structure. With the exception of the one-dimensional case, all these variants have been experimentally proven to exist (Table 1). In the field of the rhenium chalcogen halides, an analogous sequence of structures can be observed.[31For the Re6X4Ylo = [Re&4Y4]Y6 (X = chalcogen and Y = halogen) composition, isolated structural groups are formed. For the compositions R ~ ~ X= S {Re6X5Y-j]Y4Y2/2, Y ~ RefjX6Y6 = [Re6X6Y2]Y2Y4/2, and Re6X7Y4 = [Re6X7Y]Y6/2 chain, layer and, finally, framework structures, respectively, result. A similar sequence of structural changes as with the alkali-metal rhenium chalcogenides on the one hand and rhenium chalcogen halides on the other can also be observed in the quaternary compounds. This results in the KRe6SesC19 compound having a crystal structure which corresponds to the arrangement in NaCl - with K+ cations and [Re6SesC13]C16- anions. The structural system of the ternary rhenium chalcogenides can be expanded given that the linkage provided by the chalcogenide bridges in the framework structure can be substituted by dichalcogenide or even trichalcogenide bridges. The combinations of these various bridges that have been observed are also listed in Table 1.
'-
5.6 Ternary Rhenium and Technetium Chalcogenides
1593
Table 1. Structures of the ternary chalcogenides of rhenium and technetium A,M6X, where A alkaline earth or alkali metal and X = sulfur or selenium. Linkage scheme in the anionic M-X sub-structure
Ternary sulfides of rhenium and technetium
=
Ternary selenides of rhenium and technetium
With regard to the alkali-metal cations A which are incorporated in these framework structures, it is evident there is strong correlation between the size of the cations and the type of linkage formed by the [ResXs] structural units. The isolated [Re&&]& groups that are known are exclusively formed with the largest cations Rb+ and Cs+. This also appears to be valid for the layer structures. The types of bridge that result in the framework structures are clearly dependent upon the size of the alkali-metal cations (Table 1). Consequently, with Li4ResS11 = Li4 { [ResSs]S6/2}[61 containing the smallest cation, the Re6 octahedra are linked only by S2- bridges whereas in the spacious framework structure of Cs4Re6S13.5 = containing the largest cation, two S2- bridges,
1594
5 Solid-state Cluster Chemistry
three S22- bridges, and one S3*- bridge are present. Between these two extremes are many levels in which the selenides with their larger cavities can also be systematically classified (Table 1). In the row of corresponding alkaline-earth metal compounds, only the sulfides of strontium and barium and analogs with europium are known. Because the number of cations incorporated in the structure halves, the charge being doubled as a result, a linkage which is as compact as possible is favored; this corresponds to the formula A2Re6S11 = A2{[Re&,]S6p} where A = Ba, Sr or Eu.[~,'] The corresponding technetium compounds synthesized so far are isotypic with the analogous rhenium compounds (Table I). A framework structure in which the linkage is exclusively achieved via disulfide bridges is plainly an arrangement that is too spacious for Cs+ - a suitable compound Cs4Re6S14 = Csq { [Re6S8](S2)6p}has never been observed. A ternary sulfide CssResS15 does, however, exist['41 and also the corresponding ~elenide;~'~] their structures contain dichalcogenide bridge linkages in which, however, not four but six cesium ions are incorporated in addition to an X2- ion corresponding to CS6X{[Re6XxI(X2)6/21. In the following sections, the sequence of structures of the ternary chalcogenides of rhenium and technetium are presented in detail.
5.6.2.1 Ternary chalcogenides of rhenium and technetium containing isolated clusters To prepare the compounds Rb 1oTe6S14, Cs10Te6S14, Rb 1o Re6 S 14, and Cs 10 Re6 S14,I4] a mixture of the alkali carbonate and rhenium or technetium in 5 : 1 molar ratio was in each case converted at 800 "C in a stream of hydrogen doped with sulfur. The duration of these reactions was between 10 and 16 h. The preparation of the technetium compounds was performed in a radiochemical laboratory of the Forschungszentrum Julich. The 99Tc isotope used decomposes by ,K emission. The compounds were obtained as black, lustrous crystals embedded in the partially solidified melt (Fig. 2). All four sulfides are unstable in air. X-ray diffraction studies of the single crystals revealed an isotypic atomic arrangement. The cubic unit cells (space group Fmjm) contain four isolated [M6S14] structural units whose centers form a fcc lattice. Together with the alkalimetal ions in the 4a site (000), they form an arrangement analogous with that in NaCl (see Fig. 3). The remaining 36 alkali-metal ions statistically occupy the sites 8c, 32f, and 48h. The composition could be confirmed by measurement of the magnetic susceptibilities which indicate the +3 oxidation state for rhenium and technetium. The final structural refinements converged in each case for an occupation density of the alkali-metal sites corresponding to a total of 39 to 40 atoms. Therefore, the occupation factors determined for each individual structure were multiplied by a constant factor in every case so that the sum of the alkali-metal atoms in the unit cell came to 40 (see Table 2).
5.6 Ternary Rhenium and Technetium Cliakogenides
1595
Figure 2. A photograph ofCs,OTc& crystals grown from a melt taken using an electron microscope.
The crystal structure determined is characterized by regular [M&4] structural units in which the sulfide ions surround the Mg metal cluster in a trisoctahedron. The cations occupy the corners, edges and faces of a rhombic dodecahedron around the [ M ~ S Iunits ~ ] (see Fig. 4), thereby achieving optimum shielding of these complex anions. Potential energy calculations indicated that, with the exception of the 4a site, for which the potential is distinctly higher, all the cations lie on an approximately equipotential surface. The pronounced statistical distribution of the cations over the various sites is evidently connected with the spherical shape of the complex anion, its high charge, and the size ratio of the ions. The structure of T12MogBr14 can be taken as an example of an analogous system;[161in this example the [MogBrI4l2-anion occupies the same topology as the [ M ~ S Istructural ~] units we have found. Its charge is, however, lower so that, in contrast to the compounds Al0TcgS14 and AloRegS14, only half and not 9/10 of the cations are statistically distributed. In each case the ions have comparable sizes.
5.6.2.2 Cs&e& - a ternary rhenium chalcogenide with two-dimensionally linked clusters One-dimensionally linked Tcg or Reg clusters have hitherto not been observed among the ternary chalcogenides discussed here (however, see Section 5.6.4). The compound CsgRegS12 is the only example so farc5]of a two-dimensionally linked
1596
5 Solid-state Cluster Chemistry
Figure 3. Isolated [M&1S6 trisoctahedra and cesium atoms forming a rock-salt-type structure in A10M& with A = Rb or Cs and M = Tc or Re.
cluster in this field. The preparation resulted from a melt reaction in which the cesium carbonate and rhenium powder in the molar ratio 5 : 1 were heated to 800 "C in an argon stream doped with sulfur. After reaction for 70 h the mixture was maintained at 400 "C for 5 h before cooling to room temperature. Red, almost square plates resulted; these slowly decompose in air. The determination of the structure was performed using single crystals. The twodimensional linkages of the [Re6S14] structural units are depicted in Fig. 5; Fig. 6,
Table 2. Population parameters of the alkali metal atom positions 4a, 8c, 32f, and 48h ("A,). Compound
4a
8c
32f
48h
5.6 Ternary Rhenium and Technetium Chalcogenides
1597
Figure 4. Coordination of a [M&] unit by alkali-metal ions in A I o M & ~ The . occupancy of the different positions are given in Table 2. For simplification only two positions of 48h are shown in one face of the rhombic dodecahedron.
in a diagram viewed perpendicularly to Fig. 5 , shows the linkages of the Re6 clusters via sulfur bridges. Fig. 7 shows the complete atomic arrangement. An analogous linkage principle is evident in the molybdenum and tungsten halides MsY12 = [ M ~ Y ~ ] Y ~ (YY~=/ C1, z Br, I)"'] and the isotypic compound Ret;Se,jClb = [Re,jSe&12]C12C14/2 from the area of the rhenium chalcogen halides described In contrast with Cs6Re6S12, however, a less pronounced layer structure exists in these compounds because here the planes of the bridged units mutually penetrate by means of their terminal outer ligands. Comparison of the b axis lengths reflects this very clearly (Table 3 ) . The layer-like construction of the
Figure 5. Two-dimensionally linked trisoctahedra in Cs6Re&?.
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5 Solid-State Cluster Chemistry
Figure 6. Linkage of the Re6 clusters in CsgRe&. Projection along the b axis.
Cs6Re6S12structure is expressed in the anisotropic chemical properties of the compound. The addition of water to the plate-shaped CsgRe6S12 crystals causes them to split parallel to the planes of the plates. This type of reaction is not known for the molybdenum and tungsten halides M6Y 12.
Figure 7. Atomic arrangement in Cs6ResS12. The sulfur atoms forming the Sg cubes around the Re6 clusters are omitted.
5.6 Ternary Rhenium and Technetium Chalcogenides
1599
''
Table 3. Lattice constants for the compounds M0gC112,'~ ResSe6C16,"81 and CS6Re&.[5'
MOsc112 RebSefjCls CsgRe&
11.277(3) 11.310(4) 1 1.663(4)
14.068(3) 13.892(2) 21.45(1)
11.253(3) 1 1.286(5) 1 1.5 17(2)
5.6.2.3 Ternary chalcogenides of rhenium and technetium with three-dimensionally-linked clusters Ternary chalcogenides containing M6 clusters linked three-dimensionally by sulfur bridges are currently only known for M = Re. The four such compounds which have been characterized are Li4ResS11,I6]Ba2Re&l ,I7]Sr2ResS11,I7] and EuzRe&~1 The syntheses of these compounds, that is analogous to that described above, involved the employment of the carbonates of the base metal components, although the oxide Eu203 was used for the europium compound, with elemental rhenium. The conversion was accomplished in the temperature range between 1200 and 1450 "C in a stream of hydrogen sulfide. We obtained the lithium compound in the form of red crystals whereas the remaining sulfides form grey-black crystalline products. The compounds are stable in air and also stable towards dilute acids. The crystal structure of Li4Re6Sl1 is depicted in Fig. 8. In addition to the X-ray single crystal structure investigations, the positions of the lithium atoms were determined by neutron diffraction experiments on samples of Li4Re&1 powder. A
Figure 8. Atomic arrangement in Li4ResS11 omitting the s8 cubes.
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5 Solid-state Cluster Chemistry
characteristic of the atomic arrangement is again the Re6 clusters coordinated by eight sulfur ligands occupying the corners of a cube. A three-dimensional anionic framework structure {: [Re(&]S6,24-$ results from the sulfur bridging between the Re6 clusters in which the lithium ions are incorporated as layers perpendicular to the b axis. The framework structure corresponds to the atomic arrangement in Nb6111= ~{[Nb61s]16/2).[19,201 The same sites are occupied in the space group PCCn with very similar positional parameters. Analysis of the distances reveals, however, characteristic differences. In agreement with the assumption that the Re6 structural units in Li4ResS11 have 24 valence electrons, only a slight distortion of the rhenium octahedra is observed (further details are given in Section 5.6.3). For Nb6I11 with only 19 valence electrons per niobium octahedron, the distortion is considerably greater. The magnetic properties are also in agreement with these observations and will likewise be reported in a subsequent section. The compounds BazResS11, SrzRe6S11,and EuzRe&1 all crystallize with an isotypic atomic arrangement. The arrangement of the [ResSs] structural units in the [ Re6Ss]S6/24- framework structure can be derived from cubic close-packing of spheres. The c/a ratio is distinctly smaller than the ideal value. This compression is caused by the linking between the Re6 clusters, because each of the clusters has six unlinked cluster neighbors perpendicular to the c axis. Above and below this plane however, all three neighbors are linked by sulfur atoms at a consequently shorter distance (Fig. 9). The A atoms in the A2Re6S~1series of compounds are coordinated by ten sulfur atoms and the resulting coordination polyhedron can be described in terms of a threefold capped trigonal prism. One triangular plane is also capped. Two of these units are linked by their non-capped triangular plane. The pairwise filling by the A atoms results in a short A-A distance. In the europium compound it is 3.84 A. A consequence of this arrangement is that one can observe interactions, evidently antiferromagnetic coupling, between the europium ions at very low temperatures.[*] These types of framework structure presented, in which the Re6 clusters are three-
Figure 9. Linkage of the Re6 clusters in EuzRe&1. Projection along the c axis.
5.6 Ternary Rhenium and Technetium Chalcogrnides
0
1601
e
Figure 10. Atomic arrangement in A&e&12
= A4[Re6Xa]X4/2(X2)2,2
dimensionally linked by sulfur bridges, contain holes in which, per formula unit, either four cations of the smallest alkali-metal ion Li+ or two of the larger alkalineearth metal ions Sr2+ or Ba2+ can be incorporated. The incorporation of sodium, which is slightly larger than lithium, causes the holes in the framework structure to be adapted so that two of the six sulfur bridges originating from the Mg cluster are replaced by disulfur bridges['] (Fig. 10). This type of structure can also be obtained with potassium[91and rubidium.['01 For rubidium a second compound exists in which four of the sulfur bridges are now substituted by disulfur bridges["] (Fig. 11). The same structure exists with cesium,[l3] although as an alternative variant a compound that is even richer in sulfur has been found in which yet another disulfur bridge is replaced by a trisulfur bridge['31(Fig. 12). The separate structural changes can be followed using the sequence of volume increments of the alkali-metal ions. This reveals that the expected volume increase in the unchanged framework structure does not occur with the increasing size of the alkali-metal ions. The change in the structure results in this shortfall being 'restored'[''] (see Table 4). The sequence indicates a certain rigidity within the framework structure. The different types of structure and their representatives are listed in detail in Table 1. One can also infer from the list that the hitherto known technetium compounds maintain an analogous sequence. When one considers that larger holes are available for the incorporated cations with the corresponding selenium compounds, it is understandable why these compounds can incorporate larger cations. One can also infer this from the details in Table 1. Finally, another framework structure also exists in which disulfur or diselenium bridges are exclusively formed.['4,' 51 Here,
1602
5 Solid-State Cluster Chemistry
not only are four large cesium ions incorporated per formula unit, but two additional cesium ions and a sulfide or selenide ion are also included (Fig. 13). It should be added that syntheses of the group of compounds with the common, distinctive structural feature of the occurence of dichalcogen bridges, largely follows the method outlined above; quantities of the relevant alkali carbonate and rhenium or technetium serving as the starting materials. The chalcogen component is transported to the reaction area by hydrogen or argon and the reaction temperature is between 800 and 950 "C. Single crystals could always be isolated from the solidified melt; this enabled complete structural determinations. The compounds are stable in air and the crystals have a black, lustrous appearance although they appear red under transmitted light.
5.6 Ternary Rhenium and Technetium Chalcogenides
1603
Table 4. Volume of the unit cell V E , Z= 4; volume per formula unit V F ,as well as the observed volume differences ( V,,) and the calculated volume differences according to Biltz ( Vc). Compound
VE
(A')
VF
(cm')
V , (cm')
V , (cm')
38
16 12* 24 5+ ~~~
~
~
*The given value is equivalent to the difference between the volume of K2S2 ( V F= 73 cm3) and that of K2S ( V r = 61 cm3). t The given value is equivalent to half the difference between the volume of K2S' ( V F= 82 cm') and that of K2S2 ( V F= 73 cm3).
Figure 13. Atomic arrangement in Cs,jRe,jXl*= Cs&[Re6X8](X2)6/2.
1604
5 Solid-state Cluster Chemistry
5.6.3 Experiments and calculations for the characterization of the bonding in clusters of the ternary chalcogenides of rhenium and technetium 5.6.3.1 Magnetochemical and vibrational spectroscopic investigations The magnetic susceptibilities of almost all of the ternary rhenium and technetium chalcogenides listed in Table 1 have been measured in the temperature range between -269 "C and room temperature. Almost temperature-independent diamagnetism was observed (an exception is the europium compound[*]). Deviations from this behavior were only evident at the lowest temperatures when the susceptibilities increased with decreasing temperature; this we assign to the presence of minor impurities. By use of the diamagnetic ion increments to correct the molar susceptibilities, paramagnetic values in the range 100 x lop6 to 600 x lop6 cm3 mol-' were obtained. Any correlation between the weak temperature-independent paramagnetism and the various linking models of the clusters is not recognizable. Corresponding magnetic properties were also observed for other compounds containing Mg clusters, in fact always when 24 valence electrons are present per Mg cluster for the metal-metal bonds in analogous structural units. Where other electronic configurations occur with corresponding structural units, one mostly finds complicated paramagnetic behavior such as is observed, for example, with NbgI11 which has 19 valence electrons per niobium cluster. We have observed 24-electron-clusters only for the ternary rhenium and technetium compounds. As examples, the IR and Raman spectra of the cluster compounds Rb4RegS13 and Cs4RegS13.5 were measured and interpreted.r211The observed S-S stretching vibrations (425-480 cm-' ) correspond closely to those characteristic of a simple S-S covalent bond. Weakening of the bond as a result of interactions with the Re-S bonds, similar to those observed with the pyrite-type compounds of the 4d and 5d transition elements,r221does not occur. One can recognize the substitution of an S2 bridge by an S3 bridge during the structural change from Rb4RegS13 to Cs4RegS13 5 from the appearance of the S3 deformation vibration at 222 (IR) and 214 cm-' (Raman). In comparison with other sulfides, the shift to lower wavenumbers of this vibration is probably a result of the significant broadening of the S--S-S angle (124.5 O [ 1 3 ] ) . The vibrational bands of the {[Re&]Sg} group are shifted to higher wavenumbers than those of the analogous {[MgCl8]Clg}units (M = Mo, W)r23-251 this is contrary to and those of the Chevrel phases containing [MogSg] the sequence of the molar masses of the M atoms. This indicates that comparatively stronger bonds exist in the rhenium-sulfur structural units.
5.6 Ternary Rhenium and Technetium Chulcogenides
1605
5.6.3.2 Molecular orbital calculations for the determination of the relative stabilities of M6 clusters The investigations presented so far have shown that a 24-valence-electron configuration for the Re6 and T c octahedra ~ seems to constitute a relatively stable state. Comparison with the analogously constructed compounds of the neighboring elements molybdenum, niobium, and tungsten reveals in addition the existence of a correlation between the number of valence electrons per M6 cluster and the geometry of the M6 octahedra. Thus almost completely undistorted octahedra are found for M06 clusters containing 24 valence electrons, as seen for the Re6 and T c clus~ ters, whereas the Nb6 octahedra contain fewer than 24 electrons and are considerably distorted. The molecular orbital calculationsr281performed begin by using the currently accepted Also, when addressing the matter of collective structures in which the [M6Xs] structural units are linked in different solid-state structures, the simple quantum mechanical methods chosen are evidently suitable for relative comparison of the bonding ratios. This is shown by the good agreement with the experimental results. From the relative orbital energies of undistorted and distorted M6 clusters, the relative total energies for various valence-electron configurations were calculated dependent upon the parameter p = ( . d where ( is the Slater exponent1301and d is the distance in atomic units between nearest neighbors in the M6 clusters. The latter values are calculated from the equation d ( n ) = d(1) - 0.60logn as given by Pa~ling.[~'] This leads to good agreement with experimentally determined distances. The relative total energies are shown in Fig. 14 relative to p for various symmetries and electron configurations. The results show that for the 4d elements and for p < 7, the M6 clusters are stable when the number of valence electrons is lower than 24. Current experimental data confirm the predicted sequence in that for the Nb6 cluster with less than 24 electrons, a reduction in the 01,symmetry should not only be favored but would gain additional energy. One observes exactly this for the 19-electron cluster present in Nb6111where the distortion results in a lowering of the symmetry to D3. A deviation from Oh symmetry for M06 clusters with 24 electrons leads to no gain in energy. Not until the M06 cluster contains fewer than 24 electrons ( p > 7) can symmetry distortion be expected. In fact, all known M06 clusters occur with 24 electrons in undistorted octahedra. One finds deviations from the Oh symmetry in clusters with fewer electrons such as appear in the so-called Chevrel phases. The model calculations presented here should not, however, be applied automatically because several representatives of these compounds are beginning to show metallic properties. Clusters of TC6 with 24 valence electrons are stable as undistorted octahedra.
1606
5 Solid-state Cluster Chemistry
Figure 14. Upper: relative total energies of Mg clusters with Oh and D 3 d symmetry containing 19 or 24 electrons as a function of p . Principal quantum number n = 4; effective quantum number n e= ~ 3.7. Lower: relative total energies of Mg clusters with 22 or 24 electrons as a function of p . Principal quantum number n = 5; effective quantum number neff = 4.
5.6 Ternary Rhenium and Technetium Chalcogenides
1607
Electron-poor configurations and/or lowering of the symmetry produce no energy gain. The fully characterized T c cluster ~ compounds all have a 24-valence-electron configuration and show no significant deviation from Oh symmetry. The situation is similar for the energy curves of W6 clusters, because of the diagonal relationship between tungsten and niobium, as exists for the niobium compounds. The w6 clusters containing fewer than 24 electrons should be stable. In addition, one should expect a further stabilization by reduction of the symmetry. Tungsten compounds containing Wg clusters and 24 valence electrons form undistorted octahedra whereas one compound with 22 valence electrons per W6 cluster, W6Brl6 = [W6Br8]Br4[Br4]2,2,should, by contrast, contain w6 octahedra with a symmetry lower than O h . An earlier structural investigation did not supply sufficiently precise data to enable confirmation or denial of the existence of distorti~n.[~’I On the basis of the calculations submitted here, a new determination of the crystal structure was undertaken and a distorted tungsten cluster, as previously stated, was observed to exist in WsBr16.[~~] Finally, the energy curves for the Re6 cluster confirm the stability of the 24valence-electron configuration which arises. As with the technetium compound, electron-poor configurations and/or a reduction of symmetry bring no gain in energy.
5.6.4 The 24 valence electron configuration of the Re6 cluster and the explanation of contradictory experiments was reported. This sulfide has In Section 5.6.2.3 the compound already been discussed in our first publication, although the composition was stated to be Cs&e6S13.[~lDetermination of the crystal structure led to the formula C S ~ [ R ~ ~ S X ] S ~ , ~i (e S . the ~ ) ~[ResSs] , ~ S , structural units are linked by two sulfur bridges and by three disulfur bridges, and each unit contains a non-bridging sulfur ligand. The atomic arrangement was plausible because two sulfide and four disulfide bridges exist in the corresponding rubidium compound Rb4Re6S13 so the explanation followed that the site requirements of the larger Cs ions force the splitting of a disulfide bridge and thereby create a considerably larger hole. Measurements of magnetic susceptibilities performed later revealed a temperature-independent, weak paramagnetism which indicated a 24-electron system and, consequently, was incompatible with the 23 electrons present per Re6 cluster in the structural formula given above. A very careful new determination of the crystal structure was performed in which a complete Ewald-Kugel sphere was measured; this revealed the extra bridging between two sulfur ligands to form an S32p bridge. The discovery of +
I608
5 Solid-state Cluster Chemistry
the additional sulfur atom in the Fourier synthesis had consequently been impeded because it occupies a split site and so was hidden in two equivalent sites with an occupation factor of one half (Fig. 12). A compound with the composition K2Rb2Re& is included in Table 1. There were also inconsistencies for this compound. Synthesis was achieved by conversion of rubidium carbonate (purest form, Merck) with rhenium powder at 800 "C in a stream of hydrogen sulfide. Determination of the structure pointed to the formula Rb3Re6S13. Although one of the rubidium sites was only 50% occupied, it was obvious that the Rb-S distances for this position were somewhat shorter than those in the fully occupied sites, which contradicted previous experience. Additionally, the magnetic behavior also indicated a 24-electron cluster, despite the above formula suggesting the availability of only 23 electrons. To explain this contradiction, an energy-dispersive X-ray spectrum was required. Rather than half occupation by one rubidium ion, it was discovered that a site was fully occupied by an isoelectronic potassium ion. From where did the potassium originate? The rubidium carbonate used was reported by the manufacturer to contain 0.2% potassium carbonate as an impurity. Evidently, this small proportion accumulated in the resulting crystals, limited by the framework structure, to fill the holes. In 1986 we successfully synthesized a previously unknown cesium-rhenium sulfide. Investigation of the single-crystal structure pointed to the composition Cs3Re6S11. The atomic arrangement found could be described as a framework structure of [Re&] structural units linked by sulfur bridges and into which cesium ions were incorporated.[341The corresponding formula is Cs3 { [Re6S8]S6/2}(Fig. 15). The composition suggests an as yet unknown stable Re6 octahedron contain-
Figure 15. Atomic arrangement in Cs3ResOsS1I = C S ~ [ R ~ ~ O S S ~ ] S ~ / ~ .
5.6 Ternary Rhenium and Tecknetium Chalcogenides
1609
ing 23 valence electrons per octahedron. At that time the expected paramagnetism could not be observed because it was found to be impossible to reproduce the preparation of the compound. We observed the formation of this compound again 10 years later when, in addition to experiments trying to discover new rhenium sulfides, the synthesis of corresponding osmium compounds was attempted. It was soon shown that the results ‘ C S ~ R ~’ & ~ when rhenium and osmium metal are used simultaneously. The ensuing investigations finally revealed[341that the quaternary compound concerned was Cs3ResOsS11. But how was this compound obtained ten years earlier? Investigations indicated that during the series of experiments conducted at that time, osmium was once employed as the transition metal. This led to the conclusion that residues of volatile osmium compounds in the reaction area are sufficient to enable the formation of the compound Cs3ResOsSl I during experiments in which supposedly only rhenium was employed as the transition metal. Now that the preparation of this compound could be reproduced, its magnetic properties were also measured. Size and temperature variations correspond to a 24-valence-electron configuration referenced to the [ResOs] cluster. During the later attempts at the synthesis, another compound was discovered. Single-crystal investigations yielded the composition C S ~ M ~ Si.e.I ~Cs4{[M6S8]S2pS4}, , the first example in the field of the ternary chalcogenides of a one-dimensional [M&] cluster arrangement linked by sulfur bridges (Fig. 16).[351The clusters are again almost regular. Energy-dispersive X-ray analysis
Figure 16. Atomic arrangement inCs4M&i = C S ~ [ M & ] S ~ , ~ S ~ .
1610
5 Solid-state Cluster Chemistry
revealed, in addition to the rhenium content, an osmium content that was considerably larger than that in Cs3ResOsS11. Applying the underlying theme of the 24electron-rule as seems to be valid here, the compound should have the composition C S ~ R ~ ~ O Further S ~ S I ~investigations . are planned.
Acknowledgment I thank the Fonds der Chemischen Industrie and the Bundesminister fur Bildung, Wissenschaft, Forschung und Technologie for financial support of this work, and Dr. Paul Muller and Dr. Christopher Puxley for assistance in preparing the manuscript.
References [ 11 H. Schafer, H. G. von Schnering, Angew. Chem. 1964, 76, 833. [2] M. Spangenberg, W. Bronger, Angew. Chem. 1978, YO, 382; Angew. Chem. Int. Ed Engl. 1978, 17, 368. [3] A. Perrin, M. Sergent, New J. Chem. 1988, 12, 337. [4] W. Bronger, M. Kanert, M. Loevenich, D. Schmitz, Z. Anorg. Allg. Chem. 1993, 619, 2015. [ 5 ] W. Bronger, M. Loevenich, D. Schmitz, J. Alloys Comp. 1994, 216, 25. [6] W. Bronger, H.-J. Miessen, P. Miiller, R. Neugroschel, J. Less-Common Met. 1985, 105, 303. [7] W. Bronger, H.-J. Miessen, J. Less-Common Met. 1982, 83, 29. [8] W. Bronger, H.-J. Miessen, D. Schmitz, J. Less-Common Met. 1983, 95, 275. [9] W. Bronger, M. Spangenberg, J. Less-CommonMet. 1980, 76, 73. [lo] W. Bronger, M. Loevenich, J. Alloys Comp. 1994,216, 29. [ 111 W. Bronger, M. Kanert, M. Loevenich, D. Schmitz, K. Schwochau, Angew. Chem. 1990, 587, 91. [12] W. Bronger, H.-J. Miessen, R. Neugroschel, D. Schmitz, M. Spangenberg, Z. Anorg. Allg. Chem. 1985, 525,41. [13] W. Bronger, M. Loevenich, D. Schmitz, T. Schuster, Z. Anorg. Allg. Chem. 1990, 587, 91. [14] W. Bronger, T. Schuster, Z. Anorg. Allg. Chem. 1990, 587, 74. [15] W. Bronger, C. Koppe, D. Schmitz, Z . Anorg. Allg. Chem. 1997, 623, 239. [ 161 S. Boschen, Dissertation, Universitat Kiel 1991. [17] H. G. von Schnering, W. May, K. Peters, Z. Kristallogr. 1993, 208, 368. [18] A. Perrin, L. Ledue, M. Sergent, Eur. J. Solid State Inorg. Chem. 1991, 28, 919. [19] A. Simon, H. G. von Schnering, H. Schafer, Z. Anorg. Allg. Chem. 1967, 355, 295. [20] H. Imoto, A . Simon, Inorg. Chem. 1982,21, 308. [21] H. D. Lutz, B. Miiller, W. Bronger, M. Loevenich, J. Alloys Comp. 1993, 190, 181. [22] B. Miiller, H. D. Lutz, Phys. Chem. Miner. 1991, 17, 716. [23] D. Hastley, M. J. Ware, Chem. Commun. 1967, 912. [24] R. Mattes, 2. Anorg. Ally. Chem. 1968, 357, 30.
5.6 Ternary Rhenium und Terhnetiuni Chu1coymidc.s
161 1
[25] W. Preetz, K. Harder, H. G. von Schnering, G. Kliche, K. Peters, J. Les.r-Con?n?on Met. 1992, 183, 413. [26] D. J. Holmgren, R. T. Demers; M. V. Klein, D. M. Ginsberg, Phys. Rev. B., 1987, 36, 1952. [27] M. Ishii, K. Shibata, W. Wada, Phys. Stofus M i d i B, 1989, 152, K 63. [28] W. Bronger, J. Fleischhauer, H. Marzi, G. Raabe, W. Schleker, T. Schustcr, J. Solid Stofe Cliem. 1987, 70, 29. 1291 B. E. Bursten, F. A. Cotton, G. G. Stanley, Isr. J. Chcm. 1980, I Y , 132. [30] J. C . Slater, Plzys. Rco. 1930, 36, 57. [31] L. Pauling, Nutur der chemischen Bindung', 3rd ed. p. 376, Verlag Chemie, Weinheim/ BergstraBe (1 968). [32] R. Siepmann, H. G . von Schnering, Z. Anorg. Ally. Clwrn. 1968, 357, 289. [33] H. G. von Schnering, private communication. [34] W. Bronger, C. Koppe, M . Loevenich, D. Schmitz, T. Schuster, Z. Anorg. Ally. Chem. 1997, 623, 695. [35] W. Bronger, C . Koppe, D. Schmitz, unpublished.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.7 Discrete and Linked Homoatomic Clusters of the Elements Ge, Sn, and Pb Thomas F. Fassler
5.7.1 Introduction In chemistry the term ‘cluster’ generally denotes a group of atoms held together by chemical or physical forces. A unit is defined as ‘isolated’ or ‘discrete’, when intracluster bonding is much stronger than intercluster interactions. Often weak van der Waals interactions separate ‘naked’ clusters or clusters with ligand shells and the atoms belonging to the cluster unit are held together by stronger chemical bonds. One feature which distinguishes homoatomic clusters from ‘normal’ molecules or ions is that interatomic bonds may be ‘non-classical’, or in other words, do not always follow ‘classical’ valence rules.[’,21This often manifests itself as higher numbers of nearest neighbors than expected from the valence. When the number of electron pairs available is insufficient for the formation of two-center-two-electron (2c-2e) bonds between the atoms, delocalized bonds are required. On the one hand, the terminology is applicable to small transition-metal compounds such as Fe3(CO)gTe2, which contains a central cluster unit of three iron atoms and two main-group element atoms, and to molecules like P4 and C,jo that perfectly fulfil the eight(teen)-electron rule. On the other hand, a group of atoms in a solid polymer can be formally grouped together and regarded as a cluster unit to emphasize structural details. This is frequently done for intermetallic compounds or metal-rich oxides and h a l i d e ~ . [ ~The - ~ ] situation is straightforward in solid-state compounds where molecular bonding concepts can be applied to a subgroup of atoms as it is for salt-like compounds - the so called Zintl phases. In these intermetallic compounds - which consist of at least two metals or semimetals - all valence electrons are formally transferred to the more electronegative component. The resulting anionic (molecular or polymeric) subunit is then described according to the octet rule by forming 8 - N covalent bonds ( N = group number)[71between the atoms or by applying Wade’s rules[*]to cluster units.
5.7 Discrete and Linked Homoatomic Clusters of the Elements Ge, Sn, und Ph
1613
The topological transition (Fig. 1) from polymeric solids in compounds containing linked polyhedral homoatomic clusters to discrete molecular (soluble) clusters can be conveniently studied by using as examples compounds rich in elements of the main-groups 13 to 15. It is possible for main-group elements in the middle of the periodic table to form homoatomic molecules or ions with localized homonuclear 2c-2e bonds. At higher valence-electron concentrations, fewer bonding and more free-electron pairs are formed. As electron deficiency increases, however, the formation of delocalized bonds becomes necessary, a situation typical of elements on the left of the periodic table. A molecular cluster containing an A177 2- unit''] has recently been characterized by X-ray structure analysis; this shows impressively the tendency of group-1 3 elements to form very large homoatomic clusters. The inner shell of this cluster consists of a central atom surrounded by 12 atoms, similar to a small fragment of an fcc metal. The outer shell is constructed from 20 A1 atoms, each binding one N(SiMe3)z ligand. This example demonstrates nicely that discrete main-group element clusters can easily reach the size of the known and more frequently studied transition-metal clusters and that delocalized and localized bonds can coexist in a single unit. Interestingly, similar but linked cluster units were discovered long ago in intermetallic compounds containing Al. A fullerene-like (A1,Zn)ho unit and a A176 cluster are present as parts of the three-dimensional frameworks in Mg32(Al,Zn)49 and LiMgAlz, respectively." O1 A larger number of compounds with linked and discrete homoatomic clusters are known for the group-14 elements. The structures of the different modifications of the elements themselves are indicative of a tendency to form clusters. For carbon, several crystalline modifications are known the polymeric diamond structure, the two-dimensional structure of graphite, and the isolated units of the fullerenes." There are two modifications of Si-Sn - the a or the diamond structure in which all atoms are four-connected and the metallic p-tin structure with coordination number six (four shorter and two longer distances). A coordination number greater than ~
three-dimensional network of e.g. a-Ge
bond scission e.g. in K8Ge,,
isolated cluster anion e.g. G e j - i n KGe
Figure 1. Topological transition from a diamond network (left) to polyhedral homoatomic clusters (right). Some bonds between the tetravalent atoms break by adding electrons (middle). The formation of progressively more and more lone pairs eventually results in discrete cluster anions (right).
1614
5 Solid-State Cluster Chemistry
n=4
n=5
n=7
n=8
n = 10
n =I1
n=9
n = 12
Figure 2. Possible structures of deltahedral closo-clusters.
four is required for formation of a deltahedral cluster (Fig. 2). We can conceptually rationalize cluster formation by starting from the polymeric diamond structure and gradually adding electrons (Fig. 1). The first electrons will occupy non- and antibonding states (crystal orbitals). At a certain level bonds break and the formation of non-bonding electron pairs (lone pairs) is favored. The atoms then reassemble and holes form in the three-dimensional framework of the element structure. For negative charge compensation, cations fill the cavities in the framework. Finally, anionic cluster units, separated from each other by lone pairs and cations, are formed. Several steps of this hypothetical process are illustrated by the existence of stable compounds - several compounds crystallize in the clathrate-I structure type AsE44 0 2 (A = alkali metal; E = Ge, Sn; 0 =vacancy). The framework of E atoms consists almost entirely of four-connected atoms forming a network of face-sharing polyhedra with 14 and 20 vertices and A atoms at each center (Fig. 5). There are two defects per 46 framework atoms creating eight three-connected atoms with dangling bonds or eight lone pairs per unit cell. Further transfer of electrons to the framework, e.g. compounds richer in A atoms, leads to the formation of more lone pairs as is the case in K6Sn25 (below). Isolated E44- and Eg4- clusters are found in the binary phases AE and A4E9, respectively. Other discrete molecular clusters not yet observed in intermetallic phases exist in solution with a larger variety of sizes
5.7 Discrete and Linked Hornoutomic Clusters of the Elements Ge, Sn, and Ph
1615
n
1 4
3 4
3
Figure 3. (a) Classification of deviations from ideal Dolvhedra t h o - (A) and mdo-type B (b) Right, relative energies of the frontier orbitals under D3hI
9
9
A
B
I
-
symmety (20 skeletal electrons for Eg2-) and the degenerate HOMO under Cq\. -symmetry (22 skeletal electrons for Eg4-). Left, LUMO of the cluster type A corresponding to 20 skeletal electrons
b)
:"='I \
anti-bondmg
LUMO
frontier orbitals
and charges, e.g. E102-, E93-, Eg4-, and E52- clusters that have been structurally characterized. They are generally synthesized by the extraction of binary phases with amines. Apart from their interesting structures, molecular clusters are intensively studied because of their physical properties. It has been shown for transition-metal clusters that small uniform particles of a certain size show interesting electronic, sizedependent effects (quantum size effects). In AUgj(PRl)lZC16 a few electrons are trapped in a metallic state and single electrons tunnel between the cluster units. (SET, single electron tunneling).['21Such effects have yet to be observed in main-
1616
5 Solid-state Cluster Chemistry
group element clusters. There are, however, compounds with other intriguing properties e. g. alkali-metal fullerides A3C60 are high-temperature superconductors[’31 for which the critical temperature depends on the type and extent of alkali-metal doping and thus on the distance between the fulleride anions.[l4]A similar observation has been made for thin films of lead prepared by gas-phase condensation of lead vapor and showing a broad particle size-distribution from 2 to 100 lead atoms. The critical temperature of the superconducting films depends on particle size.” A further indication of correlation between the presence of group-14 element polyhedra and superconductivity is found for the silicide clathrate-I (Na, Ba),Si46.[l6] The exact composition of this material is, however, uncertain. ~
5.7.1.1 Scope Several excellent articles review homonuclear main-group clusters.r2,17-201 In this article we will concentrate on homoatomic polyhedral clusters of the elements Ge, Sn, and Pb with special emphasis on the relationship between soluble and linked clusters, and on certain physical properties. For these elements, several soluble anions and polymeric solid structures with different valence concentrations are known. In the first part attention is turned to structures and properties of ‘isolated’ molecular clusters synthesized by solution methods. In the second part, linked polyhedra and the increased formation of lone pairs with increasing valence-electron concentration in solid-state compounds is discussed. The influence of lone-pair interactions on electronic structures and on the superconductivity found in some of the compounds will also be discussed. Related aspects of compounds containing elements adjacent to Ge, Sn, and Pb in the periodic table are mentioned.
5.7.1.2 Historical background - Zintl ions and Zintl phases In 1889 Joannis discovered that some elements (E) of main groups 14 and 15 react with sodium in liquid ammonia to form intensely colored solutions and binary residues of unusual composition ( E = Pb,r211Bi, Sb[221).This finding has excited the interest of chemists for more than 100 years.[23’The knowledge gathered in the 30 years after Joannis’s discovery led to the picture of polyanions and the assumption that, for lead, the complex anion ‘Pbg4-’ exists.[241The systematic work of Eduard Zintl using potentiometric titration methods finally demonstrated the validity of these ideas. The synthesis of intermetallic compounds such as K4Pb9[251via a solution route suggested that ionic descriptions are also valid for intermetallic phases and extended the ideas of chemical bonding.[26p291 These ideas finally culminated in the concept of Zintl, Klemm, and Busmann (the ZKB concept) for intermetallic compounds consisting of an electropositive metal and a semi meta1.[30-321 The saltlike description and the formulation of anionic polymeric substructures for inter-
5.7 Discrete and Linked Homoatomic Clusters of the Elements Ge, Sn, and Pb
1617
metallic compounds was first discussed for the example of the diamond lattice of T1 atoms in NaTl (with T1- isosteric with C).[331A structural proof for isolated polyhedral anions was, however, found only two decades later with the discovery of lead tetrahedra in NaPb.[341The early postulate of a solid of composition K4Pb9 has only recently been justified for A4E9 with A = Rb, Cs and E = Ge, Sn.[35,36,1591 The strong influence of Zintl on the description of chemical bonding in compounds at the border of salts and i n t e r m e t a l l i c ~ [ ~led ~ -to~ ~the ~ ~nomenclature ~] ‘Zintl for soluble polyanions (as part of a ‘polyanionic salt’) and ‘Zintl phase’r411for compounds with anionic substructures obeying the (8 - N ) rule. Further development and the perception that the salt-metal transition is not abrupt led to a continuous extension of these t e r r n ~ . ~ ~ , Soluble ~,~’,~ polycations, ~] discrete units, and low-dimensional substructures in Zintl phases are called Zintl ions. These ‘ions’ commonly consist of metal- or semi metal-atoms, or of atoms of semiconducting elements. Clearly, they must be distinguished from ‘classical’ ions as elucidated by a comparison of SnTe44- and the iso(va1ence)electronicion so42-.
5.7.2 Molecular clusters Homoatomic clusters of group 14 are in many ways a remarkable class of compounds. The dismantling of the elemental structure and the formation of isolated units can lead to compounds with remarkable physical and chemical properties.
5.7.2.1 Wade’s Rules
+
+
+
Wades rules predict n-vertex deltahedra to be stable if 2n 2, 2n 4, or 2n 6 skeletal electrons are present.[” From this one can conclude that the formation of ‘naked’ (ligand-free) clusters is especially favored for group- 14 elements. Each E atom contributes two of its four valence electrons to the skeletal bonds of a deltahedral cluster, leaving two electrons (as a lone pair) at each cluster vertex. Thus, for any given number of vertices n, E, ‘-anionic clusters with realistically low negative charges of x = 2 or 4 should be stable. Molecular clusters of the type En2- and En4correspond to 2n + 2 and 2n 4 skeletal electron clusters, respectively, and should adopt the appropriate closo and nido-polyhedra. Consequently, the charge of homoatomic clusters of group 13 and 15 elements depends on the cluster size (Table 1). The increasing charge on the group- 13 and group- 15 cluster ions must be counterbalanced by a larger number of counter ions. A large value of n requires more and thus smaller counter ions. For this reason, polyhedral Zintl ions of group-13 elements occur preferentially in Zintl phases with alkali- or alkaline-earth metal
+
1618
5 Solid-state Cluster Chemistry
Table 1. Effect of compound-type on the the charge on homoatomic clusters of groups 13 to 15 elements with n vertices. Group 13 closo-type nido-type
(n (n
+ 2)+ 4)-
Group 14
Group 15
24-
( n 2)+ ( n - 4)+ -
atoms. So far no homoatomic clusters with group-13 atoms have been detected in solution (Table 2). On the other hand, positively charged ions of group 15 can form more electron-rich variants, owing to the formation of localized 2c-2e bonds. Therefore polycationic polyhedral clusters occur only rarely in this group.
5.7.2.2 Structurally characterized polyhedral clusters of group 13-15 elements Clusters of group-14 elements usually form soluble ions in salts with large cations (e.y. [K-crypt]+ complexes).[431Of the many possible clusters only the five-, nine-, and ten-atom units have been structurally characterized (Table 2). Homoatomic tetrahedral units have been found only in Zintl phases. Discrete tetrahedral E4 clusters are isosteric with P4; these are listed in Table 3. A nine-atom representative has recently been characterized in the A4Ge9 (A = Rb, Cs) Zintl phases.[351In the A12E17 (E = Ge, Sn) phases four- and nine-atom clusters are present in the same compound (see below).[361
5.7.2.3 Soluble polyhedral Zintl ions of group-14 elements After the discovery of the solubility of Pb and Hg[”] in liquid ammonia in the presence of alkali metals, similar investigations on Bi and Sb followed.[221This technique has met with little success when applied to B and Si.[931Electrochemical experiments were used to determine the ionic character of the soluble species. For Pb, Pb2- anions and Na+ cations were postulated.[941Improved electrochemical analyses resulted in the corrected assignment of P b ~ . 2 6 - [as ~ ~the ] soluble species. In a review article by Kraus on the constitution of metallic substances, the first formulation of Pb2.26- as the complex ion in Na4PbPbs1961was suggested, and two years later the formulation Pb94-[241was proposed. Zintl and co-workers succeeded in the detection of a large number of ammoniasoluble polyanionic salts: Na4Sng (red), Na4Pb.r (green), Na4Pb9 (red), K4Pb9 (red), Na3E, (As: x = 1, 3 (yellow), 5 , 7 (red-brown); Sb: x = 1, 3 (dark red), 7 (redbrown); Bi: x = 3 (violet), 5 (brown)).[251 For the synthesis of larger quantities of
n
15
I
14 14 15 13 13/14 14"
15 13
I
14
I I
15 16 13
!
14/15
I
I
14
1
13
E b'
Cluster
Compound and Ref.
X X
X X X
X X X X X X
X X X X
X X X X X
zc)
X X X
X
X
X
X
X X X X
X
Sd)
Table 2. Homoatomic group 13-1 5 polyanions and polycations with polyhedral structures.""' ~~
-
Dih
D4h
-
(disordered)
Td (disordered)
D4h
A
c -Td 2 v
Td
T dTd (NaPb-Typ)
-Td
Symmetry
22 (2n + 6) 18 (2n) 20 (2n + 2) 22 (2n + 4) 22 (2n + 4) 22 (2n + 4)
14 (2n + 4) 12 (2n + 2) 16 (2n + 6) 12 (2n + 2) 14 (2n + 2) 12 (2n) 14 (2n + 2) 14 (2n + 2) 14 (2n + 2)
14 (2n + 6) 12 ( 2 n f 4 ) 12 (2n + 4) 14 (2n + 6) 14 (2n 46) 12 (2n + 2)
12(2n + 4) 12 (2n + 4) 12 (2n + 4)
Electronse'
13
13
Cluster
+
Compound and Ref.
X
X
X X
zcl
X
Sdl
C3"
-
-
Th( Ih) Th( Ih)
D3d
D3h D3h D3h D3h D3h
- D4d
-
C3"
D4d
Symmetry
a)
+
25 (2n
+ 1)
3
2.
%26 ( 2 n + 2 ) 26 ( 2 n + 2 ) 26 (2n + 2)
0 k)
5
F 2
$ 6
g
2
bl
N 0
o\
18 (2n-4) 18 (2n-4)
22 ( 2 n + 2 ) 18 (2n - 4) 18 (2n-4)
I8 (2n-2) 18 (2n-2) 18 (2n-2)
Electrons"'
Excluding the elements Si, P, As, cp = n = number of cluster atoms; b)groupnumber; "'found in Zintl phases; d'found in solution; "'number of skeletal electrons, 2n 2 closo- and 2n + 4 nido-type; 'disorder; "Hg disordered over the corners of the trigonal prism; h)T11214centered with Tl'+; i1T11214centered with Na+; jITl12'~-centered with Mg2+-Hg2+; k'Au-Au bond, does not follow Wade's rules, "four and nine atom clusters of group 14 elements (see Table 3 and Table 5).
13
~
14 13
12
11
13
10
I
Eb,
n
Table 2 (continued)
'
5.7 Discrete and Linked Homoatomic Clusters oftlie Elements Ge, Sn, and Pb
1621
Table 3. Alkali and alkline-earth metal/group-14 element phases with tetrahedral group-14 clusters (structure types in bold).
RbSi 13'] CsSi L3'1
RbGe [ 3 2 1 CsGe 1321
NaSn KSn
Is4]
NaPb [341 KPb 186,871
RbSn CsSn
rs61
RbPb [861 CsPb 1861
the polyanionic salts, a reaction route involving the extraction of the solids Na4Sng or Na4Pb4 in liquid ammonia[381was introduced. Sn and Pb polyanions were also made by electrolysis using the respective metals as electrodes.[3y1Ga, In, and TI polyanions could not be synthesized electrochemically, nor by extraction of the respective Na or Cs alloys. The extraction of Ge-containing alloys generated only pale brown solutions, perhaps indicative of the formation of polyanions in poor yields. Whereas use of ammonia as the extraction solvent led to amorphous solids, the use of e t h ~ l e n e d i a m i n e resulted ~ ~ ~ ] in the isolation of crystalline materials such as Na4(en)sGe9 and Na3(en)4Sb, and led to the first crystal-structure determination of the Sng4- polyanion in Na4(en)-iSng.ry73981 Finally, the introduction of the crown ether [2.2.2]crypt by C ~ r b e t t [ ~ gave ' , ~ ~rise ~ to the structural characterization of many other polyanions." In addition to titration methods, the solutions have been characterized on the basis of the composition of the precursor alloys, color, and occasionally by NMR methods (Table 4). The structures of mixed-cluster anions of group-14 elements are only poorly characterized.['"-' 06] M'ixed anions such as Sn,PbgPx4(0 I x I 9) can be synthesized electrochemically by using a 1 : 1 Sn/Pb alloy as the Ool All nine-atom clusters are fluxional in solution. cathode.[39*' ethylenediamine
Scheme 1
Dark red Dark red Red brown Red brown Red brown Red brown
Red orange Brown Green Red browng) Dark red
Red orange
YellowJred brown YellowJred brown YellowJred brown Red (less intense) Yellow/red brown
Color of the extract
Sn94PbS Pb74-k' Pb9 4Ge,Sn9-,4-
Polyanions
(x = 0-9)
Black Black
Red brown
Dark red KPbg3
Black KSng3 Ruby red
Red Dark red Red brown Sng4-
Dark red Violet Dark red
Color of the [A-crypt]-saltsh)
"'Ethylenediamine extractions were used and the ions were characterized by NMR"00-'031 if not otherwise noted. Other solvents in brackets. b)Compositionsof the alloys are nominal. c)[741 d)Chargedistribution e)Extraction at higher temperature.[561f)[1061g)NaPb2.25and KPb2 25 form also red brown ammonia solutions.[2s1h)A = alkali metal, [Na47en]Sng copper color, [Na45en]Ge9 transparent red.[981Further literature references are given in Table 5. "Both ions are present.[661"Charge allocation unclear. k)Detectedby potentiometric titration.[2s1
KPb2, NaPb2, NaPb3, LiPb2.25, LiPb2. KPb20sf NaSnGe, KSnGe, K2SnGe3, RbSnGe LiSnPb, NaSnPb, Na4Sn3Pb6, Na4Sn6Pb3, KSnPb, RbSnPb, CsSnPb KTlSn NaSnTll5 Na2SnPbT13, KzSnPbT13 LiSnBi, NaSnBi KPbSb
Alloyb)
Table 4. Extraction of alloys containing group-I4 elements and resulting polyanions.")
Q E
E m
N N
m
c-)
5.7 Discrete and Linked Homoatomic Clusters of the Elements Ge, Sn, and Pb
1623
5.7.2.4 Structures of homoatomic nine-atom clusters According to Wade's rules, nine-atom clusters form closo and nido structures as they are shown as A and B, respectively in Fig. 3. The structures of nine-atom clusters of formula Eg4- (four counter-ions) are well understood. These 22 skeletal electron clusters generally adopt structure type B (Table 5 ) . When only three cations per cluster are present, paramagnetic Eg3- clusters with 21 skeletal electrons and structures between A and B are expected. Clusters with 20 skeletal electrons should adopt cluster type A. Eg3- clusters have been reported for Ge, Sn, and Pb. Owing to co-crystallized solvent molecules, one and two crystallographically independent clusters have been observed in single crystals ('small' and 'large' unit cells, respectively). When only one cluster and three counter ions are present in the asymmetric part of the unit cell, a cluster with an odd electron count and thus paramagnetism should be observed. When there are two clusters and six counter-ions in the asymmetric part of the unit cell, two paramagnetic Eg3- or diamagnetic Eg2- and Eg4clusters can occur. In the latter circumstance the two clusters should adopt two different structures, such a formulation has consequently been postulated for [K(2,2,2-crypt)]6GegGeg(en)2.5,although no magnetic data were available.[1071For [K-(2,2,2-crypt)]6GegGeg(en)o.j and [K-(2,2,2-~rypt)]~SngSng(en)1,j(tol)0.~, however, EPR measurements on single crystals show paramagnetic behavior." 09,1 91 Temperature-dependent magnetic susceptibility (x)measurements of micro-crystalline powders of Sn and Pb compounds, that are probably a mixture of the 'small' and 'large' unit cells show Curie-Weiss behavior. Analysis of the linear Curie-Weiss regions always leads to smaller peff than expected from one unpaired electron per cluster and corresponds with the presence of about 50% paramagnetic Ei- species. This can be taken as evidence of partial disproportionation into Eg2- and Eg4species. However, protonation of the co-crystallized ethylenediamine molecules cannot be excluded and these might also cause the lower magnetic moments." The correlation of orbital filling with observed structures is summarized in Fig. 3 and Table 5. If 21-electron clusters adopt the ideal D3h-symmetric species A, the singly occupied molecular orbital (SOMO) is anti bonding along the heights of the prisms (Fig. 3b). This should manifest itself in larger prism heights compared with the triangle edges, e, of the central prismatic unit ( h / e ratio).""] On the other hand, nine-atom clusters with ideal C4v symmetry B and 21 framework electrons have a doubly degenerate HOMO filled with three electrons, thus giving rise to JahnTeller The system can stabilize itself by undergoing a Czv distortion towards type A and destroying the degeneracy of the orbitals. When the energy gain is small compared with the total energy of the system, fluctional behavior is expected (dynamic Jahn-Teller distortion)." ' 31 Careful investigation of the structures of the nine-atom clusters might therefore give an indication of their electron counts. The deviations of the structures of nine-atom clusters relative to the ideal polyhedra A and B can be expressed by various distances and angles (see Fig. 3, and Tables 5 and 6):
1624
5 Solid-State Cluster Chemistry
")
0
. 0 0
Y
Y
Figure 4. Ion packing in compounds of composition [K-(crypt)]3Eg. (a) and (b) Two views of la, (c) 2a, (d) 2a' (after transformation of the lattice constants), (e) lc, and (f) 2c. Compound numbering is as for Table 5 . For clarity only the Ge, Pb, and K atoms are shown. In (b) the large circles represent the average sizes of the [K-crypt]-complexes.The carbon atoms of the cryptand molecules are shown as black points.
1.12 1.11 1.29
1.29
22
22 22 22
1 1.02 1.oo 1.07 1.16 1.14 1.12 1.21 1.12 1.21 1.19 1.01 1.07 1.14 1.11 1.05 1.06 1.05 1.14 1.07 1.30
18
20 22 22 21 21 21 21 20" 22" 22 21 21 21 21 20 21 21 21 21 22
hl
1.04 1.02 1.02
1.03
1.oo 1.01 0.96 1 .oo 1.01
1.oo 1.oo
0.96
1.oo
1 1.02 1 .oo 1.oo 1.10 1.12 0.99 0.97 0.99 0.97 1.05 1.01
hz
1 1 1
1
0.99 0.97 0.98 1
1
0.98 1 .oo 1.oo 1 0.98 0.96 0.98 1
1.oo
1 1 1 0.98 0.97
1
1
IQC'
4 8 21.21Y'
20,24Y'
0 0 0 6 10 10 10 17,2SY' 9 1 8,24Y) 13,31y' 1 6 14 9 3 4 4 12 6 22,22')
yd'
1.08 1.08 1.07 1.09 1.03 1.08 1.07 1.05 1.07 1.19 1.02"' 1.19 1.02"' 1.14 1.15 1.16 1.01"'
1.15 0.96 1.15 1.19 1.17 1.17 1.10 1.13 1.11 1.21
hie"
30
29
3
31
29 1
13
29 20 24
28 17 22
2 15
18
23 31 25 19 21 23 21 20
20(17) 22 22 24 31 32 24 25 25 32 27
20(17) 22 22 24 18 17 23 27 23 24 23 18 19 20 18 17 20 18 30 19
20(17) 25 22 14 16 14 5 3 8 5 11 17 13 7 15 16 14 16 12 18
Xf'
C2"
c4,
C2" C2" C2"
c2c
C4"
--
C4" C2" C2b
C4"
C2Y
- C2" - C,
C2" Cs Cs C2" C2"
D3h
-------
D3h D3h D3h C2" -C,
Sym.
~~
~~~
106,I59
123 124
122
121
66 118 109
35" I20 109
107
114,115 116,117 71 69,70 118 74 108,119
Ref.
a)
I, 11, Illa, IIIb denote the different cluster isomers E9; b)skeletalelectron count; "relative prism heights; d*e)see Fig. 3. "Dihedral angle 180 - cc; g'Biio(HfCI,); h)Bi12C114; i'[K-(crypt)]3Gey(en)o.s (la); j)[K-(crypt)]3GeyPPh3 (lar); k'[K-(~rypt)]~GeyGey(en)os(2a); "[K(crypt)]~GeyGe9(en)2.s,charge allocation for the clusters in 2a' based on structural arguments; ""[K-(crypt)]3Sn9(en)l,s (Ib); ")[K(crypt)]~SnySny(en),.~(tol)~.~ (2b); "'the disordered clusters IIIa and IIIb are superimposed; P'cluster disordered and superimposed with the anion T1Sny3-; q)'[K-(crypt)]3Pbg(en)o.s(lc); "[K-(crypt)]hPb9Pb9(en)ls(to1)o.s (2c); "[Na-(crypt)]4Sny (3a); ') [K-(crypt)]3KSng (3b) with two KSn contacts; "'[K-(18-crown-6)]4Sn9(4a); "[K-( 18-crown-6)]3KSn~(en)(4b);")[K-(crypt)]3KPb~ (5) with two K-Pb contacts; "'only two heights considered, one height of the prism becomes a diagonal in cluster type B ((3"). Y ' T ~choices o of trigonal prisms are possible. "'There are four independent clusters in the unit cell. The other three are closer to C4" symmetry.
Sng4- (4a)"' K+Sng4- (4b)" KCPby4- (5b)"'
Cluster"'
1626
5 Solid-state Cluster Chemistry
i) prism heights h (hl to h3 are d(1-2), d(5-6), d(7-8)) and the corresponding angle y (between opposite triangle faces) of the prism A. Equal heights results in coplanarity ( y = 0). To enable comparison of the distances of the nine-atom clusters independently of the elements, the distances are scaled to the shortest prism heights of one of each (distorted) trigonal prism; ii) distances, e, along the edges of the triangular faces and relationship to the heights h ( h / e ratio). iii) angle a between planes defined by one prism height and two adjacent capping atoms (e.g. 1-2-3/1-2-4). All three angles are equal in the case of A (ideal D3h symmetry). For B (C4" symmetry) two angles are the same and the third is 0". a characterizes the deviation from planarity of the open plane 1-2-3-4 in the monocapped quadratic antiprism B. The dihedral angle is then 180" - M . iv) length, d, of the two diagonals of the rectangle 1-2-3-4 (open side). Under ideal C4v symmetry the distances along the two diagonals in B are of equal length. The structure of the undistorted B9Hg2- dianion has three equal heights and consequently the triangular edges of the prisms are co-planar. The three equal bond angles a represent the ideal 3-fold symmetry of the capping atoms 3, 4,and 9. The value of the h / e ratio (0.96) agrees with the MO picture, the LUMO which is antibonding along the prism heights is not occupied (Fig. 3). The series B9Hg2- > Sng3- > Big5+ with h / e = 0.96, 1.08, 1.15 nicely shows the trend of increasing prism heights with increasing electron count. Big5+ with 2n + 4 skeletal electrons is an exception from Wade's rules but consequently shows the largest h / e ratio in the closo structure A. In the case of the non-Wade clusters B9X9 ( X = C1, Br; 2n skeletal electrons) fewer bonding orbitals along the prism heights are occupied and thus the h / e ratio is also larger. The amount of deviation from three-fold symmetry of cluster type A is expressed by the different relative prism heights, the deviation from co-planarity y of the opposite three-cornered faces, and the differences between the dihedral angles a. When 21-electron clusters are clearly present (compounds of type 1 in Table 5) the variation in the heights of the three edges of the prism is as high as 16% (Geg3-). For all compounds with two independent cluster isomers (compounds of type 2) the apparent deviation in the Ge structure is between 15 and 20%. For comparison, in ideal C4" symmetric clusters where one of the diagonals of the open square is taken as prism height, the deviation is as high as 30%. The h / e ratios calculated from averaged prism heights and edges can be correlated with the number of electrons which is, in turn, related to the occupation of the LUMOs. The deviation from ideal trigonal prismatic structures tends toward CzVsymmetry for all 21-electron clusters with the exception of Sng3- which retains almost ideal D 3 h symmetry. For Geg2-, formulated as a 20-electron species, distortion is similar to that of other 21-electron clusters. The two clusters I11 in 2a and 2a' might be assigned as Ges4-; they show, however, gradual distortion from C4" symmetry as a function of the co-crystallized solvent molecules (Table 6).
3.730 3.624
3.485 3.858
Gesn(Za)")
3.580 3.638
Geg n- ') (2a')
4.163 4.205
Sn94(34
4.129 4.229
KSn9'(3b)
3.531 4.613
Sng 4(4a)
3.122 4.506
KSng3(4b)
4.316 4.334
KPbg4(5b)
4.095 4.204
Sn I11 (Zb)"
4.480 4.011
Pb I11 (2~)"'
"'For compound numbers see the legend of Table 5 ; b)charge allocation (3- or 4-) unclear; "cluster Ill corresponds to the average picture of the two superimposed disordered clusters IIIa and IIIb.
~
d(1 2) d(3 - 4 )
Cs4Ge9
Table 6. Diagonal lengths of the open square of cluster type B (d in Fig. 3).a)
1628
5 Solid-state Cluster Chemistry
All compounds with four cations per cluster clearly contain 22-electron clusters and should adopt structure B. The planarity of the open rectangular faces 1-2-3-4 is assumed by clusters with [A-crypt]+ counterions - one of the CI angles is close to 0" (3", 2" and 1" for Sng4- (3a), KSng3- (3b), and KPb43- (5b), respectively). This correlates with similar lengths of each pair of diagonals (Table 6). Deviations are, however, larger for the recently discovered 22 electron clusters in compounds 4a and 4b and these structures are better described as compressed monocapped square antiprisms. These clusters have a higher number of direct contacts with K atoms.[ 123,1241 The nine-atom clusters that have been investigated have structures intermediate between the boundary structures A and B; the paramagnetic Sng cluster in l b forms an almost perfect tricapped trigonal prism and one of the Ge9 clusters in 2a forms an almost ideal monocapped quadratic antiprism. When the charge allocation is ambiguous, one cluster always has the structural characteristics of 2 1-electron clusters. Likewise, 22-electron clusters can deviate from CllVsymmetry. As a result we find that the relatively large deviations of the clusters from the expected ideal point symmetry makes it difficult to correlate their structures directly with their numbers of skeletal electrons.
5.7.2.5 Crystal packing in compounds with nine-atom clusters The variation in the structural distortions of the ions and their reported fluxional behavior in solution suggests that crystal packing might influence the structures of nine-atom clusters. The unit cells of the compounds having three [K+-crypt] ions per cluster have very similar arrangements of the cations and anions. The packing of the cationic [K+-crypt]units creates channels filled with the anionic clusters. The orientation of the clusters in these channels is, however, variable. Despite differences in crystal systems and thus ion packing, such channels are found in seven crystal structures with the composition [K-crypt]3E9( E = Ge, Sn, Pb; disregarding co-crystallized solvent molecules). Both pairs of compounds l a , l c , and 2b, 2c are almost isomorphous. lb," 201 however, shows deviations of the lattice constants after transformation into the standard triclinic cell, in comparison with l a and l c . Certain modifications can be clearly distinguished by their macroscopic properties (color and crystal morphology) even ion packing is similar (see examples in Fig. 4). The expected increase in cell volume along the column Ge, Sn, and Pb is found for the series of compounds 1 and 2. The unit cells of l a to l c as compared with 2a and 2c are strikingly smaller although occasionally lattice constants were determined at higher temperatures. The trend of the unit cell volumes corresponds to the number of co-crystallized solvent molecules. (la: 4 x 4084A3 = 16336A3 (193 K), 2a: 2 x 8047A3 = 16094A' (233 K), 2a': 2 x 8322A3 = 16644A3 (293 K), lb: 4 x 4435A3 = 17740A3 (213 K), 2b: 18244A3 (193 K); Ic: 4 x 4591A3 = 18364A3 (293 K), 2c: 18739w3 (253 K)).
Figure 5. Examples of polyhedral units and the resulting three-dimensional networks of compounds with linked clusters. Triangular units (A) form face-sharing octahedral chains (B, C ) in BaSn3. Quasi bcc-arrangement of pentagonal dodecahedral units (D) creates fourteen-faced polyhedra (E). The two building blocks build up a three-dimensional network (F) in K4E23 ( E = SikSn). Face sharing pentagonal dodecahedral units ( G ) form chiral chains (H) as part of a three-dimensional structure ( I ) in KsSn25. Each hexagonal face of the truncated tetrahedral unit of Pb atoms (K) is capped with nine Pb atoms to give four interpenetrating Frank-Kasper polyhedra. This ‘supertetrahedron’ (L) is the repeating unit in KsPb24 (M).
1630
5 Solid-state Cluster Chemistry
Since ion packing is similar, different orientations and thus different local environments of the clusters must be the origin of the structural variations of the polyhedra. This indicates a high flexibility of the cluster skeletons, even in the solid state.
5.7.3 Cluster units in Ge-, Sn-, and Pb-rich intermetallic phases 5.7.3.1 Intermetallic phases containing discrete polyhedra Tetrahedral clusters in the solid state have long been known as units in intermetallic compounds (Table 3). The recent discovery that nine-atom clusters are present in binary solids is the first example of homoatomic clusters of the elements Ge, Sn, and Pb that can exist in solution and in the solid state with very similar structures. In A4Ge9 (A = CS,[~’]K[361)the occurence of discrete Eg4- clusters with CilV symmetry were established by single-crystal X-ray analysis. The isomorphous phases A4E9 (A = Na, K, Rb and E = Ge; A = Rb, Cs and E = Sn) were identified from powder diagrams. In compounds of composition A12E17 (A = Na-Cs and E = Ge; A = K-Cs and E = Sn) each nine-atom cluster crystallizes with two E44- tetrahedral units.[361 Recent single-crystal investigations of Ba3Sn5 and B a s P b ~ [have ~ ~ ] shown that Sn56- and Pb56- units are present with inter-cluster distances approximately 15% longer than the intra-cluster distances. The arachno-clusters have C2., symmetry and can be derived from a pentagonal bipyramid by removing two adjacent vertices. In earlier investigations Ba3Pb5,[”’] Ce3Sn5,“ and L a 3 S n ~ [ ’ were ~ ~ ] assigned to the Pu3Pd~structure type on the basis of X-ray powder data; no inter-atomic distances that would enable evaluation of the presence of discrete cluster units were given.
5.7.3.2 Intermetallic phases containing linked polyhedra Triangular units are the building blocks of the Wade clusters, but they are also present in the intermetallic phase BaSn3. This phase had been previously described as distorted hexagonal close packing in which the positions of each hexagonal layer is occupied in a I : 3 ratio by Ba and Sn atoms. The layers stack in an ABAB Distortion of the Sn atoms within sequence as depicted in Fig. 5, C.[128,1291 the layers leads, however, to shorter contacts between Sn atoms within a layer (Sn-Sn = 3.058 A) and longer Sn-Sn interlayer contacts (3.266A). If only the shortest Sn-Sn contacts are considered to be bonding, the tin atoms can be viewed involves the transfer as forming triangular Sn3 units (Fig. 5, A). The formula s1-13~-
5.7 Discrete and Linked Homoatomic Clusters o j the Elements Ge, Sn, and Pb
1631
of Ba valence electrons to the Sn polyanion. Such units are isoelectronic with aromatic 2n-electron cyclopropenyl cations C3R3+. The triangular units do not form discrete clusters but stack in one-dimensional chains running along the c axis with strong interactions between the component n-systems. The chains can be also thought of being constructed from elongated face-sharing octahedra and can be 5, B). The close topological relationship between the described as = ' [ S I I ~ ~ (Fig. -] polyhedral units of the soluble nine-atom clusters and the Sn-substructure in BaSn3 is addressed in Fig. 6. Three-dimensional linked polyhedra are frequently found in solids that are rich in E elements. The clathrate-type structures Nags& (clathrate-I) and Na,Sil~j (clath1 l)['3n1 have been known for many years. Further examples of the rate-11, 3 5 x I first type are K~Si46['~'], KsE46 ( E = Ge, Sn),['321and C~gSn46.['~~] All frameworks of E atoms are tetrahedrally connected and form pentagonal dodecahedra together with 14-face- (Fig. 5, D F ) and 16-face-polyhedra (clathrate-11). Each polyhedron has and alkali- or alkaline-earth metal atom at its center. The composition of Each clathrate-I type was corrected from AgE46 to AgE4402 ( E = Ge, SII).['~~] vacancy creates eight three-bonded (3b) atoms per 44 remaining E atoms, thus these phases are also 'electron precise' Zintl phases ([A']~[EM]'~ 0 2 ) . RbgSn44 6 and Kl 6Cs6 4Sn44[13s1were recently found to adopt the clathrate-I structure. This structure type is also observed for several ternary phases: AgE',E46pr (A = Na, K, Rb, Cs; E' = Al, Ga, In; E = Si, Ge, Sn),['361MgE'16E30 (M = Sr, Ba; E' = Al, Ga; E = Si, Ge, Sn),['371and phases containing halogen atoms X8Y8Ge38 ( X = C1, Br, I; Y = P, As, Sb)r1381 and Ilo67Ge4333.[1391 B a 6 I n 4 G e 2 1 , ~ ' ~€3~a402Ge25,['411 *'~~~ KgSn23Bi2, and KgSn25[13s,1421 ( Pearson symbol cP124) crystallize in a novel clathrate-like structure. The structure is built from distorted pentagonal dodecahedra of the elements Ge, Sn/Bi, and Sn, respectively (Fig. 5 , G I ) . The pentagonal dodecahedra have A or M atoms at their centers and are linked by sharing three pentagonal faces and one external bond ( G ) to four other polyhedra forming a three-dimensional, chiral zeolite-like network (H, I ) . The connectivity of the 20 atoms of the pentagonal dodecahedra prompts the formulation: [3b-E]4[4b-E][4b-E]3x5p or, relative to the unit of 25 atoms, [3bE]g[4b-E]2[4b-E]15, e.g. there are 17 four and 8 three-connected atoms out of every 25 framework atoms. The resulting voids and channels are occupied by electropositive atoms. If the frameworks obey the valence rules, the 3b atoms must be isosteric with an element of main-group 15. This is fulfilled in the ternary phases BasIn4Ge21 and KgSnz3Bi2. For the binary phase K~+~Sn25, a phase width of x I 2 is described. There are two single-crystal analyses - one has a K con-2 I tent close to 8 (7.4 K atoms) and very short K-K contacts (-2.9A), the other refines to the composition K6Sn25 and represents an electron-poor Zintl phase. The electron deficiency can be understood on the basis of lone-pair interactions, and is discussed in more detail below.['431 K5Pb24crystallizes in the a-Mn structure type (Pearson symbol C I P ~ ~ ) where ['~~~, 5 of 29 positions are occupied by potassium. The Pb network can be described by -
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5 Solid-state Cluster Chemistry
Sn Sn-
octahedron
Sn
face octahedra
chain of face sharing octahedra
Figure 6 . Topological relationship between discrete and linked clusters based on triangular building blocks and the electron counts in known structures. Two units build up an octahedron (left). After a distortion, three such units form a tricapped trigonal prism (middle). An infinite number of triangular units builds up a chain of face-sharing octahedra (right).
linked deltahedral Pb units centered with K atoms. The K2 atoms form a bodycentered cubic lattice (Fig. 5, L). Each K2 atom is surrounded by 12 Pb atoms, which form a truncated tetrahedral unit (Fig. 5, K ) . Four K1 atoms are located above the four hexagons of this unit forming a Friauf-polyhedron together with the 12 Pb atoms. Each of the Pb hexagons is capped by a Pb polyhedron consisting of nine Pb atoms. This cap is now centered by K1. The nine Pb atoms of the cap,
5.7 Discrete and Linked Homoatomic Clusters of the Elements Ge, Sn, and Pb
1633
together with the six Pb atoms of the hexagon and the central K2 atom, form a Frank-Kasper polyhedron with 1 6 - ~ e r t i c e s . [Four ' ~ ~ ~of these Frank-Kasper polyhedra share the common K2 atom. The identification of the tetrahedrally arranged caps and their K1 centers leads in a hierarchical sense to a 'supratetrahedral' unit of 48 Pb atoms (Fig. 5, L). This arrangement of polyhedra is the repeating unit of the structure (Fig. 5, M ) . Space filling of these units is not perfect and the resulting 3.6A in diameter) to host K atoms. voids are too small (channels of
-
5.7.3.3 Superconductivity Investigation of the properties of clathrate-type phases has accelerated since the discovery of superconductivity in alkali-metal-doped c 6 0 fullerene compounds." 31 In NaxSi136, an insulator-to-metal transition was observed as the Na concentration was increased. Superconductivity is absent in NaxSi136 and NasSi46,"451 whereas indications of superconductivity in a clathrate-I type compound had been reported for the composition Na2.9Ba4.5Si46.[161 There is, however, some concern about the exact composition of the latter phase because it was established by atomic absorption in the presence of BaSi2 impurity. Also the possibility of vacancies and thus the formation of lone pairs, as established for the aristo type, were not considered. A correlation between the occurence of lone-pair electrons in intermetallic compounds and superconducting properties has been examined for several intermetallic phases. Further details are given in the next section. BaSn3 and K5Pb24 are both superconductors."08.' 281 Investigation of their lowfield temperature-dependent magnetic behavior leads to curves typical of diamagnetic shielding and the Meissner effect, corresponding to T, values of 4.3 K and 7.0 K, respectively. These phases are type-2 superconductors and their magnetic effects can be clearly differentiated from those of the pure elements tin ( T , = 3.7 K) and lead ( T , = 7.4). The examination of the electronic structures of both compounds reveals the presence of lone pairs with states close in energy to the Fermi level.
5.7.3.4 Theoretical investigations of the electronic structures Several theoretical investigations have been performed on silicon-based clathrate structures. Whereas early publications discuss an insulator-to-metal transition in Na-doped Na,Sil36 with electrons moving in a band with predominantly Na con471 recent calculations predict a narrow conduction band and a distributions,[' tribution of the charge density over the Si a t o r n ~ . [ ' ~ ~ , ~ ~ ~ ' There has been less interest in phases of the heavier homologs of the group-14 elements. There are indications that P-NaSn, which contains tetrahedral Sn4 units, is metallic[201and augmented spherical-wave calculations reveal a small indirect 4631
1634
5 Solid-state Cluster Chemistry
b a n d - g a ~ . [ ' ~Tight-binding ~I calculations of the Extended-Huckel type show that the pentagonal dodecahedra1 framework of A6+xE25 has a band gap of several eV for a total of 108 valence electrons as expected for the Zintl phases Ba6ImGe21 and K6Sn23Bi2.['431In KgSn23Bi2, there is a second but smaller band gap in the density of states below the Fermi level. This might explain the stability of the electrondeficient phase KgSn25 (Fig. 7a). Further analyses of the electronic structure by use reveal that the destabilization of of the electron localization function ( bands at the Fermi edge arises from interactions of non-bonding (lone) electron pairs of the three-bonded atoms of the framework (Fig. 8, A and B). A remarkable cluster of eight lone pairs is established around the cation position K2 in KgSnz$i2 (Fig. 3, I ) . The lone pairs are located at the corners of a distorted cubic cavity of three-connected framework atoms, each belonging to a different pentagonal dodecahedron. The lone pairs are all oriented towards the center of the cube. The orientation and the shape of the lone pairs was also visualized by calculating the partial electron densities ( PED)['531close to the Fermi edge.['431The closeness of eight lone pairs leads to the destabilization of one orbital (band) or four bands for the 100 E atoms of the unit cell as shown in Fig. 7a. In K6Sn25 these bands are empty and thus K6Sn25 can be described as Zintl phase which is deficient by two-electrons. Weaker interactions are calculated for Ba6ImGezl owing to more contracted orbital coefficient^."^^] The band structure of BaSn3 features a large dispersion in the bands running along the symmetry lines perpendicular to the distorted hexagonal layers; most bands parallel to the layers are comparatively flat. The density of states and a section of the band structure obtained from LMTO calculations are shown in Fig. 7b. It shows the characteristics of the 'fingerprint' of a superconductor in the density of metallic conductivity is apparent in BaSn3 with the s t a t e ~ . [ ' ~ ~An " ~anisotropic ~] special situation that flat bands and a degeneracy (in the zone center r) appear at the Fermi level. This leads to an increased density of states at EF (Fig. 7b, right). The interaction between the free-electron pairs at the Fermi level was analyzed by use of the ELF. Three outer areas with half-moon shapes correspond to the freeelectron (lone) pairs of the tin atoms, three smaller regions to the Sn-Sn bonds of the triangular units of the octahedral chain. The lone pairs are clearly flattened, owing to interactions with neighboring electron pairs (Fig. 8, C). The results obtained with the ELF show that interaction of the lone pairs parallel to the layers are responsible for the band dispersion and for the elevation of certain bands at the Fermi The LMTO-band structure also reveals the metallic character of the intermetallic phase K5Pb24."081 The Fermi level cuts at low DOS, but flat bands also occur at E F (Fig. 7c) showing up as a small shoulder in the DOS; E F consequently cuts the DOS at a local maximum. The ELF analyses show areas of high localization (ELF > 0.8) close to the Pb atoms. The regions are located at the largest cone angle described by the connections to next-nearest neighbors of the respective Pb atom, e.g. 12 lone pairs are located around K2 inside the truncated tetrahedral unit, three
5.7 Discrete and Linked Homoatomic Clusters of the Elements Ge, Sn, and Pb
1635
Figure 7. Band structure along specific symmetry lines in reciprocal space and density of states (DOS). Dashed horizontal lines indicated the Fermi level of the compounds as noted in the figures. a) Extended-Huckel calculation, b) and c) LUTO calculation with EF shifted to 0.0 eV.
1636
5 Solid-state Cluster Chemistry
Figure 8. Cluster units as part of three-dimensional structures and ISO-surfaces of the electron localization function (ELF). Pentagonal dodecahedra1 unit in KsSnzs (A) with ELF-regions indicating 2c-2e-bonds 1 along the edges of the polyhedra and lone pairs 2 located at the threebonded atoms. Orientation of eight lone pairs (B) belonging to eight different pentagonal dodecahedral units in KsSn25 (see Fig. 5, I). (C) Bonding electron pairs 1 and lone pairs 2 in BaSn3. Twelve lone pairs 1 are located inside the truncated tetrahedral unit (D) in KSPb24. Further lone pairs 2 are located at the Pb atoms that are not part of a truncated tetrahedral unit (E).
5.7 Discrete and Linked Homoatomic Clusters of the Elements Ge, Sn, and Pb
1637
inside the Frank-Kasper type polyhedra around K1. The 3D-isosurfaces are represented for ELF = 0.78 in the region of K2 and K1 in Fig. 8 D and E, respectively. The values of the ELF along direct Pb-Pb contacts are much lower. A qualitative picture of the electron-phonon interactions in BaSn3 and KsPb24 may be deduced from these results. Lattice vibrations have a strong effect on the interactions between the lone pairs. Raising the free-electron-pair states above the Fermi level results in an electron transfer to the conduction band (and uice uersu), i.e. transitions between localized and conduction electrons might be dependent on lattice vibrations. This working hypothesis for understanding superconducting behavior follows the ideas Simon and others developed for superconductors containing units with C-C and B-C bonds. Interestingly the coincident occurence of and lone pairs is also apparent in RhBi4. Here the lone pairs occur in parabolic
5.7.4 Conclusion In 1981 von Schnering expressed that nine-atom clusters such as Sng4- “form one of the most beautiful clusters known”.121More recently increasing attention in cluster chemistry has focused on the lighter group-I4 homologues - the buckyballs and the discovery of superconductivity in the alkali-metal fulleride phases. But the extraordinary variety of the different naked clusters of the heavier elements still generates substantial interest. It is intriguing that the formation both of discrete cluster units and of polymeric network structures, and the gradual transition from the one to the other is possible for compounds containing the elements Ge, Sn, and Pb. One might, moreover, expect these compounds to have interesting physical properties because the transition from insulating to metallic behavior of group-14 elements occurs going down the periodic table. When Wade’s electron-counting rules are applied for clusters one might expect stability for closo-clusters En2- for all n if E is a group-14 element. In cluster syntheses via solution methods ligand-free nine-atom cluster anions turn up most frequently. Three examples of five-atom anions and only one ten atom cluster, Gelo2-, have been reported. Six-atom clusters are only known in transition-metal complexes. Many structural investigations on the nine-atom clusters reveal that they carry 3- and 4- charges, respectively. Disproportionation reactions of trianions into clusters with 2- and 4- charges are likely, although unambiguous proof is lacking. The various structural distortions of the nine-atom clusters show that several conformations on the topological path between the C4,-symmetric monocapped quadratic antiprism and the D3h-symmetric tricapped trigonal prism have a minimum on the energetic hypersurface. Direct correlation between the observed structures of the discrete anions in the solid state and electron counts is not obvious. Although
1638
5 Solid-state Cluster Chemistry
EPR measurements show the existence of paramagnetic clusters, magnetic susceptibility data reveal magnetic moments indicative of fewer than one unpaired electron per cluster. Nine-atom Eg4- clusters also occur in binary phases, but tetrahedral four-atom E44- clusters are predominant beside the arachno-type E5 6- clusters. Larger homoatomic units are known as building blocks of three-dimensional networks in binary and ternary alkali or alkaline-earth metal phases. Face sharing clusters with 14-, 16-, and 20 pentagonal or hexagonal faces, and generally with alkali or alkaline-earth metal atoms at their centers, build up three-dimensional, polymeric frameworks. Resulting voids and channels are filled with electropositive atoms. Many of these intermetallic compounds are Zintl phases, e.g. semiconducting valence compounds. Compounds containing networks of triangular units are mostly metallic. Theoretical investigations show that lone pairs can play a crucial role in the electronic structures of these compounds and thus their physical properties. Some examples of superconductivity have already been observed. Even though the first reports on this extraordinary class of cluster compounds were published more than a century ago, detailed measurements of the physical properties of compounds containing discrete or linked clusters have just begun. Further efforts to achieve a deeper understanding of the relationships between structure and properties in this field are certainly foreseen.
Acknowledgments The author wishes to thank M. Pel1 and G. Mullen for editing the manuscript and is further indebted to Professor R. Nesper for his generous support and for valuable discussions. He also thanks the Eidgenossische Technische Hochschule Zurich for supporting some of this work.
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5.7 Discrete and Linked Hotnoatomic Clusters of the Elements Ge, Sn, and Pb
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5.7 Discrete and Linked Homoatomic Clusters of the Elements Ce, Sn, and Ph
1641
[93] A. Joannis, Ann. Chim. Phys. 1906, 7181, 5. [94] C. A. Kraus, J. Am. Chem. SOC.1907,2Y, 1557. [95] F. H. Smyth, J. Am. Chem. SOC.1917, 3Y, 1299. [96] C. A. Kraus, J. Am. Chem. SOC.1922, 44, 1216. [97] D. Kummer, L. Diehl, Angew. Chem. 1970, 82, 881; Angew. Chem., Int. Ed. Engl. 1970, Y, 895. [98] L. Diehl, K. Khodadadeh, D. Kummer, J. Striihle, Chem. Ber. 1976, ZOY, 3404. [99] D. G. Adolphson, J. D. Corbett, D. J. Merryman, J. Am. Chem. SOC.1976, Y8, 7234. [loo] B. S. Pons, D. J. Santure, R. C. Taylor, R. W. Rudolph, Electrochim. Acta 1981, 26, 365. [loll R. W. Rudolph, W. L. Wilson, F. Parker, R. C. Taylor, D. C. Young, J. Am. Chem. SOC. 1978, 100,4629, [lo21 R. W. Rudolph, W. L. Wilson, R. C. Taylor, J. Am. Chem. SOC.1981, 103, 2480. [I031 W. L. Wilson, R. W. Rudolph, L. L. Lohr, R. C. Taylor, P. Pyykko, Inorg. Chem. 1986, 25, 1535. [lo41 M. Bjorgvinsson, H. P. A. Mercier, K. M. Mitchell, G . J. Schrobilgen, G. Strohe, Inorg. Chem. 1993, 32, 6046. [I051 R. C. Bruns, L. A. Devereux, P. Granger, G. J. Schrobilgen, Inorg. Chem. 1985,24, 2615. [I061 J. Campbell, D. A. Dixon, H. P. A. Mercier, G. J. Schrobilgen, Inorg. Chem. 1995, 34, 5798. [I071 C. H. E. Belin, J. D. Corbett, A. Cisar, J. Am. Chem. SOC.1977, 99, 7163. [ 1081 T. F. Fassler, C. Kronseder, U. Work, Z. Anorg. Allgem. Chcm. 1999, 625, 15. [lo91 T. F. Fassler, M. Hunziker, Z. Anorg. Allg. Chem. 1996, 622, 837. [ 1101 T. F. FBssler, M. Hunziker, M. Spahr, unpublished results. [ 11I ] M. E. O'Neill, K. Wade, Polyhedron 1983, 2, 963. [112] H. A. Jahn, E. Teller, Proc. Roy. SOC.1937, A161, 220. [ 1131 J. K. Burdett, Molecular Shapes, John Wiley & Sons, New York 1980. [114] M. B. Hursthouse, J. Kane, A. G. Massey, Nature 1970, 228, 659. [I151 W. Honk, Y. Grin, A. Burkhardt, U. Wedig, M. Schultheiss, H. G. von Schnering, J. Sol. State Chem. 1997, 133, 59. [ 1161 L. J. Guggenberger, Inorg. Chcm. 1968, 7, 2260. [117] L. J. Guggenberger, Inorg. Chcm. 1969, 8, 2771. [ 1181 T. F. Fassler, M. Hunziker, Inorg. Chem. 1994, 33, 5380. [I191 T. F. Fassler, U. Schutz, Inorg. Chem. 1999, 38, 1866. [I201 S. C. Critchlow, J. D. Corbett, J. Am. Chem. SOC1983, 105, 5715. [I211 J. D. Corbett, P. A. Edwards, J. Am. Chcm. SOC.1977, 99, 3313. [122] R. Burns, J. D. Corbett, Inorg. Chem. 1985, 24, 1489. [123] [K-(18crown6)]4Sn9 with two K-Sn contacts,['s81T. F. Fassler, R. Hoffmann, Angew. Chem. 1999, 111, 526; Angew. Chem. Int. Ed. Engl. 1999, 38, 543. [ 1241 [K-(l8-crown-6)]3KSng x len with five K-Sn c o n t a ~ t s . [ ' ~ ~ ~ ' ~ ~ ~ [ 1251 G. Bruzzone, E. Franceschi, J. Less-Common Met. 1977, 52, 21 1. [I261 G. Borzone, A. Borsese, R. Ferro, J. Less-Common Met. 1982, 85, 195. [127] G. Borzone, A. Borsese, R. Ferro, Z. Anorg. Allg. Chem. 1983, 501, 199. [128] T. F. Fassler, C. Kronseder, Angew. Chem. 1997, 23, 2800; Angew. Chem. Int. Ed. Engl. 1997,36, 2683. [ 1291 R. Kroner, Dissertation, Universitat Stuttgart 1989. [130] J. S. Kasper, P. Hagenmuller, M. Pouchard, C. Cros, Science 1965, 1713. [I311 J. Gallmeier, H. Schafer, A. Weiss, Z. Naturforsch. 1967, 22b, 1080. [132] J. Gallmeier, H. Schafer, A. Weiss, Z. Naturforsch. 1969, 246, 655. [ 1331 Y. N. Grin, L. Z. Melekhov, K. A. Chuntonov, S. P. Yatsenko, Sov. Phys. Crystallogr. 1987, 32, 290. [ 1341 H. G. Schnering, Nova Acta Leopoldina 1985, 59, 168. [ 1351 J. Zhao, J. D. Corbett, Inorg. Chem. 1994, 33, 5721. [ 1361 J. Llanos, Dissertation, Universitat Stuttgart 1984.
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5 Solid-state Cluster Chemistry
[137] B. Eisenmann, H. Schafer, R. Zagler, J. Less-Common Metals 1986, 118, 43. [I381 H. Menke, H. G. von Schnering, Z. Anorg. Allg. Chem. 1973,395,223. [ 1391 R. Nesper, J. Curda, H. G. von Schnering, Angew. Chem. 1986, 98, 369; Angew. Chem. Int. Ed. Engl. 1986, 25, 367 [ 1401 R. Kroner, R. Nesper, H. G. von Schnering, Z. Kristallogr. 1988, 182, 164. [ 1411 H. G. von Schnering, pers. communication 1996. [142] T. F. Fassler, C. Kronseder, Z. Anorg. Allg. Chem. 1998, 624, 561. [143] T. F. Fassler, Z. Anorg. Allg. Chem. 1998, 624, 569. [144] W. Hume-Rothery, R. E. Smallman, C. W. Haworth, The Structure of Metals and Alloys, Antony Rowe Ltd, Chippenham, Wilts 1969. [145] S. B. Roy, K. E. Sim, A. D. Caplin, Philosoph. Mag. B 1992, 65, 1445. [146] C. Cros, M. Pouchard, P. Hagenmiiller, J. Solid State Chem. 1970, 2, 570. [I471 N. F. Mott, J. Solid State Chem. 1973, 6, 348. [148] A. A. Demkov, 0. S. Sankey, K. E. Schmidt, G. A. Adams, M. O’Keeffe, Phys. Rev. B, 1994, 50, 17002. [149] F. Springelkamp, R. A. de Groot, W. Geertsma, W. van der Lugt, F. M. Mueller, Phys. Rev. B 1985,32, 2319. [ 1501 A. D. Becke, E. Edgecombe, J. Chem. Phys. 1990, 92, 5397. [151] A. Savin. R. Nesper, S. Wengert, T. F. Fassler, Angew. Chem. 1997, 109, 1892; Angew. Chem. Int. Ed. Engl. 1997,36, 1808. [I521 T. F. Fassler, A. Savin, Chem. unserer Zeit 1997, 31, 110. [I531 T. F. Fassler, U. HauBermann, R. Nesper, Chem. Europ. J. 1995, 1, 625. [I541 R. Micnas, J. Ranninger, S. Robaszkiewicz, Rev. Mod. Phys. 1990, 62, 113. [155] A. Simon, A. Yoshiasa, M. Backer, R. W. Henn, C. Felser, R. K. Kremer, H. Mattausch, Z. Anorg. Allg. Chem. 1996, 622, 123. [156] N. E. Alekseevskii, G. S. Zhandov, N. N. Zhuravlev, Zh. Eksp. Teor. Fiz. 1955, 28, 237. [157] Y. Grin, U. Wedig, H. G. von Schnering, Angew. Chem. 1995,107, 1318; Angew. Chem. Int. Ed. Engl. 1995,34, 1204. [158] 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane. [159] After the submission of the manuscript the phases A4Pb9 (A = K, Cs) were structurally characterized: V. Queneau, S. C. Sevov, Inorg. Chem. 1998,37, 1358 and E. Todorov, S. C. Sevov, Inorg. Chem. 1998, 37, 3889.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.8 Hexacapped Cubic Transition Metal Clusters
and Derivatives - a Theoretical Approach Rdgis Guutier, Jean-Frangois Halet and Jean- Yves Saillard
5.8.1 Introduction Although the octahedral M6 structural arrangement is by far the most abundant in n-donor ligand-stabilized transition-metal cluster chemistry,"] nowadays a fascinating class of compounds based on the alternative cubic Mg architecture is growing quickly. With the impetus provided by research groups such as those of Fenske and Pohl in Germany, Dahl and Holm in the US, a large variety of different empty, non-metal-centered or metal-centered cubic clusters with Fe, Co, Ni, and Pd are now available. Such species are particularly important because of their relevance to catalysis and to miniaturization of solid materials to nanometer scale. Compounds which have been structurally characterized are listed in Table 1. The aim of this contribution is not to present an exhaustive compilation of their chemistry, which has been comprehensively reviewed elsewhere,['] but rather to analyze their structural arrangement and stability with regard to their electron count. It turns out that the usual electron counting rules, such as the effective atomic number (EAN) approach or the polyhedral skeletal electron pair (PSEP) theory,[31are usually inappropriate for rationalizing the structure and bonding of this kind of compound.r41 This is because electron-counting rules are based on the principle of closed-shell requirement which attributes to a given molecular structure a single favored electron count associated with a significant HOMO-LUMO gap. This energy gap is expected to indicate stability. As we will see below, however, most cubic architectures are not characterized by a single 'magic' electron number, but rather by a large range of allowed electron counts, most of them corresponding to small HOMO-LUMO gaps and often associated with open-shell electron configurations. In this paper we use results coming from theoretical calculations (extended Huckel ( EH)[51and density functional theory (DFT)['] methods) to analyze and rationalize the bonding in these cubic cluster compounds.
1644
5 Solid-Stute Cluster Chemistry
Table 1. Molecular cubic clusters characterized by X-ray diffraction. Compound and Ref.
~(M~-MJ~)
MVE~)
2.65 2.61 2.66 2.10 2.10 2.68 2.61 2.61 2.58 2.68 2.61 2.66 2.65 2.72-2.65 2.12 2.12
120 120 120 120 120 118 116 I I6 1I5 110 109 108 108
104 100
99
2.51-2.54 2.42-2.51
116 114 114 112 112 110
2.12-2.78
114
2.86 3.11 3.26 2.67 2.85 2.99 3.00 2.80 2.81 2.81
130 124 124 124 124 124 124 122 122 121
2.88-3.13 2.47
130 127
-
2.54 2.53 -
5.8 Hexacapped Cubic Transition Metal Clusters and Derivutives
1645
Table 1 (continued) ~
Compound and Ref,
LI(M~-M~)~)
Muin group elemen I -cen tered M8 (p8-El )(p4-E )6 Ls Nig (p8-As)(p4-As)6(PPPh3)s[251 2.85-2.95
-
2.48-2.65 -
a)
Averaged surface metal-surface metal distance
M VEb'
119
178 192 290
(A). b1Totdl valence metallic electron count.
5.8.2 Empty hexacapped cubic species Nig(,u4-PPh)6(CO)8(l),characterized by Lower and Dahl20 years is a typical representative example of an empty cubic inorganic transition-metal cluster of formula M ~ ( , U ~ - E )(M ~ L=. ~transition metal, E = main group element or ligand, L = two-electron terminal ligand (CO, PR3, C1-. . .) and x I 8). Compound 1 consists of a regular cube of Ni atoms, with Ni-Ni distances of 2.65 A, the six faces of which are capped by phenyl-phosphido groups. Every metal atom also bears a terminal carbonyl ligand, which is directed outwards along one of the Ni8 body-diagonals, resulting in a tetrahedral ligand environment about each nickel atom.[71According to Johnston and Mingos, compound 1 is a capped threeconnected polyhedral cluster which should be associated with 3n skeletal electrons (n represents the number of metallic centers of the cluster cage) or 15n metallic valence electrons (MVE).[301Thus, the actual count of 24 skeletal electrons or 120 MVEsfor1 [ 1 0 ( N i ) x 8 + 2 ( C O ) x 8 + 4 ( P P h ) x 6 = 1 5 ~ 8 = 1 2 O ) i s i n a g r e e ment with these PSEP electron-counting rules. It is also in agreement with the 18electron metal (EAN) rule. Different empty hexacapped cubic compounds similar to complex 1 have been structurally characterized with Ni, Co, and Fe (Table 1). They can be divided into two families. The first comprises compounds such as 1 with a terminal ligand on each metal atom (x = 8). The second class consists of compounds with an incomplete shell of terminal ligands ( x < 8) such as Ni8(,u4-Se)6(PPri3)4(2).["' Except for a few examples which, in common with 1, have the expected electron count of 120, most have fewer. A wide range from 99, as in [Fes(~4-S)618]3-,['81 to 120 MVEs has been observed for the same relatively regular cubic architecture (Table 1). Bond breaking or bond making is generally associated with addition or removal of electrons in molecular transition metal chemistry.[31Surprisingly, only slight overall
1646
5 Solid-state Cluster Chemistry
1
2
shrinkage of the metallic core occurs when MVE decreases in Mg(p4-E)6L, clusters (Table 1). The bonding in this type of cluster has been studied by several groups. An EH investigation of related solid-state materials has been performed by Burdett and Miller.[3 Karplus et al. used EH and density functional self-consistent field-multiple scattering-Xa (Xa) calculations to examine the electronic structure of molecular cubic clusters containing a coS(p4-s)6 core.[321More recently, Rosch et al. studied the metal-metal bonding in Nig(p4-PPh)6(CO)s with the help of DFT calculat i o n ~ . [We ~ ~ ]recently performed EH and Xa calculations on the whole series of M8 (p4-E)6Lxcompounds to rationalize their structural arrangement.[49341 With a count of 120 MVEs, cluster 1 is electron precise with one metal-metal edge of the cluster polyhedron corresponding to a bond pair. Such a description If' ]within such a localized bonding scheme makes it analogous to cubane C X H ~ . [ ~ the nickel atoms obey the 18-electron rule, however, the pentavalent phosphorus atoms do not follow the EAN octet rule. It is known that 2-electron-2-center bonding cannot properly describe the bonding in hypervalent molecules.[351Indeed, phosphorus has only four valence orbitals to ensure five bonding contacts. Consequently, a delocalized MO description is needed to account for the nature of the bonding in the 120-electron cubic Mg(p4-E)6L8species. We have shown that such a delocalized MO picture could easily be understood by
5.8 Hexac~uppedCiihic Trunsition Metul Clusters and Deriuutiues
1647
deriving the MO diagram for a 120-electron species M8(p4-E)~L8from the interaction of the FMOs of a [ M ~ L x ] 'cube ~ + with the FMOs of a capping [E6]'*- oct a h e d r ~ n . [ For ~ ~ ' a molecule of 01,symmetry, the interaction between the two moieties results in the MO pattern illustrated in the middle of Fig. 1. A set of 8 lowenergy, low-lying M-L bonding MOs (not indicated in Fig. 1) is found; these correspond to the eight M-L contacts. Slightly higher in energy is a set of 18 M-E bonding MOs, corresponding to the 24 M-(p4-E) bonding contacts (note the delocalized bonding picture here). These levels are not shown in Fig. 1. Higher in energy, a block of 34 MOs that are mainly metallic (d-type) in character is found, separated from M-E antibonding orbitals by a significant energy gap. The complete filling of this d-block, which is essentially non-bonding, weakly bonding, or weakly antibonding in character, leads to a closed-shell count of 120 MVEs. All calculations reported for 120-electron Mg (p4-E)6Lg complexes are in agreement with these qualitative considerations. A HOMO-LUMO gap larger than 1 eV is computed for the 120-MVE Nig(p4-PH)6(CO)g The occupation of the strongly antibonding LUMO would probably result in the breaking of the hexacapped cubic framework. Accordingly, no M8(p4-E)6L8 species with more than 120 MVEs has been reported. For the 'magic' closed-shell 120-MVE count, M-(p4-E) bonding is maximized.[341 On the other hand, the M-M bonding is not at its maximum value, because the weak antibonding character of the top of the occupied d - b l ~ c k . ' ~ ' It . ~follows ~] that in 1 and in the other 120-MVE clusters, all of which are nickel species, the M-(p4-E) bonding dominates the M-M interaction. When the metal atom is less electronegative, the M-M overlap is larger, so the balance between M-M and M-(p4-E) bonding favors the former. Such a situation favors depopulation of the top of the d-block, i.e. MVE counts lower than 120. Moreover, calculations indicate that the depopulation of the top of the d-block is also favored by the presence of TCdonor terminal ligands L on the metal and/or when the capping ligand E is a bare heavy main-group Indeed, this type of ligand tends to destabilize some of the highest level of the d-block. Examination of the compounds listed in Table 1 confirms these predicted trends. In particular, MVE counts lower than 120 are only observed in M8(p4-E)& species if Fe or Co is substituted for Ni, and/or if n-donor terminal ligands (C1-, Br-, I-, SR-. . .) are present. The hexacapped cubic framework of electron-deficient Mg (p4-E)6Lg clusters are only moderately distorted away from the ideal 01,symmetry. Compared with that of the 120-MVE compounds, the slight reduction of the average Ni-Ni separations observed in the electron-deficient nickel species (Table 1) is consistent with the weak M-M antibonding character of some of the MOs which are depopulated. Most of these clusters have a small HOMO-LUMO gap and/or a high-spin configuration. Xa calculations performed by Karplus and coworkers have shown that the singlet and triplet states in electron-deficient species with a C0g(p4-S)6 core and an even number of electrons are nearly isoenergetic, and that the distortion away from
0
Figure 1. Qualitative MO diagrams for the empty Mg(,u4-E)6L8(center), metal-centered M9(,u4-E)6L8(left), and main group elementcentered M8(,u8-E’)(p4-E),jL8(right) models for the particular case of 120 MVEs.
0
5.8 Hexacapped Cubic Transition Metal Clusters and Derivatives
1649
the cubic oh symmetry is very weak.[321The same conclusions were reached for clusters based on the Fex(p4-S)6 core.[341 It seems that the large connectivity between the atoms of the cluster prevents significant distortion from the hexacapped cubic arrangement. Such resistance to (first- or second-order) JahnTeller distortion, which often favors high-spin configurations, is reminiscent of solid state chemistry. The count of 76 MVEs might constitute the lowest hypothetical limit for these cubic species, in which the M-M cubane-type bonding mode is preserved.[341Cubic architecture containing electron-poor metal atoms and with diffuse atomic orbitals have been predicted.r341It is worth mentioning here the existence of a related cubic lanthanide chalcogenido species Ln8(pu,-E)6(p-EPh)12(THF)8 (Ln = Pr, Nd, Gd, Sm, E = S, Se) recently reported.[361Unlike the title compounds, these 96-MVE hexacapped metallic cubes are edge-bridged. In these species, the Ln atoms are fully oxidized and no electrons are available for metal-metal bonding. This is also true for the related 128-MVE copper species such as ( NPhMe3)4Cux(i-S2CC(CN)2)6, which contains a cubic [ C u g ( p - S ) 1 2core ] ~ ~ made of 16-electron Cu centers.[371 We turn now to discussion of compounds of general formula M8(p4-E)6L, (x < 8) which, like cluster 2, have an incomplete terminal ligand shell and fewer than 120 MVEs (Table 1). Calculations performed on different models in which one or several terminal ligands are missing indicate that the M-L bonds result primarily from combination of a high-lying metallic sp hybrid and the (7 lone pair orbital of L. When a metal atom loses its terminal ligand, its sp hybrid becomes non-bonding but cannot be occupied because of its high energy, leaving a large HOMO-LUMO gap for a ‘magic’ electron number which is reduced by two. This situation is analogous to that observed for 16-electron square-planar ML4 complexes, the MO pattern of which has a vacant non-bonding metallic p orbital at high energy.[351All the Mg(p4-E)6L, (x < 8) clusters listed in Table 1 satisfy the closed-shell requirement with MVE = 120 - 2 x (8 - x). It is probable that lower electron counts could occur with metal atoms which are more electropositive than nickel and/or have terminal n-donor ligands. ~(PR~)~ The coordination deficiency of some Ni atoms in the N ~ X ( ~ ~ - E ) species leads to some distortion of the cube. Additional Ni-Ni bonding occurs along the diagonals of the cube between Ni atoms with no terminal ligand. This attractive interaction between ‘non-bonded’ metals is because of second-order mixing of inphase combinations of the free sp Ni hybrids into occupied MOs of appropriate symmetry. Using DFT results obtained on different ligated Ni8(p4-E)&,, Rosch et al. have shown that there is a direct relationship between the number of terminal ligands and the M-M bond lengths.13 Ni-Ni separations between ligated metal atoms are larger than in the bulk. This is nicely illustrated in the 110-MVE compound Nig(p4-PBut)6(PPh3)2 for instance.[’] Three-coordinated Ni atoms, i.e. not coordinated to a terminal ligand are separated by 2.42 A, whereas three-coordinated and tetrahedrally four-coordinated Ni atoms are separated by 2.57
1650
5 Solid-state Cluster Chemistry
5.8.3 Species containing interstitial transition-metal atoms Several hexacapped cubic clusters containing a transition metal atom lodged at the center of the cubic core have been structurally characterized. These compounds of formula M9(p4-E)6L8, listed in Table 1, are known for M = Ni or Pd. A typical example of this new class of electron-rich compound is the 124-MVE compound Ni9(p4-GeEt)6(CO)g (3) characterized by Dahl and With MVE counts varying from 121 to 130, they are more electron-rich that their non-centered Mg(p4-E)6Lgparents, which contain a maximum of 120 MVEs (uide supra).
3
This family of compounds has been extensively studied in recent years. Wheeler has analyzed the electronic structure of Nig(p4-Te)6(PEt3)g with EH calculations.[381 Hoffmann and coworkers have also used EH calculations to examine the electronic structure of this molecular cubic cluster to draw relationships with nickel-tellurium extended structures.[391More recently, we have performed EH and Xa calculations to rationalize the bonding in these clusters, as a function of different parameters such as the electron count or the nature and the size of the different elements constituting the cluster ~ a g e . [ ~ ’For ~ ’ ] symmetry reasons, a localized two-center-two-electron
5.8 Hexacapped Cubic Transition Metal Clusters and Derivutives
1651
bonding scheme does not apply to these M9(p4-E)6L8 species.[401A delocalized approach is necessary to describe the metallic bonding mode in these compounds. According to the inclusion principle developed by Mingos and coworkers,[411these M9 compounds should have either 114 or 120 MVEs. The actual electron counts for the metal-centered cubic clusters show that these general electron-counting rules cannot be used reliably for this class of compounds. The qualitative electronic structure of the M9(p4-E)gLgcompounds can be described as resulting from the interaction of the interstitial metal atom (Mi) with its M8(p4-E)&g cage.[401The nine AOs of Mi atom span alg(s) tl,(x,y,z) eg(x2- y2, z2) tZg(xy,xz, yz) under o h symmetry. Strong bonding interactions always occur between its diffuse s (alg) and p (tlu)AOs and some corresponding levels of the d-block of the metallic M8 cage of proper symmetry. Consequently, these high-lying s and p AOs of Mi are strongly destabilized and cannot be populated. Therefore, assuming a 120-MVE M8(p4-E)6L8empty cage with a significant HOMO-LUMO gap (vide supra), these particular alg and tl, interactions with the interstitial Mi atom will not change this closed-shell electron count. The result of the interactions involving the low-lying and more contracted d AOs of M, is not so straightfonvdrd. In principle, there are four different theoretical possibilities, leading to four hypothetical closed-shell MVE counts depending upon the strength of the interaction of the Mi d orbitals with corresponding FMOs of the cubic metallic fragment.[401If both eg and t2g AOs of Mi interact strongly with some corresponding metallic levels of the cage, the resulting out-of-phase combinations will be sufficiently antibonding to lie at high energy and will not be occupied. The resulting closed-shell electronic configuration (denoted { 120)) then remains the same as that of the empty M8 parent. This is depicted on the left handside of Fig. I . Such an electron count for cubic M9 species is in agreement with Mingos’s rules.[411Another possibility is that both the eg and tzg orbitals of Mi interact weakly with the metallic cage. consequently, the out-of-phase combinations, denoted 2e, and 2t2g in Fig. I, will remain at a low energy. Under such circumstances, satisfying the closed-shell requirement would lead to their occupation, leading to the MVE count of 120 10 = 130, corresponding to the {120}(eg)4(t2g)6 configuration. With a strong t2, interaction and a weak e, interaction, the favored closed-shell electron count would be 120 4 = 124 (configuration { 120>(eg)4). On the other hand, if the e, interaction is strong and the t2, interaction is weak, the preferred closed-shell electron count corresponds 120 6 = 126 (configuration { 120)(t2g)6). These different electron counts should be associated with different M,-Mi and M,-M, bond distances. It turns out that most of the metal-centered cubic species listed in Table 1 do not fit with these four hypothetical closed-shell configurations. Only the 124-MVE compound 3 has an electronic structure which agrees with one of these situations. Both EH and Xa calculations on the 124-electron model Ni9(p4-GeH)6(C0)g indi~. of the EH bond cate a closed-shell electron configuration { ~ ? o } ( e , )Examination overlap populations shows that neither the M-M nor the M-E bonding contacts
+
+
+
+
+
+
1652
5 Solid-state Cluster Chemistry
are maximized for such an electron count.[401The computed EH and Xcc electronic populations on the central nickel atom reflect the rather strong tzg interaction. Conversely, the eg population of Nii is close to 4,indicating that these orbitals do not play any significant role in the Ni,-Nii interaction. In fact, calculations indicate that, among the four closed-shell configurations proposed above, the two which correspond to the 126- and 130-MVE counts are unlikely to exist.r401Indeed, both correspond to a weak tzg interaction (vide supra), a situation difficult to satisfy because the corresponding overlap is always significant. The reason lies in the favorable directional properties of the tzg AOs of Mi. Consequently, it appears impossible to cancel fully the antibonding character of the t2g out-of-phase combination. Even when the M-M distances are rather long, as in Pdg(p4-Sb)6(PPh3)g,[231 this MO is still somewhat antibonding and lies in the middle of an energy gap.[4o1This situation favors its non-occupation or partial occupation (vide infra). In contrast, 120-MVE M9 compounds for which both eg and t2g interactions are strong, should be attainable with transition metals which are less electron-rich than Ni and which have small-size E capping ligand~.[~O] Open-shell configurations are preferred for the other M9(p4-E)6Lgcompounds.[401 Although isoelectronic with the diamagnetic compound 3, the 124-MVE clusters M9(p4-E)6(PR3)g(M = Nil Pd; E = As, Sb, Bi, R = PBu"3, PPh3),[22,23,251 which all have a bare Group 15 atoms as the p4-E ligands, adopt a high-spin ground state. In contrast to expectations, this Their electron configuration is { 12O}(t2g)4(eg)0. electron configuration difference does not depend on the magnitude of the eg interaction between the interstitial metal atom and the cubic cage, and this interaction is always weak.[401The crucial parameter is rather the particular nature of the p4capping ligands. Bare atoms as capping ligands tend to destabilize the highest eg level of the d-block (vide supra), whether the cube is centered or not. This effect tends to disfavor the occupation of this eg level. For MVE counts greater than 124, the highest levels which can be populated are of t2g, eg and tl, symmetry. This is so for the 130-MVE cluster Nig(p4-Te)6(PEt3)g,[211for which the ground-state openshell configuration { 120}(eg)4(t1g)4(t2g)2 is computed.[401The alternative configurais computed to be considerably less stable suggesting that, as tion { 120}(eg)4(t2g)6 mentioned above, too many electrons in the antibonding t2, level induce an important loss of M,-Mi bonding, rendering the cluster unstable. The partial population of the M,-Mi antibonding t2g MO in these clusters induces some swelling of the cubic framework (Table 1). For instance, surface Ni atom separations are 2.67 A and 2.85 A in Ni9(p4-GeEt)6(C0)g ({120}(e,)4(t2g)0)and Ni9(p4-As)6(Pn-Bu3)8 ({ 120)(t2g)4(eg)0), r e s p e c t i ~ e l y . [ ~The ~ , ~open-shell ~] configurations encountered for these M9 species, the most common, are preferred over closed-shell structures which would lead to strong Jahn-Teller distortions. As stated above for the noncentered clusters, nearly regular cubic architecture is preserved for all these species because of the high connectivity of the different atoms constituting the cluster.r21p261 Making new bonds induces lengthening or breaking of other bonds. This situation is reminiscent of that encountered in body-centered-cubic metals, for instance.
5.8 Hexacapped Cubic Transition Metal Clusters and Derivatives
1653
5.8.4 Distorted metal-centered cubic M9 architectures Highly distorted metal-centered M9 cubic architectures can occur, however, when the ligand environment of the metallic cage is asymmetric, as for the cluster Pd9(p : q 5 ,q2-As2)4(PPh3)s (4).[231 In this compound, the nine Pd atoms form a distorted body-centered cube of D2d symmetry, with four short (average 2.88 A), four long (average 3.16 A), and four very long (average 4.45 A) Pd,-Pd, distances. Four faces of this highly distorted cube are asymmetrically capped by p : q 5 ,q2-As2 dumb-bells. Although the presence of several weak Pd-Pd and Pd-As bonding contacts is consistent with some delocalization in this molecule, EH and DFT calculations clearly indicate a closed-shell electronic structure which to a first approximation can be rationalized on the basis of two-electron-two-center bonding.[421
4
Within this simplified localized picture, the weak Pdi-Pd, bonding interactions (3.0-3.1 A) are neglected, leaving the interstitial d'O Pdi atom as a 16-electron center situated in a distorted square-planar environment of As atoms. The Pd, atoms can be described as 16-electron centers lying in a distorted trigonal-planar environment of ligands (two As atoms and one PPh3 ligand), with an additional 2-electron bond with its closest Pd, neighbor. Examination of the EH overlap populations confirms that the long bonding contacts can be discarded to a first approximation, suggesting that in cluster 4 the As2 units behave rather more as asymmetrical p : q2 - q2 bridges than p : q5 - r2 bridges.[421Taking away the Pd, atom of 4 leaves the 120MVE empty Pds(p : q 5 ,q2-As2)4(PPh3)s cage which has a closed-shell configuration associated with 16-electron Pd centers. This constitutes an alternative architecture for the 120-MVE M8(p4-E)6L8species described above.
1654
5 Solid-state Cluster Chemistry
Another distortion of a body-centered M9 cube is encountered in the related 127-MVE clusters [Co9(p,-Bi)4(C0)16l2- (5) recently characterized by Eveland and Whitmire.[27,2x1 If the structure of cluster 5 is viewed as a metal-centered elongated Cog cube with four Bi atoms capping the four rectangular faces, it bears some structural resemblance to the hexacapped cubic Mg(p~q-E)& species such as 3 mentioned above.r21p261 EH and DFT calculations were recently performed on compound 5 and related models for comparison with the M9(p4-E)& cubic clusters.[281
P
5
The closed-shell requirement for such a metal-centered tetracapped elongated cube can be easily derived from that of a regular tetracapped non-centered cube.[281 The ‘magic’ number for a non-centered Mg(p4-E)4L16system with regular cubic architecture (12 M-M bonds) is 120 MVEs, as for the hexacapped oh Ms(&-E)& clusters (vide infra). Stretching the tetracapped Ms(p4-E)4L16 species along its fourfold axis, i.e. breaking four M-M bonds, leads to the stabilization of three M-M antibonding MOs (not four as one might expect from a localized viewpoint). Therefore, the favored closed-shell MVE count becomes 126.[2x1 When the interstitial Mi atom is lodged in the middle of the elongated Ms(p4-E)4L16cage, its d-type AOs do not interact significantly and therefore have to be occupied. This leads to the hypothetical closed-shell MVE count of 136 for a tetracapped elongated Ms(p4-E)4L16 species. As for the oh M8(p4-E)& clusters, this ‘magic’ number corresponds to the highest limit of a large range of allowed electron counts. Calculations on cluster 5 indicate that the upper part of the d-block MOs is slightly Co,-Coi and Coi-Bi antibondingL2 Its non-occupation in this 127-MVE compound contributes to strengthening of the bonding of Coi with the CosBi4 fragment. DFT calculations
5.8 Hexacapped Cubic Transition Metal Clusters and Derivatives
1655
(alg)2for the groundindicate an open-shell electron configuration { 122}(b2g)2(eg)' state of 5 under D4h syrnmetry.fz8J
5.8.5 Interstitial main group atoms Hexacapped cubic species with an interstitial main group element instead of a transition metal are rather scarce. Hitherto, only one example, namely the 119MVE compound N&(pz-As)(p4-As)6( PPh3)s (6) has been prepared.[25JThe structural arrangement of this cluster reported by Fenske and coworkers is strongly related to that of the metal-centered Nig(p4-As)6(P - B u " ~ ) with ~ [ ~ comparable ~~ NiS-Nis and Ni,-As separations. The possibility of encapsulating main group atoms in the middle of the metallic M8(p4-E)6L8cube was initially examined by Wheeler few years ago.[38JUsing EH
6
calculations performed on the Te-centered model Nis(ps-Te)(p4-Te)6(H)I, he predicted two favorable closed-shell electron counts, 110 and 126, for these hypothetical cubic species. Although these electron counts are in disagreement with the inclusion principle[40Jwhich predicts 114 or 120 MVEs, the strong bonding of the interstitial Te atom with both Ni and Te atoms of the framework computed for the 126-MVE count led him to propose that such Ni8(ps-Te) species should be stable. He pointed out, however, that the close grouping of MOs in the HOMO region, and the variety of electron counts observed for the empty cubic Ms(p4-E)6Ls clus-
1656
5 Solid-state Cluster Chemistry
ters (vide supra), might imply the possibility of other electron counts depending on the size and nature of the l i g a n d ~ . [Since ~ ~ ] the characterization of 6, we have recently performed EH and DFT calculations on a wide series of Mg(pg-E‘)(p4-E)6Lg models.[431Our conclusions differ somewhat from those drawn by Wheeler. Indeed, two possible closed-shell electron counts are found, 120 and 122, depending on the nature of the interstitial main group element. As for the metal-centered M9(p4-E)6L8 species, the bonding in the cluster Mg(p8-E’)(p4-E)& can be envisiged as resulting from the interaction of the interstitial E’ atom with the metallic Mg(p4-E)&8 cubic host. Two favorable closedshell electron configurations are expected. The first arises when both the s and p AOs of E’ (which span alg and tl, under oh symmetry, respectively) interact strongly with metallic MO counterparts of the cubic unit. This situation, which leads to the expected closed-shell count of 120 MVEs as the empty parent molecule, is illustrated on the right of Fig. 1. If, however, E’ is a heavy main group atom, occasionally its c A 0 might be too contracted and too low in energy to interact significantly with its alg counterpart on the cubic cage. Then the closed-shell requirement requires the occupation of the resulting alg, an out-of-phase combination (2a1, in Fig. 1) leading to the 122-MVE count. DFT calculations performed on different Nig(pg-E’)(p4-E)&8models indicate that the 120-MVE count is favored with small and/or rather electropositive E’ atoms such as phosphorus, sulfur, or germanium, whereas the 122-MVE count is preferred with large and/or electronegative E’ atoms such as As or Te.[431 Because of the large variety of electron counts encountered in empty and metalcentered cubic compounds, we expect that main group atom-centered clusters based on the same architectural array should be observed with a similar large range of MVE counts, most being open-shell systems. This is demonstrated by Fenske’s cluster, 6 . DFT calculations suggest the open-shell electronic configuration { 116}(a1E)2(eE)1 with three unpaired electrons as the ground-state configuration for the 119-MVE As-containing species 6.[431The occupation of the slightly Ni,-Asi antibonding alg cluster MO is in agreement with the rather long Ni,-Ni, bonds compared with the Ni-Ni bonds measured in the 124-MVE metal-centered cluster Different Nig(p4-GeEt)6(CO)gfor instance (average 2.89 8, compared to 2.67 structural optimizations performed with the DFT method lead to several electron configurations which are very close in energy, indicating a rather flexible N&(p,-As) cluster
5.8.6 Cubic cluster condensation The reduction of the ligand-to-metal ratio leads to either spherical or non-spherical clusters with metal atoms shared between clusters. As examples of non-spherical
5.8 Hexacapped Cubic Transition Metal Clusters and Derivatives
1657
clusters, oligomers and polymers made of condensed octahedral metal units linked via corners, edges, or faces is well-documented both in molecular and solid-state c h e m i ~ t r y . [ ~On , ~ ~the ] other hand, the one-dimensional growth of metal cubic clusters resulting from the stacking of cubic arrays is very seldom observed, and is based on distorted rather than regular cubic units. The species Ni12(,u4-PPh)2(,u6-P2Ph2)4(PHPh)~(Cl)2 (7) provides a relevant example of this category of large condensed clusters.["] Its crystal structure can be described as resulting from the condensation of two empty and distorted cubic M8 units through an expanded metallic square. According to the polyhedral fusion principle,[451cluster 7 should have 184 (128 x 2 - 72) MVEs. Counting the (p6P2Ph2) bridging ligand as a 6-electron donor and the terminal PHPh ligand as 3electron donors,[461it contains 178 MVEs, i.e. six fewer than expected.
4
7
The compound [C014Bi8(C0)20]*- (8) recently reported by Eveland and Whitmire, constitutes another example of condensed cubic species. Its structure consists of two Cox tetragonal prisms as found in [CosBi4(C0)16l2- fused about one square face.r27,281 Assuming that the closed-shell favored electron count for cluster 5 is 136 (vide supra), application of the principle of polyhedral fusion[451gives an electron count of 208 MVEs (136 x 2 -64 = 208) or 210 MVEs (136 x 2 - 62 = 210) for compound 7 (64 and 62 being the usual electron counts for square M4 comp o u n d ~ [ ~ ~These , ~ ~ ]are ) . 16 and 18 electrons, respectively, more than the observed
1658
5 Solid-State Cluster Chemistry
count of 192 MVEs. We have recently analyzed the bonding in this species by EH and DFT calculations[281and confirmed the ‘magic’ number of 210 MVEs, which probably constitutes the largest possible electron count which can be accommodated by this architecture. As for 5, however, partial depopulation of the upper Coi-Co, and Coi-Bi antibonding MOs enhances the stability of the cluster by strengthening the bonding between Coi atoms and the C012Bi8 fragment in the 192MVE cluster, 8. A very small HOMO-LUMO gap (0.03 eV) is computed for this actual electron
uI
8
The same kind of condensation can be recognized in the Chinese lantern-shaped cluster Ni21 Se14(PEt*Ph)I*(9),characterized by Fenske and Its structure can be described as a selenium-tetracapped metal-centered Nil3 cuboctahedron fused with two rather regular Nis cubes along the four-fold axis. Application of the polyhedral fusion principle[4s1leads to a favored closed-shell MVE count of 286 (170 + 120 x 2 - 62 x 2), in relatively good agreement with the actual number of 290 MVEs counted for 9. EH calculations suggest, however, a ‘metallic state’ with no significant HOMO-LUMO gap for any reasonable electron count.[281Here again, as already mentioned for compounds 5 and 8, the HOMO-LUMO region is made of MOs which are slightly antibonding between the interstitial Nii atom and its surrounding Ni, atoms of the cuboctahedral part of the cluster. Such examples of empty or metal-centered condensed cubic clusters suggest that larger clusters with additional layers could be prepared, leading ultimately to an
5.8 Hexacapped Cubic Transition Metal Clusters and Derivatives
1659
t
9
infinite one-dimensional network. EH calculations that we have performed on hypothetical polymer and oligomers made of fused metal-centered tetragonal prismatic M9E4 units show that a full metallic situation (ie. no significant HOMO-LUMO gap for any realistic electron count) is reached as soon as three such units are fused.[281
5.8.7 Conclusions and comments In this article we have shown that the electronic structures of transition metal compounds based on a cubic architecture can be rationalized, by use of results obtained from molecular orbital calculations, leading to some interesting extensions of the electron-counting PSEP rules. Indeed, the cluster topology of the different cubic cluster categories is highly dependent on several parameters which can be calibrated. The empty hexacapped Ms(p4-E)& architecture is permitted for a broad range of electron counts. The upper limit (120 MVEs) corresponds to a closed-shell elec-
1660
5 Solid-state Cluster Chemistry
tronic configuration secured by a significant HOMO-LUMO gap. Because of the weak metal-metal antibonding character of the HOMO region, partial depopulation is possible leading to lower electron counts. The lowest hypothetical limit (76 MVEs) might be approached with early transition metals. In contrast, electron counts greater than 120 MVEs are unlikely. They would destabilize and break the metal cubic core. With a metal atom or a main-group atom at the center of the metal cube, the M9(,u4-E)6and Mg(,uu,-E’)(,u4-E)6 cubic species have electron counts generally higher than those of the empty cubic-related structures. A wide range of counts up to 130 MVEs have been reported so far depending on the magnitude of the interaction of the interstitial atom with its metallic cubic host and the nature of the capping E ligands (either bare or substituted). Among these electron counts, few such as 120 and 124 with an interstitial metal atom, or 120 or 122 with an interstitial main group atom, correspond to closed-shell electronic configurations, but the majority of the reported clusters have an open-shell configuration. Most ‘electron-deficient’ centered or non-centered cubic clusters have an openshell electronic configuration, or at least a small HOMO-LUMO gap. The high connectivity of the different atoms constituting the cluster core prevents any strong Jahn-Teller distortion. Making new bonds induces a lengthening, or breaking, of other bonds. This is reminiscent of structural changes encountered in solid-state chemistry. Thus, it is interesting to note that two situations can coexist from the viewpoint of electronic structure for these cubic species. The first situation is that generally observed for stable molecular systems, i. e. closed-shell electron configurations corresponding to ‘magic’ MVE counts associated with a significant HOMO-LUMO gap. The second, common in extended structures, allows a range of possible electron counts with no significant gap between the skeletal frontier orbitals and consequently, open-shell electronic configurations are often preferred. It is predicted that the occurence of variable electron counts should become more frequent for the same architecture as the nuclearity of the cluster increases. Bonding delocalization increases, HOMO-LUMO gaps decrease, and a progressive change from discrete to quasi-continuous energy levels occurs. This is particularly true for the condensed cubic compounds. Most research on transition metal cubic clusters has concentrated on their synthesis and their structural characterization. Little attention has yet been devoted to their reactivity and physical properties.[’] Elucidation of their bonding might in the future contribute to a deeper understanding of these aspects. The stability of the cubic architecture for a large gamut of electron counts is now well understood and largely confirmed experimentally. Fenske has shown, for instance, that some of the empty Mg(p4-E)6 species can undergo redox processes without altering their cubic metal arrangement.[’] This capability of behaving either as an electron sink or an electron reservoir could be exploited for catalysis and for designing new materials with interesting properties. New conducting and/or magnetic extended structures resulting from the assem-
5.8 Hexacapped Cubic Transition Metal Clusters and Derivatives
1661
blage, linking, or condensation of cubic units can be envisaged, as it is commonly observed in the solid-state chemistry of octahedral clusters.[443481 From this point of view, it is worth mentioning that cubic clustering is already documented in the solid state. M8(p4-S)6entities have been initially observed in natural (Fe, Co, Ni)9S8r491 pentlandites, and in djerfisherites such as K6LiFe24S26C1[511 and synthetic c09s8[501 or Ba6M2&7 (M = Co, Ni).[521Metallic behavior is usually observed in these compounds. The electronic properties of this material were examined in detail by Burdett and Miller some years ago.[311The 11 1-MVE c08(p4-s)6units can be compared with the 108 and 109-MVE molecular analogs [ C O ~ ( ~ ~ - S ) ~ ( S P ~ ) ~ ] ~ - ’ ~ - . Although the Co-Co separations are slightly shorter in the former (by ca. 0.16 A), the bonding is rather similar in both species.[311 Stable cubic complexes with either a complete or an incomplete terminal ligand sheath might serve to exchange labile terminal ligands during reactions. This has already been illustrated by Fenske who has shown that ligands such as (CN)-, (SCN)-, or (N3)- can replace terminal C1- ligands in the 116-MVE Nis(p4PPh)6(PPh3)4C14 compound.[’] Cubic clusters are characterized by an electronic structure which is either ‘molecular’ or ‘metallic’ in nature. Accurate measurements of their physical properties may provide some indication of the particle size at which the typical properties of a bulk metal begin to appear.[531
Acknowledgments Professor D. Fenske, Dr. E. Furet, Dr. A. Le Beuze, Professor K. H. Whitmire, and B. Zouchoune are gratefully acknowledged for their participation in some of the work discussed in this article. Molecular diagrams were based on X-ray crystallographic data were prepared with the Ca.R.Ine Cristallographie 3.0.1 program (C. Boudias and D. Monceau, 1989-1994).
References [ l ] See for example: (a) Cotton F A, Haas T E, Inorg Chem 3 (1964) 10. (b) Johnston R L, Mingos D M P, Inorg Chem 25 (1985) 1661. (c) Mealli C, Lopez J A, Sun Y, Calhorda M J, Inorg Chim Acta 213 (1993) 199. (d) Lin Z , Williams I D, Polyhedron 15 (1996) 3277. (e) Lin Z, Fan M.-F., Struct Bond. 87 (1997) 35 [2] Fenske D, In: Schmid G (ed) Clusters and Colloids: VCH, Weinheim (1994), p 212 [3] The term PSEP was first introduced by Mason R, Thomas K M, Mingos D M P, J Am Chem SOC95 (1973) 3802. For a complete description of the PSEP theory see for instance: (a) Wade
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K, Adv Inorg Chem Radiochem 18 (1976) 1. (b) K Wade, In: Johnson B F G (ed) Transition Metal Clusters: John Wiley & Sons, New York (1981), p 193. (c) Mingos D M P, Acc Chem Res 7 (1984) 311. (d) Mingos D M P, Johnston R L, Struct Bonding 68 (1987) 29. (e) Mingos D M P, Wales D J, Introduction to Cluster Chemistry. Prentice-Hall Englewood Cliffs, New Jersey (1990) Halet J-F, Saillard J-Y, Struct Bonding 87 (1997) 81 (a) Hoffmann R, J Chem Phys 39 (1963) 1397. (b) Hoffmann R, Lipscomb W N, J Chem Phys 36 (1962) 79 See for example: (a) Ziegler T, Chem Rev 91 (1991) 651. (b) Rosch N, Pacchioni G, In: Schmid G (ed) Clusters and Colloids. VCH, Weinheim (1994), p 5. (c) Ziegler T, Can J Chem 73 (1995) 743. (d) Kohn W, Becke A D, Parr R G, J Phys Chem 100 (1996) 12974 Lower L D, Dahl L F, J Am Chem SOC98 (1976) 5046 Fenske D, Basoglu R, Hachgenei J, Rogel F, Angew Chem Int Ed Engl23 (1984) 160 Fenske D, (unpublished results) Fenske D, Magull J, Z Naturforsch 45b (1990) 121 Fenske D, Krautscheid H, Miiller M, Angew Chem Int Ed Engl31 (1992) 321 Fenske D, Hachgenei J, Ohmer J, Angew Chem Int Ed Engl24 (1985) 706 Fenske D, Ohmer J, Hachgenei J, Merzweiler K, Angew Chem Int Ed Engl27 (1988) 1277 Saak W, Pohl S, Angew Chem Int Ed Engl30 (1991) 881 Christou G, Hagen K S, Bashkin J K, Holm R H, Inorg Chem 24 (1985) 1010 Junghans C, Saak W Pohl S, J Chem SOCChem Commun (1985) 2327 (a) Pohl S, Opitz U, Angew Chem Int Ed Engl32 (1993) 863. (b) Pohl S, Barklage W, Saak W, Opitz U, J Chem SOCChem Commun (1993) 1251 Pohl S, Saak W, Angew Chem Int Ed Engl23 (1984) 907 Fenske D, Hachgenei J, Rogel F, Angew Chem Int Ed Engl23 (1984) 982 Arif A M, Jones R A, Heaton D E, Nunn C M, Scwab S T, Inorg Chem 27 (1988) 254 Brennan J G, Siegrist T, Stuczynski S M, Steigerwald M L, J Am Chem SOC111 (1989) 9240 Fenske D, Fleischer H, Persau C, Angew Chem Int Ed Engl28 (1989) 1665 Fenske D, Persau C, Z Anorg Allg Chem 593 (1991) 61 Zebrowski J P, Hayashi R K, Bjarnason A, Dahl L F, J Am Chem SOC114 (1992) 3121 (a) Fenske D, Vogt K, (unpublished results). (b) Vogt K, PhD Dissertation University of Karlsruhe Germany ( 1994) Fenske D, Ohmer J, Merzweiler K, Angew Chem Int Ed Engl27 (1988) 1512 Whitmire K H, Eveland J R, J Chem SOCChem Commun (1994) 1335 Zouchoune B, Ogliaro F, Halet J-F, Saillard J-Y, Eveland J R, Whitmire K H, Inorg Chem 37 (1998) 865 Fenske D, Magull J, Z Anorg Allg Chem 594 (1991) 29 Johnston R L, Mingos D M P, J. Organomet Chem 280 (1985) 407 Burdett J K, Miller G J, J Am Chem SOC109 (1987) 4081 Hoffman G G, Bashkin J K, Karplus M, J Am Chem SOC112 (1990) 8705 Rosch N, Ackermann L, Pacchioni G, Inorg Chem 32 (1993) 2963 Furet E, Le Beuze A, Halet J-F, Saillard J-Y, J Am Chem SOC116 (1994) 274 See for example: Albright T A, Burdett J K, Whangbo M H, In: Orbital Interactions in Chemistry. John Wiley & Sons, New York (1985), p 258 (a) Freedman D, Emge T J, Brennan J G, J. Am. Chem. SOC.119 (1997) 11 112. (b) Melman, J H, Emge T J, Brennan J G, J. Chem. SOC.Chem. Commun. (1997) 2269. (a) McCandlish L E, Bissell E C, Coucouvanis D, Fackler J P, Knox K, J Am Chem SOC90 (1968) 7357. (b) Hollander F J, Coucouvanis D, J Am Chem SOC96 (1974) 5646. (c) Hollander F J, Caffery M L, Coucouvanis D, 167th National Meeting of the American Chemical Society Los Angeles Calif Avril INOR 73, (1974). (d) Avdeef A, Fackler J P, Inorg Chem 17 (1978) 2182. See also: Liu C W, Stubbs T, Staples R J, Fackler Jr J P, J Am Chem SOC117 (1995) 9778
5.8 Hexucupped Cubic Transition Metal Clusters and Derivatives
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[38] Wheeler R A, J Am Chem SOC112 (1990) 8737, ibid 113 (1991) 4046 [39] Nomikou Z, Schubert B, Hoffmann R, Steigenvald M L, Inorg Chem 31 (1992) 2201 [40] Furet E, Le Beuze A, Halet J-F, Saillard J-Y, J Am Chem SOC117 (1995) 4936 [41] (a) Mingos D M P, J Chem SOCChem Commun (1985) 1353. (b) Mingos D M P, Lin Z, J Chem SOCDalton Trans (1988) 1657 [42] Gautier R, Halet J-F, Saillard J-Y, Eur J Inorg Chem (1999) 673 [43] Gautier R, Ogliaro F, Halet J-F, Saillard J-Y, Baerends EJ, Eur J Inorg Chem (1999) 1161 [44] Simon A, In: Schmid G (ed) Clusters and Colloids. VCH, Weinheim (1994), p. 373 [45] Mingos D M P, J Chem SOCChem Commun (1983) 706 [46] Bohle D S, Jones T C, Rickard C E F, Roper W R, Organometallics 5 (1986) 1612 [47] (a) Halet J-F, Hoffmann R, Saillard J-Y, Inorg Chem 25 (1985) 1695. (b) Halet J-F, Coord Chem Rev 143 (1995) 637 [48] See for example: Prokopuk N, Shriver D F, Inorg Chem 36 (1997) 5609 and references therein [49] (a) Vaughan D J, Craig J R, Mineral Chemistry of Metal Sulfides: Cambridge University Press, New York (1978). (b) Rajamani V, Prewitt CT, Can Mineral 12 (1973) 178. (c) Ibid Am Mineral 60 (1975) 39 [SO] (a) Rajamani V, Prewitt C T, Can Mineral 13 (1975) 75. (b) Kim K, Dwight K, Wold A, Chianelli R R, Mater Res Bull 16 (1981) 1319. (c) Pasquariello D M, Kershaw R, Passaretti J D, Dwight K, Wold A, Inorg Chem 23 (1984) 872 [51] (a) Tani B S, Am Mineral 62 (1977) 819. (b) Czamanske G K, Erd R C, Am Mineral 64 (1979) 176 [52] (a) Snyder G J, Badding M E, DiSalvo F J, Inorg Chem 31 (1992) 2107. (b) Gelabert M C, Ho M H, Malik A-S, DiSalvo F J, Deniard P, Brec R, Chem Eur J 3 (1997) 1884 [53] See for example: Schmid G, In: Schmid G (ed) Clusters and Colloids. VCH, Weinheim (1994), p 178
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
5.9 Metallocarbohedrenes M&12 (M = Ti, Zr, Hf, V, Nb, Cr, Mo, Fe) and Ti,M’,,Cl2 (M‘=Y, Zr, Hf, Nb, Ta, Mo, W, Si, x + y = 8 ) - from Mass Spectrometry to Computational Chemistry Marie-Madeleine Rohmer, Marc Bknard, and Josep-M. Poblet
5.9.1 Metallocarbohedrenes - organometallic fullerenes? Since the very first report by Guo, Kerns and Castleman entitled Ti&lz+-MetalloCarbohedrenes: a new class of molecular clusters?[’] a link has clearly been established between those organometallic complexes and the class of fullerene molecules. The origin of this link should be traced to the similar development of the two discoveries. As for C60L2], Ti&12+ broke into the world of chemistry as a ‘supermagic’ peak in a mass spectrum (Fig. 1). That the peak associated with Ti&12+ largely dominates the distribution of Ti,&,+ cluster cations generated from the reaction of titanium with the vapor of various hydrocarbons was immediately interpreted by Guo and coworkers as the sign of a special stability assigned to a cage structure.[’] Hence comes the second element, of topological and chemical importance, which has been tying together the new class of metallocarbohedrenes and the famous fullerene series - on the basis of the carbon-like nature of the titanium valence-shell, a pentagonal dodecahedron structure was proposed for Ti&12+ (Fig. 2a). This conformation, made of 12 TiC-C-Ti-C pentagonal rings, is similar to that of known and stable species such as dodecahedrane C ~ O H ~or O dodecahedral [~] water clusters.[41It is also isostructural and isoelectronic with the smallest possible element of the fullerene series C20, which will probably remain elusive because of ring and cage strain. Thus it is not surprising that the dodecahedral structure initially proposed for ‘met-cars’ has been included as fullerene-like in the application field of magic number rules and other systematics devised to account for the occurrence of fullerene isomers and to predict their geometrical shapes, point groups, electronic structures, vibrational and NMR spectroscopic signatures,[5361 or to give proper representations of those large Since then, metallocarbohedrenes have gained recognition as an original class of cage-like organometallic clusters, as manifested by the publication of review articles
1665
5.9 Metallocarhohedrenes Mac12 and TixM’,C12
I
1
I
I
I
TisCta +
I
n
I
c60
kt
W
w e
s
-40.0
160.0
h--
280.0
400.0
520.0
640.0
MASS (AMU) Figure 1. Mass distribution of Ti,&+ clusters generated from the reactions of titanium with CH4.[’] (Reproduced with permission from Professor A. W. Castleman, Jr.) Smaller frame: Timeof-flight mass spectra of carbon clusters prepared by laser vaporization of graphite.’’]
devoted either exclusively[*”]or in a large part[8b1to MsClz clusters. This recognition has been acquired through a series of landmarks that have punctuated the brief, rich, and still unfolding history of these molecules - extension to most transition metals of groups IVA, VA, and VIA and to Fe,[9.101production of mixedmetal met-cars,[lll production of TisC12 and V8C12 in macroscopic quantities,“’] investigation of growth and dissociation pathways,“ 3, 14] competition with facecentered cubic clusters with a 1:1 metal-carbon ratio,“ ’] discovery of collective electronic properties upon ionization,[161and investigation of an active and varied chemistry originating at the metal ~ e n t e r s . ~ ~At ’ , ~the ~ ]same time, an impressive effort has been made to explore the electronic structure, the spatial conformation and physical and chemical properties of met-cars by quantum chemical methods. It is clear that early theoretical studies, including our work, were triggered by attractive similarities between these clusters and the fullerenes. It is remarkable from this point of view that the first communication to propose, on the basis of local density functional calculations, a credible alternative to the structure of pentagonal dodecahedron was entitled “Geometric and Electronic Structures of [Ti8C12]: Analogies with C , j ~ . ” [As ~ ~ for l experimental investigations, intense theoretical and computational effort (36 publications to date) has progressively shown that met-cars are
1666
5 Solid-state Cluster Chemistry
......................
.................
A
(a
w
n
Figure 2. Two conformers proposed for met-cars: (a) the structure of the pentagonal dodecahedron with eight equivalent metal sites ( T h symmetry), proposed by Castleman's group;"] (b) the structure of the tetracapped tetrahedron, with two sets of metal sites each occurring four times (Td symmetry), first proposed by Dance from DFT calculations.[' 91
species of their own, with a specific cage structure, probably different from the pentagonal dodecahedron, and an electronic structure suggesting the crucial importance of unpaired electrons and metal-to-carbon back-donation interactions. The goal of this article is to review present knowledge on metallocarbohedrenes and related molecules, the arguments relating to the still pending question of the cluster structure, and the comings and goings between experimental investigation exclusively relying up to now on mass spectrometric detection, and the pictures and properties proposed by quantum chemistry and computer modeling.
5.9.2 Formation, growth and dissociation of met-cars 5.9.2.1 Methods of preparation The laser-induced plasma reactor initially used by Castleman's group to produce titanium-carbon and vanadium-carbon cluster cations has been described in de-
5.9 Metallocarbohedrenes Ma Clz and Ti,MIy CIZ
1667
tai1.[20,211 It is derived from the technique developed by the groups of Smalley and Bondybey for producing gas-phase clusters of refractory metallic or non-metallic e l e r n e n t ~ . [ ~ ~The . ~ ’ ]material is irradiated with a high power pulsed laser and the vapor produced at the laser impact is transported and cooled in a high-pressure helium stream, thus resulting in the formation and growth of clusters. As long as pure helium is employed as carrier gas, the clusters are made of the initial material. When hydrocarbons, either pure or mixed with helium, are used as carrier gas, metal-carbon and/or pure carbon clusters may also be generated in large amount depending on the type of metal and on the experimental conditions. Group IVA and VA transition metal carbon clusters were produced with this method and the peak characteristic of single-cage met-cars was observed with all metals of those groups with the exception of t a n t a l ~ m . [ ~ L ’ ater, ~ ’ ~ Pilgrim ~ ~ ~ ~ and ] Duncan used the same technique to generate met-cars of Cr, Fe, and Mo.”’] The experiments by Castleman’s group have been reproduced by Yu et UI.[~’]and by Lee et a1.[281It should be noted that the charged species that are detected and characterized in the mass spectrometer can either be extracted directly from the laser vaporization (LAVA) source, or obtained by photoionizing neutral clusters, resulting in markedly different cluster distributions.[’ ’] TigCIZf and other met-cars were also synthesized by using only metal and pure carbon as reactants - eliminating the use of hydrocarbon gas and using graphite powder as the carbon source.[291This technical improvement by O’Keefe et al. was first applied to the production of carbon clusters and f~llerenes.[’~] The so-called direct laser vaporization method removes the need for carrier gas by applying the laser source to a powdered mixture of metal and graphite directly below the center of the time-of-flight extraction region.I3 Both the direct laser vaporization method and the standard technique have been used to generate Ti,Cy+ and ZrxCy+clusters, and the two series of spectra are in good qualitative accord, showing more specifically the expected magic peaks corresponding to MsC12+.[”] Both methods were also used to produce binary met-cars associating in the same clusters x atoms of titanium with y = 8 - x atoms of either zirconium or hafnium.[”] A series of six peaks of steadily decreasing intensity was obtained by direct laser vaporization of a 4:l mixture of TIC and Zr powders and assigned to the mixed metal met-cars TixZryCIZ ( y = 0 to 5) (Fig. 3). A similar regularity in the mass distribution of the various Zr-substituted metcars was observed for different Ti/Zr molar ratios and was used as an argument in favor of a pentagonal dodecahedron structure in which all metal sites are equivalent.[33]In subsequent studies, binary metal metallocarbohedrenes of titanium and other metals TixMyCIZ (x + y = 8) have been obtained either by the standard method using pure metal powders of Ti and M as a target for laser vaporization and a mixture of helium with 10% methane as carrier gas (M = Nb, Ta, Y, Si),[341or by direct laser vaporization of a mixture of titanium carbide and pure M metal (M = Y, Nb, Mo, Ta, W).[351 In an attempt to follow the fullerene route for obtaining met-cars in macroscopic
1668
5 Solid-state Cluster Chemistry
I i 42.8
J
151.4
168.4 Flight time (ps)
159.9
177.0
185.6
Figure 3. Mass spectrum of Ti*C12+ and zirconium-substituted metallocarbohedrenes produced from the direct laser vaporization of a 4 : 1 molar mixture of TIC and Zr powders." (Reproduced with permission from Prof. A. W. Castleman, Jr.)
quantities, Cartier et al. modified the common arc discharge technique by using metal-graphite electrodes of different composition and obtained soot samples con1% of Ti8C12 or V8C12.r12a1 Very recently, Selvan taining an estimated yield of and Pradeep reproduced this experiment, but at variance with the first report, the clusters were found to be extremely air-sensitive and underwent complete degradation within a few minutes.['2b1Unfortunately, it has not yet been possible to extract or isolate met-cars in the pure state. Recently, Lu et al. replaced the common arc by a pulsed arc cluster ion source (PACIS) coupled to a reflection time-of-flight mass spectrometer.r361In contrast with the contact arc discharge, characterized by a high temperature and low voltage difference at the electrodes ( - 3000 K, 30-40 V), the pulsed arc discharge occurs at low temperature (<500 K) and high voltage difference (>500 V) and results in ejection of matter at the cathode rather than at the anode. The mass distribution of vanadium cluster cations obtained from PACIS experiments indeed peaks near V&12+ but the met-car itself does not seem especially stable relative to other species such as V,C12+, V,C11+ or V&11+. The reasons why this spectrum seems different from those obtained by laser vaporization are not completely understood, but might be attributable either to the presence of oxygen originating in the VzOs sample at the cathode, or to the presence of unquenched plasma in the spectrometer.[361 Met-cars are also generated by the photochemical fragmentation of larger spe-
-
5.9 Metallocarbohedrenes Ma C12 and TixM’,C12
Figure 4. The 3 x 3 x 3 and 3 x 3 x 4 fcc crystallite structures proposed by Pilgrim and Duncan[381for intermediate sized Ti/C and Zr/C magic number clusters and the structure of the Til4Cl3 neutral cluster optimized by Dance[741from DFT calculations (Ti5-C6 = 2.13 A, Ti5-C4 = 2.17 A, Ti3-C4 = 2.05 A; Ti5-Ti5 = 3.02 A, Ti5-Ti3 = 2.89 A; the superscripts are the coordination numbers, that is the number of bonds with atoms of the other element). Reproduced with permission from the authors.
14/13
1669
18/18
14/13
cies. Pilgrim and Duncan initiated this technique[l0’l4]and were first to produce a new family of metal-carbon systems, larger than met-cars and with 1 : 1 M/C stoichiometries characteristic of cubic lattice structures.[’51 The stability of these ‘nanocrystals’, especially those of Til4C13+ and V14C13+, which are assumed to have the 3 x 3 x 3 cubic structure (Fig. 4), was found to be comparable with that of the corresponding M&12 species under the reported experimental conditions.“ 51 It was later shown in two independent s t ~ d i e s ~ ~that ~ , ~the ’ ]relative importance of Nb,C, met-cars and nanocrystals in the mass spectrum is critically dependent on the experimental conditions. It has been proved that the concentration of hydrocarbon in the carrier gas, the nature of the laser used as the vaporization source, the laser power selected, and, finally, the direct detection of cluster ions or the photoionization of neutral species drastically influence the relative proportion of met-cars to n a n o c r y ~ t a l s . 71 [ ~This ~ . ~ selectivity has been attributed to distinct mechanisms of cluster growth for met-cars and nanocrystals (Section 5.9.2.3). Laser-induced photodissociation of 3 x 3 x 3 nanocrystals M14C13+ (M = Ti, V) and larger clusters assumed to have a fcc crystal structure has been reported by Pilgrim and Duncan.[381The titanium carbide cluster corresponding to the 1044-amu peak in the mass spectrum has been assigned to the 3 x 3 x 4 fcc fragment in which one tita-
1670
5 Solid-state Cluster Chemistry
8,13
414
828
Mass (amu)
h
4
.d
4 (d 4
a,
p=.
+
Ti irC19
cage with an endohedral carbon atom.[38,39,561 The lower spectrum has prominent fragments at Ti&12+
nium has been replaced by a carbon atom. Laser-induced photodissociation of that nanocrystal generates the mass spectrum reproduced in Fig. 5a where the most intense peaks resulting from dissociation products are assigned to the metallocarbohedrene TigC12+and to the 3 x 3 x 3 fcc crystallite Til4C13+. Results obtained from the photodissociation of the latter nanocrystal are more surprising, because the most prominent dissociation peak does not correspond to the expected met-car cage, but to TigC13+ (Fig. 5b). An explanation proposed for the formation of this cluster suggests as a first step the elimination of the six metal atoms at the center of the cubic faces followed by the reorganization of the facial
5.9 Metallocarbohedrenes Mac12 and Ti,MIy Clz
1671
carbons into CZ units. A met-car cage is formed and the central carbon remains V14C13+ has trapped inside, yielding the encapsulation complex (C c TigC12)+.[~~] the same dissociation pattern, but the dissociation spectrum of Zr14C13+ is not indicative of any endohedral structure. Theoretical investigations confirm that metcar cages can accommodate an endohedral carbon without any significant loss of stabilization energy per The mass spectrum obtained by Yu and ~ o - w o r k e r s [from ~ ~ ~ the laser-induced plasma reaction of titanium vapor with dehydrogenated CH4 shows, around the dominant peak corresponding to Ti&12+, two satellite peaks assigned to the endohedral met-cars (C c TiSC12)+ and (C2 c TisC12)+. The assignment of TigC14+ as an encapsulation species is hardly compatible, however, with the facile loss of CZpreviously observed from metastable dissociation experiments performed by Castleman's In their report, however, the great stability of the TisC13+ species was observed and related for the first time to the possible existence of endohedral m e t - ~ a r s . [This ~ ~ ] hypothesis was later supported by the abundance of the VgC13+ species obtained from the metastable decay of VyC,+ (n = 14-17).rs61 Laser-induced photofragmentation of transition-metal-coated fullerenes C60Mx and C70Mx (M = Ti, V) was also shown to yield M&12 clusters as the most abundant product as soon as the laser intensity is sufficiently high to break the fullerene cage and remove the carbon s u r p l u ~ . [ ~ ~ , ~ ~ ~
5.9.2.2 Crystal growth What is the mechanism of met-car formation? Collecting information on that problem requires identification of the metal-carbon subspecies sufficiently stable to be considered as intermediates in the construction of met-cars. Since the beginning of met-car history, the mass spectrum obtained from photoionization of the neutral M,C, clusters (M = Ti, V ) has provided information about the most prominent n) = (4,8),( 5 , lo), peaks in the region of intermediate mass - combinations at ( m , (6,12) and (7, 13) are noticeably more abundant than the adjacent metal-carbon clusters (Fig. 6b).r13,391 In the range of smaller clusters, MC2 seems to be the most abundant for both metals, and the combinations (2, 4) and (3, 6) are also p r ~ m i n e n t . ~The ' ~ ] consistent importance of peaks corresponding to metal-carbon clusters of composition ( M C Z )(n ~ = 1 to 6) strongly suggests that MC2 is the building block used to produce clusters of intermediate size up to M6C12. Final cage closure is achieved by addition either of two metal atoms or of one metal and one carbon. In such a circumstance the locally dominant peak, corresponding to M7C13 stoichiometry, should be assigned to a met-car cage structure in which one metal site is occupied by a carbon atom. The recent observation of a peak that could be assigned either to TiyC12 or to TisC16 suggests that the latter species, made of Tic2 fragments only, could be the ultimate precursor to met-car formation.["' A puzzling result was ob-
1612
5 Solid-state Cluster Chemistry
b
a
MASS UNITS
MASS UNITS
Figure 6. (a) A typical mass spectrum of ionic clusters Ti,&+. (b) A typical photo-ion spectrum of neutral clusters Ti,C, at 355 (Reproduced with permission from Prof. A. W. Castleman, Jr.)
tained, however, from gas phase ion chromatography of small iron-carbon cluster anions Fe,C,- (n = 1-3, m = 2-8).r431Those experiments tend to indicate that the carbon atoms maintain a continuous chain in such small clusters and that no carbon dimers are formed that would be characteristic of met-car precursors.[431 If the growth of metal carbide clusters with approximate 2 : 3 metal-to-carbon ratio stops when the number of metal atoms is equal to 8, except for M = Zr, another series of peaks was discovered by Pilgrim and and assigned to clusters with 1 : 1 M/C stoichiometries (M = Ti, V). A magic peak with an intensity comparable with that of M&12 was assigned to M14C13+, and other peaks with significant intensities were obtained at higher masses and attributed to M17C19+, M23C25' and M30C30+.[153381 By analogy with the previously observed and Ta-Cr3'] clusters of similar stoichiometries, those titanium and vanadium carbide molecules were predicted to adopt a structure with the fcc crystal lattice with nearly equal x,y , z proportions - 3 x 3 x 3 for M14C13, 3 x 3 x 4 for M17C19, 3 x 4 x 4 for M23C25, and 3 x 4 x 5 for M30C30. Pilgrim and Duncan then characterized fcc nanocrystals of zirconium carbider3@in the region of the mass spectrum where large
5.9 Metallocarbohedrenes McyCl2 and Ti,M’,
Ci2
1673
In clusters assigned as multicage met-cars were reported by Castleman’s contrast with met-car type clusters, fcc crystallites are expected to grow by successive addition of MC fragments. The relative abundance of the MC and MC2 elementary fragments in the process of large cluster formation, critically dependent upon the experimental conditions, was put forward to explain the growth of the two completely different cluster distributions. More specifically, the effect of the power of the vaporization laser on the dehydrogenation rate of hydrocarbons in the plasma is expected to determine the relative concentrations of metal and carbon and to influence the mechanism of cluster formation either toward carbon-poor species fcc nanocrystals - or toward carbon-rich cages - met-~ars.[~’] This explanation gained credibility when it was found that Nb,C, clusters with either the met-car or the cubic structures could be selectively obtained by adjusting the experimental conditions.[261 Unusual behavior has been observed for the growth pattern of zirconium carbide clusters Zr,C,. If the most important peak corresponds, as expected, to ZrgC12+, subsequent cluster growth is observed with an approximate metal/carbon ratio of 2 : 3 , eventually leading to a series of magic peaks assigned until recently to metallocarbohedrene multicage structures.r24~25.32~441 Apart from Nb13C22+,r261no such pattern was found with the other metals of groups IVA and VA, for which the series of cluster corresponding to an approximate M2 : C3 stoichiometry generally stops when the number of metal atoms reached eight. With zirconium several series of peaks are observed, each characterized by the presence of a prominent ‘magic peak’. Beyond Zr8C12, those peaks have been assigned to the stoichiometries Zr13C22, Zr14C23, Zr18C29, and Zr22C35, and explained by the formation of stable clusters made of several dodecahedra each sharing one or two faces (Fig. 7c). Note, however, that the magic peaks observed do not correspond, as for M8C12, to ‘perfect’ met-car clusters. A regular, face-sharing double cage would correspond to the stoichiometry Zr14C21. The assignment of the most intense peak of that series to Zr13C22 implies the presence of a carbon atom at one metal site (Fig. 7c).13’] The hypothesis of a multicage structure for Zrl3C22 and larger Zr,C, clusters has, however, been questioned in a recent letter by Wang and Cheng, who raise a fascinating conjecture based upon the observation of new magic numbers for large titanium carbide cluster anions.[461The mass spectra of cluster anions are quite different from those reported for cationic and neutral species. More specifically, the met-car Ti8C12 and the cubic Ti14C13 are missing in the anion mass spectrum. Instead, the anions are distributed bimodally with magic peaks corresponding to Ti&, Ti6C13, Ti7C13, and TigC15 in the first series, and to Ti13C22 and Ti14C24 in the second series. The Ti13C22- cluster, which is expected to have exceptional stability, has the same stoichiometry as Zrl3C22+ and for which a double-cage structure was assigned by Castleman. Wang and Cheng suggest for those high nuclearity clusters a layered structure combining the cubic framework of fcc crystallites with the presence of Cz ligands at the corners of the cube for M13C22 (Fig. 7a,b) and along the vertices of more extended one-dimensional, wire-like clusters with stoichiometry
1674
5 Solid-State Cluster Chemistry
Figure 7. Three structures optimized by Wang and Cheng for the Ti13C22 cluster[461 (reproduced with permission from Professor L. S. Wang.) (a) D4h, cubic structure with eight C2 dimers at the cube corners In slightly vertical positions (computed binding energy (b.e.) 263.6 eV). (b) D 4 h , cubic structure with eight Cz dimers at the cube corners in horizontal positions (b.e. 261.6 eV). (c) C,, distorted structure resulting from an ideal double cage structure (b.e. 262.3 eV).
M9,,+4C13,,+9.The Zr22C35 species, characterized by Wei et al.1321and to which a quadruple cage structure was first assigned, would then correspond to the smallest wire-like cluster corresponding to y2 = 2. Assessment of the proposed structure on the basis of local spin density calculations yielded binding energies of 263.63, 261.63, and 262.30 eV for structures (a), (b) and (c) (Fig. 7), respectively.[461The mass spectrum of the V,C,- clusters, recently obtained by confirmed the prominence of the V13C22- and of the v14c24- magic peaks. At lower masses, VsC15- appears as a magic peak like its titanium analog, but V8cI2- was also characterized and could appear as the most important peak for clusters of intermediate sizes, depending on the experimental conditions.[471Cr8C12- appears as a magic peak irrespective of the experimental conditions.[481Interestingly, the stability of the met-car anions continues to increase with metal nuclear charge, because the enhanced stability of v8c12- relative to T&C12-, and of Cr8C12- relative to v&12- can be correlated with the adiabatic electron affinities reported for that series of met-cars - 1.05 eV (Ti~C12);1.80 eV (v8c12); and 2.28 eV ( V ~ c 1 2 ) . [ ~ ~ ]
5.9.2.3 Dissociation pathways Another way of obtaining insight into the bonding of met-cars, nanocrystals, and other cluster systems consists in investigating their mechanisms of decomposition. The method most widely used for met-cars and nanocrystals is photodissociation, in
5.9 Metallocarbohedrenes Mac12 and Ti,M',,C12
1675
which mass-selected cluster ions are laser-excited and dissociate. The mass distribution of the resulting photofragment ions is then obtained, by time-of-flight determinations. The photodissociation spectra obtained from MgC12+parent ions were found to be rather sensitive to the nature of the transition metal. The TisC12+ and VSc12+ photodissociation spectra contain 5/12, 6/12, and 7/12 fragment ions only, suggesting a dissociation channel based on the loss of metal atoms.['41For Zr&12+, the fragment ions are 7/12, 6/10, 5/8, and 4/6, indicating that the initial loss of one metal atom is followed by the successive elimination of several ZrCz units.['41The spectra obtained for Cr&12+ and FegC12+ reveals a fragmentation pattern based upon the successive loss of as many as six metal atoms.["] For MO&12+, initial loss of one MCz fragment yields the 7/10 cluster as the most intense fragmentation peak."'] The next peaks correspond to the 6/9, 5/7 and 4/6 ionic clusters, suggesting that M2C3 and MC2 fragments have been lost from the parent ion. The fragmentation pattern of NbsC12+ is more difficult to interpret because of imperfect selection of the parent ion.I371 The most important dissociation peak corresponds to Nb4C4, consistently assigned as having a 2 x 2 fcc crystallite s t r ~ c t u r eWhen .~~~~~~~ applied to larger nanocrystals (Ti14C13+,Ti1&19+, Zr14C13+, ZrlsC18+, Zr24C23+)1381 this technique consistently yields either MsC13+or MgC12+ as the most prominent photodissociation products thus providing evidence of reorganization of the fragmented fcc crystallites into met-car clusters. Kerns et al. of Castleman's group investigated the collision-induced dissociation of a series of Ti-C and V-C clusters ranging from 8/11 to 9/14 and either 10/13 or 9/17 for Tir5'] and from 6/8 to 9/14 for V.I5l1For TisClz+, this technique confirms the loss of three metal atoms in a multi-step process. Some of the larger clusters were found to undergo a one-step dissociation into Ti8C12+.[501The loss of one metal atom was also the primary dissociation pathway observed for vanadium metcars with the exception of v&14+ which loses C.1 and v&14+ which loses both V and VC2. The collision-induced dissociation technique also confirmed the special stability of Ti*C12+, VsC12+ and VsC13+ by providing an upper limit as high as 9 eV to the bonding energy of those clusters. The minimum collision energies needed to fragment the clusters were also found to be significantly higher for V&12+ and for the supposedly endohedral met-car VXcI.1+ than for the neighboring v8cl 1+ cluster ~pecies.1~ '1 Recently, the photodissociation technique has been improved by identifying the photofragments by energy analysis of the fragment ions in a reflection time-of-flight mass spectrometer, thus enabling more accurate identification of the photofragment This technique confirmed the earlier studies for single metal met-cars and was then applied to binary metal clusters of formula Ti7MC12+ (M = Y, Zr, Nb). For those three cluster species, the dominant mechanism involves the loss of at least two neutral titanium atoms.[521No dissociation pathway implying loss of M atoms was observed, even for M = Y despite the low propensity of yttrium to be included into a titanium-based binary met-car, and unsuccessful attempts to characterize YSC12+ as a magic or even an important peak.[531
1676
5 Solid-state Cluster Chemistry
5.9.3 Physical properties of met-cars and related clusters 5.9.3.1 Ionization potentials Mass spectrometry, which is the only technique that can be used to characterize met-cars and related metal-carbide clusters, implies that the detected clusters are ionized. This requirement opens a route to a variety of experimental procedures enabling insight to be gained into physical properties such as ionization energies, electron affinities, structure, and collective electronic properties such as thermionic electron emission and delayed atomic ion emission. Collision-induced dissociation (CID) experiments break the parent cluster ion into a neutral fragment and a charged one, thus enabling insight to be gained into the relative ionization energies of the two fragments. CID experiments conducted on M9C12+ (M = V, Ti) yielded a neutral metal atom and the met-car cation, thus indicating that the ionization energy of M&12+ is lower than that of M (6.74 eV for V, 6.82 eV for Both cluster fragment ions and metal cations were characterized among the charged fragments observed from photodissociation experiments on TisCl2+ and ZrsC12+; this might be interpreted as indicating that the ionization energy of the metal is similar to, or lower than, that of the cluster fragment~.[’~] This latter conclusion is somewhat puzzling because it is in contradiction with the general trend for clusters that their IPS decrease regularly from the atomic to the bulk limits. Another explanation of the presence of atomic metal ions, based on the delayed emission of such ions by the photoexcited MsC12 cluster^,['^^'^^ seems more promising (see Section 5.9.3.3). In a recent experiment, Brock and Duncan investigated the laser wavelength and power dependence of the photoionization signals to obtain information on the ionization potentials of various metal-carbon clusters.[5’] No signal was observed for Zr, V and Nb met-cars and it was concluded that those clusters have vertical ionization energies greater than the laser energy at 215 nm, namely 5.76 eV. Although the IPS calculated for vSc12 (6.74 eV;[8415.53 eV;r5715.5 e V 8 ] )and for NbsC12 (5.4 eV[581),both with the Td structure, are close to the upper limit of the laser wavelength, a problem arises with the IP of ZrgC12, which is consistently computed to be extremely low (3.99 eV,[5714.1 eV[591).The discrepancy might be explained by a poor cross-section at the observed wavelengths, perhaps because of bad FranckCondon factors between the neutral species and the ion.[601 Although a value of 4.9 k 0.2 eV was obtained experimentally for the vertical ionization potential of TisC12,[’’I ionization of TiSC12 occurred with approximately the same efficiency throughout the region 4.5-5.7 eV. The strong absorption probability observed within this region was interpreted in terms of a high density of excited electronic states. Even though photoionization studies of met-car clusters had previously been performed by Castleman and coworker^,^'^,'^^ the experiments reported by Brock and Duncan were the first to avoid multiphoton absorption and
5.9 Metallocarbohedrenes Ma Cl2 and TiXM’,C,2
1677
subsequent cluster fragmentation. The mass spectra obtained with this technique are, then, expected to reflect as much as possible the nascent distribution of neutral clusters. Rather surprisingly, neither Ti8C12 nor M14C13 (M = Ti, V, Zr, Nb) were as abundant under those conditions as in the nascent cation distribution or as resulting from the photofragmentation of larger clusters. It was therefore inferred that the enhanced stability of those clusters could be characteristic of the cations, but does not extend to the neutral species. This conclusion is strongly questioned by Castleman’s group, who proved that the mass distribution of neutral Ti-C clusters is strongly dependent upon the power of the vaporization laser.[421When the laser power is high enough to induce efficient dehydrogenation of the hydrocarbons in the plasma without inducing cluster fragmentation through multiphoton absorption, the peak corresponding to neutral TigC12 remains dominant.[421Faced with this recent controversy, theoretical models suggest that the di-cation (TigC12)~+ might be particularly stable, assuming a closed-shell structure (see Sections 5.9.5 and 5.9.6). The anomalously low value obtained for the I.P., lower than that of the metal atom by almost 2 eV, also seems in accord with the greater stability of the cationic species. After this work had been completed, Sakurai and Castleman conducted a photoionization spectroscopy study of the pure and mixed titanium/zirconium met-cars to determine their IP’s with a more direct approach, based on the investigation of the photoionization efficiency curves (PIE) near threshold.[”81 For pure metal metcars the obtained IP’s are 4.40 f 0.02 eV for TigC12 and 3.95 0.02 eV for ZrgC12. Those values are in excellent agreement with the computed potentials reported for the Td conformation of met-cars by Dance (4.37 eV for Ti8C12, 3.99 eV for ZrgC12, vertical I P ‘ S ) ~ and ~ ~by~ Poblet et al. (4.6 eV or 4.43 eV for T&C12, 4.1 eV for ZrgC12, adiabatic I P ’ S ) [ ~ Sakurai ~]. and Castleman noted that agreement and discussed its consequences on the preferred conformation of met-cars, concluding that the Td symmetry met-cars are present in considerable amounts.“ *I
*
5.9.3.2 Electron affinity Mass spectrometry obviously enables the detection of negative cluster ions, provided that such anions can be produced in the LAVA source. Guo et al. have published the mass spectrum of vanadium-carbon cluster anions extracted directly from the LAVA source.[2o1This spectrum is very similar to that of the cationic species and is characterized by the predominance of the met-car peak. In contrast to ] reported the negative ion mass spectrum those observations, Wang et u Z . [ ~ ~recently obtained by laser vaporization of either pure Ti or solid T i c in hydrocarbon-seeded helium carrier gas. The first experiment yielded small Ti,C,- clusters, with practically no TigC12- species. The met-car anion could, however, be produced with significant abundance from Tic, even though the corresponding peak was not the most prominent. Photoelectron spectra (PES) of all characterized cluster ions were The PES spectra of Ti8C12- at three different photon obtained by Wang et energies is reproduced in Fig. 8.
1678
5 Solid-state Cluster Chemistry
VJ
i wf 1
0
h
Y
.d
200
3 E 150 -
H
2
C
-
4
50
I
D
.-g 100 Y
3
266 nm
d o
1 " " I " " I " " I '
3 300- 193 nm
200 100-
0
- ~ , , , 1 , , , , , 1 , , , , , 1 , , , , , 1 , , , , , ,
0
1
2
3
4
Binding Energy (eV)
Figure 8. Comparison of the photoelectron spectra of TigC12- at three photon energies. Top, 355 nm (3.49 eV); middle 266 nm (4.66 eV); bottom 193 nm (6.42 eV).[611 (Reproduced with permission from Prof. L. S. Wang,)
Five bands can be clearly identified. X (1.16 eV), A ( - 1.56 eV) and B (- 1.81 eV) are most clearly observed at 355 nm, whereas signals labeled as C (-2.5 eV) and D ( - 2.9 eV) show up at 266 nm. The adiabatic value of the electron affinity deduced from the threshold peak is 1.05 f 0.05 eV. The non-magic peak characterized for Ti&12- can be related to this unusually low value of the electron affinity, as was the non-magic peak obtained for neutral Ti8C12 explained by the low ionization potential. The low values found for both the EA and the IP could be explained by the electronic structure proposed from recent calculations in which the HOMO is a non-bonding, triply degenerate orbital accommodating either one, two, or three electrons in Ti&12+, TigC12 or TigC12-, respectively. The same authors reported in a recent work the PES spectra of five M&12 metcars (M = Ti, V, Cr, Zr, Nb).[481In the 3d transition series, the observed electron affinities (EA) increase with the metal nuclear charge (Ti, 1.05 eV; V, 1.80 eV; Cr, 2.28 eV). The PES spectra of Zr8C12 and NbgC12 are highly similar to those of their respective 3d counterparts, especially concerning the EA values (Zr, 1.02 eV; Nb, 1.65 eV). All PES spectra have a high density of states near the Fermi
5.9.3.3 Collective electronic properties: delayed ionization and delayed atomic ion emission Photoionization of neutral clusters conducted at the fixed wavelengths of 1064, 532, 355 and 266 nm by Castleman's group are expected to result at least partly in multiphoton absorption, so that no direct information can be obtained about ionization potentials. By extracting and removing 'prompt' ions generated by the direct multiphoton ionization of the species present in the cluster beam, it was possible to detect the process of delayed ionization for single-metal met-cars (Ti8C12, V ~ C I ~ ) [ ' ~ ] and for binary metal clusters TirMYCl2 (M = Zr, Nb, 0 I y I 4, x y = 8).[541 Delayed ionization, attributed to a thermionic emission process, had been previously observed for transition metal o ~ i d e , [ ~and ~ ,metal ~ ~ ] carbider641 clusters, and also, possibly, for fullerenes.r651Thermionic emission results from competition between ionization and dissociation leading to delayed electron emission.[661It implies the presence of a high density of states between the ionization potential IP and the dissociation energy Ediss of the cluster, assumed to be higher than the IP. If multiphoton absorption provides the cluster with energy between those two limits, the electronically excited cluster might decay non-radiatively and populate closely spaced states, both electronically and vibrationally excited. Delayed ionization then results from a slow coupling of those excited Even though TigC12 was a good candidate for delayed ionization, because of its low IP and to the much higher estimates reported for Ediss, the specificity of that molecule appears from the lack of any other Ti&, cluster in the delayed ionization mass spectrum. The delayed emission of atomic metal ions was obtained by another ionization pathway implying successive multiphoton absorption. Highly excited species having energies in the region 12-15 eV could undergo energy redistribution resulting in the delayed appearance of TisC12+ or Ti+. The occurence of a broad high-energy surface plasmon resonance at 12 eV, located in the continuum of states, had been predicted by Rubio et al. from a theoretical Huckel-like model considering the hollow met-car cage as a spherically averaged 'super-atom' accommodating on the spherical surface either 20 delocalized electrons only or the complete set of titanium and carbon valence electrons (80 electrons).[671
+
-
5.9.3.4 Ion chromatography studies In the experiments discussed in earlier sections, the separation of the metal carbide cluster ions produced either directly in the source or in the carrier gas was exclusively performed on a mass criterion. Ion chromatography experiments performed by Bowers and co-workers introduce another selection criterion by allowing a mass-selected ion cloud to undergo collisions with a He buffer gas in a drift ce11.[28,681 The drift time of a given species will depend on its collision cross section with He, and the various species present in the cloud will be separated as a function
1680
5 Solid-state Cluster Chemistry
Castleman
5.78(+1.2%) Khan
4.90( - 14.2%)
Bdnard
5.86(+2.6%)
Pauling
5.25(-8.1%)
Figure 9. Prediction of the ion mobility of the Ti&12+ ion (in cm2 V-' s-' ) a s a function of the structure, by means of a Monte-Carlo technique. Also shown is the percentage deviation from the experimental mobility (5.71 f 0.05 cm2 V-' s - ' ) . ~ (Reproduced ~~] with permission from Prof. M.T. Bowers.)
of their arrival time at the detector. Assuming constant experimental conditions, the arrival time of a given species is a specific property that can be related to the mobility K of the cluster ion as it passes through the He buffer gas. The value of K in cm2 V-' s-l can be calculated, by means of a Monte-Carlo simulation technique as a function of the structure and geometry of the cluster ion.[681 For Ti&12+, three structural isomers proposed as a result of theoretical investigations were compared with the structure of the pentagonal dodecahedron proposed by Castleman's group. The values of K obtained from Monte-Carlo simulation were then compared with the experimental mobility of 5.71 f 0.05 cm2 V-' ssl (Fig. 9).[@] Although the pentagonal dodecahedron structure gives the best fit with experiment (+1.2%), the tetracapped tetrahedron structure proposed in independent work by Lin and Hall[691and Rohmer et U Z . , [ ~ ~ ] is also in reasonable agreement with the experimental value. Two isomers based on a cubic assembly of metal atoms with carbons atoms lying either on the faces of the cube ( P a ~ l i n g ) ~or ~'] clustered inside the cube ( Khan)17 yield mobility values significantly below the acceptable range. A similar study performed on Ti&13+ yielded excellent agreement between the experimental K value of 5.49 cm2 V-' s-l and a computed value based upon a pentagonal dodecahedron structure with an exohedral extra carbon.[281The value of K associated with an endohedral structure deviates from experiment by as much
5.9 Metallocarbohedrenes M8Cl2 and TixM',C12
1681
as 7.5%. Because the thermodynamic stability of the endohedral structure seems well established by various theoretical the experimental conditions leading to the observed cluster ion had to be considered. Bower's experimental K values were obtained for Ti&13+ ions directly created in the LAVA source, for which exohedral growth of the cluster seems most probable. This interpretation is supported by reactivity experiments performed on TisC13' and Ti8C14' ions generated under similar conditions.r751 In contrast with met-car clusters which generate only association products with polar reactants, these carbon-rich ions undergo a reactive process with acetone leading either to TigC12+ or to the replacement of the extra carbon(s) by an acetone molecule.[751The TixC13' ions created by photoninduced loss of six Ti atoms from Ti14C13+1151 might have a different structure, possibly endohedral, and a different mobility value.[281
5.9.4 Chemical reactivity of met-cars and related M,C, clusters 5.9.4.1 Methods of investigation Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman's group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas.['71The FTICR (Fouriertransform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers['81basically rely on the same principle even though the total pressure of the reaction chamber is torr, compared with 0.7 torr in Castleman's experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al.; this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases.[761 The dominant features of the reactivity of TisC12+ toward various molecules were reported by Castleman et al. very soon after the discovery of those molecules." 71 They have since been completed and extended to other met-cars such as Ti7NbC12+ and NbsC12+.Byun, Freiser and co-workers have investigated the reactivity of VSc12+, NbsC12+ and M-C nanocrystals.['81The results are summarized below.
-
5.9.4.2 Association reactions of Ti*C12+ i) Ti&12+ is very reactive toward polar molecules such as CH30H, H20, and NH3 (or ND3). The mass spectrum of the products arising from the reaction is composed of eight peaks corresponding to the association products Ti&12+(Q, 1 I n I 8 (Fig. lo).["]
1682
5 Solid-state Cluster Chemistry
I 0
478 0
553 4
628 8
MASS (AMU) I
478 0
I
553 4
628.8
704 2
779.6
855.0
MASS (AMU)
Figure 10. Mass spectra of products arising from reactions of TiXCI? with methanol, (a) obtained at very low partial pressure of methanol; (b) obtained at a much higher methanol pressure. The number of methanol molecules associating onto Ti&12+ is indicated. The reaction terminates a t the eighth step."'] (Reproduced with permission from Prof. A. W. Castleman, Jr.)
An increase of the reactant pressure merely displaces the relative peak heights toward higher values of n. N o additional peak is observed. It can be easily deduced from such a spectrum that the reaction proceeds through multi-step attachment of the reactant to each metal center:
5.9 Metallocarbohedrenes Mac12 and Ti,M',C12
1683
ii) Stepwise association reactions are also observed between TisC12+ and molecules, such as benzene and ethylene, which do not have a permanent dipole moment but do have a 71-bonding system. In contrast to polar molecules, no more than four such n-bonded molecules can be attached to Ti&,2+.[17,761As for many observed properties of met-cars, the termination of the sequential reaction at n = 4 has been used, though inconclusively, as an argument in the controversy that has developed about the structure of these c l u s t e r ~ . [ ~ ~ ~ ~ ~ ] iii) Association reactions have also been observed between TigC12+ and pyridine terminating at TisC12+(~ y r i d i n e ) 4 . [With ~ ~ ] acetone, five adducts are observed, namely Ti&12+ (acetone)1-5 .I7 53781
5.9.4.3 Reactions of niobium-containing met-cars and titanium carbide clusters with acetone The partial or complete replacement of Ti by Nb strongly modifies the reactivity of the met-car cage, specifically with respect to acetone. Ti7NbCl2+ and NbgC12' react with acetone to give adducts with one and two oxygen atoms, thus implying that niobium-containing met-cars can induce carbon-oxygen b ~ n d - b r e a k i n g . [ ~ ~ , ~ ~ ] Ti7NbC12+ and NbsC12+ and their oxygen adducts can undergo further associations with a limited number of acetone molecules (four for Ti7NbC12+; two for NbxC12+, Nb&12+0, Ti7NbC12+0, and Ti7NbC12+02; one for N b ~ C 1 2 + 0 2 ) . [ ~ ~ ~ ~ ~ ] The chemical stability of TisC12+ was proved by comparing its reaction pattern with acetone with that of the neighboring titanium carbide clusters TisCl I + , Ti&l3+, and Ti8C14+.[7s1In contrast with TisC12+ and other met-cars, Ti&I1+ is able to break the chemical bonds of acetone to give Ti&11+(COCH3). Association reactions of this latter product with 1-3 acetone molecules, and Of Ti&11+ with 1-4 acetone molecules are also observed.[7s1The breaking of the stable C-C bond of acetone, which was not observed with Nb-containing met-cars is attributed to the chemical activity of a vacancy site in the incomplete cage structure of TixC11'. Carbon-rich clusters Ti&13+ and TigC14' undergo three reaction pathways: i) association with one acetone molecule; ii) abstraction of extra carbon atom(s), assumed exohedral, to give TisCl2+; and iii) replacement of the exohedral carbon(s) by an acetone molecule.[7s1
5.9.4.4 Reaction of Ti&12+ and other met-cars with methyl iodide The only abstraction reaction of TisC12+ characterized to date occurs with methyl iodide and involves the breaking of the I-C bond to give the mono adduct Ti&12fI:[781
1684
I 600
5 Solid-State Cluster Chemistry
I
I
650
700
I
'750 800 MASS ( a m u )
I
I
850
900
950
Figure 11. Mass spectrum of TixC12+(1) with methanol. The numerals indicate the number of methanol molecules associating with TixC12+. The association reaction terminates at the seventh step.r761(Reproduced with permission from Prof. A. W. Castleman, Jr.)
The monoiodine adduct can react further with polar molecules such as methanol to give the distribution TisC12+(I)(CH30H)1-7. It should be noted, however, that the intensity distribution of the seven peaks corresponding to methanol adducts is not regular. The two adducts TisC12+(I)(CH30H)4and Ti&12+(I)(CH30H)7 seem to be particularly stable (Fig. 1 l).[761 Once again, niobium-containing met-cars seem to be more reactive than TigC12+. Ti7NbC12+ and NbsC12+ are capable of taking up multiple iodine atoms to form Ti7NbC12+(1)~-~ and NbsC12+(I)1-5.[781 In contrast, it seems that replacement of one, two, or four Ti atoms by zirconium, which has the same electronic configuration, does not modify the reactivity of Ti&l2+ toward methyl iodide.[781One should, therefore, expect the reactivity of vSc12+ toward CH3X to resemble that of NbsC12. A difference is observed, however, because the reaction stops at four adducts, and not at five as for n i o b i ~ m . [ ' ~The ' ~ ~diversity ] in the response of met-cars toward halide abstraction has led to the conclusion that this reaction is monitored by the electronic configuration of the metal elements. A more thorough investigation of the electronic structure of those molecules will show that the reactivity of MsC12+ with CH3X can be correlated with the number of weakly coupled metal electrons that are available to the formation of strong covalent bonds with I' (see Section 5.9.6).
5.9 Metallocarbohedrenes Mac12 and Ti,yMI, C12
1685
5.9.4.5. Oxidation-induced reactions of Ti8C12 and other metal-carbide clusters TisC12+ seems to be inert to molecules, such as oxygen and methane, which have neither a permanent dipole moment nor a 71-bonding Neutral metalcarbon species produced in a LAVA source, however, rearrange in the presence of oxygen to yield selectively and exclusively Ti&12+.[801To better understand this oxygen-induced ionization reaction, a systematic investigation of the reactivity of ionic and neutral metal carbide clusters (M = Nb, Zr, V, Ta) toward dioxygen has been conducted.[811As for Ti-C clusters, it was noticed that neutral carbide clusters of Nb and Zr undergo ionization in the presence of oxygen. Structural reorganization of the clusters was also observed. With zirconium, three intense mass peaks were assigned to the cluster ions Zr8C12+, Zr&+, and Zr4C4+. The oxidation of neutral niobium carbide clusters yields a more complex spectrum characterized by the dominance of cubic-like cluster ions. Reaction of ionic Nb,C,+ clusters with dioxygen follows a markedly different path.[811The reaction proceeds through sequential loss of C2 units eventually yielding either the naked metal cluster Nb,+ or Nb,C+, depending on whether y is even or odd. No pathway for loss of a single carbon atom is observed either from met-car or from fcc-type cluster ions, including Nb4C4+.In view of these results, it was proposed that the 2 x 2 x 2 cubic structure of Nb4C4+ be reconsidered;[491an alternative structure based on C2 building blocks has been suggested.["]
5.9.4.6 Reactivity of V&l2+ and NbsC12+ In contrast to Ti&12+, VSc12+ reacts with oxygen to generate first V8Clo+. This new cluster then reacts with oxygen to give additional metal-carbon clusters and oxidation products.["] The difference between the reactivities of T&CQ+ and V8C12+ has been interpreted as arising from the larger number of metal electrons in the vanadium cluster which tend to populate near-degenerate non-bonding orbitals.[I8l This picture is supported by the most recent calculations (Section 5.9.6). The reactivity of v8c12+,investigated by Byun, Freiser, and colleagues, is also characterized by association reactions with 71-bonding molecules that terminate sharply at vSC12+L4 ( L = C6H6, CH3CN), as for Ti&12+. The vanadium met-car reacts with traces of water by association of one H20 molecule, followed by dehydrogenation of a second water molecule to give, presumably, V ~ C I ~ ( O H )Fur~+. ther association reactions are observed, with termination at four attachments which eventually yields V8CI2+(O)( H20)3.[183771 With methyl halides, sequential abstraction reactions terminate sharply at V8C12X4+ (X = C1, Br, I).[77i Freiser and coworkers completed their study of the reactivity of VSc12' by systematic investigation of NbsC12+.[~~' Most reactions previously observed by Castleman's were reproduced at a different pressure of the He expansion gas
1686
5 Solid-state Cluster Chemistry
containing the reagents ( to torr). Stepwise association reactions with both polar and Iz-bonding molecules (H20, NH3, alcohols, CH3CN, C6H6) were observed to stop at Nb&12L4+. If L is polar, more ligands can be added to Nb&12L4+, but addition is very slow after the first four. Addition of water to Nb&12+ initially proceeds as for v&12+ - addition of H20 followed by the fixation of a second water with elimination of H2. The same process is then repeated, yielding NbgC12(OH)2(H20)+ and eventually Nb&12(0H)4+. The reaction does not proceed further. Reaction with alcohols proceeds in exactly the same way, leading to NbgC12(0R)4+. Note that this second dehydrogenation is typical of Nb8Clz+ and is not observed for v&12+. Further association reactions eventually leading to NbgC12(OR)4(R O H ) I can ~ also be observed after longer reaction times. Reaction with methyl halides CH3X terminates at Nb&,2X4+ for X = C1, but fivefold adducts can be observed after long reaction times for X = Br, I. At higher reagent pressure, Nb&1215+ further reacts with CH31 to give an association product N ~ ~ C I ~ I ~ ( C H ~InI )agreement + . [ ~ ~ ] with the electronic structure proposed for NbgC12,[~~] this behavior confirms that the cluster cation has no more than five unpaired electrons and cannot abstract more iodine atoms from CH3I. Assuming that the number of iodine abstractions observed for the various met-cars implies an equal number of accessible unpaired electrons, those numbers should be 1, 4 and 5 for M&12+, with M = Ti, V and Nb, respectively.[791Because Ti7NbC12+ can abstract up to four iodine atoms, it should be assumed that the substitution of one titanium by a niobium atom has modified the electronic structure of the cluster to make three more d metal electrons available for covalent bonding.
5.9.4.7 Reactivity of vanadium-carbon nanocrystals Similar experiments performed on V14C13+ nanocrystals reveal the reactivity pattern expected from a cluster with eight active metal sites and four accessible unpaired electrons.[831Sequential reactions with up to eight H20 molecules result in the formation of V14C13 (OH)2(H20)6f and V14C13 (OH)4(H20)4+.Association reactions are also observed with up to eight acetonitrile molecules.[831The main product of reaction of low nuclearity cluster ions V,C,+ with dioxygen is VxCy-2+.[s11At variance with observations for titanium, niobium and tantalum clusters, no ionic products are detected for reaction of neutral V,C, species with dioxygen.[811
5.9.5 Theoretical models of the pentagonal dodecahedron 5.9.5.1 The earliest theoretical studies The assumption that a pentagonal dodecahedra1 cage structure was required to explain the special stability of Ti8C12 and other met-cars had a double impact on the
5.9 Metallocarbohedrenes
M8cI2
and TiXM’,C~2 1687
early theoretical studies that were immediately started on these clusters. Firstly, the analogy with fullerenes attracted considerable interest in the modeling community and induced a large number of calculations, which seemed to be easy because of the relatively modest size of the system and because of its high postulated symmetry (Th). Secondly, the apparently convincing relationship correlating three ‘magic’ parameters, namely the number of atoms (20), the stability of the cluster (the ‘supermagic’ peak), and its postulated fullerene-like structure induced most early models, including ours, to remain hindered by the symmetry constraints of the Th point group. In their earliest reports, published March 13”l and April 24[911992, Guo et al. based their conjecture of a dodecahedral conformation upon the ability of such a structure composed of carbon and ‘carbon-like’ atoms such as titanium to generate 24 a bonds and a delocalized n-bonding system similar to that envisioned for C20. The very first ‘theoretical’ communication devoted to Ti8C12 and related complexes, submitted May 26, 1992, was that of Linus Pa~ling.[~’] It does not contain any calculation, but relies on qualitative valence-bond arguments to suggest a structure topologically similar to the pentagonal dodecahedron in which each C-C bond is contained in a face of the Ti8 cube, thus generating folded pentagons. Such a structure has, indeed, the advantage of maximizing C = C n bonding, but enhances the electrostatic and van der Waals repulsive interactions between the negatively charged C2 ligands. Subsequent computational studies apparently failed to find an energy minimum corresponding to that conformation - when the conformation is constrained to remain cubic, the total stabilization energy is 17 eV less than that of the dodecahedron.r841Monte-Carlo simulation of the observed ion mobilities based on Pauling’s structure also failed to reproduce the experimental Shortly afterwards came two communications, one by Lin and Hall, based on Hartree-Fock calculations (submission date, May 28, 1992),[851the other by Grimes and Gale (submitted June 4, 1992) relying on the density functional theory (DFT) in its local density approximation (LDA).[861 Lin and Hall used as a model the elusive Y8Clz cluster which has not since been characterized, even though some mixed metal met-cars do include yttrium atoms.[521The Th symmetry was assumed. Because in the dodecahedral conformation each carbon is a-bonded to two metal atoms, the six C2 units can be formally described as ‘ethylene-like’ and formally assigned the charge 4-. This ascribes to yttrium the oxidation state I11 and the capability of accepting 0 lone pairs from the three surrounding carbons. Being formally stripped of its valence electrons, the metal orbitals are not, however, included in a n system delocalized over the whole molecule and merely contribute to the further stabilization of strongly localized C-C n bonds. The DFT-LDA study by Grimes and GaleLx6] was performed on two M&12 clusters assumed to have Th symmetry and metals with a ‘carbon-like’ electronic structure TigC12 and Si8C12. This work was completed later by similar calculations on other M8C12 systems (M = Zr, V, Fe).[”] The optimized geometries confirm that the metal-metal distances in the dodecahedral form are somewhat beyond the range expected for M-M bonds, at least for transition metals of groups IVA (Zr-Zr = 3.29.k, -
1688
5 Solid-state Cluster Chemistry
Ti-Ti = 3.06 A) and for silicon (Si-Si = 3.03 A). Carbon-carbon bond lengths C-C I 1.44 A) are characteristic of an important n bonding interaction. (1.36 A I The question of the stability of met-cars in their dodecahedral structure is discussed. Large stability is indeed found relative to either isolated atoms (6.67 eV/atom for Ti&12; 6.90 eV/atom for Zr8C12; 6.80 eV/atom for V8C12) or cluster fragments M8 and 6C2 ( 58 eV for Ti, V, and Zr), but for the three metals able to form solid metal carbides (Ti, Zr, Si), the dodecahedral M&12 clusters are unstable relative to the reaction: N
M8C12
-+
8MCsolid carbide f 4Cgraphite
It is also worth noting the very small HOMO-LUMO gap especially for neutral Ti&12 (0.16 eV), a quasi-degeneracy that will be found in all DFT and other calculations involving a single-particle Hamiltonian. Next in chronological order comes the communication by I. Dance (submitted July 3, 1992).[191This report based on DFT-LDA calculations was the first to propose an alternative to the dodecahedral structure; it has since been gaining both theoretical and experimental credibility. This important contribution, together with related theoretical work making similar structural assumptions, and their consequences, will be discussed in detail in Section 5.9.6. Several other theoretical studies of Ti&l2 using variants of the DFT-LDA methodology and based upon the dodecahedral structure have been published since 1992 and analyzed in terms of density of states and physical properties. Most of those studies predict a high density of states at the Fermi level and conclude that the Ti8Cl2 cage is 'metallic'.[88p951Methfessel et al.[961conclude, however, that the Fermi level falls in a low-density region, between the bonding and the non-bonding states. The optimized geometries are generally close to those reported by Grimes and Gale[86*871 but Reddy et al.[881end up with relatively long C-C bonds especially when using the DVM-Xa method (1.51 A). Charge-density profiles displayed in the average pentagonal planer88~89~93p961 consistently show an important density accumulation along the C-C bond and, to some extent, along the Ti-C bonds, but practically no interaction in the metal-metal d i r e ~ t i o n . ~Most ~ ~ ~calculations ~~,~~] predict for T& i !12 a binding energy higher than 6 eV/atom (6.1 or 6.62,[8816.64,[891 6.12,[9116.25,[9316.62[961).According to Reddy et al.[881and to Methfessel et aZ.[961 the stabilization computed per atom remains lower than that computed for fcc titanium carbide (6.62 eV compared with 7.16 eV), in agreement with the result of Grimes and Gale.r871 The computed ionization potentials are generally high when compared with the experimental value of 4.9 eVr5'I (5.33 eV,[8715.97 eV, or 6.02 eV,["] 5.92 eVr931).The value computed by Grimes and Gale for the electron affinity was also too high (1.71 eV)r871but the EA predicted by Li et al. proved surprisingly good (1.10 eV).[931 Despite general agreement on the high binding energy per atom computed for dodecahedral Ti&12 and related systems, a sense of discomfort emerged from those
5.9 Metallocarbohedrenes Ma C12 and Ti,M’,
C12
1689
early papers, most clearly expressed by Grimes and Gale: “the question as to why the M8Cl2 molecular form is so stable still remains. In their original paper, Guo et al. went some way toward an explanation.. . . Nevertheless, a full explanation is still lacking.”[871The main reasons for this statement were: i) the difficulty of correlating the cluster structure, assumed to be exceedingly stable, with a n system delocalized over the whole cluster as for graphite or fullerenes. In the opposite sense, all calculations predicted the localized character of n electrons confined in the C2 ligands. It will be seen below that dodecahedral met-cars involving transition metals of groups IVA and VA are clearly missing the back-donation interactions that could tighten the bonds between the transition metal atoms and the n-acceptor C2 ligands; and ii) the surprising ‘metallic’ character of the discrete Ti8C12 cluster, implying the lack of discontinuity between the occupied and the virtual sets of orbitals. The high reactivity suggested by those orbital or band - diagrams once again seems to contradict the fullerene-like model suggested by the dodecahedral structure. ~
5.9.5.2 The quest for Jahn-Teller distortion Two contributions focused on Jahn-Teller distortions of the pentagonal dodecahedron that would be induced by the probable degeneracy of a low-spin ground state for TiXCIZ. The qualitative reasoning of Ceulemans and is based upon the assumed similarity between the valence shell of the Ti8C12 dodecahedron and that of the hypothetical C20 cluster. Both systems have 80 valence electrons, of which 60 are allocated to the thirty G bonds, leaving 20 for surface 71 bonding.[671 According to an extension of Huckel’s (4k + 2) counting rule applicable to n systems delocalized over a ring, such a n system fully delocalized over a sphere should be accommodated in a stable, closed-shell configuration when matching a 2(k 1)2 counting rule ( k = 0,1,2,. . .).I9’] The magic number for an ideal n system would, therefore, be 18 electrons ( k = 2) to provide the di-cations C202+ or Ti8Cu2+ with a particularly stable, closed-shell configuration. This idea, proposed as early as July 14, 1992, has since been confirmed in several orbital diagrams for Ti8C12, not based on the dodecahedral conformation, but accommodating 20 delocalized d-metal (see Figs 17 and 19 in Section 5.9.7). This electrons in the frontier view is echoed in one of the most recent analyses of experimental results: “It is also conceivable that both of those clusters (Ti8Cl2 and Ti&l3) have enhanced stability as cations, but not as neutrals”.[551 According to the Huckel model proposed by Ceulemans and Fowler, the two extra n electrons of the neutral Ti8C12 are accommodated in a triply degenerate orbital with t, symmetry. Such a situation is susceptible to geometric instability of the Jahn-Teller type. Rather than considering a small deformation toward the trigonal symmetry predicted from the epikernel principle to be the most stable,
+
1690
5 Solid-state Cluster Chemistry
Figure 12. The C3" structure of M B Cproposed ~~ by Ceulemans and Fowler[971and optimized by Lin and Hall with M = Y and Nb.[691(Reproduced with permission from Professor M. B. Hall.)
Ceulemans and Fowler propose a quite different conformation for the met-car cage, namely an anti-aromatic C12 ring bicapped by electropositive metal tetrahedra (Fig. 12). The energy of this D3d structure was later computed by Lin and Hall[691and compares quite well at the ab initio Hartree-Fock level with that of the dodecahedron for Y ~ C I(-20.1 Z kcal mol-') and for Nb8C12 (+3.6 kcal mol-I). The problem of degenerate low-spin states for neutral Ti8C12 inducing an important Jahn-Teller distortion was also encountered at the Hartree-Fock level by Rohmer et al.['oll Those calculations were performed assuming an outer valence shell of 20 electrons delocalized over the system, as in the qualitative scheme of Ceulemans and Fowler. A difficulty encountered in that work, and ubiquitous in the Hartree-Fock treatment of transition metal complexes was the impossibility of adequately describing those 20 interacting electrons by means of a single Slater determinant. The stabilization obtained by shifting the symmetry constraints from Th to D2h was, at least in part, a response of Hartree-Fock formalism to a problem of electron correlation.
5.9.5.3 The electronic ground state of dodecahedral met-cars This problem was overcome by P. Jeffrey Hay who proposed a conclusive description of the electronic structure of dodecahedral met-cars within the framework of the one-determinant, open-shell Hartree-Fock methodology.['021The key to Hay's explanation is the pyramidal Ti(CH3)3 fragment, taken as a model for the environment of each metal atom in dodecahedral Ti8C12. The negatively charged (CH3)ligand is a g donor and the three doubly occupied frontier orbitals of Ti(CH3)3 describe the Ti-C 0 bonds. Titanium has the oxidation state I11 and is left with one d electron, accommodated in a singly occupied, high-energy, pure dZ2 orbital (Fig. 13). The coalescence of eight such fragments will give rise to the pure metal and to the metal-carbon frontier orbitals of Ti8C12. The 30 doubly occupied, a-bonding Ti-C and C-C MOs transform according to
5.9 Metallocarbohedrenes
and Ti,M’,CIz
1691
-2
-3 -4
-5
g
-6
Y
$ m
C
w
-9
-10
e
-
-
Ti C -11
a
+
-12
Figure 13. Schematic diagram of the molecular orbitals in Ti(CH3)3 from Hartree-Fock calculations[’021(reproduced with permission from Dr. P. Jeffrey Hay). Singly-occupied or unoccupied 3d levels are taken from the positive ion.
+
+
+
+ +
the T h group as 2a, la, 2e, leu 3t, 4t,, to which the C-C n-bonding orbitals of each Cz fragment would contribute six additional orbitals of ag,e,, and t, symmetry. Note that this assignment implies that the Cz ligands donate all o lone pairs and should be formally considered as (C2)4-. Then, the metal framework is left with eight d electrons accommodated each in a pseudo-d,z orbital collinear with the local C3 axis. Because those singly occupied MOs should also transform according to the T h point group, this localized picture is equivalent to the orbital set a, a, t, t, represented in Fig. 14. If the metal atoms are too far away for significant overlap between the pseudo-d,z
+ + +
1692
5 Solid-State Cluster Chemistry
Figure 14. Molecular orbitals arising from linear combinations of localized 3d,2 -like orbitals on each Ti oriented along the body diagonals (C, axes) of the Tig cube in dodecahedra1 TigC12.['021 (Reproduced with permission from Dr. P. Jeffrey Hay.)
orbitals, the metal electrons then remain localized and generate a high-spin ground state, of symmetry 9A,: 9Ag -- ...4a,'2a,'4t,36t,3 This nonet state corresponds to a single Slater determinant and is fairly well described at the open-shell Hartree-Fock level. The possibility of through-space spin coupling should, however, be investigated by means of configuration interaction (CI) or complete active space (CAS)-SCF calculations on the complete manifold of singly occupied orbitals. The 14 singlet-coupled spin states were all found higher in energy by 2 to 5 kcal mol-' than the high-spin configuration.[lo2]
5.9 Metallocarbohedrenes Ma C12 and TixMlyC I ~ 1693
Careful analysis of the ground state configuration obtained by Hay for met-cars of groups IIIA, IVA, and VA shows, in agreement with the DFT studies, that the stabilization of the dodecahedra1 cage molecules should be attributed only to the 30 s bonds and to the six localized n bonds of the C2 fragments. The orientation of the pseudo-d,z orbital combinations along the C? symmetry axes prevents any possibility of metal-to-ligand back donation even when those orbitals are partly or fully occupied, as shown in the orbital diagram of the Ti(CH3)3 fragment (Fig. 13).
5.9.6 A conformer proposed by theory - the tetracapped tetrahedron of metal atoms 5.9.6.1 Looking for different cage structures Apart from Pauling's perfect cube[711and from the bicapped C12 chain proposed by Ceulemans and other structures have been assumed to be local energy minima. The metal-decorated C12 cage structures proposed by Khan on the basis of semi-empirical ZINDO-UHF calculations comprise an icosahedral C12 cluster encapsulated in a box of eight titanium atoms in a cubic or antiprismatic arrangement.[72,1031 A similar structure, with an extended titanium box has been proposed by the same author for Ti14C13.[1041The possibility of obtaining three-dimensional cage carbon clusters of that size under the experimental conditions used to produce ] experimental results on the ion met-cars was dismissed by Cartier et U Z . [ ~ ~ The mobility of TisC12+ could not be reproduced with that model, moreover.[681It is, however, interesting to note that a structure similar to Khan's metal-decorated cage cluster has been proposed for the bcc crystal structure of K A ~ I ~ . [ ~ O ~ ] Reddy and Khanna[841observed that the stabilization energy was provided by the C-C and Ti-C bonds and reduced by the Coulombic repulsion between metal atoms. They proposed a structure in which the top and bottom C2 ligands are diagonally oriented such that they link each C atom to three metals, thus formally providing four extra Ti-C bonds (Fig. 15b). As predicted, the total stabilization energy associated with that structure was 3.3 eV more than that of the dodecahed~0n.I~~~
5.9.6.2 The tetracapped tetrahedron of metal atoms As mentioned above, the first publication to propose a credible alternative to the pentagonal dodecahedron was that of I. Dance." 91 This communication reported geometry optimization performed by the DFT method in its local approximation
1694
5 Solid-state Cluster Chemistry
(
122.14 eV
)
(
(
120.91 eV 1 (C)
125.46 eV 1 (b)
(a)
(
105.10 eV 1 (d)
Figure 15. Geometrical structure of Ti8Clz optimized within the DFT formalism, and associated binding energiesLs4](reproduced with permission from Professor B. V. Reddy).
on two geometrical isomers of TigC12. Isomer 1 is the pentagonal dodecahedron. Geometry optimization performed under the symmetry constraints of the D2h point group yielded a local minimum with geometrical parameters close to those reported by Grimes and Gale[86,871 and other calculations (Ti-Ti = 3.02 A,Ti-C = 1.99 A, C-C = 1.40 A). The second isomer was described by Dance as a tetracapped tetrahedron of titanium atoms containing six Ti4 faces in butterfly topology, with each face accommodating a C2 unit. At variance with the tetrahedral structure, the metal atoms are no longer all equivalent. In the tetrahedral conformation, which belongs to the T d point group, there are two distinct metal sites, each occurring four times -
5.9 Metallocarbohedrenes Mac12 and Ti,M',
Cl2
1695
the four innermost atoms labeled Ti' by Dance form the small tetrahedron referred to as thn in our group, and the four outer Ti atoms ( Tio) compose a 'large tetrahedron' (THN . Optimized geometric parameters are: Ti'-Ti' 2.86 A; Ti'-Ti" 2.90 A; Ti'-C 2.19 ; Ti"-C 1.93 A; C-C 1.34 A. This conformation was found to be more stable than the dodecahedron by as much as 350 kcal mol-' (- 15 eV).['91 No other conformation has yet been found which is more stable than this by use of either loca1[19*731 or non-local['OO1DFT calculations, or from the H a r t r e e - F ~ c k ~and ~~,~~] post Hartree-Fockr82.1061levels of theory. The energy gap between the tetracapped tetrahedron and the pentagonal dodecahedron has since been estimated to be 190 kcal mol-' at the Hartree-Fock levelr1061and 300 kcal mol-' from non-local density functional methods.[1001 Note that the energy gap of 190 kcal mol-' obtained from Hartree-Fock calculations corresponds to the difference between the energy of the high-spin state proposed by Hay for the dodecahedron['021 and the quintet 5A2 state which is the single-configuration ground state for the bicapped t e t r a h e d r ~ n . [ ~The ~ , ~energy ~.~~~] comparison is biased in favor of the Th form because, in contrast to the tetrahedral conformer, no correlation can arise from the shell of eight d electrons entirely accommodated in singly occupied orbitals. Correlation of the 20 d electrons in the Td form yields an upper limit of 414 kcal mol-' for the energy difference.['061Calculations made at the Hartree-Fock level by Lin and Hall[691confirmed that the Td form is also more stable than the dodecahedron for Y8c12 (176 kcal mol-I), Zr8C12 (255 kcal mol-I), and M0&12 (203 kcal mol-I). In his first communication,r191Dance discussed neither the electronic structure of the Td conformer, nor the potential energy hypersurface of the M&12 clusters. It might be of interest in that respect to mention how we were led in independent to propose a hypersurface with seven local minima, including the bicapped tetrahedron. After the conclusions of Ceulemans and Fowler, advocating a distorted structure with trigonal symmetry,r971we started a C3,-constrained HartreeFock optimization of Ti8C12. At the same time, we performed an optimization of the same cluster at the DFT level, using the DMOL program with no symmetry constraints. Both calculations eventually converged toward structures characterized by the six C2 fragments oriented diagonally with respect to a distorted cubic metal framework. Both energy minima were therefore characterized by a formal number of 36 Ti-C contacts at bonding distance (instead of 24 for the dodecahedron) and by an acetylene-like conformation of the CZligands. The structures were, however, topologically different, respectively belonging to the symmetry point groups D3d (Hartree-Fock, Fig. 16D) and CzV(DFT, Fig. 16F). As an immediate generalization of these results, the number of geometric permutations for diagonal orientation of six linear fragments along the faces of a cube is 26 = 64, seven of which are topologically non-equivalent. Those seven conformations are displayed in Fig. 16, as optimized at the Hartree-Fock level.['061The relative energies are scattered at the SCF level, because of the lack of correlation and because of electron localization problems. Correlation of the 20 d metal electrons yielded a remarkable clustering of
d
-
1696
5 Solid-state Cluster Chemistry
LL
5.9 Metallocarbohedrenes Mac12 and TixM',C,2
1691
1698
5 Solid-state Cluster Chemistry
the relative energies of the conformers: the energies of the six non-tetrahedral forms range between +153 and +179 kcal mol-' relative to the Td form, which unambiguously remains the minimum with lowest energy (Fig. 16). In an independent study Chen et aZ.r1071had their optimization process directed towards one of the non-tetrahedral minima, with D2d symmetry. This D2d conformation was investigated later by Khanna,['0831091 by Lou and N ~ r d l a n d e r , and ~~~] by Xia et and was always found to be more stable (by -250 kcal m01-')['~~] than the T h dodecahedral structure. Finally, Dance has recently proved that the dodecahedral form was not a minimum on the potential energy surface.['oo1 How can the increased stability induced by the diagonal orientation of the C2 fragments, together with the specificity of the Td form among the seven conformers displaying this orientation, be explained? According to arguments proposed by Reddy and Khanna,[s61the diagonal orientation of all C2 ligands maximizes to 36 the number of Ti-C bonds, but also tightens the metal framework, thus increasing Coulombic repulsion between metal atoms, unless metal-metal bonding interactions can occur. Evidence of a favorable orbital interaction within the metal framework can be obtained specifically for the bicapped tetrahedron. Lin and Hallr691published an interaction diagram between the d orbital levels of the inner tetrahedron, already scattered because of the small Mi-Mi distances, and the clustered levels of the large tetrahedron (Fig. 17). The resulting orbitals of the M8 framework, labeled according to the Td point group, are divided into three sets: i) a set of nine 'bonding' levels stabilized relative to the orbitals of an isolated metal atom; ii) a set of nine 'non-bonding', slightly destabilized levels; and iii) the metal-metal antibonding orbitals. Lin and Hall[691and relied on this orbital diagram to predict a closed-shell structure with especially high stability for met-cars with either 18 or 36 metal electrons to be accommodated in the set bonding orbitals, or in the bonding and non-bonding orbitals, respectively. Note that the magic number of 18 electrons does not correspond to any neutral M&IZ cluster but, assuming acetylenic ( C Z ) ~ fragments, it fits perfectly the electronic structure of the di-cations of met-cars with metals belonging to group IVA, namely (TisC12)~' and (ZrsC~z)~+. The second magic number, 36 electrons, corresponds to the neutral form of Cr8C12 and M0&12. Such an inversion in the relative stabilities of cations, on the one hand, and neutrals and anions on the other hand, when increasing the number of d-valence electrons has been noted by Wang et a1.r483611 and questioned in other experimental rep0rts.1~'1 Another hint of the importance of bonding interactions in the tetrahedral metal framework can be obtained from H F calculations performed on neutral Ti8C12.
5.9 Metullocurbohedrenes Ma Clz and Ti,M',
C12
1699
4t?
Figure 17. Schematic interaction diagram for a tetracapped tetrahedral metal (reproduced with permission from Professor M. B. Hall) The orbital energy ordering in the center column is based on the result of ab initio H F calculations on the Td MosClz cluster.
inner tetrahedral moiety
capped tetrahedral cluster
outer tetr@edral moiety
Transition metal clusters of high symmetry cannot be adequately described at the H F (one-configuration) level unless all equivalent metal centers can be formally attributed the same integer number of electrons, either 0, 1, or 2. For this reason the single electronic configuration lowest in energy for Ti8C12 does not follow the orbital energy ordering (Aufbau principle) of Fig. 17, adapted by Lin and Hall from a calculation on the electron-rich MO8C12 cluster.[691The Aufbau principle would yield for TigC12 the electronic configuration:
In that configuration, the two electrons of the la, orbital are equally distributed among the four metal atoms of the small tetrahedron thn (Fig. 18a) whereas the electrons of the 2al orbital are similarly distributed over THN (Fig. 18b). As for such a distribution generates important hole-states in symmetric correlation effects and cannot yield a stable electronic configuration. In fact, two
1I00
5 Solid-state Cluster Chemistry
Figure 18. M I C ~ Zmetal , framework of the tetracapped tetrahedron conformer: combinations with a1 and t2 symmetry of d,l-like orbitals centered either (a) on the apices of the inner tetrahedron of metal atoms, or (b) on the apices of the outer tetrahedron of metal atoms.
configurations only can separately satisfy the electron localization requirements imposed by the Hartree-Fock approximation (Fig. 18): 1tz6le41tl 1a1 2t2
(2)
1t2 le41tl 62al 3t2
(3)
and
Both are open-shell quintet configurations which localize four electrons over four metal atoms either on thn (2, Fig. 18a), or on THN (3, Fig. 18b). The triply degenerate orbitals 2t2 and 3t2 are intrinsically non-bonding by symmetry. Orbital 2al is no more than weakly bonding because of the large M-M distances in THN. Mo-
5.9 Metallocarbohedrenes Ma C12 and Ti,M',
C12
1701
lecular orbital lal develops an important stabilizing overlap between the four inner lobes of the pseudo-d,z orbitals. The energy difference between quintets (2) and (3) is 104 kcal mol-* if (3) is computed at the optimum geometry of (2), and 78 kcal mol-I when both states are separately optimized.r821Although this energy difference should not be attributed in totality to the bonding character of orbital lal, the energy gap separating the eigenvalues of the fully symmetric open-shell orbitals: lal
-
2al
= 2.31
eV = 53 kcal mol-'
is not too far away from the computed difference. A spin-coupling of the four unpaired electrons by means of a multireference configuration interaction (CI) expansion limited to all spin- and symmetry-allowed permutations of four electrons over four orbitals yields a totally symmetric singlet state ('A1) as the ground state, separated by 2.44 kcal mol-I from a 3T1 triplet state and by 8.02 kcal mol-I from the 5A2 quintet state characterized at the Hartree-Fock level. Those energy gaps are slightly increased after a more extended CI. They are in reasonably good agreement with the Heisenberg model[''11 which predicts that for a single coupling constant J describing the antiferromagnetic coupling: E(3Tl) - E('A1) = l/2[E('A2)
-
E(3T1)] = -25
The natural orbital (NO) analysis of the singlet-coupled ground state reflects the relative stability of the lal orbital (NO population: 1.86e) with respect to the metal-metal non-bonding 2t2 orbitals (NO population: 2.14e). This analysis is basically in agreement with the results obtained by Dancerloo]within the DFT framework predicting a low-energy, doubly occupied al level and a triply degenerate HOMO populated with two electrons. The orbital energies reported by Dance display an important energy gap ( 1.5 eV) between the highest doubly occupied MO and the partly occupied HOMO. The HOMO itself is the lowest term of a cluster of closely spaced levels allowing for low-energy excitation of the two upper electrons. A similar orbital analysis was performed in 1994 on the basis of extended Huckel calculations by Srinivas et a1.[991The orbital diagram reproduced from their work shows evidence for the large energy gap separating the highest doubly occupied MO from the partly occupied level for the tetrahedral geometry (Fig. 19b). A similar diagram constructed from metal and dicarbon fragments in the dodecahedral symmetry has no such energy gap either below or above the doubly degenerate e, level. As already mentioned above, the orbital diagrams obtained from single particle treatments of the tetrahedral conformer agree with the observed trend of Ti8C12 to be especially stable as a cation. The exceptional stability computed for the tetrahedral conformer cannot be assigned only to the bonding interactions in the metal framework. In contrast to the dodecahedral conformer, the formal counting of electrons and their accommodation in the d orbitals of the bicapped tetrahedron allow for considerable back-
-
1702
5 Solid-State Cluster Chemistry
-1o.c
-12.c
-14.(
Figure 19. (a) This page: interaction diagram between cubic Ti8 and 6C2 to give the dodecahedra1 conformer. (b) Next page: energy levels of the tetracapped tetrahedron conformer for Ti&, and 4c2 units. From extended Hiickel calculations by Srinivas, Srinivas and J e m m i ~ [(reproduced ~~] with permission from Professor E. D. Jemmis).
5.9 Metallocarbohedrenes Ma C12 and Ti,M',
-10-0
-12-0
- 14.0
Figure 19 (continued)
C12
1703
I704
5 Solid-state Cluster Chemistry
donation toward the n* orbitals of the C2 ligands. An illustration of the importance of back-donation effects in the structure of the MgX1.2 clusters is provided by experimental work reported by Hollander and Coucouvanis,ll12] and, more recently, by the group of Kanatzidi~.["~IThe first group characterized two cluster compounds with a c U g s 1 2 core with the pentagonal dodecahedral structure.[''21 Kanatzidis et al. reported the structure of the polytelluride compounds &CugTel1, A3CugTelo (A = Rb, Cs), AA'zCugTelo (A, A' = K, Rb, Cs), and A2BaCugTelo (A = K, Rb, CS).[''~]The building blocks of those two- or three-dimensional structures are also pentagonal dodecahedral Cug (Te2)6 clusters sharing Te-Te edges and encapsulating an alkali metal or a Ba2+ ion. Ab initio RHF calculations performed on model conformers of an isolated (CugTe12)~-cluster unambiguously confirmed that the pentagonal dodecahedron is the most stable structure in this case because of the lack of 71 acceptor orbitals in the Te-Te fragments.[''41
5.9.6.3 Pentagonal dodecahedron or capped tetrahedron - the controversy Despite the increasing inclination within the computational community in favor of the capped tetrahedron model, the image of met-cars remains dominated by the dodecahedral picture of MgC12. In fact, since the early suggestion of a dodecahedral structure, arguments have been developed by Castleman's group in favor of that organization of the cluster, on the basis of the necessity for all metal atoms of a given cluster to be indistinguishable, i.e. accommodated in eight symmetrically equivalent sites. The association reaction of ND3 with TigC12+ provided the first justification for a perfect cubic metal framework because the reaction continues until eight ammonia molecules are accommodated - one for each Ti site.['] Another strong point in favor of the pentagonal dodecahedron came from the production of a series of binary metal met-cars with the general formula TixZryC12 (x + y = 8; y = 1 to 5).r32,331 The regular variation of the peak intensities associated with y increasing from 0 to 5 was interpreted as providing evidence that the eight metal sites are actually in a similar coordination environment. It was argued that conformers with non-equivalent sets of metal sites would yield unfavorable stoichiometries of Ti5Zr3C12 and Ti3Zr5C12 compared with Ti4Zr4C12, resulting in abrupt truncation in the mass distribution at this One can consider that both types of argument have been refuted since then, either from the interpretation of met-car reactivity, or from subsequent calculations on mixed metal clusters. Association reactions of TigC12+ and Vgc12' with 71-bonded molecules (ethylene, benzene, pyridine) have been shown to terminate at four adducts." 7,761 Although this result could be straightforwardly interpreted in favor of a structure with two distinct sets of metal atoms, Deng et al. suggested that each 71bonding ligand could be attached to two metal atoms of the cubic frame~ork."~]
5.9 Metallocarbohedrenes Ma C ~ and Z Ti,MIy C ~ Z 1105
More difficult to interpret with a unique set of metal sites is the termination or 71 slowing down at four adducts of most associative reactions involving V8C12+[1s37 or NbgC12+.[791 The number four is also characteristic of the reactivity of TigC12+(I ) with methanol[761as the peak corresponding to T&C12+(I)(CH30H)4 has an especially large intensity which is difficult to explain if all metal atoms are assumed equivalent (Fig. 11). The reactivity of met-car cations with methyl iodide seems more readily correlated with the number of weakly coupled metal electrons available in the tetrahedral conformers. Assuming that neutral Ti8C12 accommodates four metal electrons into pseudo-d,z orbital combinations not involved in backbonding interactions and that the la1 level is sufficiently stabilized by metal-metal interactions:
then one single electron remains available in the cation, leading to the mono-adduct Ti&12+( I).[759781For neutral NbgC12 eight additional electrons should be accommodated in the metal-metal non-bonding, quasi-degenerate levels 2t2, 3t2 and 2a1, which yields the electronic structure:
Assuming a maximum spin for the cation leads to five uncoupled, or weakly coupled electrons likely to yield the observed product NbgC12+(I)5.[781The termi] difficult to interpret nation of the reaction of v&12+ at four a d d u c t ~ [is~ ~more because the electronic configuration of the metal is similar to that of niobium. It is possible that stronger localization of the metal electrons on the inner tetrahedron makes those electrons less accessible for the abstraction reaction. That yields the following electron distribution for the neutral cluster: (la12t2)8(3t22al l4
(6)
This configuration, which yields for the cation three accessible electrons and a low-lying empty orbital, nicely explains the observed formation of a water adduct VsC12( I)3H20+ with the triiodide compound.[771The abstraction reaction of VgC12+ with iodine should, however, stop there, because no more than three electrons are available in the cluster cation. Despite this puzzling case, the interpretation of the halogen abstraction reaction provided by the electronic structure of the tetrahedral conformers seems more satisfactory than predictions obtained assuming the dodecahedral form. According to Hay’s interpretation of the electronic structure of the dodecahedron,[lo2]the number of accessible electrons should be seven for TigClz+ and for ZrgC12+, but only one for v&12+ and Nb&12+. Calculations recently performed by our group[”s1 show that experimental ob-
1706
5 Solid-state Cluster Chemistry
servations of the reactivity of T&C12+with NH3, CO or C6H6 can be reproduced with neutral met-cars assuming the cluster is tetrahedral in shape. DFT calculations predict that the attachment of four NH3 molecules on the outer tetrahedron is exothermic by 96 kcal mol-I. The formation of Ti8C12(NH3)8 is, moreover, thermodynamically favored with a total exothermicity of 163 kcal molt'. The fixation of 4 rc-bonded molecules on THN is also easy with stabilization energies of 88 kcal mol-' for four CO and 107 kcal mol-' for four C6H6. Those ligands are, however, difficult or impossible to attach to the metal atoms of the inner tetrahedron, a result that could explain the termination of the observed reactions at four adducts. The statistical variation of the peak intensities corresponding to y varying from 0 to 5 in the TixZryC12series has also been used as an argument to dismiss the tetrahedral conformer.[323331 This interpretation implies that the bonding energy of one or several Zr atoms in a mixed-metal met-car with 36 M-C bonds is different when Zr occupies the sites of either the capped or the capping tetrahedron. That assumption has been recently questioned by a series of DFT calculations performed on TixZryC!2 clusters with y = 0 to 5 and 8, assuming a tetracapped tetrahedral structure with C3" symmetry for the metal framework.[591Two conformations, which differ by attributing in priority the Zr atoms either to the thn, or to the THN sites, have been considered for each stoichiometry and their geometries optimized. The stabilization energy per substituted atom, -AE/ y , steadily declines when y increases from 0.68/0.72 eV for y = 1 to 0.63 eV for y = 8. But, more importantly, the total energy of the complex depends very little on the site, thn or THN, selected for substitution. For a given value of y , the energies associated with the two isomers considered never differ by more than 1.2 kcal mol-' (0.6 kcal mol-' for y = 4), which means that the two isomers considered, and possibly others, could effectively coexist. This series of results then tends to indicate that the relative peak heights of the Ti,ZryC12+ species should not be significantly influenced by the existence of two distinct metal atom sites. To summarize, it can be said that most current experimental knowledge about met-cars, including ion-mobility can be reasonably interpreted by assuming the cage structure to be either dodecahedra1 or tetrahedral. It seems to the authors of this review, however, that in the present situation where no conclusive argument based on experimental results can be put forward in favor of one or other structure, the consensus obtained from the most recent theoretical work makes the tetracapped tetrahedral structure most probable. One year after these lines have been written, it seems that the ionization potentials represent the discriminating property that was looked after to obtain convincing evidence about the structure of met-cars. The excellent agreement obtained between the recently determined IPS of TigC12 and Zr8C12 (see section 5.9.3.1)r"81 and the values computed assuming the tetrahedral conformation 7 , 5 9 1 have apparently put an end to the controversy.[' 18] The observation of methane Ti8C12 cluster complexes with a dominant peak for Ti8C12 (CH4)4was also interpreted in favor of the tetrahedral cage structure with Td ~ymmetry.[''~]
5.9 Metallocarbohedrenes MKCl2 and Ti,M',.C12
1701
5.9.7 Conclusion - more than a peak in a mass spectrum? As at the beginning of the fullerene story, the next development needed in met-car chemistry is the characterization of pure products in macroscopic quantities. In fact, macroscopic quantities of TigC12 and VgCl2 have already been obtained by a modification of the common arc discharge used to produce fullerenes.r'21 Soots were obtained in which the presence of met-cars and fullerenes was unambiguously characterized by laser desorption time-of-flight mass spectroscopy. The quantity of met-cars present in the soot was estimated to 1%."21 Unfortunately, chemical desorption and isolation from the raw soot has not yet been reported. The isolation of fullerenes was obviously made easier by their relative chemical inertness. It seems from experimental data and from theoretical models that native met-cars, despite their intrinsic stability relative to atoms and fragments, are highly unsaturated and reactive species. One might imagine stable complexes with met-car cores obtained by saturating the metal sites with o-donor ligands. This would be the beginning of another story, this time reminiscent of copper chalcogenide cluster ions ( C U ~ ~ - I E "()E- = S, Se) detected in the mass spectrometer by Dance et L Z ~ . , [ ' ' ~ ] which opened the way to the synthesis, isolation, and characterization of neutral clusters of stoichiometry [ C U ~ ~PR3),], E ~ ( stabilized by an external coating of phosphine ligands." ' I
-
References 111 Guo, B. C., Kerns, K. P., Castleman, Jr., A. W. Science255, 1411-1413 (1992). [2] Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F., Smalley, R. E. Nature 318, 162-163 (1985). [3] Paquette, L. A,, Ternansky, R. J., Balogh, D. W., Kentgen, G. J. Am. Chem. SOC.105, 54415446 and 5446-5450 (1983). [4] (a) Wei, S. Q., Shi, Z., Castleman, Jr., A. W. J. Chem. Phys. 94, 3268-3270 (1991). (b) Yang, X., Castleman Jr., A. W. J. Phys. Chem. 94, 8500-8502 and 8974 (1990). [5] Fowler, P. W. Phil. Truns. R. SOC.Lond. A 343, 39-52 (1993). 161 Tenne. R. Adu. Muter. 7,965-995 (1995). [7j Chiu, Y.-N., Ganelin, P., Jiang, X., Wang, B.-C. J. Mol. Struct. (Theochem) 312, 215-250 f 1994). [8] (a) Pradeep, T., Manoharan, P. T.: Metallocarbohedrenes: A new class of molecular clusters with cage structure Current Science, 68, 1017-1026 (1995). (b) Duncan, M. A,: Synthesis and Characterization of Metal-Carbide Clusters in the Gas Phase, J. Cluster Sci. 8, 239 (1997). (c) Selvan, R., Pradeep, T. Current Science 74, 666 (1998). [9] Guo, B. C., Wei,-S., Purnell, J., Buzza, S., Castleman, Jr., A. W. Science, 256, 515-516 (1992). [lo] Pilgrim, J. S., Duncan, M. A. J. Am. Chem. SOC.115, 6958-6961 (1993). [Ill Cartier, S. F., May, B. D., Castleman, Jr., A. W. J. Chem. Phys. 100, 5384-5386 (1994). ~I
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5 Solid-state Cluster Chemistry
[12] (a) Cartier, S. F., Chen, Z. Y., Walder, G. J., Sleppy, C. R., Castleman, Jr., A. W. Science, 260, 195-196 (1993). (b) Selvan, R., Pradeep, T. Chem. Phys. Lett., in press. [13] Wei, S., Guo, B. C., Purnell, J., Buzza, S., Castleman, Jr., A. W. J. Phys. Chem. 96, 41664168(1992). [14] Pilgrim, J. S., Duncan, M. A. J. Am. Chem. SOC.115, 4395-4396 (1993). [15] Pilgrim, J. S., Duncan, M. A. J. Am. Chem. SOC.115, 9724-9727 (1993). [16] May, B. D., Cartier, S. F., Castleman, Jr., A. W. Chem. Phys. Lett. 242, 265-272 (1995). [17] Guo, B. C., Kerns, K . P., Castleman, Jr. A. W. J. Am. Chem. SOC.115, 7415-7418 (1993). [18] Yeh, C. S., Afzaal, S., Lee, S. A,, Byun, Y. G., Freiser, B. S. J. Am. Chem. SOC.116, 88068807 (1994). [19] Dance, I., J. Chem. Soc., Chem. Commun. 1779-1780 (1992). [20] Guo, B. C., Wei, S., Chen, Z., Kerns, K. P., Purnell, J., Buzza, S., Castleman, Jr., A. W. J. Chem. Phys. 97, 5243-5245 (1992). [21] Chen, Z. Y., Walder, G. J., Castleman, Jr., A. W. Phys. Rev. B, 49, 2739-2752 (1994). [22] Dietz, T. G., Duncan, M. A., Powers, D. E., Smalley, R. E. J. Chem. Phys. 74, 6511-6512 (1981). [23] Bondybey, V. E., English, J. H. J. Chem. Phys. 76, 2165-2170 (1982). [24] Castleman, Jr., A. W., Guo, B. C., Wei, S., Chen, Z. Y. Plasma Phys. Contr. Fusion 34, 2047-2051 (1992). [25] Castleman, Jr., A. W. Z. Phys. D 26, 159-161 (1993) [26] Wei, S., Guo, B. C., Deng, H. T., Kerns, K. P., Purnell, J., Buzza, S. A., Castleman, Jr., A. W. J. Am. Chem. SOC.116,4475-4476 (1994). [27] Yu, H., Huber, M. G., Froben, F. W. Appl. Surf: Sci. 86, 74-78 (1995). [28] Lee, S., Gotts, N. G., von Helden, G., Bowers, M. T. Science, 267, 999-1001 (1995). [29] Chen, Z. Y., Walder, G. J., Castleman, A. W., Jr. J. Phys. Chem. 96, 9581-9582 (1992). [30] O’Keefe, A., Ross, M. M., Baronavski, A. P. Chem. Phys. Lett., 130, 17-19 (1986). [31] Cartier, S. F., May, B. D., Toleno, B. J., Purnell, J., Wei, S., Castleman, Jr., A. W. Chem. Phys. Lett. 220, 23-28 (1994). [32] Wei, S., Guo, B. C., Purnell, J, Buzza, S., Castleman, Jr., A. W. Science, 256, 818-820 (1992). [33] Cartier, S. F., May, B. D., Castleman, Jr., A. W. J. Phys. Chem. 100, 8175-8179 (1996). [34] Deng, H. T., Guo, B. C., Kerns, K . P., Castleman, Jr., A. W. Int. J. Mass Spect. Zon Proc. 138, 275-281 (1994). [35] Cartier, S. F., May, B. D., Castleman, Jr., A. W. J. Am. Chem. SOC.116, 5295-5297 (1994). [36] Lu, W., Huang, R., Ding, J., Yang, S. J. Chem. Phys., 104, 6577-6581 (1996). [37] Pilgrim, J. S., Brock, L. R., Duncan, M. A. J. Phys. Chem. 99, 544-550 (1995). [38] Pilgrim, J. S., Duncan, M. A. Int. J. Mass Spec. Ion Proc. 138, 283-296 (1994). [39] Wei, S., Guo, B. C., Purnell, J., Buzza, S.A., Castleman, Jr., A. W. J. Phys. Chem. 97, 95599561 (1993). [40] Tast, F., Malinowski, N., Frank, S., Heinebrodt, M., Billas, I. M. L., Martin, T. P. Phys. Rev. Lett. 77, 3529-3532 (1996). [41] Tast, F., Malinowski, N., Frank, S., Heinebrodt, M., Billas, I. M. L., Martin, T. P. 2. Phys. D 40,351-354 (1997). [42] Sakurai, H., Castleman, A. W., Jr. J. Phys. Chem. A , 101, 7695-7698 (1997). [43] von Helden, G., Gotts, N. G., Maitre, P., Bowers, M. T. Chem. Phys. Lett. 227,601-608 (1994). [44] Wei, S., Castleman, Jr., A. W. Chem. Phys. Lett. 227, 305-31 1 (1994). [45] Chen, Z. Y., Castleman, Jr., A. W. J. Chem. Phys. 98, 231-235 (1993). [46] (a) Wang, L.-S., Cheng, H. Phys. Rev. Lett. 78, 2983-2986 (1997). (b) Wang, L.-S., Wang, X.-B., Wu, H., Cheng, H. J. Am. Chem. SOC.120,6556-6562 (1998). [47] Wang, L.-S. private communication. [48] Li, S., Wu, H., Wang, L.-S. J. Am. Chem. SOC.119, 7417-7422 (1997). [49] Yeh, C. S., Byun, Y. G., Afzaal, S., Kan, S. Z., Lee, S., Freiser, B. S., Hay; P. J. J. Am. Chem. SOC.117,4042-4048 (1995).
5.9 Metallocarbohedrenes
Mac12
and TixM',C12
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[50] Kerns, K. P., Guo, B. C., Deng, H. T., Castleman, Jr., A. W. J. Chem. Phys. 101, 8529-8534 [ 1994). [5l] Kerns, K. P., Guo, B. C., Deng, H. T., Castleman, Jr., A. W. J. Phys. Chem. 100, 1681716821 (1996). [52] May, B. D., Kooi, S. E., Toleno, B. J., Castleman, Jr., A. W. J. Chem. Phys. 106, 2231-2238 (1997). [53] Kan, S. Z., Lee, S. A., Freiser, B. S. J. Muss. Spectrom. 31, 62-68 (1996). [54] (a) Cartier, S. F., May, B. D., Castleman, Jr., A. W. J. Chem. Phys. 104, 3423-3432 (1996). (b) Kooi, S. E., Castleman, Jr., A. W. J. Chem. Phys. 108, 8864-8869 (1998). [55] Brock, L. R., Duncan, M. A. J. Phys. Chem. 100, 5654-5659 (1996). [56] Purnell, J., Wei, S., Castleman, Jr., A. W. Chem. Phys. Lett. 229, 105-110 (1994). [57] Dance, I. unpublished. [58] Poblet, J.-M. unpublished [59] Muiioz, J., Pujol, C., Bo, C., Poblet, J.-M., Rohmer, M.-M., Benard, M. J. Phys. Chem. A, 101, 8345-8350 (1997). [60] Duncan, M. A,, private communication. [61] Wang, L. S., Li, S., Wu, H. J. Phys. Chem. 100, 19211-19214 (1996). [62] (a) Leisner, T., Athanassenas, K., Kreisle, D., Recknagel, E., Echt, 0. J. Chem. Phys., 99, 9670-9680 (1993); (b) Leisner, T., Athanassenas, K., Echt, O., Kandler, 0. K., Kreisle, D., Recknagel, E., 2. Physik D, 20, 127 (1991); (c) Collings, B. A,, Amrein, A. H., Rayner, D. M., Hackett, P. A. J. Chem. Phys., 99,4174-4180 (1993). [63] (a) Nieman, G. C., Parks, E. K., Richtsmeier, S. C., Liu, K., Pobo, L. G., Riley, S. J. High Temp. Sci. 22, 115 (1986); (b) Athanassenas, K., Leisner, T., Frenzel, U., Kreisle, D. Ber. Bunsenges. Physik. Chem. 96, 1 192- 1 194 (1992). [64] Amrein, A,, Simpson, R., Hackett, P. J. Chem. Phys., 95, 1781-1800 (1991). [65] (a) Campbell, E. E. B., Ulmer, G., Hertel, I. V. Phys. Rev. Letters 67, 1986 (1991);(b) Ding, D., Huang, J., Compton, R. N., Klots, C. E., Haufler, R. E. Phys. Rev. Letters73, 1084-1087 (1994); (c) Hertel, I. V., Steger, H., de Vries, J., Weisser, B., Menzel, C., Kamke, B., Kamke, W. Phys. Rev. Letters 68, 784-787 (1992); (d) Lin, H., Han, K. L., Bao, Y., Gallogly, E. B., Jackson, W. M. J. Phys. Chem. 98, 12495-12500 (1994); (e) Wurz, P., Lykke, K. J. Phys. Chem. 96, 10129-10139 (1992); (f) Zhang, Y., Stuke, M. Phys. Rev. Letters 70, 3231-3234 (1993). [66] Klots, C. E. Chem. Phys. Letters 186, 73-76 (1991). [67] Rubio, A,, Alonso, J. A., Lopez, J. M. An. Fix 89, 174-179 (1993). [68] Bowers, M. T. Acc. Chem. Res. 27, 324-332 (1994). [69] Lin, Z., Hall, M. B. J. Am. Chem. Soc. 115, 11165-11168 (1993). [70] Rohmer, M.-M., Benard, M., Henriet, C., Bo, C., Poblet, J.-M. J. Chem. Soc., Chem. Commun. 1182-1 185 (1993). [71] Pauling, L. Proc. Nutl. Acad. Sci. USA 89, 8175-8176 (1992). [72] Khan, A. J. Phys. Chem. 97, 10937-10941 (1993). [73] Lou, L., Nordlander, P. Chem. Phys. Letters 224, 439-444 (1994). [74] Dance, I. J. Am. Chem. SOC.118, 2699-2707 (1996). [75] Kerns, K. P., Guo, B. C., Deng, H. T., Castleman, Jr., A. W. J. Am. Chem. Soc. 117, 40264029 (1995). [76] Deng, H. T., Kerns, K. P., Castleman, Jr, A. W. J. Am. Chem. SOC.118, 446-450 (1996). [77] Byun, Y. G., Freiser, B. S. J. Am. Chem. Soc. 118, 3681-3686 (1996). [78] Deng, H. T., Guo, B. C., Kerns, K. P., Castleman, Jr., J. Phys. Chem. 98, 13373-13378 (1994). [79] Byun, Y. G., Lee, S. A,, Kan, S. Z., Freiser, B. S. J. Phys. Chem., 100, 14281-14288 (1996). [80] Deng, H. T., Kerns, K. P., Castleman, Jr., A. W. J. Chem. Phys. 104, 4862-4864 (1996). [81] Deng, H. T., Kerns, K. P., Bell, R., Castleman, A. W., Jr. Znt. J. MussSpec. Zon Proc. 167,615625 (1997).
1710 [82] [83] [84] [85] [86] [87] [88] [89] [90]
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Benard, M., Rohmer, M.-M., Poblet, J.-M., Bo, C. J. Phys. Chem. 99, 16913-16924 (1995). Byun, Y. G., Yeh, C. S., Xu, Y. C., Freiser, B. S. J. Am. Chem. Soc. 117, 8299-8303 (1995). Reddy, B. V., Khanna, S. N. Chem. Phys. Lett. 209, 104-108 (1993). Lin, Z., Hall, M. B. J. Am. Chem. SOC.114, 10054-10055 (1992). Grimes, R. W., Gale, J. D. J. Chem. Soc., Chem. Commun. 1222-1224 (1992). Grimes, R. W., Gale, J. D. J. Phys. Chem. 97, 4616-4620 (1993). Reddy, B. V., Khanna, S. N., Jena, P. Science, 258, 1640-1643 (1992). Lou, L., Guo, T., Nordlander, P., Smalley, R. E. J. Chem. Phys. 99, 5301-5305 (1993). Han, R.-S., Wang, S., Yin, D. L., Zheng, Q. Q., Pan, W. Solid State Commun. 86, 313-315 (1993). [91] Rantala, T. T., Jelski, D. A., Bowser, J. R., Xia, X., George, T. F. 2. Phys. D, Suppl. 26, S2556257 (1993). [92] Liu, J-N., Gu, B.-L. J. Phys.; Condens. Matter 5, 4785-4792 (1993). [93] Li, Z.-Q., Gu, B.-L., Han, R.-S., Zheng, Q.-Q. 2. Phys. D 27, 275-279 (1993). [94] Xia, H. B., Tian, D. C., Jin, Z. Z., Wang, L. L. J. Phys.; Condens. Mutter 6, 4269-4276 (1994). [95] Xiao, C.-Y., Deng, K.-M. Commun. Theor. Phys. 26, 263-272 (1996). [96] Methfessel, M., van Schilfgaarde, M., Scheffler, M. Phys. Rev. Lett. 70, 29-32 (1993); 71,209 (1993). [97] Ceulemans, A,, Fowler, P. W. J. Chem. SOC.Faraday Trans. 88, 2797-2798 (1992). [98] (a) Fowler, P. W., Woolrich, J. Chem. Phys. Lett. 127, 78-83 (1986). (b) Stollhof, G. Phys. Rev. B 44, 10998 (1991). [99] Srinivas, G. N., Srinivas, H., Jemmis, E. V. Proc. Indian Acad. Sci. (Chem. Sci.) 106, 169181 (1994). [IOO] Dance, I. J. Am.Chem. SOC.118, 6309-6310 (1996). [loll Rohmer, M.-M., de Vaal, P., Benard, M. J. Am. Chem. SOC.114, 9696-9697 (1992). [I021 Hay, P. J. J. Phys. Chem. 97, 3081-3083 (1993). [I031 Khan, A. J. Phys. Chem. 99,4923-4928 (1995). [I041 Khan, A. Chem. Phys. Lett. 247, 447-453 (1995). [I051 Khanna, S. N., Jena, P. Phys. Rev. B 51, 13705-13716 (1995). [I061 Rohmer, M.-M., Benard, M., Bo, C., Poblet, J.-M. J. Am. Chem. SOC.117, 508-517 (1995). [I071 Chen, H., Feyereisen, M., Long, X. P., Fitzgerald, G. Phys. Rev. Lett. 71, 1732-1735 (1993). [I081 Reddy, B. V., Khanna, S. N. J. Phys. Chem. 98,9446-9449 (1994). [I091 Khanna, S. N. Phys. Rev. B51, 10965-10967 (1995). [I101 (a) Cederbaum, L. S., Domcke, W. J. Chem. Phys. 66, 5084-5086 (1977). (b) Cederbaum, L. S., Tarantelli, F., Sgamellotti, A,, Schirmer, J. J. Chem. Phys. 85, 6513-6523 (1986); 86, 2168-2175 (1987). [ I l l ] (a) Heisenberg, W. 2. Physik 38, 411 (1926); 49, 619 (1928). (b) Hay, P. J., Thibeault, J. C., Hoffmann, R. J. Am. Chem. SOC.97,4884-4899 (1975). [I121 Hollander, F. J., Coucouvanis, D. J. Am. Chem. Soc. 96, 5646-5648 (1974); 99, 6268-6280 (1977). [I131 (a) Park, Y., DeGroot, D. C., Schindler, J., Kannewurf, C. R., Kanatzidis, M. G. Angew. Chem., Int. Ed. Engl. 30, 1325-1328 (1991). (b) Zhang, X., Park, Y., Hogan, T., Schindler, J. L., Kannewurf, C. R., Seong, S., Albright, T., Kanatzidis, M. G. J. Am. Chem. SOC. 117, 10300-10310 (1995). [I141 Poblet, J.-M., Rohmer, M.-M., Benard, M. Inorg. Chem., 35, 4073-4075 (1996). [I151 Poblet, J.-M., Bo, C., Rohmer, M.-M., Benard, M. Chem. Phys. Lett. 260, 577-581 (1996). [I161 El Nakat, J. H., Dance, I. G., Fisher, K. J., Willet, G. D. Znorg. Chem. 30, 2957-2958 (1991). [I171 Dehnen, S., Schafer, A,, Fenske, D., Ahlrichs, R. Angew. Chem. Int. Ed. Engl. 33, 746-749 (1994). [I181 Sakuzai, H., Castleman, Jr., A. W. J. Phys. Chem. A 102, 10486-10492 (1998). [ 1191 Sakuzai, H., Castleman, Jr., A. W. J. Chem. Phys. in press.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
6 Metal Clusters in Chemistry - Bibliography of Reviews 1988-1997 Michael I. Bruce
6.1 Introduction In the well-known book on aspects of metal cluster chemistry by Shriver et al. I contributed a bibliography of reviews of various aspects of this area of chemistry."] The Editors of this three volume book considered that a similar updated survey of areas included therein would be appropriate and kindly invited me to update and expand my earlier contribution. The following lists, arranged by subject, attempt to summarize sources of both general and detailed reviews of the various topics in the foregoing articles. By restricting the period of publication to the last decade, a more-or-less contemporary view has been achieved. Given restrictions on space, only the more readily accessible journals and books have been consulted - the coverage follows on from the previous survey (where relevant) and has been taken, where possible, to the end of 1997. The many books on the subject of metal clusters and on various aspects of their chemistry and applications to synthesis, catalysis, and surface chemistry are listed in Section 6.2. Collected contributions to conferences are mentioned in Section 6.3 and annual surveys of aspects of transition metal chemistry, which contain more-or-less work on cluster chemistry, in Section 6.4. The bibliography proper is organized by topic, within which the various citations are ordered alphabetically by author. Individual contributions to edited collections are also listed in these tables. A limited amount of cross-referencing is also included.
6.2 Books Several general inorganic chemistry texts contain sections providing an introduction to cluster chemistry. These include:
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6 Metal Clusters in Chemistry - Bibliography of Reviews 1988-1997
F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th ed., Wiley: New York, 1988: Chapter 23: Metal-metal bonds and metal atom clusters; 6th ed., 1999. D.F. Shriver, P.W. Atkins and C.H. Langford, Inorganic Chemistry, 2nd ed., Oxford UP: Oxford, 1994: Chapter 16.10-12: Metal-metal bonding and metal clusters; 18.8: Chevrel phases; 19.5: Iron-sulfur proteins and non-heme iron; 19.8: Nitrogen fixation; 3rd ed., 1999.
6.2.1 Books on metal clusters For earlier books see the previous bibliography."] Several texts on various aspects of cluster chemistry have appeared over the last decade. The contents are listed briefly below although contributions to edited collections have been listed in the main table of reviews. R.D. Adams and F.A. Cotton (eds.), Catalysis by Di- and Polynuclear Metal Cluster Complexes, Wiley-VCH: New York, 1998 Concepts and models for characterizing homogeneous reactions catalysed by transition metal cluster complexes (E. Rosenberg and R. Laine)/Activation of ruthenium clusters for use in catalysis: Approaches and problems (G. Lavigne and B. de Bonneval)/Catalysis by mixed-metal clusters containing gold phosphine groupings (L. Pignolet)/Catalysis by sulfido bridged dimolybdenum complexes (M. Rakowski DuBois)/Catalytic applications of dimolybdenum and ditungusten complexes containing multiple metal-metal bond (M. McCann)/Synthesis of organic compounds catalyzed by transition metal clusters (G. Siiss-Fink and M. Jahncke)/Catalysis with dirhodium(I1) complexes (M. Doyle)/Catalytic macrocyclization of thietanes by metal carbonyl cluster complexes (R.D. Adams)/Catalysis of Rh, Rh-Co and Ir-Co multinuclear complexes and its applications to organic syntheses (I. Ojima and Z. Li)/Bimetallic homogeneous hydroformylation (G. Stanley)/Catalysis by colloids (L. Lewis)/Catalysis with palladium clusters (I. Moiseev and M.N. Vargaftik)/Heterometallic clusters for heterogeneous catalysis (P. Braunstein and J. Rose)/Metal cluster catalysts dispersed on solid supports (B. Gates) G. Benedek, T.P. Martin and G. Pacchioni (eds.), Elemental and Molecular Clusters, Springer: Berlin, 1988 V. Bortolani, N. March and M. Tosi (eds.), Interaction of Atoms and Molecules with Suvfaces, Plenum: London, 1990 M.H. Chisholm (ed.), Early Transition Metal Clusters with rc-Donor Ligands, VCH: New York, 1995 F.A. Cotton and R.A. Walton, Multiple Bonds Between Metal Atoms, Oxford UP: Oxford, 1993 Introduction and survey/Multiple bonds in dirhenium and ditechnetium compounds/ Multiple bonds in dimolybdenum and ditungsten compounds/Multiple bonding between chromium atoms/X3 M-MX3 compounds of molybdenum and tungsten/ Diruthenium and diosmium compounds/Dirhodium compounds/Dimetal com-
6.2 Books
1713
pounds of other Group VIII elements: Co, Ir, Ni, Pd, Pt/Metal-metal bonds in other structural contexts/Physical, spectroscopic and theoretical results/Postscript on some recent developments L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds, Kluwer: Dordrecht, 1994 J.P. Fackler (ed.), Metal-metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990 T.P. Fehlner (ed.), Inorganometallic Chemistry, Plenum: New York, 1992 Introduction/Main group fragments as ligands to transition metals/Transition metal-main group cluster compounds/Bonding connections and interrelationships/ Experimental comparison of the bonding in inorganometallic and organometallic compounds by photoelectron spectroscopy/Transition metal-promoted reactions of main group species and main group-promoted reactions of transition metal species/ The metal-non-metal bond in the solid state/Molecular precursors to films/Ceramics B.C. Gates, Catalytic Chemistry, Wiley: New York, 1992 G. Gonzalez-Moraga, Cluster Chemistry, Springer-Verlag: Berlin, 1993 Current concepts in modern chemistry/Transition metal cluster chemistry/Main group-transition metal mixed clusters/Cluster compounds of main group elements/ Synthetic analogues of the active sites of iron-sulfur proteins H. Haberland (ed.), Clusters of Atoms and Molecules, Springer: Berlin, 1994 Introduction/Theoretical concepts/Experimental methods/Across the Periodic Table R. Hoffmann, Solids and Surfaces: a Chemist’s Viewof Bonding in Extended Structures, VCH: New York, 1988 C.E. Housecroft, Cluster Molecules of the p-Block Elements, Oxford Science Publications: Oxford, 1994 C.E. Housecroft, Metal-metal Bonded Carbonyl Dimers and Clusters, Oxford Science Publications: Oxford, 1996 P. Jena, B.K. Rao and S.N. Khanna, Physics and Chemistry of Small Clusters, Plenum: New York, 1987 R.B. King, Applications of Graph Theory and Topology in Inorganic Cluster and Coordination Chemistry, CRC Press: Boca Raton, FL, 1993 D.A. King and D.P. Woodruff (eds.), Chemical Physics of Solid Surfaces, Elsevier: New York, 1993 K.J. Klabunde, Free Atoms, Clusters and Nanoscale Particles, Academic: San Diego, 1994 Introduction/New laboratory techniques and methods/Alkali and alkaline earth elements (Groups 1 and 2)/Early transition metals (Groups 3-7)/Late transition metals (Groups 8- 1O)/Copper and zinc group elements (Groups 1 1 and 12)/Boron group (Group 13)/Carbon group (Group 14)/Phosphorus and sulfur groups (Heavier elements of Groups 15 and 16)/Lanthanides and actinides D.M.P. Mingos and D.J. Wales, Introduction to Cluster Chemistry, Prentice Hall: Englewood Cliffs, 1990 Survey of cluster chemistry/Closed-shell electronic requirements for cluster compounds/Introduction to tensor surface harmonic theory/Clusters where radial
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6 Metal Clusters in Chemistry Bibliography of Reviews 1988-1997 ~
bonding predominates/Clusters where s and p interactions are important/Skeletal rearrangements in clusters/Condensed and high nuclearity clusters/Clusters where d orbitals must be considered explicitly D.M.P. Mingos (ed.), Structural and Electronic Paradigms in Cluster Chemistry [Structure & Bonding, vol. 871, Springer Verlag: Berlin 1997 Mathematical cluster chemistry/Metal-metal interactions in transition metal clusters with donor ligands/Electron count versus structural arrangement in clusters based on a cubic transition metal core with bridging Main Group elements/Metalloboranes/ Clusters with interstitial atoms from the p-block: How do Wade’s rules handle them?/Diverse naked clusters of the heavy Main Group elements: Electronic regularities and analogies S.R. Morrison, The Chemical Physics of Surfaces (2nd ed.), Plenum: New York, 1990 Introduction/Space charge effects/Experimental methods/Adsorbate-free surfaces/ Bonding of foreign species at the solid surface/Non-volatile foreign additives on the solid surfacelAdsorption and desorption/The solid/liquid interface/Photoeffects at semiconductor surfaces U. Muller, Inorganic Structural Chemistry, Wiley: Chichester, 1993 [ch. 12. Polyanionic and polycationic compounds. 12.4. Cluster compounds] G.A. Olah, R.E. Williams and K. Wade (eds.), Electron Deficient Boron and Carbon Clusters, Wiley-Interscience: New York, 1991 H.W. Roesky (ed.), Rings, Clusters and Polymers of Main Group and Transition Elements, Elsevier: Amsterdam, 1989 G. Schmid (ed.), Clusters and Colloids - From Theory to Applications, VCH: Weinheim, 1994 General introduction/Electronic structure of metal clusters and cluster compounds/ Clusters in ligand shells/Clusters in cages/Discrete and condensed transition metal clusters in solids/Chemistry of transition metal colloids/Perspectives G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley: New York, 1994 Surfaces - an introduction/Structure of surfaces/Thermodynamics of surfaces/ Dynamics at surfaces/Electronic properties of surfaces/The surface chemical bond/ Catalysis by surfaces/Mechanical properties of surfaces M.T. Weller, Inorganic Materials Chemistry, Oxford Science Publications: Oxford, 1994
6.2.2 Compilations The second edition of the encyclopedic work Comprehensive Organometallic Chemistry (COMCII) contains extensive coverage of organometallic clusters, as summarized in Table 1. Volumes in the Gmelin compilation dealing with metal clusters are listed in Table 2.
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Table 1. Organometallic clusters in COMCII.* Volume 1: Li, Be and B Groups 8.2.6 Metal-rich metallaboranes Volume 3: Cu and Zn Groups 1.2.2 Au(1) complexes containing the structural building blocks C(AuL), (n > 2) Di- and polynuclear Au ylide complexes 1.2.4.2 Homo- and heterometallic Au clusters containing Au-C bonds 1.3 2 Organo-Cu and -Ag compounds (many have aggregates of Cu and Ag atoms) 2.3.2 X-ray structures of organo-Cu and -Ag compounds Volume 6: Mn Group 9.2.2 Re clusters 9.2.2.1 Re3 and Re4 clusters 9.2.2.2 High nuclearity Re clusters Volume 7: Fe, Ru and 0 s Simple carbonyls and carbonyl anions 1.2 1.2.3 FedCO)12 1.2.4.3 [Fe3(COhI 1'1.2.4.4 [Fe4(C0)131 1.3.4 Polynuclear Fe carbonyl hydrides Fe clusters with B atoms 1.8.1 1.9.1 Fe clusters with ketenylidene or 'naked' C atoms Fe3 carbonyl complexes with PR3 and/or PR2 ligands 1.10.2.7 Fed carbonyl complexes with PR3 and/or PR2 ligands 1.10.2.8 1.11.1 Fe clusters with 0 ligands 1.11.2.3 Fe3 and Fe4 complexes with S ligands 1.11.2.4 Fe-Mo-S clusters Fe clusters with Group 11 metals 1.13.1 1.14 Fe clusters with other transition metals Polynuclear Fe compounds with hydrocarbon ligands 4 4.1 Clusters derived from alkynes Carbide and related cluster compounds 4.2 Clusters containing q ' - or p2-carbon ligands 4.3 Fe clusters containing q-C5R5 groups 4.4 Ru3 and Os3 clusters: Introduction, ligand types and simple neutral, anionic and 12 hydrido clusters Ru3 and Os3 clusters: Hydrocarbon ligands on metal clusters 13 Ru3 and Os3 clusters: Clusters with M-C bonds to heteroatom ligands 14 15 Ru4 and Os4 clusters Medium- and high-nuclearity clusters of Ru and 0 s 16 Volume 8: Co, Rh and Ir Cluster complexes of Co, Rh, Ir 4 Volume 9: Ni, Pd and Pt Higher cluster complexes of Ni 1.4 1.4.1 Clusters with CO ligands
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6 Metal Clusters in Chemistry - Bibliography of Reviews 1988-1997
Table 1 (continued) 1.4.2 1.4.3 2.10 2.10.1 2.10.2 3.5.6 3.5.6.2 3.5.6.3 4.2.5 4.2.6 4.3.3 6.3.3.4 7.1.2 7.1.3 7.1.5 1.2.2 9.2.8 9.3.5
Clusters with isocyanide ligands Clusters with carbene and carbyne ligands Ni clusters containing Ni-C 0 bonds Homometallic clusters Heterometallic clusters Clusters with Ni(q-CsR5) groups Homometallic clusters Heterometallic clusters Pd carbonyl clusters Pd carbonyls containing other metals [Pd3(~3-CNR)(~c-dppm)3I2+ Synthesis of Pd-alkyne clusters Pt carbonyl clusters Pt carbonyl cluster anions Triangular dppm-bridged Pt clusters Pt isocyanide clusters Polymetallic alkene complexes of Pt Polymetallic alkyne complexes of Pt
Volume 10: Heteronuclear M-M Bonds Synthesis of compounds containing heteronuclear M-M bonds 1 2 Heterodinuclear compounds 3 Heteronuclear clusters containing C I ,C2, C3, . . . C, acyclic hydrocarbyl ligands 4 Binary CO, CO + H, CO + PR3, cyclic hydrocarbyl and Main Group ligands without cyclic hydrocarbyls 5 Cluster complexes with bonds between transition metals and Cu, Ag, and Au 6 Cluster complexes with bonds between transition metals and Zn, Cd, and Hg I Catalysis and related reactions with compounds containing heteronuclear M-M bonds Volume 13: Structure Index
* E.W. Abel, F.G.A. Stone and G. Wilkinson, Comprehensive Organometallic Chemistry ZZ, 14 vols., Elsevier: Oxford, 1995.
6.2.3 Synthetic methods In addition to the well-known compilations Inorganic Syntheses and Organometallic Syntheses, some useful synthetic approaches to metal cluster compounds have been listed in: W.A. Herrmann (ed.), Synthetic Methods of Organometallic and Inorganic Chemistry, vols. 1-10, Thieme: Stuttgart, 1996-1999
6.3 Conferences
17 17
Table 2. Organometallic clusters in Gmelin.* Elementt
Volume
Contents
Date
Ti (47) Co (58) Ni (57) Fe (59)
Organo-titanium compounds, Pt 5 Organo-cobalt compounds, Pt 2 Organo-nickel compounds, Pt 2 Organo-iron compounds, Pt B2
Di- and polynuclear compounds Tri- to polynuclear compounds Tri- to polynuclear compounds Cluster compounds containing one Fe(CO)3 group Trinuclear compounds
1990 1973 1974 1978 1991, 1992
Tri- to polynuclear compounds Trinuclear compounds
1986 1994, 1995
Trinuclear compounds (including heterometallic) Tetranuclear clusters Osg to 0 ~ 4 clusters 0
1993
0 s (66)
Organo-iron compounds, Pt C6a, 6b Organo-iron compounds, Pt C7 Organo-osmium compounds, Pt B3, B4A, B5 Organo-osmium compounds, Pt B6 Organo-osmium compounds, Pt B8 Organo-osmium compounds, Pt B9
1997 1995
* Gmelin Handbook of Inorganic Chemistry, 8th ed., Springer, Berlin. +Thesystem no. is given in parentheses.
6.3 Conferences The proceedings of many conferences on aspects of cluster chemistry have appeared as special publications or in special issues of journals. Not all of these have been tracked down for this bibliography, but the following (listed in chronological order) have been noted. Individual contributions are listed in the main table. M.H. Chisholm (ed.), Reactivity of Bridging Hydrocarbyl Ligands, containing several contributions on cluster chemistry [Polyhedron Symposium-in-Print]. Polyhedron, 1988, 7 (10/1 l), pp. 757-1052 R.D. Adams and W.A. Herrmann (Eds.), Chemistry of Heteronuclear Clusters and Multimetallic Catalysts, Konigstein, Germany, September 1987 [Polyhedron Symposium-in-Print No. 81. Polyhedron, 1988, 7 (22/23), pp. 225 1-2463 J.-M. Basset, B.C. Gates, J.P. Candy, A. Choplin, M. Leconte, F. Quignard and C. Santini (Eds.), Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis [NATO AS1 Ser., vol. C2311, Kluwer: Dordrecht, 1988 P. Braunstein (ed.), Recent Advances in Di- and Polynuclear Chemistry, New J. Chem., 1988, 12 (6/7), pp. 305-720 L. Que, Jr (ed.), Metal Clusters in Proteins, ACS Symp. Ser., vol. 372, 1988 E. Recknagel and 0. Echt (eds.), Small Particles and Clusters [Proc. 5th Int. Mtg Small Particles Inorg. Clusters, Konstanz (1990)], Springer Verlag: Berlin, 1991 A. Zecchina and C. Morterra (eds.), Workshop on Clusters and Surfaces, Turin, Italy, January 1991, Muter. Chem. Phys., 1991, 29
1718
6 Metal Clusters in Chemistry Bibliography of Reviews 1988-1997 ~
Special issue on Physics and Chemistry of Metal Clusters in Biology, New J. Chem., 1991,15 (6), pp. 407-507 L.J. de Jongh, J. Baak, H.B. Brom, D. van der Putten, J.M. van Ruitenbeek and R.C. Thiel (eds.), Physics and Chemistry of Finite Systems: From Clusters to Crystals [NATO AS1 Ser., vol. C3741, Plenum: New York, 1991 Proceedings of the US-Italy International Conference on Organic Chemistry at Clusters and Surfaces, Piemonte, Italy, July 1992, J. Cluster Sci., 1992, 3 , pp. 259497(3, 4); 1993, 4, pp. 1-88 Chemistry and Physics of Small Metallic Particles, Faraday Discussion No. 92 (1991) Cluster Models for Surface and Bulk Phenomena, [NATO ASI, vol. B2831, Plenum: New York, 1992 D.R. Salahub and N. Russo (eds.), Metal-Ligand Interactions: from Atoms to Clusters to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992 Special volume on Materials Science, J. Organomet. Chem., 1993, 449 Special issue dedicated to Lawrence Dahl, J. Cluster Sci., 1995, 6 (l), pp. 1-215 L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. 4651, Kluwer: Dordrecht, 1995 Dalton Discussion on Metal Clusters, Southampton, UK, January 1996, J. Chem. SOC.,Dalton Trans., 1996 ( 5 ) , pp. 555-799 Special issue on cluster complexes containing sulfide and thiolate ligands, J. Cluster Sci., 1996, 7 (3), pp. 223-500 T.P. Martin (ed.), Large Clusters of Atoms and Molecules [NATO AS1 Ser., vol. E3131, Kluwer: Dordrecht , 1996
6.4 Annual surveys The publication of annual surveys of organometallic chemistry, whether by periodic group or by ligand, has slackened over the past decade, as the volume of published material has grown. Fortunately, the appearance of COMCII has eased the task of searching for earlier material about organometallic clusters. The Royal Society of Chemistry continues to publish its Specialist Periodical Reports on organometallic chemistry, containing sections on organo-transition metal cluster chemistry: E.W. Abel and F.G.A. Stone (eds. for vols. 18-20), E.W. Abel (ed. for vols. 21-25), M. Green (ed. for vol. 26 and following), Organometallic Chemistry, Roy. SOC. Chem.: Cambridge, vols. 18-28 (surveys of the literature 1988-1998; continuing) Chapter 4 (vols. 18-220, ch. 3 (vols. 23-26): Carbaboranes, including their metal complexes; Chapter 8 (vols. 18-22), ch. 7 (vols. 23-26): Metal carbonyls [including polynuclear species];Chapter 8 (vol. 25), Ch. 10 (vols. 26 and following): Chemistry
6.5 Bibliography of reviews 198811997
1719
of organo-transition metal cluster complexes: Chapter 9 (vols. 18-22), ch. 8 (vols. 23, 24): Organometallic compounds containing metal-metal bonds; Chapter 16 (vols. 18-21), ch. 15 (vol. 22; discontinued after this volume): Structures of organometallic compounds determined by diffraction methods. The last chapter, continuing until 1992, contains a summary of structural studies of organometallic compounds, as determined by X-ray, electron, and neutron diffraction methods. Annual surveys of organometallic cluster chemistry have appeared in Coord. Chem. Rev., 1996, 156, 91-138 (251 references) [for 19941, 1997, 160, 237-294 (281 references) [for 19951, 1998, 168, 177-231 (304 references) [for 19961; 1998, 175, 271-322 [for 19971. Annual surveys of inorganic and organometallic chemistry of the transition elements in Coord. Chem. Rev. (CCR) and J. Organomet. Chem. (JOMC*) [volume and page only given]. In the former journal, a new series of annual reviews started in 1990 and was completed with the 1996 reviews; many of these accounts contain significant (but not complete) coverage of cluster compounds. Sections or chapters on cluster chemistry are to be found in such reviews as are extant, as listed in Table 3. Further annual surveys are to be found on the www at http://mebos.com
6.5 Bibliography of reviews 1988/1997 Table 4 consists of a bibliography of reviews of various aspects of metal cluster chemistry, divided into categories which largely reflect the topics covered earlier in this book. An alphanumeric system of referencing has been employed to facilitate cross-reference. Although this survey follows the earlier bibliography"], the reader should note that categories have been rearranged and relettered. Much of this bibliography is taken up with material appearing in review journals, etc., which enable access to the major advances in this area over the last decade. Most of entries relate to the chemistry of organotransition metal clusters and their applications to catalysis and reactions occurring on surfaces. The coverage given to solid-state and other inorganic clusters is limited, and the important role of metal cluster complexes in biological systems is hardly touched upon. Although the length of the article (inclusive pages given) and number of references (in parenthesis after the citation) indicate the depth of coverage, no account has been taken of multiple citations in the same reference number, or of separate references appearing under tables, for example. Grouping in the various categories has been determined by title and content - most articles have been sighted. The various sections are:
1720
6 Metal Clusters in Chemistry
~
Bibliography of Reviews 1988-1 997
A. Introductory and general accounts of metal cluster chemistry B. Transition metal-main group clusters B1. General B2. Main group clusters B3. Clusters containing boron: metallaboranes, metallacarboranes B4. Metal-containing carbon clusters B5. Clusters containing Group 15 heteroatoms B6. Clusters containing oxygen atoms B7. Clusters containing chalcogenide atoms (S, Se, Te) C. Ligands on clusters C1. General surveys C2. Carbenes, carbynes, olefins C3. Alkynes C4. Cyclopentadienyls, arenes D. High-nuclearity metal clusters E. Metal cluster complexes by Periodic Group El. Group 6 (Cr, Mo, W) E2. Group 7 (Mn, Tc, Re) E3. Group 8 (Fe, Ru, 0 s ) E4. Group 9 (Co, Rh, Ir) E5. Group 10 (Ni, Pd, Pt) E6. Group 11 (Cu, Ag, Au) F. Heterometallic clusters G. Synthesis and reactivity of metal clusters G1. Synthesis of metal cluster complexes G2. Dynamic processes in metal clusters G3. Reactions of metal-metal bonds in metal clusters G4. Chemistry of metal clusters H. Physical properties and spectroscopy of metal clusters H1. Physical properties of metal clusters H2. Spectroscopic properties of metal clusters H3. Electrochemistry of metal clusters I. Structure and bonding in metal clusters 11. Molecular structures of metal cluster complexes 12. Electronic structures of metal cluster complexes J. Clusters and colloids, surfaces; catalysis J1. Clusters and colloids 52. The cluster-surface analogy: surface organometallic chemistry 53. Supported clusters: surfaces, zeolites J4. Metal clusters in catalysis K. Solid-state clusters L. Naked metal clusters (gas phase, matrix isolated)
Nb/Ta Nb Ta Cr, Mo, W Cr Mo W Mnt Tct Ret Fe Ru, Ost Ru 0s cot Rht Ir+ Nit Pdt pt+
v
115 1 457* 121 134 (2) 133 124 183 124 107 115 191 115 163 134 (2) 1 124 1
134 (2) 75
146 (1) 203 138 161 138 181 477* 45 138 121 138 39 138 1 142 21 142 1 142 43 477* 219 138 219 138 87 152 1 (92-93) 146 ( I ) 235 146 ( I ) 269 146 (2) 1 138 195 142 123
127 155 127 171
134 ( I ) 1 127 65 131 95 131 153 131 127 134 (1) 91; 142 257 477* 173 131 35 127 1 134 (2) 195 134 (2) 307 131 177 142 153 127 99
146 (2) 91 146 (2) 141
134 (1) 189 134 (1) 171
152 107 152 141 146 (1) 115 146 (2) 409 146 (2) 431
146 (2) 167 146 (2) 191 146 (2) 207 146 (2) 225 141 63 146 (2) 307 146 (1) 99
146 ( I ) 43
146 ( I ) 17
146 ( I ) 37 146 (2) 155 147 373 146 (1) 1
142 147 138 27 141 1 138 71
127 133 127 139 457* 41 127 39
124 41 124 51 442* 225
sc Y Ln, Ant Ti Zr, Hf Zr Hf
1993
1992
1991
1990
Element
Table 3. Annual surveys of chemistry of transition metals.
164 415 164 483 169 237
162 417 152 157 164 27 162 477 152 393 152 175
164 503
164 261 164 289 164 345 169 153 169 187 169 201 172 3
164 203 169 129
164 183 164 189 165 163
1995
152 359 152 309 152 251 162 275 162 305 162 319 162 345
162 255
152 467 162 241 156 1 164 5 152 411
1994
172 389 172 437
172 357
172 157 172 181 172 247 172 319
172 111
172 99
1996
124 63 115 141 115 117
162 155 (91-94) 131 1 127 187 146 (I) 211 142 101 152 87 146 (2) 385
162 495 164 161
164 575 169 363 164 667
172 457
+Previous coverage: organo-Ln, An chemistry in JOMC 416 201 (for 1984-86) 442 83 (for 1987-89); orgdno-Mn, Tc, Re chemistry in JOMC 357 25 (for 1986), 416 291 (for 1987); Ru, 0 s chemistry in CCR 404 213 (for 1988), 432 215 (for 1989); organo-Ru, 0 s chemistry in JOMC 351 145 (for 1986); organo-Co, Rh, Ir chemistry in JOMC 351 215 (for 1986); organo-Ni, Pd, Pt chemistry in JOMC 442 271 (for 1986).
cu Ag Au
U
10 10
k
N
h,
4
Title
Chiral transition metal clusters. Relationships between stereochemistry, electronic structure and circular dichroism spectra Development of metallacrown ethers: a new class of metal clusters Metal clusters revisited Low-valent organometallic clusters
A7
Organometallic chemistry of hexanuclear carbouyl clusters General introduction [to cluster chemistry]
Perspectives [in cluster chemistry]
Clusters in ligand shells: introduction
A12
A13
A14
A1 1
A9 A10
A8
A6
A5
Physics of metal clusters Metal clusters: a personal perspective Molecule-cluster-microcrystal-crystal: elements of condensed matter Introduction to metal cluster compounds: from molecule to metal! Introduction [to metal cluster chemistry]
A2 A3 A4
A. Introductory and general accounts of metal cluster chemistry A1 Metal clusters and ligands
No.
Table 4. Bibliography of reviews 1988-1997.
M.S. Lah and V.L. Pecoraro, Comments Inorg. Chem., 1990, 11, 59-84 (33) J. Lewis, Chem. Brit., 1988, 24, 795-800 G. Longoni and M.C. Iapalucci, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 91-177 (412) L. Ma, G.K. Williams and J.R. Shapley, Coord. Chem. Rev., 1993, 128, 261-284 (83) G. Schmid, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 1-4 G. Schmid, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 545-546 G. Schmid, G. Longoni and D. Fenske, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 89-91
L.J. de Jongh, in L.J. de Jongh (ed.), Physics and Chemistry ofMetal Cluster Compounds, Kluwer: Dordrecht, 1994, 1-39 (48) H.D. Kaesz and D.F. Shriver, in D.F. Shriver, H.D. Kaesz and R.D. Adams (eds.), Chemistry of Metal Cluster Complexes, VCH: New York, 1990, pp. 1-10 (26) A.P. Klyagina and I.F. Golovaneva, Metulloorg. Khim., 1990, 3, 503515 (41)
P. Braunstein, in A.F. Williams, C. Floriani and A.E. Merbach (eds.), Perspectives in Coordination Chemistry, Verlag Helv. Chim. Acta: Basel, 1992, pp. 67-107 (139) M.L. Cohen and W.D. Knight, Phys. Today, 1990 (12), 42-50 (23) J. Evans, J. Chem. Soc., Dalton Trans., 1996, 555-559 (27) L. Genzel, Nova Acta Leopold., 1990, 63, 105-110 (13)
Citation
Metal clusters, an aid in the study of the formation of metals Low-valent metal clusters - an overview Tetranuclear planar clusters with capping ligands Metal cluster compounds - chemistry and significance. I. Synthesis, structure and bonding. 11. Clusters with isolated atoms of main group elements, large metal clusters and fluxionality of clusters. 111. Homogeneous and heterogeneous catalysis
A15
B2.3
B1.4
Cage compounds with main group metals
Clusters with interstitial atoms from thep-block: How do Wade’s rules handle them? B1.5 The interface of main group and transition metal chemistry B2. Main group clusters B2.1 Strain and resonance energies in main group homoatomic rings and clusters B2.2 Cluster compounds of boron in coordination chemistry
B1. General [see also: E3.241 B1.l Transition metal complexes incorporating atoms of the heavier main group elements B1.2 Synthesis and reactivity of small mixed clusters formed by transition metal and main group elements B 1.3 Transition metal-main group cluster compounds
B. Transition metal-main group clusters
A16 A17 A18
Title
No.
Table 4 (continued)
B.M. Gimarc and M. Zhao, Coord. Chem. Rev., 1997, 158,385-412 (80) N.T. Kuznetsov and K.A. Solntsev, Koord. Khim., 1991, 17, 1157 1194 (105) M. Veith, Chem. Rev., 1990, 90, 3-16 (114)
K.H. Whitmire, J. Coord. Chem., 1988, 17, 95-203 (413)
N.A. Compton, R.J. Ernngton and N.C. Norman, Adv. Organornet. Chem., 1990,31,91-182 (192) M. Di Vaira and P. Stoppioni, Coord. Chem. Rev.,1992, 120, 259-279 (57) C.E. Housecroft, in T.P. Fehlner (ed.), Znorganometallic Chemistry, Plenum: New York, 1992, pp. 73-178 (441) C.E. Housecroft, S t r u t . Bond., 1997, 87, 137-156 (66)
B.H.S. Thimmappa, Coord. Chem. Rev., 1995,143, 1-34 (91) B.H.S. Thimmappa, J. Cluster Sci., 1996, 7, 1-36 (102) B. Walther, Z. Chem., 1986, 26, 421-430 (122); 1988, 28,81-92 (122); 1989,29, 117-129 (166)
G . Schmid, Chem. Uns. Zeit, 1988, 22,85-92 (14)
Citation
a
3
B4.3
B4.2
Metallo-carbohedrenes: a new class of molecular clusters Metal-carbon clusters: the construction of cages and crystals
B4. Metal-containing carbon clusters [see also: L.21 B4.1 Metallo-carbohedrenes: a new class of cluster materials
A.W. Castleman, Jr, Proc. Electrochem. SOC.,1994, 94-24, 346-359 (51) B.C. Guo and A.W. Castleman, Jr, Adv. Met. Semi-cond. Clusters, 1994,2, 137-164 (52) J.S. Pilgrim and M.A. Duncan, Adv. Met. Semi-cond. Clusters, 1995,3, 181-221 (51)
B3. Clusters containing boron: metallaboranes, metallacarboranes [see also: E3.271 T.P. Fehlner, Ado. Inorg. Chem., 1990, 35, 199-233 (149) B3.1 The metallic face of boron T.P. Fehlner, Struct. Bond., 1997, 87,111-135 (125) B3.2 Metallaboranes T.P. Fehlner, New J. Chem., 1988, 12, 307-316 (83) B3.3 Metal-rich ferra- and cobalta-boranes. Mimics of organometallic clusters R.N. Grimes, Chem. Rev., 1992, 92, 251-268 (92) Boron-carbon ring ligands in organometallic synthesis B3.4 R.N. Grimes, Coord. Chem. Ret.., 1995, 143, 71-96 (37) B3.5 Cluster-forming and cage fusion in metallacarborane chemistry N.S. Hosmane and J.A. Maguire, Adv. Organomet. Chem., 1990, 30, B3.6 Syntheses, structures, bonding and reactivity of main 99-150 (91) group heterocarboranes N.S. Hosmane and J.A. Maguire, J. Cluster Sci., 1993, 4, 297-349 B3.7 Metallacarboranes of the CzBd-cage system (138) C.E. Housecroft, Adv. Organomet. Chem., 1991, 33, 1-50 (100) B3.8 Boron atoms in transition metal clusters C.E. Housecroft, Chem. SOC.Rev., 1995, 24, 215-222 (30) B3.9 Denuding the boron atom of B-H interactions in transition metal boron clusters C.E. Housecroft, Coord. Chem. Rev., 1995, 143, 297-330 (191) B3.10 Transition metal boride clusters at the molecular level A.K. Saxena and N.S. Hosmane, Chem. Ret.., 1993, 93, 1081-1124 B3.11 Recent advances in the chemistry of carborane metal complexes incorporating d and f block elements (216) F.G.A. Stone, Adv. Organomet. Chem., 1990, 31, 53-89 (40); S.A. B3.12 The interplay of alkylidyne and carbaborane ligands at metal centres. I. Synthesis of electronically unsatuBrew and F.G.A. Stone, Adv. Organomet. Chem., 1993,35, 135-186 rated mixed-metal complexes. 11. Proton-mediated (69) reactions Y.-K. Yan and D.M.P. Mingos, Chem. SOC.Rev., 1995,24, 203-213 B3.13 Structural, magnetic and conductivity properties of charge-transfer salts derived from metallacarboranes (34)
Title
‘Short-bite’ ligands in cluster synthesis Organo-transition metal clusters incorporating bismuth [Meldola medal lecture] Complexes with substituent-free acyclic and cyclic P, As, Sb and Bi ligands Metal-rich large clusters with P and N ligands
Emerging chemistry of polynuclear metal hydride alkoxides, H,M,(OR), Alkoxide clusters of molybdenum and tungsten
Magnetism of large iron 0x0 clusters
B6.4
B6.6
B6.5
High nuclearity 0x0-molybdenum(V) clusters
B6.3
B6. Clusters containing oxygen atoms [see also: B5.3, E3.121 B6.1 Cyclopentadienylmetal oxides B6.2 Organometallic compounds containing oxygen atoms
B5.7
B5.6
B5.4 B5.5
B5. Clusters containing Group 15 heteroatoms [see also: C3.51 B5.1 Structural chemistry of transition metal complexes with arsenic-arsenic bonds B5.2 Transition metal clusters with bridging main group elements B5.3 New transition metal clusters with ligands from main groups 5 and 6
No.
Table 4 (continued)
M.H. Chisholm, in M.H. Chisholm (ed.), Early Transition Metal Clusters with n-Donor Ligands, VCH: New York, 1995, pp. 165-215 (1 17) D. Gatteschi, A. Caneschi, R. Sessoli and A. Cornia, Chem. Soc. Rev., 1996,25, 101-109 (36)
F. Bottomley, Polyhedron, 1992, 11, 1707-1731 (152) F. Bottomley and L. Sutin, Adv. Organomet. Chem., 1988, 28, 339-396 (266) H.K. Chae, W.G. Klemperer and T.A. Marquart, Coord. Chem. Rev., 1993,128,209-224 (23) M.H. Chisholm, Chem. SOC.Rev., 1995,24, 79-87 (49)
O.J. Scherer, Angew. Chem.,1990, 102, 1137-1155; Angew. Chem., Znt. Ed. Engl., 1990, 29, 1104-1 122 (95) G. Schmid, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 178-211 (111)
A,-J. Di Maio and A.L. Rheingold, Chem. Rev., 1990, 90, 169-190 (105) D. Fenske, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 212-297 (304) D. Fenske, J. Ohmer, J. Hachgenei and K. Merzweiler, Angew. Chem., 1988, 100, 1300-1319; Angew. Chem., Znt. Ed. Engl., 1988,27, 12771296 (94) J.T. Mague, J. Cluster Sci., 1995, 6, 217-269 (256) N.C. Norman, Chem. SOC.Rev., 1988, 17, 269-281 (39)
Citation
0x0-vanadium and 0x0-molybdenum clusters and solids incorporating oxygen-donor ligands Organophosphate and organoarsenate complexes with oxovanadium and oxomolybdenum cores
B6.8
M.T. Pope and A. Miiller, Angew. Chem., 1991, 103, 56-70; Angew. Chem., Int. Ed. Engl., 1991, 30, 34-48 (165) S.K. Saha, M. Ali and P. Banerjee, Coord. Chem. Rev., 1993, 122, 4162 (50) T. Shibahara, Adv. Znorg. Chem., 1991, 37, 143-173 (101)
R.E. McCarley, in M.H. Chisholm (ed.), Early Transition Metal Clusters with x-Donor Ligands, VCH: New York, 1995, pp. 27-61 (90) R.C. Mehrotra and A. Singh, Chem. Soc. Rev., 1996,25, 1-13 (31) E. Papaconstantinou, Chem. Soc. Rev., 1989, 18, 131 (63)
M.I. Khan and J. Zubieta, in M.H. Chisholm (ed.), Early Transition Metal Clusters with rr-Donor Ligands, VCH: New York, 1995, pp. 247-283 (61) J. Kohler, G. Svensson and A. Simon, Angew. Chem., 1992,104, 14631482; Angew. Chem., Int. Ed. Engl., 1992, 31, 1437-1456 (137) R. Manchandra, G.W. Brudvig and R.H. Crabtree, Coord. Chem. Rev., 1995, 144, 1-38 (95)
R.C. Haushalter, L.M. Meyer and J. Zubieta, in M.H. Chisholm (ed.), Early Transition Metal Clusters with x-Donor Ligands, VCH: New York, 1995, pp. 217-246 (47) M.I. Khan and J. Zubieta, Prog. Inorg. Chem., 1995, 43, 1-149 (241)
tungsten oxo/sulfido clusters B7. Clusters containing chalcogenide atoms (S, Se, Te) [see also: B5.3, B6.17, C3.5, E3.1, H3.7, J4.131 B7.1 R.D. Adams, J. Cluster Sci., 1992, 3, 263-273 (10) Nucleophilic ring-opening transformations of bridging thietane ligands in metal carbonyl cluster complexes R.D. Adams and S.B. Falloon, Chem. Rev., 1995, 95, 2587-2598 (52) Chemistry of thietane ligands in polynuclear metal carB7.2 bony1 complexes Some synthetic and theoretical aspects of the chemistry A. Bencini and S. Midollini, Coord. Chem. Rev., 1992, 120, 87-136 B7.3 of polynuclear transition metal complexes [chalcogen (149) clusters]
B6.17
B6.16
B6.15
B6.13 B6.14
Electron exchange and transfer reactions of heteropolyoxometallates Cubane and incomplete-cubane-type molybdenum and
High-valent 0x0-manganese clusters: structural and mechanistic work relevant to the oxygen-evolving center in photosystem I1 Clusters and metal-metal bonded chains in molybdenum oxide systems Chemistry of 0x0-alkoxides of metals Photochemistry of polyoxometallates of molybdenum and tungsten and/or vanadium Polyoxometallate chemistry: new dimensions
B6.11
B6.12
Oxoniobates containing metal clusters
B6.10
B6.9
Metal 0x0 clusters in molybdenum and vanadium phosphate solids
B6.7
\o
Ir
3.
2C
%
3-
2
c3
5 6'
B
b
9
Nitrosyl complexes of iron-sulfur clusters
Use of pre-assembled Fe/S and Fe/Mo/S clusters in the stepwise synthesis of potential analogs for the Fe/Mo/ S site in nitrogenase Mo/Co/S clusters: models and precursors for hydrodesulfurisation (HDS) catalysts Electron-deficientmolybdenum/cobalt/sulfide clusters: chemistry related to hydrodesulfurisation (HDS) catalysts Metal-chalcogenide cluster chemistry Metals in the nitrogenases
An organometallic approach to the synthesis of high nuclearity molybdenum-iron-sulfur clusters as potential models for the iron-molybdenumcofactor of nitrogenase Structure, bonding and electron counts in cubane-type clusters having M4S4, M2M12S4and MM1& cores Subsite-specific structures and reactions in native and synthetic (4Fe-4s) cubane-type clusters Trinuclear cuboidal and heterometallic cubane-type iron-sulfur clusters: new structural and reactivity themes in chemistry and biology Sulfide aggregates and clusters of platinum Mo(W, V)-Cu(Ag)-S(Se)cluster compounds
Structure and reactivity of molybdenum clusters with loose coordination sites, M03S4 { S2P(OEt)2}4L
B7.4
B7.5
B7.10
B7.16
B7.14 B7.15
B7.13
B7.12
B7.11
B7.8 B7.9
B7.7
B7.6
Title
No.
Table 4 (continued)
T.S.A. Hor, J. Cluster Sci.,1996, 7,263-292 (75) H.-W. Hou, X.-Q. Xin and S. Shi, Coord. Chem. Rev., 1996, 153, 2556 (168) J.Q. Huang, J.L. Huang, M.Y. Shang, S.F. Lu, X.T. Lin, Y.H. Lin, M.D. Huang, H.H. Zhuang and J.X. Lu, Pure Appl. Chem., 1988, 60, 1185-1192 (16)
R.H. Holm, S. Ciurli and J.A. Weigel, Prog. Znorg. Chem., 1990, 38, 1-74 (233) R.H. Holm, Adu. Znorg. Chem., 1992,38, 1-71 (257)
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L
u
v)
v)
4
2 9"
4
B
h, W
4
B7.29
B7.28
B7.27
B7.26
B7.25
B7.24
B7.23
B7.21 B7.22
B7.20
B7.19
B7.18
B7.17
Cobalt (iron) thiolato complexes containing the co-ligand phosphine and reaction products of the structural fragment ML2L’ (L = I , 2-bidentate thiolate, L’ = tertiary phosphine) Transition metal thiolates: molecular fragments to active centres of biomolecules Enumeration and structural classification of clusters derived from parent solids: metal-chalcogenide clusters composed of edge-sharing tetrahedra Addition of organic and inorganic moieties to FedC0)6(p-EE’) and Fes(C0)9(p,-E)(p~-E’) ( E = Se, Te; E’ = S, Se, Te) Chalcogen-bridged metal carbonyl complexes CdS nanoclusters stabilised by thiolate ligands: a minireview Orbital interactions, electron delocalisation and spincoupling in iron-sulfur clusters New developments in the coordination chemistry of inorganic selenide and telluride ligands Chalcogenide cluster complexes of the early transition metals Chalcogen clusters of chromium, molybdenum, tungsten and rhenium Group 6 metal chalcogenide cluster complexes and their relationships to solid-state cluster compounds Synthesis of sulfur-bridged molybdenum and tungsten coordination compounds [includes M3, M4 and mixed-metal cubanes] Designed syntheses for some transition metal clusters involving bridging-sulfido ligand
X. Wu, P. Chen, S. Du, N. Zhu and J. Lu, J. Cluster Sci., 1994, 5, 265-285 (52)
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T. Saito, in M.H. Chisholm (ed.), Early Transition Metal Clusters with n-Donor Ligands, VCH: New York, 1995, pp. 63-164 (202) T. Saito and H. Imoto, Bull. Chem. Soc. Jpn, 1996, 69, 2403-2417 ( 100) T. Saito, Adv. Znorg. Chem., 1997, 44, 45-91 (130)
L. Noodleman, C.Y. Peng, D.A. Case and J.-M. Mouseca, Coord. Chem. Rev., 1995, 144, 199-244 (148) L.C. Roof and J.W. Kolis, Chem. Rev., 1993,93, 1037-1080 (254)
P. Mathur, Adv. Organomet. Chem., 1997,41, 243-314 (145) C.J. Murphy, J. Cluster Sci., 1996, 7, 341-350 (34)
P. Mathur, D. Chakrabarty and I.J. Marvunkal, J. Cluster Sci., 1993, 4, 351-375 (53)
B. Krebs and G. Henkel, Angew. Chem., 1991,103, 785-804; Angew. Chem., Int. Ed. Engl., 1991, 30, 769-788 (212) J.R. Long and R.H. Holm, J. Am. Chem. SOC.,1994,116,9987-10002 (100)
B.-S. Kang, Z.-N. Chen, Z.-Y. Zhou, H.-Q. Liu, H.-R. Gao, B.-M. Wu, T.C.W. Mak, Y .-B. Cai, Y .-J. Xu and Z.-T. Xu, J. Cluster Sci., 1996,7, 317-340 (51)
Title
C2. Carbenes, carbynes, olefins [see also: E3.2, E3.3, E3.181 c2.1 Metal cluster complexes containing hetero-atomsubstituted carbene ligands c2.2 Organometallic chemistry of vinylidene and related unsaturated carbenes C2.3 Hydrocarbyl ligand derivatives of Group 6-Group 9 heterometallic clusters C2.4 Homometallic and heterometallic transition metal allenyl complexes: synthesis, structure and reactivity C2.5 Interaction of ketenes with organometallic compounds: ketene, ketenyl and ketenylidene complexes C2.6 CC and CO transformations in ketenylidene cluster compounds Bis-alkylidyne cluster compounds of iron C2.7 0 , z-Bridging ligands in bimetallic and trimetallic C2.8 complexes Cluster-stabilised cations: synthesis, structures, moleC2.9 cular dynamics and reactivity
C1. General surveys [see also: G4.31 Bonding modes adopted by organo-fragments on metal CI. 1 cluster surfaces Perpendicular vs parallel alkynes, monohapto vs C1.2 dihapto vinyls, aromatic vs nonaromatic heterocycles: access to different types of bonding of these ligands in clusters
C. Ligands on clusters
No.
Table 4 (continued)
D. Lentz, Coord. Chem. Rev., 1995, 143, 383-406 (63) S. Lob, P.H. van Rooyen and R. Meyer, Adv. Organomet. Chem., 1995, 37, 219-320 (362) M.J. McGlinchey, L. Girard and R. Ruffolo, Coord. Chem. Rev., 1995, 143, 331-381 (134)
g
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u
v)
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1
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5
2
b
%
2
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M.I. Bruce, Chem. Rev., 1991, 91, 197-257 (378)
R.D. Adams, Chem. Rev., 1989,89, 1703-1712 (60)
A.J. Deeming, A.J. Arce and Y. De Sanctis, Muter. Chem. Phys., 1991, 29, 323-331 (25)
G. Conole, Muter. Chem. Phys., 1991, 29, 307-322 (45)
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Cyclisation reactions in the inner coordination sphere of transition metals involving carbyne complexes Transformations of carbon monoxide and related ligands on metal ensembles Transition metal propargyl complexes: versatile reagents in synthesis Heterobinuclear and heterotrinuclear transition metal p-allenyl complexes
M.J. Went, Adv. Organomet. Chem., 1997, 41, 69-125 (186)
M.J. Went, Polyhedron, 1995, 14, 465-481 (42)
R.D. Adams, J.-C. Daran and Y. Jeannin, J. Cluster Sci,, 1992, 3, 154 (75) M. Akita and Y. Moro-oka, Bull. Chem. Soc. Jpn, 1995, 68, 420-432 (39) S. Deabate, R. Giordano and E. Sappa, J. Cluster Sci., 1997, 8, 407459 (1 57) J. Fornies and E. Lalinde, J. Chem. Soc., Dalton Trans., 1996, 25872599 (43)
A. Wojcicki and C.E. Shuchart, Coord. Chem. Rev., 1990, 105, 35-60 (57) A. Wojcicki, J. Cluster Sci., 1993, 4, 59-75 (48)
D.F. Shriver and M.J. Sailor, Acc. Chem. Rex, 1988, 21, 374-379 (65)
A.Z. Rubezhov, Metalloorg. Khim., 1991, 4, 249-291 (159)
C4. Cyclopentadienyls, arena [see also: E3.5, E3.11, E3.15, E4.3, E4.41 C4.1 Arene clusters D. Braga, P.J. Dyson, F. Grepioni and B.F.G. Johnson, Chem. Rev., 1994, 94, 1585-1620 (112) Ruthenium clusters containing the [2.2]paracyclophane C4.2 P.J. Dyson, B.F.G. Johnson and C.M. Martin, J. Cluster Sci., 1995, 6, ligand: recent developments in arene-cluster chemistry 2 1-37 (25) Coordination of benzene in clusters: the face-capping B.F.G. Johnson, J. Lewis, C. Housecroft, M. Gallup, M. Martinelli, C4.3 mode D. Braga and F. Grepioni, J. Mol. Catul., 1992, 74, 61-72 (25) Benzene in p3-qz: q 2 : q2 face-capping coordination B.F.G. Johnson and P.J. Dyson, Transition Met. Chem., 1993, 18, C4.4 mode 539-544 (26) C4.5 Arene-cluster compounds B.F.G. Johnson, J. Organomet. Chem., 1994,475, 31-43 (38)
C3. Alkynes [see also: C1.2, E3.2, E3.16, E3.25, F.51 C3.1 Organometallic cluster complexes containing ynamine ligands C3.2 Structure and reactivity of Cz species on polymetallic surfaces C3.3 Metal carbonyl clusters with alkynes bound in “parallel” or “perpendicular” fashion C3.4 Synthesis, structure and reactivity of homo- and heteropolynuclear complexes of platinum bearing C=CR groups as unique bridging ligands Multidentate ligands bound via alkyne and Group 15 or C3.5 16 donor sites Synthesis and reactions of polynuclear cobalt-alkyne C3.6 complexes
C2. I3
C2.12
C2.11
C2.10
2
4
w
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h
s
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-3 (h
%
9 2
0
g
bcr
bl
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Arene cluster compounds
Linked arene clusters
Benzene and its derivatives as bridging ligands in transition metal complexes
C4.6
C4.7
C4.8
B.F.G. Johnson, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 317-333 (26) B.F.G. Johnson, C.M. Martin and P. Schooler, Chem. Commun., 1998, 1239-1246 (30) H. Wadepohl, Angew. Chem., 1992,104,253-268; Angew. Chem., Int. Ed. Engl., 1992, 31,247-262 (107)
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El. Group 6 (Cr, Mo, W) [see also: B7.271 El. 1 Reactivity of dinuclear and tetranuclear clusters of molybdenum and chromium
E. Metal cluster complexes by Periodic Group M.H. Chisholm, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 55-74 (75)
D. High-nuclearity metal clusters [see also: E3.13, E5.13, E6.2, E6.10, E6.11, E6.121 D. 1 High-nuclearity carbonyl metal clusters A. Ceriotti, R. Della Pergola and L. Garlaschelli, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds, Kluwer: Dordrecht, 1994, pp. 41-106 (189) L.J. de Jongh, J.A.O. de Aguiar, H.B. Brom, G. Longoni, J.M. van D.2 Physical properties of high-nuclearity metal cluster Ruitenbeek, G. Schmid, H.A. Smit, M.P.J. van Staveren and R.C. compounds Thiel, Z. Phys. D, 1989, 12, 445-450 (24) D.3 High nuclearity clusters and beyond B.F.G. Johnson, J. Lewis, M. Gallup and M. Martinelli, Faraday Disc., 1991, 92, 241-254 (80) Ligand-stabilised metal clusters: structure, bonding, D.4 K.C.C. Kharas and L.F. Dahl, Adu. Chem. Phys., 1988,70 Pt 2, 1-43 fluxionality, and the metallic state (143) Large transition metal clusters - bridges between D.5 G. Schmid, Aspects Homogen. Catal., 1990, 7, 1-36 (58) homogeneous and heterogeneous catalysis? D.6 Two-, four- and five-shell clusters and colloids G. Schmid, N. Klein, B. Morun and A. Lehnert, Pure Appl. Chem., 1990, 62, 1175-1177 (10) G. Schmid, Polyhedron, 1988, 7, 2321-2329 (20) D.7 Metal clusters and cluster metals
Title
No.
Table 4 (continued)
3'
h,
w
4
e
S.V. Kryutchkov, Topics Curr. Chem., 1996, 176, 189-252 (174) A. Perrin and M. Sergent, New J. Chem., 1988, 12, 337-356 (181)
T.J. Henly, Coord. Chem. Rev., 1989, 93, 269-295 (100)
E3. Group 8 (Fe, Ru, 0s) [see also: B7.20, C4.2, D.3, G3.3, G3.5, H2.4. 54.9, 54.181 R.D. Adams and M. Tasi, J. Cluster Sci., 1990, 1, 249-285 (33) E3.1 Synthesis and reactivity of sulfido-ruthenium carbonyl clusters R.D. Adams, J.E. Babin and H.-S. Kim, Polyhedron, 1988,7,967-978 E3.2 Structure, bonding, and transformation behavior of iminium, aminocarbene, and aminocarbyne ligands in (39) triosmium cluster complexes S.L. Bassner, G.L. Geoffroy and A.L. Rheingold, Polyhedron, 1988,7, Generation and reactivity of ketene ligands on E3.3 791-805 (22) triosmium clusters D. Braga, F. Grepioni, E. Parisini and S. Righi, Muter. Chem. Phys., Dynamic processes in crystals of transition metal E3.4 1991,29, 165-173 (21) clusters [Rug, Osd] D. Braga, F. Grepioni, P.J. Dyson and B.F.G. Johnson, J. Cluster Sci., E3.5 Molecular and crystal structures of ruthenium and 1992,3, 297-31 1 (17) osmium arene clusters M.I. Bruce, Pure Appl. Chem., 1990, 62, 1021-1026 (25) Carbenes, carbides and carbon. Ten years of transition E3.6 metal-acetylene chemistry [Burrows lecture] M.I. Bruce, M.P. Cifuentes and M.G. Humphrey, Polyhedron, 1991, Ruthenium clusters containing N-donor ligands E3.7 10, 277-322 (206) M.I. Bruce, in M. Chanon, M. Julliard and J.C. Poite (Eds.), PuraE3.8 Electron transfer-catalysed substitution reactions of magnetic Organometallic Species in ActivutionlSelectivity,Catalysis metal cluster carbonyls [NATO AS1 Ser., vol. C2571, Kluwer: Dordrecht, 1989, pp. 407-422 (46) J.F. Corrigan, M. Dinardo, S. Doherty and A.J. Carty, J. Cluster Sci., Chemistry on rhomboidal R u faces ~ of E3.9 1992, 3, 313-332 (31) R Q ( C O ) I ~ ( , U - P Rnovel ~ ) ~ : small molecule, ligand and skeletal transformations A.J. Deeming, J. Cluster Sci., 1992, 3, 347-360 (23) E3.10 Bridges between trinuclear metal clusters [of Ru, Os] P.J. Dyson, B.F.G. Johnson and C.M. Martin, Coord. Chem. Rev., E3.11 Synthesis of ruthenium and osmium carbonyl clusters 1996, 155, 69-86 (45) with unsaturated organic rings G.R. Frauenhoff, Coord. Chem. Rev., 1992, 121, 131-154 (86) E3.12 Oxyligand derivatives of triosmium dodecacarbonyl
E2. Group 7 (Mn, Tc, Re) [see also: K.101 Rhenium carbonyl clusters: synthesis, structure, E2.1 reactivity Chemistry of technetium cluster compounds E2.2 Rhenium clusters in inorganic chemistry: structures and E2.3 metal-metal bonding
Substitution reactions of the trimeric dodecacarbonyls M3(C0)12 (M = Fe, Ru, 0s) and their derivatives M3(C0)l2-,Ln (n = 1-3) Arene cluster compounds [of Ru, Os]
E3.14
Synthesis and reactions of edge double-bridged triruthenium and triosmium cluster complexes Reactivity of methylidyne ligands on trinuclear clusters of Group 8 metals Base-promoted ruthenium carbonyl cluster complexes: from fundamental reactions to catalysis Reflections on osmium and ruthenium carbonyl compounds Compounds of iron with increased coordination numbers [much cluster chemistry] Tetranuclear carbonyls of osmium Ligand dynamics and reactivity in trimetallic clusters [of Ru, Os] Hydride mobility and its relation to structure and reactivity in polymetallic clusters
E3.17
Reactions of ruthenium carbonyl clusters with alkynes Butterfly cluster complexes of Group 8 transition metals
Borane-osmium cluster chemistry
E3.25 E3.26
E3.27
E3.24
E3.22 E3.23
E3.21
E3.20
E3.19
E3.18
Reactions of acetylenes with edge double-bridged triruthenium complexes
E3.16
G. Lavigne, N. Lugan, S. Rivomanana, F. Mulla, J.-M. SouliC and P. Kalck, J. Cluster Sci., 1993, 4, 49-58 (28) J. Lewis and P.R. Raithby, J. Organomet. Chem., 1995,500,227-237 (22) A.I. Nekhaev and B.I. Kolobov, Koord. Khim., 1992,18,262-286 (137) [Engl. pp. 227-2481 R.K. Pomeroy, J. Organomet. Chem., 1990,383, 387-411 (81) E. Rosenberg, W. Freeman, Z . Carlos, K. Hardcastle, Y.J. Yoo, L. Milone and R. Gobetto, J. Cluster Sci., 1992, 3, 439-457 (33) E. Rosenberg, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 125-140 (8) E. Sappa, J. Cluster Sci., 1994, 5, 21 1-263 (202) E. Sappa, A. Tiripicchio, A.J. Carty and G.E. Toogood, Prog. Inorg. Chem., 1987,35,437-525 (222) S.G. Shore, Pure Appl. Chem., 1993, 65, 263-272 (22)
J.B. Keister, Polyhedron, 1988, 7,847-858 (62)
B.F.G. Johnson, P.J. Dyson and C.M. Martin, J. Chem. Soc., Dalton Trans., 1996, 2395-2402 (27) H.D. Kaesz, Z . Xue, Y.J. Chen, C.B. Knobler, W. Krone-Schmidt, W.J. Sieber and N.M. Boag, Pure Appl. Chem., 1988,60, 1245-1250 (6) H.D. Kaesz, J. Organomet. Chem., 1990, 383, 413-420 (38)
B.F.G. Johnson, L.H. Gade, J. Lewis and W.T. Wong, Muter. Chem. Phys., 1991, 29, 85-96 (3) B.F.G. Johnson and Y.V. Roberts, J. Cluster Sci., 1993,4,231-244 (31)
High nuclearity clusters and beyond [Os clusters]
E3.13
E3.15
Citation
Title
No.
Table 4 (continued)
Reactions and dynamics of ruthenium clusters
(Cyc1opentadienyl)metalcluster complexes of the Group 9 transition metals Static and dynamic stereochemistry of the organometallic cluster complexes (CpCo)3(p,-arene)
E4.3
The chemistry of Group 10 metal triangulo clusters
Formation of platinum and palladium clusters with carbonyl and phosphine ligands Reactivity and flexibility in platinum metal clusters Excited state properties of the low-valent bi- and trinuclear complexes of palladium and platinum Interstitial nickel carbonyl clusters Structural systematics in nickel carbonyl cluster anions Palladium clusters: stoichiometric and catalytic reactions Synthesis and catalytic activity of carbonyl palladium clusters
E5.3
E5.4
E5.10
E5.7 E5.8 E5.9
E5.5 E5.6
Palladium cluster compounds
E5.2
E5. Group 10 (Ni, Pd, Pt) [see also: B7.14, C3.4, 12.251 E5.1 Nickel carbonyl cluster complexes
E4.4
Intramolecular exchange in d9 metal carbonyl clusters
E4.2
E4. Group 9 (Co, Rh, Ir) [see also: C3.61 E4.1 Polynuclear iridium hydrido complexes
E3.28
G. Longoni, Pure Appl. Chem., 1990, 62, 1183-1186 (34) A.F. Masters and J.T. Meyer, Polyhedron, 1995, 14, 339-365 (52) 1.1. Moiseev, T.A Stromnova and M.N. Vargaftik, J. Mol. Catal., 1994,86, 71-94 (46) 1.1. Moiseev, Pure Appl. Chem., 1989, 61, 1755-1762 (27)
J.K. Beattie, A.F. Masters and J.T. Meyer, Polyhedron, 1995, 14, 829868 (153) A.D. Burrows and D.M.P. Mingos, Trans. Met. Chem., 1993,18, 129148 (145) A.D. Burrows and D.M.P. Mingos, Coord. Chem. Rev., 1996, 154, 1969 (98) N.K. Eremenko and S.P. Gubin, Pure Appl. Chem., 1990,62, 11791182 (8) L.J. Farrugia, J. Cluster Sci., 1992, 3, 361-383 (35) P.D. Harvey, J. Cluster Sci., 1993, 4, 377-402 (89)
T.M. Gomes Carneiro, D. Matt and P. Braunstein, Coord. Chem. Rev., 1989,96,49-88 (61) R. Roulet, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 159-173 (50) H. Wadepohl and S. Gebert, Coord. Chem. Rev., 1995, 143, 535-609 ( 142) H. Wadepohl, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 175-191 (16)
K. Vrieze and C.J. Elsevier, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 95-1 11 (30)
2C
ch
W
4
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2
Lc
5
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%
g
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2
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P
Coordinatively unsaturated tripalladium and triplatinum clusters: models for reactions on metal surfaces Chemistry of carbonylpalladium acetate
Giant palladium clusters: synthesis and characterisation
E5.11
E5.13
R.J. Puddephatt, L. Manojlovic-Muir and K.W. Muir, Polyhedron, 1990,9, 2767-2802 (100) T.A. Stromnova, I.N. Busygina, M.N. Vargaftik and 1.1. Moiseev, Metalloorg. Khim., 1990, 3, 803-813 (27) M.N. Vargaftik, 1.1. Moiseev, D.I. Kochubey and K.I. Zamarev, Faraday Disc., 1991, 92, 13--29 (24)
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E6. Group 11 (Cu, Ag, Au) [see also: F.12, F.15, F.16, F.18, H1.5, 12.11 E6.1 Recent developments in homo- and heteronuclear clusD.M.P. Mingos and M.J. Watson, Transition Met. Chem., 1991, 16, ter compounds of gold 185-187 (37) E6.2 D.M.P. Mingos, J. Cluster Sci., 1992, 3, 397-409 (27) High-nuclearity clusters of the transition metals and a re-evaluation of the cluster-surface analogy E6.3 Recent developments in the cluster chemistry of gold D.M.P. Mingos, in A.J. Welch and S.K. Chapman (eds.), Chemistry of the Copper and Zinc Triads, Roy. SOC.Chem.: Cambridge, 1993, pp. 189-197 (27) E6.4 D.M.P. Mingos, J. Chem. SOC.,Dalton Trans., 1996, 561-566 (45) Gold - a flexible friend in cluster chemistry E6.5 Database analysis of Au. . .Au interactions [693 examS.S. Pathaneni and G.R. Desiraju, J. Chem. SOC.,Dalton Trans., 1993, 319-322 (29) ples] I.D. Salter, Adv. Dynamic Stereochem., 1988, 2, 57-110 (84) E6.6 Skeletal rearrangements in cluster compounds containing the Group IB metals E6.7 H. Schmidbaur, Pure Appl. Chem., 1993,65, 691-698 (39) Some new concepts in the chemistry of the p-block elements [poly-goldclusters] E6.8 High-carat gold compounds [Ludwig Mond lecture] H. Schmidbaur, Chem. SOC.Rev., 1995,24, 391-400 (36) E6.9 Synthesis and structures of copper and silver cluster K. Tang and Y. Tang, Heteroat. Chem., 1990, 2, 345-370 (38) complexes with sulfur-containingligands E6.10 Clusters of clusters: self-organisation and self-similarity B.K. Teo and H. Zhang, Proc. Nut. Acad. Sci. USA, 1991,88, 5067in the intermediate stages of cluster growth of gold5071 (46) silver supraclusters E6.11 Clusters of clusters: coining coinage metal clusters B.K. Teo, H. Zhang and X. Shi, in A.J. Welch and S.K. Chapman (eds.), Chemistry of the Copper and Zinc Triads, Roy. SOC.Chem.: Cambridge, 1993, pp. 211-234 (108)
E5.12
Title
No.
Table 4 (continued)
Polyicosahedricity: icosahedron to icosahedra of icosahedra growth pathway for bimetallic (Au-Ag) and trimetallic (Au-Ag-M; M = Pt, Pd, Ni) supraclusters; synthetic strategies, site preferences and stereochemical principles
Heteronuclear clusters containing platinum and the metals of the iron, cobalt and nickel triads Structural variations in tetranuclear platinum-ruthenium clusters
F.7
F.ll
F.10
F.9
F.8
F.6
Synthesis and characterisation of heterometallic carbony1 cluster anions Mercury, a structural building block and source of localised reactivity in extended metal-metal bonded systems Heteropolymetallic clusters
Chemistry of heterometallic clusters prepared by condensation of Group 6 metal acetylides and Group 8 binary carbonyl complexes Unusual ligand transformations and rearrangements in heterometallic clusters
F.5
F.4
F.3
Interplay between bridging groups and metal-metal bonds in heterometallic clusters Metal-metal and metal-ligand interactions in heterometallic complexes Heterometallic clusters in catalysis
F.2
F. Heterometallic clusters [see also: C2.13, E6.12, H3.71 F. 1 NMR studies on the dynamic behaviour in solution of rhenium-platinum mixed metal clusters containing P-donor ligands
E6.12
S.P. Gubin, Koord. Khim., 1994, 20, 403-428 (79) [Engl. pp. 379-4031
L.H. Gade, Angew. Chem., 1993,105, 25-39; Angew. Chem., Int. Ed. Engl., 1993, 32, 24-40 (106)
L.J. Farrugia, D. Ellis and A.M. Senior, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 141-157 (31) A. Fumagalli, Mater. Chem. Phys., 1991, 29, 211-230 (28)
Y. Chi, S.-J. Chiang and C.-J. Su, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 113-124 (26) L.J. Farrugia, Adv. Organomet. Chem., 1990, 31, 301-391 (220)
P. Braunstein and J. Rose, Stereochem. Organomet. Inorg. Compounds, 1988,3, 3-138 (320) Y. Chi, J. Chin. Chem. SOC.,1992, 39, 591-601 (49)
P. Braunstein, New J. Chem., 1994, 18, 51-60 (20)
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B.K. Teo and H. Zhang, Coord. Chem. Rev., 1995, 143,611-636 (34)
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P.C. Ford, J. Organomet. Chem., 1990, 383, 339-356 (42) Yu.N. Kukushkin and N.S. Panina, Zh. neorg. Khim., 1991, 36, 355374 (63) [Engl. pp 197-2081 N. Leadbeater, J. Chem. SOC.,Dalton Trans., 1995, 2923-2934 (50)
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3
B -. 5-
R.J.H Clark, Chem. Soc. Rev., 1990, 19, 107-131 (103)
b
P
M.H. Chisholm, Pure Appl. Chem., 1991,63, 665-680 (69)
G4. Chemistry of metal clusters [see also: E3.14, E3.17, E3.28, F.61 (34.1 Associative reactions of metal carbonyl clusters: L. Chen and A.J. Poe, Coord. Chem. Rev., 1995, 143, 265-295 (62) systematic kinetic studies of some ruthenium and other clusters
G3. Reactions of metal-metal bonds in metal clusters [see also: E3.81 Developing the reactivity of multiple bonds between G3.1 metal atoms: inorganic functional groups Synthesis, structure and spectrometry of metal-metal G3.2 dimers, linear chains and dimer chains [Nyholm Lecture] (33.3 Mechanistic studies of photoreactions of M3(CO)l2 G3.4 Reactions of complexes with rupture of metal-metal bonds Generation and reactivity of versatile ruthenium G3.5 carbonyl organometallic intermediates by cluster photochemistry
G2. Dynamic processes in metal clusters [see also: E3.4, E3.23, E3.24, E3.28, E4.2, E6.6, F.l, 12.351 Dynamical processes in crystalline organometallic D. Braga, Chem. Rev., 1992, 92, 633-665 (207) G2.1 compounds [including clusters] Dynamics and fluxionality in metal carbonyl clusters: G2.2 L.J. Farrugia, J. Chem. Soc., Dalton Trans., 1997, 1783-1792 (83) some old and new problems Cluster equilibria. Relevance to the energetics and (32.3 T.P. Fehlner, in L.J. Farrugia (ed.), Synergy between Dynamics and reactivity of surface-bound fragments Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, Kluwer: Dordrecht, 1995, pp. 75-94 (41) G2.4 Polyhedral rearrangements and fragmentation reactions B.F.G. Johnson and A. Rodgers, in D.F. Shriver, H.D. Kaesz and in cluster complexes R.D. Adams (eds.), Chemistry of Metal Cluster Complexes,VCH: New York, 1990, pp. 303-327 (36) G2.5 Mechanistic features of carbonyl cluster rearrangements B.F.G. Johnson, A. Bott, R.E. Benfield, D. Braga, E.A. Marseglia and A. Rodger, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 141-160 (13) G2.6 Ligand dynamics and reactivity in trimetallic clusters E. Rosenberg, W. Freeman, Z. Carlos, K. Hardcastle, Y.J. Yoo, L. Milone and R. Gobetto, J. Cluster Sci., 1992, 3, 439-457 (33)
Systematic kinetics of associative reactions of metal carbonyls [including clusters] Chemistry and reactivity of metal cluster carbonyl radical anions
G4.4
H1.4
H1.3
Electron microscopy of transition metal carbonyl clusters Magnetic properties of complexes with short metalmetal bonds
H1. Physical properties of metal clusters [see also: 12.261 Magnetic properties and UV-visible spectroscopic H1.l studies of metal cluster compounds Specific heat studies on metal cluster compounds H 1.2
H. Physical properties and spectroscopy of metal clusters
G4.6 G4.7
New interconversions in the ligand sphere of clusters Ligand sphere reactivity of organometallic clusters
Cluster-assisted ligand transformations
G4.3
G4.5
Ligand substitution reactions
~~
Title
G4.2
~~
No.
Table 4 (continued)
R.E. Benfield, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds,Kluwer: Dordrecht, 1994, pp. 249-275 (99) H.B. Brom, J. Baak and L.J. de Jongh, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds,Kluwer: Dordrecht, 1994, pp. 21 1-226 (25) R.W. Devenish, P.J. Goodhew, B.T. Heaton, C. Jacobs and S. Mulley, Sci. Prog. (Oxford), 1990, 74, 513-528 (29) O.G. Ellert, I.L. Eremenko, Y.V. Rakitin, A.A. Pasynskii and V.T. Kalinnikov, Zh. neorg. Khim., 1991, 36, 2255-2265 (21) [Engl. pp. 1272-12771
B.H. Robinson and J. Simpson, in M. Chanon, M. Julliard and J.C. Poite ( Eds.), Paramagnetic Organometallic Species in ActivationJSelectiuity, Catalysis [NATO AS1 Ser., vol. C2571, Kluwer: Dordrecht, 1989, pp. 357-374 (62) H. Vahrenkamp, Pure Appl. Chem., 1989,61, 1777-1782 (15) H. Vahrenkamp, Pure Appl. Chem., 1991,63, 643-649 (28)
D.J. Darensbourg, in D.F. Shriver, H.D. Kaesz and R.D. A d a m (eds.), Chemistry of Metal Cluster Complexes, VCH: New York, 1990, pp. 171-200 (93) G. Lavigne, in D.F. Shriver, H.D. Kaesz and R.D. Adams (eds.), Chemistry of Metal Cluster Complexes, VCH: New York, 1990, pp. 201-302 (323) A.J. Po&,Pure Appl. Chem., 1988,60, 1209-1216 (33)
Citation
u
v3
v3
‘2 I.
22 00
‘2.
9’
Photochemical and photophysical properties of tetranuclear and hexanuclear clusters of metals with d'' and s2 electronic configurations [Cu, Tl] Magnetic properties of metal cluster compounds
Structural, bonding and mechanistic rearrangement information on transition metal carbonyl clusters from multinuclear magnetic resonance studies Electron spin resonance studies of organotransition metal reactive intermediates [Co, Fe, Ru clusters]
X-ray absorption edge studies of the electronic structures of metal catalysts Application of Mossbauer effect spectroscopy to cluster research
X-ray photoelectron spectroscopy applied to pure and supported molecular metal clusters
H2.3
H2.5
H2.7
H3. Electrochemistry of metal clusters H3.1 Progress in cluster electrochemistry H3.2 Importance of paramagnetic organometallic species in cluster electrochemistry
H2.6
H2.4
Solid-state carbon-13 NMR of metal carbonyls
H2.2
of metal clusters H2. Spectroscopic . properties . _ H2.1 NMR in sub-micron particles
HI .6
H1.5
P. Lemoine, Coord. Chem. Rev., 1988,83, 169-197 (146) P. Lemoine, in M. Chanon, M. Julliard and J.C. Poite (Eds.), Paramagnetic Organometallic Species in ActivationJSelectivity,Catalysis [NATO AS1 Ser., vol. C2571, Kluwer: Dordrecht, 1989, pp. 391-405 ( 184)
R.C. Thiel, H.H. Smit and L.J. de Jongh, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds, Kluwer: Dordrecht, 1994, pp. 183-209 (69) R. Zanoni, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds, Kluwer: Dordrecht, 1994, pp. 159-182 (69)
P. Rieger, in M. Chanon, M. Julliard and J.C. Poite (Eds.), Paramagnetic Organometallic Species in ActivationlSelectivity, Catalysis [NATO AS1 Ser., vol. C2571, Kluwer: Dordrecht, 1989, pp. 375-389 (96) J.H. Sinfelt and G.D. Meitzner, Acc. Chem. Rex, 1993, 26, 1-6 (70)
H.B. Brom, D. van der Putten and L.J. de Jongh, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds, Kluwer: Dordrecht, 1994, pp. 227-247 (43) B.E. Hanson, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 231-247 (54) B.T. Heaton, Pure Appl. Chem., 1988, 60, 1757-1761 (28)
J.M. van Ruitenbeek, D.A. van Leeuwen and L.J. de Jongh, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds, Kluwer: Dordrecht, 1994, pp. 277-306 (45)
P.C. Ford and A. Vogler, Acc. Chem. Rex, 1993, 26, 220-226 (74)
Some aspects of organometallic cluster electrochemistry Some old and new redox reactions of polynuclear organometallic complexes
Stereochemical aspects associated with redox behaviour of heterometallic carbonyl clusters Stereochemical aspects of the redox propensity of homometal carbonyl clusters Electrochemistry of metal-sulfur clusters: stereochemical consequences of thermodynamically characterised redox changes. I. Homometal clusters. 11. Heterometal clusters
H3.3 H3.4
H3.5
P. Zanello, in P. Zanello (ed.), Stereochemistry of Organometallic and Inorganic Compounds, 1994, 5, 163-407 (403) P. Zanello, Coord. Chem. Reo., 1988, 83, 199-275 (162); 87, 1-54 (98)
D. Osella, Muter. Chem. Phys., 1991, 29, 117-131 (29) H Vahrenkamp, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, Kluwer: Dordrecht, 1995, pp. 297-316 (37) P. Zanello, Struct. Bond., 1992, 79, 101-214 (201)
Citation
11. Molecular structures of metal cluster complexes [see also: E3.5, E6.51 11.1 Structural variability in metal cluster compounds V.G. Albano and D. Braga, in A. Domenicano and I. Hargittai (eds.), Accurate Molecular Structures - Their Determination and Zmportance, Oxford Univ. Press: Oxford, 1992, pp. 530-553 (54) 11.2 Metal-ligand distances in n-complexes A.S. Batsanov and Yu.T. Struchkov, Metalloorg. Khim., 1992, 5, 53 D. Braga and F. Grepioni, Acc. Chem. Res., 1994, 27, 51-56 (34) 11.3 From molecules to molecular aggregation: clusters and crystals of clusters 11.4 D. Braga and F. Grepioni, Acc. Chem. Rex, 1997, 30, 81-87 (53) Hydrogen-bonding interactions with the CO ligand in the solid state 11.5 Ligand polyhedral model and its application to the B.F.G. Johnson and Y.V. Roberts, Polyhedron, 1993, 12,977-990 (47) binary carbonyls R.B. King, Znorg. Chim. Acta, 1992, 198-200, 841-861 (105) 11.6 The icosahedron in inorganic chemistry R.B. King, J. Cluster Chem., 1995, 6, 5-20 (37) 11.7 Metal triangles as building blocks in metal cluster chemistry Z. Lin and I.D. Williams, Polyhedron, 1996, 15, 3277-3287 (88) 11.8 Structure and bonding in face- and edge-bridged octahedral transition metal clusters
I. Structure and bonding in metal clusters
H3.7
H3.6
Title
No.
Table 4 (continued)
9"
Q
R
-4
i
Structural systematics in molecular inorganic chemistry [including clusters] Structural chemistry of organometallic compounds: 7~ complexes and clusters of transition metals. Geometric criteria of closed electron configurations [Results from X-ray structures at INEOS over preceding five years]
11.10
Yu.T. Struchkov, A.S. Batsanov and Yu.L. Slovokhotov, Sou. Sci. Rev., B, Chem., 1987, 10, 385-442 (144)
D.M.P. Mingos and A.S. May, in D.F. Shriver, H.D. Kaesz and R.D. Adams (eds.), Chemistry of Metal Cluster Complexes, VCH: New York, 1990, pp. 11-119 (181) A.G. Orpen, Chem. Soc. Rev., 1993, 22, 191- 197 (26)
12. Electronic structures of metal cluster complexes [see also: 11.8: 11.91 12.1 Electronic structures of clusters [Cu, with CO, CN P.S. Bagus, in G. Benedek, T.P. Martin and G. Pacchioni (eds.), ligands] Elemental and Molecular Clusters, Springer: Berlin, 1988, pp. 286 306 (72) 12.2 D.E. Ellis, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Theory of electronic properties of metal clusters and particles Cluster Compounds,Kluwer: Dordrecht, 1994, pp. 135-157 (59) 12.3 Development of the concept of the chemical bond from N.P. Gambdryan and I.V. Stankevich, Usp. Khim., 1989, 58, 1945hydrogen to cluster compounds 1970; Russ. Chem. Rev., 1989, 58, 1103-1118 (136) 12.4 Limitations of the polyhedral skeletal electron pair J.-F. Halet, Coord. Chem. Rev., 1995, 143, 637-678 (170) theory in organometallic cluster chemistry: examples in tri- and tetrametallic systems 12.5 Electron-count versus structural arrangement in clusters J.-F. Halet and J.-Y. Saillard, Struct. Bond., 1997, 87, 81-109 (67) based on a cubic transition metal core with bridging main group elements 12.6 M.B. Hall, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Electronic structure of metal dimers and metal clusters: the 18-electron rule versus skeletal electron-pair Chemistry and Catalysis, Plenum: New York, 1990, pp. 265-274 (12) counting 12.7 T. Hughbanks, Prog. Solid State Chem., 1989, 19, 329-372 (148) Bonding in clusters and condensed cluster compounds that extend in one, two and three dimensions 12.8 B.F.G. Johnson and A. Rodger, Inorg. Chim. Acta, 1988, 145, 71-75 Principles of bonding and reactivity in transition metal cluster compounds (18) R.L. Johnston, Struct. Bond., 1997, 87, 1-34 (108) 12.9 Mathematical cluster chemistry 12.10 R.B. King, Isr. J Chem., 1990, 30, 315-325 (75) Graph-theory derived models for the skeletal chemical bonding in organometallic metal carbonyl clusters
11.1 1
Structural and bonding aspects of metal cluster chemistry
11.9
Mathematical inorganic chemistry: from gas-phase metal clusters to superconducting solids Mathematical methods in coordination chemistry: topological and graph theoretical ideas in the study of metal clusters and polyhedral isomerisation Topological and geometrical aspects of icosahedral structural units Metal-metal interactions in transition metal clusters with donor ligands Theoretical models of cluster bonding
Bonding models for ligated and bare clusters
Theoretical aspects of metal cluster chemistry Moments of inertia in cluster and coordination chemistry Bonding connections and inter-relationships
Molecular orbital analysis of main group clusters with interstitial transition metal atoms Catalytic reactions of transition metal clusters and surfaces from ab initio theory
Density-functional theory of spin polarisation and spin coupling in iron-sulfur clusters Stereochemical aspects of organometallic clusters. A view of the polyhedral skeletal electron pair theory
12.11
12.15
12.16
12.17 12.18
12.20
12.22
12.23
12.21
12.19
12.14
12.13
12.12
Title
No.
Table 4 (continued)
D.M.P. Mingos and R.L. Johnston, Struct. Bond., 1987, 68, 29-87 (183) D.M.P. Mingos, T. Slee and L. Zhenyang, Chem. Rev., 1990,90, 383402 (130) D.M.P. Mingos, Pure Appl. Chem., 1991, 63, 807-812 (40) D.M.P. Mingos, J.E. McGrady and A.L. Rohl, Struct. Bond., 1992, 79, 1-54 (72) D.M.P. Mingos, in T.P. Fehlner (ed.), Inorganometallic Chemistry, Plenum: New York, 1992, pp. 179-221 (43) D.M.P. Mingos, S. Weisberger and S. Heeb, New J. Chem., 1993, 17, 531 H. Nakatsuki, H. Nakai and M. Hada, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Interactions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992, pp. 251-285 (50) L. Noodleman and D.A. Case, Adv. Inorg. Chem., 1992,38, 423-470 (122) D. Osella and P.R. Raithby, Stereochem. Organomet. Inorg. Compounds, 1988,3, 303-362 (109)
R.B. King, in G.W. Kabalka (ed.), Current Topics in the Chemistry of Boron, RSC: Cambridge, 1994, pp. 379-382 (9) Z . Lin and M.-F. Fan, Struct. Bond., 1997, 87, 35-80 (246)
R.B. King, Coord. Chem. Rev., 1993,122,91-107 (40)
R.B. King, Acc. Chem. Rex, 1992,25,247-253 (86)
Citation
Electronic structures and reactivities of metal cluster complexes Isoelectronic relationships in the chemistry of multinuclear complexes Quantum chemical models of chemisorption on metal surfaces
12.29
12.36
12.35
12.34
12.33
12.32
12.31
12.30
Three-dimensional Huckel molecular orbirtal correlation diagrams for polyhedral rearrangements General energy decomposition scheme for study of metal-ligand interactions in complexes, clusters and solids
Clusters in inorganic and molecular beam chemistry. Some unifying principles What can calculations employing empirical potentials teach us about bare transition metal clusters? Electronic structure theory for transition metal systems
Electronic structures of metal clusters and cluster compounds Density functional model calculations for homogeneous and heterogeneous catalysis
12.27
12.28
Electron counting in clusters: a view of the concepts Carbonylated nickel clusters: from molecules to metals Molecular metal clusters: structures and bonding
12.24 12.25 12.26
T. Ziegler, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Interactions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992, pp. 367-396 (51)
(41) D.J. Wales, D.M.P. Mingos, T. Slee and L. Zhenyang, Acc. Chem. Res., 1990, 23, 17-22 (33) D.J. Wales, L.J. Munro and J.P.K. Doye, J. Chem. Soc., Dalton Trans., 1996, 611-623 (90) M.C. Zerner, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Interactions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C378], Kluwer: Dordrecht, 1992, pp. 101-123 (63) M. Zhao and B.M. Gimarc, Polyhedron, 1995, 14, 1315-1325 (55)
U. Wahlgren and P. Siegbahn, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Interactions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992, pp. 199-249
H. Vahrenkamp, J. Organornet. Chem., 1990,400, 107-120 (48)
S.M. Owen, Polyhedron, 1988, 7, 253-283 (202) G. Pacchioni and N. Rosch, Ace. Chem. Rex, 1995, 28, 390-397 (25) G. Pacchioni, in G. Benedek, T.P. Martin and G. Pacchioni (eds.), Elemental and Molecular Clusters, Springer: Berlin, 1988, pp. 364--376 (43) N. Rosch and G. Pacchioni, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 5-88 (324) N. Russo, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Znteractions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992, pp. 341-366 (109) W.C. Trogler, Ace. Chem. Res., 1990, 23, 239-246 (45)
5
LI~
$
9. rp
e2
5
2
Q
0
2
6
bJ
b
9
Title
Ligand stabilised metal clusters and colloids: properties and applications Colloidal metals: past, present and future
51.10
G. Schmid, in L.J. de Jongh (ed.), Physics and Chemistry of Metal Cluster Compounds, Kluwer: Dordrecht, 1994, pp. 107-134 (55) G. Schmid, V. Maihack, F. Lantermann and S . Peschel, J. Chem. Soc., Dalton Trans., 1996, 589-595 (18) J.M. Thomas, Pure Appl. Chem., 1988,60, 1517-1528 (52)
G. Schmid, Chem. Rev., 1992, 92, 1709--1727 (96)
G. Schmid, Muter. Chem. Phys., 1991, 29, 133-142 (23)
L.N. Lewis, Chem. Rev., 1993,93, 2693-2730 (402) G. Schmid, Endeavour, 1990, 14, 172-178 (12)
H. Bonnemann, G. Braun, W. Brijoux, R. B r i n h a n n , A.S. Tilling, K. Seevogel and K. Siepen, J. Organomet. Chem., 1996,520, 143-162 (121) J.S. Bradley, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 459-544 (293) J.S. Bradley, J.M. Millar, E.W. Hill, S. Behal, B. Chaudret and A. Duteil, Faraday Discuss., 1991, 92, 225-268 (20) A. Henglein, Chem. Rev., 1989, 89, 1871-1873 (44)
Citation
52. The cluster-surface analogy: surface organometallicchemistry [see also: 12.21, 12.311 J2.1 From clusters and surfaces to clusters on surfaces: an J.-M. Basset, J.P. Candy, A. Choplin, C. Nedez, F. Qignard, C.C. Santini and A. Theolier, Muter. Chem. Phys., 1991, 29, 5-32 (45) opening toward surface organometallic chemistry
51.11
J1.9
51.8
51.7
J1.5 51.6
51.4
Surface chemistry on colloidal metals: spectroscopic study of adsorption of small molecules Small-particle research: physicochemical properties of extremely small colloidal metal and semi-conductor particles Chemical catalysis by colloids and clusters Clusters and colloids: bridges between molecular and condensed material Clusters and colloids - bridges between molecular and condensed materials Large clusters and colloids. Metals in the embryonic state Ligand-stabilised giant metal clusters and colloids
51.3
J1. Clusters and colloids [see also: 12.21 J1.l Nanoscale colloidal metals and alloys stabilised by solvents and surfactants. Preparation and use as catalyst precursors J1.2 Chemistry of transition metal colloids
J. Clusters and colloids, surfaces; catalysis
No.
Table 4 (continued)
52.14
52.13
52.12
52.1 1
52.10
J2.9
52.8
52.7
52.6
52.5
A surface organometallic approach to the process of formation of bimetallic particles from bimetallic supported molecular clusters Cubane-type clusters as potential models for inorganic solid surfaces Surface catalytic reactions assisted by gas-phase molecules [ 0x0 surfaces] Molecular surface chemistry: reactions of gas-phase metal clusters An atomic view of diffusion on metal surfaces
Restructuring and self-organisation in reactions on metal surfaces Extending the metal cluster-metal surface analogy [commentary] Dynamics of the desorption of carbon monoxide from size-selected supported platinum clusters
Surface organometallic chemistry on metals: a novel and effective route to custom-designed bimetallic catalysts Surface organometallic chemistry: new perspectives for the synthesis of metal carbonyl clusters NMR investigations of binding of aromatics at catalytic surfaces
52.4
52.3
Mimicking aspects of heterogeneous catalysis: generating, isolating and reacting proposed surface intermediates on single crystals in vacuum Can we put the cluster-surface analogy on a sound structural basis?
52.2
A. Kaldor, D.M. Cox and M.R. Zakin, Adu. Chem. Phys., 1988,70 Pt 2, 211-261 (135) G.L. Kellogg, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 21-35 (36)
Y. Iwasawa, Acc. Chcm. Res., 1997,30, 103-109 (23)
K. Isobe and A. Yagasaki, Acc. Chem. Rex, 1993, 26, 524-529 (49)
B.C. Gates, Angew. Chem., 1993, 105, 240-241; Angew. Chem., Int. Ed. Engl., 1993, 32. 228-229 (15) U. Heiz, R. Sherwood, D.M. Cox and A. Kaldor, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 37- 47 (27) L. Huang, A. Choplin, 5.-M. Basset, U. Siriwardane, S.G. Shore and R. Mathieu, J. Mol. Catal., 1989, 56, 1-19 (26)
C. Dossi, A. Fusi, R. Psaro, D. Roberto and R. Ugo, Mater. Chem. Phys., 1991, 29, 191-199 (29) C. Dybowski and M.A. Hepp, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., VOI. C4651, 1995, pp. 49-61 (33) G. Ertl, J. Mol. Catal., 1992, 74, 1-9 (32)
A.M. Bradshaw, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, 1995, pp. 1-20 (64) 5.-P. Candy, B. Didillon, E.L. Smith, T.B. Shay and J.-M. Basset, J. Mol. Catal., 1994, 86, 179-204 (62)
B.E. Bent, Chem. Rev., 1996,96, 1361-1390 (480)
Ligands on clusters adsorbates on surfaces Monolayer surface structure analysis
Modern surface science and surface technologies: an introduction Surface coordination chemistry of monometallic and bimetallic electrocatalysts Model studies of the desulfurisation reactions on metal surfaces and in organometallic complexes An organometallic guide to the chemistry of hydrocarbon moieties on transition metal surfaces
52.20 52.21
52.22
53. Supported clusters: surfaces, zeolites Surface organometallic chemistry on oxides, on zeolites 53.1 and on metals Supported metal catalysts: some unsolved problems 53.2 Techniques for the electronic and structural investiga53.3 tion of copper clusters on graphite
52.25
52.24
52.23
52.19
52.18
-
Clusters, alloys and poisoning. An overview Molecular organometallic chemistry on surfaces: reactivity of metal carbonyls on metal oxides Mechanisms of skeletal rearrangements of hydrocarbons on metals: elementary steps Surface-bound metal hydrocarbyls. Organometallic connections between heterogeneous and homogeneous catalysis Ligands on clusters - adsorbates on surfaces
52.15 52.16
52.17
Title
No.
Table 4 (continued)
5.-M. Basset, J.P. Candy, A. Choplin, M. Leconte and A. ThColier, Aspects Homogen. Catal., 1990, 7, 85-1 15 (46) G.C. Bond, Chem. Soc. Rev., 1991,20,441-475 (150) M. De Crescenzi, M. Diociaiuti, L. Lozzi, P. Picozzi and S. Santicci, in G. Benedek, T.P. Martin and G. Pacchioni (eds.), Elemental and Molecular Clusters, Springer: Berlin, 1988, pp. 96-104 (19)
F. Zaera, Chem. Rev., 1995, 95, 2651-2693 (899)
B.C. Wiegand and C.M. Friend, Chem. Rev., 1992,92,491-504 (140)
M.P. Soriaga, Chem. Rev., 1990, 90, 771-793 (85)
M. Moskovits, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Interactions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992, pp. 1-15 (32) M. Moskovits, J. Mol. Catul., 1993, 82, 195-209 (32) G.A. Somorjai and U. Starke, Pure Appl. Chem., 1992,64, 509-527 (77) G.A. Somorjai, Chem. Rev., 1996,96, 1223-1235 (50)
T.J. Marks, Acc. Chem. Rex, 1992, 25, 57-65 (45)
K.J. Klabunde and Y.-X. Li, ACS Symp. Ser., 1993,517, 88-108 (97) H.H. Lamb, B.C. Gates and H. Knozinger, Angew. Chem., 1988, 100, 1162-1 179; Angew. Chem., Int. Ed. Engl., 1988, 27, 1127-1 144 (71) G. Maire and F. Garin, J. Mol. Catal., 1988, 48, 99-116 (83)
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Supported metal clusters: synthesis, structures and catalysis Supported metals and supported organometallics From molecular carbonyl clusters to supported metal particles: synthesis, characterisation, catalysis
Preparation of silica- and alumina-supported clusters of platinum group metals Clusters in cages
Chemical and photochemical reactions of metal carbony1 cluster compounds on solid surfaces [analysed by EXAFS] Intrazeolite organometallics and coordination complexes: internal vs. external containment of metal guests Solid-state NMR studies of supported organometallics [including clusters] Transition metal clusters and isolated atoms in zeolite cages Metal clusters in zeolites: an intriguing class of catalysts Zeolite-supported transition metal catalysts Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides Silver clusters and chemistry in zeolites Structure and reactivity of surface species obtained by interaction of organometallic compounds with oxidic surfaces: infra-red studies
53.5
53.8
53.10
54. Metal clusters in catalysis [see also: D.5, F.4, 12.281 54.1 Clusters and their implications for catalysis
53.17 53.18
53.14 53.15 53.16
53.13
53.12
53.11
53.9
53.6 53.7
Metal clusters and supported metal catalysts
53.4
R.D. Adams, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 75-91 (24)
T. Sun and K. Seff, Chem. Rev., 1995,95, 857-870 (127) A. Zecchina and C.O. Arean, Catal. Rev. Sci. Eng., 1993,35, 261-317 (235)
W.M.H. Sachtler, Acc. Chem. Rex, 1993, 26, 383-387 (39) W.M.H. Sachtler and Z. Zhang, Ado. Catal., 1993, 39, 129-220 (374) S.L. Scott and 5.-M. Basset, J. Mol. Catal., 1994, 86, 5-22 (82)
W.M.H. Sachtler, Springer Ser. Surf: Sci., 1990, 22, 69-85 (85)
L. Reven, J. Mol. Catal., 1994, 86, 447-477 (65)
G.A. Ozin and C. Gil, Chem. Rev., 1989, 89, 1749-1764 (74)
B.C. Gates and H.H. Lamb. J. Mol. Catal., 1989, 52, 1-18 (58) R. Giordano, E. Sappa and G. Predieri, in L.J. Farrugia (ed.), Synergy between Dynamics and Reactivity at Clusters and Surfaces [NATO AS1 Ser., vol. C4651, Kluwer: Dordrecht, 1995, pp. 63-73 (19) R.D. Gonzalez and H. Miura, Catal. Rev. Sci. Eng., 1994,36, 145-177 (88) S. Kawi and B.C. Gates, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 299-372 (248) H. Kuroda, Pure Appl. Chem., 1992,64, 1449-1460 (31)
B.C. Gates, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 127-140 (40) B.C. Gates, Chem. Rev., 1995,95, 511-522 (97)
4
$
-
00 Q
\o
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5
9. rp
2
$-
9
5 F’
B
b
P
Cluster complexes as homogeneous catalysts and catalyst precursors
Catalysis - where science and industry meet Concepts in heterogeneous catalysis
Homogeneous catalysis by transition metal-oxygen anion clusters Metal cluster compounds as molecular precursors for tailored metal catalysts Molecular clusters as models of metallic catalysts
Hydroformylation catalysed by ruthenium complexes [includes clusters] Metal clusters and particles as catalyst precursors and catalysts
54.3
54.4 54.5
54.6
54.9
54.13
54.11 54.12
54.10
54.8
Homogeneous catalysis of the water gas-shift reaction Metal carbonyl catalysts of the synthesis of organic compounds from carbon monoxide and molecular hydrogen Catalytic applications of transition metal complexes with sulfide ligands
Uses of metal clusters in homogeneous and heterogeneous catalysis
54.2
J4.7
Title
No.
Table 4 (continued)
M. Rakowski DuBois, Chem. Rev., 1989, 89, 1-9 (93)
B.F.G. Johnson, M. Gallup and Y.V. Roberts, J. Mol. Catal., 1994, 86, 51-69 (78) P. Kalck, Y. Peres and J. Jenck, Adv. Organomet. Chem., 1991, 32, 121-146 (70) H. Knozinger, in G. Pacchioni, P.S. Bagus and F. Parmigiami (eds.), Cluster Modelsfor Surface and Bulk Phenomena [NATO AS1 Ser., vol. B2831, Plenum: New York, 1992, pp. 131-149 (45) R.M. Laine and E.J. Crawford, J. Mol. Catal., 1988, 44, 357-387 (87) A.L. Lapidus and M.M. Savel'ev, Usp. Khim., 1988, 57, 29-49; Run. Chem. Rev., 1988, 57, 17-28 (165)
D.J. Darensbourg, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 41-54 (34) W.L. Gladfelter and K.J. Roesselet, in D.F. Shriver, H.D. Kaesz and R.D. Adams (eds.), Chemistry of Metal Cluster Complexes, VCH: New York, 1990, pp. 329-365 (112) 5. Haber, Pure Appl. Chem., 1994, 66, 1597 G.L. Haller and R.S. Weber, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Interactions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992, pp. 71-100 (93) C.L. Hill and C.M. Prosser-McArtha, Coord. Chem. Rev., 1995, 143, 407-455 (213) M. Ichikawa, Ado. Catal., 1992, 38, 283-400 (244)
Citation
9
d
4 ul 0
e
-
New catalysts reactions through transition metal clusters Novel catalytic applications of ruthenium clusters
Some relationships between metal cluster chemistry and heterogeneous catalysis Molecular aspects of catalytic reactivity. Application of EPR spectrometry to studies of the mechanism of heterogeneous catalytic reactions Transition metal clusters in homogeneous catalysis
Bonding patterns in intermetallic compounds
K.8
K.7
Non-molecular metal chalcogenide/halide solids and their molecular cluster analogues Metal clusters in the solid state
K.6
K. Solid-state clusters Chemical clusters from solid state systems at high K. 1 temperatures. Interstitials as a means to stability and versatility Coordination chemistry in the solid state: cluster and K.2 condensed cluster halides of the early transition metals Diverse solid-state clusters with strong metal-metal K.3 bonding. In praise of synthesis Diverse naked clusters of the heavy main group K.4 elements: electronic regularities and analogies Halide-supported octahedral clusters of zirconium: K.5 structural and bonding questions
54.18
54.17
54.16
54.15
54.14
F.A. Cotton, T. Hughbanks, C.E. Runyan, Jr, and W.A. Wojtczak, in M.H. Chisholm (ed.), Early Transition Metal Clusters with n-Donor Ligands, VCH: New York, 1995, pp. 1-26 (45) S.C. Lee and R.H. Holm, Angew. Chem., 1990, 102, 868-884; Angew. Chem., Znt. Ed. Engl., 1990, 29, 840-856 (194) R.E. McCarley, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 91-102 (30) R. Nesper, Angew. Chem., 1991, 103, 806-834; Angew. Chem., Znt. Ed. Engl., 1991, 30, 789-817 (199)
J.D. Corbett, Struct. Bond., 1997, 87, 157-193 (112)
J.D. Corbett, J. Chem. Soc., Dalton Trans., 1996, 575-587 (79)
J.D. Corbett, Pure Appl. Chem., 1992, 64, 1395-1408 (50)
J.D. Corbett, E. Garcia, Y.-U. Kwon and A. Guloy, Pure Appl. Chem., 1990, 62, 103-1 12 (39)
G. Suss-Fink, in H. Werner and G. Erker (eds.), Organometallics in Organic Synthesis, Springer: Berlin, 1989, vol. 2, pp. 127-136 ( 1 1 )
G. Suss-Fink and G. Meister, Ado. Organomet. Chem., 1993, 35, 41134 (410) G. Suss-Fink, Nachr. Chem. Tech. Lab., 1988, 36, 1110-1 113 (25)
Z. Sojka, Catal. Rev. Sci. Eng., 1995, 37, 461-512 (141)
D.F. Shriver, J. Cluster Sci., 1992, 3, 459-467 (19)
2
4
-
00
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4
s
g.
2
%
$
6
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r/,
P
Cluster intermediates in the molecular synthesis of solidstate compounds Centred zirconium chloride clusters: synthetic and structural aspects of a broad solid-state chemistry
-
R.P. Ziebarth and J.D. Corbett, Acc. Chem. Rex, 1989, 22, 256-262 (43)
Engl., 1988,27, 159-183 (226) J.H. Sinfelt, in J.P. Fackler (ed.), Metal-Metal Bonds and Clusters in Chemistry and Catalysis, Plenum: New York, 1990, pp. 103-112 M.L. Steigenvald, ACS Symp. Ser., 1990, 437, 188-196 (25)
A. Simon, Angew. Chem., 1988, 100, 163-187; Angew. Chem., Znt. Ed.
A. Simon, Pure Appl. Chem., 1995,67, 311-312 (6)
M. Sergent, C. Perrin, S. Ihmaine, A. Perrin, H. Ben Yaich, 0. Pena, R. Chevrel, P. Gougeon and M. Potel, J. Chim. Phys. Phys.-Chim. Biol., 1991, 88, 2123-2142 (17) A. Simon, in G. Schmid (ed.), Clusters and Colloids, VCH: Weinheim, 1994, pp. 373-458 (250) A. Simon, Muter. Chem. Phys., 1991, 29, 143-152 (38)
0. Peiia and M. Sergent, Prog. Solid State Chem., 1989, 19, 165-281 (250) A. Perrin and M. Sergent, New J. Chem., 1988, 12, 337-356 (181)
Citation
L. Naked metal clusters (gas phase, matrix isolated) [see also: 12.11, 12.16, 12.32, 12.33, 52.131 L.l Collision-induceddissociation of transition metal cluster P. Armentrout, D.A. Hales and L. Lian, Adu. Met. Semi-cond. Clusions ters, 1994, 2, 1-39 (81) M.T. Bowers, Ace. Chem. Rex, 1994, 21, 324-332 (60) L.2 Cluster ions: carbon, metcars and cT-bond activation L.3 Chemistry on molecular surfaces: reactions of gas phase D.M. Cox, M.R. Zakin and A. Kaldor, in G. Benedek, T.P. Martin clusters and G. Pacchioni (eds.), Elemental and Molecular Clusters, Springer: Berlin, 1988, pp. 329-349 (45)
K.18
K.17
K.16
K.15
K. 14
K.13
K.12
K.ll
Discrete and condensed transition metal clusters in solids Similarities and differences between cluster compounds of d and f metals Discrete and condensed clusters - a link between molecular and solid-state chemistry Clusters of valence electron poor metals structure, bonding and properties Nature of bimetallic clusters
Rare earth-based Chevrel phases REMo6Xg: crystal growth, physical and superconducting properties Rhenium clusters in inorganic chemistry: structures and metal-metal bonding Cluster compounds in solid state chemistry
K.9
K.10
Title
No.
Table 4 (continued)
Structural models for clusters produced in a free jet expansion Chemical reactions of trapped metal clusters Transition metal clusters: physical properties [gas-phase]
L.5
Chemistry of transition metal clusters [gas-phase]
L.11 L.12
L.13
L.9 L.10
L.8
Recent advances in the chemistry of gas-phase transition metal clusters Experimental studies of gas-phase main group clusters Formation, stability and reactivity of gas-phase bimetallic clusters Clusters - between atoms and solid state [free clusters] Chemistry of metal and semi-metal cluster ions
L.6 L.7
Electronic and geometrical structures of small elemental clusters
L.4
M.M. Kappes, Chem. Rev., 1988,88, 369-389 (199) K. Kaya and A. Nakajima, Adv. Met. Semi-cond. Clusters, 1994, 2, 87-1 14 (57) K.-H. Meiwes-Broer and H.O. Lutz, Phys. Bl., 1991, 47, 283 D.C. Parent and S.L. Anderson, Chern. Rev., 1992, 92, 1541-1565 (133) S.J. Riley, in D.R. Salahub and N. Russo (eds.), Metal-Ligand Znteractions: From Atoms to Clusters, to Surfaces [NATO AS1 Ser., vol. C3781, Kluwer: Dordrecht, 1992, pp. 17-36 (35)
P. Fantucci and J. Kontecky, in G. Benedek, T.P. Martin and G. Pacchioni (eds.), Elemental and Molecular Clusters, Springer: Berlin, 1988, pp. 125-147 (53) J. Farges, M.F. de Feraudy, B. Raoult and G. Torchet, Adv. Chern. Phys., 1988, 70 Pt 2, 45-74 (38) M.P. Irion, Lect. Notes Phys., 1992, 404, 201-213 (21) M.F. Jarrold, in H. Haberland (ed.), Clusters ofAtorns and Molecules, Springer: Berlin, 1994, pp. 315-330 (53) A. Kaldor and D.M. Cox, Pure Appl. Chern., 1990,62, 79-88 (19)
cr,
P
1754
6 Metal Clusters in Chemistry - Bibliography of Reviews 1988-1997
Reference [ l ] M.I. Bruce, in D.F. Shriver, H.D. Kaesz and R.D. Adams (eds), The Chemistry ofMetcl-l Cluster Complexes,VCH: New York, 1990, ch. 8, pp. 367-419.
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
7 Retrospective and Prospective Considerations in Cluster Chemistry Jack Lewis
7.1 Introduction It is apparent from the vast range of chemistry covered in the previous chapters that compounds containing metal-metal bonds are extensive and are wide spread throughout the periodic table. The stability of many of the compounds toward air or moisture is, however, low and although the range of complexes that can be produced is very extensive, the techniques in handling and preparing them are often very specific. Much of the work in this area of chemistry has been directed at establishing the existence of cluster compounds and their chemistry has not been explored in any detail. There are undoubtedly applications of these compounds across broad areas that will only be recognized when detailed studies on the chemistry has taken place and this must be a prime aim in this field. With the exception of polynuclear boron hydrides, cluster chemistry has developed mainly over the past 40 years. Prior to this a number of compounds that would now be classified as clusters had been but their nature was not recognized. It is perhaps interesting to ask the question of why cluster compounds, that is molecules containing metal-metal bonds, were considered prior to 1940 to be a relatively uncommon feature in inorganic chemistry? The problem was in part associated with the dominance of organic chemistry coupled with the difficulties in the rapid determination of the structure of compounds of this type. The determination of the X-ray structure of relatively simple compounds could take months or years to accomplish. As stated above, for the s and p block elements the compounds were also generally found to be unstable to air and moisture, whilst for the d and f block the compounds were often very complex in structure and the stoichiometry difficult to assess. This led to a general lack of appreciation of the occurence and stability of metal-metal or element-element bonds. The subject developed as a coherent field in the mid 1950s. The majority of work on transition elements during this period was associated with metal carbonyl com-
1756
7 Retrospective and Prospective Considerations in Cluster Chemistry
pounds and, to a lesser extent, with halides of the early transition elements. The following discussion will be directed primarily at problems related to the polynuclear compounds of the metal carbonyls, an area that was initiated by the classical work of Hieber from 1930 onwards. This field may be used to illustrate many of the problems associated with cluster chemistry in general, but it is important to emphasize that the area of “cluster chemistry” is now extensive, and each aspect of the field has unique problems. The definition of cluster compounds that has become most widely accepted is due to F.A. Cotton[’] who defined a cluster as “A discrete molecule containing at least three metal atoms in which there is a substantial and direct bonding interaction between the metal atoms even though some non-metal atoms may be associated intimately with the cluster”. This definition requires a cluster to be a unit of three or more metal atoms, and emphasises the importance of the bonding between metal centres. It allows for the fact that this bonding may be viewed from the two extremes of (i) a direct bond between the metal centres, with or without the participation of bridging ligand groups, or (ii) a metal-metal interaction occurring solely via the agency of the bridging groups. It was the latter form of interaction that first attracted my attention to this field using the magnetic properties of the paramagnetic transition elements as a potential measure of this effect. However, the magnetic criteria leads to a measure of the degree of interaction between the metal centres without any necessary insight into the mechanism or detail of the form of this interaction.
7.2 Critique of metal-metal bonds One of the main difficulties that presented itself in the interpretation of the bonding in these compounds was the general belief, prevalent until the 1950s, which was that for the transition series the bonds between metals were relatively weak. This was an extension of the belief that bonding between elements, other than for the first row of the periodic table, was weak. Much research was directed towards attempting to prepare compounds equivalent to the vast array of carbon compounds found in organic chemistry. Naturally emphasis was laid on the study of the elements in the same part of the periodic table, boron, silicon and related elements. It was then recognized that the element-element bond strengths decrease on descending a group. The supposition that the same bonding pattern applied to the transition elements was perhaps a natural, if not correct, extension to the chemistry of these compounds and reflected the paucity of experimental data then available. In the case of carbonyl compounds, the work of Sidgwick[’] on the structure of the polynuclear complexes of iron and cobalt illustrates this point well. Sidgwick considered that the bonding in these compounds did not involve the presence of any
7.2 Critique of metal-metal bonds
-c-
‘
1157
”
U
Fe._C*O
I c*o
s
0
I
co
Figure 1. Is0 Carbonyl Structure of [Fe~(C0)9], [Fe3(CO)lz]and [Cod(CO)12]
metal metal bonds. In order to attain the inert gas configuration around the metal centres he used in addition to the two electrons from the carbon of the co-ordinated carbon monoxide one of the lone pairs on oxygens to form a bridging carbonyl group to another metal centre. The proposed structures of the iron and cobalt compounds are shown in Fig. 1. The bridging carbon monoxide is bonding as an isocarbonyl group, a mode that has been recognized recently in a number of cluster carbonyls. This led to the determination of the structure of [Fe:!(C0)9] by Powell and E ~ e n s . [The ~ ] problem was suggested by Sidgwick, who had doubts about the earlier X-ray work of Brill. This previous study had not solved the stucture of the
1758
7 Retrospective and Prospective Considerations in Cluster Chemistry
carbonyl but had established that the molecule had D3h symmetry which would not have been in keeping with the isocarbonyl structure suggested by Sidgwick. The molecule was shown to have D3h symmetry and involved the presence of bridging carbonyls with bonding solely to the carbon centres. However, the presence of bridging carbonyl groups was interpreted as implying that the bonding between the metal centres was weak. The presence of metal-metal intraction was invoked to account for the diamagnetism of the compound. The Powell and Ewens paper also brought out for the first time the difficulty of assigning oxidation state to the metal atoms in compounds of this type. They assigned an oxidation state of +I11 to the iron atoms on the basis of the bridging carbonyl groups only donating one electron to each metal centre. The nature of the interaction between the metal centres in bridging carbonyl compounds is still a point of controversy. The interpretation that the interaction occurs via the bridging carbonyl groups rather than via a direct metal bond is possible. The real breakthrough came with the determination of the structure of [Mn2(CO)lo] by Rundle and Dahl.14] The structure established the presence of a molecule in which there were no bridging carbonyl groups and in which the two ‘‘Mn(C0)S” fragments were held together by a manganese-manganese bond. Dahl has continued working in the structural field of metal cluster chemistry and to him we must attribute some of the most significant structural advances in this area. As often happens in science the solution of one problem poses another, in the case of [Mnz(CO)101 the manganese-manganese bond length was significantly longer than had been anticipated. The bond distance between the metals in the iron carbony1 dimer was 2.46A) which was approximately the distance that would have been expected from the separation between iron atom in the metallic state (2.44A) and allowing for the variation in the co-ordination number between that in the metal (12) and that in the compound (7). However, in the manganese carbonyl the Mn-Mn distance was in considerable excess of the anticipated distance, being 2.92A) whilst for the metal the separation was 2.74A, giving an anticipated distance of 2.63 8, for a six coordinate environment. This difference in the anticipated and observed bond length has been accounted for in a variety of ways but it does leave open the question of whether data from the metallic phase should be used as a reference point for interpretation of metal distances in compounds of this type. There is little doubt that the bonding in metals and low nuclearity cluster complexes are different in character. The observed metal distances in cluster complexes cover a broad range of values. Thus for osmium compounds, single bond distances are reported to vary from 2.66 to 3.10 & whilst the separation expected using the distance in the bulk metal as a marker would be 2.60 A.The values observed for these bond distances reflect the connectivity of the metal centre, the distance increasing with the increase in the connectivity of the metal atom, as may be expected on a simple basis of packing, but is also associated with the nature of other ligands in the complex. What is clear is that the simple bond length-bond order relationship that applies to the bond distance of first row elements of the periodic table does not apply to
7.3 Metal-metal bond energies
1759
complexes of this type. This of course reflects the possibility of a contribution from d and f electrons to the bonding and in particular their potential interaction with s and p electrons of the same symmetry. In the larger cluster compounds there is the obvious possibility of a contribution from "metallic " bonding and the effect of this on the observed bond lengths will be of interest.
7.3 Metal-metal bond energies Some attempts have been made to determine or assess the bond energy of metalmetal bonds in carbonyl cluster compounds. Cotton,[5' in an early study, was able to illustrate the difficulties in determining the values of metal-metal bond energies in polynuclear carbonyl systems, by considering the bond energy of the metal-metal bond in manganese decacarbonyl. However, he was able to arrive at a value for the bond strength of the Mn-Mn bond in manganese decacarbonyl in the order of 130 f.20 kJ mol-' indicating a relatively weak bond. Further work which is summarked by Connor[61gave a range of values for the bond energy dependent upon the type of measurement carried out and the assumptions made. The bond energy for the Mn-Mn bond was found to vary from 35 kJ mol-' to > 176 kJ mol-' . The value that seems to be accepted at the moment is 159 f 21 kJ mol-I. Values of the same order of magnitude have been obtained for metal-metal bonds in a variety of carbonyl compounds for other metals. It has been established that the bond energy increases on descending a transition metal triad, but the bond energies of the metalcarbon bonds of the associated carbonyl groups are also stronger. The data imply that in contrast to the enthalpy of the metal bonds in the metals themselves, which rise to a maximum towards the middle of the transitions series, bond energies in complexes are more variable. Work by WadeL7]using various approximation methods relating bond strength to bond length, also indicated that the bond energy of metal bonds in the metal carbonyls increases on descending a triad. This trend is of course in contrast to the behaviour of the s and p block elements. The strength of metal-metal bonds in mixed metal systems has also been studied, but with no clear pattern emerging. Thus, in the mixed manganese-rhenium decacarbonyls the Mn-Re bond energy is less than the mean of the Mn-Mn and Re-Re bond energies of the parent carbonyls whilst for the mixed iron-ruthenium carbonyls [FezRu(CO)121 and [FeRu2(CO)lz], the Fe-Ru bond energy is greater than the mean of the metals in the parent carbonyls. Clearly a lot more work is necessary to place the thermodynamic data in this field on a much firmer footing but the intial work has established a fundamental differences between the transition elements and the s and p block metals.
1760
7 Retrospective and Prospective Considerations in Cluster Chemistry
7.4 Structure of carbonyl clusters An obvious problem that was of interest to workers in the field of metal carbonyl clusters was the prospect of predicting the structures of the compounds obtained. The principle of the attainment of the inert gas configuration by the metal atoms was one of the guiding rules applied in all the early considerations of the structure of these compounds. A major development occurred when Wade[*]drew an analogy between the structures of the metal carbonyls and boron hydride compounds whose structure were well established. Using this approach Wade was able to predict the correct structure for a number of polynuclear metal carbonyls many of which had been rationalised in terms of the effective atomic number rule. One of the early successes of the Wade approach was the structure of [Osg(CO)18],Fig. 2, which was
0 s = ae
K O = 5e M- M
ILr Ce
Figure 2. Structure of [Osg(CO)18]
7.4 Structure of carbonyl clusters
1761
found to have a structure with symmetry C2". The simple application of the 18 electron rule would have predicted an octahedral of Oh symmetry with all the 0smium atoms in equivalent sites, rather than as three pairs of osmiums in three different environments. This approach has been extended by M i n g ~ s [who ~ ' has formulated a series of rules which have wide applcation in accounting for the structure of metal clusters. These and related methods of structure prediction depend upon the allocation of specific electron counts to different framework geometries and these applications have been very successful in assigning structures to cluster carbonyls and their derivatives. However, for the higher polynuclear carbonyls as the molecularity of the compounds increases the predictive power of the theories becomes less decisive in differentiating between alternative structures. In essence the Wade-Mingos approach assumes that the frontier orbitals of the complex primarily involve metal orbitals so that any variation in the electron occupation will be reflected in a structural change in the metal framework. The Wade theory also requires that the structure of the complexes are based on triangulated polyhedra as found for the boron hydrides. Thus the application of a very simple bonding approach to the structure of [Osg(CO)18],may be used to illustrate a problem associated with the Wade methodology in interpreting the chemistry of this molecule. From the structure shown in Fig. 2 there are three different osmium environments each associated with two Os(CO)3 units. The electron count for this unit is 14 electrons and the group requires four more electrons for each metal centre to attain the inert gas configuration. For the atoms denoted (1) this is possible with the formation of the four metal-metal bonds associated with the normal bonding pattern. However, atoms (2) have only three metal bonds between the metal centres, whilst atoms (3) have five metal bonds if each edge of the polyhedron corresponds to a bond. The postulation of a donor bond from metal centre (3) to (2) would allow for the correct electron count at each metal centre. This donor bond would then imply a difference in charge distribution within the molecule as indicated in Fig. 3. This difference in the polarity between the metal centres should be and is reflected in the chemical behaviour of this and related molecules. A similar analysis of the bonding in the ion [ O S ~ ( C O ) ~the ~]~ hydride -, complex [H20s5(CO)15] and the neutral carbonyl [Os5(CO)16],all of which have the same electron count, and have a trigonal bipyramidal metal core, also involves the formation of donor bonds; in this instance between the axial and equatorial osmium atoms. If we consider the addition of a nucleophile such as iodide or a phosphine to these molecules the structure of the resulting compounds would be based on a square based pyramidal structure of osmium atoms according to Wade's rules, and involve nucleophilic attack at the equatorial osmium centres."'] The actual structure of the products is shown in Fig. 4. This may be rationalised by considering that the nucleophilic reaction involves attack at the electron deficient axial metal centre of the metal donor bond, resulting in an opening of one of the edges of a tetrahedron.
1162
7 Retrospective and Prospective Considerations in Cluster Chemistry
0
Figure 3. Charge distribution in Osg(CO)18
For the compound [oS5(co)16], using carbon monoxide as the nucleophile, this metal bond breaking proceeds with the addition of three carbon monoxide mole~ . [ was ~ ~ ]shown to have a “bow tie” cules to give the compound O S ~ ( C O ) ~This structure, Fig. 4, and may be viewed as being formed by the progressive addition of carbon monoxide to metal-metal bonds. A corresponding reaction occurs with the compound [osg(co)18]. In this instance many of the intermediate molecules have been identified. The various compound are also illustrated in Fig. 4,and represent a group that is referred to as “raft” compounds. This group once again provides a series of molecules that do not conform to the prediction of Wade’s rules but may be readily rationalised in term of nucleophilic addition to metal donor bonds. A further difficulty that arises with the use of Wade’s rules in predicting the structure of polynuclear carbonyls is that for the higher nuclearity carbonyl species a variety of isomers are possible. This arises from the fact that for boron hydride systems that were used as the basis for the Wade approach, the compounds considered were found to have the same or fewer boron atoms in the molecule than corresponded to the vertices the polyhedral predicted by the theory. For the carbony1 complexes it was often found that there was an excess of metal atoms over the number required to satisfy the polyhedra predicted. Mingos suggested a simple solution to this problem. Thus for osmium compounds he noted the compatibility of the three empty orbitals of the [ O S ( C O ) ~ ]ion ~ + with the triangular face of the polyhedral of metals predicted from Wade’s rules, see Fig. 5. Thus the compound [oSg(co)18may ] be considered as [ O S ( C O ) ~ ~ + ] [ O S ~ and ( Cthe O ) structure ~~]~-, corresponds to a “capping” of one of the faces of the trigonal pyramidal structure of the [ O S ~ ( C O ) I ~by] ~the - [ O S ( C O ) ~ ]ion ~ + to give the structure observed for [ 0 ~ 6 ( c 0 ) 1 8 ](Fig. 2). For structures of higher nuclearity with a number of different triangulated metal faces the capping of the metal polyhedron can clearly occur in more than one way.
7.4 Structure of carbonyl clusters
Figure 4. Reaction of carbon monoxide with [0s~(CO)~,j] (a) and [Osg(CO)18](b)
1763
1764
d8, 0s
7 Retrospective and Prospective Considerations in Cluster Chemistry
=
6e i n 3. bonds t o CO 2e I n 0 bonds t o meiol
Figure 5. Frontier orbitals for [os(C0)32+l
Thus for the ions [H40slo(CO)24l2- and [Osl0(CO)26]~-,which on the basis of electron counting should have the same structure, and from Wade's theory would be based on a central octahedral array of osmium atoms, the capping of the trigonal faces of the basic octahedron occurs in two different way leading to the structures shown in Fig. 6.[l2]These polyhedra may be derived from the structure of the [HOsg(C0)24]-, see Fig. 7,[13]by capping the faces Os(2), Os(5), Os(6) or the Os(4), Os(6), Os(l), respectively. A further complication that arises in considering higher nuclearity cluster compounds is the apparent lack of variation in the stereochemistry of the cluster with change in oxidation state. As the electron count of a molecule is associated with a basic metal framework redox behavior should be associated with a change in the metal poyhedron. Thus the polynuclear carbonyl anion [Os20(C0)40]~-, which has a tetrahedral array of metal (Fig. 8), exhibits at least eight different oxidation states. The structures of these different species do not appear to involve any apparent change in the metal geometry although the evidence for this is solely IR spectroscopy. However, in terms of the electron count the stereochemistry of the parent ion would not be predicted to be tetrahedral. Clearly there is a necessity to consider the basis for the electron counting methods that have been applied to these type of compounds and in particular the approach to higher nuclearity clusters. There is little doubt that the initial theories that were successful with compounds of low metal nuclearity were an important stimulant in the development of the subject, however some of the more recent developments in this area of chemistry involve the preparation of clusters of higher metal nuclearity and new theoretical approaches to their structure and properties will be necessary.
7.4 Structure of carbonyl clusters
01321
n
Figure 6. Structures of
[H20sl0(C0)24l2- and
[OS,O(CO)~~]~-
1165
1766
7 Retrospective and Prospective Considerations in Cluster Chemistry a52)
Figure 7. Structure of [HOsg(CO)26]-
7.5 High nuclearity polynuclear carbonyls One of the major factors leading to the explosion, during the 1950s, in the chemistry of the metal carbonyls and other cluster compounds was the availability of instrumentation and experimental techniques to deal with these compounds. Many of the preparative techniques used in the formation of these compounds are often not difficult or sophisticated. The main problem was the identification of the compounds once they had been prepared. Even the basic analytical data could often be ambiguous when dealing with high molecular weight compounds, and the determi-
7.5 High nuclearity polynuclear carbonyls
1767
Figure 8. Structure of ( O S ~ O ( C O ) ~ O ] ~
nation of the molecular weight of the complex was often not easy because of solubility problems. Thus the compound “Rh4(CO)11” isolated by Hieber was shown by X-ray analysis to be [Rh6(C0)16].[~~] Perhaps the three most useful techniques that became available in the study of the carbonyls were:
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7 Retrospective and Prospective Considerations in Cluster Chemistry
i) The rapid improvement in the rate of determination of X-ray structures. This in part arose from the improvement in the instrumentation but also the huge advances in computer power. ii) The development of mass spectrometry. This allowed for the rapid determination of molecular weight from the parent ion and composition from the fragmentation pattern. Initially the method was only applicable to the neutral molecules but with the advent of FAB and other techniques this has become extended to ionic species. iii) The occurence of effective separation techniques such as the different types of chromatography. The preparation of the osmium carbonyl clusters by thermolysis of [Os3(CO)12]produces a mixture of carbonyls with a high conversion of the initial carbonyl. These can be separated in a matter of minutes using thin layer chromatography. Recent work in the field of the higher nuclearity carbonyl clusters is perhaps at the limit of the techniques currently available, particularly that of X-ray crystallography. Much of the excellent work form the Dahl group on the higher clusters of platinum and palladium has been troubled with the limitations of the X-ray crystallographic technique. These include the frequent occurence of small weakly diffracting crystals that may exhibit disorder, also the high absorption coefficients of the systems that contain heavy Pd or Pt metals makes the location of light atoms difficult. The advent of synchrotron radiation and new detectors such as imaging plates and CCD devices has aided the determination of a number of the high nuclearity platinum clusters that were not previously possible, and provides the potential for new experimental procedures to establish the structures of even larger clusters. In the field of cluster chemistry one of the more exciting groups of complexes that were isolated by Chini, in the early years, were a series of rhodium compounds. This established for the first time, in the [Rh13(C0)24H3I2-and the [Rh15(C0)30l3- ions, a metal atom that was surrounded only by other metal centres, see Fig. 9. The development of the cluster chemistry of rhodium and the group 10 elements by Chini and his coworkers, particularly Longoni, has been a major impetus for the development of the chemistry of polynuclear carbonyls. The elucidation of the structure of these compounds depends largely on the use of X-ray structural analyses. The structure of these rhodium clusters with the unique rhodium centre,“ posed a general question that has underpinned much of the research in this area, namely, when do compounds of this type begin to exhibit the properties of metals? This of course involves the question of what criteria are required to define “metal” properties. No satisfactory solution has as yet been found to the initial question but work in this area has established that there is a gradual variation in properties as the size of the metal polyhedra increases. It would now appear that a spectrum of properties are possible, and that for the very high nuclearity clusters, of the types discussed below, new and distinctive behaviour may be anticipated. If this proves to be the case then this enhances the possible application of these materials.
7.5 High nuclrurity polynucleur curhonyls
1769
Figure 9. Structure of the metal framework of the ion [RhI 3 (C0)24H3]
’-
An interesting series of polynuclear carbonyl anions that relates to variation in property with build up in cluster nuclearity is [ O S ( C O ) ~ ] ~[ -O, S ~ ( C O ) ~ ~ ] ~ - , The structures are given in Fig. 10. The [OsloH4(C0)24I2-and [Os20(C0)40]~~. complexes may be considered to develop the tetrahedral structure of the series from a single metal centre by progressively adding layers of three, six and ten metal “rafts”. The next member of the series would involve the addition of a fifteen metal ~ ~A compound of this metal stoichiometry has been atom plane to yield an O S unit.
n
Figure 10. Tetrahedral metal carbonyl anions with cores built up in layers from 0 s to Os20
I770
7 Retrospective und Prospective Considerations in Cluster Chemistry
isolated, but only identified on the basis of mass spectrometry, the crystal being too weakly diffracting for X-ray measurements. The structure of this Os35 unit is of particular interest because with the addition of a fifteen atom layer, one of the osmium atoms is surrounded only by other osmium atoms. A number of higher nuclearity platinum and palladium clusters have been identified by Dahl.[’71Some of the more recent compounds that have been reported involve close packed cores of Pt35, Pt44 and Pt5o units, which occur in the carbonyl anions [Pt35(C0)40l2-, [Pt44(C0)47l4-and [Pt50(C0)4,l4-. Palladium does not give rise to pure carbonyl clusters, but a series of carbonyl phosphine derivatives have been isolated: [Pd29(CO)28(PPh3)7I2-, [I’d35 (CO)23(PMe3)151, [Pd37(C0)28(PPh3)121, [Pd39(C0)23(PMe3)16] and [Pd59(C0)32(PMe3)21]. The Pd59 cluster is the largest known metal-metal bonded system prepared to date from a monopalladium precursor. The structure involves a bicapped face-fused bioctahedral geometry and has 11 interior palladium atoms.[”] The development of logical synthetic pathways to clusters of this type will be of major importance if these compounds are to be utilised in chemistry. They involve completely different metal systems and their potential in fields such as catalysis is an obvious extension of their chemistry. From the work already discussed on the higher nuclearity osmium clusters it would be of interest to examine their redox properties. Another area of major interest in the field is that of mixed metal cluster systems. Hieber[l9]very early in the study of metal carbonyl chemistry was able to prepare mercury complexes of the type Fe(C0)4(HgC1)2. The related gold1201compounds were prepared in the 1960s by reaction of phosphine gold halides with carbonyl anions. (Ph3P) AuCl+ [M(C0),I2-
+
(Ph3PAu),M(CO),
+ 2C1
(M = Fe, Ru, 0 s ) Bruce[”] has utilised the gold oxonium ion, [(Ph3PAu)30]+, as a powerful reagent for the production of a range of mixed-metal, gold-containing clusters. Gold compounds have proved to be a rich source for the preparation of mixedmetal systems and many of the compounds prepared have been used for comparison with the corresponding hydride derivatives, since the [Ph3PAu]+ and [HI+ ions may be considered as isolobal if the 4p orbitals are not used in the bonding of the gold compounds. A recent X-ray structure of the iron-containing compound, [( Ph3PAu)2Fe(C0)4] indicates the presence of metal bonding between the gold atoms, and so the complex [(Ph3PAu)zM(CO)4](M = Fe, Ru, 0 s ) represents the first mixed-metal cluster compound to have been isolated. The nature of the bonding interaction between the gold centres in these polynuclear gold compounds has been a point of contention. In many of the compounds the gold-gold distances are within what could be considered as “bonding” distance, but the nature of the bonding in these molecules would then involve the use of the d orbitals of the gold.
7.5 High nuclearity polynuclear carbonyls
1I11
The problem has in part been resolved by the recognition that for the heavier elements, such as gold, relativistic effects must be considered in the interpretation of the bond length data.’221 Using gold derivatives it has been possible to prepare clusters in which other transition metal units are associated with a central gold cluster core. Thus the reaction of the [Ni6(C0)12l2- dianion with [(Ph3P)AuCl] gives the dianion [Au6Ni12(C0)24l2- in which a central A u ~octahedron is encompassed by four triangulated Ni3(CO)3(pu-CO)3ligand~.[’~I Complexes of this type emphasis the considerable potential for the production of arrays of different types of metal atom within the same cluster molecule. Related to this work is the preparation of large gold and silver clusters by T ~ O . [ ~ ~ ] These silver/gold clusters have been used to develop the idea of “clusters of clusters” or “supra-clusters”. The basis of the work is the assembly of large metal cores by the fusion of subclusters. The structural units can differ substantially from the arrangement of metal atoms in the bulk metal. Thus for the group 11 metals it has been possible to prepare a series of gold/silver clusters involving vertex-sharing gold-centred icosahedra. Many of these complexes involve the Ml3 unit, a fundamental building block in gold cluster chemistry. A variety of structures have been established for this series of compounds. In [Aul3Ag12(Ph3P)12C16In+, the two (Au7Ag6) icosahedra are linked through a common Au atom to generate a metal pattern similiar to that observed in the Pt19 polynuclear carbonyl anions, whilst in the ion [Aul8Agl9(PTol3)12Br11]” three icosahedra are linked by common Au vertices with one or two Ag atoms occupying central capping positions. Teo has postulated a general scheme for predicting the nuclearity and structures for higher species in this series. Bimetallic alloys have been of importance in catalytic industrial chemistry, and bimetallic ruthenium-copper alloys have a major role in catalytic refining process in the petrochemical industry. Clusters of ruthenium and copper have recently been obtained by reaction of ruthenium carbonyl anions with copper cations. The structure of some of these compounds are shown in Fig. 11. The anions [ H ~ R U ~ C(CO)24I2-, U ~ C ~ ~[H ~ R u I ~ C U ~ C ~ ~ ( Cand O)M [ H] ~ -R, u ~ ~ CcO)48l4U~C~( involve the fusion of two sets of ruthenium cluster units with a central copper core.[’ 5l All complexes involve copper atoms that are only bonded to other metal atoms. These complexes have recently been incorporated into zeolitic arrays which on thermolysis appear to lose carbonyl groups to yield materials that are catalytically active.[26]Compounds of this type are clearly going to be a very useful source for deposition of controlled mixed-metal systems, particularly if they can be deposited in restricted environments such as nanotube materials which may control the size of the resulting metal particle.‘271 Bimetallic cluster compounds have also been isolated for the group 10 metals. There is a wide range of metal ratios in the compounds that have been characterised, thus for the palladium-nickel series the following metal core systems have been isolated: Pd7Ni18, Pd13Ni13, Pd6Ni4, Pd20Ni20 and Pd33Nig.[18*281 The core
1772
7 Retrospective and Prospective Considerations in Cluster Chemistry
a
Figure 11. Structure of mixed-metal ruthenium-copper carbonyl ions
7.5 High nucleuvity polynuclear curhonyls
1773
Figure 11 (continued)
unit Pdl6Ni4 has a similar metal framework to the tetrahedral arrangement in the [ O S ~ O ( C O )dianion ~ O ] ~ ~with the four nickel atoms occupying the four tetrahedral vertices. A more restricted range of compounds has also been isolated for the nickelplatinum combination. Recently a palladium-platinum cluster has been obtained involving a Pd28 core with 12 platinum phosphine groups, [H12Pd28(PtPMe3). (PtPPh3)12(C0)27].It has been suggested that the hydrides occupy octahedral PdsPt sites within the cluster unit. The cluster has been proposed as a potential model for the study of the absorption of hydrogen in palladium metal as the hydride ligands in this cluster readily exchange with deuterium.[291 A further advance in higher nuclearity cluster chemistry is the work of Schmid in Germany and Moiseev in Russia,[301who have isolated a series of remarkable compounds with central metal cores units of Pd55, Pd309, and Pd561, and more recently Pd1415 and Pd2057. These clusters are stabilised by an outer shell of organic ligands such as phen, PPh3, dipy, oxides and halides. They provide a very interesting new regime of materials that are intermediate between the small metal clusters, metal colloidal particles and metals. It has not been possible to determine the structure of these complexes by X-ray crystallography. However, many of the techniques used in surface chemical studies such as STEM, EELS, EXAFS and HREM have been applied to elucidate the structure of these compounds. The exact nuclearity of these materials has not been fully established and the stoichiometry is often based on the assumption of closed packed structures for the metal core leading to “magic” atom numbers when analysed in terms of a shell model. These idealised shell models correspond to core units of M55, M309, M561, MI415 and M2057. A related series of compounds has also been isolated for platinum but the nature and the nuclearity of the compounds are not as well established as for palladium. This group of com-
1114
7 Retrospective and Prospective Considerutions in Cluster Chemistry
pounds provides samples for studies of a new regime in solid state structures and the related data on magnetism, surface energy and other properties are of interest for direct comparison with other solid state systems. As perhaps may have been anticipated these giant clusters have been intensively studied for their possible catalytic activity. The extension of this work to the preparation of the related mixed metals systems will be of interest. These compounds provide potentially important systems not only for their use as sources of possible catalytic systems but their physical properties will be of importance in theoretical studies on clusters and in determining the variation in properties with nuclearity. It is now clear that compounds with a wide range of nuclearity can be prepared but the difficulties in structure determination has restricted progress in the study of their chemistry. As mentioned above, these mixed clusters of high nuclearity are more or less at the present limits of the techniques available for structure determination. All work that has been carried out indicates that these compounds have a rich chemistry that is different from that of the smaller nuclearity metal cluster systems that have been the main concern to date. A related field that illustrates the difficulties in attempting to define the demarcation between molecular and solid state chemistry is the work of Fenske.13’] This group have synthesised molecular complexes with a range of heavy atoms varying from dozens to several hundreds. In particular, the copper-sulphide, selenide and telluride systems have provided a wealth of new compounds with a variety of structures, many of which have been determined by X-ray crystallography. The compounds have been prepared using mild synthetic conditions, in contrast to the high temperature “ceramic” methods generally employed in solid state chemistry. The complexes involve a variety of environments for the metal and chalcogen with a progression of “structures” within the “molecule”. The form of interaction between metal centres in these molecules is open to the same doubts that were voiced for the polynuclear carbonyls. They involve the possibility of both direct bonding between metal centres and interaction via the chalcogen. The situation is complicated as there is often a variety of metal environments within a given compound. These compounds are one of the more exciting developments within the area of cluster chemistry and provide a bridge between molecular compounds and solid state chemistry. The coupling of the main group and transition metals in the same high nuclearity metal cluster must be one of major future developments in this field of chemistry.
7.6 Interaction of clusters and organic molecules An underlying theme in many studies in this area is the possibility of using cluster compounds as catalysts and/or giving an insight into an understanding of the
7.6 Interaction of clusters and organic molecules
1775
mechanism of the catalytic activity in heterogeneous systems. Clearly the behaviour of organic molecules with clusters is of prime importance in solving these problems. A vast amount of work has been carried out on the form and nature of the interaction of a wide variety of organic molecules with cluster systems. The work has been mainly confined to systems of relatively low metal nuclearity, normally three or four metal atoms per molecule. This is a satisfactory model in many cases as the polyhedral structure of many of these molecules involves triangular faces. However, the bonding of an organic species to some of the bigger clusters involves the coordination of more than one molecule and the effects of interactions between these different molecules is of importance in elucidating the chemistry on metal surfaces. The little work that has been done has indicated that the chemistry of these polyligand molecules is different from that when one organic group is present. Thus in the complex[321[ R u ~ C ( C O ) ~ O ( C ~the H ~benzene ) ~ ] , groups are bonded to the metal polyhedra in two different modes, one bonding as a terminally bound group whilst the other caps a triangular metal face, Fig. 12. One point of interest in the early work in organometallic chemistry was the use of the structure of organometallic compounds as models for the bonding of unsaturated organic molecules to a metal surfaces. One of the very early applications of this approach was the use of Zeise’s salt K [ P ~ C ~ ~ ( C Z H ~ as ) ] a. Hmodel ~ O for the coordination of ethylene on a metal surface. The appearance and the variation in the C=C stretching frequency in the IR spectrum was compared with that obtained for molecules of ethylene absorbed on metal surfaces, and the close similiarity was taken as indicative of a similiar pattern of bonding. However, if the structure of the corresponding ethylene- metal cluster compounds are considered more complicated interactions have been observed. Thus with triruthenium or triosmium decacarbonyl, in addition to coordination of the carbon-carbon double, carbon-hydrogen bond cleavage occurs with the formation of metal hydrogen and metal carbon bonds. This has provided a more satisfactory model for the nature of the product of the interaction of alkenes with metal surfaces. The use of systems with only one metal has proven to be a very limiting model. In the majority of cases that have been reported, the interaction of saturated organic molecules with polynuclear systems occurs with bonding to more than one metal centre, and often with coordination of more than one site in the organic molecule. In many instances more than one organic molecule is coordinated to the polynuclear species and/or fission of carbon-carbon bonds in the organic molecule occurs. The latter type of interaction is particularly common for alkyne compounds, and has been observed for a wide variety of transition metal clusters. Some of the bonding modes are illustrated in Fig. 13. The archetypal molecule in a study of the interaction of organic molecules with clusters must be benzene. Fig. 14 illustrates some of the variations of structure that have been observed for benzene with metal ions or metal clusters. In many of the cluster systems the benzene molecule is bonded to more than one metal centre and the rationalisation for calling these “benzene” compounds poses the problem of
1176
7 Retrospective and Prospective Considerutions in Cluster Chemistry
how far the reactions have reduced/modified the aromaticity which is the characteristic property of the molecule. In the case of the complex [ ( C ~ H ~ ) O S ~ ( C O ) ~ ] [ ~ ~ the evidence favors coordination of the benzene as a localised double bonded system to the three metal centres, i.e. as a cyclohexatriene species with alternating double bonds in the carbon ring and the carbon hydrogen bonds being pushed out of the carbon ring plane.The identification of the structure of benzene on a rhodium metal surface has been shown by S ~ m o r j a i [to ~ ~involve ] a similiar bonding patten to that of the osmium compound discussed above. Clearly, the utility of these compounds as potential models for the bonding of organic molecules to metals or other sufaces is useful. However, it is also clear that a large range of bonding patterns are possible and that the true nature of the adduct may not relate to any of the “models” that have been observed. Nevertheless, the compounds do provide a very useful source for the study of any changes in the chemistry of the organic species that may arise upon coordination to a metal or other potential surface material. It is equally
7.6 Interaction of clusters and organic molecules
R
R
I
R
R
\ P
C
II
C E C
f hl
HI?
M-
\ /
M-
M1
/
c 111
\ /R
1777
hl
M
M3
M2
M-I
\M/ M7
M6 R
R
\
/
/c=c
M-
\
hl-
\
h?
\
prl
M10
M8 R
R'
\
R
/
hlM11
/
M h112
Figure 13. Some examples of bonding patterns of alkynes coordinated to metal clusters
important to study the variation in the properties of these molecules in their new coordination environment, an area that has been little explored for cluster systems but has proved of major interest with mononuclear compounds with direct application in synthetic organic chemistry.
1778
7 Retrospective and Prospective Considerations in Cluster Chemistry
I
M
I
M
M
<&>
M
M
Figure 14. Some examples of bonding of benzene to metal centres
w
7.7 Prepurution of cluster compounds
M
m
1779
M
Figure 14 (continued)
7.7 Preparation of cluster compounds An important aspect of the chemistry of cluster compounds is the development of systematic methods of synthesis. Much of the early work was dependent upon serendipity, rather than logical synthesis. Thus in the case of the polynuclear metal carbonyls either thermal condensation reactions of lower nuclearity clusters to produce higher clusters was employed or redox condensation reactions between ionic species were the two most common entries to the synthetic field.[341The first depends on the favourable entropy of a condensation reaction which proceeds via the elimination of carbon monoxide and the formation of metal-metal bonds, which is therefore favored by an increase in temperature (as discussed above metal-metal bonds are significantly weaker than metal-carbonyl bonds and hence in the case of the lower cluster systems the enthalpy term will normally be unfavorable). In general, higher temperatures favour the formation of clusters of higher nuclearity. In the case of the halide clusters thermal or highly reducing conditions were employed to give the polynuclear compounds, but the nature of the resultant products are, in general, not predictable. Recently, attempts have been made to devise more rational synthetic procedures and although this is still in its early stages it is obviously a prime goal in many fields of cluster chemistry. For systems involving multiple bonding between metal centres, compounds have been prepared in a logical manner by addition to these centres. At the moment the occurence of multiple bonding within carbonyl cluster compounds is relatively rare, but is a more common feature of the early transition element clusters. For the osmium and ruthenium carbonyl systems, replacement of carbonyl groups by effective leaving groups, such as acetonitrile, has resulted in the development of synthetic procedures leading to the designed synthesis of cluster derivatives. Thus for
1780
7 Retrospective and Prospective Considerations in Cluster Chemistry
the triosmium dodecacarbonyl one or two carbonyl groups may be replaced by acetonitrile on reaction of the osmium complex with amine oxide in a~etonitri1e.I~ 51 The resultant products are stable compounds and may be used as the starting materials for the synthesis of a whole range of derivatives formed by replacement of the acetonitrile groups by other nucleophiles. A range of nucleophilies have been used which range from inorganic ligands to organic groups, and include carbonyl anions
Reaction with carbonyl anions provides a facile route to either mixed-metal polynuclear carbonyls or to the extension of the nuclearity of the osmium series. This approach has been expanded to include a number of the higher polynuclear carbonyl species and provides a very effective method of controlled synthesis. In many instances the reactions with the acetonitrile derivatives take place under mild conditions and allow for the preparation of complexes that normally would be unstable using thermal methods. Reaction of pyridine with [Osg(CO)11(CH3CN)I affords [Os3(CO)ll(py)]in good yield. The pyridine bonds as a unidentate group via the nitrogen atom. In contrast, direct reaction with pyridine and the tricarbonyl leads to co-ordination of the pyridine by the nitrogen and the insertion of a metal into the CH bond of the pyridine. The monosubstituted complex is readily converted to the other product on warming in an organic solvent. Recently a range of new synthetic intermediates, termed “capping” groups, have been developed for controlled synthesis, on the basis of a simple cation-anion interaction, and utilising acetonitrile as a good “leaving” A range of cationic metal species of the type [M(arene)(CH3CN)3In+have been synthesised and are found to react with a variety of metal carbonyl anions. This leads to mixed compounds with the introduction of a variety of arene ligands into the co-ordination sphere in a controlled manner. [RU(C6H6)(CH3CN)3i2++ [OS5(CO),512-
[R~O~~(CO)I~(C~H~)]
The use of singly charged cations, i.e. [ R U ( C ~ H S ) ( C H ~ C Nin ) ~reactions ]+, with cluster dianions, allows for the introduction of two organometallic units 2 [Ru(C5H5)(CH3CN)3]++ [o% (co)1512 -
+
[RuzOss(co)15 (C5H5)2]
The resulting organometallic species can be further reduced to a carbonyl anion with Na/Hg alloy or potassium ketyl, which can be reacted with other cationic species giving rise to a whole range of substituted derivatives. A problem that can arise, however, is that, particularly for the carbonyl anions of the larger cluster complexes, reaction with the cationic species may lead to oxidation of the cluster
7.8 Conclusions
178 1
unit rather than addition of the capping group. This reflects the redox activity of the larger cluster compounds.
7.8 Conclusions There is little doubt that the area of cluster chemistry will provide ranges of new compounds that will yield insights into many aspects of solid state chemistry and have application to many fields. The chemistry of many of these compounds has not been investigated in any detail and the main emphasis has been on the determination and rationalisation of the structure of the complexes. The recognition of the stability of metal-metal bonds makes it apparent that there is a rich chemistry for many of these elements, particularly the 2nd and 3rd-row transition elements, and brings a new dimension into the potential range of compounds that can be prepared. The rich chemistry of carbon compounds that reflects the stability of the carbon-carbon bond may now be extended to the chemistry of many other elements, and although the extent of compound formation will not compare with that of organic chemistry there are rich fields to be harvested. A challenge I feel that will be a major development is the production of metal clusters with extended multiple bonded metal components which will lead to a new dimension in the reactivity of these systems. However, it must be emphasised that we are still in a very early stage in the development of this subject. I believe that the future holds much promise in the area of catalysis and solid state properties and these areas will be enhanced when more systematic methods of synthesis have been developed. The designed synthesis, particularly of the non-transition element clusters, will enable a more extensive approach to the utilisation of these compounds in chemistry and the overall potential in this area is clearly very exciting.
References [ l ] F.A. Cotton, Quut. Ref:. Chem. Soc., 1966, 416 [2] N.V. Sidgwick and R.J. Bailey. Proc. Roy Soc., Lon~lon.Scv., A , 1934, 521 [3] H.M. Powell and R.V.G. Ewens, J. Cheni. Soc., 1939, 286 [4] L.F. Dahl, E. Ishishi and R.E. Rundle. J. Cliern. Phj... 1957, 26, 1750 [5] F.A. Cotton and R.R. Monchamp. J. Cliern. Soc., 1960, 533. [6] J.A. Connor, Energetics qf Organonictcdlic Species. N A TO .scv%,.s, Kluwer Acad. Publishers, 1991, p 189 [7] C.E. Housecroft, K. Wade and B.C. Smith, J. Clicw~.Soc., C’hetn. C ’ o n i n i . , 1978, 765 [8] K. Wade, Electron Deficient Compoidc, Nelson. London. 1971
1782
7 Retrospective und Prospective Considerations in Cluster Chemistry
[9] D.M.P. Mingos and D.J. Wales, Introduction to Cluster Chemistry, Prentice Hall, 1990, p 99 [ l o ] B.F.G. Johnson, J. Lewis, P.R. Raithby and M.J. Rosales, J. Organomet. Chem., 1983, 259, c9 [ 1 I ] D.H. Farrar, B.F.G. Johnson, J. Lewis, P.R. Raithby and M.J. Rosales, J. Chem. Soc., Dalton Truns., 1982, 2051. [ 121 A. Bashall, L.H. Gade,. J. Lewis, B.F.G. Johnson, G.T. McIntyre and M. McPartlin, Angew. Chein.Int. Ed, Engl., 1991, 30, 1164: J. Chem. Soc., Chem. Commun., 1982, 640; D. Coughlin, J. Lewis, J.R. Moss, A.J. Edwards and M. McParlin, J. Organomet. Chem., 1993, 444, C53; A.J. Amoroso, B.F.G. Johnson, J. Lewis, P.R. Raithby and W.T. Wong, Anyew. Chem. Int. Ed En(//.,1991, 30, 1505 [ 131 A.J. Amoroso, B.F.G. Johnson, J. Lewis, P.R. Raithby and W.T. Wong, J. Chem. Soc., Chem. Co~nmun.,1991, 814 [ 141 A.J. Amoroso, L.H. Gade, B.F.G. Johnson, J. Lewis, P.R. Raithby and W.T. Wong Angew. Cheni., Int. Ed. Engl., 1991, 30, 107 [ I S ] E.R. Corey, L.F. Dahl and W. Beck, J. Am. Chem. Soc., 1963, 85, 1202. [ 161 V.G. Albano, A. Ceriotti, P. Chini, G. Ciani, S. Martinengo and W.M. Anker, J. Chent Soc., Chem. Commun., 1975, 859 [ 171 M. Kawano, J.W. Bacon, C.F. Campana and L.F. Dahl, J. Am. Chem. Soc., 1996, 118, 7869; A. Ceriotti, P. Chini, G. Longoni, M. Marchionna, L.F. Dahl, R. Montdg and D.M. Washecheck, X V Cong. Nus. Chim. Inory., Buri (Italy) 1982, A28 [ 181 L.F. Dahl, Private Communication [ 191 W. Hieber and U. Teller, Z. Anorg. Chem., 1952, 269, 306 [20] C.E. Coffey, J. Lewis and R.S. Nyholm, J. Chem. Soc., 1964, 1741 [21] M.I. Bruce and B.K. Nicholson, Organometullics, 1984, 3, 101. [22] P. Pyykko, K. Angermaier, B. Assmann and H. Schmidbaur, J. Chem. Soc., Chem. Commun., 1995, 1889 [23] A.J. Whoolery Johnson, B. Spencer and L.F. Dahl, Inorg. Chim. Acta., 1994,227, 269 [24] B.K. Teo, H. Zhang and X. Shi, J. Am. Chem. Soc., 1993,115, 8489; B.K. Teo and H. Zhang, Polyhedron, 1990, 9, 1985 [25] M.A. Beswick, J. Lewis, P.R. Raithby and M.C. Ramirez de Arellano, Angew. Chem., Int. Ed. Engl., 1997, 36, 291 [26] D.S. Shepard, T. Mashmeyer, G. Sankar, J.M. Thomas, D. Ozkaya, B.F.G. Johnson, R . Raja, R.D. Oldroyd and R.G. Bell, Chem. Eur. J., 1998,4, 1214 [27] S. Kawi and B.C. Gates, “Cluster and Colloids”, Ed., G. Schmid, VCH Publishers, Weinheim, 1994 p. 354 [28] M. Kawano, J.W. Bacon, C.F. Campana and L.F. Dahl, J. Am. Chern. Soc., 1996, 118, 7869 [29] J.M. Bennis and L.F. Dahl, J. Am. Chem. Soc., 1997, 119, 4545 [30] 1.1. Moiseev, M.N. Vargaftik, V.V. Volkov, G.A. Tsirkov, N.V. Cherkashina, V.M. Novotortsev, O.G. Ellert, I.A. Petrunenko, A.L. Churilinaw and A.W. Kit, Mendeleev Comm., 1995, 87; 1.1. Moiseev, R.J. Rudy, N.V. Cherkashina, L.K. Shubochkin, D.I. Kochubey, B.N. Novgorodov, G.A. Kryukova, V.N. Kolomiychuk and M.N. Vargaftik, Inorg. Chim. Acta, 1998,280, 339; G. Schmid, Chem. Rev., 1992, 1709; G. Schmid, M. Harms, J.O. Malm, J.A. Bovin, J. van Ruitenbeck, H.W. Zandbergen and W.F. Fu, J. Am. Chem. Soc., 1993,115,2046 [31] D. Fenske, G . Longoni and G. Schmid, ref. 27, p. 212 [32] M.P. Gomez-Sal, B.F.G. Johnson, J. Lewis, P.R. Raithby and A.H. Wright, J. Chem. Soc., Chenz. Commun., 1985, 1682 [33] G.A. Somorjai, Pure Appl.Chem., 1988, 60, 1499 [34] B.F.G. Johnson and J. Lewis, Adu. Inorg. Chem. and Radiochem., 1981,24, 225 [35] B.F.G. Johnson, J. Lewis and D. Pippard, J. Organomet. Chem., 1978, 160, 263 [36] J. Lewis, C.K. Li, M.C. Ramirez de Arellano, P.R. Raithby and W.T. Wong., J. Cheni. Soc., Dalton Trans, 1993, 1359; J. Lewis, C.A. Moorewood, P.R. Raithby and M.C. Ramirez de Arellano, J. Chem. Soc., Dalton Trans., 1996, 4509
Metal Clusters in Chemistry Voliime 3
Nanomaterials and Solid-state Cluster Chemistry Ed ted by P. Braurstein, L. A. 01-0 & P. R. Raithby CopyrightOWILCY-VCti Verleg GmhH, D-69469 Weinhcim (Federal Republic of Germany), 1999
Index
ab-initio calculations 146, 157 f, 483, 580, 585. 1160, 1493, 1704 absolute zero 1378, 1388, 1457, 1464 acid-base reactions 137, 497 acid-catalysed reactions 687 acido-basic condensation I35 acido-basic titration 130 acid stabilization 563 activation volume 1006 acyl chlorides 112 adatoms 787, 789 addition 632 adiabatic approximation 1265 adiabatic electron affinities 1674 adiabatic electron potential 1266 adsorption 621, 635, 741, 924, 1080, 1291, 1411, 1424 adsorption energy 1165 aerosol techniques 1436 A-frame complexes 402 Ag(100) surface 1161 agostic interactions 777 AgTe nanoclusters 1305, 1308 alcoholysis 703, 705 alkali-metal sulfides 163 alkali-metal thiolates 163 alkenylidene ligands 283 alkylidyne ligands 223, 290 alkyl lead compounds 94 alkyl in compounds 94 alkyl transfer 106 alkyne addition 737 alkyne cyclotrimerization 239 alkyne hydrogenation 715 alkyne insertion 242 alkyne metathesis 223
alkyne-substituted clusters 221 alloys 572, 784, 791, 877, 907, 917, 1195, 1205, 1227, 1239, 1281, 1286, 1288, 1332, 1417, 1621, 1771 allylic distortions 949 alumina-supported synthesis 866 aluminium clusters 0x0 complexes 21 1 aluminium complexes - cobalt containing 100 - iron containing 96 molybdenum containing 97 tungsten containing 99 amidocarbonylation 657 amine oxides 194, 206, 224, 800, 812, 824, 1037, 1041, 1042 amino ligands 73 ammonia solutions 572, 573 ammonia synthesis 652 amorphous binary phases 1615 amphoteric behaviour 406 angle dependent 1 182 anion doping 1538 anisotropic chemical properties 1598 anisotropic metallic conductivity 1634 anti-coordination chemistry 564 anticubeoctahedron 41, 938 antiferromagnetic coupling 1600, 1701 antiferromagnetic materials 1437 antimon clusters 564 anti-phase boundaries 1207, 1210 Arbuzov rearrangement 112 arc discharge synthesis 1668 arene clusters site exchange 257 - substituted clusters 1038 ~
~
~
~
1784
Index
arene migration 1041 arene-transfer reagents 252 aromatic substitution 692 aromatization 664 Arrhenius relationship 1353 aryl effect 968 aryl migration 243, 245 aryl rearrangements 249 aryl transfer 256 association reactions 1683, 1686 asymmetric carbonyl bridge 174 atomic force microscopy 1225, 1336 atomic orbital projection 1518 atom-solvent interactions 1217 Au(100) surface 1167 Aufbau principle 1699 Auger spectroscopy 1190 augmented spherical-wave calculations aurophilicity 478, 493, 500, 556 autocatalytic cluster growth 1243 azide ion 901
bonds 6bonds 310 zbonds 310 bond valence scheme 944 borido clusters 10 Born-Haber cycles 564 boron complexes - manganese containing 95 - molybdenum containing 95 - tungsten containing 95 boundary scattering 1450 Bragg orientation 1195 bridging carbonyl ligands 203, 315 bridging halide ligands 423 bridging hydride 65 bridging imido ligand 381 bridging selenido ligands 488 bridging sulfido ligands 482 Buckminster-fullerene complexes 588 bulk exciton 1267 bulk-like regime 1395 bulk metals 1075, 1077 bulk-molecule transition 1327 butterfly structure 167, 385, 391, 883, 896, 1033, 1117 -
1634
B3LYPmethod 1162, 1164, 1167 ballistic transport 1349 bandgap 1634 band model 1326 band structure 349, 1435, 1512 band-structure calculations 1517, 1534, 1538 bent metal-metal bond 150, 172 benzotriazole synthesis 7 12 berry pseudo-rotation 519, 1061 bimetallic complexes 91 bimetallic particles 1205 bimolecular elimination 93 binary metal carbonyls 163 binary-phase materials 1302 binary phases 907, 1098 bioinorganic chemistry 124 biomimetic materials 1491 biphenyl 296 bipyramidal voids 1513 bis(amin0)germylenes 74 bis(amin0)stannylenes 74 bis(p-oxo)complexes 146 Bloch wave 1462 Bohr radius 1263, 1267, 1463 bond length-bond order relationship 1758 bond-order-bond-strength relationship 363 bond-order conservation 1411 bond-order summations 1538
c 6 0 fullerene compounds 1633, 1665 caesium chloride type structure 1558 calcination 783, 1273 calorimetric studies 1073 calorimetry 1439 Cambridge Structural Database 3 15, 3 17 carbamate ligands 698 carbamate synthesis 697, 816 carbazole synthesis 707 carbene 49, 73, 17 carbene complex 65 carbide ligands 223 carbohydrate oxidation 920 carbon-carbon bond cleavage 293, 791, 826, 1775 carbon-carbon bond formation 301, 1298, 1388 carbon dioxide elimination 1098 carbon-hydrogen bond activation 261, 269, 283 carbon-hydrogen bond breaking 283 carbon-hydrogen bond cleavage 293 carbon-hydrogen insertions 692 carbon migration 232 carbon-oxygen bond cleavage 814 carbon-sulfur bond breaking 767
Index carbon-sulfur bond cleavage 224, 745, 769, 772 carbon-sulfur bond formations 771 carbonyl activation 802 carbonylation 381, 623, 627, 632, 644 f, 648, 650 f, 654, 659, 703, 1281, 1287 carbonyl disproportionation 897 carbonyl dissociation 255 carbonyl elimination 1030, 1033, 1039 carbonyl exchange 980 carbonyl insertion 1031 carbonyl ligands 698, 730, 1035 carbonyl migration 978, 983, 992 carbonyl scrambling 982, 1004 carbonyl transfer 1061 CASSCF calculations 309 catalyst precursors 716 catalytic properties 110 catalytic relay 1241 cation doping 1538 cation metathesis 867 CdSe nanoparticles 1302 cementite 1099 ceramic oxocuprates 1104 chain-like polymers 499 chalcogenides - nakedatoms 143 chalcogenometalates 124 charge delocalization 962 charge-density studies 1688 charge-density waves 1546 charge distribution 1171 charge effects 1583 charge equalization 951 charge transfer 1137, 1167, 1169, 1412, 1429, 1491 charge-transfer adducts 173 charge-transfer bands 448 charge-transfer interaction 587 charging energy 1355 chemical transport reactions 1565 chemisorption 236, 399, 808, 847, 884, 908, 919, 1297, 1434 chemoselectivity 693 Chevrel phases 776, 1579, 1604 chiral complexes 1 18 chloride abstraction 989 chromatography 546 chromium clusters 0x0 complexes 21 1 circular dichroism 1 19 clathrate-I type structure 1614 clathrate-I1 type structure 1631 -
1785
Claus process 742 closed-packed structures 549, 1046, 1075, 1773 closed-shell configuration 1418 cluster aggregation 609 cluster-arene interactions 588 cluster condensation 1486, 1521, 1656 cluster definition 3 cluster distortion 1579, 1605 cluster fragmentation 726 cluster growth 193, 204, 870 cluster growth kinetics 1222 cluster packing 1486 cluster rearrangements 20, 86, 356, 360 cluster-size effect 1164 cluster-substrate interactions 1181 cluster/surface analogy 261, 275, 327, 348, 399, 606, 621, 639, 715, 745, 796, 814, 877, 908, 935, 1067, 1073, 1155, 1159, 1273, 1325, 1434, 1505, 1652, 1775 CNDO calculations 585, 1160 cobalt clusters - aluminium containing 98 - carbon containing 890, 1040 - germanium containing 77 - gold containing 540 - molybdenum containing 773 - nitrogen containing 326, 329, 887 - phosphorus containing 888, 1108 - selenium containing 184, 189 - silicon containing 890 - sulfur containing 182, 184 - tellurium containing 177, 187, 189 - tin containing 77 cobalt complexes - arene containing 238 - cyclopentadienyl containing 79 - gold containing 540 cobaltocene 279 co-catalysts 700 co-condensations 1310 co-cyclooligomerizations 270 collision cross section 1679 collision-induced dissociation 1398, 1675 f Collman’s reagent 115 colloidal 1462 colloidal dispersions 621 colloidal metals 9 13 colloids 1216, 1364, 1370, 1388, 1436, 1439, 1445, 1454 complete active space calculations 1692 concerted processes 153 condensations 549, 809, 1064
1786
Index
conductimetry 1214 conduction band 1267, 1441 conduction electron wavepacket 1462 conductivity 1474 configuration-interaction calculations 1692 conical fragments 27 I cooperative effect 624 coordinative unsaturation 360, 606 copper clusters - arene containing 244 - 0x0 complexes 21 1 core extraction 130 correlation diagrams 146 correlation effects 478, 1699 corrosion 1159, 1241 cortex-catalysts 914 Coulomb attraction 1267 Coulomb blockade 1345, 1349 f, 1356 Coulomb charging energy 1347, 1447 Coulomb effects 1463 Coulomb energy 1470 Coulomb gap 1357, 1473 Coulomb interactions 1264, 1361, 1445, 1467, 1583, 1693 Coulomb potential 1435 Coulomb repulsion 1419 Coulomb shift 1190 Coulomb term 1493 Coulomb-type interactions 1172 critical radius 1267 cryptands 572 cryptate compounds 1618 cryptate effects 952 crystal engineering 844, 1486, 1505 crystal-field theory 1141 crystal growth 1671 crystal orbital overlap population 1516 crystal structure prediction 1493 CS2 activation 134 Cu(100) surface 1167 cubane structures 771, 774 cubeoctahedral 41 cubeoctahedral holes 1543 Curie effects 1447 Curie paramagnetism 1458 Curie-Weiss behavior 1587, 1623 Curie-Weiss law 1292 CuTe nanoclusters 1310 cyclic voltammetry 159 cycloadditions 645 cyclometalations 684 cyclopentadienyl bridge 83
cyclopentadienyl-carbonyl metal complexes 78 cyclopentadienyl ligands 724 cyclopropanation 692 cyclotrimerization 625
d + n* interactions 942, 946 DDQ 463 deboronation 28 de Broglie wavelength 1342, 1464 Debye function analysis 925 Debye-Waller factors 1019 decarbonation 865 decarbonylation 230, 255, 335 defect rock-salt structure 1509 defects 1520 defect structures 1487 deformation isomerism 1059 dehydration 633, 652, 655 f, 810, 812, 816, 819, 828 dehydrocondensation 1297 dehydrogenation 270,633, 643, 790, 1039, 1489, 1677, 1686 dehydrohalogenation 259 dehydroiodination 249 dehydroxylation 812, 865 deintercalation 1546 delayed atomic ion emission 1676 delayed ionization mass spectroscopy 1679 delocalized bonds 1612 delocalized electrons 1326 delocalized valence-hole states I186 delocalized valence states 1186 demethylation 647 dendrimeric structures 1381 density functional theory 480, 485, 580, 962, 1095, 1147, 1153, 1160, 1163, 1170, 1392f, 1441, 1443, 1489, 1494, 1566, 1643, 1687, 1693, 1706 density of states 1348, 1394, 1398, 1512, 1676, 1688 deoxygenation 216, 824 dephenylation 1367 deprotonation 31 desorption 1244, 1707 desulfurization 753 Dewar-Chatt-Duncanson model 1405 DFT calculations 146, 158, 309, 485 AGt 515, 517, 520, 525, 527 AH: 683, 686 ASt 515, 520, 683, 686 dialkylcarbamato ligands 209
Index diamagnetism 589 diauracycles 459, 467 a-diazo decompositions 678 dicobalt octacarbonyl 176, 224, 227, 312, 821 dielectric relaxations 1585 differential scanning calorimetry 452 differential thermal analysis 452 diffuse reflectance spectroscopy 448 up’-diimine ligands 699 diiron nonacarbonyl 75, 315 dimanganese decacarbonyl 75, 169, 179 dimensional-reduction method 1486 dimetallacycloalkanes I00 dimethyldibenzothiophene 742 diorgano-group 14 compounds 49 diradical 76 direct laser vaporization method 1667 direct profile imaging 1195, 1202 dispersion effects 1424 disproportionation 167, 196, 563, 589, 639, 1231, 1244, 1623 disruption enthalpy 1078 dissociative adsorption 897 distortional perturbations 146 disulfide bridges intermolecular 173 disulfide ligands 13I , 175 disulfido complexes 132 dithiocarbamate ligands 468 dithiolene 134 divided metals 1461, 1464 djerfisherites 1661 DNA strands 1361 domains 1529 donor-acceptor bonds 41 7 double bond shft reactions 285 dynamic behaviour 5 11 dynamic electron-pair formation 1545 dynamic equilibrium 241, 1056 dynamic processes 368 -
effective atomic number rule 105 EHMO calculations 146 electrical capacitances 1342 electrocatalysis 1159 electrochemical oxidation 159 electrochemistry 126, 154, 239, 301, 327, 332, 338, 349, 371, 376, 387, 401, 447, 455, 573, 925, 1042, 1105 f, 1108, 1111, 1114, 1138 f, 1145, 1150, 1423, 1618 electrode position 1361
1787
electrode potentials 1122 electron affinity 1088 electron chain transfer catalysis 327 electron-containing rules 1612 electron-correlated level calculations 1 I62 electron correlation 1 180, 1690 electron-counting rules 323, 329, 349, 383, 405, 575, 577, 583, 935, 944, 1032, 1048, 1054, 1090, 1105, 1304, 1485, 1617, 1643, 1659, 1760 electron-counting schemes 10 electron-deficient bonding 773, 1420 electron-deficient clusters 1647, 1660 electron delocalization 1467 electron depopulation 1647 electron diffraction 1194, 1290, 1365, 1546 electron-electron interactions 1451 electron flow 1412 electron-hole pair 1457 electron-localization function 1634 electron localized-delocalized transition 1545 electron microscopy 593, 647, 783, 915, 925, 1194, 1225, 1365, 1439, 1487, 1520, 1773 electron-phonon coupling 1327 electron-phonon interactions 1637 electron rearrangement 1408 electron spin-echo modulation 1217 electron sponges 1 138, 1150 electron transfer 410, 411, 704, 916, 1228 electron transfer mechanism 1241 electron wave function 1268 electron wave packet 1462 electronic buffer systems 1514 electronic devices 1274 electronic spectrum 132 electronic thermodynamic properties 1447 electrophilic attack 128 electrophoresis 927, 1351 electrostatic correction 1162 electrostatic effects 1170 electrostatic image forces 1263 eliminations 92 empirical packing energy calculations 1492 enantioselectivity 922 encapsulated metal atom 188 energy dispersive X-ray spectroscopy 1608 energy-level separation 1457 energy minimum distance 1171 energy relaxation rate 1350 enthalpy 1073 enthalpy of combustion 1073, 1077 enthalpy of disruption 1082, 1099 enthalpy of formation 1081, 1083, 1099
1788
Index
enthalpy of vaporization 1076, 1081, 1093 entropic factors 1092 environmental protection 782 epikernel principle 1689 epitaxial growth 1207, 1424 epitaxial relationships 1198 EPR spectroscopy 450, 985, 1140, 1144, 1414, 1623, 1638 equal potential surface model 940 etching 1437 ethanethiolate 181 ethanethiolate ligands 176 ethylidyne 271, 275 Ewald-Kugel 1607 EXAFS 100, 151,633, 785 f, 791, 808, 915, 936, 1001, 1010, 1019, 1021, 1194, 1261, 1276, 1281, 1284, 1286, 1294, 1365, 1369, 1371, 1376, 1380, 1382, 1385, 1439, 1773 excess molecular susceptibility 1471 exchange anisotropy 1437 exchange processes 479, 517, 853 excitation dampening 1328 excited-state structures 1002 exciton Hamiltonian 1264 excitons 1263 exogenous carbonyl ligands 185 extended Hiickel calculations 121, 160, 309, 319, 393,405,471,474, 485, 581, 585, 1141, 1148, 1150, 1160, 1487 f, 1494 f, 1503, 1512, 1544, 1566, 1634, 1643, 1679, 1689
face-capping carbonyl ligands 276 fast precipitation 1259 faujacite 1274, 1296 Fe(100) surface 1160 Fermi cut-off 1185 Fermi distribution 1348 Fermi energy 1394, 1399, 1408, 1416, 1440, 1457, 1512, 1517, 1633 f, 1678, 1688 Fermi potential 1231 ferrocene 269 fine chemicals 794 first-order perturbation theory 1265, 1269 first-order transition 1464 Fischer carbenes 624 Fischer-Palm cluster 298 Fischer-Tropsch reactions 636, 640, 642, 649, 798,803, 897, 908 fluxes 1510, 1544 fluxionality 362, 395, 510, 988, 1008 force-field approach 943
force-field calculations 708 formal local charge 945 Fourier-transform ion cyclotron resonance 1681 fractal aggregate 1371 fractional occupation numbers 1394 fractional oxidation states 1257 framework structures 1593 Franck-Condon factors 1676 Frank-Kaspar polyhedra 1633 Frank-Kaspar-type polyhedra 1637 Frank-van der Menve mechanism 1426 free-electron-pair states 1637 Friauf-polyhedron 1632 frontier orbitals 556 fuel cells 924 fullerene-like 1613 fullerene-type compounds 1671, 1687, 1707 functionalized supports 1020
gallium complexes molybdenum containing 93 - nickel containing 95 gas chromatography 783 gas-phase catalysis 1336 gas-phase clusters 1667 gas-phase condensation 1616 gas-phase ion chromatography 1671, 1679 gas-phase metal clusters 1179 gas sensors 858 Gaussian-type calculations 1394, 1441 gels 926 geometry change 128, 131 germanium clusters - rhodium containing 81 giant clusters 1367, 1774 Gibbs distribution function 1450 gold-carbon bonds 461 gold clusters 372 azide containing 536 cobalt containing 548, 551 - iridium containing 551 iron containing 540 manganese containing 541 molybdenum containing 539, 544 - nitrogen containing 494 oxygen containing 478, 494 - palladium containing 546 - platinium containing 546 - rhenium containing 543, 551 rhodium containing 548, 551 selenium containing 488, 502 -
-
-
-
-
Index silver containing 487 sulfur containing 481, 494 f tellurium containing 490 tungsten containing 540 - vanadium containing 539, 551 gold colloids 1327 gold complexes 459, 463 - sulfur containing 477 gold-gold interactions 459, 467, 477, 486, 495, 1770 gold-phosphine fragments 14, 23, 315, 319, 344, 355, 389, 407, 422, 477, 486, 509, 528, 535, 556, 894, 977, 989, 1057, 1059, 1068, 1182, 1770 Green’s function 1351, 1357 Group 13 complexes cobalt containing 92, 94 -
-
-
-
Haber process 908 halide ion adsorption 116I , 1 168 Hamiltonian 1447, 1449 Hammett scale 567 Hartree-Fock calculations 1162, 1394, 1687, 1690, 1695 heat capacity 1378 Heck reactions 921 Heisenberg antiferromagnetism 1415 Heisenberg model 1701 hessite 1309 heterocycles 706 heterogeneous catalysis 126, 270 heterometallic nitrido clusters 335 heteronuclear group 11 clusters 509 heteropolytungstates 444 heterotrimetallic clusters 83 HFS calculations 153 Hg(l11) surface 1161 high-field magnetization 1442 high-pressure infrared spectroscopy 703 high-resolution electron microscopy 1332 high-vacuum evaporation 1553 Hofmann degradation 705 hole-states 1699 homogeneous catalysis 290 homologation 637, 640, 642, 650 f, 654, 658, 1293, 1298 host-guest interactions 412 hot electrons 1327 hot injection techniques 1302 HPLC 196 Huckel calculations 1354 Hund’s rule 1443
1789
hydrations 810, 828 hydride ligands 766 hydride migration 1034 hydridocobalt clusters 100, 105 hydrocarbonylation 650 hydrodenitrogenation 127 hydrodesulfurization 124, 608, 627, 629, 632, 634 f, 651, 741, 776 hydroformylation 622, 626, 628 ff, 634 f, 640, 642, 644, 646, 648,650, 657 R, 660, 662, 664, 1049, 1286 y-hydrogen abstraction 791 hydrogen activation 737 hydrogenation 126, 348, 624 ff, 628 ff, 632 ff, 642, 644,647 ff, 651 ff, 659 R, 665, 721, 723, 726, 730, 788, 914, 922, 1289, 1297, 1329, 1332, 1367, 1388 hydrogen adsorption 302 hydrogen bonding 807 - intramolecular 683 hydrogen elimination 294 f, 303, 791 hydrogen migration 22 1, 992 hydrogenolysis 126, 631, 638 f, 641, 643, 645, 652 f, 658, 745, 784, 1286, 1292 f, 1368 selective 792 hydrogen peroxide 456 hydrogen shift 291 hydrogen transfer 737, 768 hydrolytic processes 209 f hydrosilation 633 ff, 643, 645, 658 f hydrosilylation 1388 hydrosols 9 18 hydrothermal synthesis 164 hyper-aurated compounds 48 1 hypervalent molecules 1646 -
I- V curves 1474 IGLO calculations 36 imido ligands 381 impurity scattering 1450 incommensurate phases 586 indoles synthesis 710 inert gas aggregation 1194 infrared spectroscopy 448, 453, 547, 716, 846, 850, 1338, 1604 insertion 113 insulator-to-metal transition 1633 intensity profiles 1206 interaction energies 1159 inter-band transitions 1267 inter-cluster bonding 589
1790
Index
inter-cluster interactions 587 inter-exchange processes 965 intergrowth compounds 1526 intermetallic contacts 1404 intermetallic phases 1630 intermolecular forces 1068 intermolecular interactions 9.51, 1498 interstitial atoms 878, 880, 889, 972 interstitial boron atoms 894, 972 interstitial carbon atoms 896, 974 interstitial hydrogen atoms 894 interstitial nitrogen atoms 901, 978 interstitial oxygen atoms 904 interstitial phosphorus atoms 904 interstitial sulfur atoms 905 intra-cluster bonding 579 intra-exchange processes 965 intramolecular cyclization reactions 708 intramolecular hydrogen exchange 255 intramolecular migration 1034 intramolecular site-exchange 519 intra-unit bonding 1563 inversion processes 527 ion-exchange reactions 1275 ionic coupling reactions 353 ion mobility studies 1706 iridium clusters - boride containing 14, 21 - iron containing 93 - lead containing 64 - tin containing 59 iridium complexes 764 - arene containing 238 iron clusters - bismuth containing 593 - boride containing 12, 21 - carbide containing 3 18 - diphosphine substituted 199 - germanium containing 76 - imido containing 701 - lead containing 63 - ligand effects 31 1, 314 - molybdenum containing 773 - nitride containing 325, 327 - oxygen containing 21 1 - phosphorus containing 1108 - rhodium containing 972 - ruthenium containing 736 - selenium containing 184, 194 - sulfur containing 165, 180, 182, 748, 1106 - tellurium containing 187 - tin containing 56, 76
triphenylphosphine substituted 194 iron complexes - arene containing 238 cyclopentadienyl containing 79 ironpolymetal sulfides 128 y-irridation 1224, 1231 isocyanate synthesis 697 isolobal character 482 isomerization 19, 625 f, 629, 633 ff, 639, 641, 643, 648, 652 ff, 659, 973 isopropanethiolate ligands 176 isotopic exchange reactions 855 -
-
Jahn-Teller distortions 586, 1402, 1623, 1649, 1652, 1689 jellium model 1402, 1441 Jonas reagent 270
Keggin anions 137, 444 Keggin structure 136 ketene ligands 1031 ketenylidene 216 kinetic studies 400, 726 f, 738 Kohn-Sham formalism 1393 k-space data 1022 Kubogap 1459
labeling 946 lacunary anion 445 lacunary polyanions 133, 136 lamellar crystals 1195 Landauer resistance 1353 Langmuir-Blodgett techniques 1350, 1359 Langmuir technique 1476 lantern type structure 1255 lanthanide complexes 453 laser evaporation 1423 laser-induced photodissociation 1669 laser-induced plasma synthesis 1666 lattice energies 1088 lattice strains 1195 layered clays 1273 layer structures 1593, 1597 LCGTO calculations 1147, 1153 lead clusters chromium containing 592 ‘least-motion’ pathway 1012 length-energy correlation constants 1077 Lewis acid complex 100
-
Index Lewis acids 111, 136, 566 libration 953, 1010, 1016 Lifshiz-Slezov distribution 1265 ligand effects 1218, 1329 ligand eliminations 1028 hgdnd exchange reactions 278 ligand-induced transfer 1442 ligand isomerism 1053 ligand-metal interactions 1437 ligand migration 241 ligand polarization 1186 ligand polyhedral model 935, 938, 1001, 1008, 1010 ligand-stabilized particles 1326 ligand stereochemistry 937 ligand substitution 128 Lindlar catalysts 921 liposomes 926 liquid ammonia 571 liquid drop model 1469 liquid sulfur dioxide 569, 591 lithography 1345, 1357, 1359, 1437 LMTO calculations 1541 long-range interactions 971, 1491 Loose clusters 494 low-temperature isomerization 34, 44 luminescence 504
mean free path 1342, 1462 Meissner effect 1633 merry-go-round fluxionality 1491 ‘merry-go-round’ mechanism 1007 mesoporous cavities 1273 mesoporous materials 621, 913, 1284 mesoporous oxides 61 3 mesoscopic systems 1461 mesoscopic tunnel junctions 1351, 1361 metal blacks 1388 metal-ceramic interactions 1423 metal clusters benzyne containing 250 metal colloids 605 metal core rearrangements 51 1 metalhydrides 113 metal-insertion reactions 753 metal-insulator transition 1454, 1467, 1473 metallacarborane clusters 26 copper containing 33 gold containing 33 intermediates 43 indium containing 41 - isomerization 39 mercury containing 32 molybdenum containing 43 palladium containing 33 platinium containing 40 rearrangement 28 rhodium containing 34 ruthenium containing 37 metallacyclopentadiene complexes 229 metallacyclopropenes 25 1 metallation 28, 679, 1056 metallic behavior 1538 metallic bonding 1326, 1759 metallic character 1689 metallic divided state 1454 metallic foil 1565 metallic phase 1758 metallic properties 1521, 1605 metallocarbenes 692, 1664 metallocenes 799 metallo-cyclopropane model 158 metallodiphosphines 114 metalloligands 1 17 metallophosphines 110 metal-molten salt systems 565 metal-metal bond cleavage reactions 716 metal-metal bond energies 1759 metal-non-metal transition 1327 metal-organic interactions 348 -
-
-
-
-
-
macroscopic metals 1456 macroscopic phases 1237 macroscopic systems 1461 magic-number clusters 1438 magicnumbers 1375,1487, 1647,1654,1689,1773 magic peak 1673 magnetic measurements 1574 magnetic quenching 1407, 1427, 1443 magnetic rearrangements 1623 magnetic suppression 1441 magnetic susceptibility 915, 1292, 1378, 1447, 1471, 1594, 1604, 1607 manganese clusters germanium containing 75 tin containing 75 manganese complexes selenium containing 169 tellurium containing 166 mass spectroscopy 984 material science 107 matrix effects 1581 matrix isolation 1148, 255 McMurry coupling reactions 92 1 -
-
-
-
-
1791
1792
Index
metal-oxide interface 1424 metal particle dispersion 1019 metal surfaces 605 metal to ligand charge transfer 1220 metastability 145 metastable dissociation experiments 1671 methanation 629, 631, 636, 640, 642, 649, 655 methaneselenolate ligand 174 methoxy ligands 384 MgO(001) surface 1196, 1423 micelles 915 microelectronics 1423 microemulsions 926 micropores 1277, 1281 microporous materials 621 microwave absorption 1473 mineralizers 1510, 1544 mixed chalcogen/carbonyl ligand spheres 163 mixed charge clusters 1555 mixed-valence compounds 461, 471, 1137 MO calculations 202 Mossbauer spectroscopy 174, 743, 1010, 1296, 1365, 1439 f MOCVD 107 model clusters 746 model potential calculations 1398 model reactions 746 Moirk fringes 1202 molecular beams 1435 molecular energetics 1394 molecular mechanics 940, 1009, 1011, 1498 molecular mechanics calculations 280, 940, 948 f, 956, 1011 molecular modeling 145, 789 molecular orbital calculations 143, 31 1, 349, 1056, 1058, 1441, 1486, 1605, 1659 molecular orbital (MO) studies 26 molecular orbital theory 580 molecular recognition 1486 molten salt solutions 564 molybdenum clusters - imido complexes 232 - nitrogen containing 880 - 0x0 complexes 231 - selenium containing 805 - sulfur containing 771, 805 molybdenum complexes - antimony containing 591 - sulfur containing 747 Monsanto process 199 Monte-Carlo calculations 1680, 1687 mordenite 1274
Mott-Hubbard correlation energy 1466 Mott-Hubbard energy 1470 Mott transition 1464, 1476 Mulliken charges 1428 multiphoton absorption 1677, 1679 multiple quantum transitions 962 multi-slice technique 1209 M0ller-Plesset corrections 1 162
naked boron atom 894 naked clusters 561, 587, 1395 naked post-transition element clusters 577, 584 nanoclusters 621 nanocomposites 193 nanocrystals 1669 nanomaterials 914, 1274, 1281 nanoparticles 348, 638, 653, 1179, 1274, 1287, 1434, 1437, 1476 nanoparticle self-assembly 1343, 1356 nanotubes 927, 1336, 1771 nanowires 1274, 1281, 1289, 1294 naphthylene coordinated clusters 259 naphthalene substituted clusters 262 N-donor ligands 724 f, 732 negative rate-dependence 728 neutron diffraction 883, 886,991, 1073, 1526 nickel clusters arene containing 239 - carbon containing 890, 1422 cyclopentadiene containing 290 - germanium containing 892 molybdenum containing 773 - 0x0 complexes 21 1 - tin containing 893 nickel complexes - aluminium containing 99 - arene containing 237 nickelocene 291 nitrene ligand 336 nitride clusters 15 nitrogenase cofactor 627 nitrosonium ion 324, 344, 407, 901 NMR spectroscopy 745, 936 - I'B 18, 21, 29, 35, 449, 972 - *lBr 1552 I3C 74, 80, 225, 228, 231, 233, 291, 298, 303, 337, 343, 354, 384, 392, 512, 690, 751, 850, 852, 883, 960, 962, 974, 978, 983, 984, 1002, 1004, 1008 f, 1015, 1057, 1095 35C1 587, 1552 - I9F 424, 437, 690 ~
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Index ' H 60, 62, 80, 118, 199, 277, 295, 303, 337, 354, 384, 392, 498, 511, 518, 683, 733, 756, 847, 882, 887,960, 979, 981,991, 995, 1037, 1057, 1369, 1371, 1376, 1553 - "N 337 f, 343, 854, 978 - 1 7 0 454 31P 15, 116, 118, 199, 354, 358, 361, 368, 372, 375, 449, 454, 456, 484, 490, 514, 518, 680, 682, 685, 689 f, 733, 801,960, 970, 973, 984, 989, 1059 I9'Pt 354, 1029, 1285 In3Rh 354, 690, 960, 970, 974, 979, 984, 992, 994, 1002 - 29Si 74 '19Sn 65 lS3W 137, 449, 454 2 dimensional 961 - 2DEXSY 519 - COSY 36, 970 dynamic 1003 EXSY 199, 962, 1003 fluxionality 199, 277, 286, 298, 412, 512, 585, 949, 1062 - HMQC 961 - INDOR 960 line shape analysis 1003 low temperature 358, 395, 511, 685, 970, 976, 983, 986, 991, 995, 1030, 1057 magnetization transfer 1003 multinuclear 960 nuclear Overhauser effect 848 - solid state 783, 846 f, 850, 852, 953, 989, 1009, 1013, 1015, 1018, 1371 - spin relaxation studies 1003, 1009 spin saturation experiments 257 temperature dependent 80 variable temperature 85, 118, 247, 354, 396, 498, 514, 518, 683, 774, 883, 953,962, 993 f, 1013, 1015, 1029 non-bonding interactions 434, 1249, 1321, 1492 non-classical bonds 1612 non-dissociative adsorption 1160 non-linear optic materials 8 19 non-linear optical properties 1263 non-radiative decay 1679 nucleation mechanism 1213 nucleophilic attack 134, 470 nucleophilic substitution 1I3 -
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octahedral cavity 886 octahedral holes 1512, 1543
1793
odd-even electron-number effects 1445 Ohm's law 1343 olefin hydrogenation 8 12 olefin metathesis 624, 631 olefin polymerization 622 one-electron energy gap 1461 one-electron oxidation 148 optical extinction spectroscopy 1327 orbital correlation 309 orbital mixing 1424 osmium clusters 846, 851, 862, 865 arene complexes 237 boron containing 884 carbide containing 366, 374 - carbon containing 882 - germanium containing 52 - gold containing 355 phosphorus containing 889 - platinum containing 1048 sulfur containing 750 tin containing 51, 55, 65 osmium complexes arene containing 238 osmium dodecacarbonyl 56 osmometry 681 oxidation 625, 637, 664, 1281 oxidation catalysts 455 oxidation-induced reactions 1685 oxidative additions 221, 255, 410, 461, 471, 750, 756 oxidative transfer reactions 193 oxide supports 606 0x0 ligands 209, 1060 oxomolybdates 1509 oxoniobates 1509 oxonium salts 479 oxotantalates 1542 ~
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packing defect 1353 packing effects 585 packing potential energy 1493 palladium complexes arene containing 238 palladium particles 1196 paracyclophane complexes 238 paramagnetism 173, 273, 290, 582, 984, 1141, 1149, 1394, 1400, 1471, 1513, 1538, 1551, 1566, 1574, 1586, 1609, 1623, 1628, 1756 partial electron densities 1634 Pauli exclusion principle 1405 Pauling's electroneutrality principle 945 ~
1794
Index
Pauling’s perfect cube 1693 Pauli paramagnetism 1457 f, 1471 Pauli spin-paramagnetism 1443 P-donor ligands 721 pentlandites 1661 permutational isomerism 1053 perovskite-type structures 1487, 1521 petrochemicals 794 phase relaxation length 1342 phase transfer agents 447 phase transfer chemistry 804 phase transition 1013, 1497, 1503 phase transitional behavior 1492 phasoid 1526 pH dependence 447 phosphane ligands 185 phosphido complexes 110 dibridged 116 phosphine ligands 732 phosphine stabilizing sphere 1302, 1438 phosphonium salts 113, 496 phosphorus atoms 904 phosphorus-carbon bond cleavage process 904 phosphorus-oxygen bond formation 805 photoelectron spectroscopy 777, 920, 1179, 1677 photoemission spectroscopy 1400, 1539 photoionization 1213 photoisomerization 2.55 photolysis 12, 74, 116 f, 206, 237, 336, 353, 399, 493, 504, 535 f, 539, 543, 629 f, 632 f, 641, 681, 686, 820, 853, 1039, 1043, 1289, 1291, 1294, 1435, 1668 chemically 179 photovoltaic application 163 physicochemical properties 1104 physisorption 641, 654, 784,861, 866, 1434 pillared clay 1297 plastic deformations 952 platinium blue 1257 platinium complexes - gold containing 436 - lead containing 436 f - mercury containing 436 - silver containing 418 - thallium containing 436 tin containing 436 platinum layer 787 pnictide metallates 559 f points on a sphere model 938 Poisson distribution 1447 polarization effects 1424 polychalcogenides 581 polyhalides 58 1 ~
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polyhedral deformation 32, 34, 44 polyhedral rearrangements 1028, 1032, 1045 polyhedral skeletal electron pair (PSEP) 26 polyhedranes 577 polymorphs 1565 polyoxometalates 134 polyoxothioanions 139 polyphosphanes 581 polyphosphides 581 polysulfide anions 133 polytopal rearrangement 32 population analysis 1403 post-transition elements 561, 569 potential energy calculations 1093, 1595 potential energy curves 1607 potential energy hypersurface 1492, 1695 potential energy surfaces 363, 937, 945, 1427, 1501, 1698 potentiometric titration 571 powder diffraction 1526, 1630 probability distribution functions 1449 protective ligand shell 1326, 1347, 1365, 1376, 1393, 1404, 1437, 1612 protolysis 206 proton transfer reactions 681 PSEP theory 206 pseudomorphic growth 1426, 1428 pseudopolymorphism 952 pseudo-potentials approximation 1163 Pt( 100) surface 1161 Pt( 1 11) surface 1160 pulse radiolysis 1213, 1220, 1222, 1227, 1243 pulsed arc cluster ion source 1668 pyrazolyl ligands 679 pyrite type structure 1604 pyrochlore type structure 1547 pyrolysis 273, 329, 331, 336, 353, 359, 362, 364, 613, 661, 869, 896,904, 1042, 1046, 1288, 1296, 1564
quadrupole splitting parameter 1440 quantum calculations 1002 quantum dots 927, 1348, 1360, 1435 quantum size behavior 1339 quantum size effects 926, 1292, 1326, 1342, 1348, 1350, 1378, 1434, 1440, 1443, 1445, 1449, 1459, 1615 quantum tunneling 1437 quantum wells 1263, 1435, 1445 quantum wires 1351 quasi-atomic state 1213 quasi-degenerate levels 1705
Index quasi-particles 1267 quasireversibility 11 18, 1143 quaternary phase nanoclusters quinolone synthesis 71 1
1795
palladium containing 390 phosphorus containing 890, 984 platinum containing 982 ruthenium containing 965 sulfur containing 890, 994 ring opening metathesis polymerization 61 I ring opening reactions 470, 760 ring opening oligomerization 769 ruthenium clusters 722, 734 boride containing 10, 12, 17, 20 boron containing 884 carbide containing 18, 366, 518 carbon containing 883, 1036, 1043 diphosphine substituents 200 gold containing 357, 511, 540 iron containing 736 lead containing 64, 69 - mercury containing 390 nickel containing 54 - nitride containing 18, 325, 328, 329 - nitrogen containing 883 - oxygen containing 799 - palladium containing 390 phosphorus containing 1108 rhodium containing 972 selenium containing 194 sulfur containing 530, 750, 776, 1106 tin containing 55, 69 triphenylphosphine substitutes 196 ruthenium complexes - arene containing 238 rutile-type structure 1534 -
1312
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r-space data 1022 radial bonds 553, 1418 Radial Distribution Function Method 1248 radiolytic reduction 1213 Raman spectroscopy 448, 454, 1285, 1604 - surface-enhanced 1 169 random-matrix theory 1447, 1449 Raney nickel 790 rate-determining step 733 reaction intermediates 130 reaction rate 735 rearrangements 527 - pathways 995 torsional barrier 146 redox condensation 328, 353, 873, 1779 redox-dependent effects 186 redox potential experiments 1228 redox processes 128, 158 reduction 381, 623, 627 f, 632, 634, 641, 645, 649, 659, 1281 reductive carbonylation 697, 870, 874, 1286 reductive elimination 536, 701, 733, 751 refractory carbides 907 regioselective addition 1 18 regioselective substitution 202 relativistic effects 478,483,493, 1395, 1399, 1418, 1426, 1581 reversible metalation 684 rhenium clusters carbon containing 982 oxygen containing 805 sulfur containing 1591 rhenium complexes - arene containing 238 rhodium clusters - antimony containing 893 - boride containing 14, 19, 21 - boron containing 972 - carbon containing 974 imide containing 391 - imido containing 382, 391 iron containing 965 - mercury containing 390 nickel containing 979 nitride containing 326, 329 - nitrogen containing 887, 978 osmium containing 965 -
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salt-metal transition 1617 sandwich compounds 137 SAXS 915, 1365, 1368, 1371, 1380 scalar-relativistic calculations 1411, 1415 scanning microscopy 1161 scanning tunneling microscopy 1349, 1359, 1365, 1376, 1437, 1473 Schrock carbenes 624 Schrodinger equation 1392 Schulz-Flory-Anderson distribution 654 Schulz-Flory distribution 1297 second harmonic generation 1491 second-order mixing 1649 seed-germ process 1332 selenium clusters 568 - tungsten containing 591 selenophene 760 self-assembly 504 self-condensation 139
1796
Index
semi-bridging carbonyl ligands 175, 943 semi-conducting materials 1303 semiconductor materials 1263, 1342, 1423, 1435; 1454, 1463, 1470, 1513, 1518 semiconductor nanocrystals 1263 semi-heavy atoms 1252 semi-quantitative calculations 3 12 silanolate complexes 872 silica-mediated synthesis 863, 873 silica-supported species 861 silver complexes 463 single electron devices 1345 single electron tunneling 1342, 1344, 1615 single electron tunneling devices 1356 single-domain particles 1437 singlet ground state 145 singlet state 76 site-isolation model 1287 size-dependent electronic relaxation 1327 size quantization I266 skeletal electron pairs (seps) 26 skeletal isomerism 1053, 1065 Slater determinant 1690 Slater exponent 1605 slipping parameter 32 slow precipitation 1259 small molecule activation 145 sodium benzophenone 353,800 sodium chloride type structure 1592 sol-gel processing 348 solid-gas conditions 803 solid-gas reactions 844 ff, 851 solid-state devices 444 solid-state disorder 1494 solid-state dynamics 1503 solid-state reactions 607, 844, 1286, 1509, 1564 solid-state synthesis 1573, 1594, 1774 solid-state theory 1456 sols 913, 922 solute speciation 567 solvated electrons 1217, 1233 solventothermal synthesis 574 solvothennal conditions 165 solvothennal reaction 169, 185 specific adsorption 1159 specific conductance experiments 1552 spectromicroscopy 1192 spherical droplet model 1401 spherical potential well 1263 spin density plots 1414 spin-orbit coupling 1443, 1447, 1449 spin-orbit interaction 1398
spin-ordering 1441 SQUID magnetometer 1443 stacking effects 1568 stacking factors 1631 stacking faults 1195, 1210 stannylene 49 stannyne complex 68 'Star of David' disorder 1009, 1013, 1015, 1017 static dielectric constant 1217 statistical distribution 1595 stereochemical non-rigidity 510 stereochemical regularities 941 stereospecificaddition 136 stereospecificring opening 407 Stokes-Einstein law 1372 structural compression 1600 structural correlations 308 structure correlation theory 937 structure-property relationships 1393 sub-halides 590 sublimation energy 1213 substitution reactions 463, 482 sub-valent species 562 sulfidation 639 sulfide complexes polymetallic 128 sulfido ligands 209 sulfoxonium salts 496 sulfur abstraction 773 sulfur-containing compounds 124 sulfur-hydrogen bond cleavage 745 superacids 567 f superconducting transitions 1539 superconductivity 1491 superconductor materials 1538, 1542, 1545, 1616, 1633 supercritical nuclearity 1233, 1238 supercritical solvents 572, 574 superparamagnetic materials 1437 superparamagnetic properties 927 superparamagnetism 1297 supersensitive electrometry 1361 supersonic molecular beams 1214 supersymmetric field theory 1451 supported catalysis 743 supported cluster reductions 1230 supported clusters 1019, 1202, 1273, 1325, 1388, 1393, 1422, 1436 supramolecular aggregation 483, 504 surface-mediated synthesis 860, 866, 869 surface metallation 593 surface migration 784 surface pinning 1437
Index surface plasmon 1469 surface plasmon spectrum 1223 surfactants 1216 Suzuki reactions 921 symmetry allowed processes 155 symmetry forbidden processes 152, 156 symmetry operators 1493 symproportionation reactions 564 synchrotron radiation 1180, 1374 synergistic effects 921, 1104 synthesis gas 65 1
tantalium clusters 0x0 complexes 215 TCNQ 463 technetium clusters - sulfur containing 1591 tellurium-carbon bond cleavage 1307 tellurophene 760 temperature-dependent impedance 1350 temperature-independent paramagnetism 1378 temperature-induced transformations 1382 temperature-programmed decomposition 635 ternary phase nanoclusters 1312 tetra-alkylammonium salts 447 tetracobalt dodecacarbonyl 98 tetrahedral holes 88 1, 1511 tetrahydrothiophene 420, 427, 472, 501 tetrairidium dodecacarbonyl 59 tetrarhodium dodecacarbonyl 58 tetrathiometallates molybdenum containing 125 - tungsten containing 125 therapeutic action 493 thermal analysis 1380 thermal desorption studies 745 thermionic electron emission 1676 thermionic emission process 1679 thermogravimetric analysis 1551 thermolysis 106, 224, 229, 237, 244, 256, 898 - high pressure 215 thietane 769 thiirane 769 thin films 1616 thiocarbonate 134 thiolate complexes 163 thiolato ligands 747, 1058 thiophyne coordinated clusters 259 through-space spin coupling 1692 time-of-flight mass spectrometer 1675 time-resolved spectroscopy 1214, 1221, 1244 ~
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1797
time-reversal symmetry 1449 tin clusters - chromium containing 74, 584, 592 - nickel containing 82, 86 - palladium containing 82 - platinium containing 83 - rhodium containing 81 Tolman parameters 968 p-tolylacetylene ligands 282 topological transitions 1613 torsion angles 308 trans-annular bonds 149 trans effect 942 transesterification 705 transient species 128 trans-influence 62, 65, 466 transition metal bonds to group 13 elements 92 transition metal-boron bonds 95 transmetallation reactions 389 transmission electron microscopy 1332, 1336, 1338, 1365, 1380 trifluoromethanesulfonic acid 172 trigonal prismatic cavity 887 trigonal twist mechanism 973 triiron dodecacarbonyl 50, 58, 194, 224, 325, 748, 803, 824, 938, 1008, 1104, 1495, 1757 trimethylamine N-oxide 50 triorganophosphines 51 triosmium dodecacarbonyl 353, 826, 861, 938, 1008, 1080, 1254, 1494, 1780 tripalladium clusters 405 triphenylphosphine 85, 118 triple-decker complexes 239 triplet ground state 148 triplet-singlet crossing 1407 tripodal ligand 157 tripodal rotation 1002, 1004 triruthenium dodecacarbonyl 51, 68, 194, 225, 259, 326, 328, 337, 609, 699, 720, 751, 803, 826, 828, 938, 1008, 1104, 1279, 1296, 1494 T-shaped structure 468 tunable donor properties 387 tunable optical properties 1435 tungsten bronze-type structure 1542, 1547 tungsten carbide 1098 tungsten clusters oxygen containing 807 sulfur containing 771 tungsten hexacarbonyl 74 tungsten sulfide complexes gold containing 128 nickel containing 126
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1798
Index
tungsten sulfide complexes (cont.) platinum containing 128 - ruthenium containing 128 tunneling electron microscopy 1194, 1288, 1290, 1371, 1476 tunneling resistance 1344 tunnel junctions 1342, 1348 twisting processes 527
water activation 799 water-cluster interactions 1159 water-copper interactions 1166 water-gas shift reaction 400, 640, 650 f, 657, 659, 798, 804, 824, 874, 1292 f Wigner-Dyson level statistics 1450 Wigner-Seitz radius 1459 Wilkinson’s catalyst 81 work function 1469
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ultra-divided metals 1213, 1241 a,,&unsaturated aldehydes 789 uranium clusters - 0x0 complexes 210 urea synthesis 697 UV-vis spectroscopy 130, 750, 1284, 1338 vacuum induction 927 valence band 1267 valence-bond theory 1687 valence-electron concentrations 1566, 1613 van Vleck contribution 1471 van Vleck paramagnetism 1378 van Vleck term 1586 vapor deposition 1436 vertex extrusion 34 vesicles 926 vibrational frequency calculations 1168 vibrational motion 974 vibrational spectroscopy 342, 354 Vollmer-Weber mechanism 1426 volumetry 783 Wade-Mingos rules 275 Wade’srules 26 Walsh diagrams 150
XANES 925 Xcc-calculations 1488, 1646 X-ray absorption spectroscopy 1018 X-ray crystallography 745 X-ray diffraction studies 846 X-ray photoelectron spectroscopy 1291, 1296 X-ray powder diffraction 451 X-ray powder diffraction curve 1248
ylide formation 692 ylide ligands 461, 466, 496
Zeeman effect I47 1 Zeise’s salt 158, 1775 zeolite A 575 zeolites 613, 913, 1020, 1273, 1293, 1436 zinc clusters 0x0 containing 215 Zintl ions 571, 573, 1488 Zintl-Klemm-Busman concept 1616 Zintl phases 574, 577, 582, 587, 590, 1485, 1488, 1612, 1616, 1631, 1634, 1638 zirconium clusters - 0x0 containing 215 ZnO(001) surface 1198 ~
