Organometallic Chemistry, Volume 31 (Specialist Periodical Reports)

Organometallic Chemistry Volume 31

A Specialist Periodical Report

Organometallic Chemistry Volume 31 A Review of the Literature Published During 2001 Senior Reporter M. Green, University of Bristol, UK Reporters Simon Aldridge, University of Cardiff, UK John G. Brennan, State University of New Jersey, USA A.J. Bridgeman, University of Hull, UK Ian R. Butler, University College of North Wales, Bangor, UK Marie P. Cifuentes, Australian National University, Canberra, r\ustra a Kevin R. Flower, UMIST, Manchester, UK Matthew D. Francis, University of Sussex, UK Mark G. Humphrey, Australian National University, Canberra, Australia Paul A. Jelliss, St Louis University, USA Philip J. King, University of Hull, UK Richard A. Layfield, University of Cambridge, UK D.J. Linton, University of Cambridge, UK, Guy C. Lloyd-Jones, University of Bristol, UK Jason M. Lynam, University of York, UK Andrea Sella, University College London, UK A.E.H. Wheatley, University of Cambridge, UK Dominic S. Wright, University of Cambridge, UK

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advancing the chemical sciences

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ISBN 0-85404-338-1 ISSN 0301-0074

A catalogue record for this book is available from British Library 0 The Royal Society of Chemistry 2004

All rights reserved Apart from any fair dealing for the purposes of research or private study, or criticism or review as permitted under the terms of the U K Copyright, Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the U K , or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the U K . Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org Typeset by Vision Typesetting, Manchester, UK Printed by Athenaeum Press Ltd, Gateshead, Tyne & Wear

Preface

Organometallic chemistry continues to flourish with important developments which once would have been viewed as belonging to the separate domains of inorganic and organic chemistry. I would like to thank the contributors for capturing the excitement of the subject. A planned chapter which was to describe progress during the year 2001 of our understanding of the Reactions of Transition n-Complexes of Alkenes, Alkynes and Dienes will now be combined with the corresponding work in the year 2002 and will appear as one chapter in Volume 32. Michael Green

V

Contents

Chapter 1 Theoretical OrganometallicChemistry B y A.J. Bridgeman 1 Introduction 2 s-Block Metals 2.1 Structural, Spectroscopic and Mechanistic Studies 3 p-Block Metals 3.1 Structural and Spectroscopic Studies 3.2 Mechanistic Studies 4 d-and f-Block Metals 4.1 Structural and Spectroscopic Studies 4.2 Mechanistic Studies References Chapter 2 Groups 1 and 11: The Alkali and Coinage Metals B y D.J. Linton and A.E.H. Wheatley 1 Alkali Metals 1.1 Introduction 1.2 Alkyl Derivatives 1.3 Alkenyl, Allyl, Vinyl, Alkynyl and Related Derivatives 1.4 Aryl Derivatives 1.5 Cyclopentadienyl and Related Derivatives 2 Copper, Silver and Gold 2.1 Introduction 2.2 Copper Compounds 2.3 Silver Compounds 2.4 Gold Compounds References

Organometallic Chemistry, Volume 3 1 0 The Royal Society of Chemistry, 2004 vii

1 1 1 1 4 4 10 13 13 26 38 48

48 48 48 52 53 55 56 56 57

60 62 64

...

Contents

Vlll

Chapter 3 Group 2 (Be-Ba) and Group 12 (Zn-Hg) By Dominic S. Wright 1 Scope and Organisation of the Review 2 Group2 3 Group 12 References

Chapter 4 Scandium, Yttrium and the Lanthanides B y John G. Brennan and Andrea Sella 1 Introduction 2 Synthetic Studies 2.1 Cyclopentadienyl Ancillaries 2.2 Indenyl, Naphthalene and Other Aromatic n-Complexes 2.3 Cyclooctatetraenyl Chemistry 2.4 Miscellaneous Ancillaries and Hydrocarbyls 3 Organometallics in Materials Synthesis 4 Polymerization Chemistry 4.1 Non-Polar Mono-Olefin Polymerization 4.2 Diene Polymerization 4.3 Polymerization of Acrylate Monomers 4.4 Polymerization of Lactide 5 Spectroscopic and Theoretical Studies 6 Lanthanides in Organic Synthesis References

Chapter 5 Carboranes, Including Their Metal Complexes By Paul A. Jelliss 1 Introduction 2 Theoretical and Computational Studies 3 Carboranes 3.1 (CBH) 3.2 ( G & ) 3.3 (CzB9) 3.4 (C2Bm) 4 Metallacarbaboranes 4.1 {MCBlo) 4.2 (exo-MCBI1) 4.3 (MC2B8) 4.4 (MC2B3) 4.5 {MC2B4) and (MC2B8) 4.6 (MC2B9) 4.7 (exo-MCJ39)

70 70 70 76 81 85

85 85 85

93 95 96 100 101 101 102 103 104 104 105 107 112 112 113 113 113 113 114 114 117 117 117 117 117 118 118 121

ix

Contents

4.8 (MC2BlO) 4.9 (exo-MC2Blo) 4.10 {MC3B?) 5 Biological Carborane Chemistry and BNCT 6 Crystal Engineered Supramolecular and Polymeric (Metal1a)carborane Materials References

Chapter 6 Group III: B, Al, Ga, In and TI By Simon Aldridge 1 General 2 Boron 2.1 B(C6F& and Related Boranes 2.2 Borate Anions 2.3 Boron Hydrides 2.4 Boratabenzenes and Related Ligands 2.5 Boron-containing Materials 2.6 Boron-based Materials 2.7 Boron-based Ligand Systems 2.8 Boronic Acids 2.9 Suzuki and Other Coupling reactions 2.10 Diboron(4) Reagents 2.1 1 Borane Functionalized Cyclopentadienyl Ligands 2.12 Miscellaneous 3 Aluminium 3.1 Sub-valent Aluminium and Aluminium Clusters 3.2 Aluminoxanes, M A 0 Models and Aluminium in Olefin Polymerization 3.3 Aluminium Derivatives Containing Bonds to Group 15 Elements 3.4 Aluminium Derivatives Containing Bonds to Group 16 Elements 3.5 Aluminium Organometallics in Organic Synthesis 3.6 Miscellaneous Examples 4 Gallium 4.1 Sub-valent Gallium and Gallium Clusters 4.2 Gallium Derivatives Containing Bonds to Group 15 Elements 4.3 Gallium Derivatives Containing Bonds to Group 16 Elements 4.4 Gallium Organometallics in Organic Synthesis 4.5 Gallium Hydrides 4.6 Miscellaneous 5 Indium 5.1 Sub-valent Indium and Indium Clusters

121 122 123 123 125 126

130 130 130 130 132 133 134 135 135 136 138 138 140 141 143 145 145 146 148 153 155 156 157 157 160 161 162 163 163 165 165

Contents

X

5.2 Indium Derivatives Containing Bonds to Groups 15 or 16 5.3 Indium Organometallics in Organic Synthesis 5.4 Miscellaneous Examples 6 Thallium References Chapter 7 Group 1 4 Silicon, Germanium, Tin and Lead By Richard A. Layfeld 1 2 3 4 5 6

Overview Silylenes and Silyl Anions Germylenes, Stannylenes and Plumbylenes Multiply Bonded Compounds n-Bonded Compounds Group 14 Organometallics as Ligands at Transition Metal Centres References

Chapter 8 Group 15: Phosphorus, Arsenic, Antimony and Bismuth By Matthew D. Francis 1 2 3 4

Phosphorus Arsenic Antimony Bismuth References

Chapter 9 Organic Aspects of Organometallic Chemistry By Guy C. Lloyd-Jones

166 167 167 167 168 177 177 177 180 182 185 186 188 191 191 199 203 204 205 209

209 1 Introduction 209 2 Organomagnesium Reagents 3 Reactions Involving Organozirconium and Organotitanium Intermediates 211 3.1 Zirconium Species 21 1 3.2 Titanocene Species 216 218 3.3 Zirconium and Titanium Ally1 Complexes 3.4 Alkoxy- and Aryloxy- Titanium and Zirconium Species 218 4 Oxidation and Reduction 222 4.1 Oxidation 222 4.2 Reduction 226 5 Isomerism, Cycloisomerisation, Cyclisation and Cycloaddition 227 5.1 Isomerisation 227

xi

Contents

5.2 Cycloisomerisation 5.3 Cyclisation 5.4 Cycloaddition 6 Conjugate Addition and Allylic Substitution Reactions 7 Cross Coupling Reactions 8 Metathesis 8.1 Ring Opening and Ring Closing Metathesis of Allienes 8.2 Cross-Metathesis and Isomerisation of Alkenes 8.3 Alkyne Metathesis 8.4 Miscellaneous 9 Transition Metal Catalysed Hydrodgenation and Hydrometallation 9.1 Use of Gaseous Hydrogen and Bidentate Phosphorus Based Ligands 9.2 Use of Monodentate Phosphorus Based Ligands 9.3 Transfer Hydrogenation 9.4 Transition Metal Catalysed Hydroboration, Hydrosilylation and Hydroarylation 10 Organozinc and Mai Group Reagents 10.1 Organozinc Species 10.2 Organoaluminium Reagents 10.3 Organotin Reagents 10.4 Organobismuth Reagents 10.5 Organoboron Reagents References

Chapter 10 Complexes Containing Metal-Carbon o-Bonds of Groups 4 and 5 (Including Multiple Bonded Species) By Jason M . Lynam References

228 229 234 237 24 1 248 248 250 252 255 256 256 259 262 264 267 267 269 271 27 1 272 275 279 287

Chapter 11 Complexes Containing Metal-Carbon o-Bonds of the Group 7 (Including Multiple Bonded Species) 289 By Jason M . Lynam References

Chapter 12 Organo-Transition Metal Cluster Complexes By Mark G. Humphrey and Marie P . Cifuentes 1 2 3 4

Introduction General Reviews Spectroscopic Studies Theory

295 297 297 297 297 298

xii

Contents

5 Structural Studies 6 Large Clusters 6.1 Homonuclear High-nuclearity Clusters 6.2 Heteronuclear High-nuclearity Clusters 7 Group6 8 Group7 8.1 Homometallic Clusters 8.2 Mixed-metal Clusters Containing Only Group 7 Metals 9 Group8 9.1 Iron 9.2 Ruthenium 9.3 Osmium 9.4 Mixed-metal Clusters Containing Only Group 8 Metals 10 Group9 10.1 Cobalt 10.2 Rhodium 10.3 Iridium 10.4 Group 9 Clusters as Catalysts 11 Group 10 11.1 Nickel 11.2 Palladium 11.3 Platinum 11.4 Mixed-metal Clusters Containing Only Group 10 Metals 12 Group 11 12.1 Copper 12.2 Silver 12.3 Gold 13 Mixed-metal Clusters 13.1 Group 4 13.2 Group 6 13.3 Group7 13.4 Group 8 13.5 Group9 13.6 Group 10 13.7 Group 11 13.8 Clusters Containing Three Different Metals References

298 298 298 300 302 303 303 304 304 304 305 313 3 19 319 319 319 32 1 32 1 329 32 1 32 1 322 323 324 324 324 324 325 325 325 330 331 336 337 338 338 340

...

Contents

Xlll

Chapter 13 Complexes Containing Metal-Carbon o-Bonds of the Groups Iron, Cobalt and Nickel, Including Carbenes and Carbynes 349 B y Philip J . King 1 Introduction 2 Reviews and Articles of General Interest 3 Metal-Carbon s-Bonds Involving Group 8,9 and 10 Metals 3.1 The Iron Triad 3.2 The Cobalt Triad 3.3 The Nickel Triad 4 Carbene and Carbyne Complexes of Groups 8,9 and 10 References

Chapter 14 Transition Metal Complexes of Cyclopentadienyl Ligands By Ian R. Butler General Introduction General Chemistry, Main Group, Lanthanides and Actinides Titanium, Zirconium and Hafnium Vanadium, Niobium and Tantalum Chromium, Molybdenum and Tungsten Manganese, Rhenium and Technetium Iron, Ruthenium and Osmium 7.1 Ferrocenes, Ruthenocenes and Osmocenes Cobalt, Rhodium and Iridium Nickel, Palladium and Platinum References

349 349 350 350 359 363 372 372 393 393 393 397 399 400 403 406 408 433 434 435

Abbreviations

Ac acac acacen Ad AIBN amPY Ar Ar* Ar‘r arphos ATP Azb 9-BBN BHT Biim BINAP biPY Bis bma BNCT BP bpcd bPk BPz, Bu‘2bPY t-bupy Bz Bzac cbd 1,5,9-~dt chd chpt CIDNP CCOl (CO) cod coe cot CP/MAS CP CPR

acetate acetylacetonate N,N’-ethylenebis(acety1acetone iminate) adamantyl azoisobutyronitrile 2-amino-6-meth ylp yridine aryl 2,4,6-tri(tert-buty1)phenyl 33-bis(trifluoromethy1)phenyl 1-(diphenylphosphino)-2-(diphenylarsino)ethane adenosine triphosphate azobenzene 9-borabicycloC3.3.llnonane 2,6-dibutyl-4-methylphenyl biimidazole 2,2’-bis(dipheny1phosphino)1,l’-binaphthyl 2,2’-bipyridyl bis(trimethylsily1)methyl 2,3-bis(diphenylphosphino)maleicanhydride boron neutron capture therapy biphenyl 4,5-bis(diphenylphosphino)cyclopent-4-ene1,3-dione benzophenone ketyl (diphenylketyl) tetra(1-pyrazolyl)borate 4,4’-di-tert-bu t yl-2,2’-bipyridine tert-but ylpyridine benzyl benzoylacetona te cyclobu tadiene cyclododeca-1,5,9-triene cyclohexadiene cycloheptatriene chemically induced dynamic nuclear polarisation cobalamin cobaloxime [Co(dmg), derivative] cycloocta-1,5-diene cyclooctene cyclooctatriene cross polarisation/magnetic angle spinning $-c yclopen tadienyl $-alk ylcyclopentadienyl xv

XVI

CP* CP’ Cp”

cv

CVD CY Cyclam CYm CYttP dab dabco dba dbpe DBU DCA depe depm DFT diars diarsop dien diop

DIPAMP diphos diPP dipyam DMAD DMAP dmbPY DME DMF dmg dmgH dmgH2 DMP dmpe dmpm dmpz DMSO dpae dpam dPPa dPPb dPPbZ dPPe dPPf dPPm dPPP

Abbreviations

q5-pentamethylcyclopentadienyl trimethylsil ylcyclopentadienyl tet ramethyleth ylcyclopentadienyl cyclic voltammetry(ogram) chemical vapour deposition cyclohexyl 1,4,8,11-tetraazacyclo tet radecane p-cymene PhP(C H ~ C H ~ C H ~ P C Y ~ ) ~ 1,4-diazabutadiene 1,4-diazabicyclo[2.2.2]octane dibenzylideneacetone 1,2-bis(dibutylphosphino)ethane 1,s-diazabicyclo[5.4.01undec- 7-ene 9,lO-dicyanoanthracene 1,2-bis(diethylphosphino)ethane 1,2-bis(diethylphosphino)methane density functional theory o-phenylenebis(dimethy1)arsine { [(2,2-dimethyl- 1,3-dioxolan-4,5-diyl)bis(methylene)] bis[diphenylarsine]} dieth ylenetriamine { [(2,2-dimethyl- 1,3-dioxolan-4,5-diyl)bis(methylene)]bis- 1[diphenylphosphine]} 1,2-bis(phenyl-o-anisoylphosphino)ethane 1,2-bis(diphenylphosphino)ethane 2,6-diisopropylphenyl di-(2-pyridyl)amine dimet h yl acet ylenedicarbox ylate 2-dimethylaminopyridine dimet h yl bipyridine 1,2-dimethoxyethane N,N-dimethylformamide dimethylglyoximate monoanion of dimethylglyoxime dimet hylgly oxime dimet hylpiperazine 1,2-bis(dimethylphosphino)ethane bis(dimet hy1phosphino)methane 1,3-dimethylpyrazolyi dimethyl sulfoxide 1,2-bis(diphenylarsino)ethane bis(dipheny1arsino)methane 1,2-bi~(diphenylphosphino)ethyne 1,4-bis(diphenylphosphino)butane 1,2-bis(dipheny1phosphino)benzene 1,2-bis(dipheny1phosphino)ethane 1,l’-bis(dipheny1phosphino)ferrocene bis(dipheny1phosphino)methane 1,3-bis(diphenylphosphino)propane

Abbreviations

DSD edt EDTA ee EELS EH MO ELF en ES EXAFS F6acac Fc Fe* FP FP’ FTIR FVP glyme GVB HBpz3 HBpz*3 H4cyclen HEDTA hfa hfacac hfb HMPA HNCC HOMO IGLO im IS*

ISEELS KTP LDA LiDBB LMCT LNCC MA0 MezbPY Me6[14]dieneN4 Me6[141N 4,7-Me2phen 3,4,7,8-Me4phen Mes Mes* MeTHF mcpba MLCT

xvii diamond-square-diamond ethane- 1,2-dithiolate eth ylenediaminetetraacetate enantiomeric excess electron energy loss spectroscopy extended Hiickel molecular orbital electron localisation function ethylene-1,2-diamine MS electrospray mass spectrometry extended X-ray absorption fine structure hexafluoroacetylacet onate ferrocenyl Fe(CO),Cp * Fe(CO)*CP Fe(C0)2q5-(C5H4Me) fourier transform infrared flash vacuum pyrolysis ethyleneglycol dimethyl ether generalised valence bond tris(pyrazoly1)borate tris(3,5-dimethylpyrazolyl)borate tetraaza- 1,4,7,10-cyclododecane N-hydrox yet hylethylenediaminetetraacetate hexafluoroacetone hexafluoroacetylacetonato hexafluorobutyne hexamethyl phosphoric triamide high nuclearity carbonyl cluster highest occupied molecular orbital individual gauge for localised orbitals imidazole 2,4,6-triisopropylphenyl inner shell electron energy loss spectroscopy potassium hydrotris( 1-pyrazoly1)borate lithium diisopropylamide lithium di-tert-butylbiphenyl ligand to metal charge transfer low nuclearity carbonyl cluster methyl alumoxane 4,4’-dimethyl-2,2’-bypyridyl 5,7,7,12,14,14-hexamethyl1,4,8,11-tetraazacyclo tetradeca4,lldiene 5,5,7,12,12,14-hexamethyl-l,4,8,1l-tetraazacyclotetradecane 4,7-dimethyl-1,lO-phenanthroline 3,4,7,8,-tetramethyl-1,lO-phenanthroline mesityl 2,4,6-tri bu tylphenyl methyltetra hydrofuran metachloroperbenzoic acid metal-ligand charge transfer

Abbreviations

XVlll

meth ylrhenium t rioxide MTO 1-napht hyl nap norbornene nb norbornadiene nbd N-bromosuccinimide NBS N-chlorosuccinimide NCS neutron capture theory NCT neopentyl Neo 1-naphthyl NP N(CH$H2PPh2)3 nP3 nitrilotriacetate nta octaet hylporph yrin OEP trifluoromethanesulfonate (triflate) OTf p - t oluenesulfonate (tosylate) OTs phthalocyanin Pc photoelectron spectroscopy PES pentameth ylenediethylenetetramine PMDT pentane-2,4-dionate Pd 1,lo-phenanthroline phen pyridine-2-carboxylic acid pic ( )-pinany1 Pin pentamethyldiethylenetriamine Pmedta P(CH2CH2PPh& PP3 [PPN] "Ph,P)2NI + pyridine PY p yridazine PYdZ pyrazolyl PZ (R)-(+ )- 1,2-bis(diphenylphosphino)propane R-PROPHOS R,R-SKEWPHOS (2R,4R)-bis(diphenylphosphino)pentane radial distribution function RDF ring opening metathesis polymerisation ROMP salicylaldehyde sal N,N'-bis(salicyla1dehydo)ethylenediamine salen N,N-bisalicylidene-o-phenylenediamine saloph self consistent field SCF tetracyanoethylene TCNE 7,7,8,8-tetracyanoquinodimethane TCNQ 2,2',2"-terpyrid yl terPY 1,1,4,7,10,10-hexaphenyl1,4,7,10-tetraphosphadecane tetraphos trifluoroacetic acid TFA tetrafluorobenzobarrelene tfbb trifluoroacetylacetonato tfacac tetrahydrofuran THF thiosalicylate (Zthiobenzoate) thsa tetrahydrothiophen tht NNN'Nt'-tetramethyl-2-butene-1,4-diamine TMBD (tmena) tetramethylet hylenediamine TMEDA 2,2,6-6-tetramethylpiperidino tmP tetramet hylsilane TMS tolyl to1

+

+

Abbreviations

TP TP* TPP Trip Triph triphos TRIR Tsi TTF vi WGSR XPS XYl

XlX

hydrotris( 1-pyrazoly1)borate hydrotris(2,5-dimethylpyrazolyl)borate meso-tet raphenylporphyrin 2,4,6- t riisopropylphenyl 2,4,6-( t ripheny1)phenyl 1,1,l-tris(diphenylphosphinomethy1)ethane time resolved infrared (spectroscopy) tris(trimethy1silyl)methyl(Me3Si)3C tetrathiafulvalene vinyl water gas shift reaction X-ray photoelectron spectroscopy XYlYl

1 Theoretical Organometallic Chemistry By A.J. BRIDGEMAN

1

Introduction

This chapter aims to cover theoretical and computational studies on organometallic molecules. Section 2 covers the s-block elements, Section 3 covers the p-block metals and Section 4 covers the d- and f-block metals. Clusters, carbonyls and metal-metal bonded systems containing M-C bonds are included. Cyanide complexes, metal fullerene derivatives, extended systems and organic species on metal surfaces are excluded except where calculations have been performed on model complexes designed to mimic solid state and surface chemistry. Only a brief mention of the computational method is given. Standard abbreviations for computational methods are employed throughout. Given the plethora of basis sets available in modern computational chemistry programs and the variety of basis set designations employed by authors in this field, no description of basis sets is given. The reader should consult the original work for further details of the computational method and the basis set.

2

s-Block Metals

2.1 Structural, Spectroscopic and Mechanistic Studies. - 2.1.1 Metal AZkyls. B3LYP calculations have been used' to study the structures and oligomerisation of methyllithium and tert-butyllithium (RnLi,, n = 1-4; R = Me, t-Bu) and phenyllithium (Ph,Li,, n = 1,4) leading to good agreement with available experimental and previous computational results. Aggregation energies, computed at the B3LYP/6-3 11+ G(2d,p) + ZPC//B3LYP/6-3 1+ G* level, for the tetramers of methyllithium, t-butyllithium, and phenyllithium are -124.4, -108.6, and -117.2 kcal mol-*, respectively. B3LYP studies2 of the inversion of methyllithium in both tetrameric and dimeric aggregates show that inversion occurs either via dissociation of the tetramer into the dimers, passage of a four-membered-ring transition state, and association of the dimers to form the inverted tetramer, or via a nonconcerted route involving an eight-memberedring transition state. Both routes are predicted to have similar activation energies. Organometallic Chemistry, Volume 3 1 0 The Royal Society of Chemistry, 2004 1

2

Organometallic Chemistry

The structure of the lithiated 4-isopropyl-3-methylthiomethyl-5,5diphenyloxazolidin-2-one derivative features coordination of Li to carbonyl oxygen and an antiperiplanar arrangement of the C, Li and S, CH3 bonds, according to a B3LYP study3. The Si-Li bond lengths and the unusual 29Si chemical shifts in amino-functionalized silyllithium compounds are influenced by the electronegativity of substituents located at the nitrogen centre according to RI-DFT and IGLO calculations4. B3LYP, CBS-Q, CBS-QB3, G1, G2MP2, G2, G3, and G3B3 methods have been used5to calculate the vibrational spectra and ionization energies of BeCH3, MgCH3 and CaCH3. The B3LYP method when used with a large basis set such as 6-311+ + G (3df, 3pd) is accurate for both vibrational frequencies and ionization energies. G2M(MP2 calculations show6 that the BeO+CH4 reaction proceeds by barrier-less formation of the CH4Be0complex followed by isomerization to a CH3BeOH molecule. This can dissociate without an exit barrier to BeOH + CH3 or rearrange through a high barrier to a weakly bound CH30HBe complex. The most stable structure of lithium dimethylaminoborohydride is a dimer in which the lithium and boron atoms were bridged by two hydrogen atoms, similar to the three-center twoelectron bonds in diborane according to a B3LYP study7.The rotation barrier of the C-N bond in lithium acetamide is less than 10 kcal mol-' with conjugation effects comparable to that in vinylamine, according to an MP3 and B3LYP studf. The possible structures of dimers of 2-(lithiomethyl)-l-methylimidazole, and the role of THF and 1,2-dimethylimidazolein the solvation of the dimers have been investigated at the PM3 and B3LYP levels' indicating that 1,2dimethylimidazole will replace THF in the salvation sphere. The rate-limiting activated complex for the deprotonation of epoxide by chiral lithium amides is composed of two molecules of the monomer of lithium amide and one molecule of epoxide according to a B3LYP study". The structures and vibrational frequencies of [MgC3] and [MgC3H] have been investigated at the MP2 and B3LYP levels" and their energies have been computed at the G2 and CCSD(T) levels. The 2A1ground state of [MgC3] has a rhombic structure. Its four-membered ring is maintained upon protonation to give the ground state of [MgC3H] +. +

+

+

2.1.2 Interactions with Unsaturated Organic Systems. The stable isomers of the ferrocene-lithium cation gas-phase ion complex have been studied'* using the B3LYP method. The most stable isomer 1 has a Li+ bonded on top of a cyclopentadienyl ring, while in the other isomer 2 lithium binds to the central iron atom. The energy difference between the isomer is estimated to ca. 8 kcal mol-' with an activation energy for their interconversion of ca. 2.6 kcal mol-'. The structure of ferrocene is not greatly affected by either type of coordination. The interaction energies of the Li+, Na+ and K + cations with the z systems benzene, toluene, ethylbenzene, and tert-butylbenzene have been calc~lated'~ at the MP2 level. Induction and electrostatic interactions are the major source of the attraction. B3LYP methods have been used to mode114the protonation and the binding of Li+ to corannulene. A proton attaches preferentially to one carbon atom, forming a a-complex. The lithium cation positions itself

1: Theoretical Organometallic Chemistry

3

4'' I

Fe

I

2

1

Scheme 1

preferentially over a ring. MP2 and MPWlPW91 calculations have been reported" for the interaction of Na+ with phenylalanine and alanine leading to a higher affinity for the former by 5-7 kcal mol-I. The ground-state geometries of the complexes of C2H2 and C2H4with H+, Li+, and Na+ ions have been optimized16 at the B3LYP and MP2 levels. Bond indices and localized MOs indicate the presence of three-centre bonding in all the complexes. In the protonated species the bonding is found to be predominantly covalent; in the Li+ and Na+ complexes the covalent interaction also plays a fairly important role. The main characters of the potential energy surface of the methylenelithoflurosilylenoid (H2C= SiLiF) have been studied at the G2(MP2)leve117revealing four equilibrium structures corresponding to a n-complex, a three-membered ring, a a-complex and a silene, and three isomerization transition states. The non-planar n-complex has the lowest energy. The cation-n complexation of oligo[(dimethylsilylene)phenylene] with alkali metal cations has been studied at the MP2 leve1I8showing that %Me3group but not the SiH3group substantially increases the attraction between the cation and the n system. Significant conformational changes on the silamacrocycles occur due to complexation. The structures and vibrational spectra of alkali metal cyclopentadienyl(CpM, M = Li, Na, K) and pentamethylcyclopentadienyl (Cp*M, M = Li, Na) complexes have been studied at the BLYP leve1I9leading to a reassignment of the IR spectra. The apparent non-VSEPR shapes of do complexes including the ($C5R5)2M(M = alkaline earth) have been reviewed20identifying the roles of the metal d-orbitals and metal-ring n bonding in controlling the shape. Li bonds to pyrenes, anthracenes and phenanthrenes with binding energies of 143,211 and 146 kJ mol-' respectively on interstitial and edge sites according to a B3LYP study2'. Li dimers attached to anthracene and phenanthrene with binding energies of 200 and 146 kJ mol-', respectively, are also predicted. The aromatic rings lose their planarity when they accommodate Li atoms. The complex formed between Na and C2H2is very weak with large metal-ligand distances and can be considered as effectively a ground state ligand and a metal atom according to CEPA calculations22.The nominally hypervalent complexes LiXCH2( X = 0 or NH) are stable, residing in three potential energy minima, according to HF calculation^^^. Two states resemble C-centered radicals carry-

4

Organometallic Chemistry

ing an ion pair, Li+[XCH2]-, and can be viewed as lithiated derivatives of hydroxymethyl (HOCH2) or aminomethyl (H2NCH2)radicals. The third state is a conventional, electrostatically bonded Li-X = CH2 complex with an essentially intact X-C double bond and the unpaired electron located at the metal atom. The most stable structure of phenyl calcium hydride in donor solvents is dimeric, although monomers or tetramers may be present at very low or very high concentrations respectively according to a B3LYP Hydride bridging is favoured over phenyl bridging and the coordination number of six is predicted to be dominant for these calcium species in solution. The interaction between alkaline earth metal ions and benzene is very strong according to B3LYP and MP2 calc~lations~~. Charge transfer and induction make a significant contribution to the bonding through metal s-ring rc and metal p-ring rc orbital interactions. The gas-phase and solvated structures of fluorescent indicators, used for the intracellular determination of Ca2+ and Mg2+ and their coordination to these metals has been studied at the B3LYP Binding to the metal is enhanced by electron-donating substituents and weakened by electron-withdrawing groups on the indicator. Conversion of carbon monoxide to formaldehyde can be catalyzed by beryllium oxide in the gas phase, according to a MP2 study2’. One possible mechanism involves Be0 reacting with CO to form a OBeOC complex which interacts with H2 to give a Be0 / H2CO complex. This decomposes to Be0 and formaldehyde without an exit barrier. A second possible mechanism involves reaction of Be0 with HZfollowed by CO insertion into the Be-H bond of HBeOH to form HCOBeOH. This undergoes a 1,3-hydrogen shift from carbon to oxygen yielding the OBeOCH2 complex which decomposes to the final products. 3

p-Block Metals

3.1 Structural and Spectroscopic Studies. - 3.1.1 Metal Alkyls and Analogues. Alkyl, silyl, and phosphane ligands contain sp3-hybridized C, Si and P donor atoms respectively and are related to each other by the isolobal analogy. This analogy has been reviewed enabling the reactivity and bonding of these ligands to be consistently described28. BP86 calculations have been used to calculate29the energies and structures of 36 different methylaluminoxane cage structures with the general formula (MeAlO),, where n ranges from 4 to 16. By fitting to the calculated energies, a formula for the total energies of these clusters, thought to be activators in Cp2ZrMe2/A1Mesethene polymerization, has been devised. Topological arguments show that the clusters contain a limited amount of square faces as compared to octagonal and hexagonal ones. The lack of square faces, with their strained Al-0 bonds, may explain the high molar A1:catalyst ratio required for activation. BP86 calculations30have been used to obtain structures and energies of over 30 different structures with the general formula (A1OMe);(AlMe3), where n ranges from 6 to 13 and rn ranges between 1 and 4, depending upon the structure of the parent (AlOMe), cage. Trimethylaluminum does not bond to

1 :Theoretical Organometallic Chemistry

5

methylaluminoxane for cages where n = 12 or n 2 14. A comparison of inorganic and organometallic fluorides in the framework of the hard and soft arid and base principle, has been made using HF, MP2 and B3LYP calculations on alkylaluminium fluorides3'. A new theoretical model is proposed to put in equation form the qualitative statements of the Bent rule. The model allows the rationalization of the tendencies of bond angle variation in R2MX2 systems containing a main group metal, in terms of hybridization of the central atom and the reciprocal influence of hard and soft ligands. The structural, electronic, and thermochemical properties of indium compounds which are of interest in halide transport and organometallic chemical vapor deposition processes, including In(CH3),In(CH3)H,In(CH3)H2,In(CH3)*, In(CH3)2H, In2(CH3)4, In(CH3)3, In(CH3)2CH2, In(CH3)CH*, In(CH2), (CH3)31n:NH3, (CH3)31n:N(CH3)3, (CH3)31n:N(H2)N(H2),In(CH3)2(NH2)and In(CH3)(NH),have been studied by B3LYP calculations and statistical thermodynamic methods32.The electronic structures of the ground and lowest lying excited state of the silicon methylidyne radical (HCSi) have been investigated at the SCF, CISD, CCDS and CCSD(T) levels33.The ground and first excited electronic states, 211and 2X+ respectively, it is linear with C-Si triple bond character. The linear excited 2Z+ state has a real degenerate bending vibrational frequency, whereas the groundstate is subject to the Renner-Teller effect and presents two distinct real vibrational frequencies. The HCSi radical is an A-type Renner-Teller molecule.Thegeometries, energies and vibrational frequencies of the X 2Hand A 'X+ states of the HCGe radical have been investigated at the SCF, CISD, CCSD, and CCSD(T) levels34.The ground state is linear with C-Ge triple bond character. The dipole moment of HCGe is 0.122 D according to HF

calculation^^^.

H,Ge(H2)+(n = 0,l) play an important role in the unimolecular dissociation of the metastable [Ge,H,]-'''+ (n = 2, 3) cations according to combined mass spectrometry and CCSD(T,full) and B3LYP studies3? [Ge,H2] + could exist in one of three low lying states, 2A1HGeH+, 2B2Ge(H2)+and 2B1Ge(H2)+.The vibrational frequencies of the gauche, ortho, transoid and anti conformations of the tetrasilanes SiMe3SiX2SiX2SiMe3 (X = H, F, C1, Br, I) have been studied at the B3LYP level3'. Silanes are weak Lewis acids towards neutral N and 0 donor Lewis bases as the energy gained by interaction is not much bigger than the energy necessary to change the geometry of the silane from the ground state to that in the complex according to a B3LYP The interaction between Si and the donor atoms exhibit an highly ionic character, increasing from SiH3Xto SiH2X2to Six4and from X = Br to C1 to F. [Me4Ga4(p3-Te)2(p3-0)2] and [Me4Ga4(p3-Te)2(p3-0)2]are not energetic minima according to B3LYP calculation^^^ but are possible intermediates in the oxidation of [MeGa(p3-Te)I4.Themolecular structure of 1,2-di-tevt-butyltetrachlorodisilane, ButC12SiSiC12Butcalculated at the MP2 levela has a conformation distorted slightly from the fully staggered. The structure and EPR spectra of models of the phosphaallene fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butylpheny1)phosphane have been calculated at the B3LYP level4' suggesting that the anion is readily protonated and that the experimental EPR spectra are due to the

6

Organometallic Chemistry

phosphaallylic radical, Bonding and structural studies on heterobimetallic complexes in which a Group 1 metal centre counteracts the negative charge placed on a Group 12 or higher Group 13 metal have been reviewed42. to optimize the structures of SnMe2X2, H F calculations have been [SnMe2X3]-, trans-[SnMe2X4I2-(X = F, C1, Br, I), cis-[SnR2Cl4I2-(R = Me, Et), SnEt2C12,[SnEt2C13]-, and trans-[SnEt2Cl4I2-. The formation of [SnR2X3]anions from SnR2X2and X- is an exothermic process in the gasphase but [SnR2X4I2- anions are unstable in the gas phase toward dissociation into [SnR2X3]- and X-. cis-[SnR2X4I2-species (R = Me, Et) are unstable with respect to their trans isomers by ca. 79 kJ mol-’. H F and B3LYP calculated charge distributions in [(CH3Sn)12014(OH)6]2f the hexacoordinated tin atoms to be harder and the pentacoordinated ones softer. The molecular properties of diorgano- and triorgano-tin(1V) complexes of [protoporphyrin-1x1 and [meso-tetra(4-carboxyphenyl)porphine]derivatives, determined at semi-empirical and ab initio levels45,are essentially independent of the presence of the peripheral tin atoms. The coordination mode, the structures and the effect of methyl substitution upon complexation of [H3AlX(CH3)2]- (X = N, P, and As) and H3AlY(CH3)2(Y = 0, S, and Se) donor-acceptor complexes have been studied at the G2(MP2) The interaction of the alane with the donor ligand is stronger in the anionic complexes than in the neutral ones and the methylated complexes are more stable than the hydrogenated ones. Alane-[X(CH3)3]- (X = C, Si, and Ge) and alane-Y(CH3)3(Y = N, P, and As) have also been investigated as donoracceptor complex types at the G2(MP2) level4’ again showing that the anionic complexes are more stable than the neutral ones. This stability decreases when going from carbon to germanium for [H3AlX(CH3)] - complexes and from nitrogen to arsenic for H3AlY(CH3)3complexes. The equilibrium constant for the reaction of trimethylgallium and ammonia has been determined at the H F level is -15.9 kcal mol-’ in good agreement with experiment4*.The thermochemistry of dissociation and elimination reactions of organogallium precursors for the GaN chemical vapor deposition has been studied at the B3LYP leading to geometries, relative energies, vibrational frequencies of R,GaNR’, species, and their dissociation products (NR,, GaR,, x = 1-3; (R, R’ = H, CH3). Methane elimination from the source adducts is exothermic at standard conditions, while hydrogen elimination is endothermic. For R = H, CH3, elimination reactions are predicted to be more favourable compared to dissociation into components. Molecular mechanics studies have been performed” on helical polysilenes and indicate a double well potential energy curves corresponding to almost enantiomeric helices. 3-aminopropyltrimethoxysilane and N,N-dimethylamino-propyltrimethoxysilane exist as open-chain structures according to an analysis of their NMR spectra using GIAO calculations of chemical shifts5’. [Me3Ge]+ binds with the 0 atom of the carbonyl group of acetophenones leading to a Ge-0 bond with pronounced covalent character, leading to the development of positive charge at the benzylic carbon atom according to B3LYP

calculation^^^.

1: Theoretical Organometallic Chemistry

7

3.1.2 Clusters. A generally applicable electron-counting rule called the mno rule that covers macropolyhedral boranes, metallaboranes, and metallocenes and any combination thereof has been presented53.According to this rule, rn + n o number of electron pairs are necessary for a macropolyhedral system to be stable where m is the number of polyhedra, n is the number of vertices, and o is the number of single-vertex-sharingcondensations. The origin of the mno rule is explored by using fragment molecular orbitals. The ground state of C3Si is 'X- and linear whilst the first excited state is either 'A or 'X+ according to B3LYP and MP2 ~ a l ~ ~ l a t iThe o n ground ~ ~ ~ . 'X- state of C4Si2is also linear. A CASSCF of Sic3 and Si2C2predicts a linear 'Xground state for Sic3 with a terminal Si atom. The global minimum of Si2C2is predicted to be a rhombic structure with the triplet state of linear Si-C-C-Si lying 1.0 kcal molt' higher inenergy at this level. The generalized tight-binding molecular dynamics scheme has been extended to treat heteroatomic interactions in covalent clusters56.The calculated structures of small and intermediate size Si,C, clusters show good agreement with experiment. The equilibrium geometries of two isomers of cyclic Sic3with rhombic or 'butterfly' shapes distinguished by a Si-C and a C-C transannular bond have been determined by HF, CCSD and CCSD(T) calc~lations~~. At all levels of theory, the isomer with the transannular C-C bond is predicted to be more stable, with a likely energy separation of around 5 kcal mol-*. There are nine local minimum isomers on the potential energy surface of the Si4H2cluster, according to B3LYP and MP2 calculationss8, in which the Si4frame is slightly distorted by the dissociative adsorption of H:! on it. The most stable isomer of Si4H2is a classical structure with both hydrogen atoms bonded to a single silicon atom. The germanium-carbon cluster GeC3Ge has a linear 'Zg+ ground state with two terminal Ge atoms according to B3LYP calculationss9.The energetics and the electronic structure of A1,C clusters (n = 3,4,5; 11,12,13)have been studied6' at the BPW91 level. Ionization potentials decrease steadily from A13C to Al5C while that of Al12Cis higher than its neighbours. In small clusters, carbon adopts planar coordination and adopts an interior site in the larger clusters. The nonclassical compounds BB6(CH)3and [BB6X2]- (X = NH, 0)containing a hexacoordinated central boron atom have stable planar structures according to MP2 and B3LYP calculations61.The minimum energy conformations and transition states for bowl-to-bowl inversion of the heterobuckybowls C&H6 (X = 0,NH, CH2, BH, S, PH, PH3, Si, SiH2,and AlH) have been calculated at the B3LYP The size of the heteroatom seems to exclusively control the bowl depth and rigidity but the bond length alternation is controlled by electronic factors. Structural and electronic properties of semiconductor binary microclusters A,B, (A,B = Si,Ge,C) have been investigated at the B3LYP for s = m + n 5 10. The binary clusters are have lower symmetries than the elemental Si and Ge clusters of this size and to have either singlet or triplet ground states, depending on specific cluster size, cluster composition, and configurations. The ground states of [Gh]-, [In4]- and K2Ga4(C6H5)2 possess two delocalized 3t electrons and are considered to be aromatic, according to MP2, CCSD(T) and B3LYP calculation^^^.

+

8

Organometallic Chemistry

3.1.3 Cyclopentadienyl Complexes and Analogues. A variety of density functionals have been to study the structures of unsubstituted Group 14 metallocenes (C5H5)2M(M = Si, Ge, Sn, Pb). The minimum energy structure for each metallocene is bent, but the preference for a bent over a linear geometry is slight and decreases from 3.1 kcal mol-' for M = Si to 0.27 kcal mol-' for M = Pb. The energy difference between the two forms of stannocene found crystallographically is very small. The effect of intramolecular packing effects on the structures of main group cyclopentadienyl has been reviewed66and the throughspace coupling concept, a molecular orbital representation of van der Waals repulsive-attractive forces, introduced as a model for explaining bending and slipping of the Cp rings. The geometries and energetics of the borocenium cation, [BCp2]+, and the decamethylborocenium cation, [BCp*J +,have been studied by using B3LYP and ONIOM ~ a l ~ ~ l a t iThe o n ground-state ~~~. structure of [BCP*~] has a q' and a q5ring which is more stable than the q5:q5structure by around 55 kcal mol-'. The ring exchange process for the [BCp2]+ cation has an activation energy barrier of 14.7 kcal mol-'. +

3.1.4 Carbonyls. The properties, bonding and vibrational frequencies in the monocarbonyl complexes of the s- and p-block main group elements of the first five periods have been investigated at the BP86 level? The bonding is described in terms of synergistic M t C O a-donation and M-CO a-backbonding, shown in (a)(Scheme 2), analogous to that used for transition metal carbonyls, shown in (b), revealing that the bonding in s-block monocarbonyls is dominated by the repulsive a-interaction and that there is significant involvement of the filled n-level on CO for electronegative elements. B3LYP studies have identified6' the ground states of AsCO and AsCO- to be 'a and 'X+,respectively. 3.1.5 Low Valent and Multiply Bonded Systems. B3LYP and B3PW91 calculations have been reported7' on the nature of Ga-Ga bond in Na2[Ar*GaGaAr*] (Ar* = C6H3-2,6-(C6H2-2,4,6-i-Pr3)2). The short Ga-Ga bond length is found to be the result of several factors, including Na-terphenyl and terphenyl-terphenyl ionic interactions, direct Ga-Na-Ga bridge bonding, and adjustments in the C-Ga-Ga angles due to the steric requirements of the i-Pr groups on the bulky rn-terphenyl ligands. B3LYP calculations have also been used to study7' the effects of bulky aryl groups, C6H2-2,4,6-{CH(SiMe3)2}3 and C6H3-2,6-(C6H2-2,4,6i-Pr3)2,on germanium-germanium and tin-tin triple bonds. Although the large groups weaken the M=M bonds, they are predicted to prevent dimerization. The nature of bonding and the origin of the structural variations in the ethyne analogues [M2R2I2-, Li2[M2R2Jand X2RZ (M = B, Al, Ga, In, T1; X = C, Si, Ge, Sn, Pb; R = H, Me, Ph) have been studied using B3LYP calc~lations~~. Transbending is predicted for all systems with M # B or X ;t C as a result of minimisation of steric repulsions between the two ends of the molecule whilst enhancing the bonding to the substituent even though this results in a weakened metal-metal bond as evidenced by bond lengthening and a reduction in the bond order.

1 :Theoretical Organometallic Chemistry

9

1 donation

n-backdonation (in two planes)

C

0

o-donation n

n

x-backdonation (in two planes)

Scheme 2

Valence bond studies of silynes and disilynes lead to similar conclusions73:the 0-framework is more stable in the trans-bent geometry, but this comes at the expense of n-bond weakening. B3LYP studies of the bond strength of C-Si and Si-Si triple bonds that there is no overall strengthening of the bond in the trans-bent structure. SiCNN possesses a SisC triple bond and is predicted to reside in a very deep potential, at least 53.2 kcal mol-', on the potential energy surgace of S E N 2according to B3LYP and CCSD(T) calculation^^^. HF, MP2, CISD, CCSD, CCSD(T) and CCSDT calculations have been to investigate the two lowest-lying electronic states of the disilaethynyl (SiSiH) radical. The cyclic *A1state is predicted to be the ground state at the CCSD(T) and CCSDT levels, in agreement with experiment. The remaining levels predict the H-bridged 2B1state to be more stable. The isomerization of triplet and singlet HCGeF to C=GeHF and to HFC = Ge has been studied at the B3LYP and QCISD levels77and it is reduced by electronegative halogen groups. The relative stability of doubly bonded and doubly bridged disilenes containing heteroatom substituents have been calculated at the MP2, CCSD(T) and B3LYP levels7'. The conformation and bulkiness of the substituents controls the interplay between the Si=Si and bridged structures and in the bond order of the Si-Si bond. Substituent effects on the disubstituted heavier analogues of olefin A2X= YH2 (X, Y = Si, Ge; A = H, CH3, NH2, OH, F, and C1) have been in~estigated~~ at the B3LYP level suggest-

10

Organometallic Chemistry

ing that the the strength of the n-bond and the tendency to trans-bending are strongly related to the singlet-triplet energy gap (AE,,) of the corresponding disubstituted divalent species A2X - as AEst becomes larger, the mbond is weakened, the distortion increases, and the thermodynamic stability of the doubly bonded molecule is reduced. electronic properties of stable non-annelated and annelated heterocyclic carbenes and carbene analogues (silylenes, germylenes and nitrenium ions) have been investigated at the B3LYP and MP2 levels" indicating that they all have singlet ground states. The geometries, rotational constants, vibrational frequencies, proton affinities and heats of formation of the ground state planar structures of linear 1,4-diphosphabutadiyne P = C-C = P have been calculated at the B3LYP and CCSD(T) levelsg1leading to a PC-CP bond strength of 606 kJ mol-l. Cyanophosphapropyne N = C-C = P is the most stable isomer on the (C2NP) potential energy surface, followed by isocyanophosphapropyne C =N-C = P, the linear azaphosphadicarbon C = C = N = P and the bent isocyanophosphavinylidene N = C-P = C, according to B3LYP and CCSD(T) calculationsg2. NMR chemical shielding tensors for the Si=C bond in model compounds have been calculatedg3using the GIAO-MP2 and GIAO-B3LYP leading to isotropic 13Cand 29Sisignals with errors of 1-5 ppm compared to experiment. Larger errors in the anistropic values are ascribed to the experimental difficult in their measurement. B3LYP and QM/MM ONOIM methods have been usedg4to study the effects of bulky aryl groups on silicon-silicon triple bonds. It is predicted that the bulky groups could limit dimerization whilst maintaining sufficiently strong Si-Si bonds to make such molecules realistic synthetic targets. The n-donor ability of the pnicogen substituents on cations of the general type [A(XH2)3] + (A = C, Si; X = N, P, As, Sb, Bi) follows the order N c P < As c Sb c Bi whereas the intrinsic stabilization energies of the planar carbenium ions follows the reverse order according to MP2 and CCSD(T) calculationsp5. Orbital phase theory, HF and DFT calculations have been used to investigateg6the stabilities of the branched isomers of (E = C, Si, Ge, Sn) relative to the normal ones. Preferential stabilization of the branched isomers was shown to be general and controlled by the orbital phase. The nature and origin of the n-H interaction in both the ethene (olefinic)and benzene (aromatic) complexes of the first-row hydrides (BH3, CH4, NH3, H 2 0 , and HF) has been investigatedg7 using MP2 calculations. The strength of the n-H interaction increases from CH4 to HF due to changes in the attractive (electrostatic, inductive, dispersive) and repulsive exchange components of the total binding energy. B3LYP calculations indicateggthat the boroxine rings (HBO)3and (HB0)4possess strain enthalpies of 11.4 and 31.6 kJmol-', respectively and that the absence of eight-membered (RB0)4 rings is due to a combination of ring strain and the lability of the B-0 bond. When excited to a 4s state, the interaction of A1 with Ne and H2, DZ,NZ, CH4may be attractive or repulsive interactions, according to ab initio potential energy curvesg9,in contrast to higher states where the interactions are attractive. 3.2 Mechanistic Studies. - A new hybrid density-functional method KMLYP

1: Theoretical Organometallic Chemistry

11

which predicts transition state barriers with the same accuracy as CBS-APNO, and transition state barriers and enthalpies of reaction with smaller errors than B3LYP, BH and HLYP, and G2 has been presented and applied to a wide variety of reactionsw. B3LYP calculations have been used to study'' conjugate addition of lithium diorganocuprzte to acrolein, cyclohexenone, and 4,4-dimethylcyclohexenone in the presence of Me20 molecules coordinated on the lithium atoms. Solvation does not change the mechanism of the conjugate addition but increases the activation energy of the C-C bond-forming step through attenuation of the Lewis acidity of the lithium atoms. The substituent on the enone electrophile are found to have considerable influence on the activation energy. Nucleophilic addition of methyl magnesium bromide to a C-(2-pyrrolidinyl) nitrone is exothermic and proceeds via precomplexation of the nitrone with the organometallic reagent. Chelation is the main factor governing the experimentally confirmed preference for the Si attack leading to syn adducts according to H F and B3LYP calculation^^^. The unimolecular isomerization of the three membered (Me3SiN)2SiF2ring into the four membered cyclodisilazane ring has been studied using B3LYP and MP2 calculation^^^. The reaction enthalpy is calculated to be -72.1 kcal mol-' and the mechanism involves reductive insertion into a N-N single bond accompanied by migration of a phenyl group from Si to N. Potential energy surfaces for systems consisting of SiH3F and SiH3Cl molecules with addition of two water molecules (neutral hydrolysis), with one water and one ammonia molecule (base-catalyzed hydrolysis), and with two ammonia molecules (racemization), calculated at the B3LYP and MP2 levels94,show stationary points for tightly bound, low-entropy complexes with SiO and SiN bonds of length ca. 2.0 A. For SiH3F, the structures of these complexes are close to octahedral with NH3 or H20 in axial positions, while those for silyl chloride are closer to trigonal bipyramidal zwitterions formed by the SiH3cation with two nucleophiles and the C1 anion separated from Si by ca. 3 A. MP2, B3LYP, and CBS-Q calculations have been to study the mechanism of 1,2-addition reactions of water, methanol, and trifluoromethanol to Si = Si, Si = C , and C = C bonds. Two mechanisms emerge. In one, the reagent attacks a doubly bonded silicon atom leading to a small activation barrier and a strongly exothermic reaction. In the second mechanism attack is at a doubly bonded carbon and the barrier is high and the reaction is moderately exothermic. Homolytic (1,2)translocation reactions of SiH3, GeH3 and SnH3 groups between silicon and other Group 14 centres proceed via homolytic substitution mechanisms involving frontside attack at the heteroatom undergoing translocation according to a B3LYP study". HF, MP2 and higher level calculations have been performed97to explore the reaction potential energy surfaces of singlet silylene and germylene with water, methanol, ethanol, dimethyl ether, and trifluoromethanol revealing two new reaction channels involving H2 elimination following the initial formation of an association complex. The chemistry of gaseous germane and ethene has been investigated9*using a combination of experimental and B3LYP studies. Formation of an adduct between GeHi+ and H2C= CH2 is the initial step and is fairly

12

Organornetallic Chemistry

exothermic. The free energy of this reaction allows formation of species such as GeC2H;+, GeC2H5+,GeC2Hi+and GeCH3+.B3LYP calculations suggest99that silenes are probably not involved as intermediates in the reaction of the trichlorovinylsilane/t-BuLi reagent with alkynes. Ring opening of disilacyclopropylidenes provides a possible route to the yet unknown 1,3-disilaallenes according to an MP2 studylooof the potential energy surfaces for the elimination of LiCl from cyclo-CSi2H4C1Li. HF, AMl, B3LYP and first principles molecular dynamics calculations have been used"' to study the role of aluminum amidinate species in ethylene polymerization. Dinuclear amidinate structures are very stable toward decomposition but with sterically crowded substituents the insertion step is inhibited. With small substituents, chain termination by P-hydrogen transfer is predicted to dominate over insertion. MP2 calculations have been usedlo2to study the balance between olefin insertion and P-hydrogen transfer to monomer for all 'well-defined' aluminum polymerization catalysts reported to date. The balance is predicted to be significantly worse than for Me2A1Et,implying that nune of the proposed active species should give a high-molecular-mass polymer. Olefin polymerization at a single aluminium centre is predicted to be rather unlikely. Propane adds to coordinatively unsaturated aluminium, as in the clusters (H0)3Al(OH2),(x = 0, l), by aluminium insertion into a C-H bond, followed by hydrogen migration to an oxygen atom according to B3LYP calc~lations~~'~. Two pathways for the reaction of LiAlH4with formaldehyde and four transition states corresponding to axial and equatorial attack at cyclohexanone have been located in a HF and B3LYP studylo4.Analysis of the transition state structures suggests that electronic effects are more important than torsional effects in controlling stereoselectivity. The uncatalyzed hetero-Diels-Alder reaction of benzaldehyde with Danishefsky's diene proceeds as a concerted reaction with an unsymmetrical transition state. The catalytic reaction, studied using (Me0)2AlMe,followed by (S)-BINOL-A1Meas the catalysts at the AM1 and H F levels1o5, follows a two-step process. The first step involves nucleophilic attack of the activated diene to the carbonyl carbon atom, with an activation energy of up to 13 kcal mol-', depending on the catalyst and calculation method used, leading to an aldol-like local energy-minimum intermediate. The second step involves ring-closure, has a significantly lower activation energy and leads to the heteroDiels-Alder adduct. B3LYP calculations have been usedIo6to study the 10-X-2 ate complexes formed by reaction of vinyllithiums with vinyl halides. The results indicate that trifluoroethene is 35 kcal mol-' more acidic than ethene. The ate complexes generated by reaction of vinyllithium with vinyl bromide or iodide are calculated to be transition states, but the corresponding perfluorinated complexes are found to be minima on the potential energy surface. The interaction between s2atoms and methane has been studiedio7using MP2 and CCSD(T) calculations on the B+/nCH4hypersurfaces (where n = 1,2). The electrostatic complexes show a strong variation in the sequential binding energy. Insertion transition states can be stabilized by allowing the B+ ion to interact with multiple C-H o-bonds. Highly coordinated organotin (IV) enolates have higher nucleophilicity to organic halides and lower reactivity to carbonyl com-

1 : Theoretical Organometallic Chemistry

13

pounds than four-coordinated reagents due to increase in nucleophilicity and a decrease of Lewis acidity, according to a H F study"'. 4

d- and f-Block Metals

Structural and Spectroscopic Studies. - 4.1.I Bonding Models. An energy partitioning analysis and its application to three classes of transition metal complexes - neutral and charged isoelectronic hexacarbonyls TM(CO)6Y (TMq=Hfz-, Ta-, W, Re+, Os2+,Ir3+),Group-13 diyl complexes (C0)4Fe-ER (E = B, Al, Ga, In, TI; R = Cp, Ph), Fe(ECH3)5and Ni(ECH& and complexes with cyclic x-donor ligands F ~ ( C Pand )~ F~(v'-N~ has ) ~ been presented"'. The model addresses the question of ionic versus covalent bonding as well as the relative importance of 0 and n bonding contributions. The analogy between metaldihydrogen o-bonding and the metal-olefin x-bonding model has been discussedlloand extension to coordination by Si-H and C-H bonds outlined. Group theory forbids"' either Oh octahedral or D 3 h trigonal prismatic geometry for a six-coordinate early transition metal complex using a six-orbital sd5 manifold but intermediate C3bicapped tetrahedral geometries are allowed and are found in a few metal tris(dithio1enes) complexes. The stereochemical non-rigidity of trigonal prismatic metal tris(dithio1enes)may occur through a 'rotary electric switch' mechanism or through bicapped tetrahedral intermediates. The n-acceptor character of an isoelectronic series of ligands has been analyzed quantitatively112leading to a numerical scale in which the ligands are ordered according to their donor/acceptor character and their chemical stability. Computational studied of the occurrence of metalloaromaticity in metal chelate rings has been reviewed' 13.

4.1

4.1.2 Complexes Containing Metal-Carbon Bonds and Analogues. The use of molecular mechanics to calculate the structures and conformations of first-row transition metal complexes in the Cambridge Structural Database has been reviewed114. The interaction between first-row transition metals and carbenes has been studied at the BP86 level through cal~ulations"~ on the relative energies of spin-states, on the geometries and on the vibrational frequencies of M-CH2, M-CHF and M-CF2.The M-CF2 binding energy is about 30% smaller than that for the M-CH2 complexes. For M = Sc - Mn, the high-spin state is predicted to be the ground state but for Co - Cu lower spin multiplicity is preferred. The Co-C bond dissociation energy in six-coordinate cobalamins, models of coenzyme BI2have been calculated at the B3LYP level'". The bond energies do not depend on the trans axial ligand and correlate linearly with the Co-C bond length. B3LYP has been used1I7in the first study of B12models that contain the entire corrin ring. Geometry optimizations on eight octahedral corrin systems with various axial ligands reveal a systematic cis-steric effect and a less systematic trans induction. The corrin framework is predicted to be fairly inert toward the size of the axial R-ligands. PM3(tm) semi-empirical geometry optimizations

14

Organometallic Chemistry

have also been used"* to study the equilibrium structures of cobalamins which suggest that the nucleotide loop and the amide chains have minor effects on the corrin geometry. B3LYP calculations have been used to study119the structures and spin states of dialky and dithiolate d4 osmium/ruthenium(IV) porphyrin complexes The bent structural feature found in the dialkylosmium(1V) porphyrin complex is a result of first-order Jahn-Teller distortion. The instability of a paramagnetic (triplet) [Os(porphyrin)R2] with a linear C-0s-C unit is related to the strong trans influence of the alkyl ligands. H F geometry optimizations and B3LYP energies have been used to characterizeI2' a series of systematically varied (q3-1,3-dialkylallyl)palladium complexes of (4S)-[2-(2-diphenylphosphanyl)phenyl]-4,5-dihydrooxazole(PHOX)ligands. The electronic effects on the stability of metal-alkyl bonds have been studied121using B3LYP calculations on a variety of alkyl-transition metal complexes, including zirconocene complexes [Cp2Zr(H)R], [Cp2Zr(Cl)R], [Cp2ZrR] , and [Cp* 2Zr(Cl)R], iron compounds [CpFe(C0)2R] and [CpFe(CO) { P(CH3)3}],dimethylamino-dithiocyanato-palladium complexes [{ (CH3)2NCS2}Pd{ P(CH3)3}R];and cationic diimino palladium complexes [{NN}Pd(L)R] + (with {NN} = H N = CH-CH = N H or N,N'(o,o'-bis-diisopropylphenyl)diiminoacenaphthalene,and L = nothing, C1-, (CH3)2O, (CH3)2S, C2H4, or CH3CN). It is shown that primary alkyl complexes are usually more stable than secondary and tertiary ones and that this is an electronic effect, due to the partial carbanionic character of the alkyl group. Steric effects are shown to play only a minor role in many cases. Notable exceptions occur in the case of extremely bulky compounds or for systems in which the metal-carbon bond is less polar. The performance of various density functionals (SVWN, BLYP, BPW91, B3LYP and B3PW91) in comparison with H F and post-HF methods has been investigated'22 using the set of copper complexes CuH, CuO, CuS, Cu2, [ c u c l ~ -, ] CuCH3, CuC2H2,C U ~ ( H C O ~ ) ~ ( Hand ~ OC) ~U, ~ H ~ ( P.H The ~ )SVWN ~ performances slightly better than the other methods for bond lengths but the gradient-corrected and hybrid functionals perform better with respect to vibrational frequencies and, particularly, to bond energies. B3LYP calculations on known and hypothetical doubly bridged Cu(1)-Cu(1)dimers and other dlo-d10 analogues have been reported'23 including bridging ligands that are o donors, x-donors and x-acceptors. For 0-donors, electron density is driven into the bonding combinations of empty metal s and px orbitals. For n-donors, population of the corresponding o* and x* levels occurs and the M-M bond vanishes. Insufficient back-donation from copper d orbitals prevents the formation of bridged carbonyl dimers and trigonal-planar monomers are favoured. Bond valence calculations on 26 binuclear carboxylic complexes with Cu04Nchromophores the non-bonding character of the Cu(I1) . . . Cu(I1) contact. B3LYP calculations have also been to study phosphido-bridged trinuc(M, M', M" = lear M3(II)compounds [(CF3)2M(pPH2)2M'(p-PH2)2M''(CF3)2]2Pd(II), Pt(I1))and the oxidation product [Pt3(p-PH2)4(CF3)4]. B3LYP calculations have been used126to study the structures of platinum(I1) hydride silanone complexes [PtH(dipe)(R2SiO)]+(R = H, F, CH3,CF3,or SiH3; +

1 : Theoretical Organometallic Chemistry

15

dipe = H2PCH2CH?PH2), their acetone and ethylene analogues, and cylic trimers of silanone, (R2SiO)3.,Theplatinum(I1) hydride silanone complex does not involve a usual q2-Si0 coordinate bond with the platinum center but two bonding interactions, one between the 0 atom and the platinum center and the other between the Si atom and the hydride ligand. In the acetone analogue, only the 0 atom interacts with the platinum centre. The W-W bond in W 2 ( p H)2(0'Pr)4(DMPE)2is essentially a a2z26*,bdouble bond with extensive mixing of the W-W and W-H o bonds according to Fenske-Hall CASSCF calculations128indicate that the ground and first excited states of OsC are '2- and 'A respectively. RhC has a '2+ground state with the unpaired electron housed in a a orbital with Rh 4dZ2,5s,and C 2pz character according to a BP86 and BPW91 of its EPR spectrum. Inclusion of spin-orbit coupling, which mixes the Rh d, and d, orbitals, leads to good agreement with the experimental EPR A-tensor and reasonable agreement with the experimental g-tensor. [PuO]', [PuH]+, [PUN]+ and [PuC]' are stable with %, '2-, 'Zand 'X- ground states respectively, according to a B3LYP study'''. The ground state of FeC is 'A2 which lies 3528 cm-' below a 'A state and 7248 cm-' above a '112 state according to MR-SDCI calculations'3'. The electronic structure of PdC is to be extremely complicated with a good treatment of relativistic effects are shown for a proper examination of its low-lying states. B3LYP, CCSD(T) and CASCF calculations have been used1" to study the structures and stability of methylidenetungsten WCH2+ and hydridomethylidynetungsten HWCH+,to probe the likely products of the methane dehydrogenation by W in the gas phase. The isomers are predicted to be nearly degenerate. HWCH+ features a very strong W-C triple bond with a bond energy of about 158 kcal mol-I. WCH2+ exhibits a strong agostic distortion. The structure, bonding, relative isomeric stability and oxidation potentials of cationic isocyanide alkenyl-carbyne tungsten complexes have been investigated at the RHF and MP2 level~''~. Oxidation potential follows the order of the net n-electron acceptor minus a-donor character of the ligands. A study using BP86 and hybrid QM/MM-DFT calculations has demon~ t r a t e d ' 'the ~ importance of x-x stacking interactions in determining the structural features of two exemplary d8 palladium complexes, PdBr(p-NCC6H4)({S}MeO-Biphep), and PdBr(C6F,)({S}-MeO-Biphep). The planar structure of the latter is a consequence of an additional stacking interaction between one Pphenyl ring and the pentafluorophenyl o-ligand suggesting that n-n stacking interactions are important in the stabilization of structural features in transition metal compounds. An MP2 of the manganese complexes [(c0)3(dppe)Mn(OH2)]BF4 and [(Co),(dppe)Mn(oH2)...FMn(dppe)(CO)3lBF4 shows that they are held together by unusually strong 0-H-F hydrogen bonds. HF and B3LYP calculations have been to study the products of the reactions of 1 and 2 equivalents of dimethylzinc with 10 different model ligands, each containing two heteroatoms. As the heteroatom combination is varied, the stability of the complexes with acidic ligands decreases with the combination of heteroatoms as NS > SS > NO > NN> 00.With non-acidic ligands, the stability decreases as NN > NO > NS > 00 > SS. +

16

Organometallic Chemistry

The factors affecting the relative stability of metal-alkyl, alkylidene and alkylidyne complexes, MCX2R,X-M = CXR and X2MCR(M = Os, Ru) have been identified13*in DFT studies showing the major influence of x-donation by OR and the preference of 0 s for saturation and higher oxidation state. The complexes (PH3)2(H)Pt-Si(SH)2 , (PH3)2(H)Pt-Si( SMe)*+, (PMe3)2(H)Pt-Si( SMeh , (P’Pr3)2(H)Pt-Si(SEt)2+ and ( P C Y ~ ) ~ ( H ) P ~ - S ~possess ( S E ~ ) both ~ Si---S and Si--Pt multiple bond character with twisted geometries as a result of steric factors according to AIM and CDA analyses’39of B3LYP calculations. The binding energy of Au+-furan complexes is much higher than that of Cu+-furan and Ag+-furan according to an MP2 and CCSD(T) study’40of the energies and structures of Group 11 furan complexes. The structures and binding energies of Cu+, Ag+ and Au+ complexes of propanol, acetone, propene, and H20, have been calculated at the MP2 and BLYP levels141. Gold forms the strongest complexes. Dissociation energies of dimethoxyethane (DXE) complexes with copper ions, Cu+(DXE), (n = 1,2) have been calculated at the MP2 +

+

+

4.1.3 Non-Classical Metal-X-H Interactions. B3LYP has been to obtain binding energies for metal cation-oligomer complexes of Na+, Li+, Co+, Cu+, Zn’, and Zn2+ with straight chain aliphatics (CnHln+2,n = 1-12) and poly(ethy1ene glyco1)s (HO-[C2H4Oln-H,n = 1-5).For the alkanes, the strength of the complexation increases with an increasing degree of polymerization and with a decreasing size of the metal ion. Transition metals give stronger complexes than the main group metals. The poly(ethy1ene glyco1)s bind significantly stronger to the metal cations (Na+ and Cu+)than the aliphatics. The formally d2 silylamido derivatives [NbCp{q3-N(Ar)SiMe2H}C1(PMe3)] and [NbCp{q3-N(Ar’)SiMe2H}(PMe3)(C1)] have a stretched P-agostic Si-H-Nb interactions which are only reproduced by BP86 calculations144if the phosphine groups are modelled adequately. When PH3 is used in place of PMe3 the calculated structures are better described as silanimine-hydrido derivatives. The nature of the nonclassical interligand interactions of silyl groups with two and more hydrides has been studied and reviewed145.These can involve Si-H ointeractions or more complex structural patterns such as those based on Si3-H o-interactions.

Metal Carbonyls. B3LYP calculations on the compounds (C0)3Fe(pCO)3.,(p-InCH3)xFe(C0)3 (x = 0, 1,2,3) the specific influence of stepwise substitution of CO by RIn(1) fragments on their electronic, structural, and bonding properties. This influence is determined by the order of the 0 donor [RIn(I) >> CO(bridge)] and TG acceptor [RIn(I) < CO(bridge) << CO(terminal)] capabilities of these ligands. BP86, B3LYP and CCSD(T) calculations predict 14’ that the borylene ligand in (OC)4Fe-B(NH2)occupies the equatorial position, while the axial and equatorial forms of the parent compound (OC)4Fe-BH are energetically nearly degenerate. Axial isomers of O~~(CO)~~(a-diimine) clusters with a-diimine = DAB (1,4-diaza-1,3-butadiene), PYCA (a-N,a-N’-pyridine-2carbaldehyde-imine) and BIPY (2,2’-bipyridine)are the most stable according to a BP86 study14*.The LUMO becomes increasing localized on the a-diimine 4.1.4

I: Theoretical Organometallic Chemistry

17

ligand as this is expanded leading to increasing MLCT character for the HOMO-LUMO transition. The geometries of the formal 18-valence-electron complexes [W(CO)5L] with the a-and a-bonded monodentate ligands L = N2,NCH, C2H2,C2H4, OH2,SH2, NH3, F-, C1-, OH-, SH- and those of [W(CO)4L]2- with the bidentate ligands L2- = O2C2H?-, S2C2H?- and the 16-valence electron complexes [W(CO),L] and [W(CO)3L]’- have been optimized149at the B3LYP level. A B3LYP study on Fe(CO)’, in conjunction with electron diffraction has ree~arnined’’~ the gas-phase structure and confirms that the equatorial Fe-C bonds are longer than the axial ones. The history of experimental and computational studies of the shape of Fe(C0)4 has been re~iewed’~’. The geometrical and vibrational properties of CoCO in its (’A) ground state and in its @ ‘ and 4A” at the BPW91 level. excited states have been Nineteen homoleptic binary cobalt carbonyls with multiple cobalt-cobalt bonds have been e~arnined”~ using a variety of density functionals. For Co2(C0)*,three energetically low-lying structures are found, in agreement with experiment: C2v(dibridged) 3, D3d (unbridged) 4, and D2d (unbridged) 5 symmetry. At the BP86 level, the energy ordering is 3 < 5 -c 4. Co-Co bond orders up to 4 are postulated for the lower carbonyls. B3LYP and BP86 calculations on the thermodynamic stability of dichromium carbonyls have also been reported”‘. The homoleptic chromium carbonyl structures of the formula Crz(CO)llappear to be thermodynamically unstable with respect to dissociation to the fragments Cr(C0)6and Cr(C0)’ and only slightly metastable with respect to the transition state leading to these dissociated fragments with the potential energy surface in the region adjacent to these minima appearing to be very flat.

3

5

4

Scheme 3

Relativistic PW91 calculations have been carried out”’ to study the structure, energetics and vibrational spectra of the products of the reaction of Th atoms

18

Organometallic Chemistry

with CO. CThO is an unprecedented actinide-containing carbene molecule with a triplet ground state and an unusual bent structure with a bond angle of 109". OThCCO molecule has a bent structure while its rearranged product OTh(q3CCO) is found to have a unique exocyclic structure with side-bonded CCO group. Th(CO):!and Th(CO):!- are highly bent, with bond angles close to 50" due to extremely strong Th-CO back-bonding which causes significant three-centered bonding among the Th atom and the two C atoms. MP2 calculations have been perf~rmed'~'on the structure, bonding and vibrational frequencies of OCCuF, OCCuC1, and OCCuBr showing evidence of n backbonding from the formally Cu(I1) ion to CO. The carbonyl complexes OC-AuX (X = F, C1, Br) have CO bond lengths close to that of free GO and relatively long Au-C bonds, according to a MP2 There is significant a-donation from CO to Au, plus some nback-donation from Au to CO. B3LYP computations have been performed158to determine equilibrium geometrical structures, magnetic properties, and vibrational frequencies of a series of Rh,(CO), (x 5 4, y 5 2) clusters to compare with experiments on CO adsorption on small rhodium aggregates. The monocarbonyls are predicted to be paramagnetic and there is a more pronounced dependence of the CO-stretching frequency on the magnetic properties than on the number of rhodium atoms. A range of post-HF computational techniques including MP2 and CCSD(T) has been used'j9 to study Au(CO),,(n = 1-3) complexes. The Au-C distance and Au-CO binding energy are sensitive to the electron correlation potential. These effects were evaluated using several levels of theory, ranging from MP2 to CCSD(T). The long-distance behaviour of the AuCO interaction is related to simple induction and dispersion expressions involving the individual properties of both gold and CO. The dispersion interaction is the principal contribution to the stability at long distances and an important term at short distances. The structures and vibrational frequencies of nickel, copper, and silver metal carbonyl chloride molecules have been studied"@using B3LYP calculations leading to the prediction that M(C0)Cl (M = Ni, Cu, Ag) molecules are linear with CO binding energies of 37.7, 34.2, and 17.8 kcal mol-' respectively. Clusters. A quasi-classical linearized Thomas-Fermi model has been usedl'l to study the structure of the iron-hydrocarbon cluster Fe13(C2H& and indicates that a centred Fe13-icosahedronwith C2H2-radicalsarranged symmetrically on the icosahedron sides is the most stable form. The bonding and electron counting in triruthenium clusters containing a methylphenylsulfoximido cap or bridge, R U ~ ( C Op2-H)[p3-NS(0)MePh], )~( RU~(CO)~O(P~-H)[ P3-NS(0)MePh17 Ru3(CO)&3-qL-CPhCHB~)[p3-NS(O)MePh], Ru3(CO),(p3-q2-PhCCCCHPh)[pL-NS(O)MePh], and Ru3(C0)7(p2CO)(p3-q2-PhCCCCHPh)[p3-NS(0)MePh] have been examined162 by EHT and DFT calculations. A p3-sulfoximidogroup is not a 3e- ligand but rather acts as a three-orbital / 5e- system. It should therefore be considered as isolobal to an NR- ligand. In a p2 coordination mode, the group retains a lone pair on its pyramidalized N atom and becomes a two-orbital / 3e- ligand. 4.2.5

4.2.6 Interactions with Unsaturated Ligands. The relationship between the

1 :Theoretical Organometallic Chemistry

19

metal-olefin 3c-2e a-bonding and the bonding in B2H and B3 fragments of borane structures has been e~plored'~'.Incorporation of metals in boron deltrahedra often changes their shapes and electron count but the 3c-2e B2M bonds are related to the C2M a-bonding in the Dewar-Chatt metal-olefin bonding model. The majority of n-acceptor ligands can be categorized into two types - double-face and single-facen-accepting ligands - with unique structural preferences. B3PW91 calculations have been wedla to identify and systematize the structural consequences for q '-alkenyl, q2-silane,q2-alkeneand boryl octahedral complexes. The appropriateness of the Dewar model in describing the metalmain group q2-bonding interactions and their relationship with metal-olefin n-complexes has been explored165revealing similarities and differences in the mutual perturbation of the metal and main group atom centres. The variety of metallaborane analogues of larger hydrocarbon n-complexes is used to illustrate the scope of possible new systems. Cation-n interactions between ligands coordinated to a metal cation and aromatic groups have been predicted and identified from B3LYP calculations and analysis of structures of metalloproteins in the Protein Data Bank'66.The energy of the proposed 'metal ligand aromatic cation-n' (MLAC n) intramolecular interactions is estimated to be about 4 kcal mol-'. The phenyl group in Zr(q5-C5H5)(q5-C5HqCR2C6H5)Me]+ (R = H) is coordinated via one of the phenyl carbon atoms, rather than via an agostic Zr-H contact according to a BP86 and B3LYP A stationary point with such an agostic interaction is the transition structure for phenyl rotation with a barrier higher than 50 kJ mol-'. The structural assignments are supported by the good agreement between the GIAO and experimental NMR chemical shifts. The rotational barriers about the M-S bonds of 16e- bent metallocene monothiolates (q5-C5H5)2ZrC1(SR) (R = CH3, CH2CH3, CH(CH3)2, C(CH3),] arises from ground-state orientation about the Zr-S bonds that maximizes S(pn)-M(dn) bonding but also maximizes Cp R steric interaction and transition-state orientation that minimizes the former but maximizes the latter, according to B3LYP study'68. The atoms in molecules approach has been used'69to investigate the nature of the short Ti-C contacts in a crystalline compound postulated to contain Ti bonded to cyclopentadienyl and a substituted dienyl fragment. No bond paths were found to link the Ti to the carbons exhibiting the short contacts. Studies of the bonding and structure of high spin transition metal cyclopentadienyl have been reviewed170showing that maximum spin is possible in Cr and Ni systems as well as for the more common high spin metal complexes. in comprehensive study of the geometries, The PW91 method has been energetics, and electronic structure of neutral and charged 3d transition metal atoms interacting with benzene molecules. The variation of the metal-benzene distances, dissociation energies, ionization potentials, electron affinities, and spin multiplicities across the 3d series in MBz complexes differs qualitatively from those in M(Bz)~.In multidecker complexes involving Vz(Bz)3 and Fe2(Bz)3,the metal atoms are found to couple antiferromagnetically. The geometries and bonding in a series of [M(q6-C6H6)]"+complexes where M = Ti, Cr, c o (n = o),

-

20

Organometallic Chemistry

V ( n = 3), Fe (n = 0,2), Ni ( n = 0,2,4), and Cu (n = 1)has been at the B3LYP level revealing a correlation between the total benzene charge and the frequency shift of the El ring vibration. to study the structures and bond energies BP86 calculations have been of Cr(C6H6)2 and Cr(C6H6)and their cations. Neutral Cr(C6H6)has a low bond energy (0.36 eV) whereas that for cationic Cr(C6H6)’ is much higher (1.84 ev). Dissociation of the first ligand from Cr(C6&)2needS much higher energy than the second one, and depends on the intersystem crossing relaxation (ISC) of the intermediate Cr(C6H6)compound. Dissociation for Cr(C6H6)2+may occur with and without ISC. The photochemistry of Group VI cyclopentadienyl metal carbonyl compounds (q5-C5Hs)2M2(C0)6, (M = Cr, Mo, and W) and the relationship with their M O schemes has been The lack of high-level calculations in the literature on these systems is a hindrance to understanding their photochemistry. Very small spin densities on the carbon atoms of [Fe(q5-C5Me5)2]+are predicted at the BP86 with the correct signs, but with magnitudes which are three times larger than the experimental ones. The possibility of forming monoand 1,l’[C5Me5MC5Me4CH2] and dications [1,2(CH2)2C5Me3MC5Me5]2+ [M(C5Me4CH2)2]2+ starting from iron subgroup decamethylmetallocenes (M = Fe, Ru, 0 s ) has been studied at the BLYP and suggests that the stabilities increase in the order Fe c Ru c 0 s . Ab initio calculations on the complexes [OS($-C~H~)(= C = CH2)(PH3)2]+, [Os(q5-C5H5)(q2-HCCH)(PH&]+, [OsH(q5C5HS)-(CCH)(PH&] and suggest that their thermodynamic stability decreases in this ~ r d e r ” ~ . LMCT transitions of racemic d o x y substituted ethylene bridged bis(indeny1)type zirconocenes have been studied17*at the H F and B3LYP levels revealing clear correlations between the experimental LMCT absorption energies and theoretical HOMO-LUMO energy gaps. Hydrogenation of the indenyl ring and the position of the siloxy substituent have strong influences on HOMO-LUMO energy gaps, and consequently on the observed absorption energies. The structures and bonding of bis(indeny1)derivatives of Fe, Co, and Ni at the BP86, MP2 and EH level have been s t ~ d i e d ” Bis(indeny1)iron ~. is an 18-electron compound, with an almost perfect q5coordination of the indenyl ring, bis(indeny1)cobalt is paramagnetic but the ring in bis(indeny1)nickel is slipped, folded and between y1* + q 3 and q3. The molecular structures of 1,l‘-di-tert-butylferroceneand isopropylferrocene have been optimized’*’ using B3PW91 and MM calculations, suggesting a mixture of C2 eclipsed isomers. The eclipsed ring-ring and the ring-isopropyl conformations are essentially identical. The bonding in the triple decker complex [{ (qS-Me4EtCS)Ni}2(~-q3:q3-decacyclene)] 6, which features an arene sandwiched between NiCp fragments, has been studied at the EHMO level’” revealing that the arene lability can be directly related to the filling of metal-arene antibonding orbitals and hence the Ni-arene overlap population. +

+

21

1 : Theoretical Organometallic Chemistry

I

> ?

Ni

0

O O

Ni

I

6

Scheme 4

LDA calculations have been used'82to study the bonding of the P4 unit in the Ru4(C0)&-PF2)(p4-P)2-/o/2+ series and indicate that the phosphorus atom is best described as being a three-electron donor. Its unexpected pyramidalization is the result of atomic size and of the pinch effect of the pPF2 ligand. BP86 calculations have been to study iron-olefin bond energies for the monoolefin iron tetracarbonyl complexes Fe(C0)4(C2X4) (X = H, F, Cl, Br, I, CN). A bond energy decomposition analysis indicate that the attractive electronic interactions of the haloolefins and percyanoethylene with iron are stronger than those of ethylene. The net bond energy depends on the energy needed to deform the Fe(C0)4and olefin fragments from their equilibrium geometries to the geometrical conformation they adopt in the complex and this leads to bond energies for the substituted olefins which are similar to or smaller than that of the Fe-C2H4 bond. The deformation energy involves deforming the olefin with a change in hybridization of the carbon atoms from sp2in the free olefin toward an sp3-like carbon in the bound olefin. Metal-Nz bond energies for Fe(C0)j.,(N2), (n = 1-5) and Cr(C0)6 n(N2)n (n = 1-6)complexes obtained at the BP86 leve11g4are in good agreement with experiment. The metal-N2 bond is weaker than the metal-CO bond because CO is both a better donor and a better acceptor of electron density. The exo and endo isomers of [ C ~ M O ( C O ) ~ ( ~ ~ - have C ~ Hvery ~ ) ]similar energies according to a B3LYP studyls5. Equilibrium geometries, bond dissociation energies and relative energies of axial and equatorial iron tetracarbonyl complexes of the general type Fe(C0)4L (L = CO, CS, NZ, NO+, CN-, NC-, q2-C2H4, q2-C2H2,CCH2. CH2, CF2NH3, NF3, PH3. PF3, q2-H2)have been studied at the CCSD(T) and B3LYP levels'86 showing that the ligand site preference of these ligands does not correlate with the ratio of their cs-donor/nacceptor capabilities. The strongest Fe-L bonds are found for complexes involv-

22

Organometallic Chemistry

ing NO+, CN-, CH2 and CCH2. The smallest bond dissociation energies are found for the ligands NF3, N2 and q2-H2. BP86 calculations have been ~ s e d " to ~ ,optimize ~ ~ ~ the structures and calculate the ionization energies of [Fe(qS-P2C3H3)2], [ F ~ ( T ~ ' - P ~ C ~ [Fe(q5H~)~], P2C3H3)(q '-P3C2H2)], [RU(q5-P3C2H3)2], [Mn(q5-P3C2H2)(Co)31and [Mn(r15P3C2H2)(CO),],leading to good agreement with experiment and showing that the rings are better acceptors than Cp rings. Fe(q5- N5)2is a strongly bonded complex with similar bonding to that in ferrocene, according to a B3LYP The metal - ligand bonds are roughly half ionic and half covalent with the covalent bonding derived mainly from elg(q5-N5)+Fe2+x-donation. B3LYP geometry o p t i m i s a t i ~ nof~ [Cp2Fe2S4I4 ~~ complexes (q = 0, 2, 1 and -2) lead to good agreement with experimentally determined structures. Studies of the bonding in M2(EH2)2(P2)2 (M = Pd, Pt; E = Si, Ge; P2 =(PH3)2or diphosphinoethane (H2PCH2CH2PH2; dipe)) at the B3LYP leve119' suggest that the Pt species should be considered as di(y-sily1ene)- and di(y-germy1ene)bridged complexes with M2Si2and M2Ge2four-member ring structures, respectively whilst the Pd species should be considered as y-disilene- and p-digermenebridged complexes in which the Si-Si and Ge-Ge bonding interactions are maintained. The effects of the silicon substituents on the Si-Ru bond for four transition metal-substituted base-stabilized silylene complexes has been studied at the H F leve1Is2.The methyl is predicted to be the best at strengthening the Si-Ru bond whilst S(To1-p) substituents lead to the weakest Si-Ru bond. The structures and bonding in the q3 x complexes of A3H3+(A = C, Si, Ge) with Co(CO)3, Rh(C0)3, II'(CO)~,Ni(COh, Co(PH3)3,and Ni(PH3)3have been at the B3LYP and B3P86 levels. In Si and Ge complexes there is a ligand to metal charge transfer, making Si3H3and Ge3H3 cationic ligands, whereas in C complexes there is a small charge transfer from metal to ligand, making C3H3ligands anionic. The molecular and electronic structure of [M(q3by means of quasi-relativistic C3H&] (M = Ni, Pd, Pt) has been in~estigated'~~ BP86 calculations leading to binding energy differences between isomers which are smaller than 0.2 kcal mol-'.The variable energy photoelectron spectra have been reassigned by assuming that the trans:cis ratio in the gas phase is close to one. Olefin pyramidalisation strain energies have been calculated at the B3LYP level'y5for 16 highly pyramidalised alkenes leading to a useful correlation the binding energies of the platinum complexes and either the donation/back donation ratio, the pyramidalisation angle, or the strain energy1 of the free alkene. Benzene platinum complexes (Bz-Pt2, Bz2-Pt,Bz2-Pf2, and Bz3-Pt2) have been studied at the CASSCF, SDCI , MP2 and B3LYP levels'96. DFT and MP2 favour chemisorbed structures whilst CASSCF favours physisorbed structures. The low-energy staircase structures of Bz2-Pt, BzZ-Pt2, and Bz3-Pt2 complexes are predicted to be electrically conducting LDA calculations197 on unusual phosphaalkenes cis and trans[CP~(OC)~MO~( y-ql:q 2-P(Ph)= C(H)Me}] suggest that the x-bond of the phosphaalkene moiety is essentially lost on coordination, with both the P = C x and x* orbitals being used in bonding to the dimetal core. LDA calculations have also been usedts8to investigate the bonding in [Cp2(OC),Mo2(p-PH2)(y-H)].

+ +

1 : Theoretical Organometallic Chemistry

23

B3LYP calculations have been used''!' to probe the electronic structure of the polyynyl complexes [Ru{(C=C),R}(PH3)2Cp] (n = 1-6; R = H, CH3, C6H5, C&NH2-p, CsH4N02-p, CN), the diynyl compounds [Ru{ (C=C)2R}(C0)2Cp], and the oxidized species [Ru{(C=C),C6Hs}(PH3)2Cp]+. The bonding is best described in terms of a strong o-bonding component and a weaker interaction between the filled metal d orbitals and filled polyyne n orbitals. A model has been developed200for understanding the shapes of transition metal complexes containing multiple bonds focusing on Lewis-like structures and the balance of forces arising from a- and n-bond frameworks. Potential energy expressions suitable for implementation in molecular mechanics computations have been derived from consideration of orbital hybridizations. Dinuclear complexes of transition metal ions of type [M2(p,,q1-XY)2L4],where XY is an unsaturated ligand that can act as a four-electron or a two-electron donor through the X atom, appear in two molecular conformations depending on whether the coordination planes around the two metal atoms are coplanar or bent, according to an MP2 calculations on an extensive set of In both structures the geometry of the X atom is planar, corresponding to sp2 hybridization. B3LYP calculations on the fulvene-Mn(C0)3 complex 7 lead202to a nonplanar fulvene due to bending of the exomethylene group. This structure is more stable than the system with a planar fulvene moiety by 23 kJ rnol-'. B3LYP calculations predict203exocyclic q3coordination for the rings in the fluroenyl ( = Flu), cyclopenta[def]phenanthrenyl ( = cpp) and 8,9-dihydrocyclopenta[def] phenanthrenyl ( = H2cpp) complexes IndMo(q3-Flu)(CO)2(Flu),(q5-Ind)Mo(q3~pp)(CO)~(cpp) and (q5-Ind)Mo(q3-H2cpp)(CO)2(H2cpp) complexes.

7 Scheme 5

MP2 and DFT studies of the interactions between P substituents and the ally1 group in P-substituted (q3-ally1)palladium complexes have been reviewed204. B3LY P and BLYP calculations on [(q5-C5H5)2Ta(q2-H2)(CO)] revea1205that there is a large barrier to rotation of the H-H bond as the transition state involves H2 being a-coordinated only. The dinuclear sandwich complex [(C5H5BMe)2Y(p-Cl)l2exists in three minima corresponding to the rotational position of the boratabenzene ligands, according to B3LYP calculations206. The nature and strength of the q6-n;-arenebonding in monomeric europium(I1) and ytterbium(I1)thiolates has been studied by MP2 and B3LYP calculations207 on the model q6-x-arene systems [M(arene)12+(M = Eu, Yb; arene = C6H6, +

24

Organometallic Chemistry

C6FG,1,3,5-Me3C&3, 1,3,5-iPr3C6H3)and Yb(SH)2(C6H6),(n = 1, 2) and the o-donor adducts [M(THF)]’+, [M(dme)12+,and [M(imidaz01-2-ylidene)]~+(M = Eu, Yb). The nature of bonding and energetics in trivalent rare earth n-donor ligand complexes [NdC13-nL] and [NdCp2-nL] +,where nL represents a ndonor ligand, have been investigated at the HF, BP86 and B3LYP levels which indicate208that the binding is essentially electrostatic and weak. Bonding studies of substituted cyclooctatetraenes as ligands in f-metal chemistry have been reviewed209.Dinitrogen complexes of trivalent uranium feature a U-N2-U core which is supported by U+N2 n-donation with no significant N2-U bonding according to relativistic BP86 calculations210.The structures and vibrational properties of t r ~ n s - [ V ( N ~ ) ~ ( P H ~ tran~-[Cr(N2),(PH3)~], )~l-, [Mn(H)(N2)(PH3)4], LF~(NI)(PH~)~], [Fe(H)(N2)(PH3)4] and [FeC1(N2)(PH3),] have been calculated at the BLYP leve1211showing a good correlation between increasing bond strength and decreasing v(N-N). +

+

SpectroscopicProperties. Local density functional calculations have been carried out212to determine the photoionization cross section and the asymmetry parameter profiles of Fe(C5H&leading to very good agreement with experimental data except for autoionization resonances. The dynamics of ultrafast fragmentation processes during resonant multiphoton ionization of c ~ M n ( C 0 ) ~ with femtosecond laser pulses has been studied using adiabatic ab initio potentials and the related components of the transition dipole matrix elements213.The outer valence photoionization cross-section profiles of cobaltocene and nickelocene have been calculated at the LB94 leve1214allowing the experimental data to be assigned. The cross section and asymmetry parameter profiles of (q6-C6H&Crhave been calculated at the DFT level2I5with an explicit treatment of the continuum wave function and with an exchange-correlation potential with the correct Coulomb asymptotic behaviour leading to good agreement with available experimental data. The structures of bis(penta1ene) complexes of titanium, zirconium, and hafnium have been calculated216at the H F level and the orbital sequence of the highest lying orbitals used to assign the photoelectron spectra. The relativistic two-component zeroth order regular approximate Hamiltonian has been applied2I7to the calculation of one-bond metal-ligand coupling contants of coorand 199Hg-C.Relativistic dinatively unsaturated compounds containing 195Pt-P DFT calculations, including spin-orbit coupling or coordination effects by solvent molecules are able to reproduce the experimental findings with good accuracy for the systems under investigation. Quasi-relativistic ab initro calculations have been performed218for 199Hgnuclear magnetic shielding constants and chemical shifts in a series of Hg(XH3)2 (X = C, Si, and Ge) compounds. The calculated I9’Hg chemical shifts are in reasonable agreement with experimental data only if the spin-free relativistic, spin-orbit, and relativistic magnetic interaction terms are included and tight s basis functions of mercury are used. The trends in the chemical shifts in Hg(XH3)2(X = C, Si, and Ge) originates from the sum of the Fermi contact and paramagnetic terms which in turn are related to the effects of relativity and the ligand electronegativity, respectively.

4.1.7

25

1 : Theoretical Organometallic Chemistry

CASSCF/CASPT2 and TD-DFT calculations of the UV-Visible absorption spectra of [Ru(E)(E’)(C0)2(iPr-DAB)](E = E’ = SnPh3or Cl; E = SnPh3or C1, E’ = CH3; iPr-DAB = N,N’-Di-isopropyl-1,4-diaza-1,3-butadiene have been reported219.The agreement of the more costly CASSCF/CASPT2 approaches with the TD-DFT results is very good for the non-halide complexes but the approaches lead to different assignments for the halides. The electronic structures of W Z ( ~ - E ) ( ~ - O C H ~ ) ~ ((E O= C 0, H ~S,) Se, ~ or Te) complexes studied at the B3LYP level’” reveal M-M o-type molecular orbital as HOMO and that the energy of the E pn lone pair approaches this level with increasing mass of E. A tentative assignment of the electronic absorption spectra is made using TD-DFT calculations. The low-lying excited states of HRe(CO)5have been calculated at the CASSCF/CASPT2 and TD-DFT levels2” leading to assignments which are comparable for the lowest excited states. The assignment of the higher energy state is more problematic with the TD-DFT method. A weak feature in the UV/Vis spectrum of [PtCl(SeCH2CH2NMe2)(PR3)] is assigned to a ligand (Se)-to-ligand (PR3) charge transfer (LLCT) according to TD-DFT calculations222.The performance of TD-DFT and INDO/S in the prediction of the electronic spectra of [R~(bqdi),(bpy)~-,]*+ (bpy = 2,2’-bipyridine, BQDI = o-benzoquinonediimine)is very ~irnilar”~ (errat~rn”~) suggesting that INDO/S is a good model for reproducing TD-DFT calculations. Low-energy emission from [W(C0)4(phen)]and [W(C0)4(tmp)] (phen = 1,lOphenanthroline, TMP = 3,4,7,8-tetramethyl-l,lO-phenanthroline) is due to the radiative decay of two unequilibrated W-+phen/tmp 3MLCTstates whilst highenergy emission is due predominantly to W+CO 3MLCT state(s) according to TD-DFT ~ a l ~ ~ l a [Cr(C0)4(tmp)] t i ~ n ~ ~ ~ ~ has . three low-lying, nearly degenerate electronic states which differ in the symmetry of the singly occupied Cr 3d-based MO. Oxidation of [Cr(C0)4(tmp)] leads to lengthening of the Cr-C bonds and shortening of CO and Cr-N bonds according to BP86 calculations226. A method for calculating the EPR g-tensor based on coupled perturbed HF, D F and hybrid methods has been presented227in which unrestricted orbitals are calculated up to first order in the applied magnetic field and the g-tensor is evaluated as a mixed second derivative property with respect to the applied field and the electron magnetic moment. Its application to small complexes, including paramagnetic organometallics, suggests it is among the most accurate of those available. The difficulty in accurate determination of the nuclear quadrupole moment of the first I = 3/2 excited nuclear state of 57Fefrom electronicstructure calculations of the iron electric field gradient combined with Mossbauer measurements of the nuclear quadrupole splitting in the isomershift has been addressed by comparing ab initio with density functional calculations”’ for iron pentacarbonyl, Fe(C0)5and ferrocene, Fe(C5H5)’.The iron electric field gradient is reproduced poorly by single reference many-body perturbation theory and is sensitive to the specific density functional used. The influence of near neighbouring effects and relativistic effects onthe calculated iron electric field gradient is small. to calculate EPR g and A-tensors ZORA calculations have been for bis(maleonitriledithiolato)nickelate(III) and nickeltricarbonylhydride +

26

Organometallic Chemistry

(Ni(C0)3H) leading to deviations in the calculated g-tensor values from the experimental data which are proportional to the deviation from the free electron value and very good agreement between calculated and experimental hyperfine tensors when scalar and spin-orbit relativistic effects are included. BLYP calculations of the geometries and EPR g-tensors of [M(C0)2(S2C2Me2)2]0/'-/2(M = Mo, W) have been to study the electron distributions and electroactive orbital. This orbital is predominantly ligand in character so that reduction is essentially ligand-based, and is thus the source of the noninnocent nature of the dithiolene ligands. The spin arrangements for adsorbed organic radicals in nanoporous crystals of a metal-organic framework have been studied at the HF, B3LYP, MM and MC levels231suggesting that the interactions between radical pairs may exist in the nanoporous cavity. The electronic and geometric structures of qs-CpMn(C0)3in the near-UV region have been investigated232using CASSCF/CASPT2 and TD-DFT methods. The metal to ligand charge transfer (MLCT) states (3dMln-+n*~~) are well separated from the metal centered (MC) (3dMn-3d~")states. The timedependent DFT excitation energies and related assignments compare well to the multistate-CASPT2 results for the lowest MC excited states. Vertical excitation energies for FeCp2,RuCp2and CpNiNO have been calculated233at the TD-DFT level. Comparison with experiment suggests that these systems are well described by TD-DFT. The low-lying excited states, associated potential energy curves and UV/Vis spectrum of Ru(S~H~)~(CO)~(M~-DAB), a model for the R~(SnPh~)~(CO)~(a-diirnine) complexes, have been calculated and assigned at the CASSCF and CASPT2 levels234.The presence of a 3SBLCT state with a predissociative character has been proposed as a cause for the Ru--Sn bond homolysis by irradiation. The vibrational frequencies in the 100-400cm-I region of Cp2MoC12and the structure of crystalline 1:l inclusion complexes from its reaction with P-cyclodextrin have been calculated at the B3LYP The most stable interaction complex has one Cp ligand inside the host cavity with the Mo-Cl bond intact. The inclusion compounds between P-cyclodextrin and the tetrafluoroborate salts [ C ~ ' M O ( ~ ~ - C ~ H ~ ) ([BF,] C O ) ~and ] the neutral derivatives Cp'Mo(q3)C6H7)(C0)2[Cp' = Cp, Ind] have been studied at the B3LYP The Cp derivatives form stable two-to-one (host-to-guest) channel-type inclusion compounds but the indenyl analogues form only weak complexes probably due to steric hindrance arising from the presence of the indenyl ligand. 4.2 Mechanistic Studies. - 4.2.1 Alkene Polymerization. The formation of = CH2)H] and [(C5Me5)Ir(PMe3)(H2C = CH2)] when [(C5Me5)Ir(PMe3)(CH [(C5Me5)Ir(PMe3)]is thermolytically generated in the presence of ethylene has been studied at the B3LYP suggesting that both singlet and triplet spin surfaces need to be considered. Thermolysis of the singlet alkyl hydride precursor directly forms triplet [(C5Rs)Ir(PR3)]and the weak van der Waals adduct of the latter with ethylene is proposed to be the key intermediate in the overall reaction. The low activity of [ C ~ M O ( ~ ~ - C ~ H ~ )for ( Cethylene H ~ ) ~ Jpolymerization as compared to pentamethylcyclopentadienyl derivatives and the niobium"' com-

1 : Theoretical Organometallic Chemistry

27

pounds is due to the low polarity and high strength of the Mo-alkyl bond according to B3LYP ~ a l ~ ~ l a t iThe o nactivation ~ ~ ~ ~ . barrier for ethylene insertion is essentially equivalent for the Nb and Mo systems but the presence of the extra electron for the Mo system opposes the formation of M-H-C agostic interactions and strengthens the back-bonding M-ethylene interaction. BP86 calculations has been to study the elementary reactions for the copolymerization of ethylene with methyl acrylate catalyzed by Pd-based diimine catalysts revealing a strong electronic preference for the 2,l-insertion paths, with a barrier that is 4.5 kcal mol-' lower than any other studied insertion pathway. The activity of metal catalysts with d" (n > 0) electron counts in olefin polymerization has been studied at the BP86 for d' V, d2 Cr, and d3 Mn systems with an amine and two amide ligands, together with some second-row analogues. When the ligands are tethered with ethyl bridges, the possible conformers are restricted and the catalytic properties change dramatically. DFT and MP2 calculations have been used to the ethylene polymerization catalyst C5H5Nb(butadiene)C12/ methylaluminoxane with C5H5Nb(butadiene)Rf assumed to be the active catalyst. Ethene coordination is predicted to very weak. Insertion into the metal-alkyl bond has an energy barrier of 4 kcal molt' for R = CH3 and 6 kcal mol-' for R = C2H5.The ethene insertion transition state is stabilized by an agostic interaction with metal-hydrogen distances of 2.07-2.16 A. In alkyl conformations these bonds are longer and correspond to only weak agostic interaction. Termination via hydrogen transfer to a coordinated ethene molecule ejecting a terminal alkene has a high energy barrier of 17 kcal mol-'. The effect of counterion on the kinetics and mechanism of ethylene polymerization by the zirconocene-boron catalytic system was examined242using the Perdew, Burke, and Ernzerhof GGA calculations on the model complex Cp2ZrEt+and the ion-pairs Cp2ZrEt+A- (A- = CH$(CbF5)3-, B(C6F5),-).The results predict that the weaker the nucleophilicity of the counterion, the higher the fraction of the P-agostic isomer produced, the higher the exotermicity, the lower the activation barrier to ethylene addition to Cp2ZrEt+A-, and the lower the activation energy of chain propagation. to study the mechanisms of B3LYP and IMOMM methods have been chain propagation and P-hydride transfer termination stages of poly- and oligomerization of ethylene by catalysts of general formula [2,6-(CR' = N((2R2)(4-R4)(6-R3)C6H2)2C5H3N]FeC12. For a model low steric bulk system the lowest (singlet) P-hydride transfer transition state is 5.7 kcal mol-' lower in energy than the lowest (quintet and singlet) chain propagation transition state. For a system with higher steric bulk, the f3-hydridetransfer transition state is 17.6 kcal mol-' higher than the lowest chain propagation transition state. 4.2.2 X - H Activation, Oxidative Addition and Reductive Elimination Processes. Key computational studies of alkane C-H activation and functionalization with homogeneous transition metal catalysts have been reviewed244.The molecular mechanism of the cleavage of the C-H bond at oxide surfaces has been modelled245using LDA calculations with a methane molecule and a small

28

Organometallic Chemistry

vanadium oxide particle. The study suggests that both fragments of the cleaved C-H bond become attached to the surface oxide ions to form OH and alkoxy groups with two electrons entering the conductivity band of the solid. BP86 calculations have been performed246on the C-H activation step in the Catalytica process. The most likely catalyst is either (bipyrimidine)Pt(OSO3H)+ or (bipyrimidine)PtCl+.In the former case C-H activation takes place by a-bond metathesis, whereas the latter involves C-H activation by an oxidative addition mechanism. BP86 calculations have also been carried on competing processes occurring in solution during ethylene polymerization by the catalyst (1,2-MelCp)2ZrMe+.The values of the enthalpy of dormant product formation showed that the precatalyst (1,2-Me2Cp)2ZrMe2andAlMe3may be strong competitors for the vacant coordinate site in the catalyst. The possibility of forming such complexes decreases after insertion of one or more monomer units into the zirconium-alkyl bond. B3LYP calculations have been performed2'*to investigate the oxidation addition of methane to M(q5-C5H5)2,M(qs-CjHq)*CH*,M(q5-C5H5)2C2H4and M($C5H5)*SC2H4(M = Mo, W) and suggest that a methane a-complex is an intermediate. Configurational inversion of both free methane and methane bound to first-row transition-metal ions occur via C, M+(CHq)(M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) transition states in which one pair of C-H bonds is about 1.2 A in length and the other pair is about 1.1A, according to B3LYP calculations249.The barrier height decreases significantly from 109.4 kcal mol-' for free methane to 17-23 kcal mol-' for the late transition-metal complexes and is thus thermally accessible. An atom-molecule complex NiCH4 acting as precursor in the breaking of C-H bond is predicted in the singlet state potential energy curve of the reaction between nickel and methane by B3LYP calculations*". The reaction of Ni+ with propane leads to CH4 elimination via two elementary steps according to a B3LYP study2s1.Initial C-C bond activation in propane is mediated by Ni+ and this is followed by subsequent isomerization of the inserted intermediate. Reductive elimination, the final step of the Monsanto and Cativa processes, has been studied for elimination of CH3COOI from isomers of the anionic dicarbonyls [Rh(C0)2(COCH3)13]- and [Ir(C0)2(COCH3)13] - at the B3LYP For the Rh system, elimination from the rner,trans isomer is most likely. For the Ir system, elimination from thefac,cis isomer is probably dominant. An alternative pathway for the Heck reaction involving a Pd"/Pd'" redox system has been investigated at the B3LYP and proceeds via oxidative addition as the rate determining step and requires ligand detachment and re-attachment. The pathway is only feasible if a weakly coordinating ligand is present. B3LYP calculations have been used to study254the production of endo or exo q3-allyl complexes from the reaction of cyclopropane with [CpIr(PH3)CH3] . The reaction proceeds through C-H bond activated Ir" intermediates and CH4 elimination, followed by ring opening of the iridium cyclopropyl complexes through an iridium carbene vinyl intermediate to their respective q3-allyl products. The mechanism breaks two C-C bonds simultaneously and then re-forms one en route from the iridium cyclopropane complex to the iridium ally1 products. +

29

1 : Theoretical Organornetallic Chemistry

The importance of relativistic effects for oxidative addition reactions of palladium to C-H, C-Cand C-Cl bonds has been investigated using BP86 non-relativistic, quasi-relativistic and ZORA calculation^^^^ on the reactions of dl0 Pd atoms with CH4, C2H6 and CH3CI. Inclusion of relativistic effects lowers activation barriers are reduced by 6-14 kcal mol-I whilst reaction enthalpies become 15-20 kcal mol-' more exothermic. Pronounced potential wells are observed in the reaction profiles and are associated with collisionally stabilized reactant complexes. to study the thermodynamics and the B3LYP calculations have been mechanism of the reaction between Pt and CHc The bond dissociation energies of the Pt+-H, Pt+-CH,, Pt+-CH2, Pt+-CH, Pt+-C, and H-Pt+-CH3bonds are larger than those for the first and second-row metals and this is ascribed to the accessibility of the s'd8 electronic configuration and effective sd hybridization as a consequence of relativistic effects. Oxidative addition of CH4to Pt + produces a hydrido-methyl platinum cation intermediate, H-Pt +-CH3 and is essentially barrierless. The oxidative addition of imidazolium cations to zerovalent Group 10 metals, to afford heterocyclic carbene complexes has been using B3LYP calculations. Addition of imidazoliums to Ptoand Niois more exothermic than to Pd', and Nio is predicted to react with a much lower barrier than either Pto or Pdo. B3LYP calculations have been performed258on the effects of replacing PH3 with PMe3 ligands in the reductive elimination of methane from cis-hydridomethyl-bisphosphine platinum(I1) and platinum(1V) model complexes. In both the Pt(I1) and Pt(1V) complexes, the replacement of PH3 ligands by the more strongly basic PMe3ligands is predicted to favour a direct mechanism for reductive elimination over one in which the initial step is phosphine ligand loss. However, the effect of the increased platinum-phosphine binding enthalpy on the ligand-predissociation mechanism is partially canceled by an increase in the barrier height for the direct mechanism. B3LYP and mPW 1PW91 calculations have been used259to characterize stationary points on the doublet potential energy surface for the reaction Y + C2H4 -+ YC2H2 H2 and suggests there is no energy barrier to the formation of a long-range Y-ethylene complex. A new, low-energy path for the reaction is proposed involving concerted rearrangement of the HYC2H3 insertion intermediate directly to a weakly bound, product-like complex with no exit channel barrier to elimination products. The intermediates in the Si-H bond-activation reaction of triplet Fe(C0)4and triplet CpCo(C0) with triethylsilane have been studied at the B3LYP and compared with the those from the reaction with the singlet species CpRh(C0). The triplet organometallics have a greater overall reactivity than singlet species due to a change in the Si-H activation mechanism, involving only very weak coordination of triplet intermediates with the ethyl groups of triethylsilane. As a result, the triplet species do not become trapped in alkyl-solvated intermediate states. The origin of regioselectivity in rhodium diphosphine catalyzed hydroformylation has been investigated261using hybrid QM/MM calculations with the IMOMM method. The regioselectivity is governed by the nonbonding interac+

+

30

Organometallic Chemistry

tions between the diphenylphosphino substituents and the substrate whereas the effects directly associated to the bite angle seem to have a smaller influence. A two-layer ONIOM model, using both B3LYP and mPWlK, has been used262to compare the steric and electronic requirements for C-C and C-H activation by Rh catalysts with chelating and non-chelating ligands. Steric requirements are predicted to differ significantly and chelation appears to play an important role in C-C bond activation. A QM and QM/MM study of steric and electronic effects in the main steps of Rh-catalyzed carbonylation reactions has been reported263.The energy barrier of the CO insertion reaction is lowered by the presence of substituents on the chelating ligands. B3LYP and QM/MM ONIOM calculations have also been reported2@for the mechanism of competitive intramolecular C-C and C-H bond activation in PCN pincer complexes of rhodium@).The C-C activation product is the most stable one by ca. 50 kJ mol-' relative to the C-H activation product due to insufficient stabilization of the C-H activation product due to strain. The potential energy hypersurface for ethylene hydroformylation catalyzed by HRh(PH&(CO) has been calculated265at the B3LYP and CCSD(T) levels. Reaction pathways originating from the trans isomer of the active catalyst are favoured. Oxidative addition of H2 to the unsaturated Rh-acyl complex occurs on the same side as the ethyl moiety of the acyl ligand. CO insertion step is predicted to rate-determining step with predicted activation barriers of 20.4 and 14.9 kcal mol-' at the CCSD(T)//B3LYP and B3LYP//B3LYP levels of theory. The rate determining step in the carbonylation of methanol to acetic acid catalyzed by a Rh complex is oxidation addition of CH31 with an activation energy of 216 kJ mol-', according to a H F study266.The carbonyl insertion and CH3COI reductive elimination steps have activation energies of 128 and 127 kJ mol- ', respectively. of the reaction profiles for the BP86 calculations have been oxidative addition of NH3 to a number of unsaturated CpM(C0) (M = Rh, Ir), tr~ns-M(PH3)~X (M = Rh, Ir; X = H, C1) and ML2 (M = Pd, Pt; L = PH3, L2 = H2PCH2CH2PH2,dpe). Reactions with the d8 species are characterized by the formation of strongly bound ammine complexes from which computed activation energies for oxidative addition are in excess of 16 kcal mol-I. For the d'O M(PH3)2species computed ammine adducts are weak, activation barriers are in excess of 23 kcal mol-', and the overall reaction is endothermic for both M = Pd and Pt. Comparison of the computed reaction profiles for analogous second- and third-row complexes shows the NH3 oxidative addition reaction to be more favourable with the third-row species. Oxidation of methanol to formaldehyde by [Fe04J2- occurs via activation of the 0 - H and C-H bonds of methanol through an addition-elimination mechanism that involves coordination of methanol to diprotonated ferrate or through H atom abstraction from the 0 - H or C-H bond of methanol. The C-H bond cleavage is the most likely initial reaction according to a B3LYP study268.The regioselectivity in [FeO] and [Fe02] +-mediated hydroxylation reactions of 2-methylbutane has been studied at the B3LYP H-atom abstraction involves a concerted mechanism via a four-centred transition state C . . . H . . . +

1 : Theoretical Organometallic Chemistry

31

0-Fe. Regioselectivity in the concerted mechanism arises from secondary C-H bond dissociation whereas regioselectivity in the radical mechanism arises from tertiary C-H bond dissociation. The lowest energy pathway, at the B3LYP for the reaction of [FeS]+ with methane involves a formal 1,2-addition of H3C-H across the Fe+-Sbond to generate a [CH3FeSH]+ insertion intermediate. This bond activation step involves spin inversion from the sextet to the quartet surface en route to the products. Oxidation of substituted alkenes by [MnO4I2- follows a (3 + 2) cycloaddition mechanism rather than a (2 + 2) pathway through a metallaoxetane with a difference activation energy of ca. 40 to 45 kcal mol-' at the B3LYP Remote substituents affect the regioselectivity in the oxymercuration of 2-substituted norbornenes according to a B3PW9 1calculations272. B3PW91 calculations have been used to calculate273the energy required to activate the H-H bond in the entire series of Cp2LnH complexes leading to activation energies which vary from 0.5 to 8.0 kcal mol-l, indicating an overall facile reaction. The transition state is best viewed as an almost linear H3- ligand with short H-H distances and strong M-H interaction, through the wingtip H centers, with Ln. The exchange reaction is CJ bond metathesis. 4.2.3 Cyclopropanation Reactions. The palladium-catalyzed cyclopropanation of olefins has been investigated274at B3LYP level. The active catalytic species is reported to be a carbenoid complex in one or both of the two almost degenerate forms (PH3)2Pd(CH2Cl)Cland (PH3)C12Pd(CH2PH3) rather than a metal-carbene. Cyclopropanation can proceed either along concerted or multistep reaction pathways. The nonconcerted paths involve the formation of palladacyclobutane intermediates. The coordination of a range of substituted alkenes (C2H3X;X = H, CH3,t-Bu, CN, C02CH3,CF3,OCH3, CHCH2,C6HS,F, C1, Br) to a cationic methyl- or phenylpalladium(I1)diiminecomplex and the subsequent migratory insertion into the methyl-palladium and phenyl-palladium bonds have been using B3LYP calculations. In general, electron-rich alkenes coordinate more strongly, whereas electron poor alkenes insert more readily. In all cases the alkenes have a weaker coordination to the phenylpalladium(I1)diimine compared to the methylpalladium(II)diimine complex and a lower insertion barrier. Copper carbene complexes are intermediates in the copper(1)-catalyzed olefin cyclopropanation with diazomethanes with rate-determining barriers of the order of 25 kcal mol-' according to a BP86 The mechanism for the Kulinkovich hydroxycyclopropanation reaction has been explored277with B3LYP on the reactions between R'COOMe and Ti(OMe)2(CH2CHR2) (where R' and R2 are H and alkyl groups). Addition of ester to titanacyclopropane is found to be fast, exothermic, and irreversible. The cyclopropane-forming step is the rate determining, and affords the experimentally observed cis-R'/R2 diastereoselectivity in the a-addition manifold by generating cis-R'/R2 1,2-disubstituted cyclopropanol when R' is a primary alkyl group. the mechanism of the B3LYP/6-31G(d) calculations have been used to copper(1)-catalyzedcyclopropanation reaction. The starting ethylene complex of

32

Organometallic Chemistry

the copper (I)catalyst undergoes a ligand exchange with methyl diazoacetate to yield a reaction intermediate, which subsequently undergoes nitrogen extrusion to generate a copper-carbene complex. The cyclopropanation step takes place through direct carbene insertion of the metal-carbene species to yield a catalystproduct complex, which falls apart to regenerate the starting complex. 4.2.4 Migratory Insertion Reactions. BP86 and C P MD calculations have been

reported279on the multistep migratory insertion reaction of CO into the zirconium-carbon bonds in [calix[4](OMe)z(O)zZrMeJ has been investigated by means of both static and dynamic density functional calculations. A relatively stable facial CO complex is predicted with a negligible barrier for CO insertion into one of the Zr-Me bonds, leading to the formation of an q2-acylcomplex. The insertion of the residual alkyl group into the acyl moiety, leading to an q2-bound acetone, has a small energy barrier of 2.3 kcal mol-'. BPW91 and CP M D calculations have been used280to study the migratory insertion of CO into the titanium-methyl bond in T ~ ( C P ) ~ ( C H showing ~ ) ~ that lateral and central CO approaches are kinetically equivalent. Ethylene insertion into the metal-methyl bonds of Group 4 C(C5H5)2MCH3]and [H2Si(C,H4)((BuN)-'Bu)MCH3]catalyst cations (M = Ti, Zr) proceeds via intermediate n-complexes and subsequent Cossee-Arlman four-center transition state structures, according to MP2, MP3, MP4-SDQ and CCSD calculations2". BWP91 calculations have been used282to study ethylene coordination and insertion into the transition metal-methyl bonds for the lowest spin states of [(q',q5-H2NC2H4C5H4)M(III)Me] (M = Sc-Co) compounds. A single low-lying unoccupied frontier orbital available for bond formation in the n complex and the transition state region is required for the reaction. The empty orbital can be created by spin pairing which allows the formation of a n complex with a covalent metal-ethylene bond. As bond must be broken during insertion, high activation barriers for the low-spin complexes are to be expected. B3LYP calculations have been to study direct insertion of ethylene into the chromium-carbon bond in singly charged bis(imido)chromium(VI) cations for n-alkyl and benzyl as starting polymer chains. Frontside coordination of ethylene takes place without activation to give a stable complex but the subsequent insertion into the Cr-alkyl bond requires a free energy of activation of at least 15 kcal mol-'. Ethylene coordination in the backside mode requires considerable activation. CO insertion into Pd-C is much easier than into Pt-C for neutral and cationic complexes, [MR(Cl){ P(CH3)3}2]and [MR{ P(CH3)3}2(s)]+BF4- (M=Pt, Pd, R=CH3, C6H5,S=coordinated solvent) as a result of relativistic stabilization of Pt-C bonds according to a MP2 B3LYP/63lLAN calculations have been carried out285to study the mechanism of the Pauson-Khand reaction, a one-step C~~(CO)~-catalyzed synthesis of cyclopentenone through [2 2 + 11assembly of one molecule each of alkene, alkyne, and carbon monoxide. The reaction consists of olefin insertion, CO insertion, and reductive elimination steps. The olefin insertion step is found to be an irreversible step that determines the stereo- and regiochemistry of the overall reaction and the following steps are low activation energy processes and revers+

+

+

+

1 : Theoretical Organometallic Chemistry

33

ible. The regioselectivity of this reaction has also been modelled2*6in a separate B3LYP study. In this work it is reported that ethylene does not appear to react principally from an equatorial position but rather through facile pseudorotation, from an axial position. This route is found to be the one of lowest activation energy for C-C bond formation. Propene insertion into the Rh-H bond of HRh(PPh&(CO)(q 2-CH2= CHCH3) occurs via two separate reaction channels, according to a QM/MM study2*'.The linear insertion product is derived from an adduct with an equatorial-axial arrangement of the two PPh3 co-ligands and the branched insertion product is derived from a propene adduct with a bis-equatorial arrangement. The latter is more stable by 1.0 kcal mol-' leading to a roughly three-fold excess for the branched product. Hydrido migration to the C 0 2 and CS2carbon of Fe, Ru, and 0 s complexes which have an intramolecular N-Ha-H-M H-bond has been studied288at the (q5B3LYP level using the model complexes C5H4(CH2)3NH3f)MH(H2PCH2PH2) (M = Fe, Ru, and 0s). The reaction follows an abstraction path of each metal and polar solvents increase the activation energy. H F and B3LYP calculations have been used to the carbon dioxide insertion process into the W-N bond in [Na][W(C0)5(2AP)] and [Na]2[W(C0)4(2AP)]2 (AP = amidopyridine). C02 insertion into the W(CO)4(2AP)-anion to provide the chelated carbamate, W(C0)4(0C(0)2AP)-, was thermodynamically favoured by >110 kJ molt' over insertion in into [W(C0)4(2A P)]2 - . The associative reaction of (q5-C5H4X)M(C0)2 with PR3 (X = H, Me, C1; M = Co, Rh, Ir) has been studied290at the B3LYP level. Steric factors appear to be less important than electronic effects except when very large substituents are present on the cyclopentadienyl (Cp) and the entering ligand are sizable enough to hinder the reaction. Electron-withdrawing substituents on the Cp ring and electron-donating substituents on the entering ligand facilitate CO substitution. Although the Group 9 systems clearly react by an associative mechanism, Group 7 systems, such as CpM(CO)3(M = Mn, Tc, Re), undergo substitution reactions by a dissociative mechanism, with significantly higher barriers as they are too crowded for the associative route and do not have a readily available empty orbital. 4.2.5 Other Reactions ofdkenes. BP86 calculations have been used291to study the complexes involved in the rearrangements undergone by ethylene on the calixC41arenetungsten fragment {(C2H30)4}W leading to geometrical and thermodynamical data which agree well with the observed reaction trend. [ R u H ~ C I ~ ( P ' Preacts ~ ~ ) ~ with ] terminal alkynes to give [RuC12( = CHR)(P'PP~~)~] as a side product either by the addition of acetylene and two subsequent 1,3-H shifts from the metal center to the C2 carbon of the acetylene ligand or by dissociation of HCl from the starting compound, rearrangement of acetylene to vinyl and the formation of the carbene by addition of HCl. Both routes are possible according to B3LYP ~ a l ~ ~ l a t iThe o n ~formation ~ ~ ~ . of coordination compounds of phosphanyl carbenes with a transition metal in a high oxidation

34

Organometall ic Chemistry

state involves a facile distortion to a new type of an anionic metallaalkylidene donor-acceptor if one of the ligands at the carbene center possesses a leaving ability, according to a B3LYP BP86 and MP2 calculations have been to model W(0) carbene mediated homogeneous metathesis reaction of propylene and show that the rate determining step of the metathesis is the initiation. The low stereoselectivity of the olefin metathesis reaction is due to the close matching of activation energies for cis and trans isomer formation and the fast cis-trans isomerization caused by the catalysts. to study scalar-relativistic effects on the BP86 calculations have been transition states for ethylene epoxidation with Mimoun-type diperoxo complexes [MO(q2-02)2(OPH3)] (M = Cr, Mo, W) showing that spin-orbit effects are small and that scalar-relativistic effects shift the extent of reaction at the transition state toward the reactants and decrease the activation barrier. 1,2hydrogen shifts in [W(q-C5H5)2(CH3)]+,studied296at the BP86 level, proceed via a hydridocarbene intermediate and an agostic methyl complex, with the former as the being more stable by 42 kJ mol-’, leading to [W(q-C5H5)2(CH3)(PH3)J+ thermodynamic product. The relative rates of PdC12 oxidation of functionalized acyclic alkenes have been correlated297with alkene ionization iotentials, and the HOMOS, and LUMOs. The work indicates that the mechanism has similarities to that for hydroboration, although there is a nucleophilic rate-determining step. B3PW91 calculations have been carried for the reaction of vinyl fluoride with Cp,ZrHCl to give Cp2ZrFC1and C2H4. The calculated reaction enthalpy of -55.1 kcal mol-1 is dominated by the highly exothermic formation of a Zr-F bond. Studies of the transition states indicate that the approach of either the slender F-C bond or the bulkier face (C/C mystem) of vinyl fluoride is significantly easier in the ‘inside’orientation, as the ‘inside’unoccupied orbital is a better Lewis acid. The regiochemistry of addition of Zr-H across the CH2 = CHF bond is kinetically controlled by a 3.5 kcal mol-’ difference in activation energy, which calculates to a 0.3% predicted yield for the a-fluoroethyl product. B3LYP calculations have been used299to investigate the mechanism of transfer dehydrogenation of ethane catalyzed by (PCP’)Ir(H)2 [PCP’ = q3C6H3(CH2PH2)2-1,3] with ethylene as the hydrogen acceptor. Transfer dehydrogenation of ethane by (PCP’)Ir(H)2involves dehydrogenation of (PCP’)Ir(H)2 by the hydrogen acceptor to produce the key intermediate (PCP’)Ir followed by dehydrogenation of ethane by (PCP’)Irto produce the product ethylene and to regenerate the catalyst (PCP’)Ir(H)2.The origin of enantioselectivity in transfer hydrogenation of aromatic carbonyl compounds catalyzed by chiral q6-areneruthenium(I1) complexes has been investigated at the B3LYP level3’’ suggesting that attractive interaction between C-H bonds and rc rings is significant enough to stabilize key transition states. B3LYP calculations have been performed301to investigate the retro DielsAlder reaction of norbornadiene with a Fe+ catalyst. The reaction is predicted to be stepwise with an activation barrier of 18.8 kcal mol-l. The Fe+-catalyzed retro reaction of norbornene is also calculated to be stepwise with an activation barrier of 24.9 kcal mol-I. The [2 31 addition of tetramethylethylene (TME) to

+

1: Theoretical Organometallic Chemistry

35

the M n 0 2moiety of MnO3C1is thermodynamically favoured over [2 + 11 addition (epoxidation), while the kinetic barriers for both reactions are of comparable height according to a B3LYP BP86 calculations have also been used303 to study the polar monomer binding mode in complexes with Ni- and Pd-based Brookhart-diimine (cationic) and Grubbs-salicylaldiminato (neutral) catalysts. Binding of the polar monomer by its olefinic functionality is preferred for the Pd-based Brookhart system whereas 0 complexation is preferred for the Ni system. For the Grubbs catalyst, n-complexes are strongly preferred for both the Niand Pd-based systems. The vinamidinium ligand in [PdCl(PH3){C(CHNH2)2}]+, a model of the likely Pd(I1) intermediate in the Suzuki-Miyaura cross-coupling reaction of P-chlorovinamidinium salts using a Pd(0) catalyst, has a large trans effect and the Pd-C is predominantly of 0symmetry according to a BP86 study304.There is therefore free rotation around the Pd-C bond. The ligand HOMO is employed to form the Pd-C a-bond and interacts strongly with the metal-based LUMO which is mainly Pd d,2 in character. Protonation of [RuH(CI)(PP~~)~(NBD)] (NBD = norbornadiene) is favoured leading to the formation of norbornene whereas protonation of [OSH(CI)(PP~~)~(NBD)] is not favoured due to the stronger osmium-olefin interaction, according to a B3LYP The interaction between Ag+ and olefins has been studied using BLYP ~ a l ~ ~ l ato tmodel i ~ nolefin ~ ~ transporta~ tion through membranes containing AgBF4. Reaction of Ag + with ethylene occurs through addition of one ethylene molecule to a silver cation followed by replacement of the bound ethylene molecule with another. The first reaction is a non-activated, pre-equilibrium process and the second reaction occurs through an associative transition state. 4.2.6 Reactions of Carbonyls. B3LYP calculations have been reported307on the structure of [Fe2(co)8]2-, and its adducts with electrophiles. The geometries and vibrational frequencies of three energy minima and four transition states of [Fe2(C0)8]2- have been characterized, and the relative energies of several alternative structures have been evaluated. The proton transfer reaction between H20 and FeH(C0)4- has been studied308at the H F and CASSCF levels. The process in solution is predicted to be less endothermic than in the gas phase by ca. 50 kcal mol-I. The results predict that the mechanism involves a single structure in which the transferred hydrogen is simultaneously bonded to iron and oxygen with a different polarization depending on the reaction coordinate. B3LYP calculations have also been used309 to study the reactions of [(q5C5H5)Fe(C0)2(PR3)](R = H, OMe ) with the hydride anion H-. The energetically most favourable sites of attack are to the CO ligands leading to the formyl species (q5-C5H5)Fe(CHO)(CO)(PR3) (R = H, OMe) and to the cyclopentadienyl ring leading to the ring adducts ( V ~ - C ~ H ~ ) F ~ ( C O(R ) ~ (=P R H~,)OMe ). The formyl species undergo loss of the carbonyl, phosphine, or phosphite ligands to give the respective hydrides. Reaction of metal fragments M(CO), (M = Cr, Mo, W) with methylenephosphanes forms mononuclear M-P adducts without considerable geometrical change of the original phosphaalkene moiety or binuclear +

Organometallic Chemistry

36

complexes with a distorted bipyramidal arrangement at the phosphorus atom and a lengthening of the P-C bond, according to B3LYP 4.2.7 Reactions of Alkynes, AlEenes and Ketenes. HF/3-21G, HF/HW3, and B3LYP/HW3 methods have been used3” to study the addition of alkynes (R = H, Me, Ph; R’ = CH3, CF3). Addition HC=CR to MO(NH)(CH*)(OR’)~ leads to distorted trigonal bipyramidal transition states. The calculated activation enthalpy for HC=CH addition to Mo(NH)(CHz)(OR’)2 is about 10.3 kcal/mol for R’ = CH3 and about 2.3 kcal/mol for R’ = CF3, indicating a significant preference for acetylene addition to Mo(NH)(CH2)(0CF3)2over Mo(NH)(CH~)(OCH~)~. The a-addition of HC=CR (R = Me, Ph) is found to be considerably more favorable than the P-addition to Mo(NH)(CH~)(OR)~ by over 4 kcal molt’ although this preference is reduced by solvent effects. A B3LYP study of the activation of the C=C n bond of ethyne, the 0 - H bond of water, and the N-H bond of ammonia at the Pd = X (X = Sn, Si, C) bonds by (PH&Pd = XH2 complexes to produce the products (PH3)2Pd(CH=CH)XH2, (PH3),Pd(H)X(0H)H2and (PH&Pd(H)X(NH2)H2 respectively, has been reported312.Activation of the C=C n bond of ethyne at the Pd = Sn bond involves a weak q2-interaction of the C=C n-bond with Sn with a small energy barrier of only 6.2 kcal mol-’. Reaction at the P d = Si bond follows the same mechanism but is barrierless. Ammine and the amide proton transfer to the Ti-C bond have activation energies that are more than 50 kJmol-’ higher than the cycloaddition step in the hydroamination of allenes, alkynes, and alkenes catalyzed by cyclopentadienyltitanium-imido complexes according to a B3LYP study”’. B3LYP and BP86 calculations on the butatrienylidene complex trans[Cl(L2)2Ru= C = C = C = CH2] and the product with addition of amine trans[Cl(L2)2RuC3{ N(CH&}CH3] where L2 is a chelating diphosphine have been reported3I4.Attack of the amine is at the C3 position as a result of orbital and steric effects. B3LYP calculations have been performed3I5to study the formation of dinuclear platina-P-diketones [Pt2(v-C12{(COR)2H}2]from reactions of hexachloroplatinic acid with silylated alkynes R’C=CSiMe3 (R = CH2R’)via terminal alkyne complexes of Zeise’s salt type [PtC13(p2-R’C=CH)]- . Reaction occurs via alkyne-vinylidene equilibration, substitution of C1- by propyne, addition of water to the vinylidene ligand yielding a hydroxycarbene unit, and its deprotonation, affording an acyl ligand. B3LYP and QM/MM modelling of the chemistry of the ketene complex {K’-(~-BU)~PCH~P(~-BU)~}I~C~[~~-(C,C)-P~~C = C = 01 has been reported3“ = C = 01, ( K ~ through structural studies of (K2-H2PCH2PH2)IrCl[q2-(C,C)-H2C H2PCH2PH2)Ir[q2-( C,C)-CH2= C = 01 and (K2-H2PCH2PH2)Ir(CH2)(CO)+, and a study of their interconversion by intramolecular C = C double bond cleavage/formation. Ammine and the amide proton transfer to the Ti-C bond have activation energies that are more than 50 kJmol-’ higher than the cycloaddition step in the hydroamination of allenes, alkynes, and alkenes catalyzed by cyclopentadienyltitanium-imido complexes according to a B3LYP study3I7.Reaction of HgC12 +

+

+

1: Theoretical Organometallic Chemistry

37

with acetylene proceeds via a n-complex with an activation energy of 31 kcal mol-' according to an MP2 study318.Coupling reactions of formaldehyde with RuBr(q3-C3H5)(CO)3 takes places via coordination of formaldehyde with the Ru(I1) centre to give an (q'-allyl)ruthenium(II) formaldehyde complex followed by C-C bond formation between the ql-ally1 ligand and the formaldehyde with activation energies of 19.9 kcal mol-I and 12.5 kcal mol-' respectively according to an MP2-MP4(SDQ), CCSD(T), and B3LYP study319. In Ru(q3C3H5)(HCHO)(CO)3] + and R U B ~ ( ~ ~ - C ~ H ~ ) ( H C H O coupling ) ( C O ) ~ proceeds through one transition state, to afford [Ru(OCHlCH2CH = CH,)(CO)3] and RuBr(OCH2-CH2CH= CH2)(C0),respectively with considerably larger activation energies of 50.7 and 34.8 kcal mol-', respectively. The dimerizations of metallacyclocumulenes to metal substituted radialenes are symmetry allowed processes320for Ti and Ni. Due to the do electron count and hence the lack of backbonding, the C = C double bond is intact in Ti cyclocumulenes and the C-C bonds are essentially equal in length due to n delocalization over the carbon and Ti atoms in the a2 and bl orbitals shown below (Scheme 6). +

bl

a2

Scheme 6

4.2.8 Other Reactions of Complexes With Cyclopolyene Rings. B3LYP calculations have been used321to investigate the meta regioselectivity of the nucleophilic addition to methoxy-substituted arene-Cr(C0)3complexes and show that the reaction proceeds kinetically controlled nucleophilic attack at the meta position. The addition of electrophiles, nucleophiles and radicals to tricarbonylchromium-complexed arenes, studied at the B3LYP proceeds via reactive intermediates which are stabilized by the tricarbonylchromium group. Inter and intramolecular radical addition leads to exclusive reaction on the complexed arene ring. For electrophilic addition to complexed arenas, a pathway in which the cation initially adds to the metal centre rather than to the arene ring is predicted. BP86 calculations have been performed323to study the electrode potentials and the effect of one-electron oxidation on the Fe-X Bonds in 17- and 18-electron Cp*Fe(dppe)X Complexes (X = F, C1, Br, I, H, CH3). The electrode potential is reported to depend on destabilizing electrostatic effect caused by X rather than on the Fe(d,)-X(p,) antibonding character. BLan 1 calculations have been performed324to study the interaction between the a-dicarbonyls 2,3-butanedione, methyl pyruvate, and pyruvic acid and a single nickel atom For 2,3

38

Organometallic Chemistry

butanedione, the chelate complex is slightly more stable than the singly-bonded keto-bonded form whereras the latter is more stable for the other systems. A B3LYP analysis325of the formation of the terminal nitrido complex (q3CP*)~MO(N)(N~) by elimination of N2from C ~ * , M O ( Nhas ~ ) ~been reported and indicates that the observed exolendo conformation of Cp*,Mo(N3)2 is lower in energy than the exolexo and endolendo isomers by 2.2 and 2.7 kcal mol-I respectively. 4.2.9 Ring Slippage and Ring Opening Processes. Studies of the ring slippage reaction, such as in [(q5-X)Mn(C0)3](where X is a variety of unsaturated rings), [ M O ( ~ ~ - I ~ ~ ) ( C O(L ) ~= L ~CO, ] phosphite), [Mo(q5-Cp)(q5-Ind)L212 have been reviewed326.The slippage process is driven by a tendency to decrease the antibonding character of the HOMO after two electrons are added to the system and, usually, a structural rearrangement of the complex (or the ring) takes place, depending on the characteristics of the polyene in the ring. Larger rings are found to slip more easily. Electrochemical reduction of [Fe(q5-C6H7)(CO)3] [PI?,] involves solvent coordination and qSto q3 ring slippage of the cyclohexadienyl ligand according to a B3LY P The coordination geometry of shifted cyclopentadienyl ligands resulting from two-electron reduction of q5-Cphas been studied using B3LYP ~ a l ~ ~ l a t i oon n ~ ~piano-stool ~* mono(cyclopentadieny1) complexes, [q5CP)M(CO)~]and bent metallocenes, [qj-Cp)2M(C0)2l3+ with first (M = Mn) and third (M = Re) transition row metals and the reduced analogues. Reduction yields haptotropic shifts in all cases, but the resulting (q-Cp)-M coordination geometry depends on the complex geometry and the metal size. For the bis(cyclopentadieny1)species folded q3-Cp compounds are found while the reduced mono(cyclopentadieny1)complexes have slipped planar Cp ligands. B3LYP calculations have been to compute the structures and relative energetics of competing disrotatory and conrotatory transition states for ringopening of Fe(C0)3-complexed methylenecyclopropanes. Disrotatory ringopening toward the metal is accompanied by a large activation barrier of approximately 40 kcal mol-' whilst the barrier for conrotatory ring-opening is only 28.8 kcal mol-'. However, both barriers are significantly higher than the barrier for disrotatory ring-opening away from the metal 12.2 kcal mol-'. +

+

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230. D.V. Fomitchev, B.S. Lim and R. H. Holm, Inorg. Chem., 2001,40,645 231. T. Kawakami, S. Takamizawa, Y. Kitagawa, T. Maruta, W. Mori and K. Yamaguchi, Polyhedron, 2001,30,1197. 232. J. Full, L. Gonzalez and C. Daniel, J . Phys. Chem. A, 2001,105, 184 233. P. Boulet, H. Chermette, C. Daul, F. Gilardoni, F. Rogemond, J. Weber and G. Zuber, J . Phys. Chem. A, 2001,105,885 234. M. Turki and C. Daniel, Coord. Chem. Rev., 2001,216,31. 235. S.S. Braga, I.S. Goncalves, M. Pillinger, P. Ribeiro-Claro, J.J.C. Teixeira-Dias, J . Organomet. Chem., 2001,632,ll. 236. S . Lima, I.S. Goncalves, P. Ribeiro-Claro, M. Pillinger, A.D. Lopes, P. Ferreira, J.J.C. Teixeira-Dias, J. Rocha, C.C. Romao, Organometallics, 2001,20,2191 237. K.M. Smith, R. Poli, J.N. Harvey, Chem. -Eur. J., 2001,7,1679 238. E. Le Grognec, R. Poli, Chem. - Eur. J., 2001,7,4572 239. A. Michalak and T. Ziegler, J . Am. Chem. Soc., 2001,123,12266 240. T.K. Firman and T. Ziegler, J . Organomet. Chem., 2001,635,153 241. A.J. Sillanpaa and K.E. Laasonen, Organometallics, 2001,20,1334 242. I.E. Nifant’ev, L.Y. Ustynyuk and D.N. Laikov, Organometallics, 2001,20,5375 243. D.V. Khoroshun, D.G. Musaev, T. Vreven and K. Morokuma, Organometallics, 2001,20,2007 244. R.H. Crabtree, J . Chem. Soc. Dalton Trans., 2001,2437 245. E. Brocawik, J. Haber and W. Piskorz, Chem. Phys. Lett., 2001,333,332 246. T.M. Gilbert, I. Hristov and T. Ziegler, Organometallics, 2001,20, 1183 247. K. Vanka and T. Ziegler, Organometallics, 2001,20,905 248. M.-D. Su and S.-Y. Chu, J . Phys. Chem. A, 2001,105,3591 249. K. Yoshizawa, J . Organomet. Chem., 2001,635,100 250. H.Q. Yang, Y.Q. Chen, C.W. Hu, H.R. Hu, M.C. Gong, A.M. Tian, N.B. Wong, J . Mol. Struct. Theochem, 2001,574,57. 251. D.J. Zhang, C.B. Liu, Acta. Chim. Sinica., 2001,59, 1406. 252. T. Kinnunen and K. Laasonen, J . Organomet. Chem., 2001,628,222 253. A. Sundermann, 0.Uzan, J.M.L. Martin, Chem. - Eur. J., 2001,7, 1703 254. C.E. Webster and M.B. Hall, Organometallics, 2001,20,5606 255. A. Diefenbach, F.M. Bickelhaupt, J . Chem. Phys., 2001,115,4030. 256. X.-G. Zhang, R. Liyanage and P. B. Armentrout, J . Am. Chem. SOC.,2001,123,5563 257. D.S. McGuinness, K.J. Cavell, B.F. Yates, B.W. Skelton and A.H. White, J . Am. Chem. Soc., 2001,123,8317 258. K.L. Bartlett, K.I. Goldberg and W.T. Borden, Organometallics, 2001,20,2669 259. M. Porembski, J.C. Weisshaar, J . Phys. Chem. A., 2001,105,6655. 260. P.T. Snee, C.K. Payne, K.T. Kotz, H. Yang, C.B. Harris, J . Am. Chem. Soc., 2001, 123,2255. 261. J.J. Carbo, F. Maseras, C. Bo and P.W.N.M. van Leeuwen, J . Am. Chem. Soc., 2001, 123,7613 262. B. Rybtchinski, S. Oevers, M. Montag, A. Vigalok, H. Rozenberg, J.M.L. Martin and D. Milstein, J . Am. Chem. Soc., 2001,123,9064 263. L. Cavallo and M. Sola, J . Am. Chem. Soc., 2001,123,122294 264. A. Sundermann, 0.Uzan and J.M.L. Martin, Organometallics, 2001,20, 1783 265. S.A. Decker and T.R. Cundari, 2001,20,2827 266. M. Lei, W.L. Feng, M.R. Ha0,Y.Q. Ji, Z.F. Xu, Sci. China. Series B Chem., 2001,44, 465. 267. S.A. Macgregor, Organometallics, 2001,20, 1860 268. T. Ohta, T. Kamachi, Y. Shiota, J. Yoshizawa, J . Org. Chem., 66,4122

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269. T. Yumura, K. Yoshizawa, Organometallics, 2001,20,1397. 270. S. Barsch, D. Schroder, H. Schwarz, P.B. Armentrout, J . Phys. Chem. A , 2001,105, 2005. 271. T. Strassner, M. Busold, J . Org. Chern., 66,672. 272. P. Mayo, G. Orlova, J.D. Goddard, W. Tam, J . Org. Chem., 66,5182 273. L. Maron and 0. Eisenstein, J . Am. Chem. Soc., 2001,123,1036. 274. F. Bernardi, A. Bottoni and G.P. Miscione, Organometallics, 2001,20,2751 275. H. von Schenck, S. Stromberg, K. Zetterberg, M. Ludwig, B. Akermark and M. Svensson, Organometallics, 2001,20,2813 276. M. Buhl, F. Terstegen, F. Loffler, B. Meynhardt, S. Kierse, M. Muller, C. Nather, U. Luning, Eur. J . Org. Chem., 2001,2151. 277. Y.-D. Wu and Z.-X. Yu, J . Am. Chem. SOC.,2001,123,5777. 278. J.M. Fraile, J.I. Garcia, V. Martinez-Merino, J.A. Mayoral and L. Salvatella, J . Am. Chem. Soc., 2001,123,7616 279. S. Fantacci, F. De Angelis, A. Sgamellotti and N. Re, Organometallics, 2001, 20, 403 1 280. F.D. Angelis, A. Sgamellotti and N. Re, J . Chem. SOC. Dalton Trans., 2001,1023 281. G . Lanza, I. Fragala, T.J. Marks, Organometallics, 2001,20,4006. 282. V.R. Jensen and W. Thiel, Organometallics, 2001,20,4852 283. V.R. Jensen and K.J. Barrve, Organometallics, 2001,20,616 284. Y. Kayaki, H. Tsukamoto, M. Kaneko, I. Shimizu, A. Yamamoto, M. Tachikawa and T. Nakajima, J . Organomet. Chem., 2001,622,199 285. M. Yamanaka and E. Nakamura, J . Am. Chem. Soc., 2001,123,1703. 286. T.J.M. de Bruin, A. Milet, F. Robert, Y. Gimbert and A.E. Greene, J . Am. Chem. SOC.,2001,123,7184 287. S.A. Decker and T.R. Cundari, J . Organornet. Chem., 2001,635,132 288. T. Matsubara and K. Hirao, Organometallics, 2001,20,5759 289. D.J. Darensbourg, B.J. Frost and D.L. Larkins, Inorg. Chem., 2001,40,1993 290. H.-J. Fan and M.B. Hall, Organometallics, 2001,20, 5724 291. S . Fantacci, A. Sgamellotti, N. Re and C. Floriani, J . Chem. Soc. Dalton Trans., 2001,1718. 292. N. Dolker and G. Frenking, J . Organomet. Chem., 2001,617,225 293. W.W. Schoeller, A.J.B. Rozhenko and A. Alijah, J . Organomet. Chem., 2001, 617, 435 294. M. Tlenkopatchev and S . Fomine, J . Organomet. Chem., 2001,630,157 295. D.V. Deubel, J . Phys. Chem. A, 2001,105,4765 296. J.C. Green and C.N. Jardine, J . Chem. Soc. Dalton Trans., 2001,274 297. D.J. Nelson, R. Li and C. Brammer, J . Am. Chem. Soc., 2001,123,1564. 298. L.A. Watson, D.V. Yandulov and K. G. Caulton, J . Am. Chem. Soc, 2001,123,603. 299. S . Li and M.B. Hall, Organometallics, 2001,20,2153 300. M. Yamakawa, I. Yamada, R. Noyori, Angew. Chem. Int. Ed. Eng., 2001,40,2818 301. M.L. McKee, J . Am. Chem. Soc., 2001,123,9426 302. T. Wistuba, C. Limberg, Chem. Eur. J., 2001,7,4674. 303. A. Michalak and T. Ziegler, Organometallics, 2001,20, 1521 304. I.W. Davies, J. Wu, J.F. Marcoux, M. Taylor, D. Hughes, P.J. Reider, R.J. Deeth, Tetrahedron, 57,5061 305. S.H. Liu, S.Y. Yang, S.T. Lo, Z.T. Xu, W.S. Ng, T.B. Wen,Z.Y. Zhou,Z.Y. Lin, C.P. Lau, G.C. Jia, Organomet., 2001,20,4161. 306. C.K. Kim, C.K. Kim, B.-S. Lee, J. Won, H.S. Kim and Y.S. Kang, J . Phys. Chem. A, 2001,105,9024

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307. G. Aullbn and S . Alvarez, Organometallics, 2001,20,818 308. C. Amovilli, F.M. Floris, M. Sola and J. Tomasi, Organometallics, 2001,20, 1310 309. D.A. Brown, J.P. Deignan, N.J. Fitzpatrick, G.M. Fitzpatrick and W.K. Glass, Organometallics, 2001,20,1636 3 10. W.W. Schoeller, A.B. Rozhenko, S . Grigoleit, Eur. J . Inorg. Chem.,2001,2891. 311. Y.-H. Sheng and Y.-D.Wu, J . Am. Chem. Soc., 2001,123,6662 312. T. Matsubara, Organometallics, 2001,20, 1462 313. Bernd F. Straub, Robert G. Bergman, Angew. Chem. Int. Ed. Eng., 2001,40,4632. 314. R.F. Winter, K.-W. Klinkhammer and S. Zalis, Organometallics, 2001,20,1317 315. K. Nordhoff and D. Steinborn, Organometallics, 2001,20,1408 3 16. H. Urtel, G.A. Bikzhanova, D.B. Grotjahn and P. Hofmann, Organometallics, 2001, 20,3938 317. Bernd F. Straub, Robert G. Bergman, Angew. Chem. Int. Ed. Eng., 2001,40,4632. 318. Y.A. Borisov, A.S. Peregudov, Russ. Chem. Bull., 2001,50,958. 319. S. Sakaki, T. Ohki, T. Takayama, M. Sugimoto, T. Kondo, T. Mitsudo, Organometallics, 2001,20,3145. 320. E.D.Jemmis, A.K. Phukan and U. Rosenthal, J . Organomet. Chem., 2001,635,203 321. A. Pfletschinger, W. Koch, H.-G. Schmalz, New. J . Chem., 2001,446 322. C.A. Merlic, M.M. Miller, B.N. Hietbrink, K.N. Houk, J . Am. Chem. Soc., 2001,123, 4904. 323. M. Tilset, I. Fjeldahl, J.-R. Hamon, P. Hamon, L. Toupet, J.-Y. Saillard, K. Costuas, and A. Haynes, J . Am. Chem. Soc., 2001,123,9984 324. A. Rochefort and P. McBreen, J . Phys. Chem. A, 2001,105,1320 325. J.H. Shin, B.M. Bridgewater, D.G. Churchill, M.-H. Baik, R.A. Friesner and G. Parkin J . Am. Chem. Soc., 2001,123,10111 326. M.J. Calhorda, L.F. Veiros, Cornrn. Inorg. Chem., 2001,22,375. 327. M. Fernanda, N.N. Carvalho, M. Amelia, N.D.A. Lemos, L.F. Veiros, G.R. Stephenson, J . Organomet. Chem., 2001,632,49. 328. L.F. Veiros, Organometallics, 2001,20,5549. 329. D.J. Tantillo, B.K. Carpenter and R. Hoffmann, Organometallics, 2001,20,4562

2

Groups 1 and 11: The Alkali and Coinage Metals BY D.J. LINTON AND A.E.H. WHEATLEY

1

Alkali Metals

1.1 Introduction. - As has been the case with this review article in recent years, Part 1 deals with alkali metals (M+) and is thereafter categorised primarily according to the organic anion component ( R ) of the organometallic species R-M+. The review will concentrate on compounds that contain at least one carbon-alkali metal interaction except in cases of extreme interest. A broad overview of the synthetic uses and mechanistic probes of alkali metallated organics resides at the start of each section. After this, structural studies are arranged according to the analytical method used. Solid-state investigations (for the most part by single-crystal X-ray diffraction but, in certain cases, by powder diffraction or solid-state NMR spectroscopy) dominate and are followed by solution NMR spectroscopy and molecular orbital calculations, in that order. 1.2 Alkyl Derivatives. - 2001 has seen many reports of the employment of alkyllithium compounds as synthetic tools both in their own right and in order to afford new lithium reagents in situ. Hence, MeLi has been used in conjunction with Me2Zn and RSi(OH)3 (R = 2,6-Pri2C6H3NSiMe3)in the presence of N,N,N',N'-tetramethyl- 1,4-phenylenediamine ( = tmpda) to yield the soluble zinc silicate (RSi03ZnLi.tmpda)4,the core of which is defined by a Si4012Zn4Li4 polyhedron.' The rearrangement and/or substitution of one/two pentadienyl groups on (4R,5R)-N,N'-bis [( 1S)- 1-phenylethyl]-3,6-divinyl- 1,7-octadiene-4,5-diaminehas been achieved by treatment with 2-4 equivalents of Bu"Li (or PhLi) in thf with exact reaction conditions dictating whether C -1- or C-2-symmetric 1,2-disubstituted 1,2-diamines result.2 n-Butyllithium has also lately been used to regioselectively metallate 3- and 4-chloropyridines at the 2-positi0n.~It has been employed in the preparations of 3,4- and 3,4,5-substituted piperidines-including (-)-paroxetine-via enantioselective carbon-carbon bond formation reactions during which intermediate lithio-N-Boc allylamines react with nitroalk e n e ~ . ~ Readily prepared 4-isopropyl-3-(methylthiomethyl)-5,5diphenyloxazolidin-2-one undergoes metallation at the exocyclic methylene group in the presence of BunLiand so acts as a chiral formyl anion equivalent. As Organometallic Chemistry, Volume 31

0 The Royal Society of Chemistry, 2004

2: Groups 1 and 11: The Alkali and Coinage Metals

49

such it has lately been used in the enantioselective preparations of 1,2-diols, 2-amino alcohols, 2-hydroxy esters, and 4-hydroxy-2-alkenoatese5The preparation of 4- and 5-substituted 1-hydroxyimidazoles has been achieved by using Bu"Li to regioselectively metallate the corresponding 4-bromo-2-chloro- 1-(benzoxy)imidazole or 1-(benzyloxy)imidazole,respectively.6It has also been used to lithiate P-diphenyl(alkyl)(N-methoxycarbony1)phosphazenes which have, in turn, been treated with Michael acceptors and aldehydes. The result has been the generation of lH-1,2-azaphosphinin-6-ones, P-hydroxy(N-methoxycarbony1)phosphazenes and 5,6-dihydro-1,3,4-0xazaphosphinin-2-ones! nButyllithium has been reacted with (RHN)3PS to yield [(RHN)(RN)PSLi.thf]; (R = But) with (R = Pr') and { [(RHN)2(RN)PSLi][(RHN)(RN)2PSLi.thf}}2 further lithiation resulting either in formation of the dianion [(RHN)(RN)2PS]2(R = Pr') or in sulfur extrusion (R = Bu').~ Various butyllithium isomers have been employed in the a-metallation of selected bicyclic carbamates that have a range of accessible angles and distances between the carbonyl group and the site of metallation? In the presence of Bu'Li-(-)-sparteine, racemic 3-(indeny1)alkyl carbamate undergoes cyclocarbolithiation, which is accompanied by kinetic resolution during the cyclisation step. After addition of electrophiles, stereohomogeneous optically active benzobicyclo[3.3.0]octenols result." The same chiral organolithium complex is utilised in the first stage of the asymmetric metallation/trapping-reductive elimination strategy which has been reported to allow the formation of (SJ-methylphosphine-borane derivatives." Meanwhile, the application of tmeda ( = N,N,N',N'-tetramethylethylenediamine)instead of (-)-sparteine leads to the formation of a tricyclic cyclopropane." s-Butyllithium has also been used to selectively metallate 1-(2,2-diethylbutanoyl)indole. Hence, Bu'Li-tmeda has been noted to deprotonate the 3-position whereas Bu'Li-Bu'OK-thf reacts at the 2-po~ition.'~ Conditions that affect the regioselectivity with which t-butyllithium adds to cinnamic acid have been in~estigated.'~ The same organolithium substrate has also been used to metallate 1-naphthamides, the dearomatising cyclisations of which have allowed the generation of a potent ( + /-)-4-(2-hydroxyphenyl) analogue of the acromelic acids14and substituted lactams which could be converted into kainoid-like polyglutamates. Bu'Li has found use in the synthesis of the superlithiated cluster (Li4Am3)+ (AmH = N,N'-diphenylbenzamidine), the charge on which is countered by a lithium bis(a1uminate) ion.16 The same organolithium substrate has also been employed in conjunction with ZnMe2 and AmH to form new lithium zincates, the solvent-dependent oxophilicities of which have been probed." The regioselective addition of benzylic metallating agents to a-oxoketene dithioacetals has lately been discussed.18 Reaction of lithiated nitriles with aamidoalkylphenyl sulfones has been the subject of study, the resultant synselective process having yielded @-amino nitriles.'' The lateral lithiation of o-methyl and o-ethyl benzamides has been followed by (-)-sparteine-mediated condensation with imines to enantioselectively yield tetrahydroisoquinolin-lA variety of cyclic, P-functionalised organolithium species have been

50

Organometallic Chemistry

prepared and synthetically utilised, their relative stabilities being computed using the principle of microscopic reversibility and Baldwin's rules whereby their p-eliminative decompositions were viewed as the reverse of n-endo-trig cyclisations.22A variety of organolithium substrates have been reacted with electrophilic olefins in the presence of Cu(1)in thf-hmpa (hmpa = hexamethylphosphoramide) to yield conjugate addition products23 and, lastly, lithiated phosphane oxides have been employed in the preparation of single enantiomers of (E)-1,5-diarylpentene-4,5-diol~?~ The single crystal structure of [(MeLi)4-1.5dem], (dem = diethoxymethane) has lately been reported to reveal analogous interactions to those which have long been known to give rise to the polymeric structure of solvent-free [(MeLi)4]m.25 This contrasts with the fully characterised (methyllithium monomer-containing) species Me[(B~'N)~S]Li,.3thf,which results from the reaction of methyllithium with [ ( B u ' N ) ~ S ] L ~In~a. ~similar ~ vein, the new alkylenediazasulfite systems { [(RN)2SCH2]Li2-thf}2 (R = But, Me3Si)and { [(BU'N)~SC(M~)E~)Li2*thf},have been prepared and structurally a ~ t h e n t i c a t e d .Their ~ ~ , ~formation ~ proceeds by the addition of an alkyllithium to a sulfur diimide (to yield a diazaalkylsulfinate) with the a-carbon then being metallated using methyllithium. These sulfur-ylides can be rationalised as sulfite congeners wherein two oxygen atoms are isoelectronically replaced by a Bu'N group and the third oxygen is replaced by CR2.27 Recently reported were a series of tetra- and hexanuclear co-complexes in each of which BunLi was associated with lithium anisyl fencholates. The enantiopure preparation of these co-complexes followed from direct combination of BunLi with the corresponding anisyl fenchol, though the ratio of butyl to fencholate ligands in the product was found to depend on the ortho substituents in the anisyl fragments.28As part of an investigation into the polymerisation of L-lactide, 2,2'-ethylidene-bis(4,6-di-teut-butylphenol) ( = edbpH2) has been treated with excess Bu"Li to yield [(p~-edbp)Li2]2[(p3-B~~)Li-OSOEt~]~. The solid-state structure of this co-complex comprises a core of two stacked C02Li3rings.29Mesitylene has been lithiated by Bu"Li in tmeda to give an ion-separated product .30 The simple lithium salt wherein the anion is [(3,5-Me2C6H3CH2)2Li.tmeda] PhP(CH2Ph)(CHPh)2Li.tmedahas been characterised, with the metal being supported by direct bonding with a deprotonated benzyl centre and the ql~~ the interaction of an aryl ring as well as by tmeda s o l ~ a t i o n .Meanwhile, deprotonation of Me2Si(Bz)(CH2NC5Hlo)(Bz = benzyl; CH2NC5Hlo = piperidinomethyl) has yielded the monolithiated (aminomethy1)benzylsilane Me2Si[CH(Li)Ph](CH2NC5H10). This exhibits a cyclic tetrameric structure in the solid state.32It has been established that But2P-P = P(Me)But2reacts with BunLi in thf to give Bu'2P-P = P(CH2Li.2thf)But2wherein completion of the metal coordination sphere is achieved through chelation by the terminal P(II1)donor centre.33Reaction of Bu'NP(~-NBu')~PNH~ with excess Bu"Li gives [Bu'NP(pNBU')~P]L~~.(BU"L~)~, the solid-state structure of which has been reported.34 Furthermore, Bu"Li has been employed in the synthesis of a novel mixed anion-dianion cluster. Hence, the deprotonation of 6-MeC5H3N-2-OHhas given the alkoxo-cage complex (6-LiCH2CgH3N-2-OLi)4( 6-MeC5H3N-2-OLi)2.9thf.35

2: Groups I and 1 I : The Alkali and Coinage Metals

l

51

The tertiary phosphine (2-Me2NCH2C6H4)2PMe has been treated with ButLi (from petroleum/toluene) and to yield both (2-Me2NCH2C6H4)2PCH2Li-3PhMe 2-Me2NCH2C6H4P(Me)[2-Me2NcH(Li)C6H4] (from toluene only). The former product has been fully characterised and is found to be based on a cyclic tetramer? The tris(phosphinomethy1)-substituted alcohol (Me2PCH2)3COH has been reacted with Bu"Li in the absence of toluene to yield a mixed butyllithium/lithium alkoxide dimer that reveals a distorted octahedron of metal centres, two faces of which are alkoxide-capped and two of which are capped by Bun groups. Moreover, the same reaction conducted in the presence of toluene results in formal elimination of Li20 and the generation of a cluster in which a trigonal array of Li+ ions is capped by both a tris(phosphinomethy1)-substituted alkoxide and a Y-conjugated, symmetrically tris(dimethylphosphin0)-substituted trimethylmethane d i a n i ~ n . ~ ~ The first structurally authenticated R3PRb and -Cs systems have been pres{ [(Me3S&C(2ented, with alkali metal stabilisation in Me2NCH2C6H4)2PM.nPhMe]m)m (M = Rb, n = 0, rn = 2; R = Cs, n = 1, rn = 1) being provided by the carbanion centre only in the case of M = CS.~' Concerning heterobimetallic systems, the 1:2 reaction of [Ni(acac)12with RLi (R = Me, Ph) has given the binuclear complexes (RN$L [L = N1,N2-bis(2pyridylmethyl)-N3,N4-bis(2,4,6-trimethylphenyl)oxalamidinate]. This species reacts with an excess of MeLi to give both Me8Ni2Li2(Li.thf)2 and thf-enriched A recent study of tetrasubstituted guanidinate ions as supportMesNi2(Li.thf)4.39 ing ligands in organoyttrium chemistry has reported the 1:4 reaction of dimeric { [(Me3Si)2NC(NPt)2]2Y(p-Cl)}2 with MeLi in the presence of tmeda. Single crystal diffraction reveals that the product, [(Me3Si)2NC(NPri)2]2Y(pMe)zLi-tmeda,is based on a YC2Licyclic core in the solid state? The anionic triazacyclononane complex (Pr'-tacn)TaCl&=N(2,6-Pri2C6H3)] (tacn = triazacyclononane)has been reacted with Me3SiCH2Lito give (Me3SiCH2)2[(2,6Pri2C6H3)N =]Ta(CH2SiMe3)(p-q':q3-Pri2-tacn)Li.This, in turn, undergoes thermal elimination of SiMe4 uia a-abstraction to give the alkylidene (Me3SiCH2)[(2,6-Pri2C6H3)N =]Ta(CHSiMe3)(p-q':q3-Pr'2-tacn)Li.Each of these last two lithiated species have been structurally analy~ed.~' In the context of heterobimetallic complexes of higher Group 1 metals, weak 1 potassium-carbon interactions are noted in the solid-state structure of the mixed K-Ga system (tmeda)potassium [cis-ethene-1,2-di(tert-b~tylamido)]gallate(I)?~ The effect of hmpa on the ratio of 1,2- to 1,4-addition of sulfur-substituted organolithium reagents to cyclohexenones and hexenal has been studied using low-temperature, multinuclear NMR techniques, with quantitative information about the ratio of contact (CIP) and solvent-separated (SIP) ion pairs in solutions of dithianyllithiums and phenylthiobenzyllithiums in thf-hmpa having been obtained. Results suggest that complexation of hmpa to lithium causes ion pair separation and lowers the Lewis acidity and catalytic effectiveness of the lithium cation: both of these effects favour 1,4-additi0n:~ DFT methods have recently been applied to [2-(Me3Si)2C(Li)C5H4N]2 and have revealed that delocalisation of the C-Li bonding electrons over the agostic alkyl group controls the geometry of the Six-Li bond angle.44

52

Organometallic Chemistry

1.3 Alkenyl, Allyl, Vinyl, Alkynyl and Related Derivatives. - A variety of lithiated cyclic compounds were tested as alkenyllithium components in the dianionic oxy-Cope rearrangement that occurred upon their addition to (q6-benzocyclobutenedione)tricarbonylchromium~5Results reveal that, with 5-lithio2,3-dihydrofuran, rearrangement and subsequent intramolecular aldol addition is diastereoselective. Meanwhile, reaction with phenyllithium, 2-lithio-Nmethylpyrrole, 2-lithiofuran, or 2-lithiothiophene result in the formation of single adducts and proximally or distally ring-opened d i a d d u ~ t sThe . ~ ~reaction of organolithium species with enaldimines has been probed, with imines derived from naphthalene- 1-carbaldehyde and acyclic cQ-unsaturated aldehydes which bear N-centred electron-withdrawing groups favouring 1,2-reaction and those which bear N-alkyls or bulky N-aryls preferring 1,4-addition Stereogenic allyllithium compounds have been employed in the enantioselective syntheses of cyclopentanes. The mechanism by which these cycloalkylation reactions proceed has been found to represent a completely regioselective antiSN’SE’-transformation.“7 Similarly, the asymmetric syntheses of C3.3.01-, C4.3.01-, [5.3.01-, and [5.4.0]-carbocycles and heterocycles have been achieved using configurationally stable allylic organolithium species.48Bis(2-1ithioallyl)amines have been derived from bis(2-bromoally1)aminesand have, in turn, undergone intramolecular carbometallation of a lithiated double bond in the presence of tmeda, to give dilithiated dihydropyrroles. Reaction of these intermediates with electrophiles has yielded a variety of new fused and non-fused five-membered functionalised hetero~ycles.4~ Lastly, vinyllithium reagents have been used in conjunction with Cu(1) to effect the formation of 3-phenyl-5-vinyl-substituted pr01ines.~~ Two dilithium salts of tetrakis(trimethylsily1)butatriene have recently been fully characterised, one as a monomer and one as a dimer.5’ Lithium benzamidinates which reveal q3-NCN coordination of the metal centre have been prepared by the reaction of p-toluonitrile with lithium hexamethyldi~ilazide.~~ The monomeric Li, Na and K complexes of bis(iminophosphorano)methanides have lately been presented. Hence, (Me3SN= PCy2)2CH2 (Cy = cyclohexyl) has been reacted with MeLi, NaH or KH to yield either or [(Me3SN= PCY~)~-KN,KN’][(Me3SN= PCy2)2-~C,~N,~N’]CHLi-OEt2 CHM.2thf (M = Na, K).53In a similar vein to the benzamidinate structures noted above,52q3-PCN coordination of the metal is manifest in the caesium = di(2-pyridy1)phosphide [C5H4N(p-PC5H4N)Cspmdetal2 (pmdeta N,N,N’,”’,N’’-pentameth yldiet h~lenetriarnine)’~ The M-Mn complex Mn(dm~e)2(C=CSiMe~)~M [M = Li, Na; dmpe = 1,2bis(dimethy1phosphino)ethanel has been prepared via the simple reaction of Mn(dm~e)~(C=CSiMe~)~ with the corresponding elemental metal. The sodium product has been fully authenticated and it has been found that the alkali metal ion bisects the two cis-organised acetylide l i g a n d ~ . ~ ~ Reaction of acetonitrile solutions of MesMo(C0)3 (Mes = mesityl) with Et4NCNand either LiOTf (Tf = triflate) or NaSbF6 has afforded the tetrahedral tetrahedranes (Et4N)5[MCMo4(p-CN)6(CO)12] (M = Li, Na), the sodium form of which has been fully characterised. Moreover, it has been established that either

2: Groups 1 and 1 1 : The Alkali and Coinage Metals

53

species can be converted to the trigonal prismatic Cs-containing equi~alent.'~ 1.4 Aryl Derivatives. - The directed lithiation of benzamides has been followed by treatment with sulfur and phosgene to give benzo[e][ 1,3]thiazine-2,4diones, while thiophosgene instead affords 2-thioxo-2,3-dihydro-benzo[e][l,3] t hiazin-4-ones. In a like vein, benzenesulfonamidesgive the previously unknown 1,l-dioxo-1,2-dihydro-1,4-dithia-2-aza-naphthalen-3-0ne with phosgene and the 1,2-dioxo-2-pheny1-2,3-dihydrobenzo[e] [1,4,2]dithiazine-3-one with thiophosgene. The employment of selenium in place of sulfur affords the analogous selenium heterocycles, notwithstanding the fact that for benzamides the use of selenium and phosgene yields benzo [d]isoselenazol-3-0nes.5~ Reductive lithiation of a,a-dibromo esters using lithium naphthalenide has afforded ester dianions and thence ynolate anions,58while the formation of 4-arylisochroman-3-acetic acids has also been achieved from benzoic acids employing reductive m e t a l l a t i ~ n .It~ ~has been suggested that naphthalenecatalysed lithiation reactions of chlorinated precursors probably involve the participation of a dilithium naphthalenide species as the active electron carrier agent during the chlorine-lithium exchange.60For the first time, the lithiation of a soluble chloromethyl-containing polymer has been achieved-this using a 4,4'di-tert-butylbiphenyl-catalysed lithiation process.61 Reaction of (Me3Si)(2-PhOC6H4)NHwith Bu"Li in Et2O has yielded the which dilithiate-containing co-complex [(Me3Si)(2-PhOC6H4)NLi2-OEt2]Bu"Li reveals a dimeric hexalithium aggregate in the solid state.62 The first full structural characterisations have recently been carried out on ortho-lithiated aromatic tertiary amides. The dimeric products go a long way towards clarifying the nature of (amide)O-Li coordination in ortho-lithiated amides and hence how ortho-activation of benzenoid aromatics proceeds.63 Ortho-lithiated benzyl diorganophosphines have also been investigated. Hence, (2-Me2PCH2C6H4Li.0Et2)2 has been both structurally probed and found to isomerise to give { [Me2PCH(Li-OEt2)]Ph}2.64 A decalithium aggregate has been noted to form from the reaction of N-(2-phenoxyphenyl)-N-(trimethylsilyl)amine with excess Bu"Li. The product aggregate, { [2-C6H4(2'-OC6H4)](Me~si)N}Li2, contains carbanions derived from abstraction of the proton from the ortho position of the phenyl ~ubstituent.~~ Lithium, sodium and potassium have all been employed to reduce 1,3,5-triboracyclohexanesand related 1,3,5-triboraalkanes. The resulting trishomoaromatics have been structurally characterised in both the solid and solution states.66 Metallation of the (previously described)monolithiated (aminomethy1)benzylsilane Me2Si[CH(Li)Ph](CH2NC5H10) has yielded the (hexameric) 1,3-dilithiate Me2Si[CH(Li)(2-C6H4Li)](CH2NC5Hlo) wherein the second deprotonation has been achieved at the ortho position on the phenyl ring.32The solid-state structure of polymeric lithium (S)-1- [2'4 methoxymethy1)pyrrolidin-1'-yl] - 3,5-dimethylboratabenzene has been presented.67 The sodium salt But2PhSiNa has been crystallised and revealed to be a polymer based on intermolecular ($-Ph)-Na Lithium and sodium salts containing carbons-adjacent arachno-C2Blo carbor-

54

OrganometalEic Chemistry

ane tetraanions with both hexagonal (q6-C2B4)and pentagonal (q5-C2B3)metalbonding faces have been prepared by directly reducing 1,2-o-xylylene-1,2-carboranes with excess Moreover, two examples of potassacarboranes have recently been structurally characterised. [closo-exo-{ p-1,2-[0-C6&(CH2)~]- 1,2C2B10H10}2K3-2(18-crown-6)][( 18-crown-6).K.2NCMe] represents the first fullsandwich potassacarborane while its protonation yields [{ p-1,2-[0C6H4(CH2)2]-1,2-C2B10Hll}K-( 18-~rown-6)],.~'The first fully authenticated potassium salts of benzenoid radical anions without stabilising substituents have lately been reported. Hence, the toluene radical anion reveals a monomer while the congeneric benzene radical anion is observed to dimerise via the formation of a carbon-carbon bond.71Monomeric potassium disupersilylsilanides have been presented, with R ( B U ' ~ S ~ ) ~ S ~ K (R- ~=C H, ~ HMe) ~ revealing bis(q6-benzene) A potassium fluorosilicate polymer based on the reacsolvation of the tion of Mes2C6H3SiF3 with KF has recently been characterised wherein the metal ions reveal q2-interactionswith the encapsulating terphenyl l i g a n d ~The . ~ ~rubidium salt of TippPH2 (Tipp = 2,4,6-P1j~C~H~) has been shown to exhibit a polymeric Rb-P ladder wherein each metal centre is $-supported by an aryl group bonded to an adjacent P - ~ e n t r e . ~ ~ Several mixed-metal aromatic species have been crystallised recently. The lithium gallate complex [(Me3Si)3CGa12( p-I)I3Li3.PhMe.4(0H2)has revealed a spirocyclic Li(p-O)2Li(p-O)2Licore about which the hydrocarbon solvent adopts an q2-complexationmode to one of the terminal metal centres.75In a similar vein, (Ph3E),Ga(p-I)Li.2thf(E = Si, Ge) reveals weak q2-interactionbetween the alkali metal and a single phenyl ring.76Moving to potassium salts, the lanthanoidocene 18-crown-6))2.C4Hs02 reveals a complex ({(BH4)2[Ph2(C13Hs)C(C5H4)]Nd}K-( dinuclear arrangement incorporating q2 fluorenyl-potassium linkages.77An In06K3 core defines the solid-state structure of In[(S)-bin0late)3(K.2PhMe)~ [(S)-binolate = (S)-(-)-l,l'-bi(2-naphthoate)]. This species, which results from the 1:3 reaction of InC13 with [(S)-binolate]&, demonstrates variability in the The complex caesium tris(bihapticities of the solvating toluene m01ecules.~~ imidazolato)nickelate(II) has been crystallised, with two of the three 2,2'-biimidazolate ligands being Cs-bonded via electrostatic n-intera~tions.~~ The solution behaviour of peri-lithiated N,N-dimethyl- 1-naphthylamine has recently been studied. The thf-solvate of l-(dimethylamino)-8-naphthyllithium-a dimer in the solid state-retains its integrity in deuterated thf at -90°C and reveals 1J~3c7L1 by 13CNMR spectroscopy." The tandem addition of phenyllithium to (E)-cinnamaldehyde has recently been reported to provide a route to @-substituteddihydrochalcones, with both isotopic exchange reactions and solution NMR spectroscopic studies having yielded insights into the route by which this transformation occurs. Results suggest that the aggregation features of aryllithium reagents and extended charge delocalisation effects promote g-selectivity!' Theoretical studies into the stability of sodiated phenylalanine have revealed that the relative stabilities of charge solvated and zwitterionic structures are dependent on the influence of the amino acid chain. It transpires that cation-n interactions confer greater stability on the former of these two structure-types.p2

2: Groups 1 and 11: The Alkali and Coinage Metals

55

1.5 Cyclopentadienyl and Related Derivatives. - The synthesis and addition of 9-deaza-9-lithiopurine to a carbohydrate-derived cyclic imine has lately facilitated the preparation of biologically active aza-C-nu~leosides.8~ Reaction of hydrotris(3-ethyl-imidazolin-2-ylidene)borate(tris"') with Bu"Li leads to the elimination of LiBF, and Bu"H. The result is the homoleptic carbene-lithium complex (trisE')Liwhich is revealed by crystallography to be a (CLi)2-based dimer in the solid The lithium complex (F5C&B(NC4H4)Li.0Et2has been full4 characterised and reveals q5-coordination of the alkali metal. This species represents a Cp-anion equivalent and is of interest in the context of preparing homogeneous metallocene Ziegler catalystP The X-ray powder-diffraction structure of (Cp*hNa (Cp* = pentamethylcyclopentadienyl) has been recently reported to reveal disordered organic residues which are rotated away from the eclipsed by IfI 13.8°.86 Unsolvated 1,2,4-tris(trimethylsilyl)cyclopentadienylpotassium has been shown to polymerise in the solid state in a manner already known to pertain for related lithium The formation of minteractions between Group 1 and Group 2 metals and a variety of meso-octaalkylporphyrinogens (R8N4H4)has been probed.88Potassium- and caesium-containing three-dimensional networks have been prepared wherein the metal centres experience a variety of interactions with pyrrolyl moieties. Also fully characterised has been the mixed Li-Ca octaalkylporphyrinogen (R = Et) wherein the Group 2 metal centre is q5-bonded by two trans-pyrroles and ql-bonded to the other pair of pyrroles which concomitantly act as q3-donors for the two Li centres.88As part of a study into the redox chemistry and photolability of Ru-nitride fragments, ruthenium(I1) meso-octamethylporphyrinogen (R = Me) has been transformed into the nitride-bridged dimer. This, in turn, has undergone single-electron reduction to give a species in which two Na+ ions are each q5:q5-sandwichedbetween pyrrole groups in either half of the structure and also closely contact the bridging nitride centre.89In a related study, the rearrangement of acetylenes of Ru(I1) porphyrinogen complexes to give vinylidenes and carbenes has been investigated?O In this context, Ru2Na-and Ru2Na-containing systems have been structurally characterised. In either case the ruthenium centres are acetylene-bridged and the Na+ ions are q5:q5-sandwichedby pyrrole units in each of two porphyrinogen macrocycles whilst also interacting weakly with the acetylene bridges." Mixed Li-Sm species have been structurally characterised as part of a study into calix-tetrapyrrole Sm(I1) and Sm(II1)complexes with acetylene. Results suggest that the reactivity of calix-tetrapyrrole tetraanions ({ [R2C(C4H2N)]4}4-) is greatly dependent on the nature of substituents present." Hence, for R = [--(CH2)5-]0.5dehydrogenation yields the dinuclear complex ({ [-(CH2)5-]4-calix-tetrapyrrole}Sm(III)~(pC2L&)-thfwherein the Group 1 metals bind to the acetylide but are also sandwiched by the two calix-tetrapyrrole systems. This contrasts with the observation that, for R = Et, dehydrogenation is accompanied by acetylene coupling and isolation of the dimeric butatrienediyl enolate {(Ets-calixtetrapyrrole)Sm(III)[Li(Li.thf)~(p3-0CH = CH2)]}2(p:q2:q2-HC= C = C = CH) wherein the lithium centres lie peripheral to the core. Related reactions utilising

56

Organornetallic Chemistry

the trivalent hydride {(Ets-calix-tetrapyrrole~thf)Sm(III)[(p-H)Li~thf]}~or the (Ets-calix-tetrapyrro1e)terminally bonded methyl derivative (Me)Sm(III)[(Li.thf)2(Li.2thf)(p3-C1)] yielded a collection of carbide, dimerisation and isomerisation products.” Mixed Li-Sm systems have also been probed as part of a study into organolanthanide complexes derived from the ligand Me2Si(C5Me4H)(C2B1~H11).92 In this context, the 1:l reaction of Me2Si(C5Me4H)C1with C2BI0Hl0Li2 in toluene/Et20 has given the versatile From this, various alkali monoanionic salt Me2Si(C5Me4)(C2BloH11)Li~0.50Et2. metal-lanthanide complexes have been produced, including { [(p-q5):aMe2Si(C5Me4)(C2B loHlo)Li-thf ]2Li}Li-4thf.t hf, which features p:q’:q 5-C5Me4R bridges between Group 1 metal Concerning other alkali metal-containing heterobimetallics, the reaction of N - [2-N’,N’’-(dimethylaminoeth y1)-N-methylaminoethyl] ferrocene (FcN,NH) with Bu”Li has yielded the corresponding lithium organyl wherein metallation has occurred at the 2-position of the already-functionalised ring, the solid-state nature of the tmeda-solvated dimer having been ~btained.’~ The structure of (F~N)~Br2Li~Mgm20Et2 { FcN = Fe(q5-Cp)[q5-CsH3CH2NMe2-2]} has been reported as part of a wider study into the dynamics of complexes containing both magnesium and N,N-dimethylaminomethylferrocenyll i g a n d ~ Ferrocene .~~ has recently been derivatised as a series of trimagnesium-bridged trinuclear ferrocenophanes co-complexed with alkali metal amides. Species which have been characterised in this conte~t-[Fe(C~H~)~]~M~Mg~(tmp)~-L~ (M = Li, Na; L = Htmp, C5H5N;tmp = tetramethy1piperdide)-all reveal Group 1-Group 2 p2:q‘:q1-CsH4bridges.” Reaction of Tm12.2thf with (Cp*),K gives the Tm(II1) complex (C~*)~TmI.thf which has, in turn, been used to generate the sodium complex (p2:q5:r5-Cp*)[Na(p2-OMe)2Tm(~5-Cp*)(~2:r5:~S-Cp*)Na.2dme] (dme = dimethoxyethane)?6(The same study also reports the solid-state structure of Me2NCH2CH2C5H4K.thf.)96 Recently, (q2-Cp)3MnK.l.5thfhas been prepared from the reaction of CpK with Cp2Mn;the product revealing p2:q2:q2-Cpbridges between different metal types.97A study seeking to probe the similarities between Cp and Tp { = tris[pyrazolyl(borate)]} has recently presented the first solidstate characterisation of an $-interaction between a Tp-type ligand and potassium. Accordingly, [Tp’CuK(p&03)KTp’]2 (Tp’ = 3-Me-5-CF3-Tp)comprises both ql- and $-modes of metal ~tabilisation.~~ Lastly, a p-q5:q5-bondedpolymer has been noted for the diphosphatiboylyl potassium complex [( 1,4,2P2SbC2But2)K-dme] in the solid 2

Copper, Silver and Gold

2.1 Introduction. - Section 2 of this review is sub-divided into three parts. Each considers a different coinage metal. In the same way as for Section 1 the second part of the article is dominated by compounds that contain at least one carbonmetal interaction. Synthetic and mechanistic studies are reported first in each part and are followed sequentially by solid-state, solution and theoretical investigations.

2: Groups 1 and 11: The Alkali and Coinage Metals

57

2.2 Copper Compounds. - Organocuprate reagents have lately been employed to effect the Suzuki cross-coupling reaction of olefinic aziridines,'(" in the stereoselective preparation of functionalised (E)-alkene dipeptide isoesters of L-amino acid-L-Glu and L-amino acid-D-Glu,'" to effect conjugate addition to 3-arylsulfinylchromones en route to homochiral 2-substituted chromanones,102 and also to create new metal cyclopentadienides from fullerene~.'~~ Just as BunLi has lately been employed in the synthesis of peroxetine, so the lithiocuprate compound (4-&H&CULi has also been used.'04 The effect of Cu(1)salt (CuCN, CuCN.2LiC1, CuI), cuprate reagent, organolithium substrate, solvent, and temperature upon the reaction of a-(N-carbamoy1)alkylcuprates with various alkenes, ketones, esters, and an acid chloride has been p r ~ b e d . ' ~Meanwhile, ~~'~~ lithium cyanocuprates have been noted (like organolithiums themselves) to oxidise to the corresponding alcohols with loss of stereochemistry at the nucleophilic C-centre in cyclopropyl metal compounds. However unlike organolithiums, the cuprates form dimer~."~ Lithium cyanocuprates have also been employed (amongst other organocuprates) in conjunction with (S)-N,N'bis(p-methoxybenzyl)-3-methylene-6-isopropylpiperazine-2,5-dione to yield cis3-isopropy1-6-alkyldiketopiperazines.108 The lithiocuprate Me2CuLi has been utilised in the synthesis of enantiopure (3S,4R)-3-amino-4-alkyl-2-piperidinone derivatives, with reaction proceeding via highly diastereofacially selective 1,4-addition of the organocuprate to the chiral oxazolidine a$-unsaturated ester.'" From optically pure L-serine a new N-Fmoc 3-iodoalanine tert-butyl ester derived organozinc reagent has been prepared. This has been reacted with CuCN-2LiC1 to give an organocopper reagent capable of undergoing coupling with ally1 chloride or ethyl oxalyl chloride to yield Fmoc-protected amino acids suitable for use in automated solid phase peptide synthesis."' Unsaturated lactams have been found to undergo the stereoselective conjugate addition of lower order cyanocuprates to ultimately afford enantiopure cis-3,4-disubstituted and 3,4,5-trisubstituted piperidines."' The importance of conjugate addition to unsaturated carbonyls has led to the development of a new synthetic approach whereby an umpolung of the Michael acceptor reactivity occurs. In this vein, unsaturated acetals and ketals have been shown to undergo hydroboration and conversion to secondary alkylzinc reagents, with the application of CuCN.2LiCl and then allylic bromides, alkynyl halides or propargyl bromide yielding the products of formal Michael addition.'12 Phosphoramidite ligands have been shown to induce high enantioselectivities in the Cu-catalysed conjugate addition of dialkylzinc reagents to various Michael acceptor^."^ Moreover, Arduengo-type carbenes have been noted to enhance the rate at which unsaturated ketones undergo Cu-catalysed Et2Zn conjugate addition.'14 A variety of allylic alcohols have been resolved by Sharpless kinetic resolution and their hyroxyl functions derivatised as diethyl phosphates whereupon their reactivities towards R2(NC)CuLi2have been probed.'15 Just as the preparation of certain (E)-alkenedipeptide isoesters has been reported,'O' so lithiocuprates have been employed to achieve a-substituted (2)-fluoroalkane dipeptide isoe s t e r ~ . " ~The * ~ ~enantiopure ~ N-acyliminium ion (4RS,5S)-5-chloromethyl-4-

58

Organometallic Chemistry

methoxy-1,3-oxazolidin-2-onehas undergone substitution of the 4-methoxy group using a variety of organometallic nucleophiles (including R3Cu2Li,R = Bun,Ph). This gives trans-diastereoselective 4,Sdisubstituted 2-oxazolidinones, with significant flexibility in the choice of 4-position substituent.118The addition of a series of lithiocuprate reagents to 3-acyl-N-alkylpyridinium salts, followed by acylation of the intermediate 1,4- or 1,2-dihydropyridines has been reported.'19 The transmetallation of various tellurides have been achieved using lithiocuprates while organocuprates have also been utilised to effect the reaction of vinylic tellurides with alkenes in the presence of catalytic Pd.120 Magnesium cuprates have been used to diastereoselectively convert 2siloxycyclopentene and -hexene carboxylates into syn-anti cyclopentanols and -hexanols.12' The conjugate addition of magnesium divinylcuprate (amongst other organocuprates) to 4-0-crotonyl derivatives of methyl a-D-glucopyranoside affords the corresponding adducts, from which (3-C-substituted butanoic esters can be obtained in significant enantiomeric excess.'22The 1,4-addition of the same cuprate to some methyl a-D-manno- or a-D-galactopyranosidic substrates incorporating a crotonyl group has also been probed. Various magnesium organocuprates have been utilised to install an alkyl group onto cis-4cyclopentene-1,3-diol monoa~etate.'~~ Reaction of an allyl ester and a magnesium diallylcuprate, or an allyl Grignard reagent in the presence a cata-lytic copper salt, has recently been found to yield mixtures of homo- and crosscoupled 1,Sdienes. The product ratios recorded are close to those expected for a reaction involving a triallylcopper(II1) intermediate and it has been suggested that the ability of the allyl ligands to ql- or q3-coordinate the metal facilitates stabilisation of the Cu(II1) species.124 Lately, a review has been published detailing the ways in which the addition of catalytic or stoichiometric quantities of H2O to organic and organometallic processes can result in beneficial effects on reaction rate, product yield, and regio-, diastereo- and enantioselectivity. In this context, the use of water as an internal quenching agent in the stannyl- and silylcupration of alkynes has been

The solid-state structures of two 2-alkylpyridine-bridged dimers have been (R = H, Me3Si)has been reported. Hence, [6-RCH2C5H3N[p-2-C(SiMe3)2]Cu}2 structurally authenticated. 26 Internal q2-alkene coordination has been reported in the monocationic, mononuclear complex between Cu(1) and a derivative of (S,S)-bis(pinene)bipyridine which incorporates two terminal alkene functions and in which the metal is rendered A,A-isomeric by virtue of being complexed by two bipyridine N-centres and the two C-centres of a single pendant alkene.127Reaction of the thallium P-diketiminate (Me2NN)Tl{ Me2NN = H2C[C(Me)N(2,6-Me2C6H4)I2} with CuBr-SMe2in the presence of either ethylene or styrene in hydrocarbon media has yielded thermally stable (Me2NN)Cu(q2-alkene)complexes. For the use of ethylene, subsequent reaction with O2 has been shown to give a (pOH)2[C~(II)]~-based dimer.12' The q2-interactionof an allylammonium fragment Also with copper has been noted in (H~O)[(O,SNH~)-)~(HN~CH~CHCH~)+CU.'~

2: Groups 1 and 11: The Alkali and Coinage Metals

59

recently reported are the solid-state structures of [(atsc)Cu]H2NS03 and its hydrolysis product [(atsc)Cu12S04(atsc = 4-allylthiosemicarbazide)wherein the The synthesis, metals are stabilised by sulfur and also q2by the ally1 luminescence and structural properties of a trinuclear copper(1) acetylide complex have been probed. A trigonal array of metals reveals p3-capping by the terminal carbon centres of both acetylide moieties in [(p3-q'-C-C-benzo- 15PF6[dppm = bis(dipheny1phosphino)methane] .l 31 The crown-5)2(p-dpp~n)~Cu~] encasement of an acetylenediide ion by a butterfly-shaped tetranuclear copper cluster has also resulted in the isolation and characterisation of a luminescent complex: [(p-Ph2Ppyp~)(p~-q',q~-C=C)Cu41(ClO~)~ [PhzPpypz = 2-(diphenylphosphin0-6-pyrazol-l-yl)pyridine].'~~ Reaction of 1,4,7-tri(5-phenyl-4-pentyny1)- 1,4,7-triazacyclononanewith CuI and NaBPh or with 4MeCN.CuPF6 has yielded the corresponding monocationic tacn complexes. For the former system, the solid-state structure reveals one Cu-bound and two free a1k~nes.I~~ Copper halides and pseudohalides have been generated hydrothermally. The copper(I1) pseudohalide [(meso-cth)(NC)Cu]C1O4.H20(cth = 5,5,7,12,12,14hexamethyl-1,4,8,11-tetraazacyclotetradecane) has been structurally elucidated.134 The complex Pz(NC)Cu3C12 (Pz = pyrazine)polymerises via the formation of a C12Cu2backbone in which the two copper atoms are bridged by the It~has nitrile charge centre, the nitrogen component of which bonds to P z C U . ' ~ been noted that the co-complex between 2,3-dihydroxyquinoxalineand CuCN adopts a one-dimensional chain structure in the solid state that is based on Cu-NC-Cu units.'36 Hydrothermal reactions of CuCN and KCN with the appropriate non-linear bipodal organodiimines (e.g., 2,2'-bipyridine, 1,lOphenanthroline and 2,2'-biquinoline) have given several one-dimensional polymers based on CuCN ba~kb0nes.l~~ In a similar vein, a variety of two- and three-dimensionalpolymeric copper complexes have been obtained by the treatment of CuCN with linear bipodal organodiimines such as pyrazine, alkylpyrazines, quinoxaline, phenazine, 4,4'-bipyridine and trans-4,4'-bipyridylethylene.138 has The mixed-metal Cu(1) acetylide (q5-C5H4SiMe3)Ti(C&But)2Cu(C=C)2Et been structurally characterised and the Group 11 metal has been found to bisect the titanium bis(acety1ide)bite angle.'39Polymeric arrays based on similar nitrile bridges to those alluded to above'36have been noted in {[WS4(NC)2CuBr2] (NEt4)2}mand { [Zn(NC)6Cu4.4dmf](NEt4)2}m (dmf = dimethylformamide).140 The syntheses of new heterotetrametallic (d6-d1'-d8)polyalkynyl complexes have been reported. In this context (Et3P)Cp*M(p-1~Ca:q2-C=CPh)2M'2(p-4~Ca: C=CPh)2Pt(CbF5)2 has been prepared for M = Rh, Ir and M' = Cu, Ag with the solid-state structure of the Rh-Cu complex having been obtained. This species reveals two very different bis(alkyny1)metal fragments connected through the copper centres.141 In relation to arylcopper derivatives, the straightforward tetranuclear complexes (dmpC~)~(1Cu)(Bu'OCu)'~~ (dmp = 2,4-Mes2C6H3) and 1,2:3,4-(hmdsCu)21,4:2,3-(MesCu)z(hmds = hexamethyldisilazide) have been shown to incorporate near planar C2OICu4 and C2N2C&cyclic cores, re~pective1y.l~~ The solidstate structure of the copper(1) thioether [(Ph2Bt)CuI4 [Ph,Bt =

60

Organometallic Chemistry

Ph2B(CH2SMe)2]reveals an 8-membered (SCU)~ core with the metals also being supported by q2-coordinationfrom one of the two phenyl rings on each ligand.l4 The use of functionalised epichlorohydrin has allowed the generation of a zwitterionic diimidazolium salt which has been subsequently employed as a proligand in conjunction with Ag20 to form a silver alkoxy-carbene complex. This has, in turn, been treated with CuI to form both AgI and the corresponding dimeric, neutral copper alkoxide-carbene, the solid-state structure of which reveals a C2Cu(p-O)2CuC2-core.'45 Encapsulation of Cu(I1) by an N-confused calyx[4]phyrin derivative has been the subject of study by both X-ray crystallography and EPR spectros~opy.'~~ Variable-temperature NMR spectroscopy has been employed to probe the solution behaviour of the tacn-type Cu-complexes afforded by reaction of 1,4,7tri(5-phenyl-4-pentynyl)- 1,4,7-triazacyclononanewith CuI and NaBPh4 or with 4MeCNCuPF6.The revelation that only one of the three alkynes is Cu-bound in the solid state is also reflected in dueterated acetonitrile Treatment of (R)-N-methyl-1-phenyl-2-(1-pyrrolidinyl)ethanamine with Bu"Li and CuI has afforded a dimeric lithium amidocuprate. In the presence of cyclohexanone this has been shown, by NMR spectroscopy, to give a monomeric copper a-complex between ketone and lithium amid~cuprate.'~~ Extensive NMR methods have also been applied to the question of the structure adopted by MezCuLisLiCN in EtlO solution. Results suggest a homo- [(Mezc~Li)~] rather than a heterodimeric (Me2CuLi.LiCN)core.*48The cuprate n-complex that results from the application of the 2-en-4-ynoate Bu'C&CH = CHC0,Et to ( 13CH3)2CuLi-LiCN in dueterated thf has been probed by I3C,NOESY and ROESY methods with results indicating that the cuprate-alkene core deviates from the ideality of a square planar arrangement.'4y The conjugate addition of lithium diorganocuprates to acrolein, cyclohexanone and 4,4-dimethylcyclohexanone in the presence of solvating MezO has been theoretically probed. Solvation evidently does not change the mechanism of addition but does cause the activation energy of carbon-carbon bond formation to be raised.150

2.3 Silver Compounds. - Whereas organosilver species have recently found little use in synthetic chemistry, silver acetylides have been prepared and employed in Pd-catalysed coupling reactions in order to generate enynes.'" The lithium aluminates (RF0)4AlLi [RF = (CF&CH-, (CF3)2C(Me)-, (CF3),C-] react with AgF to quantitatively give the corresponding silver aluminates. The solid-state structures of these products reveal both n-interactions with solvent (toluene) molecules and also weak cation-anion c~ordination.'~~ The crown ether assembly of one-dimensional silver clusters that encapsulate acetylenediide fragments has been investigated. Two sandwich complexes have been revealed in each of which seven silver centres describe a distorted pentagonal bipyramid within which the C22 ion Silver dodecahedra that encase C? have been revealed to polymerise in the solid state. A zigzag chain of such dianions has been noted in the anionic component of H20[H20.(CF3C02)5(C,")Ag6][(BF4)(tmc)Ag] (tmc = 1,4,8,1l-tetramethyl-

2: Groups 1 and 1 1 : The Alkali and Coinage Metals

61

1,4,8,1l-tetraazacyclotetradecane).’54Moreover, as part of a related study, a series of double salts of silver(1) have been prepared by treating an aqueous solution of RFC02Ag(RF = CF3, C2F5) and AgBF4 with C2Ag2.The resulting polymeric complexes incorporate polyhedral metal arrays in which C2- ions are embedded.’55Recently a new quadruple salt that incorporates acetylenediide has been synthesised. Hence, the first silver(1) salt containing four types of anions, 2C2Ag2-3NCAg15CF3C02Ag.2AgBF4.9H20,has been found to exhibit a columnar structure based on the polymerisation of (C2)2Ag13 cl~ster~.’~~ New concave n-prismand hydrocarbon [2.2.l]rn,p,p- and [2.2.l]p,p,p-cyclophanes have lately been employed in the encapsulation of Ag+ ions, with both cyclophanes behaving similarly. Hence, [2.2.1]rn,p,p-cyclophane-Ag+ triflate reveals interactions between the metal and one double bond in each of two phenyl rings along with one carbon centre in a third. Meanwhile, in [2.2.1]p,p,pcyclophane-Ag+ triflate each ring acts as an q ’ - d o n ~ r . ’In ~ ~a similar vein, the nickel catalysed oligomerisation of trans- 1,2-dibromobenzocyclobutene followed by dehydrogenation has yielded a concave annulene which has been A polymeric structure has shown to act like a n-prismand with respect to ~i1ver.l~~ been revealed for [(~2-benzene)silver(I)]-~-trifluoromethanesulfonate0;O’;O”:O” in the solid Reaction of 2,4,6-triphenoxy-1,3,5-triazine (tpotz) with silver perchlorate and trifluoromethanesulfonate has yielded both [( C104)2( tpo t ~ ) ~ A and g ~ ][(~CF3S03)2( t p o t ~ ) ~ A hf g ~]m.. t These incorporate support of the metals by phenoxy groups through q2- and ql-bonding, respectively.l6’ The reaction of C104Ag-H20with 7-methylbenzo[a]pyrene (L) has yielded the two-dimensional sheet structure [(C104)2LAg2-0.5PhMe]m while CF3SO3Ag reacts with dibenzo[b,deflchrysene (L’) to give a two-dimensional, neutral, lamellar chain and also a one-dimensional polymer as the co-crystalline product { [(CF3S03)2L’Ag2][(CF3S03)2Ag2.2PhMe]},. The use of AgC104.H20 in conjunction with benzo[e]pyrene (L”) in p-xylene instead gives the closed tripleThis can be regarded as a polymer decker compound (C104)4(p-xylene)(L”)4Ag4. by virtue of intermolecular n-n stacking.I6’ A dicationic imidazolium-linked cyclophane has been reacted with AgzO to generate a new dimeric complex in the X-ray crystal structure of which nbonding interactions are noted between the Ag centres and N-heterocyclic carbenes.162 In the context of silver-containing heterometallic systems a tetranuclear Ti2Ag2 complex (see Ti-Cu species, above)I3’that incorporates a Ag-NC-Ag core has been prepared and characterised. Hence, the silver centres in [(q5C5H4SiMe3)Ti(C-CSiMe3)2Ag]2(CN) bisect the angles described by Ti-bonded a~ety1ides.l~~ The compound (nit~py)~Mn[(NC)~Ag]~ [nitppy = 2-(4-pyridyl)4,4,5,5-tetramethylimidazoline-l-ozyl-3-oxide] is based on one-dimensional chains which form as the result of [(NC)2Ag]- ions bridging between Mn A polymeric array incorporating Cd-NC-Ag motifs has also been re~0rted.I~’ Moreover, helical polymer chains based on [Pt(Phpy)2Ag.0CMe2]+ and [Pt(thpy)2Ag-OCMe2] building-blocks [HPhpy = 2-phenylpyridine, Hthpy = 2-(2-thienyl)pyridine] have been reported to be based on dative PtAg bonds with the silver centres also being supported by n-interactions with the +

62

Organornetallic Chemistry

1-position of the phenyl or 2-thienyl rings.'66 Extensive HF-DFT calculations have been employed to probe the nature and strength of cation-anion interactions in the silver aluminates (R~0)dAlAg [RF = (CF3)2CH-,(CF3),C(Me)-, (CF3)3C-] alluded to above.'52 2.4 Gold Compounds. - Just as [6-MeC5H3N(p-2-C(SiMe3)2Cu]2has been structurally characterised, so too has the congeneric gold complex been observed.'26A series of azo-containing phosphine complexes of Au(1) have been prepared with a representative example having been fully characteri~ed.'~~ Reaction of the gold isocyanates LClAu (L = 2,6-Me2C6H3NC;2,4,6-Me3C6H2NC) with KSCN has yielded the corresponding species L(NCS)Au.These are noted to be centrosymmetric dimers in the solid state.'68The simple, antiparallel isocyante polymer (C3H7NCAuCl),has also been r e p ~ r t e d . A ' ~variety ~ of dinuclear cycloaurated complexes incorporating bridging 2-(dipheny1phosphino)phenyl or 2-(diethy1phosphino)phenylligands have been structurally chara~terised.'~~ The has been reacted with [(p cycloaurated complex [(p-C6H3-2-PPh2-5-Me)Au]2 S2CNBun2)Au12to give heterobridged dinuclear (p-C6H3-2-PPh2-5-Me)(vS2CNB~"2)A~2.171 This species has been shown to undergo oxidative addition with iodine to afford the heterovalent gold(II1)-gold(1) compound I(S2CNBun2)Au(p-C6H3-2-PPh2-5-Me)AuI.'7' Reaction between (Ph2PS2Au)2 and [Me2P(CH2)2AuI2has resulted in ligand exchange and the formation of (Ph2PS2)[Me2P(CH2)2]A~2. This last compound has been used in conjunction with Clz to prepare the oxidative addition product (Ph2PS2)[Me2P(CH2),1 (ClAu),-both of these dithiophosphinate gold(1)compounds having been structurally probed.'72 Several new gold(II1) thiosalicylate complexes incorporating 2-arylpyridine, 2-anilinopyridine and 2-benzylpyridine ligands have been generated and that which utilises a 2-p-tolylpyridine moiety has been characterised in the solid The linkage of gold(1) diphosphines via aurophilic interactions has led to the crystallographic study of co-complexes between { [ P ~ ~ P ( C H ~ ) , P P ~ ~ ]and ~ A (NC)2Au U ~ } ~ +, a linked ring complex (n = 3) and a polymer (n = 5) having been e1~cidafed.l~~ Recently, ttf ( = tetrathiofulvalene) vinylogues have been prepared. In this context, an infinite network has been achieved by synthesising the adduct between one such ttf derivative and (NC)~AU-.'~~ A series of trinuclear Au(1) complexes of the type (p-triphos)(X3Au)[triphos = bis(2-diphenylphosphinoethy1)phenylphosphine;X = C1, Br, C6F5] have been prepared and, in the case of X = C6F5, been structurally authenticated. The triphosphine has been noted to reside in a conformation that incurs long inter-metal interaction^.'^^ Formation of the (2-Ph2PC6H4NH2)Cl(C6F5)2A~ monomer from [(p-C1)(C6F5)2A~]2and 2-Ph2PC6H4NH2has been reported. Moreover it has been converted into both the cationic species [(2Ph2PC6H4NH,)(C,F5)2Au] and the neutral amido complex (2Ph2PC6H4NH)(C6F5)2A~.'77 The straightforward monomeric terphenyl complex dmpAuPPh3 (dmp = 2,6-dimesitylphenyl) has been reported. ''* Reaction of C12(C6H4CH2NMe2-2)Au with acetoacetanilide has afforded an aurolactam based on a 4-membered NC2Au cyclic core.'79In a similar vein, an expanded +

2: Groups 1 and 1 1 : The Alkali and Coinage Metals

63

(8-membered) ring core has been achieved in {(5-MeOC6H4CH2NMe2~)[N(CO~E~)C(O)CHCN]AU}~.~CDC~~; this species having formed by slow dimerisation of the corresponding m~nomer."~ Further concerning arylgold(II1) species, the reaction of anhydrous gold(II1) chloride with various aromatics (benzene,toluene, xylenes, mesitylene, cumene, methoxybenzeneand chlorobenzene = Ar) and thereafter with 2,6-lutidine (lut)has yielded a series of complexes of the type C12(lut)ArAu.X-ray diffraction confirms the trans configuration at the metal centre that is suggested by infra-red spectroscopy.'*0 As part of a study into homoleptic carbene complexes in general and bis(imidazo1in-2-ylidene-1-y1)boratecomplexes of Pd(II), Pt(I1) and gold(1) in particular, the solid-state structure of [(y-bisE')Aul2[bisEt = a bis(imidazolin-2ylidene-1-y1)borate ion] has been obtained. It reveals an 8-membered dimetallocyclic core based on two y2-q1:q1-bondedN-heterocyclic carbenes.lS1A recent investigation of trinuclear Au(1) compounds with aromatic-substituted imidazolate or carbenate bridging ligands has led to the combination of the former with tcnq ( = 7,7,8,8-tetracyanoquinodimethane) and the latter with C6F6 to yield acid-base adducts which adopt extended-chain structures and may be of optoelectronic interest.lg2 The heterometallic polymer [Bun4N(Me3Sn)Cu2(CN)&has been fully characterised for the first time and found to incorporate ammonium guest ions between sheets in which tri-coordinate Cu(1) centres are linked by cyano and CN.SnMe3.NC bridges.lg3The study that reported on the reaction of N-[2N',N"-(dimethylaminoethy1)-N-methylaminoethyl] ferrocene (FcN,NH) with BunLihas also presented the straightforward complex (FcN,N)AuPPh3,wherein the ring which bears the amine is aurated at the 2-po~ition.~~ The ferrocene derivative F c ( ~ - S C ~ H ~ formed N ) ~ , by the reaction of FcLi2.tmeda with (2SC5H4N)2,has been treated with excess (C6F5)3AuOEt2to yield Fc[(2SC5H4N)Au(C6F5)3]2, within which each of the gold(II1) centres are N-coordinated by a different pyridine ring.lg4 Three-dimensional Au-Cu coordination polymers have been prepared recently, with [(NC)2Au]- having been employed to enhance the dimensionality of aggregation via Au - - - Au interactions in both [(NC)2Au]Cutmeda'85 and PZ[(NC)~AU]~CU.~~~ As part of an electrospray MS study of heterometallic sulfide aggregates of Au(III), Hg(I1) and Sn(1V)with Pt(II), the solid-state structure of the [(Ph3P)4Pt2(p3-S)2Au(pap-C',N)]2 ion [pap = ~ - C ~ H ~ N H C S H ~ N - ~ ] has been reported to comprise a trigonal array of metal centre^.'^^^'^^ Mixed Au(1)-Au(II1)complexes with bridging selenido ligands have been reported. The treatment of (Ph3PAu)2Sewith [Cl(C6F5)2AU]2 has given [(Ph3PAu)Se]2[p(C6F5)2A~]2 while 1:l reaction of [(p-dppf)Au2]Se [dppf = 1,l'-bis(dipheny1phosphino)ferrocene] with (C6F5)3A~.0Et2 gives the mixed-valence compound [(C6F5)3A~]Se[(p-dppf)Au2]. The experimentally observed geometries of these species have been rationalised by quasi-relativistic pseudopotential calculation~.~*~ The recent report of the single crystal structure of [Bun4N(Me3Sn)Cu2(CN),1, has been augmented by a powder diffraction and solid-state 13C,15Nand l19Sn NMR spectroscopic comparison with the new homologue +

64

Organometallic Chemistry

{ B U " ~ N [ M ~ ~ S ~ ( C H ~ ) ~ S ~ M ~ ~On ] ~the . ~ basis C U ~of ( Csimilarities N)~}~. be~~~ tween the two, a three-dimensional host framework within which tetrabutylammonium ions are trapped has been predicted for the latter species. The solid-state NMR spectra of { [Me2Sn(CH2)3SnMe2]o.5Cu(CN)2}m have also been ~ e p 0 r t e d . l ~ ~ References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

25. 26. 27. 28.

29. 30.

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2: Groups 1 and 1 I : The Alkali and Coinage Metals 31. 32.

33. 34.

35. 3 6. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.

50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65.

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66 66. 67. 68. 69. 70. 71. 72. 73. 74.

75. 76. 77. 78. 79. 80. 81. 82. 83. 84.

85. 86. 87. 88. 89. 90.

91. 92. 93. 94. 95. 96. 97. 98. 99.

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2: Groups 1 and 11: The Alkali and Coinage Metals

67

100. D. J Lapinsky and S. C. Bergmeier, Tetrahedron Lett., 2001,42,8583. 101. S . Oishi, H. Tamamura, M. Yamashita, Y. Odagaki, N. Hamanaka, A. Otaka and N. Fujii, J. Chem. SOC.,Perkin Trans. 1,2001,2445. 102. K. J. Hodgetts, K. I. Maragkou, T. W. Wallace and R. C. R. Wootton, Tetrahedron, 2001,57,6793. 103. E. Nakamura and M. Sawamura, Pure Appl. Chem., 2001,73,355. 104. J. Cossy, 0. Mirguet, D.-G. Pardo and J. R. Desmurs, Tetrahedron Lett., 2001,42, 7805. 105. R. K. Dieter, C. M. Topping, K. R. Chandupatla and K. Lu, J. Am. Chem. SOC., 2001,123,5132. 106. R. K. Dieter, C. M. Topping and L. E. Nice, J. Org. Chem., 2001,66,2302. 107. M. Moller, M. Husemann and G. Boche, J. Organomet. Chem., 2001,624,47. 108. S. D. Bull, S. G. Davies, A. C. Garner and M. D. O’Shea, J. Chem. Soc., Perkin Trans. I , 2001,3281. 109. C . Flamant-Robin, Q. Wang and N. A. Sasaki, Tetrahedron Lett., 2001,42,8483. 110. H. J. C. Deboves, C. A. G. N. Montalbetti and R. F. W. Jackson, J. Chem. Soc., Perkin Trans. 1,2001, 1876. 111. M. Amat, M. Perez, N. Llor, J. Bosch, E. Lago and E. Molins, Org. Lett., 2001,3, 61 1. 112. E. Hupe and P. Knochel, Angew. Chem. Int. Ed., 2001,40,3022. 113. A. Alexakis, S. Rosset, J. Allamand, S. March, F. Guillen and C. Benhaim, Synlett., 2001,1375. 114. P. K. Fraser and S. Woodward, Tetrahedron Lett., 2001,42,2747. 115. J. L. Belelie and J. M. Chong, J. Org. Chem., 2001,66, 5552. 116. A. Otaka, H. Watanabe, E. Mitsuyama, A. Yukimasa, S. Oishi, H. Tamamura and N. Fujii, Tetrahedron Lett., 2001,42,285. 117. A. Otaka, H. Watanabe, A. Yukimasa, S. Oishi, H. Tamamura and N. Fujii, Tetrahedron Lett., 2001,42,5443. 118. K. Schierle-Arndt, D. Kolter, K. Danielmeier and E. Steckhan, Eur. J. Org. Chem., 2001,2425. 119. M. L. Bennasar, C. Juan and J. Bosch, Tetrahedron Lett., 2001,42,585. 120. R. E. Barrientos-Astigarraga,P. Castelani, J. V. Comasseto, H. B. Formiga, N. C. da Silva, C. Y. Sumida and M. L. Vieira, J. Organomet. Chem., 2001,623,43. 121. V. Dambrin, M. VilliCras, P. Janvier, L. Toupet, H. Amri, J. Lebreton and J. VilliCras, Tetrahedron, 2001,57,2155. 122. K. Totani, T. Nagatsuka, S. Yamaguchi, K. Takao, S. Ohba and K. Tadano, J. Org. Chem., 2001,66,5965. 123. M. Ito, M. Matsuumi, M. G. Murugesh and Y. Kobayashi, J. Org. Chem., 2001,66, 5881. 124. A. S. E. Karlstrom and J.-E. Backvall, Chem. Eur. J., 2001,7, 1981. 125. S. Ribe and P. Wipf, Chem. Comrnun.,2001,299. 126. T. R. van den Anker, S. K. Bhargava, F. Mohr, S. Papadopoulos, C. L. Raston, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 2001,3069. 127. S. Fraysse, A. v. Zelewsky and H. Stoekli-Evans,New J. Chem., 2001,25,1374. 128. X . Dai and T. H. Warren, Chem. Commun., 2001,1998. 129. 0.P. Melnyk, D. Schollmeyer,V. V. Olijnyk and Y. E. Filinchuk, Acta Crystallogr., Sect. C , 2001,57,151. 130. 0.P. Mel’nyk, Y. E. Filinchuk, D. Schollmeyer and M. G. Mys’kiv, 2. Anorg. Allg. Chem., 2001,627,287. 131. V. W.-W. Yam, C.-H. Lam and K.-K. Cheung, Inorg. Chim. Acta, 2001,316,19.

68

Organometallic Chemistry

132. H.-B. Song, Q.-M. Wang, 2 . Z Zhang and T. C . W. Mak, Chem. Commun., 2001, 1658. 133. M. V. Baker, D. H. Brown, N. Somers and A. H. White, Organometallics, 2001,20, 2161. 134. C. Diaz, J. Ribas, M S. El Fallah, X. Solans and M. Font-Bardia, Inorg. Chim. Acta, 2001,312, 1. 135. N. S. Persky, J. M. Chow, K. A. Poschmann, N. N. Lacuesta and S . L. Stoll, Inorg. Chem., 2001,40,29. 136. M. Heller and W. S. Sheldrick, 2.Anorg. Allg. Chem., 2001,627, 569. 137. D. J. Chesnut, A. Kusnetzow and J. Zubieta, J . Chem. Soc., Dalton Trans., 2001, 258 1. 138. D. J. Chesnut, D. Plewak and J. Zubieta, J. Chem. Soc., Dalton Trans., 2001,2567. 139. W. Frosch, S. Back, H. Muller, K. Kohler, A. Driess, B. Schiemenz, G. Huttner and H. Lang, J . Organomet. Chem., 2001,619,99. 140. C.-P. Cui, P. Lin, W.-X. Du, L.-M. Wu. 2.-Y. Fu, J.-C. Dai, S.-M. Hu and X.-T. Wu, Inorg. Chem. Commun., 2001,4,444. 141. I. Ara, J. R. Berenguer, E. Eguizabal, J. ForniCs, E. Lalinde and A. Martin, Eur. J . Inorg. Chem., 2001,1631. 142. M. Niemeyer, Acta Crystallogr., Sect. E , 2001,57, m416. 143. M. Niemeyer, Acta Crystallogr., Sect. E , 2001,57, m491. 144. C. Ohrenberg, L. M. Liable-Sands, A. L. Rheingold and C. G. Riordan, Inorg. Chem., 2001,40,4276. 145. P. L. Arnold, A. C. Scarisbrick, A. J. Blake and C. Wilson, Chem. Commun., 2001, 2340. 146. H. Furuta, T. Ishizuka, A. Osuka, Y. Uwatoko and Y. Ishikwa, Angew. Chem. Int. Ed., 2001,40,2323. 147. J. Eriksson and 0.Davidsson, Organometallics, 2001,20,4763. 148. R. M. Gschwind, X. Xie, P. Rajamohanan, C . Auel and G. Boche, J . Am. Chem. Soc., 2001,123,7299. 149. J. Canisius, T. A. Mobley, S. Berger and N. Krause, Chem. Eur. J., 2001,7,2671. 150. M. Yamanaka and E. Nakamura, Organometallics, 2001,20,5675. 151. S . Dillinger, P. Bertus and P. Pale, Org. Lett., 2001,3, 1661. 152. I. Krossing, Chem. Eur. J., 2001,7,490. 153. Q.-M. Wang and T. C. W. Mak, Angew. Chem. Int. Ed., 2001,40,1130. 154. Q.-M. Wang and T. C. W. Mak, Chem. Commun., 2001,807. 155. Q.-M. Wang and T. C. W. Mak, J . Am. Chem. SOC.,2001,123,7594. 156. Q.-M. Wang and T. C. W. Mak, J . Am. Chem. Soc., 2001,123,1501. 157. T. Lahtinen, E. Wegelius and K. Rissanen, New J . Chem., 2001,25,905. 158. Y. Kuwatani, T. Yoshida, A. Kusaka, M. Oda, K. Hara, M. Yoshida, H. Matsuyama and M. Iyoda, Tetrahedron, 2001,57,3567. 159. H. Wadepohl and H. Pritzkow, Acta Crystallogr., Sect. C , 2001,57,383. 160. M. Munakata, M. Wen, Y. Suenaga, T. Kuroda-Sowa, M. Maekawa and M. Anahata, Polyhedron, 2001,20,2321. 161. J. C. Zhong, M. Munakata, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga and H. Konaka, Inorg. Chem., 2001,40,3191. 162. J. C. Garrison, R.S. Simons, W. G.Kofron, C. A. Tessier and W. J. Youngs, Chem. Commun.,2001,1780. 163. T. Stein and H. Lang, Chem. Commun., 2001,1502. 164. I. Dasna, S. Golhen, L. Ouahab, N. Daro and J.-P. Sutter, Polyhedron, 2001,20, 1371.

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69

165. H.-X. Zhang, B.-S. Kang, L.-R. Deng, C. Ren, C.-Y. Su and Z.-N. Chen, Inorg. Chem. Commun., 2001,4,41. 166. T. Yamaguchi, F. Yamazaki and T. Ito, J . Am. Chem. SOC.,2001,123,743. 167. M. J. Alder, K. R. Flower and R. G. Pritchard, J . Organomet. Chem., 2001,629,153. 168. T. Mathieson, A. Schier and H. Schmidbaur, J . Chem. SOC., Dalton Trans., 2001, 1196. 169. R. E. Bachman, M. S. Fioritto, S. K. Fetics and T. M. Cocker, J . Am. Chem. SOC., 2001,123,5376. 170. M. A. Bennett, D. C. R. Hockless, A. D. Rae, L. L. Welling and A. C. Willis, Urganometallics, 2001,20,79. 171. S. K. Bhargava, F. Mohr, M. A. Bennett, L. L. Welling and A. C. Willis, Inorg. Chem., 2001,40,4271. 172. W. E. v. Zyl, J. M. Lopez-de-Luzuriaga, J. P. Fackler Jr., and R. J. Staples, Can. J . Chem., 2001,79,896. 173. W. Henderson, B. K. Nicholson, S. J. Faville, D. Fan and J. D. Ranford, J . Organomet. Chem., 2001,631,41. 174. M.-C. Brandys and R. J. Puddephatt, Chem. Commun., 2001,1280. 175. Y. Yamashita, M. Tomura and K. Imaeda, Tetrahedron Lett., 2001,42,4191. 176. M. Bardaji, A. Laguna, J.Vicente and P. G . Jones, Inorg. Chem., 2001,40,2675. 177. E. J. Fernandez, M. Gil, M. E. Olmos, 0.Crespo, A. Laguna and P. G. Jones, Inorg. Chem., 2001,40,3018. 178. G. W. Rabe and N. W. Mitzel, Inorg. Chim. Acta, 2001,316,132. 179. W. Henderson, B. K. Nicholson and A. G. Oliver, J . Organomet. Chem., 2001,628, 182. 180. Y. Fuchita, Y. Utsunomiya and M. Yasutake, J . Chem. SOC.,Dalton Trans., 2001, 2330. 181. R. Frankel, J. Kniczek, W. Ponikwar, H. Noth, K. Polborn and W. P. Fehlhammer, Inorg. Chim. Acta, 2001,312,23. 182. M. A. Rawashdeh-Omary, M. A. Omary and J. P. Fackler Jr., J . Am. Chem. Soc., 2001,123,9689. 183. E. M. Poll, J.-U. Schutze, R. D. Fischer, N. A. Davies, D. C. Apperley and R. K. Harris, J . Organomet. Chem., 2001,621,254. 184. E. M. Barranco, 0.Crespo, M. C . Gimeno, P. G. Jones, A. Laguna and C. Sarroca, J . Chem. SOC., Dalton Trans,, 2001,2523. 185. D. B. Leznoff, B.-Y. Xue, B. 0. Patrick, V. Sanchez and R. C . Thompson, Chem. Commun., 2001,259. 186. D. B. Leznoff, B.-Y. Xue, C. L. Stevens, A. Storr, R. C. Thompson and B. 0.Patrick, Polyhedron, 2001,20, 1247. 187. S.-W. A. Fong, J. J. Vittal, W. Henderson, T. S. A. Hor, A. G. Oliver and C. E. F. Rickard, Chem. Commun., 2001,421. 188. S.-W. A. Fong, W. T. Yap, J. J. Vittal, T. S . A. Hor, W. Henderson, A. G. Oliver and C. E. F. Rickard, J . Chem. Soc., Dalton Trans., 2001,1986. 189. S. Canales, 0. Crespo, M. C. Gimeno, P. G. Jones, A. Laguna and F. Mendizabal, Organometallics, 2001,20,4812.

3

Group 2 (Be-Ba) and Group 12 (Zn-Hg)

DOMINIC S. WRIGHT

1

Scope and Organisationof the Review

This review presents a perspective of the important structural and synthetic studies reported in the year 2001. As with the previous few years reports, the strict definition of an organometallic compound as one containing at least one C-metal interaction or bond has been used in this survey (with some exceptions, where appropriate). Although based on an extensive literature search, the text is not intended to be fully comprehensive but to highlight major areas of current and potential future interest. In order to facilitate rapid access to a particular subject heading, individual topic headings have been placed in bold script in the text. It can be noted that the far more extensive literature concerning structural aspects of Group 2 and 12 organometallics reported for the year 2001, than for the previous few years, has meant that this aspect dominates this review.

2

Group2

As has been increasingly the case in recent years, the magority of the structural reports of a-bonded organomagnesiumcompounds'-" have concerned their applications to a number of technologically important fields, including polymerisation catalysis and the development of novel reagents for stereo- and regioselective organic synthesis. Sterically-demanding P-diketimate ligands of the type [RC( -NDipp) -CH -C( -NDipp)R]- (Dipp = 2,6-'Pr-CsH3) have the effect of shielding coordinated Mg2+ cations and reducing the tendency for dimerisation.'.2The monomeric complex rBuC( -NDipp) - CH - C( NDipp)'Bu]MgMe.thf (l),prepared from simple deprotonation of the organic acid with Me2Mg, readily loses coordinated thf on prolonged exposure to a vacuum at elevated temperature ( torr, 150°C).The resulting product rBuC ( -NDipp) -CH I . . C( - NDippYBulMgMe (2) is a rare example of an organomagnesium compound containing a three-coordinate Mg2+cation.' Although this strategy represents a promising way of obtaining coordinatively unsaturated Mg species for polymerisation catalysis, apparently the steric demands of the Dipp groups also have the effect of rendering 2 inactive to polymerisation of ethene. In contrast to the desolvation of 1, desolvation of the Organometallic Chemistry, Volume 3 1 0The Royal Society of Chemistry, 2004

70

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

71

monomeric ally1 magnesium complex [{'BuC( - NDipp) - CH - C( NDipp)'Bu}MgCH2CH =CH2thf] (3) under similar conditions leads to an increase in the aggregation ~ t a t eThe . ~ remarkable hexamer [{'BuC( -NDipp) CH -C( -NDipp)fBu}MgCH2CH= CH2I6(4) (Figure 1)is formed as a result of ql-, ql-bridging of the Mg2+cations. Relavent to future developments in the area of alkene polymerisation is the first report of a diorganomagnesium compound containing three-coordinate Mg2+.4 The complex [DetpMg(y-Detp)12(5) (Detp = 2,6-Et2C6H3) is obtained by desolvation of the thf adduct [Detp2Mg(thf),], and exists as an arene-bridged dimer in the solid state. This arrangement is in stark contast to diorganomagnesium compounds containing less sterically-demanding organic groups (like Me2Mg)which exist as polymers with four-coordinate metal ions. NMR spectrocopic investigations of 5 in arene solvents suggest that a monomer-dimer equilibrium is occurring, and give a unique insight into the solution dynamics of unsolvated diorganomagnesium species. Aspects of the applications of heterometallc, alkoxy Mg2+complexes in Ziegler-Natta catalysis have also been the subject of a review.12

4 Figure 1

The developing area of so-called 'Inverse Crowns' has been highlighted by a recent review,I3 and has continued to be a fascinating area of study. These species, possessing heterometallic cationic amide rings of alkali or alkaline earth metals (or zinc), are capable of synthetic transformations which could not be accomplished using a reagent containing one type of metal only. One of the highlights of 2001 was the extension of this methodology to reactions involving transition metal precursors. The reaction of 'Pr2NH (three equivalents) with an equimolar mixture of "BuNa and ""Bu2Mggenerates an intermediate (postulated to be '[NaMg(N'Pr2)3]') which smoothly deprotonates ferr~cene.~ Remarkably,

72

Organometallic Chemistry

this is not the normal one- or two-fold deprotonated product expected using a single-metal reagent (such as "BuLi alone) but an unprecedented (1,1',3,3') fourfold deprotonation (the product 6 is shown in Figure 2). This method provides a unique synthon for potential functionalisation of ferrocene. Interestingly, using a mixture of "BUM, ""Bu2Mgand C(Me2)(CH2)3C(Me2)NH ( = tmpH) (2:1:3 equivalents) only results in double-deprotonation of ferrocene in the heterometallic [M = Li (7),Na (8);L = Lewis base products [{(q5-C5H4)2Fe}3Mg3(tmp)2(M.L)2 donor] (Figure 3).6 The related heterometallic Li/Mg co-complex [{ (q5C5H3CH2NMe2)(q5-C5H4)Fe}2Mg(LiB~Et20)2] (9) has also been characterised recently, obtained by single-deprotonation of the Me2NCH2-substitutedferrocene [(q5-C5H4CH2NMe2)(q5-C5H4)Fe] with "BuLi, followed by reaction with MgBr2.' Of note also in these studies was the structural characterisation of the [{ (q5-C5H3CH2NMe2)(q5unusual diorganomagnesium complexes C5H4)Fe}2MgL](L = thf, Et20)

6

7 and 8

Figure 3

Figure 2

A study of 'Cluster Grignard Reagents' has provided the first direct evidence for these species.14The phenylpolymagnesium halides [PhMgX] (X = F, C1, Br) (11)are obtained by cocondensation of Mg and PhX at 100-130K,and have been characterised by MALDI-TOF mass-spectroscopy. Unlike classical Grignard reagents hydrolysis with H20 gives H2, as well as benzene (equation 1). PhMg4X

+

7HZO-PhH

+

Mg(0H)X

+

3Mg(OH)2 +3H2

(1)

Not surprisingly, investigations involving Grignard reagents (and related species) in r e g i ~ - ' and ~ - ~stereo~elective~~--~~ ~ organic synthesis have been numerous and highly varied in 2001. Full discussion of this area is beyond the scope of this review. Rather, the discussion is focused on fundamental studies involving the nature of Grignard reagents in organic reactions and the development of new organomagnesium reagents. Further evidence of the involvement of radicals during the formation of Grignard reagents has come from a study of the

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

73

formation of cyclopropyl magnesium bromide ('PrMgBr) in E t 2 0in the presence of dicyclohexyl phosphine (CY~PH)."Addition of the latter to the reaction mixture leads to reduction in the yield of 'PrMgBr by as much as 75% and the disappearence of solvent-derived products. At the same time, ("Pr)Cy2P and (Cy,P)2 appear as products. These results can be rationalised by a reaction scheme involving the formation of alkyl ('Pr) and phosphine radicals (Cy2P) which can combine. There has been continued interest in the synthesis of highly functionalised magnesium reagents. A new, highly regioselective method has been reported for Mg-halogen exchange of various aryl halides using the 'ate complexes R"BuzMgLi [R= "Bu (12); R = 'Pr (l3)J.I6Smooth iodine-Mg exchange of functionalised aryl iodides (ArI) occurs using 12 at temperatures as low as -78"C, producing the Grignards ArMgI. More reactive 13 is even capable of bromine-Mg exchange under similar conditions. The exchange of alkenyl halides using this method proceeds with retention of configuration of the double bond. 12 has also been found to be a highly useful reagent in the synthesis of asilylalkyl-magnesium reagents (14) (Scheme l).'*Reaction of Ph2(Me)SiCH2Br with 12 occurs rapidly at - 78"C,with subsequent migration of an "Bu group at 0°C in the presence of CuCN2LiCl catalyst giving the reagents 14. Addition of acid chlorides or a$-unsaturated ketones provides an excellent method for the preparation of a-silyl ketones. A further application of organomagnesium reagents in organosilicon chemistry is also worthy of note. Normally the use of Grignard reagents in addition reactions to vinylsilanes is fraught with problems, and primary alkyl Grignards do not react at all. By using a 2-pyridyl-dimethylsilyl directing group in 15 (Scheme 2), however, carbomagnesation is observed for the a range of alkyl, aryl and ally1 Grignards." The short reaction times and the highly solvent-dependent rates of reaction suggest that a pre-equilibrium complex between 15 and RMgX is involved, making subsequent carbomagnesation intramolecular. The further reactions with electrophiles, followed by oxidation at the Si-C bond with H202, generates unsymmetrical alcohols. Thus, the 2-pyridyl-dimethyl-silyl group can be removed once the series of transformations is complete.

Ph2(Me)SiCH2Br2

a

"Bu3Mg-Li'

Br

"BU

14

Scheme 1

SiA Q

Me2

15

Mg'kua

Ph2(Me)Siy

Ph2(Me)Siy

Scheme 2

14

Organometallic Chemistry

Investigation of the addition of the chiral secondary Grignard PhCH2C*H(MgC1)Et(16)24ato a range of electrophiles (including ketones, aldehydes and allyl halides) suggests that the reactions occur by competing concerted (SN2)and single-electron transfer (SET) mea~hanisms?~~ Reactions with the majority of substrates result in retention of configuration of the chiral centre (*) of 16.24b,25 However, with electron-poor aromatic aldehydes or ketones and allyl halides significant racemisation occurs. Interestingly, for the allyl halides CH2= CH-CH2Xracemisation increases in the order I> Br > Cl (in-line with their decreasing reduction potentials). Although not strictly speaking organometallics, the applications of magnesium amide bases in a range of organic transformations has been of considerable interest re~ently.~' The first report of a polymersupported chiral Mg-amide base provides an efficient and recyclable reagent to affect enantiomeric deprotonation. The active reagent (R-17) is readily obtained by reaction of the tethered chiral amine with "."Bu2Mg(Scheme 3). Deprotonation of a range of ketones, followed by quenching with Me2SiCI,produces the corresponding (S)-silylenols with moderate to high levels of asymmetric induction (up to 93:7).

Z" -

!e

H N

A

Ph

N

R-17

Scheme 3

0i

s I

Ph

The syntheses and structures of organometallics of the heavier Group 2 elements has continued to be an active area of A major focus of studies of the o-bonded organometallics of the heavier Group 2 metals has been the development of new ligands that can be used as alternatives to cyclopentadienyl ligands in the stabilisation of organometallic derivatives. The benzyl complexes [(DMAT)2Ca.2thf] (18)33and [(DMAT)&-2thf] (19)34[DMAT(H) = 2-Me2N(Me3Si)CH2Ph)]are obtained by the reactions of the K complex [(DMAT)K] (20) with the metal iodides. 18 is the first structurally characterised benzyl strontium complex. Both 18 and 19 contain six-coordinate metal centres, being chelated by the Me2Ngroups and the benzylic a-carbons of DMAT as well as by two thf ligands. The presence of two chiral benzylic centres in 18 and 19 results in diastereoisomers in apolar solvents at room temperature for both. Higher temperatures or the addition of further thf results in rapid inversion of the chiral centres via a dissociative process involving cleavage of the metal-C bonds. Both complexes were found to be active in the anionic living polymerisation of styrene (the rate for 19 being ca. three times that for 18). The related, functionalised benzyl ligand [2-MeO-Ph2PCH2Ph] has also been employed in the stabilisation of two other benzyl Ca c~mplexes.~' Reaction of the Na precursor C(2-MeOPh2PCHPh)Na-2Et20]2 (21)with Ca12in E t 2 0gives the 'ate complex [{(2-Me0Ph2PCHPh)3CaNa-EtzO](22).However, remarkably demethylation of the M e 0

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

75

group occurs in the reaction of the Li precursor [(2-MeO-Ph2PCHPh)Li.Et20I2 (23) with Ca12in thf, giving the novel heterocubane [[(2-0-Ph2PCHPh)Ca.thfl4 (24) (containing a Ca404core).Also worthy of mention in this area are the reactions of distilled Sr and Ba with [Zn(CH2SiMe3)2] (1:3 equivalents, respecti ~ e l y ) The . ~ ~ products [(thf)2M{Zn(CH2SiMe3)3}2] [M = Sr (25a), Ba (25b)l (obtained in thf) and [($-tol~ene)2M{Zn(CH2SiMe3)~}~](26) (obtained in toluene) all contain two [Zn(CH2SiMe3)3]-zincate ions which chelate the Sr2+or Ba2+ ions using two of the CH2 groups of each of the zincate ions in agostic interactions. The reaction of [Zn(CH2SiMe3)2]with Ba in heptane initially gives the unsolvated species [Ba{Zn(CH2SiMe3)3}2] (27).Addition of thf gives a small amount of the hexanuclear complex [(thf)3Ba(p-O)(Me3SiCH2ZnCH&SiMe2]2 (28) (as well as 25b), in which extensive rearrangement of the [Zn(CH2SiMe3)3]anion to a neutral [(Me3SiCH2ZnCH2)2SiMe2] ligand has occured. The 0 centres of 28 presumably result either from ether cleavage or from the presence of trace oxygen or H20. Interestingly, treatment of the unsolvated zincate 27 with further distilled Ba with high-energy ultrasound leads to even more extensive rearrangement in the double-cubane product [Ba4{((Me3SiCH2)2ZnCHSiMe3)2ZnCHSiMe3}2](29).37The formation of [{ (Me3SiCH2)2ZnCHSiMe3}2ZnCHSiMe3I4- tetraanion of the latter can be explained by the intermediacy of highly reactive [Ba(CH2SiMe3)2](30), which is preumably responsible for the deprotonation of Zn-bonded CH2SiMe3groups. Structural studies of n-complexes of the Be38and Mg39have been relatively rare in 2001. The major focus has remained on n-complexes of the heavier Group 2 metals (Ca-Sr).39-49 Structural and NMR spectroscopic studies of the insertion reaction of 2,6-Me2C6H3C=N(XylNHC) into [ B ~ C P * ~(31) ] (Cp* = C5Me5)give unique direct evidence for ql-,q5-bonding of the Cp* ligands for 31 in solution (as is observed for 31 in the solid The formation of the structurally characterised product [(q5-Cp*){q1-(Cp*)XylN=C}Be] (32) is reversible at higher temperatures. The insertion reaction with [CP‘~B~] (33) (Cp’= Me4C5H) gives [(q’-Cp’)(ql-(Cp’)XylN = C}Be] (34) as isomer A at - 78°C and isomer B at +25°C (Scheme 4). Although 34A is converted into 34B at room temperature in solution, surprisingly 34A is stable in the solid state under these conditions.

34A

348

Scheme 4

Studies of the heavier Group 2 metals involving the cyclopentadienyl family of ligands have involved a variety of species, including carbene complexes of metallo~enes,3~ ~nsu-rnetallocenes~~~~~~~~ as well as h a l f - ~ a n d w i c hand ~ ~ . unsym~~

76

Organometallic Chemistry

metrical metallocene compounds.44Two results stand out as particularly important to future developments. The addition of Ph3P=O to the half-sandwich (35) displaces the Ca-bonded thf and I- ligands and complex [C~*CaI(thf)~] gives [Cp*Ca(O = PPh3)3]+I- (36), a rare example containing a cationic metal10cene.~~ This route may provide general access to related heavier Group 2 cations of this type and, as such, represents an important advance towards applications as Lewis acid catalysts. The heteroleptic, half-sandwich complex [(DMAT)Ca(9-Me3Si-F1)Ca.thf] (37) (DMAT = 2-MeO-Me3SiCH2Ph; Fl = fluorenyl), obtained by the reaction of [(DMAT)2Ca.2thf] (18)33with the neutral ligand 9-Me3Si-Fl(H),illustrates the potential of heavier Group 2 metallocenes as polymerisation catalysts.4 Uniquely in this area, it is found that 37 polymerises styrene in a largely stereoselective manner, producing syndiotactic polystyrene with molecular weights of CLE.lo5by a living polymerisation process. Other structural studies of n-complexes of the heavier Group 2 metals have involved more exotic alternatives to the classical cyclopentadienyl l i g a n d ~ . ~ ~ - ~ ~ These studies include new sandwich and half-sandwich derivatives of phospholes, arsoles and stiboles,46a full account of work on the metallation of the rneso-octaalkylporphyrins [EtsN4H4]with Ca, Sr and Ba,47and the first report of a barium carborate c0mplex.4~

3

Group12

Extensive X-ray structural studies of a-bonded organozinc compounds were reported in 2001.36,37-50-62 These concerned a number of diverse areas, including applications to organic ~ynthesis,~' biological polymerisation catalysis51-53heterometallic complexes containing Zn,36337,59,60,62 as well as the investigation of novel inorganic reactions involving zinc o r g a n ~ m e t a l l i c s . ~ ~ , ~ ~ Motivated by previous evidence for Lewis acid activation of the SimmonsSmith reagent (IZnCH21)by ZnC12, a recent important study has concerned the development of carbenoids containing internal Lewis acid groups.50 The acyloxymethylzinc reagents [{ RC( = O)CH2}2Zn]can be regarded as the methylene transfer equivalents of peracids in the epoxidation of alkenes. The reagent [{ C6F5C(= O)CH2}2Zn](38) has been shown to have enhanced reactivity with a range of alkenes, e.g., readily reacting with trans-stilbene, which is often unreactive using most Simmons-Smith protocols. In contrast to other carbenoid complexes, which contain tetrahedral Zn, the X-ray structure of the model reagent [{ PhC( = O)CH2}2Zn](39) reveals a square-bipyramidal environment for Zn. The first investigations of the reactions of [H(OEt2)] [B(C6H5)4]-(40) and B(C6H& (41) with ZnR2 are relavant to applications in the activation of hightemperature alkene p~lymerisation.~' These reactions are highly solvent-dependent, the reactions of 40 or 41 with ZnR2(R = Me, Et, 'Bu) in the presence of Et2O giving salts of the type [RZn(OEt2)3]+[B(C6H5)4]- (42) and [RZn(OEt2)3Jf [RB(C6H5)3]- (43), respectively, whereas reactions of ZnR2 with 41 (R = Me, Et) or [Ph3C)+[B(C6H5)4]- (44) (R =Et) in toluene give Zn(C6H5)2(45) (uia alkyl/C6F5exchange or p-hydride elimination). The structure of [EtZn(OEt2)3] +

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

77

+[B(C&)4]- (42a) was obtained, showing a tetrahedral geometry for the Zn2+ cation. Also worthy of note in regard to applications in catalysis is the synthesis of a range of new f3-diketimate complexes [MeC( -NDipp)- CH -C( -NDipp)Me]ZnR (R = Me, ‘Bu, Ph) (46).52All of these species are monomeric in the solid state and contain two coordinate Zn2+. Interestingly, they resist coordination by strong Lewis base donors (such as thf) in which they are prepared. This behaviour for the Me derivative [MeC( -NDipp) -CH -C( -NDipp)Me]ZnMe (46a) is in marked contrast to the closely related Mg complex 2l (mentioned previously) and appears to reflect the shorter metal-N/C bonds in 46a (and consequent greater steric shielding of the Zn centre). A series of alkyl-zinc complexes obtained by metallation of the tris(3,5-dialkyl-2-hydroxypheny1)methane ligands [{ 3,5-R2-2(OH)C6H2}3CH]display interesting structural chemistry, which depends on the organic substituents (R) and the solvent employed in reactions.53The structures of the aesthetically-pleasing S6-symmetric cages [{ 3,5-R2-2(0)C6H~}3CH}z(ZnR’)6] (47) (R = Me, ‘Bu; R‘ = Me, Et) are especially worthy of mention. However, all of these species show poor activity in the copolymerisation of C02 with epoxides, partly as a result of their relatively low solubilities. A novel oxidative-coupling reaction occurs upon thermolysis of the dimeric complex [{ 2-(Me~BuSi)NCHzpy}ZnMelz(48) (py = pyridyl) in the solid state, yielding the C-C coupled product 49 (Figure 4) and Zn metal (among other specie^).^' 49 is obtained more efficiently (67%) by addition of excess ZnMe2to 48 in toluene solution at elevated temperature. This reaction allows direct and efficient access to a [2-pyCH(NSi‘BuMe2)]*-dianion without going through a diazabutadiene [RN = C-C =NR] (50). Of particular note is the fact that unlike previously reported species of this type the formation of the C-C bond in 49 is not reversible, allowing the study of this framework in future without the complication of radical formation.

I

I

Me

Me

49 Figure 4

Heterometallic complexes containing Zn have been obtained using a number of approaches. The sequential reactions of ZnMez with 2-pyridylamines 2-RNpy (R = Ph, 3,5-Me2CsH3,CH2Ph, Me)59or N,N’diphenylbenzamine (= HAm),@ followed by reaction with tBuLi and molecular oxygen, give a range of new Li

Organometallic Chemistry

78

zincate complexes. The structures of these species reveal a variety of 0-capture modes, resulting from 0 insertion into metal-C bonds or from encapsulation of 02-. The competition between the oxophilic nature of Li and Zn is suggested by the formation of M e 0 and ‘BuO ligands in these species. A further influence on the nature of the species obtained is the solvent employed in these reactions. The reaction of HAm with ZnMe2 then ‘BuLi and O2in the presence of thf gives the pseudo-dimer [(Am)2(Li.thf)2Zn(p3-OMe)]2 (51), in which formal insertion into Zn-Me bonds has occurred, whereas the same reaction in apolar solvents pro(52), composed of an octahedral duces the elaborate cage [(Am)6Zn2Li4(p6-O)] arrangement of the metals, and the heterometallic cubane [Li(MeZn)3(p3O ( B U ) ~(53).60 ] ~ Of some relevance to these studies is a recent NMR spectroscopic investigation of the stoichiometry of formation of organo-alkoxy zincates from ZnR2 and ROM (M =alkali The formation of the Cu(II)/Zn adducts [( Me3Si)3CZnC1Cu{ OCH(R)CH2NMe2}2](54) (R = Me, CH2NMe2)from reactions of [Cu{ OCH(R)CH2NMe2}2](55) with [(Me3Si)3CZnClJ(56) is a further way by which mixed-metal complexes with well-defined stoichiometries can be accessed.62These species represent single molecular precursors for the preparation of metal/metal- oxide catalysts, their pyrolysis in dry N2/H2 giving Cu and ZnO and in Ar/02 oxidised Cu species. a-Bonded organometallics of the heavier Group 12 elements have been dominated by those of Hg(II).64-79 The greater Lewis acidity of Hg(I1) than the other metal(I1) ions of Group 12 and its predominantly linear geometry in organometallics of the types [RHgX] and [R2Hg] have lead to a variety of interesting supramolecular architectures in the solid state. The ‘acetonyl’ complex [Hg{CH2C( = O)CH3}] (57),a key ligand-transfer reagent in the synthesis of transition metal derivatives, exhibits an elaborate three-dimensional network The presence of aldehyde (rather than ketone) involving 0 * Hg intera~tions.6~ functionality in [{ 2-(CH = O)C6H4}HgCl] (58) results in a more simple zig-zag chain arrangement in which monomer units are linked via 0 - - - Hg and H - * C1 interaction^.^^ The X-ray structures of the Hg(I1) amino thiolate complexes [{ 3-(NH2)-C6H4S}HgMe] (59) and [{ 3-(NH2)-C6H4S}HgPh](60)make an interesting comparison.66In both complexes linear monomers are associated into dimer pairs by intermolecular S Hg interactions. The lower steric congestion of the Hg(I1) centres in 59 results in additional secondary N - - - Hg interactions, resulting in a sheet structure. However, in 60 dimers associate into an interesting polymeric, ladder arrangement exclusively uia S - Hg bonding. The structure can be seen as arising from the association Hg& broken-cubane units sharing opposite faces. In the carbene complex [{ Me2CNO}2CHg(02CMe)](61),the first characterised C(2)-derived heavy metal nitrosyl nitroxide, separate pseudocubane units (akin to those in 60) result from intermonomer 0 - - - Hg bonding.67 The syntheses and structures of several Hg(I1) complexes containing more conventional carbenes are also worthy of note.68 Studies aimed at utilising the Lewis acid character of Hg(I1) in the coordination of anions or polar molecules are of great current i n t e r e ~ t . Investigation ~~-~~ of the coordination characteristics of the dimercury acceptor [1,2(HgCI)2{CsF,)] (62) represent a simple example of this effect.73Although 62 does *

*

* *

* *

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

79

not form stable complexes with aliphatic aldehydes, the benzaldehyde adduct 62.C6HsCH = 0 has been obtained. The solid-state structures shows that Hg 0 bonding of C6HsCH = 0 occurs with only one of the Hg centres of each molecule of 62. In contrast, actonitrile and propylene oxide form chelate complexes with 62, involving the interaction of the N and 0 centres of these ligands with both Hg centres. The propylene oxide complex 62-O(CH2)CHMerepresents a model intermediate for the observed polymerisation of propylene sulfide using 62. Investigation of coordination and supramolecular chemistry of mercuracarborands has remained an important area of r e s e a r ~ h .Whereas ~ ~ - ~ ~ the trimeric, macrocylic 'anti-crown' [9,12-Me2-C2BlOHsHgl3 (63) forms sandwich adducts with C1-, Br- and I- ions (X-), containing [(63)2-X]- ions in which the X- ions are coordinated by six Hg(I1) centres in an q3-type fashion,74the tetrameric acceptor [9, 12-12-C2BloHsHg14 (64) coordinates two I- ions in the complex [Li2(Me2C= 0)6]2f[64.1z]2-.4H20.75 The novel, microporous lattice structure of the latter results from B-I Li I-B linkages. The structure of the first benzene/H20 complex [(63.H20)2.C6H6](65) is of particular interest, in which a benzene molecule is trapped by two 63+H20units in an H20-C6H6--H20 sandwich arrangement.76 n-Complexes of Group 12 elements remain relatively The first example of a Zn(I1) monomeric species stabilised by q2-bonded alkynes, 66 (Figure 5), contains a ZnBr2monomer which is coordinated in a side-on fashion by a Ti x-tweezer ligand." The unusual decanuclear cluster [Pt4Cd6(C=cPh)&C=CPh)12(p3-OH)4] (67), exhibits related q2-alkynyl bonding to Cd2+ions, the complex being composed of four, square-planar [ P t ( c ~ C P h ) ~ligands l ~ - which support a central [Cd6(p3-OH)4]8+cage cation.82A full account of novel areneHg(I1) complexes stabilised by Group 13 halides has also been reported.83The complexes [Hg(arene)2(MC14)2](arene = C6H&k, M = Al, Ga; arene = C6H5Et, M = Al) (68), obtained from the reactions of M = Al, Ga; arene = 1,2,3-Me3C6H3, MC13 with HgC12 in arene solvents, exist as ion-paired complexes in the solid state, in which the Hg2+cations are x-bonded to two arene ligands and two C1 centres of separate MC4- anions. Surprisingly, a very different ion-separated structure is found in the case of 1,2-Me2C6H4, the complex [Hg( 1,2-Me2CsH4)2(pC1)2AlC12] [A1C14]- (69) being obtained. The applications of Group 12 organometallics in organic synthesis continues to be dominated by those of Zn.50384-93 Organic reactions involving Hg(I1) are more limited and will not be discussed here.69,72,94 As with the literature concerning Grignard reagents, that on Zn reagents is too extensive to be covered in detail in this review. Although much work concerning allenyl- and propargyl-metal compounds has been reported, few studies have previously concerned proparylic carbenoids. Inherent problems involved with Li species of this type are their low thermal stabilty and tendency to self-couple. However, it was found recently that the Zn reagent 70 is far more stable (decomposing above ca. -1OOC) and can be generated readily according to Scheme 5.8470 reacts with aliphatic aldhedydes [RC(H) = 01 to give the chlorohydrins R(H)C(OH)CH(C1)C=CSiMe3(71). The observed preference for the anti-isomers (as shown in Scheme 5) as products can *

+

*

Organometallic Chemistry

80

66

Figure 5

be explained by a chelate-type transition state in which steric replusion between the R group of the aldehyde and C1 of 70 is a decisive factor. The synthetically useful (blue-green) pyridylzinc reagent [2-Br-6-ZnCl-C5H3N](72), prepared by lithiation of 2,6-Br2C5H3Nand addition of ZnBr2, is also worthy of mention in regard to the development of new reagents."

SiMes

=

.# IV

A d S i M e 3

71

Scheme 5

Of interest also in this area is the observation of regioselective allylzincation of the alkenylborate 72 (Scheme 6).86The use of Group 13 elements in the activation of C = C bonds is comparatively rare. DFT calculations suggest that this reaction occurs via the zincio-ene reaction.

72

Scheme 6

Finally, a recent study has investigated the use of ionic liquids as greenThe reactions of a range of solvents for the well known Reformatski aldehydes [RC(H) = 01 with a-bromo-esters BrCX2C(= 0)Et (X = F, H) give the products in 4593% yields, depending on the temperature and R group. For a given reaction, the yields obtained in the ionic liquids employed were similar to

3: Group 2 (Be-Ba) and Group 12 (Zn-Hg)

81

those in thf solvent at the same temperature. In the case of X = F the formation of p-hydroxy-esters means that the ionic solvents can be recycled (with no loss in activity).The use of zinc organometallicsin ionic liquids has a great deal of future potential.

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85. 86. 87. 88. 89. 90. 91. 92. 93. 94.

4

Scandium, Yttrium and the Lanthanides JOHN G. BRENNAN AND ANDREA SELLA

1

Introduction

This review covers all organometallic complexes of Sc, Y and the lanthanides reported in the year 2001 and their reactions.' Endohedral fullerene complexes of the lanthanides have, as usual, been excluded. This year's survey has uncovered fewer examples of blatant fragmentation and/or re-publication of earlier results. Highlights this year include reports of the development of the chemistry of the non-classical ion thulium(II), a thought-provoking study of divalent ytterbium carbonyls and isocyanides which raises the possibility of metal-ligand a-backbonding, the isolation of some unusual hydrolysis products, and the first direct observation of olefin alkyl bond insertion at a do centre. The first annual SellaBren prize for distinguished contribution to organo-rare earth chemistry in the year 2001 is, however, awarded to Dr Zhaomin Hou and his colleagues for their isolation of a soluble Sm6H,cluster, a development certain to spawn a wide variety of exciting new chemistry. 2

Synthetic Studies

2.1 Cyclopentadienyl Ancillaries. - 2.1.1 C p Complexes. The ubiquitous Cp ligand continues to serve as a stabilizing influence in organolanthanide chemistry, with Birmingham and Wilkinson's classic LnCp3system continuing to serve as the starting material of choice for many authors. Treatment of Cp3Ln (Ln = Pr, Nd) with stoicheiometric amounts of the classic Schiff base bis(acet ylacetone)ethylenediamine (H2acacen) hemihydrate (H2L'.0.5H20) or bis(salicy1idene)trimethylenediamine (H2saltn) hemihydrate (H2L2-0.5H20)in THF affords three examples of the currently fashionable hydroxo-bridged tetranuclear organolanthanide clusters [CpLn2L2(p-OH)I2,L = L', n = 4, Ln = Abbreviations: Ln = lanthanide; Cp = q-C5H5;MeCp = q-C,H,Me,Cp* = -q-C,Me,; Cp' = q-C,H,(SiMe,); Cp" = q-C5H3(SiMeJ2;Ind = q-C9H7;Flu = Y - C , ~ Hfluorenyl; ~, COT = C,H,; COT' = 1,4-CSH,(SiMe,),; TMEDA = tetramethylethylenediamine (1,2bis(dimethy1amino)ethane); HMPA = hexamethylphosphorustriamide; DME = 1,2(dimethoxykthane; MMA = methylmethacrylate; M A 0 = methylaluminoxane; DFT = density functional theory.

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Pr, Nd; L = L2,n = 2, Ln = Nd).’ The benzotriazolyl complexes Cp2Ln(Btz)(Ln = Yb, Gd, Tm) have been obtained from the reaction of LnCp3with benzotriazole.*. A similar protonolysis reaction with 2-mercaptobenzothiazole gave dimeric complexe~.~In another example, the crystal structure of [Cp2Tm(SBT)(THF)] was reported separately.’” An analogous synthesis using pPzMe2)] 3,5-dimethylpyrazole gave the binuclear complexes [Y Cp(K2-PzMe2)( and [ Y ( K ’ - P Z M ~ ~ ) ~ ( ~ P Z M ~ ~ ) Insertion ( ~ . - T H Fof) ]dimethylsiloxane ~. was also reported: The structures of [TmCp3L2](L = CH3CN)5and [Cp2YbX(THF)] (X = C1, Br) have been determined, the latter study at last completing the series of halides6 Finally, potentially useful new starting materials for anhydrous lanthanide chemistry have been reported. Reaction of hydrated lanthanide bromides with pyrazinamide (P)in THF gave good yields of LnBr3P3which may be converted to the half-sandwich complexes, [LnCpBr2P], by reaction with three equivalents of NaCp. The rather unconventional stoicheiometry was not explained by the authors but may be related to the insolubility of these complexes in all common solvents. Some polymerization activity for ethylene was observed in the presence of M A 0 . 7

2.1.2 Cp* Chemistry. The permethylated cyclopentadienyl ligand remains a key ancillary in lanthanide chemistry and a number of impressive developments have occurred within the year. At last, divalent thulocene chemistry has been investigated. Tm12, prepared directly from Tm and I*, reacted with KCp* in Et*O under nitrogen forming a white precipitate and reddish orange solution from which [ ( C ~ * ~ T r n ) ~ ( p -(1) N~)l was obtained in 55% yield. Analogous complexes were prepared using Cp’ and Cp”. Under argon the reaction proceeded differently, forming a purple solution that changed to orange when nitrogen was added. In the absence of nitrogen, the purple solution slowly changed to yellow-orange giving yellow crystals of a complex which indicated attack of Tm(I1) on the solvent, [(Cp*2Tm)2(pOEt)2(Cp*Tm)(p-O)(TmCp*2)].* By contrast, Bochkarev found that reaction of Tm12(THF)* and KCp* react in THF to yield the Tm(II1) complex [Cp*2TmI(THF)]. Attempts to isolate a Tm(I1) derivative by reduction of the trivalent complex in DME resulted in solvent degradation and the isolation of [(DME)*Na(p-q5:q5-Cp*)Tm(q’-Cp*)(p2-OMe)2(Na-p-q5:q 5-Cp*)Na(pOMe)2Tm(q5-Cp*)2].Treatment of TmI2(THF)2 with Cp”MgC1 in THF yields the dimeric Tm(II1) complex [Cprr2TmC1]2,while Tm13(THF)3reacts with the pendant Cp derivative [C5H4CH2CH2NMe2]K(THF)to yield solvent-free [(C5H4CH2CH2NMe2)2TmI]. Most of the new Tm(II1) complexes were characterized by X-ray diffraction.’ In a surprising study of divalent ytterbium, the IR spectra of methylcyclohexane solutions of the base-free ytterbocenes [YbCp”,], [Yb( ~ , ~ - B U ‘ ~ C ~ H ~ ) ~ ] , and YbCp”2 in the presence of CO show that carbonyl adducts are formed reversibly at room temperature (2). The YCO values for the first two complexes are lower than that of free CO indicating that CO is acting as a net a-acceptor ligand in those molecules, much as was noted in U(II1) chemistry.”>l 1 By contrast the YCO value for the adduct of YbCp”, is slightly higher than that of free CO, which

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demonstrates that changing the substituents on the cyclopentadienyl rings can alter the electron richness of the ytterbocene. The isocyanide complexes of these ytterbocenes and of [Yb(C5Me4H)2]were isolated. The X-ray structures of [C~*~Yb(CN-2,6-xyl)~] (3),[Cp”2Yb(CN-2,6-xyl)2],and [Yb( 1,3-But2C5H3)2(CN2,6-x~1)~] were determined. The solution electronic spectra (350-1000 nm) for both the base-free ytterbocenes and their CNR/CO adducts were measured and interpreted according to a molecular orbital model. All of the data are consistent with the rather surprising notion that Yb(I1) metallocenes can act as net rc-donor fragments.12

Evans and others have continued to develop the chemistry of decamethylsamarocene. The interaction of the substituted diene monomers isoprene, C5H8, and myrcene, C10H16, with [SmCp*2] forms the bimetallic complexes [(Cp*2Sm)2(p-q2:q4-CH2CHC(Me)CH2)] and { CCp*2Sml2[p-q2:q4CH2CHC(CH2)CH2CH2CHCMe2]} which have closely related stru~tures.’~ In an unusual electron transfer approach to the preparation of nitroxide complexes, the free radical 2,2,6,6-tetramethylpiperidinyl-l-oxy (TMPO) reacts with [SmCp*3] to form (C5Me5)2 and the per-nitroxide complex [(qlONCsHhMe4)2Sm(~-q1:q2-ONCsH6Me4)]2.’4 Electron transfer has also been used to prepare phospholyl and arsolyl complexes of Sm of the type [SmCp*2(E’)]by cleavage of the P-P or As-As bonds of 1,l’-biphosph/arsolyls (E’-E’) with [SmCp*2] or [SmCp*2(Et20)] in toluene. The solid state structures of the complexes depend on the substitution pattern on the phospholyl ligand. They can be ql- or q5-coordinated monomers, a p:q1,q5-symmetrical dimer, or even unsymmetrical dimers where both the q’- and the p:q1,q5-coordinationmodes are found (4)(5).15Controlled hydrolysis of the divalent organosamarium complex [Cp*2Sm(THF)2]in THF forms [ ( C P * ~ S ~ ) ~ Owhich ~ H ~has ] a solid-state structure consisting of a distorted octahedral array of six decamethylsamarocene units connected by eight triply bridging and one central oxygen (6).16 A mixed ligand Cp* samarium (11) alkyl complex has been prepared by reaction of KCH(SiMe3)* with [Cp*2Sm(THF)2], yielding [ C P * S ~ ~ ( C H ( S ~ M ~ ~ ) ~ ) ( C ~ M in ~ ~9)1K OO / (yield. T H FWhen ) ~ ] stirred with H3SiPh in THF, a hexasamarium cluster complex [cp*sm(p-H)2]6[(p-H)K(THF)2]3. (7) Both structures were determined by X-ray diffraction. The alkyl was shown to be active for hydrosilylation of several types of 0lefin.l’ Similarly, the half sandwich { c~*yb[N(siMe~)~](THF)~} is generated from K[N(SiMe3)2] and [Cp*Yb(THF)2(p-I)I2in T H F and crystallizes from toluene with a distorted piano stool geometry.” In an elegant investigation of a-bond activation, Tilley examined the stability

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THF‘ THF

of [ c ~ * ~ S m P hThermolysis ]~. resulted in the formation of the phenylene-bridged species [Cp*2Sm(p-1,4-C6H4)SmCp*2] (8) which shows significant agostic interactions with the hydrogens atoms of the bridging group. The observed rate law indicated two parallel pathways, uni- and bimolecular, respectively. In the presence of silane substrates both Si-C and Si-H activation occurs. Thus [Cp*,SmPh12 reacts with PhSiH3to give PhzSiH2and [C~*~sm(p-H)]~. With the perfluorinated analogue C6F5SiH3, silane formation is observed and [Cp*2Sm(C6F5)]2is isolated. Finally, Si-H activation could be observed exclusively in the reaction of o-anisoylsilane with [C~*~Srn(p-H)l~ which generates [Cp*,SmSiH2(o-MeOC6H4)]as an unstable intermediate which decomposes to [Cp*2Sm(o-MeOC6H4)]and other products. The outcomes of these reactions may all be rationalized in terms of the relative bond strengths.” In closely related work using the smaller ion lutetium, very similar reactivity was observed. In this case, however, the neutral chelated lutetium silyl complex [cyclic]Cp*2LuSiH2(o-MeOC6H4) could be structurally characterized?’ In efforts to make sterically crowded t ris(pera1kylcyclopentadienyl) complexes of lanthanum for the exploration of sterically-induced reduction chemistry with a diamagnetic system, the synthesis of [La(C5Me4R)3]complexes has been pursued with R = Me, Et, iPr, and SiMe3.The complexes were synthesized in four steps: reaction of Lac13 with KC5Me4R to form [(C5Me4R)2LaC12K(THF)z], addition of allylmagnesiumchloride to make [(C5Me4R)2La(C3H5)], protonolysis with Et3NHBPh4to make [(C5Me4R)2La][BPh4],and finally the replacement of BPh- with C5Me4R-using KC5Me4Rto make [La(C5Me4R)3].The X-ray crystal

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structures for all four complexes were obtained and the M-C distances were shown to depend on the bulk of the substituents. Interestingly, despite the steric crowding, ligand exchange could be observed by NMR spectroscopy.21 After several years of effort, Casey et al., have reported the first non-chelated do metal-alkylpropene complex. The reaction of [CP*~YH]~ with 3-methyl-l-b~tene in a 1:l mixture of methylcyclohexane-d14and pentane-d12produced the expected insertion product, C P * ~ Y C H ~ C H ~ C H ( C inHquantitative ~)~ yield within minutes at -78". Because the complex does not insert a second equivalent of 3-methyl-1-butene up to -30" (at which point decomposition begins) it was possible to add propene to generate the alkyl-olefin complex (9). The kinetics of propene insertion into the Y-C bond to give [Cp*2YCH2CHMeCH2CH2CHMez] could be measured directly.22 Marks et al. have continued comprehenive studies of intramolecular cyclization reactions, focusing on the use of phosphino-alkenes and -alkynes as substrates to give the corresponding heterocycles. Detailed mechanistic studies show significant parallels between the behaviour of [ c ~ * ~ L n E ( S i M e(Ln ~ ) ~= l La, Sm, Y, Lu; E = CH, N) for these processes and the corresponding hydroamination/cyclizations. The rate/turnover limiting step was proposed to be the insertion of the carbon-carbon multiple bond into the Ln-P link. The catalyst resting state is likely a lanthanocene phosphine-phosphido complex, and the dimeric complex [Cp*2YP(H)Ph]2 was isolated and crystallographically characteri~ed.~~ A study by Barbier-Baudry and coworkers showed that samarium(II1)monoalkoxides bearing two, one, or no cyclopentadienyl-type spectator ligands are easily synthesized by one-electron oxidation of the samarium(I1)precursors with t-butylperoxide. The precursors included the metallocenes [Sm(C5Pr'4H)2]and [Sm(C4Me4P)J which gave products of the type [SmCp2(0But)(THF)]. [Sm(C5Pr'4H)I(THF)2]2was generated by the comproportionation of the samarocene with Sm12 and structurally characterized. These monoalkoxides are single-site initiators for ring-opening polymerization of lac tone^.^^ YR3(THF)3(R= CH2SiMe3)has been used to generate half sandwich complexes of the pendant alkene-substituted cyclopentadiene = CH2), LH. This protonolysis reaction formed the (C5Me4H)SiMe2(CH2CH bright yellow complex [LYR2(THF)2] which was found to be rather unstable, with further metallation occurring to give an unprecedented trianionic cyclopentadienyl ally1 ligand, (C5Me4SiMe2C3H3)3- in the complexes [(C5Me4SiMe2C3H3)YL']2(L' = THF, DME) (10). Addition of C02 to LYR2(THF)2resulted in insertion to give [LY(02CR)2]2,a lantern-like complex with four bridging carb~xylates.~~ The -ate complexes [{ (C5HR4)2Ln(p-C1)(p3-C1)Na(OEt2)}2] (R = CHMe2)(Ln = La, Nd) can be isolated in high yield by reaction of LnC13 with Na(C5HR4). With the smaller ion Yb, the half-sandwich complex [(C5HR4)YbC12], was obtained. It was also prepared by oxidation of octaisopropylytterbocene with hexachloroethane.26Tin reagents have been used for the preparation of ytterbocene derivative^.^'

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(9)

(10)

2.1.3 R-Substituted C p Complexes. Reactions of organolanthanides with the unsaturated bonds in PhNCO were numerous, and product diversity is clearly a reflection of the different ancillary ligands present. In a reactivity study involving Ln-S bonds, nucleophilic attack of the thiolate in [ ( M ~ C P ) ~ N ~ ( ~ - S P ~ ) ( T H F ) ] ~ with PhNCO was found to give dimeric [(MeCp)2Nd]2[p-q2-OC(SPh)NPh]2, which has an interlinked tricyclic structure formed via two bridged N d - 0 bonds. Similarly, the thiolate reacts with PhNCS to give (MeCp)2Nd[q2-SC(SPh)NPh] (THF), forming an q2-coordinated ligand of SC(SPh)NPh?' Divalent [ ( M ~ C P ) ~ S ~ ( T H Fand ) ] Sm12(THF), reduce Ph isocyanate to give [( MeCp)2(THF)Sm]2[p-q4-(PhN)OCCO(NPh)] and [SmI2(THF)5] [Sm14(THF)2]respectively. Both Cp complexes polymerize phenylisocyanate. The Cp dimer can be considered to be the real active species for the PhNCOolig~merization.~~ Finally, [(MeCp);?LnR(THF)]reacted with PhNCO to form the insertion products [(MeCp),Ln(0C(R>NPh)l2[R = Bu, Ln = Sm, Dy, Er ;R = a-naphthyl, Ln = Dy ] (11). An excess of PhNCO did not affect the nature of the final complexes, a single insertion only being observed, and excess PhNCO forming a cyclotrimer. The reaction of [ ( M ~ C ~ ) H O C ~ ~ ( Twith H F )BuLi ~ ] and subsequently with 2 PhNCO in T H F gave Ho[OC(Bu)NPh13 and [(MeCp)Ho(0C(Bu)NPh)l2,which can be rationalized by the rearrangement reaction of the di-insertion product (MeCp)Ho[0C(Bu)NPhl2(THF),. In the structures of these reaction products there is an unusual bonding mode of the amido groups arising from the insertion of PhNCO into the Ln-C o-bonds, and the 0-C-N fragment of the OC(R)NPhligand acts as both a bridging and side-on chelating Caprolactam (HN(CH2)5C0)polymerizations continue to capture attention. Methods to incorporate the strong ligation capacity of E-caprolactam into monoanionic ligands were outlined. c-Caprolactam was deprotonated by (CSH4Me)2Y[N(SiMe3)2]to form the dimeric complex [(C5H4Me)2Y(pNC6H100)I2(12). Each caprolactamate anion, forms one Y-N bond and coordinates to both Y centers via a bridging 0 such that Y is formally 9-coordinate. c-Caprolactam was also deprotonated by (C5Me5)2Y(C3H5)(THF), to form the monomeric, pentamethylcyclopentadienyl analogue [(C5Me5)2Y(NC6H100)] (13), in which the caprolactamate anion chelates the formally eight-coordinate yttrium; C 0 2inserts into this Ln-N bond to form [(C5Me5)2Y(p-02CNC6H100)12. The tridentate monoanionic (02CNC6H100)-ligand forms an eight-membered YOCOYOCO ring via carboxylate oxygen atoms and also coordinates to each Y center via the oxygen originating from c-caprolactam. By contrast to the 0 = C = 0 reaction, PhN = C = 0 inserts into the Y-N bond to form Cp*,Y[(p2-

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OC(NPh)NC6Hlo0)],which contains a planar YOCNCO ring and eight-coordinate yttrium. Tert-Bu isocyanide does not undergo similar insertion chemistry, In other work, the but instead forms the adduct [Cp*2Y(NC6HIoO)(CNCMe3)].31 reaction of (MeC5H4)2LnCl(Ln = Yb, Y, Er) with lithiated E-caprolactam [LiNCO(CH2)4CH2]yielded {(MeC5H4)2Ln[OCN(CH2)4CH21>Z[Ln = Yb, Y, Er]. In the Yb compound, the two trivalent (MeCsH4)2Ybunits are bridged by two deprotonated E-caprolactam ligands via Yb-N (2.374(4) A) and Yb-O* (2.277(3) A) bonds. All the complexes exhibited catalytic activity for the ringopening polymerization of ~-caprolactone.~~ Protonolysis of the new linked amino-cyclopentadiene (C5Me4H)CH&Me2NHCMe3 with [Y(CH2SiMe3)3(THF)2] gave the alkylyttrium complex [Y(q5:q1-C5Me4CH2SiMe2NHCMe3)(CH2SiMe3)(THF)]. Hydrogenolysis of this product gave the dimeric hydride [Y(qs:qlC5Me4CH2SiMe2NCMe3)(THF)(p-H)I2, which contains a Y2H2 core connected to two [Y(q5:q1-C5Me4CH2SiMe2NCMe3)(THF)] fragments in a skewed manner. Two diastereomers were noted in low temperature NMR experiments. This dimer will efficiently catalyze the hydrosilylation of 1-decene with PhSiH3 to give the terminal silane n-CloHzlSiH2Phexclusively. The yttrium-hydride [Y (q5:q 'complex reacts with styrene to give C5Me4CH2SiMe2NCMe3){ CH(CH3)Ph}(THF).33

In a cluster experiment reminiscent of many in the recent past, the halfsandwich complexes { [(q5-tBuC5H4)Ln(THF)]2(p2-C1)2(p3-C1)3Na(THF)}, [Ln = Nd, Sm, Gd, Yb] were prepared from reactions of LnC13with N a ( ~ l ' - ~ B u c ~inH ~ ) THF, and the Nd compound was found to react with M2Se5to give [Na(THF)6]2 [(qs-tBUC~H4)6Sm6( P6-Se)(P-Se2)6], Or [Li(THF)4]2[ Cp6Nd6(p&+)( P - S ~ ~ )Structural ~]. characterization of both clusters revealed an octahedrally coordinated Se atom at the center of the compound.34 A number of structurally characterized Cp/halide compounds were noted. The syntheses and characterization of [C5H4C(C2H5)2CH2CH= CH21 LnC12-MgC12-THF (Ln = La, Nd, Sm, Gd) was described.35The complex (CH3C5H4)2SmClhas been synthesized by the reaction of SmC13 with CH3C5H4Nain THF.36Anhydrous SmC13reacts with two equivalents of Li(1,3'Bu2C5H3) to give the heterometallic product [( 1,3-'B~~C~H~)~Srn(pC1)2Li(THF)J. Two 1,3-'Bu2C5H3-ringcentroids and two bridging chloride atoms around Sm atom form a distorted t e t r a h e d r ~ n . ~ ~ Mixed ligand complexes with oxygen centered anions were also described. Orange-red crystals of [Yb(MeCp)2(02CPh)]2were obtained from the redox

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reaction of Yb(MeCph with T1(02CPh).The dimeric structure is held together with bridging bidentate (0,O’)-benzoate groups connecting the two 8-coordinate Yb(II1)ions.38.Weirdly the authors make no comment of why this Yb(II1)species with no exceptional chromophore is not colourless. A related thiolate dimer [(MeCp),La(SPh)(THF)J was also described. The La& core is planar, and the La(II1) ions are nine-~oordinate.~~ Four bisaryloxo lanthanide chlorides [Ln(Ar0)2Cl(THF)2](Ln = Sm, Er, Yb, Y) (ArO = 2,6-di-t-butyl-4-methylphenoxo) were synthesized by the reaction of LnC13 with NaOAr. These chlorides can be used as starting materials to synthesize heteroleptic lanthanide aryloxides. The Yb compound reacts with MeCpNa in T H F to form [(MeCp)Yb(ArO),(THF)]. The divalent aryloxo lanthanide complexes [Ln(OAr)2(THF)3](Ln = Sm, Yb) were obtained in good yield by reducing the Sm and Yb compounds with Na metal in THF.40 2.1.4 Functionalized Cp Complexes. The functionalized Cp ligands represent a continuously expanding array of ligands with tailored steric/electronic properties that will, eventually, facilitate the preparation of Ln compounds with preordained chemical or physical properties. The fluorinated alcohol-substituted ligand CjHj(CH2CArF20H) (ArF = 3,5-C6H3(CF3),), LH2 was prepared by ring opening of the epoxide ArF2C(0)CH2with NaCp. Reaction of Li2L with YC13 afforded the -ate complex LYLi(THF)2which partially desolvated under vacuum. It was only possible to prepare the lithium-free chloride, [YLCl(THF)2] by protonolysis of { Y [N(SiMe3)2]2(THF)(p-Cl)}2 with LH2. Metathesis reactions generated [YLX(THF),] (X = N(SiMe3)2, Cp, and CH(SiMe&; n = 1, 2), and -ate complexes could also be prepared!’ In an unusual synthetic approach to the preparation of heterobimetallics, denoted ‘yttrate metathesis’ by the authors, an -ate complex based on the Bercaw constrained geometry Cp/amide system was used as a starting material. Thus reaction of [Y(C5Me4Si(Me2)N(2-pyridyl))2Li]with CuCl resulted in the replacement of the lithium by copper giving [Y(C5Me4Si(Me2)N(2-pyridyl))2Cu] (14).42 Linked Cp-carborane ligands continue to attract attention, although the carborane group does not always bind to the metal. The reaction of closo-exoLi(TMEDA)-1-Li(TMEDA)-2,3-(SiMe3)2-2,3-C2B4H4 and CeC13, in a molar ratio of 2:1, gave a high yield of the novel orange Ce(II1) bent-sandwich complex, [Li(TMEDA),] [1-Cl- 1-(p-C1)-2,2’,3,3’-(SiMe3)4-5,6[( v - H ) ~ L ~ ( T M E D A ) ] - ~ , ~ ’ , ~ ’ [(p-H)3Li(TMEDA)]-l,1’-commo-Ce(2,3-C2B4H4)2].43 The reaction of LnC13 with 2,4-dimethyl-pentadienyl K affords (2,4-C7H1&Ln(Ln = Dy, Er) which are structurally similar to Nd and Gd [(2,4-C7HI1)2Yb.DME]was synthesized by the analogous reaction of YbC13. Single crystal X-ray diffraction showed that the complex exists in a cis-staggered conformation. The complex can be used as a catalyst for the polymerization of MMA.44 A linked tetramethylcyclopentadienyl/carboraneligand was prepared by reaction of Me2Si(C5Me4H)Clwith Li2C2B10H the monoanionic salt ‘LiHL1’which could be conveniently converted into the dianionic salt [{ [(p-q5):0Me2%(C Me4)(C2BloH o)] Li(TH F)} 2Li][Li(TH F)4], ‘Li2L’ by treat men t with n-

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BuLi. LnC13 reacted stepwise with ‘LiHL1’in THF to give the -ate complex L1HLnC13Li(THF)3 (Ln = Nd, Sm) followed by [(L1H)2Nd(yC1hLi(OEt2)(THF).Partial hydrolysis resulted in the isolation of a remarkable cluster [{(L1)Sm}2(yZ-C1)3(y3-C1)4Li(OEt2)2Li(THF)2Sm(y4-0)]2 containing an 02Sm6core (15) supported by bridging chlorides. Treatment of ‘LiHLl’ with 4 equivalents of NaNH2 in THF, followed by reaction with YbC13, resulted from Reaction the cleavage of the Si-C bond giving (q5-C5Me4H)2Yb(y-C1)2Li(THF)2. between ‘Li2L1’and a single equivalent of NdC13 yielded the anionic compound [L2Nd][Li(DME)3], one of the few complexes in which the carborane is more than just a sterically demanding appendage to the Cp ligand. When two equivalents of lanthanide were used anionic chloro-bridged dimers [Li(S),12[(L1)2LnCl(y-C1)]2(Ln = Y, Sm, Yb; S = THF, DME) were obtained. Metathesis with NaN(SiMe3)2 yielded the unsolvated compound [LYN(SiMe3)2]. Ionic organolanthanide (11)compounds were prepared from Ln12with 0.5 equivalents of ‘Li2L1’to give the ionic organolanthanide(I1) compounds [Li(DME)3]2[(L1)Ln12](Ln = Sm, Yb).45

(14)

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2.2 Indenyl, Naphthalene, and Other Aromatic n-Complexes. - The facility with which napthanide anions can be displaced encourage the use of naphthalene complexes as starting materials for organometallic chemistry. The reaction of the naphthalene ytterbium(I1)complex {(p-CloH8)[YbI(DME)2]2} with indene proved to be a convenient synthetic route to the novel indenyl half-sandwich ytterbium(I1) complex [(q5-C9H7)YbI(DME)2], which is a monomer in the solid state and stable in DME solution, although it disproportionates in THF to give [(q5-C9H7)2Yb(THF)2] and [Yb12(THF)2].The reaction of this half sandwich with NaCp in DME also results in disproportionation, giving [YbCp*(DME)] and [(q5-C9H7)2Yb(DME)] >6 Parallel work on fluorene proceeded somewhat differently. Novel fluorenyl complexes (q5-C13H&Yb(THF)2 and q5CI3H9)*Yb(DME) were isolated from metathetical reactions of YbIZ(THF)2 with C13H9K.The compound can also be prepared by reaction of [(CIoH8)Yb(THF)2] with fluorene in THF. The THF complex reduces tBuN =CHCH = NBu, giving (C13H9)2YbfBuN= CHCH =NBu). By contrast, the reaction of [YbI(DME)&(y-CloHg) with fluorene gave disproportionation products. The first mixed-ligand sandwich complex of a divalent lanthanide metal [(Flu)Cp*Yb(DME)] was isolated from the reaction of [ Y ~ I ( D M E ) ~ ] ~ ( ~ - C I O H ~ ) with C5MesH and KFlu in DME. ansa-Me2Si(C13H&Yb(THF) and ansaMe2Si(C13H8)2Sm(THF)4 formed in reactions of Ln(I1) iodides with ansaMe2Si(C13H9)2K2 in THF>6

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The use of naphthalene as a co-reductant has resulted in the preparation of an unexpected bridging napthalenide complex. The reaction of the diamidophoswith LnC13(THF)3(Ln = Y, Sm, Ho, Yb, phine PhP(CH2SiMe2NPh)2Li2(THF)2 Lu) in refluxing toluene generates mononuclear five coordinate [PhP(CH2SiMe2NPh)2] LnCl(THF). At tempts to prepare a1k yl derivatives were partially successful. MeMgCl reacts with [PhP(CH2SiMe2NPh)J YCl(THF) to give [PhP(CH2SiMe2NPh)2]YMe2MgC1, while the reaction with LiAlH4results in complete ligand exchange and the formation of the tetranuclear { [PhP(CH2SiMe2NPh)2]AlH2Li(THF)}2. Reduction of the Lu chloride complex with potassium graphite and naphthalene generated the remarkable bimetallic naphthalene-bridged compound { [PhP(CH2SiMe2NPh)2]Lu}2(pq4:q4-C~oH8) (16)in which each Lu has q4-coordination to opposite sides of the arene moiety.47 In related work, attempts to prepare hydrocarbyl complexes supported by the macrocyclic bis(amidophosphine) ligand PhP(CH2SiMe2NSiMe2CH2)2PPh, [P2N2] have resulted in some unusual coupling reactions which depend on the size of metal centre. With small ions such as Yb and Lu, reaction of {[P2N2] M } ~ ( V - Cwith ~ ) ~ ,PhLi results in the formation of the corresponding monophenyl derivative, isolated as [P2N2]LnPh (Ln = Yb) or mixed with the -ate complex CP2N21LnPh-LiCl(Ln = Lu). By contrast, with Ln = Y, Ho, the unexpected biphenylide-bridged dimer is isolated (17). Interestingly, the coupling reaction is entirely suppressed in THF, and using tolyl lithium, the THF adduct [P2N2] Y(C6H4-p-Me)(THF)is isolated. The analogous biphenylide-bridged Lu complex could also be prepared but only by a reductive Nicotine adducts of tris-indenyl complexes, [Ln(Ind)3.Nic] [Nic = (S)-(-)nicotine] (Ln = La, Pr, Nd) were synthesized and characterized by spectroscopic and X-ray techniques. Analogous Pr complexes with pyridine and P-picoline donors were also isolated, but compounds with larger pyridine donors could not be prepared. The chiral LII(C~H~)~=N~C adducts are virtually isostructural, although subtle but systematic conformational variations emerge from the crystal structure analyses. While the unit cells of the nicotine compounds contain two epimeric diastereomers, pyridine and picoline compounds contain genuine optical antip0des.4~ In redox chemistry, Tm diiodide reduces cyclic aromatic hydrocarbons that have reduction potentials more positive than -2.0 V vs. SCE. For example, cyclooctatetraene, acenaphthylene, and Li anthracenide are reduced to give Tm13 and [(q8-C8H8)TmI(THF)2],rac-ansa-[(q5-C12H&TmI(THF)], or [(q2C14Hlo)TmI(DME)2], respectively.” The reductive coupling of acenaphthylene (C12Hg) by iodine-activated metallic ytterbium and samarium, yields the C2symmetric trans-rac-ansa-‘lanthanoacenes’ [(q5-C12H8)2Ln(THF)2] as dark red (Ln = Yb) (18) or dark brown crystals (Ln = Sm) in yields of about 90%.’l Calcium behaved similarly. The ansa-biacenyl ligand was found to be easily cleaved. Reaction of [(q’-C,2H8)2Yb(THF)2]with ZrCL in T H F gave 86% [(a:q2C12H8)ZrC12(THF)3]. Decoupling of the ligand also takes place when the ansaacenes are reacted with Me3SiC1 in T H F to give 1,2-bis(trimethylsilyl)-substituted acenaphthene, (Me3Si)2C12H8.52 Tm diiodide also couples acenaphthylene, to give the analogous rac-an~a-[(q’-C~~H~)~TmI(THF)].~~

4: Scandium, Yttrium and the Lanthanides

95

Chiral 1,1'-(3-0xapentamethylene)-bridged bis(indeny1) ansa-lanthanidocenes were synthesized stereoselectively. The reaction of YbC13 with O(CH2CH2C9H6K)2 in THF provided a rac/meso mixture of [O(CH2CH2C9H6)2] YbCl(THF); the major diastereomer formed is racemic. The rac[O(CH2CH2C9H&]YbCl(THF) was isolated after recrysallization of a rac/meso mixture of [O(CH2CH2C9H6)2]YbCl(THF) in THF. Direct amidation (or alkylation) of the rac/meso lanthanidocene chloride mixtures in toluene provided pure racemic ansa-lanthanidocene amides [O(CH2CH2C9H6)2]LnN(SiMe3) 2 (Ln = Y, Pr, Nd, Y b, Lu) and pure racemic ansa-lanthanidocene hydrocarbyls [O(CH2CH2C9H6)2]LnCH2SiMe3 (Ln = Dy, Yb). The amide and hydrocarbyl complexes are efficient single-component catalysts for MMA polymerization. The effects of solvent and reaction temperature on the polymerization were studied. Very high molecular weight (Mn > 106) iso-rich poly(MMA) was obtained at lower temperature, while low molecular weight moderately syndiotactic poly(MMA) was obtained at higher temperature The polymerization behavior probably is associated with rac/meso interconversion of the active center.53

2.3 CyclooctatetraenylChemistry.- Some work on chemistry of non-classical oxidation states has been described in section 2.1.2 above. In addition, with a view to establishing the 'true' reducing power of Tm12,its reactions with a range of aromatic hydrocarbons have been investigated. Thus reaction of COT or acenaphthylene in THF, or with Li anthracenide in 1,2-dimethoxyethane(DME) were found to give Tm13 and the Tm(II1) complexes [ ( T ~ " - C ~ H ~ ) T ~ I ( T H F ) ~ ] , ra~-ansa-[(q~-C~&)~TrnI(THF)], or [ ( T ~ ~ - C ~ ~ H ~ ~ ) T ~ respectively. I ( D M E ) ~ The ], molecular structures of these complexes were determined by X-ray diffra~tion.~' The half-sandwich complex [Nd(COT)(HMPA)3][BPh4] was prepared by substitution of the corresponding half sandwich THF complex. The crystal structure allowed comparison with the corresponding U and Sm complexes. The variations in the M-ligand bond lengths were found to simply reflect the differences in the ionic radii of the Mixed troponiminate COT complexes have been prepared. Reaction of Li2[COT"], with anhydrous Y or Lu trichloride and K[(iPr),ATI}], leads to the K N-isopropyl-2-(isopropylamino)troponiminate, complexes [(q8-COT")Ln{(i-Pr)2ATI}(THF)] (19). The solid-state structure of the Y complex was established by X-ray diffraction. The analogous Sm complex

96

Organometallic Chemistry

was prepared by reaction of [(q*-COT”)SmI(THF)] and K{ (iPr)2ATI}. The chiral complex [(qs-COT”)Ln{(S-PhCHCH3)2ATI}(THF)] can be prepared analogously?’ Two sets of closely related sandwich complexes, ($MeOCH2CH2C9H6)Ln(qs-COT)(THF), [Ln = La, Nd, n = 0; Sm, Dy, Er, n = 11 and [(q’-C4H70CH2C9H6)Ln(q8-COT)(THF)] [Ln = La, Nd, Sm, Dy, Er], were synthesized by the reactions of LnC13with 1equivalent of K2CsHs,followed by treatment with the potassium salt of ether-substituted indenide. The molecular structures of [(q’-MeOCH2CH2C9H6)Sm(qs-COT)(THF)] and [($C4H70CH2C9H6)Ln(q8-COT)(THF)] were determined by X-ray diffractions6 Finally, the unusual inverse sandwich complexes [{(Me3Si)2N}Sm(THF)2}2(pCOT)], in which a planar COT dianion is sandwiched between two samarium amide units, has been prepared by reaction of [(Me3Si)2N]2Sm(THF)2, Sm12(THF)2,and K2COT in THF. The Yb complex has also been prepared?

2.4 Miscellaneous Ancillaries and Hydrocarbyls. - Evans et al. have at last crystallized YR3(THF)3(R = CH2SiMe3)which exists as the facial isomer . The compound desolvates readily and is more conveniently available as the bis-THF adduct, YR3(THF)z.2S A related bis-THF complex was synthesized independently by Niemeyer. Reaction of ICH2SiMe3with Yb chips gave (THF)2Yb(CH2SiMe3)3, a slightly distorted trigonal-bipyramidal compound with two axial THF and the three alkyl ligands in equatorial positions (av. Yb-C 2.374 A).57 Pyrazole-based ligands continue to provide a stabilizing influence in lanthanide chemistry. The homoleptic pyrazolate complexes [Sc(tBu2pz)3], [ L ~ ~ ( % u ~ P(Ln z ) ~ ]= La, Nd, Sm, Lu), [Eu4(%u2pz)8] and the heterovalent [Yb2(fBu2pz)S](‘Bu2pz = 3,5-di-tert-butylpyrazolate) were prepared by reactions of Ln with 3,5-di-tert-butylpyrazole, in the presence of mercury. In addition [Yb2(‘B~~pz)~] was prepared by redox transmetalation/ligand exchange between Yb, HgPh2 and tBu2pzHin toluene, while the same reactants in Et20 gave the heterovalent compound. The trivalent Ln compounds are dimeric, with chelating q2-terminaland q2:q2-bridgingpyrazolate coordination, whereas the smaller Sc forms monomeric [S~(‘Bu~pz)~] q2-bonded pyrazolates. The divalent tetrametallic cluster [ E u ~ ( ~ B u ~has ~ z )two ~ ] types of europium centers in a linear array. The outer two are bonded to one terminal and two bridging pyrazolates, and the inner two are coordinated by four bridging ligands. Unprecedented p-q5:q2pyrazolate ligation was noted, with each outer Eu2+sandwiched between two q5-bondedpyrazolate groups, which are also q2-linkedto an inner Eu2+.The two inner Eu2+ ions are linked together by two half occupancy, symmetry related, disordered pyrazolate groups with one component q4:q2bridging and one q3:q2bridging. [La2(tB~2p~)6] also is a Tishchenko reaction catalyst.’8 Addition of stoicheiometric KR (R = CH2C6H4-o-NMe2,C6H4-o-CH2NMe2, CH2CsH5) to solutions of Sm(TpMe’)2Cl (TpMe2= hydrotris(3,5-dimethyl-pyrazoly1)borate)in toluene or THF led to reduction and the precipitation of purple s r n ( T ~ M e ~In) ~the . presence of more acidic substrates (HZ; HZ = HOPh, HCp, HCCPh, HOC6H2-2,4,6-tBu3,HNPh2 and 3,5-Me2pzH) the corresponding Sm(TpMe2)2Z compounds (20) form in good yield^.'^

4: Scandium, Yttrium and the Lanthanides

97

In a continuation of last year's dinitrogen reduction work, the first cyclic lanthanide clusters containing only highly reactive Sm(I1) centers were described. The silylamide [Sm{ N(SiMe3)2}2(THF)2]reacts with 1,l-di(a-pyrroly1)cyclohexane in THF/Ar to give dark red, octameric [{ [lJ-(aC4H3N)2C6Hlo]Sm}8(THF)4]. The crystal structure contains eight Sm atoms bridged by eight ligands arranged to form a neutral macrocycle in which the Sm atoms adopt a flattened boat-like conformation. The analogous reaction carried out with Ph2C(a-C4H3N)2H2 gave hexanuclear [{ [Ph2C(a-C4H3N)2] Sm}6(THF)3], which contains six Sm atoms that form a regular, flat hexagon with Sm-Sm distances ranging from 4.36( 1)-4.23(1)A. Both clusters reduce N2 to give previously reported dinitrogen compounds [{ [R~C(CX-C~H~N)~] Sm}4(THF)2](p-N2). The reduction stops at the hydrazido tetraanion stage, presumably because encapsulation of N2 into the small tetranuclear cluster inhibits further reduction!' Conversely, reduction of { [1,1-H10C6(a-C4H3N)2] Sm}4(THF)2(p-N2) with Na in THF afforded the linear polymeric divalent Sm where each samarium atom complex { [1,1-HIOC6(a-C4H3N)2]2Sm[Na(THF)]2}., is surrounded by four q5-bonded pyrrolide rings, thus giving the metal center a formal 30-electron configuration61 Other small molecule reductions gave equally stimulating products. The nature of the substituents present on the calix-tetrapyrrole tetraanion ligand {[R2C(C4H2N)]4}4- (R = {-(CH2)5-}0.5, Et) were shown to have a dramatic influence on the reactivity of the corresponding Sm(I1) compounds with acetylene. Dehydrogenation of [{ [-(CH2)5-]4-~ali~tetrapyrrole}(THF)Sm(II){Li2[Li(THF)](p3-OCH= CH2)}] occurred upon reaction with acetylene to yield the nearly colourless dinuclear diacetylide complex [{ { [-(CH2)5-]4-calix-tetrapyrrole}Sm(III)}2(~-C2Li4)] (21) as the only detectable reaction product. With R = Et, dehydrogenation was accompanied by acetylide coupling to give [{ (Et8-calix-tetrapyrr~le)Sm(III){ Li[Li(THF)I2(p3OCH = CH2)}}2(p,q2,q'2-HC = C = C =CH)] (22). Reaction of the trivalent hy(p-H)[Li(THF)]}I2 or methyl derivative dride [(Et8-calix-tetrapyrrole)(THF)Sm{ [(Et8-ca1ix-tetrapyrrole)SmMe{[Li(THF)I2[Li(THF)2](p3-Cl)}] with acetylene resulted in a three component mixture of the carbide [{ (Et8-calix-tetrapyrrole)Sm}2(P-C2Li4)], dimerization product [{ (Et8-calixtetrapyrrole)Sm{Li[Li(THF)I2(p3-OCH=C H ~ ) } ) ~ ( P , ~ ~ , = ~C ' ~=- C H= CCH)], and [{ (Et8-calix-tetrapyrrole*)Sm[Li(THFh]}2],in which the macrocycle was isomerized by shifting the ring attachment of one of the four pyrrole rings.62 A series of carborane derivatives were prepared and structurally characterized. New metallacarboranes [{q5:q1:q6-Me2Si(C9H5CH2CHzX)(C2B IOHlo)Sm}z(pCl)]

Organometallic Chemistry

98

[Li(THF)4] (X = NMe2,OMe), were isolated from reactions of Sm12(THF)5with [Me2Si(C9H5CH$2H2X)(C2BloH10)]Li2(OEt2)2 in THF via unexpected Sm-mediated tandem reaction^.^^ Similarly, the reaction between the THF solvated 'carbons apart' dinatracarborane, closo-exo-4,5-Na(THF)z-1-Na(THF)2-2,4(SiMe3)?C2B4H4 with GdC13 in a 1:l molar ratio gave off-white crystals of the dimeric half-sandwich dichlorogadolinacarborane, { 1,l-Cl2[p,p'-Na(TMEDA)] - 1-(THF)-2,4-(SiMe3)2-closo-q51-Gd-2,4-C2B4H4}2.64 Finally, in a completely different incorporation of boron into organolanthanide chemistry, the reaction of YC13 with lithium 1-methylboratabenzene in toluene afforded high yields of the pale yellow dinuclear sandwich complex [(C5H5BMe)2Y(p-C1)]2 (23).Three conformational polymorphs of this compound that differ in the rotational position of the boratabenzene ligand were noted in the solid state. A DFT analysis indicated that the three solid state structures correspond closely to three minima on the gas-phase potential energy surface.65 There were a considerable number of synthetic investigations that focused on structural characterization of the final product. In divalent chemistry, the Yb(I1) tetraalkylaluminate complexes [Yb(AlR4)2]nwere obtained from reactions of Yb[N(SiMe3)2]2(THF)2with excess AIR3 (R = Me, Et, i-Bu). While the tetramethylaluminate compound is an insoluble pyrophoric material, the Et and 'Bu congeners dissolve readily. Per-ethylated polymeric [Yb(A1Et4)& forms an intricate three-dimensional network in the solid state, with bridging a-C atoms and secondary Yb H-C 'agostic' interactions combining p,ql, p,q2, and p,q3 coordination modes which result in remarkably short Yb - - - A1 (2.809(2) A). A theoretical analysis of the [Yb(AlEt4)3]- and [Yb(A1Et4)] polymer components reproduced the X-ray geometry remarkably well and predicted a highly fluxional aluminate coordination (AE(q2-+q3) = -- 8 kcal/mol), that was in agreement with VTNMR measurements. A topological analysis of the total electron density of the p,q2-bonded aluminate ligands suggests a hypervalent character of the bridging C atom.66Similarly, the first structure of a ytterbium diary1 compound, bis( 1,1':3', 1 "-terphenyl-C2')bis(tetrahydrofuran-0)yt terbium( 11) was described. The Yb atom shows a strongly distorted tetrahedral environment formed by the C atoms of two aryl groups (av. Yb-C 2.520 A) and the 0 atoms of two T H F ligands. Completing the Yb coordination sphere are two weak q'-n-arene interactions (av. Yb . . . C 3.138 A.) involving ortho-C atoms of the rn-terphenyl Ph groups.67The molecular structures of novel donor-functionalized terphenyl derivatives of trivalent Yb, Y, and Sm [DanipYb(p2-C1)2(p3-C1)Li(THF)l2 and [DanipLn(p2-C1)2(p2-C1)Li(THF)2]2 (Ln = Y; Ln = Sm) were determined [Danip = 2,6-di(o-anisol)phenyl].The complexes, obtained from the reaction of * *

+

4: Scandium, Yttrium and the Lanthanides

99

DanipLi and LnC13 in THF at room temperature in 60% yield, are the first examples of donor-functionalized terphenyl complexes of these elements. In the structures LiCl bridges two DanipLnC12moieties.68 Numerous trivalent compounds were obtained from metathesis reactions. For example, the first dinuclear anionic lanthanocene compound was obtained when the reaction of [Nd(BH4)3(THF)3]with K2[FluCPh2Cp] in the presence of 18-crown-6 gave the anionic complex [K( 18-crown6){ F ~ U C P ~ ~ ( C ~ H ~ ) N ~The ( B Hcrystal ~ ) ~ }structure ]~. of this complex revealed a binuclear arrangement consisting of three parts: the two discrete { F ~ u C P ~ ~ ( C ~ H ~ ) N- ~anions ( B H ~connected )~} by a [K( 18-crown-6)] cation through weak q2interactions of the Flu moieties with the K+.69 Reactions of K{ CH(PPh2NSiMe3)2} with LnC13 gave [{CH(PPh2NSiMe3)2}LnC12]2 (Ln = Y, Sm, Er, Dy, Yb, Lu). The single-crystal X-ray structures of these isostructural complexes reveal a methine carbon bound to Ln. This is supported by DFT calculations, which show a comparable interaction. Further reaction of the Y and Sm compounds with KNPh2afforded [{ CH(PPh2NSiMe3)2}Ln(NPh2)2], in which the { CH(PPh2NSiMe3)2}-ligand again coordinates through the methine carbon in the solid state and in solution.’” Chiral (racemic)terphenyl complexes DnpLnC12(THF)2[Dnp = 2,6-di(l-naphthy1)phenyll (Ln = Yb, Tm, Y) were synthesized by reactions of DnpLi with LnC13. The molecular structures exhibit distorted trigonal-bipyramidal coordination environments at the metal centre, with the two 0 atoms of the THF ligands occupying the axial positions of a trigonal-bipyramidal coordination polyhedron. An attempt to prepare a Sc derivative gave [(THF)3Sc20C15Li(THF)]2in low yield.’l A reaction of the dianion Li2[Me&NC(Ph)N(CH2)3NC(Ph)NSiMe3] with YC13(THF)3.5 gave [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]YCl(THF)2, which was converted by metathesis to the alkyl complex [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3] Y[CH(SiMe3)2](THF)(24). A structural analysis of this alkyl showed that linking together the amidinate functionalities opens up the coordination sphere to allow the bonding of an additional THF not present in the unbridged bis(amidinate) analogue [PhC(NSiMe3)2]2Y[CH(SiMe3)2]72 Attack at ‘PrN= C =N‘Pr by (Me$i)2Nfollowed by reaction with YC13 generated dimeric { [(Me3Si)~NC(NiPr)2]2Y(pC1)}2, which was derivatized metathetically to give [( Me3Si)2NC(NiPr)2] 2Y CH(SiMe&, [( Me3Si)2NC(NiPr)2] 2YN(%Me&, p-Me)2Li[Me2NCH2CH2NMeJ, and [(Me3Si)2[(Me3Si)2NC(NiPr)2]2Y( NC(NiPr)J2YCMe3. These compounds are apparently the first reported examples of organoyttrium complexes supported by a guanidinate ligand.73

+’

100

Organometallic Chemistry

The tris-ally1 Nd(II1) complex [Nd(n-C3H5)3-dioxane]was obtained from a Grignard reaction. Similarly, neutral [Nd(n-C5Me5)(n-C3H5)2.dioxane] and ionic [Nd(n-C3H5)CI(THF)5]B(C6H5)4 were isolated and fully characterized. Whereas the allyl-Cp* compound was a highly active catalyst for butadiene polymerization in the presence of MAO, the salt was ~ n r e a c t i v e . ~ ~ There was also one arene complexes of Ln(II1) described: reaction of NdCl3 with AlC13 and mesitylene in benzene gives [Nd(q6-1,3,5-C6H3Me3)(A1C14)3] (C6H6)which was characterized by conventional techniques. The structure of this compound is a distorted pentagonal bipyramid. A comparison of bond parameters for all the reported Ln(~f-Ar)(AlCl~)~ complexes indicates that the Ln-C bond length decreases along with the number of alkyl substitutions in the arene ligand.75Interestingly, this compound is a very effective precursor to supported catalysts for the polymerization of b~tadiene.'~

3

Organometallicsin Materials Synthesis

Driven in part by interest in doping lanthanide ions into sulfido matricies for electroluminescence applications, numerous groups have continued to use organometallic ligands as supports in the synthesis of compounds with direct bonds to the more electropositive atoms S, Se, and P. In a continuation of earlier cluster chemistry, the half-sandwich complexes { [(q5--'BuC5H4)Ln(THF)]2(p2Cl)2(p3-CI)3Na(THF)}. [Ln = Nd, Sm, Gd, Yb] were prepared by the reaction of LnC13with NatBuCSH4in THF. The Sm derivative reacts with Na2Se5to give the hexanuclear salt [Na(THF)6]2[(q5-tBuC5H4)6Sm6(p6-Se)(p-Se2)6]. Similarly, [Li(THF)4]2[C~~Nd~(p~-se)(p-Se2)~] was isolated from the reaction of CpNdCl2 with Na2Sesin THF. Both structurally characterized clusters contain an interstitial Se coordinated to six Ln(II1) ions.34 was In sulfur chemistry, the structure of [(CH3C5H4)2La(THF)(p-SC6H5)]2 determined to be a centrosymmetric dimer bridged through the S atoms of the benzenethiolate ligands. The La2S2ring is planar, while the geometry around the nine-coordinate La is best described as a distorted trigonal b i ~ y r a m i d The .~~ related Nd derivative [(CH3C5H4)2Nd(p-SPh)(THF)I2 reacts with PhNCO and and (CH3C5H4)2Nd[q2PhNCS to give [(CH3CsH4)2Nd]2[p-q2-OC(SPh)NPh]2 SC(SPh)NPh](THF), respectively, with the thiolate ligand acting as a nucleophile. Both reaction products were characterized by X-ray diffraction, and the latter compound was found to contain an q2 SC(SPh)NPh ligand.28Base-free, hydrocarbon-soluble thiolate complexes Ln(SAr*)2(Ln = Eu, Yb; Ar* = 2,6Trip2C6H3;Trip = 2,4,6-iPr&H2) (25) (result from the protonolysis reaction of (2-CF3C6H4)LnI with HSAr*. Similarly, the purple YbIII complex [Yb12(SAr*)(THF)3]and the yellow DME adduct Yb(SAr*)2(DME)2were synthesized and characterized completely. The base-free compounds are monomeric, with the Ln bonded to two terminal thiolate ligands and $-n;-arene interactions between Ln and two of the Trip substituents. VTNMR experiments were used to estimate the binding energy for these Ln-arene interactions at ca. 55 kJ/mole. The nature and strength of the $-x-arene bonding was also studied by

4: Scandium, Yttrium and the Lanthanides

101

ab initio ~ a l ~ ~ l a t i Finally, o n ~ . ~[Tm(C5H5)3] ~ reacts with one equivalent of 2mercapto-benzothiazole in THF to give the monothiolate that crystallized as a molecular complex with each thulium ion coordinated by two q5-cyclopentadienyl groups, one oxygen atom of THF, one sulfur and one nitrogen atoms from the chelating benzothiazole-2-thiolateligand to form a distorted trigonalbipyramidal ge~metry.~

(25)

In actual materials synthesis, a systematic evaluation was made of the performance and efficiency of CeCp3 as a Ce dopant source in atomic layer epitaxy and CVD of Ce-doped strontium sulfide (SrS:Ce).In situ growth and characterization studies were carried out, without a vacuum break, of the adsorption and decomposition mechanisms of CeCp3at industrially useful temperatures. Results were compared to those of Ce(2,2,6,6-tetramethyl-3,5-heptadionatob, which was used as a comparative performance baseline. The Cp3Ce source decomposed more efficiently than did the dionato compound, in terms of reduced hydrocarbon-based surface contamination and a cleaner Ce phase.78Organoerbium compounds were also used to prepare Er doped InP, and the effects of co-doping with 0 2 was examined with by X-band ESR measurements at low temperatures. The ESR at around g = 6, which corresponds to a trivalent Er site with Td symmetry, lost its intensity quickly and disappeared above 12 K. ESR intensity also decreased upon addition of 02.79

4

Polymerization Chemistry

A number of reviews have been published in the past year. Yasuda has published a review of lanthanide catalyzed copolymerization of polar monomers with 1-olefins.8' A review of the polymerization activity half-sandwich complexes for the homo- and copolymerization of ethylene and styrene has been published.81 The same authors have published a short review of the activity of [Ln(q5:q1CsMe4SiMe2NCMe3)(THF)(p-H)]2 (Ln = Y, L u ) . ~A ~review of the activity of half-sandwich divalent complexes for the polymerization of a range of monomers has a~peared.8~ Lastly, a review appeared in Chinese, covering the synthesis and molecular structure of lanthanocene amides, and their applications in polymerization of polar m0nomers.8~ 4.1 Non-polar Mono-olefin Polymerization. - In a fairly comprehensive study a detailed comparison the structures and polymerization behaviour of a series of

102

Organometallic Chemistry

samarium and yttrium -ate complexes with rac, meso, and C1 symmetries and with a variety of bridging groups has been carried out. The complexes were: ra~-Me~Si(2,4-(Me3Si)2C5H2)2SmC12Li(THF)~] (rac-Sm-Cl), C1-Me2Si[2,4(Me3Si)2C5H2] [3,4-(Me3Si)2C5H2] SmC12Li(THF)2 (CI-Sm-Cl), C 1-Ph2Si[2,4(Me3Si)2C5H2][3,4-(Me3Si)2CsH2]SmC12Li(THF)2 (C1-(PhzSi)-Sm-Cl),meso-[ 1,2(Me2Si)(MezSiOSiMe2)](4-Me3CC5H2)2SmC12Li(THF)2 (meso-Sm-Cl), racMe2Si[2,4-(Me3Si)2C5H2]2YC12Li(THF)= (ruc-Y-Cl), Me2Si[2,4-(Me3Si)2C5H2] [3,4-(Me3Si)2C5H2]YC12Li(THF)2 (C,-Y -Cl), meso-[ 1,2-(Me2%)(Me2SiOSiMez)] (4-Me3SiC5H2)2YC1(THF) (meso-Y-Cl).Isomeric mixtures could be separated by fractional crystallization from hexane. The halide complexes were converted to the corresponding hydrocarbyls by reaction with LiCH(SiMe3)2.Only the C1 isomeric alkyls were found to polymerize ethylene to high molecular weight. Structural data on several of the complexes was ~ b t a i n e d . ~ ~ A similar set of trivalent neodymium -ate complexes [rac-{ Me2Si(y5-2-SiMe34-t-Bu-CSH2),}Nd(p-C1)2Li(THF)z], [roc-{ Me2Si(q5-2,4-( SiMe3)2CSH2)2}Nd( pCl)2Li(THF)2], and [{ Me2Si(q5-2,4-(si Me3)2CSH2)(q ’-3,4-( SiMe3)2C5H2)} Nd(pC~)ZL~(THF)~] have been prepared and structurally characterized. When combined in situ with a dialkylmagnesium co-catalyst, they were found to initiate the polymerization of ethylene and 1-octene to yield di(oligoalky1)magnesiumspecies. The ansa-bridged species were significantly more active catalysts for 1octene oligomerization than non-bridged analogues.86 A range of complexes including Sm(01Pr)3,Sm(acac)3,Sm(OAc),, Sm12(THF)2 or SmC13,coupled with Et3Al or M A 0 were found to polymerize styrene. The Sm(OPr)3/A1Et3system shows the highest catalytic activity, suggests that the polymerization proceeds by a radical mechanism.87 A witches brew method for the generation of ethylene/hexene copolymers has been patented. This involves mixing supported or unsupported Mg/Ti precursors with a mixed ligand cyclopentadienyl lanthanide In a study of polymerization by divalent complexes, a wide range of compounds was tested for styrene and propene polymerization. The highest activity for styrene polymerization was found for hydrides, naphthalene and stilbene complexes of samarA patent has been filed for the block ium(II), europium(I1) and ytterbi~m(II).*~ copolymerization of vinyl aromatic compounds with both polar and non-polar monomers simply in the presence of metallocenes of Sc, Y, La, or lanthanide metals.90 The half-sandwich metallocene complex [LaCp*(CH(SiMe&)(THF)2 polymerized both non-polar monomers such as ethylene and styrene as well as polar monomers like MMA, hexyl isocyanate, and acrylonitrile in high yields. By contrast, the metallocene complexes, [(C5H4SiMe3)2Y(p-Me)]2 and [Cp*2YMe(THF)], were significantly less effective?’ Finally, in a departure from the usual Cp-based catalysts, the Y dialkyl complexes [N,N’-R,-tacn-N”(CH2)2NBu‘]Y(CH2SiMe3)2 (R = Me, Pri; tacn = 1,4,7-triazacyclononane) were prepared. When activated with [PhNMe2H][B(C6F5)4]these complexes form cationic alkyl species that are active ethene polymerization catalyst^.^^. 4.2

Diene Polymerization.- Diene polymerization using a range of complexes,

4: Scandium, Yttrium and the Lanthanides

103

not all of them metallocenes, continues to attract research attention. In an extension of previously reported work kinetic studies on the polymerization of butadiene by neodymium allyl complexes indicate that these are true living systems. The single site catalyst is proposed to consist of [Nd(q3-RC3H4)(qC4H6)2(X-[AlOR]n)2] as the single-site catalyst, the propagation step involving the insertion of coordinated butadiene into a n-ally1 groups.93The same group has filed yet another patent for the copolymerisation of conjugated dienes with non-conjugated olefins using tris-allylneodymium.Y4Supported catalysts were prepared by the reaction of [Nd(q6-ChH5CH3)(AlCls)3] with modified silica. These compounds were also very effective for the polymerization of b~tadiene.~' Non-hindered ansa-dicyclopentadienyl allyl complexes of samarium, [(CMe2C5H4)2Sm(allyl)]n, and (CMe2C5H4)2Sm(allyl)L (L = THF or allylLi) polymerize isoprene without an aluminium co-catalyst. The polymerizations are highly stereospecific, affording very regular 1,4 trans-polyisoprene. Addition of linear 1-olefins led to copolymers with markedly different viscoelastic (elastomeric) proper tie^.'^ Dy12is also capable of polymerizing isoprene to produce cis-I,4-polyisoprene elastomer.96 In a follow-up paper to one reported last year, the microstructure of butadiene-ethylene copolymer generated using a number of neodymocene catalysts has been reported. Intriguingly, the presence of a substantial number of trans1,2-cyclohexaneunits, resulting from intramolecular cyclization, was detected.97 Copolymers of isoprene with C6-CI8a-olefins by a single component organolanthanide catalyst affords poly(trans-l,4-isoprene) containing between 6-1070 of inserted olefin. The additional alkyl chains result in quasi-amorphous viscoelastic material^.^^ A broad patent for the preparation of ethylene-conjugated diene copolymers has been filed covering both linked and classical metallocenes with a range of co-catalysts selected from alkylmagnesiums, alkyllithiums, alkylaluminums, or Grignard reagents.99 The heterobimetallic complex [Cp*2Sm(pMe)2A1Me2],when combined with A ~ ( ~ - Band u ) ~[Ph3C][B(C6F5)4],was shown to be an excellent catalyst system for the living 1,4-cis stereospecific polymerization of 1,3-butadiene with styrene.'" The method has been patented."', lo2

4.3 Polymerization of Acrylate Monomers. - The complexes [Ind2Y(pEt)2A1Et2]and [Ind2LnN(i-Pr)2] (Ln = Y, Yb) exhibit extremely high catalytic activity in the polymerization of MMA and good activity for a~rylonitrile.'~~~ '04 A patent has been filed for the polymerizatiion of (metha)acrylic acid esters and cyclic lactones by a range of metallocene alkyls [M(C5R5)2MR']2,where R' = CH2CH(R1)nCHCH2where R' = C1-4 alkylene and n = 0 or 1. The polymers are useful as pressure-sensitive adhesive and moulding material^.'^^ The ansa-Cp complex [Me2Si(C5H3SiMe3)2NdCl] activated by n-BuMgCl at low molar ratio (1:1.2) was successfully used to polymerize MMA. The results indicated that Grignard reagents may be better co-catalysts than alkyl aluminiums.'06 The polymerization of MMA by silicagel-supported [Cp*2SmMe(THF)] has been studied. Activity was lower than that observed for homogeneous catalyst syst e m ~ .Polymerization *~~ of MMA by samarocenes support on rneso-porous silica

104

Organometallic Chemistry

gave higher molecular weights than the corresponding homogeneous systems.'" Polymerization reactions of butylacrylate (BuA) were studied using an organosamarium complex, [Cp*2SmMe(THF)] as initiator and gave high number average polymers (M, > 200,000)with narrow polydispersity (1.07).'09Lastly, an unusual patent has been filed this year claiming the preparation of highly isotactic Bu-MA/MMA block copolymers using the surprising divalent precatalyst M[C(SiR3)3]2(M = Sc, Y, lanthanides; R = H, C1-', or Si-containing hydrocarbyls).'lo A more likely patent claims that highly isotactic polymerization of broad range of acrylates can be achieved using dimethylsilylene-bridged bis-indenyl yttrium compounds." A study of acrylonitrile polymerization has been reported using a precatalyst comprising [ ( ' B U C ~ ) ~ N ~ Min~ combination );! with a quaternary ammonium salt, or sodium aryloxide. The conversion of acrylonitrile was reported to be as high as 65%.lt2 4.4 Polymerization of Lactide. - Although polyesters are normally generated using alkoxide catalysts two organometallic examples have been reported. Random copolymerization of L-lactide with (R)-, (S)-, or rac-1-methyltrimethylene carbonate using [ C P * ~ S ~ M ~ ( T H as F ) the ] initiator provided high molecular weight polymers with low polydispersities. Biodegradation of the resulting polymers was ~tudied."~ Complexes containing phosphorane iminato ligands, including [La2(NPPh3)4(p-NPPh3)2(p-THF)] and [Yb(NPPh3)J2 were found to initiate the polymerization. of E-caprolactone. The activity was found to be higher than corresponding Cp systems. Interestingly a mixed ligand system [Cp3Dy*(NPPh3)3] was found to be particularly effective. Block copolymerization indicates that these are living systerns.'l4

5

Spectroscopic and TheoreticalStudies

This has been a lean year for theoretical/spectroscopic studies of lanthanide organometallics. Amberger and Edelstein have continued their combined in depth magnetic/spectroscopic survey of lanthanide Cp systems with a detailed examination of the [TmCp3L2] (L = CH3CN).The crystal structure has been determined and optical spectra have been fitted to successfully to both relativistic and non-relativistic MO schemes.' The refractive indices of [LnCp3] (Ln = Nd, Sm, Er) have been measured in the vapour phase. The electronic static dipole-polarizabilities were derived and the trend found to opposite to those measured in THF ~olution.~'' The nature of bonding and energetics in trivalent rare earth n-donor ligand complexes [NdC&-nL] and [NdCp2-nL]+ (L; L = HC=CH, H2C=CH2, H2C = CHCH = CH2, C6H6)was investigated with Hartree-Fock (HF) and density functional theory (DFT) methods. Geometries and binding energies were reported. The analysis presented in this study clearly indicates the essentially electrostatic character of the binding interaction in terms of a cation-digand interaction. The lanthanide to n-ligand bonding was predicted to be weak,

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105

accompanied by a slight distortion of the ligand’s geometry upon coordination. Electron correlation was not crucial to correctly predicting the metal-ligand interaction energy and the ligand distortion.’16Niemeyer has used quasi-relativistic ab initio methods to understand the nature of the Ln(I1)-n-arene interactions. The calculated dissociation energies compare quite well with those measured by NMR methods. The electronic and molecular structures of selected zerovalent d and f metal bis-$-benzene sandwich complexes M(C6H&(M = Sc, Ti, V, Cr, Y, Zr, Nb, Mo, La, Ce, Gd, Lu, Hf, Ta, Nb, Th) were investigated at the scalar-relativistic level using a combination of energy-consistent ab initio pseudopotentials and gradient-correlated density functionals. The results were compared to previous pseudopotential coupled-cluster and all-electron density functional results based on the zero-order regular approximate and to experimental data.117 The energy required to activate the H-H bond in the entire series of Cp2LnH complexes was calculated by DFT methods. The activation energies vary from 0.5 to 8.0 kcal mot’, indicating an overall facile reaction. The electronegativity of the lanthanide in its most stable oxidation state is suggested to be a leading factor for interpreting the trends in activation energy. The geometry of the transition state is best viewed as an almost linear H3(151) ligand with short H-H distances and strong M-H interaction, through the wingtip H centers, with Ln.”*

6

Lanthanides in Organic Synthesis

A number of catalytic reactions have been studied and will discussed sequentially. The hydrosilylation activity of a number of Lu and Sm hydride complexes have been discussed in Section 2.1.2.17*19, 2o The dimeric rare earth (Ln = Y, Tb, Yb, Lu) hydrides, [(fB~Cp)~Ln(p-H)l~ and hydrocarbyls [(‘BuCp)2Ln(p-Me)12as well as compounds with mixed methyl/hydride bridges, [(‘B~cp)~Ln(p-H)(pMe)Ln(tBuCp)2Ln],are efficient and selective catalysts of 1-octene hydrosilylation. The study concludes that dissociation is not important in this system and that binuclear hydride-bridged complexes are the key intermediates. Kinetic studies of the hydrosilylation of 1-octene with PhMeSiHz suggest that hydride transfer from silane to Ln is fast while the addition of Ln-H to olefin is ratelimiting. By contrast, the methyl-bridged complexes exhibit much higher initial catalytic activity. In this case, the hydrosilylation seems to involve the formation of a highly active monomeric hydride, [(‘BuC~)~L~H]. After several minutes, however the hydrosilylation rate decreased as a result of catalyst association.”’ Divalent lanthanide-imine complexes and a related species catalyzed the hydrosilylation of olefins with PhSiH3 or Ph2SiH2.In contrast, conjugated dienes were converted to 1,4-bis-silyl-2-butenesand 3-~ilacyclopentenes,accompanied with hydrogen evolution, under similar conditions.120 Marks and his group have continued to develop hydroamin-/phosphin-ation. Further examples of intramolecular hydroamination/cyclization of amines tethered to 1,2-disubstituted alkenes have been described. These systems yield

106

Organometallic Chemistry

the corresponding mono- and disubstituted pyrrolidines and piperidines. Precatalysts included ansa and unlinked metallocenes, as well as constrained (Ln = Sm, geometry complexes such as [MezSi(q5-Me4C5)('BuN)]LnE(SiMe3)2 Y, Yb, Lu; E = N, CH). Yields and diastereoselectivities were found to be high.121 Molander et al., have applied a diastereoselective, lanthanocene-catalyzed, intramolecular hydroamination reaction to the preparation of 2,6-disubstituted piperidines and in a short synthesis of pinidol. Diastereoselectivity was optimized by varying both the metal and the ligand. The complex [Cp*2NdCH(TMS)2] was found to convert 2-substituted 8-nonen-4-amines to 2,6-disubstituted piperidines with greater than 100:1 selectivity for the formation of the cis isomer.122 Metallocenes of the general formula Cp*2LnE(SiMe3)l(Ln = La, Sm, Y, Lu; E = CH, N) serve as effective precatalysts for the rapid intramolecular hydrophosphination/cyclization of the phosphinoalkenes and phosphinoalkynes RHP(CH2)nCH= CH2 (R = Ph, H; n = 3,4) and H2P(CH2),C( 186)C-Ph (n = 3, 4) to afford the corresponding heterocycles CH3CH(CH2),PR and Ph(H)C = C(CH2)nPH,respectively. Detailed kinetic and mechanistic data allow comparison with the corresponding hydroamination/cyclizations. The catalyst resting state is believed to be a lanthanocene phosphine-phosphido complex. Dimeric [ C P * ~ Y P ( H ) Pwas ~ ) ~isolated and crystallographically characterized. 23 The ytterbium-imine complex, [Yb(q'-Ph,CNPh)(hmpa)6] was found to be active for catalytic intermolecular hydrophosphination of alkynes with Ph2PH. Both terminal and internal alkynes were converted in high yields to the corresponding alkenylphosphines or phosphine oxides after oxidative workup. The method was found to be applicable to conjugated diynes and dienes, allenes and styrene derivative^.'^^ The related cyclization/boration can be carried out for 1,5and 1,6-dienes with a catalytic amount of [Cp*2Sm.THF] in the presence of 1,3-dimethyl-1,3-diaza-2-boracyclopentane. The resulting organoboranes can be oxidized to the corresponding primary cyclic alcohols using standard condition~.~~~ Finally, organolanthanide complexes have been applied to a variety of other transformations. Catalytic isomerization of 1,5-hexadiene by Cp2Ln Schiff base/NaH (Schiff base = 2-(2-HOC6H4CH= N)C6H40CH3;Ln = Sm, Dy, Y, and Er) systems was studied. The isomerization results in a mixture of 1,4hexadiene, 2,4-hexadiene, 1,3-hexadiene, methylene cyclopentane and methylcyclopentene. The effects of nature of catalyst, temperature, catalyst concentration, and time on isomerization rate and product composition were also studied. The ratio of linear to cyclic product in reaction depends upon the amount of catalyst used.Iz5 Evans has filed a patent the preparation of Tm12or Dy12claiming that DME adducts are more powerful reducing agents and catalysts for reductive alkylation reactions of ketones and alkyl halides compared with the classic Sm12(THF),/HMPA analogue.96 In the presence of methyl iodide, presumably acting as an initiator, Yb reacts with ally1 bromide smoothly to form bromo(propenyl)ytterbium, which further reacts with diselenides, aldehydes and ketones to afford allylselenides and

4: Scandium, Yttrium and the Lanthanides

107

homoallylic alcohols. respectively in good yields under mild and neutral conditions.'26The new Yb(I1) thiocyanate complex [Yb(NCS)Z(THF)z],synthesized by redox transmetalation between [Hg(SCN)2] and Yb metal in THF at room temperature, was found to reduce benzophenone, Ph2C0, in THF giving the binuclear [{Yb(NCS)2(THF),}2(p-OC(Ph)2C(Ph)20)], in which two octahedral Yb's are bridged by a 1,1,2,2-tetraphenylethane-1,2-diolateligand, derived from reductive coupling of the benzophenone reagent.'27 Lastly, a patent has been filed for the preparation of siloxymalononitriles. Cyclohexanone oxime acetate was treated with Me3SiCNin PhMe in the presence of Cp*2Sm(THF)2at 25" for 15 h to give 88% MeC(CN)20SiMe3.128

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P. W. Roesky, J . Organomet. Chem., 2001,621,277. J.-L. Huang, X.-Q. Shen, Q.-C. Liu, Y.-L. Qian, and A. S.-C. Chan, Chin. J . Chem., 2001,19, 102. M. Niemeyer, Acta Cryst. E , 2001, E57, m553. G. B. Deacon, A. Gitlits, P. W. Roesky, M. R. Burgstein, K. C. Lim, B. W. Skelton, and A. H. White, Chem. Eur. J., 2001,7, 127. I. Lopes, B. Monteiro, G. Lin, A. Domingos, N. Marques, and J. Takats, J . Organomet. Chem., 2001,632,119. M. Ganesan, S. Gambarotta, and G. P. A. Yap, Angew. Chem. Int. Ed. Engl., 2001, 40,766. M. Ganesan, M. P. Lalonde, S. Gambarotta, and G. P. A. Yap, Organometallics, 2001,20,2443. T. Dube, J. Guan, S. Gambarotta, and G. P. A. Yap, Chem. Eur. J., 2001,7,374. S. Wang, H.-W. Li, and Z. Xie, Organometallics, 2001,20, 3624. N. S. Hosmane, S. Li, C. Zheng, and J. A. Maguire, Inorg. Chem. Commun., 2001,4, 104. X. Zheng, B. Wang, U. Englert, and G. E. Herberich, Inorg. Chem., 2001,40, 31 17. M. G. Klimpel, R. Anwander, M. Tafipolsky, and W. Scherer, Organometallics, 2001,20,3983. M. Niemeyer, Acta Cryst. E , 2001, E57, m578. G. W. Rabe, C. D. Berube, and G. P. A. Yap, Inorg. Chem., 2001,40,4780. C . Qian, W. Nie, and J. Sun, J . Organomet. Chem., 2001,626, 171. M. T. Gamer, S. Dehnen, and P. W. Roesky, Organometallics, 2001,20,4230. G. W. Rabe, C. D. Berube, and G. P. A. Yap, Inorg. Chem., 2001,40,2682. S. Bambirra, A. Meetsma, B. Hessen, and J. H. Teuben, Organometallics, 2001, 20, 782. Z. Lu, G. P. A. Yap, and D. S. Richeson, Organometallics, 2001,20, 706. R. Taube, S. Maiwald, and J. Sieler, J . Organomet. Chem., 2001,621,327. Y.-M. Yao, Y. Zhang, Q. Shen, Q.-C. Liu, Q.-J. Meng, and Y.-H. Lin, Chin. J . Chem., 2001,19,588. F. Barbotin, R. Spitz, and C. Boisson, Macromol. Rapid. Commun., 2001,22, 141 1. M. Niemeyer, Eur. J . Inorg. Chem., 2001,1969. J. E. Lau, G. G. Peterson, D. Endisch, K. Barth, A. Topol, A. E. Kaloyeros, R. T. Tuenge, and C. N. King, J . Electrochem. Soc., 2001,148, C427. H. Ohta, C. Urakawa, Y. Nakashima, J. Yoshikawa, T. Koide, T. Kawamoto, Y. Fujiwara, and Y. Takeda, Physica E: Low-Dimensional Systems & Nunostructures (Amsterdam, Netherlands), 2001,10,399. H. Yasuda, J . Polym. Sci. A , 2001,39,1955. J. Okuda, S. Arndt, K. Beckerle, K. C. Hultzsch, P. Voth, and T. P. Spaniol, Organometallic Catalysts and Olefin Polymerization, 2001, 156. J. Okuda, S. Arndt, K. Beckerle, K. C. Hultzsch, P. Voth, and T. P. Spaniol, Pure Appl. Chem., 2001,73,351. Z . Hou, S. Kaita, and Y. Wakatsuki, Pure Appl. Chem., 2001,73,291. Q. Shen and Y.-m. Yao, Youji Huaxue, 2001,21,1018. E. Ihara, S. Yoshioka, M. Furo, K. Katsura, H. Yasuda, S. Mohri, N. Kanehisa, and Y. Kai, Organometallics, 2001,20, 1752. S. Bogaert, T. Chenal, A. Mortreux, G. Nowogrocki, C. W. Lehmann, and J.-F. Carpentier, Organometallics, 2001,20, 199. T. Hayakawa, Y. Nakayama, and H. Yasuda, Polym. Int., 2001,50,1260. D. M. Brown, 2001, Ep 1101777.

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89. E. N. Kirillov, E. A. Fedorova, A. A. Trifonov, and M. N. Bochkarev, Appl. Organomet. Chem., 2001,15,151. 90. M. Gepraegs, J. Queisser, J. Okuda, K. C. Hultzsch, and K. Beckerle, 2001, De 19959252. 91. K. Tanaka, M. Furo, E. Ihara, and H. Yasuda, J. Polym. Sci.A, 2001,39,1382. 92. S. Bambirra, D. van Leusen, A. Meetsma, B. Hessen, and J. H. Teuben, Chem. Commun., 2001,637. 93. S . Maiwald, C . Sommer, G. Mpller, and R. Taube, Macromol. Chem. Phys., 2001, 202,1446. 94. H. Windisch, G. Sylvester, R. Taube, S. Maiwald, J. Giesemann, and T. Rosenstock, 2001, Wo 0185814. 95. D. Barbier-Baudry, F. Bonnet, A. Dormond, A. Hafid, A. Nyassi, and M. Visseaux, J . Alloys Cmpds., 2001,323-324, 592. 96. W. J. Evans and N. T. Allen, 2001, Wo 0121667. 97. M. F. Llauro, C. Monnet, F. Barbotin, V. Monteil, R. Spitz, and C. Boisson, Macromolecules, 2001,34,6304. 98. M. Visseaux, D. Barbier-Baudry, F. Bonnet, and A. Dormond, Macromol. Chem. Phys., 2001,202,2485. 99. F. Barbotin, 2001, Ep 1092731. 100. S. Kaita, Z. Hou, and Y. Wakatsuki, Macromolecules, 2001,34, 1539. 101. S. Kaita, Z. Hou, and Y. Wakatsuki, 2001, Wo 0177199. 102. S. Aida, C.-m. Hou, and Y. Wakatsuki, 2001, Jp 2001288234. 103. L.-Q. Ying, X.-W. BayY.-Y. Zhao, G. Li, T. Tang, and Y.-T. Jin, Chin. J . Polym. Sci., 2001,19,89. 104. L.-q. Ying, G. Li, Y.-y. Zhao, X.-w. BayD.-m. Cui, T. Tang, and Y.-t. Jin, Zhongguo Xitu Xuebao, 2001,19,275. 105. Y. Kawaguchi and H. Yasuda, 2001, Jp 2001081129. 106. J. Sun, Z. Pan, and S. Yang, J. Appl. Polym. Sci., 2001,79,2245. 107. Y.-f. Zhang, X.-f. Ni, Z.-q. Shen, and H. Yasuda, Gaodeng Xuexiao Huaxue Xuebao, 2001,22, 1778. 108. X. F. Ni, Z. Q. Shen, and H. Yasuda, Chin. Chem. Lett., 2001,12,821. 109. Y. Kawaguchi and H. Yasuda, J . Appl. Polym. Sci., 2001,80,432. 110. Y. Kawaguchi and H. Yasuda, 2001, Jp 2001261762. 111. R. Anwander, H. W. Goerlitzer, W. A. Herrmann, K. Dorn, E. Osthaus, and H. Schwind, 2001, De 10010511. 112. Y.-j. Luo, Y.-m. Yao, and Q. Shen, Yingyong Huaxue, 2001,18,392. 113. C . Tsutsumi and H. Yasuda, J. Polym. Sci. A, 2001,39,3916. 114. P. Ravi, T. Grob, K. Dehnicke, and A. Greiner, Macromol. Chem. Phys., 2001,202, 264 1. 115. U. Hohm and A. Loose, Chem. Phys. Lett., 2001,348,375. 116. S. Tobisch, T. Nowak, and H. Bogel, J . Organomet. Chem., 2001,619,24. 117. M. Dolg, J . Chem. Inf: Cornp. Sci., 2001,41, 18. 118. L. Maron and 0.Eisenstein, J . Am. Chem. SOC.,2001,123, 1036. 119. A. Z. Voskoboynikov, A. K. Shestakova, and I. P. Beletskaya, Organometallics, 2001,20,2794. 120. K. Takaki, K. Sonoda, T. Kousaka, G. Koshoji, T. Shishido, and K. Takehira, Tetrahedron Lett., 2001,42,9211. 121. J.-S. Ryu, T. J. Marks, and F. E. McDonald, Org. Lett., 2001,3, 3091. 122. G. A. Molander, E. D. Dowdy, and S. K. Pack, J . Org. Chem., 2001,66,4344. 123. K. Takaki, M. Takeda, G. Koshoji, T. Shishido, and K. Takehira, Tetrahedron

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Lett., 2001,42, 6357. 124. G. A. Molander and D. Pfeiffer, Org. Lett., 2001,3,361. 125. Y. Muhammad, J.-1. Huang, Z.-f. Feng, and Y.4. Qian, Huadong Ligong Daxue Xuebao, 2001,27,211. 126. W.-K. Su, Y.-M. Zhang, and Y.-S. Li, Chin. Chem. Lett., 2001,19, 381. 127. G. B. Deacon, C. M. Forsyth, and D. L. Wilkinson, Chem. Eur. J., 2001,7,1784. 128. Y. Ishii and T. Nakano, 2001, Jp 2001253887. 129. L.-x. Zhang, X.-g. Zhou, Z.-e. Hwang, R.-f. Cai, L.-b. Zhang, and Q.-j. Wu, Jiegou Huaxue, 2001,20,40.

5 Carboranes, Including Their Metal Complexes BY PAUL A. JELLISS

1

Introduction

This review covers the 2001 literature of carboranes and metallacarboranes,* and is formatted in a similar manner to previous years. Section 2 addresses theoretical and computational articles, while Sections 3 and 4 deal with the bulk of carborane and metallacarborane papers, respectively. In Section 3 carboranes are ordered in terms of their C,B, formula (increasing x, y) with metal complexes, M,C,B,, similarly ordered in Section 4. Articles concerning em-metal complexes are listed separately from those for endo-metal species, where the metal forms an integral part of the polyhedral cage structure and bonding. The nomenclature and formulae adopted are those used in the cited articles, though readers should be aware of inconsistencies, e.g. M(q"-nido-C,BJ us. closo-MC,B,. Articles directly related to biological applications, particularly boron neutron capture therapy (BNCT), are covered in Section 5, with any crystal engineering and materialsrelated work in Section 6. The chemical literature has been surveyed using Chemical Abstracts OCLC First Search and I S 1 Web of Science. Two specific reviews have appeared in 2001 on carboranes and related compounds: Alkylated Carborane Anions and Radicals;' Polyhedral Boron Derivatives of Porphyrins and Phthalocyanines? Both are concerned with emerging fundamental applications of carboranes and their metal complexes: the former with carboranes and their derivatives as weakly coordinating anions to counter catalytic cationic metal centers without inhibiting substrate access; the latter with the coupling of carborane moieties with porphyrin or phthalocyanine molecules to give dual BNCT/photodynamic tumor therapy agents. A brief report has appeared in Chemical and Engineering News entitled: Perfluorinated Carborane: A 'Teflon Ball'! Throughout this review, the following key is used to describe the cage vertex atoms in the figures used:

Organometallic Chemistry, Volume 3 1

0 The Royal Society of Chemistry, 2004

112

5: Carboranes, Including Their Metal Complexes

2

113

Theoretical and Computational Studies

Perhaps the simplest possible carborane H2C(H2)BH2 +,an analogue of diborane, has been the subject of ab initio analysis to evaluate the stability of hydrogen atoms bridging C-B conne~tivities.~ Minimum energy structures have been located and characterized by second order MP2 perturbation theory and vibrational energies and infrared absorption intensities also calculated. The formation of the subject molecule has been analyzed by considering the interaction of orbitals between the tetrahedral CH4 molecule and the linear BH2+ fragment. The article concluded by stating that any molecules comprising the CH2 unit should interact favorably with BH2+ and then threw down the challenge that formation of further new cationic and neutral carboranes should be feasible by replacing B atoms, which participate in two 2-centre-2-electron B-H bonds together with two 3-centre-2-electronB-H-B bonds, with C+.As if to echo these sentiments, ab initio calculations at the B3LYP/6-31G* level coupled with NBO charge distribution analysis of the tricarbon series [C3Bn-3Hn]f(n = 5,6, 7, 10, 12) have indicated an increase in stability for larger cage series.6The positive charges are dispersed throughout the cages, making them suitable candidates as weakly coordinating electrophilic cations. The C-H-B bridge bonding mode has also been addressed in a blanket ab initio study at the MP2(fc)/6-31G*level for a plethora of carboranes with nido pentagonal pyramidal ge~rnetry.~ Certain structural features have been assigned specific energy penalties, which could be treated additively and used to predict the relative stabilities of the carboranes with reasonable accuracy compared to MP2 analysis. An ab initio study at the HF/6-3 1G level on icosahedral dicarbon carboranes has verified observations that substitution of CH vertices with heteroatoms (Si, P, Ge, As) leads to longer, weaker B-B bonding due to increased boron electron density and higher boron skeletal antibonding orbital population.' Finally, fluorination of [c~oso-CB&i n d [closo-CB9Hlo]-by H F has been simulated at the B3LYP/6-31G(d) and 6-311 G(2d,p)levels and shown to proceed either ionically (H+,F-)or covalently (H-F) via attack at a BBB deltahedral face, the energies of the two alternative processes being comparable.'

+

3

Carboranes

3.1 {CBI1}. - The ion [~loso-CB~~H1~]has been shown to raise measured detection limits for ion-selective electrodes compared to established lipophilic tetraphenylborate ion-exchange salts, which undergo undesirable leaching from the membrane phase into the sample aqueous solution."

3.2 { CzBs}. - The synthetic, structural, conformational, thermal, and spectroscopic characterization of C-diheptyl and C-diheptynyl derivatives of p-C2B8Hlo have been reported and have revealed significant electronic communication between the cage and acetylenic groups in the latter compared to analogous p-CzBloH~derivatives.* The p-C2BS systems, however, unexpectedly showed

114

Organometallic Chemistry

lower phase transition temperatures than their p-C2BI0counterparts. 3.3 {CzB9}.- The deboronation of o-C2BIOH12 and its C-alkyl derivatives by (R = H, alkyl) has CsF in refluxing ethanol to give [7,8-R2-~zido-7,8-C2B9H10]been reported.12 A similar set of reaction conditions employed for the corresponding rn-carborane have been shown to be ineffective. An ongoing debate regarding the position of the endo-H atom in the anion [nido-7,8-C2BgH12]- has been further tackled by X-ray and neutron diffraction studies of [PSH] + [nido7,8-C2B9H12](PS = proton sponge).13The neutron study revealed asymmetric facial B-H . - - B bridging and this has been supported by MP2/6-31G* ab initio calculations. ~

3.4 { C2BI0}.- This facet of carborane chemistry is dominated almost entirely by the now-ubiquitous closo-C2BI0icosahedral motif. The chemical inertness of the C-H bonds in 1,12-(H)2-closo-1,12-C2Blo(OH)lo has prompted studies to functionalize the carbons with alternative oxidatively resistant groups, namely sulfinates and sulfonates, prior to perhydroxylation of the boron vertice~.'~ This has been achieved by treatment of the dilithiated p-carborane with SO2 to give the sulfinate and subsequently with SO2Cl2and A1Cl3to give the sulfonate. The bis(C-sulfinic acid) and bis(C-sulfonic acid) p-carboranes are then isolated following H + cation exchange and perhydroxylated with 30% H 2 0 2to give 1,12(H03S)2-cZoso-1,12-C2B10(OH)10. Perfluorination of 1,12-(H)2-closo-1,12C2BIOMe10 with F2/N2mixtures at elevated temperatures (> 35 "C) has yielded 1,12-(F)2-closo-1,12-C2Blo(CF3)lo exclusively, while a similar reaction with closo1,12-C2BloMe12 gave inseparable mixtures of closo- 1,12-C2B10(CF3)12, l-CF3-12F-closo- 1,12-C2B10(CF3)10, and 1,l2-(F)2-closo-1,12-C2Blo(CF3)lo.15 The monohalogenated compounds l-R-3-X-closo-1,2-C2BloHlo (R = Me, Ph; X = Br, I) have been reported as have their aryl dehalogenation reactions at the B(3) vertex.16This synthetic work has been supported by density functional (B3LYP) and H F calculations, which show that the iodo species (among F, C1, Br, and I) is the most suitable for derivatization. The C-H vertices of o-carborane have been cyanoethylated with acrylonitrile, in the presence of benzyl(triethy1)ammonium hydroxide, in a two-phase dichloromethane/water or 1,2-dimethoxyethane/water ~ystem.'~ The reaction is ortho-specific and does not transfer to m- or p-carboranes. and N-(2The reaction between Li[ l-Ph-cZoso-1,2-C2BloH~o] bromoethy1)phthalimidein anhydrous toluene has yielded l-phenyl-2-[2,3-benzobicylco[ 3,3,0]- 1-oxo-4-oxa-7-aza-8-y1]- 1,2-dicarba-closo-dodecaborane( 12), the product of an unprecedented coupling reaction (Figure l).'* In related work, Li2[closo-1,2-C2BloH10] has been shown to react with two molar equivalents of N-(bromomethy1)phthalimide in an anhydrous benzene/diethyl ether mixture to produce the expected bis(C-substituted) o-carbor12),which has ane, 1,2-bis(phthalimidomethyl)-1,2-dicarba-closo-dodecaborane( been used as a precursor for polyamine molecule^.^^ X-ray structural characterization of a,a'-bis(2-phenyl-1,2-carboran-lyl)lutidine,formed by treatment of 1-phenyl-1,2-carborane with LiBu" followed

5: Carboranes, Including Their Metal Complexes

115

Br

Figure 1

by 2,6-bis(chloromethyl)pyridine,has revealed a herringbone structural motif, held together by weak (pheny1)C-H - n(pheny1) bonds.20 A kinetic study of the solvolysis of o-, rn-, and p-(2-aryl-closo-carborany1)benzyl-p-sulfonates has revealed that rates of hydrolysis positively correlated with increasingly electron-releasing character of the aryl substituents on the rn- and p-carborane cages, verifying their ability to transmit electronic effects.21A negative correlation has been established for the o-carborane systems and attributed to an interaction between the attacking nucleophile and the cage B(3) vertex. A novel macrocyclic arrangement incorporating two alternate rn-carboranyl and two N,N'-dimethyldiphenylurea groups has been synthesized and structurally characterized by X-ray diffraction? Meanwhile, assemblies of two and three o-(2-phenyl-cZoso-carboranyl)groups bound via their cage carbon atoms to 1,3- and 1,3,5-substituted benzene rings, respectively, have been shown by X-ray structural characterization, to exhibit syn oriented carboranyl phenyl groups, despite anticipated steric crowding.23 Structural characterizations of 3-Ph-~Zoso-1,2-C~B~~H~~ and l-Ph-closo-l,7CZBloH11, which have B- and C-phenyl groups, respectively, have indicated through differences in bond angles about the phenyl ips0 carbon atoms, that there is a discrepancy in electron donation of the carborane cage for the B- and C-substituted moieties.24 In seminal work to employ carboranes as electronic bridges in molecules with significant non-linear optical responses, the compound 1-(4-~ycloheptatrienyl)12-(3,4-dimethylcyclopentadienyl)-cZoso-l,l2-carboranehas been synthesized and characterized as the first such non-metallic bis(o1efin)-substitutedcage sysThe ultimate target of this work is the promising zwitterionic tropylium-

-

116

Organometallic Chemistry

carborane-cylcopentadienide compound 1-(1-C7H&12-(CsH2-3,4-Me2)-closo1,12-C2BloHlo(Figure 2). Compared to classical aryl bridges, which act as efficient electronic conduits, the carborane cage is expected to limit electronic redistribution sufficiently to give a highly polarized cluster molecule. Me Me

Figure 2

The acyclic species O - B ~ ~ H ~ ~ C ~ has ( S ~been H Mshown ~ ~ ) ~to react, in the presence of a nickelacarborane catalyst, with propionitrile to give an N-silyl enamine.26In general, use of nitriles with no a-Hatoms resulted in the formation of six-membered cyclic imines, while other nitriles with a-H atoms afforded N,N-bis(sily1)enamines. An unusual exception involved reaction with 9-anthracenecarbonitrile to give a five-membered N,N-bis(sily1)aminevia a sequential hydrosilylation of the nitrile C=N bond. The benzyne analog, 1,2-dehydro-o-carborane, continues to attract attention. An efficient precursor, phenyl[o-(trimethylsilyl)carboranyl]iodonium acetate, has been reported and used for the synthesis of benzyne-type 2 + 4 cycloadditions with dienes such as anthracene, naphthalene, and thiophene, in the presence of a desilylating reagent.27The same authors have used the o-carboranyl-disilacyclobutene, B10H10C2(SiEt2)2, as a valuable reagent for palladium-catalyzed oxygen, sulfur, aldehyde, and alkyne insertions into the Si-Si bond.28The same molecule has also undergone a C-H bond activation when refluxed in benzene to give 1-(diethylphenylsilyl)-2-(diethylsilyl)-c~o~~-1,2-carborane, but with trans-cinnamaldehyde, double insertion into a cage C-Si bond has occurred to give 6,7carboranylene- 1,5-bis(styrenyl)-2,4-dioxa-3,3-diethyl-3-silacyclohept-6-ene (Figure 3).

0

\

/””

0

Figure 3

5: Carboranes, Including Their Metal Complexes

4

117

Metallacarboranes

4.1 {MCBlo}.- A series of experiments have been carried out to functionalize the cage boron atoms in the coordinating face of the anionic metallacarboranes [2,2,2-(C0)3-2-L-closo-2,1-MCBloHll]2(M = Mo, W; L = CO, PPh3, CNBU').~~ The hydridic nature of exopolyhedral BH hydrogen atoms has permitted hydride abstraction by addition of strong acid or methylating agent, followed S-, N-, or C-donor molecules. Most intriguing are the by attachment of 0-, nitriles and isonitriles, which have yielded mixtures of N-donor cis- and transiminium substituents on a p-boron vertex (Figure 4).

0p3

H

-N

R'

M=MO,W

R' H Me

R2 R3

Me MeorBu'

H

MeorBu'

Figure 4

4.2 {eno-MCBl,}. - The utility of closo-CBll monoanions as weakly coordinating ligands has been further exploited by use of the silver complex [(PPh3)Ag(CB11H6Y6)] (Y = H, Br) to catalyze a hetero-Diels-Alder reaction between N-benzylidene and Danishefsky's diene?' and also the bis(oxazowhich has 1ine)rhodiumcomplex, [Rh(COD){(NC3H3B~t0)2CMe2}][CBllH12], catalyzed the addition of arylboronic acids to aldehydes with high activity at low catalyst loading compared to traditional counterions such as PF6-and BF4-.31 The silver complex has been structurally characterized and the Ag(PPh3) fragment found to bind to the cage by three exo B-H Ag interactions. A series of molybdenum-silver complexes employing the carboranes [cIoso-CBl1Hl2] and [ c l ~ s o - C B , ~ B r ~have H ~ ] ~been isolated.32The former cage binds to silver or cyclopentadienylmolybdenum centers via B-H 3-electron-2-centre agostic bonds, while the latter functions as a weakly coordinating 4-electron donor through its exopolyhedral Br atoms.

-

4.3 { MC2B2). The ruthenacarborane [1,2-(C5Me5RuH)2-3,4CHC(C02Me)B2HiJ has been synthesized by cage cluster degradation of the ruthenaborane [l,2-(C5Me5R~H)2-B3H7], concomitant with expansion by addition of the alkyne H C P C C O ~ M ~ . ~ ~

4.4 { MC2B3}. - The use of metalladicarbapentaborane-derived building blocks for the construction of molecular wires has been described.34Thus, through substituent alkyne coupling, the complexes 1,4-[(nido-Et2C2B3Hs)Co(q5C5Me4)-C=CI2C6H4,(nido-Et2C2B3H5)Co(q5-C5Me4)-C=C-Pd(NHEt2)2CI, and

118

Organometallic Chemistry

1,4-[(rl5-CsMes)Co(~ido-Et2C2B~H3)(C-CSiMe3~C~C]2C~H4 have been synthesized. The last of these is characterized by alkynyl groups directly bonded to the carborane cages, which gives rise to meso and d,l stereoisomers (Figure 5). Me

M Me

Figure 5

4.5 {MC2B4}and {MC2Bg}.-The syntheses and X-ray crystal structures of the first half-sandwich ‘carbons-apart’ gadolinacarborane, { 1,1-C12[p,p’Na(TMEDA)] - 1-(THF)-2,4-(SiMe3)2-cEoso-r151-Gd-2,4-C2B4H4}2, and full-sandwich ‘carbons-together’ ceracarborane, [Li(TMEDA)*][ 1-C1-l-(p-C1)-2,2’,3,3’(SiMe3)4-5,6-[( p-H)2Li(TMEDA)]-4,4’,5’-[(P-H)~L~(TMEDA)]1, 1’-cornrnoCe(2,3-C2B4H4)2],have been r e p ~ r t e d . ~ ’ . ~ ~ with [RuC12(PPh&] have reToluene-refluxed solutions of nido-5,6-C2B8HI2 sulted in the degradative removal of four BH vertices from the carborane to give [l,1-(PPh~)~-1-H-l-Cl-closo-1,2,3-RuC2B4H~] as the major product, albeit in low (17 %) yield.37Two other minor products, [n-C1-1-(q6-C6H5Me)-1,2,4-RuC2B8H9 ( n = 3, 6) have also been isolated from this reaction in even lower yields. The major product is believed to be formed by a series of metal-promoted BH vertex excisions accompanied by molecular H2 elimination.

4.6 {MC2B9}.- The employment of cobaltacarborane sandwich complex anions in a variety of structural roles and applications in sensor technology continues to be an ongoing theme of research. In particular, polymeric (33 % PVC, 64 YO2-nitrophenyl octyl ether) ion-selective electrodes, specific for alkali metal ions, have been shown to function more selectively using [ C O ( C ~ B ~ H ~as~a) ~ ] lipophilic coreceptor than currently used tetraarylborate ion additives.’8 Not only do the electrodes demonstrate improved sensor capability, but also a low level of leaching into the aqueous phase of the sample medium has been observed. Treatment of the imido-amido complex [W(NBU‘)~(NHBU‘)~] with nid0-7,8C2B9HI3 has yielded a stable, high valent complex [W(NBU‘)(NHBU~)~(C~B~H~ ,)I, itself a precursor to several amido-substituted species.39Most notable is the product of acetonitrile insertion, the amidine { W(NBu‘)2[N(H)C(Me)NHBut] (C2B9Hll)}(Figure 6), which comprises a distorted six-membered ring with one imido tethered to the amidine by an unusual intramolecular N-H - - N hydrogen bond.

5: Carboranes, Including Their Metal Complexes

119

Figure 6

The family of bidentate q5:q1or q5:acarboranyl metal complexes has been further expanded by the synthesis of [(C2B9H1~-CH2NMe2)FeL2] (L = CO, CNBu', PMe3;L2 = COD) and [(q5:q1-C2B9Hlo-CHzNMe2)RuL(C0)] (L = CO, NCMe) by routine cage degradation of 1-NMe2CH2-cEoso-1,2-C2Bl0Hllfollowed by facile metal insertion.4O While these cages bear C-bound G bonded amino groups, the q5cages in the constrained geometry compounds l-M[(Cl)(THF),]2-(l-o-NHCH2)-cZoso-2,3-q5-C2B9HIo (M = Zr, n = 1; M = Ti, n = 0) carry G donor amido ligands on their carbon The incorporation of planar chiral carborane ligands into enantioselective rhodium hydrogenation catalysts has been achieved with the formation and resolution of bis(phosphine)[exo-nido-(R or S)-(7-diphenylphosphanyl-8-phenyl7,8-dicarbundecaborato)]rhodium, using chiral and achiral bis(phosphine) ligands, via internally diastereotopic palladium complexes of the carboranyl anion?2 Also in the platinum metal group, treatment of the COD complex [{ Rh(C8H12)C1}2]with the sterically encumbered Na2[7,8-Ph2-7,8-nido-C2BgHsI has yielded two conformers of [1,8-Ph2-2-(1-3-q3-:5,6-q2-C8Hll)-closo-2,1,8RhC2B9H9]and an unexpected tetramer [{ (HO)RhPh2C2B9H9}4]which comprises an &-symmetrical cubane-like { Rh4(p3-OH)4}central c0re.4~The formation of both products involved low temperature polyhedral 1,2 -+ 1,7 carbon vertex isomerization. Incipient cage C-C separation has also been demonstrated in derivatives of the pyrrolyl carboranyl complex [3-Co(q5-NC4H4)l,2-Rz-closo1,2-C2B9H9],particularly with sulfide derivatives (R = SMe, SPh) on the cage carbon atoms.44Such plasticity of the C-C connectivity has been attributed to sulfur lone pair donation into a cage LUMO orbital, which is antibonding with respect to the carbon vertices. This has lead to a decrease in C-C bond order, and consequently an increase in C-C bond distance, and has challenged the notion that steric effects alone control C-C bond lengthening. The complex C S [ R ~ ( C O ) ~ ( $ - ~ , ~ - C ~ Bpreviously ~ H ~ ~ ) ] , thought to be somewhat chemically unreactive, has yielded further neutral adducts upon combination with [AgL]+ cations [L = bipyridyl, terpyridyl, tetramethylethylenediamine, bis(pyrazolyl)methane, and tris(pyrazolyl)methane]. In all cases the cages have been shown to bridge the Re-Ag bonds in the solid state structure^.^^ The tris(pyrazoly1)methane ligand has additionally been

Organornetallic Chemistry

120

shown to adopt an unusual K ' , K ~ bridging mode between two [ReAg(p-10-H-q57,8-C2B9H10)(C0)3] fragments, irrespective of ligand-to-metal ratio during synthesis (Figure 7). In the absence of any N-donor ligand on the silver, a novel tetrameric complex [R~A~(~P~,~,~O-(H)~-~~-~,~-C~B~H~)( has been shown to assemble from CHzClzsolutions of C S [ R ~ ( C O ) ~ ( ~ ~ - ~ , ~ - C and ~ BAgBF4. ~HI~)]

H H

H I C

Figure 7

A

series of

binuclear

sulfide-bridged metallacarborane complexes, = 0, -1, -2) have been systematically synthesized and Reversible stepwise reduction from the neutral species to the dianion can be achieved chemically and has been shown to lengthen and ultimately cleave carborane C-C bonds to give semi-closo cage frameworks (Figure 8).

[(C2B9H11)M(p-SPh)2]2n(PPN)~nl (M = Mo, W; n

2-

PhS phs@: W

A C-Cbondedcage carbon

B

c-c partially cleaved -9

carban

Figure 8

C

5: Carboranes, Including Their Metal Complexes

121

4.7 { exo-MC&}. - The exo-nido osmacarborane complex, [C1(Ph3P)20s5,6,10-([p]-H)3- 10-H-7,8-C2B9H8], has been synthesized by reaction of [ O S C ~ ~ ( P Pwith ~ ~ )K[7,8-R2-C2B9H10] ~] (R = H, alkyl). The O S C I ( P P ~frag~)~ ment is bound to the cage through three B-H- 0 s interactions, although a mixture of isomers results, depending on whether the phosphine ligands are cis or trans 0riented.4~It has been reported that addition of bromine to the related ruthenacarborane complex [Cl(Ph3P)2Ru-5,6,lo-([p]-H)3- 1O-H-7,8-C2BgH*] surprisingly did not result in cage boron bromination, but rather yielded the bromo-substituted analogue [Br(Ph3P)2Ru-5,6,lO-([p]-H)3- 10-H-7,8-C2BgH8].48 4.8 { MCzBlo). - The first structurally characterized full-sandwich potassacarborane [( 1~ - C ~ O W ~ - ~ ) K ( C H [ex.-{~ C N p-)1,2-[o-C6H4(CH2)2]-~ho~] 1,2C2BloHlo}2K3( 18-Cr0wn-6)~]has been synthesized by treatment of the closocarborane p-1,2-[o-C6H4(CH2)]-1,2-C2B10H10 with excess potassium in THF?9 The key feature of the structure is the q6bonding of one of the potassium atoms to each of the two cage CCBBBB faces, with the cage carbons notably adjacent. The synthesis and structural characterization has been reported for [{ p-1,2[ o - C ~ H ~ ( C H ~ ) ~ ] - ~ ~ ~ ~ ~ ~ ~ - ~ the ,~tetraanionic C ~ B ~ ~ Hcage ~~} of ~ L ~ ( T which has bothB-C hexagonal and m B pentagonal open faces, fused about the same C-C connectivity and bound to lithium ions (Figure 9).” The isolated sodium salt, by contrast, displayed only a nido structure with a ‘carbonsadjacent’ CCBBBB hexagonal face.

Figure 9

A samarium-mediated C-C oxidative coupling of two carborane cages has been achieved by treatment of [Me2Si(C9H5CH2CH2G)(closo-C2B~oH1o)] Li2(0Et2)2sLiCl (G = NMe2, OMe) with Sm12(THF)x.51 The resulting product consists of two Sm atoms, each q6bound to two ‘carbons-apart’ nido-C2BloHlo cage m B faces, conjoined by a direct cage-C-cage-C bond (Figure 10). A novel ytteracarborane sandwich complex has been reported, which contains ligands with the cages Yb(II1) centers between two [q7-arachno-R2C2BloH10]4-

Organometallic Chemistry

122

Figure 10

subtended at an angle of 160.2 to give a bent sandwich A further Yb(I1) metal center engages in agostic bonding to eight B-H bonds, two each from four carborane cages, which belong to two adjacent ytteracarborane sandwich molecules, an unusual coordination mode for lanthanides. O

4.9 {exo-MC2Blo}.- The reversible oxidative addition of the B-I bond of 9-

iodo-m-carborane to [Pd(PPh3),] (n = 3, 4) has been shown to be reactantfavored in THF, but nevertheless, leads to substitution reactions such as halide I n situ formation of [Pd(PPh&] by decomposition of phenylpale~change.’~ ladium formato complexes in the presence of this iodo-carborane, however, has lead principally to the synthesis of m-carborane and iodophenylbis(tripheny1phosphine)palladium. Treatment of the dianionic salt [Li(THF)4] [{ [(p-qS):o-Me2Si(CsMe4)(C2B10H10)]Li(THF)}2Li].THF with a variety of lanthanide chlorides (Sm, Nd, Yb) has produced a number of lanthanacarboranes, primarily with the cyclopentadienide fragment q 5coordinated to the lanthanide and the tethered carboranes o coordinated to the metal center via the cage carbon vertex.s4 Related constrained geometry cyclopentadienyl and indenyl carborane molecules Me2(R)A-closo-l,2-C2BloH1 l (A = C, Si; R = CsHs,C9H7)have undergone reaction with Z T ( N E ~and ~ ) ~Ti(NMe& to yield precursors to highly active polyethylene catalysts.55 Nickel-mediated double germylation of alkene, alkyne, and nitrile substrates has been achieved using the complex [o-(GeMez)zCzBloHlo]Ni(PEt3)2.s6 Ethynylferrocene molecules have been coupled to give a mixture of (E)- and (2)-1,4diferrocenylbuten-3-yne by the o-carboranyl rhodium complex, Rh(CSMeS)[Se2C2(B,oHlo)], while in the presence of the analogous sulfido complex, Rh(CSMeS)[S2C2(BloH10)],insertion of one of the ferrocene molecules into the Rh-S bond has been observed, followed by intramolecular hydroboration across the metal center.57The 16-electron complexes Ir(CSMes)[E2C2(B1~H10)] (E = S, Se) have undergone selective stepwise substitution at the cage B(3,6) vertices, resulting in insertion of acetylenecarboxylate into one of the Ir-chalco-

5: Carboranes, Including Their Metal Complexes

123

gen bonds.58Furthermore, Ir-induced B-H activation can occur to give an Ir-B o bond to the cage. The employment of p-carborane as an electronic linker has been investigated with the synthesis of the tricluster complex [{C~C~(SiMe~)(CO)~(dppm)}~(pCBloHloC)](Figure 1l).59 Cyclic and squarewave voltammetry experiments have revealed a significant degree of electronic communication between the acetylidebridged Co2cluster termini. 0

o

c

SiMe3

Figure 11

4.10 (MC3B7). - A variety of complexes incorporating the tricarbollide ligand, [6-Me-nido-5,6,9-C3B7H9]-,have been isolated and structurally characterized.60 Most notable was the isolation of five isomers of the vanadium sandwich complex V(C3B7H9)2 following treatment of VBr2with two molar equivalents of Li[6-Me-nido-5,6,9-C3B7H9]in refluxing toluene.

5

Biological Carborane Chemistry and BNCT

The reaction of Li[ l-ButMe2Si-1,2-C2BloHlo] with N-(bromoalky1)phthalimides has produced carboranyl heterocycles.6l For the bromoethyl and bromopropyl congeners, subsequent insertion of the phthalimide carbonyl into the carboraneC-Si bond has also occurred. These molecules are precursors to aminoalkyl closo-C2Bloand nido-C2B9BNCT candidates. The union of photodynamically active porphyrins with readily functionalized boron-rich carboranes has lead to promising oncologically therapeutic materials for a new generation of BNCT agents. The syntheses of 3- and/or 4-substituted o-carboranylpyrroles have been described, in addition to their tetramerization to yield mixtures of P-carboranylporphyrins.62The first structurally characterized covalently bonded carboranylporphyrin species has been reported and afforded unusual intermolecular B-H - Zn - - H-B bonding to give a pseudo-hexacoordinate zinc center (Figure * *

12).63

Interactions of these carboranylporph yrins, specifically meso-tetrakis[n-(nidocarborany1)phenyllporphyrin (n = 3,4) with DNA have been investigated using electronic absorption spectroscopy and resonance light scattering experiment^.^^ Carboranylporphyrins have also been synthesized by reaction of the carboxylic

124

OrganometaZlic Chemistry

Me

I

Figure 12

acid groups of deuteroporphyrin IX and protoporphyrin IX with l-hydroxymethyl-closo-monocarboncarborane c a e s i ~ m The . ~ ~ resulting dianionic salts, 1,3,5,8-tetramethy1-6,7-di[2’-(cEoso-monoca~boncarborane1’-yl caesium)methoxycarbonylethyl]porphyrin and 1,3,5,8-tetramethyl-2,4-divinyl6,7-di[2’-(closo-monocarboncarborane-l’-yl caesium)methoxycarbonylethyl] porphyrin are expected to show improved water-solubility, thus increasing their pharmaceutical value. Syntheses of the carboranyl amino acids, 1-amino-3-[2{ 7-( 6-deox y-x-galactopyranos-6-y1)- 1,7-dicarba-closo-dodecaboran( 12)-1yl}ethyl]cyclobutanecarboxylic acid (x = alpha, beta), have also been reported and improved aqueous solubility is again expected to augment their therapeutic value.66Novel carboranyl isonitrile derivatives, 3-(isonitrile)-l,2-dicarba-closododecaborane(12) and 1-(isonitrilemethyl)-1,2-dicarba-closo-dodecaborane(12) have been synthesized for the purpose of coordination to transition metals to give materials for radiopharmaceutical BNCT and synovectomy treatment^.^^ Carboranes have not been limited in their biological role as BNCT agents. The dicarba-closo-dodecaborane moiety has been employed as a potent estrogen agonist by virtue of its spherical hydrophobic structure.68A luciferase reporter gene assay revealed a ten-fold improvement in activity of the most potent of these species over that of 17P-estradiol. The molecule 4-[4-(2-propyl-l,2-dicarbacloso-dodecaboran-1-yl)phenylamino]benzoic acid, a recognized retinoidal agonist, has been the subject of a structure-activity study, which suggested that the planarity of the phenyl-N-phenyl moiety is critical to its biological functiona l i t ~ The . ~ ~same authors have experimentally determined the Hansch-Fujita hydrophobic parameters for a variety of pharmacophoric dicarba-closododecaboranes using an HPLC method.’O A biomimetic tyrosine analogue,

5: Carboranes, Including Their Metal Complexes

125

3-[ 1-hydroxy-1,12-dicarba-cZoso-dodecaboran( 12)-12-yllpropionic acid, has been synthesized and shown to behave as a hydrophobic surrogate for N terminal tyrosine residues in insect and mammalian neuropeptides?' The carborane moiety has been used as an environmentally benign non-aromatic hydrophobic constituent in amphiphilic pseudopeptide analogues of insect pyrokinin/PBAN ~ e p t i d eThis . ~ ~ could lead to the possibility of replacing harmful bezenoid components in insecticides. Two diamminechloroplatinum (11)centres have been linked by a bridging 1,7-bis(3-aminopropy1)-1,7-carborane ligand.73 DNA binding studies have shown improved inhibition of plasmid DNA cleavage by Eco RI restriction endonuclease compared to cis-[PtCl2(NH&]. 6

Crystal Engineered Supramolecular and Polymeric (Metal1a)carborane Materials

The class of mercuracarboranes continues to expand and provide new examples of self-assembledmicroporous solids. In particular, the X-ray crystal structure of Li2[(HgC2BioH812)4-12] has been The structure consists of four divalent 9,12-Iz-1,2-C2BloHgcages linked at carbon by four Hg atoms in a cyclic tetramer, with iodide ions located above and below the tetramer cavity. The assembly is directed by novel B-I - - Li - ..I-B linkages and results in microporous channels, which can be occupied by small electron-rich guest molecules. A relatively simple species, 1-carboxy-1,2-dicarba-closo-dodecaborane(11) has been engineered to give centrosymmetric dimers, held together by hydrogen bonding between the carboxylic acid groups, resulting in eight-membered rings.75The crystalline complexes (1,2-dicarbadodecaborane(12))(1,10-phenanthroline)i,5and (1,2-dicarbadodecaborane( 12))2(1,2-dimethoxybenzene) have been grown from toluene solutions, and both display bifurcated cage C-H hydrogen bonding within the 1,2 arrangement of acceptor atoms from the planar aromatic m01ecules.~~ In conjunction with the calixerene-type molecule cyclotriveratrylene and alkali metal ions, the closo carboranes and [CB11H12]-have yielded a collection of similar inclusion complexes, held together by a two-dimensional network of extended coordinate and hydrogen bonding intera~tions.7~ All the structures described comprise intracavity complexation of alkali metal cations and guest DMF molecules by the cyclotriveratrylene, with the carborane cages occupying the same channels within the structure, irrespective of whether they are charged or neutral. Together with cationic (MC [2.2.2]cryptate)+ (M = Na, K), anionic cobaltacarborane sandwich molecules, [CO(C~B~H&]-,have been encapsulated within a multicomponent supramolecular array of macrocyclic 5,7,12,14-tetramethyldibenzo[b,i]- 1,4,8,11-tetraazacyclotetr adecinenickel (11) heterotopic receptor^.^' The network is characterized by non-classical carborane C-H - - - TC hydrogen bonds to the aromatic rings of the receptor Ni complex. Both [CB11HI2]- and [ C O ( C ~ B ~ H ~ions, ~ ) ~respectively, ]have been engineered to give crystalline materials with cyclotriveratrylene and (NaC [2.2.2]cryptate)+ .79 The cobaltacarborane molecules, in particular, stack in columns along one crystallographic axis and have revealed short intercage C-H - - - H-B dihydrogen bond*

126

Organometallic Chemistry

ing. Tetrahedral (cyclotriveratrylene)4clusters with acetonitrile host-guest interactions and surrounded by an adamantoidal assembly of [Sr(H20)8]2+complex ions, have been shown to network together with a-Po topology.80The resulting rectangular channels are occupied by [Co(C2B9H12)2]ions. Pairs of Mo2(formidinate)3 units have been linked by a multitude of organic hyperconjugated and saturated groups, including p-carborane-C-dicarboxylate and studied by X-ray structural characterization, cyclic and differential pulse voltammetry.**The same carborane moiety has been used to build supramolecular squares with Mo;+ corners, which in the solid state assemble to give interstitial sites capable of accommodating solvent Carboranes have been shown to play a role in synthetic polymer chemistry with the grafting of methylenebutanedioic acid to low-density polyethylene during the course of reactive extrusion, a process initiated by carboranyl peroxi d e ~ . * ~A dissymmetric hafnacarborane complex, [Hf (q5:q1-C5H4CMe2CB10HIOC)2] has been embedded in a hole-transporting polymer and shown to be both mechano- and electr~luminescent.~~ Octamethylated 0- and rn-carboranes have been functionalized to give Caminopropyl and C-aminobutyl groups, respectively, by a series of simple steps with a view to creating amphiphilic carboranes, which will be modules for supramolecular c o n s t r ~ c t i o nIn . ~ ~particular, ultrasonication of freshly filtered solutions of the hydrochloride derivative of the C-aminobutyl-B-octamethyl-ocarborane gave self-organized supramolecular aggregates up to 1 pm in diameter, which are stable for at least five days at ambient temperature.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14.

For the previous review in this series, see: A. S. Weller in Organometallic Chemistry; ed. M. Green, Vol. 29, The Royal Society of Chemistry, Cambridge, 2001. B. T. King, I. Zharov and J. Michl, Chern. Inov., 2001,31,23. V. I. Bregadze, I. B. Sivaev, D. Gabel and D. Wohrle, J . Porphyrins Phthalocyanines, 200 1,5,767. Science Concentrates, Chem. Eng. News, 2001,79,47. L. E. MacDonald and G. I. Gellene, Mol. Phys., 2001,99,377. E. D. Jemmis, M. Ramalingam and E. G. Jayasree, J . Cornp. Chern., 2001,22,1542. M. Hofmann, M. A. Fox, R. Greatrex, P. v. R. Schleyer and R. E. Williams, Inorg. Chem., 2001,40,1790. M. Zhang and Y. Zhao, J . Mol. Struc. Theochern, 2001,73,5582. M. L. McKee, Inorg. Chem., 2001,40,5612. M. Telting-Diaz and E. Bakker, Anal. Chem., 2001,73, 5582. P. Kaszynski, S. Pakhomov, K. F. Tesh and V. G. Young, Jr., Inorg. Chem., 2001, 40,6622. J. Yoo, J.-W. Hwang and Y. Do, Inorg. Chem., 2001,40,568. M. A. Fox, A. E. Goeta, J. A. K. Howard, A. K. Hughes, A. L. Johnson, D. A. Keen, K. Wade and C. C. Wilson, Inorg. Chem., 2001,40,173. A. Herzog, C. B. Knobler and M. F. Hawthorne, J . Am. Chem. SOC., 2001, 123, 12791.

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H. Brunner, A. Apfelbacher and M. Zabel, Eur. J. Inorg. Chem., 2001,4,917. B. E. Hodson, D. Ellis, T. D. McGrath, J. J. Monaghan, G. M. Rosair and A. J. Welch, Angew. Chem., Int. Ed. Engl., 2001,40, 715. 44. J. Llop, C. Vifias, F. Teixidor, L. Victori, R. Kivekas and R. Sillanpaa, OrganometalEics, 2001,20,4024. 45. D. D. Ellis, J. C. Jeffery, P. A. Jelliss, J. A. Kautz and F. G. A. Stone, Inorg. Chem., 2001,40,2041. 46. J.-W. Hwang, J.-H. Kim, H. Lee, H. Lee, S. Kim, J. Kwak and Y. Do, J . Am. Chem. Soc., 2001, 123,9054. 47. S. V. Timofeev, I. A. Lobanova, P. V. Petrovskii, Z. A. Starikova and V. I. Bregadze, Russ. Chem. Bull, 2001,50,2245. 48. G. D. Kolomnikova, P. V. Petrovskii, P. V. Sorokin, F. M. Dolgushin, A. I. Yanovsky and I. T. Chizhevsky, Russ. Chem. Bull, 2001,50,706. 49. G. Zi, H.-W. Li, Z. Xie, J. Chem. Soc., Chem. Commun., 2001,1110. 50. G. Zi, H.-W. Li and Z. Xie, Organometallics, 2001,20,3836. 51. S. Wang, H.-W. Li and Z. Xie, Organometallics, 2001,20,3624. 52. S . Wang, H.-W. Li and Z. Xie, Organometallics, 2001,20,3842. 53. W. J. Marshall, R. J. Young, Jr. and V. V. Grushin, Organometallics, 2001,20, 523. 54. G. Zi, Q. Yang, T. C. W. Mak and Z. Xie, Organometallics, 2001,20,2359. 55. H. Wang, Y. Wang, H.-W. Li and Z. Xie, Organometallics, 2001,20,5110. 56. J. Lee, C. Lee, S. S. Lee, S . 0.Kang and J. KO,J. Chem. Soc., Chem. Commun., 2001, 1730. 57. M. Herberhold, H. Yan, W. Milius and B. Wrackmeyer, J. Organomet. Chem., 2001, 623, 149. 58. M. Herberhold, H. Yan, W. Milius and B. Wrackmeyer, J. Chem. Soc., Dalton Trans., 2001, 1782. 59. M. A. Fox, M. A. J. Paterson, C . Nervi, F. Galeotti, H. Puschmann, J. A. K. Howard and P. J. Low, J. Chem. Soc., Chem. Cornrnun., 2001,1610. 60. M. D. Wasczak, Y. Wang, A. Garg, W. E. Geiger, S. 0.Kang, P. J. Carroll and L. G. Sneddon, J. Am. Chem. Soc., 2001,123,2783. 61. A. S. Batsanov, A. E. Goeta, J. A. K. Howard, A. K. Hughes and J. M. Malget, J. Chern. Soc., Dalton Trans., 2001, 1820. 62. S . Chayer, L. Jaquinod, K. M. Smith and M. G.H. Vicente, Tetrahedron Lett., 2001, 42,7759. 63. M. G. H. Vicente, D. J. Nurco, S . J. Shetty, C. J. Medforth and K. M. Smith, J. Chem. Soc., Chem. Commun., 2001,483. 64. R. Lauceri, R. Purello, S. J. Shetty and M. G. H Vicente, J. Am. Chem. Soc., 2001, 123,5835. 65. V. A. Ol’shevskaya, R. P. Evstigneeva, V. N. Luzgina, M. A. Gyul’malieva, P. V. Petrovskii, J. H. Morris and L. I. Zakharkin. Mendeleev Cornrnun., 2001,1, 1. 66. B. C. Das, S. Das, G. S. Li. W. L. Bao and G. W. Kabalka, Synlett, 2001,9,1419. 67. J. F. Valliant and P. Schaffer, J. Inorg. Biochem., 2001,85,43. 68. Y. Endo, T. Iijima, Y. Yamakoshi, H. Fukasawa, C. Miyaura, M. Inada, A. Kubo and A. Itai, Chem. Biol., 2001,8,341. 69. Y. Endo, T. Iijima, K. Yaguchi, E. Kawachi, N. Inove, H. Kagechika, A. Kubo and A. Itai, Bioorg. Med. Chem. Lett., 2001,11, 1307. 70. K. Yamamoto and Y. Endo, Bioorg. Med. Chem. Lett., 2001,11,2389. 71. I. Ujvary and R. J. Nachman, Peptides, 2001,22,287. 72. R. J. Nachman, P. E. A. Teal and I. Ujviry, Peptides, 2001,22,279. 73. S. L. Woodhouse and L. M. Rendina, J. Chem. Soc., Chem. Comrnun., 2001,2464. 42. 43.

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74.

75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85.

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H. Lee, C. B. Knobler and M. F. Hawthorne, Angew. Chem.,Int. Ed. Engl., 2001,40, 2 124. A. J. Welch, U. Venkatasubramanian, G. M. Rosair, D. Ellis and D. J. Donahoe, Acta Cryst., 2001, C57, 1295. M. J. Hardie and C. L. Raston, Cryst. Eng. Commun.,2001,39. M. J. Hardie and C. L. Raston, Cryst. Growth Design, 2001,1,53. M. J. Hardie, N. Malic, C. L. Raston and B. A. Roberts, J. Chem. SOC., Chem. Commun.,2001, 865. M. J. Hardie and C. L. Raston, J . Chem. SOC.,Chem. Commun.,2001,905. M. J. Hardie, C. L. Raston and A. Salinas, J. Chem. SOC., Chem. Commun., 2001, 1850. F. A. Cotton, J. P. Donahue, C. Lin and C. A. Murillo, Inorg. Chem., 2001,40,1234. F. A. Cotton, C. Lin and C. A. Murillo, Inorg. Chem., 2001,40,478. Yu. M. Krivoguz, A. P. Yuvchenko, T. D. Zverava and S. S. Petrovskii, Russ. J. Appl. Chem., 2001,74,845. E. Hong, H. Jang, Y. Kim, S. C. Jeoung and Y. Do, Adu. Mater., 2001,13,1094. A. Maderna, A. Herzog, C, B. Knobler and M. F. Hawthorne, J. Am. Chem. SOC., 2001,123,10423.

6

Group 111 - B, Al, Ga, In, TI SIMON ALDRIDGE

1

General

This account covers highlights from the chemical literature in the year 2001, and has been compiled from a review of papers published in the primary and review literature. The scope of organometallic chemistry of group 13 is immense with vast uses in organic synthesis stemming for example from the use of hydroboration in functionalization chemistry, of Suzuki-Miyaura and Barbier couplings in C-C bond forming reactions and of tri-coordinate group 13 species in Lewis acid catalysis. Similarly, the use of boron containing Lewis acids in the activation of olefin polymerisation catalysts continues to attract much attention. Given the scope for exploitation of group13 organometallics, a comprehensive review is beyond the scope of this report, and some topics (e.g. the use of boranes in hydroboration chemistry, group 13 hydrides as reducing agents and arylboronic acids in coupling reactions) have been touched on only where significant new developments have been reported. A number of reviews covering topics within the remit of this report have appeared during the year 2001. These include reports on indium trihydrides,' organometallic compounds containing the extremely bulky -C(SiMe3)3ligand,' main group cyclopentadienyl complexe~,~ the chemistry of Group 13/15 compounds (111-V compounds) with the higher homologues of group 15, Sb and Bi? perthe chemistry of 1,3,2-diazaborolines (2,3-dihydro-lH-1,3,2-diazaboroles)~ fluoroalkyl borates,6 transition metal complexes of b o r ~ n heavier ,~ group 13 metal 'ate' complexes: amido compounds of gallium and indium,' group 13 compounds incorporating Salen ligands," hydrides of the main group metals," organoboranes as radical sources," the interaction of water with group 13 organometallic boron-bridged group 4 ansa-metallocene complexes,I4the B-alkyl Suzuki-Miyaura coupling r e a ~ t i o n ,multiple '~ bonds involving aluminium and gallium atoms,16 and sub-valent aluminium and gallium cluster~.'~

2

Boron

2.1

B(C6F& and Related Boranes. - Bochmann and co-workers have reported

Organometallic Chemistry, Volume 3 1 0 The Royal Society of Chemistry, 2004 130

6: Group I I I

- B,

Al, Ga, In, TI

131

that warming mixtures of (CpR)Zr(q3-C4H,)(q4-C4H6) [CpR = C5H3(SiMe3)2, C5H5, C5H4Me or C5H4SiMe3]and B(C6F5)3 leads to complete transfer of all three C6F5 substituents of a B(C6F5)3 molecule to give borole-bridged triple-decker complexes with a Zr2C4Bcore, a zwitterionic structure and an unusually strong Zr-F donor interaction." The reactions of titanium and zirconium complexes of the type (q5-C5Me4SiMe2NtBu)M(diene) (1,3-diene = 1,3-butadiene, 2-methyl1,3-butadiene, 2,3-dimethylbutadiene, 1,3-pentadiene, 2,4-hexadiene) with B(C6F5)3 or A1(C6F5)3have also been reported. Such reactions yield zwitterionic borate or aluminate products typified by (q5-C5Me4SiMe2NtBu)Zr[q3CH3(CH)3CH2B(C6F5)3](1) formed from the zirconium 1,3-pentadiene precursor and B(C6F5)3.I9Ion pairs of the type Cpx2ZrMe+ A- containing various ansazirconocene methyl cations in contact with MeB(C6F& or B(C6F5)4-anions have been studied with regard to their anion exchange kinetics in benzene or toluene solutions. These results imply that anion exchange occurs by way of ion quadruples or higher ionic aggregates, rather than via dissociation to solventseparated ions. These findings imply that solvent-separated (i.e. anion-free) alkyl zirconocene cations are unlikely to be relevant intermediates in reaction systems containing Cpx2ZrMe+- A- ion pairs and, hence, also in zirconocene-based catalyst systems for the polymerization of a-olefins.20 The use of tris(pentafluoropheny1)borane as a one-electron oxidant has been investigated both from a fundamental structural perspective and in synthetic organometallic chemistry. The anion derived from B(C6F5)3 has been detected by EPR, and the boron and fluorine hyperfine splitting constants have been determined. Treatment of B(C6F5)3 with the reductant CP*~CO in thf at -50 'C rapidly results in a dark blue (h,,, = 603 nm) paramagnetic solution containing the anion radical. The half-life for disappearance of Amax, about 10 min at room temperature, is consistent with that of the EPR signal2*The synthetic use of B(C6F5)3as a one-electron oxidant has been demonstrated; the q2(3e)-vinyl complex [Mo{=C(Ph)CHPh}{ P(OMeb}2Cp] is oxidized to form the 17-electron cation [Mo{=C(Ph)CHPh}{ P(OMe)3}2Cp]+,which on warming loses H to form the cationic q2(4e)-alkynecomplex [Mo(q2-PhC=CPh){P(OMe)3}2Cp]+. In this system there is also evidence for a competing reaction between the q2-vinyl complex and the acid (H20)B(C6F5)3, resulting in the formation of a labile trans-stilbene complex.22 A series of 1:l adducts of B(C6F5)3 with organophosphoryl ligands of the types R3P0, (R0)3PO, (R0)2(R)P0and (R0)2(H)P0(R = alkyl, aryl groups) have been synthesized and characterised by spectroscopic and analytical techniques, and in the case of B(C6F5)3.0PPh3 by single-crystal X-ray diffraction. Variations in 31PNMR chemical shifts and v(P0) IR stretching frequencies have been related to the nature of the substituent at phosph~rus?~ Titrations of B(C6F5)3 with water, in toluene-d8solution, monitored by 19Fand 'H NMR have revealed initial formation of the adduct [(C6F5)3B(OH2)] and then its stepwise transformation into the two aqua species [(C~F~)~B(OH~)].HZO and [(C6F5)3B(OH2)]*2H20 containing, respectively, one or two water molecules hydrogen-bonded to the protons of the B-bound water molecule.24 Density functional studies of model triboramacrocyclic Lewis acids 2 and 3 * * *

132

Organometallic Chemistry

indicate extremely strong binding of the methyl anion, which in some cases exceeds that of B(C6F5)3.In addition, anion selectivity for the fluoride ion is implied by binding energies ca. 200 kJ mol-' greater than those of other halides2' F

3

2

Finally, the polyfluoroalken- 1-yldifluoroboranes RCF=CFBF2 (R = F, cis-, trans-C1, trans-C4F9,cis-C2F5,cis-C6F13,trans-C4H9,trans-C6H5)have been prepared by defluoridation of the corresponding fluoroborate salts K[RCF=CFBF3] (using boron trifluoride in dichloromethane or fluorotrichloromethane) and characterised by 'H, ''B and "F NMR spectroscopy. Their reactivity towards ether, water and anhydrous H F was also investigated.26 2.2 Borate Anions. The formation of adducts of tris(pentafluoropheny1)borane with strongly coordinating anions such as CNand [M(CN),I2-(M = Ni, Pd) has been reported to yield the bulky, very weakly coordinating anions [CN{ B(C6F5)3}2]-and [M{ CNB(C6F5)3}4]2which have been isolated as stable NHMe2Ph+and CPh3 salts. Furthermore, zirconocene dimethyl complexes LzZrMez [L2 = Cp,, rac-MezSi(Ind)2] are found to react with these salts in benzene solution at 20 "C to give the salts of binuclear methyl-bridged cations, [(L2ZrMe)2(p-Me)][CN{B(C6F5)3}2] and [(L2ZrMe)2(pMe)l2[M { CNB(C6F5)3}41.27 A new perfluorinated tetraaryl borate, [B{ C6F4C(C6F5)2F}4]- (4), has been prepared and characterized, with a single crystal structure X-ray study of the + salt confirming the tetrahedral geometry at boron. The [C6H5N(H)(CH3)2] performance of 4 as a weakly coordinating anion in ethylene polymerization was evaluated and compared with that of B(C&5)4- and B(3,5-(CF3)2CbH3)4-.28 Salts of the tetrakis(trifluoromethy1)borate anion, M[B(CF3)4] (M = Li, K, Cs, Ag) have also been prepared for the first time. The colourless compounds are thermally stable up to 425 "C (Cs salt) and soluble in anhydrous HF, water, and most organic solvents. The anion is resistant to strong oxidizing (e.g. F2)and reducing agents (e.g. Na) and is not affected by nucleophiles like C2Hs0- or electrophiles such as H 3 0 + .It is very weakly coordinating, as demonstrated by the low-equilibrium CO pressure over the [Ag(CO)J[B(CF3)4] (x = 1, 2) coadducts and the formation of [Ag(CO),][B(CF3)4] (x = 3,4) at higher CO pressure.29Additionally N-pyrrolyllithium has been shown to add to B(C6F5)3 to yield the salt [(C4H4N)B(C6F5)3]Li (5).Treatment of 5 with HCl in ether leads to addition of H at a pyrrolyl a-position to yield the neutral dipolar Brarnsted acid +

+

6: Group I I I - B, AZ,Ga, In, TI

133

system [(C~HSN)B(C~F~)~], which can in turn be used to protonate C P ~ Z ~ ( C H ~ ) ~ to yield [Cp2Zr(CH3)]+[(C4H4N)B(C6F5)3] - and methane?’ Reaction between ZnR2 and [H(OEt2)2][B(C6F5)4] has been reported to be strongly solvent dependent. The reaction in ether leads to the salts of the type [RZn(OEt2)3][B(C6F5)4],while mixtures of ZnR2(R = Me, Et) and B(C&)3 in toluene-dg undergo facile alkylfC6Fs group exchange to give Zn(C6F5)2*(t~1~ene). Furthermore, mixtures of ZnR2and B(C6F5)3in mixed hydrocarbon/diethyl ether solvent systems react via alkyl transfer to afford the ion pairs [RZn(OEt2)3] [RB(C6F5)3]1 :

2.3

Boron Hydrides. - Neutral boron hydride adducts continue to attract attention, not only as precursors to poly- and oligomeric species, but also as ligands to transition metals through coordinated B-H bonds. Contreras and co-workers have reported multinuclear NMR studies of Nborane cyclic adducts in solution, including details of ‘H-TI relaxation times, conformational behaviour, and intramolecular H8+***H*contacts.32 The dehydrocoupling of the phosphine-borane adduct ‘Bu2PH-BH3above 140 “C, catalysed by the rhodium complexes [Rh( 1,5-~od)~][OTf] (cod = 1,4-cyclooctadiene) or Rh6(C0)16,has been reported to give the four-membered chain (6).Thermolysis of 6 in the temperature range 175-180 tBuzPH-BH2-fBu2P-BH3 “C led to partial decomposition and the formation of ‘Bu2PH-BH3.When the dehydrocoupling was performed in the presence of [{ Rh(p-Cl)(1,5-c0d))~]or (7)was RhC13 hydrate, the chlorinated compound ‘Bu2PH-BH2-‘Bu2P-BH2C1 formed which could not be obtained free of 6.33In a separate study, flash vacuum thermolysis of cyclic phosphine-borane complexes has been reported to generate transient phosphinoboranes, which have been analysed by ultraviolet photoelectron spectroscopy to provide information about their electronic properties. Ab initio calculations enabled the authors to assign the different ionizations of the PE spectra and provided more information about the character of the B-P bond.34 Photolysis of C ~ M n ( c 0 in ) ~ the presence of BH3.L (L = NMe3, PMe3) produced borane CJ complexes, 8 and 9 which have been characterized by X-ray crystallography. The bonding between the borane ligand and metal is considerably different from those of other CJ complexes. It consists exclusively of electron donation from the BH orbital to metal with negligible back-donation into the BH o* orbital.35 -

H

OC\”Tn\,,,BYpMe, I ;/H

oc 9

The rational and high-yield synthesis of [(cod)Rh{(q2-BH3)Ph2PCH2PPh2}] [PF6] (lo), a compound which bears a chelating monoborane phosphine, has also been reported. The solid-state structure shows that the borane coordinates to the metal via two 2e-3c B-H-Rh bonds. Furthermore, the applicability of 10 as

134

Organometallic Chemistry

a catalyst in organic synthesis has been demonstrated in the coupling of boronic acids with en one^.^^

10

A computational study of the protonation of BXH2and BX2H (X = F and Cl), including the structures of BXH3+ and BX2H2+and of the dihydrogen complexes BXH5+and BX2H4+has been reported by Rasul and Olah.37 2.4 Boratabenzenes and Related Ligands. - Boratabenzene and related complexes of transition metals continue to attract comparison with related cyclopentadienyl species. The reaction of Li[C5H5BR](R = Ph, 11; R = NMe2, 12) with [RhC1(C2H4)2I2 is reported to yield boratabenzene complexes [C5HSBR]Rh(C2H4)2in excellent yield, both compounds being shown crystallographically to possess a molecular geometry analogous to that of the corresponding Cp and Cp* complexes. The use of 11 and 12 in promoting alkane boration was evaluated against the activity of Cp*Rh(C2H4)2;the boratabenzene complexes displayed faster initiation, but yielded less thermally stable catalyst^.^' The reaction of yttrium trichloride with lithium 1-methylboratabenzene in a 1:2 molar ratio afforded the dinuclear sandwich complex [(C5H5BMe)2Y(p-Cl)]2 (13), for which single crystal and powder diffraction methods revealed three conformational polymorphs. The molecules in the three phases vary remarkably in the rotational position of boratabenzene ligands with differences of 91.1,133.1, and 24.9' between each pair. DFT calculations revealed that the three observed molecular structures correspond closely to three minima on the gas-phase potential energy surface.39The reaction of the 1-chloro-1-boracyclohexadiene14 with N,N'-dimethyl-N,N'-bis(trimethylsilyl)ethylene-1,2-diaminefollowed by LDA afforded the dilithium salt of N,N'-bis( 1-boratabenzene)-N,N'-dimethylethylene1,2-diamine(15). Subsequent reaction of 15 with ZrC14afforded a boron-bridged dimethylethylenediamino bis(boratabenzene) zirconium dichloride complex, while reaction of 15 with FeC12 afforded the corresponding dimethylethylenediamino-bridged iron(I1) complex. The structures of the zirconium and iron complexes have been determined by X-ray diffraction; in each case the boratabenzene units are bridged by an approximately C2-symmetric dimethylethylenediamino bridge thereby making the complexes chiral.4' Herberich and Zheng have reported a synthesis of the boratabenzene ligand precursor, lithium (S)-1- [2'-(me t hoxymet hy1)pyrrolidin-l'-yl]-3,5-dimet hylboratabenzene [Li(L)] and its treatment with [(C5Me5)Fe(NCMe)3]PF6to generate the enantiopure ferrocene analogue (C5Me5)Fe(L).In addition, crystallizawhich consist of tion of Li(L)from thp afforded colourless rods of [Li(L)(th~)o.~]~;

6: Group I11 - B, Al, Ga, In, TI

135

infinite chains with alternating bis(b0ratabenzene)lithiate sandwich units and tetrahedral LiN202 Main group boratabenzene derivatives have also been reported in 2001. The reaction of Ce12, SnC12, or PbC12 with two equivalents of lithium aminoboratabenzenes in diethyl ether affords the new sandwich compounds Pb(3,5-Me2C5H3BNMe2)2 (16) and E[3,5-Me2C5H3BN(SiMe3)2]2 (E = Ge, 17; E = Sn, 18; E = Pb, 19).While compound 16 undergoes facile coupling at ambient temperature, 17-19 are thermally stable, and have been shown crystallographically to possess a monomeric bent-sandwich geometries with facially bonded boratabenzenesP2 1,2-Dihydro-1,2-azaborines have been prepared from the corresponding 2,3dihydro-1H-1,2-azaborol-3-yllithium reagents. Further reaction with molybdenum and chromium reagents of the type L3M(C0)3(20) affords complexes in which that the metals are q6-bound to the 1,2-dihydro-1,2-azaborine rings.43

20

2.5 Boron-containing Materials. - A boron-ruthenium conjugated system has been prepared by the hydroboration polymerization reaction between mesitylborane and a ruthenium-phosphine complex containing a tetrayne. In the UVVis absorption spectrum of this polymer, the absorption maximum of the MLCT band (dn-px" transition) was highly bathochromically shifted by 141nm relative to that of the tetrayne rnon0mer.4~ 2.6 Boron-based Sensors. - The use of boron-based reagents in the sensing of anionic and charge neutral molecules continues to attract enormous attention. Only a brief discussion of selected developments is therefore possible here. Modular and modular polymer supported fluorescence photoinduced electron transfer (PET) sensors containing two boronic acid receptor units, a pyren1-yl fluorophore, and hexamethylene linker have been reorted by James and co-workers. These species show selective saccharide binding in aqueous methanolic solution at pH 8.21.45However, in a follow-up to a recent report claiming the first example of fluorescence-based sensing of boronic and boric acids at sub-micromolar concentrations (using anthracene functionalised tertiary amines), James and co-workers have shown that the observed sensor response is for protons, rather than the boron-based acids originally claimed.46 Boron-containing n-conjugated systems, including tris(9-anthry1)borane (21) and tris[( 1O-dimesitylboryl)-9-anthryl]borane(22), have been investigated as a new type of fluoride chemosensor. Upon complexation of 21 with F-, a colour change from orange to colourless was observed and, the 470 nm band characteristic of 21 disappeared to be replaced by bands around 360-400 nm assignable to n-x* transitions of the anthryl moieties. This change can be rationalized as a

Organometallic Chemistry

136

22

result of the interruption of the mconjugation extended through the vacant p-orbital of the boron atom by the formation of the corresponding four-coordinate flu~roborate.~’ 2.7 Boron-based Ligand Systems. - Boryl and borylene complexes of transition metals have attracted much attention both from a structure and bonding viewpoint and on account of their implication in extraordinary organic transformations, such as the catalytic, selective functionalization of hydrocarbons. The synthesis and characterization of the first Ir(V) complexes containing boryl ligands have been reported by Hartwig and Kawamura. The reaction of such species with hydrocarbons was investigated with a view to a greater understanding of the functionalization of alkanes catalysed by group 9 organometallics!8 Additionally, the synthesis, spectroscopic and structural characterisation of the first transition metal complex containing a bis(pentaf1uorophenyl)boryl ligand, C P F ~ ( C O ) ~ B ( C(23), ~ F ~have ) ~ been described. DFT calculations revealed a significantly enhanced n-acceptor role for the -B(C6F& ligand compared to its non-fluorinated counterpart, -B(C6H5)2.49

23

Reaction of the chloroborane C1B[NMe(CH2CH2)NMe] with Pd(q3C3Hs)(q5-CsH5) and PMe3 (two equivalents) generates trans-

6: Group I I I - B, Al, Ga, In, Tl

137

PdC1{ B[NMe(CH2CH2)NMe]}(PMe3)2(24),which has been characterized crystallographically as a rare example of a palladium boryl complex. Treatment of complex 24 with alkynes or methyl vinyl ketone is reported to give chloro(1boryl- 1-alken-2-y1)palladiumcomplexes or a chloro(3-boroxy-2-buten-1-y1)palladium complex, re~pectively.~'

24

Braunschweig and co-workers have reported the synthesis and structural characterization of two new chromium complexes containing terminally bound borylene ligands (BR), 25 and 26.25 has been prepared by photolytic transfer of the BN(SiMe3)2ligand from the corresponding tungsten complex, whereas 26 was synthesized by the salt elimination reaction between Na2[Cr(CO)5] and 26 contains an electronically unsaturated boron centre, and C12BSi(SiMe3)3. features a Cr-B distance [1.878(10) A] consistent with a Cr= B double

\

OC ~~ ,-=B=N

oc

SiMe3

co 25

-Si.,

CO

fiMe3 LSiMe3 "'SiMe3

26

The triruthenium hexahydride complex [{(q5-CSMe5)Ru}3(pH)6](BF4) is reported to react with NaBH3X (X = H, CN) to yield the novel trinuclear p3-borylene complex { (qs-C5Me5)R~}3(pH)3(p3-BX)r 27, which can be further converted into a p3-B(OR)analogue (R = Me, Et) by treatment with MeOH or EtOH, re~pectively.~~ Borylene complexes have also been the subject of significant computational effort. Quantum chemical calculations reported by Chen and Frenking at the NL-DFT (BP86, B3LYP) and CCSD(T) levels of theory have predicted that the borylene ligand in (OC)4Fe[B(NH2)]occupies the equatorial position, while the axial and equatorial forms of the parent compound (OC)4Fe(BH)(28 and 29) are energetically nearly degenerate. Furthermore, charge and energy analysis of the bonding situation suggests that the borylene ligands are rather strong n acceptors. The strengths of the Fe-BR (R = NH2 or H) n-back donation in the axial and equatorial plane are very different from each other, leading to very different bond lengths and bond angles for the axial and equatorial CO ligands. Additionally, the calculations show that B(NH2) is a weaker n-accepting ligand than BH, which contradicts the qualitative rule that the equatorial position is occupied by the better ~t acceptor.54The metal-ligand bonds in the homoleptic complexes Fe(EMe)5 and Ni(EMe),, (E = group 13 element) have been found to possess

138

Organometallic Chemistry

28

29

higher ionic character than in (CO),Fe(ER). Furthermore, the contribution of the TM-ER a back-donation to the AEorbterm is greater in the homoleptic complexe~.~~

2.8 Boronic Acids. - The reaction between arylboronic acids and various salen ligands (in a 2:l molar ratio) has been investigated in ethanol, toluene, and acetonitrile solution. In all cases bimetallic boronates with chiral boron atoms could be isolated with the difference that in ethanol mostly open bimetallic boronic esters were obtained, while in toluene or acetonitrile closed bimetallic complexes with a central seven- or eight-membered heterocyclic ring were formed.56The formation of macrocyclic boronates, from the reaction of 3aminophenylboronic acid and either salicylaldehyde, 2-hydroxyacetophenone and 2-hydroxybenzophenone has also been reported.57 2.9 Suzuki and Other Coupling Reactions. - Although a discussion of Suzuki coupling reactions is beyond the scope of this report, several papers reporting advances in B-alkyl coupling reactions deserve menti~n.~*-~'The first report of an alkyl-alkyl Suzuki-type cross-coupling reaction at ambient temperature involving alkyl bromides which possess p hydrogen atoms has been reported by Fu and co-workers.61Lautens and co-workers have reported the coupling of arylboronic acids [ArB(OH)2, Ar = Ph, 4-MeOC6H4, 4-MeC(0)C6H4,3-1C6H4, 2-MeC&, 2-BrC6H4,2-napthylJ to a range of olefins in water using [Rh(cod)Cl]2 as a catalyst.62Rhodium-catalysed hydroarylation of alkynes with a range of arylboronic acids or anhydrides has been reported by Hayashi et al. A 1,4-shift of rhodium from a 2-aryl- l-alkenylrhodium species to a 2-alkenylarylrhodium intermediate is proposed to account for the observed product distribution^.^^ Interestingly, Marder and co-workers have reported the functionalization of C-H bonds to boronate esters by the reaction with pinacolborane (HBpin, HB02CMe2CMe2) in the presence of ca. 1 mol % RhCl(P'Pr3)2(N2). Functionalization of benzene yielded PhBpin (scheme l), whereas toluene yielded a number of products, with methyl activation competing with attack at the aromatic C-H linkages. The 14-electron species R ~ ( P ' P I - ~ has ) ~ Hbeen postulated to be the active species in the catalytic The reaction of l,l-diethoxybut-2-ene and trialkylboranes in the presence of

Scheme 1

6: Group III - B, Al, Ga, In, TI

139

Schlosser's superbase {and catalysed by iodobenzene and tetrakis(tripheny1phosphine)palladium(O),[(C6H5)$]4Pd) has been shown to offer a new synthetic pathway to l,l-dialkylbuta-1,3-dienes(scheme 2).65Additionally, triethylborane has been reported to promote the Pd-catalysed selective C-diallylation of ohydroxyacetophenone and the C-monoallylation of o-hydroxypropiophenone with a variety of allyl alcohols. The reaction proceeds smoothly at 25-50°C and provides the allylation products in excellent Suginome, Ohmori and Ito have reported a convenient preparation of alkenylboranes through the reaction of p-borylallylsilanes with acetals (scheme 3).67Oshima and co-workers have reported triethylborane-induced radical cyclization reactions using the Schwartz Reagent Cp2Zr(H)C1(scheme 4).68 The radical addition of various xanthates to allyl or vinyl boronates has been shown to occur smoothly in the presence of a small amount of lauroyl peroxide as initiator to give good yields of usefully functionalised boronates (scheme 5).69

/\

BFgOEt2/H202

PhI/(Ph3P)dPd

R

I

R

I

R = "Bu, CPent,"Hex, CHex

Scheme 2 Bpin

Lewis acid, CHFI,

L

pin = 02CMe&Me2

R

Scheme 3 Et3B,CpzZr(H)Cl,thf X = Br, I

Scheme 4

R"

R

140

Organometallic Chemistry

+

Peroxide

~

( n = 0,1)

, rB<x R

SCSOEt

0

Scheme 5

The boron-mediated aldol reactions of a range of chiral a-(N,N)-dibenzylamino ketones with aldehydes can be controlled to provide stereodefined adducts. Complementary induction can be achieved with ‘HexZBCl/Me2NEt leading to preferential formation of the 1,2-anti-2,4-syn adducts, while Bu2BOTf/’Pr2NEtprovides 192-syn-2,4-antiadduct~.~’ Furthermore, Akibo and co-workers have reported the synthesis of the first doubly borylated enolate as an intermediate in the double Aldol reaction.” The reaction of 1-alkynyltrialkyl borates with sulfenyl, selenenyl and tellurenyl halides has been reported to yield P-chalcogeno alkenylboranes in good yields, with a cis relationship between the boron and the chalcogen moities. Protodeborylation of these compounds by acetic acid, or by a transmetalation-protonolyais sequence, leads to vinyl chalcogenide~.’~ Dialkoxo-substituted monoborylacetylenes (e.g. 30) are obtained from the reaction of bis(ciiisopropy1amino)borylacetylene with catechol derivatives and 2,2’-biphenol. Furthermore, catalytic trimerization with [(q5-C5H5)Co(CO),] yields isomeric mixtures of the triborylbenzene derivatives 31 (scheme 6).73

CpCo(CO)2. toluene, A

H

CatB

Cat = 1,2-02C6H4 CatB

30

BCat 31

Scheme 6

2.10 Diboron(4) Reagents. - Diboration of unsaturated substrates has become an increasingly popular methodology for introducing functionality in organic molecules. The use of automated parallel screening procedures and a series of in situ generated platinum(0) phosphine complexes has allowed the identification of improved catalysts for the diboration of alkynes using bis(pinaco1ato)diboron [B2(pin),] (scheme 7). The optimum phosphine:platinum stoichiometry was identified as 1:1, with the best phosphines employed in the study [PCy3 and PPhz(o-Tol)] giving activities orders of magnitude greater than the worst [P(C6F5)3and PBu‘J. The monophosphine catalysts were reported to function

6: Group III - B, Al, Ga, In, Tl

141

much more efficiently than previous catalysts for a range of alkynes, thereby allowing diborations to be performed at ambient temperature^.^^

H

Bpin

Bpin

Scheme 7

The cross-coupling reaction of B2pin2with chloroarenes to yield pinacol arylboronates has been carried out in the presence of KOAc (1.5 equivalents) and Pd(dba)2/2.4PCy3(3-6 mol %). The catalyst was also reported to be effective in carrying out analogous coupling with aryl bromides or triflates under milder conditions than those of the previous procedures catalysed by PdClz(dppf) in DMS0.75 Bpin

Bpin

B2pin2, Pt(PPhd.4 toluene

Scheme 8

B2pin2 has been reported to add to a,P-unsaturated carbonyls, yielding the products of 1,4-addition after hydrolysis of the intermediate boron enolates (scheme 8). In addition, it is reported that diazomethane reacts with B2pin2via methylene insertion to give pinBCH2Bpin in 83 % yield.76Furthermore, the addition of B2pin2to a,P-unsaturated carbonyl compounds and to terminal alkynes (yielding either 2-boryl-1-alkenes or 1-boryl-1-alkenes)has been carried out at room temperature in the presence of CuCl and AcOK. Transmetalation between diboron and [Cu(Cl)OAc]K generating a borylcopper species has been proposed as the key step in the reaction mechanism.77Additionally Yang and Cheng have reported an unusual diboration reaction which offers a novel and efficient route to bis-boronic compounds. Diboration of allenes catalysed by palladium complexes and organic iodides is reported to generate 1,2-diborated species of type 32 in high yield.78

32

2.1 1 Borane Functionalized Cyclopentadienyl Ligands. - Cyclopentadienyl ligands bearing pendant borane or borate functions have been investigated as single component olefin polymerisation catalysts, and as materials with novel

Organometallic Chemistry

142

electronic properties. High yielding syntheses of the precursor amino(dior(34), and gany1)boranes 'Pr2NB(q1-C5H5)2(33), 'Pr2NB(q1-C5H5)(q'-C9H7) Me2NB(q1-C13H9)2(35), containing C5Hj (cyclopentadienyl), C9H7 (indenyl), C13Hg (fluorenyl)moieties have been reported by Braunschweig and co-workers. With the exception of 35, these compounds show the expected presence of different constitutional isomers.79 Treatment of [C5H4B(C6F5)3]Na,Li.Et20 with zirconocene dichloride has been reported to give the neut r a1 t ris(cyclopent adieny1)Zr-bet aine-t ype complex (q'C P ) ~ [ ~ ~ - C ~ H ~ B ( C(36). ~ F The ~ ) ~crystal ] Z ~ structure of 36 contains the three q'-cyclopentadienide ligands adopting a near trigonal planar geometry around zirconium, with a pronounced Zr-F-C(ary1)coordination perpendicular to

F

36

Bochmann and Lancaster have reported several new examples of boratebridged ansa-metallocenes. The ansa-borane complex (Me2S)(C6H5)B(C5H4)2ZrC12 reacts selectively with two equivalents of LiC6F5to (37) and with three equivalents of LiC&5 to give (Me2S)(C6H5)B(C5H4)2Zr(C6F5)2 form the borat o-bridged complex [Li(Et20)3] [( C6F5)(C&€$3( C5H4)2Zr(C6F5)2] (38).The C6F5groups can then be exchanged for Me by reaction with AlMe3to (39). The activity of these comform [Li(Et20),] [(C6F5)(C6H5)B(C5H4)2ZrMe2] plexes as catalysts for ethene polymerization when activated with M A 0 or A~'BuJ[CP~~][B(C~F~)~] has been investigated.8l Reactions of the niobocene cyclic organohydroborates Cp,Nb{(p-H)2BR2} (R2 = C4H8, C5H10, C8HI4),with B(C6F5)3 in toluene and diethyl ether give different products. In toluene, the salt [Cp2Nb(pH)(q5-q1-C5H4)Nb(q5-q1-C5H4)2Nb{( pH)( q5-C5H4B(C6F5)2)}] [HB(C6F5h]-, 40, which contains a triniobocene cation, is formed. On the other hand in diethyl ether, the reaction of Cp2Nb{(yH)2BR2} with B(C6F5)3produces the covalent complex CpNb(C6F5){(p-H)(q5C5H4B(C6F5)2)}, 41. In both cases Nb(II1) is oxidized to Nb(IV).82 The ansa-ferrocene complex [l,l'-F~(Bbipy)~o](PF~)~ [42, Fc = (q5C5H4)2Fe] has been synthesized and structurally characterized. Electronic communication between the two 2,2'-bipyridylboronium substituents was observed, suggesting the reduced forms [42] and [42]- to be partially delocalized redox intermediate^.^^ A ferrocene-based macrocyclic system has also been synthesized in high yield from 2,5-di(pyrazol-1-y1)hydroquinoneand 1,l'-Fc[B(Me)NMe212 { Fc = Fe(C5H4)2}.The molecule incorporates two redox-active 1,l'-ferrocenylene units in its backbone and contains four chiral boron centres, each possessing the same c~nfiguration.~~ +

+

6: Group I I I - B, Al, Ga, In, T1

143

2.12 Miscellaneous. - Computational studies have probed the origin of the regio- and stereoselectivity of the Diels-Alder reactions of dialkylvinylboranes with substituted dienes. B3LYP/6-3lG* calculated energies of the transition structures for the reactions of dimethylvinylborane and vinyl-9-BBN with transpiperylene and isoprene yielded calculated ratios which were in very good agreement with experimental values. Nonclassical carbon-boron [4 31 secondary orbital interactions seem to account for the high endo stereoselectivity of these reaction^.'^ The relative stability of classical versus nonclassical mono- and bis-boron analogues of the 2-norbornyl cation have been investigated by ab initio and density functional methods. Nonclassical structures were preferred in carbon bridged isoelectronic 1,2-diboranorbornyl anions having H, CH3 or CN substituents on boron, whereas the difluoro analogue preferred the classical structure. The mono-boron analogue preferred an open structure.86 1-Boraadamantane has been shown to react with di(1-alkyny1)siliconand -tin compounds, Me2M(C=CR)2,in a 1:1 ratio by intermolecular 1,l-alkylboration, followed by intramolecular 1,l-vinylboration, to give siloles (such as 43) and stannoles, respectively, in which the tricyclic 1-boraadamantane system is enlarged by two carbon atoms.”

+

43

The reaction of 2,6-(4-t-B~CsH~)~C~H&i with BH2C1SMe2in hexane or E t 2 0 solution is reported to generate the terphenyl-substituted unsymmetrical 9borafluorene l-(4-tert-butylphenyl)-7-tert-butyl-9-(bis-2,6-(4-tert-butylphenyl)phenyl)-9-borafluorene (44), which is readily reduced to the deep red heteroaromatic dianionic (p2-q5,q5-l-(4-tert-butylphenyl)-7-tert-butyl-9-(bis-2,6-(4-tertbutylphenyl)phenyl)-9-borafluorenyl)bis(diethy1ether)dilithium(45) with excess lithium powder in E t 2 0 solution. Reactions of the dianionic 45 with various metal salts leads to reduction of these salts, and bright yellow 44 is recovered in essentially quantitative yields.88 Reaction of Cp2Ta(=CH2)CH3with two equivalents of HB(C6F5)2has been shown to result in the formation production of the dihydride Cp2Ta[CH2B(C6F5)2](pH)(H) (45), plus one equivalent of H3CB(C6F5)2. The reaction pathway is shown to involve stepwise attack of borane first at the methylene group, followed by attack at the methyl group, which then undergoes alkyl/hydride exchange with the second equivalent of HB(C6F5)2. The product of HB(C6F5)2addition to the methylene ligand, methyl hydride complex Cp2Ta(CH2B(C6F5),)(p-H)(CH3) (46), can be intercepted by carrying out the reaction in hexane at low temperat~re.’~

Organometallic Chemistry

144

44

Reaction of equimolar amounts of Me3SiC(N2)Liwith the 2-bromo-2,3dihydro- 1H- 1,3,2-diazaborole derivatives tBuNCH=CHN(tBu)BBr, EtNC(CH=CHCH=CH)CN(Et)BBr, and tBuNCH=C(CH=CHCH= CH)NBBr has been shown to afford the 1,3,2-diazaborolyldiazomethanes 'BuNCH=CHN('Bu)BC(N2)SiMe3 (47), EtNC(CH=CHCH=CH)-CN(Et)BC(N2)SiMe3(48), and 'BuNCH=C(CH=CHCH=CH)NBC(N2)SiMe3 (49). The X-ray structure analysis of 47 revealed a planar five-membered heterocycle which is linked to the carbon atom of the diazomethyl unit via a B-C single bond. x-Interactions between this carbon atom and the boron atom can thus be excluded?O

47

The reaction of boriranylideneboranes with tetrahalogenodiboranes(4) has been reported to yield two types of products, cyclic tetraborylmethane derivatives and isomeric linear diborylmethyleneboranes. The composition of the products was determined from spectroscopic data and in some cases by X-ray structure analyses." The reactions of LiSnR3(R = Me, Ph) with Cl(Me2N)B-B(NMe2)Cl have been investigated by Noth and co-workers and lead to the corresponding 1,2-(triorganylstannyl)diboranes(4). Similarly, when the triborane Cl(Me2N)B-BNMe2B(NMe2)Cl is reacted with Me3SnLi at low temperature the corresponding bis(trimethylstanny1)triborane 50 is formed. In contrast, when the dibromide Br(Me2N)B-BNMe2-B(NMe2)Bris allowed to react with MesSnLi only small amounts of 50 are formed; the main product being the cyclic isomer 51.92

6: Group I I I - B, Al, Ga, In, Tl

145 Me3Sn

\

/B\

Me3Sn\

I

B

I

NMe2

NM%

jB\

Me2N-B,N/B--NMe2

jSnMe3

B

SnMe3

.$

I

4'55. Me

Me

Hexamethyl-1,3,5-tribo~cyclohexanecan be reducgj by alkali metals in donor solvents. Two electrons are added to the combination of p-orbitals at the boron centres forming trishomoaromatic dianions. X-ray structure analyses show a variety of arrangements, depending on the alkali Titanium and niobium complexes containing diorgano-hydroborate ligands, Cp2M{(pH)2BC4H8}, Cp2M{(p-H)2BGHlo} and C ~ ~ M { ( ~ - H ) ~ B Chave BH~~} been prepared from the reactions of Cp2MC12with the potassium salts, K[H2B2(p-H)(p-C4H&], K[H2BC5Hlo], and K[H2BC8H14],respectively, with concomitant reduction of the metal centre from M(1V) to M(III).94

3

Aluminium

3.1 Sub-valent Aluminium and Aluminium Clusters. - The compound ($CsMe5)A1+Al(C6F5)3 (52), which is the first valence isomer of a dialane, was prepared by Cowley and co-workers by treating [Al(q5-C5Me5)]4 with A1(C6F5)3. 52 was characterized by NMR spectroscopy and X-ray crystallography, the latter revealing an Al-A1 distance of 2.591(2)1$?

52

Disproportionation of a metastable AlCl solution with concomitant Cl/N(SiMe3)2ligand exchange leads to the formation of the (crystallographically characterized) cluster N(SiMe3)2} (53) which can be regarded as an intermediate on the way to bulk metal formation. Although the packing density of the A1 atoms in 53 is similar to that in the previously reported species [A177N(SiMe3)2}2~]2(as well as that in A1 metal), the X-ray structural analysis shows significant differences in topology and distance proportion^.^^ The synthesis and structural investigation of the first chlorides of the type A122C120.12L (L = thf, thp) have been reported by Schnockel and co-workers. These polyhedral aluminium subhalides exhibit interesting multicentre bonding which have been further probed by quantum chemical calculation^.^^ The hydroalumination of Me2AlC=CMewith a large excess of Me2AlH afforded the arachno-carbaalane (AlMe)8(CCH2Me)5H(54) by the release of

146

Organometallic Chemistry

AlMe3, On recrystallization from thf solution, the adduct (A1Me)s(CCH2Me)5H.2(thf) (55) was isolated as the first stable ether adduct of a carbaalane. A crystal structure determination for 55 revealed a cube of eight aluminium atoms, five faces of which are bridged by C-CH2Megroups. The sixth face is p2-bridged by a hydrogen atom, and two opposite aluminium atoms of this face are each coordinated by one thf ligand.98The carbaalane cluster (AlEt)7(C= CHC6H5)2(CCH2C6H5)3H has been reported to react with HBF4.0Et2 to yield the fluorine derivative (A1Et)7(C= CHC6H5)2(CCH2C6H5)3(p3-F) (56) by the replacement of its bridging hydrogen atom and the cleavage of a B-F bond. By contrast, use of HCl as a proton donor did not result in substitution of the bridging hydrogen atom; rather a methyl group at the bottom of the cluster was replaced by release of methane. The compound (AlMe)7(A1Cl)(CCH2C6H5)5(p-H) (57), which possesses a terminally bonded chlorine atom, was f0rmed.9~Treatment of Me3N*AlH3with one equivalent of CNtBu in refluxing toluene has been reported to generate [(p3-A1H)(p3-CH2NtBu)l4 (58) which has been shown crystallographically to be based around an A14C4N4cage, in which the carbon atoms adopt exo positions and can be formally viewed as inserting into four of the Al-N bonds of an A14N4 cube.'00Treatment of 1,1,4,4-tetramethyl-2,3-diazabutadiene with the alane adduct AlH3NMe2Et has been shown to yield the hydrazine derivative (AlH2)2(AlH)2(N2iPr2)3(59) by the hydroalumination of both C-N double bonds. 59 has a complicated cage structure formed by three hydrazido groups and four aluminium atoms, containing an unusual N-N group 'side-on' coordinated to one aluminium atom through its lone pairs of electrons.'*' Geometries, energies, vibrational frequencies, and magnetic properties have been computed by Schleyer and co-workers for a family of endohedral closoboranes, -alanes, and -gallanes Ng/AI2HL?-(A = B, Al, Ga) with noble gas atoms (Ng) located in the centres of the icosahedral clusters. The endohedral structures of most of the systems are minima lying below separated Ng A12H122by at least 30 kcal mo1-1.102

+

3.2 Aluminoxanes, M A 0 Models and Aluminium in Olefin Polymerization. Gibson and co-workers have reported the synthesis and structural characterization of mono- and dinuclear methylaluminium complexes (60 and 61) containing boryloxide (-OBR2) ligands, with a view to modelling the behaviour of catalytically relevant MAO-type activator^."^

60

61

The reaction between 'Bu3Al and ArB(OH)2 (Ar = 2,6-diisopropylphenyl) leads, via the intermediate ~ B u ~ A I ( O ) B A ~ ( O (62), H ) ] ~to the boralumoxane A ~ ( ' B u ~ A ~ ) ( ~ B u A ~ ) ~(63). ( O ~The B Alatter ~ ) ~ compound has been structurally char-

6: Group I I I - B, Al, Ga, In, TI

147

acterized by X-ray diffraction and has been shown to activate CpzZrMe2asa catalyst for ethene polymeri~ation.'~~ The reaction of trimethylaluminium with hexameric tert-butylalumoxane, [('B~)Al(p~-o)]~, has been investigated by Barron and co-workers. Reaction of [('B~)Al(p~-o)]~ with one equivalent of AlMe3 results in the formation of two isomers of the hybrid tert-butylmethylalumoxane, [A17(p3-o),(tBu),Me3], 64. The activity of 64 for the [Me2C(Cp)(Flu)]ZrBzzcatalysed polymerization of 1,5-hexadiene, is significantly increased in comparison to [(tB~)Al(p~-o)]~. The effect of additional equivalents of AlMe3 on the co-catalytic activity of 64 suggests that a maximum activity is obtained at a [(fB~)AI(p3-0)]~ to AlMe3 ratio of 1:6.105 Furthermore, the thermal decomposition of [Me2Al(pOCPh3)I2,to yield Ph3CMeand methylalumoxane [(MeAlO),, MA01 has also been studied by Barron. The reaction is initially catalysed by the addition of AlMe,; however, it is also found to be catalysed by the M A 0 product.106In arelated study, the reaction of [Me2A1(p-OEPh3)I2with thf has been shown to yield AlMe(OEPh3)2(thf)(E = C , 65; Si, 66), although the dimethyl compounds, A1Me2(0EPh3)(thf)(E = C , 67;Si, 68),have been detected in thf-ds solution. The reaction of [Me2Al(p-OCPh3)]2with thf was followed by 'H NMR and found to occur by a two-step process. First, the A1202 core of [Me2A1(p-OEPh3)I2is cleaved by thf to form compound 67.Second, two molecules of AlMe2(0CPh3)(thf)react with each other, resulting in ligand redistribution and the formation of 65 and A1Me3(thf).'O7 Density functional theory (DFT) has been used by Ziegler and co-workers to calculate the energies of 36 different methylaluminoxane (MAO) cage structures with the general formula (MeAlO),, (n = 4 - 16). Ultimately the authors used these results to predict the percentage of each oligomer found in the mixture of species at a given temperature."' In a related study, similar methods were to calculate the energies of over 30 different structures with the general formula (A10Meb.(AlMe3), (n = 6 - 13, m = 1 - 4). The way in which trimethylaluminium bonds to M A 0 was determined as well as the location of the acidic sites present in M A 0 cage structures. Topological arguments were used to show that Me3Aldoes not bind to M A 0 cages where n = 12 or n 2 14.'09Thereactivity and acidity of alkylaluminoxanes of relevance to catalyst activation has also been examined by Rytter and co-workers by computational methods.lloA computational study by Talarico, Busico and Budzelaar has examined the balance between olefin insertion and p-hydrogen transfer to monomer for all 'welldefined' aluminium polymerization catalysts reported to date, concluding that olefin polymerization at a single aluminium centre appears unlikely."' Finally, Chen and co-workers have investigated the activation of both constrained geometry and ansa-bridged metallocene complexes of group 4 with B(C6F5)3and Al(C6F5)3.In contrast to the borane analogue, A1(C6F5)3is capable of doubly activating dialkyl derivatives, thereby yielding dicationic species (e.g. 69) which are far more active in the polymerisation of olefins.'12

148

Organometallic Chemistry

'B u

3.3 Aluminium Derivatives CoHaining Bonds to Group 15 Elements. - 3.3.1 Alkyl aluminium species. The reactions of Me3Al with 1,2diaminobenzene [1,2-(H*N)2CbH4] or anthranilic acid, [1,2-(H2N)(H02-C) C6H4], followed by treatment with acetonitrile has been reported to yield tetranuclear and hexanuclear aluminium-containing ring systems. A single crystal X-ray study of the hexametallic product reveals the construction of quinazoline ligands arising via insertion of acetonitrile into A1-N bonds.113Reaction of N,N'bis(mesity1)ethylenediamine with triethylaluminium in benzene leads to the formation of an aluminium diamide derivative of formula [AlEt(pMesNCH2CH2NMes)],(70). Upon treatment with pyridine or thf, 70 is converted into dinuclear aluminium amide derivatives of general formula [AlEt( pMesNCH2CH2NMes)I2.2L(L = pyridine or thf). Single crystal X-ray analysis indicates that the two adducts exist as ten-membered heterocycles with tetracoordinated aluminium centre^."^ Orthometalated aluminium-nitrogen (71)have been synthesized by the dimers of the type [RA1N('Bu)-y-(CH2C6H4)l2 thermolysis of 1:1 mixtures of R3Al and HN('Bu)CH2Ph.The reaction times and temperatures required for orthometalation, the molecular structural data, and the 'H and I3CNMR spectral parameters of the dimers were then compared with the data for the series [RAlN(CH2Ph)-p-(CH2C6H4)]2.11s Organometallic monomeric and dimeric, neutral and cationic, K ~ -and K ~ coordinated mono-pendant arm triazacyclononane complexes of aluminium and indium have been reported by Mountford, Schroder and co-workers along with three new mono-pendant arm triazacyclononane ligand precursors1 The synthesis and structural characterization of the trimetallic complex [Me2Al(hpp),TiCl2,A1Me3](hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[ 1,2-a) pyrimidinate) containing [hpp] - ligands which chelate to titanium and also bridge to an AlMe2unit via nitrogen have also been reported.'17The reactions of cis-['Bu(H)N(E=PNtBu)2N(H)'Bu (E = 0, S, Se) and cis[R(H)N(Ar=PN'Bu)zN(H)R] (R = 'Bu; Ar = Ph, p-Tol) with two equivalents of Me3Al have been reported to generate the bis(dimethyla1uminium) complexes {(Me2A1)[RN(E=PNtBu)2NR](AlMe2)}[R = 'Bu, E = 0 (72), E = S (73), E = Se (74); R = Ph, E = S (75), Se (76), and R = 'Bu, E = Ntol-p (77), E = NPh (78)]. Compounds 72,73, and 77 have been characterized by single-crystal X-ray studies and shown to be trispirocyclic complexes in which the ligands coordinate both dimethylaluminium moieties in an q2-fashion, as dimeric aminophosphoranates.' l 8 The 1:1 reaction of 4-(dimethy1amino)pyridine (dmap) with several Al-P and

'

6: Group I I I - B, Al, Ga, In, T i'

149

Al-As heterocycles of the type [R2AlE(SiMe3)2],(R = Me, x = 3; R = Et, x = 2) and Ga-Sb heterocycles [R2GaSb(SiMe3)2]x (R = Me, x = 3; R = Et, x = 2) has been shown to offer a general pathway for the formation of monomeric, Lewis (78).'19 base-stabilized compounds of the type dma~mR~ME(SiMe~)~ 3.3.2 Amine and Phosphine Adducts. Thermal and photolytic reactions of the Group 13 metal atoms (M = Al, Ga, In) with EH3 (E = N, P) in solid argon matrices yield species with the general formula H,MEH,. These molecules have been characterised structurally, energeticallyand vibrationally by Density Functional Theory (DFT) calculations, and species of the types M-EH3and HMEH2 (E = N, P) and H2MNH2, H2MPH, and MNH2 identified experimentally by their IR spectra.12*A second paper reported in detail on the reaction of metal atoms with PH3.12'Thereaction of [Me2NH2]C1with LiAlH4in Et2O solution at ca. 250 K yields the adduct Me2(H)N.AlH3(79). Single crystals of this and the corresponding gallane Me2(H)N*GaH3 (80) contain dimeric units Me2(H)N.H2M(p-H)2MH2.N(H)Me2 featuring highly unsymmetrical M-H * * M hydrogen bridges. There is also evidence for significant intermolecular NH-a-H-M interactions, particularly in the case of M = A1.'22 Sheer and coworkers have reported the syntheses of Lewis acidbase stabilized phosphanyl alanes and gallanes by the reaction of W(CO)5(PH3)with the adducts Me3NMH3 (M = Al, Ga). The structure of the aluminium complex (181) is dimeric and features asymmetric bridging hydrogens, Lewis base stabilized aluminium centres and W ( C 0 ) 5 - ~ ~ ~ r d i n aphosphide ted ligand~.'~~ *

Amidoh ydridomet alates containing the anions [H Al(N Ph2)3]- , [HAl{ N(CH2Ph)2}3]-, and [H~AI(NC~Z)~](Cy = cyclohexyl) have been prepared by reaction of the corresponding amines with LiA1H4in thf. Single crystal X-ray diffraction reveals that in each case the aluminate anions possess distorted tetrahedral coordination 3.3.3 Pyrazolato and Related Derivatives. The synthesis of a range of 3,5-di-tertbutylpyrazolato (3,5-tBu2pz) derivatives of aluminium containing alkyl, halide and acetylide ligands have been reported by Roesky and co-worker~.'~~ In addition, the novel alumoxane hydride [(p-q': q'-3,5-'B~2pz)2(q'-3,5-~BuzpZ)z(CL30)(p-A1)3H3].2(thf) (82) has been reported, stemming from the reaction of the aluminium dihydride [(p-q': q'-3,5-'Bu2pz)( p-Al)H2J2 (83)with one equivalent of water. The core of compound 82 consists of two tetra- and one hexacoordinated A1 atoms with short A1-0 bonds.'26The same pyrazolato ligand was also exploited in the formation of the unusual monomeric alkenyl-substituted pyra-

150

Organometallic Chemistry

zolato aluminium dichloride [(3,5-'Bu2-N-CH=C(SiMe3)-pz)A1Cl2] (84). The addition of one and two equivalents of K[3,5-'Bu2pz] to 84 resulted in the formation of two novel complexes, [(3,5-'Bu2-N-CH=C(SiMe3)-pz)AlCl(3,5'Bu2pz)l (85) and [(3,5-'Bu2-N-CH=C(SiMe3)-pz)Al(~'-3,5-'B2-3,5'Bu2pz)l (86), respectively. The coordination of the pyrazolato ligand in 85 represents an extreme example of a 'slipped' q2-coordination.'27

But

H

84

Treatment of diisobutylaluminium hydride with 3,5-disubstituted pyrazoles in a 2:l molar ratio has been shown to afford bridging hydride complexes of the type [(p,ql: q'-R2pz)(ApB~2)2(p-H)]. A theoretical study of the reaction of these complexes with methanol predicts that the reaction proceeds by way of an intermediate containing a strong dihydrogen bond, suggesting that dihydrogen bonding is an important feature of the reactivity of organoaluminium hydrides.12*The 2: 1 reaction between triethylaluminium and diphenylpyrazole has been reported to yield a pyrazolate-bridged dialuminium complex that contains a bridging ethyl group between the two aluminium centres. This complex has been structurally characterized and its reactivity and properties described.*29 3.3.4 @-Diketiminatesand Related Systems. - A series of group 13 metal com- (i.e. plexes featuring the P-diketiminate ligand [{(C6H3-2,6-'Pr2)NC(Me)}2CH] [Dipp2nacnac]-, Dipp = C6H3-2,6-'Pr2)have been prepared. These include the methyl, chloride and iodide derivatives DippznacnacMX2[X = Me, C1, I; M = Al, Ga, In (87)], the latter being useful precursors for reduction to M(I) derivatives. X-ray crystal structures for these compounds feature nearly planar C3N2 arrays in the Dippznacnac ligand backbone with short C-C and C-N distances that are consistent with a delocalized Reduction of [Dipp2nacnac]A112with potassium in the presence of alkynes

87

6: Group I I I - B, Al, Ga, In, T1

151

C2RR' has been reported to generate the first neutral cyclopropene analogues of aluminium LAl[q2-C2RR'] (R = R' = SiMe3,88; R = R' = Ph,89; R = Ph, R' = SiMe3,90). Reduction of [Dipp2nacnac]Al12 in the presence of Ph2C0, on the other hand, generated the aluminium pinacolate LA1[02(CPh2)2](91), irrespective of the amount of Ph2CO employed. Furthermore, insertion of the unsaturated molecules C02, Ph2C0, and PhCN into one of the Al-C bonds of 88 led to ring enlargement and the formation of novel aluminium five-membered heterocyclic systems.131A similar sterically bulky framework has allowed Power and Roesky to generate stable monomeric imides of aluminium and gallium of the type (Dipp2nacnac)MN(C6H3Trip2-2,6) (Trip = 2,4,6-'Pr3CsH2). The crystal structure of the gallium compound (89) reveals a Ga-N distance [1.742(3) A] consistent with multiple bond c h a r a ~ t e r . ' ~ ~

Me3Si*"'

SiMe3

88

The synthesis, structures, and reactivity of cationic aluminium complexes containing the N,N-diisopropylaminotroponiminateligand ('Pr2-ATI-) have been described by Jordan and co-workers. The reaction of ('Pr2-ATI)A1R2with [Ph3C] [B(CbF5)4] yields ('Pr2-ATI)AlR+species whose fate depends on the properties of the R ligand. For R = Me the dinuclear monocationic complex [{('Pr2-ATI)A1Me}2(p-Me)] [(C6F,)4](90) is formed, the cation of which contains two fPr2-ATI)AlMe+ units linked by an almost linear Al-Me-A1 bridge.133Stable group 13 metal chlorobenzene complexes of general type [('Pr2-ATI)MR] [B(C6F5)4](ClPh) (M = Al, Ga, In) have also been reported. X-ray crystallographic studies show that the PhCl ligands in these compounds are coordinated by dative M-C1Ph bonding and suggest that x-stacking interactions between the PhCl phenyl ring and the 'Pr2-ATI ligand may also contribute to the PhCl co~rdination.'~~ The reaction of m-terphenyl-substituted amidines with trimethylaluminium to form dialkylaluminium amidinate complexes has also been reported. In the crystalline state, these species are robust complexes that can be easily handled in air for short periods of time without noticeable d e c o m p ~ s i t i o nThe . ~ ~sequential ~ reaction of PhN(H)C(Ph)NPh (AmH) with AlMe3and 'BuLi has been reported to generate both the cluster [L&Am3] { Li[(p-Me)2A1(Me)'BuI2} - (91) and the aluminium tris(amidinate) AlAm3 (92). In the solid state, 91 has a polymeric structure based on tetranuclear Li4 cluster cations and lithium bis(a1uminate) anions which associate by the formation of weak Li - - MeAl +

3.3.5 Heavier Group 15 Elements. The reaction between Bi2Et4and 'Bu3M (M

152

Organometallic Chemistry

= Al, Ga) has been investigated by Schulz and co-workers. In each case the (crystallographically characterized) product contains a dibismuthine ligand, in which both bismuth centres are coordinated to a group 13 metal centre ( 9 ~ 2 ) . ’ ~ ~

92

Analogous reactions between distibanes and trialkylalanes and -gallanes R’3M in 1:2 stoichiometry have been shown to yield similar bis-adducts, [Sb2R4] [MR’3]2,93. These have been characterized by multinuclear NMR and elemental analysis and in four cases by single crystal X-ray diffra~ti0n.l~’ Reactions of trialkylalanes AIR3(R = Me, Et, ‘Bu)and triorganylbismuthanes BiR’3(R’ = ‘Pr, SiMe3)were performed and the products investigated both in solution and in the solid state. Et3A1’Bi(SiMe3)3(94), ‘Bu3A1’Bi(SiMe3)3(95), and tBu3Al*Bi(‘Pr)3 (96) are stable Lewis acid-base adducts in the solid state but only 95 and 96 remain intact in solution. Their dissociation enthalpies, as determined by temperaturedependent NMR spectroscopy, have been estimated to be 6.3 (95) and 6.9 kcal mol-’ (96).139 3.3.4 Miscellaneous Examples. Aluminium and gallium ‘constrained geometry’ complexes of type 97 (M = Al, Ga) have been reported by Cowley and coworkers from the reaction of MeMC12 with the dianionic precursor [Me2Si(CsMe4)(NtBu)l2-. Crystallographic results show an q1 mode of attachment of the cyclopentadienyl ligand in each case, although NMR measurements at room temperature imply rapid fl~xiona1ity.l~~

‘Bu 97

Ready replacement of the amine hydrogens of the cyclic trimeric silazane [Me2SiNH]3 by the H2A1.NMe3group has been reported to afford the aluminium amides (Me2SiNA1H2*NMe3),(Me2SiNH)3-, (98,n = 3; 99, n = 1; 100,n = 2). The monosubstituted amide 94 could not be isolated, as it underwent condensation to the tricyclic compound 1,1’,2,2’-(HA1NMe3)2 (NSiMe2NSiMeZN(H)SiMe&.By contrast, the analogous reactions of the more flexible cyclic tetrameric silazane CMe2SiNH14 with H3Al.NMe3did not give

153

6: Group I I I - B, Al, Ga, In, TI

simple aluminium amides, but gave instead complicated mixtures from which polycyclic species could be isolated in low ~ i e 1 d s . l ~ ~ The reaction of di(tert-buty1)aluminium hydride with hydrazine N2H4 has been reported to yield the hydrazide (Me3C)2A1N2H3 (loo),which is dimeric both in solution and in the solid state and possesses a six-membered A12N4heterocycle in a twist conformation with two intact N-N bonds. Further reaction of 100 with an excess of HA1(CMe3)2yielded the tricyclic aluminium and nitrogen rich A4N4 2[ A1(CMe3)2I2. 142 compound [( Me3C)2AlN2H2]

100

The reaction of AlC13 with one equivalent of Li{ C4H2N(CH2NMe&2,5} or two equivalents of Li{C4H3N(CH2NMe2)-2}in diethyl ether at -78 "C afforded A1C12{C4H2N(CH2NMe2)2-2,5} (101) and AlCl{ C4H3N(CH2NMe2)-2)(102), respectively, which have been characterized by NMR spectroscopy and X-ray structure deterrninati~n.'~~ The related hydride-bridged tetranuclear aluminium complex { A1H2[C4H2N(CH2NMe2)2-2,5]AlH3}2 (103) has been synthesized by the reaction of 101 with two equivalents of LiAlH4.The crystal structure of this complex reveals an unusual structure in which the four aluminium atoms are in a square-planar arrangement.I4 Finally, Hoskin and Stephan have reported the synthesis and structural characterization of an unusual phosphorus/aluminium macrocycle formed by the reaction of an organo-tetraphosphine with A1Me3.145

3.4 Aluminium Derivatives Containing Bonds to Group 16 Elements. - A series of 1,l'-N-substituted ferrocenediyl ligands has been synthesised from 1,l'-diaminoferrocene by Schiff base condensation reactions. The coordination chemistry of these ligands has been probed by reaction with Zr and A1 reagents and the unusual dimetallic, bis-chelate A1 complex 104 has been structurally characteri~ed.'~~ Reaction of 'Bu3M (M = Al, Ga) with anthranilic, salicylic and ortho-toluic acids yields [ ( ' B U ) ~ M ( ~ - O ~ C C ~ H ~ - ~[M - N H= ~ ) ]A ~1 (105), Ga (106)], [ ( ' B u ) ~ G ~ ( ~ - O ~ C C ~ H ~ (107), - ~ - O and H ) ] ~[ ( ' B U ) ~ G ~ ( ~ - O ~ C C ~ H(108), ~-~-M~)]~ respectively. Reaction of anthranilic acid with two equivalents of 'Bu3Al on the other hand leads to the isolation of ('Bu)~A~( p-02CC6H4-2-NH2)Al(tBu)3 (109), which has been proposed as a Lewis acid stabilized complex of the intermediate in the synthesis of compound 105.147 Furthermore, treatment of salicylaldimine ligands of the type 3,5-But2-2-(OH)C6H2CHNR with Me3Alin toluene has been reported to yield complexes of the type [3,5-But2-2-(O)C6H2CH=NR]A1Me2 (106). Reaction of these systems with B(C6F5)3in the presence of thf leads

Organometallic Chemistry

154

Me-At-0

104

smoothly to [{ 3,5-Buf2-2-(0)C,H2CH=NR}A1Me(thf)]isolated as the B(C6F5)3Me- salts (scheme 9). By contrast, the same reaction performed in dichloromethane solution without thf gives complex mixtures including products from Me/C6FSexchange r e a c t i o n ~ . ' ~ ~ +

'Bu

/

Me

'3

Me

Scheme 9

The reaction of 2,2'-methylenebis(4-chloro-6-isopropyl-3-methylphenol) (MCIMP)H2 with 1.2 molar equivalents of AlMe3 has been reported to yield [(MCIMP)AlMe(thf')] (107). 107 further reacts with a stoichiometric amount of 2-propanol, affording the isopropoxy-bridged dimer [(MC1MP)A1(p,-olPr)l2 (108) which shows excellent catalytic activity toward the ring-opening polyrnerization of E-caprolactone and &valer~lactone.'~~ The reactions of various amino alcohols with AlH3.NMe3has been shown to yield the corresponding aminoalkoxyalanes R2N(X),OAlH2 (X = aliphatic linker) (109) which are dimeric in the solid state, featuring a characteristic A1202 ring system with pentacoordinated A1 atorns.15* The reaction of AlCl3 with 2-dimethylaminoethanol in a 1:l molar ratio yields ( l l O ) , which further reacts with two equivalents of [C12Al(p-OCH2CH2NMe2)]2 vinylmagnesium chloride to give the divinylalane [(CH2=CH)2Al(pOCH2CH2NMe2)]2(111).Compound 111 was found to be a useful reagent for the transfer of a vinyl group to 1,3-dipheny1-2-propene-1-one. The 1,4-addition product is formed regioselectively, both in a non-catalysed as well as in a

6: Group III - B, Al, Ga, In, TE

155

nickel-catalysed r e a ~ t i 0 n . lReactions ~~ of thiols RSH (R = Et, ‘Pr, ‘Bu) with A1H3-NMe3have been reported to generate compounds of the types RSAlH2.NMe3(112) or (RS)2AlH.NMe3(113), depending on the stoichiometry. Similarly, the 1:l reaction of LiA1H4with RSH (R = Et, ‘Pr)produces LiA1H3SR (114). By contrast, the reaction of NaAlH4 and ‘PrSH results in ligand exchange from NaAlH3(SR)to generate NaA1H4,NaAlH2(SR)2and NaA1H(SR)3.’52 The hydroxysilsesquioxanes ( C - C ~ H ~ ) ~ S ~ @ (115) ~ ~ ( ~ Hand ) (c(116) have been studied by Duchateau and coC5H9)7Si709(OH)20SiMePh2 workers as model supports for silica-grafted aluminium alkyl species. Treatment of AlMe3 with 115 yields polymeric { [ ( C - C ~ H ~ ) ~ S ~ ~ O ~ ~(117), ]AIM which ~ ~ } ,is readily transformed into the corresponding monomeric adduct, { [(cC5H9)7Si8013]AlMe2.py (118) on treatment with pyridine. The corresponding reaction with 116 generates noticeable amounts of the 2:l product {[(cC5H9)7Si7011(OSiMePh2)J(A1Me2)2)2 (119) and the Brarnsted acidic 1:2 product ( [ ( C - C ~ H ~ ) ~ S ~ ~ O ~ ~ ( O S ~ M ~ P ~(120) ~ ) ] ~in A Iaddition - ) ( H + }to the main reaction product, { [(C-C~H~)~S~~O~~(OS~M~P~~)]A~M~}~ (121).ls3 Roesky and co-workers have reported the synthesis of tris(trimethylsilyl)methylaluminium dihydride, (Me$i)3CA1H2.thf (122), by the reaction of RLi*2(thf)[R = C(Me3Si)3]with H3A1.NMe3.Furthermore, the reaction of 122 with elemental Se or Te in toluene afforded the novel organoaluminium chalcogenideheterocubanes [RA1(p3-E)I4(E = Se, 123;Te, 124) in good yield. In a similar manner [R’GaH(p-H)12 (125) [R’ = (Me3Si)zC( Ph)C(Me3Si)N] was obtained by -the reaction of R’Li-thf with H3Ga-NMe3.‘54 PY

(Me3Si)&

\At-E

I

/ I / ‘C(SiMed3

IE ’A -I

(Me3Si)&

/E-I/A11C(SiMe3)3

E = Se, 123; Te, 124

118

The synthesisand structural characterization of the first examples of functionalized dimeric four-ring type aluminophosphonates have been reported. Reaction of ‘BuP(0)(OSiMe3)(OH)with Me3Alleads to the formation of [Me2Al(p0)2P(OSiMe3)(fBu)]2 (126) whereas Me2AlClreacts with Ph2P(O)(OH) to yield [(Cl)(Me)Al(~ - 0 ) ~ P P(127).155 h~l~ 3.5 Aluminium Organometallics in Organic Synthesis. - The asymmetric methylation, ethylation and allylation of aldehydes using trialkylaluminium reagents catalysed by titanium(1V)complexes of N-sulfonylated amino alcohols

156

Organometallic Chemistry

R

LH

Ti(O'Pr)4/Ligand

+ AIR'S

tM

+ R

CI

HO

Ligand =

""\

H/N- so2

Scheme 10

has been reported (scheme 10). Enantiomeric excesses of up to 99% were reported in thf ~ o l u t i o n . ' ~ ~ Intramolecular trans-vinylsilylation using silicon-tethered alkynylvinylsilanes has been reported to be catalysed dramatically by Lewis acids such as EtA1Cl2.ls7 The cyclopropanation of mono- and disubstituted acetylenes by a CH212/Et3Al reagent has been studied. The structure of the products was stated to depend on the reagent ratio and the solvent New inexpensive aluminium-based bidentate and tridentate chelates have been reported to be found to be efficient catalysts for the Tischtschenko reaction of aldehydes. The conversion of nbutanal to n-butyl n-butyrate using catechol-derived catalysts at room temperature was found to be complete in two Lewis bases such as phosphines, arsines, stibines, and sulfides have been found to catalyze the alkylation of symmetrical epoxides with trialkylaluminium compounds very effectively at a 5 mol % level. Coordination of the Lewis base to the Lewis acidic aluminium reagent has been demonstrated by 27Al and 31PNMR spectroscopy and is proposed to form a more nucleophilic alkylating agent.'60 Regio- and stereospecific substitution at l-(phenylthi0)-2,3-epoxyalkaneshas been achieved using nucleophilic organoaluminium reagents. Under the influence of trimet hyl- or t riet hylaluminium, a 1-(phenylthio)-2,3-epoxyalkaneunderwent substitution at the C2 position to give a product with retention of configuration.161 The mechanism of the hetero Diels-Alder reaction of benzaldehyde with Danishefsky's diene catalysed by various types of achiral and chiral aluminium complexes has been studied using semi-empirical and ab initio calculations, taking (Me0)2AlMeand (S)-BINOL-AlMe as model catalysts.162

3.6 Miscellaneous Examples. - The Zr(II1) complex [Cp2Zr(p-H)I2(y-H)A1Cl2 (128)and the Zr(IV) complex [Cp2ZrH(p-H)2]3Al(129) have been isolated from solutions containing zirconocene(1V)compounds and LiAlH4and characterized by X-ray structural analysis. The basic structural element of 128 is the six-atom ring Zr2AlH3,in which the metal atoms are linked by bridging h ~ d r 0 g e n s . l ~ ~

6: Group III - B, Al, Ga, In, TI

157

A synthetic strategy for organometallic fluorides from metal alkyl derivatives and the HF2- anion has been reported and exemplified by [Me4N][fBu)2AlF2] (130), [P~~P][CBU)~AIF~] (131), and [Ph4P][AlF4] (132).la The syntheses of [((Me3Si)3CAlF2)2(p-O)Li2(thf)4] (123) and [{ Li(Me3Si)3CA1F3(thf)}3LiF(thf)] (134)have been reported. Compound 133 was obtained either by the reaction of (Me3Si)3CA1Me2.thf with Me3SnF in the presence of Li20 or by the reaction of [(Me3Si)3CAlF2]3 with Li20, and is an interesting example of an oxygen- and fluorine-containing alkyl aluminium

128

Multiple C-H bond activation occurs upon reaction of phosphinimide complexes of the form Cp'(R3PN)TiMe2(Cp' = Cp, indenyl; R = i-Pr, Cy, Ph) with excess AlMe3, affording the carbide complexes of the type Cp'Ti(p2-Me)(p2NPR3)(p4-C)(AlMe2)3(135) or in some cases [CpTi(p2-Me)(p2-NPR3)(p5C)(A1Me2)3-(AlMe3)] (136). These species contain four- and five-coordinate carbide centres; VT-NMR studies have established that such species exist in equilibrium. The four-coordinate carbide complexes retain Lewis acidity at a planar three-coordinate A1 centre, as evidenced by their reaction with diethyl ether, thf, or PMe3.166 The homoleptic ytterbium(I1) tetraalkylaluminate complexes [Yb(A1&)2]. (137) have been obtained via a silylamide elimination reaction between Yb[N(SiMe3)2]2(thf)2and excess AIR3 (R = Me, Et, 'Bu). Solid [Yb(AlEt&]. contains [Yb(AlEt4)] and [Yb(AlEt4)3]- fragments, which form an intricate three-dimensional network in the solid state. Both fragments are linked by bridging a-carbon atoms and secondary Yb. - .H-C agostic interactions combining p,$, p,q2, and p,q3 coordination modes which result in remarkably short Yb . - - A1 [2.809(2) A] and a large range of Yb * - C distances [2.649(5)-3.364(6) +

Ay7

The reaction of LiGePh3 with Me2AlClin ether yields the trisgermylaluminate [Li(OEt2),][(Ph3Ge)3AlMe], 138, which has been shown by single-crystal X-ray diffraction to contain a near-tetrahedral aluminium centre and a Ge-A1 bond length of 2.520

4

Gallium

4.1 Sub-valent Gallium and Gallium Clusters.- The organometallic chemistry of low-valent gallium systems has seen considerable advances in 2001, with a

158

Organometallic Chemistry

variety of interesting systems incorporating cluster aggregation, binuclear GaGa entities and even discrete Ga centres (albeit stabilized by a donor/acceptor interaction). The reaction of (q5-pentamethylcylopentadienyl)gallium(Cp*Ga) with B(C6F&, GatBu3, and Cp*GaX2 (X = C1, I) gives rise to the new complexes Cp*GaB(C6F& (139), Cp*GaGatBu3 (140), and C ~ * G a G a ( x ) ~ c p[X* = C1 (141), I (142)], respectively. Complex 141 was also formed in the reaction of Cp*Ga with InCl.169Ga/B compound 139 has a structure analogous to the diallane analogue reported by Cowley,g5and the structures of 141 and 142 are also consistent with Ga(I)/Ga(III) donor/acceptor complexes. Similar com143; L = pounds of the type LGa-*B(C6F& { L = HC[MeC(2,6-iPr2C6H3)N]2, (q5-CsMes),144) containing gallium/boron donor/acceptor bonds, have been prepared by treatment of the free gallanediyls (LGa) with B(C6F5)3. The structures of 143 and 144 were determined by X-ray crystallography revealing in each case Ga-B distances of the order of 2.15 Me

Me I

Me

Me @ ' Me

rx /

.Ga-Ga.,

X

Me

X = CI, 141; I, 142

Di(tert-buty1)gallium hydride has been reported to undergo ligand redistribution in solution forming tri(tert-buty1)gallium and the sesquihydride [(Me3C)2GaH]2[H2GaCMe3]2 (145). The loss of tert-butyl radicals upon irradiation of this mixture with day light or a UV lamp led to the formation of the hexagallium compound (Me3CGaGaCMe3)2(y-H)2[y-H2Ga(CMe,),l2 (146), which possesses two Ga-Ga single The novel neutral and anionic gallium clusters GaloR6, [Gal&*6]- and

rMe3

Me3C\ Ga

6: Group III- B,Al, Ga, In, TE

159

[Ga13R*6]-have been obtained by the reaction of ‘GaI’ with Li(thfhR [R = Si(SiMe3)3] and Na(thf)2R*(R* = Si‘Bu3),respectively. Both of the Galo compounds may be considered as conjunct0 clusters consisting of two edge-sharing 0 ~ t a h e d r a . Similarly, l~~ the gallium clusters [Ga18R*8](147) and [Ga22R*8](148) have been obtained by warming a solution of gallium(1)bromide after addition of an equimolar amount of NaR*. From X-ray structure analyses, the observed arrangements of the 18 and 22 Ga atoms in 147 and 148, respectively, are comparable with an 18 atom section of the p-Ga modification, or show at least some kind of relationship to a 22 atom section of the Ga-I11 modification. This allows a description of both the clusters as metalloid. The topology of the Ga atoms in 148 is also well explained by the Wade-Mingos rules as an eightfold capped closo-Ga14 Uhl and co-workers have reported the synthesis of GastBu9(149) in ca. 4 YOyield from the reaction of tert-butyllithium with gallium trihalides. The crystal structure of 149 revealed a tricapped trigonal prism of gallium atoms and three distinct ranges of Ga-Ga bond lengths: short distances of 2.588 A (av) were detected to the capping Ga atoms, while longer distances of 2.670 A were found on the triangular faces of the prism. Very long Ga-Ga separations were observed for the edges of the prism perpendicular to these triangles (2.988 A), indicative of very weak Ga-Ga interaction^.'^^ A novel Ga8R6 cluster [R = C(SiMe3)J (150) has been reported by Schockel and co-workers from the reaction of metastable GaBr with LiC(SiMe3)3in thf at -78°C. The structure of 150, features two tetrahedral gallium units joined by an exo-polyhedral Ga-Ga bond and has been described as a model for a metal atom contacts in the form of a nan0-wi~e.I~~

150, R = C(SiMe&

Much larger gallium containing clusters have also been reported. Schnockel and Schnepf have reported the synthesis of a Ga84R204 cluster [R = N(SiMe3)2] from the reaction of metastable GaBr and Li[N(SiMe&] in a 4:1 mixture of toluene and thf. In the crystal structure the anionic Gag4 units adopt a slightly distorted, cubic close packing, in which the tetrahedral holes are completely occupied by [Li(thf)4]+ ions and the octahedral holes with the dication [( thf)3LiBrLi(t hf)3]? .176 Reduction of cis-ethene-1,2-di(tert-butylamido)gallium(III) chloride with metallic potassium in the presence of N,N,N‘,N’-tetramethylethylenediaminehas been reported to yield (N,N,N’,N’-tetramethylethylenediamine) potassium [cisethene-1,2-di(tert-butylamido)]gallate(1)(151) which has a novel dimeric structure in which the potassium cations are q5-coordinated to the pseudo-carbene ring system of one monomer and q’-bonded to the gallium atom of the other.’77 The novel digallanes [GaN(R)CH = CHN(R)12(R = 2,6-diisopropylphenyl, 152; +

160

Organometallic Chemistry

2,6-diethylphenyl,153) have been obtained in quantitative yield by photolysis of the corresponding l-galla-2,5-diazoles [(q'-Cp*)GaN(R)CH = CHN(R)]. This alternative approach to the synthesis of digallanes demonstrates the efficiency of the Cp* moiety as the leaving group in the formation of metal-metal bonds.'78 R

\

\

R

I

R

R = 2,6-diisopropylphenyI, 152; 2,6diethylphenyl, 153

The reduction of [R*2Ga-GaR*]' in organic solvents with Na, NaCloH8,or NaR* leads to deep-red [R*2Ga-GaR*Na(thf)3] (154), which transforms in the [R*2Gapresence of 18-crown-6 into the deep-blue "a( 18-~rown-6)(thf)~] GaR*-GaR*]- (155). Oxidation of the latter anion with R*Br or TCNE leads to deep-green tetra(supersily1)cyclotrigallanyl [R*Ga-GaR*2-GaR*]' (156). The latter radical thermolizes at 100 "C yielding R*4Ga4and the digallanyl R*3Gai. The Ga-Ga bond orders in these compounds have been interpreted on the basis of crystallographicallymeasured Ga-Ga distance^."^ Finally, theoretical calculations have been carried by Nagase and co-workers to probe the GaGa bond in Na2[Ar*GaGaAr*] [Ar* = 2,6-Trip&H3], whose description as the first example of a triple bond between group 13 elements has previously sparked considerable interest. The short Ga-Ga bond length is found to be the result of several factors, including Na-terphenyl and terphenyl-terphenyl ionic interactions, direct Ga-Na-Ga bridge bonding, and adjustments in the C-Ga-Ga angles due to the steric requirements of the 'Pr groups on the bulky m-terphenyl ligands.'" +

4.2 Gallium Derivatives Containing Bonds to Group 15 Elements. - Two studies have reported applications of organogallium azide derivatives. A combined FTIR/matrix-isolation study of the pyrolysis of the single-source gallium nitride precursor Me2N(CH2)3Ga(N3)2 (156) has been reported by Muller and Bendix, who identified monomeric Ga(N3)as a reactive intermediate.181Furthermore, Bittner and Zink have reported studies of the luminescence of dimethylgallium azide both as a solid and in solution. The origins of the observed features are reported to lie in a low energy electronic transition which is of predominantly LMCT character.182 Gallane complexes bearing amido-amine ligands -N(R)CH2CMe2CH2NMe2 (R = H or SiMe3),{ H2Ga[N(H)CH2CMe2CH2NMe2]}2 (157), H2Ga[N(SiMe3)CH2CMe2CH2NMe2] (158), { H(Cl)Ga[N(H)CH2CMe2CH2NMe2]}2(159), {[(Me3Si)2N](H)Ga[N(H)CH2CMe2CH2NMe2]}2 (160), and HGa[N(SiMe3)(161), have been synthesized by the reactions of the CH2CMe2CH2NMe2I2 quinuclidine adducts of mono- and dichlorogallane with the corresponding

6: Group I I I - B, Al, Ga, In, TI

161

lithium amides. Structural determinations of compounds 158, 160, and 161 showed that all three were dimeric with bridging amido groups.183In addition, the reaction between GaC13and various secondary amines and amides has been reported by Carmalt and c o - w ~ r k e r s . ' ~ ~ Alkyl- and phenylbis(dimethy1amido)gallium derivatives, [RGa(NMe2)2]2 have been synthesized by the reaction of LiNMe2 with (RGaC12)2.The crystal structure of methyl compound (162, R = Me) revealed a disordered mixture of anti and syn isomers, consistent with 'H NMR studies. Furthermore, 'H NMR and mass spectra indicated that a species with the general formula of R3Ga2(NMe2)3 was formed when these compounds were heated at high temperat u r e ~ . The ~ * ~syntheses of the gallium/nitrogen compounds, EtzGaNMe(C6H22,4,6-'Bu3) (163), Me2GaNMe(C6H2-2,4,6-'Bu3)(164), MeGa[NH(C6H2-2,4,6'Bu3)]2 (165), and Ga[N(H)C6H2-2,4,6-'Bu3I3 (166), have been reported. In particular, the monomeric derivatives 163 and 164 are of interest because their 'H NMR spectra at room temperature are consistent with restricted rotation about the Ga-N bond. Variable-temperature 'H NMR studies of a toluene solution of 164 quantified the rotational barrier as approximately 71 kJ mol-', suggesting that steric effects might be responsible.'86 The heterocycles [($-Cp*)GaN(Dipp)CH=CHN(Dipp)] (165) and [($Cp*)GaN(Dep)CH=CHN(Dep)](166) have been obtained by Jutzi and coworkers from the oxidative addition of (q5-pentamethylcyclopentadienyl)gallium(1) (Cp*Ga) to Dipp-DAB [1,4-bis(2,6-diisopropylphenyl)-1,4-diazabuta1,3-diene] and Dep-DAB [1,4-bis(2,6-diethylphenyl)-1,4-diazabuta-1,3-diene], respectively. Compounds 165 and 166 represent the first examples of monomeric gallium-containing heterocycles of the diazabutadiene (DAB) type, the former being structurally characterized by X-ray ~rystallography.'~~

4.3 Gallium Derivatives Containing Bonds to Group 16 Elements. - The ability of an epoxide ligand to coordinate to a group 13 metal centre through one or both of the oxygen lone pairs has been demonstrated by Lewinski and coworkers. Alkoxide-tethered epoxide complexes containing two or four gallium atoms were synthesized by the reaction of GaMe3 with 3-hydroxypropylene oxide in the appropriate molar ratio (scheme 11).188 The same group has also examined the structural chemistry of organogallium alkoxides containing tethered amine donors. Me2Ga(OCH2CH2CH2NH2) (167) and [Me2Ga{p-OCH2CH(CH3)NH2)l2 (168),demonstrate the influences of hydrogen bonding, ligand conformational changes and steric bulk on network morphology and molecular aggregation.'" The reaction of equimolar quantities of 1,8-bis(trimethylst anny1)naphthalene and GaC13 at 25 "C and in the presence of water, led to the formation of a 12-membered macrocycle (169) containing three gallium atoms linked by 1,8naphthalenediyl ligands and arranged about a central oxygen atom.'g0 Reactions of GaH3*L (L=NMe3 or quinuclidine, quin) with one or two equivalents of 'BUSH and GaH3-,Cl,(quin)with one or two equivalents of LiS'Bu have been investigated as routes to amine adducts of tert-butylthiolate gallium hydrides of possible use as precursors to gallium sulphide films. The complexes

162

Organometallic Chemistry

-MeH

Scheme 11

169

G~H(S'BU)~(NM and ~ ~GaH@'Bu)(quin) ) were isolated and characterized by X-ray crystallography. In the solid state, GaH(StBu)*(NMe3)is best described as having a trigonal pyramidal geometry.'" Similarly, the reaction of GaMe3 with one equivalent of (S)-BINOL in toluene generates the chiral organogallium alkoxide [(thf)MeGa((S)-BINOLate)12(170) after recrystallization from thf. By contrast, treatment of PhCH21nC12with one equivalent of Li2((S)-BINOLate)in thf at reflux and recrystallization from dimethoxyethane (dme) led not to the desired organoindiurn alkoxide but to [{ (dme)Li}3{In((S)-BINOLate)3}].(2dme), 171. Compound 170 is dimeric with a Ga202 four-membered ring, while 172 consists of a tetranuclear In06Li3skeleton in which every Li+ ion is coordinated by one additional DME ligand.'92The reactions of [RGa(p3-Te)14(R = 'Bu, SOzor Se02have been reported to generate oligomeric species CMeEt,) with 02, of the type [RGa(p3-O)], (R = 'Bu, n = 9; R = CMeEt,, n = 6) and the mixed cubanes [hGa4(p3-O),(p3-Te)4 (x = 1, 2).'93 4.4 Gallium Organometallics in Organic Synthesis. - Aromatic hydrocarbons have been alkenylated with silylallene in the presence of GaC13 at -90°C. Organometallic electrophiles generated from the allene and GaC13 are the active

6: Group III- B, Al, Ga, In, T1

163

species in this reaction and a modest level of ortho-selectivity has been observed.194Allylgallium reagents have also been found to be effectivefor the radical allylation of a-iodo or a-bromo carbonyl compounds. The addition of water as a co-solvent improved the yields of allylated products, implying that the allylgallium species resists immediate decomposition on exposure to

4.5 Gallium Hydrides. - The chemistry of binuclear gallane derivatives continues to be investigated, notably by Downs and Pulham. The structures of dimethylgallium tetrahydroborate (172) and its 1:l adducts with diethyl ether and tetrahydrofuran have been investigated by single crystal X-ray diffraction. The structure of the parent compound has been shown to consist of discrete Me2Ga(p-H)2BH2units, whereas the adducts are composed of molecules in which the gallium centre is pentacoordinate and asymmetrically bound to a bidentate BH4unit. The adducts display enhanced thermal stability compared to that of uncoordinated Me2Ga(BH4),but reversibly dissociate in the gas phase at ambient temperat~re.’~~The related compound gallaborane (GaBH6, 173), synthesized by the metathesis of LiBH4 with [H2GaC1In at ca. 250 K, has been characterized by chemical analysis and by its IR and ’H and “B NMR spectra. The IR spectrum of the vapour at low pressure implies the presence of a single diborane-like species H ~ G ~ ( P - H ) ~and B Hthe ~ structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements constrained by the results of ab initio molecular orbital calculations. Aggregation of the molecules occurs in the condensed phases. X-ray crystallographic studies of a single crystal at 110K reveal a polymeric network with helical chains made up of alternating pseudotetrahedral GaH4and BH4units linked through single hydrogen bridges.I9’ I

R = Me, 172; R = H,173

Downs and co-workers have reported several studies on the reactivity of monochlorogallane, [H2GaC1In (174). Thermal decomposition at ambient temperatures releases H2 and results in the formation of gallium(1) species, including the new compound Ga[GaHC&], which has been characterized crystallographically (scheme 12). Symmetrical cleavage of the Ga(~1-Cl)~Ga bridge in 174 occurs in its reaction with NMe3 but unsymmetrical cleavage, giving [H2Ga(NH&] C1-, in its reaction with NH3. Ethene inserts into the Ga-H bonds to form first [Et(H)GaC1I2and then [Et2GaC1]2.’98 +

4.6 Miscellaneous. - The unsupported gallylene-bridged diiron complexes [C~’Fe(C0)~]~(p-GaMes) (Cp’ = q5-C5Me5,175; q5-C5H5,176) have been synthesized by the reactions of M[Cp’Fe(C0)2] (M = K, Na) with MesGaC12in thf. Photolysis of a toluene solution of 175 resulted in the formation of the trans isomer of the gallylene- and carbonyl-bridged diiron complex [Cp*Fe(C0)I2(p-

164

Organometallic Chemistry

Scheme 12

CO)(p-GaMes); by contrast photolysis of a toluene solution of 176 afforded a mixture of the cis and trans isomers of [CpFe(C0)]2(p-CO)(p-GaMes).'99

176

An alternative route to Fe-Ga bonds involves the use of gallium(1) and gallium(I1)halides. Ga2C14.2(dioxane)and GaI were reacted with K[CpFe(C0)2] and [Cp(C0)2Fe]2,resulting in all cases in disproportionation of gallium(1) and gallium(I1) species into elemental gallium and gallium(II1) compounds. Novel complexes of the types Cp(C0)2FeGaX2(S)and [Cp(CO)2Fe]2GaC1(S)(S = solvent) containing Fp-Ga linkages were isolated from the reaction mixtures and structurally characterized?@ The trinuclear gallium-bridged ferrocenophane 177 has been reported by Jutzi and co-workers from the thermolysis of the donor stabilized bis(gally1)ferrocene precursor [(q5-C5H4GaMe2*py)2Fe] (py = pyridine). The crystal structure of 177 reveals three ferrocene-1,l'-diyl units linked together by two pyridine-stabilized gallium(II1) centres to form a highly unusual 'carousel' structure.201The related gallium bridged [l,l]ferrocenophane (178)has been obtained by the reaction of 1,l'-dilithio ferrocene with the alkyltrichlorogallate [Li(thf)] [RGaC13][R = CH(SiMe3)z-j. The crystal structure determination revealed two trigonal planar, bridging gallium atoms, each of which is attached to two carbon atoms of different ferrocene molecules and the inner carbon atom of the bis(trimethy1sily1)meth yl subst ituen t .202 The syntheses of two tetramethylcyclopentadienyl gallium(II1) compounds Ga(C5Me4H)3and Cl2(C5Me4H)Ga.pyhave been reported. The attempted syntheses of Ga(C5Me4H)2Cl,Ga(C5Me4H)C12 and Me2Ga(C5Me4H)by metathetical and/or ligand redistribution reactions, however, led instead to a mixture of compounds rather than single species.203 +

6: Group III - B, Al, Ga, In, TI

165 C(SiMe3)3 I

AGa&

I

I

Fe

I

I I C(SiMe3)3

177

5

178

Indium

5.1 Sub-valent Indium and Indium Clusters. - The synthetic, structural and reaction chemistry of subvalent indium species continues to excite much attention. Such studies have generated a range of new indium cluster compounds as well as species containing indium-transition metal linkages, typically formed by reaction with binuclear metal carbonyl reagents. The syntheses of the first main group triple-decker cations including [($C7He)In(~-~5-CsMes)In(~4-C,H~)][(C6F5)3BO(H)B(C6F5)3] (179), have been described by Cowley and co-workers. 179 was prepared by treatment of [In($CsMes)I6with an equimolar mixture of B(C6F5)3 and H20-B(C6F5)3and characterized crystallographically.2M Treatment of the tetrahedral indium(1)cluster compound 1n4[C(SiMe3)3l4with a mixture of Al13 and I2 has been shown to generate the diiodotriindium com(180)in 73% yield. 180 contains a chain of three indium pound In3I2[C(SiMe3)3I3 atoms connected by In-In single bonds. A trigonal bipyramidal structure resulted in the solid state by two iodide bridges between the terminal indium Partial oxidation of I I I ~ [ C ( S ~ M ~ with ~ ) ~halogen ]~ donors or with mixtures of bromine and aluminium tribromide has also been shown to afford novel alkylindium halides. These include the dimeric In(II) species In2X2Rz(X = Cl, Br), and the mixed-valence tetrahedral cluster In4Br2R4(181) which features an average indium oxidation state of 1.5.206

I

C(SiMe3)3 In

180

181

166

Organometallic Chemistry

The monomeric fragment InC(SiMe3)3has been inserted into the Ni-Ni bond of Ni2Cp2(p-C0)2upon treatment of the carbonyl complex with In4[C(SiMe3)3]4 in a molar ratio of 4 to 1. The product [CpNi(C0)]JnC(SiMe3)3 (182) contains an indium atom coordinated to one alkyl substituent and two Ni(Cp)CO groups in a planar coordination sphere. Reaction of the starting compounds in a molar ratio of 2 to 1 led to the formation of a second Ni/In complex (183) in which groups had occurred. The replacement of both CO ligands by two tr~C(SiMe~)~ insertion product was not observed with the gallium derivative Gas[C(SiMe3)3]4; instead, a nickel gallium complex analogous to 183 containing two bridging GaC(SiMe3)3ligands was isolated as the only product regardless of the ratio of the starting comp0unds.2~~

oc! 182

183

A similar structural motif is achieved in the reaction of decamethylsilicocene with trimethylindium, which has been reported to yield the disilyl indium compound [Cp*zSi(Me)]21nMe (184) in nearly quantitative yield. 184 has been characterised by multinuclear NMR, microanalysis data and by X-ray crystallography, the latter revealing In-Si distances of 2.640(3) and 2.642(2)A.2o8

FCP*)

(+cp*)\

184

Chaudret and co-workers have reported the first synthesis of indium nanoparticles under mild conditions using the organometallic precursor [InCp],. The particles can be stabilized by a polymer or by ligands and, whatever the synthesis and stabilization method, the particles generally have a uniform size. They also adopt an amorphous (or disordered) structure derived from the bct structure of bulk indium.209 5.2 Indium Derivatives Containing Bonds to Groups 15 or 16. - The structures of trimethylindium (Me31n)adducts with the bidentate tertiary amine N,N,N',N'tetramethyl-4,4'-methylenebis(aniline)(MBDA) and the macrocyclic amines 1,4,8,11-tetramethyl- 1,4,8,11-tetraazacyclotetradecaneand 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane have been determined by single crystal X-ray diffraction. The adduct Me31n.2MBDA (185) contains a fivecoordinate trigonal bipyramidal indium centre and displays the longest In-N

6: Group I I I - B, Al, Ga, In, T1

167

bond lengths yet reported (2.720 and 2.865 A).21oBy contrast, the X-ray structure analysis of [Et2In(p,h-O2C)C6H4NH2-2In (186) has revealed an extended onedimensional chain comprised of five-coordinate indium centres. The chain unit repeated along two-fold screw axis is defined by the diethylindium moiety chelated asymmetrically by the carboxylate group with one of the two carboxylate oxygens acting as bridge to the next indium centre in the chain.211

5.3 Indium Organometallics in Organic Synthesis. - The use of organoindium reagents in C-C bond forming reactions continues to generate much interest, and a comprehensive treatment of such reactivity is beyond the scope of this report.212-228 In an interesting study, dichloroindium hydride (CI2InH),generated in situ from tributyltin hydride and indium trichloride, has been reported to act as a novel radical initiator for the reduction of organic halides with tributyltin h~dride.~~~ 5.4 Miscellaneous examples. - InMe3 has been shown to react with isonitriles CNR (R = CsH40Me-p, CsH4Me-p) to afford the corresponding adducts Me31nCNR,even in the presence of an excess of CNR. Furthermore, Me31nCNR reacts with pyrrolidine to give products of the type [Me21nC(=NR)(Pyrr)](Pyrr = conjugate base of pyrrolidine), via insertion of CNR into the In-NPyrr bond.23o The bis- 1,4-dioxane (diox) adduct of bromomethyl-dibromo-indium(III), Br21n(diox)2CH2Br, has been reported by Tuck and co-workers to react with triphenylphosphine oxide, triphenylphosphine sulfide and benzyl sulfide to produce the compounds Br21n[OP(C6Hs)3]2CH2Br and Br31nCH2L(L = benzyl sulfide, triphenylphosphine sulfide). The crystal structures of these new organoindium compounds have been solved by X-ray diffraction The bisphosphavinyl indium complex, [CyIn{ C(tBu)=PCy}2], has been prepared from the reaction of two equivalents of [CyP=C('Bu)MgCI(OEt2)] with CyInBr2 and shown to be monomeric with a trigonal planar indium centre in the solid state. The reactions of [CyP=C('Bu)MgCl(OEt2)] with MX3 (M = Al, Ga, In; X = CI or Br), on the other hand, leads to facile phosphavinyl coupling reactions and the formation of the diphosphametallobicyclo[ 1.1.llpentane complexes, [M{ C$Bu)2P2Cy,} { C('Bu)=PCy}], which contain terminal phosphavinyl ligand~.~~~

6

Thallium

As in previous years, the organometallic chemistry of thallium is scarce in comparison with the lighter elements of group 13. However, 2001 has witnessed, for example, the first N-heterocyclic carbene adducts of thallium trihalide~.*~~ Reaction of the N-heterocyclic carbenes :CN(Mes)C2R2N(Mes), (R = H, Imes; R = Br, ImesBr; Mes = 2,4,6-Me3C6H2),with TlX3, (X =C1, Br), has been reported by Jones and co-workers to yield the thallium trihalide carbene adducts, [T1X3{CN(Mes)C2R2N(Mes)}],one of which, [TIC13(IMes)] (187), was characterised crystallographically. In addition, further reaction of [T1C13(IMes)]

168

Organometallic Chemistry

with one equivalent of :CN(Me)C2Me2N(Me)afforded the mixed bis-carbene complex [T1C13(IMes){CN(Me)C2Me2N( Me)}]F3' Lithiation of the tripodal amine HC{ SiMe2NH(p-Tol)}3with "BuLi in the presence of TlCl has been shown by Gade to give access to the mixed valence T1(I)/Tl(III) complex [HC{ SiMe2N(p-Tol)}3(T1nBu)(Tl)], 188. An X-ray diffraction study has established that the central "BuTl(II1)unit is coordinated by the amido tripod in which two of the amido functions are additionally bridged by the Tl(1) atom.234A homoleptic phosphine adduct of thallium(1) supported by a tris(phosphin0)borate ligand has also been isolated and structurally character-

187

ized. The complex, [PhB(CH2PPh2)3Tl](189),was obtained from the reaction of [Li(tmeda)][PhB(CH2PPh2)3]with Tl[PF6] in water and characterized spectroscopically and crystall~graphically.~~~

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72. 73. 74. 75. 76. 77. 78. 79.

80. 81. 82. 83. 84.

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Organometallic Chemistry

222. J.H. Cha, A.N. Pae, K.I.I. Choi, Y.S. Cho, H.Y. Koh and E. Lee, J . Chem. SOC., Perkin Trans. 1,2001,2079-2081. 223. J.A. Shin, K.I. Choi, A.N. Pae, H.Y. Koh, H.-Y. Kang and Y.S. Cho, J . Chem. SOC., Perkin Trans. 1,2001,946-948. 224. R. Yanada, A. Kaieda and Y. Takemoto, J . Org. Chem., 2001,66,7516-7518. 225. B.C. Ranu, S. Samanta and A. Hajra, J . Org. Chem., 2001,66,7519-7521. 226. W. Lu and T.H. Chan, J . Org. Chem., 2001,66,3467-3473. 227. Y. Canac, E. Levoirier and A. Lubineau, J . Org. Chem., 2001,66,3206-3210. 228. T. Hirashita, H. Yamamura, M. Kawai and S . Araki, Chem. Commun., 2001, 387-3 88. 229. K. Inoue, A. Sawada, I. Shibata and A. Baba, Tetrahedron Lett., 2001, 42, 4661-4663. 230. R. Bertani, L. Crociani, G. DArcangelo, G . Rossetto, P. Traldi and P. Zanella, J . Organomet. Chem., 2001,626,ll-15. 231. C. Peppe, J.A. Nobrega, M.Z. Hernandes, R.L. Longo and D.G. Tuck, J . Organomet. Chem., 2001,626,68-75. 232. C. Jones and A.F. Richards, J . Organomet. Chem., 2001,629,109-113. 233. M.L. Cole, A.J. Davies and C. Jones, J . Chem. SOC.,Dalton Trans., 2001,2451-2452. 234. C.H. Galka and L.H. Gade, Chem. Commun., 2001,899-900. 235. I.R. Shapiro, D.M. Jenkins, J.C. Thomas, M.W. Day and J.C. Peters, Chem. Commun., 2001,2152-2153.

7

Group 14: Silicon, Germanium, Tin and Lead BY RICHARD A. LAYFIELD

1

Overview

Space limitations preclude a comprehensive review of every aspect of the 2001 literature on Group 14 organometallic chemistry. This article will concentrate mainly on low-coordinate and multiply bonded compounds, Group 14 organometallics as ligands to transition metal centres, and novel technological applications. Consequently, the scope of the account is limited mainly to the elements in their subvalent (formal) oxidation state of + 11, although important compounds containing the elements in other oxidation states are occasionally included.

2

Silylenesand Silyl Anions

The thermal decomposition reactions f a range of disupersilylsilanes, R*2SiX2 (R* = supersilyl = 'Bu3Si,X = H, halogen) have been reported.' The intermediate existence of the silylenes R*SiX, formed via elimination of R*X, was inferred from subsequent trapping (insertion) reactions (Scheme 1).

160°C -R*X

R*$iX2

Et,SiH

[R*SiX]

SiEt, I

R* = Si'Bu3

si:HR*

Scheme 1

The rate of R*X elimination was found to increase in the order R*2SiH2 < R*2SiMeH < R*2SiBrH < R*2SiF2c R*2SiBr2< R*2Si12.In the same study, thermal decomposition of the supersilylsilanidesR*XSiHalM (M = Na, MgHal) were found to proceed with elimination of MHal at low-temperatures, and concomitant formation of the silylenes R*SiX was similarly inferred from their follow-up reactions. Thermolysis of R*SiX2Na resulted in formation of the disilanide R*X2Si-SiR*XNa, which, at -20" C, decomposes with elimination of NaX to form cis-R*XSi= SiR*X. The silanides R*2SiXLi, where X = F, Cl, Br, eliminate LiF, LiCl and LiBr at room temperature, -20" C and -78" C respectively, resulting in the formation of [R*2Si], which subsequently decomposes according to Scheme 2. ~

_

_

_ ~~

~

~

Organometallic Chemistry, Volume 31 0 The Royal Society of Chemistry, 2004 177

Organometallic Chemistry

178

A

R*?Si(X)Li

-- LiX

[R*?Si ]

15%

R*(H)Si- CH2 I

I

Bu‘?Si-- CMez

Scheme 2

85%

Accounts of the chemistry of stable silylenes (of general formula R2Si) continued to grow in number during 2001. Two important review articles have summarized the recent literature.’,’ The stable dialkylsilylene 1 was shown to react with (chloromethy1)cyclopropane unexpectedly under double insertion (Scheme 3). R = %Me3

lill

I’Me

Scheme 3

In contrast to the more commonplace oxidative addition reactions of silylenes with substrates such as iodomethane (Scheme 3), the unusual reactivity of 1 is thought to be dependent on the concentration of silylene and has been explained on the basis that a carbon chlorine bond is activated through complexation of the silylene, which facilitates nucleophilic attack by a second ~ilylene.~ The first stable silylborane-isocyanide complexes, (Tbt)(Mes)Si(H)-BH,.CNAr (Tbt = 2,4,6-tris[bis-trimethylsilyl)methylJphenyl; Mes = mesityl; Ar = Tbt, 2,4,6‘Bu3C6H2, 2,4,6-’Pr3C6H2),were obtained from the 1:l reactions of a range of stable silylene-isocyanide complexes, (Tbt)(Mes)Si-CNAr, with BH3-THF.’The reaction of (arylsily1)boranesR3Si-BR’2 (R3Si = Ph3Si,PhMe2Si;BR’? = BMes2, catecholboryl) with reagents such as MeLi, KO‘Bu and MeMgBr in hydrocarbon solvents (hexane or toluene) have provided an alternative route to silyl anions of the type R&M +.6 Although not organometallics in the strictest sense, the chemistry of the bis(amino)silylenes does overlap significantly with that of the diorganosilyl e n e ~ . Reduction ~.~ of the marginally stable silylene 2 with either one or two equivalents of Na/K leads to the disilyl dianion 3 or the silyl dianion 4 (Scheme 4).

7: Group 14: Silicon, Germanium, Tin and Lead

179

The dianion 4 slowly deprotonates THF with a reaction tl12 of ca. 3 hours to afford the hydrosilyl monoanion 5. The anions 3-5react with a range of electrophilic and Brarnsted acidic substrates.10 The d o l e and germole dianions 6 and 7,formed by alkali metal reduction of the corresponding dicholorosilane and -germane, have been shown for the first

Bu'/

\[Bu

\

2

1

'

'Bu

THF

'BU

4

24h

Scheme 4 Ph Ph

c1' %I

Ar

a

R

-2LiCI

Ph E = Si, 6 ph

E

Ph

0 0

E=Ge, 7

1) 0.5 equiv 6 or 7 2) MeOH, Hi(aq))

HO

Scheme 5

R

time to induce reductive coupling of aryl aldehydes and ketones under very mild conditions (Scheme 5)'' A review of the literature describing the use of anionic siloles, germoles and stannoles as n-bonded ligands to a range of Main Group and transition metals was published in 2001.12Synthetic routes to siloles are attracting greater attention due to their potential usage as single-layer electroluminescent devices.13 The synthesis and structure of the first isolable silyl radical (8) was r e p ~ r t e d . ' ~ %I.[Bu

1.-

Si, 'Bu,MeSi--S
's1.'

8

I

SiMe'Bu2

The second example of a free acyclic silylium cation [Dur3Si]+ (9) (Dur =

180

Organometallic Chemistry

duryl, 2,3,5,6-tetramethylphenyl)was obtained as the salt of [B(CbF5)4]-. The ion-separated nature of 9 was deduced from the 29SiNMR spectrum, which exhibited a signal at 6 = 226.8~pm.l~

3

Germylenes, Stannylenes and Plumbylenes

The chemistry of low-coordinate germanium, tin and lead compounds has been reviewed? The kinetically stabilized organotin(I1) halide (2,6-Trip2C6H3)SnCl (Trip = 2,4,6-triisopropylphenyl) was found to exist as both monomers (10a)and chloride-bridged dimers (lob) in the crystalline state. Solution-phase Il9SnNMR spectroscopy revealed that 10b readily dissociates to the monomeric form in C6D6.Addition of pyridine to 10b afforded the 3-coordinate, monomeric adduct [pySn(C1)(2,6-Trip2C6H3)](ll).” The less sterically hindered dimer [Sn(pCl)(2,6-Mes2C6H3l2 (12)was reduced by an excess of potassium metal to result in the unusual octanuclear cluster (13)in which the Sn-Sn bond order is formally 0.7.18 13 =

R

Sn

R = 2,6-Mes2C6H3

The formation of germylene intermediates in the thermally induced ring contraction of cis,cis-1,6,7-trigerma-1,6,7-tris(tri-t-butylsilyl)-7-mesityl-3,4dimethylbicyclo[4.l.0]-hept-3-ene (14) in the presence of diphenylacetylene was 21 cycloaddition products.’’ The first indicated by the formation of [2 synthesis and solid-state structural characterization of a stable, monomeric (15), and -stannylene, (2,6aryl(germyl)germylene, (2,6-Mes2C6H3)Ge(Ge‘Bu3) Mes2C6H3)Sn(Ge‘Bu3) (16), have been reported. Significant electron-donating effects in 15 and 16 were evidenced by red shifts in the n --+ p transition bands.” Intramolecular coordination by pendant Lewis bases group to the E(I1) centres was considered to be the origin of stability in the series of structurally characterized 1,3-dimetallacyclobutanes 17,18 and 19 (E = Ge, Sn, Pb, respectively).

+

..

I

Me3Si

17

Me3Si

E = Sn (18), Pb (19)

The qualitative mechanistic pathway proposed to account for the formation of 17-19 suggested that these metallacyclobutanes may have formed as a result of

7: Group 14: Silicon, Germanium,Tin and Lead

181

cycloaddition between two unstable metallavinylidene fragments?l The reactions of GeClydiox with sterically encumbered, alkoxy-functionalized phenyllithium reagents (1:2equivalents) proceeded smoothly under nucleophilic substitution to afford the donor-stabilized germylenes 20 and 21 (Scheme 6). 2[2,4-'Bu,-6-(ROCH&H2]Li

-I- GeCIydiox

-2LiCl

R = Me (22)

R = 'Pr (20) R = 'Bu (21)

Scheme 6

In contrast, however, the reaction of GeC12.dioxunder similar conditions with a methoxy-functionalized phenyllithium afforded the product of oxidative addition (22) even at low-temperatures. The synthesis and structure of the 3-coordinate, donor-stabilized complex [2,4-'Bu2-6-('BuOCH2)C6H2)GeCl (23)was also reported in this study.22In the solid-state molecular structures of 24-26 intramolecular coordination of the pyridyl moiety was E = Ge (24), Sn (25), Pb (26)

The zwitterionic carbene-stannylene adduct 27 was formed from the C = C double bond cleaving reaction between a bis(amin0)stannyleneand a dibenzotetraazafulvalene according to Scheme 7.24 I

\

N

Scheme 7

27

182

4

Organometalfic Chemistry

Multiply Bonded Compounds

Stabilization of Si=C bonds by means of intramolecular coordination of a dialkylamino group has permitted structural characterization of the silenes 28 and 29. In both 28 and 29 strong donor-acceptor interactions were revealed, resulting

in pyramidalization of the silicon centres, with retention of planarity about the silene ~ a r b o n .The ~ ~ reaction - ~ ~ of ‘ B U ( M ~ , S ~ ) ~ S ~ -with C H CTripLi ~ ~ (1:2 equivalents) afforded, on hydrolytic workup, the silanol ‘ B u ( T ~ ~ ~ ) S ~ ( O H ) - C H ( aS ~ M ~ ~ ) ~ , reaction which was assumed to proceed via the silene intermediate ‘Bu(Trip)Si= C(SiMe3)z(30).28Measurements and theoretical calculations of = (2-Ad) (31) (2-Ad = NMR chemical shielding tensors for (‘BuMe2)(Me3Si)Si 2-adamantyl) indicated the genuine existence of a Si = C n bond in ~ i l e n e s . ~ ~ Dehalogenations of a series of lY2-disupersilyldisilanes,R*HalzSi-SiHalHR*, by supersilylsodium afforded the corresponding disilanides R*Ha12Si-SiNaHR* which subsequently eliminate NaHal, resulting in formation of the unstable disilenes R*HalSi = SiHR. The fleeting existence of these disilenes was inferred from cycloaddition trapping reaction^.^' Reduction of the stable tetrakis(trialky1sily1)disilenes(R3Si)2Si= Si(SiR3)2(R3Si = ‘BuMezSi, 32; ‘Pr2MeSi,33) with lithium metal in THF solvent afforded the corresponding 1,2-dilithiodisilanes [(R3Si)2SiLi]2 (34 and 35 respectively), whereas reduction of (iPr3Si)2Si = Si(SifPr3)2 resulted in formation of the 1,l-dilithiosilane (‘Pr3Si)2Si(Li)z(36).,’ The reaction of the cyclotrisilene 37 with phenylacetylene gave the unusual product (38)with a bicycl0[3.2.0]hepta-3~6-diene framework (Scheme 8).

Ph 37

38

Scheme 8

Mechanistic studies involving deuterium labelling revealed that 38 formed following a series of [2 + 21 cycloadditions and i~omerizations.~~ Photolysis at h > 340nm of the silacyclopropenes (R3Si)2SiC2(SiMe3)2 (R3Si = ‘Pr3Si,39; R& = ‘Bu2MeSi, 40) afforded the thermally stable but photolabile 1-silaallenes (R3SQ2Si = C = C(SiMe3)2.33

183

7: Group 14: Silicon, Germanium, Tin and Lead

The tetrasila-1,3-butadiene 41 was shown to react with a variety of small molecule substrates. Reactions with Clz, NH334and N2H435 furnished the products of addition across the Si=Si bonds, whereas the reaction with maleic anhydride afforded 42 as the product of formal [4 + 41 cycloaddition (Scheme 9).36A rare example of [2 + 23 cycloaddition between a silene and a C = C double bond was observed in the photolysis reaction between the cyclic imine 43 and cyclo-CfBu2Si13(Scheme A novel approach to the synthesis of Si=Si and Si=Ge double bonds by treating the dilithiosilanes (R3Si)2Si(Li)z (R3Si = ‘Pr3Si,‘Bu2MeSi)with dichlorosilanes and -germanes Ar2EC12(Ar = Mes, Trip; E = Si, Ge), respectively, was described.38In contrast to the well established [2 + 21 cycloaddition chemistry of dirnetallene~,~~ the reactions of the 3H-disilagermirene

,R

R,

Si-Si SiRz R2Si,’ Cl c1

\,

R, ,R Si-Si\ R2Si’ SIR2 \ I NH NH I I NEI, NH2

I

N2H4

R,

Si-Si

R2S?

,R \\

2NH3

SIR2

41

42

Scheme 9

Scheme 10

P

R, Si-Si\ SiR? R2Si’ \

I

H2N.. H, NH

184

Organornetallic Chemistry

( " B ~ ~ M e s i ) ~ G(44) e S iwith ~ acetophenone and butane-2,3-dione yielded the cis enol ethers 45a and 46a, both of which subsequently isomerize to the thermodynamically favoured trans isomers 45b and 46b (Scheme 11). Concomitantly, the second carbonyl group in 46b inserts into the endocyclic Ge-Si bond to afford the norbornane derivative 47.4' The reaction of tetrakis[di-t-butyl(methyl)silyl]-2-disilagermirene (48) with phenylacetylene furnishes l,l ,2,3-tetrakis[di-t-butyl(met hyl)silylJ-4-phenyl-l ,2-disila-3-germacyclopenta-2,4-diene (49), a previously unknown heterocyclopentadiene type!' (which exists as the analogous digerThe reaction of (2-'B~-4,5,6-Me~c~H)~Ge mene in the solid-state) with 'BuC= P resulted in the formation of the unusual 2:2 product 50, which comprises a germadiphosphacyclobutene ring with an additional exocyclic Ge = C double bond (Scheme 12).42

Ar2Ge=GeAr2

= 2Ar2Ge:

2'BuCzP:+

Scheme 12

Ar2Ge-P I

t

50

The digermenes Ar(R)Ge = Ge(R)Ar (Ar = 2,6-TripzC6H3;R = Me 51, Et 52, Ph 53) adopt centrosymmetric structures in the solid-state that differ from the valence isomeric structures of the corresponding Sn and Pb compounds, the difference being attributable to the stronger nature of Ge=Ge double bonds relative to double bonds involving Sn and Pb.43Intramolecular coordination is responsible for the stability of the first germavinylidene (54).44

Recent progress in the structure, bonding and reactivity of so-called 'heavy ketones' has been reviewed!' The attempted syntheses of the heavy ketones Ar2M= 0 (M = Ge, Sn; Ar = 2,6-Mes2C6H3)by oxidation of the analogous germylene and stannylene, Ar2M,did in fact generate the gem-hydroxygermane and -stannane Ar2M(OH)2(M = Ge, 55; = Sn, 56), possibly due to adventitious water in the reaction mixture!6

185

7: Group 14: Silicon, Germanium, Tin and Lead

The synthesis, structure and reactivity of the intramolecularly donor-stabilized diazogermene ArGe = C(N2)SiMe3(57) (Ar = 2,6-['Pr2NCH2]2C6H3)has provided the first chemical evidence for the existence of a germyne. Photolysis of 57 at -50' C and h = 300nm resulted in the formation of a polymeric material of empirical formula [ArGeCSiMe3], and liberation of dinitrogen. Similar photolysis of 57 in the presence of alcohol trapping reagents afforded the gem-dialkoxygermanes ArGe(OR)2CH2SiMe3(R = Me, 'Bu) in reactions which are believed to feature a germyne intermediate.47Although triple bonds between two silicon atoms have evaded capture by experimental chemists up to 2001, insights from theory have strongly suggested that 'disilynes' (RSiaSiR)are indeed realistic synthetic

5

x-Bonded Compounds

The remarkable structural diversity exhibited by cyclopentadienyl (Cp) complexes of the Main Group elements is well known.%In particular, it has become apparent in recent years that the structures of Group 14 metallocenes may depend on a subtle interplay of factors. A theoretical treatment of the influence of intramolecular packing forces on the structures and properties of main group Cp compounds has been summarized in a comprehensive review in terms of the through-space coupling (TSC)concept, which is essentially the molecular orbital treatment of van der Waals repulsive/attractive forces.s1The uncertain relationship that exists between the molecular and electronic structures of Group 14 metallocenes was the stimulus for a reinvestigation of the structures and energetics of the parent compounds Cp2E (E = Si-Pb) using the density functional theory (DFT).52These calculations agreed with an earlier that the preference for bent instead of linear geometries in the gas-phase is slight, and highlighted that the metal-centred lone-pair of electrons may indeed be stereochemically inactive. Consequently, factors such as crystal packing forces and core polarization effects may be important in determining the molecular structures of Group 14 metall~cenes.~~ The first example of a n-bonded triple-decker main group cation, [($Cp*)Sn(pCp*)Sn(q'-Cp*)]+(58) (Cp* = C5Mes)as the salt of [Ga(C6Fs)4]-was synthesized and structurally characterized.

58

The two Cp,-Sn-Cp,

angles in 58 are 154.6 and 151.8' , respectively (Cp, =

186

Organometallic Chemistry

centroid of Cp ring).54A series of Sn(I1) half-sandwich complexes were also rep~rted."?'~ The reaction of Cp*,Si (59)with an excess of Me31nwas shown to proceed via insertion of Si lone pairs into two of the In-Me bonds according to Scheme 13, providing an alternative synthetic route to the relatively rare indium silyl compound~.'~ A strong suggestion as to the structure of protonated decamethylsilicocene, [Cp*&H] (61),was provided by DFT calculations of 29SiNMR chemical shifts for various Cp* hapticities, revealing that an q3:q2arrangement of the ligands is probable. Facile fluxionality in 62 via a series of ligand haptotropic shifts at temperatures as low as -80°C was also p r e d i ~ t e d The . ~ ~ increased interest in cyclopentadienylsilanes, - d i s i l a n e ~and ~ ~ -germane@' has stemmed from their potential use as precursors for the deposition of thin silicon and germanium films in the MOCVD process. +

60 Scheme 13

The series of bis(amino)boratabenzene complexes of Ge (63), Sn (64) and Pb (65) reveal monomeric bent sandwich structures with a large extent of ring slippage from idealized $-bonding.

(Me3SihN-13g$ E = Ge (63), Sn (64), Pb (65)

An appreciable electrostatic contribution to the metal ligand bonds in 63-65 was identified on the basis of "B NMR spectroscopic data.6' 6

Group 14 Organometallicsas Ligands at Transition Metal Centres

There is a relatively large and ever-growing volume of literature on the elaboration of Group 14 elements other than carbon at transition metal centres due to the potential alternatives that the heavier congenors may provide to range of established catalytic processes. The dinuclear Pd(0) complex 66, containing

187

7: Group 14: Silicon, Germanium, Tin and Lead

p-silylene ligands, was the first metal silylene complex to employed as a catalyst in Suzuki coupling reactions of bromoarenes and arylboronic acids.62The first example of a silylene-bridged dinuclear nickel complex (67) was also reported in 200 1.63

67

66

Synthetic and structural studies on a series of mono- and dinuclear bis(amino)silylene metal(0) carbonyl complexes was reported, and it was concluded that the electronic properties of the silylene ligands are similar to those of triarylphosphines. The cone angle generated by the silylene ligands in these complexes was also shown to be strongly dependent on the N - s ~ b s t i t u e n tA. ~ ~ theoretical treatment of the effects of various silylene silicon substituents on metal-ligand bond orders found that methyl and triflate promoted an appreciable degree of M = Si double bond ~haracter.6~ Synthesis and structural characterization of the tungsten silylene complex 68 revealed a very short W-Si bond distance of 2.354(3)A.66367

68

The first silabenzene complex (69) was obtained and characterized by multinuclear NMR spectroscopy, which suggested that the bonding of the ligand is best described as q5:q1,signifying an important contribution from the resonance form 69b.68The attempted synthesis of a 1,4-disilabenzene complex did in fact result in the metalladisilanorbornadiene (70) which was structurally authenticated by X-ray crystallography. A theoretical study of a related model complex revealed that the Ru-Si interaction in 70 may be described as being intermediate between the two resonance forms 70a and 70b.68 The reactions of the donor-stabilized plumbylene 71 with the Group 6 carbonyls M(C0)5L(L = THF, NMe3)afforded the trimetallic complexes with M = Cr (72),Mo(73)and W(74).

188

*

Organometallic Chemistry

Ru 0 e S i - ' B u

-

Ru 8

<=>Si-'Bu

69b

69a

I

7Qa

70b

Approximate trigonal bipyramidal geometry is adopted by the lead centres in

72-73,and the lengths of the Pb-M bonds are likely to indicate some nbonding.69 References 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13.

14.

N. Wiberg and W. Niedermeyer, J . Orgunomet. Chem., 2001,628,57. M. Haaf, T. A. Schmedake and R. West, Acc. Chern. Rex, 2000,33,704. B. Gehrus and M. F. Lappert, J. Orgaraomet. Chem., 2001,617-618,209. S. Ishida, T. Iwamoto, C . Kabuto and M. Kira, Chem. Lett., 2001,1102. N. Takeda, T. Kajiwara and N. Tokitoh, Chem. Lett., 2001,1076 A. Kawachi, T. Minamimoto and K. Tamao, Chem. Lett., 2001,1216. B. Gehrus, P. B. Hitchcock and M. F. Lappert, 2. Anorg. Allg. Chem., 2001, 627, 1048. Y. Ding, H. W. Roesky, M. Noltemeyer, H. -G. Schmidt, and P. P. Power, Organometallics, 200 1,20, 1190. M. Stender, A. D. Phillips and P. P. Power, Inorg. Chem., 2001,40,5314. R. West, T. Schmedake, M. Haaf, J. Becker and T. Mueller, Chem. Lett., 2001,68. Y. Liu, D. Ballweg and R. West, Organometallics, 2001,20, 5769. A. P. Sadimenko, A h . Heterocyclic Chem., 2001,79, 115. S . Yamaguchi, T. Endo, M. Uchida, T. Izumizawa, K. Furukawa and K. Tamao, Chem. Lett., 2001,98. A. Sekiguchi, T. Matsuno and M. Ichinohe, J . Am. Chem. Soc., 2001,123,12436.

7: Group 14: Silicon, Germanium, Tin and Lead

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

35. 36. 37. 38.

39. 40. 41. 42. 43. 44. 45. 46. 47. 48.

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J. B. Lambert and L. Lin, J. Org. Chem., 2001,66,8537. M. Weidenbruch, Main Group Met. Chem., 2001,24,621. B. E. Eischler, L. Pu, M. Stender and P. P. Power, Polyhedron, 2001,20,551. B. E. Eischler and P. P. Power, Angew. Chem. Int. Ed. Engl., 2001,40,796. N. Fukaya, M. Ichinohe, Y. Kabe and A. Sekiguchi, Organometallics, 2001, 20, 3364. W. Setaka, K. Sakamoto, M. Kira and P. P. Power, Organometullics, 2001,20,4460. W. -P. Leung, Z. -X. Wang, H. -W. Li, Q. -C. Yang and T. C. W. Mak, J. Am. Chem. Soc., 2001,123,8123. P. Jutzi, S. Keitemeyer, B. Neumann, A. Stammler and H. -G. Stammler, Organometallics, 2001,20,42. S. S. Al-Juaid, A. G. Avent, C. Eaborn, M. S. Hill, P. B. Hitchcock, D. J. Pate1 and J. D. Smith, Organometallics, 2001,20, 1223. F. E. Hahn, L. Wittenbecher, M. Kuhn, T. Lugger and R. Frohlich, J. Organomet. Chem., 2001,617-618,629. M. Mickoleit, R. Kempe and H. Oehme, Chem. Eur. J.,2001,7,987. K. Schmohl, H. Reinke and H. Oehme, Eur. J. Inorg. Chem., 2001,481. M. Potter, U. Baumer, M. Mickoleit, R. Kempe and H. Oehme, J. Organomet. Chem., 2001,621,261. K. Schmohl, H. Reinke and H. Oehme, 2. Anorg. Allg. Chem., 2001,627,2619. J. J. Buffy, R. West, M. Bendikov and Y. Apeloig, J. Am. Chem. Soc., 2001,123,978. N. Wiberg, H. Auer, S. Wagner, K. Polborn and G. Kramer, J . Organomet. Chem., 2001,619, 110. M. Kira, T. Iwamoto, D. Yin, T. Maruyama and H. Sakurai, Chem. Lett., 2001,910. M. Ichinohe, T. Matsuno and A. Sekiguchi, J. Chem. Soc., Chem. Commun., 2001, 183. M. Ichinohe, T. Tanaka and A. Sekiguchi, Chem. Lett., 2001,1074. S. Boomgaarden, W. Saak, M. Weidenbruch and H. Marsmann, 2. Anorg. Allg. Chem., 2001,627,349. S . Boomgaarden, W. Saak, H. Marsmann and M. Weidenbruch, 2. Anorg. Allg. Chem., 2001,627,805. S . Boomgaarden, W. Saak, M. Weidenbruch and H. Marsmann, Organometallics, 2001,20,349. N. G. von Keyserlingk, J. Martens, D. Ostendorf, W. Saak and M. Weidenbruch, J. Chem. Soc., Perkin Trans. 1,2001,706. M. Ichinohe, Y. Arai, A. Sekiguchi, N. Takagi and S. Nagase, Organometullics, 2001,20,4141. V. Y. Lee, M. Ichinohe and A. Sekiguchi, Chem. Lett., 2001,728. V. Y. Lee, M. Ichinohe and A. Sekiguchi J. Chem. Soc., Chem. Commun., 2001,2146. V. Y. Lee, M. Ichinohe and A. Sekiguchi, J. Organomet. Chem., 2001,636,41. F. Meiners, W. Saak and M. Weidenbruch, J. Chem. SOC., Chem. Comrnun., 2001, 215. M. Stender, L. Pu and P. P. Power, Organometullics, 2001,20, 1820. W. -P. Leung, Z. -X. Wang, H. -W. Li and T. C. W. Mak, Angew. Chem. Int. Ed. Engl., 2001,40,2501. N. Tokitoh and R. Okazaki, Adu. Organomet. Chem., 2001,47,121. L. Pu, N. J. Hardman and P. P. Power, Organometallics, 2001,20,5105. C. Bibal, S. MaziGres, H. Gornitzka and C. Couret, Angew. Chem. Int. Ed. Engl., 2001,40,952. K. Kobayashi, N. Takagi and S. Nagase, Organometallics, 2001,20,234.

190 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69.

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N. Takagi and S. Nagase, Chem. Lett., 2001,966. P. Jutzi and N. Burford, Chem. Rev., 1999,99,969. V. N. Sapunov, K. Kirchner and R. Schmid, Coord. Chem. Rev., 2001,214,143. J. D. Smith and T. P. Hanusa, Organometallics, 2001,20,3056. D. R. Armstrong, M. J. Duer, M. G. Davidson, D. Moncrieff, C. A. Russell, C. Stourton, A. Steiner, D. Stalke and D. S. Wright, Organometallics, 1997,16,3340. A. H. Cowley, C. L. B. MacDonald, J. S. Silverman, J. D. Gorden and A. Voigt, J . Chem. Soc., Chem. Commun., 2001,175. G. M. de Lima, H. G. L. Siebald, J. L. Net0 and V. D. de Castro, Main Group Met. Chern., 2001,24,223. G. M. de Lima, D. J. Duncalf and S. P. Constantine, Main Group Met. Chem., 2001, 24,675. T. Kiihler, P. Jutzi, A. Stammler and H. -G. Stammler, J . Chem. Soc., Chem. Commun., 2001,539. T. Miiller, P. Jutzi and T. Kuhler, Organometallics, 2001,20, 5619. A. Klipp, S. H. A. Petri, B. Neumann, H. -G. Stammler and P. Jutzi, J . Organornet. Chem., 2001,628,57. K. Dittmar, P. Jutzi, J. Schmalhorst and G. Reiss, Chem. Vap. Deposition, 2001,7, 193. X. Zheng and G. E. Herberich, Eur. J . Inorg. Chem., 2001,3013. A. Furstner, H. Krause and C. W. Lehmann, J . Chem. Soc., Chem. Commun., 2001, 2372. S. Shimada, M. L. N. Rao, T. Hayashi and M. Tanaka, Angew. Chem. Int. Ed. Engl., 2001,40,213. T. A. Schmedake, M. Haaf, B. J. Paradise, A. J. Millevolte, D. R. Powell and R. West, J . Organomet. Chem., 2001,636, 17. H. P. Hratchian, T. Prendergast and M. C. Milletti, Polyhedron., 2001,20,209. B. V. Mork and T. D. Tilley, J . Am. Chem. Soc., 2001,123,9702. H. Sakaba, T. Hirata, C. Kabuto and H. Horino, Chem. Lett, 2001,1078. J. M. Dysard, T. D. Tilley and T. K. Woo, Organometallics, 2001,20, 1195. N. Seidel, K. Jacob and A. K. Fischer, Organometallics, 2001,20,578.

8

Group 15: Phosphorus, Arsenic, Antimony and Bismuth ~

~~

~~

BY MATTHEW D. FRANCIS

1

Phosphorus

Due to space limits, a comprehensive review of organo-phosphorus and phospha-organometallic chemistry is not possible. Consequently the first part of this article will focus predominantly, but not exclusively, on low coordinate phosphorus chemistry. Reports which are not specifically based on low coordinate phosphorus have been included where it is felt they are relevant or are closely related to the theme of the main subject. Several reports dealing with the chemistry of phosphaalkynes have appeared including a review of the comparative behaviour of phosphaalkynes and alkynes towards electron rich phosphinic metal centres.' The preparation of a range of kinetically unstabilised phosphaalkynes has also been described. This involved the chemoselective reduction of a-dichlorophosphonates, RCC12-P(= O)(OPri)2, with AlHC12 followed by bis-dehydrohalogenation of the resultant a-dichlorophosphines, RCC12PH2,using a strong Lewis base such as DBU. Yields of 60-81% were attainable for the phosphaalkynes R-C=P (R = H, Me, Et, Bun, PhCH2CH2-,CH2= CHCH2-, CH2= CHCH2CH2-,C6HI1).Most of these phosphaalkynes were stable for prolonged periods of time at -20" C in solution in the presence of duroquinone as a radical inhibitor.2 The reaction of the bulky germylene Ar2Ge: (Ar = 2-tert-butyl-4,5,6-trimethylphenyl) with ButC=P was explored in a communication by Weidenbruch and c o - ~ o r k e r s Unlike .~ the reactions of phosphaalkynes with less hindered dialkyl germylenes which generally yield [I2+ 11 cycloaddition products, this reaction afforded a 59% yield of a compound 1 (X-ray) comprising a germadiphospha-cyclobutene unit with an additional exocyclic germanium-carbon double bond. NMR spectroscopic data were consistent with the presence of two conformers of 1 in solution as a result of the different orientation of substituents at the exocyclic carbon. No interconversion of the two was observed even at elevated temperatures however, as a consequence of the steric encumberance of the bulky aryl groups. The ability of phosphaalkynes to bond in an q2(4e)manner was demonstrated with the synthesis, in 85% yield, of a d2 tantalum complex [TaCl2(q5-C5Mes){q2(4e)Bu'C=P}] 2 from [TaCl2(q'-C5Me5)(CO)2(thf)] and ButC=P? An X-ray structural analysis of 2 showed a short distance of 2.079(9)A between the Ta centre Organometallic Chemistry, Volume 3 1 0The Royal Society of Chemistry, 2004 191

192

Organometallic Chemistry

and the phosphaalkyne carbon indicating participation of the nl orbital and consequent 4e involvement, a conclusion which was supported by Extended Hiickel MO calculations. Complex 2 was found to slowly react with an excess of Bu'CsP to afford the 1,2-diphosphacyclobutadiene complex [TaC12(q5CSMe~)(o,o,n;-1,2-P2C2But2)]. An X-ray structural analysis of this complex indicated a o,o,xbonding mode rather than an q4 bonding mode through a delocalised n system, The reactivity of the amino-substituted phosphaalkyne Pr'2N-C=P with the N-heterocyclic Arduengo-type carbene 1,3,4,5-tetramethylimidazol-2-ylidene has been e ~ p l o r e dThis . ~ reaction led to the azaphosphole 3 in quantitative yield after crystallisation from pentane. Support for the existence of the canonical form 3A came from the reaction of the azaphosphole with BH3.thf which yielded a product with two coordinated BH3 groups solely at the phosphorus centre. In addition to the carbon-phosphorus triple bond of phosphaalkynes, a report appeared this year concerning the chemistry of the tungsten-phosphorus triple bond in a reactive intermediate! Thermolysis of [Cp*P{ W(CO)s}2] enabled formation of a reactive intermediate with a W-P triple bond namely [CP*(CO)~W=P{ W(CO),}] which was trapped with alkynes to afford a number of novel compounds which were spectroscopically and structurally characterised. Furthermore, the photolysis of [Cp*P{ W(CO)s}2] was also explored. In this case, the intermediate phosphido species [Cp*(CO)2W=P{W(CO)5)jwas again formed by Cp* migration but also the intermediate [P{ W(CO)s},] was generated by elimination of Cp*. Dimerisation of the two intermediates afforded the novel complex [{ C~*(Co)~w~}(p-H)(pq :q :q - P2){ W(CO),} 2] (X-ray).

1

2

3A

Several reports appeared concerning the chemistry of systems containing 02,h3 phosphorus environments such as phosphaalkenes and diphosphenes and the relatively new class of phosphavinyl Grignard reagents RP = CBu'(MgC1). Treatment of [CyP = C(But)MgC1(OEt2)]4 (Cy = cyclohexyl) with in situ generated CyInBrz afforded the first group 13 phosphavinyl complex 2,Z[CyIn{ C(Bu')= PCY}~]which was characterised spectroscopically and by X-ray crystallography! Reaction of the same phosphavinyl Grignard with the group 13 trihalides MX3 (M = Al, Ga, In ; X = Cl, Br) led unexpectedly to the diphosphametallobicyclo[ 1.1.llpentane species 5-7 by an unknown mechanism. The first endo:endo-2,4-diphosphabicyclo[1.l.Olbutane was reported to result from the reaction of 4 with PbC12 possibly via an oxidative coupling mechanism. No valence isomerisation was found to occur by thermal or photochemical means. Interestingly, DFT calculations on the simpler tetramethyl-2,4-diphosphabicyclo[ l.l.O]butane suggested the endo:endo isomer to be 25 kJmol-' less stable than the endo:exo isomer. The formation of this less stable valence isomer

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193

was attributed to stereochemical constraints during the oxidative coupling stage? Two reports appeared concerning the coordination properties of systems bearing both a &A3 and a &,A3 phosphorus centre. 2-[Mes*P = C(Cl)PPh2] (Mes* = 2,4,6-tri-tert-butylphenyl)forms an adduct with the W(C0)5 fragment via interaction of the lone pair on the saturated phosphorus centre. Irradiation of this adduct or treatment of the initial uncoordinated material with [W(CO)4(COD)](COD = cycloocta- 1,5-diene) yielded a chelate complex with the W(C0)4 moiety bound to both phosphorus cenfres.l0 The chelating species &[ortho-(Ph2P)C6H4-CH= PMes*] can be isomerised to its 2 isomer by irradiation in benzene solution and forms chelate complexes with Mo(O), Pd(I1) and Pt(I1) involving coordination of both phosphorus centres.'' P/Q

M = Al,5 ; Ga6 ; In 7

Mes*

8

hes*

Studies of phosphaalkenes and their complexes include the chemistry of the { p-q':q '-P( Ph) = C(H)Me}] with unusual compound ci~/trans-[Cp~(CO)~Mo~ both inorganic and organic reagents.12 The reactivity of inversely polarised phosphaalkenes RP =C(NMe2)'(R = But, Me3%,H) towards ethoxyaryl Fisher type carbene complexes, [(CO),M = C(OEt)Ar] (Ar = Ph, M = Cr, W ; Ar = 2-MeC6H4,2-Me0-C6H4, M = W), has been reported. This resulted in the formation of complexes of the type [{ RP = C(NMe2)2}M(CO)5]but also complexes of the type [{ RP = C(OEt)Ar}M(C0)5] resulting from a metathesis reaction.13 Excellent catalytic activity for the hydroamination of 1,3-dienes was observed for q3-allylpalladium complexes bearing diphosphinidene-cyclobutene ligands 8. Product yields of 66-96% were reported with the best results in aromatic solvents and poor activity in polar solvents such as dimethylformamide and dich10romethane.l~A quantum mechanical investigation has been carried out on the formation of complexes between electron rich phosphaalkenes and &-transition metal fragments? In the case of mononuclear complexes, in which the metal is coordinated via the P-lone pair, there is little change in the geometry of the phosphaalkene unit compared with the uncoordinated ligand. Substitution of the doubly bonded carbon with an amino group alters this situation however. In such a case the P =C bond is lengthened because of the stronger N-C n bonding. This causes development of negative charge and slight pyramidalisation at the phosphorus centre. A slight lengthening of the P-M bond is seen as a result. Phosphinidenes generated photochemically from phosphaWittig reagents have a different fate depending on the nature of the substituent at the cr2 phosphorus.16Thus Mes*P = PMe, when photolysed yields the transient phosphinidene { Mes*P} which undergoes an intra-molecular insertion of the phosphorus into a vicinal C-H group of the ortho But group. In contrast photolysis of ArP = PMe3 (Ar = 2,6-(2,4,6-trimethylphenyl)phenyl) afforded the

194

Organometallic Chemistry

diphosphene ArP = PAr. In the latter case, where such an intra-molecular insertion cannot take place, the generated phosphinidene {ArP} may dimerise to form the resulting diphosphene, or more likely, attack unreacted ArP = PMe3to form the diphosphene with concomitant elimination of trimethylphosphine. The strong basic character of phosphaallenic radical anions was investigated by studying the electrochemical reduction of fluoren-9-ylidenemethylene-(2,4,6-tritevt-butyl-pheny1)phosphane in THF. The study showed that the EPR spectrum of the reduced species was due to the phosphaallylic radical formed by protonation of the initially formed radical anion,” A highly persistent diphosphanyl radical [Mes*(Me)P-PMes*]’ with a t1,2of 90 minutes at room temperature was reported to form from the chemical reduction of the phosphonium salt [Mes*(Me)P = PMes*][03SCF3] with tetrakis-(dimethylamino)ethylene.l*The synthesis of the first l-aza-3-phosphabuta-1,3-diene complexes [(Me3Si)2C= PC(pip)= N-P(C1)R{W(C0)5}] (pip = piperidino R = CH(SiMe3)2,C5Me5)have been reported this year.19 Several reports appeared concerning systems with a P = P bond in which one phosphorus centre is trivalent and the other pentavalent e.g. But2P-P= P M ~ ( B u ‘ ) ~22. ~ ’ Phospholyl and polyphospholyl anions [C,R,P5-,] - and complexes derived from them have featured extensively in this year’s literature and a review appeared on the use of such ligands and other related heterocyclopentadienyls in the formation of complexes of the group-3 metals (Sc, Y, Ln series and U).23In particular, numerous reports have appeared concerning phospha-metallocene complexes and their use in a variety of catalytic processes. Zakrzewski and co-workers described the resolution of rac-3,3’,4,4-tetramethyl-1,l’-diphosphaferrocene-2-carboxylic acid by preparation of diastereomeric salts with homochiral brucine. The salts could be separated by fractional crystallisation and then separately converted back to their enantiomerically pure carboxylic acids by hydrolysis with dilute HCl.24Conversion of the same racemic phosphaferrocene to diastereomeric amides with (S)-a-phenylethylamine was achieved and the amides separated by column ~ h r o m a t o g r a p h y Copper .~~ (I) mediated oxidative coupling of (-)-menthylacetylene followed by treatment with PhPHLi afforded a good yield of a novel chiral phosphole.26This phosphole could be converted readily to a number of chiral complexes including its corre(R = (-)-menthyl). sponding phosphaferrocene [Fe(q5-C5H5)(q5-PC4H2-2,5-R2)] The synthesis of the chiral phosphino-phosphaferrocene [Fe(q5CsH4{ CH2PPh2})(q5-PC4H2-2,5-R2)] 9 was also reported.27This acts as a ligand forming an in situ complex with [ ( P ~ C ~ ( T C - C ~ Hwhich ~ ) } ~is] a highly active catalyst for the asymmetric allylic alkylation of rac- 1,3-diphenyl-2-propenyl acetate with dimethyl malonate. Yields of alkylation product and enantiomeric excesses ranged from 97-99 YOand 77-99 YOrespectively depending on the reaction conditions. The use of phosphaferrocenes as ligands has been further explored by a number of research groups. Reaction of 2-phenyl-3,4-dimethylphosphaferrocene with [ R u H ~ ( H ~ ) ~ ( Pled C ~to ~ )evolution ~] of one equivalent of dihydrogen and formation of the ruthenium (11) phosphaferrocene complex [RuH~(q’-H2)(Pcy,),( FeCp(q’-PC4H-2-Ph-3,4-Me)}] 10 which is fluxional in solution. Variable temperature NMR studies of 10 yielded a AGt value of 46.2

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195

kJmol-I at 263 K for the exchange between the hydrides and the dihydrogen ligand.28The phosphaferrocene [FeCp(q5-PC4Et4)]reacts with [Pd(COD)C12] to give cis-[Pd{ FeCp(q5-PC4Et4)}2C12]which decomposes slowly in solution to give the novel dimeric species [{ Pd[FeCp(q5-PC4Et4)]C1)21incorporating a Pd-Pd bond. A similar scenario was observed with the related phosphaferrocene [FeCp(q5-PC4H-2-Ph-3,4-Me)]?9 Two reports appeared concerning phosphaferrocenes with additional pendant Cp functionalities viz. [FeCp(q5-PC4-3,4Me2-5-{C5Me4H})]and [Fe(q5-C5R5){q5-PC4-3,4-Me2-5-(C5H4)}][Na] (R = H, 31 In the latter case the pendant cyclopentadienyl group was used for further pentahapto coordination to a ruthenium centre resulting in a bimetallic complex.31

9

10

11

A direct way of introducing acetoacetyl groups into ferrocenes and phosphaferrocenes was reported which avoided the need for a Claisen condensation type reaction. This was especially important for phosphaferrocenes which are sensitive to the strong basic conditions needed for the Claisen r e a ~ i o n The .~~ knowledge that strained [1]- and [2]-ferrocenophanes can be ring opened to give ferrocene based polymers led to the motivation for the synthesis of a phosphaferroceneophane namely 3,3’,4,4’-tetramethyl-l,l‘diphospha[2lferrocenophane 11. Complex 11 was obtained as a single isomer.33Fu and co-workers reported the synthesis of two early transition metal based phospholyl complexes, ra~-[MCl~(q~-PC~-3,4-Me-5-Ph)~] (M = Zr 12, Hf 13).34Interestingly, diastereomerically pure samples of each racemic mixture are converted to mixtures of the rac and meso isomers. This happens with a t1/2of 12 hours in benzene 12 but much more rapidly for 13 which reaches equilibrium within minutes. Furthermore, addition of polar Lewis bases such as PMe3 or thf rapidly accelerates the rate at which the isomerisation occurs presumably by stabilising ring-slipped intermediates. Complex 12 could be converted to its dimethyl derivative 12A by treatment of 12 with a methyl Grignard reagent. Both of these complexes showed high activity as catalysts for the co-polymerisation of ethylene and hexene. The synthesis of q5-2,5-di-tert-butylphospholyl gallium from the corresponding lithium phospholyl and a solution of metastable GaBr was described. The solid state structure of this novel main group half sandwich complex revealed zigzag chains of gallium centres with alternating phospholyl rings on both sides The 3,5-di-tert-butyl-1,2,4-triphospholyl anion [P3C2But2]- 14 of the featured in a number of publications this year. The pyridine adducts of the zinc and cadmium complexes, [ M ( ~ ’ - P ~ C ~ B U ~ ~ ) ~(M ( N= C Zn, ~ Hn~=) ~2];M = Cd, n = 3) derived from 14 were described by Nixon and c o - w ~ r k e r sIn . ~both ~ cases the triphospholyl ligand is bonded in a mono-hapto fashion to the metal centre.

196

Organometallic Chemistry

Both complexes are fluxional in solution with the metal rapidly migrating between the two adjacent phosphorus centres of the triphospholyl ring, a process which was deemed to be extremely rapid in both cases since no coalescence could be observed even at -95 "C.Complexes of 14 have also been synthesised by the reaction of the phosphaalkyne B u G P with metal atoms generated by the metal vapour synthesis (MVS)procedure. In this manner, reaction of cobalt atoms with ButC=P yielded the complexes [Co(q5-P3C2But2)(q4-P2C2But2) 1, [C0(q5P2C3But3)(q4-P2C2But2)]and the unusual protonated tetraphospha-barrelene complex [Co(q4-P4C4But4H)(q4-P2C2But2) 15 which was structurally characterised as its adduct with W(C0)5.37Phosphines and isonitriles could be used to substitute carbonyl groups in [Co(qS-P3C2Bu'2)(C0)2] and [Mn(qSP,C~BU'~)(CO)~] via thermal or photochemical means. Variable temperature NMR studies of one of the products, [Mn(q5-P3C2B~t2)(PMe3)2(CO)], enabled the activation energy of rotation of the triphospholyl ring to be evaluated as 53.5 kJm01-l.~~ In two publications, density functional theory (DFT) and photoelectron spectroscopy (PES) were used to probe the electronic structures of a number of complexes derived from 14 and the related diphospholyl anion (M = Mn, Re)39 [P2C3But3]-. These complexes included [M(q5-P3C2But2)(C0),] and [M(qS-P3C2B~'2)2] (M = Fe, Ru), [Fe(qS-P2C3B~t3)2] and [Fe(q5P ~ C ~ B U ' ~ ) ( ~ ~ - P GInBthe U ~latter ~ ) ] . complexes ~~ the presence of phosphorus centres in the ring increased their acceptor properties compared with carbocyclic analogues. However in the tricarbonyl manganese and rhenium complexes the presence of electron withdrawing CO groups led to ring-to-metal donation being more significant than back donationfrom the metal. But P

4

Neutral phospholes and polyphospholes have also featured in the literature year. Reports include the synthesis of the first 1,3,4-triphosphole complex [P{ CH(SiMe,),}C(NHPr') = P-P{W(CO)5}], prepared from the regiospecific insertion of Pr'(SiMe3)NC=Pinto the P-P bond of the 1H-diphosphirene complex [(Pr'HN)C = P-P{CH(SiMe3)}{W(CO)5}]. A partially unrefined X-ray analysis indicated a weakly pyramidal d,h3phosphorus centre (X angles = 322°).41The neutral triphosphole [P = C(Bu')P =C(Bu')P{CH(SiMe3)}] derived from the triphospholyl anion 14 was found to react with cobaltocene to afford two novel organo-phosphorus cage compounds: P&But4CH(SiMe3) 16 and the diamagnetic cobalt complex [CoCp(q4-C4H4CHCHP&But4)]17 (X-ray)?2Interestingly, both these complexes are members of an emerging family of cages of general

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

197

formula P6C4Buf4Ederived from the parent cage P6c&& 18. Compound 18 itself reacts directly with Se or Te resulting in facile insertion of the chalcogen into the P-P bond between the two five membered rings.43Theoretical studies on the parent compound P6C4H418A were carried out to understand why only the unique central P-P bond of 18 is involved in such insertions. It was found that this P-P bond is involved both in the HOMO and the LUMO but also the HOMO-1. Furthermore the lowest harmonic frequency in 18A (224cm-') involves a considerable lengthening of this bond while the rest of the cage hardly moves. Other recent studies have shown that 18 also reacts in an analogous manner with other carbene like species including the stable germylene GeR2, stannylene SnR2(R = N(SiMe3)2)and plumbylene PbAr, (Ar = C6H3(NMe2)2).44 The importance of 7-phosphanobornenes in asymmetric catalysis led to an improved route to trivalent 7-pho~phanobornenes~~ This was achieved from the [4 21 cycloaddition of a dienophile to a phosphole modified in such a way as to enhance dienic activity. In this report, it was found that the presence of an electron withdrawing group such as -CN or -OR at the trivalent phosphorus enabled such a modification by reducing the delocalisation of the lone pair. Thus 1-cyano-3,4-dimethylphosphole reacted with acrylonitrile giving an inseparable mixture of the corresponding syn and anti phosphanobornenes, endo-2-cyano5,6-dimethyl-(anti)- and (syn)-7-cyano-7-phosphabicyclo[2.2.l]hept-5-enens. The endo stereochemistry of the -CN substituent was not proven directly but was inferred from the analogous reactivity of the isopropoxyphosphole. The reaction of the alkoxy phosphole l-isopropoxy-3,4-dimethylphospholewith acrylonitrile proceded with better stereochemical control and only the corresponding anti,endo 7-phosphanobornene was formed exclusively. The stereochemistry was determined from an X-ray crystal structure of the P-sulphide derivative of the product. The difficulty of functionalising phospholes directly was addressed in an article by Mathey and Deschamps and co-workers."6 They reasoned that a possible approach would be the functionalising of a corresponding phosphacymantrene followed by liberation of the free phosphole. Unlike phospholes, phosphacymantrenes can be readily functionalised because of their readiness to undergo electrophilic substitution. They reported that the phosphacymantrene [Mn(q'-PC4-2,5-Ph)(CQ3] could be photochemically degraded in methanol affording a 64% yield of the corresponding phosphole as its dimer, 2,2',5,5'-tetraphenyldiphosphole.This method however was unsuccessful for the related phosphacymantrenes [Mn(q5-PC4-3,4-Me)(C0)3]and [Mn(q5PC4-2-Ph-3,4-Me)(CO)3]. Six membered heterocycles containing one or more low coordinate phosphorus centres have been the focus of a number of studies this year. In particular, further studies have appeared concerning the 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene [P3C3Buf3]19 which has been the focus of much attention in recent years. Motivated by the unusual and contrasting behaviowr of 19 towards silylenes and Arduengo-type carbenes, Lammertsma and co-workers examined the reactivity of 19 towards the in situ generated phosphinidene PhPW(C0)S. This resulted in two unexpected products 20 and 21 (both X-ray characterised)?' A proposed mechanism, examined with DFT calculations at the B3LY P/6-3 1G*

+

198

Organometallic Chemistry

level suggested that initially the phosphinidene adds to one of the P = C bonds of 19 forming an intermediate with possible syn and anti isomers, an intramolecular rearrangement of the anti isomer leading to 20. In contrast, a 1,3-sigmatropic rearrangement of the syn isomer could give an intermediate tetraphosphanorbornadiene which, by means of an intramolecular [2 + 21 cycloaddition, would lead to 21. This hypothesis was further examined in a second publication in which 19 was reacted with the transient phosphinidene MePW(C0)5, thus eliminating the possibility of a product such as 20.48Pleasingly this led to a separable 1%mixture of the tetraphosphanorbornadiene 22 (X-ray) and 23, the methyl analogue of the quadricyclane 21. Heating a solution of isolated 22 or 23 led to the same 1:8 equilibrium mixture of both compounds.

20

22

21

A family of q6 phosphinine (phosphabenzene) cationic complexes were described this year. Synthesis was achieved by treatment of the appropriate phosin the presence of AgBF4.49One of the phinine with [Ru(q5-C5Me5)(q4-C6HlO)Cl] complexes, [Ru(q5-CsMe5){q6-PC5H3-2,6-(SiMe~)z}][BF4], was characterised by X-ray crystallography. In contrast, when similar reactions were carried out with less sterically hindered phosphinenes the products, which depended on the reaction conditions, contained only monohapto bonded phosphinines. Incorporating two phosphinene units into the macrocycle 24 enabled the two phosphorus centres to be brought into close proximity such that a P-P bond could be formed by chemical or electrochemical r e d ~ c t i o nBoth . ~ ~ a single reduction to the radical anion and the dianion were possible. The corresponding radical anion was studied in solution using EPR which, coupled with theoretical calculations, suggested the presence of a one electron bond (2.763 A), the electron localised in a 0 P-P orbital. The further reduced dianion was isolated and characterised by X-ray crystallography which clearly showed a P-P linkage of 2.305(2)A.

\

24

Three membered rings have also featured in the literature this year including a report describing the synthesis of polyphosphirene chains.51Also described was the gas phase detection of the first azaphosphirenium cation [MezN = PNMe2]+ which was generated in a mass spectrometer by the degredation of the

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

199

ferriphosphaalkene [Fe(q5-C5Me5)(CO)2{ P = C(NMe2)2}]and cyclisation of the transient phosphavinyl cation [P = C(NMe2)2]+.52 Such species have not yet been observed in solution but have been predicted by DFT calculations. Numerous other reports dealing with phospha-organometallic chemistry have appeared including some dealing with P(V) systems as well as P(II1). A review of the chemistry of N-heterocyclic carbenes described the synthesis of carbenephosphinidene and -arsinidene complexes from the reaction of Arduengo type carbenes with homocyclic group 15 systems (ER), (E = P, As ; R = Ph, CF3, C6Fs; n = 4,5,6).53 Two bonding situations can be envisaged in these complexes i.e. a low valent Fischer-type situation or a high valent Schrock-type alkylidene bonding mode. Spectroscopic analysis and X-ray crystallography suggested the predominance of the Fischer type bonding, a conclusion which was supported by the reaction with BH3 leading to coordination of the complex to two BH3 units. The use of the phosphoraneiminato group R3PN- as a ligand with early transition metals has been explored by Niecke and c o - w o r k e r ~They . ~ ~ considered, experimentally and theoretically, the factors influencing the tautomerisation between the group 4 complexes [(R2N)2PN(H)-MCp,C13-,,] and [(R2N)2P(H)=N-MCpnCl3_,] (M= Ti, Zr, Hf). It was found that the PH tautomer was favoured by a more Lewis acidic metal centre i.e. one which contained fewer Cp groups, which compete with the phosphorane-iminato group for the metal’s acceptor orbitals. Several reports have appeared concerning phosphoniobenzo-phospholides including a study of the structural and electronic properties of neutral phosphoniobenzo[c]phospholides55 and the use of a chelating bis-phosphoniobenzo-phospholide cation in the formation of rhodium complexes with catalytic proper tie^.^^ The first catena-trihydrogen triphosphide [Na(NH3)5][Na(NH3)3(P3H3)]has also been synthesised from the reaction of P d with sodium in liquid ammonia and characterised by X-ray ~rystallography.~~ 2

Arsenic

An interesting essay appeared in the literature this year entitled ‘Cadet’sfuming arsenical liquid and the cacodyl compounds of Bunsen’, charting the historical development of organo-arsenic ~hemistry.~’ Cadet’s fuming liquid, then of unknown composition, was one of the first organo-arsenic compounds to be prepared. Work by later experimentalists including that of Robert Bunsen in the mid 1800s enabled the main components of this noxious material to be established as (CH3)2AsOAs(CH3)2 and (CHJ2As-As(CH&. A number of reports have appeared this year concerning the chemistry of low-coordinate arsenic systems including a new route to diarsenes and arsaphosphenes from fluorinated arsines published by Escudie and c o - w ~ r k e r sThey .~~ found that treatment of Mes*AsF2with Mes*As(SiMe3)Lior Mes*P(H)Li led to the corresponding diarsene Mes*As =AsMes* 25 and arsaphosphene Mes*As = PMes* 26 (both X-ray). The double bond lengths of 2.2634(3) A in 25 and 2.141(5) A in 26 were similar in length to those seen in a number of other structurally characterised compounds of this type and showed a shortening of

200

Organome tallic Chemistry

approximately 8 - 8.5% compared with a single bond. The steric bulk of the Mes* group expectedly led to both molecules exhibiting an E rather than a 2 configuration. Notable spectroscopic data include the 31P{'H) NMR spectrum of 26 which showed a typical low field signal at 524.5 ppm. The gas phase synthesis of a compound with As=N bond has also been reported this year as well as the corresponding P = N analogue.60Using C12As-N(SiMe3)2 and C12PN(SiMe2But)2 as precursors, flash vacuum thermolysis enabled generation of the corresponding low coordinate arsenic and phosphorus species cisClAs = NSiMe3 and cis-C1P = NSiMe2Butwhich were characterised in the gas phase from their photoelectron spectra. DFT calculations were also performed to establish the effect of the silyl and chloride substituents and the thermodynamic stability of the species was attributed to an antiperiplanar interaction between the nitrogen lone pair and the o * E - ~ (E = P, As) orbital. High ionization energies for the n E = N orbitals were observed as a result of negative hyperconjugation despite relatively poor pn-pn overlap. A number of phospholyl and arsolyl complexes of samarium have been described this year by Nief and Ricard.61They found that the reaction of [Sm(q5-C5Me5)2] with the bisphospholyls (C4Me4P)2,(C4H2Me2P)*,(C4B~t2H2P)2 and the new bis-arsolyl (C4Me2H2A~)2 afforded the corresponding bis-(pentamethylcyclopentadienyl) phospholyl and arsolyl samarium(II1) complexes in yields from 34 YOto 60 YO. The complex derived from the parent phospholyl anion [PC4H4]- was synthesised in 56 YOyield from the reaction between [Sm(qS-C5Mes)2(Et20)] and [Tl(PC4H4)]. X-ray diffraction studies of the complexes indicated two pentahapto bound Cp* ligands in each case, but the bonding of the heterocyclopentadienyl ligand depended on the substituents around the ring. The structures of the complexes derived from both the 3,4-dimethylphospholyl and -arsolyl ligands showed dimeric structural units with one heterocyclopentadienyl ring coordinated to both samarium centres in a p:q1,q5manner and the other bonded only to one samarium in an q1mode. Conversely the complex derived from the 2,5-di-tert-butyl phospholyl anion viz [Sm(q5-CsMe5)(q5-PC2H2But2)] is monomeric with all three rings q5bound to the central samarium atom. This study highlights the importance of electronic as well as steric factors on the bonding modes of heterocyclopentadienyl anions. A multistep route to 1-arsanaphthalene 27 and its 2-trimethylsilyl derivative 28 was described this year as well as the conversion of 28 to an q 4 complex of Mo(C0)3 29.62The parent 1-arsanaphthalene 27 appeared to be less robust than its silyl counterpart, giving a mixture of Diels-Alder dimers in solution whereas 28 could be isolated in 79% yield. Treatment of 28 and M o ( C O ) ~ ( P(Py ~ ) ~= pyridine) in the presence of BF30Et2afforded a 97% yield of 29. The X-ray crystal structure of 29 showed the Mo(C0)3 moiety to be bound via the arsaben-

R=H27 SiMe, 28

29

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

201

zene section of the 1-arsanaphthalene in an asymmetric q6 mode. The molybdenum-carbon distances which range from 2.36 to 2.53 are shorter than the molybdenum-arsenic distance of 2.65 A in line with the larger size of the arsenic atom. An examination of the chemistry of 2-arsa and 2-stiba-1,3-dionato lithium complexes with late transition metals also appeared this year. These lithium salts are closely related to the P-diketonate class of compounds which have long been important in inorganic coordination chemistry. Treatment of [{(solv)Li{ E[C(0)R]2}}2] (E = As, Sb ; R = alkyl, aryl) with [CpFe(CO)J] or [Cp*Fe(C0)2(NCMe)][BF4] led to a range of complexes of the formulation [(q5-C5R5)Fe(CO)2-E{C(0)R}2] (R’ = Ph, E =As, R = H, Me ; R’ = But, E = Sb, R = H).63The X-ray structure of [(q5-C5Me5)Fe(C0)2-As{ C(O)Ph}2] 30 was carried out and showed the diacylarsenido ligand to be q1bound though the pyramidal arsenic with localised As-C and C = 0 bonds. In contrast treatment of [{(dme)Li{A~-[C(O)Bu~12>}~] with FeClz or CoCl2 led to the q 2 - 0 , 0 bound paramagnetic species [M(drne){A~[C(o)Bu~]~}~] (M = Fe, 31 ; Co 32) with coordination of the diacylarsenido unit through the oxygen centres.

M=Fe31 Co 32

Two articles this year detailed the use of triphenylarsine in catalytic processes.64’ 65 The often undesirable aryl/aryl exchange observed between a palladium bound aryl group and a phosphorus bound aryl group in phosphinepalladium catalysed cross coupling reactions was capitalised on in the synthesis of a range of substituted arsines under solventlessconditions.64Thus treatment of a substituted aryl triflate R-C6H4-OTfwith triphenylarsine in the presence of a 10 mol % of Pd(OAc)*led to the corresponding arsine AsPh2-C6H4Rin yields of around 50%. The mechanism suggested involves initial reduction of the Pd(0Ac)z to Pd(0) by AsPh3 followed by oxidative addition of the aryl triflate. Reductive elimination then occurs to give an aryltriphenylarsonium salt. This salt undergoes As-C oxidative addition to afford the substituted arsine and the catalytic cycle is completed by reductive elimination of triphenylarsine with a Pd bound phenyl group to give the tetraphenylarsonium triflate as side product. This procedure was particularly useful for aryl triflates bearing redox sensitive aldehyde or ketone side groups which needed no protection and deprotection measures. The second of these catalytic based reports involved a study of the rate and mechanism of oxidative addition of phenyl iodide to Ph3Asligated Pd(0).65 This is important in the Stille coupling reaction in which the presence of AsPh3 leads to an acceleration of the reaction rate by several orders of magnitude

202

Organometallic Chemistry

compared with PPh3. This is thought not to be due to thermodynamic factors but to the higher reactivity of the active species [(solv)Pdo(AsPh3)2] towards oxidative addition. Electronic reasons were believed to be responsible given the similarity in the cone angles of AsPh3 and PPh3. The X-ray structure of the diarsane R2As-AsR2(R = (SiMe3)2CH-)has been reported this year along with the structure of its corresponding arsinyl radical, R2As' determined by gas-phase electron diffraction.66The radical adopts a Vshape in the gas phase with a syn,syn conformation in which the methine hydrogens point towards the centre of the V-shape thus reducing interactions between the bulky silyl ligands. Upon dimerisation to the diarsane a syn,anti conformation is adopted, the flexibility of the bis-(trimethylsily1)methylsubstituent allowing this necessary conformational change to occur. Similar conclusions were reached for the related phosphorus system which was also reported in this paper. The reaction of aluminium pnicitines (dmap)AlMe2E(SiMe3)z(dmap = 4(dimethylamino)pyridine,E = P, As, Sb) with Ni(C0)4led, in high yield, to the formation of the pnictogen bridged complexes (dmap)AlMe2E(SiMe3)2Ni(C0)3 (E = P 33, As 34, Sb 35 X-ray except 33).67The latter two represent the first examples of complexes with a transition metal and a group 13 element bridged by an arsine or stibine. The infra red spectra of 33, 34 and 35 show a slight decrease in the stretching frequency of the A, CO vibration compared with Ni(C0)4,this being ascribed to the slight increase in 0 donor / JC acceptor ratio with increasing atomic weight of the pnictogen.

Q I

E=P33

The properties of some transition metal complexes bearing arsine ligands has been explored. A study of the correlation between triboluminescence (luminescence upon application of mechanical force) and structure of tetrahedral manganese (11) compounds revealed a correlation between triboluminescent activity and space group acentricity. Of the compounds explored, (Ph3As0)2MnC12and (Ph3As0)2Mn12showed no triboluminescent activity whereas (Ph3As0)*MnBr2 showed triboluminescence at 80 Kelvin.68Anionic tris-dicyanamide complexes of the type [M(d~a)~][AsPh4](dca = N(CN)>-)where M = Co(I1) orNi(I1) have been synthesised and their crystal structures and magnetic behaviour The nickel complex exhibits long range magnetic ordering with an ordering temperature of around 20 Kelvin whereas the cobalt analogue shows no such behaviour but does display a field dependent magnetic moment below 20 Kelvin. The synthesis and structural analysis of dithiocarboxyarsines (RCS2),AsPh3-, (x = 1-3)(R = range of alkyl and aryl groups) have been described including their reactivity with amines leading to the first isolation of the organo-trithioarsonate dianion RAsS,~-'O and the crystal structure of the double butterfly iron carbonyl

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

203

complex [{(p-Phse)(p-MeA~)Fe~(CO)~}~] was also been rep~rted.~' The thermolysis of [Co4(p3-AsPh)(p4-q2:q2:q1-As4Ph4)(p-C0)2(CO)~] has been studied as well as its reactivity towards Fe2(CO), and P(OMe)3with particular attention being paid to the degradation of the tetraarsine In the case of thermolysis or reaction with Fe2(C0),the tetraarsine unit is disrupted to give complexes with a bridging As,Ph2 unit whilst the reaction with P(OMe)3yields [Co4(p3-AsPh)(p4q2:q2:q1-A~4Ph4)(p-CO)2(C0)5{ P(OMe)3}3]with the As4Pk unit remaining intact. Diarsanes (and diphosphanes) have been used as ligands in the preparation of complexes of Sb(II1)and Bi(II1) halides.73

3

Antimony

Recent developments in the chemistry of low coordinate organo-antimony (and bismuth) compounds have been reviewed this year including 2-stiba-193-dionato complexes, systems with localised h3,a2-Sb= C and h3,01-Sb=Cas well as five and six membered heter0cycles.7~ Other reports of low coordinate antimony systems include the synthesis of the potassium salt of the diphosphastibolyl anion [{ [K(dme)]( 1,4,2-P2SbC2But2)},]36 (X-ray)(dme = dimethoxyethane)from the reaction of K[Sb(SiMe3)2] and the phosphaalkene Me3Si-P= C(OSiMe3)B~t.75 The solid state structure consists of infinite polymeric chains of alternating potassium cations and diphosphastibolyl anions. Reaction of two equivalents of 36 with SiMe2C12led to the novel organoantimony cage compound 37 (X-ray) viu a mechanism most likely involving initial formation of [Me2Si(q '-P2SbC2But2)2] followed by a sequence of further steps including elimination of antimony and proton abstraction from the solvent.

H

37

Two new stable hydrides of antimony, namely RSbH2 38 and RHSb-SbHR 39 R = -CH(SiMe&, were reported this year by Breunig and co-corkers, the latter being the first organo-distibane with Sb-H bonds.76Both compounds show notable thermal stability, the mono-stibane decomposing at 100 "C to R4Sb4,the distibane melting at 69 "C. Synthesis of 38, a distillable liquid, was achieved in 69% yield from the addition of RSbC12 to LiAlH4 in ether. In contrast 39 was synthesised in 93 % yield by a similar procedure but with reverse addition of reagents. In solution 39 exists as a mixture of its d, 1 and meso isomers. Ninety percent of molecules of 39 in the solid state were shown by X-ray to exist in the meso form in an antiperiplanar arrangement, the di-stibane units being linked in chains by weak S b . - S b interactions. A study of the ring-chain equilibria of

204

Organometallic Chemistry

organo-antimony compounds was also reported this year. Both cyclo(Me&CHzSb), where n = 4 or 5 react with Me4Sb2or Ph4Sbz with formation of equilibrium mixtures containing as major components, the tristibanes Me2SbSbR-SbMez and PhzSb-SbR-SbPh2(R = Me3SiCH2)re~pectively.~~ Attempts to isolate these tristibanes were unsuccessful. Interestingly, treatment of the equilibrium mixtures with [Cr(C0)4(nbd)] (nbd = norbornadiene) led to the tetrastibane complexes [Cr(CO)4{R’2Sb-(SbR):-SbR’Z)I(R’ = Me, Ph ; R = Me3SiCH2)(the yields depending on the stoichiometry of the equilibrium reaction) despite the relatively low proportion of the tetrastibane in the mixture. This was though to reflect the good match between the bite of the chaleating tetrastibane and the chromium centre and also the high thermodynamic stability of five membered ring systems. Further reports of cyclic organo-stibanes include the reaction of ~yclo-(Me3SiCH2Sb)~ with [W(CO),(thf)] which afforded the 2: 1 adduct cycZo-[p-(Me3SiCH2Sb),-Sb’,Sb3-( W(CO)5}2]40 (X-ray). The related but smaller ring system cyclo-[(Me3Si)2CHSb]3undergoes an insertion reaction with Fe2(C0)9 to form the four membered metallocycle cyclo[{ (Me3Si)2CHSb}3Fe(CO)4] (X-ray).78Trialkyl-alanes and -gallanes were found to react with di-stibanes, the products depending on the conditions of the reaction.79Bis-adducts of the type [Sb2R4][{MR’3}2]were observed when a 2:l ratio of alane or gallane to di-stibane was used for a short reaction time (R = Me, Et ; R’ = Me, Et, Bu‘). The adducts [Me4Sb2][{EB~t3}2] (M = Al, Ga) and [Et4Sb2][{ E B u ~ ~ }(M ~ ] = Al, Ga) were characterised by X-ray crystallography which showed no evidence for lengthening of the central Sb-Sb bond despite the increased steric pressure upon adduct formation. All of the bis-adducts are unstable in solution, however, decomposing to form heterocycles of the type [{ R2SbMR’2},]. [{ Me2SbGaBut2}3]and [{ EtzSbGaButz}z]were characterised by X-ray and contained a six and four membered Sb-Ga cyclic core motif respectively with distorted tetrahedral Sb and Ga A report appeared this year describing the use of organo-antimony compounds as anti-tumour agents. A series of Sb(V) triphenylgermanylpropionates of formula (Ph3GeCHR’CHR2C02),SbAr5-,,(R’ = H, Ph ; R2 = H, Me ;n = 1,2) were prepared, characterised and their in vitro anti-tumour activity studied, the activity being dependent on the nature of the aryl group on antimony and the triphenylgermanylpropionic acid used.*’

S,b’

Sb-1-Sb sb+*S

I

I

b

R

40 R = Me3SiCH2-

4

E = A l 41 Ga 42

Bismuth

The chemistry of trialkyl-bismuthane and di-bismuthane ligands has been explored this year by Schulz and co-workers.81In one report they described the

205

8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

synthesis and characterisation of a range of Lewis acid-base adducts formed between trialkyl-alanes and bismuthane ligands.82Both Et3Al-Bi(SiMe3)3and B u ' ~ A ~ - B represent ~ P ~ ' ~ the first structurally characterised alane-triorganylbismuthane Lewis acid-base adducts. Of the compounds synthesised, only But3AlBi(SiMe3)3and B u ' ~ A I - B ~are P ~ adducts '~ in solution, the rest showing extensive dissociation. Variable temperature NMR studies enabled the dissociation energies to be calculated as 6.3 kcal mot' for B~~~Al-Bi(siMe3)~ and 6.9 kcal mol-' for But3Al-BiPri3.Reaction of Et4Bi2with But3E(E = Al, Ga) in a 2:l molar ratio (E = A1 afforded the corresponding di-bismuthane adducts [Bi2Et4][{ EBu'~}~] 41, Ga 42).83 Both 41 and 42 show significantly different stabilities both in solution and in the pure solid state. Adduct 42 shows the greater stability and can be stored for weeks at -30 "Cin the dark under inert gas whereas under the same conditions 41 shows slow decomposition. A similar scenario was observed in solution: at -30 "C the 'H NMR spectrum of 41 shows resonances in a ratio inconsistent with the presence of a bis-adduct. In the solid state, the subsitutents bound to bismuth are staggered and the Bi-Bi bond distances are almost the same as in the uncomplexed di-bismuthane. A similar situation was observed in the related di-stibane adducts (vide supra).79 Practical applications of organo-bismuth compounds have been described in the literature this year. The Bi(V) compound bis-(2-methoxyphenyl)-bismuthane oxide 43 was synthesised as a yellow solid from the reaction between tris-(2methoxypheny1)-bismuth dichloride 44 and KOBu' in methylene dichloride in the presence of water at 0 0C.84This dimeric bismuth oxide was shown to be a remarkable oxidising agent for the conversion of primary and secondary alcohols to aldehydes and ketones in very high yield and in reaction times of the order of a few minutes. The oxide was however of limited thermal stability, decomposing in benzene at 60 "C over 15 hours. A rare example of the use of organo-bismuth compounds in asymmetric catalysis was demonstrated in a report by Uemura and co-workers. They found that Ph3Bi(OAc)2was useful as a phenylating agent in the Pd(I1)catalysed kinetic resolution of racemic secondary alcohols using C0.85 Ar

CI

I

Ar-Bi...ll

Ar

I 'Ar

Ar

43

Cl

44

Ar = 2-MeOC&

5 1. 2. 3. 4.

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8: Group 15: Phosphorus, Arsenic, Antimony and Bismuth

38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67.

207

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70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85.

9

Organic Aspects of Organometallic Chemistry BY GUY C. LLOYD-JONES

1

Introduction

This chapter lughlights some of the major advances in the application of organometallic reagents to the synthesis of organic compounds which have been reported in the literature during the period 2000-2001. As is evident from the vast nature of the field, this review is not and cannot be comprehensive. Consequently, the author has selected a number of areas in which there has been, in his opinion, outstanding developments or sustained interest. Inevitably, personal preferencesbias the selection process. Furthermore, emphasis is placed on those reactions in which there is an issue of selectivity (chemo- disatero- regio- or enantio-selectivity)or unusual reactivity and those in which mechanistic understanding is developing rapidly. Catalytic and stoichiometric organometallic species are considered with equal preference and the coverage is divided into 10 sections. Each section contains examples which are related either by reaction type or by element (individually or in closely related transition or main-block groups).

2

OrganomagnesiumReagents

The Grignard reagent has been a mainstay nucleophilic organometallic reagent in organic synthesisfor well over a century. Its predictable behaviour, reasonable functional group tolerance, low cost and ready preparation from the corresponding organohalide often make it the reagent of choice. Nonetheless, in certain cases, Grignard reagents can be hard to form or unstable. This latter problem becomes apparent when the organohalide bears functionality that will react with the organomagnesium component under the conditions usually employed for formation of the reagent. Consequently there has been substantial effort to find new routes to such species using conditions (usually low temperature) that will allow compatibility between the functionality and the R-MgX unit. Knochel et al. made significant developments in this area by exploiting the thermodynamic favourability of exchange of aryl-iodides with hindered dialkyl Grignard reagents generated by Schlenk equilibrium. However, one is limited to use of the iodide and thus there has been continued development of methods that utilise the Organometallic Chemistry, Volume 3 1

0 The Royal Society of Chemistry,2004

209

210

Organometallic Chemistry

cheaper and more readily synthesised aryl bromides. A number of major advances have been reported. For example, Rieke et al. have made use of highly activated Mg (so-called Rieke Magnesium, 'Mg*') to generate functionalised aryl Grignard reagents. The high surface area and small particle size of the Mg* facilitates rapid formal insertion of the Mg into the C-Br at temperatures as low as - 78 "C.For example, aryl bromide 1 bearing a tert-butyl ester was smoothly converted into Grignard 2 and subsequently reacted with benzaldehyde at - 40 "C to give the ester-substituted benzhydrol3 in good yield, Scheme 1.' c02t-Bu

Mg*, THF

*

nco2t-B PhCHO

-78 "C, 15 min. B ~ M ~

Br 1

-78 "C, 15 min. Ho then H+

I

2

Scheme 1

The ester group, whilst generally immune to attack by the Grignard under these conditions, is best deployed as a tert-butyl ester. Less hindered esters, such as methyl and ethyl were found to inhibit formation of the Grignard at - 78 "C, possibly by passification of the Mg* surface, requiring warming to -50 "C before being formed. A more general system employing trialkylmagnesiates was reported by Oshima et al. Thus a range of aryl bromides, including for example 1, undergo ready halogen magnesium exchange at - 78 "C with i-PrBu2MgLi.' Alkenyl iodides and bromides also undergo smooth exchange. For example, reaction of alkenylsilyl bromide 4 (84% E ) with i-PrBu2MgLi at -0 "C, was accompanied by near-perfect isomerisation (equilibration) to generate 5 on trapping with D 2 0(E = D, 96% E ) or ally1 bromide (E = CH2-CH=CH2,94% E), Scheme 2. TMS

i-PrBu2MgLi

E+

W

Br 4

TH F 0 "C, 1 h.

E

*

TMS

0 "C, 30 min.

Scheme 2

€4

Elimination of halogen altogether is feasible when aromatic groups bearing suitable ortho-metallating moieties are employed with dialkylmagnesium species acting as 'base'. Queguiner et al. have developed useful pyridine functionalisation methodologies via these techniques3 The pyridine ring imparts its rather unique reactivity to the process and thus attempts to functionalise the ring at various positions can result in rather different outcomes. For example, the 2-amidopyridine 6 undergoes high yielding orthometallation (as evidenced by trapping with electrophiles, or deuteration to give 7 in 97% yield), Scheme 3. In stark contrast, the 3-amidopyridine 8 undergoes nucleophilic attack at the 4-position by the reagent to yield (on aerobic oxidation during work-up) the butylated pyridine 9.

9: Organic Aspects of Organometallic Chemistry

21 1

6

7

8

9

Scheme 3

Magnesium amide bases (‘Hauser bases’) have also proved highly effective for the generation of aryl Grignard reagents by deprotonation. For example, ethyl thiophene-2-carboxylate 10 undergoes moderate to high yielding magnesiation, to generate the 5-iodide 11 on quenching the arylmagnesium intermediate with 12, Scheme 4.4 2 eauiv.

10

11

Scheme 4

Two equivalents of the metallating reagent (i-PrzNMgCl) are required, possibly due to strong complexation of one equivalent of the reagent with the ester. However, the ethyl ester itself is not further attacked by the reagent or by the Grignard generated on addition of the second equivalent of i-PrzNMgCl.

3

Reactions Involving Organozirconium and Organotitanium Intermediates

Zirconocene Species. - The interest in the synthetic organic community in both stoichiometric and catalytic application of organozirconocenes continues unabated. The high reactivity of coordinatively unsaturated species such as ‘Cp2Zr’towards pi-systems such as alkenes, alkynes and imines, has resulted in the continuing discovery, design and development of new addition and cyclisation C-C bond-forming processes. The use of ‘Cp2Zr’in the development of an ll-step total synthesis of the natural product (-)-a-Kainic acid (17, Scheme 5) by Taylor et u Z . , ~ illustrates this well. The enantiomerically pure ‘Garner aldehyde’ 12 was converted into allylic ether 13 which underwent a stoichiometric cyclisation-elimination sequence, viu 14, on reaction with ‘CpzZr’generated in situ from

3.1

212

Organometallic Chemistry

‘Cp2ZrC12and 2 BuLi. The syn stereochemistry required at C3 / C4 is set up elegantly by this process and trapping the elimination product 15 with I2 allows a subsequent carboxylation, oxidation and deprotection to yield 16. Although the cyclisation proceeds with high stereoselectivity, the yields with the ‘Cp2Zr’ reagent were lower than those obtained on model systems and ultimately a Ti-based reagent, operating by an analogous mechanism, was employed with good effect, Scheme 5.

An 12 ?Ph

13

‘2

___c

in OMe 15

01

Ti(0-C Pr)4, 2 CPrMgCl EtpO, -50 “C

OMe

.

14

.

0 N

.,#,,,(OMe

I

Bn 16

OMe

17 (-)-a-Kainic acid

Scheme 5

The use of zirconocenes in the reaction of organosilanes with organometallics and with alkynes is an area that is attracting much attention. An interesting example of the former comes from Takahashi et al. in the development of a Zr-catalysed reaction of Grignard reagents with H2SiPh2.The reaction of primary Grignard reagents (e.g. n-PrMgBr) with H2SiPh2 (18) occurs smoothly in THF at ambient temperature without the need for catalysis. However, secondary reagents, e.g. i-PrMgBr (19)do not react at all. However, Takahashi et al. found that addition of a stoichiometric quantity of Cp2ZrC12to H2SiPh2 with two equivalents of i-PrMgBr results in quantitative generation of n-PrSi(H)Ph2(20) together with propane, 2 MgX2 and, formally, ‘Cp2Zr’. Consideration of the mechanism led to the co-addition of a simple organic halide (RX)to reoxidise the ‘Cp2Zr’thus allowing the use of catalytic quantities of Cp2ZrC12,see boxed section in inset to Scheme 6. A simple screening exercise found 1,3-dibromopropane to be most effective and under these conditions the reaction proceeds with as little as 2 mol% Cp2ZrC12 giving 93% yield of 20, via the mechanism outlined in Scheme 6.6 The reaction of acetylenes with lithiated silanes is mediated by zirconcene reagents. For example, cyclohexenylphenylacetylene (21) reacts with Me2PhSiLi (22) in the presence of one equivalent CpzZrC12to give, after aqueous work-up,

213

9: Organic Aspects of Organometak Chemistry 2 mol Yo

H I

Si(H)Ph,

+

18

>M@r

19

cp2zr'

Cp2ZrCl2,t

-Si(H)Ph2

THF, 20 "C Br-Br

20

(= RX)

93 Yo

R

A

CP27 H

I

Si(H)Ph2

20

Scheme 6

the formal products of lithio-silation of the alkyne unit, (23) with 75% regioselectivity. Mori et al. have shown that the mechanism of this rather general and readily applied reaction, most likely proceeds via generation of di-silylated zirconocene (24) which undergoes p-H transfer to yield a silazirconacyclopropane type reagent (25). This intermediate can then insert an alkyne to generate a silazirconacyclopentene(26) prior to quenching, Scheme 7.' The intermediate 26 can also undergo ring expansion on reaction with CO and analogously with isocyanides. Evidence to support 25 (which is very unstable and formally a silene-zirconocene)and 26 include NMR detection of both species (R = 4-anisyl) and trapping of 26 with D20to give 27 (R = Ph). Trapping of 26 (R = Ph) with ally1 chloride gives the 6-siladecal,4,9-triene28, which is efficiently converted to the l-silacycloocta-2,5-diene 29 by Ru-catalysed alkene metathesis. The reaction of ethyl-based Grignard reagent with imines is another example of an organometallic process where zirconocene catalysis has been recently developed. In the absence of a catalyst, there is no reaction between ketimines and EtMgCl (although aldimines do react slowly). Szymoniak et al., have demonstrated that addition of Cp2ZrC12allows reaction to proceed in good to excellent yield to give secondary amines bearing a tertiary alkyl group. For example, the acetophenone derived imine 30 yields chiral (but racemic) amine 31 in 78% yield on reaction with 2 equiv. EtMgCl in THF at 20 "C with 10 mol% Cp2ZrClz,Scheme 8.*A series of deuterium labelling experimentswere performed to determine the partioning of the likely intermediate 33, generated by cleavage of the Zr-N bond in azazirconacyclopentane32, between fl-H transfer (to give 34) and Grignard mediated C-Zr cleavage to give 35 and Cp2ZrEt2.Support for both pathways was obtained, with kinetic isotope effects modulating the partition ratio. For example, using a deuterated Grignard reagent and quenching with

214

Organometallic Chemistry

I

21

THF, 20 "C

23

SiMe2Ph

R

Ph

27

26

I

24

Ph ph* Ph

allylCl

-

Ph

Ru-cat ph$

si,M Ph-''\ 28

29

-

Scheme 7

D 2 0gave d3-31as a mixture of two regioisotopomers in equal ratio. Overall it was concluded that the latter pathway (via 35)dominates and indeed, 0.6 equiv. ethane could be detected in the reaction head-gas during the catalytic turnover. Bergman et al. have reported in depth on the intriguing chemistry of imidozirconocenes, CpzZr=NR , as intermediates in stoichiometric and catalytic addition and metathesis procesess. For example, the addition of imines R'C(H) = NR" (36) to Cp2Zr=NR (37) generated in situ from Cp2Zr(THF)= NR (which can be prepared by reaction of Cp2Zr(Me)C1with RN(H)Li in THF) results in a diazazirconocyclobutane (38)which undergoes retro addition, thereby regenerating either the original imine and imidozirconocene (36 and 37) or the products of imine metathesis (39 and 40), Scheme 9. In principle this process is catalytic, however, by detailed study of the mechanism of this reaction (NMR and kinetics) it was found that the dimerisation of the imidozirconocene intermediates (37 and 40) is competitive with imine addition and gives inert hetero and homo-dimers, 41. The co-addition of diphenyl acetylene (42) to the system depletes the standing concentration of 37 and 40 by reversibly forming cycloaddition products 43. Whilst this thereby reduces the rate of reaction of 37 and 40 with the imine, since the kinetics of the dimerisation processes are second order in 37 and 40, the relative rate of dimerisation versus metathesis is substantially reduced. In this very elegant manner, an efficient catalytic system was developed.' The reaction of chiral imidozirconocenes with allenes has also been studied in detailby Bergman and Andersen et al., Scheme 10." Using the C2-symmetric

215

9: Organic Aspects of Organometallic Chemistry

Scheme 8

I

cP2zr+ph

43

Ph

Scheme 9

R“ I

~ p , ~N r lZrCp2 < N I RIR”

41

216

Organometallic Chemistry

chiral 'ebthi' ligand, introduced by Brintzinger, imidozirconocene complexes of the type 45 (Ar = 2,6-xylyl and 4-tolyl) were prepared in from the corresponding lithiated amine, ArN(H)Li, and [ebthiJZr(Me)C1(44) in both racemic and enantiomerically pure form. Reaction of 45 with 1,3-disubstituted allenes (symmetrical (46)and non-symmetrical (not shown))again in racemic and enantiomerically pure form to give azazirconacyclobutylidene type cycloaddition products (47) was studied in detail. The stereochemical issues attending these reactions are complex in nature. Considering only the symmetrically 1,3-disubstituted allenes (which can only give one regioisomer of product 47), it emerges that a reasonably efficient kinetic resolution occurs. With 1,2-cyclononadiene(46, R = -(CH2)6') the s-value (the relative rates of matched and mismatched reactant pairings) is ca. 12. However, the product (47) of the reaction of both enantiomers of 46 with either enantiomer of 45 was found to be essentially a single diastereoisorner, which is not consistent with the previously assumed concerted nature of such cycloaddition reactions. Consequently, reaction was proposed to proceed via a diradical intermediate (49)generated on THF displacement from 45 by the allene (46). The precursor to intermediate 49 would be disastereoisomeric complexes (48 and 48) in which there will be steric clash between one allene substituent and the [ebhti] ligand in one diastereoisomer, but not in the other, thereby facilitating kinetic resolution. However, since both 48 and 48' give 49, essentially perfect stereochemical convergence is achieved on formation of 47.

Titanocene Species. - The titanocence methylidene reagent, Cp2Ti= CH2, pioneered by Tebbe for deoxygenative methylidenation of carbonyl compounds has now been applied by Nicolaou et d.to reactions involving heteroatom oxides X = O (X = S, Se and N)." Using the Petatsis procedure to generate the reagent (thermolysis of the readily prepared Cp2TiMe2)selenoxides and sulfoxides (50) are cleanly deprotected to give the corresponding selenide and sulfide (53) respectively. The reactions are proposed to proceed viaformation of an intermediate non-stabilised carbene (52) resulting from collapse of the oxatitanacyclobutyl intermediate 51. The spontaneous decomposition of 52 would then yield the product (53),Scheme 11.In contrast, pyridine N-oxides (54) undergo methylation, rather than deprotection, to give 56 via an analogous but diverted methanism (at 55) to the reactions involving 50. The simple conditions and exclusive delivery of the methyl group to carbon 2 on the pyridine ring makes this a rather useful procedure. The Petasis reagent (Cp2TiMez)has also been used as a catalyst for the hydroamination of alkynes. Thus, for example, the secondary amine 58 is readily prepared by reaction of aniline with diphenylacetylene (44) and then reductive work-up using NaBH3CN to convert the imine product (57) to the amine, Scheme 12.12The use of microwave irradiation (2.45 GHz, 300 W) makes the process occur much more efficiently. Monocyclopentadienyl titanium species, such as the dimethyltitanium amide 59, have also proved of interest. Piers et al., have conducted extensive mechanistic studies (NMR, kinetics and labelling) into the mechanism of activation of such complexes, by Lewis acidic B(C6F5)3(60, Ar = C6F5),for in situ generation of

3.2

9: Organic Aspects of Organometallic Chemistry

217

Zr

L

J

u

fast, 23 "C

Scheme 10

-"-?l , 0 R

R'\x/R

0,

Cp2Ti=CH2 R'\

0:'

50 (X = Se, S)

54

I

0 -Ti.

0. / Ti \ c$ CP

O 0

OO

R

/o CP

CP

51

RI-X-

R 1

___t

R',

00 52

X'R

53 (X = Se, S )

I

0.Ti \

cp'

cp

0.Ti,

55

cd

D

CP

Scheme 11

dj-56

Organometallic Chemistry

218

0""'

aN P h r P h

3 mol % Cp2TiMep e

PhCCPh (44) toluene, MW

P h y P h

NaBHSCN, THF pTsOH, 25 "C

57

58

Scheme 12

Ti-catalysts allowing 'living' polymerisation of eth~1ene.l~ The process involves generation of an intimate ion-pair (61) which, in the absence of monomer, undergoes unimolecular elimination of methane (with no cross-over) by a 0methathesis mechanism, to generate 62,Scheme 13. The latter process is postulated to be a major and irreversible deactivation pathway under catalytic turnover of monomer to polymer.

59

""'9

\6U/

62

Scheme 13

3.3 Zirconium and Titanium Ally1 Complexes. - Ti(II1) and Zr(II1) allyl complexes (63) have been prepared by reaction of two equivalents of a 1,3-bisTBDMS-ally1lithium reagent with the M(1V) halide in the presence of TMEDA. Both complexes were fully charactersised, including structural data from X-ray diffra~tion.'~ This represents the first example of a Zr(II1) allyl complex to be characterised in this manner. Both complexes proved to be inactive for polymerisation of ethylene and propylene, however activation with M A 0 , which is proposed to generate M(1V) complexes of the type 64, proved possible and active catalysts were obtained. Independent syntheses of 64, as shown in Scheme 14, confirmed that the M(1V) species are active catalysts. 3.4 Alkoxy- and Aryloxy- Titanium and Zirconium Species. - Zirconium (IV) alkoxides bearing chiral diol ligands have proven to be highly succesful in a number of catalytic asymmetric reactions. Two reports from Kobayashi involve ) ~ two different the use of BINOL-based ligands. Reaction of Z r ( 0 t - B ~ with brominated BTNOL species in a 1 / 2 ratio respectively in the presence of N-methyl imiadzole ( = L), generates the NMR-characterised, complex 65.15Use

9: Organic Aspects of Organometallic Chemistry

219

Z

Z

Scheme 14

of alternative stoichiometries also gives rise to 65, but less efficiently, and thus 65 is thermodynamically preferred for some, as yet undetermined, reason. The binuclear complex 65 proves to be highly effective as a catalyst for asymmetric Strecker reactions with enantioselectivities reaching 94%. For example, imine 66 is converted to 67 in 98% yield and 91% ee using Bu3SnCN as the cyanide source, Scheme 15. The cyanoamine products from such reactions are readily hydrolysed to the corresponding enantiomerically enriched amino-acid derivatives. HO

-Q

HO

3 mol O h 65 Bu3SnCN

-Q NH

H

T C N

0 "C,12h tol-benzene

-

66

67

Scheme 15

The same imine (66) can be allylated enantioselectively to give 69, again using a stannane as the source of nucleophile, but in this case the mononuclear complex 68 proved extremely effective, with the dichloroBINOL based catalyst more enantioselective than the dibromo analogue, Scheme 16.l6 The related atropisomeric ligand 'VAPOL', developed by Wulff et al., forms mononuclear bis-chelates with Zr and these complexes catalyse the iminoaldol reaction of imines analogous to 66. For example, imine 70 is alkylated by the ketene acetal71 to give aminoester 72 in 91O/O ee, when VAPOL complex 73 is present, Scheme 17. The same reaction with the analogous bis(3,3'-dibromo-

220

9

HO

Organometallic Chemistry

"&SnBu3

HO

-Q

O-t-Bu

10mol%68 0 "C, 12h toluene

66

*

68

69

Scheme 16

BINOL) complex gives 72 with lower selectivity ( 86% ee) whilst with complexes of BINOL itself the ee is 28Y0." Of note is the observation that the opposite enantiomer of 72 is induced by the (S)-forms of BINOL and dibromo BINOL as compared to (S)-VAPOL. OTMS

-Q

HO

71

?OMe 20 mol % 73t -45 "C, 20 h

HoQ H

0

YNH&OMe

toluene

o/o

ee

72

-

Ph

'Ph

Ph

,

Ph

Scheme 17

The 1,2-addition of diethyzinc to benzaldehyde remains ever-popular as a benchmark reaction for testing and comparing new ligand chiral systems. The mannitol-derived diol74 forms a dinuclear Ti alkoxide complex (75) on reaction with one equivalent Ti(i-Pr0)4, which has been characterised by X-ray crystallography, Scheme 18. l 8 The complex catalyses ethylation of benzaldehyde with moderate ee. However, addition of a further 0.8 equiv. of Ti(i-Pr0)4,increases the selectivity to 85% ee and the yield of secondary alcohol (PhC(OH)(H)Et)) becomes quantitative. Detailed NMR studies revealed that the addition of excess

221

9: Organic Aspects of Organometallic Chemistry

Ti(i-PrOb converts 75 into a non-symmetric complex 76, which serves as a more active and more selective catalyst.

,ytoH

Ti(i-P rO)4

0

74

U

76

Scheme 18

Similar bridged dinculear titanium alkoxide species are formed from chiral Salen ligands. Belokon' and North et al., have applied these complexes (e.g.77, R = H or t-Bu) to the catalysis of the addition of TMS-CN to aldehydes to generate the TMS-cyanohydrin.19Extensive mechanistic studies, predominantly based on detailed kinetic analysis, suggests that the reaction proceeds via a dinuclear difunctional intermediate, 79 (uia 78) in which one Ti-centre activates the aldehyde (typically benzaldehyde) whilst the other acts as a (mono)-tethered, but still highly orientated, source of nucleophile, giving asymmetric inductions of up to 92% ee, Scheme 19. 1,3-nonadienes bearing a terminal (C(9)) ester group, e.g. 80, undergo high yielding cyclisation, Grignard-capture sequence on reaction with in situ generated low valent Ti alkoxides. For example, reaction of 80 with excess (3-4 equiv.) cyclopentyl Grignard and 1.1 equivalent MeTi(i-Pr0)3in THF at 0 "Cgives the trans-2-vinylcyclohexanol 82 in 81 % yield.20When the excess of Grignard reagent is reduced to 1.8, the dominant product is 81 (54% yield). Quenching the reactions with D 2 0 affords dl-81 and d2-82 and the mechanism outlined in Scheme 20 is suggested by Quan and Cha to account for these observations, in particular the high diastereoselectivity, which establishes 4 stereocentres and a double bond geometry. Analogous reactions can be performed withl,3-octadienes bearing a terminal (C(8))ester group to give the corresponding cyclopentyl products, albeit in lower yields. A new variant of the Reppe reaction (the cyclotrimerisation of three alkynes to generate a benzene ring) has been developed by Sat0 et al., allowing direct construction of titanated aryl rings, which can subsequently trapped with simple electrophiles, Scheme 21. 21 For example, the octynoate 83 undergoes smooth reaction with octyne (84) and (q2-propene)Ti(O-i-Pr)2(85) to generate titanacyc-

222

Organometallic Chemistry

PhCHO ___)

1

TMSCN

-+

R

I

R I

79

lopentadiene 86. This is regioselectively trapped in situ by the tosylacetylene 87 via one of two mechanisms to generate 88 or 89. The a-elimination of sulfinite anion from either species then generates the titanated aromatic ring 90, which can be captured by benzaldehyde. The addition products spontaneously lactonises and the final four-component coupling species 91 is obtained in 50% yield.

4

Oxidation and Reduction

4.1 Oxidation. - The epoxidation of alkenes is an area that continues to attract significant interest. This is likely to be due to the fact that the substrates are readily prepared, that the reaction introduces the electrophilic epoxide function which has high synthetic utility, the potential for asymmetric induction and the potential for use of ‘green’ systems for cheap and clean oxidation. Ruthenium porphyrins, well known oxidation catalysts, have been immobilised by cross-polymerisation, thus generating conveniently separable and recyclable systems. For example, copolymerisation of a ruthenium porphyrin, bearing 4-styryl substitutents, with ethylene glycol dimethyl acrylate, gave a dark red

c..-

9: Organic Aspects of Organometallic Chemistry

'I-

0-i-Pr

i-Pr-0 \

i-Pr - 0-Ti

-

"Ti(jOPr)$ /

80

223 ,0-i-Pr

,0-i-Pr

C02Me f---

QP

D

0 c3

81

d2-82

J

Scheme 20

polymer (92) which did not leach Ru on grinding and sieving.22Using 2,6dichloropyridine N-oxide as terminal oxidant, styrene could be epoxidised in > 99% yield with as little as 0.02 mol% Ru, Scheme 22. Additionally, benzylic hydrocarbons could be oxidised selectively, stepwise, via the benzyalcohol to the ketone. Epoxidation using peroxo-Zr-tartrate complexes, generated in situ, allows highly selective asymmetric oxidations to be effected. In this vein, a nice example of the 'meso trick' comes from Spivey et aZ.23Thus the achiral diene 93 can be selectively desymmetrised, using ( +)-diisopropyl tartrate (DIPT), thereby establishing eight unique and contigous chiral centres in one step, Scheme 23. The ee of the product (94) was as high as > 95% under optimised conditions and the structure resembles the core ring structures of the polyhydroxylated CeZastraceae sesquiterpenes. The Jacobsen type chiral enantiomerically pure salen-manganese catalysts which have proven very effective for the asymmetric epoxidation of pro-chiral cis-alkenes, have now been applied by Adam et al. to the kinetic resolution of chiral, racemic, allylic The cis alkenes are again best for selectivity and

Organometallic Chemistry

224

; ? i-Pr-0 o

CO2t-BU

-i-Pr

Hex __t

H Hex

I

Hex

H Hex 88

a7

Hex

84

Hex

86

1

87 I Hex

90

Hex

I

Hex

Scheme 21

with cinnamyl type substrate 95, the selectivity factor (s) is ca. 13, thereby allowing the generation of epoxy alcohol 96 in high enantio and diastereoselectivity and the recovery of unreacted 95 with enantioenrichment. The stereoselection (diastereo and enantio) has been rationalised on the basis of allylic strain, hydrogen bonding and the alkene approaching the Mn-oxo species (generated in situ from PhIO) along the Katsuki trajectory, Scheme 24. The oxidation of alkenes by dioxygen can be effected in a simple and 'green' manner by use of supercritical C 0 2 as the solvent, an aldehyde as a sacrificial oxidant, and a stainless steel reaction vessel as p r ~ m o t o r .With * ~ certain substrates, the reaction is very efficient. For example, cyclooctene (97) is oxidised with 99% selectivity and 99.6% conversion to the corresponding epoxide (98) in 18h at 55 "C,Scheme 25. The complex and surprising reactivity of the very simple metal 0x0 compound Cr02C12has been overviewed, 26 highlighting, for example elegant studies on the oxidation of alkenes by this do-complex using matrix isolation techniques, Scheme 26.

9: Organic Aspects of Organometallic Chemistry

0.02mol % 92

225

.O

Scheme 22

93

CH2C12, -20"C

94

Scheme 23 (R)-95 Ph H-, ,OH1

(s)-95 H

Ph

Ph

H

96

Scheme 24

Brown and Keily have devloped a remarkable asymmetric oxidative cyclisation of 1,5-dienes using a simple permangante oxidant and a cinchonidiniumbased phase transfer catalyst?' For example, diene 99 is oxidatively cyclised to

Organometallic Chemistry

226

RCHO +

0

02,

scco2

stainless steel 55 "C,

D

97

Scheme 25

CI

C?

1

0

CI' "0

0 0

+

RC02H

98

10 K

Ar, 411 nm'

Scheme 26

100, in the presence of enantiomerically pure 101, creating three stereocentres with high control over relative and absolute stereochemistry, Scheme 27. KMn04, AcOH Ar

99

-30 "C, CH2C12 * 10 mol Ol0 101

A *r OH

100

Scheme 27

4.2 Reduction. - Li and Chan have reported on the use of aluminium powder to effect the reduction and the reductive coupling of ketones to give the corresponding alcohols or diols (pinacols) respectively.2*The passifying oxide layer that coats aluminium surfaces obviously makes the reaction a serious challenge. However, Li and Chan noted earlier literature that described how traces of fluoride in water can dramatically increase the rate of aqueous corrosion of the metal surface and turned this to their advantage by deliberate use of metal fluoride salts as promoters for the reaction. The outcome of the reduction is cleanly controlled by the identity of the added metal fluoride. Thus, for example, para-trifluoromethyl benzaldehyde (102) can be reduced to the benzyl alcohol (103) with 100% selectivity by using iron, cobalt or nickel (11) fluorides or reductively coupled to the pinacol(lO4) using simple fluoride sources such as KF or Bu4NF,Scheme 28. Maruoka et al., have reported a significant improvement in the catalyst system for the venerable Meerwein-Ponndorf-Verley reduction of ketones using isopropanol as r e d ~ c t a n tUsually, .~~ the redox partner (acetone) must be removed

9: Organic Aspects of Organometallic Chemistry

227 OH

F3C

At, H20

102

\

(M= K,Bu~N) 104

Scheme 28

continuously from the reaction medium by distillation to allow complete reduction of the ketone - particularly if the ketone is an acetophenone derivative. However, by fine-tuning a novel perfluoroalkyl-based bidentate ligand systemfor an in situ-generated A1 catalyst, it is now possible to effect complete reduction in the presence of 10 equivalents iso-propanol without removal of acetone. Thus for example, ketone 105 is reduced essentially quantitatively to 107 in l h at 25 "C using 10 mol% 106, Scheme 29. 10 eq. tPrOH, 10 mot YO106 t

CH2C12, 25"C, l h

107

105

106

Scheme 29

The highly E-selective reduction of a-halo-P-hydroxy-y,6-unsaturatedesters (e.g. 108) to P,y-unsaturated esters (109)by Sm12has been reported by Concellon et ~ 1 . ~By ' quenching the reaction with D20, two deuteria are introduced (with no diastereoselectivity) to give, e.g. d2-109. An allyl-samarium, samarium enolate intermediate (110) is proposed, with a syn-syn-ally1 stereochemistry to account for the E-selectivity, Scheme 30.

5

Isomerisation, Cycloisomerisation, Cyclisation and Cycloaddition

5.1 Isomerisation. - The asymmetric Rh-catalysed isomerisation of allyic amines to enamines is a reaction that has been advanced over many years to the point of commercial production on the multi-ton scale for the provision of feedstock to the fragrance and perfume industries. The analogous reaction involving allylic alcohols has recently been developed by Fu et d3'Using a chiral bidentate phosphaferrocene ligand (112), a range of allylic alcohols undergo

228

Organometallic Chemistry OH

0

0

D

i. Sm12

R' +oMR"

___)

X

R

ii. D20

108 (X = halogen)

-

c---

Scheme 30

isomerisation to the aldehyde in high ee (up to 93%) and in high yield. Stereochemicallabelling studies were conducted to explore the reaction mechanism. Of significant interest are the results obtained with enantiomerically enriched substrate (S)-111 which undergoes isomerisation with opposite enantiomers of 'Rh-112' to give products (113)in which the H-migration (which is strictly intramolecular) occurs stereospecifically and with a kinetic isotope effect augmenting or attenuating the enantiotopic group selectivity, Scheme 3 1.

94

ee h:eao

Ph

D

5mol%

Me H

Rh-(+)-112

'ao Me

66%ee

5 rnol %

87

p

Rh-(-)-llZ*

6

Mea 46 % ee

Ph

4

,3

THF

1000~

Me

Ph

111

D

Ph

113

113

L

Me

i

Scheme 31

5.2 Cycloisomerisation. - The cycloisomerisation of dienes, particularly hepta-l,ti-dienes, has undergone somewhat of a resurgence in recent years. The reaction is a popular testing ground for new catalyst systems and the goal is usually the control of regioselectivity. When one of the alkene termini is monosubstituted (see 'R' in scheme 32), the cycloisomer that is obtained, usually has the alkene unit on or adjacent to what was the more susbtituted alkene unit. In stark contrast, a Ru-based catalyst system developed by Itoh, generates the exomethylene type cycloisomer with high ~electivity.~~ Thus, for example, diene 114 is cycloisomerised to exomethylene product 115. The mechanism outlined in Scheme 32 is proposed to account for the unusual regeioselectivity. What is

9: Organic Aspects of OrganometaElic Chemistry

229

unusual about the mechanism is that although a Ru-H is required for turnover, the reaction does not proceed via an alkene hydrometallation step. Instead an oxidative cyclisation occurs and the key step for the selectivity in the overal process is the reductive-elimination type hydride transfer from the ruthenacyclopentane intermediate which is driven by steric decompression to place the H on the more substituted carbon.

EqTMs '%iMS 5 mol % [Ru(COD)C12],

E

\

i-PrOH

D

90 "C,24 h

E

(*)-115

E-d 1-114

Scheme 32

Enynes are also popular substrates for such reactions and Furstner et a1 have developed some remarkable cyclisations involving simple platinum salts as catalysts. For example, enyne 116 is cycloisornerised to 117 and 118 with PtC1,. The nature of the alkyne (internal versus terminal) strongly controls the ratio of 117 / 118, Scheme 33.33 A mechanism involving a non-classical carbocation generated by attack of the alkene on the Lewis-acid activated alkyne is proposed and supported by interconversion of some of the products by other Lewis acids and by deuterium labelling experiments. The use of Rh-catalysts has also been explored extensively, with simpler enyne substrates. A highly enantioselective version has been developed by Zhang.34 Although the products are suggestive of a hydrorhodation, carborhodation-P-H elimination sequence, as yet there has not been any mechanistic investigation into these reactions. Nonetheless, under the correct conditions, the enantioselectivities are spectacular, as demonstrated by the example in Scheme 34.

5.3

Cyclisation. - A tandem isomerisation-cyclisation of epoxypropargyl esters, promoted by samarium and catalysed by Pd has been developed by Aurrecoechea et ~ 1 The . reaction ~ ~ generates substituted furans in good to excellent

230

Ts-N

g 5x

Organometallic Chemistry Ts I

R

4 mol O h PtC12 toluene 60-80 "C

87-91Oo/

V

116

117

R=Bu R=H

9 5

118

c15 65

35

Scheme 33

98 YOee

I

OPPh2 OPPh2

Scheme 34

yield under relatively mild conditions. For example, 119is converted to furan 120 in 78% yield, possibly via the mechanism outlined in Scheme 35.

R

R = (CH2)sCN

i) Sml2 ii)H20, AcOH

119

iii) 5 mol % Pd(OAc)*

120

HX __t

Scheme 35

120

9: Organic Aspects of OrganometaEEic Chemistry

23 1

The cyclohydrocarbonylation reaction has long been of interest for the generation of ketones and esters. Alper et al have now found conditions for the cyclocarbonylation of ynones using Rh-catalysis under a CO / HZatmosphere. 36 An interesting feature is the mechanism, of which there are a number of possibilities emanating from the initial generation of a rhodium-hydridro species from an $-arene rhodium zwitterion (122). The reaction is efficient over a very broad range of substrates, for example ynone 121 is cyclohydrocarbonylated to yield butyrolactone 123 in 88% yield, Scheme 36.

*,a,3 [ TBPh3 1 2 mol Yo 122 32 mol O/O (Ph0)3P

121

38.5atm CO 3.5 atm H2 CH&12,120 "C

123

0

122

Rh( 1,5-COD)

Scheme 36

Yamamoto have developed a two-component tandem cycloetherification process catalsed by Pd, Scheme 37.37The reaction mechanism involves the generation of a pi-ally1 intermediate from a linear allylic carbonate (124) bearing a pendant alkoxyalkyl group and a Pd(0) complex. Decarboxylative proton transfer then generates an alkoxide (125) which undergoes conjugate addition to an enoate type coupling partner (126). The resulting carbanion (127) then attacks the Pd-ally1 complex to yield the cyclisation product (128) and regenerates the Pd(0) catalyst. By using chiral enantiomerically enriched bidentate phosphine ligands, products of type 128 (n = 1, 2, R = Ar) were obtained with moderate diastereoselectivity (favouring trans) and with ee's of up to 92%. In a similar addition-cyclisation approach, Grigg and Savic have coupled vinylic bromides with propiolate and sulfonyl acetylenes to generate pyrroles. 38 For example, bromoallylamine 129 undergoes addition to tosylacetylene (130)to give the enamine 131, which cyclises to pyrrole 132 on exposure to Pd, Scheme 38. A number of mechanistic possibilities for the process are discussed. The internal trpping of a rhodium carbenoid by a carbonyl gives rise to a carbonyl ylide. Padwa et al. have developed this process further such that the ylide undergoes addition to dimethyl acetylene dicarboxylate (DMAD), in effect facilitating a 1,3-dipolarcy~loaddition.~~ A good example is 133 which undergoes cyclisation and 1,2-H shift in the absence of DMAD (to give 134) but is smoothly captured when DMAD is present, to give 135, Scheme 39. The Pauson-Khand reaction has evolved over many years from a stoichiometric to a Co-catalysed process. Hyeon and Chung et aL4' have now found that cobalt metal which has been deposited on a mesoporous silica support performs well in this reaction and is easily recovered and recycled simply by filtration.

232

Organometallic Chemistry

k

CPrOC02

L2Pd(O)

OH

0

-pd12

127 R

L

dEE 126

i'

0

0

i-PrOC02

-PdL2

(Cy2h

iPrOH, CO2

0

0

125

Scbeme 37

129

130

132

131

Scheme 38

&

(E = C02allyl)

E

133 E

134

E DMAD

135

Scheme 39

124

9: Organic Aspects of Organometallic Chemistry

233

Careful analysis of reaction mixtures by atomic absorption spectroscopy confirmed that the catalyst was entirely hetereogeneous. Although high temperatures and pressures are required, good yields of cyclocarbonylated enynes (e.g. 137 from 136) are obtained, Scheme 40.

#

-0

/

cat CoBBA-15 Ph

136

20 atm CO CH2C12, 130 "C 18h, 98 Yo

137

Ph

Scheme 40

The same group have also reported on the Co-catalysed cyclisation of triynes, this time using C02(C0)8 (2.5 mol%) to give fused biscyclopentenones. The reaction is proposed to proceed viaa CO insertion and then a Pauson-Khand reaction? Thus, for example, 138 forms 140 (tlia cobaltacyclopentadiene 139) which then undergoes a second cyclisation to give 141 in 71% yield, Scheme 41. 2.5 mol % C02(CO)8 t

30 atm CO

I

CH2C12, 130 "C 3 d, 71%

TIPS

138

TIPS

L

139

-

d'

140

Scheme 41

TIPS

0

141

0

I

0

An intermolecular Ru-catalysed Pauson-Khand procedure (alkyne + alkene + CO) has been discovered and extensively developed Chatani and Murai et aZ.42The reaction is rather versatile and has now been extended to include (ketone + alkene + CO) cyclisations to yield lactones. For efficient turnover, the

reaction requires the ketone be an a-keto carbonyl or an N-hetereocyclic ketone. Furthermore, the use of a catalyst system comprising R u ~ ( C O/)PAr3, ~ ~ where Ar = p-CF3-Ph,was found to give the best results. For example, cyclooctene (142)is coupled with dipyridyl ketone (143) and CO to give lactone 144 in 94% yield, Scheme 42. Murai has also applied a similar catalyst system to the carbonylative rearrangement of cyclopropyl imines (145) to generate enamido type lactams (147).43 A side product from the process (formed in small quantities) was found to be the linear cx,p-unsaturated aldehyde (148) and this is accounted for by the mechanism outlined in Scheme 43, where partitioning occurs at intermediate 146.

Organometallic Chemistry

234 2.5 mol Yo Ru~(CO)Q

7.5 mol % PAr3

142

143

*

5 atm CO (at RT) CH2Cl2,160 "C 20 h, 94 Yo

(10 equiv.)

144

0

Scheme 42

NR

145

2 mol Yo Ru3(C0)12 * 2 atm CO (at RT) toluene, 160 "C 60 h

Scheme 43

The transition metal catalysed hydration of alkynes is a well known process. Trost et al. have now developed a Ru-catalysed hydration-cyclisation reaction based on the hypothesis that one might effect intramolecular capture of the vinylruthenium (or oxoallyl ruthenium) intermediate by an enone. 44 In the event the reaction proved to be rather different with pyrans being isolated in the absence of water and 1,5-dicarbonyls in the presence of H 2 0 / acid. Thus, by suitable choice of conditions, eneyneones of type 149 can be converted to pyrans (150) or diones (151) in good to excellent yields, possibly via the mechanisms depicted in Scheme 44, which begins with a oxidative cyclometallation step and not attack by water on the alkyne. 5.4 Cycloaddition.- A number of very interesting papers describing transition metal catalysed cycloadditions have appeared. It is salient to note that although described as 'cycloaddtions' they are not (in most cases) cycloaddition type processes in the strictest sense, but they do effect the same net transformation in a stepwise rather than synchronous manner. The classic Diels-Alder reaction has long been a target for asymmetric catalysis. A number of highly selective examples have been reported, here we highlight only two. The first is from Aggarwal et al. on the addition of cyclopentadiene to thioacrylates using Cu-

235

9: Organic Aspects of Organometallic Chemistry 5 mol Yo R uCp(MeCN)3PF6 t

x40

X

acetone 4h

R 150

10 mol% RuCp(MeCN)3PF6

= (CH2)nl OPTsN* 1 49

10 mol% CSA acetone, H20 12h

t

x??

151

Scheme 44

catalyst bearing chiral bisoxaline ligands. 45 By careful tuning of the acrylate moiety, the endo-selectivityand enantioselectivityof the reaction was optimsed to very high levels. For example, acrylate 152 reacts with cyclopentadiene in the presence of 20 mol% Cu-catalyst generated in situ from 153 to give Diels-Alder product 154 in 92% yield with >%YO ee and 15 / 1 endo / exo selectivity, Scheme 45.

*

20 mol % Cu(OTff2

PhS

30 mol ' 7 153

152

CH2C12, -78°C

L

Scheme 45

/Q

SPh

C02Et

154

236

Organometallic Chemistry

The aza-Diels Alder reaction has also received attention, since it provides a route to enantiomerically enriched nonproteinogenic a-amino acids. Jrargensen et al. report the highly effective use of the tol-BINAP ligand (157) in the Cucatalysed reaction of a range of imines with a range of dienes?6 Under optimsed conditions, ee's of up to 99% were attained. In certain cases (e.g. with imine 155 and diene 156), the products corresponding to Diels-Alder type reaction were not obtained and instead, the Mannich adducts were isolated (c$ 158). In THF, the enantioselectivity and diastereoselectivity of this process reaches synthetically useful levels, as shown in Scheme 46. Gevorgyan, et al. have constructed a two-step synthesis of coumaranones employing a Pd-catalysed [2 + 41-cycloaddition of enynes (e.g. 159) with bisalkoxydiynes (e.g. 160).47By using a tert-butoxy diyne (as in 160), acid work-up (TsOH) allows a deprotection-cyclisation of the resulting phenolic ether (e.g. 161) to yield the coumaranone (162) in good yield, Scheme 47. 10 mol% C ~ Cc I O o~ QH EtOpC 155

TMSO

10 rnol O/o 157

156

THF, -78°C

158 TsN

C02Et

82 % yield 96 % ee

Ar = p T o l

Scheme 46

5 mol % Pd(PPh& c

159

'f"

TsOH

0 "C

- k 0

0 "C, THF, 48 h

162

Scheme 47

The [2 + 2 + 21 cycloaddition of diynes to nitriles has opened new avenues for the controlled synthesis of pyridine rings. Itoh et al. have now developed significantly improved Ru-based catalysts that engender useful levels of regioselectivity in this process!' Using dinitriles (such as malononitrile 163) and Cp*Ru(COD)Cl as catalyst, diynes undergo cycloaddition under mild condi-

9: Organic Aspects of Organometallic Chemistry

,<

237

tions. For example, diyne 164 gives pyridine 165 in 92% yield and with essentially perfect regioselectivity, Scheme 48. //N

Me02C<+TMS MeO&

-

N

164

163

5 mol YO Cp*Ru(COD)CI 60 OC,DCE, 5.5 h, 92 %

.t

165

Scheme 48

The cyathane class of natural products are a popular and challenging target in organic synthetic methodology. Wender et al. have developed a large number of Rh-catalysed cycloaddition reactions which are outstanding methods to set up fused ring ~ystems.4~ They have now turned their attentions to this class of diterpenes and report a method for the rapid assembly of the cyathane tricyclic core (cJ: 169).The mechanism is proposed to involve an oxidative cyclometallation (166 to 167) followed by a strain-driven cyclopropyl-methylene rhodium to homoallyl rhodium rearrangement (yielding 168) prior to reductive elimination, Scheme 49. 0.

168

169

Scheme 49

In the final example in this section on cycloaddition, Cheng et al. have generated a low valent Co-catalyst in situ using Zn as reductant [2+2] cycloaddition of strained bicyclic alkenes to alkynes.” Two different bridgehead systems (X, = 0,NC02Me)were examined and found to react smoothly with a range of simple alkynes. An example is given in Scheme 50.

6

Conjugate Addition and Allylic Substitution Reactions

The conjugate addition reaction (‘l,rl‘-addition and ‘Michael reaction’) and allylic substitution reaction have long been favourite methods for homologation

238

Organometallic Chemistry

4

amongst synthetic organic chemists. In recent years there have been significant advances, especially in the field of asymmetric transition metal-catalysed conjugate additions. This section will focus on this area in particular.

5 mol 32 mol O h CoI2(PPh3): % PPh3 /

\

2 eq. Zn powder tol., 90 "C,24h

I'

/

07 % , X = NC02Me Scheme 50

Bimetallic catalysis is a burgeoning area and has been put to good use in conjugate addition reactions. For example the salen-Ni type complex 172 bears two phenoxy caesium units which are proposed to generate a malonate nucleophile (from 170) which will then be held in an organised manner in close proximity to the Ni-centre which activates the cyclohexenone (171)" Under optimsed conditions, the conjugate addition product (173) is generated in up to 90% ee, Scheme 5 1.

(co2Bn bOpBn

171

170

-

10 mol % 172

THF, -40 O C , 48 h

Scheme 51

A 173

90 % ee

co2&7

Whilst most workers focus on generating asymmetric addition protocols based on conjugate addition of a nucleophilic carbon or heteroatom bearing group to an enone (as in Scheme 51) an alternative route to the same product would be to add a hydride in a conjugate sense to a substituted enone (e.g. 174). Buchwald et aZ.52have used tol-BINAP (157)as a very effective ligand system for a Cu-catalysed conjugate addition of 'H-' from a silane. The product enolate is captured with alkylating agents after activation of the silyl centre with the fluoride source [ B Q N ] [ P ~ ~ S ~ F('TBAT') ~] with high diastereoselectivity. The

9: Organic Aspects of Organometallic Chemistry

239

product (e.g. 175) de can be upgraded by equilibration of the a-keto centre, Scheme 52.

b,'0 0

5 mol O/O CUCl 5 mol Oh (5157) 5 mol% t-BuONa Ph2SiH2

tol. 0 "C, 24h

0

-

A

1.2 equiv. TBAT

'0

2.0 ea. BnBr CH2C12, rt 24h.

174

175

94 o/o de 95 O h ee

Scheme 52

The conjugate phenylation of linear and cyclic enones can be effected by use of PhSiC13and a Rh-catalyst in refluxing water containing a simple fluoride source, such as NaF, to generate the silate in sit^.^^ Analogous reaction with Ph-SiMe3, PhGeC13did not proceed. The use of Ph2SiC12(in an excess, possibly due to the instability of the silyl chloride species under aqueous conditions) proved to most efficacious. Using this reagent, a broad range of substrates underwent phenylation in good to excellent yield. The example given in Scheme 53, demonstrates the use of the process in the construction of N- and C-protected benzyl glycine derivative 177 from a dehydroamino acid derivative (176).

xo

5 mol Oi0 [Rh(COD)$ 20 eq. NaF

0

Q

0

4eq. Ph2SiCI2

H20.100 "C, 12h

176

Scheme 53

177

Thiol addition to enones and enals has been explored by Nakajima et al. using Cd(I1) as a catalyst with bispyridine N-oxides as ligands. 54 The enantiomerically pure atropisomeric bipyridine N,N'-dioxide ligand 179 proved rather interesting; giving up to 70% ee in the addition of PhSH to simple aliphatic enals, e.g. 178, Scheme 54. Cumulated enones are also interesting substrates for conjugate addition reactions. In this area most work has been performed with cuprates since the reactions of these species have been extensively explored and understood with the simpler enone type systems. Dieter et al. have found that a-aminoalkylcuprates are particularly effective reagents in conjugate addition and undergo quite diastereoselective additions to a l l e n ~ a t e s For . ~ ~ example, the cuprate reagent generated in situ from Boc-protected amine 182, through deprotonation and

240

Organometallic Chemistry

then trapping with 0.5 equiv. Cu source, adds with 92% facial selectivity to the allenyl ester 181, generating an enolate (183)with on protonation and Lewis acid 1 mol Yo 179

1m ~ l % Cdlp 2eq.PhSH

JH

tol. RT 12h.

178

180

then NaBH4

Scheme 54 R

R

Pi

\ BOC

182 t

s-BuLi / sparteine THF, -78 "C then CuCN.2LiCI -55 "C

185

dr92/8

0

184

Scheme 55

(184) mediated cyclisation yields the lactam 185 with defined alkylidene geometry. The use of Cu and Pd-catalysts for allylic substitutions is commonplace and the bulk of the stereochemical intricacies for these systems have been explored. As such these reactions can represent excellent methods for controlling or inducing stereochemical outcomes in substitution reactions - especially in cyclic systems. Less work has been conducted on acyclic systems. Belelie and Chong have found that allylic phosphates undergo clean S~Z-antireaction with a range of simple organocopper reagent^.'^ By using enantiomerically enriched allyic phosphates (derived from Sharpless kinetic resolution of the corresponding allylic alcohol) and then oxidative cleavage of the allylic substitution product, one can thus access a-branched aldehydes or acids in high enantiomeric excess with predictable configuration, Scheme 56.

Scheme 56

9: Organic Aspects of Organometallic Chemistry

241

The use of Ni-catalysts for allylic substitution is undergoing a revival, particularly for the reaction of allylic electrophileswith organoboron reagents. Uemura et al. have obtained moderate, but promising, enantioselectivities in the Nicatalysed addition of arylboronic acids to allylic esters. 57 Using a ferrocene based phopshinoaryl oxazoline(187), thus incorporating both planar and central chiral elements, cyclic esters are arylated with up to 50% ee and linear systems up to 20% ee. The mechanism probably proceeds via a Ni-ally1 species, generated by oxidative ionisation of a Ni(0) complex with the allylic substrate, which undergoes arylation at the Ni-centre followed by reductive elimination. The DIBAL-H serves to reduce the Ni(I1) procatalyst to start the cycle and the KOH serves to activate the boronic acid as a borate. As an example, the phenylation of cyclohexenyl acetate (186)is given in Scheme 57.

-$

5 mol PhB(0H)Z % Ni(acac),

DIBAL-H (*)-186

KOH

THF, 67 “C

188 50 *Aw

&

187

5 mot % 187

0

-

PhB(0H)j

Scheme 57

7

Cross Coupling Reactions

The cross-coupling reaction of an organometallic with an organic electrophile, catalysed by a transition metal, is one of the major success stories in organic synthesis in recent decades. The reaction, once considered exotic outside of academic labs, has reached the point where it is almost ubiquitious in, for example, pharmaceutical medicinal chemistry research labs. The high levels of geometric control in alkene synthesis and the ability to construct aromatic ring substitution patterns with ease make it a very useful transformation. We start this section by highlighting recent advancs in organoindium chemistry - an area that has received growing attention in recent years. Sarandeses et al. have found conditions whereby a broad range of organo indium species R31n can be cross coupled with aryl halides, aryl triflates, vinyl triflates, benzyl halides and acid chlorides, in good yield and with high tranfer of ‘R,under Pd-catalysis.” The latter point is valuable, inasmuch as only 34 mol% R31nis required for complete consumption of the cross-coupling partner. The identity of ‘R’can be aryl, alkynyl,vinyl or alkyl. Ironically, when R = allyl, these are the easiest organoindium reagents to form but they do not undergo analogous cross-coupling

242

Organometallic Chemistry

reactions. The mechanism is the process is, in the broadest sense, most likely to be very similar to that of other Pd-catalysed cross-couplings:oxidative addition, transmetallation, reductive elimination. An example reaction is given in Scheme 58.

0""'

1 mol o/o Pd(dppf)Cl2

0.34 equiv. (vinyl)3I n

* 96 %

THF, reflux, 1 h

Scheme 58

Much of the interest in organoindium chemistry has derived from the perceived stability of such reagents to aqueous media. Oshima et al. have performed analogous complings to those outlined above, but using water as a co-solvent to the THF.59The diorganindium halide species appear to be the organometallic of choice and trifuryl phosphine was employed as ligand for Pd. Using such conditions, excellent yields were obtained, Scheme 59. NMR experiments suggested that the active transmetallating agent is R21n(OH)and reaction without water did not proceed as efficiently. 2.5 mol YO Pd2(dba)~.CHC13 15(panisyl)21nCI mol 0.65 Ol0 (2-furyl)3P equiv. OH

-

$:o). /

97%

OH

THF, H20 8 / 1 reflux, 1 h

Scheme 59

There has been much attention given to the development of methods for the cross-coupling of aryl and vinyl chlorides instead of their more expensive bromide and iodide analogues. The Pd-catalysed 'Negishi coupling' of organozinc species with organohalides has been the subject of such development and Fu and Dai have reported that the commercially available Pd(t-Bu3P)zcomplex acts as a general and efficient catalyst for such processes.60The systems comprises of a THF / N-methyl pyrollidinone solvent mixture at 100 "Cand employs 2 mol% Pd(t-Bu3P)2.Under these conditions, even very hindered biaryls (e.g.191)can be prepared in good yield from an aryl chloride (e.g. 189) and the requisite aryl zinc (190), Scheme 60. CN

OMe

Me

189

190

THF / NMP, 100 "C

76%

OMe 191

Scheme 60

9: Organic Aspects of Organometallic Chemistry

243

Using a vanadium (V) coupling agent, organozinc compounds can be homoand hetero-coupled, with significant selectivity for the latter. 61 Whilst it is not clear whether these processes occur via inter- or intra-molecular coupling, both zinc and zincate species may be employed. Furthermore, the Schlenk equilibrium can be exploited. For example, the mixed organo zinc species 193, generated in situ from bromonaphthalene 192 undergoes smooth oxidative heterocoupling to generate the methylated naphthalene 194 in 92% yield.

?'

192

ZnMe

Me

193

194

Scheme 61

As an alternative bis-electrophile to diketene in the synthesis of 3-substituted but-3-enoic acids, Duch2ne et al. have explored the use of unprotected 3iodobut-3-enoic acid with organozinc reagents under Pd Remarkably, the acid can be used directly and under optimised conditions employing 3 equiv. of organozinc species and a phosphine free catalyst system the coupled product is obtained in excellent yield and with very little double bond isomerisation, Scheme 62. The requirement for 3 equiv. of organozinc reagent was explored in some detail. Quenching with D 2 0resulted in no D-incorporation at the a-carbon and thus there is no zinc ketene acetal intermediate. Furthermore, addition of first 2 equiv. of MeZnBr and then 1equiv. benzyl ZnBr afforded only the benzyl coupled product. On this basis, the intermediacy of a dizincate (195) was proposed. R

195

PdC12(MeCN)2

Scheme 62

Despite the toxicity of organotin species, they remain very popular crosscoupling partners for organic electrophiles under Pd-catalysed conditions, the so-called 'Stille coupling'. Maleczka Jr. et al. have found that microwave irradiation dramatically accelerates the cross-coupling of in situ-generated vinyl stannanes with alkenyl and aryl halides, thereby generating dienes and styrenes re~pectively.~~ Interestingly, Pd is utilised twice in one pot, first to catalyse alkyne (e.g. 196) hydrostannylation and secondly to effect the cross coupling. An example is given in Scheme 63. The generation of small quantities of the Z isomer of the product (197) was ascribed to the competition of a radical hydrostannylation induced by the high-temperature microwave conditions. The in situ-generation of the tributyl tin hydride from Bu3SnC1 / aq. K F / PMHS

244

Organometallic Chemistry

(polymethylhydrosiloxane) results in the presence of water in the reaction medium. On the basis of control experiments, the authors suggest that this water is an essential component in the form of a microwave 'heat sink'. 197

wOH

Ph

\ L o t i THF, Bu3SnCI aq. KF, PHMS 140 W, 3 min

196

14 11.6 E I Z

then PhBr,

198

Ph

140 W, 10 min.

Scheme 63

The versatility and reliability of the Stille coupling, which operates under mild conditions and is amenable to scale-up, is exemplified by the ready generation of nine examples of mono and bis-methylated 2,2'-bipyridine ligands through C(1)-C(1') coupling, Scheme 64.64The complexation of these ligands with Cu(I1) and Fe(I1) salts was explored and their potential as ligands in atom transfer radical polymerisations considered. cat. Pd(PPh314

Br R, R' = H or Me

Sn&r3

b

toluene refluxah

9 examples

50-87'Yo

Scheme 64

The rapid and mild carbonylative coupling of diaryliodonium salts with aryltributylstannanes has been explored by Al-Qahtani and Pike as a new route for the generation of benzophenones with labelled carbonyl carbon^.^' Such compounds are likely to be of use in the preparation of "C-labelled radiotracers for positron emission tomography, an important medical imaging technique. The very short half life of carbon-11 (ca. 20 min) means that routes must be very rapid. The use of a cyclotron to produce radio-labelled 'lCO2 allows ready access to "CO and the high radiochemical yields in these reactions, e.g. Scheme 65, thus make them of significant utility. [Ph21JBt

1 mi% SnBu3

F

DME-HpO Pd(C12 * CO-N~ (1 atm) 1 min, RT

98 % yield

Scheme 65

Fy-qcQ f

F

9: Organic Aspects of Organometallic Chemistry

245

As alluded to earlier, the toxicity of tin is an issue in the generality of application of Stille type couplings. Maleczka Jr. et al. have reported that the tin content can be reduced by 94% in hydrostannylation-Stillecoupling through use of catalytic quantities of tin and regeneration of a tin-hydride? An example of this remarkably efficient process is given in Scheme 66. 6ml% Me3SnCI 1 ml% Pd(PPh314

+

w

I mol % Pddba3 4 mol % (furyl)3P

PHMS, aq, Na2C03 Et20, 37OC, 15h

Ph*Br

91 Yo

Scheme 66

Copper is emerging as a cross-coupling catalyst that in some cases equals or surpasses Pd. The cross-coupling of Grignard reagents is most often accomplished using Ni (‘Kumada coupling’) or Pd-catalysts, however Backvall has developed the use of Cu(1) and applied this in an early stage of the total asymmetric synthesis of Paeonilactone A (201), Scheme 67.67Interestingly, the relatively acidic methine group in the substrate (199) does not interfere and the desired methylated butadiene (200) is obtained in excellent yield, by using an excess of the Grignard reagent. OTf

2.4 mol YOCul

Me

3 equiv.

Me‘

199

200

201

Scheme 67

Copper has also been exploited (stoichiometrically)in the cross-coupling of allylic silanes with allylic halides. Takeda et al. have found that CuOt-Bu smoothly couples fl-silyl allyic alcohols (e.g.202) with allyl chlorides (e.g. 204) to afford the corresponding allylated allyl alcohol (205)afterwork-up, Scheme 68.68 The reaction is proposed to proceed via deprotonation of the hydroxyl group by the CuOt-Bu and then C to 0 migration of silicon resulting in a vinylic copper species, e.g. 203. This is then captured by the allylic halide and the use of TBAF in the work-up cleaves the Si-0 bond to yield 205. Copper (I) is often employed as a co-catalyst in the Sonogashira crosscoupling of terminal alkynes with organic halides. It emerges from work by Kang et al. that Cu(1) is also essential in a similar process involving a range of TMSacetylenes (206) and triarylantimony diacetates (207).69The reaction has been extended to include carbonylative conditions (CO atmosphere) and thereby

Organometallic Chemistry

246

SiMe3 OH ph%

202

205

203

Scheme 68

obtain propynones (208) in good yields. The mechanism proposed for this process bears much similarity to that of the Sonogashira reaction and is outlined in Scheme 69.

R

Me3SiX SiMe3

206

x x R-CU

[Pd(COAr)][SbAr2(OAc),]

R-CuPd(C0Ar)

x

Scheme 69

207 [SbArdOAc)d

R

co

COAr

208

Raston et al. have studied the reaction of triarylgallium species with water and found it co-solvent de~endent.~' For example in a mixture of toluene and water, tri(bipheny1)gallium (209) undergoes reductive homocoupling yielding bis(biphenyl) (210) whereas in a mixture of THF and water, protolysis results in the generation of biphenyl (21l),Scheme 70. The homocoupling process is exclusively intramolecular and is proposed to proceed through the generation of a Ga(II1) bishydrate species at the toluene water interface. This process is blocked by THF coordination and protolysis results. One remaining aryl group remains attached to Ga in both cases and p - 0 Ga clusters are generated.

210

Scheme 70

211

211

The use of supercritical carbon dioxide (scC02)as an environmentally benign solvent for organic synthesis is growing in popularity and applicability. Use of

9: Organic Aspects of Organometallic Chemistry

247

traditional catalyst systems in this low polarity medium has been hampered by issues of solubility of ligands. Holmes et al. have now demonstrated that Heck and Suzuki cross couplings proceed reasonably efficiently in scCO2 by use of the P ~ ( O A Cand )~ P(~-Bu)~.~'

DIPEA 5 mol %Pd(OAc)2 10 mol %P(f-Bu)3

68 O/O

Scheme 71

Biaryl formation through Suzuki coupling is a powerful synthetic process. However, until very recently the potential for this reaction to be employed in an asymmetric manner was not explored. Using naphthyl based coupling partners, Cammidge and Crepy have performed the first examples of an asymmetric variant of this process. 72 Although the selectivities are not outstanding, they certainly demonstrate proof-of-concept and undoubtedly refinement of ligand systems will eventually lead to highly selective processes. A n example, affording the highest selectivity, is given in Scheme 72. Me2N

fir

Me

ph2pkd

3 rnol % PdCI2

6mol%L

L=

CsF, DME 6 d, reflw 50 %

I

Fe I

05%-

Scheme 72

As mentioned earlier, for cost and environmentalreasons there has been much interest in the development of methods for the Pd-catalysed cross coupling of aryl chlorides as opposed to the more reactive bromides and iodides. The analogous reaction of an aryl fluoride, whilst perhaps less useful than that of the chloride, is unprecented. The activating effect of the Cr(CO), unit on the reaction of aryl rings with nucleophiles is well known and Wilhelm and Widdowson have applied this to effect the Suzuki cross coupling of aryl fl~orides.?~ Interestingly, the reaction becomes selective for fluoride over chloride, Scheme 73. However, the analogous reaction using bromo aryl boronic acid led to polymeric material suggesting that ArBr is more active than [Crl-ArF. The direct use of carboxylic acids , as opposed to e.g. acid chlorides, in Pdcatalysed coupling reactions has recently been reported by Goossen et ~ 1

.

~

~

Organometallic Chemistry

248

5 mol % Pd2(dba)3

WH),

oc -c;- co co

20 mol % PMe3 DME, reflux, 16h 64 %

I

OC-Cr.

I

co

co

Scheme 73

Pivalic anhydride, or better, di(N-succinimidyl)carbonate, activates the acid in situ and allows the high yielding synthesis of ketones from carboxylic acids and boronic acids through a Suzuki type coupling that has a very broad scope. The example given in Scheme 74 shows the remarkable functional group tolerance of the procedure.

Ho NJOH 10

3 mOl% Pd(F@CaC)2 1.2 eq. PhB(OH);! 9 mol % PCy3 2 eq. NanCOa THF, 60 "C, 20 h. 78 ole

*

HoA J P10h

Scheme 74

8

Metathesis

8.1 Ring Closing and Ring Opening Metathesis of Alkenes. - Alkene metathesis is, without doubt, the fastest growing area in organic synthesis. The emergence of efficient, easily handled pro-catalysts with predictable reactivities and selectivities has made this chemistry available to and indeed used by the regular synthetic chemist. The reaction is so dissimilar to most other organic transformations that it has opened new avenues for synthetic strategy and indeed may well have changed the way in which synthetic chemists approach 'disconnection'. For example, macrocylic peptides (cJ: 214) are usually made by closing the ring through a macrolactamisation process. However, the use of the 'first generation' Grubbs type catalysts (213) with 1,n-terminal dienes (cf:212) allows ring closing metathesis (RCM) to be used, Scheme 75.75The ready hydrogenation of alkenes allows a 'traceless' nature to such transformations. An analogous approach has been employed by Ojima et al. in the preparation of macrocyclic taxoids, one example of which is shown Scheme 76.76The high density of functionality in the substrate (215) yet smooth conversion to product (216) highlights the selectivity of the catalyst for alkenes and thus the power of the reaction. The rapid access provided by RCM allowed the generation of a very diverse library of such structures such as 216 and the testing of their cytotoxicity provided further support for the common pharmacophore theory in the design of paclitaxel and epothilone hybrids.

9: Organic Aspects of Organometallic Chemistry CI p R )I,,

10 mol%

OMe O

212

249

Tcy3y 9

Ph

PCY3 213

1,1,2-trichioroethane 16h,83%

O

Scheme 75

213 20 mol X

CHPCIZ

20 h, 82 X

Scheme 76

Much progress has been made towards the development of efficient and selective catalysts for asymmetric RCM processes. Unfortunately, thus far, the reactions require the more reactive and less easily handled Mo-based systems (e.g. 218). Nontheless, the selectivities attained in some cases are outstanding. Many approaches to such reactions may be envisaged, e.g. desymmetrisation, kinetic resolution and asymmetric ring-opening cross metathesis. Hoveyda and Schrock have been key players in this area and the example in Scheme 77, which delivers outstanding selectivity under solvent-free conditions, comes from their laborato~ies.7~

217

(no solvent) 22 OC,5 min R = CPr

Scheme 77

t

219 99 % ee

250

Organometallic Chemistry

There has been much effort devoted to making RCM reactions even more reliable, robust and thus simpler to perform. Jafarpour and Nolan have impregnated polydivinylbenzene(Poly-DVB)with Ru carbene procatalysts, such as 213 and the ‘newer generation’ 220 and 221, to give heterogenous catalyst systems of type 222 that can be recovered by simple filtration and then recycled, Scheme 78.78 Addition of CuCl, to act as a ‘phosphine sponge’ results in increased activity. A ‘boomerang’ type mechanism is proposed whereby solution-phase catalyst re-metathesises back onto the polymer.

213,220,221

I

213

220

n

221

Scheme 78

The use of scC02in Mo- and Ru-catalysed RCM reactions has been explored by Furstner and Leitner et al., and found to be equally as efficient as chlorinated The high reactivity of the second generation catalyst systems, e.g. 220, is not attenuated in scCO2 and thus di- and tri-substituted alkenes can still be employed whilst enjoying the benefits of the rather unique properties of the solvent. Through a series of careful experiments in which diene 223 was reacted in a vessel in which a previous RCM (also of 223) had been conducted, it was demonstrated that complexes of the type 220 are not soluble in scC02 and thus act as heterogenous catalyst systems, Scheme 79.

223

* SCCO~, 40 OC, 70 h

Scheme 79

I

Ts

224

8.2 Cross-Metathesisand Isomerisation of Alkenes. - The range of alkene units open for metathesis reactions is expanding rapidly. Previous work with W and Mo-based carbene catalyst systems had shown that a,b-unsaturated carbonyls

9: Organic Aspects of Organometallic Chemistry

25 1

(enones, acrylates etc.) did not turn over due to the stability of the metallocyclobutane because of carbonyl chelation. Furthermore, Ru-based enoic carbenes were found to preparable but very unstable and unable to enter into turnover in ring-opening metathesis (ROM) with cyclohexene.However, Grubbs et al. have now found that the electron rich carbene 221 will undergo metathesis with acrylates (e.g. 224) and the resulting enoic carbene enter into ROM-cross metathesis (ROM-CM) with alkenes and dienes to generate enoates.80For example, cyclohexene (in excess) reacts with tert-butyl acrylate to produce the deca-2,8diendioate (225) in 88% yield, Scheme 80.

lreflux, 3 h, 88 %

225

Scheme 80

224

Rutjes et al. have found that alkene-enamidesalso undergo methathesis, albeit RCM and most likely uia an 'alkene-first' type reaction mechanism.81Nonetheless, the reaction provides a useful entry point to e.g. dihydropyrroles and homologues. On an attempted extension to the analagous process but employing allenamides, an interesting isomerisation process was observed. Thus, alkeneallenamide 226 (R = H) failed to undergo RCM, but 227 (R = Me) underwent smooth conversion to triene 228 in 100% yield. Such isomerisations are not unprecedented in methathesis and may well be due to the competing generation of a Ru-H species. Such a process would also account for why diene 229 underwent RCM to give a six, not seven- membered ring product 231, most likely uia initial isomerisation to 230. cat. 220

/

<'N-

(227 only)

I

TS

Ts

226 (R = H) 227 (R = Me)

229

I

Ts

228

230

I

Ts

%heme 81

I Ts

231

252

Organometullic Chemistry

The CM of allenes has been studied by Barrett et al. and found to be a reasonably useful method for the generation of 1,3-disubstuted allenes.8* The reaction suffers from a competing polymerisation process, which is the exclusive pathway when the allene subsrate bears a (directly attached) aryl ring. Thus for example, allene 232 (R = hexyl) undergoes CM in 85% yield to give allene 234 whilst 233 (R = Ar) undergoes polymerisation to give 235 (Mw 3600-5700), Scheme 82. Although a standard metathesis mechanism was suggested for the CM process, no mechanism was proposed for the polymerisation, although a simple one involving a Ru-H intiated process could be envisaged. PCY3

Clh, I c 1 - y

5 mol o/o 232, R = hexyl

Ph

pcy3

213

t

CH2C12 20 "C, 20 h

233, R =Ar

R

1 ;-JR

234, R = hexyl

235'

=Ar

Scheme 82

The generation of E/ 2-mixtures in alkene cross-metathesis (CM) can be a major problem. Taylor et al. have found that homoallylic alcohols (cf: 236) undergo CM with allyltrimethyl silane to give ally1 silanes (237) with high kinetic E selectivity, providing that the homoallylic alcohol has an anti substituent at the allylic position." The selectivity can be rationalised by a Ru-chelate intermediate where the anti substitutent (R) undergoes clash with the TMS group ( c . 239), Scheme 83. The mechanism thus also accounts for the loss of selectivity with non or syn-substituted substrates (c. 238). It is important to note that these selectivities are kinetic and that equilibration by reverse CM can attenuate selectivity. The mechanism of the Ru-catalysed (213) CM reaction of styrene with vinylsilane to give 240 has been studied in depth by Fischer et al. using isotopic labelling and kinetic techniques and found to proceed via a carbene mechanism. 84 A key feature of the study was the dissociative nature of the mechanism involving generation of the silylcarbene intermediates. Thus a PCy3ligand must dissociate from 213 to generate the highly reactive intermediate 241 which then undergoes oxidative cyclisation, Scheme 84. 8.3 Alkyne Metathesis. - The cross-metathesis of alkynes has become a general and useful reaction through catalyst systems based on Mo. For example, Fiirstner has introduced the tris aniline complex 242 which effects alkyne CM under remarkably mild conditions and without the need for promoters as had previously been the case with other systems.85A key point to these discoveries was that 242 is activated in situ, by reaction with CH2C12.For example, alkyne 243 undergoes smooth CM with just 1.5 equiv. 244 to give 245 in 82% yield after heating to 80 "C in toluene. The intramolecular combination of a terminal alkene with a terminal alkyne allows a reaction analogous to diene RCM to proceed, but with skeletal reor-

9: Organic Aspects of Organometallic Chemistry

253

n

Ides’

N

y:

Mpehs

5mol% (c12Ru/ J I pcy3 221

Ph+TMS

*

237

CbC12 reflux

Ph+

236

(excess)

R

m /T

M

Phd

80%E

OH T

M

S

92%E

S

Scheme 83

6 h, quant. r

240 1

Scheme 84

ganisation. The number of catalyst species known to effect such transformations is growing rapidly and the reaction is of particular synthetic interest since the product is a conjugated diene. The reaction has been applied to the synthesis of medium rings, traditionally rather challenging to prepare. For example, Mori et al. have cyclised enyne 246 using the first generation catalyst 213 to give diene 250 in 99% yield, Scheme 86.86An ‘yne-then-ene’mechanism, involving attack of the in situ generated methylidene ruthenium carrier first at the alkyne (to generate 247) before ring-opening (248) ring closing (249) and diene (250) liberation, is suggested for this process. It should be noted that an ‘ene-then-yne’ mechanism is also a possibility and indeed may well be favoured when the alkyne is hindered.

Organometallic Chemistry

254

OMe

L

'MO'

,!

L

242

10 mol %

243

I.=

OMe

CH2 c 12 toluene, 80 "C I

245

CN

scheme 85

CH2C12

I

Ts

7 h, rt, quant.

/

Ts

246

246

I

I

Ts 247

Ts

250

248

1 scbeme 86

Not all enyne metathesis reactions involve turnover via the classic 'Chauvin' mechanism in which a carbene is propagated intermolecularly ( i e . as Scheme 86). However they may still involve carbene or carbenoid intermediates. Such processes, whilst still formally metathesis reactions, are often termed 'reorganisations'. For example, Oi and Inoue et al. have reported extensively on the use of

9: Organic Aspects of Organometallic Chemistry

255

cationic platinum phosphine complexes to catalyse the metathesis of enynes (c.f. 251 and 253) to give dienes (e.g. 252 and 254).87Using a combination of I3Cand *Hlabelling (in separate experiments) two basic reaorganisation modes (‘A’ and ‘B’) were identified and the generation of a cationic platinum carbene (255) proposed as an intermediate from which divergence occurs through a platinavinylcyclopropanerearrangement (256), Scheme 87.

I

2 mol X

251

252 (>95%€)

253

254 ( 7 2 % Z )

Scheme 87

8.4 Miscellaneous.- The use of heteroatom ‘alkenes’, e.g. imines in metathesis is an area of growing interest. An example of this process was described earlier (Scheme 9). Bruno and Li have developed a very efficient niobium based catalyst system for the metathesis of aldimines with formaldehyde imine trimers.88The reaction proceeds without need for a niobium imine pre-catalyst, indeed 5 mol% of the simple complex NbC13(DME)functions well, Scheme 88. The mechanism of the process is suggested to be analogous to that outlined in Scheme 9 and indeed a niobium imine complex (rner-(DME)C13Nb=NPh) was prepared and found to be active. Furthermore, kinetic studies demonstrated that the reaction does not proceed via attack of free amine on the imine. Alkane metathesis is an area with huge potential and one that is being

256

Organometallic Chemistry

Scheme 88

vigorously explored in a number of laboratories. Thivolle-Cazat andBasset et aE. have reported that tantalum (V) complexes supported on a silica-gel pellet surface effect a-bond cross-metathesis of alkanes. 89 Using r n ~ n o - ' ~labelled C ethene it was demonstrated that the 2,2-dimethylpentane (258) detected in the MS analysis of the reaction of ethene with complexes of type 257 must be generated by C-C cross-metathesis. Two mechanisms were considered to account for such a process: apparent and real a-metathesis, the former proceeding via the neopentylidene moiety of 257 and the latter via the neopentyl moiety, Scheme 89.

Etti

500-600equiv. +

>r,w)(0-Si),

+MeH, EtH, PrH,BuHetc

3-x

258

257

Scheme 89

9

Transition Metal Catalysed Hydrogenation and Hydrometallation

9.1 Use of Gaseous Hydrogen and Bidentate Phosphorus Based Ligands. - Hydrogenation remains an extremely well used reaction in both academia and industry. The 'clean' nature of the reagent (Hl), the high selectivities induced by modern catalyst systems and the ready preparation of unsaturated substrates for

9: Organic Aspects of Organometallic Chemistry

257

the reaction make it a transformation of choice for commercial production, natural product synthesis and ligand testing and design. For example, RajanBabu et al. have developed a highly versatile asymmetric synthesis of functionalised 1,2,3,4-tetrahydroquinolines(e.g.260)exploiting the reliable and predictable Rh-catalysed hydrogenation of dehydroaminoacids(259),Scheme 90?

259

260

L=

Ar = 3,5-C&(Me)2)

The asymmetric hydrogenation of dehydroamino acids continues to be the 'benchmark' for the testing and comparison of new classes of chiral ligand. Claver et al. have developed phosphine-phosphite ligands based on enantiomerically pure carbohydrate scaffolds. 91 Some of the ligands (e.g. 262) displayed near-perfect selectivities in the asymmetric hydrogentaion of the standard acetamidoacrylate substrate 261 to 263, Scheme 91.The mechanism for this process was studied by 31PNMR and by reaction kinetics, which suggested that the turnover limiting step is the oxidative addition of the dihydrogen.

Scbeme91

258

Organometallic Chemistry

The group of Gridnev and Imamoto have introduced two remarkable chiral bidentate ligand systems based on two tert-butyl(methy1)phosphine units linked by an ethyl or methyl group. The former (264) is given the abbreviated name 'BisP*' and gives extremely high enantioselectivities in asymmetric hydrogenation of dehydroamino acids.92Interestingly, this ligand does not obey the 'quadrant rule' inasmuch as Rh-chelate complexes of 264 induce formation of R-amino acids but have a h-conformation. Detailed NMR and deuterium labelling studies support a dihydride mechanism in which the substrate dehydroamino acid binds ufer oxidative addition of H2.Indeed a pair of isomeric dihydrides were spectroscopically characterised after reaction of complex 265 with H2in CD30D, Scheme 92. This stands in stark contrast to diary1 phosphine rhodium chelates (e.g. dppe) where the dihydride is not spectroscopically observed (but has been detected using para-hydrogen).

Scheme 92

The mechanism of asymmetric Rh-catalysed hydrogenation by complexes of both BisP* (264) and its relative 'MiniPhos' were further studied by 13C, 2H labeling and NMR studies which, on the basis of isotopic fractionation, suggested that there are two modes of substrate coordination / migratory insertion, Scheme 93.93The major pathway is proposed to be 'A', except when the dehydroamino acid bears a bulky substituent and then pathway 'B' is favoured. In both cases the major enantiomer of product is the same.

r

NHAc

NHAc

264

D

(0

NHAC

NHAC

Scheme 93

Aryl-alkyl ketones have long been excellent substrates for asymmetric Rucatalysed hydrogenation and the range of useful substrates has now been ext-

259

9: Organic Aspects of Organometallic Chemistry

ended by Noyori et al. to include hetereoaryl alkyl For example, using the optimsed catalyst system 267, a range of pyridyl and thiophene based substrates can be reduced with high TON and good to excellent selectivities.For example bis ketone 266 is reduced to the corresponding non-meso diol in ‘100%’ ee in 17h with 10,OOOturnovers of 267, or the catalyst derived thereof, Scheme 94. 0.01 mol% 267

CPrOH-DMF 0

266

0

8 atrn H2 100 % ee

* OH

6H

OMe

Scheme 94

9.2 Use of Monodenate Phosphorus Based Ligands. - The history of chiral ligand design for asymmetric transition metal catalysed reactions, particularly asymmetric hydrogenation, displays interesting trends in concept and application. Thus the rather poor selectivities induced by monodentate ligand systems such as ‘PAMP were much improved by the introduction of bidentate C2symmetricligands such as DIOP, DIPAMP, DUPHOS, BINAP. A move away from C2-symmetry,particularly with a heavy electronic asymmetry then became fashionable(cJ:ligand 262). However, recently there has been an almost full-circle return to monodentate ligand systems, sometimes with great su~cess?~ Chiral biaryls (atropisomeric chirality)have always been popular frameworks on which to mount phosphine ligands. Sannicolo et al. have reported the preparation and resolution of ligand 268 which induces 74% ee in the hydrogenation of 261 under standard conditions, Scheme 95? Ruthenium complexes of 268 are moderately effective in catalysing asymmetric Diels-Alder reactions of acrylamide derivatives with cyclopentadiene. Whilst the selectivity is not outstanding, the new scaffold may well facilitatedesign of new improved ligands based on this building block. In hydrogenation, monodentate chiral P-based ligands where the P is connected to heteroatoms (phosphites, phosphoramidates etc.) seem to be much more successful at inducing high enantioselectivities than their simpler phosphine analogues. For example, Orpen and Pringle et al. have prepared phosphonite ligands based on binol and homologues, such as 269 which induce up to 92% see in the hydrogenation of 261.9’ Although they were unable to obtain crystals of any of the catalytically active and spectroscopicallycharacterised Rh-complexes, PtC12complex 270 was prepared, Scheme 96, and its X-ray structure compared

260

Organometallic Chemistry

Br

SNa

Br

BuLi

PPh2CI then H202

/ S

/

then C13SiH

268

Scheme 95

with that of complex 271 bearing a similar, but bidentate, ligand. The major difference in the bis monomeric system (270) appeared to be restricted P-M rotation and the generation of a dominant rotamer that induces better enantioselectivity.

270

269

Scheme 96

Reetz and Mehler have prepared and tested similar ligands that are phosphites rather than phosphonites but again based on binol? It was found that a chiral centre on the non-binol derived P-OR group lead to enhanced selectivity,

9: Organic Aspects of Organometallic Chemistry

26 1

although the match-mismatch effect with the binol was negligible, for example itaconic acid dimethyl ester (272) was hydrogenated to give 273 with 99.2 and 98.2% ee using ligands 274 and 275 respectively, Scheme 97. When a ca. 1 / 1 mixture of 274 and 275 was employed, the ee was 98.8%, although using pure 274, substrate to catalyst ratios could be increased to ca. 5000.

Me02C

JL

COZMe

272

0.1 mot%

[Rh(L)$OD]BF4 c 1.3 atm H2 CH2C12,20 "C, 20h

Me02CL C O z M e

273

100 %, 98.8 % ee

L = 2741 275

Scbeme 97

Similarly high selectivities are reported for the Rh-catalysed hydrogenation of itaconates (up to 96.6% ee) and dehydroamino acids (e.g. 261)in up to 99.8% ee by de Vries and Feringa et al. using their 'MonoPhos' phosphoramidite ligand 276.99Interestingly, with most bidentate ligands, higher hydrogen pressures lead to lower selectivities, however, with 276 higher pressure leads to a slightly higher selectivity and furthermore, by suitable choice of solvent, substrate catalyst ratios can be increased whilst still maintaining fast reaction times, Scheme 98.

276

Scheme 98

262

Organometallic Chemistry

9.3 Transfer Hydrogenation. - In a complementary manner to the reduction of C = C units by Rh complexes, Ru-complexes are very efficient at mediating the reduction of C =X units (X = 0, N), especially under transfer hydrogenation conditions. van Koten et al. have recently added Ru-catalysed tranfer hydrogenation catalysts to their extensive range of 'pincer' complexes. loo These new systems are of interest since they engender very high rates of turnover, thereby achieving high TON with high TOF. For example, complex 277 will transfer hydrogen from isopropanol to cyclohexanone with TON and TOF up to 10,OOO and 27,000 h-' respectively, Scheme 99. A possible intermediate (278) was detected by NMR on heating 277 in KOH / i-PrOH.

PPh2

0

OH

0.01 mol %

CPrOH, KOH

Scheme 99

Faller and Lavoie have found that using the same enantiomer oi aminoindanol 279, but switching between Ru and Os-based catalyst sytems for kinetic resolution in oxidation and in asymmetric induction in reduction allows access to both enentiomers of secondary arylethyl alcohols in high enantiomeric excess,'O1 for example 280 from either rac-280 or from ketone 281, Scheme 100. Although the yield is much lower in the kinetic resolution, the method does in principle allow the preparation the product as essentially a single enantiomer if conversions are allowed to proceed to high values. 0.1 M f-BuOK

i-PrOH, acetone OH

t

cat. (CyRuCI&

OH

rac-280

q 0

281

S-280

279

R-280

NH2

0.1 M t-BuOK b

i-PrOH cat. (CyOsC12)2 (Cy = cymene)

Scheme 100

OH

263

9: Organic Aspects of Organometallic Chemistry

Asymmetric Ru-catalysed transfer hydrogenation in water has been reported by Chung et a1.lo2Although the selectivities are not as high as in conventional pure organic systems, they are still quite reasonable and since the addition of hexane at the end of reaction allows separation of products (in the organic phase) and then recycling of the catalyst in the aqueous phase, the method bodes well for the development of environmentally friendly procedures. Indeed, after 5 recycling events, the catalyst system derived from 284 still gave essentially the same enantioselectivity with reasonable run-time in the reduction of ketone 282 to alcohol 283,as illustrated in Scheme 101. OH

0

H20,30°C

run 1: 4 h, 95.3% ee run 5:7 h, 94.2 % ee

284

Scheme 101

Grubbs et aE. have exploited the mutifaceted nature of Ru-catalysis to conduct tandem Ru-catalysed RCM-transfer hydrogenation reactions, with remarkable effect.’03 For example, dieneone 285 undergoes RCM and then transfer hydrogenation to yield alcohol 286 in > 85% yield, Scheme 102.

:a K 0

/

285

n OH

3 mot Yo 221

DCE

then KOH, CPrOH

286

Scheme 102

The mechanistic aspects of Ru-catalysed transfer hydrogenation have been the subject of much experimental and experimental study. A pivotal player in the development of the reaction has been Noyori and the key mechanistic aspects of the ‘nonclassical’ mechanism have been given in a ‘Perspective’ The unusual feature of the mechanism is that instead of both ‘H’ groups being delivered in a step-wise manner from the metal centre (as in the ‘classical’ mechanism) one is delivered from the ruthenium amido group, in a pericyclic type process, Scheme 103. Thus the activation of the pro-catalyst (287) involves

264

Organometallic Chemistry

deprotonation and loss of a counter-ion (Cl) to give 288 which hydrogen bonds through the ruthenium amide to the alcohol to be oxidised. Pericyclic deprotonation-hydride abstraction (289) then gives the coordinated oxidation product (ketone or aldehyde) which is released to generate the ruthenium hydride species 290. The reverse process then occurs to reduce a ketone of aldehyde to the corresponding alcohol.

GD I

-HCI

422 I

H

287

288

m I

I

RU H-~’ \ 288

a

WNTS

ti I

$-OH

aI 289

Scheme 103

Casey et al. have observed an analogous ‘H,H’ delivery from the Ru-H and Cp-OH groups in complex 291 to carbonyls, e.g. benzaldehyde, Scheme 104. lo’ The ruthenium co-product 292 from the reduction process is coordinatively unsaturated and undergoes reaction with 291 to generate dimeric 293. The reduction process was studied in detail by NMR, kinetics and isotopic labelling and found to be concerted with polarised double bond units (aldehydes and ketones) undergoing much more rapid reaction than alkenes.

9.4 Transition Metal Catalysed Hydroboration, Hydrosilylation and Hydroarylation. - Since its introduction in the mid eighties by Noth, Rh-catalysed hydroboration has been developed into a powerful asymmetric process. ‘P,N’ ligands have proved to be particularly effective for the hydroboration of styryl

9: Organic Aspects oj OrganometaIIic Chemistry

H:*lo T To1

PhCHO To'*>H, -* To1

m,. .

265

.

D..

ww

291

Dh

-

Ru"-Y! W'AO

Scheme 104

,y

H

+

To1

co

OH

292

CCFO

293

type substrates. Guiry et al. have modified the 'Quinap' ligands of Brown (vide infia) to generate quinazoline based ligands (e.g.296) that are very effective with certain styrene substrates.106For example, f3-methyl styrene (294) undergoes hydroboration-oxidation to generate alcohol 298 in up to 96% ee and with 91 YO regioselectivity for generation of the benzylic borane (297) and thus alcohol, Scheme 105. The mechanistic aspects of Rh-catalysed hydroboration are still a matter of debate, however there is a general consensus that oxidative addition of the catecholborane (295)followed by insertion of the alkene into the Rh-H bond (in this case probably generating an q3-benzyl complex) and then reductive elimination of the rhodium-alkyl boryl complex is the general pattern. 295

0" 294

THF, 2h, RT 1 mot% [Rh(296)(COD)][OTr]

Q I

\

0, B/O

NaOH, H202 f

297

298

Scheme 105

The oxidation of the organoborates which are the primary products from Rh-catalysed hydroborations (e-g. 297) to the corresponding alcohol by basic peroxide is stereospecific and proceeds with retention. Brown et al. have developed an analogous process that generates amines instead of alcohols. Used after asymmetric hydroboration of styrenes catalysed by Rh-complexes bearing the Quinap ligand (300) the process generates primary and secondary benzylic amines (e.g. 305) in high enantiomeric excess, Scheme 106.'07Prior conversion of

266

Organometallic Chemistry

the primary hydroboration product (301) to the trialkyl borane (302)by reaction with a Grignard reagent is essential. Furthermore, it was found that the N-chloro or sulfonato-amine reagent must bear an N-H group. On this basis it was suggested that the zwitterionic adduct (303)is unreactive towards the [1,2)-shift and must first be deprotonated to give the anionic borate (304) which undergoes benzylic migration selectively.

295

THF, 2h,FIT MeO 1 mol O h [Rh(300)(COD)][OTfl

Me0 299

30min. 301

MeO

30: 2

3 eq. H2NOS03H t

THF, 10h.

Me0

303

Me0

304

98 YO ee

Scheme 106

Rhodium catalysed hydroboration is not restricted to styrene based substrates, but they do tend to give the best control of regioselectivity and generally the highest enentioselectivity. Consequently, such substrates are generally used to 'benchmark' new ligands. For example, Knochel et al. Have found that the tris cyclohexyl diphosphine ligand 307 compares reasonably well with, e.g. Quinap (300) or Binap, in the hydroboration of styrene."' The analogous diary1 alkyl diphosphine 306 is much less effective, Scheme 107. Asymmetric hydrosilylation of alkenes has been reasonably well explored, with the Pd-MOP complexes of Hayashi proving very effective in the reaction of styrene type substrates with chlrosilanes. More recently, the application of early transition metals for the reduction of imines through hydrosilylation has been developed. For example, Hansen and Buchwald have demonstrated that the

9: Organic Aspects of Organometallic Chemistry

261

295

L

'

Scheme 107

EBTHI-based titanocene difluoride 309 catalyses the hydrosilylation of N-aryl imines (e.g. 308) with very high selectivity, Scheme 108.'09 To maintain catalyst turnover, and high enentioselectivityfor 310, it was found to be necessary to add a primary amine (generally isobutylamine) slowly during the course of reaction. NJofMe pyrollidine, MeOH D

308

PhSiH3 2mol%309 slow addition IBuNH;!

eo HN D

O

M

0

99%ee

Scheme 108

Hayashi et al. have reported on a mechanistically intriguing hydroarylation of arynes, catalysed by Rh and employing aryl boronic acids or boroxines as the arylating agent."' For example, dec-5-yne (311)undergoes Rh-catalysed reaction with triphenyl boroxine in dioxane / water to give 5-phenyldec-Sene (314) in 86-87% yields. The deuterium labelling experiments shown in Scheme 109 provide convincing evidence to support the concept that reaction proceeds via a 11PI-Rh-H exchange in intermediate 312 generated through aryl rhodation, to generate aryl-rhodium species 313 which undergoes protolysis.

10

Organozinc and Main Group Reagents

10.1 Organozinc Species. - The generation of organozinc species, either directly using activated zinc powder or indirectly via transmetallation, is an area that is growing steadily. Jackson et al. have developed an efficient synthesis of arylethylamines, in protected form, through generation and then cross-coupling of organo-zinc species."' For example, iodide 315 undergoes smooth conversion, via organozinc 316 to the naphthylethylamine 317 in 61% yield with complete stereofidelity, Scheme 110.

Organometallic Chemistry

268 3m

~ l %

Rh(acac)(C2H4)2 + dPPb

Bu-Bu

L

1.5 equiv. (PhB0)3

dioxane, H20

311

100 OC,3 h

I

[Rhl

-

(PhB0)3

OH I [Rh]

-

ph>_
BU

4

Bu

314

Bu

in dioxane, D20 Bu

D

H20

Bum Bu

I

312

Bu

313

Scheme 109

NHBOC

315

316

Scheme 110

The palladium catalysed reaction of organozincs with ally1 suplhone 317 has been found to result in either a formal SN2'displacement of the sulfonyl group by the organo-group on the zinc (to give 318), or by 'hydride' (to give 319), Scheme 11 l.'I2 The source of the 'H' was ultimately traced to the presence of water in the reaction medium as evidenced by addition of D20.In the absence of Pd there is no reaction with benzylic halide, suggesting that reaction proceeds via a pi-ally1 Pd intermediate. COzEt

R = vinyl, aryl R%ph 318 COZEt

317

&fh '

Scheme 111

R = ethyl, benzyl

319

The configurational stability of organo zinc and organomagnesium com-

9: Organic Aspects of Organometallic Chemistry

269

pounds where the C-M unit is at a stereocentre, has for obvious reasons of synthetic utility (or not) become a subject of a number of recent studies. For example, Knochel et al. have found that organozinc species (e.g. 322)prepared by transmetallation from organoboranes ( e.g. 321) which are stereochemically defined (derived from e.g. alkenes of type 320) are configurationally stable at close to room temperature, Scheme 1l P 3The rate of boron-zinc exchange was found to be a crucial factor in minimising erosion of stereochemistry, with fast exchange giving high degrees of retention and stability.

TH F Zn(i-Pr) 78 oc

-40 "C

320

321

322

323

Scheme 112

Poisson and Normant have prepared enantioenriched allenyl zinc reagents by a kinetic resolution procedure, Scheme 1 13.114 Treatment of racemic allenyl zinc species 324 which is configurationally stable up to - 10 "C,with activated imine 325 at - 60 "C results in kinetic diastereoselection and more rapid consumption of the M-isomer of 324 (to give 326 in up to > 95% de). Trapping of the unreacted P-324with t-BuCHO gives 327 in up to 88% ee. Pr Pr*znBr

SiMe3

OTBDMS

-

THF

f-BuCHO SiMe3

327

+

___c

-60"C TBDMSO

Ph%

*llBr

SiMe3

(p)-324

(*)-324

+

+ Pr

NBn SiMe3

R-325

Scheme 113

10.2 Organoaluminium Reagents. - Other than the ubiquitous dibal-H ,the use of organoaluminium species in general synthetic organic chemistry, is relatively rare. This is most likely because of the perceived difficulty in their preparation, instability towards ligand exchange and radical fragmentation and their high reactivity making handling difficult. However, such reagents do offer some unusual reactivity patterns and often with very high selectivity. Miyashita et al. have made recent contributions to this field through study of the reactions of epoxy alcohols and epoxy sulfides. For example, the reaction of epoxy sulfides with triorganoaluminium species R3A1results in ring opening of the epoxide with delivery of ' R at the fJ-carbon of the sulfide. Curiously these reactions proceed with clean net retention of configuration at both carbons of the epoxide. A neighbouring group participation mechanism, involving an episulfonium intermediate (cf 329)is suggested to account for the stereochemistry at the P-carbon

270

Organometal lic Chemistry

where double inversion results in net retention and thus diastereomeric epoxy sulfides 328 give Me-ring opened products 330, with opposite stereochemistry, Scheme 114.115 .

Me3AI t

CH2C12, -30 "C

O P S P h Pr cis328

@ AIMe2

/ '

Pr

15 min

@

Pr

-m

syn-329

1

AIMe2 P r G S P h

Me

CH2C12, -30 "C 15 rnin

trans-328

anti-330

Me

Scheme 114

anti329

syn-330

In a similar manner, the stereochemical outcome of the reactions of epoxy alcohol substrates with organoaluminium reagents can be controlled. By deliberately forming aluminium 'ate' complexes, the regiochemistry of the attack of R3Al on epoxy alcohol 331 can be switched from giving 332 to giving regioisomeric334 via intermediate 'ate' complex 333, Scheme 115.'16When 'R is an alkynyl group, the 'ate' complex method provides access to products that are not available using more conventional Cu-alkynyl reagents. R3AI

CH2C12, 0 "C trans331

t

30 min then H20

TBSo+oH

OH

anti332

BuLi CH2C12 -30 "C

T B S O L O H 30 rnin then H20

Scbeme 115

R

anti334

9: Organic Aspects of Organometallic Chemistry

271

10.3 Organotin Reagents. - The Pd-catalysed reactions of organotin species (Stille cross-coupling) was covered earlier in Section 7. However, organotin species can also be employed in non-catalysed reactions displaying useful stereoselectivitieswhich make the toxicity of the reagents a tolerable issue until other reagents are developed. For example, Roush et al. have developed a range of allenyl metal reagents, including those based on tin, which can be generated in situ from propargylic pre-cursors (e.g. 335) by transmetallating with SnCl+*” These allenic stannane reagents (e.g. 336) undergo highly diastereoselective reaction with aldehydes. The intramolecular example shown in Scheme 116, which leads to the tetrahydrofuran derivative 337 in > 96% de, displays this well.

335

63 Yo

Scheme 116

In allyic tin reagents, the Sn-C bond is readily cleaved under radical conditions. This has been exploited by Sibi et al. in the development of novel 6-endo [4 21 and 7-endoC5 + 21 cyclisations to generate carbocyles from alkenes and allylic tin species.”’ Triethyl borane is used as radical initiator with Yb(OTf)3or Sc(OTf)3as Lewis acid activators. The example shown in Scheme 117 generates a seven membered ring with good control over the relative stereochemistry of three centres. The 5,7-fused ring system in 340 is a common structural motif in a number of terpenoid phytochemicals and thus the methodology may well find application in natural product synthesis.

+

u

0

Y b(OTf),

338 +

BEt3, 0 2 CH2C12, THF -78°C

Ico

340

Bu3Sn

339

Scheme 117

10.4 Organobismuth Reagents. - The low toxicity of and cost organobismuth species make them ideal candidates for development as reagents for synthetic organic chemistry. However, surprisingly little has be explored outside of amine

272

Organometallic Chemistry

and alcohol arylation reactions. Nonethless, there have been major devlopments within this reaction class with respect to control of absolute stereochemistry through the application of chiral ligand systems. For example, Uemura et al. have reported that kinetic resolution of secondary alcohols (e.g. 341) occurs on Pd-catalysed phenylation using the commericially available Ph3Bi(OAc)2reagent.' l9 The selectivities are promising but not yet synthetically useful. However, this would appear an ideal testing ground for novel ligand systems, Scheme 118. 10.5 Organoboron Reagents. - The use of organoboron reagents under Pd- and Rh- catalysis (Suzuki cross-coupling, Rh-catalysed hydroboration and hydroarylation reactions) has already been covered in Sections 7 and 9. However, uncatalysed hydroboration reactions still remain a synthetically powerful technique with improved reagents, modifications to reaction conditions and new substrates being reported frequently. Knochel et al. have been studying the Ph3Bi(OAc), *

THF, -30 "C

(*)-cis341

a:h 0 +

cat. PdC12(L) AgOAc

342

48 % ee

,Ph ""OH 341

5 YOee

Scheme 118

stereochemical aspects of the thermolysis of organoboranes generated through hydroboration and found these to be highly regio- and stereo-selective.'20For example, alkene 343 undergoes hydroboration with BH3.THF to initially give organoborane 344. However, on thermolysis (60 "C) rearrangment via B-H elimination, alkene rotation, and then reinsertion (hydroboration) yields borane 345.By transmetallation with Zn and then Cu before trapping with ally1 bromide, hydrocarbon 346 is obtained with very high diastereoselectivity, Scheme 119. Chiral hydroborating reagents are the original method for asymmetric hydroboration and are not yet superceded by Rh-catalysed methods. Furthermore, they can also be applied to the reduction of keto and imino functionalities. For example, Ramachandran et al. have reported the use of diisopinocampheylborane (348) in the preparation of &lactones (e.g. 350) in up to 98% ee from ketoacids of the type 347.12' Reaction proceeds via cyclic intermediates of type 349 which undergo deborylative lactonisation on exposure to reasonably strong acid, Scheme 120. Pre-complexation of the borane reagent (348) to the acid is undoubtedly the source of such high asymmetric induction. Organoboranes readily undergo radical fragmentation, indeed Et3B / O2 is often used as a radical initiator (as in Scheme 117). Dalko et al. have developed

9: Organic Aspects of Organometallic Chemistry

273

allyl-Br

CPr2Zn ____)

Ph

-

Et

then

CuCN. 2 LiCl

Ph

:

Et

Et

346

345

Scbeme 119

the use of alkyl boronates as precursors to radicals that can be trapped with phenylcarbonyloxy(pyridine-2-thione) (‘Bartons ester’, 356) to generate thiopyridyl ethers in good yield and with reasonably high diastereoselectivity.’22 THF 0 “C, 17 h 83% 9a % w

&OH 3470

‘ 7 CF3C02H CH&2

349

350

Scbeme 120

The boronate (354) is prepared in situ by alcoholysis (with e.g. catechol, 353)of the primary hydroboration product (352) from a trisubstituted alkene (e.g. 351) and then photoloysed in the presence of the trap. Most interestingly, by conducting the reaction in the presence of acrylates and analogous activated alkenes, products of type 357 isolted in good yield, as shown in the example in Scheme 121. Since catechol boronates turn out to be the most effective, this suggests that the process could be coupled to an asymmetric Rh-catalysed hydroboration (see Section 9.4) There is a continued interest in the presence of highly fluorinated, molecules for use in the generation and exploitation of ‘fluorous’phases and in compounds for biological study. Ramachandran et al. have explored the hydroboration of alkenes bearing perfluorinated alkyl groups and found that with trisubstituted alkenes (e.g. 358), ‘Markovnikov’ products are obtained with HBC12 as hydroboration reagent.12’ Thus tertiary alcohols (e.g. 359) can be prepared by this method with > 99% regioselectivity, Scheme 122. The selectivity is ‘normal’ for 1,l-disubstituted alkenes (e.g. 360) and 1,2-disubstituted alkenes (e.g. 362) give products analogous to the trisubstituted, confirming the directing power of the perhoroalkyl group.

274

Organometallic Chemistry

352

351

hv

F C N

benzene,

___c

0-5 "C,2h

354

dCN SQ

357

L

355

02CPh 356

Scheme 121

. BHC12

hexane then H202 / NaOH

358

359

F3C 360

F3c&

nHex

362

0c4Fg

BHC12

noct

hexane then H202 / NaOH

361

. &

hexane then H202 / NaOH

Scheme 122

nHex

F3C

363

9: Organic Aspects of Organometallic Chemistry

275

References 1.

2. 3. 4. 5. 6. 7. 8. 9.

10. 11.

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3 1. 32.

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Organometallic Chemistry

33. 34. 35. 36. 37.

A. Furstner, F. Stelzer, and H. Szillat, J. Am. Chem. Soc., 2001,123, 11863. P. Cao and X. M. Zhang, Angew. Chem.-lnt. Edit., 2001,40,278. J. M. Aurrecoechea, E. Perez, and M. Solay, J. Org. Chem., 2001,66,564. B. G. Van den Hoven, B. El Ali, and H. Alper, J. Org. Chem., 2000,65,4131. M. Sekido, K. Aoyagi, H. Nakamura, C. Kabuto, and Y. Yamamoto, J. Org. Chem., 2001,66,7142. R. Grigg and V. Savic, Chem. Commun., 2000,873. A. Padwa, 2. J. J. Zhang, and L. Zhi, J. Org. Chem., 2000,65,5223. S. W. Kim, S. U. Son, S. I. Lee, T. Hyeon, and Y. K. Chung, J. Am. Chem. Soc., 2000, 122,1550. S. U. Son, Y. A. Yoon, D. S. Choi, J. K. Park, B. M. Kim, and Y. K. Chung, Org. Lett., 2001,3, 1065. M. Tobisu, N. Chatani, T. Asaumi, K. Amako, Y. Ie, Y. Fukumoto, and S. Murai, J. Am. Chem. SOC.,2000,122,12663. A. Kamitani, N. Chatani, T. Morimoto, and S. Murai, J. Org. Chem., 2000,65,9230. B. M. Trost, R. E. Brown, and F. D. Toste, J. Am. Chem. Soc., 2000,122,5877. V. K. Aggarwal, D. E. Jones, and A. M. Martin-Castro, Eur. J. Org. Chem., 2000, 2939. S. L. Yao, S. Saaby, R. G. Hazell, and K. A. Jorgensen, Chem.-Eur. J.,2000,6,2435. V. Gevorgyan, L. G. Quan, and Y. Yamamoto, J. Org. Chem., 2000,65,568. Y. Yamamoto, R. Ogawa, and K. Itoh, J. Am. Chem. Soc., 2001,123,6189. P. A. Wender, F. C. Bi, M. A. Brodney, and F. Gosselin, Org. Lett., 2001,3,2105. K. C. R. Chao, Dinesh Kumar; Wang, Chun-Chih; Cheng, Chien-Hong, J. Org. Chem., 2001,66,8804. E. F. DiMauro and M. C. Kozlowski, Org. Lett., 2001,3,1641. J. Yun and S. L. Buchwald, Org. Lett., 2001,3, 1129. T . S. Huang and C. J. Li, Chem. Commun., 2001,2348. M. Saito, M. Nakajima, and S. Hashimoto, Chem. Commun., 2000,1851. R. K. Dieter, K. Lu, and S. E. Velu, J. Org. Chem., 2000,65,8715. J. L. Belelie and J. M. Chong, J. Org. Chem., 2001,66,5552. K. G. Chung, Y. Miyake, and S. Uemura, J . Chem. Soc.-Perkin Trans. 1,2000,2725. 1. Perez, J. Perez Sestelo, and L. A. Sarandeses,J. Am. Chem. Soc., 2001,123,4155. K. Takami, H. Yorimitsu, H. Shinokubo, S. Matsubara, and K. Oshima, Org. Lett., 2001,3, 1997. C. Y. Dai and G. C. Fu, J. Am. Chem. SOC.,2001,123,2719. T. Hirao, T. Takada, and A. Ogawa, J. Org. Chem., 2000,65,1511. M. Abarbri, J.-L. Parrain, M. Kitamura, R.Noyori, and A. Duchene, J. Org. Chem., 2000,65,7475. R. E. Maleczka, Jr., J. M. Lavis, D. H. Clark, and W. P. Gallagher, Org. Lett., 2000, 2, 3655. U. S. Schubert, C. Eschbaumer, and M. Heller, Org. Lett., 2000,2, 3373. M. H. Al-Qahtani and V. W. Pike, J. Chem. Soc.-Perkin Trans. 1,2000,1033. W. P. Gallagher, I. Terstiege, and R. E. Maleczka, Jr., J. Am. Chem. Soc., 2001,123, 3 194. C. Jonasson, M. Ronn, and J. E. Backvall, J. Org. Chem., 2000,65,2122. H. Taguchi, K. Ghoroku, M. Tadaki, A. Tsubouchi, and T. Takeda, Org. Lett., 2001,3,3811. S. K. Kang, H. C. Ryu, and Y. T. Hong, J. Chem. Sm.-Perkin Trans. 1,2001,736. P. J. Nichols, S.Papadopoulos, and C. L. Raston, Chem. C o m u n , 2000,1227. T. R. Early, R. S. Gordon, M. A. Carroll, A. B. Holmes, R. E. Shute, and 1. F.

38. 39. 40.

41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

9: Organic Aspects of Organometallic Chemistry

72. 73. 74. 75. 76. 77. 78. 79. 80.

81. 82. 83. 84. 85. 86. 87. 88. 89.

90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107.

277

McConvey, Chem. Commun., 2001,1966. A. N. Cammidge and K. V. L. Crepy, Chem. Commun., 2000,1723. R. Wilhelm and D. A. Widdowson, J. Chem. Soc.-Perkin Trans. I , 2000,3808. L. J. Goossen and K. Ghosh, Chem. Commun., 2001,2084. J. F. Reichwein, C. Versluis, and R.M. J. Liskamp, J. Org. Chem., 2000,65,6187. I. Ojima, S. N. Lin, T. Inoue, M. L. Miller, C. P. Borella,X.D. Geng, and J. J. Walsh, J. Am. Chem. Soc., 2000,122,5343. A. H. Hoveyda and R. R. Schrock, Chem.-Eur. J.,2001,7,945. L. Jafarpour and S. P. Nolan, Org. Lett., 2000,2,4075. A. Furstner, L. Ackermann, K. Beck, H. Hori, D. Koch, K. Langemann, M. Liebl, C. Six, and W. Leitner, J. Am. Chem. Soc., 2001,123,9000. T . L. Choi, C. W. Lee, A. K. Chatterjee, and R. H.Grubbs, J. Am. Chem. SOC.,2001, 123,10417. S. S. Kinderman, J. H. van Maarseveen, H. E. Schoemaker, H. Hiemstra, and F. Rutjes, Org. Lett., 2001,3,2045. M. Ahmed, T. Arnauld, A. G. M. Barrett, D. C. Braddock, K. Flack, and P. A. Procopiou, Org. Lett., 2000,2,551. F. C. Engelhardt, M. J. Schmitt, and R. E. Taylor, Org. Lett., 2001,3,2209. C. Pietraszuk, B. Marciniec, and H. Fischer, Organometallics, 2000,19,913. A. Furstner and C. Mathes, Org. Lett., 2001,3,221. M. Mori, T. Kitamura, N. Sakakibara, and Y. Sato, Org. Lett., 2000,2,543. S . Oi, I. Tsukamoto, S. Miyano, and Y. Inoue, Organometallics, 2001,20,3704. J. W. Bruno and X . J. Li, Organometallics, 2000,19,4672. C. Coperet, 0.Maury, J. Thivolle-Cazat,and J. M. Basset, Angew. Chem.-lnt. Edit., 2001,40,2331. I. Gallou-Dagommer, P. Gastaud, and T. V. RajanBabu, Org. Lett., 2001,3,2053. 0. Pamies, M. Dieguez, G. Net, A. Ruiz, and C. Claver, J. Org. Chem., 2001,66, 8364. 1. D. Gridnev, N. Higashi, K. Asakura, and T. Imamoto, J. Am. Chem. SOC.,2000, 122,7183. I. D. Gridnev, M. Yasutake, N. Higashi, and T. Imamoto, J. Am. Chem. SOC.,2001, 123,5268. T. Ohkuma, M. Koizumi, M. Yoshida, and R. Noyori, Org. Lett., 2000,2,1749. I. V. Komarov and A. Borner, Angew. Chem.-lnt. Edit., 2001,40,1197. T. Benincori, S. Gladiali, S. Rizzo, and F. Sannicolo,J. Org. Chem., 2001,66, 5940. C. Claver, E. Fernandez, A. Gillon, K. Heslop, D. J. Hyett, A. Martorell, A. G. Orpen, and P. G. Pringle, Chem. Commun., 2000,961. M. T. Reetz and G. Mehler, Angew. Chem.-lnt. Edit., 2000,39,3889. M. van den Berg, A. J. Minnaard, E. P. Schudde,J. van Esch, A. H. M. de Vries, J. G. de Vries, and B. L. Feringa, J. Am. Chem. Soc., 2000,122,11539. P. Dani, T. Karlen, R. A. Gossage, S. Gladiali, and C. van Koten, Angew. Chem.-lnt. Edit., 2000,39,743. J. W. Faller and A. R. Lavoie, Org. Lett., 2001,3, 3703. H. Y. Rhyoo, H. J. Park, and Y. K. Chung, Chem. Commun., 2001,2064. J. Louie, C. W.Bielawski, and R.H. Grubbs, J. Am. Chem. SOC.,2001,123,11312. R. Noyori, M. Yamakawa, and S. Hashiguchi, J. Org. Chem., 2001,66,7931. C. P. Casey, S. W.Singer, D. R. Powell, R. K. Hayashi, and M. Kavana, J. Am. Chem. SOC.,2001,123,1090. M. McCarthy, M. W. Hooper, and P.J. Guiry, Chem. Cornrnun., 2000,1333. E. Fernandez, K. Maeda, M. W. Hooper, and J. M. Brown, Chern.-Eur. J., 2000,6,

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1840. 108. S. Demay, F. Volant, and P. Knochel, Angew. Chem.-Int. Edit., 2001,40,1235. 109. M. C. Hansen and S. L. Buchwald, Org. Lett., 2000,2,713. 110. T. Hayashi, K. Inoue, N. Taniguchi, and M. Ogasawara, J. Am. Chem. SOC.,2001, 123,9918. 111. C. Hunter, R. F. W. Jackson, and H. K. Rami, J. Chem. Soc.-Perkin Trans. 1,2000, 219. 112. M. Woods, N. Monteiro, and G. Balme, Eur. J. Org. Chem., 2000,1711. 113. A. Boudier, C. Darcel, F. Flachsmann, L. Micouin, M. Oestreich, and P. Knochel, Chem.-Eur. J., 2000,6,2748. 114. J. F. Poisson and J. F. Normant, J. Am. Chem. SOC.,2001,123,4639. 115. M. Sasaki, K. Tanino, and M. Miyashita, J. Org. Chem., 2001,66,5388. 116. M. Sasaki, K. Tanino, and M. Miyashita, Org. Lett., 2001,3, 1765. 117. B. M. Savall, N. A. Powell, and W. R. Roush, Org. Lett., 2001,3, 3057. 118. M . P. Sibi, J. Chen, and T. R. Rheault, Org. Lett., 5,2001,3,3679. 119. Y. Miyake, T. Iwata, K.-G. Chung, Y.Nishibayashi, and S. Uemura, Chem. Comrnun., 2001,2584. 120. L. 0. Bromm, H. Laaziri, F. Lhermitte, K. Harms, and P. Knochel, J. Am. Chem. SOC.,2000,122,10218. 121. P. V. Ramachandran, H. C. Brown, and S. Pitre, Org. Lett., 2001,3,17. 122. C. Cadot, J. Cossy, and P. I. Dalko, Chem. Commun., 2000,1017. 123. P. V. Ramachandran and M. P. Jennings, Org. Lett., 2001,3,3789.

10

Complexes Containing Metal-Carbon 0 Bonds of Groups 4 and 5 (Including Multiple Bonded Species) BY JASON M. LYNAM

The use of Ti(CH2Ph)4, 1, as a polymerisation catalyst precursor has been described.' Reaction of 1 with B(C6F5)3 results in the formation of [Ti(CH2Ph)3(q6-PhCH2B(C6F5)3],whereas the products of the reaction with [PhC] [B(C6F5)4], [Ti(CH2Ph)3] [B(C6&)4] or [(PhCH2)3Ti(pCH2Ph)Ti(CH2Ph)3][B(C6F5)4], depend on the stoichiometry of the reaction. The complex { [ ( M ~ S N C H ~ C H ~ ) ~ N M ~ ] Z ~ M ~ ) [will B ( initiate C ~ F ~ )the ~ ] polymerisation of ethene? but has been found to decompose under the reaction conditions to give methane and 2: the formation of 2 is believed to proceed via an ortho C-H activation on one of the mesityl groups. In contrast, the related species { [ ( A ~ c I N C H ~ C H ~ ) ~ N M ~[B(GF5)4] ] Z ~ M ~ }(Arc,, = C&-2,6-C12) is relatively stable and will polymerise hexane in a living manner. Me

I

A series of ansa half sandwich complexes of titanium, zirconium and hafnium with cyclopentadienyl and indenyl substituents have been prepared and their polymerisation activity in~estigated.~ The complexes with o-alkenyl substituents show self-immobilisation in ethene polymerisation. Reaction of the o-phenylalkyl complex 3 with two equivalents of BuLi results in the formation of the metallacycle 4.4 Organometaliic Chemistry, Volume 3 1 0 The Royal Society of Chemistry, 2004 279

280

Organometallic Chemistry

3

4

The hafnium complexes (MesNPy}HfR2 {R = alkyl; MesNPy = [(M~sNCH~)~C(CH,)(~-C~H~N)]~-} have been shown to react with [Ph3C] [B(C~FS)~].~ The resulting products depend on the nature of the alkyl group, for example, in the case where R = 'Bu, the formation of Ph3CH, isobutene and [(M~SNP~}H~('BU)][B(GF~)~], 5, was observed; the latter was characterised by NMR spectroscopy and was shown to be stable in solution for two hours at 0 "C. Complex 5 acts a catalyst for the polymerisation of 1-hexene to produce atactic poly(hexane), the resulting system is living and does not exhibit any 6-hydride elimination after two -three hours at 0 "C. A study of the reaction of silanes with hafnium alkyl complexes has revealed that Hf(q5-C5H5)(q5-C5Me5)Me2 does not react with PhSiH3 in C6D6solution over a period of one week.6 In contrast, the zwitterionic complex Hf(q5-C5H5)(q56, reacts quantitatively in three hours to give Hf ($C5Me5)Me(p-Me)B(C6F5)3, C5Hs)(~5-C5Me5)H(y-H)B(c6F5)3 and PhSiMe2H.The results suggest that 6 functions as an ion pair and that its enhanced reactivity towards o-bond metathesis may be explained in terms of its Hf (q5-C5Hs)(q5-C5Me5)Me+ character. Titanium and zirconium complexes containing a chelating diamide ligand with a diboron bridge have been ~ r e p a r e d Reaction .~ of TiC13(THF)3with the [CsH3-2-6-Pf-N(Li)B(NMe2)B(NMe2)N(Li)C6H3-2-6-Pr'] followed by oxidation with PbC12gave 7:the chloride ligands could easily be exchanged in this species for methyl groups by treatment with MeMgBr. The zirconium complex 8 could be prepared in a similar fashion. The complexes, after treatment with a suitable activator, are catalysts for the copolymerisation of ethene/octane, but the activities produced were low, with a broad polydispersity index. An interesting new variant on the 'constrained geometry' type catalyst has been reported.' The carborane [nido-7-NMe2CH2-7,8-RC2B9Hlo](R = Me or H) reacts with TiCb and LPBu to give [q5:q1-RC2B9H9CH2NMe2TiC12], 9; in contrast reaction with either ZrC14or HfC14resulted in the formation of M(q5:q1RC2B9H9CH2NMe2)2. This difference in reactivity was ascribed to the greater ionic radii of zirconium and hafnium. Complex 9, with the addition of MAO, acts as a catalyst for the polymerisation of ethene. The activity of the system was low and the polymers produced were of a high molecular weight with a broad polydispersity index. The reaction of a range of zirconium metallocene complexes ZrCp2C12(Cp = q5-C5H5, q5-C5Me5or q5-C5H4Me)and Zr(q5-C5H5)(q5-C5Me5)C12 with 1-1ithio-2-

10: Complexes Containing Metal-Carbon a Bonds of Groups 4 and 5

28 1

7 M = Ti, R =C1 8 M = Zr, R = CH2% Me2N

I

[Zr] =

10

methylpentane results in substitution of both chlorine atoms and formation of bis-alkyl complexes? These species have an unprecedented thermal stability, particularly in the case of the Zr(q5-C5Hs)(q5-C5Me5) derivative. The reactivity of zirconium dimethyl complexes possessing singly and doubly ansa-bridges with hydrogen has been reported.'* Reaction of, for example, { rneso-Me2Si(q5-C5H3-3CMe3)2}ZrMe2with H2results in the formation of isomeric complexes [{mesoMe2Si(q5-C5H3-3-CMe3)2}ZrH]2(p2-H)2, whereas the reaction of the doublybridged complex {(Me2Si)z(q5-C5H3)2}ZrMe2 under forcing hydrogenation conditions gives { (Me2Si)2(q5-C5H3)2)Zr(p.3-H)2(p.2-H)3, 10. Reaction of Zr(q5C5Me5)2H2with fluorobenzene yields both Zr(qS-C5Me5)2HFand Zr(q5C5Me5)2(C6H5)F.'1 It appears that the mechanism for this reaction does not proceed via a radical mechanism but rather by a dual hydrodefluorination. A thorough study into the chemistry of C3chiral amido ligands has been reported.'* For example, the zirconium alkyl complexes 12 can be prepared from 11 by reaction with an appropriate organolithium or Grignard reagent. Complex 12 reacts with prochiral ketones and aryl aldehydes. In the latter case, after hydrolysis alcohols with an enantiomeric excess upto 82 % could be obtained. Treatment of MC14(M = Ti, Zr, Hf)with two equivalents of n-butyllithium results in the formation of species MCl2Bu2.I3This species acts as a strong base

Organometallic Chemistry

282

/

Me2SiAc

i

I\SiMe, \

R

Ph

11R=C1 12 R = alkyl or Ph

and has been employed in the synthesis of a range of metal precursors. The reaction of titanium precursor Ti(q5-C5H5)(N = CBut2)Me2with [Ph3C] =C B U ~ ~ ) M ~ } ~ ( ~ - M ~ ) ] [B(C6F5),] has been reported.', Dimers [{ Ti(q5-C5H5)(N [(C6Fs),] were obtained as a mixture of meso and rac isomers. These dimers underwent methane loss in solution to give [{ Ti(q5-C5H5)(N =CBut2)}z(p-Me)(pCH2)] [(c6F5)4]Reaction of with LiCH2SiMe2CH2CH = CH2 results in the formation of Zr(q5-C5H5)2(CH2SiMe2CH2CH =CH2)2.I5Protonation of this species with [HNMePh2][B(C6F5)4]results in the formation of 13. A line-shape analysis allowed for the determination of the alkene dissociation energy barrier. Treatment of the zirconium complexes Zr(q5-C5H5)2(THF)(q2-R'N-CHR2) (R' = Ph or C6H4-4-OMe,R2 = Ph or C6&-3-But) with carboimides R3-N= C =N-R3 (R3= %Me3or C6H4-4-OMe)results in the formation of 14.16Reaction of 14 with HCl results in the displacement of R3NH-C(= NR3)-CHR2-NHR'.The reaction was subjected to a mechanistic study.

13

14

Ally1 ligands in complexes Ti(q5-C9H6R)2(q3-allyl) (R = NMe2 or piperidino) react with organic radicals generated from Pr'I/Sm12to give titanocyclobutane complexes Ti(q5-C9H6R)2(CPhHCHPiCH2). The crystal structures of these complexes were reported.'' The products from the reaction of Ti(q5-C5Me5)2C1with RCmCLi (R = SiMe3, But, Ph) strongly depend on the nature of the solvent.'8 For example, reaction in THF solution affords the lithium tweezer complexes Ti(q5-C5Me5)2(-C

10: Complexes Containing Metal-Carbon CJ Bonds of Groups 4 and 5

283

=CR)2{Li(THF),,}.In contrast reaction in hexane solution (in the case where R = Me or But) mono-substituted complexes, Ti(q5-C5Mes)2(-C=CR)2, were formed. The reactions of these complexes with C 0 2were reported. The 'tweezer' effect in similar complexes has also been reported to hold Zn(I1) groups.'' Reaction of T ~ ( T - ~ ~ - C ~ H ~ S ~ M ~ (R ~ ) ~=( -SiMe3, C=CR or) ferrocene) ~ with anhydrous Zn halides results in the formation of Ti(q5-C5H4SiMe3)2(-C=CR)2(ZnX2). In a similar fashion to that described above, reaction of (q5-C5H4SiMe3)2TiC12 with lithium acetylides (LiC-CR, R = CMe = CH2; C6H4CN-4;CH2NMe2) affords (q5C5H4SiMe3)2Ti(-C=CRh.20 The formation of pincer complexes with a range of late metal systems was described. Reaction of (q5-Cs&SiMe3)2TiC1(CH2SiMe3) with LiC-CC5H4N-4 gives (qS-C5H4SiMe3)2Ti(C=CC5H4N-4)(CH2SiMe3). The synthesis and reactivity of the copper acetylide complexes [($CsH4SiMe3)2Ti(C=CR')2]CuC=CR2 (R' = SiMe3, But; R2 = CCCH2CH3, CMe = CH2, CsH,CN-4] were also reported. The electrochemical behaviour of these complexes was described. Similarly, complexes Ti(q5-C5H4SiMe3)2(-C -CPhhMX(MX = CuBr, NiPPh3 and PdPPh3) have been reported2' and the structure of the PdPPh3 complex has been determined by single crystal X-ray diffraction. Reaction of the complexes Ti($-C5H4SiMe3)2(-CICRI)Z(CuR2)(R' = %Me3, R2 = Me; R* = But, R2 = Me; R' = But, R2 = C-CSiMe3) with acid chlorides R'COCl (R3 = Ph or Me) result in the formation of ketones R3COR2.22 The reactions of acid anhydrides were also investigated. A review of the chemistry of titanium group carbene complexes has appeared, giving information about structure and reactivity patterns.23 The potentially tridentate ligand [q'-C5H3-1,3-(SiMe2-CH2PPri2h], [PzCp], has been shown to be a useful ligand in the synthesis of hafnium carbene c0mplexes.2~Reaction of [P2Cp]HfC13with two equivalents of KCH2Ph results in the formation of [P2CpJHf(=CHPh)Cl which exhibits an a-agositc interactions. The structure of this complex was compared to its zirconium analogue. The reaction of titanallene [Ti(qs-C5Mes)-x=C =CH2)J + (generated from [Ti($CSM~= ~ )C~=CH2)Me)) ( with a range of isonitriles has been reported?' For example, reaction with cyclohexylisonitrile results in the formation of 15. Interesting coupling reactions with 2,6-dimethylphenylisonitrilewere also described

284

Organornetattic Chemistry

The behaviour of the fascinating agostic interactions in the niobium alkyl complexesNbTp*ClR(q2{4e}-PhC=CMe)has been In the case where R = lPr a single crystal X-ray structure indicated that a p-agostic interaction was present between the isopropyl substituent and the metal. In solution however, an equilibrium between this species and one possessing an a-agostic interaction was observed. Thermodynamic data were obtained by NMR spectroscopy and the system was subjected to a theoretical study. Reaction of the metal imido complexes [ M ( N R ) C ~ ( K ' - N ~ N ~ (M ~ ) (= ~ ~Nb )] or Ta, R = But or Ar, N2Npy = MeC(2-C5H4N){CH2NSiMe3}2)with either LiCH(SiMe3)2 or PhCH2MgCl(M = Nb) results in the formation of complexes CM(NR)(CH{siMe3}2)(k3-N2NPY)(pY~l and " b ( ~ ~ ~ ~ ~ ~ 2 ~ ~ ~ ~ ~ ' - ~ respectively?' In the case of [Nb(NR)(CH{SiMe3}2)(~3-N2Npy)(py)] the structure was confirmed by a single crystal X-ray study. The niobium imido complexes Nb(q5-C5Me5)2(NR)R' (R = But or C6H4Me-4; R1 = Me, Et, CH2Ph, CH2SiMe3,CH2CH=CH2) have been prepared from the reaction of Nb(qsCsMes)2(NR)Clwith the appropriate Grignard reagent?8 Hydrolysis of Nb(qs= 0)Me and C5Mes)2(NC6H4Me-4)Me gave the 0x0-complex Nb(q5-C5Me5)2( H2N-C6H4Me-4. Reaction of the tantalum species complex [Ta(q5C5Me5)(NBu')(NMe2)(Me)] with C 0 2results in the formation of a q2-carbamate complex [Ta(qs-CsMeS)(NBu')(q2-02CNMe2)(Me)] which in turn reacts with 2,6-dimethylphenylisonitrile to give [Ta(qS-C5Mes)(NBut) (q2-C(Me)=NAr)(q'O2CNMe2)].29 Supported tantalum carbene complexes [(=SiOXTa(= CHCMe3)(CH2CMe3)(3.,,1 (X = 1 or 2) can act as precursors to the active species in alkane metathe~is.~'The reaction is thought to proceed via a a-bond metathesis pathway and, importantly, evidence for cross-metathesis products was obtained. The synthesis of tantalum alkyl complexes containing the triazacyclononane 'Pr2-tacn ligands has been described. Reaction of Li'Pr2-tacn with (THF)2TaC13( =NR) (R = C6H3-2,6-'Pr2) results in the formation of 16 (Scheme l).31 Reaction of 16 with three equivalents of Me3SiCH2Liresults in the formation of 17, which undergoes a slow reaction to lose SiMe4 in toluene to give 18. Further reactions of 18 with a series of reagents were subsequently reported.32 For example, treatment of 18with CO results in the formation of 19, whereas the reaction with benzophenone results in 20: this reaction may be viewed as being similar to classical reaction of metal carbenes. Complex 18 was also shown to react with metal complexes via salt metathesis-type pathways. For example, reaction with [RhCl(COD)]2 resulted in the formation of 21 which possesses a monodenate 'Pr2-tacnfragment. Reaction of Ta(qs-C5HS)z( =CH2)(Me)with two equivalents of HB(C6F5)2 results in the formation of 23 and MeB(C6Fs)2.33 The mechanism for this reaction appears to proceed via initial coordination of a HB(C6F5)2 to one of the methyland addition of a ene groups to give 22 followed by elimination of MeB(C6Fs)2 further equivalent of HB(CsF5)2.Reaction of 23 with CO allows for the trapping of the interesting q2-boratalkenecomplex Ta(qs-CsH5)2(q2-HC2B{ C6F5}2)(CO). = CHR) Treatment of the tantalum carbene complexes Ta(CH2R)3(PMe3)( (R = %Me3)with SiPhH2R' (R1= Me or Ph) results in the formation of com-

10: Complexes Containing Metal-Carbon o Bonds of Groups 4 and 5

285

16

17

I t

I

(ii) I

Ti

/ qrn

Me3SiH2C

18 19

I

+

Scheme 1 Ar = C6H3-2,6-iPr2 (i) 3 Me3SiCH2Li,- 2 LiC1; (ii)A, - SiMe,; (iii) (iv) Ph2C= 0,- Ph2C=CHSiMe,, (v) [Rh(COD)Cl),, - LiCI.

+

+

+ CO,

plexes Ta(CH2R)3(PMe3)( =C{ SiHPhR*}R).34 Interestingly, reaction of Ta(CH2R)(PMe3)2( = CHR)2 with SiPhH2Rresults in the formation of 24, whereas the reaction of PhH2SiCH2SiH2Phaffords 25. Photolysis of Ta[P2N2]Me3 ([P2N2] = PhP{ CH2SiMe2NSiMe2CH2)2PPh) affords the thermally unstable complex Ta[P2N2]Me( = CH2),26, as the major

Organometallic Chemistry

286

@

/C6F5

23 Me3P

\

-

%Me3

24

25

(silox),Nb-,

27

28

Scbeme 2 silox = But3Si0(i) - H2;(ii) H2or D2(A, 1 day),

Nb(silox),

- H2or HD.

10: Complexes Containing Metal-Carbon (J Bonds of Groups 4 and 5

287

product? In the presence of ethene, 26 is converted into [P2N2]Ta(C2Hs)Etwith [P2N2]Ta(C2H4)Me observed as a minor product. X-ray crystallographic and NMR studies indicate the presence of agostic interactions in [P2N2]Ta(C2H4)Et and these were investigated by deuterium labelling studies. Thermolysis of the diniobium cyclooctatetrenecomplex 27 resulted in the loss of dihydrogen and the formation of 28 (Scheme 2).36Treatment of a benzene solution of 28 with hydrogen at elevated temperatures gave the niobium carbene 29, the structure of which was confirmed by X-ray diffraction. Repeating the reaction with D2 resulted in the formation of HD and the incorporation of deuterium in the eight-membered ring in the CH2group flto the metal.

References 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

M. Lin and M. C. Baird, J. Organometal. Chem., 2001,619,62. R. R. Schrock, P. J. Bonitatebus and Y. Schrodi, Organometallics, 2001,20,1056. H. G. Alt, A. Reb, W. Milius and A. Weis, J. Organometal. Chem., 2001,628,169. H. G. Alt, A. Reb and K. Kundu, J. Organometal. Chem., 2001,628,211. P. Mehrkhodavandi and R. R. Schrock, J. Am. Chem. SOC.,2001,123,10746. A. D. Sadow and T.D. Tilley, Organometallics, 2001,20,4457. J. T . Patton, S. G. Feng and K. A. Abboud, Organometallics, 2001,20,3399. D.-H. Kim, J. H. Won, S.-J. Kim, J. KO, S. H. Kim, S. Cho, and S. Kang Organometallics, 2001,20,4298. Ola F. Wendt and J. E. Bercaw, Organometallics, 2001,20,3891. P. J. Chirik, L. M. Henling and J. E. Bercaw, Organometallics, 2001,20,534. B. M. Kraft, R. J. Lachicotte and W D. Jones, J. Am. Chem. SOC.,2001,123,10973. L.H. Gade, P. Renner, H. Memmler, F. Fecher, C. H. Galka, M. Laubender, S. Radojevic, M. McPartlin and J. W. Lauher, Chem. Eur. J., 2001,7,2563. J. J. Eisch, F. A. Owuor and P. 0.Otieno, Organometallics, 2001,20,4132. S. Zhang and W. E. Piers, Organometallics, 2001,20,2088 C. P. Casey, D. W. Carpenetti I1 and H. Sakurai, Organometallics, 2001,20,4262. J. A. Tunge, C. J. Czerwinski, D. A. Gately, and J. R. Norton Organometallics, 2001, 20,254. G. Greidanus, R. McDonald and J. M. Stryker, Organometallics, 2001,20,2492. F. G. Kirchbauer, P.-M. Pellny, H. Sun, V. V.Burlakov, P. Arndt, W. Baumann, A. Spannenberg, and U. Rosenthal. Organometallics, 2001,20,5289. H. Lang, N. Mansilla, and G. Rheinwald Organometallics, 2001,20,1592 W. Frosch, S. Back, H. Muller, K. Kohler, A. Driess, B. Schiemenz, G. Huttner and H. Lang, J. Organometal. Chem., 2001,619,99. S . Back, T. Stein, W. Frosch, I.-Y. Wu, J. Kralik, M. Buchner, G. Huttner, G. Rheinwald and H. Lang, Inorg, Chim. Acta, 2001,325,94. W. Frosch, S. Back and H. Lang, J. Organometal. Chem., 2001,628,140. R. Beckhaus and C. Santamaria J . Organometal. Chem., 2001,617,81. M. D. Fryzuk, P. B. Duval, B. 0.Patrick, and S. J. Rettig, Organometallics, 2001,20, 1608. C. Santamaria, R. Beckhaus, D. Haase, R. Koch, W. Saak, and I. Strauss, Organometallics, 2001,20, 1354. J. Jaffart, M. Etienne, F. Maseras, J. E. McGrady and 0.Eisenstein, J. Am. Chem. SOC.,2001,123,6000.

288

Organometallic Chemistry

27.

S. M. Pugh, D. J. M. Trosch, M. E. G. Skinner, L. H. Gade and P. Mountford, Organometallics, 2001,20,353 1. A. Antiiiolo, I. Lopez-Solera, A. Otero and S . Prashar, J. Organometal. Chem.,2001, 631,151. J. Sanchez-Nieves and P. Roy0 J. Organometal. Chem., 2001,621,299. C. Copkret, 0.Maury, J. Thivolle-Cazat, J.-M. Basset, Angewandte Chemie, Int. Ed, 2001,40,2331. J. A. R. Schmidt, Stephen A. Chmura and J. Arnold, Organometallics, 2001, 20, 1062. J. A. R. Schmidt and J. Arnold, J. Am. Chem. SOC.,2001,123,8424. K. S. Cook, W. E. Piers, T. K. Woo, and R. McDonald, Organometallics, 2001,20, 3927. J. B. Diminnie, J. R. Blanton, H. Cai, K. T. Quisenberry, and Z. Xue, M. D. Fryzuk, S. A. Johnson and Steven J. Rettig, J. Am. Chem. SOC., 2001,123, 1602. A. S. Veige, P. T. Wolczanski, and E. B. Lobkovsky, Angew. Chem. Int Ed., 2001,40, 3629.

28.

29. 30. 3 1.

32. 33. 34. 35. 36.

11 Complexes Containing Metal-Carbon c3 Bonds of Group 7 (Including Multiple Bonded Species) BY JASON M. LYNAM

Reaction of Mn(CO)sR(R = Me of Ph) with imines TolR'C = NR2(R' = H R2 = Me, R' = H R2 = CH2Ph,R' = But R2 = H)results in the formation of the metallated imine 1via the acyl complex 2 (Scheme l).' Reaction of Mn(CO)sMe with TolHC =NMe in the presence of AlC13leads to the formation of 3 (Scheme l), the metallocycle unit be isolated by reaction of 3 with PPh3 to give 4. In contrast, reaction of [Na][Mn(C0)5] with the N-acyl imminium salt [TolCH =N(Me)- C(O)Ph][Cl] results in the formation of the metallocycle 5, which on heating liberated CO and gave 6 which has a structure analogous to 4. The pK, values of the carbene complexes [Re(q5-C5H5)(NO) (PPh3)(= &eCh =CHCH2)[BF4],7, and 8 (see scheme 2) have been measured in aqueous acetonitrile, deprotonation takes place on the heterocyclic rings (Scheme 2).2These complexes appear to be much more acidic that the corresponding neutral carbene complexes such as Cr(CO),( =C{ 0Me)Me). Reaction of substituted aryl ketones and aryl esters with Mn(C0)5(CH2Ph),9, results in cyclomanganation to yield complexes such as Mangantion appears to occur at the 2-position, reaction of these complexes with ICl results in decomplextion and formation of an aryliodide. Reaction of 9 with 1,Sdiarylpentan-1,4-dienones results in the formation of cyclomanganted complexes l l.4 Reaction of complex 11 with alkynes resulted in the formation of complexes containing either $-pyranyl or q5-oxocycloheptadienylcomplexes (Scheme 3), Reaction of the q5-pyranylwith iodine resulted in the salts 14, whereas reaction of the $-oxocycloheptadienyl complex with Ce(1V)nitrate resulted in the formation of the tropone 15. Photochemical reaction of [Re(q5-C5R5)(C0)2(N2)](R = H or Me) in the presence of 1,4-difluorobenzeneresults in the formation of two products, 16 and 17.5A thorough NMR (includingNOESY and EXSY) and computational study of these two species was undertaken. This revealed that 17 is the initial photoproduct and 16 is formed subsequently. Complexes containing two rhenium end-groups linked by an unsaturated chain of carbon atoms have been prepared.6 For example, Re($C5H5)(NO)(PTo13)( - CSC -)2Re(q5-C5H5)(NO)(PTo13), 18, was prepared by the Cu(OAc)2 mediated coupling of Re(q5-C5H5)(NO)(PTo13)( - C-C - H). Compounds with longer chains were prepared using similar methodologies. The Organometallic Chemistry, Volume 3 1 0 The Royal Society of Chemistry, 2004 289

Organometaliic Chemistry

290

AICI,

I

I

Rl

To1

2

R =Me. R1= H, R2 = Me

To1

-RH

- co

3

Scheme 1

Ph

Ph

5

6

redox chemistry of these compounds was investigated. Oxidation with either one or two equivalents of AgSbF6gave 18+SbF6and 182+(SbF6)2 respectively. The reaction of a series of propargyl alcohols with the [Re(CO)2(triphos)(OTf)],19,unit have been r e p ~ r t e dFor . ~ example, reaction of 19 with HC=CC(OH)(C6H1 results in the formation of the alkenylvinylidene complex [Re(CO)2(triphos)(= C = C{ H } x = C{ H}-{ CH2}3cH2)][OTf] 20. This reaction was shown to proceed via a hydroxlvinylidene complex

11 :Complexes Containing Metal-Carbon a Bonds of Group 7

29 1

I

Base Acid

8

Scheme 2

10

I

14

I,

F% 15

292

Organometallic Chemistry

I

16

17

[Re(COh(triphos)(= C = C{H}~{OH}{CH2}4~H2)][OTf]21. Interestingly, reaction of 20 with NEt3, results in the formation of the enylyl compex [Re(CO)z(triphos)(= C=C{H}X = C{H}-{CH2}& H2)], 22. A review of the luminescent properties of transition metal complexes,featuring Re(1) carbene and Re(V)benylidene complexes, has appeared?

24

Scheme 4

The reaction of Re(qS-CsHs)(C0)2( = C(To1) -C=C -Tol), 23,with a series of phosphine ligands has been investigated? For example, reaction of 23 with dppe results in the formation of the remarkable complex 24 containing two five membered 3,4-dihydrophosphoilumunits. A mechanistic study of the reaction of 23 with PPh2Me showed that the reaction proceeded by initial nucleophilic

11: Complexes Containing Metal-Carbon u Bonds of Group 7

293

attack by the phosphine at the carbene carbon atom to give 25 (Scheme 4), on warming from - 78 "C to - 40 "C the two starting materials were generated. On further warming to -20 "C a new complex 26 was observed, and on final warming to room temperature the final product of the reaction 27 is obtained. A study of the [1,3] metal shift in rhenium alkynyl carbene complexes Re(q5C5H5)(CO),(= C{Tol) - C=C - GH4-p -X) has shown that the rate of shift is significantly enhanced in the case where X is the electron withdrawing S02CF3 group." The mechanism for the reaction is proposed to proceed via a q5-q3shift in the cyclopentadienylring and formation of a dehydrometallocyclobutadiene. Replacing the q5-C5H5group by q5-C5Me5results in an increase in the rate of the [1,3] shift." Heating the complex Re(q5-C5H5)(C0)2( = C{ Ph} -C=C - Ph) results in a C-H insertion reaction to give 28. Reaction of Mn(q5C5H4R)(C0)2( = C{ Ph} - C-C - Ph) (R = H or Me) with CO,(CO)~ results in the formation of two products 29 and 30 (Scheme 5).12 The CO group within the coordinated ketene ligand of 29 was shown to undergo an intramolecular exchange with terminal metal carbonyls bound to rhenium. A reinvestigation of the chemistry of Mn(q5-CsH5)(C0)2(CPh2 =C =0) showed that this complex exhibited similar behaviour.

28

Ph

Ph

29 Ph

Scheme 5

294

Organometallic Chemistry

The chiral-at-metal complexes [Re(q5-C5H5)(NO)(PPh3) (= C - 0 CH2CH2CH2)][BF4] have been prepared by the photolysis of [Re($C5H5)(NO)(PPh3)(C0)] [BF4] in the presence of HC=CCH2CH20H. l 3 The tetrahydrofuranyl ligand is found to align itself with the NO ligand so that donation into the empty p-orbital on the carbene by a filled metal d-orbital is maximised. Reaction of Re(=CBu')(= CHBu')(CH2But)2with silica results in immobilisation of the Re(=CBu')( = CHBut)(CH2Bu') unit onto the silica surface.14 The resulting heterogenous system is an effective catalyst for both alkene and alkyne metathesis. A new type of metal carbene complexes containing a diazo function have been reported.'' Reaction of [Mn(CO)3(q5-C5H4Me)]with an organolithium compound LiR followed by treatment with the electrophilic silyl triflate TfOSi(P$)2C(N2)C02Meresults in the formation of [Mn( = C{ R}OSiPr'2C(N2) C02Me)(CO)2(qs-CsH4Me)J. Deprotonation of the carbene complexes Mn(q5C5H4Me)(C0)2( =C{OEt}CH2R) (R = Me or H) results in the formation of the anions Mn(q'-CSH4Me)(C0)2(= C{ 0Et)CHR) 31.16 Treatment of 31 with an oxidising agent such as &(I), Cu(I1) or Fe(II1) salts results in a coupling reaction to give 32. The chemistry of complex 32 was fully explored, for example treatment with two equivalents of LiBu results in double deprotonation of the bridging ligand, whereas treatment with BC13 results in the formation of two products, one with a carbene/carbyne bridge 33, one with a bis-carbyne bridge 34 (Scheme 6). (q-5MeC5H4)(OC)2Mn

Mn(C0)2(q5-C5H4Me)

32

qR

(q-'MeC5 H4)( OC), Mn

33

)VMn(C0),(q5-C,H4Me) R

)-MII(CO)~(~~-C~H,M~)

34 Scheme 6

1 1: Complexes Containing Metal-Carbon cs Bonds of Group 7

295

Reaction of the carbene complex Mn(q5-C5H5)(CO)2( = C(0Ac)Ph) with chiral alcohols HOR* results in the formation of Mn(q5CsHs)(C0)2(=C{OR*}Ph), 35.’’ Photolysis of 35 in the presence of either PPh3 of P(OMe)3 results in the diastereoselective formation of the chiral-at-metal = C{ OR*)Ph) (R = Ph, OMe). complexes Mn(q5-C5H5)(CO)(PR3)( The reaction of the carbene complexes [M(C0)2(q5-C5H5)(=Ph)][BBr](M = Mn 36 or Re, 37)with anionic diiron species [E][Fe2(p-CO)(p-SeR)(C0)6] (E = Et3NH or MgBr; R = Ph, Tol, Et,) results in a range of clusters compounds.’* For example, reaction of [Re(CO)2(q5-C5H5)(=Ph)][BBr]with [MgBr][Fe2(pCO)(p-Se-n-C4H9)(C0)6]affords both [Fe2(p-Se-n-C4H9)2(CO)6], 38, and 39, whereas reaction of the same precursor with [Mn(CO)2(q5-CsH5)(=Ph)] [BBr] also gave 38 and the manganese hydride cluster 40.

I

Ph

40

39

42

Oxidation of Mn(C0)2(q5-C5H5)( = C = CHPh), 41, with AgBF4 results in the formation of the dimeric complex 42.19Interestingly, reaction of 41 with [CPh3] [PFb] results in the formation of Mn(CO)2(q5-C5H5)( =C = C{CPh3)Ph). References 1. 2.

D. Lafrance, J. L. Davis, R. Dhawan and B. A. Arndtsen, Organornetallics, 2001,20, 1128. C . F. Bernasconi and Mark L. Ragains, J. Am. Chem. SOC,2001,123,11890.

296

Organometallic Chemistry

3.

J. M. Cooney, L. H. P. Gommans, L. Main and B. K. Nicholson, J. Organometal. Chem., 2001,634,157. W. Tully, L. Main and B. K. Nicholson, J. Organometal. Chem., 2001,633,162. J. J. Carbo, 0.Eisenstein, C. L. Higgitt, A. H. Klahn, F. Maseras, B. Oelckersand R. N. Perutz, J. Chem. SOC.,Dalton Trans., 2001,1452. W. E. Meyer, A. J. Amoroso, C. R.Horn, M. Jaeger and J. A. Gladysz, Organometallics, 2001,20, 1 11 5. C. Bianchini, N. Mantovani, L. Marvelli, M. Peruzzini, R. Rossi and Antonio Romerosa, J. Organometal. Chem., 2001,617,233. S.-W. Lai, M. C. W.Chan, Y. Wang, H.-W. Lam, S.-M. Peng and C.-M. Che, J. Organometal. Chem., 2001,617,133. C. P. Casey, S. Kraft, D. R. Powell and M. Kavana, J. Organometal. Chem., 2001, 617,723 C. P Casey, S. Kraft and D. R. Powell, Organometallics, 2001,20,2651 C. P. Casey, S. Kraft and M. Kavana, Organometallics, 2001,20,3795. Y. Ortin, Y. Coppel, N Lugan, R. Mathieu and M. McGlinchey, Chem. Commun, 2001,2636. M. F. Semmelhack,A. Lindenschmidt and D. Ho, Organometallics, 2001,20,4114. M. Chabanas, A. Baudouin, C. Copkret and J.-M. Basset, J. Am. Chem. SOC,2001, 123,2062. G. Maas and D. Mayer, J. Organometal. Chem., 2001,617,339. A. Rabier, N. Lugan and R.Mathieu, J. Organometal Chem., 2001,617,681. K. WeiDenbach and H. Fischer, J. Organometa/. Chem., 2001,621,344. R. Wang, Q. Xu,Y.Souma, L.-C. Song, J. Sun, and J. Chen Organometallics, 2001, 20,2226. L. N. Novikova, M. G. Peterleitner, K. A. Sevumyan, 0. V. Semeikin, D. A. Valyaev, N. A. Ustynyuk, V. N. Khrustalev, L. N. Kuleshova and M. Yu. Antipin, J. Organometal Chem., 2001,631,47.

4. 5.

6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

12

Organo-transition Metal Cluster Complexes MARK G. HUMPHREY AND MARIE P. CIFUENTES

1

Introduction

This chapter covers the chemistry of metal carbonyl and organometallic clusters containing three or more metal atoms. The treatment is in Periodic Group order, homometallic compounds being followed by heterometallic clusters. The numbered compounds are illustrated. Ligands are not shown for high-nuclearity clusters, emphasis being placed on core geometry.

2

General Reviews

A textbook on transition metal carbonyl chemistry has appeared,' and the chemistry of organometallic clusters summarised for the year 1999.2 A survey of the preparation and reactivity of transition metal carbonyl clusters with benzyne (including benzene and diene) ligands3 and a brief review of the chemistry of tetra- and trinuclear platinum(I1)clusters" have been published.

3

Spectroscopic Studies

The IR spectroscopic properties of heterothiometallic cluster complexes, including organometallic examples, have been outlined: and the spherical harmonic model has been applied to the interpretation of the vibrational spectra of some carbonyl clusters containing Fe(C0)4fragments.6 A comparison of laser desorption ionisation and electrospray ionisation (ESI) mass spectrometry in the characterisation of cluster anions suggests ESI may be the more reliable method.' The use of energy-dependent electrospray ionisation mass spectrometry in the characterisation of reaction mixtures of metal carbonyl clusters has been described.* Raman spectra of a number of doubly-bridged triosmium clusters have been interpreted using a modified Plastic Cluster Model in which the mass of all the ligands is incorporated?

Organometallic Chemistry, Volume 3 1

0 The Royal Society of Chemistry, 2004

297

298

4

Organometallic Chemistry

Theory

The diversity of structural types observed for a series of tetrametallic cluster complexes with thirteen carbonyl ligands has been rationalised using the Ligand Polyhedral Model," and a unifying electron counting rule, the mno rule, has been developed to encompass bonding patterns displayed by single and condensed polyhedral borane and metallaborane clusters." Extended Huckel and density functional (DFT) calculations on a series of methylphenylsulfoximido triruthenium clusters indicate the p3-ligand coordinates in a 3-orbital/5 electron fashion, and the p2- ligand as a 2-orbital/3 electron system.I2DFT and extended Huckel calculations have also been reported on a variety of Os3(CO)lo(a-diimine) clust ers.'

5

Structural Studies

X-ray structural studies of WFe2(p-H)(p3-S)(CO)8{q5-C5H4C(Me) = NNHC6H3(N02)2-2,4),l4 W21r*(CO)lo(r'-C~H4Me)2,~~ C5H4Me)2,16Re4Pt(p-PCy2)2(CO)1817 (Cy = cyclohexyl) and MnReAg(pPCy2)(C0)7(PPh3)2'8 have been published.

6

Large Clusters

This section covers transition metal clusters containing nine or more metal atoms. Diagrams of complexes in this section show core interactions only. 6.1 Homonuclear High-nuclearity Clusters. - The decaosmium pentahydrido cluster anion [H50~10(C0)24]has been obtained in high yield (65 YO) by hydrogenation (1 atm, 200 "C) of silica-supported { Os(C0)3(0H)2},, and thermolysis of the same precursor in ethylene glycol at 160 "C gave the tetrahydrido species [H40slo(C0)24]2-in excellent yield (79 %).19 C O ~ ( C Owas ) ~ reacted with C P ' ~ N ~ ( T ~(Cp' ~ ) H= q5-C5H4Bu')in refluxing toluene to give the body-centred cubic cluster Cog(~-Te)~{ p4TeNb(CO)Cp'2}3(CO)sin good yield. A structural determination of the Cr(C0)5 adduct confirmed the structure.20 Condensation of the monocapped square antiprismatic cluster anion [co&8-P)(c0)21]'- with [CO(CO)~]- afforded the related bicapped cluster [co10(~8-P)(co)22]3-,a process which was reversed under a CO atmosphere. Electrochemical studies showed an absence of stable redox congeners for the complexes.21Electrospray mass spectral analysis of [ C O ( C O ) ~ ] - / ( C O ~ ( C O ) ~reaction } ~ ( ~ - mixtures G~) has led to a route into the decacobalt cluster dianion [ C O , ~ ( ~ ~ - G ~ ) ~ ((1). C OThe ) ~ ~complex ]~consists of an unusual metal core with semi-encapsulated Ge atoms.22 Attempts to stabilise an incompletely characterised Ni-Pd carbonyl cluster by CO substitution with PMe3 afforded a range of high-nuclearity palladium clusters, Pd16(C0)13(PMe3)9, Pd35(C0)23(PMe3)15, Pd39(C0)23(PMe3)16 and

12: Organo-transition Metal Cluster Complexes

299

(1)

Pd59(C0)32(PMe3)21, along with a small amount of the mixed-metal complex Ni3Pd29(C0)22(PMe3)13. Each cluster contains a centred icosahedral metal core, with the four largest showing unprecedented core ge~metries.~~ The selenophosphate cluster Cu12(IL8-r\3:r12:r12:r13:r12:112-P2se6){ Se2P(OEt)2)8 was obtained in 11 % yield from the reaction of [CU(NCM~)~] with NH4Se2P(OEt)2. The complex is the first example of a neutral molecule containing an ethane-like P2Se:- bridging Reaction of CuSCN or CuCl with A ~ P h ( s i M ein ~ )the ~ presence of tertiary phosphines has afforded a range of high-nuclearity copper clusters with cores containing from eight to fourteen copper atoms? Reaction of Ag2C2ina concentrated aqueous solution of RC02Ag(R = CF3, C2Fs)and AgBF4 gave a series of silver(1) double salts consisting of cage structures with encapsulated C22-fragments. The cage geometry and the interconnectivity between cages are dependent on the nature of the ancilliary solvent present.26 The first silver salt containing four different types of anion, 2Ag2C2.3AgCN 15CF3C02Ag2AgBF49H20,has been prepared from a mixture of Ag2C2,AgCN, AgCF3C02and AgBF4.The columnar structure is made up of three units consisting of a Ag13(pe-q2-C2)2 double cage (2), a cyanide-bridged Ag4 fragment and a Ag2{(w-q2-CN)2Ag2}2 fragmentF7The silver-alkynyl cage complexes [Ag14(C=CB~t)12X]Y (X = CI, Br; Y = OH, BF4)have been prepared via an anion-templated synthesis using AgBF4,Bu'CmCH and NEt3 in the presence of a halide salt such as NMe4Cl or NBu4Br. The presumably polymeric [Ag(C=CBu')], formed in the absence of halide can be converted to the cage by the templating effect of the spherical, monoanionic halide species? +

1,2-Bis(diphenyphosphino)ethane (dppe) has been effectively utilised to stabilise polynuclear silver/chalcogenide cores, with Ag18Te(TePh)15(dppe)3Cl, Ag38Te13(TeBut)12(dppe)3 and the polymeric { [Ag5(TePh)6(Ph2PCH2CH2PPh3)] (dppe)} rep0rted F9 The optical properties and electronic structure of [C&(pSePh)6C14]2-, Cdlo(C13-Se)4(CL-SePh)l2(PPr3)4, [Cd17(p3-Se)4(~-

300

Organometallic Chemistry

SePh)24(PPh2Pr)4]2+, and Cd32(~-Se)1~~3-Se)4(~-SePh)36(PPh3)4 have been examined. The absorption and low temperature photoluminescence excitation onset undergo a blue shift on proceeding to the smaller

6.2 Heteronuclear

High-nuclearity Clusters. - Condensation of [Fe2Au3(CO)s(L2)]-[L2 = dppe, bis(dipheny1phosphino)methane (dppm)] with AuCl(SEt2)or with CuCl gave dodecanuclear complexes of core geometry 3 (M = Au, Cu). The copper atoms in the latter occupy the sites with the greatest metal connectivity, suggesting that site exchange between Cu and Au atoms occurs during the rea~tion.~' The hexanuclear cluster dianion [ppn]2[Rh(bc)(co),6]was reacted with AgNO3 at room temperature to form [ppn] [Ru&(~-c)(co),& (4,consisting of infinite chains of alternating ruthenium clusters and silver ions separated by layers of ppn cations; the silver ions possess distorted tetrahedral geometries. Although the ppn salt 4 is stable at 60 "C in acetone, the analogous PPh+ salt reacted in methanol at 60 "C with CO loss and Ru-Ru bond scission to give the dimeric complex 5, where the Ag atoms now occupy face-capping sites.32

r

1-

The hexarhodium cluster dianion [Rh6(C0)15]2- was reacted with { PtCl(dppm)}, to give the Rh8Pt2cluster 6, while Rh6(CO)ls(NCMe)reacted with Pt2(C0)3(dppm)2to afford the RbPt4 complex 7; both clusters consist of an Rh6 octahedron linked to a tetrahedral fragment via a single (RhRh or RhPt) bond. Electron counting suggests the presence of a dative M-M bond in each complex, where the Rh6 unit acts as a two electron acceptor. The former product was accompanied by Rh2Pt2(C0)6(dppm)2.33 Reduction of PdC12(PPh3)2 with [Ni6(CO)12]2- afforded [N4Pd16(CO),2(PPh,)4]2- and[Ni9Pd33(C0)41(PPh3)6j4-, shown to consist of a cubic close-packed (c.c.P.) and hexagonal close-packed (h.c.p.) arrangement of metal atoms, respectively. The minor products were identified as Pd12(C0)12(PPh3)6 and [Pd29(C0)28(PPh3)7]2-, the former consisting of a hexa-

22: Organo-transition Metal Cluster Complexes

301

capped Pdb octahedron and the latter an unprecedented Pd-centred 28-atom polyhedron with a mixed h.c.p./c.c.p.sequence?' Reaction of [Ni&O),2]2- with K2PtCl4 gave [Ni24Pt14(C0)44J4(8)with a C.C.P. arrangement, together with the Ni/Pt disordered cluster CNildr\Ti6-,Pt,)Pts(CO)~o]4(x = 1.92) (9) with an unprecedented h.c.p. sequence of M,, MI,, and M7 The preparation of [Ni36Pt4(C0)45j6-(10) and [Ni33Pf4(C0)46]6(11) from [Ni6(CO)lz]z- and K2PtCld has been described; the former contains a Pts tetrahedron enclosed in a shell of 36 nickel atoms, while the fatter possesses an extra Ni(C0) fragment capping the unique triangular face of the metal polyhedron.36The first highnuclearity NiCu carbonyl cluster has been prepared from [N&(CO)lZ J 2 - and CuBrz; the complex has been formulated as [Ni35-xCux(C0)40]5(x = 3 or 5) and contains an unprecedented 35-atom three-layer h.c.p. stacking?' 1

4-

(9)

(10)

a

Pt

0 Ni

Ni Opt

(11)

pt

0 Ni

The decanuclear Pt-Cd complex Pt&d&-0H)4(pL-CHCPh)~$C~CPh)4 (12) was formed in low yield from the reaction of the dianion [Pt(C=CPh)4]2- with Cd(C10&.6Hz0, together with an insoluble solid identified as { PtCd(c~cPh)~),. The latter was treated with NB&X (X = CI, Br, CN) to give

Organometallic Chemistry

302

the soluble, tetranuclear complex dianions [Pt2Cd2(p-q1-C2Ph)8x2]2-. Solid complexes are strongly luminescent, while solutions display weak emissions at room temperature. 38

Ag20Au&l14(PPh3)~2has been shown to display efficient optical limiting prope r t i e ~The . ~ ~use of macrocyclic multidentate polyphosphine ligands with nanosized cavities in stabilising or encapsulating large metal cluster complexes has been demonstrated by the preparation of a series of Ag-Au clusters containing 5-phenyldibenzophosphole ligands (PDBP), [AgloAu15Cls(PDBP)lo] *, Ag18A~18CI12(PDBP)12 and its bicapped derivative [ A ~ , o A ~ ~ ~ C ~ ~ ~ ( P D B P ) ~ ~ The neutral cluster Pt2Ag12A~11C17(PPh3)lo was formed in 25 YOyield from the reduction of a mixture of [ P ~ A Q ( P P ~ ~ and ) ~ ]Ag4C&(PPh3)4 ~+ with NaBH4. The complex consists of two Pt-centred Ag6Au6 icosahedra sharing a common Au atom?I 7

Group6

Reaction of Cr(C0)6 with Se02under basic conditions afforded the first selenium-capped trichromium carbonyl cluster, [Cr3(p3-Se)2(CO)lo]2-, which underwent facile vertex exchange upon reaction with Mo(CO)~to give the mixed-metal cluster dianion [Cr2Mo(p3-Se)2(CO)lo]2-.42 Electrochemical studies on the cubane clusters {Cr(p.3-O)(q-CSR5)}4 (RS = H5, Me5, H4Me)have been reported, with structural analysis of the cationic [{Cr(p3-O)(q-C5Me5)}4] showing average bond lengths shorter than those of the neutral p r e c ~ r s o r . ~ ~ The incomplete cubane [M3S4(qs-CsH4Me)3][pts](M = Mo, W; pts = p toluenesulfonate) has been prepared from [M3S4(H20)9] [ptsI4 and reacted with group 10 rnetal(0) complexes in the presence of triphenylphosphine to produce a homologous series of heterometallic cubane clusters [M3M’(p3-S)4(PPh3)(qs(M = Mo, C5H4Me)3] (M’ = Ni, Pd, Pt).”?45 Excision of polymeric { M3S7X4}x W; X = Cl, Br) with diphosphines [bis(diphenylphosphino)- and bis(dimethy1phosphino)-ethane] afforded a one step, high-yield route to the incomplete +;a structural study of the hydrido cubane cluster cations [M3S4X3(diphos)3] derivative [W3H3(p3-S)4(q2-dppe)3] showed a triangular metal core.46 with Reaction of the incomplete cubane cluster [W3(p3-S)(p-S)3(NCS)9]5acetylene afforded [W3(p3-S)(p3-SCH= CHS)(p-SCH = CH2)(NCS)9]4- (13), +

+

+

12: Organo-transition Metal Cluster Complexes

303

along with proposed intermediates 14 and 15 (X = S).47A similar reaction with gave 15 (X = the related oxygen-bridged cluster [W3(p3-S)(p-O)(p-S)2(NCS)9]5o).48 Ligand exchange studies on W&L6 with a variety of Lewis bases have afforded a series of new homoleptic clusters where L = PBun3,PPh3, ButNC, morpholine, MeNH2 and B u ' N H ~ . ~ ~

/"

\I

H

(15) X = S , 0

8

Group7

8.1 Homometallic Clusters. - Reaction of Mn2(CO)lowith Mo(SPh)zCpz afforded the tetrathiolate-bridged [Mn3(p-SPh)4(C0)9]-as the [MoH(CO)Cp2] salt; an X-ray structural study confirmed the incomplete cubane-type ~ t r u c t u r e . ~ ~ Tellurium-bridged manganese cluster dianions 16 and 17 were produced from the reaction of Mn2(CO)loand appropriate ratios of K2Te03;oxidation of 16 with [CU(NCM~)~]+ afforded 18.51 A series of trirhenium hydrido clusters bearing a coordinated c 6 0 , Re3@H)3(p3-q2:q2:q2-C60)(C0)8L (L = CO, PPh3, CNCH2Ph)has been prepared from trirhenium precursor^.^^ The preparation of a series of three- and four-membered rhenium ring complexes from the reaction of Re2(C0)8(thf)2with carbonylrhenates bearing terminal hydrides has been described; the known trinuclear cluster [Re3(p-H)2(C0)12]-was obtained from [ReH2(C0)4]- and was protonated to give the neutral complex Re3(p-H)3(C0)12, while reaction with [Re2H2(pH)(CO)& afforded the novel puckered square cluster [Re4(p-H)@0)&, with two of the hydrides bridging Re-Re edges inside the metal ring.53 Reaction between the octahedral hexarhenium cluster trianion [Re6(p3+

304

Organometallic Chemistry

&)&I3- and the

bidentate phosphines dpph [Ph2P(CH&PPh2J and dpppen [Ph2P(CH2)5PPh2] have afforded clusters with 1 - 3 bridging diphosphine ligands ([Re6(p3-Se)s(p-dpph~16-2x]2X-4 (x = 1 - 3) in the case of dpph, and a monodentate diphosphine complex in the case of the smaller dpppen? Molecular squares made up of octahedral [Re6(p3-Se)8(PEt3)4]2+ units linked by bidentate ligands 4,4‘-bipyridine, (E)-1,2-bis(4-pyridyl)ethene and 1,2-bis(4pyridy1)ethanehave been reported.55

Mixed-metal Clusters Containing Only Group 7 Metals. - The reaction of Re(03SCF3)(C0)5, EMS4]’- (M = Mo, W) or [WOS3J2-,and Li2E(E = S, Se) afforded a series of heterometallic Mom-Re cubane clusters [MRe3(p3-E)(p3S)3(CObY]- (M = Mo, W; Y = 0,S; E = S, Se; various corn bin at ion^).^^

8.2

9

Group8

Polymer-supported amine-N-oxides have been prepared, and their utility in the preparation of M3(C0)12.,(NCMe),(M = Ru, 0s; n = 1, 2) has been demon~trated.5~

Iron. - 9.1.1 Trinuclear Clusters. C- ligands. Treatment of Fe3(CO)12with 1-ethynylcyclohexanol afforded binuclear metallacyclic derivatives along with small amounts of triiron clusters 19 - 21, containing a ‘parallel’ alkynol, an allenylidene and a dimeric metallacyclic ligand, respectively. In contrast, a similar reaction with Co2(CO)*afforded small amounts of the methylidyne cluster co3{p~-CCHt(C6HloOH)}(C0)9.58

9.1

n

B-ligands. Proton competition reactions have been used to determine the Fe3(p-H)(p3relative aciditiesof the ferraborane clusters Fe4(pH)(p4-BH2)(CO)12, BH2)(p-CO)(C0)9and Fe3(p-H)(p3-BH4)(C0)9, with the second-mentioned the most acidic.59 Group 15/16 ligands. Reaction between Fe3(p3-Y)2(CO),(Y = S, Se) and Cr(CO)2(q3-P3)Cpafforded diiron complexes containing mixed chalcogenide/phosphorus ligands, (Fe2(C0)6}(p3-PY3)(Cr(CO)zCp} .60

305

12: Organo-transition Metal Cluster Complexes

9.1.2 Hexanuclear Clusters. Treatment of the nitrido cluster [Fe4(p4-N)(CO)12]- with Mo(CO)~(NCE~)~ afforded the nitrosyl cluster [Fe6(bN)(CO)14(NO)]2-,which consists of an octahedral metal core and a linear nitrosyl ligand formed by oxidation of the starting nitrile, the oxygen presumably derived from a carbonyl group; cyclic voltammetric data suggest that no stable redox congeners exist.61 9.2 Ruthenium. - 9.2.1 Trinuclear Clusters. C- ligands. Activation of the C = C bond in 1,l-disubstituted alkenes has been achieved by reaction with the Reactrinuclear pentahydrido cluster R u & ~ - H ) ~ ( ~ - H(Cp' ) ~ C= ~ ' q5-C5Me5). ~ tion at 80 "C afforded 22 together with the correspondinghydrogenated organic compound, whilst reaction under slightly milder conditions (60 "C) afforded the intermediate p-vinylidene complex 23.62 R'

(22)

R'= C02Me,R" = H R'= Ph, R" = H R' = R" = -(CH2)3-,-(CH&, -CO(CH2)2-

(23)

R' = C02Me,R" = H R' = R" = -CO(CH2)2-

The triruthenium azulene complex RU3(y3-~':q3:r13-C10H~)(CL-CO)(C0)6 has been carbonylated to give the metastable R ~ ~ ( y ~ - q ~ q ~ : q ' - C ~ ~ Hwhich ~)(C0)8, A afforded R u ~ ( C O and ) ~ ~ a binuclear complex on further ~arbonylation.~~ number of tri- and tetra-nuclear clusters (24 - 27) derived from triruthenium precursors have been described. Complex 24 was prepared from the reaction of R U ~ ( C O ) ~ ~ ( Nwith C M PhC=CC=CPh, ~)~ the bridging hydroxyl ligand thought to be derived from traces of water present in the reagent. Cluster 25 was obtained in trace amounts from R ~ ~ ( y - d p p m ) ( Cand O ) ~HC=CCPh2(0H). ~ Thermolysis of the mixed-metal RU~CO~(~~-P~C~C~P~)(~.-CO)~(CO)~~ gave 26 in minor yield, whilst 27 was obtained from reaction of R u ~ ( ~ ~ - H C ~ H ) ( ~ - C Owith ) ( CPMe3.64 O)~ Group 13 and other group 14 ligands. Thermolysis of the hydrogen-rich metallaborane commo-{ ~ - ( R u C ~ ' ) ( ~ - H ) B ~ in H ~benzene ) ~ R U afforded 28, an example of a rare closed cluster with (n - 1) skeletal-electron-pairs!' Reaction of the triangular cationic cluster [(Cp*R~)3(y-H)~]+with NaBH3X (X = H, CN) gave the y3-borylene derivative 2 9 treatment of the X = H complex with alcohols afforded the alkoxy analogues (X = OMe, OEt). These complexes are unusual in having only a single face-cappingborylene ligand on the triangular metal framework. The X = H complex ring-opened the S-containing ring in ben-

Organometallic Chemistry

306 Ph H-C

I

\\

(24)

zothiophene.66Thermolysis of R U ~ ( C Oand ) ~{~( ~ l ~ - c ~ M e ~ H ) M afforded e ~ G e } 30, ~ containing a novel Ru3Ge2trigonal-bipyramidal core, along with a binuclear complex; in contrast, a similar reaction with { C P M ~ ~ gave G ~ }bimetallic ~ and ruthenium dimer products, with no cluster complexes is0lated.6~

Group 15 Zigands. The chemistry of R~~(p-H)(p~-q~-ampy)(CO)~ (Hampy = 2-amino-6-methylpyridine) has been further elaborated. Reaction with diynes RC4R (R = CH20Ph, Ph) afforded ynenyl derivatives 31, with 32 also obtained in the case of R = Ph. The organic ligand in 31 can be considered a resonance

12: Organo-transition Metal Cluster Complexes

307

hybrid of the 3 - and 5-electron donor forms. Further reaction of 31 (R = Ph) with diynes and alkynes gave 33, containing a ruthenacyclopentadienyl fragment formed by coupling of the coordinated ynenyl of 31 with a triple bond on the Reaction of Ru3(p-H)(p3-q2-ampy)(C0), with hexa-2,4-diyne gave the corresponding ynenyl species which could be reacted further to give 34, containing a novel diynedienyl ligand formed from coupling of the diyne with an ynenyl ligand.69

k (33) R = various combinations

(34)

The imido-bridged cluster R u ~ ( ~ ~ - N P ~ ) ( ~ ~ - Creacted O)(CO with ) ~ activated alkynes to give tetranuclear derivatives bearing a p4-phenylimido ligand, 35, together with binuclear complexes containing acrylamido ligands, 36. Similar reactions with diynes afforded analogous products, with the tetranuclear complex containing a pendant alkyne group (35, R = alkyne). In the case of diphenylbutadiyne, isolation of trinuclear complex 37, which could be thermolysed to the corresponding 35 and 36 products, indicates the intermediacy of trinuclear species in these reactions." Thermolysis of R U ~ ( ~ - H ) ~ ( ~ ~ - N O M with ~)(CO two ) ~ equivalents of Co(I)2(CO)Cp*afforded a series of mixed-metal complexes (38 - 40) along with a trihydrido nitrene cluster containing a terminal iodo ligand, Ru3(p-H)3(p3NOMe)I(CO)s.71A similar reaction of Ru3(p3-NOMe)(p3-q2-PhC,Ph)(C0)9 gave

308

Organometallic Chemistry Ph

(35)

R,R’ = various combinations

(36)

(37)

41, while reaction with two equivalents of {Co(CO)2Cp*}2 gave a series of products, including the 4-electron-deficient spiked-square 42 (15%), the metallapyrrolidone complex 43 resulting from C-H activation of a methyl group of the Cp* ligand, and the mixed-metal complex Ru2Co(p3-C0)(p3-NH)(p-q3C~H~P~RU(CO)~COC~*)(CO)~C~*.~~

(411

(42)

(43)

NMR studies on triruthenium clusters Ru~(CO)~OL~ (L = PMe2Ph,PPh3)have shown three isomeric forms in solution. Hydrogen addition to the phosphine complexes gave products containing a bridging and a terminal hydride ligand; mechanisms for hydride exchange involving Ru-Ru bond scission and CO loss have been New mono- and di- phosphine derivatives of Ru3(CQI2 with bulky diphosphine ligands 1,2-bis[bis(pentafluorophenyl)phosphino] ethane and bis(dicyclohexy1phosphino)methane have been described these clusters catalyse the hydroformylation of ethylene and propylene to the corresponding aldehydes.74Reaction of the duster anion [Ru3(p-H)(p-CO)(CO),,1- with

12: Organo-transition Metal Cluster Complexes

309

dicyclohexylphosphine in dry thf at room temperature afforded the anionic ( C Owas ) ~ ~protonated -, to give substitution product [ R U ~ ( ~ . - H ) ( ~ - P C Y ~ ) ~which a mixture of R u ~ ( ~ - H ) ~ ( ~ - P C Y and ~ ) ~R ( CUO ~ )( ~ - H ) ( ~ - P C Y ~ In ) ~ contrast, (CO)~. reaction in refluxing methanol afforded the electron deficient linear cluster 44 along with H~RU~(~-PCY~)~(CO)~(PHC~~)~.~~ A comparative study of the reactivity of dihydrido clusters Ru&-H)~(~-PR~)~(CO)~ (R = But,Cy) towards a variety of mono- and di-phosphine ligands has been described; whereas the former reacted spontaneously at room temperature, the latter was unreactive under similar conditions, requiring elevated temperatures for reaction to pro~eed.~~Ru~(p-H)~(p-Pcy~)~(p-dppm)(Co)~ reacted with CS2 under mild conditions with Ru-Ru bond cleavage and insertion of CS into a phosphido bridge to give 45.77

The novel alkynylphosphines Ph2PC6H4-4-CrCR(R = SiMe3,H) have been prepared and used to link mononuclear R U ( P P ~ ~units )~C to~a central Ru3(C0)9 core, affording a triruthenium cluster-centred hexaruthenium 'star' Reactions of R U ~ ( C Owith ) ~ ~phosphine selenides have afforded a number of new phosphine-substituted ruthenium-selenido clusters. Reaction with Ph2(pyth)PSe[pyth = 5-(2-pyridyl)-2-thienyl]gave the oxidative addition product 46 (R = pyth) and triangular 47 through ligand fragmentati~n?~ whereas reaction with Ph2(2-C5H4N)PSe afforded 46 (R = C5H4N),tetranuclear R*(p4Se),{ p-P,N-Ph2(CsH4N)P}(C0)9with a metal-square geometry, and the linked complex 48.80 Reaction with 1,l'-bis(dipheny1phosphino)ferrocene diselenide (dppfSe,) afforded both the chelating and bridging isomers of Ru3(p3Seb(dppf)(CO)7, together with Ru3(p3-Se){p-P(Ph)C5H4FeC5H4PPh2)(pOCPh)(C0)6obtained as a minor product; the bridging benzoyl group is derived from fragmentation of the dppfSe2ligand and migratory insertion of a Ph ring into a CO bond!

se

Se (46) R = pyth, C5H4N

Se

3 10

Organometallic Chemistry

R ~ ~ ( p d p p m ) ( C was O ) ~reacted ~ with ethyne to give triruthenium clusters 49 51, whereas reaction of R u ~ ( ~ ~ - H C ~ H ) ( ~ - C Owith ) ( Cdppm O ) ~ gave 52.82

Reaction of Ru3(p-H)(p3-C2CH2(0H)}(COkwith dppm in the presence of acid afforded cationic 53 which was deprotonated to 54; a similar reaction with dppe gave the analogue of 54.83

Treatment of the linked hexanuclear cluster { Ru3(p-PPh2)(C0)2)2(p3:p3-C4) with K[BHBus3] followed by acidification yielded 55, whereas reaction with dihydrogen gave pentanuclear 56; the osmium analogue of 55 has also been ~repared.8~ Group 16 ligands. 18f3-Glycyrrhetinicacid was reacted with R U ~ ( C Oto) ~give ~ a binuclear complex, probably the tetracarboxylate-type Ru2(p02CC29H4502)2( CO),(t hf)z, whereas the same reaction with Os3(CO)lo(NCMe)2

311

12: Organo-transition Metal Cluster Complexes

\I

(55)

afforded Os3(p-H)(p02CC29H4502)(CO)lo, in which the oleanic framework is coordinated exo to the triangular metal plane.85A route into triruthenium sulfido clusters containing labile MeCN ligands has been reported, photolysis of [R~~(p~-S)~(q~-cymene)~]*+in MeCN giving 57. Treatment with excess PPh3 in MeCN gave the monosubstituted derivative; however, the bis-substituted product formed slowly, requiring the more weakly-coordinating solvent acetone. Treatment of 57 with the tridentate ligand 1,4,7-trithiacyclononaneafforded the expected nitrile-replacement product in 46 YOyield, and prolonged irradiation of 57 in MeCN afforded the pentanuclear bow-tie cluster 58 in 15 YOyield amongst decomposition products.86

1

2+

I

2+

(57)

Complex 59 was formed as the sole product from the reaction between R u ~ ( C Oand ) ~ ~S ( c ~ C s i M e ~Thermolysis )~. of 59 at 115 "C gave a mixture of products including the proposed open-triangular cluster, Ru3(p3-S)(p3-q2C ( S ~ M ~ ~ ) C ( C E C S ~ M ~ ~and ) ( C60, O ) ~the , latter containing coupled alkynyl groups." 9.2.2 Tetranuclear Clusters. The aqueous chemistry of the electron-deficient cluster cation C R U ~ ( C ~ ~ - H ) ~ ( ~+~has - C ~been H ~ )elaborated ~]* to include reaction with a number of simple nucleophiles. Reaction with CO afforded 61, whereas the NaN3-catalysed reaction with water or reaction with alcohols gave 62 (R = H) and 62 (R = Me, Et, PhCH2, Ph, 4-EtC6H4),respectively.88 DNA binding activity of [RU~(C~~-H)~(~~-C~H~)~]~+ has been assayed, both this cluster and Ru3(CO),(PTA), (PTA = 1,3,5-triaza-7-phosphatricyclo[3.3.l.l~decane) causing damage to DNA; the former is believed to cross-link DNA, while

3 12

Urganometallic Chemistry SiMe3

I

e I

l+

\I

1

2+

the latter is thought to i n t e r ~ a l a t e The . ~ ~ cubane-type chalcogenido cluster Ru&3-Se)4(C0)12has been prepared from a selenium-saturated xylene solution?’ 9.2.3 Hexanuclear Clusters. Thermolysis of Ru3(p-H)(p3-C2SiMe3)(ydppm)(C0)7in refluxing methanol in the presence of KF gave the hexanuclear raft cluster 63 in 80 % yield; the same reaction in the presence of RuCl(PPh3)zCp gave trinuclear 64 (23 %), together with 63.82

9.2.4 Ruthenium Clusters in Catalysis. R U ~ ( C Ocatalysed )~~ the cleavage of sp3 C-H bonds adjacent to a nitrogen atom in N-2-pyridylalkylamines to give coupling products with alkenes; the reaction was applied to a variety of alkenes, including terminal, internal, and cyclic alkenes?l Comparative studies of the

12: Organo-transition Metal Cluster Complexes

313

effect of various phosphine-substituted triruthenium clusters for the isomerisation of olefins (including the influence of nitrogen, argon, xenon or helium on the reaction rate)P2 and the effect of excess PPh3 on the catalytic activity of R U ~ ( C Oand ) ~ ~ R u ~ ( C O ) ~ ( P for P~~ the ) ~hydrogenation and isomerisation of l - h e ~ e n e ?have ~ been reported. The intramolecular hydroamination of aminoalkynes by R U ~ ( C Ohas ) ~ been ~ investigated; the process is highly regioselective, the nitrogen atom selectively attaching to an internal carbon of the alkynes to give five-, six- or seven-membered heterocycles, as well as ind0les.9~Chelating diimines of the type bis(ary1imino)acenaphtene and bis(pheny1imino)phenanthrene have been found to be efficient promoters for the R~~(CO)~~-catalysed reduction of nitroarenes to anilines by CO/H20.95 R~~(CO)~~-catalysed ring opening of W(C0)A1-phenyl-2-isopropylidenephosphirane) in toluene afforded a mixture of two isomers of R~(cO)~[q~-3,3dimethyl- 1-phenyl-1-{W(CO)5}-1-phospha-trimethylenemethane]? The hydrogenation of benzene and derivatives has been catalysed under biphasic conditions by the water soluble triruthenium cation [ R ~ ~ ( p - H ) ~ ( p ~ - o ) ( q ~ - C ~ H ~ ) ( q " C6Me6)2]+;the open triangular cluster [Ru3(p-H)2(p3-o)(p-0H)(T16_CgH6)(q6C6Me&] was isolated from the reaction with ethylbenzene under hydrogen pre~sure.9~ +

9.2.5 Surface-bound Ruthenium Carbonyl Clusters. Bimetallic catalysts combining K2[Fe2(C0)8]- or K2[R~(C0)13]-with 'Sibunit' carbon-supported Co or Ir have been prepared, and the subsequent effect on the catalytic efficiency in the synthesis of ammonia assessed. The activity of the ruthenium-based catalysts was found to decrease in the presence of both Co and Ir; in contrast, an acceleration in the reaction rate was observed for the Fe-Ir catalysts, with little effect being observed in the Fe-Co case?* - has been The hexanuclear carbido cluster anion [R~&~-C)(p-SnC13)(C0)16] supported on mesoporous silica, and selectivity in the solvent-free hydrogenation of 1,5,9-cyclododecatriene and other alkenes has been demon~trated.9~ The hexanuclear cluster RU&~-C)(CO)~~ has been attached to ArgoGel polymer beads via interaction of the crown ether-substituted analogue Rh(p6C)(C0)14(r16-C16H2406)with acid-generated terminal ammonium groups in the ArgoGel, or via derivatisation of the chloro- or amine- Argogel systems with diphenylphosphine and subsequent treatment with Ru6(b-C)(C0)17.The crownether system exhibited high activity and selectivity for the hydrogenation of olefins and transfer hydrogenation of ketones.laO

9.3 Osmium. - 9.3.1 Trinuclear Clusters. H2/D2 exchange experiments and para-H2 effects have been used to identify two possible exchange pathways in OS3(p-H)2(C0)10.lo' Hydrocarbon ligands. The chemistry of triosmium clusters with bis(ferrowith ceny1)oligoyneshas been further elaborated. Reaction of Os3(CO)11(NCMe) 1,8-bis(ferrocenyl)octatetrayne gave 65 - 68, while a similar reaction using OS~(CO)~~(NC gave M ~the ) ~ linked cluster 69. Electrochemical studies suggest

3 14

Organometallic Chemistry

that there is significant electronic communication between the ferrocenyl groups in 67.'02 In contrast, reaction with R U ~ ( C O gave ) ~ ~ the linked complex { RU~(CO)~(~-FCC~FCC=CC=C)}~, containing an almost planar [12ldehydroannulene ring containing four C=C bonds.lo3 Fc

Fc

Fc

"( ' " \

/ \-0s\ /

-0 s

/

0s. I\

Treating O S ~ ( ~ - H ) ( ~ - ~ ~ - N C ~with H ~ )1,4-bis(ferrocenyl)butadiyne (CO)~~ in the presence of Me3N0 gave isomeric 70 and 71.'04

OS~(CO)~~(NCM has~ ) ~been reacted with the symmetric diyne MeC=CC=CMe, giving three new clusters 72 - 74. The Co2(CO)6adducts and some MeCN mono-substituted analogues were described, and 0s3(p3-q':q2:q2RC3CHR)(p-OH)(C0)9 was isolated from reaction of 0s3(p3-q2RC2C2R)(COb(NCMe)(R = Me, Ph) with H2O.loS The cationic complex [Os3(p-H)(p-q':q2-C= C = CMe2)(CO)lo]+reacted with PPh3 at low temperature to give two isomers of the phosphonium derivative 75 (differing in the position of the hydride ligand), and the spectroscopically-

315

12: Organo-transition Metal Cluster Complexes Me

\ I

Ill

Me

identified PPh3 monosubstituted complex 76. The latter quickly formed the a-acetylide cluster 77, subsequent decarbonylation affording an isomer of 76.'06

Reaction of O S ~ ( ~ - H ) ~ ( with C O )diynes ~ ~ containing p-NH groups resulted in cyclisation of the alkyne to give Smembered pyrrolyl rings coordinated in an edge-bridgingp-q':q2- or p-q':ql manner. The latter were easily decarbonylated with concommitant transformations of the cluster framew~rk.'~~ Group 15 ligands. Treatment of the nitrite cluster 0s3(p-H)(p-q2-NO2)(CO)~~ with ferrocenyl phosphines dppf and ppfa (1,l'-bis(dipheny1phosphino)ferrocene and N,N-dimethyl-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethylamine, respectively) afforded triangular clusters with an Os-0s bond containing all three bridging substituents.108 Triosmium clusters containing monodentate (through the phosphorus atom), chelating and bridging coordination products were obtained from reactions of Ph2PCH2CH2SMe with OS~(CO)~~-,(NCM~), (n = 1,2); isomerisation processes were discussed.'09 Similarly, monodentate, bridging and linked clusters, including 78 and 79, were isolated from reactions of 1,2,3-triphenyl-1,2,3-triphosphaindane with OS~(CO)~~-,(NCM~), (n = 1, 2)."'

Ph

1

P-0s

\/,

qjL+< P-P Ph'

(79 1

\

Ph

316

Organometallic Chemistry

Treatment of the unsaturated cluster OS3(p-H)(p3-q3Ph2PCH2P(Ph)C6H4}(C0)8 with dithiols afforded the open triangular clusters 80 (52 electrons)and 81 (50 electrons).80 converted to 81 under forcing conditions, and both 80 and 81 were decarbonylated with CH activation of the dithiolato ligand to give 82."'

(80)n = 2,3

(81) n = 2,3

(82)n = 1 , 2

Thermolyses of O S ~ ( C O ) ~ ~ ((E E P=~ ~AS,"^ ) Sb'13) have afforded a series of new triosmium and linked clusters containing As (e.9. 83- 86)and Sb (e.9. 84,87) in a variety of coordination modes.

Treatment of the linked cluster O~~(pH)~(p~Sb)(p~-q~-C6H~)(p~-q~-CtzH SbPhz)(CO)lswith Bu'NC resulted in 0 s - 0 s bond scission to give 88 and 89, containing five-membered Os3Sb2 rings.' l4 Group 16 and 17 ligands. Studies into the reactivity of Os3(p-H)(p OSiPhzR)(CO)lotowards hydrolysis, alcoholysis and reduction by CO and H2, and the thermal behaviour of O S ~ ( ~ - H ) ( ~ - O S ~ E ~ have ~ ) ( Cenhanced O ) ~ ~ , understanding of the surface chemistry of silica-anchored O S ~ ( ~ - H ) ( ~ - O S ~ = )'I5( C O ) ~ ~ . The preparation of Os3(p3-q2-Me3SiC2C=CSiMe3)(p-OMe)~(CO)9 from Os&-

12: Organo-transition Metal Cluster Complexes

317

\

(89)

CNBu'

Me3SiC2C=CSiMe3)(p-CO)(COband o ~ B r ( P P h ~ ) ~and c p ,its crystal structure, have been reported.' l6 A series of organic transformation of coordinated diols have been demonstrated on the triosmium cluster O S ~ ( ~ - H ) ( ~ - O C H ~ C H ~ O Hone ) ( CofO a) ~ ~ number of osmium diols prepared from treatment of Os3(p-H)(p-OH)(CO)lo with appropriate glycols. Esterification with benzoyl chloride gave the corresponding ester Os3(p-H){p-OCH2CH~0C(0)Ph}(CO)lo,oxidation giving the aldehyde and then allylation giving Os3(p-H){p-OCH2CH(OH)CH2CH=CH2)(CO)lo, while reaction with PhMgBr afforded Os3(p-H){p-OCH2CH(OH)Ph)(CO)lo."7 Reaction of Os3(CO)12with (thd)H (thd = 2,2,6,6-tetramethyl-3,5-heptandionate) afforded the linked cluster 90,along with the linear tetraosmium complex 91. Treatment of 90 with acetonitrile resulted in substitution of the carbonyl of the fragment that is trans to the bridging C02 monometallic OS(CO)~

The activated triosmium clusters OS~(CO)~~-,(NCM~), (n = 1, 2) and 0s3(pH)2(C0)10have been treated with a number of thioethers to form OS~(CO)~~(L), O S ~ ( ~ - H ) ( ~ - ~ ~ - C ~ H ~ and S R )os,(p-H)(p3-C6H4SPh)(C0)9, (CO)~~ the latter two complexes containing orthometallated C6H4SR.'l9 The reactivity of the untowards thiols saturated cluster Os3(p-H){p3-q3-Ph2PCH2P(Ph)C6H4}(CO)~ showed a dependence on the steric bulk of the reagent - large thiols gave only oxidative addition products, whereas the smaller thiols such as RSH (R = Et, Pr') gave novel 50- and &electron clusters 92 and 93, respectively.12' OS~(CO)~ l(NCMe)and O S ~ ( ~ - H ) ~ (reacted C O ) ~with ~ Ph3PSeto give the selenido clusters Os3(p3-Se)(C0)9(PPh3), O S & - H ) ~ ( ~ ~ - S ~ ) ( C O ) ~and ( P hexanucP~~), lear 94, containing a p3-Seatom linking two Os3triangles.12'The 'hook' cluster 95, containing a dative bond from a linear Os3unit to a pendant 0 s 2fragment, was prepared from O S ~ ( ~ - C ~ ) ~and ( COS(CNBU')(CO)~ O)~~ at 60 0C.122

Organometallic Chemistry

318

(93)R = Et, Prl Bu'NC

PPhq

9.3.2 Higher-nuclearity Osmium Clusters. Tetranuclear O S ~ ( ~ - H ) ~ ( was C Ore)~~ acted with 1,4-bis(ferrocenyl)butadiyneto give 96; no electronic communication was detected between the cluster and the ferrocenyl groups.'23

Fc

(96)

A series of tetraosmium clusters containing ligands with an azo-linkage, including 97-102, was prepared from Os4(p-H)4(C0)12 or O S ~ ( ~ - H ) ~ ( C O ) ~ O (NCMe)2;electrochemical analysis revealed electronic communication between the two cluster units in the linked system 100.'24125 O~5(p5-C)(p3-q~:q~:q~-C~~)(CO)~~(PPh~) was prepared from Os5(p5C)(C0)14(PPh3);in contrast to the analogous pentaruthenium complex, the c 6 0 ligand on the osmium cluster underwent reversible interconversion to p-q2:q2coordination i.e. to O~~(p~-C)(p-q~:q~-Ca)(CO)~2(PPh3). The isocyanide complexes M5(p5-C)(p3-q2:q2:q2-C60)(CNCH2Ph)(CO)ll(PPh3) (M = Ru, 0 s ) were prepared to help elucidate the nature of the transformation; it is likely that the p-q2:q2-complexis formed following Os-0s bond scission on addition of the incoming ligand. In the ruthenium case, the M5C-Cainteraction is thought to be considerably stronger than in the osmium cluster, which precludes bond-scission and favours carbonyl loss.'26 9.2.4 Osmium Clusters in Catalysis. O S ~ ( ~ - H ) ~ ( promoted C O ) ~ ~ the ring-opening polymerisation of norbornene to give high molecular weight poly(1,3-cyclopentylenevinylene)in high yield.'27

12: Organo-transition Metal Cluster Complexes

(97) R = H, NMe2

319

(98) R = H, NMe2

9.4 Mixed-metal Clusters Containing Only Group 8 Metals. - The reactivity of FpC=CH and the ethynediyl complex F p C d F p [Fp = Fe(CO)2Cp, Fe(C0)2Cp*] towards R U ~ ( C Ohas ) ~ been ~ investigated. Whereas reaction with FpCsCH gave the triruthenium acetylide complex 103, a similar reaction with F p C d F p afforded 104 - 107, including a number of mixed-metal dicarbide clusters. The interconversions possible between the cluster products suggest that the permetalated ethene and ethane structures result from formal sequential addition of dimetallic fragments to the C d triple bond of the ethynediyl reagent. 128 10

Group9

10.1 Cobalt. - Reaction of C O C I ( P P ~with ~ ) ~ sodium diethyldithiocarbamate resulted in cleavage of single and double C-S bonds to give the triangular cluster 108,'29and a series of new open tricobalt clusters, 109 (two isomers) and 110, has been prepared from treatment of c ~ ~ ( p - S M e ) with ~ C pC~O ~ ( ~ - F ~ C C ~ H ) ( C ~ ) ~ . ' The triangular cluster Co3{C L ~ - C C H ~ ( C ~ H ~ ~was ~ H isolated ) } ( C ~in ) ~minor yield from C O ~ ( C Oand ) ~ HC-CC6HloOH,58and the preparation and structures of tetrahedral clusters C~~(p-dpprn)~(p-CO)~(CO)~ and Co4(1".-CO)3(C0)6($C6H6)have been r e ~ 0 r t e d . l ~ ~ 10.2 Rhodium. - Crystallisation of mononuclear Rh{q2-MeOCH2CH2N(C3H2N2Me2)}(q4-cod) (cod = 1,5-cyclooctadiene)from wet solvents gave the triangular cluster cation [Rh3(p3-OH)2(q4-~~d)3J +.132 A study of

Organometallic Chemistry

320

a

-CF3

the reactivity of the tetrahedral cluster Rh(C0)12 with 1,3,5,7-~yclooctatetraene (cot) or its bis(trimethylsily1)derivative 1,4-CsIi6(SiMe3)2 indicated a preference for facial coordination of the cot Iigand. Preparation of the mono-substituted complex, R ~ ( ~ ~ - C ~ H S ) ( C and O ) a~ , variety of bis-substituted complexes, e-g- Rh4(p3-cSH8)(c0b(r4-c8H8), and Rh4{~3-C8H6(SiMe3)2)(C0)6(r4c&k,(SiMe3)z}, has been desc15bed.I~~ Reaction of [7,8-Ph2-7,8-C~BgHg1'- with { Rh(q4-cod)Cl}2 gave minor amounts of { R ~ ( V ~ - O H ) ( P ~ ~ C ~ B consisting ~ H ~ ) } ~of , four 2,1,8-RhC2B9 icosahedra bridged by four p3-OH units to give a cubane-type Rh4(p3-OH)4 core. The complex is the first example of a tetrameric icosahedral metallacarborane, and the first example of a group 9 M4(p3-OH)4~ u b a n e . ~ ~ ~ The structures of a series of hexarhodium clusters, Rh6(C0)1& [L = CO, P(OPh)3, P(4-XC6H4)3 (X = CF3, c1, F, OMe), PBu"3, Me2S0, MeCN, C8H14] have been determined and compared, and trends in structural features on ligand replacement CO ligand fluxionality on Rh6(C0)15L(L = PR3, P(OPh)3, NCMe, I-),136and ~Rh6(~~-c)(co),dpPh3~]2-137 have been probed using variable temperature and two-dimensional X-(lo3Rh} (X = 13C, 31P)

I 2: Organo-transition Metal Cluster Complexes

321

HMQC and 13CEXSY NMR experiments. 10.3 Iridium. - Addition of equimolar PPh3 to [HIr4(CO)l,)- afforded [Ir4H(pCO)3(CO)dPPh3)]- (111) cleanly, no polysubstitution products being isolated.13*Astudy of the reactivity of y-A1203-supportedIf4 towards propene and H2showed that reactivity towards H2for propylidyne on Ir4/y-A1203is reduced compared to that on extended metal surfaces, suggesting that an ensemble of more than three metal surface atoms is required for rea~tivity.'~~

Condensation of [HIr4(CO)ll]- with [II~CO)~]-afforded the trigonal bi- (112) in almost quantitative pyramidal cluster anion [IrS(p-H)(pC0)2(C0)10] yield.13*

\I

1-

/"\ (1 12)

10.4 Group 9 Clusters as Catalysts. - Heating sol-gel entrapped C02Rhz(C0)12 afforded an efficient catalyst precursor for the hydrogenation of arenes." 11

Group10

11.1 Nickel. - Trigonal bipyramidal [C(NMe2)3]2[NiS(p-CO)3(C0)9)(113) and

octahedral co-crystallisation product [C(NMe2)3]3[N&(p-CO)s(C0)6] [02CNMe2] (114) have been isolated in minor amounts from the reaction of Ni(C0)4 with C(NMe2)4.14' 11.2 Palladium. - A paramagnetic palladium cluster, [Pd3(dppm)3CO]+,has been identified as the intermediate in the electrochemically-inducedC-X (X =

Organometallic Chemistry

322

halogen) bond cleavage reaction between [Pd3(dppm)3C0]2 and a large variety of C-X -containing substrates. The 43-electron cluster is stable on the electrochemical time scale, and has been characterised by EPR spectro~copy.'~~ +

11.3 Platinum. - A kinetic study into the redox reactions of the Chini-type clusters [{Pt3(co)6}J'- (n = 3 - 5) with H2and acid has suggested a mechanism involving the formation and disproportionation of cluster aggregate^.'^^ The triangular clusters Pt3(p-C0)3(PR3)3 [PR3 = PPh3, P(CH2Ph)Ph2,PCy3, PPri3J have been reacted with alkenes and alkynes. Addition of R'02CC=CC02R' ( R = Me, Bu') at low temperature afforded the unstable, spectroscopically identified adduct P t p3-alkyne)(C0)3(PR&; dynamic NM R data suggest that the complex undergoes a fluxional process involving Pt-Pt bond opening. Reaction at room temperature gave dinuclear complexes with a p-ql:qi-or p-q2:q2-coordinated alkyne ligand.I4 Reaction with electron withdrawing alkenes (trans-dicyanoethene and maleic acid) gave the unstable adducts Pt3(p-CO)3(alkene)(PR3)3; at higher temperatures, these clusters converted quantitatively to stable mononuclear Pt(CO)(alkene)(PR3).145 Tetracyanoethylene (tcne) was reacted with the linear cluster cation [Pt3(pdpmp)2(CNR)2]2'[dpmp = bis(diphenylphosphinomethy1)phenylphosphine; R = 2,6-xylyl, 2,4,6-mesityl] to give nitrene-bridged linked complexes in which two tcne molecules are coupled to form a (heptacyanocyclopent- 1-eny1)nitrene ligand.'46 The linear trinuclear cluster Pt3(p-H)2(p-PPh2)2(C6F5)2(PPh3)2 was obtained in 21% yield amongst binuclear products in the reaction of 1:l:l cisPt(C6F5)2(PHPh2)2, Pt(norborneneh and PPh3; the yield was more than doubled by using a 1:2:2 ratio of reagents.14' The mechanism of the protonation reaction of triangular complex P ~ ~ ( H ) ( ~ - P B u ~ ~with )~(C TfOH O ) ~(Tf = CF3S02)to give cationic [Pt3(p-H)(~ - P B U ' ~ ) ~ ( C ~ ) ~ ( P H B U ' been ~ ) ] examined; employing has TfOD afforded exclusively [Pt3(p-D)(p-PBu'2)2(C0)2(PHBu'2)]+, suggesting that the incoming proton transfers to the metal core with P-H bond formation involving the terminal hydride of the precursor.148Pt3(H)(p-PB~'2)3(C0)2 has also been used as a precursor to a series of novel hexaplatinum clusters; reaction with excess acid under a CO atmosphere afforded [Pt6(p-PBut2)4(C0)6]2+ (115, L = +

323

12: Organo-transition Metal Cluster Complexes

CO) with a Pt-dibridged tetrahedral metal core. Subsequent substitution with trimethylphosphine gave 115 (L = PMe3), and reaction with NaBH4 gave the neutral CHO-substituted analogue 116 (L = CHO) which could be decarbonylated at 60 "C to give 116 (L = H). 116 (L = CHO) is the first diformyl metal ~ 1 u s t e r . l ~ ~

But2

BUt2

(115) L = CO, PMe3

(116)

L = CHO, H

Addition of Pt(C0)2(PPh3)2 to [Pt3(p3-H)(p-dppm)3] or [Pt3(p3-CO)(pd ~ p m ) ~ ] *afforded + two new tetraplatinum clusters, 117 and 118; the latter is the first dicationic Pt4 +

PPh3

I

1'

PPh3

I I

1 2+

Dt

(118)

(117)

11.4 Mixed-metalClustersContaining Only Group 10 Metals. - Reaction of the linear cluster [PdPt2(p-dpmp)(CNXyl)2]2 with a variety of electron-deficient alkynes R C d R ' resulted in site-selective insertion of the alkyne into the Pd-Pt bond and P-C bond formation regioselectively on the terminal alkyne carbon (when present), to give a series of unsymmetrical A-frame mixed-metal clusters, 119.I5l +

1*+

XylNC /CNXy'

Organometallic Chemistry

324

12

Group 11

12.1 Copper. - The synthesis, structure and luminescence of the tricopper clushas been r e ~ 0 r t e d . l ~ ~ ter cation [Cu3(p3-q*-C=C-benzo15-~rown-5~(p-dppm)~] with BunLi/HC=CSiMe3 or Reaction of [C~~(p-PPh~pypz)~(NCMeh]~+ NaCBCH afforded a luminescent ethynediyl-bridged tetracopper complex in which the C2 ligand bridges a pair of Cu2 subunits in both q1and q2bonding modes.153 Metathesis of CuCl with Li[RCS(NR')] (R = Bun,R' = But;R = R = Me) gave a mixture of hexamer and tetramer complexes { Cu[RCS(NR')]},, with the bulkier ligand favouring the tetramer, and the smaller methyl-substituted reagent favouring the hexamer. Cu4{p3-q2-SC(Bu")N(But)}4consists of a Cud tetrahedron whereas cu6{p3-q2-SC(Me)N(Me)}6 contains a cu& pseudohexagonal prismatic +

12.2 Silver. - The triple salt 2Ag2C2.9AgN03.AgFH20has been prepared from a mixture of Ag2C2, AgN03 and AgBF4; the complex contains two types of monocapped trigonal prismatic Ag7C2cages, 120 and 1 2 P 5The preparation of a dicationic imidazolium-linkedcyclophane,and its reaction with Ag20 to give a unique dimeric silver N-heterocyclic carbene complex, has been reported; the Ag4core possesses a planar butterfly

12.3 Gold. - The cationic complex [Au(C~F~)~(PNH~)]+ reacted with two equivalents of Au(acac)(PPh3)(acac = acetylacetonate) to afford Au3(p3-q2NC6H4PPh2-2)(C6F5)2(PPh3), the structural data suggesting the presence of weak Au(1) . . .Au(II1) interaction^.'^' Au2C12{p-Ph2PN(C6H4Me-4)PPh2} reacted with H2S to afford the heterocubane 122;the product possesses an abnormally large Stokes shift, consistent with an excited state structure which is highly distorted A series of luminescent trinuclear gold comfrom that of the ground plexes containing dpmp and a variety of S-donor ligands has been r e ~ 0 r t e d . l ~ ~ Interaction of ligand-bridged, electron-rich trinuclear gold complexes, Au3{p-q2-N,C-NC2H2N(CHzPh)}3 and Au3{p-q2-EtOC= N ( ~ - c ~ H ~ h ! fwith e)}~ small electron acceptors, tetracyanoquinodimethane (TCNQ) and C6F6, respectively, afforded charge-transfer adducts with extended chain structures consisting of the organic fragment sandwiched between layers of gold complex. The extended structures can form with and without the original aurophilic interac-

12: Organo-transitionMetal Cluster Complexes

325

tions.la Similarly, Au3(p-q2-MeN=CORh (R = Me, Et) formed columnar adducts with nitr0-9-fluorenes.'~~ 13

Mixed Metal Clusters

13.1 Group 4. - Ti - Ru. - Core-expansion of the bimetallic Cp2Ti(p SH)2RuClCp*with R u C ~ A P Pgave ~ ~ )the ~ bimetallic cluster 123.'62

13.2 Group 6. - M o - Mn,Re, Fe, Ru,Co. - A series of mixed-metal complexes containing a naked phosphorus atom have been prepared from the reaction of with M3(C0)12(M = Fe, Ru; the dinuclear complex Mo2(p-H)(p-PH2)(CC))4Cp2 124 - 125) or M2(C0)10(M = Mn,Re; 126). The Mo-Mn cluster 126 has been structurally characterised and contains a planar Mo2MnP core and trigonal planar P atom; it represents the first neutral M3Pcomplex containing two M-M bonds.163 The dimolybdenum phosphaalkene complex Mo2{p-q1:q2P(Ph)=C(H)Me}(CO)&p2 has been reacted with MJ(CO)12(M = Fe, Ru) to form the mixed-metal triangular clusters Mo2M(p3-PPh)(C0),Cp2(M = Fe, Ru) = C(H)Me}(pand with CO~(CO)~ to give tetrahedral Mo2C02{p3-q2-P(Ph) C0)2(C0)6.164 Organometallic macrocyclic crown ethers containing redox-active Mo2Fe(p3-S)cluster cores have been prepared via self-assembly chain cyclisation of ether chain-bridged mononuclear and cluster reagents.165The reaction between the triangular cluster Mo2Rh(p3-S)(p-Ct)(p-S)3(PPh3)2(q2-S2CNEt2)z and phenylacetylenegave 127, wsulting from incorporation of three alkyne units into

Organometallic Chemistry

326

I

\ /

(124) M = Fe, Ru

(126) M = Mn, Re

( 125)

ph2Mep/

Ph /-7

(127) S

Ph

'CI

n (128) S S = S2CNEt2

Ph

I

/

S = S2CNEt2

,PMePh2

(129)

\

the cluster core. The structure of the reactant analogue 128 was also reported.'66 W - Fe, Ru. - Thermolysis of WFe2(p3-S)2(C0)10 with W(C-CPh)(CO),Cp afforded 129,'67 and treatment of R~cl(PPh3)~Cp with W(C=CC-CSiMe3)(C0)3Cp and then reacting the product with Fe2(CO), gave 130.168 The carbido-alkylidyne clusters 131 and 132 were formed via acetylide C-C bond cleavage in the condensation reaction of R u ~ ( C O ) ~and ~ WRu2(C=CPh)(CO),L (L = Cp, Cp*); treatment of 131 and 132 with CO afforded the original acetylide complex in good yield. The result of hydrogenation is dependent on L, thermolysis of 131 (L = Cp) under H2giving 133 (L = Cp; R = H), whereas a similar reaction for 131 (L = Cp*) gave no tractable products; hydrogenation of 132 gave 134 as the only product. Reaction of 131 with thiophenol afforded 133 (L = Cp, Cp*; R = SPh).169 Reaction of W R U ~ ( ~ - H ) ( C Oand ) ~ ~WH(CO)3L' L (L, L' = Cp, Cp*, Cp/Cp*) gave bridged-butterfly 0x0-carbido complexes 135 via cleavage of a carbonyl C - 0 bond. Interestingly, in the case of (L = L' = Cp) the trigonal bipyramidal

327

12: Organo-transition Metal Cluster Complexes

(133) L = Cp; R = H L = Cp, Cp*; R = SPh

(134)

cluster 136 was isolated as the first-formed product and formed the corresponding 135 with loss of CO on heating in toluene. The distorted square-pyramidal complex 137 was formed on extensive heating of 135 in toluene, a reaction reversed on treatment with C0.170

(135) L = L'= Cp L = L' = cp* L=Cp, L ' = c p *

(137) L = L' = Cp*

L = cp, L ' = cp*

W - Co, Ir. - The triangular p3-vinylidene cluster 138 was formed in the reaction of W(C=CFc)(C0)3Cp (Fc = ferrocenyl) and C~~(p-dppm)(CO)~.'~' The of W21r2( CO)lo(q-C5H4Me)2 with 1,2-(E)-bis { 4'-(oct - 1"reaction yny1)phenyl)ethene gave an unusual edge-bridged trigonal bipyramidal complex 139, containing a p4-q2-coordinatedcarbonyl ligand with the longest carbonyl C - 0 bond distance found thus far; the cluster is the first hexametallic group 6 group 9 mixed-metal ~ 1 u s t e r . l ~ ~ WIr3(C0)11(q-C5H4Me) reacted with diphenylacetylene to give a mixture of

328

Organometallic Chemistry Ph

P odi cts, including tetrahedral cluster 140 and the butterfly complex 141, along with a binuclear product 142.’73

Cr, Mo, W - Pd, Pt, Ag, Au, H g - Linear tetranuclear clusters Cr2Pt2&(pyphos)(143 - 144) (X4 = Me4, C12Me2;pyphos = 6-diphenylphosphino-2-pyridonate)were formed by addition of the appropriate Pt(I1)species to Cr2(pyphos)4.Magnetic and structural studies indicate Cr-Cr bond elongation induced by the axially located Pt

Pt-Cr P - N r O l

4 /t I

Me

Cr-Pt

/

O-N-P

$1

Me

(1 43)

Ligand details omitted for clarity

O-N-P (144) X = CI,Me

L~[Mo(CO)~(~’-C~&BR)] (R = NW2,Me) have been prepared and used in the synthesis of a series of heterometallic complexes with 2-boranaphthalene ligands, 145 - 147, by reaction with HgC12,AgOTf and PdC12(PEt3)2,respectiveiy.175

has been reacThe diplatinum dihalide complex Pt2(pH)(pPPh2)(I)2(PPh3)2 ted with carbonylmetalates CM(CO)3Cp]- (M = Mo, W) to give 44-electron triangular clusters MPt2(p-CO)3(p-PPh2)(PPh3)2Cp. In contrast, reaction with In each case, the [CO(CO)~]-gave homometallic Pt3(p-C0)2(pPPh2)I(PPh3)3. [M(CO)3Cp]- and [PtI(CO)2(PPh3)]- fragments can be considered as fourelectron donors to the Ptz cationic Reaction of MoH2Cp2and AgBF4 in

12: Organo-transition Metal Cluster Complexes

329

the presence of a variety of mono- and dithio reagents afforded monomeric, dirneric and polymeric complexes in which the Ma unit is coordinated to an AgS fragment, examples including linear cluster complexes 148 and 149.177

opt

The anians [Modp-H)(pdppm)(CO)& and [Mn3(p-H)(CO),2}Z-have been reacted with AuC1-terminated carbosilane dendrimers to produce dendritic complexes containing terminally bound MozAu and Mn3Au clusters.178A series of complexes containing an aminocarborane ligand have been reported, including the linear Mo-Au-Mocluster anion [2,2'-y-Au-(1,2-y-NWBu'-2,2,2-(C0)3-closo2,l -MoCBloHlo)27-. 179 Mo, W - Ru,Os, Rh, It., Cu cubane clusters. - A series of cubane-type clusters cores (M'= Ru, Os, Rh,Ir) has been prepared from reaction with MO~M'(P~-S)~ of [Mo3S&$-CsH4Meb] with the corresponding metal alkene complex, Ru(COM1,53cod),Os(CO~l,3-cod)or (hl'Cl(cyclooctene)2)2(M = Rh, fr). The ruthenium product, MqRu(~rrS)4(C0)2frl~-CsH4Me)3, lost CO to form a carbonyl-bridged dkubane cluster, [email protected] The preparation of triangular clusters [M3X3(p3-Se)(pSe)3(dmpe)3]+ [M = Ma, X = Cf, Br; M = W, X = Br (151); dmpe = 1,2-bis(dirnethylphos+

Organometallic Chemistry

330

12+

&f\

phino)ethane] has been reported. The complexes readily incorporate copper to give cubane complexes such as 152. Electrochemicaland linear optical properties vary across the series as expected, and all clusters are efficient optical limiters.'81.182 A

Me2P

PMe2

l+

13.3 Group 7. - Reaction of the carbyne complex [Mn(=CPh)(CO),Cp]+ with - gave the triangular cluster MnFe2(p-H)(p3[Fe2(p-SeC4H9-n)(p-CO)(C0)6] CPh)(p-C0)2(C0)6Cp,containing a face-capping carbyne ligand.183 The octahedwas formed from the reaction ral cluster anion [MnFe5(p6-N)(p-CO)3(CO)13-J2of [Fe&k,-N)(C0)15}3- and a threefold excess of M ~ Z ( C O )or ~ &from [Fe4(p4N)(CO)12J- and [MnFe(C0)9]-; the complex is the first nitrido cluster containing a group 7 meta1.6l The triangular complexes M2Rh(p-PCy2)(p-COb(CO)8 (M2 = Mn2, Re2, MnRe) have been prepared from [M2(p-H)(p-Pcy2)(co)8]- and [Rh(cod)] at low temperature in the presence of CO; the products underwent substitution of CO by tertiary and secondary phosphines. M,Rh(p-PR2)(~-co)~co)8 (R = Cy, Ph) reacted with diphosphines L [L = dppm, (Ph2P)2C= CH2] with loss of CO and ring opening to give unsaturated linear clusters M2Rh(p-PR2)(pCO)2(COb(L)(153); subsequent reaction of the rhenium examples with CO gave the valence saturated clusters, Re2Rh(p-PR2)(C0),dL).Deprotonation of M2Rh(p-PCy2)(p-C0)2(C0)7(dppm) followed by reaction with [Au(PPh3)] afforded ruc-M~Rh(p-PCy2)(~-CO~(CO)~{(Ph~P)~CHAuPPh~}, and reaction of LiR with M2Rh(p-PCy2)(p-CO)2(C0)7{(Ph2P)2C =CH2} gave rac-M2Rh(pPCy2)(p-CO)2(CO)7{ (Ph2P)2CHCH2R} for a variety of R. Catalytic properties in +

+

12: Organo-transition Metal Cluster Complexes

33 1

hydroformylation and isomerisation of 1-hexene have also been described.'84'185 A series of diastereomeric tetrahedral complexes Re2MM'(p-PCy2){p-(- )thi~camphanate}(CO)r(PPh~)~ [MM' = Ag,, Au2(154), AgAu, CuAu) have been prepared; the signs of long wavelength absorption bands in the CD spectra correlate with the configuration of the chiral transition metal core? Decarbony'ation of the phosphine-linked cluster [Mn2Au(p-PPh2)(CO)8]2(pP-P) [P-P = (Ph2P)X(PPh,); X = Fe(C5H& (CH2)n(n = 1 - 4)] in the presence of free phosphine afforded 155, with the phosphine now bridging the heterometallic Mn-Au bond; the process was shown to occur for a wide variety of diphosphines, with the dppf analogue proving stable towards CO and PPh3 substit~tion.'~~ PPh3 Au Ph2

- Mn/p\-Mn1, \

"\/

(153) M = Mn, Re X = CH2, C=CHz

(154) and diastereomer R = thiocamphanate

'P

AU-PJ

(155)

The heptanuclear rhenium cluster dianion [Re7(p6-C)(CO)~(J3reacted with MC12 (M = Cd, Zn) or Cd(N03)2 to form 156; treating 156 [MX = Cd(N03)] with 4- bromo thiophen01 gave [Re7(p6-C)(CO)2 Cd(SC6H4Br)12-. '" The dimercury-linked cluster tetraanion 157 was formed on reaction of [Re7(b-C)(CO)zlJ3- with Hg2(N03),;the complex represents the first example of a dimercury bridge between two cluster faces. Oxidative cleavage of the Hg-Hg bond with a variety of reagents gave 158 or with ferrocenium ion in the presence of tetramethylthiourea gave 159. Complex 159 was prepared in better yield by protonation of 158 (X = 02CMe)in the presence of tetramethylthiourea and is the first heptarhenium-mercury complex containing a neutral ligand on the mercury atom, the possibility of ligand exchange demonstrated by substitution with PEt3.'89

13.4 Group 8. - Fe, Ru - Co, Ir. Co2{p-~2-(5-alkynylcyclopentadienol)}(CO) complexes reacted with Fe(CO)5to give FeCo2 conmpounds in which the iron atom is linked to the dicobalt unit by a bridging hydride.'" A 57Feand 1931R Mossbauer spectroscopic study of the tetranuclear cluster dianion [Fe21r2(C0)12]2-,and of the cluster supported on hydrated MgO, has been reported; results show that the cluster is physisorbed without transformation or decompo~ition.~~~ Phosphine/arsine substitution on FeCo2(y3-C= CHPh)(C0)9 afforded 160 - 162, where the orientation of the vinylidene ligand varies with the incoming ligand.'92 Addition of C O ( C O ) ~ Cto ~ *the triruthenium methoxynitrido cluster Ru3(pH)2(p3-NOMe)(C0)9afforded a series of nitrido and nitrene clusters 163 - 166. The Ru2Co cluster 164 is formed via a metal-exchange reaction of a ruthenium

332

Organometallic Chemistry

(158) X = CI,NO3, SC6H4Br, 1, Br, 02CMe

(159)

vertex by an isoelectronic Co(CO)Cp* unit, whereas the pentaruthenium complex 165 is the result of the intermolecular transfer of a Cp* ligand, and represents the first example of a pq-NH ligand on a pseudooctahedral Ru5framework. Complex 163 was deprotonated to 164, and reacted with diphenylacetylene to give 167 (where the metal-bound hydride ligand has been transferred to the nitrido group) and Dipropargyl esters have been used to link triangular cluster units derived from RUCO~(CO)~ H

12: Organo-transition Metal Cluster Complexes

333

The pentanuclear cluster dianion [ R U ~ I ~ ~ ( ~ - C O ) ~ ( Cformed O ) ~ ~from ] ~ - , reaction of Ru3(C0)12 with [Ir(C0)4]-, was protonated to give [ R U ~ I ~ ~ ( ~ - H ) (-.C O ) ~ ~ ] A structural analysis of the latter showed two independent anions with different CO ligand arrangements in the asymmetric Fe, Ru, 0 s - Ni, Pd, Pt. - Decomposition of PdC12{(q1-PC4Et4-q5)FeCp}2 or reaction of PdC12{(q1-PC4HMe2Ph-q5)FeCp)z with Pd(dba)2 (dba = dibenzylidene acetone) afforded tetranuclear Fe2Pd2complexes in which the iron atoms coordinate in a dative fashion to the palladium atoms.196Reaction of [Fes(p5-C)(CO)14]2with PtC12(cod)(cod = cyclooctadiene) or PtC12(PMe2Ph)2 gave the FePt mixed-metal carbide clusters 169 and 170, re~pective1y.l~’

Hexanuclear Ru-Ni clusters 171 and 172 were obtained from reaction of nickelocene with RU~(~~-C~)(~-SM~)~(~-PP~~)~(CO)~~. The R&Ni2 cluster 172 contains a disordered C2ligand located parallel or perpendicular to the non-

334

Organometallic Chemistry

bonding Ni . . . Ni vector; extended Hiickel studies suggest that the two forms are isoenergeti~.'~'

)(\ a /\\

\I

F F e - k .--yPt'

Pt

\

'Fev /

4 _ - - c .-7 Fe 'I \

/

PMe2Ph

- *- __--- .p\ \

0

Fe

(170)

(169)

The tetraruthenium butterfly clusters RU&~-PR)(CO)~~ (R = NPri2,F) reacted with Pt(q2-C2H4)(PPh3)2 to give the mixed-metal phosphinidene complexes 173 and 174. Reaction of 173 (R = NPri2)with HBF4.H20 gave the fluorophosphinidene complex 173 (R = F), together with 175, the first mixed-metal cluster containing a p4-phosphorus monoxide ligand, and reaction of 173 (R = F) with ethanol afforded the alkoxyphosphinidene complex 173 (R = OEt).'99

;-

\

C-Ni-Cp

Cp4'"'\T PPh2

PPhc, . ..

P -.=

I

I

Ru

Ru

\

(173) R = NPS2, F, OEt

(174) R = NPS2, F

(175)

A series of Ru3Pt clusters, 176 - 180, has been prepared from the reaction of Pt(nb)3-,(PPri3), (n = 1, 2; nb = bicyclo[2.2.l]hept-2-ene) with a variety of

12: Organo-transition Metal Cluster Complexes

335

triruthenium complexes. The butterfly cluster 176 was formed from R U ~ ( C O ) ~ ~ , while 177 and 178 were prepared from Ru~(~-H)(~~-~'-M~CCHCM~)(CO and R~3(p3-q~-PhC2Ph)(CO)~~, respectively. The nitrogen-containing products 179 and 180 resulted from the reaction withRu&-H)(p-NO)(CO)lo. Complex 179 is the first spiked-triangular cluster with a p4-N ligand. Complex 180 contains a p4-q2-nitrosylgroup, and is not a reaction intermediate en route to 179;the latter could not be converted to 179 on standing under nitrogen.2wThe mononuclear carbonyl complex Ru(CO)~reacted with Pt(nbh with loss of CO to give two polymorphs (one previously unreported) of R U ~ P ~ ~ ( C O ) , ~ . ~ ~ ' The spiked-triangular cluster 181 was formed from the treatment of OS~(CO)~~(NC with M ~ci~-Pt(C=CPh)~(4,4'-R~-bipy); )~ the complex contains an unusual p4-q211 -coordinated alkynyl unit.202Mixed-metal 0s-Pt clusters containing the 1,4-bis(ferrocenyl)butadiyneligand (182 and 183) have been prepared ) ( CPt(c0d)2.~'~ O)~~ from reaction of O S ~ ( ~ ~ - F C C ~ F Cand PPi3

PPr'3

I

s

Pt

-RIA/ '

/

pj3p

/-\

Pe, Ru - Ag, Au. - The tridentate ligand Fe2(p-L)(CO)*[L = 2,6-bis(diphenylphosphino)pyridine], prepared from Fe(C0)5,reacted with silver perchlorate to give 184, with mercury(I1) chloride to give Fe2Hg2(p-L)C14(CO)s,and with mercury(I1) acetate to give 185 and 186.*"" A series of ClAu-terminated dendrimers have been reacted with [Fe2(pPPh2)(p.-CO)(CO)6]- and [Fe3(CO),1]2- to form dendrimers containing mixedmetal clusters on the periphery, the largest containing 192 Fe2Au The unstable pentaruthenium boride anion [ R U ~ ( ~ ~ - B ) ( C Ocan ) ~ ~be] -stabilised by reaction with gold phosphine agents, with 187 being isolated from the reaction with [(Ph3PAu)30] (although the structural study was sufficient to determine the metal connectivity only).206 +

Organometallic Chemistry

336

(181) R = Me, But

Fc

/

\

Me

(186)

13.5 Group 9. - Rh, fr - Pd, P t , Au. A series of RhPt, and IrPt2 complexes (188 189) containing a Pt-Pt covalent and Pt + M dative bond have been prepared from reaction of [Pt2(p-dpmp)z(CNXyl)2 J2 [dpmp = (Ph2PCH&PPh] with { MCl(cod)}z(M = Rh, Ir). Nucleophilesreact at the electrophilic Rh centre, and electrophilesat the nucleophilic Pt2 unit. Complex 188 reacted with isocyanides with addition at the group 9 metal, and 188 (M = Rh) added CO reversibly in a +

337

12: Organo-transition Metal Cluster Complexes

bridging coordination between Rh and Pt atoms, whereas reaction of 188 (M = Rh) with the electron-deficient alkyne HC=CC02Mewas site-selectiveinto the Pt-Pt bond, giving 190.207

-

Ru

Au

rh - -I -& I I

Ph2P

I XylNC Pt

Pt

&,-

Ph2P

-

Au

(187)

PPh2

12+

Ph2y

2

XylNC Pt

I ‘Pt:--;M\

CNXyl

CI’I

PPh2

(188) M = Rh, Ir

Ph2P

I

P

-

‘

P

rhI

PPh2

CI

12+

I ,CNXyl I

ph2pb PPh2 (189) M = Rh, Ir

JPPh2

Ph (190) R = C02Me

Trinuclear complexes 191 and 192 were formed from the reaction of Ir&SH)ZC12Cp*2and MClz(cod)(M = Pd, Pt) or Pd(PPh&, respectively. Phosphine substitution with PPh3 and dppe afforded a series of cationic phosphine complexes, with the dppe chelating the group 10 metal atom. Complexes 191 catalysed the addition of alcohols to alkynes to give the corresponding acetals.208

CI

.

CI ’

(191) M = Pd, Pt

(192)

The digold dithiolato complexes AU~(V-S-S)(PR~)~ (S-S = benzenedithiolate, toluenedithiolate; PR3 = PPh3, PPh2Me) reacted with [M(cod)2] to give linear clusters 193.209 +

13.6 Group 10. - Pd, P t - Ag, Au. Reaction of Pt(phpy)z (Hphpy = 2-phenylpyridine) with Ag(C104) in acetone gave [{ P t ( ~ h p y ) ~Ag(a~etone)}~f, )~{

338

Organometallic Chemistry

(C104)2,,.n(acetone),consisting of a helical chain of alternating Pt and Ag atoms connected by Pt -+ Ag dative bonds. In contrast, a similar reaction with P t ( t h ~ y ) ~

m

R3P,

Au-M-AU

/ \

s

I+ 0

PR3

s

R' ' (193) M = Rh, Ir R = PPh3, PPh2Me R' = H, Me

[Hthpy = 2-(2-thienyl)pyridine] gave [{ Pt(thpy)2}3{ Ag(acet~ne))~] (C104)2.acetone,made up of alternating Pt and Ag atoms connected similarly by Pt Ag dative bonds, but in this case terminating at the pentanuclear Pt-Ag-PtAg-Pt The complexes M{q2-S2C= C{C(0)Me}2L2 (M = Pt, Lz= cod; M = Pd, Pt, L = PPh3) reacted with Ag(C104)to give the tetranuclear species 194,*" and reaction of the heptanuclear Pd-Au cluster dication [PdAu6(PPh3)7l2+with 2,6-dimethylphenyl isonitrile gave 195.?12

-

(194) M = Pt, L2 = 1 3 -cyclooctadiene

M = Pd, Pt, L2 = 2PPh3 Ligands omitted for clarity

13.7 Group 11. - A brief review on the interaction of closed-shell d'' metal centres with thallium(1) has been p~blished."~ 13.8 Clusters Containing Three Different Metals. - Reactions of Cp2Ti(pSH)2RuClCp* with {M(cod)}&-Cl), (M = Rh, Ir) or M(PPh& (M = Pd, Pt) afforded 196 and 197, respectively, the first heterotrimetallic clusters containing Ti. The labile C1 and PPh3ligands on 197 provide sites for further reactivity and reaction of 197 (M = Pd) with H 2 0in the presence of base gave the hexanuclear complex 198, which inserted dimethyl acetylenedicarboxylate into the Pd-(p3-S) bond to give 199.162, 214 A study of metallosite-selective C O substitution at MFeCo(p,-S)(CO),(q'C5H4R)[M = Mo, W; R = H, C(O)Me, C02Me] with CyNC has afforded the series MF~CO(~~-S)(CO)~-,,(CNC~),,(~~-C~H~R) (M = Mo, n = 1-3; M = W, n =

339

12: Organo-transition Metal Cluster Complexes /

(196) M = Rh, Ir

CP

Ph3P

\ I

PPh3

(197) M = Pd, Pt

PPh3

PPh3

I

(199)

COOMe

1-2); site selectivity decreases as M(CO)2(q5-C5H4R) (M = Mo, W) > Fe(C0)3> C O ( C O ) ~The . ~ ~oxygen ~ transfer reactions of F ~ C O ~ ( ~ ~ - S ) ( MFeCo(p3CO)~, ~ ~=- C ~ H ~ R ) E)(CO)8(q5-C5H4R)(M = Mo, W) and [ M o F ~ C O ( ~ ~ - E ) ( C O ) ~ ] ~ ((E S, Se; various R) with (p-MeOC6H4)2Te0(BMPTO) have been described and shown to be selective towards the Co atoms; subsequent substitution of the 217 BMPT ligands by PPh3 was also dem~nstrated?’~, The preparation of M R U C O ( ~ ~ - S ~ ) ( C O ) ~ ( ~ (M ~ -=C Mo, ~ H ~W; R )various R), some hydrazine derivatives and a number of chiral clusters MoMCo(p3X)(CO)8(q5-C5H4R)(M = Fe, Ru; E = S, Se in various combinations) with functionally substituted Cp ligands has been reported, but the diastereoisomers could not be separated by chromatography on silica ge1.218-221 Similarly, the preparation of M F ~ C O ( ~ ~ - S ~ ) (q5-C5H4C(0)CH2CH2C02Me} CO)~{ (M = Mo, W) and some amine and hydrazine derivatives have been described.218 Cluster expansion of W R U ~ ( ~ - S ) ~ ( C Oor) ~W CR ~ ’U~~ ( ~ L , - S ) ( ~ - S ) ~ ( C O ) ~ ( S ) C ~ (Cp’ = q5-C5Me5,q5-C5Me4Et) with one or two equivalents of PtMe2(cod) afforded the series 200 - 202. Treatment of 200 and 202 with excess HCl resulted in chlorine substitution of one of the methyl groups of each of the PtMe, moieties; the monochlorinated derivative of 202 was accessible quantitatively via reaction with one equivalent of HCl. The pentanuclear cluster 203 formed from the reaction of W R U ~ ( P - S ) ~ ( C Owith )~C~ Pt(C2H4)(PPh3)2.222 ’~ The heterobimetallic octahedral cluster dianion [ M O F ~ ~ ( ~ ~ - C ) ( C Oreac),,]~ted with electrophilic metal fragments [M(PPh3)]+ (M = Au, Cu, Ag), [Au(PMe3)]+, [MHg(C0)3Cp]+ (M = Mo, W) with selective addition on the triangular MoFe2 face. Reaction with [Au2P2I2+[P2 = dppm, dppe, 1,3bis(dipheny1phosphino)propane (dppp)] afforded a linked cluster for dppe and dppp, and complex 204, containing the digold fragment in a novel (p3-q2-) bridging mode, in the case of d ~ p m . ’ ~ ~

340

Orgunometallic Chemistry Me

Me

b

Me Me \/

\

Ph2P

The trigonal prismatic [Cob(p&)-( CO)ls]2- and octahedral [Cob(p.6' C)(CO)13]2-cluster dianions reacted with equimolar [HgM] [M = W(CO)3Cp, M o(CO)3Cp,Fe(CO)zCp, Co(CO)4, Mn(CO),] to give [MCo6Hg(p6-C)(CO)15]and [ M c ~ ~ H g ( ~ - c ) ( C-,o respectively. )~~] The complexes retain the original cluster geometry on face-capping with the mercury fragment, and were interconverted on addition or elimination of CO. Reaction of both clusters with excess [HgM] gave the doubly-capped octahedral cluster dianion [M2C06Hg2(p6C)(C0)12]2 - .224 +

+

References 1. 2.

P. J. Dyson and J. S. McIndoe, 'Transition Metal Carbonyl Cluster Chemistry', Gordon and Breach, Amsterdam, 2000. M. G. Richmond, Coord. Chem. Reu., 2001,214,l.

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7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

34 1

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12: Organo-transition Metal Cluster Complexes

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343

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122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133.

134. 135.

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13

Complexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel, Including Carbenes and Carbynes BY PHILIP J. KING

1

Introduction

The format of this chapter will follow that used in previous volumes. Section 2 will deal with any review articles that are of relevance to this chapter. Section 3 will focus on articles relating to metal-carbon 0-bonds involving Group 8,9 and 10 metals, whilst Section 4 will be concerned with the carbene and carbyne complexes of those metals. Over 500 journal articles of relevance to this Chapter were published during the year 2001 and limitations of space prevent the inclusion of them all; that this involves the omission of some interesting pieces of research is of necessity rather than by desire. Concerning the articles contained within this review, descriptions are intentionally brief and where there are several references concerning similar structure types (e.g. Pt-Me bonds), these have been afforded a more general overview. In all cases, the reader is referred to the original reports for more information. 2

Reviews and Articles of General Interest

Several general review articles have appeared including annual surveys on the topics of organometallic cluster chemistry,' the application of transition metals in hydr~formylation'~and the chemistry of carbon-transition metal double and triple bonds! The topics of transition metal-carbon multiple bonds: factors governing the equilibria between metal-alkyls, carbenes and carbynes; formation of aminocarbene~~ and aminocarbynes*from isocyanide complexes,carbyne complexes derived from lithiated heterocycles by transmetallation: development of olefin metathesis catalyst precursors bearing nucleophilic carbene ligands,'O catalyst selection for metal carbene transformations," assymetricintermolecular C-H activation by rhodium carbenoid intermediates,12cyclopropenation of organometallicvinylidene complexes' and the reactivity of allenylidene and cumulenylidene c~mplexes'~ have all been reviewed. Accounts of 2-fury1 phosphines as ligands for transition-metal-mediated organic ~ynthesis'~ and complexes conOrganometallic Chemistry, Volume 31

0 The Royal Society of Chemistry, 2004

349

3 50

Organometailic Chemistry

taining trans-spanning diphosphine ligands16have appeared as have reviews on metal la benzene^,'^ non-metathesis ruthenium-catalysed C-C bond formation,18 patterns of reactivity of thiophenes in transition metal chemistry’’ and wateraccelerated organic transformations.20 Articles concerning computational studies into ethene insertion into first-row transition metal-methyl bonds;l the nature of the bonding in transition metal metal-dihydrogen and o-bond c0ordination,2~and electronic effects on the stability of isomeric alkyl complexes24have been published. Finally, reviews concerning rhodium(I1) mediated cyclizations of diazo alkynyl C-H activation and coordination chemistry of rhodium and iridium trispyrazolylborate complexes;6 platinum(1V) hydride ~hemistry;~ platinum group ‘pincer’complexes as sensors, switches and and palladium catalysed Reppe carb~nylations~~ have appeared.

3

Metal-Carbon a-Bonds Involving Group 8,9 and 10 Metals

3.1 The Iron Triad. - Hybrid density functional studies into the addition of hydride to iron carbonyl phosphine and phosphate complexes have shown that attack at a carbonyl ligand and formation of acyl complexes is energetically fa~oured.~’Addition of nucleophiles to the alkyne ligand in [Fe(q5CsH5)(C0)2(q-C2Ph2)][BF4)affords the corresponding alkenyl complexes, the product stereochemistrydepends on the basicity of the n~cleophile.~~ The alkyne complex [Fe(C0)2{P(OPh)3}2(q-C2Ph2)](1) undergoes rapid phosphite substitution upon reaction with phosphites and phosphines, whilst, reaction with CO affords ferrocyclopentenedionecomplexes 2 via insertion of CO into both Fealkyne bonds.32Thermolysis of [Fe3(C0)12]in the presence of l-ethynylcyclohexanol affords the ‘ferrole’ complexes [Fe2(CO)5(p-CO){ p-q2,q4, R3 = H; R = C ~ H ~ O OR2 H, C(R)C(R’)C(R’)C(R’)}](R = R2 = C ~ H ~ O OR’H = = C6HI0,R’ = R3 = H) and the tinuclear complexes 3,4 and 5.33Reaction of

0

(2a)L = CO (2b) 1. = P(OPh)3

*

13: Complexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel 351

I

Ph2P-Fe-

CZ

C- C E C

ph213

C E C -Fe-PPh2

-SMe3

!Ph2 (7)

(6)

[Fe(q5-C5Me5)(dppe)(Cl)] with the 1,3-diynesRC=CC=CSiMe3(R = H, SiMe3) offers a direct route to the butadiynyl complexes 6 and 7 depending on the reaction conditions? The diyndiyl complex [{ Fe2(C0)6(p-PPh2)}2(p-C~CC1C)] has been shown to react with P(OMe)3or NHEt2 to give products derived from addition to the a-carbon and facile P-C, N-C and/or C-C bond formation.35A range of a-substituted alkenyl complexes [Fe2(C0)&-PPh2)(p-RC= CH2)] (R = Ph, Me, “Pr, “Bu or ‘Bu) has been prepared and their isomerisation to the p-substituted alkenyls [Fe2(co)6(p-PPh2){ p-HC =C(R)H}] studied? The migratory insertion and CO substitution reactions of the P,y-unsaturatedesters and amides [Fe2(C0)5(p-PPh2){ p- q1,q2-NuC(0)CH2C = CH2}] (Nu = OMe, OEt, OPr, NHPh, NH‘Bu) have been studied and found to afford products containing either p-acyl or p-alkenyl ligands?’ The dinuclear complex [Fe2(C0)6{pC(Me)O}2]has been synthesised via alkylation of [Fe2(CO),(p-COLi(THF)3}2] with MeS03CF3.38 The iron complex 9, which contains two bidentate tied-back phosphonite ligands, has been synthesised through alkylation of an iron dichloride precursor 8.39

M%Mg Et

(8)

Et

(9)

A theoretical study into the participation of q3-allylruthenium(I1) complexes in C-C bond formation and bond cleavage has suggested the intermediacy of ql-ally1complexes.4oExperimental and theoretical studies into the formation of the alkynyl complexes [M(CO){CH =CH(R)}(Cl)(PiPr3)2](M = Ru, Os), through reaction of alkynes HCaCR with [M(C0)(H)(C1)(PiPr3)2],have shown the rate of product formation to be dependent on the electronic properties of the alkyne substituent? The vinylidene complexes [ R U ( C ~ ) ~ ( P C =~C~=) ~ ( C(H)’Bu}] and [Ru(C~)(H)(PC~~)~( = C =CH2)] react with 1,2-C2H4(PCy2)2 to =C = C(HrBu}] and alkynyl afford the vinylidene [Ru(C1),(Cy2PC2H4PCy2){ [Ru(Cl)(H) (Cy2PC2H4PCy2)(= CH = CH2)] chelate species, respectively!2 Reaction of propargyl alcohols HC=CC(R)(R’)OH with [Ru(q5-

352

Organometallic Chemistry

C5Me5)('Pr2PC2H4PiPr2)(Cl)] and NaBPh yields the corresponding hydroxyalkynyl species [Ru(q5-C5Mes)~Pr2PC2H4PiPr2)(H){CaCC(R)(R1)OH}][BP~], which irreversibly isomerise in solution to the vinylidene complexes [Ru(q5C5Me5)('Pr2PC2H4PiPr2){ = C = C(H)C(R)(R')OH}] [BPlq]!3 A new family of enantiopure planar-chiral ruthenium vinylidene complexes 10 has been reported in

I

H ( 1 0 ~R ) = Me, R' = Ph (lob) R = Ph, R' = Ph (1Oc) R = Me, R' = H (l0d)R = Ph, R' = H

-

(1Oe) R = Me, R' = Ph (lot) R = Ph, R' = Ph (log R =Me, R' = H (1Oh) R = Ph, R' = H

which the redox properties of the ruthenium can be finely tuned by varying the substituent on the cyclopentadienylligand.44Deprotonation of the bis(dipheny1phosphino)methane (dppm) ligands of the vinylidene [Ru(q5C9H7)(dppm){=C =C(H)R}][CF3S02] (R = 'Bu, Ph) and carbene [Ru(q5C9H7)(dppm){ = C(0Me)CH= C(H)R}][PF6] (R = H, Ph) complexes results in intramolecular attack of the methanide group at the a-carbon of the vinylidene and y-carbon of the carbene and formation of novel metallacyclic species.45 ( ~ ~ -1,lC~M~~)~ Reaction of the triruthenium complex [ R u ~ ( ~ - H ) ~ ( ~ ~ - H ) ~ with disubstituted alkenes at room temperature affords the corresponding p3-vi{ R = C02Me, nylidene complexes [Ru3(p-H)3{p3-C =C(R)CH2R'}(q5-C5Me5)3] R' = H; R = R' = COCHS2}.Thermolysis of the vinylidene species affords the corresponding alkyne-alkylidynes[Ru3(w - H ) ~ ( ~ ~=- CR')(p3-CH)(qS-CsMe5)3] RC via formation of an ally1 complex followed by carbon-carbon bond cleavage? The allenylidene complex [Ru(Cl)(= C =C = C P ~ ~ ) ( K ~ - P , O Cy2PCH2CH20CH3)2][PF6] has been prepared and found to react with acid to form the dicationic carbyne complex [Ru(Cl)(=CCH= CPh2)(~2-P,0Cy2PCH2CH20CH3)(~-P-Cy2PCH2CH20CH~)][PF6]2!7 The formation of ruthenium aminoallenylidene complexes via an ma-cope rearrangement of butatrienylidene intermediates has been studied using spectroelectrochemical and computational methods? Treatment of the acetylide complex [R~(Cl)(C&C=CSiMe~)(dppe)~l( 11)with ferrocenium hexafluorophosphateaffords the novel dinuclear complex 12 via an unprecedented [2+2] coupling reacti0n.4~The diruthenium species [ R U ~ ( C ~ ) ~ ( ~ - S M ~ ) ~ ( $ has - C ~been M~~)~I found to act as a catalyst in the propargylic alkylation of propargyl alcohols. Experimental evidence suggests the allenylidene complexes [Ru*(C~)~( pSMe)2{= C =C =C(H)R}(q5-C5Me5)2] (R = Ph, o-MeC6H4)are formed as intermediates in the reaction.MThe reactivity of the triruthenium p3-allenylidene complexes [Ru~(CO)~(~-CO){ p3-C=C =CPh(R)}] (R = Ph, Me)51 and

13: Complexes Containing Metal-Carbona-Bonds of the Groups Iron, Cobalt and Nickel 353

n

[Ru3(CO)&.-H)(p3-C=C =CH$f2 towards alkynes5', silica-gel5', and dppeS2has been reported. The reactions involving the bidentate phosphines yield complexes containing highly unusual heterocyclic phosphorus ligands. The dinuclear arylacetylide complexes [(q5-C5H5)Ru(PR3)z(p-q1,q6C~Cc6H4Me-p)Ru(rl~-C5Mes)l[Cl] (R = Ph, Me) have been synthesised from monoruthenium acetylide precursors and their reactivity towards acid, iodine and carbon monoxide ~tudied.'~The first cationic (arene)ruthenium acetylide [(cym)Ru(phen)(q'-CXR)]i-(cym = q6-4-methylisopropylbenzene;phen = 1,lO-phenanthroline; R = H, Ph, SiMe3) and (arene)ruthenium bis-acetylide [(cym)Ru(PMe3)(q'-C=CPh)2] complexes have been synthesised and characterised using x-ray crystallographic technique^.'^ A ruthenium(I1)acetylide complex connected to a barbituric acid through a n-conjugated bridge (complex 13)has

'

Ph2P

\

'PPh2

I

been synthesised and found to have two switching modes? One-electron reductions of the metallacumulenes 14 and 15 with cobaltocene afford radical species with the unpaired electron localised on the trisubstituted carbon atom of the cumulene moiety. Quenching of the radical species using Ph3SnH affords the corresponding acetylide complexes 16 and 17?6The heterobimetallic p-acetylide complexes [(~5-C5H5)Ru(PPh~)(~-q',~2-C-CPh)(p-E)Zn(rl5-C~H5)~] (E = S, Se)

354

I

Organometallic Chemistry

n

Ph2P\ C l - R u/pph2 tC=CkC, Ph2P’

L

/

u ‘PPh2

R R

(ii) Ph,SnH

(14a)R=Ph;n= I (14b) R = Me; n = 1 (15) R = Ph; n = 2

(16a)R=Ph,n= 1 (16b) R = Me; n = 1 (17) R = P h ; n = 2

have been synthesised from reaction of [(q5-C5H5)Ru(PPh3)2(EC=CPh)] with [(q5-C5H5)2Zn(C1)2] and found to undergo phosphine displacement reactions with nu~leophiles.~~ The stereoselective synthesis of chiral (E)-1,2-enynes from the aldehydes (1R)-(-)-myrtenal and (S)-(-)-perillaldehyde using the acetylidephosphonio complex [(q5-C9H7)R~(PPh3)2{ C=CCH2(PPh3)}][PF6]as a synthon has been reported.” Thermolysis of the bridging acetylide complex [Ru3(C0)9(p-q2-SC=CSiMe3)(p,-q2-C=CSiMe3)] leads to rupture of the S-C bond and C-C bond formation between the two acetylide ligands to afford [R U ~ ( C O ) ~ ( ~p3-q2-C( ~ - S ) { SiMe3)C(C=CSiMe3)}], [Ru2(C0)6(p-q3SC=CSiMe3)(p-q2-C=CSiMe3)], [RU~(CO)~~(~-S)(~-~~-C=CS~M~~ and [R&(C0)9(p-C0)2(p4-S){ w-q2-C(SiMe3)C(C=CSiMe3)}].59 Addition of tetracyanoethene to the acteylide complex [(q5-C5Me5)Ru(PPh3)2(C=CPh)] affords the allylic complex [(q5-C5Me5)Ru(PPh3){ q3-C(CN)2C(Ph)C= C(CN)2}]. Thermolysis of the allylic species affords mono-, di- and tri-ruthenium complexes each containing isomeric froms of the C4(CV4Phligand.@ The mono- and bis-acetylide complexes 18 and 19 have been structurally characterised and their

C G C -C

Cl-Ru-Ru-

C -%MeJ

(18)

Me3Si-

C

C -C G C -Ru-Ru-

C EC

-C G C: -%Me3

(19)

redox properties studied.61A facilitated dn-pn* transition has been observed in a boron -ruthenium conjugated system prepared by hydroboration polymerisation between mesitylborane and the ruthenium tetrayne complex [R~(dppe)~(qlC=CC6H4C=C)2].62 DFT calculations on a series of ruthenium(I1) polyynyl complexes suggest their electronic structures are best described in terms of a strong a-bonding component and a weaker interaction between the filled metal dorbitals and filled polyyne ~c-orbitals.~~ The acetylide complexes [(q5C5H5)Fe(CO)2(C=CH)], [(q5-C5Me5)Fe(C0)2(C=CH)] and [{(q5C5Me5)Fe(CO)2}2(p-C=C)] have been reacted with [ R U ~ ( C O )to ~ ~form ] a variety

13: Complexes Containing Metal-Carbon 0-Bonds of the Groups Iron, Cobalt and Nickel 355

of mixed metal dicarbide cluster compounds.64A novel mixed-metal dehydroannulene complex has been synthesised in low yield from reaction of [Ru3(C0),,] with 1,8-bis(ferrocenyl)-1,3,5,7-0ctatetrayne~~ The five-coordinate and six-coordinate vinyl complexes [Ru(NCMeh(PCy3)2(CH= CH2)][PF6] and [RU(NCM~)~(P'P CH ~ ~=) ~ { C(H)Ph}] [PF6]have been synthesised from vinylidene precursors. Exchange of the cation in the five-coordinate species for [BF4] followed by protonation affords [RU(NCM~)~(PCY~)~{ = C(H)CH3)][BF4]2, the first example of a dicationic ruthenium(I1) carbene A series of indenyl-ruthenium(I1) vinyl complexes has been synthesised via addition of terminal alkynes to indenylruthenium(I1)halide precursors. The reactions are found to proceed through the initial formation of an equilibrium mixture of q2-alkyne/q1-vinylidenetautomers which has been studied using AB initio te~hniques.6~ Treatment of the ruthenium cyclopropenyl complexes 20a and 20b with Me3SiN3affords the nitrile (21) and the tetrazolate (22) species, respectively.68 The rate of alkyne

I

Ph3P'

R (20a) R = Ph

(20b)R = CN

R

Ly

(Ua) R = Me (23b) R = C1 (a) R = OMe

insertion into the ruthenium-carbon bond of 23 has been studied and found to increase with increasing electron withdrawing power of the R-substituent.@' The novel diruthenium fury1 complex [Ru~(CO)~{ p-P(C(CH)30)}{p-q'-q2C(CH)30}J has been synthesised and its reactivity towards terminal alkynes investigated.'* There have been several reports concerning the reactivity of triruthenium complexes containing bridging imido71, 1-a~avinylidene~~ or amid~pyridine~~.~' ligands towards alkynes71p72-74 and d i y n e ~leading ~ ~ , ~ to ~ the

356

Organometallic Chemistry

isolation of a range of novel cluster complexes containing Ru-C a-bonds. The cycloruthenated azobenzene complexes [Ru(PPh3)2(X)(CS)(q2-C,NC6H4N=NPh)] (X = Cl, Br, I) have been synthesised and characterised. The solid state structures of the complexes X = C1 and X = I show significant changes in the bond lengths of the cyclometallated azobenzene ligand relative to free azobenzene, which may be explained in terms of a cis-push-pull effect.75The synthesis of a series of ruthenium-phenyl complexes containing phosphorus ligands has been reported and the kinetics and stereochemistryof their P-C bond making and bond breaking processes Reaction of the ruthenium aryl (R = Et, pcomplex [RU(PP~~)~(CO)(NO~)(C~H~OH-~-CHNR-~-M~-~)] MeC6H4,p-ClC6H4)with terminal alkynes H C d X (X = H, Ph, CH30H)leads to linkage isomerism of the nitrite ligand and formation of a six-membered vinyl-phenolato chelate ring to give [Ru(PP~~)~(CO)(NO~)( q2-c6H2CXCH-1-02-CHNHR-3-Me-5)]?7 A series of azophenol complexes has been synthesised by insertion of oxygen into the ruthenium-carbon bond of the corresponding azophenyl complexes.78The ruthenium complexes 24 containing q3-P,C,Pbonded ligands have been synthesised in good yield. Complex 24 is of particular interest as it is soluble in fluorinated solvents and so may find a use in fluorinated biphasic systems.79

(24a) R = Br

(24b)R = Si(n-CHzCHzCgF17)3 (24c) R = H

The complexes [Me2C(q5-C5H5)2R~2(CO)4] and [(q1,q4-C5H4)Me2C(q5C5H5)Ru2(H)(CO),] may be interconvertedphotochemically and thermally. Such a photo-thermal equilibrium is the first example of a thermo-optical organometallic switch based on recversible C-H bond breaking and making.*' The presence of a chelating tether attached to ethynylguanine leads to the formation of a metal-carbon bond at C8 of the nucleobase upon reaction with trans[RuC&(dm~o)~] (dmso = dimethylsulfoxide).81The ruthenium azirinyl complexes [(q5-C5H5)Ru(PPh3)2{ C =NC(H)R}] (R = CN, CH =CH2,Ph) are found to exhibit reversed regiospecificity in their carbonyl insertion reactions.82Reaction of the (indeny1)ruthenium complex [(~'-C,H,)RU(PP~~)~(C~)] with dichloromethane affords [(q5-C9H,)Ru(PPh3){CH2P(Ph)2C6H4}]via incorporation of a methylene moiety between the Ru-P bond and orthometallation of the triphenylphosphine ligand to form a five-membered cyclic phosphorus ylide ~tructure.8~ Two reports have appeared on the synthesis, electrochemistry and reactivity of ruthenium and ruthenium-cobalt nitrid0,8~9~~ n i t ~ e n eand ~ ~imidos5

13: Complexes Containing Metal-Carbona-Bonds of the Groups Iron, Cobalt and Nickel 357

clusters towards alkynes. Two nickel-ruthenium clusters containing C2 ligands have been structurally characterised and the bonding modes of the C2 ligands rationalised using extended Huckel and density functional calculations.86 A series of mononuclear iron and osmium complexes containing one or two methyl groups bound to the metal has been prepared with the structures of the products varying according to the nature of the metal ~entre.8~ The osmium complexes [(q5-C5Hs)Os(PiPr3)(EPh3)(H)(Cl)] (E = Ge, Si) react readily with LiCH2CN to afford the substituted cyclopentadienyl derivatives [(q5C5H4EPh3)Os(PiPr3)(CH2CN)(Hb]. In addition, employingeither MeLi or "BuLi in the reaction leads to orthometallation of one of the phenyl groups and Reacformation of the novel complexes [(q ',q5-C5H4EPh2C6H4)0s(PiPr3)(H)2].88 tion of the bis(trimethylsily1)methyl osmium complex [ O S ( C ~ ) ~ ( N ) ( C H ~ S-~ M ~ ~ ) ~ ] with two equivalents of AgBF4in acetonitrile solvent affords the lightly ligated species [o~(NCMeh(N)(CH~siMe3)~1+, which combines readily with [Pt(dppe)(SSiMe3)J to yield trimetallic [{ Os(N)(CH2SiMe3)2}2( p3-S)2Pt(dppe)].89 The osmium complex [Os(Cl)s(NH4)2] reacts with PPh2(2,6-Me2CaH3)to yield a cyclometallated osmium(I1) complex 25 containing a tridentate trans-stilbenetype ligand formed via a dehydrogenative carbon-carbon coupling of two phosphine o-methyl groups.go

of the osmium complexes [OS(PP~~)~(CI)( =N = Reaction CR2)(CH =CH(Ph)}]+ (CR2 = CMe2or C(CH2)4CH2} with carbon monoxide leads to an intramolecular coupling between the vinyl and azavinylideneligands and formation of the azaosmetine species [Os(P'Pr&ol)(CO){ = C(H)CH(Ph)N =CR2}] +?I The osmium vinyl complexes 26 have been prepared and complex 26b reacted with tetrafluoroboric acid to yield the dihydrogen species 27.92 Addition of the diynes RC-CCICCH~NHR' (R = Ph or CH2N(H)Ph; R' = Ph or CH2Ph)to [OS~(H)~(CO)~~] affords complexes containing substituted pyrrolyl rings which bridge two osmium atoms in q1,q2-or q',q'-coordination modes depending on the nature of R and R'?3 The reactivity of the imine-vinylidene complexes [OS(P'P~~)$CI)~(NH =

358

Organometallic Chemistry Ph

Ph

I

I

HBF4

R

(26a) X = C1

(26b)X = H

(28)

CR2){CH= CH(Ph))]+ {CR2 = CMe2 or C(CH&CH2} towards amines, "BuLi and HBF4 has been rep0rted.9~A family of half sandwich osmium-vinylideneand osmium actylide complexes containing a variety of substituents on the metal centre and on the cyclopentadienyl ligand has been ~ r e p a r e d ? The ~.~~ thiocarbony1 complex [OS(CS)(CO)(PP~~)~] reacts with propyne to form the acetylide species [OS(H)(CS)(CO)(PP~,)~(C=CM~)] and the osmabenzene complex 28, in A series of triosmium which two propyne molecules have linked tail to complexes containing bridging C=CCMe2PPh3units coordinated in a variety of modes has been synthesised and their isomerisation and carbonylation reactions with ~ ) ~the ] platinum bis(acety1ide) studied?' Treatment of [ O S ~ ( C O ) ~ ~ ( N C M complex [ P t ( c ~ c P h ) ~ ( L{)L ] = 4,4'-dimethyl-2-2'-bipyridine or 4,4'-bis(tertbutyl)-2,2'-bipyridine} at room temperature affords the novel, spiked triangular heterometallic cluster complexes [P~OS~(CO)~(~~-~'-C=CP~)(~'-C=CP~)( Treatment of [ O S ~ ( C O ) ~ ~ ( N C M with ~ ) ~the ] diyne MeC=CC=CMe yields the expected bridging diyne species [Os3(C0),(p-CO)(p3-MeC=CC=CMe)] and a range of complexes containing ligands derived from dimerisation of two molecules of diyne and carbonyl insertion. The reactions of the free acetylenic functionalities of the products with [co2(co)8] were investigated.'@' A series of triosmium clusters containing ligands derived from 1,8-bis(ferrocenyl)octatetrayne has been synthesised, characterised and their electrochemical properties studied."' The osmium(1V) tetraaryl complex [0s(C8H9),] (C8H9 = 2,5-dimethylphenyl) reacts with pyridinium tribromide in the presence of Fe powder to give [ O S ( C ~ H ~ Bwhich ~ ) ~ ~undergoes , Suzuki coupling with arylboronic acids to afford a series functionalised aryls of osmium.'02The synthesis and reactivity of the fiveand six-coordinate aryl-osmium(I1) complexes [Os(CO)(PPh3)2{Si(OEt)3}(aryl)] and [Os(CO),(PPh3)2{ Si(OEt),)(aryl)] (aryl =

13: Complexes Containing Metal-Carbon o-Bonds of the Groups Iron, Cobalt and Nickel 359

phenyl or o-tolyl) has been rep~rted."~The hexahydride complex [Os(H)6(P'Pr3)2] has been found to promote the C-H and C-F activation of aromatic-1wand cycloalkyl-ketones105 to afford a range of mononuclear complexes containing metal-carbon o-bonds. The syntheses, structures and redox properties of mononuclear complexes derived from cyclometallation and N-N bond cleavage of 2-(ary1azo)phenols by osmium have been reported.lo6Cyclometallated osmium complexes containing the tridentate PCP ligands 2,6(CH2PtB~2)2C6H3107 or 1,3-(CHiPr2P)2C6H3108 have been prepared and their structures and reactivity studied. Finally in this Section, the preparation, thermolysis and reactivity of a series of triosmium complexes containing bridging phenyl ligands and the p-block elements Ado9or Sbllo>lll has been reported.

3.2 The Cobalt Triad. - The sterically demanding phenyltris((tert-butylthi0)methyl)borate ligand has been used to provide a sulfur-only environment capable of stabilising tetrathedral organocobalt(I1) functional groups.'12 The cobalt complex [(T~~-C~H~)CO(CO)~] has been used as a template in the formation of pyridine-containing macrocycles from the trimerisation of triply-bonded spec i e ~ , "and ~ in the trimerisation of alkynes in supercritical carbon di0~ide.I'~ The two processes both involve the formation of cobaltacyclopentadiene intermediates. Several articles concerning the role of [CO~(CO)~] in the Pauson-Khand reaction (cyclisationof an alkyne, alkene and carbon monoxide via formation of cobalt hexacarbonyl-alkyne complexes) have appeared. Thus, a report on new advances in the development of reactive cobalt complexes has a~peared."~ Theoretical studies on cobalt hexacarbonyl-alkyne complexes have suggested that the regiochemistry of the Pauson-Khand reaction may be controlled by electronic differences in the alkyne substituents' and have also given information as to the mechanism through which the reaction Electrochemical and spectroscopic studies of the structural and electronic variations in dicobalt-alkyne complexes have been used to formulate an empirical MO scheme."' In addition, a theoretical study of reactive intermediates formed from dicobalt-alkyne complexes has provided much information about their geometries, electronic structures and coinfigurational stabilities.'20 Several novel alkyne-C~~(CO)~ complexes have been synthesised including the chiral,propeller-like species 29,121the tetra-cobalt species30 (formed through the unusual linkage of two alkyne-bridged dicobalt complexes with a urea-based ligand),'22and complexes containing ferr~cenylalkynes'~~ and br~moalkynes.'~~ 16911'

(29) A-enatiomer

Organometallic Chemistry

360 0

The p-alkyne complexes [ ~ o ~ ( ~ ~ ) ~ ( ~ - C F ~ (R C I= I I CF3, C R )H) ] have been reacted with [(q5-CsH5)2C02(pSMe)Jleading to the formation of novel thiolatoalkyne tetra- and tricobalt c l ~ s t e r sExperimental .~~~ evidence for the involvement of a decarboxylation step in the mechanism of metal exchange in propargyl al~ohol-Co~(C0)~ complexes has been obtained.'26Reaction of ($-arene)tricarbonylchromium complexes bearing propargyl substituents on the arene ring with dicobalt octacarbonyl affords the new heterotrimetallic species [Cr{($C6HsC=CC(H)(R)Y)Co2(CO),j}(CO)3] (R = Me, Y = NMe2; R = H, Y = NMe2; R = H, Y = OH).'27Reports concerning the synthesis and reactivity of dicobalt'28and tung~ten-cobalt'~~ complexes having phosphorus or sulfur metallacyclic ligands coordinated in a variety of modes, have appeared. The new mixed-metal vinylidene complexes [FeC02(CO)&-dppm){ p3-C =C(Ph)H}] and [FeCo2(CO~-,,(L),,(p3-C=C(Ph)H}] (L = PPh3, n = 1,2; L = AsPh3, n = 1) have been prepared, structurally characterised and their cyclic voltammetric behaviour investigated.lW DFT studies have been carried out into the Co-C bond energie~,'~' and steric and electronic within cobalamins (models of coenzyme B12).Mechanistic studies into the enzymatic activity of coenzyme B12 have suggested that Co-C bond cleavage is not the rate determining step for enzyme The equilibria and kinetics of the reactions of alkylcobalamins with cyanide'34and the alkyl exchange reactions of organocobalt porphyrin~'~~ have been studied. Reports concerning the synthesis of novel alkyl-cobalt porphyrin complexes containing tridentate-oxime ligand~,"~ Co-N-C three-membered rings'37and water-soluble porphyrin rings138have appeared. A number of catalytic reactions have been reported in which complexes containing rhodium-carbon a-bonds have been proposed, and in some cases identified, as intermediates. The catalytic processes include hydroformylation,139-141 activation of h y d r o ~ a r b o n s , ' ~C=C ~ ~ ' ~bond ~ cleavage,14 hydroarylation of a l k y n e ~ ,G ' ~O ~ - i n ~ e r t i o nand ~~~ allylation ~ ' ~ ~ of The synthesis and solid-state structure of the trifluoromethyl rhodium(1) complex [Rh(CF3)(PPh)3(cod)](cod = cyclooctadiene) has been r e ~ 0 r t e d . l ~ ~ Selective oxygenation of the rhodium ethene complex [Rh(C2€&)(Rbpa)]{ Rbpa = N-alkyl-N,N-di(2-Pyridylmethyl)amine where alkyl = methyl, butyl or benzyl} in acetonitrile by hydrogen peroxide affords the corresponding 2-rhodaoxetane complexes [ R ~ ( K ~ - C , O - C H ~ C H ~ O ) ( N C M ~ ) ( RThe ~ ~ conver~)] sion of the perfluorbenzyl complexes [(q'-CsMe5)Rh(CF2C6F6)(I)] to fluorinated oxarhodacycles through reaction with moist silver oxide has been i-ep~rted.'~~ Experimental studies into the cyclometallation of the chelating P,O-ligand

13: Complexes Containing Metal-Carbona-Bonds of the Groups Iron, Cobalt and Nickel 361

Ph2PC6H4(CH2N(Me)COEt)) at rhodium centres have led to the proposal of some reaction mechanism^.'^^ A rhodium complex containing the chiral dissymmetric phosphine ligand (R)-BINAP has been prepared and its conformation rationalised by DFT calculations.153A neutral 14-electron alkylrhodium complex [Rh(dtbpm){CH2C(MehCH,)J (dtbpm = bis(di-tertbuty1phosphino)methane)has been synthesised and found to be stabilised by an agostic y-C-H interaction with a saturated hydrocarbon group.1s Mononuclear rhodium complexes containing acyl ligands have been prepared via oxidative addition of aldehyde^''^ and through intramolecular migration of a methyl group to a carbonyl unit.'56 Several articles concerning the C-H activation reactions and redox activities of organo-rhodium complexes containing tridentate tris(pyra~olyl)borate'~~-'~~ or tris(pyrazolyl)methanesulfonato'a ligands have been published. The synthesis, structure and photoreactivity of the novel complexes cis,cis[Rh(R)2(I)(CO)(dmb)](R = Me, 'Pr; dmb = 4,4'-dimethyl-2,2'-bipyridine)has been described.I6l A study into the factors affecting the M-C bond strength in porphyrin complexes (including those of rhodium and iridium) has been carried out.162 Three articles documenting the synthesis and catalytic applications of mononuclear rhodium complexes containing N,C,N-163*164 and N,C,P-'65pincer-type ligands have appeared. The rhodium-aryl complex [Rh(dtbpm)(Cl)(2,2'-biphenyl)] is reported to be a catalyst for the coupling of biphenylene with unsaturated organic substrates.166Several dirhodium complexes containing bridging orthometallated aryl-phosphine ligands (complex 31,167for example) have been synthesised and found to have applications as catalysts for the cyclopropanation of styrenes,'67 l-diazo-S-penten-2-0ne,'~*~'~~ and 1-diazo-5hexen-2-0ne~'~~ enantioselective cyclisation of a-diaz~ketones'~'and hydroformylation of alkened7'

The five-coordinate rhodium(1) stibine complexes [Rh(SbPh3)3(CO)(X)](X = Cl, Br) react with a wide range of propargyl species RCdCH2Y to afford the corresponding rhodiacyclopent-3-ene-2-ones [Rh(SbPh3)3(X){q2C( =O)C(R)= C(Y)CHz)], the reactions of which were in~estigated.'~~

362

Organometallic Chemistry

The electrochemical properties of complexes containing two dirhodium units linked by axial acetylide units have been in~estigated.'~~ Regiospecific reductive coupling of the diyne MeC&C=CC&C6H&k with [Rh(PMe3)4(C=CSiMe3)] affords the stable, luminescent complex 32, in high yield.174The allenylidene complexes [Rh(PiPr3)2(X){ =C=C=CR(R')}] (X = I, OH; R = Ph, p C6H40Me;R1 = Ph, o-tolyl, 'Bu, p-C6H@Me) have been synthesised and found to undergo unprecedented insertion reactions of the unsaturated C3-units into Rh-0 and Rh-C The iridium complex [(q'-C5Me5)Ir(PMe3)(Me)(SiMe20S02CF3)] has been reacted with LiB(C6F5)4to form a mixture of the silene and silylene [(q5-C5Me5)Ir(PMe3)(H)(q2-CH2SiMe2)][B(C6F5)4] and [(q5species C5Me5)Ir(PMe3)(Me){SiMe2(OEt2)}][B(C6F5)4], re~pectively.'~~ The diiodobenzene iridium(II1) complexes [Ir(dppe)(R)(Co)(oTf)(q2-C6H412)]2+ (R = CH3, CF3; OTf = triflate) have been prepared and found to be active olefin polymerisation Several M(allyl)3(M = Rh, Ir) complexes have been reacted with a wide range of donor ligands to afford a series of substituted complexes containing a mixture of q I - and q3-allyl Mononuclear iridium complexes containing 3-butenyl ligands have been isolated as intermediates in the iridium catalysed ring-opening isomerisation of methylenecy~lopropanes.~~~ A set of iridium(II1)-methyl complexes containing the sterically demanding hydridotris(3,5-dimethylpyrazollyl)borateligand has been synthesised and their ability ro activate C-H bonds compared with that of the analogous (q5-C5Me5) species.180 The iridacyc1opentadiene,l8l iridapyryliumlS2and iridathiaben~ene'~~ com-

13: Complexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel 363

plexes (33), (34) and (35) have been synthesised and their reactivity towards a variety of reagents, including hydrocarbons, electrophiles and nucleophiles, studied. Three articles describing the synthesis, photophysics and solid-state structures of a range of octahedral IR complexes containing a variety of cyclometalated ligands have appeared.lS4-ls6The iridium PCP-pincer complexes [Ir(H)(OH){q3-C6H3-2,6-(CH2PtBu2)2}]1s7 and [Ir(H)2{q3-C6H3-l,3(CH2PH2)2]18s have been shown to be catalysts for the dehydrogenation of alkanes. The catalyticcycle of the latter has been probed using DFT calculations. P'Pr3

L Ci-Ir-C=C=C /

' PriP

Ph

/

\

R

/

(36a) R = Ph (36b) R = 'Bu

The square-planar allenylidene iridium complexes (36)have been prepared189 and found to undergo a wide range of reactions including substitution react i o n ~migratory , ~ ~ ~ insertion of the and addition of electrophiles.lW

3.3 The Nickel Triad. - Density functional studies have been used to study the dimerisation of metallcyclocumulenes to afford metal substituted radialenes, with bonding and mechanisitic models suggested.'" A series of highly reactive nickel-aryl complexes has been synthesised electrochemically and used in electrocatalytic proce~ses.'~~ The mechanism for the insertion of CO into the Nimethyl bonds of four-coordinate dimethyl-nickel complexes containing bidentate phosphine ligands has been elucidated and found to differ from that of the analogous palladium species.193A Study into the reversible fixation of carbon dioxide at nickel(0) centres has resulted in the formation of several large organometallic rings, dendrimers and tetramers.194 The dinuclear organo-nickel complex 37a has been reported and found to react with both phenylacetylene, affording complex 37b, and MeLi, yielding the Ni2Messpecies 38.19' Addition of HCl to the alkynyl-phosphines [Ni(q2-Cy2PCH2CH2PCy2)(q2Ph2PC=CR)](R = Me, C02Me, Ph) affords the corresponding nickel(I1) complexes [Ni(Cl)(q2-Cy2PCH2CH2PCy2){ K~P,C-C( =CHR)PPh2}], containing a methylenephosphanickelcyclopropane fragment.196Two metallacyclic complexes derived from the oxidative cyclisation of nickel(0) with an alkynyl enal have been isolated and structurally ~haracterised.'~~ The nickelcycles

Organometallic Chemistry

364

L1

I (374 R = Me (37b) R = CCPh

[Ni(Br)(PR3)(~2P,C-C6H4CH2PPh2)] (PR3 = PEt3 or PPhBz2) have been found A series of to support insertion of alkynes into the nickel-phenyl nickelcycles containing di-Schiff bases acting as CNN anionic ligands has been prepared and their fluxional behaviour studied using NMR spectroscopic techn i q u e ~ .A' ~series ~ of nickel-aryl complexes containing P,O-chelate ligands with bulky substituents next to the oxygen donor site has been prepared and used as catalysts for olefin polymerisation?'y201Analogous nickel-aryl complexes containing N,O-chelate ligands have also been reported as highly active olefin polymerisation ~ a t a l y s t s .The ~ ~ ~nickel-aryl . ~ ~ ~ complex (39) has recently been synthesised, via oxidative addition of a C-F bond in pentafluoropyridine to nickel@),and found to be a catalyst for the cross coupling of pentafluoropyridine with tributyl(vinyl)tin?M Complex 40, containing two 2,4,6-tris(trifluoromethy1)phenylligands, has been synthesised and structurally characterised. The complex is found to have significant nickel-fluorine interactions both in the the solid-state and in solution.2o5

F

F F+F Et3P-Ni-PEt3

I

F

(39)

Their have been a large number of articles dealing with the synthesis, reactivity and catalytic activity of platinum complexes containing Pt-CH3bonds. Square planar platinum(I1) complexes containing a bidentate nitrogen donor ligand and two methyl groups have been synthesised2M(e.g. complex 41"") and found to undergo a number of reactions including: oxidative addition of tin") halides:" mercury(I1) compounds2o8and perfluoroalkyl halides;2wprotonation

13: Complexes Containing Metal-Carbona-Bonds of the Groups Iron, Cobalt and Nickel 365

with loss of methanetlo protonation with metalation;206and formation of an organometallicpolyrotoxane with a novel architecture.211

(41)

Square planar platinum(I1) complexes containing a bidentate phosphine do(e.g. nor ligand and one or two methyl groups have also been ~ynthesised~’”~’~ complexes 42212and 432’2).These complexes have been found to activate Care,-H bonds:12 and support CO-insertion into the Pt-methyl bond?14 Ph7

The first stable five coordinate Pt(IV) complex [PtMe3Lf (L = an anionic f3-diimine ligand) has been isolated, such complexes have long been proposed as intermediates in a variety of reactions? The complex [PtMe(H)2(Tp’)] (Tp‘ = hydridotris~3,5-dimethylpyrazolyl)borate} has been shown to react with B(C6F& in aromatic solvents to yield the corresponding platinum(1V) aryi dihydride systems [PtAr(H)z(Tp’)], via borane-induced reductive elimination of Octahedral methane and subsequent C-H activation of the tris(methy1) platinum(1V)complexes have been prepared and found to undergo reductive elimination reactions to form carbon-carbon and/or carbon-oxygen The synthesis of some novel diplatinum(II1) complexes containing ketonyl ligands, RCHC( = O)R’, has bezn reported. The ketonyl ligands are found to be

366

Organometallic Chemistry

extremely reactive towards a range of nucleophiles?18A family of unsymmetrical organodiplatinum complexes containing pdppm ligands has been prepared via the combination of mononuclear diaryl- and dialkyl-platinum species, and their reactivity towards Me1 in~estigated.~'~ The monomeric methylplatinum species [(q5-C5Me4R)Pt(Me)(cod)](R = CH3, CH2CH2NMe2)have been synthesised and found to dimerise affording the corresponding cod-bridged species.220A DFT study into the mechanism of formation of dinuclear platina-P-diketones has been carried out?2f Thermolysis of [Pt(Me)2(cod)] in the presence of [($C5Me5)2R~2W(C0)2(p-S)4] affords tetra-and penta-nuclear mixed metal clusters containing bridging PtMe2moieties?22

(44)

C

I

Ph

There have been several reports concerning the synthsesis and chemistry of platinum complexes containing one, two and four acetylide ligands. Thus, monoacetylide complexes containing bidentate223.224 and tridentate225ligands (e.g. complexes 44224 and 45225) have been synthesised via the activation of alkyne C-H2249225 and C-C223bonds by platinum. Interestingly, reaction of [Pt(dppm)2(Cl)2]with terminal alkynes HCEECRhas been found to afford diplatinum complexes containing a terminal acetylide unit and an unusual bridging vinylidene unit.226Far more common than their mono-acetylide counterparts are the bis(acety1ide) platinum complexes.There have been several reports regarding the synthesis and photophysical i n ~ e s t i g a t i o n ~of~ platinum ~ - ~ ~ ' complexes containing two acetylide units (e.g. complex 46227). Other reports have detailed the synthesis and use of bis(acety1ide species in the formation of organometallic dendrimer~?~' molecular rods (e.g. complex 47),232 molecular dimer~~~ and ~ ' polymers.234 ~'' Reports concerning platinum complexes containing four acetylide ligands have focused mainly on the ability of the pendant triple bonds to co-ordinate other metals. Ag, Rh(cod), Thus, complexes containing Tl,236 Cd(halide),237CU(NCM~)?~' Ir(cod), Pd(C6F5)2 and P d ( a l l ~ 1units ) ~ ~ coordinated ~ to the acetylide triple bonds have all been synthesised and their luminescence properties investigated. Two examples of the above are seen in complexes 48236and49.239A series of complexes containing polyyne csc '1 6 chains spanning two [Pt { P(C6H4Me)3}(C6F5)] units has been synthesised including the first structurally characterised 1,3,5,7,9,11,13,15-octayne?@ There have been many reports concerning complexes containing platinumaryl a-bonds. A series of variety of complexes of general formula cis[ P ~ ( U ~ ~ ) ~ . , ( X ) , ( N - where N ) ] ~ ~X~= * ~halide ~ ~ and N-N is a bidentate, N-donor

13: Complexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel 367

ligand (e.g.complex 5024'),and tran~-[Pt(aryl)(X)(PR~)2]~~~-~~~ (e.g.complex 51243). These complexes have been reported to undergo reactions such as cleavage of peroxide oxygen-oxygen cleavage of C-H bonds in toluene and xy1ene,242isonitrile insertion into the Pt-aryl bond244and formation of aryl-bridged d i m e ~ s . 2There ~ ~ have been five article^^^^-^^' dealing with the synthesis and reactivity of platinum complexes containing bidentate N,C-donor ligands derived from the cyclometalation of aryl groups bound to a nitrogen atom (e.g.

368

Organometallic Chemistry

complex 52247).Interestingly, the use of ligands such as 3,6-diphenylpyridazines or 2,2’:6’,2”-terpyridineleads to double cyclometalation and formation of diplatinum specie^.^^^-^^ Similar synthetic strategies have been reported for the and the synthesis incorporation of platinum into tetraphenylbenzip~rphyrin~~~

Ph3P\ Pt/ I Ph

R - g

\””

of platinum complexes containing bidentate C,C-ligands derived from phosphonium Orthoplatination of diaminoarenes leads to the formation of complexes containing NCN ‘pincer’ ligands (e.g. complex 53253). There have been eight art i c l e ~concerning ~ ~ ~ - ~ many ~ aspects of these types of complexes including their potential as building blocks for organometallic and m a ~ r 0 m o 1 e ~ ~ l e ~ ~ ~ ~ their ability to promote C-C bond-f0rmation,2~~ their potential as asymmetric their ability to reversibly bond S02,2s7and their luminescenceproperties.258-259 The synthesis and molecular structure of a perfluoroalkyl complex of platinum containing a PCP ‘pincer’ligand has also been reported.26’ The synthesis of stable Pt(1V) vinylic complexes, which show unusual regioselectivity in the activation of the triple bond of methylpropiolate, has been reported.262These mechanism by which Pt(1V) vinyl species activate the triple bonds of alkynes has been studied using DFT calculation^?^^ Complexes derived from the addition of alkynes to Pt3-~I~sters,264 and the insertion of alkynes into the Pt-Pd or Pd-P bonds of A-frame Pt2Pd species265have been prepared and characterised. Several reports on the synthesis, structure and reactivity of Pd(1I) methyl complexes containing bidentate N-donor ligands have a ~ p e a r e d ? Thus, ~~.~~~ Pd-methyl complexes with pyrazolyl-containing ligands have been prepared and their fluxional behaviour studied.266A series of complexes containing imidazole based chelate ligands has been synthesised and found to readily insert CO to afford the corresponding acyl ~pecies.2~~ Diimine ligands with a pendant thienyl group have been used to prepare a cationic methyl Pd(II) complex which has

13: Complexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel 369

been used to catalyse the isomerisation of 1-hexene to internal hexenes.268A phenanthroline derived Pd-Me species has been used a starting material for a series of reactions culminating in the direct observation of the conversion of a palladium 5-hexenyl chelate complex to a palladium cyclopentamethyl com~ l e xElectrophilic . ~ ~ ~ binuclear methylpalladium(I1)complexes have been prepared and found to be catalyst precursors for the copolymerisation of alkenes and C0.270Reaction of a bipyridine derived Pd(I1)-methylcomplex with CO and ethane has allowed the formation and structural characterisation of an acyl intermediate relevant to ethene/CO polymeri~ation.2~~ The insertion aptitude and regiochemistry of various alkenes coordinated to cationic diimine Pd(I1) methyl complexes has been the subject of a theoretical s t ~ d y . 2 ~ ~ Articles concerning methyl-palladium(I1) complexes containing bidentate phosphine ligands have also appeared. A BINAPHOS based Pd-acyl complex has been synthesised from the corresponding methyl species and used in NMR studies into the insertion of styrene into the Pd-acyl bond?73Bond angle effects on the migratory insertion of ethene and CO into Pd-methyl bonds of complexes bearing bidentate phosphine ligands have been s t ~ d i e d . 2Such ~ ~ complexes have also been utilised in a study of the palladium catalysed ring-opening of oxabicyclic alkenes with dialkylzin$” and in a study of the rate of CO insertion into the Pd-methyl bond.276 The fluxional behaviour of the P,N-bidentate ligand bis(oxazo1ine)phenylphosphonite in a range of Pd(I1) complexes has been studied using NMR spectroscopic te~hniques.2~~ Palladium(I1) methyl complexes containing phosphine imine bidentate ligands have been used in the study of alkene and CO copolymerisation reaction^?^^**^^ A report concerning the synthesisand carbonyl insertion reactions of several neutral and cationic Pd(I1)-methyl complexes containing an imidazolylphosphine ligand has appeared?O Several reports concerning the synthesis and reactivity of Pd(I1)-aryl complexes containing bidentate ligands derived from bipyridine have a ~ p e a r e d . ~ ~ ’ - ~ ~ ~ Pd(I1)-aryl complexes containing bidentate phosphorus ligands have been utilised in the formation of new oxapalladacyclic ~pecies,2~’ investigations into the mechanism of palladium catalysed arylation r e a ~ t i o n s , 2 ~the ~ *characterisa*~~ tion of intermediates from a Stille and kinetic studies into the amination of arylbromides and aryl t1iflates.2~~ into the The insertion of a l k y n e ~ : sulfur ~ ~ ~ dioxide:% ~ and vinyl Pd-aryl bonds of a range of bisphosphine substituted complexes has been reported. Throughout 2001 there have been numerous articles concerning the synthesis and reactivity of palladium complexes containing bidentate N,C-donor ligands derived from the cyclometalation of aryl groups bound to nitrogen atom (i.e. palladium analogues of 52).293-308 Thus, dimeric complexes containing two different hydrazonato l i g a n d ~and ~ ~mononuclear ~ complexes containing terdentate l i g a n d ~ ~have ~ ~ ,been ~ ~ ’synthesised and structurally characterised. Interestingly, the palladation of tricarbonylchromiumderivatives of a range of arylamines has led to the formation of a series of heteronuclearspecies such as complexes 54 and 55?97Regarding their reactivity, N,C-palladacyclic species have been used in the

370

Organometallic Chemistry

isolation of enantiomerically pure asymmetric di(tertiary p h o s p h i n e ~ )the ~~~ Suzuki coupling of aryl chloride^,^" the asymmetric synthesis of iminophosphines302and in the Heck coupling of aryl halides with In addition, the cyclopalladated complexes have been used in the formation of liquid cryst a l ~ ,molecular ~ ~ ~ >t r~i c~o ~ r n ~and ~ ~dendrimem3O6 ~

(54)

Two novel variations on the N,C-cyclopallated structural form have been r e p ~ r t e d . ' ~Thus, ~ . ~ ' ~a series of novel monomeric and dimeric cyclopalladated ferrocenyl Schiff base compounds has been prepared (q. complex 563'0)and structurally characterised. Complexes containing P,C-cyclopalladated ligands derived from phosphonites311and benzylpho~phines~'~ have been prepared and found to be extremely active catalysts in the Suzuki biaryl coupling reaction. A report concerning the preparation and catalytic properties of non-racemic S,C-palladacycles has ap~ e a r e d .The ~ ' ~ sulfur containing cycles show some potential as catalyst precursors for aryl homocoupling reactions. Reaction of PdC12 with oxybenziporphyrin affords the corresponding palladium complex 57. The effects of complexation and electrophilic addition on the aromaticity of the ligand have been probed using spectroscopic technique^.^'^

13: CompZexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel 37 1

The synthesis and structure of a new dinuclear organopalladium macrocyclic complex has been reported. The structure shows two 2,6-bis(diaminomethy1)phenyl units each coordinated to a PdBr centre via N,C,N-donor atoms in a ‘pincer’type fashion.315A report describing the palladation of N,C,Npincer type ligands tethered to Cm, has appeared.316The resulting complexes have been converted into active Lewis acid catalysts for aldol condensation. The complex [PdBr(Ph2PC6H4CH = NCH2C6H4)],which contains a P,N,C-‘pincer’ type ligand, has been ~ynthesised.~~’ Unusually, the Pd(I1)-aryl bond of this complex is hydrolysed upon reaction with water. A series of Pd(I1)-alkene complexes containing the P,N,P ligand Ph2PCH2C5NH4CH2PPh2 has been prepared and found to readily undergo nucleophilic attack of the highly electrophilic alkene? The heterometallic A-frame complexes 58 readily undergo insertion of CO or isonitriles into the Pd-Me bond leading to products containing acyl or iminoacyl ligands?19 Abstraction of chloride from the acyl complexes affords cationic derivatives which support alkene insertions into the Pd-acyl bond. Two general routes to the formation of halide-bridged organopalldium A-frame complexes have been reported and a range of such complexes synthesised and structurally characterised?20

c1 (58a) X = N H (58b) X = CH2

The Pd(1) acetylide complex [Pd(q3-C3H5)(PiPr3)(C=CH)] is formed from with lithium a~teylide?~~ Further reaction reaction of [Pd(q3-C3H5)(PiPr3)(C1)] = C3H5=CH2)(PiPr3)]affords the of the Pd-acetylide species with [Pd(q2,q2-H2C p-acetylide, p-allyl-complex [Pd2(PiPr3)2(p-C3HS)(p-C=CH)].Extended one pot procedures have been employed in the formation of palladium-ethynylthiophene organometallicoligomers (e.g. complex 59).322 The formation of the luminescent The complexes are branched acetylide complexes 60 and 61 has been reported.323 emissive in EtOH-MeOH (4:1, v/v) glass at low temperature and also show some potential as building blocks for metallodendrimers. r

1

372

Organometallic Chemistry

Finally, complexes containing Pd-Cally1,)24326 Pd-C,,:27-331 or Pd-C,lky~32-340 abonds have been proposed as intermediates in a variety of catalytic processes. 4

Carbene and Carbyne Complexes of Group 8,9 and 10

Numerous articles have appeared in the literature regarding carbene complexes in the context of catalysis. Thus, there are reports on the synthesis and use of carbene complexes in ring-opening m e t h e s i ~ , 3olefin ~ ~ ' ~m ~ e t a t h e s i ~ , 3cyclo~~-~~~ propanation of a1kenes,355'357cyclisation of 2-aminobenzyl alcohol with cocyclisa.tionof diynes and C0,3s9cyclisation of dienes,360cyclisation of a-diazoketones%'and hydrosilylation of a l k y n e ~ . ~ ~ ~

Species containing N-heterocyclic carbene ligands (imidazol-2-ylidene moieties), such as complex 62,363represent a growing class of carbene complexes. Throughout 2001 their have been many reports regarding their ability to catalyse a number of processes including olefin m e t a t h e s i ~ ring-closing ~~~-~~~ reaction^:^^-^^^ cyclisametathesi~,3~~ olefin cross m e t a t h e s i ~ , 3Heck ~ ~ ~ ~coupling ~' amination of aryl chlorides378 and hydrotion of 3-methylpent-2-en-4-yn-1-01:~~ genation of

/

C1

\

Ph

(62)

A special volume of Journal of Organometallic Chemistry (Volume 617-618) is dedicated to the synthesis and reactivity of carbene complexes, which the reader may find of interest. A new general method for the preparation of metal carbene

13: Complexes Containing Metal-Carbona-Bonds of the Groups Iron, Cobalt and Nickel 373

complexes involving the use of sulfur ylides as carbenoid precursors has been reported.382Several Ru, 0 s and Rh carbene complexes have been prepared using this method. (R Reaction of ferrocenyl acetylene with [(r15-C5H5)Ru(PR3)(NCMe)~][PF6] = Me, Ph, Cy) leads to formation of the first examples of allenyl carbene complexes [(qS-C5H5)Ru(PR3){q2=C(Fc)CH = C =CH(Fc)}][PF6] (Fc = ferr~cenyl)?~ A series of metallacyclopentatriene complexes (63) has been synthesised from addition of deca-2,g-diyne with the corresponding ruthenium species [(q5-C5H5)Ru(ER3)(NCMe)2] [PF6].384,385 The PMe3 containing species

dCH PMe3

(6311) ER3 = SbPh3 (6%) ER3 = SbnBu3 (6%) ER3 = AsPh3 (63d)ER3 = PCy3

(63e)ER3 = PMe3

undergoes a metal-ligand migration to afford complex 64, whereas, the As, Sb and PCy3derived monomers rearrange via a 1,2- H shift to form the corresponding butadienyl carbene species 65. Addition of PR3 (R = Me, OMe) to the q2-=C(C6H4R')CH= cyclopentatriene complexes [(q5-C5Me5)Ru(C1){ CHC(Ca&R')} J (R' = H, Br) leads to nucleophilic attack at one of the carbene carbons and formation of the corresponding allyl-carbene species [($C5Me5)Ru(C1){ q1,q3-=C(C6H4R)CHCHC(C6H4R)PR3}].386The ruthenium amidinato-carbene complexes [(q6-C6Me6)RU(SiMe3){q2=CHNfPr)C(Me)=NpPr)}] undergo a reversible a-silyl group migration from the metal to the carbene ligand upon reaction with carbonyls or isonitReaction of [ R u ( C ~ ) ~ ( P C=~CH(Ph)}] ~)~{ with Na0C6H3Me2-2,6-THF initially forms the aryloxide complex [Ru(OC6H3Me2)(Cl)(PCy3){ =CH(Ph)}], which slowly undergoes transfer of two hydrogens from one ortho-methyl group of the aryl oxide to the benzylidene carbon and formation of 66? A study into the protonation of monoruthenium vinylidenes containing carboxylato ligands has led to the isolation of a range of five- and six-coordinate carbene and carbyne species and the observation of an unprecedented equilibrium between carbenes A novel organometallic transformation has been reported in and ~arbynes.~*~ +

374

Organometallic Chemistry

QMe

HC‘

Cl-Ru-

1

1

p

3

/

0

CY3p

(66)

which the carbene protons of the water-soluble ruthenium carbenes 67 undergo non-destructive, degenerate exchange with deuterons from solvents such as D 2 0 and CD30D.390

D2O or CD3OD w

+ H20 or CHJOH

(67a)R = CH2CH2pMe2 (67b)R = C,H,N+M%

(67c)R = CH2CH2N’Me2 (67d)R = C5H4N%le2

The tris(pyrazoly1)borate (Tp) containing complex [(Tp)Ru(PPh3)(q202CCHPh2)] has been reacted with a range of carbene and vinylidene precursors to afford products containing carbene or vinylidene ligand~.~” A novel class of ruthenium(11) polyp yridine complexes containing ort homet alat ed aminocarbene ligands has been prepared, and their luminescence and electrochemical properties Reaction of [R~(bpy)~(napy-lc~N,N)] (napy = l,%naphthyridine; bipy = 2,2’-bipyridine) with HC=CCOOH and methanol yields a Ru-carbene complex (68) with a five membered metallacycle involving a 1,8-naphthylidine framew0rk.3~~ A ruthenium porphyrinogen based on the meso-octaalkylporphyrinogen tetraanion has been reacted with terminal acetylenes to form the first examples of macrocyclic ruthenium carbene and vinylidene complexes.394 2+

U

N = 2,2’-bipyridine

13: Complexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel 375

Protonation of the diruthenium p-carbene complex [Ru2(CO)&-CH2)(pdppm)] at low temperature affords the fluxional p-methyl species [Ru2(CO)&CH3)(pdppm)]+ which contains an agostic C-H interaction with the metal centre.395Reaction of the carbene complex with acetic acid leads to the formation of products derived from coordination of acetic acid to the metal centre and insertion of CO into the Ru-carbene bond. The decomposition of 47-electron clusters, generated by electrochemical or chemical oxidation of 48-electron trihydrido(carbyne)triruthenium clusters, has been shown to proceed via a disproportionation me~hanism.3~~ Thermolysis of [Ru3(CO)&J and the acetylide (R = H, Me) affords the carbido-carcluster [(q5-C5R5)WRu2(COja(C=CPh)] byne cluster complexes [(q5-C5R5)WRh(CO)12(p5-C)(p-C=CPh)] and [(q5C5R5)WRu5(Co)l,(p6-C)(~-c~cPh)] through reversible scission of the acetylide ligand.397The hydrogenation reactions of the carbyne clusters were found to be influenced by the nature of the ancilliary ligand on the W atom.

Ar (69a) Ar = Ph

(69b) Ar = 2-thienyl

(i) ArLi (ii) Et30BF4

b

(7la) Ar = C&Is (71b) Ar = p-MeC6H4 (71c) Ar = o-MeOC& (71d) Ar = p-MeoC6H4 (71e) Ar = p-CF3C&I4

Iron aminocarbene complexes containing a double C = C bond in the Nsubstituent (e.g. complexes 69) have been prepared from reaction of a range of tertiary amides with Na2[Fe(C0)4].398Reaction of the azadiene complex 70 with aryllithium reagents affords the novel alkoxy(amino)carbeneiron species 71.399A series of half-sandwich iron aryl carbenes has been prepared and found to undergo either C-C1 or C-C bond activation reactions with alkoxides depending on the nature of the co-ligands on the metal centre.400 The nonlinear optical properties of the complexes [(q5-C5H5)Fe2(CO)2(pC0)2{p-CH =CH(RFc)}], containing electron accepting p-vinylcarbyne diiron and electron donating ferrocene donors (RFc), have been studied using the hyper Rayleigh scattering technique"' The diiron cationic carbyne complexes [($C5H5)2SiMe2Fe2(CO)2(p-CO)(p-CAr)]BBr4 react with NaSR at low temterature to afford the corresponding mercaptocarbene species [(q5-

376

Organometallic Chemistry

C5H5)2SiMe2Fe2(CO)2(pCO){p-C(SR)Ar}] (Ar = C6H5, R = Me, C6H5 or p MeC6H4; Ar = p-MeC& R = Me, C& or p-MeCsH4; Ar = p-CF3C6Hjt4, R = Me, C6H5 or p-MeC6H4)].402Iron-sulfur cluster complexes containing bridging carbene ligands have been obtained through addition of [Fe2(C0)6(p-CO)(pSC6HS)(p-SLi)]to the carbyne complexes. The heterometallic carbene complex (73) has been synthesised from reaction of 72 with 1,2-diphenylcycloprop e n ~ n eThe . ~ ~reaction involves intramolecular oxygen transfer from a ketone to CO. A series of mixed metal p-carbene complexes [(q5-C5H5)MFe(CO)5(vCO){pC(SeR))] (M = Mn or Re; R = C6H5 or p-MeC6h) has been prepared from the reaction of cationic rhenium and manganese carbyne species with diiron anions.4o4 H

I

oc

I

/Fiyy 0 c

Ah

Ph

P'h

(73)

R = 2,3,4-F,C6H,

The osmium carbene complexes [Os(Lh(H)(Cl){=C(Me)OR}] (L = PiPr3or with PtBu2Me;R = Ph, Et) have been prepared from reaction of [OS(L)~(H)~(C~)] vinyl Subsequent R- and L-dependent reactions involve Cmrk,,-OR bond cleavage and formation of either carbyne or vinylidene species. A one-pot synthesis of osmium(II1)azavinylidene-carbyneand azavinylidene-vinylcarbyne complexeshas been reported and involves the addition of alkynes to a mixture of Ag[CF3S03] and the osmium(I1) azavinylidene-hydride compound [Os(H)(Cl)(P'Pr&(=N =CMe2)]? The synthesis and structure of the p-acylhydroxycarbene complex [CO~(CO)~(~-C(~BU)OCCBU) = O}] has been reported.407 The carbene rhodium(1) complexes [Rh(C1)(L)2(=CPh2)] (L = PiPr3 or Sb'Pr3)react with PF3 via cleavage of the rhodium carbene bond to give the corresponding complexes [Rh(Cl)(L)2(PF3)].In contrast, the half sandwich carbene complex [(q5-C5H5)Rh(P'Pr3)( =CPh2)] reacts with both PF3and P(OPh)3 to afford ring substituted products derived from insertion of the CPh2 unit into one of the cyclopentadienyl C-H bondsm The reactivity of a series of half sandwich rhodium carbene complexes towards electrophiles has been studied and found to afford a range of product types depending on the nature of the carbene substituents and the ancilliary ligands on the rhodium.- Reaction of

13: Complexes Containing Metal-Carbon a-Bonds of the Groups Iron, Cobalt and Nickel 377

[{ Rh(pPz)(CWBu)2}2] (Pz = pyrazolate) with dichlorocompounds CHRC12 leads to oxidative addition and formation of the corresponding functionalised carbene compounds [{ R~(~-PZ)(CI)(CN'BU)~}~(~-CHR)] (R = H, Ph, COMe, C02Me)?' The cyclic carbene complex [II(H)~{ = cHN(Me)~y}(PPh3)~1+ (py = 2-pyridyl) has been formed from reaction of 2-dimethylaminopyridine with [Ir(H)2(OCMe2)2(PPh3b],via an oxidative addition, reversible a-elimination sequence:" A theoretical study into the interconversion between an iridium ketene complex and its carbene carbonyl isomer has calculated the process to be reversibleP12 The syntheses and structures of the carbene-bridged heterobinuc(L =CO, H2C= CH2, PMe3, lear complexes [IrR~(CO)~(L)(p-CH2)(p-dppm)2] NCMe or NCHC = CH2)have been rep0rted.4'~ p-CO)Ni(q5Reaction of methyl methacrylate with [(q5-C5H5)Mo(C0)2( C5Me5)]affords the heterometallic p3-carbynecomplex [(q5-C5H5)Mo(C0)4{ pCCH2C(0)OMe}Ni(q5-CsMe5)] as one of two pr0ducts.4'~The synthesis, structure and characterization of the p-carbene complex [Pt2{p-C(Me)H}(p-dfepe)] {dfepe = (C6F5)2PCH2CH2P(C6F5)2} has been reported!''

-+ (74)

(75a) R = Me (75b) R = Ph

The novel q3-vinylcarbene complexes 75 have been formed from reaction of 74 with electron-rich alkynes? The syntheses of monoruthenium complexes containing cationic N-heterocyclic carbene ligands417and nucleophilic chiral Nheterocyclic carbene ligands418have been reported. The unusual iridium complexes 76 have been synthesised in which the imidazole ligand is bound to the metal centre the 'wrong way' (Le., at C-5 not c-21.419

(76a) R = 'Pr (76b) R = 'Bu

3 78

Organometallic Chemistry

A series of reports has appeared concerning density functional analyses of and experimental studies into the N-heterocyclic carbenes formed from oxidative addition of imidazolium cations to Ni(O), Pt(0) and Pd(O)-~entres.~”-~~~ The syntheses and structures of the unprecedented Ni(I1) alkyl complexes 77 containing di-N-heterocyclic c a r b e n e ~and ~ ~the ~ palladium species 78 containing functionalised heterocyclic carbenes have been reported.424Finally, two reports concerning the synthesis of platinum and palladium N-heterocyclic carbene complexes in ionic liquids have

Me-Ni-Me

I P‘Bu~

Me-Ni-Me

I

P$u~ (77a)n= 1 (77b) n = 2

(78a) R = H

(78b) R = COMe

References 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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388

Orgmometullic Chemistry

2001,309. 299. S. Chatterjee, M. D. George, G. Salem and A. C. Willis, J. Chem. SOC.,Dalton Trans., 2001,1890. 300. C. Garcia-Herbosa, N. G. Connely, A. Muiioz, J. V. Cuevas, A. G. Orpen and S. D. Politzer, Organometallics,2001,20,3223. 301. R. B. Bedford and C. S. J. Cazin, J . Chem. SOC.,Chem. Commun.,2001,1540. 302. X . Liu, K. F. Mok and P-H. Leung, Organometullics, 2001,20,3918. 303. L. Diez, P. Espinet and J. A. Miguel, J. Chem. SOC.,Dalton Trans., 2001,1189. 304. L. Omnhs, B. A. Timimi, T. Gelbrich, M. B. Hursthouse, G. R. Luckhurst and D. W. Bruce, J. Chem. SOC.,Chem. Commun., 2001,2248. 305. R. F. Carina, A. F. Williams and G. Bernardinelli, Inorg. Chem., 2001,40,1826. 306. A. W. Kleij, R. J. M. K. Gebbink, P. A. J. van den Nieuwenhuijzen, H. Kooijman, M. Lutz, A. L. Spek and G. van Koten, Organometallics, 2001,20,634. 307. I. P. Beletskaya,A. N. Kashin, N. B. Karlstedt, A. V. Mitin, A. V. Cheprakov and G. M. Kazankov, J. Organomet. Chem., 2001,622,89. 308. T. Rosner, J. Le Bars, A. Pfaltz and D. G. Blackmond, J. Am. Chem. Soc., 2001,123, 1848. 309. J. M. Vila, E. Gayoso, M. Lopez-Torres, A. Fernandez, J. J. Fernandez, T. Periera, J. M. Ortigueira and M.Mariiio. J. Organomet. Chem., 2001,638,577. 3 10. S . PCrez, R. Bosque, C. Lopez, X. Solans and M. Font-Bardia, J. Organomet. Chem., 2001,625,67. 31 1. R. B. Bedford and S. L. Welch, J. Chem. SOC.,Chem. Commun., 2001,129. 312. S . Gibson, D. F. Foster, G. R. Eastham, R. P. Tooze and D. J. Cole-Hamilton, J. Chem. SOC.,Chem. Commun., 2001,779. 313. J. Dupont, A. S. Gruber, G. S. Fonseca, A. L. Monteiro, G. Ebeling and R. A. Burrow, Organometallics, 2001,20,171, 314. M. Stepien, L. Latos-Graiynski, T. D. Lash and L. Szierenberg, Inorg. Chem., 2001, 40,6892. 315. T. Tsubomura, T. Tanihata, T. Yamakawa, R. Ohmi, T. Tamane, A. Higuchi, A. Katoh and K. Sakai, Organometallics, 2001,20,3833. 316. M. D. Meijer, N. Ronde, D. Vogt, G. P. M. van Klink and G. van Koten, Organometallics, 2001,20,3993. 317. A. D. Getty and K. L. Goldberg, Organometallics, 2001,20,2545. 318. C. Hahn, P. Morvillo and A. Vitagliano, Eur. J. Inorg. Chem., 2001,419. 319. P. Braunstein, J. Durand, M. Knorr and C. Strohmann, J. Chem. SOC., Chem. Commun.,2001,211. 320. R. A. Stockland, M. Janka, G. R. Hoel, N. P. Rath and G. K. Anderson, Organometallics, 2001,20, 5212. 321. J . Krause, R. Goddard, R. Mynott and K. R. Porschke, Organometallics, 2001,20, 1992. 322. P. Altamura, G. Giardina, C. Lo Sterzo and M. V. Russo, Organometallics, 2001,20, 4360. 323. V. Yam, L. Zhang, C. Tao, K. Wong and K. Cheung, J. Chem. SOC.,Dalton Trans., 2001,1111. 324. P. B. Tivola, A. Deagostini, C. Prandi and P. Venturello, J. Chem. Soc., Chem. Cornmun.,2001,1536. 325. T. Hirashita, H. Yamamura, M. Kawai and S. Araki, J. Chem. SOC.,Chem. Commun., 2001,387. 326. K. Itami, T. Nokami and J. Yoshida, J. Am. Chem. SOC.,2001,123,5600. 327. A. C. Albkniz, P. Espinet, B. Martin-Ruiz and D. Milstein, J. Am. Chem. SOC.,2001,

13: Complexes Containing Metabcarbon cr-Bonds of the Groups Iron, Cobdt and Nickel 389

123,11504. 328. K. E. Torraca, X.Huang, C. A. Parrish and S. L.Buchwald,J. Am. Chem. SOC.,2001, 123,10770. 329. P. Nilsson, M. Larhed and A. Hallberg, J . Am. Chem. SOC.,2001,123,8217. 3 30. H. Dang and M. A. Garcia-Garibay,J. Am. Chem. SOC.,2001,123,355. 331. J. W. Herndon, Y.Zhang and K. Wang, J. Organomet. Chem., 2001,634,l. 332. A. M. M. Antunes, S. J. L. Marto, P.S. Branco, S. Prabhakar and A. M.Lobo, J. Chem.SOC.,Chem Commun.,2001,405. 333. A. Krotz, F. Vollmuller, G. Stark and M.Beller, J. Chem. SOC., Chem. Cornmun., 2001, 195. 334. L. Bagnell, U. Kreher and C. R.Strauss, J. Chem. SOC., Chem. Commun.,2001,29. 335. S . Kamijo, T. Jin and Y.Yamamoto, J. Am. Chem. SOC.,2001,123,9453. 336. M. Bao, H. Nakamura and Y.Yamamoto, J. Am. Chem. SOC.,2001,123,759. 337. H. Nakamura, K. Aoyagi, J-G. Shim and Y. Yamamoto, J. Am. Chem. SOC.,2001, 123,372. 338. K. L. Bray, J. P. H. Charmant, I. J. S. Fairlamb and G. C. Lloyd-Jones,Chem. Eur. J., 2001,7,4205. 339. C. Benedek, A. Gomory, B. Heil and S.Toros, J. Organomet. Chem., 2001,622,112. 340. N. Y. Kozitsyna, A. A. Bukharkina, M.V. Martens, M. N. Vargaftik and I. I. Moiseev, J. Orgunomet. Chem., 2001,636,69. 341. L.Delaude, A. Demonceau and A. F. Noels, J. Chem. SOC.,Chem. Commun.,2001, 986. 342. S. Aime, A. J. Are, 0.Chiantore, R. Gobetto, A. Russo and Y. De Sanctis, J. Organomet. Chem., 2001,622,43. 343. J. N. Coalter I11 and K.G. Caulton, New. J. Chem., 2001,25,681. 344. S . Randl, S. J. Connon and S. Blechert, J. Chem. SOC.,Chem. Commun., 2001,1796. 345. M. S . Sanford, M. Ulman and R.H. Grubbs, J . Am. Chem. Soc., 2001,123,749. 346. J. Dowden and J. Savovik, J. Chem. SOC.,Chem. Commun.,2001,37. 347. S . D. Drouin, F. Zamanian and D. E. Fogg, Organometullics, 2001,20,5495. 348. J. Louie, C. W. Bielawski and R.H. Grubbs, J. Am. Chem. SOC.,2001,123,11312. 349. T. Choi, C. W. Lee, A. K. Chattejee and R.H. Grubbs, J. Am. Chern. Soc., 2001,123, 10417. 350. A. Furstner, L. Ackermann, K. Beck, H. Hori, D. Koch, K. Langemann, M. Liebl, C. Six and W.Leitner, J. Am. Chem. SOC.,2001,123,9000. 351. M . A. Sandford, J. A. Love and R.H. Grubbs, J. Am. Chem. SOC.,2001,123,6543. 352. A. Fiirstner, 0. Guth, A. Diiffels, G. Seidel, M. Liebl, B. Gabor and R. Mynott, Chem. Eur. J., 2001,7,4811. 353. M. August, 0.Volland, C. Adlhart, C.A. Kiener, P. Chen and P. Hofmann, Chem. Eur. J., 2001,7,4621. 354. W. Buchowicz, F. Ingold, J. C.Mol, M. Lutz and A. L. Spek,Chem. Eur. J., 2001,7, 2842. 355. C. G. Hamaker,J-P. Djukic, D. A. Smith and L. K. Woo, Organometallics, 2001,20, 5189. 356. C. Che, J. Huang, F. Lee,Y. Li, T. h i , H. Kwong, P. Teng, W. Lee, S. Peng and Z. Zhou, J. Am. Chem. SOC.,2001,123,419. Am. Chem. SOC.,2001,123,4843. 357. Y. Li, J. Huang, Z. Zhou and C. Che, .I. 358. C. S. Cho, B. T. Kim, T. Kim and S. C. Shim, J. Chem. SOC.,Chem. Commun.,2001, 2576. 359. T. Sugihara, A. Wakabayashi, H. Takao, H. Irnagawa and M. Nishizawa, J. Chern. SOC.,Chem. Commun.,2001,2456.

390

Organometallic Chemistry

360. I. Gottker-Schnetmann and R. Aumann, Organometallics, 2001,20,346. 361. J. P. Snyder, A. Padwa, T. Stengel, A. J. Arduengo, A. Jockisch and H. Kim, J. Am. Chem. SOC.,2001,123,11318. 362. B. M. Trost and Z. T. Ball, J. Am. Chem. SOC.,2001,123,12726. 363. A. Fiirstner, L. Ackermann, B. Gabor, R. Goddard, C. W. Lehmann, R. Mynott, F. Stelzer and 0. R. Thief, Chem. Eur. J.,2001,7,3236. 364. M. S. Sandford, J. A. Love and R. H. Grubbs, Organometallics, 2001,20,5314. 365. B. Cetinkaya, S. Demir, I. Ozdemir, L. Toupet, D. Skmeril, C. Bruneau and P. H. Dixneuf, New. J. Chem., 2001,25,519. 366. J. Louie and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 2001,40,247. 367. T. M. Trnka, M. W. Day and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 2001,40, 3441. 368. T. Kitamura, Y. Sato and M. Mori, J. Chem. SOC., Chem. Commun., 2001,1258. 369. T. Choi and R. H. Grubbs, J . Chem. SOC.,Chem. Commun., 2001,2648. 370. T. Choi, A. K. Chatterjee and R. H. Grubbs, Angew. Chem., Int. Ed. Engl., 2001,40, 1277. 371. S. Imhof, S. Randl and S. Blechert, J. Chem. SOC.,Chem. Commun., 2001,1692. 372. E. Peris, J. A. Loch, J. Mata and R. H. Crabtree, J. Chem. SOC., Chem. Comrnun., 2001,201. 373. A. A. D. Tulloch, A. A. Danopoulos, G. J. Tizzard, S. J. Coles, M. B. Hursthouse, R. S. Hay-Motherwell and W. B. Motherwell, J. Chem. SOC.,Chem. Commun., 2001, 1270. 374. S. Grundemann, M. Albrecht, J. A. Loch, J. W. Faller and R. H. Crabtree, Organometallics, 2001,20,5485. 375. M. V. Baker, B. W. Skelton, A. H. White and C. C. Williams, J. Chem. SOC.,Dalton Trans., 2001, 11 1. 376. A. M. Magill, D. S . McGuiness,K. J. Cavell, G. J. P. Britovsek, V. C. Gibson, A. J. P. White, D. J. Williams, A. H. White and B. W. Skelton, J. Organomet. Chem., 2001, 618,546. 377. I. ozdemir, B. YiBt, B. Cetinkaya, D. Ulku, M. N. Tahir and C. Arici, J. Organomet. Chem., 2001,633,27. 378. L. R. Titcomb, S. Caddick, F. G. N. Cloke, D. J. Wilson and D. McKerrecher, J. Chem. SOC.,Chem. Commun., 2001,1388. 379. H. M. Lee, D. C. Smith, 2.He, E. D. Stevens,C. S. Yi and S . P. Nolan, Organornetallics, 2001,20,794. 380. H. M. Lee, T. Jiang, E. D. Stevens and S. P. Nolan, Organometallics,2001,20,1255, 381. A. C. Hillier, H. M. Lee, E. D. Stevens and S. P. Nolan, Organometallics,2001,20, 4246. 382. M. Gandelman, B. Rybtchinski, N. Ashkenazi, R. M. Gauvin and D. Milstein, J. Am. Chem. Soc., 2001,123,5372. 383. E. Riiba, K. Mereiter, R. Scmid, K. Kirchner and H. Schottenberger,J. Organomet. Chem., 2001,637,70. 3 84. E. Becker,E. Ruba, K. Mereiter, R. Schmid and K. Kirchner, Organometallics, 2001, 20,3851. 385. E. Ruba, K. Mereiter, R. Schmid and K. Kirchner, J. Chem. SOC.,Chem. Commun., 2001,1996. 386. C . Ernst, 0.Walter and E. Dinjus, J. Organomet. Chem., 2001,627,249. 387. T. Hayashida and H. Nagashima, Organometallics, 2001,20,4996. 388. J. N. Coalter 111, J. C. Huffman and K. G. Caulton, J. Chem. Soc., Chem. Commun., 2001,1158.

13: Complexes Containing Metal-Carbon 0-Bonds of the Groups Iron, Cobalt and Nickel 391

389. P. Gonzalez-Herrero, B. Weberndorfer, K. Ilg, J. Wolf and H. Werner, Organometallics, 2001,20,3672. 390. D. M. Lynn and R. H. Grubbs, J. Am. Chem. SOC.,2001,123,3187. 391. M. S . Sanford, M. R. Valdez and R. H. Grubbs, Organometallics, 2001,20,5455. 392. V. W. Yam, B. W. Chu, C. KO and K. Cheung, J. Chem. SOC.,Dalton Trans., 2001, 1911. 393. T. Tomon, D. Ooyama, T. Wada, K. Shiren and K. Tanaka, J. Chem. SOC.,Chem. Commun., 2001,1100. 394. L. Bonomo, C. Stern, E. Solari, R. Scopellitiand C. Floriani, Angew. Chem., Int. Ed. Engl., 2001,40, 1149. 395. Y. Gao, M. C. Jennings and R. J. Puddephatt, Organometallics, 2001,20,1882. 396. D. J. Bierdeman, J. B. Keister and D. A. Jelski, J. Organomet. Chem., 2001,633,51. 397. S. Hwang, Y. Chi, S. Chiang, S. Peng and G. Lee, Organometallics, 2001,20,215. 398. L. Vyklicky, H. Dvofakova and D. Dvofak, Organometallics, 2001,20,5419. 399. S. Zhang, Q. Xu, J. Sun and J. Chen, Organometallics, 2001,20,2387. 400. K. Ferrk, G. Poignant, L. Toupet and V. Guerchais, J. Organomet. Chem., 2001, 629, 19. 401. T. Farrell, A. R. Manning, T. C. Murphy, T. Meyer-Friedrichsen, J. Heck, I. Asselberghs and A. Persoons, Eur. J. Inorg. Chem., 2001,2365. 402. R. Wang, Q. Xu, J. Sun, L. Song and J. Chen, Organometallics, 2001,20,4092. 403. M. Lutz, M. Haukka, T. A. Pakkanen and L. H. Gade, Organometallics, 2001,20, 2631. 404. R. Wang, Q. Xu, Y. Souma, L. Song, J. Sun and J. Chen, Organometallics, 2001,20, 2226. 405. G. Ferrando, H. GCrard. G. J. Spivak, J. N. Coalter 111,J. C. Huffman, 0.Eisenstein and K. G. Caulton, Inorg. Chem., 2001,40,6610. 406. R. Castarlenas, M. A. Esteruelas and E. Oiiate, Organometallics, 2001,20, 3283. 407. C. Zucchi, E. Soos, V. Galamb, G. Varadi, L. Parkanyi and G. PBlyi, Eur. J. Inorg. Chem., 2001,2207. 408. U. Herber, R. G. Sanchez, 0.Gevert, M. Laubender and H. Werner, New. J. Chem., 2001,25,396. 409. E. Bleuel, P. Schwab, M. Laubender and H. Werner, J. Chem. SOC., Dalton Trans., 2001,266. 410. C. Tejel, M. A. Cirano, L. A. Oro, A. Tiripicchio and F. Ugozzali, Organornetallics, 2001,20,1676. 41 1. D. Lee, J. Chen, J. W. Faller and R. H. Crabtree, J. Chem. SOC., Chem. Cornrnun., 2001,213. 412. H. Urtel, G. A. Bikzhanova, D. B. Grotjahn and P. Hofmann, Organometallics, 2001,20,3938. 41 3. M. M. Dell’Anna, S. J. Trepanier, R. McDonald and M. Cowie, Organometallics, 2001,20,88. 414. P. Braunstein, M. J. Chetcuti and R. Welter, J. Chem. SOC.,Chem. Commun., 2001, 2508. 415. S. White, E. W. Kalberer, B. L. Bennett and D. M. Roddick, Organometallics, 2001, 20,5731. 41 6. T. M. Trnka, M. W. Day and R.H. Grubbs, Organometallics, 2001,20,3845. 417. A. Chaumonnot, B. Donnadieu, S. Sabo-Etienne, B. Chaudret, C. Buron, G. Bertrand and P. Metivier, Organometallics, 2001,20, 5614. 418. J. Huang, L. Jafarpour, A. C. Hillier, E. D. Stevens and S. P. Nolan, OrganometalZics, 2001,20,2878.

392

Organometallic Chemistry

419. S. Griindemann, A. Kovacevic, M. Albrecht, J. W. Faller and R. H. Grubbs, J . Chem. SOC.,Chem. Commun., 2001,2274. 420. D. S. McGuiness, K. Cavell and B. F. Yates, J. Chem. Soc., Chem. Commun., 2001, 355. 421. D. S. McGuiness, K. J. Cavell, B. F. Yates, B. W. Skelton and A. H. White, J . Am. Chem. SOC.,2001,123,8317. 422. D. S. McGuiness, N. Saendig, B. F. Yates and K. J. Cavell, J. Am. Chem. SOC.,2001, 123,4029. 423. R. E. Douthwaite, M. L. H. Green, P.J. Silcock and P.T. Gomes, Organometallics, 2001,20,2611. 424. H. Glas, E. Herdtweck, M. Spiegler,A. Pleier and W. R. Thiel, J. Organomet. Chem., 2001,626,100. 425. M. Hasan, I. V. Kozhevnikov, M. R. H. Siddiqui, C. Femoni, A. Steiner and N. Winterton, Inorg. Chem., 2001,40,800. 426. C . J. Mathews, P.J. Smith, T. Welton, A. J. P.White and D. J. Williams, Organometallics, 2001,20,3848.

14

Transition Metal Complexes of Cyclopentadienyl Ligands" BY IAN R. BUTLER

1

General Introduction

The review essentially follows the same style as previous years', however the focus has changed somewhat because of the sheer number of articles which contain cyclopentadienyl ligands. The focus therefore will be on interesting structural complexes which contain cyclopentadienyl ligands and on reactions where the cyclopentadienyl ligand itself is involved. Inevitably this means the emphasis will be on the metallocenes and primarily on ferrocenes. The review will begin with general references and review articles. At the outset it is worth noting that the articles reviewed in this year represent work in year of the 50th anniversary of the discovery of ferrocene and indeed to mark this milestone a whole volume of the Journal of OrganometallicChemistry has been dedicated to ferrocene research. 2

General Chemistry, Main Group, Lanthanides and Actinides

A range of primary cyclopentadienylphosphines and arsines have been obtained by the reduction of dichlorophosphino-(or arsino-)cyclopentadienes, which in turn were prepared by treatment of the tributyltincyclopentadieneswith phosphorus trichloride or arsenic trichloride? In further work by Jutzi and Muller, the nature of protonated decamethylsilicocene has been investigated by quantum mechanical methods. This complements the synthetic work that this research group have done on substituted cyclopentadienes over the past years.3 (see also references 198, 200, 244 by the same group) The reaction of [C~*CaI(thf)~] with triphenylphosphine oxide results in the formation of the stable cationic species [Cp*Ca(OPPh3)3] which has the classic piano-stool geometry! The mixed cyclopentadienyl dianion [CpSi(Mez)Cp*]*- has been reacted stepwise with different metal salts to give both mono and bimetallic complexes. In this way cobaltocenes and half-sandwich rhodium cyclopentadienyl complexes have been prepared, three of which have been structurally +

*Thefollowing abbreviations are used throughout: Cp (q5-C5H5),Cp' ($-C,H,Me), Cp' (q'-C,Me,), Cp*(general Cp), Fp (Fe(COLCp),dppf (1,l'-bisdiphenylphosphinoferrocene), dippf (1,l'-bisdiisopropylphosphinoferrocene) -

Organometallic Chemistry, Volume 3 1 0 The Royal Society of Chemistry, 2004 393

394

Organometallic Chemistry

characterised. The dialane [(Cp*Al -*A1(C6F5)3],the first valence isomer of a dialane, has been obtained simply by treating [Al(Cp)]4 with A1(CsF5)3.6 Again the synthesis of the first main group triple decker cations have been reported. These are based on stannocenyl structures. As an example one of these [Cp*Sn(p-q5-Cp*)SnCp*][Ga(C6F5)4] is shown as 1. The molecule exhibits an interesting tilted s t r u ~ t u r eThe . ~ synthesis and characterisation of a fluorinated tris(pyrazoly1)borate complex has been reported in which an (q5-Pyrazo1e)-K+ interaction is observed in the solid state.8 The alkyl Y(q5:q1complex [Y(q5:q1C5Me4CH2SiMe2NCMe3)(CH2SiMe3)(thf)] has been observed to quantitatively

1

convert to the hydride dimer [Y(q5:q1-CsMe4CH2SiMe2NCMe3)(thf)(p-H)l2 by hydrogenolysis? Half-sandwich calcium complexes have been used in both the living and stereoselective polymerisation of styrene." The calcoacene complex [(q5-C12H&Ca (thf)2] and its Yb analogue have been prepared by reduction of the parent acene activated by iodine." An interesting bent metallocene show as 2 has been obtained in the reaction of YC13with the dianion [Cs(Me)4Si(Me)2N-(2pyr)]-. Copper was subsequently bound between the two nitrogen atoms in the complex in place of the lithium shown in the figure.12 The nature of the metal-ligand bond in a trivalent neodymium complex which contain neutral x-donors has been calculated using Hartree-Fock and density functional theory-based methods. These complexes are based on [Cp2Nd] structures.13Anew approach has been taken towards the synthesis of endohedral metallocenophanes - this new method makes use of alkynes which are tethered to the x donor ligand. As an example of the method the cobalt complex shown as 3 has been prepared by reaction of [1,3-('BuC&-(CH2)nC [q5-C5H3( 1 3 (tB~C=C-(CH2)n-)2)] The 2 + 2 intramolecular cycloaddition of the alkynes occurs to assemble the second (in this case) cyclobutadienyl ring.14 In related work the double deprotonation of a cyclopentadienyl alkene has been observed. In this work the bright yellow complex [($C5Me4SiMe2(CH2CH = CH2)])Y(CH2SiMe3)2(thf)2] was prepared from the neutral cyclopentadiene [(C5Me4)SiMe2(CH2CH = CH,)] and the complex [Y(CH2SiMe3)3(thf)2] directly. In an attempt to characterise the product, it was +

14: Transition Metal Complexes of Cyclopentadienyl Ligands

395

2

observed that it changed to a dark red compound(s) over several days. These dark red products contained a new type of trianionic cyclopentadienyl alkyl ligand [Me2Si(C5Me4)(C3H3)]’-.Thus double metallation had taken place. A structure of one of these products (which is the dimethoxy methane complex rather than the thf one) is shown as 4.15 The orange-red crystalline complex [Cp* Sm(M-]H2)I6[( M-H)K(thf)2] has been obtained in the reaction of the green [Cp*Sm(CH(SiMe3)2)Cp*K(thf)2]n complex with H3SiPh.The X-ray structure of this complex has revealed it to be a polyhydride Sm(H)K cluster which consists of six Cp*SmH2 units and three HK(thf) units. For clarity this is structurally shown as 5.16

396

Organometallic Chemistry

In a general reference iron (11) to cobalt (11) charge transfer has been studied in methylene linked rnetall~cenes,'~ Vanadocene has been observed as an intermediate in the vanadium catalysed synthesis of magnesium dicyclopentadienide.'*

14: Transition Metal Complexes of Cyclopentadienyl Ligands

3

397

Titanium, Zirconium and Halfniurn

Once again the majority of articles in this section have been on complexes which have been used as catalysts in olefin polymerisations. As the number of these articles is enormous the reviewer has been selective in choosing those articles in which the molecules are structurally interesting. The zirconocene complex [Cp2Zr(p-H)]2(p-H)AlC12 and its titanium analogue have been prepared in the reducing reactions of Cp2ZrC12 and Cp2TiC12 with LiA1H4.l9The reactions of Cy7Si709(OH)3with Ti(OEt), and Cp2TiCl2 lead to the formation of [(Cy7Si7012)Ti(p-OEt)-(EtOH)]2and (p-O)-[(Cy7Si7011(0SiMe3)}TiCp]2 respectively. These complexes have been prepared as model species for silicon supported Ti catalysts. The molecular structures of these products have been determined?O The thermolysis of [(q5-C5Me4R)2Ti(q2-Me3SiC'CSiMe3)] R = Bz, Ph, p-F-phenyl, gives rise to the titanocenes of the type shown as 6 and 7 where one of the cyclopentadienyl ring has assumed the 'tucked in' geometry?

6

7

Unbridged metallocene dichlorides with indenyl ligands which have a substituent in the 2-position of the indene have been prepared and utilized in propene polymerisation - these give polymers with alternating atactic and isotactic blocks?2 The racemic isomers of ansa-zirconocene biphenolate complexes have been obtained in the reaction of o,o'-disubstituted zirconium biphenolate dichlorides with Mg or Li salts of ethene or Me&-bridged bis-indenyl ligandsF3 The reaction of [(q5-RC5H4)2Ti(Me3SiC2SiMe3)]R = H, iPr with B(C6FS)3gives rise to the formation of Zwitterionic complexes of the type [(q5-RC5H4)q51,3-R-C5H3B(C6FJ3],which can both be air oxidised to A series of titanoxanes or reacted with acetone to give acetone addu~ts.2~ zirconocenes bearing 2,2-dimethyl-2-sila-4-pentenyl substituents and related analogues have been prepared and used in olefin epimerisation? The complexes of the type [Cp*M(bis-dithiolates)] have been prepared, where M = Zr, Hf.

398

Organometallic Chemistry

These were oxidised by iodine or TCNQ in solution to give 1,1.3 or 1.8 electron oxidised complexes, for example [Cp2Zr(C8H4S8] , [CpZr(C8H4S&J0, [M(Cp*)(C8H4Ss)2]o.3etc. The oxidation of these complexes has been studied in detail and a range of conductivity measurements undertaken.26 The synthesis of the titanium compound [(MeCsH4)TiC1(p-NSiMe3)]2 in which migration of a SiMe3 group is observed during the synthesis and the preparation of the related zirconium cyclopentadienyl complex [Cp2ZrC1(q2NHNCHSiMe3)] have both been reported. These complexes are shown as 8 and 9.27 Again the synthesis and crystal structures of the interesting organotitanoxanes [CsMe5TiMe2]2(p-O) and [(C5Me5)4Ti4Me2](p-0)5, have appeared the former of which is shown as +

8

9

10

A rhodium complex has been prepared on a titania-silica model compound as a support. The synthesis of the two dimetallic titanium alkyl siloxide complexes [Cp*TiMe(02SiPh2)12and [Cp* Ti(02SiPh2)3TiCp*] are both reported in this article. The former of these is reacted with [{Rh(p-OH)(COD))2] to give the product compound [{Cp*Ti(02SiPh2)(p3-O)Rh(COD))2]F9 The mechanism of [Cp2TiMe2]-catalysed intermolecular hydroamination of alkynes has been examined in detail and the kinetics have been used to deduce the mechanism.30Similarly the mechanism of the related hydroamination of alkenes has also been studied?' A novel synthetic route has also been devised to titanium fulvene complexes. The route is a simple one in that it involves the magnesium based reductive reaction between CptTiC13 and the fulvene. The product complexes have the a-carbon on the cyclopentadieyl ligand bound to titanium. This is best shown pictorically and is shown as ll.32 In an article with an intriguing title on hexaphosphapentaprismane(PbCiBu4) chemistry the unit has been used to make new zircon~cenes.~~ It has been shown that vinyl fluoride reacts with Cp2ZrHC1to give Cp2ZrFCl and ethane as the primary product in a study which has highlighted several interesting mechanistic points during the reactions of Cp2ZrHCl with fluorobenzene, p-methoxystyrene and obviously fl~oroethene.~~ The structure of [(q5:q1C5R4SiMe2NBut) Ti (OCMe2CH2CH2CH2C = CH2)+], R = Me has been published as a model system for investigation of intermediates in constrained geometry catalyst^.^' A related article on the 'double activation' of constrained geometry and ansumetallocenes has also been published?6A study of the mechanism of zirconocene

14: Transition Metal Complexes of Cyclopentadienyl Ligands

399

aluminate methylmethacrylate polymerisation using N.M.R has been carried Finally a general article on tethered ansu metallocenes has detailed their use in olefin polymerisations? 4

Vanadium, Niobium and Tantalum

Vanadocene dichloride has been used as precursor to attach the cyclopentadienyl vanadium function to a silica surface. Following hydrolysis and calcination a silica-bound vanadium oxide is f0rmed.3~The synthesis and structure of the interesting tantalum dihydride [Cp2Ta(q2-H2)(CO)] +,as the first thermally stable dihydrogen complex from the group 5 elements, has been described? An interesting half-metallocene tantalum complex which has been appended to methylmethacrylate (MMA) has been prepared and utilized as a catalyst in polymerisation experiments The synthetic methodology made use of [Cp*TaC&] as a precursor, which was reduced with sodium amalgam then reacted with MMA to give the MMA complex [Cp*TaC12(q4-supine-MMA)]. Optionally this compound could also be prepared by treatment of the dinuclear Ta"' complex [(Cp*TaClt}2] with MMA. The further reaction of the product compound with one equivalent of the dilithium salt of 1,4-bis(p-methoxyphenyl)1,4-diaza-1,3-butadiene (p-MeOC6H4-DAD)or the dilithium salt of 1,4-dicyclohexyl-1,4-diaza-1,3-butadiene (Cy-DAD) in THF gave rise to the half-sandwich DAD complexes of tantalum. The poymerisation of methyl methacrylate were subsequently investigated using trimethylaluminium as an a c t i ~ a t o rThe .~~ reaction of the phosphaalkyne 'BuCmP with [Cp* TaC12 (C0)z (thf)] leads to the rapid displacement of the carbonyl ligands to give the interesting q2 - 4e phosphaalkyne complexes shown as 12!2 Vanadocene dichloride and vanadocene acetylacetonate have been found to act as potent antiproliferation agents in anti-cancer st~dies.4~

400

Organometallic Chemistry

12

5

Chromium Molybdenum and Tungsten

The hydrogenation, carbonylation and protonation reactions of [(q5-C5Ph5)2W] have been carried out and in this work the first reported dicationic complex of the form [(q5-C5Ph5)2W]2+ has been documented: this is [(q5-C5Ph5)2WI2+ [CF3SO3I2and is shown as 13(cation only).44The thermal reaction of Mn2(CO)lo and MoCp2(SPhb has given rise to an unexpected product in the form of [Cp2Mo(CO)H] [Mn3(C0)9(~-SC6H5)~]-.45 The complex [Cp2Mo(H)OTf] is water soluble and has been observed to undergo H/D exchange in D20. It reduced ketones and aldehydes under very mild conditions. This compound has been structurally characterised which is shown as 14.46 +

13

14

Cyclopentadienyl ligands functionalised with quinoline or N,N-dimethylaniline have been used to form rigid half-sandwich Cr-complexes which are highly active in olefin polymerisation. An example of one of these, the quinoline complex, is shown as 15.47 In a special issue dedicated to organometallic chemistry in Portugal the crystalline inclusion complex of Cp2MoC12 with fbcyclodextrin has been re-

14: Transition Metal Complexes of Cyclopentadienyl Ligands

401

ported. The complex was characterised by powder X-ray diffraction methods. The study concludes that the CpzMoC12unit is incorporated into the cyclodextrin rather than any of its hydrolysis products? Unexpected similaritiesbetween [C~MO(CO)~]units and [PtI(CO)r(PPh3)]- units to act as four electron in metal cluster complexes have been observed. These similarities are highlighted by the two structures shown as 16 and 17.49A density functional study of the highly adaptable molecular structures of Mo(V) and W(V) dithiolenes and related complexes, such as the tungsten example shown as 18 have been performed.

16

17

For example the complexes Cp2M(dithiolene)(M = Mo",W") d2 exhibit a folding of the MS2C2 metallacycle along the S-S axis upon oxidation to the d'-paramagnetic cation. A range of calculations have been performed in this study combined with the practical synthesis and structural characterisations of the products obtained? The synthesis of a cyclopentadienylwith uniquely three different chacogenido S2- and Se*-) has been reported in a synthesiswhich uses the halide ligands (02-, complex [Cp*WC&] as the precursor. This is reacted stepwise with

402

Organometallic Chemistry

18

Me3SiSCH2CH2SSiMe3,H20/Et3N/”HF and PPLBr/CH3CN to give [PPhd] [Cp*W(O)(S)2] which is then reacted in a further two steps to give the product [Cp*W(O)(S)(Se)].” A number of chromium cyclopentadienyl-basedcubanes of the type [Cpf,Cr& (C(S) Net2)2}]and [Cp6Cr8S8(S2CNEt2)2)have been prepared. The synthetic method utilises the reaction of [{CpCr(CO)8)2] with tetraethylthiuram disulfide to give [CpCr(C0)2(S2CNEt2)] in high yield (87%) at room temperature whereas at 90”C a mixture of compounds was obtained which included [{ CpCr(CO)2)2S] [Cp4Cr4S4], [CpCr(CO)2 (SCNW], [Cr(S2CNEt2)3], [Cp6CrgSs{C(S)NEt2}2}]and finally [Cp6Cr8S8(S*CNEt2)2],the latter of which is shown as 19.52

19

A series of ‘solitaire and gemini’ metallocene porphyrazines have been prepared where either biscyclopentadienyl- vanadium or molybdenum have been

14: Transition Metal Complexes of Cyclopentadienyl Ligands

403

anchored via dithioate-type linkages to the macrocyclic ligand. This is best shown illustratively as the crystal structure of one of the vanadium complexes, shown as 20.53

In a study on the reactions of the complexes [Cp2ML], M = Mo, W, L = ethene, CO the reactions with pefluoroalkyls have been investigated it has been observed that [Mo(Cp)2(C2W4)] reacts with perfluoro-n-butyl iodide or perfluorobenzyl iodide with initial loss of ethane to give the fluoroalkyl complexes [Cp2Mo(L)If, L =-(CF&CF3;-CF2CbFS, the latter of which has been crystallographically characterised. In the case of the starting complex containing a carbonyl ligand the alkylation still occurs but without CO loss therefore the product is obtained as the iodide salt e.g. [Cp2Mo(CF2C6F5)(C0)] +I-.54Whilst the reaction of methylmagnesium or methylaluminium compounds with [Cp*Mo(NO)Cl(p-Cl)$ gives rise to metathesis of the chloride for methyl to afford [Cp*Mo(N0)Me(p-Cl-J2it is also observed that the reaction with methyl lithium generates the same compound but additionally the trimethyl complex [Cp*MoMe3(NO-Li(OEt)&] is also obtained which on treatment with a source of [Me] leads to the terminal alkoxylimido species [Cp*MoMe3(=N-OMe)]. Additionally on treatment of the original precursor compound with methyl lithium in toluene results in the formation of the dimethyl complex. [Cp*Mo(NO)Me] which is a rare example of the 16-electron compound of this type. This latter compound can also form 18e complexes on reaction with a number of Lewis bases.55 +

6

Manganese Rhenium and Technetium

Chiral fLaminoalkoxy(pheny1)carbene complexes of the type [Cp(CObMn =C(0R)Ph.f have been obtained in the reaction of the 0-Ac car-

404

Organometaflic Chemistry

benes with a range of chiral alcohols, for example, N,N-dimethylalaninol, N,Ndimethylvalinol e t ~The . ~ first ~ nucleophilic carbene complexes which contain 'slipped' cyclopentadienyl ligands have been reported. These are formed when manganocene is treated with the 1,3-dimesitylimidazol-2-ylidene ligands of Arduengo and c o - ~ o r k e r sDuring . ~ ~ the thermolysis of the alkynylcarbenecomplex [Cp*(C0)2Re= C(Ph)13C=C13Tol]at 120°C a rapid equilibration is set up where the phenyl and tolyl groups obsensively change places. The labelling study showed the mechanism of this process and evidence was provided for siteselective C-H insertion?8 An interesting manganese tricarbonyl capped ruthenocene shown as 21 has been prepared and structurally characterised.

21

W

The corresponding ferrocene and osmocene have also been reported and their physical and electrochemical properties of all three compounds have been compared. The synthesis utilized the [($-naphthalene)Mn(COh] transfer reagent.59 Planar chiral tricarbonyl(cyclopentadieny1)manganese complexes have been prepared beginning with planar chiral ferrocenes. In this methodology the cyclopentadienyl ring is transferred to the manganese from the iron in dichloromethane by displacement of naphthalene in the complex [(CloH8)Mn(CO)3] BF4.Interestingly, during the transfer process complex inversion of the absolute ring configuration occurs which suggested an intermediate species where the manganese initially reacts on top of the cyclopentadienylring which is ultimately transferred.m Bridged complexes of the type [(q5-C5R4Me)L(ON)Mn(pXY)Mn(C0)2L(dppm)y, X,Y = C, N, z = 1-3 have been prepared, where the pseudo tetrahedral unit [Mn'X(NO)L(Cp')] is joined to an octahedral unit. The interest arises from the electron transfers between the metal centres which are studied in detail in this report? The straightforward synthesis of +

14: Transition Metal Complexes of Cyclopentadienyl Ligands

405

[(Cp*Re)2B7H7]from the reaction of [ ( C P * R ~ H ~ ) ~ Bwith ~ H ~BH3THF ] has been reported and the development of this reaction has allowed synthesis of the compounds [(Cp*Re)2B,H,], n = 8-10, which although they are obtained as in a mixtures because they are insensitive to air and moisture they can be separated easily by thin layer chromatography. The structures of two of these new compounds namely [(Cp*Re)2B9H9]and [(Cp*Re)zBloHlo]are shown as 22 and 23.62 Planar-chiral rhenium complexes, a typical structure of which is shown as 24, have been used as ligands in the asymmetric phase transfer of phenyl groups (from organozincs) to aldehydes. The product enantioselctivities are extremely high and are in general are better those of the related chiral ferrocene-based ligand~.6~ A one pot synthesis of cyclopentadieyl-derivativesof 99nTctriceboyl has been devised. Clearly because this is a useful nucleus in diagnostic medicine it means that the simple and effective synthesis is extremely useful. The method makes use of the reactions of acetylcyclopentadiene.64

22

23

The one part of synthesis of indenyl- and cyclopentadienyl-rhenium complexes has been reported in the reaction of CpSnMe3 or indenyl SnMe3 with BrRe(CO)Sor (EhN)2ReBr3(C0)3.6s

406

7

Organometallic Chemistry

Iron, Ruthenium and Osmium

The condensation reaction of [Cp2Fe&0)2(p-CO)(p-C-CH3)] [BF4]- with a range of bithienyl aldehydes has been used as a synthetic method to prepare further molecules for use in non-linear optical applications. A ferrocene has been appended to these molecules to investigate its donor properties in these speciesM Similarly, a range of complexes such as [FeCp(dppe)(p-NCC,H,N02)][PF6] have been prepared for use in NLO work.67The reaction of [(p-S,){CpFe(COh)] with RSCO2C1where x = 3, 4; R = CF3, CC13, C6Fsresults in a metathesis reaction to give the complexes [CpFe(COhSS(0)zR] which contain polysulfur oxide ligands. One of these which has been structurally characterised is shown as 25.6' +

25

The preparation of the p-q':q6-ary1ethynyl diruthenium complexes [Cp(PR3)2Ru(p-q':116-C~CC6H4Me-p)RuCp* J.Cl, R = Ph, Me have been reported and their reactions with 12, CO and H + have been investigated. As an example the structure of one of these complexes where R = Ph is shown as 26.69 Six doubly bridged bis-cyclopentadienyl-tetracarbonyldiiron complexes have been prepared by reaction of Fe(C0)5 with a number of doubly-bridged cyclopentadienyl precursors where the bridging atom@) was C, Si or Ge.70The hydrofunctionalisation of alkenes, with a range of nucleophiles, promoted by diruthenium complexes of the type [{(q5-CSH5)2(SiMe3)2)R~Z(C0)3(q-CH2 = CHR)(p-H)] i-has been reported by Algeli~i.~~ Intramolecular C-H insertion reactions of Fp-carbenes has been used in the synthesis of (k)sterpurene and (k) ~ e n t a l e n e . The ~ ~ photochemical reaction of the complex [(SiMe2)([q5C5H4Fe(C0)2SiMe2SiMe2R]2where R = Me, Ph results in the elimination of ~ilylene.7~ It has been observed that the substitution of acetonitrile ligands in a planar chiral ruthenium complex, which contains a cyclopentadienyl-linkedphosphine ligand with a triphenylphosphine, proceeds enantioselectively resulting in the

14: Transition Metal Complexes of Cyclopentadienyl Ligands

407

26

formation of a metal-centred chiral c~mplex.'~ The reaction of [CpRu(PR3)(MeCN)2]PF6,R = Me, Cy, Ph with nona-2,7-diyne, deca-2,8-diyne and hex-1-yne results in either a 1,2-hydride shift or phosphine migration dependent on the nature of the alkyne? The thermal reaction between [CpFe(CO)(q2-SiMe2-OtBu-SiMe2) and [(Me3Si)3EH](E = Si, Ge) at 80°C produces [(Me3Si)3ESiMe2H],whereas under photolytic conditions unexpectedly there is no The heterobimetallic complexes [Cp*Ru(p-HbMH3(Cp*)] M = Mo, W have been prepared in the reaction between [Cp*RuC1I4 and [Cp*MoC&] under reducing conditions (LiB€Q.77The hybrid density functional method (B3LYP) has been used to examine the H- migration to the carbon in C 0 2 on the complex [$(C5H4(CH2)3NMe2H+)RuH (dppm)] based on the assumption that the reaction pathway is either COzforced abstraction or C 0 2'insertion' into the Ru-H bond without pre-coordination? A series of K2-S,S-dithiocarbamatecomplexes of the type [CpRu(PPh3)S2CNR'R2](R' = SiiPr3,R2 = Ph, p-tol, 1-naphthyl;R' = H, R2 = Ph) have been prepared in the reaction between [ C ~ R U ( P P ~ ~ ) ~ SorS ~ ' P ~ ~ ] CCpRu(PPh3)SHI and appropriately substituted i~othiocyanates.7~ In a short note the synthesis of a number of ferrocenes and ruthenocenes have been prepared in the reaction of protected amino acid-substituted cyclopentadienyl anions with FeC12 or [(PPh3)3RuC12].80The synthesis of a series of sila and disila-benzene complexes of '[Cp*Ru]' have been described: in this work the reaction of [Cp*RuC1I4 with Li[C5H5SiH('Bu)] gives the neutral complex [Cp*Ru(q5-C5H5SiH('Bu))] which affords [Cp*Ru(q6-C5H5SitBu)][BH(C6F5)3] on proton abstraction by B(C~FS)~.~' Chiral half sandwich ruthenium complexes have been prepared by displacement of thiophene from [S-(thienyl) Ru(Cp)(chiraphos)] by a thionolactone which is ring opened by a tropo-dia-

408

Organometallic Chemistry

stereoselective ring cleavage.82The reactions of [(q5-indenyl)Ru(PPh3)2Cl]with CH2C12and CHC13 and base results in the formation of the complexes [($indenyl)Ru{CH2PPh2(C6H4)}(PPh$]and [(q’-indenyl)R~(PPh~)(CO)Cl].~~ The origin of enantioselectivity in transfer hydrogenation reactions of aromatic carbonyl compounds which are catalysed by chiral $-arene ruthenium complexes has been examined using molecular modelling. A detailed description of the thermodynamic and kinetic parameters is given to justify the proposed mechani~rn.~~ Continuing in the same vein the synthesis of a cyclopentadienyl ruthium complex which is capable of proton accepting and anti-Markovnikov hydration of terminal alkynes has been devised. In this work [CpRu(CH3CN)3] +CF3S03-is treated with a phosphine-substituted imidazole to give a complex which traps a water molecule in its cleft. This water can be displaced during catalysis and this provides a template for the hydration rea~tion.~’ Yet another complex which includes the P4 unit has been obtained by displacement of a chloride ligand in [Cp*-Ru(L)2Cl], L = PEt3,1/2dppe to give [Cp*Ru(L)2P4] +ClP An unusual bonding mode has been observed for an amidenate in a ruthenium cyclopentadienyl complex. Essentially what is observed is a p-amidenate ligand it is produced in the reactions of q-amidenate complexes with excess [Cp+ RuXI4. An example of this type of complex is [Cp+Ru(p2-’PrN = C(Me) N~P~)Ru(X)CP+].~~ The synthesis of nido-[ l-OMe-2,3-(Cp*Ru)z{ pP(OMe)2}B3H5]in which methoxy transfer from posphorus to boron and cluster core rearrangement is observed.88 7.1

Ferrocenes, Ruthenocenes and Osmocenes

A special issue of the Journal of Organometallic Chemistry was dedicated to ferrocene It opens with a series of personal recollections from those directly inv01ved?O-~~ This should prove useful to future historians on the subject. What follows in the volume is a demonstration of the utility of ferrocene in organometallic chemistry. The first article in this issue is a review on the cyclometallation reactions of ferrocene with 100 references on the topic. The focus, not surprisingly is on the metallations with palladium, platinum and mercury, which are predominant in this area.95There are a significant number of articles which feature the chemistry of ferrocenylphosphines: the ortho-silylated derivatives of 2,2’-bis(oxazoliny1)1,l’-bis(dipheny1phosphino)ferroceneshave been used as ligands in palladium catalysed allylic alkylations reactions.96This further demonstrates the utility of ferrocenyl oxazolines since the first report by Sammakia and Richards in 1995. Oxazolines have also been proposed as voltametric sensors in a short report.97 Continuing on in their excellent work in the area the Broussier/Meunier group have elaborated on the synthesis of a tetrakis-diphenylphosphinoferrocenebeginning with dicyclopentadiene as the starting material. The ligands obtained are 1,1’,3,3’-tetra- dialkyl- or aryl-phosphinoferrocenes. Interestingly testing this synthetic procedure in our own research group showed this to be a particularly

14: Transition Metal Complexes of Cyclopentadienyl Ligands

409

useful and facile synthetic method.98 Complexes of the general type [(cym)OsCl(L)](PF6) where L = dppfand dippf, cym = p-cymene have been obtained in the reaction of the ligands with [(cym)OsC12] by activation with TeN03. The product complexes have been fully characterised by electrochemistry.99 Further reactions of dppf and ppfa with the osmium cluster [Os3(pH)(CO),&.q2-N02)] result in the formation of the new ferrocene-containing clusters [Os3(pH)(CO),(p-q2-N02)(L)],L = dppf, ppfa.'O" The extension of the work on diferrocenyltriphosphane ligands towards ruthenium complexes has been carried out and interesting deligation properties have been observed which have potential for pH controlled switchable catalysts. An example of the one complex obtained together with a scheme from the paper are shown in 27."'

a

27

Further cluster chemistry with dppfthis time with ruthenium carbonyl clusters has produced complexes which incorporate selenium into the clusters. Again an example is shown as 28.'02 A number of crown thioether complexes have also been reported which contain dppf as a ligandlo3and the synthesis of [Rh(dppJ)]", x = 0, -1, have been reinvestigated.'@' In the latter study both complexes have been characterised by single crystal diffraction and the structures are shown as 29 and 30. Again thiocarboxylate palladium complexes which contain dppf as a ligand have been obtained by a simple synthetic procedure and these have also been crystallographically ~haracterised.'~~ A series of chiral ferrocene based ligands were used in palladium catalysed allylic substitution reactions of racemic 1,3diphenylprop-2-enyl acetate with diethylmalonate and benzylamine. The chiral ferrocene ligands were prepared from the reactions of enantiopure cyclohexyldiamines and 2-diphenylphosphinylferrocene-carboxylicacid.lMThe reactions of dppf with technetium and rhenium complexes of the type [M(N)Cl,]- and [M(NPh)C13(PPh3)2],M = Tc, Re has resulted in the formation of the complexes [M(N)Cl,(dppJ)] and [M(NPh)C13(dppj)]in which the dppf coordinates in the

410

Organometallic Chemistry

28

29

30

31

equatorial plane of a distorted square pyramid - an example is shown as 31 for the Re complex.1o7 Detailed electrochemistryhas been carried out on a number of boranato-substituted dppfligands: the main conclusions reached in this work is that a one electron oxidation leads to the formation of the more stable ferrocenium congeners. A discussion on the stability of the ferrocerium cations is discussed, however the ultimate fate of the compounds is the oxidation of the phosphorus to the oxide in the presence of trace water."* In an interesting piece of work alkyl palladium complexes have been prepared from dppfand dippfand their insertion chemistry has been studied in detail with carbon monoxide.'0g Finally in a second paper which details chemistry arising from ferrocenyl oxazolines a number of ligands have been prepared in which the ligands contain a phosphine, a carboxylic acid and an exchangeable trimethylsilyl group.'" A large number of papers in this issue are targeted at the synthesis of arrays of ferrocenes or compounds derived from ferrocenyl These are

14: Transition Metal Complexes of Cyclopentadienyl Ligands

411

individually detailed as follows: the first paper in this series is a discussion paper on the molecular architectures of ferrocene polymers which is essentially a mini review with approx 200 references which includes discussions on derivatives containing ferrocenes etc.' l' The first paper on ferrocenylacetylene itself deR = scribes the reaction of the cationic complex [q5-CpRu(CH3CN)2(PR3)]+, Ph, Cy with ferrocenylacetylene. The products obtained are q-ally1 ruthenium complexes one of which has been structurally characterised and is shown as 32, [CpRu( = C(Fc)-q2-CH=C =CH(FC)(PP~~)]PF~.''~

32

This paper is followed by one on the reaction of ferrocenylacetylene with [CpRu(l,S-C0D)Brf in which a 2,5-bis(ferrocenyl)ruthenacydopentadienecomplex is ~btained."~ The [2 2 2]cyclotrimerization reactions of bis-arylacetylenes which bear ferrocenyl units with planar chirality has been described. A synthetic scheme taken from the paper is shown as 33. This highlights the syntheticprocedure when the alkyne is cyclotrimerizedby cobalt, beginning with the chiral0- protected ferrocene carboxaldehyde which is ortho-metallated and catalytically coupled with bromoarylalkynes. The product complexes such as those shown in the scheme have been subjected to preliminary electrochemical investigations.' l4 Simpler star shaped compounds have subsequently been reported in which ferrocenes have been attached to the perifery of a triarylamine core with thiophene spacer groups."' Even simpler versions follow which are ferrocenylacetylenes appended to fluorenes.' l6 More interesting are the so-called star-shaped complexes derived from the Heck coupling of vinyl ferrocene and related diferrocenylvinyls with tribromobenzene. One such product obtained in this manner is shown as in the scheme 34 and contains six ferrocene units."' The palladium complex of the (ferrocenylacetylene-substitutedbenzonitrile) ligand shown as 35 has been obtained and fully characterised."* Semi-protected ferrocene dicarboxaldehyde has been similarly used as a key synthon in con-

++

Organometallic Chemistry

412

34

jugated ferrocene arrays. Although these is essentially nothing new in the methodology a number of potentially interesting core molecules such as masked ferrocenylvinylaryls have been obtained."' An interesting trityl diradical has been obtained essentially using divinyl ferrocene as the cement between two traditional polychlorinated triphenylmethyl radical centres. Using ESR it has been demonstrated that the ferrocene bridge acts as a ferromagnetic coupler of both radicals.120On a simpler theme a number of alkyne-substituted ferrocenylacetyleneshave been prepared and redox correlations have been made which related to the nature of the substituents.121A series of three ferroceneswhich each contain two pyridine ligands (vinyl spaced)have been prepared and their copper(1)and zinc (1I)complexeshave been studied. Again the factors which affect the redox properties and colours of these

14: Transition Metal Complexes of Cyclopentadienyl Ligands

413

35

ligands on complexation have been reported. A molybdenum complex of these ligands has also been structurally characterised in this particular report.122 The theme of using ferrocenes as redox sensors for anions is continued in a paper which reports the synthesis of new amide-containing ferrocenyl ligands. Such a group of ligands as represented in the paper are shown as 36.

36

The following anions: F-,HS04-, H2P04- and ATP2- have been used as analytes in the experiments. Tabulated data is given indicating the electrochemical response of the ferrocene to the presence of the analyte in the ligand array.'23 The 'on column' approach has been used for the synthesis of ferrocene-substituted oligodeoxynucleotides- palladium catalysed cross coupling is used in this synthesis beginning with ferrocene propargylamide and a bromo-adenosine or iodo-uridine.'" The molecules prepared were characterised spectroscopically and electrochemically before being used in duplex studies. A further paper describes the conjunction of ferrocene with fluorene; this time using a vinylspacer group. This particular paper is number 12 in a series - essentially the

414

Organometallic Chemistry

fluorenes are coupled directly to ferrocene carboxaldehydes in DMF in a simple synthetic methodology. The most interesting feature of the work is the direct correlation between the energy of the intermolecular charge transfer bond with the q,-values for the fluorene substit~ents.'~~ It has also been observed that ferrocene naphthalenediimide combined to DNA.RNA hetero duplex, which, it is claimed, will have potential use in the electrochemical detection of mRNA expression.The actual synthesis in making the tagging molecule involved the use of ferrocene carboxylic acid as a reagent which was attached to a pendant alkylamine spacer which was in turn coupled to the naphthalenediimide.'26A series of ferrocene end-capped complexes of the type [CpW(C0)3(CC),Fc] have been prepared and their reaction chemistries examined, e.g. reaction with tetra~yan0ethene.l~~ Many unusual potential estrogen receptor modulators have been obtained which are all crystallographically characterised.128Some elegant semi-masked 1,l'-diethynylferrocne chemistry has been developed by the Schottenberger group - this uses 1'-ethynyl-ferrocene carboxaldehyde as a key reagent in the synthesis. The interesting metallocenophene shown 37 in (as compound 12 in the paper) has been produced which has a very high ring strain energy. In addition a broad range of interesting ethynylferrocene have been produced.129

37

38

In a short paper the synthesis of ferrocene-substituted chlorostilbenes and butadiene is described - again the synthetic method utilises ferrocene carboxaldehyde as the key synthesis.' 30 1,l'-Bis[4-2,2':6',2''- terpyridinyl-4'-yl)phenyloctamethyl ferrocene, shown as 38 has been obtained directly from lithiated 4'-[4;2,3,4,5-tetramethylcyclopenta-1,3-dien-l-yl)phenyl]-2,2',6',2''-terpyridine this compares with the earlier direct synthesis of similar molecules beginning with ferrocene (monosubstituted compounds). This synthetic route now offers a high yielding a1ternati~e.l~' More work has been carried out on the attachment of ethyne-linked ferrocenes to phthalocyanines. These should prove to be interesting molecules for future colourists. The product molecules have been examined using UV-visible spectro~copy.'~~ There is continued interest in the chemistry of (E)-1-ferrocenyl-2-(l-methyl-4-pyridiniumyl)-ethene:in particular the charge transfer properties of these compounds have been investigated in addition to its

14: Transition Metal Complexes of Cyclopentadienyl Ligands

415

solvato- and photo-chromic ~r0perties.l~~ Again, more work has been carried out to obtain the diffusion coefficients of n-conjugated polymers which contain the ferrocene unit. The design motif is again to use ferrocenes with ethyne spacers.'34Finally on this topic, further work has been carried out on ferrocenehexanethiols appended to gold surfaces and more data on the nature of self-assembly process has been acc~rnulated.'~~ The reaction of ferrocene bis-carboxaldehyde with 2,3-bis(hydroxyamino)-2,3dimethylbutane results in the formation of a mixture of the compounds -ferrocene 1'-formylferrocene iminonitroxide and 1'-formyl-ferrocenenitronyl nitroxide when the reaction is carried out in thf, which is a variation of the previously used procedure of Lamchen and Mittag. Three crystal structures are reported in this paper, two of which are the aforementioned ferrocene-containing reaction products.136 A large scale synthesis of [FCC(NC~)~] has been reported - the synthesis involves the reaction of ferrocene with t-butyllithium in thf followed by the addition of dicyclohexylcarbodimide.The product was isolated by sublimation. The reaction chemistry of this key compound (deprotonation etc) was subsequently investigated and some of its coordination chemistry, particularly with Continuing their work on planar chiral 1,2-disubstituted ferrocenes the Manoury group have reported the synthesis of a series of new chiral pyridinium salts for use in non-linear optic applications. In this work 4 new such salts have been prepared and their Second Harmonic Generation bulk efficiences have been measured- these range from 0-0.25 times the efficiency of that of urea.138 The reaction of FpI and CpLi has been reinvestigatied with the aim of further deprotonating the sigma cyclopentadienyl ring in the product. This indeed has been achieved and this has allowed for the preparation of a range of a ferrocenes substituted with half metall~cenes.'~~ A series of Ru(I1) complexes have been prepared which contain 4-ferrocenylisocyanide ligands. The crystal structure of one of these, the trans,trans,trans- [RuCl2(POMe-P)2(F c C ~ H ~ N C ) ~ ] where (POMe = PPh2C&OCH3, has been reported." An electrochemicalstudy into the behaviour of ferrocene-containing open chain and macrocylic oxaza-, polyaza compounds has been carried out in a~etonitrile.'~' Mossbauer and structural comparisons have been made between ferrocene and its open-ring and half open ring analogues. The results obtained are consistent with previous bonding theories, which imply greater metalfligand orbital mixing in open pentadienyl l i g a n d ~ .An ' ~ ~interesting article on the synthesis of a vinylene-bridged ferrocene, in which a hydoquinone is appended to a ferrocene and proton coupled electron transfer has been investigated. 2-(2-Ferrocenylvinylhydroquinone, was synthesized using Wittig coupling with ferrocenylmethyltriphenylphosphonium bromide and 2,5-ditosylbenzaldehyde,followed by a deprotection step. This compound was then reacted with ferricinium salts as 1-electron oxidising agents. It was observed that bond rearrangement occurred on two electron deprotonation according to those shown in figure A from the a~tic1e.l~~

Organometallic Chemistry

416

I

t

A

Novel molecular assemblies have been made such as[Cr("')00SQ)3-n(&Cat)n] -n (X,Cl and Br; n = 0, 1, and 2), with metallocenium cations, [M("')Cp2] + (M ) Co and Fe). An example of a complex which has been structurally characterised is (Fe111Cp2)[Cr**1(Br&3Q)2(Br4Cat)]CS2, where SQ = semiquinonate, and Cat = catecholate),which is shown as 39.

39

These molecules form ordered arrays in the solid state and the paper documents the extended macrostructures which ensue.144In a study on the cooperative static quenching of the S1-Sofluorescence of tetrasulfonated aluminum phthalocyanine by azaferrocene and an organic dons such as imidazole, 4aminopyridine it has been shown that some other nitrogen ligands can cooperate with azaferrocene,to give non-fluorescent, mixed hexacoordinate adducts. This leads to 'co-operative' static quenching observed for imidazole and 4arnin~pyridine.'~~ The irradiation of the absorption band of the NAD (nicotinamide adenine dinucleotide)dimer analogue, 1-benzyl-1,4-dihydronicotinamidedimer, (BNAh, in acetonitrilewhich contains Fpz,[CpFe(CO)&, results in generation of 2 equiv of the cyclopentadienyliron dicarbonyl anion, [CpFe(CO)zl-, which is accompanied by the oxidation of (BNAk to yield 2 equiv of BNA+.These studies have

14: Transition Metal Complexes of Cyclopentadienyl Ligands

417

shown that the photochemical generation of [CpFe(C0)2]- by (BNAh proceeds by the photoinduced electron transfer from the triplet excited state of (BNA)2 to F ~ 2 .Following l~~ their work on rhenium complexes of dppfthe Lotz group have examined the coordination chemistry of the novel ferrocenyl ligand rac- 1,6diferrocenyl-N,N'-bis(2-hydroxypropyl)-2,5-diazahexane(H2L)which was prepared from ferrocenylcarboxaldehydeand ethylenediamine followed by the reduction of the Schiff base with LiAlH4 and subsequent N-alkylation with 1,2propyleneoxide. The dianion of H2L was observed to react with [ReO(PPh3)2C13], to give a product when treated with NH4PF6afforded the complex [ReO(LN202)PPh3]PFs.'47Continuingtheir work on electron transfer reactions involving ferrocenes the electron-transfer salts of 1,2,3,4,5-pentamethylferrocenethe Miller group have prepared a range of donor acceptor adducts and they have structurally and magneticallycharacterised two 1:1 and two 2:3Fe(C5Me5)(C5H5) electron-transfer salts of tetra~yanoethy1ene.l~~ A new route to dehydoannulenes has been described by the coupling of linear polyynes using ruthenium carbonyl. The novel dehydroannulenecomplex shown as 40 was obtained in extremely low yield (4% ) from the reaction of 1,8-bis(ferrocenyl)-1,3,5,7-octatetraynewith R u ~ ( C Oin ) ~hexane ~ solvent at reflux. This compound was characterized by a range of spectroscopic techniques. The molecule contains two Ru@0)6 groups composed of RuC(Fc)CCCFcmetallacycles formed by the coupling of the alkyne groups at both of the polyyne termini of two molecules of 1,8-bis(ferrocenyl)1,3,5,7-octatetrayne.14'

40

The synthesis and crystal structure of a cyclopentadiene-substitutedphosphaferrocenehas been reported by the Matthey group [continuing their work on phosphaferrocenes which they have pi~neered].'~ An example is shown as [41]. A macrocyclic product has been synthesized in high yield in the reaction 2,5-di(pyrazol-l-yl)hydroquinonewith 1,l'-fc[B(Me)NMe& (fc = Fe(C5H&}. This molecule incorporates two redox-active 1,l'-ferrocenyleneunits in its backbone and contains four chiral boron centres. Interestingly,it has also been shown that that the crystal structures of organometallics of moderate complexity can be solved from high-resolution X-ray powder diffraction patterns, once the connect-

Organometallic Chemistry

418

41

ivity between the functional groups is known.151The 57FeNMR spectra (11.66 MHz) of ferrocene, butylferrocene, and acetylferrocene have been measured at natural abundance of 57Feby application of the polarization transfer technique (INEPT; 1H-57Fe).It is reported that in contrast to previous assumptions this technique works very well, although it has to be based on rather small ( 0.3-0.7 Hz) long-range scalar 57Fe-'H couplings across two bonds. This should prove useful in the characterisation of numerous ferrocenes in the The reaction of 1-(a-aminoalkyl)-2-diphenylphosphinoferrocene with glyoxals has resulted in the discovery of a new class of planar chiral ferrocenes because of an unusual heterocyclization. One of the new ligands has been used in the Cucatalysed cyclopropanation of styrene by ethyl diazoacetate which exhibited complete diastereo-discrimination leading to the formation of trans-product in 100% ~ie1d.l~' The reaction of O S ~ ( C Ol(NCMe), )~ with 1,8-bis(ferrocenyl)octatetrayne, has yielded four new products: OS~(CO)~~(~~-~~-F~-C~-C=C-C Fc), O S ~ ( C O ) ~ ~ ( ~ ~ - ~ ~ - F C - C ~ X ~ - CO~SC~-(C C ~OC) ~- ~F (C~ )~,- ~ ~ - F C - C ~ - C = C d - F c ) , and OS~(CO)~~(FC-C~-C~-C-C-C=C-FC.'~~ The reaction of sodium-t -but ylcyclopentadienide with hexafluorobenzene and sodium hydride results in the formation of either the mono-pentafluorophenylsubstituted tert-butylcyclopentadienoneor the (1,2)-di-pentafluorophenyl-substituted tert-butylcyclopentadiene depending on the reaction conditions. Both these products can be further metallated and reacted with FeBr2 to afford the appropriately substituted ferrocene products. For example the rac- and mesoforms of l,l'-bis(pentafluorophenyl)-3,3'-di-tert-butylferrocene have been obtained, and the former of which has been crystallographically ~haracterised.'~~ A number of ferrocenylmethyltrialkylammoniumsalts, FcCH2NR3 where R = alkyl groups have been used as templates in order to synthesise two dimensional oxalate-bridged molecular magnets with the general formulae { [C~"'M"(OX)~] where M = Mn2+, [FcCH~NR~]},,and { [Cr"'M11(ox)3][Fc(CH3)CH2NBu3]}, Ni2+,ox = C2042-. These network compounds were characterised by circular dichroism and magnetic measurements (all are ferr~magnets).'~~ 1,l'-Bisccarbonylisothiocyanate]ferrocene has been reacted with 1,2-phenylene diamine to give both polymeric products such as -(fc-C(O)N(H)C(S)N(H)(C6H4)NC( S)N(H)C(O)-)n or cyclic Ferrocenyl imidazolide has been prepared from ferrocene carboxylic acid in a one-step reaction with N,N-dicarbonyldiimidazole in thf. The product was then converted to triferrocenylmethanol and diferrocenyl ketone by reaction with lithioferrocene. In addition ferrocenyl phenylsulfide and diferrocenoyl disulfide were prepared in this study and both were crystallo-

-

+

14: Transition MetaE Complexes of Cyclopentadienyl Ligands

419

graphically ~haracterised.'~~ A gallium bridged [l,l']-ferrocenophane shown as 42 was prepared in the first reaction of 1,l'-dilithioferrocene with [Li(THF)] [R-GaC13]-, [R = CH(SiMe3)2].'59 +

€@a

42

t53

The Knoevenagel condensation of ferrocenyl-substituted carboxaldehydes with methylene active compounds on inorganic supports has been used as an effective method of producing a range of interesting ferrocene substituted products. Also included in this report are straightforwardpreparations of some of the precursors used such as 3-ferrocenyl-acrolein. The second order non-linear optical properties of the condensed products was also reported.lmSimilarly, the condensation reactions of [Fe2(r15-Cp)&O)2(p-CO)(p-C-CH3)] [BFq]- with a series of terthienyl aldehydes yield a series of merocyanines. A ferrocene was appended to the terthienyl chain and the linear and non-linear optical properties of these products was studied? The Mannich condensation reaction catalysed by TiC14of 1,l'-diacetylferrocenewith secondary amines followed by hydrogenation of the product imines has resulted in the formation of a range of transdisubstituted [3]-ferro~enophanes.'~~ The reaction of ferrocenyl-aroylhydrazone with triethylindium has been used as a method to obtain indium complexes of ferr~cenes.'~~ Diferrocenylmercury has been used as a key precursor in palladium-catalysed coupling reactions with aryl halides, acid chlorides and electrophilic alkynes. The reaction conditions have been optimised in the iodoselective coupling reactions with bromo-iodo arenes. 164 The reaction of dilithioferroceneswith RGeC13has been used as a simple and effective method for obtaining chlorogermyl-[ 11ferrocenophanes. The electrochemistry of the product compounds has also been in~estigated.'~~ Ring closing metathesis using Grubbs catalyst has been used to produce ferrocenophanes in a simple and effective method.'66 A series of group 6 metal carbonyl complexes which contain ferrocenylpyrazoles have been synthesised by photolysis and their electrochemistry has +

420

Organometallic Chemistry

been in~estigated.'~~ The first example of a selenoaldenylidenehas been reported this molecule contains a ferrocenylselenyl-substituted ruthenium allenylidene.I6' 3-Bromo-2,4-bis(ferrocenyl)furanhas been obtained in the reaction of cpbromoacetylferrocenewith [Pd(PPh& under deprotonation condition~.'~~ The crystal structure of [Fe(Cp)(q5-C5H4CH =NNHC5H4N)].HCl has been obtained and the reaction of the free ligand with a range of metals have been r e ~ 0 r t e d . The l ~ ~ palladium-catalysed coupling of enantiopure 2,2'-diiodo- 1,l'binaphthyl with 1,l'-bis(ch1orozincio)ferrocene leads to the formation of enantiopure bi-naphthyl bridged ferrocene. This is an extremely useful preparation which when adapted to include substituted ferrocene will lead to interesting ligands.171The coordination and electrochemistry of a range of ferrocene-substituted macrocyclic dioxotetraamines has been documented and these molecules have been observed to recognise Co, Ni and Cu divalent ~ a f i 0 n s .In l ~the ~ continuing work of Henderson on ferrocene-substituted phosphoric acids the latest paper in the series relates the synthesis of the compounds Fc(CH2),-,P(0)(OH),,(n = 0, 1, 2) and the related arsenic The preparation of a range of 8 ferrocenyl oligopeptides has been reported - the synthesis makes use of the reaction of 1,l'-ferrocene bis-carboxylic acid with hydroxybenzotriazole and a series of related derivative^.'^^ Suzuki coupling has been used to prepare a series of naphthyl-substituted ferrocenes, which contain a pendant thiophenyl group on the ferrocene. Although the method is not new the strategy Chromium(I1)-imino-diacetate, towards chiral ligands is an important EDTA and 1,3-propanediamine-N,N-diacetic-N,N-dipropionte complexes have been used as reagents in the preparation of ferrocenyl ketyl radicals which have been isolated under argon in the solid state.176 1,l'-Ferrocene-dimethanol has been reacted with a range of pyrole-substituted q,o-diamines which vary in length with the number of heteroatoms between the amines to produce a series of ansa-ferrocenophanes. A correlation was then made between phosphate binding affinity and the number of heteroatoms in the diamine molecule. A solid state structure of one of these molecules (as dihydrate) where the spacer is -CH2(CH20CH2)2CH2-shown as 43 is a representative example (one water molecule is contained within the cavity and the other (not shown)is outside the The stepwise synthesis of ferrocene-labelled di- and tripeptides has been achieved beginning with ferrocenylamino acids. Key to the synthesis has been the use of the fluorenyl-methoxy-carbonyl and t-butoxycarbonyl protecting groups for the amino acids. A heterobimetallic product in which a ferrocene group is at one end of the peptide chain and a benzene-chromiumtricarbonyl is at the other end.178 A number of a-disilylferrocenylthiolshave been prepared by reduction of thioferrocenoylsilanes.Included in this work is the synthesis of new planar chiral thioferrocenylsilanes.These use Kagan's method of ortho-lithiation of chiral sulfinyl ferr0~ene.l~~ Continuing the work on S-methyl-substituted ferrocene the Long group have reported the preparation of Re(1) and Pt(I1) complexes of l,l', 2,2'-tetra(methy1thio)ferrocene. In this paper the complexes have been electrochemically characterised. The Re-complexes which were prepared make use of [Re(CO)sBr] as a precursor and the Pt complexes use [PtCi2(PhCN)2J.*80 -

14: Transition Metal Complexes of Cyclopentadienyl Ligands

421

43

After the dearth of syntheses over the past decade or so another synthesis of ferrocenyl amine has appeared following the recent report of Arnold. This method uses the reaction of ferrocenyl lithium with g-azidostyrene which on decomposition of the product salt with aqueous HCl affords the ferrocenylamine. In addition a simple and elegant synthesis of 1,l’-diisocyanoferrocene is reported in this note which is derived from 1,l’-ferrocenebis-carboxylic acid.18’Sonogashiracoupling has been used in the synthesis of bipyridine ligands containing fluorene and ferrocene. These ligands have been attached to ruthenium and rhenium centres and their luminescence properties have been determined.Is2Further work on lead complexes of ferrocenylamineshas appeared: in this work the reaction of [{ CpFe(q5-C5H3(CH2NMe2)-2}-C,N]2Pb, [FcN], has been reacted with the group 6 metal carbonyls [M(CO)sL; L = thf, NMeJ to give the heterotrimetallics [(FCN)~P~M(CO)~], M = Cr, Mo, W.183The reaction of the ferrocenophane [Fe(q5-C5H4)2SiMeH]with dicobalt octacarbonyl has been examined in which the metallation of Si occurs coupled with the ring opening of the ferrocenophane.When the reaction was conducted in the presence of an amine as a proton trap the red ferrocenophane [Fe(q5-C5H&Si[Co(CO)d] (Me)] was obtained. This was structurally characterised and is shown as 44.184

44

422

Organometallic Chemistry

Further work has been carried out on the synthesis of ferrocenes containing podand dipeptide chains. In this work the dipeptide chains -Gly-L-Pro-OEt chain has been attached to fe~r0cene.l~~ The reader is directed to related work rep5' in this chapter. Again in another work relating to a previous referen~e,'~'further work on the preparation of the ansa-ferrocene complex [1,1'-fc(Bbipy)20](PF6)2- 45 has been reported. The electrochemical investigation concludes that there is electronic communication between the two bipyridyl boron substituents.'86A new heterobimetallic pentatenyl ruthenium complex, has been prepared and tested for catalytic activity in the styrene dehydrogenation silylation of styrene.

The product complex [Cp*Ru(p-q5,q3-C8H6)Rh('r14-COD)] was prepared in the reaction of [RL(p-C1)(COD)]2with the lithium salt of [ C ~ * R U ( ~ ' - C ~ H ~ ) ] - . ' ~ ~ A range of cyclometallated palladium complexes of dppfhave been prepared for use in the construction of heteronuclear metal macrocycles. One of the macrocyclic fragments which contains two dppf units is crystallographically shown as 46.'88 Continuing their work on ferrocenyl amines the Arnold group have silylated 1,l'-diaminoferrocene to give l,l'-Fc(NHSiMe3)2 which on reaction with M(CH2Ph)4,M = Ti, Zr yields the complexes where the silylated diamine serves as a ligand to Ti and Zr. In a similar vein a Mg complex, shown as 47 has also been obtained in this Again continuing their elegant work on phosphaferrocenes, Mathey's group has reported the synthesis and characterisation of a l,l'-diphospha-[2]-ferrocenophane which was the result of a six step syn-

14: Transition Metal Complexes of Cyclopentadienyl Ligands

46

423

47

thesis." Cyclopentadienyl tethered phosphaferrocenes have been prepared and reacted with [(PPh3)3RuC12] to afford new ferrocene-containing half sandwich comple~es.~~' A convergent synthesis of a range of ferrocene dendrimers which contain vinyl ferrocenes on the extremities has been r e ~ 0 r t e d . lA~ ~synthetic method which is based on aza-Wittig chemistry of a-azidoacetyl ferrocene or its disubstituted ferroceneanalogue with a range of acid chlorides has been developed. In this work acetyl ferrocene is deprotonated then reacted with TMS chloride and NBS before treatment with a polymeric source of azide. The product is subsequently reacted with acid chlorides to give the product oxa~oles.'~~ Further work has been carried out on the reaction of dilithioferrocene with [CpFe(C0)2PPh3]Iin which the complex is substituted on the cyclopentadienyl ring of the half metallocene. A typical product [CpFe[p,$:q4-5-exo-( 1'C5H4)C5H5]Fe(C0)2PPh3] is shown as 48.It is shown that iodide substitution in FpI by PPh3 is faster than metathesis with the ferrocenyl anion which is of general i n t e r e ~ t . 'Highly ~~ soluble alkyl-substituted poly(ferroceny1ene)phenylene has been prepared and I2 oxidised. The synthesis makes use of substituted cyclopentenones to obtain the polymeric products.'95 In a special issue of the Journal of Organometallic Chemistry dedicated to carbene chemistry there are several articles dealing with cyclopentadienyl-containing carbenes, the most significant of which for the purposes of this review is a mini-review on carbenes with ferrocene sub~tituents.'~~ Further work has been carried out on the synthesis and characterisation of linear platinum alkyne complexes. The product complexes are reminiscent of those prepared by Long and co-workers reported in earlier reviews. The structures of these complexes have the form [FcC=CP~(PP~~)~-C=C-~-C~H~-~-C~H~-C= C=CFC].'~~ A range of multiply stannylated ferrocenes ranging from one to six trimethyltins on ferrocene have been prepared. These have been made from the appropriately substituted cyclopentadienes prepared by Jutzi's method. The NMR spectra of these compounds are discussed in detail in this paper.19*The

Organometallic Chemistry

424

48

insertion of bis-ferrocenylbutadiyne into an osmium hydride bond has been observed. The actual complex [Os4(C0),1(p-q2-FcCC(H)C2Fc)(p-H)3 was obtained in 90% yield in the reaction of O S ~ ( C O ) ~ ~ with (~-H Me3NO )~ followed by reaction with FCCPC-C~~C-FC.'~~ Further novel cyclopentadienes of the type CptSiH,C13.,, where Cpt = Me4EtCs,y = 1, Cpt = Me4CsH,y = 1,O; Cpf = Me3C5H2,y = 1 etc have been prepared again by the Jutzi group. These compounds are synthesised by metathesis from the correspondinglithium cyclopentadieneide with trichlorosilane or tetrachlorosilane in thf.'"O The synthesis and characterisation of the compound [Cp*Fc-dicyclopenta(9,f ) naphthaleneFcCp*] and its salt has been reported in a synthesis which sees the reaction of Cp*Fc(acetylacetonate) with dihydrodicyclopenta(9,f) naphthaleneFO' The ortho-metallated complex [(FCN)~Z~] (FcN) = (o-FcCH2NMez)has been investigated and it has been shown to crystallize solely as a (CN)-bonded rac-diasteromer, however, in solution a mixture of meso- and rac-compounds are observed which exhibit dynamic exchange properties?O2 In further work by Adams, (see also ref. 149) two new platinum-triosmium complexes of the form [Pt OS,(CO)~(COD)( b-FcC4Fc)] and [Pt20s3(C0),,(COD)(ps-FcC4Fc)]both of which have been structurally characterised have been reported.203In a special issue of the Journal of Organometallic Chemistry dedicated to Henri Brunner a report on the use of tunable ferrocene ligands which have been used in the Ir-catalysed enantioselective hydrogenation of N-aryl imines is reported. The ferrocenyl phosphines used are the standard Togni-type ligands with a range of phosphine substituents.N Also in this issue is the synthesis of 1,l'-ferrocenediyldialkoxysilaneswhich are obtained as a consequence of the ring opening of the ferro~enophanes.2~~ A range of ferrocenylquinolineshas been prepared together with their Re and Mn complexes. These have been obtained both from ferrocene by palladium-catalysed coupling and by reaction of the lithium quinolinate with substituted cyclopentenonesto give the corresponding cyclopentadienes?%Deprotonation of the conjugate acid [(FC)~CTH(FC)~] BF4- has yielded the tet+

14: Transition Metal Complexes of Cyclopentadienyl Ligands

425

raferrocenyl-C7-cumulene compound which has interesting spectroscopic properties. The evidence for this compound is based on trapping experiments. In this paper two precursor compounds [(Fc2C(OCH3)C=C)2] and [{(FchC =C(H)CiC),l have been structurally characterised - these are shown as 49 and 50 re~pectively.~~ The sulfide of the bisisopropylphosphinoferrocene ligand (dippfi has been used as a ligand to form tellurium complexes,one of which is shown as 51.208

49

51

It has been shown that ferrocenylacetylene dimerises in boiling chloroform to give a mixture of (E) and (2)-1,4-diferrocenylbutene-3-ynes, which is contrasted with the trimerisation of phenylacetyleneunder Rh catalysis.2wIn a similar vein the reaction of [OS,(CO)~~(NCM~)] with 1,8-bis(ferrocenyl)octatetrayne has given rise to four new compounds all of which exhibit unusual coordination modes of the ferrocenylalkyneligand?1° Salts of the ruthenocenylmethylium cation have been isolated in the reaction of ruthenocenylmethanol with acids.211Hayashi has reported the synthesis of new chiral phosphinophosphaferrocenes which may act as mono- or bidentate

426

OrganometaElic Chemistry

ligands. The ligand has been used in the palladium catalysed allylic alkylation of 1,3-diphenyl-2-propenylacetate and e.e’s up to 99% was achieved?12 A series of heterobimetalliccomplexes modulated by a carbonyl spacer have been prepared in which ferrocene is at one end of the chain and chromium tricarbonyl is at the other end. These complexes were prepared from acetylferroceneby condensation with aldehydes.213Two new C2symmetric 1,l’-bis(oxazoliny1)ferroceneand ruthenocenes have been prepared and their zinc complexes have been studied in phenyl to aldehyde transfer rea~tions.2~~. The complexation behaviour of 1,l’; l”,l”’-bis(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diylmethyl)bis-ferrocene has been investigated. In particular the coordination of KPF6 has been studied. The actual synthesis of the complex ligand used well established literature routes beginning with ferrocene biscarboxylic acid. Nevertheless the salt inclusion complex, which is structurally shown as 52, is fa~cinating.~’~

The step by step assembly of directly linked ferrocenes has been carried out starting with a simple pentafulvene molecule. The products which are either bior ter-ferrocenes were fully characterised by NMR spectroscopy and elect r o ~ h e m i s t r yBromination .~~~ of the inclusion complex of trans-ferrocenylchalcone derivatives with P-cyclodextrin has been studied. Interestingly, in the absence of the p-cyclodextrin no room temperature bromination is observed but its inclusion complex brominates readily to give the erythrodibromide. Again Ferrosignificantly the ferrocene itself remains untouched by the cenylketones have been catalytically added to terminal olefins using [(Ru(H),(C0)(PPh3)] catalysis. In this investigation four new ferrocenes have been prepared which are essentially derivatives of benzoylferrocene in which the ortho position of the phenyl ring has been substituted with an alkyl group. It is of general interest that the ferrocene ring itself is untouched in this Ferrocenes substituted with o-methylephedrinehave been used to direct lithiation asymmetrically to produce enantiopure 1,2-disubstituted ferr0cenes.2~~ The photochemical reaction of alkoxychromium(0) carbenes with imine-substituted ferrocenes has been used as a method to obtain ferrocenyl-2azetidinones. This synthetic method allows access to the synthesis of fi-lactams

14: Transition Metal Complexes of Cyclopentadienyl Ligands

427

with ferrocene anchored at the C-3 position?20 Samarium iodide promoted tandem addition and dehydration has been used as a general synthetic method to substituted alkenyl ferrocenes starting with acyl ferrocenes. In this method a substituted bromomethyl derivative is added in one step to the acyl ferrocene. This method should prove to be a useful alternative to traditional Wittig chemistry for such ferrocened21 An enantiopure synthesis of optically pure azaferrocene anions has been published in which chiral azaferrocenes may be obtained from the chiral anions.222Tetrapyrrole derivatives which are highly substituted with ferrocenylethynes have been prepared and characterised. The synthetic approach uses the palladium-catalysed coupling reactions of halophthalonitriles which are subsequently condensed by standard meth0ds.2~~ Again a new ferrocene-based chiral auxiliary for use in the N-alkylation of substituted indole-carboxylates has been reported.224The synthesis of a chiral aryl ferrocenyl ligands has been achieved beginning with 2-iodoferrocene carboxylic acid which was reacted with DCC and DMAP in the presence of a substituted phenol to give the ortho-iodo aryl esters which themselves were set up for intramolecular ring closure?25A series of three types of chiral ferrocenyl phosphines which incorporate chirality on the phosphorus have been used in asymmetric allylic substitution reactions. This work provides a detailed insight into the catalysis including an insight into the mechanistic pathway?26The crystal structure of the electron transfer salt [Cp*2Fe]"- DMe-DCNQIO- has been reported as an array of parallel alternating donor/acceptor stacks?*' The gas phase reaction of lanthanide and actinide cations with ferrocene leads interestingly to the production of lanthanide and actinide metallocenes [M(Cp)2] .228 A new ferrocenylphosphonodithioate with the general structure [CpFc(r15-CsH4PS20CH2c6H~N3)1has been prepared on the reaction of [(FcP(S)S)J with hydroxylmethoxybenzotriazole in the presence of triethylamine. Its rhodium and nickel complexes were subsequently prepared, the former of which was characterised by X-ray diffract i ~ nA. series ~ ~ ~of 4-ferrocenylidene-1-aminoimidazolones have been prepared which in turn were obtained beginning with ethyl-f5-ferrocenyl-a-azidoacyrates from ferrocenecarb~xaldehyde?~~ It has been observed that like ferroceneazaferrocene acts as a fluorescence-quenchingreagent. In this study a tetrasulfonated A chiral ferroaluminium phthalocyanine is used as the fluorescent cenyl oxazoline as its ortho diselenol has been used as a catalyst in asymmetric aryl transfers to aldehydes.232 An octamethylferrocenelinked to a nitrothiophene acceptor by an ethenyl link has shown to be a simple and effective NLO redox switching molecule. This compound shown as 53 was easily prepared in the Wittig reaction of the octamethylferrocenylmethyltriphenylphosphonium bromide and 5-nitro-2-thiophene ~arbaldehyde.2~~ Continuing with the materials theme the condensation of 1-ferrocenyl-1,2,2tricyanoethylene in the presence of magnesium results in the production of a tetraferrocenyl-tetraazaporphyrin with an intense broad near IR band.234A simple, yet novel, receptor for barbiturates and ureas have been prepared - these molecules are 1,3-disubstituted ferrocenes with amide linkages to a 2-pyridyl group. The synthetic method uses ferrocene-1,3-dicarbonylchlorideas an effective precursor?35The reaction of chiral tert-butylferrocenylsulfoxidefollowed by +

Organometallic Chemistry

428

53

treatment with tosylazide has been used as a general method to obtain chiral amino and sulfonamido ferrocenesfor use as ligands in the asymmetricreduction of benzaldehyde to its chiral alcoh01.2~~ Chiral oxazolinylferrocenylphosphines have been successfully used as ligands in the ruthenium-catalysed hydrosilylation of k e t o x i n e ~ . ~The ~ ~ new air stable trarylferrocene [($CsH3(SiMe3)(PPh2)Fc(Cp*)f has been used as a ligand in palladium-catalysed Suzuki-couplings of arylchlorides which give excellent yields under very mild

A ferrocene-modified bis(spiropyrid0pyran) has been used as a synthetic signalling receptor for guanine-guamine dinu~leoside.2~~ Again the kinetic resolution of secondary alcohols has been carried out semi-effectively using oxazolinylferrocenylphosphines,cJ: ref 237.240The first fused #J,fJ-metallocenylporphoryrins have been prepared in a simple yet elegant synthesis. One of these, the ruthenocenyl compound is shown as

54

The ferrocenyl Schiff base polychlorotriphenylmethyl radical, shown in the figure was prepared by the condensation of (4-amino-2,6dichlorophenyl)bis(2,4,6-trichlorophenyl)methylradical and ferrocene carboxaldehyde. Typically the condensation reaction was not stereoselective and the

14: Transition Metal Complexes of Cyclopentadienyl Ligands

429

trans and cis isomers of compound were both formed and both could be isolated and fully characterised. The cis-trans isomerisation of these compounds leads to the simple design for a switchable molecular magnetic because trans-isomer exists in solution as a monomeric species, whereas the cis-isomer aggregates in solution through the formation of hydrogen bonds to give a thermodynamically stabilized diradical dimer with strong antiferromagnetic interaction^?^^ Chiral ferrocene ligand in palladium-catalysed asymmetric alkylation of ketone enolates have also been ~ r e p a r e d . 2 ~ ~ An intriguing molecular carousel, [{ Fe(CsH4)2}3{Ga(CsHsN)}~],which is shown as 55 has been obtained from 1,l’-bis(trichlorostanny1)ferrocenewhich is reacted stepwise with excess trimethylgallium to give the hitherto unreported air sensitive orange compound 1,l’-bis(dimethylgal1yl)ferrocene which in turn was then heated in pyridine and toluene to give the product 244. The regioselective tetrametallation of ferrocene has been achieved in a single reaction; essentially the synthetic methodology uses nBuNA BnMg and iPrzNH using ultrasonic conditions. The result is the 1, l’, 3, Y-tetraani0n.2~~ The first [4, 41 ferrocenophane -1,3, 15, 17 tetrayne has been reported the synthesis uses Eglinton coupling. The buckled product molecule is shown as 56.246

55

In a redox kinetic study into alkane thiolate monolayers on gold electrodes it has been observed that when ferrocene is embedded within the monolayer the oxidation/reduction potentials are strongly shifted towards positive potentials relative to those where the ferrocene is exposed to the electrolyte solution. The results obtained conclude that electron transfer must occur concommitantly within ion tran~fer.2~’ Sonogashira coupling of diiodoferrocene with 2,6-dia-

430

Organometallic Chemistry

56

mido-4-ethynylpyridines results in the formation of the appropriately substituted ferrocene which is used as a receptor in homogeneous solution by the use of a lysophelic thymidyl-yl(3' - 5') rhymidene analogue. The seclective bonding was studied using detailed N.M.R. and UV-visible st~dies.2~' In a general review on transition metal catalysis using functionalised dendrimers there are a significant number of dendrimers which are terminated with ferrocene. This area of research is set to expand rapidly as the area of catalysis de~elops.2~~ Further work has been carried out on planar chiral ferrocene-fused pyridine-N-oxides which also contain an electron rich cyclopentadierylring. These ligands have been used in the desymmetrising ring opening of epoxides with chlorosilanes which is observed to occur in enantiomeric excurses of 91-98%?50 Further work has been carried out on ferrocenes which bear podand dipeptide chains. These have been characterised using a range of spectroscopes including 'H N.M.R., F.T.I.R., C.D. and single crystal analysis. As an example it has been shown that the ferrocene bearing (-DAla-D-Pro-OE) exhibits C2 symmetric intramolecular hydrogen bonding between the CO in the alanine residue and the NH of another alanine. Similar results were found for the other dipeptide contains ferrocenes. This is best displayed using a crystal packing model such as that shown for molecule 5. (dipeptide- GLy-L-Phe-) shown as 57.25' Light harvesting and photocurrent generation by gold electrodes has also made use of ferrocenes in a ferrocene-porplyrin-fullerene In the initial report on the work, the planar chiral ferrocenes with N-oxides has been described by Fu and coworkers.This shows preliminary arylation Fluorescence methods have been used to screen a range of ferrocene ligand based [(q5-C5Ph5)FePBu2] in palladium catalysed Heck coupling The synthesis of a further range of cyclopalladated compounds with the general formula [Pd{(q5-C5H3)-CH=N-CH2-C6H5}]Fe(Cp)(X)(PPh3)]has been reported and the influences of the monoanionic ligands (Cl-, Br-, I-, CN-, SCNor AcO-; X) on the complex has been The preparation and characterization of ruthenium(I1) monophosphaferrocene complexes,the reactivity,dynamic solution behavior, and X-ray structure of [RuH~(q2-H2)(PCy~)2(2-phenyl-3,4-dimethylphosphaferrocene)] have been re+

14: Transition Metal Complexes of Cyclopentadienyl Ligands

43 1

57

p0rted.2~~ A range of cationic bimetallic compounds of the types (E)-[CpFe(q5C5H4-CH=CH-C6H4-CN-RuCp(PPhs)2]X,X = PF4, BF4 and [CpFe(q5-C5H4)CH-C6H4-CN-FeCp(C0)2]PF6 have been studied in terms of their NLO properties. The design of these molecules is a simple one. They are obtained simply by using the ferrocenylethenyl-benzonitrile-pas a ligand to iron or ruthenium ~entres.2~~ The controversy regarding the nature of the magnetic coupling in the charge transfer salt [FeCp2*]'+ [TCNE]'- has been revisited with a detailed series of experiments and Ab initio calculations. It is concluded that the signs of the spin populations are consistent with the Cornnell I mechanism but the numerical calculations perfomed on the basis of the spin population values yielded intrachain exchange values which were lower than those experimentally determined.258 A range of palladium and platinum complexes of dppf and dippf which contain dithiolate ligands, five of which have been crystallographically characterised, have been examined electrochemicallyand the data has been correlated.259 It has been observed that the oxidation of decamethylferrocene by hexacyanoferrate results in the production of the salt. [Fe"*Cpf~]~[Fe11'(CN)6].260 In a paper examining the use of chiral ferrocenyl oxazolinyl ligands in the enantioselective Pd catalysed allylic substitution reactions attention has been focused on the importance of the planar chirality. While this has been well known to be the important chiral feature of ferrocene ligands in asymmetric

432

Organometallic Chemistry

processes it serves as a timely reminder when ligand design is under consideration. A number of single crystal structures are included in this paper to demonstrate the role of the chiral plane in these ligands.261The stable isomers of ferrocenyl lithium gas phase ion complexes have been investigated using hybrid density functional theory and the outcome of the calculations predict that there are two isomers of the ferrocenyl lithium cation separated by a barrier of 25.6kcal mot'. Interestingly,the most stable isomer has the lithium cation on top of one of the cyclopentadieryl rings and the least stable isomer binds to the central iron. The prediction is that the lithium in the latter structure can orbit the central iron. In the case of protonated ferrocenes there are two stable structures the first of which is a two electron, three centre hydrogen - bonded species (shown in the figure as 1 taken from the reference) and the second is the metal protonated ferrocene (less stable) which is shown as structure 2 in the

2

1

1.38

Figure 1

In a paper entitled ferrocenes based nanoelectrics an asymmetric pyridyl bis alkyne has been used as a linker between two ferrocenes and is proposed as a model towards reversible molecular swi~thes.*~~ An organometallic thermo-optical switch has been reported based on the complex [Me2C(q5-C5H4)2R~2(CO)4J. This photochromic complex switches or irradiation at h> 400nm between the a

14: Transition Metal Complexes of Cyclopentudienyl Ligands

433

yellow-orange isomer where here is a metal metal bond and the other isomer which is colourless, one of the ruthenium is bound to one cyclopentaienylring in a $-fashion and the other cyclopentadienyl ring in a q1- mode. The second ruthenium exists as a cyclopentadienyl dicarbonyl hydride. Both isomers were structurally characterised?" 8

Cobalt, Rhodium and Iridium

The reaction of the chiral phosphines (S)-Ph2PCH2CHMeCH20H with [($Cp*RhCl}2(p-C1)2) results in the production of the complex [Cp*RhC12(q1PPh2CH2CHMeCH20HP)]which in turn has been used to prepare metal centred chiral compoundsof Rh? Stereochemicallypure complexes of Co, Rh, Ir and Mo which contain chiral menthyl-substituted indenyl ligands have been prepared and characterised. These complexes are obtained in the simple metathesis reactions of the chiral lithium indenyl salts with, for example, the Rh or Ir cyclooctadienylchloride dimers? Cobaltocene has been used as a reductant in the preparation of the blue radical anion derived from (C&)3B.267 A range of chiral indenyl ligands with pendant side chain phosphates have been prepared by the direct reaction of the lithium salt of the chiral ligand and [Rh(C0)2C1]2.268 The X-ray characterisation of ethynylcobaltocenium hexafluorophosphate has been achieved - this molecule was prepared according to the published method (J.O.M.C. 1990)reported a decade earlier by the same group. The preparation of [Cp*Ir(PMe3)S4]is achieved using the thiol precursor [Cp*Ir(PMe3)(SH)2]and SO2 in the presence of trace ~ a t e r . 2A~ ~ novel cage compound [CpCo(q4C4H4CHCHP6C4Buf4) is formed in the reaction between cobaltocene and the triphosphole P3C2B~'2CH(SiMe3)2. This compound has been crystallographically characterised and is shown as 59.270

59

434

Organometallic Chemistry

The self assembly of CpCoCb-derived ligand scaffold has been observed. (Cb =cyclooctadienyl). In this synthesis [CpCo(CO)2] is treated with bis(3-pyridtyl acetylene to give the pyridtyl substituted cyclobutenyl cyclopentadieryl cobalt which is then treated with [enPd(N03)] to obtain the barrel c0mplex.2~'It is well known that the cation complex [Cp+(PMe3)IrMe(C1CH2Cl)] [BAr4]-, Ar = e.g. 3,5-C6H3(CF3)2or C6F5activates C-H bonds even at low temperatures. It has been observed that the addition of hydrogen (at ca-84°C)to the complex immediately results in the methane expulsion with the formation of the iridium hydride complex which is thermally unstable. The latter hydride complex reacts at low temperature with a range of reagents such as ethane, ethanal and benzaldehyde, the latter giving the alkyl-carbonyl cationic products. These initial results prompted the authors to examine the decomposition of the hydride in the presence of d12-cyclohexane which resulted in the formation of [Cp*(PMe3)IrD3], which showed that the alkane substrates are the hydrogen source in the activation reactions. These results are important to all organometallicchemistry and demonstrate the ability of careful mechanistic investigation^.^^^ Cobaltocenium functionalised polypropylene imine dendrimers have been prepared and their redox chemistry has been investigated. It has been observed that on reduction the dendrimers have a tendancy to elcectrodeposit on the electrode surface. Consistent thin films could be thus obtained which exhibited well defined electrochemicalresponses.273 +

9

Nickel Palladium and Platinum

The novel complex [Pt{q4-C5Me4(CF3)H}C12] has been obtained by the reaction of Zeise's salt dimer with 1,2,3,4-tetramethyl-5-(trifluoromethyl)cyclopentadiene. In addition it has been observed that the reaction of C5Me4(CF3)H with K2PtC14 The molecular structure of which is then catalysed gives [Pt(q4-C5Me4H2)C12]. the PF4 salt of [Pt{q4-C5Me4(CF3)H}Cp]PF6-has also been reported in this Associative ligand exchange has been observed in the complex [1-Meindenyl)(PPh3)NiCl]in the presence of PCy3to give [( 1-Me-indenyl)(PCy3)NiC1] - a bisphosphine intermediate is proposed.275 The reaction of NiC12(PPh3)2 with the lithium salts Liz[CMe2(C5H4)2],Li2[CMe2(C9H6)2]and Li2[(C5H4)CMe2(CgHs)] has been found to be an efficient route to ring coupled cyclopentadienyl and indenyl nickel complexes.276The reaction of [Ni2Cp2(p-C0)2]with alkyl gallium(1) and alkylindium(1)compounds of the type [E4(C(SiMe3)3]4,E = Ga, In, results in the insertion of a [EC(SiMe3)] moiety into the nickel-nickel bond or by replacement of the two carbonyl ligands. A representative structure of each of these products are shown as 60 and 61 re~pectively.?~~ Finally the reaction of a tetamethylpentafulvalene dianion with Cp*Ni(AcAc) has resulted in the formation of asymmetric tetradecamethylnickolocenes.278

14: Transition Metal Complexes of Cyclopentadienyl Ligands

435

3

c7

60

References 1.

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